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1. Influence of organic thiols on the work function of coinage metals

1.1. Work Function of Metals – Physical Description

Let us consider an electron at rest in a vacuum at an infinite distance from a metal surface, as depicted in Figure 1a. The total energy, E_tot, of such an electron is equal to its potential energy, which can be defined after Cahen and Kahn as the vacuum level at infinity, E_vac(∞).¹ Let us supply the electron with some kinetic energy, E_k, and allow it to travel towards the surface. The total energy of the electron outside the metal can be expressed as the sum of its potential energy, E_V, and kinetic energy, E_k:

E_tot = E_V + E_k Equation 1

As the distance between the electron and the metal surface decreases the potential energy of the electron will increase, thus slowing it down, according to Equation 1 (see Figure 1b). If the kinetic energy is sufficiently large to overcome the potential barrier at the metal / vacuum interface the electron will go over the barrier and then will rapidly lose its potential energy due to the interaction with the positively charged ion lattice within the metal, resulting in the increase of kinetic energy. Once the electron is within the metal Equation 1 no longer holds as the interactions with other electrons and the ion lattice result in energy dissipation and thermal equilibration. The electron is now trapped within the metal.

A similar thought experiment can be performed in the opposite direction. Let us choose an electron from one of the highest occupied energy levels at the particular temperature of the system. The potential energy of this electron is approximately at the Fermi level, E_F.<a href="#_ftn1" name="_ftnref1" title="">[a]</a>^,^2,3 The electron is then supplied with some kinetic energy, E'_k, sufficiently large to overcome the potential barrier existing at the metal surface. Right after escaping the metal, the greatly slowed-down electron has a potential energy which can be defined as the vacuum level close to the surface, E_vac(s).¹ The drop in the kinetic energy of the electron caused by the potential energy barrier is defined as the work function of the metal surface, F_m: ^1,2

F_m = E_vac(s) - E_F Equation 2

After escaping from the metal the electron is moving away further from the surface experiencing acceleration as the potential drops to the value of E_vac(∞).

<a name="OLE_LINK2"></a><a name="OLE_LINK1">Figure 1.</a> a) Potential energy of an electron in vacuum at an infinite distance from a metal surface, E_vac(∞), and the Fermi level of the metal, E_F. b) Potential landscape the electron experiences along a travel path towards the metal surface (thick line). The difference between the electron energy just outside the metal surface, E_vac(s), and E_F defines the work function of the metal surface, Φ_m = E_vac(s) - E_F.

It is important to describe the physical origin of the potential landscape illustrated in the above thought experiment. As already mentioned, the sharp potential drop the electron experiences when it enters the metal is related to the electrostatic attraction force between the negative charge of the electron and the positively charged ion lattice of the metal. However, at the vacuum / metal interface the electron experiences a potential energy barrier. This implies that in that region of space there must be a repulsive force that acts upon the electron. Indeed, the lack of positively charged metal ions on the vacuum side of the aforementioned interface causes a negative charge to exist just outside of the metal and, conversely, an uncompensated positive charge within the metal. The spilling of the electronic density outside of the solid causes a formation of a sheet of dipoles, which are often referred to as the surface dipoles.^1-3

1.2. Work Function of Metallic Surfaces – Methods of Measurement

The work function of solid surfaces can be determined experimentally using absolute or relative approaches. Absolute methods allow one to measure the work function value directly. Here, the electrons in the metal are supplied with sufficient kinetic energy to overcome the barrier at the metal / vacuum interface, and can thus escape the metal, and the work function can be obtained from the resulting electric current. Absolute methods include measurements based on thermionic emission, field emission, and the photoelectric effect. Briefly, in the thermionic emission method electrons are ejected from the material after receiving sufficient thermal energy to overcome the energy barrier at the metal / vacuum interface. The appropriate thermal energy is supplied by incremental heating of the sample to temperatures at which the Fermi-Dirac distribution of the electrons in the metal allows for substantial population of electrons at energies higher than the interface energy barrier. The resulting electric current is measured as a function of temperature; this allows one to extract the work function of the surface. The temperature range used in the thermionic emission method is often very high (thousands of Kelvins), making this method of limited value for studying materials and surfaces which are unstable at high temperatures.⁴ The field-emission method utilizes an electric field to accelerate the electrons inside of the metal to kinetic energies sufficiently high to overcome the interface barrier. The resulting electric current is analyzed as a function of the applied field and the work function is calculated.^4,5

Photoelectric-effect-based methods use light, typically in the UV range, as the source of energy for the electrons. As in other absolute methods, the resulting electric current (here called photocurrent) is analyzed. Figure 2 shows the energetics of a photoelectric-effect-based measurement of the work function. In this experiment the electrons are provided with a known energy, hν (red arrows in Figure 2). Electrons with sufficient kinetic energy to overcome the barrier at the interface are able to escape the metal – these are represented with the blue box in Figure 2. These photoelectrons then travel away from the metal surface experiencing the potential depicted with the thick black line in Figure 2. As the electrons move further away from the surface their kinetic energy increases, according to Equation 1. The generated photocurrent is then measured as a function of the photoelectron kinetic energy. Two important features are present in a typical plot of photocurrent – kinetic energy. First, a sharp onset at low photoelectron kinetic energy, E_min is present. As already mentioned, this onset defines the lowest energy electrons able to overcome the work function of the surface.

Figure 2. Energetics of electrons in a photoelectric-effect-based measurement of the work function. The photon energy, hν, is shown as a red arrow. After ejection from the metal the electrons experience the potential shown with the thick black line. The kinetic energies of two photoelectrons originating from different energy levels in the metal are shown with thick green lines.<a href="#_ftn2" name="_ftnref2" title="">[b]</a>

The second feature is the high kinetic energy onset of the photocurrent, E_max, and it is a manifestation of the electron population around the Fermi level of the metal; i.e., since there is an abrupt decrease in the electron population above the Fermi level, there are essentially no photoelectrons with E_tot > E_F + hν right after ejection from the surface. Using the definition of the work function in Equation 2 (see also Figure 1b), it follows that Φ_m= E_min + hν - E_max.<a href="#_ftn3" name="_ftnref3" title="">[c]</a> Thus, the work function can be obtained from the onsets of the photocurrent as a function of the kinetic energy of the photoelectrons.^1,2

Relative methods employ a reference made of a material with a known work function and focus on measuring differences in electrical quantities between the studied material and the reference. These methods include diode methods and condenser methods (Kelvin probe) and will not be discussed further.⁵

Table 1 shows experimentally measured work-function values for a variety of metallic surfaces. The presented data show a variation of the work function in the range of 2.7 – 5.65 eV, revealing that the chemical composition of the surface has a large effect on the work function value.

Table 1. Experimentally measured values of work function for different metals.*

Element	Φ_m [eV]	Element	Φ_m [eV]	Element	Φ_m [eV]
Sc	3.5 ± 0.15	Ni	5.15 ± 0.1	Ag	4.0 ± 0.15
Ti	4.3 ± 0.1	Cu	4.65 ± 0.05	La	3.5 ± 0.2
V	4.3 ± 0.1	Y	3.1 ± 0.15	Ce	2.9 ± 0.2
Cr	4.5 ± 0.15	Zr	4.05 ± 0.1	Sm	2.7 ± 0.3
Mn	4.1 ± 0.2	Nb	4.3 ± 0.15	Gd	3.1 ± 0.15
Fe	4.5 ± 0.15	Mo	4.6 ± 0.15	Pt	5.65 ± 0.1
Co	5.0 ± 0.1	Pd	5.55 ± 0.1	Au	5.1 ± 0.1

^* From photoelectric-effect-based measurements. Values taken from ref. 6.

1.3. Work Function of Metals – Implications for Organic Electronics Applications

The presence of the surface dipole on the metal surface has important implications for electronics applications. Charge-carrier injection from a metal electrode to an active layer in a variety of optoelectronic devices is considered to be one of the crucial processes essential to the overall device performance.^2,3 The energetics of the charge-injection process are defined by the relative positions of the Fermi level of the metal and the accessible energy levels of the organic active layer. This is depicted in Figure 3 for the case of a hypothetical simplified organic electroluminescent (EL) device.² Consider the right-hand side of the energy diagram. In order for an electron to undergo a transfer from the metal cathode to the organic layer it must be supplied with sufficient energy to overcome the barrier existing at the interface. The electron-injection barrier, Δ_e, is defined as the energy difference between the work function of the metal and the solid-state electron affinity, EA, of the organic layer: Δ_e = Φ_m - EA.<a href="#_ftn4" name="_ftnref4" title="">[d]</a> In the case of an EL device the energy needed to overcome the electron-injection barrier is supplied by applying a voltage between the electrodes. From the technological perspective the applied voltage must satisfy certain requirements, for example fall in a range that minimizes the power loss of the device, and allows for integration of the device into a standardized circuit. Thus, matching the solid-state electron affinity and solid-state ionization potential of the organic layers with the work functions of the metal electrodes is the key to obtaining a suitable charge injection during device operation.

Figure 3. Energy diagram of an organic electroluminescent device. The charge carriers – electrons (e-) and holes (h+) – are injected respectively from the metal cathode (right-hand side of the diagram) and the anode (left-hand side of the diagram) into the organic active layers – the electron-transport layer (ETL) and the hole-transport layer (HTL). At the ETL / HTL interface the charge carriers recombine generating photons. The Fermi levels of both the metal cathode and the anode and the energy levels of the organic layers (HOMO and LUMO energy levels) have to be matched in order to achieve an efficient charge injection into the organic layers. Based on ref. 2.

The EL device described above is only one of the examples in which the metal electrode work function plays a crucial role in the overall device performance. Matching the work function of the electrodes with the organic-layer energy levels in order to balance the charge-carrier-injection barriers is very important in organic field-effect transistors (OFETs), photovoltaic devices, or organic light-emitting transistors (OLETs).^3,7,8 Traditionally this has been addressed by choosing metals with low work function for the cathode, high-work-function materials for the anode, and matching the organic layer energy levels appropriately.³ Lately however, there has been a substantial effort to employ metal electrodes whose work function has been tuned by adsorbates.^9,10 This approach is, in principle, more versatile than the use of different, often reactive metals, as it opens the possibility of using relatively chemically stable metals (such as coinage metals) with a layer of work-function-tuning adsorbate as electrodes in organic electronic applications.

1.4. Work Function of Metals – Influence of Adsorbates

The work function of a metal is very sensitive to the presence of any adsorbates present on the surface. Experiments have revealed that even physisorbed atoms of inert gases such as argon or xenon influence the work function on a variety of metallic substrates.^11-13 Even though there is no significant charge redistribution in the inert-gas atoms near the metal surface, due to a chemical reaction, there is a substantial charge redistribution on the surface after the physisorption has taken place, which changes the surface dipole on the surface. Two major physical phenomena are responsible for this. First, the electronic density extending outside of the metal surface is pushed back by the repulsive force of the electronic cloud of the inert gas atoms; this is referred to as Pauli push-back, or sometimes as the “pillow effect”.^3,14 Second, van der Waals interactions between the inert-gas atom and the metal surface cause polarization of the otherwise highly symmetric electronic cloud of the inert-gas atom.<a href="#_ftn5" name="_ftnref5" title="">[e]</a> This results in the formation of a sheet of dipoles in the space occupied by the adsorbed atoms which alters the potential landscape at the vacuum / metal interface.^12,13

On the basis of simple electrostatic considerations one can calculate the dipole-moment surface density corresponding to the measured change of the work function.³ The work function change, ΔΦ_m, caused by a sheet of dipoles residing on the surface is expressed by the Helmholtz equation:^3,15

<img width=107 height=54 src="for%20STC%20Wiki_SAMs_files/image004.png"> Equation 3

where e is the elementary charge, μ is the dipole moment in the direction of the surface normal, <img width=18 height=18 src="for%20STC%20Wiki_SAMs_files/image005.png"> is the area of the metal surface, ε is the dielectric constant of the dipole layer, and ε₀ is the vacuum permittivity. Table 2 shows experimentally measured values of the work-function changes for a series of inert-gas / metal-surface systems together with the dipole-moment surface density induced by the inert-gas atoms calculated according to Equation 3.¹³ It can be seen that the adsorption of inert-gas atoms can lower the work function of coinage-metal surfaces by as much as 0.62 eV, which in the case of Cu(111) surface corresponds to ca. 13% drop in the work function upon adsorption of xenon. The dipole-moment surface densities calculated according to Equation 3 from the measured work-function changes in Table 2 are on the order of 1.5 D / nm². This corresponds roughly to a separation of a whole elementary charge by 1 Å in each 3 nm² of the surface.

The changes in the work function due physisorption of inert-gas atoms are interesting from the perspective of fundamental understanding of the electronic processes at the surfaces. However, inert gases do not form robust layers upon adsorption and they do not allow for much control of the dipole moment present on the surface after adsorption. The possibility of such control via synthetic design was opened with the development of the field of self-assembled monolayers (SAMs) of organic thiols on metals.

Table 2. Experimentally measured changes in the work function, ΔΦ_m, of coinage metals upon adsorption of inert gases.* Values adapted from ref. 13. The dipole-moment surface densities calculated according to Equation 3 are given in the third column.

System	rΦ_m [eV]	*μ / A* [D/nm²]
Kr on Au(111)	-0.42	1.1
Xe on Au(111)	-0.53	1.4
Kr on Ag(111)	-0.46	1.2
Xe on Ag(111)	-0.59	1.6
Kr on Cu(111)	-0.49	1.3
Xe on Cu(111)	-0.62	1.6

^* The change in the work function, <a name="OLE_LINK7"></a><a name="OLE_LINK6">ΔΦ_m,</a> is defined as the difference in the work function of the clean substrate and the work function of the surface after the adsorption of inert gas atoms.

1.5. Self-Assembled Monolayers of Organic Thiols on Metals

Self-Assembled Monolayers of organic thiols on metals have received considerable attention over the last two decades.^16-19 From a purely scientific standpoint these structurally ordered systems offer a great opportunity for studying structure–property relationships of organic-inorganic interfaces. Studies of the influence of the molecular structure on the packing of thiols on noble-metal surfaces have led to important insights into the molecular scale morphology of the monolayers. It is now understood that the thiol group binds to gold and often long-range order is observed in the resulting organic adlayer.^17-23 This understanding has further led to studies of the influence of synthetically accessible adsorbate structures on a variety of surface characteristics including wetting properties,^20,24 electronic characteristics,^25-27 and chemical reactivity.^28-30 Figure 4 shows a schematic of a structure of an organic thiolate SAM on a metal surface, together with design motifs that can be used to tune the properties of the surface.

Figure 4. Schematic diagram of a SAM of organic thiolates supported on a metal surface. The anatomy and characteristics of the SAM are highlighted. Based on ref. 19.

As discussed earlier, the work function of metals is affected by adsorbates present on the surface. The knowledge of adsorption characteristics of organic thiols on metals and the synthetic accessibility of a variety of structures makes these SAMs excellent systems to be employed in the systematic study of the influence of adsorbates on the work function of the metallic substrates.

1.6. Self-Assembled Monolayers of Alkanethiols and their Influence on the Work Function of Metals

The first study addressing the effects of organic thiol SAMs on the work function of gold was published by Evans and Ulman.³¹ The authors performed ellipsometry and Kelvin-probe measurements on a series of alkanethiol SAMs with varying alkyl spacer lengths on a gold surface. Ellipsometry revealed that the thickness of the monolayers systematically increased with the alkyl chain length, which supported the formation of dense thiolate-bound monolayers on gold. The studied alkanethiol SAMs showed a linear decrease of the work function of the metal with the number of methylene units in the alkyl chain length on the monolayer constituent, with a slope of -9.3 meV / methylene group. This has been interpreted using a model involving a dipole layer residing on top of the metal, as depicted in Figure 5. The net dipole moment of the organic SAMs was found to have the positive end at the organic / air interface, thus effectively decreasing the work function as compared to clean gold (see Equation 3). The addition of methylene units in the alkyl chain increased the magnitude of the dipole moment showing the abovementioned trend in the work-function change with alkyl chain length. Extrapolating the measured changes of the work function to a hypothetical monolayer without methylene groups revealed that Au – S layer decreases the work function of the substrate by as much as ca. 0.5 eV, which is qualitatively consistent with charge rearrangement due to the formation of a chemical bond. Additionally, the dielectric constant of the alkyl chain layer, ε₂in Figure 5, was suggested to change with the alkyl chain length thus highlighting that both the SAM dipole moment and the dielectric constant influence the metal work function, the latter parameter because it effectively depolarizes the dipole layer. The decrease of the gold work function caused by the alkanethiols studied by Evans and Ulman was as large as 0.70 eV. However, it should be stressed that the method applied to measure the work function did not take into account any adsorbates that might have been present on the reference (in this case a “clean” gold surface); thus, the absolute value of the work function depression with respect to truly clean gold surface was most likely underestimated.

Figure 5. Schematic diagram of an alkanethiol SAM on gold. The organic adlayer can be envisaged as two layers of dipoles with dipole moments μ₁ and μ₂ and the corresponding dielectric constants ε₁ and ε₂. The net dipole moment, μ_net is also shown. Based on ref. 31

Further investigations of alkyl thiol monolayers containing a polar aromatic group showed that both the direction and the magnitude of the dipole moment of the molecules forming the monolayer are important factors determining the work function of the underlying metal. Evans et al. used Kelvin probe measurements to show that the structure of the organic adsorbate had a critical effect on the magnitude and the sign of the work-function change.³² In particular, the substitution of the terminal alkyl chain in one of the studied SAMs to a fluoroalkyl chain resulted in a net dipole moment of opposite direction to that of the SAM with the terminal alkyl chain (see Figure 6). In effect, while the terminal-alkyl-chain SAM causes a depression of the work function of gold by ca. 0.45 eV, the SAM with terminal fluoroalkyl chain increases the work function by as much as 0.75 eV. Thus, the authors clearly showed that the structure of the organic SAM, and in particular the effective net dipole moment of the organic structure, has a dramatic effect on the work-function change of the underlying metal.

Figure 6. Schematic diagram of gold coated with alkanethiol SAMs containing polar aromatic groups studied by Evans et al. in ref. 32. The differences in the structures of SAMs are highlighted as well as the direction of the effective dipole moments of the organic adlayers.

Lu et al. successfully used Kelvin-probe force microscopy (KPFM) to image a gold surface patterned with a mixed alkylthiolate monolayer.³³ Alkylthiol terminated with a methyl group and another alkylthiol terminated with a carboxylic acid group were patterned via the microcontact printing technique.¹⁹ The observed contrast in the KPFM images was an effect of the difference in the work function of the gold-surface regions coated with the two adsorbates possessing different dipole moments. In the case of the two different alkyl thiolates studied by Lü the contrast was as large as 0.4 eV. Furthermore, a gold surface patterned with methyl-group-terminated alkylthiols containing different numbers of methylene units showed a trend in the measured surface potential, corresponding to a linear decrease of the work function with the slope of ca. -14 meV / methylene group, a behavior qualitatively similar to that reported by Evans.³¹

In an elegant and comprehensive study Alloway and coworkers investigated the influence of alkanethiol and partially-fluorinated-alkanethiol SAMs on the work function of gold.³⁴ In contrast to the reports Evans and Lu, the work of Alloway et al. was based on ultraviolet photoelectron spectroscopy (UPS), which is an absolute method performed under ultrahigh vacuum conditions, and thus it is anticipated to reflect the true values of the work function changes caused by thiolate SAMs on gold substrates.<a href="#_ftn6" name="_ftnref6" title="">[f]</a> All of the samples of alkylthiolate SAMs on gold showed a work function more than 1 eV lower than the work function of clean gold. Similarly to previously mentioned reports,^31,33 the authors showed that a change in the number of methylene units results in a change in the work function. Fitting the data points with a linear function yielded the slope of -19 meV / methylene unit, an absolute value larger than the values reported by both Evans et al. and Lu et al. Partially fluorinated alkyl thiolate SAMs showed an increase in the work function of the underlying substrate by as much as ca. 0.5 eV, consistent with the different direction of the dipole moment of the surface-attached molecules in the case of fluorinated and non-fluorinated chains. Analysis of the measured changes in the work function of gold as a function of the calculated projection of the molecular dipole moment onto the surface normal in the alkyl- and perfluoroalkylthiolate SAMs revealed that the Au – sulfur interaction depresses the work function by ca. 0.5 eV, which is the same value as that found by Evans et al. for similar systems.³¹ An interesting behavior was observed in a series of SAMs formed with alkylthiolates of different length terminated with a trifluoromethyl group. Figure 7 shows a schematic representation of these systems.

Figure 7. Schematic diagram of SAMs on gold studied by Alloway et al. The projections of the dipole moment of polar trifluoromethyl group onto the surface normal (dashed line) are shown by the vertical arrows. Based on ref. 34.

Due to the different orientation of the polar CF₃ groups with respect to the surface normal in odd- and even-numbered alkylthiolate SAMs, the magnitude of the effective-dipole-moment projection onto the surface normal, shown in Figure 7 with blue and red arrows, is different and manifests itself with an odd-even effect on the measured work function of the underlying gold substrate.

De Boer et al. studied the effects of alkylthiolate and perfluoroalkylthiolate SAMs on the work function of gold and silver.¹⁰ Using the Kelvin probe technique the researchers found qualitatively similar changes of the work function, caused by the studied SAMs, for both metals. In particular, the perfluorinated alkylthiolate SAMs increased the work function by 0.6 eV and 1.1 eV for gold and silver, respectively. The alkylthiolate SAMs, on the other hand, decreased the work function for both metals respectively by 0.8 eV, and 0.6 eV. Again, the changes in the work function caused by the organic monolayers were rationalized in terms of the SAM dipole layer and the resulting surface potential difference for different molecular structures. De Boer et al. took one step further and demonstrated the effect of the SAMs on the performance of organic diodes employing silver electrodes. These devices were built as models for organic light emitting diodes (OLEDs) based on the use of a semitransparent silver anode. The ionization potential of the active organic layer used in these OLEDs – poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) – is not matched with the untreated anode’s work function, resulting in a hole injection barrier of ca. 0.9 eV, which is a rather large value requiring operation of the device at high voltage. The devices described by the authors were prepared in a few configurations: a) Ag anode / MEH-PPV / Ag, b) Ag anode / perfluoroalkylthiolate SAM / MEH-PPV / Ag, and c) Ag anode / alkylthiolate SAM / MEH-PPV / Ag. Figure 8 shows the energy diagram of the Ag anode / organic layer interface for configurations a) and b). Due to the dipole layer of the perfluoroalkylthiolate SAM the work function of the silver anode surface was increased by 1.1 eV, thus effectively eliminating the hole-injection barrier, Δ_h, from the electrode to the organic layer and forming an ohmic contact. Indeed, the current – voltage characteristics for the investigated devices showed ohmic behavior in the device incorporating perfluoroalkylthiolate SAM, in contrast to the device without the SAM on top of the silver anode, which showed an onset voltage for the conduction. Interestingly, the presence of alkylthiolate SAM on the silver anode (configuration c) of the device) increased the onset voltage of the diode even further when compared to the device without any SAM. This was rationalized in terms of the reduction of the work function of silver by the alkylthiolate SAM, which resulted in the increase of the hole-injection barrier with respect to case a).

Figure 8. Schematic diagram of a) Ag anode / MEH-PPV interface and b) Ag anode / perfluoroalkylthiol SAM / MEH-PPV interface. Vacuum levels close to the surface, E_vac(s), SAM induced change of the work function, ΔΦ_m, and hole-injection barriers, Δ_h, for both devices are shown. Based on ref. 10.

In summary, alkylthiolate-based monolayers on coinage metals have been shown to reproducibly modify the work function of the underlying substrates. Additionally, the molecular structure of constituents of SAMs, together with simple electrostatic considerations, has been shown to be useful in terms of designing surfaces with a desired work-function change. It is important to stress that the modification of the work function of metal electrodes with alkylthiolate SAMs has been demonstrated independently by different research groups, and that the ability to tune the work function has been successfully applied to solve specific technological problems.^9,10

1.7. Self-Assembled Monolayers of Conjugated Thiols and their Influence on the Work Function of Metals

A limitation on the use of monolayers consisting of long-chain alkyl thiols for modifying injection barriers between metals and organics is the intrinsic electrically insulating characteristics of the alkyl chains. Monolayers consisting of π-conjugated thiols, which have been shown to exhibit considerably enhanced conductivity,^26,35 may be more promising for this type of application. Monolayers composed of π-conjugated systems based on oligo(phenylethynyl)benzenethiols and oligo(phenyl)benzenethiols have been shown to form self-assembled domains on gold.^21,22,36-38 A variety of data from different measurement techniques supports the formation of these SAMs on gold and silver with the thiol group binding to the underlying metal and the molecular backbone orienting itself nearly perpendicularly to the surface plane of the substrate.^{21,22,36,39-41} Even in the case of molecular structures based on biphenyl thiols with very polar substituents this structural arrangement seems to hold, as demonstrated by Kang et al. in their comprehensive report.²² As with alkylthiolate SAMs, the understanding of the structure of π-conjugated thiolate SAMs on metal substrates opened the possibility to study the influence of the molecular structure on the electronic properties of the underlying substrates.

Campbell et al. studied the effects of oligo(phenylethynyl)benzenethiolate SAMs on copper on the performance of organic diodes.⁴² Two molecular systems were compared, with the substitution of a terminal hydrogen atom for a fluorine atom as the only difference between the SAM constituents. The researchers compared the current – voltage characteristics of organic diodes with different structures – Cu anode / MEH-PPV / Ca, and Cu anode / SAM / MEH-PPV / Ca. The onset voltage for the conduction in the studied devices showed the expected trend, with the lowest onset recorded for the SAM composed of the fluorine-substituted molecules and the highest onset voltage for the corresponding SAM without fluorine substitution. The rationalization of the observed effect invoked the difference in the hole-injection barrier from the electrode to the organic layer due to the change in the work function caused by the formation of SAMs. Kelvin probe measurements indeed showed that the work function of the copper electrodes coated with the different SAMs qualitatively follows the prediction based on the well known effect of the sheet of dipole moments on top of the metal.

Zehner et al. were the first to systematically study the work function modification of gold by conjugated thiols.⁴³ The authors applied Kelvin-probe measurements to a series of oligo(phenylethynyl)benzenethiolate SAMs on gold. The rather impressive number of studied systems included molecular structures with different lengths of the π-conjugated backbones as well as different polar end-substituents. The trends of the work-function change reported in this work are actually not consistent with the predictions based on the molecular dipole moment. In particular, the molecular systems with dipole moments that should theoretically lower the work function actually increase it. Nevertheless, the authors attributed the observed work-function changes to the different dipole moment values of the studied molecular systems, and further conclusions followed.

Chen et al. investigated work function of gold substrates coated with a series of differently substituted terphenyl thiolate SAMs.⁴⁴ All of the studied systems showed a depression of the work function when compared with clean gold. A gold surface with an overlayer of a SAM of terphenylthiolate, characterized by only a small dipole moment, showed a work-function value 0.90 eV lower than that measured for the clean gold. Because the dipole moment of the molecule is small, this change has to be attributed to the charge redistribution caused by the formation of Au – S bond. Substitution of hydrogen on phenyl rings in the terphenylbackbone with fluorine atoms showed an increase of the work function of the underlying substrate, in accordance with qualitative predictions based on the dipole moment considerations. The authors showed their ability to tune the work function from 4.30 eV to 4.95 eV by simply using different SAMs on top of the gold electrodes. The structures of the SAMs and the corresponding values of the measured work function are shown in Figure 9. Further, the authors showed changes in the hole-injection barrier, Δ_h, into a copper-phthalocyanine (CuPc) layer deposited on top of the different SAM-coated gold substrates. This was done by measuring the position of the highest molecular energy level of CuPc (HOMO) with respect to the Fermi level for the different samples. The changes in the measured hole-injection barrier roughly follow the dependence: Δ_hµ Φ_m, where Φ_m is the work function measured for the SAM-coated gold substrates. Thus, the use of the different π-conjugated-thiolate SAMs allowed for tuning the hole-injection barrier from the gold electrode into CuPc organic layer. The values of the hole-injection barrier, r_h, are also shown in Figure 9.

Figure 9. Schematic representation of SAMs on gold studied by Chen et al. The measured work functions for the SAM-coated substrates, Φ_m, as well as the hole-injection barriers measured for Au / SAM / CuPc systems, r_h, are shown. Based on ref. 44.

Apart from other experimental reports on the metal-work-function changes induced by π-conjugated SAMs,^45,46 particular attention should be brought to theoretical work dealing with the energy level alignment of organic overlayers on gold. Among those, the reports by Heimel et al.,^3,47 and Romaner et al.^15,48 are perhaps of the highest interest in the context of this thesis. These studies focus on theoretical description of biphenylthiolate-based systems as there is a vast amount of data showing these SAMs possess two dimensional order and their structure has been studied extensively, thus making them good model systems for theoretical investigations.^19,22 Employing density functional theory (DFT), Heimel et al. showed that biphenylthiolate SAMs terminated with three different end-groups (a weak π-donor –SH, a strong π-donor –NH₂, and a strong π-acceptor –CN; see Figure 10) can result in large changes of the work function of the underlying gold substrate.^3,47

Figure 10. Schematic representation of biphenylthiolate SAMs on gold studied theoretically by Heimel et al. The calculated changes of the work function caused by the SAMs, rΦ_m, and the offsets between the HOMO and the Fermi level, r_HOMO, are shown. Results from ref. 47.

Because of the electron-donating ability of the amino group to the π-conjugated biphenyl backbone, this system exhibits a dipole moment with the positive end at the organic / vacuum interface, thus decreasing the work function of the substrate by 2.7 eV. On the other hand, a strongly electron-withdrawing substituent, the cyano group, forms a dipole with its positive end at the metal / organic interface. This results in an increased work function of the substrate by as much as 2.7 eV. Interestingly, the mercapto-substituted biphenylthiolate SAM on gold (the structure in the middle of Figure 10), a system with essentially no dipole moment along the long molecular axis, showed the work function to be decreased by 1.0 eV when compared to that of clean gold. This has been attributed to the dipole layer formed by the charge rearrangement due to the Au – S bond formation, and labeled a bond dipole. Analysis of the charge-density distribution led the authors to conclude that the bond dipole in all three studied systems is the same and, thus, that the charge rearrangement due to Au – S binding does not depend on the end-substituent of the biphenylthiolate backbone. Surprisingly, the offsets of the highest molecular orbital and the Fermi level for all three systems were calculated to be the same, despite the large differences in the gas-phase molecular ionization potentials. This has serious consequences for tuning of the hole-injection barrier from the gold electrode to the organic monolayer, as the authors clearly showed that the use of SAM constituents with clearly different molecular ionization potentials does not necessarily translate into a different offset between the HOMO and the Fermi level. However, it would be prudent not to draw general conclusions from these findings, as the results were obtained for biphenyl-based systems only and it is not clear if the used DFT methodology describes the studied systems in sufficient detail and accuracy.

Using the same DFT approach as in Ref. ⁴⁷, Romaner et al. studied the influence of the surface coverage on the work-function changes induced by substituted biphenylthiolate SAMs on gold.⁴⁸ Though some of the theoretically studied systems are not likely to be synthetically accessible, rather important conclusions were reached. Due to the polarizable nature of the biphenyl backbone of the SAM constituents the molecule – molecule distance on the gold surface has a large effect on the effective dipole-moment-layer depolarization. It was found that the increase of the surface coverage of the biphenylthiolate moieties not only increases the surface density of the dipole moments, but also leads to an increase in molecule – molecule depolarization effects, which effectively decreases the contribution of the additional dipoles to the change of the work function. Also, while the high-coverage systems showed the same behavior as those studied by Heimel et al.,⁴⁷ at lower coverages there seemed to be a dependence of the energy offset between the HOMO and the Fermi level on the molecular ionization potential of the SAM constituents.

2. References

(1) Cahen, D.; Kahn, A. Adv. Mater. 2003, 15, 271-277.

(2) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605-625.

(3) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.-L. Acc. Chem. Res. 2008, 41, 721-729.

(4) Murphy, E. L.; Good, R. H. Phys. Rev. 1956, 102, 1464.

(5) Hölzl, J.; Schulte, F. K.; Wagner, H. Solid surface physics; Springer-Verlag: Berlin, 1979; Vol. 85.

(6) Eastman, D. E. Phys. Rev. B: Condens. Matter 1970, 2, 1.

(7) Bock, C.; Pham, D. V.; Kunze, U.; Kafer, D.; Witte, G.; Woll, C. J. Appl. Phys. 2006, 100, 114517/1-114517/7.

(8) Santato, C.; Cicoira, F.; Cosseddu, P.; Bonfiglio, A.; Bellutti, P.; Muccini, M.; Zamboni, R.; Rosei, F.; Mantoux, A.; Doppelt, P. Appl. Phys. Lett. 2006, 88, 163511/1-163511/3.

(9) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Phys. Rev. B: Condens. Matter 1996, 54, R14321.

(10) De Boer, B.; Hadipour, A.; Mandoc, M. M.; Van Woudenbergh, T.; Blom, P. W. M. Advanced Materials (Weinheim, Germany) 2005, 17, 621-625.

(11) Nieuwenhuys, B. E.; Bouwman, R.; Sachtler, W. M. H. Thin Solid Films 1974, 21, 51-8.

(12) Nieuwenhuys, B. E.; Van Aardenne, O. G.; Sachtler, W. M. H. Chem. Phys. 1974, 5, 418-28.

(13) Huckstadt, C.; Schmidt, S.; Hufner, S.; Forster, F.; Reinert, F.; Springborg, M. Phys. Rev. B: Condens. Matter 2006, 73, 075409/1-075409/10.

(14) Cornil, D.; Olivier, Y.; Geskin, V.; Cornil, J. Adv. Funct. Mater. 2007, 17, 1143-1148.

(15) Romaner, L.; Heimel, G.; Ambrosch-Draxl, C.; Zojer, E. Adv. Funct. Mater. 2008, 18, 3999-4006.

(16) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-68.

(17) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-67.

(18) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-257.

(19) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103-1170.

(20) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097-105.

(21) Duan, L.; Garrett, S. J. J. Phys. Chem. B 2001, 105, 9812-9816.

(22) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, G. y. Langmuir 2001, 17, 95-106.

(23) Azzam, W.; Fuxen, C.; Birkner, A.; Rong, H.-T.; Buck, M.; Woell, C. Langmuir 2003, 19, 4958-4968.

(24) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96.

(25) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017-7021.

(26) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science (Washington, D. C.) 1996, 271, 1705-07.

(27) Zangmeister, C. D.; Robey, S. W.; Van Zee, R. D.; Yao, Y.; Tour, J. M. J. Phys. Chem. B 2004, 108, 16187-16193.

(28) Houseman, B. T.; Gawalt, E. S.; Mrksich, M. Langmuir 2003, 19, 1522-1531.

(29) Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Langmuir 2003, 19, 8141-8143.

(30) Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051-1053.

(31) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462-466.

(32) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121-4131.

(33) Lu, J.; Delamarche, E.; Eng, L.; Bennewitz, R.; Meyer, E.; Guntherodt, H. J. Langmuir 1999, 15, 8184-8188.

(34) Alloway, D. M.; Hofmann, M.; Smith, D. L.; Gruhn, N. E.; Graham, A. L.; Colorado, R.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.; Armstrong, N. R. J. Phys. Chem. B 2003, 107, 11690-11699.

(35) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2001, 123, 5549-5556.

(36) Fuxen, C.; Azzam, W.; Arnold, R.; Witte, G.; Terfort, A.; Woell, C. Langmuir 2001, 17, 3689-3695.

(37) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320.

(38) Yang, G.; Qian, Y.; Engtrakul, C.; Sita, L. R.; Liu, G. y. J. Phys. Chem. B 2000, 104, 9059-9062.

(39) Richter, L. J.; Yang, C. S. C.; Wilson, P. T.; Hacker, C. A.; van Zee, R. D.; Stapleton, J. J.; Allara, D. L.; Yao, Y.; Tour, J. M. J. Phys. Chem. B 2004, 108, 12547-12559.

(40) Arnold, R.; Terfort, A.; Woell, C. Langmuir 2001, 17, 4980-4989.

(41) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408-2415.

(42) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett. 1997, 71, 3528-3530.

(43) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir 1999, 15, 1121-1127.

(44) Chen, W.; Huang, C.; Gao, X. Y.; Wang, L.; Zhen, C. G.; Qi, D.; Chen, S.; Zhang, H. L.; Loh, K. P.; Chen, Z. K.; Wee, A. T. S. J. Phys. Chem. B 2006, 110, 26075-26080.

(45) Zangmeister, C. D.; Picraux, L. B.; van Zee, R. D.; Yao, Y.; Tour, J. M. Chem. Phys. Lett. 2007, 442, 390-393.

(46) Risko, C.; Zangmeister, C. D.; Yao, Y.; Marks, T. J.; Tour, J. M.; Ratner, M. A.; van Zee, R. D. Journal of Physical Chemistry C 2008, 112, 13215-13225.

(47) Heimel, G.; Romaner, L.; Bredas, J.-L.; Zojer, E. Phys. Rev. Lett. 2006, 96, 196806/1-196806/4.

(48) Romaner, L.; Heimel, G.; Zojer, E. Phys. Rev. B: Condens. Matter 2008, 77, 045113-9.

<a href="#_ftnref1" name="_ftn1" title="">[a]</a> In terms of Fermi – Dirac statistics the Fermi level is defined as the energy at which the probability of finding an electron is 0.5 for a system at thermal equilibrium. For metals at room temperature the probability of finding an electron below the Fermi level is close to unity.

<a href="#_ftnref2" name="_ftn2" title="">[b]</a> In a real experiment the photoelectron-kinetic-energy analyzer causes an additional drop in the potential, which adds a rigid offset to the kinetic energy of the photoelectrons. More on this can be found in Ref. 12.

<a href="#_ftnref3" name="_ftn3" title="">[c]</a> In practice the photoelectrons are additionally accelerated by an external electric field, which has to be taken account in the calculations.

<a href="#_ftnref4" name="_ftn4" title="">[d]</a> The solid-state electron affinity and solid-state ionization potential discussed here are not the same as the corresponding values measured for bulk organic layer. The contact with the metal electrode causes so-called “band bending”, which influences the values of both electron affinity and ionization potential. This is discussed in more detail in Ref. 13.

<a href="#_ftnref5" name="_ftn5" title="">[e]</a> The two described mechanisms provide an oversimplified picture of the charge-redistribution process. Contribution from orbital interactions is also invoked as another mechanism of charge redistribution. See ref. 24.

<a href="#_ftnref6" name="_ftn6" title="">[f]</a> As mentioned in section 1.4 any adsorbate will change the work function of a metallic surface. Thus, only working under ultrahigh vacuum conditions with properly cleaned metal surfaces can yield true work function values of the metal surface.

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-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>Let us consider an electron at rest in a vacuum at an infinite distance from a metal surface, as depicted in Figure 1a. The total energy, ''E<sub>tot</sub>'', of such an electron is equal to its potential energy, which can be defined after Cahen and Kahn as the ''vacuum level at infinity'', ''E<sub>vac</sub>(∞)''.</span><span style="font-weight: normal"><sup>1</sup></span><span style="font-weight: normal"> Let us supply the electron with some kinetic energy, ''E<sub>k</sub>'', and allow it to travel towards the surface. The total energy of the electron outside the metal can be expressed as the sum of its potential energy, ''E<sub>V</sub>'', and kinetic energy, ''E<sub>k</sub>''<nowiki>:</nowiki></span>
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-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>As the distance between the electron and the metal surface decreases the potential energy of the electron will increase, thus slowing it down, according to Equation 1 (see Figure 1b). If the kinetic energy is sufficiently large to overcome the potential barrier at the metal / vacuum interface the electron will go over the barrier and then will rapidly lose its potential energy due to the interaction with the positively charged ion lattice within the metal, resulting in the increase of kinetic energy. Once the electron is within the metal Equation 1 no longer holds as the interactions with other electrons and the ion lattice result in energy dissipation and thermal equilibration. The electron is now trapped within the metal. </span>
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+</span></span></i><i><span style='font-size:14.0pt;line-height:200%'>Influence
+of organic thiols on the work function of coinage metals</span></i></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>A similar thought experiment can be performed in the opposite direction. Let us choose an electron from one of the highest occupied energy levels at the particular temperature of the system. The potential energy of this electron is approximately at the ''Fermi level'','' E<sub>F</sub>''.[#_ftn1 <span class="MsoFootnoteReference"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="mso-bidi-font-size: 10.0pt; line-height: 200%; mso-fareast-font-family: Times; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">[a]</font></font></span></span></span></span>]<sup>,</sup></span><span style="font-weight: normal"><sup>2,3</sup></span><span style="font-weight: normal"> The electron is then supplied with some kinetic energy, ''E'<sub>k</sub>'', sufficiently large to overcome the potential barrier existing at the metal surface. Right after escaping the metal, the greatly slowed-down electron has a potential energy which can be defined as the ''vacuum level close to the surface'', ''E<sub>vac</sub>(s)''.</span><span style="font-weight: normal"><sup>1</sup></span><span style="font-weight: normal"> The drop in the kinetic energy of the electron caused by the potential energy barrier is defined as the work function of the metal surface, </span>''<span style="font-weight: normal"><font face="Symbol">F</font></span>''''<sub><span style="font-weight: normal">m</span></sub>''<span style="font-weight: normal"><nowiki>: </nowiki></span><span style="font-weight: normal"><sup>1,2</sup></span><span style="font-weight: normal"></span>
+<p class=MsoHeader style='margin-left:.5in;text-indent:-.5in;line-height:200%'><i><span
+style='line-height:200%;font-weight:normal'>1.1.<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+</span></span></i><i><span style='font-weight:normal'>Work Function of Metals –
+Physical Description </span></i></p>
-<span style="font-weight: normal"><span style="mso-spacerun: yes"> </span></span>''<span style="line-height: 200%; font-weight: normal"><font face="Symbol"><font size="14.0pt">F</font></font></span>''''<sub><span style="line-height: 200%; font-weight: normal"><font size="14.0pt">m</font></span></sub>''''<span style="line-height: 200%; font-weight: normal"><font size="14.0pt"> = E<sub>vac</sub>(s) - E<sub>F</sub></font></span>''<span style="line-height: 200%; font-weight: normal"><font size="14.0pt"><span style="mso-tab-count: 2">            </span></font></span><span style="line-height: 200%"><font size="10.0pt">Equation 2</font></span><span style="line-height: 200%; font-weight: normal"><font size="14.0pt"></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            Let
+us consider an electron at rest in a vacuum at an infinite distance from a metal
+surface, as depicted in Figure 1a. The total energy, <i>E<sub>tot</sub></i>, of
+such an electron is equal to its potential energy, which can be defined after Cahen
+and Kahn as the <i>vacuum level at infinity</i>, <i>E<sub>vac</sub>(&#8734;)</i>.</span><span
+style='font-weight:normal'><sup>1</sup></span><span style='font-weight:normal'>
+Let us supply the electron with some kinetic energy, <i>E<sub>k</sub></i>, and
+allow it to travel towards the surface. The total energy of the electron outside
+the metal can be expressed as the sum of its potential energy, <i>E<sub>V</sub></i>,
+and kinetic energy, <i>E<sub>k</sub></i>:</span></p>
-<span style="font-weight: normal">After escaping from the metal the electron is moving away further from the surface experiencing acceleration as the potential drops to the value of ''E<sub>vac</sub>(∞)''.</span>
+<p class=MsoHeader style='line-height:200%'><i><span style='font-size:14.0pt;
+line-height:200%;font-weight:normal'>E<sub>tot</sub> = E<sub>V</sub> + E<sub>k</sub></span></i><i><sub><span
+style='font-weight:normal'>             </span></sub></i><i><span
+style='font-weight:normal'>            </span></i><span style='font-size:10.0pt;
+line-height:200%'>Equation 1</span></p>
-<span style="font-weight: normal; mso-no-proof: yes">                    </span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            As
+the distance between the electron and the metal surface decreases the potential
+energy of the electron will increase, thus slowing it down, according to Equation
+(see Figure 1b). If the kinetic energy is sufficiently large to overcome the
+potential barrier at the metal / vacuum interface the electron will go over the
+barrier and then will rapidly lose its potential energy due to the interaction
+with the positively charged ion lattice within the metal, resulting in the
+increase of kinetic energy. Once the electron is within the metal Equation 1 no
+longer holds as the interactions with other electrons and the ion lattice
+result in energy dissipation and thermal equilibration. The electron is now trapped
+within the metal. </span></p>
-<span style="mso-bookmark: OLE_LINK2"><span style="line-height: 200%"><font size="10.0pt">Figure 1.</font></span></span><span style="mso-bookmark: OLE_LINK1"><span style="mso-bookmark: OLE_LINK2"><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> a) Potential energy of an electron in vacuum at an infinite distance from a metal surface, ''E<sub>vac</sub>(∞)'',</font></span></span></span><span style="mso-bookmark: OLE_LINK1"><span style="mso-bookmark: OLE_LINK2"><span style="line-height: 200%"><font size="10.0pt"> </font></span></span></span><span style="mso-bookmark: OLE_LINK1"><span style="mso-bookmark: OLE_LINK2"><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">and the Fermi level of the metal, ''E<sub>F</sub>''. b) Potential landscape the electron experiences along a travel path towards the metal surface (thick line). The difference between the electron energy just outside the metal surface, ''E<sub>vac</sub>(s)'', and ''E<sub>F</sub>'' defines the work function of the metal surface, ''Φ<sub>m</sub> = E<sub>vac</sub>(s) - E<sub>F</sub>''.</font></span></span></span><span style="mso-bookmark: OLE_LINK1"><span style="mso-bookmark: OLE_LINK2"><span style="font-weight: normal"></span></span></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            A
+similar thought experiment can be performed in the opposite direction. Let us choose
+an electron from one of the highest occupied energy levels at the particular
+temperature of the system. The potential energy of this electron is approximately
+at the <i>Fermi level</i>,<i> E<sub>F</sub></i>.<a href="#_ftn1" name="_ftnref1"
+title=""><span class=MsoFootnoteReference><span class=MsoFootnoteReference><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>[a]</span></span></span></a><sup>,</sup></span><span style='font-weight:normal'><sup>2,3</sup></span><span style='font-weight:normal'>
+The electron is then supplied with some kinetic energy, <i>E'<sub>k</sub></i>, sufficiently
+large to overcome the potential barrier existing at the metal surface. Right
+after escaping the metal, the greatly slowed-down electron has a potential
+energy which can be defined as the <i>vacuum level close to the surface</i>, <i>E<sub>vac</sub>(s)</i>.</span><span
+style='font-weight:normal'><sup>1</sup></span><span style='font-weight:normal'>
+The drop in the kinetic energy of the electron caused by the potential energy
+barrier is defined as the work function of the metal surface, </span><i><span
+style='font-family:Symbol;font-weight:normal'>F</span></i><i><sub><span
+style='font-weight:normal'>m</span></sub></i><span style='font-weight:normal'>:
+</span><span style='font-weight:normal'><sup>1,2</sup></span></p>
-<span style="mso-bookmark: OLE_LINK2"></span><span style="mso-bookmark: OLE_LINK1"></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'> </span><i><span
+style='font-size:14.0pt;line-height:200%;font-family:Symbol;font-weight:normal'>F</span></i><i><sub><span
+style='font-size:14.0pt;line-height:200%;font-weight:normal'>m</span></sub></i><i><span
+style='font-size:14.0pt;line-height:200%;font-weight:normal'> = E<sub>vac</sub>(s)
+- E<sub>F</sub></span></i><span style='font-size:14.0pt;line-height:200%;
+font-weight:normal'>            </span><span style='font-size:10.0pt;
+line-height:200%'>Equation 2</span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>After
+escaping from the metal the electron is moving away further from the surface
+experiencing acceleration as the potential drops to the value of <i>E<sub>vac</sub>(&#8734;)</i>.</span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>It is important to describe the physical origin of the potential landscape illustrated in the above thought experiment. As already mentioned, the sharp potential drop the electron experiences when it enters the metal is related to the electrostatic attraction force between the negative charge of the electron and the positively charged ion lattice of the metal. However, at the vacuum / metal interface the electron experiences a potential energy barrier. This implies that in that region of space there must be a repulsive force that acts upon the electron. Indeed, the lack of positively charged metal ions on the vacuum side of the aforementioned interface causes a negative charge to exist just outside of the metal and, conversely, an uncompensated positive charge within the metal. The spilling of the electronic density outside of the solid causes a formation of a sheet of dipoles, which are often referred to as ''the surface dipoles''.</span><span style="font-weight: normal"><sup>1-3</sup></span><span style="font-weight: normal"> </span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'><img
+width=575 height=221 id="Picture 4"
+src="for%20STC%20Wiki_SAMs_files/image001.jpg"></span></p>
-''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; mso-fareast-font-family: &quot;Times New Roman&quot;; font-weight: normal"><span style="mso-list: Ignore">1.2.<span style="font: 7.0pt &quot;Times New Roman&quot;">            </span></span></span>''''<span style="font-weight: normal">Work Function of Metallic Surfaces – Methods of Measurement</span>''
+<p class=MsoHeader style='line-height:200%'><a name="OLE_LINK2"></a><a
+name="OLE_LINK1"><span style='font-size:10.0pt;line-height:200%'>Figure 1.</span></a><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'> a) Potential energy
+of an electron in vacuum at an infinite distance from a metal surface, <i>E<sub>vac</sub>(&#8734;)</i>,</span><span
+style='font-size:10.0pt;line-height:200%'> </span><span style='font-size:10.0pt;
+line-height:200%;font-weight:normal'>and the Fermi level of the metal, <i>E<sub>F</sub></i>.
+b) Potential landscape the electron experiences along a travel path towards the
+metal surface (thick line). The difference between the electron energy just
+outside the metal surface, <i>E<sub>vac</sub>(s)</i>, and <i>E<sub>F</sub></i>
+defines the work function of the metal surface, <i>&#934;<sub>m</sub> = E<sub>vac</sub>(s)
+- E<sub>F</sub></i>.</span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>The work function of solid surfaces can be determined experimentally using absolute or relative approaches.<span style="mso-spacerun: yes">  </span>Absolute methods allow one to measure the work function value directly. Here, the electrons in the metal are supplied with sufficient kinetic energy to overcome the barrier at the metal / vacuum interface, and can thus escape the metal, and the work function can be obtained from the resulting electric current. Absolute methods include measurements based on ''thermionic emission'', ''field emission'', and the'' photoelectric effect''. Briefly, in the thermionic emission method electrons are ejected from the material after receiving sufficient thermal energy to overcome the energy barrier at the metal / vacuum interface. The appropriate thermal energy is supplied by incremental heating of the sample to temperatures at which the Fermi-Dirac distribution of the electrons in the metal allows for substantial population of electrons at energies higher than the interface energy barrier. The resulting electric current is measured as a function of temperature; this allows one to extract the work function of the surface. The temperature range used in the thermionic emission method is often very high (thousands of Kelvins), making this method of limited value for studying materials and surfaces which are unstable at high temperatures.</span><span style="font-weight: normal"><sup>4</sup></span><span style="font-weight: normal"> The field-emission method utilizes an electric field to accelerate the electrons inside of the metal to kinetic energies sufficiently high to overcome the interface barrier. The resulting electric current is analyzed as a function of the applied field and the work function is calculated.</span><span style="font-weight: normal"><sup>4,5</sup></span><span style="font-weight: normal"> <span style="mso-tab-count: 1">   </span></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            </span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>Photoelectric-effect-based methods use light, typically in the UV range, as the source of energy for the electrons. As in other absolute methods, the resulting electric current (here called ''photocurrent'') is analyzed. Figure 2 shows the energetics of a photoelectric-effect-based measurement of the work function. In this experiment the electrons are provided with a known energy, ''hν'' (red arrows in Figure 2). Electrons with sufficient kinetic energy to overcome the barrier at the interface are able to escape the metal – these are represented with the blue box in Figure 2. These ''photoelectrons ''then travel away from the metal surface experiencing the potential depicted with the thick black line in Figure 2. As the electrons move further away from the surface their kinetic energy increases, according to Equation 1. The generated photocurrent is then measured as a function of the photoelectron kinetic energy. Two important features are present in a typical plot of photocurrent – kinetic energy. First, a sharp onset at low photoelectron kinetic energy, ''E<sub>min</sub>'' is present. As already mentioned, this onset defines the lowest energy electrons able to overcome the work function of the surface.</span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            It
+is important to describe the physical origin of the potential landscape
+illustrated in the above thought experiment. As already mentioned, the sharp
+potential drop the electron experiences when it enters the metal is related to
+the electrostatic attraction force between the negative charge of the electron
+and the positively charged ion lattice of the metal. However, at the vacuum / metal
+interface the electron experiences a potential energy barrier. This implies
+that in that region of space there must be a repulsive force that acts upon the
+electron. Indeed, the lack of positively charged metal ions on the vacuum side
+of the aforementioned interface causes a negative charge to exist just outside
+of the metal and, conversely, an uncompensated positive charge within the metal.
+The spilling of the electronic density outside of the solid causes a formation
+of a sheet of dipoles, which are often referred to as <i>the surface dipoles</i>.</span><span style='font-weight:normal'><sup>1-3</sup></span><span style='font-weight:normal'>
+</span></p>
-<span style="font-weight: normal; mso-no-proof: yes">  </span><span style="font-weight: normal"></span>
+<p class=MsoHeader style='margin-left:.5in;line-height:200%'>&nbsp;</p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 2.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Energetics of electrons in a photoelectric-effect-based measurement of the work function. The photon energy, ''hν'', is shown as a red arrow. After ejection from the metal the electrons experience the potential shown with the thick black line. The kinetic energies of two ''photoelectrons'' originating from different energy levels in the metal are shown with thick green lines.[#_ftn2 <span class="MsoFootnoteReference"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="line-height: 200%; mso-fareast-font-family: Times; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">[b]</font></font></span></span></span></span>]</font></span><span style="font-weight: normal"></span>
+<p class=MsoHeader style='margin-left:.5in;text-indent:-.5in;line-height:200%'><i><span
+style='line-height:200%;font-weight:normal'>1.2.<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+</span></span></i><i><span style='font-weight:normal'>Work Function of Metallic
+Surfaces – Methods of Measurement</span></i></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 2">                        </span></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            The
+work function of solid surfaces can be determined experimentally using absolute
+or relative approaches.  Absolute methods allow one to measure the work
+function value directly. Here, the electrons in the metal are supplied with sufficient
+kinetic energy to overcome the barrier at the metal / vacuum interface, and can
+thus escape the metal, and the work function can be obtained from the resulting
+electric current. Absolute methods include measurements based on <i>thermionic
+emission</i>, <i>field emission</i>, and the<i> photoelectric effect</i>. Briefly,
+in the thermionic emission method electrons are ejected from the material after
+receiving sufficient thermal energy to overcome the energy barrier at the metal
+/ vacuum interface. The appropriate thermal energy is supplied by incremental
+heating of the sample to temperatures at which the Fermi-Dirac distribution of
+the electrons in the metal allows for substantial population of electrons at
+energies higher than the interface energy barrier. The resulting electric
+current is measured as a function of temperature; this allows one to extract
+the work function of the surface. The temperature range used in the thermionic
+emission method is often very high (thousands of Kelvins), making this method
+of limited value for studying materials and surfaces which are unstable at high
+temperatures.</span><span
+style='font-weight:normal'><sup>4</sup></span><span style='font-weight:normal'>
+The field-emission method utilizes an electric field to accelerate the
+electrons inside of the metal to kinetic energies sufficiently high to overcome
+the interface barrier. The resulting electric current is analyzed as a function
+of the applied field and the work function is calculated.</span><span
+style='font-weight:normal'><sup>4,5</sup></span><span style='font-weight:normal'>
+   </span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>The second feature is the high kinetic energy onset of the photocurrent, ''E<sub>max</sub>'', and it is a manifestation of the electron population around the Fermi level of the metal; i.e., since there is an abrupt decrease in the electron population above the Fermi level, there are essentially no photoelectrons with ''E<sub>tot</sub>'' &gt; ''E<sub>F</sub>'' + ''hν'' right after ejection from the surface. Using the definition of the work function in Equation 2 (see also Figure 1b), it follows that ''Φ<sub>m </sub><nowiki>= E</nowiki><sub>min</sub> + hν - E<sub>max</sub>''.[#_ftn3 <span class="MsoFootnoteReference"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="mso-bidi-font-size: 10.0pt; line-height: 200%; mso-fareast-font-family: Times; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">[c]</font></font></span></span></span></span>] Thus, the work function can be obtained from the onsets of the photocurrent as a function of the kinetic energy of the photoelectrons.</span><span style="font-weight: normal"><sup>1,2</sup></span><span style="font-weight: normal"></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            Photoelectric-effect-based
+methods use light, typically in the UV range, as the source of energy for the
+electrons. As in other absolute methods, the resulting electric current (here
+called <i>photocurrent</i>) is analyzed. Figure 2 shows the energetics of a
+photoelectric-effect-based measurement of the work function. In this experiment
+the electrons are provided with a known energy, <i>h&#957;</i> (red arrows in
+Figure 2). Electrons with sufficient kinetic energy to overcome the barrier at
+the interface are able to escape the metal – these are represented with the
+blue box in Figure 2. These <i>photoelectrons </i>then travel away from the
+metal surface experiencing the potential depicted with the thick black line in
+Figure 2. As the electrons move further away from the surface their kinetic
+energy increases, according to Equation 1. The generated photocurrent is then
+measured as a function of the photoelectron kinetic energy. Two important
+features are present in a typical plot of photocurrent – kinetic energy. First,
+a sharp onset at low photoelectron kinetic energy, <i>E<sub>min</sub></i> is
+present. As already mentioned, this onset defines the lowest energy electrons
+able to overcome the work function of the surface.</span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>Relative methods employ a reference made of a material with a known work function and focus on measuring differences in electrical quantities between the studied material and the reference. These methods include ''diode methods'' and ''condenser methods ''(Kelvin probe) and will not be discussed further.</span><span style="font-weight: normal"><sup>5</sup></span><span style="font-weight: normal"><span style="mso-tab-count: 1">   </span></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'><img
+width=575 height=369 src="for%20STC%20Wiki_SAMs_files/image002.jpg"></span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>Table 1 shows experimentally measured work-function values for a variety of metallic surfaces. The presented data show a variation of the work function in the range of 2.7 – 5.65 eV, revealing that the chemical composition of the surface has a large effect on the work function value. </span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 2.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Energetics of electrons in a photoelectric-effect-based
+measurement of the work function. The photon energy, <i>h&#957;</i>, is shown
+as a red arrow. After ejection from the metal the electrons experience the
+potential shown with the thick black line. The kinetic energies of two <i>photoelectrons</i>
+originating from different energy levels in the metal are shown with thick
+green lines.<a href="#_ftn2" name="_ftnref2" title=""><span
+class=MsoFootnoteReference><span class=MsoFootnoteReference><span
+style='font-size:10.0pt;line-height:200%;font-family:"Times New Roman","serif"'>[b]</span></span></span></a></span></p>
-<span style="font-weight: normal"> </span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>                        </span></p>
-'''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">Table 1.</font></font></span>'''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt"> Experimentally measured values of work function for different metals.* </font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            The
+second feature is the high kinetic energy onset of the photocurrent, <i>E<sub>max</sub></i>,
+and it is a manifestation of the electron population around the Fermi level of
+the metal; i.e., since there is an abrupt decrease in the electron population
+above the Fermi level, there are essentially no photoelectrons with <i>E<sub>tot</sub></i>
+&gt; <i>E<sub>F</sub></i> + <i>h&#957;</i> right after ejection from the
+surface. Using the definition of the work function in Equation 2 (see also Figure
+b), it follows that <i>&#934;<sub>m </sub>= E<sub>min</sub> + h&#957; - E<sub>max</sub></i>.<a
+href="#_ftn3" name="_ftnref3" title=""><span class=MsoFootnoteReference><span
+class=MsoFootnoteReference><span style='font-size:12.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>[c]</span></span></span></a> Thus, the work
+function can be obtained from the onsets of the photocurrent as a function of
+the kinetic energy of the photoelectrons.</span><span
+style='font-weight:normal'><sup>1,2</sup></span></p>
-{| class="MsoTableGrid" style="width: 6.0in; margin-left: 5.4pt; border-collapse: collapse; border: none; mso-border-top-alt: solid windowtext .5pt; mso-yfti-tbllook: 1184; mso-padding-alt: 0in 5.4pt 0in 5.4pt; mso-border-insideh: none; mso-border-insidev: .5pt solid windowtext" width="576" border="1"
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            Relative
-|- style="mso-yfti-irow: 0; mso-yfti-firstrow: yes; height: 27.6pt"
+methods employ a reference made of a material with a known work function and
-| style="width: 70.5pt; border: solid windowtext 1.0pt; mso-border-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
+focus on measuring differences in electrical quantities between the studied
-<center>Element</center>
+material and the reference. These methods include <i>diode methods</i> and <i>condenser
-| style="width: 70.5pt; border: solid windowtext 1.0pt; border-left: none; mso-border-left-alt: solid windowtext .5pt; mso-border-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
+methods </i>(Kelvin probe) and will not be discussed further.</span><span
-<center>Φ<sub>m</sub> [eV]</center>
+style='font-weight:normal'><sup>5</sup></span><span style='font-weight:normal'>   </span></p>
-| style="width: 70.5pt; border: solid windowtext 1.0pt; border-left: none; mso-border-left-alt: solid windowtext .5pt; mso-border-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center>Element</center>
-| style="width: 70.5pt; border: solid windowtext 1.0pt; border-left: none; mso-border-left-alt: solid windowtext .5pt; mso-border-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center>Φ<sub>m</sub> [eV]</center>
-| style="width: 70.5pt; border: solid windowtext 1.0pt; border-left: none; mso-border-left-alt: solid windowtext .5pt; mso-border-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center>Element</center>
-| style="width: 79.5pt; border: solid windowtext 1.0pt; border-left: none; mso-border-left-alt: solid windowtext .5pt; mso-border-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="106" |
-<center>Φ<sub>m</sub> [eV]</center>
-|- style="mso-yfti-irow: 1; height: 27.6pt"
-| style="width: 70.5pt; border-top: none; border-left: solid windowtext 1.0pt; border-bottom: none; border-right: solid windowtext 1.0pt; mso-border-top-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Sc</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">3.5 ± 0.15</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Ni</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">5.15 ± 0.1</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Ag</span></center>
-| style="width: 79.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-top-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="106" |
-<center><span style="font-weight: normal">4.0 ± 0.15</span></center>
-|- style="mso-yfti-irow: 2; height: 27.6pt"
-| style="width: 70.5pt; border-top: none; border-left: solid windowtext 1.0pt; border-bottom: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Ti</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.3 ± 0.1</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Cu</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.65 ± 0.05</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">La</span></center>
-| style="width: 79.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="106" |
-<center><span style="font-weight: normal">3.5 ± 0.2</span></center>
-|- style="mso-yfti-irow: 3; height: 27.6pt"
-| style="width: 70.5pt; border-top: none; border-left: solid windowtext 1.0pt; border-bottom: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">V</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.3 ± 0.1</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Y</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">3.1 ± 0.15</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Ce</span></center>
-| style="width: 79.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="106" |
-<center><span style="font-weight: normal">2.9 ± 0.2</span></center>
-|- style="mso-yfti-irow: 4; height: 27.6pt"
-| style="width: 70.5pt; border-top: none; border-left: solid windowtext 1.0pt; border-bottom: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Cr</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.5 ± 0.15</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Zr</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.05 ± 0.1</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Sm</span></center>
-| style="width: 79.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="106" |
-<center><span style="font-weight: normal">2.7 ± 0.3</span></center>
-|- style="mso-yfti-irow: 5; height: 27.6pt"
-| style="width: 70.5pt; border-top: none; border-left: solid windowtext 1.0pt; border-bottom: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Mn</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.1 ± 0.2</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Nb</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.3 ± 0.15</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Gd</span></center>
-| style="width: 79.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="106" |
-<center><span style="font-weight: normal">3.1<span style="mso-spacerun: yes">  </span>± 0.15</span></center>
-|- style="mso-yfti-irow: 6; height: 27.6pt"
-| style="width: 70.5pt; border-top: none; border-left: solid windowtext 1.0pt; border-bottom: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Fe</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.5 ± 0.15</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Mo</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">4.6 ± 0.15</span></center>
-| style="width: 70.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Pt</span></center>
-| style="width: 79.5pt; border: none; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="106" |
-<center><span style="font-weight: normal">5.65 ± 0.1</span></center>
-|- style="mso-yfti-irow: 7; mso-yfti-lastrow: yes; height: 27.6pt"
-| style="width: 70.5pt; border: solid windowtext 1.0pt; border-top: none; mso-border-left-alt: solid windowtext .5pt; mso-border-bottom-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Co</span></center>
-| style="width: 70.5pt; border-top: none; border-left: none; border-bottom: solid windowtext 1.0pt; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-bottom-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">5.0 ± 0.1</span></center>
-| style="width: 70.5pt; border-top: none; border-left: none; border-bottom: solid windowtext 1.0pt; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-bottom-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Pd</span></center>
-| style="width: 70.5pt; border-top: none; border-left: none; border-bottom: solid windowtext 1.0pt; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-bottom-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">5.55 ± 0.1</span></center>
-| style="width: 70.5pt; border-top: none; border-left: none; border-bottom: solid windowtext 1.0pt; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-bottom-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="94" |
-<center><span style="font-weight: normal">Au</span></center>
-| style="width: 79.5pt; border-top: none; border-left: none; border-bottom: solid windowtext 1.0pt; border-right: solid windowtext 1.0pt; mso-border-left-alt: solid windowtext .5pt; mso-border-left-alt: solid windowtext .5pt; mso-border-bottom-alt: solid windowtext .5pt; mso-border-right-alt: solid windowtext .5pt; padding: 0in 5.4pt 0in 5.4pt; height: 27.6pt" width="106" |
-<center><span style="mso-fareast-font-family: &quot;Times New Roman&quot;; font-weight: normal"><span style="mso-list: Ignore">5.1<span style="font: 7.0pt &quot;Times New Roman&quot;">  </span></span></span><span style="font-weight: normal">± 0.1</span></center>
-|}
-<sup><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><nowiki>*</nowiki></font></sup><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> From photoelectric-effect-based measurements. Values taken from ref. </font><font face="&quot;Times New Roman&quot;,&quot;serif&quot;">6</font><font face="&quot;Times New Roman&quot;,&quot;serif&quot;">.</font>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            Table
+shows experimentally measured work-function values for a variety of metallic
+surfaces. The presented data show a variation of the work function in the range
+of 2.7 – 5.65 eV, revealing that the chemical composition of the surface has a
+large effect on the work function value. </span></p>
-<span style="font-weight: normal"> </span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>&nbsp;</span></p>
-<span style="font-weight: normal"> </span>
+<p class=MsoNormal><b><span style='font-size:10.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>Table 1.</span></b><span
+style='font-size:10.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+Experimentally measured values of work function for different metals.* </span></p>
-<span style="font-weight: normal"> </span>
+<table class=MsoTableGrid border=1 cellspacing=0 cellpadding=0 width=576
+ style='width:6.0in;margin-left:5.4pt;border-collapse:collapse;border:none'>
+ <tr style='height:27.6pt'>
+  <td width=94 style='width:70.5pt;border:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'>Element</p>
+  </td>
+  <td width=94 style='width:70.5pt;border:solid windowtext 1.0pt;border-left:
+  none;padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'>&#934;<sub>m</sub>
+  [eV]</p>
+  </td>
+  <td width=94 style='width:70.5pt;border:solid windowtext 1.0pt;border-left:
+  none;padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'>Element</p>
+  </td>
+  <td width=94 style='width:70.5pt;border:solid windowtext 1.0pt;border-left:
+  none;padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'>&#934;<sub>m</sub>
+  [eV]</p>
+  </td>
+  <td width=94 style='width:70.5pt;border:solid windowtext 1.0pt;border-left:
+  none;padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'>Element</p>
+  </td>
+  <td width=106 style='width:79.5pt;border:solid windowtext 1.0pt;border-left:
+  none;padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'>&#934;<sub>m</sub>
+  [eV]</p>
+  </td>
+ </tr>
+ <tr style='height:27.6pt'>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:solid windowtext 1.0pt;
+  border-bottom:none;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Sc</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>3.5 ± 0.15</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Ni</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>5.15 ± 0.1</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Ag</span></p>
+  </td>
+  <td width=106 style='width:79.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.0 ± 0.15</span></p>
+  </td>
+ </tr>
+ <tr style='height:27.6pt'>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:solid windowtext 1.0pt;
+  border-bottom:none;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Ti</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.3 ± 0.1</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Cu</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.65 ± 0.05</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>La</span></p>
+  </td>
+  <td width=106 style='width:79.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>3.5 ± 0.2</span></p>
+  </td>
+ </tr>
+ <tr style='height:27.6pt'>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:solid windowtext 1.0pt;
+  border-bottom:none;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>V</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.3 ± 0.1</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Y</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>3.1 ± 0.15</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Ce</span></p>
+  </td>
+  <td width=106 style='width:79.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>2.9 ± 0.2</span></p>
+  </td>
+ </tr>
+ <tr style='height:27.6pt'>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:solid windowtext 1.0pt;
+  border-bottom:none;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Cr</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.5 ± 0.15</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Zr</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.05 ± 0.1</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Sm</span></p>
+  </td>
+  <td width=106 style='width:79.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>2.7 ± 0.3</span></p>
+  </td>
+ </tr>
+ <tr style='height:27.6pt'>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:solid windowtext 1.0pt;
+  border-bottom:none;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Mn</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.1 ± 0.2</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Nb</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.3 ± 0.15</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Gd</span></p>
+  </td>
+  <td width=106 style='width:79.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>3.1  ± 0.15</span></p>
+  </td>
+ </tr>
+ <tr style='height:27.6pt'>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:solid windowtext 1.0pt;
+  border-bottom:none;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Fe</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.5 ± 0.15</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Mo</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>4.6 ± 0.15</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Pt</span></p>
+  </td>
+  <td width=106 style='width:79.5pt;border:none;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>5.65 ± 0.1</span></p>
+  </td>
+ </tr>
+ <tr style='height:27.6pt'>
+  <td width=94 style='width:70.5pt;border:solid windowtext 1.0pt;border-top:
+  none;padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Co</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:none;border-bottom:
+  solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>5.0 ± 0.1</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:none;border-bottom:
+  solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Pd</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:none;border-bottom:
+  solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>5.55 ± 0.1</span></p>
+  </td>
+  <td width=94 style='width:70.5pt;border-top:none;border-left:none;border-bottom:
+  solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;
+  height:27.6pt'>
+  <p class=MsoHeader align=center style='text-align:center;line-height:200%'><span
+  style='font-weight:normal'>Au</span></p>
+  </td>
+  <td width=106 style='width:79.5pt;border-top:none;border-left:none;
+  border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;
+  padding:0in 5.4pt 0in 5.4pt;height:27.6pt'>
+  <p class=MsoHeader align=center style='margin-left:.25in;text-align:center;
+  text-indent:-.25in;line-height:200%'><span style='font-weight:normal'>5.1<span
+  style='font:7.0pt "Times New Roman"'>&nbsp; </span></span><span
+  style='font-weight:normal'>± 0.1</span></p>
+  </td>
+ </tr>
+</table>
-''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; mso-fareast-font-family: &quot;Times New Roman&quot;; font-weight: normal"><span style="mso-list: Ignore">1.3.<span style="font: 7.0pt &quot;Times New Roman&quot;">            </span></span></span>''''<span style="font-weight: normal">Work Function of Metals – Implications for Organic Electronics Applications</span>''
+<p class=MsoFootnoteText style='line-height:200%'><sup><span style='font-family:
+"Times New Roman","serif"'>*</span></sup><span style='font-family:"Times New Roman","serif"'>
+From photoelectric-effect-based measurements. Values taken from ref. </span><span
+style='font-family:"Times New Roman","serif"'>6</span><span style='font-family:
+"Times New Roman","serif"'>.</span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>The presence of the surface dipole on the metal surface has important implications for electronics applications. Charge-carrier injection from a metal electrode to an active layer in a variety of optoelectronic devices is considered to be one of the crucial processes essential to the overall device performance.</span><span style="font-weight: normal"><sup>2,3</sup></span><span style="font-weight: normal"> The energetics of the charge-injection process are defined by the relative positions of the Fermi level of the metal and the accessible energy levels of the organic active layer. This is depicted in Figure 3 for the case of a hypothetical simplified organic electroluminescent (EL) device.</span><span style="font-weight: normal"><sup>2</sup></span><span style="font-weight: normal"> Consider the right-hand side of the energy diagram. In order for an electron to undergo a transfer from the metal cathode to the organic layer it must be supplied with sufficient energy to overcome the barrier existing at the interface. The electron-injection barrier, Δ<sub>e</sub>, is defined as the energy difference between the work function of the metal and the solid-state electron affinity, ''EA'', of the organic layer: ''Δ<sub>e</sub> = Φ<sub>m </sub><span style="mso-spacerun: yes"> </span>- EA''.[#_ftn4 <span class="MsoFootnoteReference"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="mso-bidi-font-size: 10.0pt; line-height: 200%; mso-fareast-font-family: Times; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">[d]</font></font></span></span></span></span>] In the case of an EL device the energy needed to overcome the electron-injection barrier is supplied by applying a voltage between the electrodes. From the technological perspective the applied voltage must satisfy certain requirements, for example fall in a range that minimizes the power loss of the device, and allows for integration of the device into a standardized circuit. Thus, matching the solid-state electron affinity and solid-state ionization potential of the organic layers with the work functions of the metal electrodes is the key to obtaining a suitable charge injection during device operation.</span>
+<p class=MsoHeader style='margin-left:.5in;line-height:200%'><span
+style='font-weight:normal'>&nbsp;</span></p>
-<span style="font-weight: normal"> </span>
+<p class=MsoHeader style='margin-left:.5in;line-height:200%'><span
+style='font-weight:normal'>&nbsp;</span></p>
-<span style="mso-no-proof: yes">  </span><span style="font-weight: normal"></span>
+<p class=MsoHeader style='margin-left:.5in;line-height:200%'><span
+style='font-weight:normal'>&nbsp;</span></p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 3. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">Energy diagram of an organic electroluminescent device. The charge carriers – electrons (e-) and holes (h+) – are injected respectively from the metal cathode (right-hand side of the diagram) and the anode (left-hand side of the diagram) into the organic active layers – the electron-transport layer (ETL) and the hole-transport layer (HTL). At the ETL / HTL interface the charge carriers recombine generating photons. The Fermi levels of both the metal cathode and the anode and the energy levels of the organic layers (HOMO and LUMO energy levels) have to be matched in order to achieve an efficient charge injection into the organic layers. Based on ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">2</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">. </font></span><span style="font-weight: normal"></span>
+<p class=MsoHeader style='margin-left:.5in;text-indent:-.5in;line-height:200%'><i><span
+style='line-height:200%;font-weight:normal'>1.3.<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+</span></span></i><i><span style='font-weight:normal'>Work Function of Metals –
+Implications for Organic Electronics Applications</span></i></p>
-<span style="font-weight: normal">The EL device described above is only one of the examples in which the metal electrode work function plays a crucial role in the overall device performance. Matching the work function of the electrodes with the organic-layer energy levels in order to balance the charge-carrier-injection barriers is very important in organic field-effect transistors (OFETs), photovoltaic devices, or organic light-emitting transistors (OLETs).</span><span style="font-weight: normal"><sup>3,7,8</sup></span><span style="font-weight: normal"> Traditionally this has been addressed by choosing metals with low work function for the cathode, high-work-function materials for the anode, and matching the organic layer energy levels appropriately.</span><span style="font-weight: normal"><sup>3</sup></span><span style="font-weight: normal"> Lately however, there has been a substantial effort to employ metal electrodes whose work function has been tuned by adsorbates.</span><span style="font-weight: normal"><sup>9,10</sup></span><span style="font-weight: normal"> This approach is, in principle, more versatile than the use of different, often reactive metals, as it opens the possibility of using relatively chemically stable metals (such as coinage metals) with a layer of work-function-tuning adsorbate as electrodes in organic electronic applications.</span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            The
+presence of the surface dipole on the metal surface has important implications
+for electronics applications. Charge-carrier injection from a metal electrode
+to an active layer in a variety of optoelectronic devices is considered to be
+one of the crucial processes essential to the overall device performance.</span><span style='font-weight:normal'><sup>2,3</sup></span><span style='font-weight:normal'>
+The energetics of the charge-injection process are defined by the relative
+positions of the Fermi level of the metal and the accessible energy levels of
+the organic active layer. This is depicted in Figure 3 for the case of a
+hypothetical simplified organic electroluminescent (EL) device.</span><span
+style='font-weight:normal'><sup>2</sup></span><span style='font-weight:normal'>
+Consider the right-hand side of the energy diagram. In order for an electron to
+undergo a transfer from the metal cathode to the organic layer it must be
+supplied with sufficient energy to overcome the barrier existing at the
+interface. The electron-injection barrier, &#916;<sub>e</sub>, is defined as
+the energy difference between the work function of the metal and the
+solid-state electron affinity, <i>EA</i>, of the organic layer: <i>&#916;<sub>e</sub>
+= &#934;<sub>m </sub> - EA</i>.<a href="#_ftn4" name="_ftnref4" title=""><span
+class=MsoFootnoteReference><span class=MsoFootnoteReference><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>[d]</span></span></span></a>
+In the case of an EL device the energy needed to overcome the
+electron-injection barrier is supplied by applying a voltage between the
+electrodes. From the technological perspective the applied voltage must satisfy
+certain requirements, for example fall in a range that minimizes the power loss
+of the device, and allows for integration of the device into a standardized
+circuit. Thus, matching the solid-state electron affinity and solid-state
+ionization potential of the organic layers with the work functions of the metal
+electrodes is the key to obtaining a suitable charge injection during device
+operation.</span></p>
-''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; mso-fareast-font-family: &quot;Times New Roman&quot;; font-weight: normal"><span style="mso-list: Ignore">1.4.<span style="font: 7.0pt &quot;Times New Roman&quot;">            </span></span></span>''''<span style="font-weight: normal">Work Function of Metals – Influence of Adsorbates </span>''
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>&nbsp;</span></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span>The work function of a metal is very sensitive to the presence of any adsorbates present on the surface. Experiments have revealed that even physisorbed atoms of inert gases such as argon or xenon influence the work function on a variety of metallic substrates.</span><span style="font-weight: normal"><sup>11-13</sup></span><span style="font-weight: normal"> Even though there is no significant charge redistribution in the inert-gas atoms near the metal surface, due to a <span style="mso-spacerun: yes"> </span>chemical reaction, there is a substantial charge redistribution on the surface after the physisorption has taken place, which changes the surface dipole on the surface. Two major physical phenomena are responsible for this. First, the electronic density extending outside of the metal surface is pushed back by the repulsive force of the electronic cloud of the inert gas atoms; this is referred to as Pauli push-back, or sometimes as the “pillow effect”.</span><span style="font-weight: normal"><sup>3,14</sup></span><span style="font-weight: normal"> Second, van der Waals interactions between the inert-gas atom and the metal surface cause polarization of the otherwise highly symmetric electronic cloud of the inert-gas atom.[#_ftn5 <span class="MsoFootnoteReference"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="mso-bidi-font-size: 10.0pt; line-height: 200%; mso-fareast-font-family: Times; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">[e]</font></font></span></span></span></span>] This results in the formation of a sheet of dipoles in the space occupied by the adsorbed atoms which alters the potential landscape at the vacuum / metal interface.</span><span style="font-weight: normal"><sup>12,13</sup></span><span style="font-weight: normal"></span>
+<p class=MsoHeader align=left style='text-align:left;line-height:200%'><img
+width=433 height=325 src="for%20STC%20Wiki_SAMs_files/image003.jpg"></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">On the basis of simple electrostatic considerations one can calculate the dipole-moment surface density corresponding to the measured change of the work function.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>3</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> The work function change, ''ΔΦ<sub>m</sub>'', caused by a sheet of dipoles residing on the surface is expressed by the Helmholtz equation:</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>3,15</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 3. </span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'>Energy diagram of an organic electroluminescent
+device. The charge carriers – electrons (e-) and holes (h+) – are injected respectively
+from the metal cathode (right-hand side of the diagram) and the anode (left-hand
+side of the diagram) into the organic active layers – the electron-transport
+layer (ETL) and the hole-transport layer (HTL). At the ETL / HTL interface the
+charge carriers recombine generating photons. The Fermi levels of both the
+metal cathode and the anode and the energy levels of the organic layers (HOMO
+and LUMO energy levels) have to be matched in order to achieve an efficient
+charge injection into the organic layers. Based on ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>2</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>. </span></p>
-<span style="line-height: 200%; mso-ascii-theme-font: minor-latin; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-font-family: &quot;Times New Roman&quot;; mso-bidi-theme-font: minor-bidi; position: relative; top: 26.5pt; mso-text-raise: -26.5pt; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Calibri&quot;,&quot;sans-serif&quot;"><font size="11.0pt">  </font></font></span><span style="line-height: 200%; mso-fareast-font-family: &quot;Times New Roman&quot;; mso-fareast-theme-font: minor-fareast"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><span style="mso-tab-count: 1">         </span><span style="mso-tab-count: 1">            </span></font></font></span>'''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">Equation 3</font></font></span>''''''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%"></span>'''
+<p class=MsoHeader style='line-height:200%'>&nbsp;</p>
-<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; font-weight: normal">where ''e'' is the elementary charge, ''μ'' is the dipole moment in the direction of the surface normal, </span><span style="position: relative; top: 2.0pt; mso-text-raise: -2.0pt">  </span><span style="mso-spacerun: yes"> </span><span style="font-weight: normal">is the area of the metal surface,</span><span style="mso-bidi-font-size: 12.0pt; line-height: 200%; font-weight: normal"> ''ε ''is the dielectric constant of the dipole layer, and</span> ''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; font-weight: normal">ε<sub>0</sub></span>''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; font-weight: normal"> is the vacuum permittivity. Table 2 shows experimentally measured values of the work-function changes for a series of inert-gas / metal-surface systems together with the dipole-moment surface density induced by the inert-gas atoms calculated according to Equation 3.</span><span style="mso-bidi-font-size: 12.0pt; line-height: 200%; font-weight: normal"><sup>13</sup></span><span style="mso-bidi-font-size: 12.0pt; line-height: 200%; font-weight: normal"> It can be seen that the adsorption of inert-gas atoms can lower the work function of coinage-metal surfaces by as much as 0.62 eV, which in the case of Cu(111) surface corresponds to ca. 13% drop in the work function upon adsorption of xenon. The dipole-moment surface densities calculated according to Equation 3 from the measured work-function changes in Table 2 are on the order of 1.5 D / nm<sup>2</sup>. This corresponds roughly to a separation of a whole elementary charge by 1 Å in each 3 nm<sup>2</sup> of the surface.</span>
+<p class=MsoHeader style='text-indent:.5in;line-height:200%'><span
+style='font-weight:normal'>The EL device described above is only one of the
+examples in which the metal electrode work function plays a crucial role in the
+overall device performance. Matching the work function of the electrodes with
+the organic-layer energy levels in order to balance the charge-carrier-injection
+barriers is very important in organic field-effect transistors (OFETs),
+photovoltaic devices, or organic light-emitting transistors (OLETs).</span><span style='font-weight:normal'><sup>3,7,8</sup></span><span style='font-weight:normal'>
+Traditionally this has been addressed by choosing metals with low work function
+for the cathode, high-work-function materials for the anode, and matching the
+organic layer energy levels appropriately.</span><span
+style='font-weight:normal'><sup>3</sup></span><span style='font-weight:normal'>
+Lately however, there has been a substantial effort to employ metal electrodes
+whose work function has been tuned by adsorbates.</span><span
+style='font-weight:normal'><sup>9,10</sup></span><span style='font-weight:normal'>
+This approach is, in principle, more versatile than the use of different, often
+reactive metals, as it opens the possibility of using relatively chemically
+stable metals (such as coinage metals) with a layer of work-function-tuning
+adsorbate as electrodes in organic electronic applications.</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">The changes in the work function due physisorption of inert-gas atoms are interesting from the perspective of fundamental understanding of the electronic processes at the surfaces. However, inert gases do not form robust layers upon adsorption and they do not allow for much control of the dipole moment present on the surface after adsorption. The possibility of such control via synthetic design was opened with the development of the field of self-assembled monolayers (SAMs) of organic thiols on metals. </font></font></span>
+<p class=MsoHeader style='line-height:200%'>&nbsp;</p>
-'''<span style="mso-fareast-font-family: Times"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> </font></span>'''
+<p class=MsoHeader style='margin-left:.5in;text-indent:-.5in;line-height:200%'><i><span
+style='line-height:200%;font-weight:normal'>1.4.<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+</span></span></i><i><span style='font-weight:normal'>Work Function of Metals –
+Influence of Adsorbates </span></i></p>
-<span style="line-height: 200%"><font size="10.0pt">Table 2.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Experimentally measured changes in the work function, </font></span>''<span style="font-weight: normal">ΔΦ<sub>m</sub></span>''<span style="line-height: 200%; font-weight: normal"><font size="10.0pt">,</font></span><span style="line-height: 200%"><font size="10.0pt"> </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">of coinage metals upon adsorption of inert gases.* Values adapted from ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">13</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">. The dipole-moment surface densities calculated according to Equation 3 are given in the third column.</font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            The
+work function of a metal is very sensitive to the presence of any adsorbates
+present on the surface. Experiments have revealed that even physisorbed atoms
+of inert gases such as argon or xenon influence the work function on a variety
+of metallic substrates.</span><span
+style='font-weight:normal'><sup>11-13</sup></span><span style='font-weight:
+normal'> Even though there is no significant charge redistribution in the
+inert-gas atoms near the metal surface, due to a  chemical reaction, there is a
+substantial charge redistribution on the surface after the physisorption has
+taken place, which changes the surface dipole on the surface. Two major physical
+phenomena are responsible for this. First, the electronic density extending
+outside of the metal surface is pushed back by the repulsive force of the
+electronic cloud of the inert gas atoms; this is referred to as Pauli push-back,
+or sometimes as the “pillow effect”.</span><span
+style='font-weight:normal'><sup>3,14</sup></span><span style='font-weight:normal'>
+Second, van der Waals interactions between the inert-gas atom and the metal
+surface cause polarization of the otherwise highly symmetric electronic cloud
+of the inert-gas atom.<a href="#_ftn5" name="_ftnref5" title=""><span
+class=MsoFootnoteReference><span class=MsoFootnoteReference><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>[e]</span></span></span></a>
+This results in the formation of a sheet of dipoles in the space occupied by
+the adsorbed atoms which alters the potential landscape at the vacuum / metal
+interface.</span><span
+style='font-weight:normal'><sup>12,13</sup></span></p>
-{| class="MsoTableGrid" style="border-collapse: collapse; border: none; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; mso-table-overlap: never; mso-yfti-tbllook: 1184; mso-table-lspace: 9.0pt; margin-left: 6.75pt; mso-table-rspace: 9.0pt; margin-right: 6.75pt; mso-table-anchor-vertical: paragraph; mso-table-anchor-horizontal: margin; mso-table-left: left; mso-table-top: .05pt; mso-padding-alt: 0in 5.4pt 0in 5.4pt" border="1" align="left"
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
-|- style="mso-yfti-irow: 0; mso-yfti-firstrow: yes"
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>On
-| style="width: 99.9pt; border: solid black 1.0pt; mso-border-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="133" |
+the basis of simple electrostatic considerations one can calculate the
-<center>'''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">System</font></font></span>'''</center>
+dipole-moment surface density corresponding to the measured change of the work
-| style="width: 1.0in; border: solid black 1.0pt; mso-border-themecolor: text1; border-left: none; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="96" |
+function.</span><span
-<center>'''''<span style="line-height: 200%; mso-ascii-font-family: &quot;Times New Roman&quot;; mso-hansi-font-family: &quot;Times New Roman&quot;; mso-bidi-font-family: &quot;Times New Roman&quot;; position: relative; top: 2.0pt; mso-text-raise: -2.0pt; mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;"><font face="&quot;Wingdings 3&quot;"><font size="12.0pt"><span style="mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;">r</span></font></font></span>''''''''''<span style="line-height: 200%; position: relative; top: 2.0pt; mso-text-raise: -2.0pt"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Φ<sub>m</sub></font></font></span>''''''''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><span style="mso-spacerun: yes"> </span>[eV]</font></font></span>'''</center>
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>3</sup></span><span
-| style="width: 81.0pt; border: solid black 1.0pt; mso-border-themecolor: text1; border-left: none; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="108" |
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
-<center>'''''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">μ / A </font></font></span>''''''''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">[D/nm<sup>2</sup>]</font></font></span>'''</center>
+The work function change, <i>&#916;&#934;<sub>m</sub></i>, caused by a sheet of
-|- style="mso-yfti-irow: 1"
+dipoles residing on the surface is expressed by the Helmholtz equation:</span><span
-| style="width: 99.9pt; border: solid black 1.0pt; mso-border-themecolor: text1; border-top: none; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="133" |
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>3,15</sup></span></p>
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Kr on Au(111)</font></font></span></center>
-| style="width: 1.0in; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="96" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">-0.42</font></font></span></center>
-| style="width: 81.0pt; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="108" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">1.1</font></font></span></center>
-|- style="mso-yfti-irow: 2"
-| style="width: 99.9pt; border: solid black 1.0pt; mso-border-themecolor: text1; border-top: none; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="133" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Xe on Au(111)</font></font></span></center>
-| style="width: 1.0in; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="96" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">-0.53</font></font></span></center>
-| style="width: 81.0pt; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="108" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">1.4</font></font></span></center>
-|- style="mso-yfti-irow: 3"
-| style="width: 99.9pt; border: solid black 1.0pt; mso-border-themecolor: text1; border-top: none; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="133" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Kr on Ag(111)</font></font></span></center>
-| style="width: 1.0in; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="96" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">-0.46</font></font></span></center>
-| style="width: 81.0pt; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="108" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">1.2</font></font></span></center>
-|- style="mso-yfti-irow: 4"
-| style="width: 99.9pt; border: solid black 1.0pt; mso-border-themecolor: text1; border-top: none; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="133" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Xe on Ag(111)</font></font></span></center>
-| style="width: 1.0in; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="96" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">-0.59</font></font></span></center>
-| style="width: 81.0pt; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="108" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">1.6</font></font></span></center>
-|- style="mso-yfti-irow: 5"
-| style="width: 99.9pt; border: solid black 1.0pt; mso-border-themecolor: text1; border-top: none; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="133" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Kr on Cu(111)</font></font></span></center>
-| style="width: 1.0in; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="96" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">-0.49</font></font></span></center>
-| style="width: 81.0pt; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="108" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">1.3</font></font></span></center>
-|- style="mso-yfti-irow: 6; mso-yfti-lastrow: yes"
-| style="width: 99.9pt; border: solid black 1.0pt; mso-border-themecolor: text1; border-top: none; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="133" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Xe on Cu(111)</font></font></span></center>
-| style="width: 1.0in; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="96" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">-0.62</font></font></span></center>
-| style="width: 81.0pt; border-top: none; border-left: none; border-bottom: solid black 1.0pt; mso-border-bottom-themecolor: text1; border-right: solid black 1.0pt; mso-border-right-themecolor: text1; mso-border-top-alt: solid black .5pt; mso-border-top-themecolor: text1; mso-border-left-alt: solid black .5pt; mso-border-left-themecolor: text1; mso-border-alt: solid black .5pt; mso-border-themecolor: text1; padding: 0in 5.4pt 0in 5.4pt" width="108" |
-<center><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">1.6</font></font></span></center>
-|}
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><br clear="all" /></font></font></span><sup><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><nowiki>*</nowiki></font></sup><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> The change in the work function, <span style="mso-bookmark: OLE_LINK7">''ΔΦ<sub>m</sub>'',</span>'' ''is defined as the difference in the work function of the clean substrate and the work function of the surface after the adsorption of inert gas atoms.</font>
+<p class=MsoNormal style='text-autospace:none'><span
+style='font-size:11.0pt;line-height:200%;font-family:"Calibri","sans-serif";
+position:relative;top:26.5pt'><img width=107 height=54
+src="for%20STC%20Wiki_SAMs_files/image004.png"></span><span style='font-size:
+.0pt;line-height:200%;font-family:"Times New Roman","serif"'>                     </span><b><span
+style='font-size:10.0pt;line-height:200%;font-family:"Times New Roman","serif"'>Equation
+</span></b></p>
-<span style="font-weight: normal"><span style="mso-tab-count: 1">            </span></span><span style="mso-bidi-font-size: 12.0pt; line-height: 200%; font-weight: normal"></span>
+<p class=MsoHeader style='line-height:200%'><span style='line-height:200%;
+font-weight:normal'>where <i>e</i> is the elementary charge, <i>&#956;</i> is the
+dipole moment in the direction of the surface normal, </span><span
+style='position:relative;top:2.0pt'><img width=18 height=18
+src="for%20STC%20Wiki_SAMs_files/image005.png"></span> <span style='font-weight:
+normal'>is the area of the metal surface,</span><span style='line-height:200%;
+font-weight:normal'> <i>&#949; </i>is the dielectric constant of the dipole
+layer, and</span> <i><span style='line-height:200%;font-weight:normal'>&#949;<sub>0</sub></span></i><span
+style='line-height:200%;font-weight:normal'> is the vacuum permittivity. Table
+shows experimentally measured values of the work-function changes for a
+series of inert-gas / metal-surface systems together with the dipole-moment
+surface density induced by the inert-gas atoms calculated according to Equation
+.</span><span
+style='line-height:200%;font-weight:normal'><sup>13</sup></span><span
+style='line-height:200%;font-weight:normal'> It can be seen that the adsorption
+of inert-gas atoms can lower the work function of coinage-metal surfaces by as
+much as 0.62 eV, which in the case of Cu(111) surface corresponds to ca. 13%
+drop in the work function upon adsorption of xenon. The dipole-moment surface
+densities calculated according to Equation 3 from the measured work-function
+changes in Table 2 are on the order of 1.5 D / nm<sup>2</sup>. This corresponds
+roughly to a separation of a whole elementary charge by 1 Å in each 3 nm<sup>2</sup>
+of the surface.</span></p>
-''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; mso-fareast-font-family: &quot;Times New Roman&quot;; font-weight: normal"><span style="mso-list: Ignore">1.5.<span style="font: 7.0pt &quot;Times New Roman&quot;">            </span></span></span>''''<span style="font-weight: normal">Self-Assembled Monolayers of Organic Thiols on Metals</span>''
+<p class=MsoNormal style='text-indent:.5in;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>The
+changes in the work function due physisorption of inert-gas atoms are interesting
+from the perspective of fundamental understanding of the electronic processes
+at the surfaces. However, inert gases do not form robust layers upon adsorption
+and they do not allow for much control of the dipole moment present on the
+surface after adsorption. The possibility of such control via synthetic design
+was opened with the development of the field of self-assembled monolayers
+(SAMs) of organic thiols on metals. </span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Self-Assembled Monolayers of organic thiols on metals have received considerable attention over the last two decades.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>16-19</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> From a purely scientific standpoint these structurally ordered systems offer a great opportunity for studying structure–property relationships of organic-inorganic interfaces. Studies of the influence of the molecular structure on the packing of thiols on noble-metal surfaces have led to important insights into the molecular scale morphology of the monolayers. It is now understood that the thiol group binds to gold and often long-range order is observed in the resulting organic adlayer.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>17-23</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> This understanding has further led to studies of the influence of synthetically accessible adsorbate structures on a variety of surface characteristics including wetting properties,</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>20,24</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> electronic characteristics,</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>25-27</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> and chemical reactivity.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>28-30</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> Figure 4 shows a schematic of a structure of an organic thiolate SAM on a metal surface, together with design motifs that can be used to tune the properties of the surface.</font></font></span>
+<p class=MsoNormal><b><span style='font-family:"Times New Roman","serif"'>&nbsp;</span></b></p>
-<span style="line-height: 200%; mso-no-proof: yes"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">  </font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Table 2.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Experimentally measured changes in the work function,
+</span><i><span style='font-weight:normal'>&#916;&#934;<sub>m</sub></span></i><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>,</span><span
+style='font-size:10.0pt;line-height:200%'> </span><span style='font-size:10.0pt;
+line-height:200%;font-weight:normal'>of coinage metals upon adsorption of inert
+gases.* Values adapted from ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>13</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>. The
+dipole-moment surface densities calculated according to Equation 3 are given in
+the third column.</span></p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 4.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Schematic diagram of a SAM of organic thiolates supported on a metal surface. The anatomy and characteristics of the SAM are highlighted. Based on ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">19</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">.</font></span><span style="font-weight: normal"></span>
+<table class=MsoTableGrid border=1 cellspacing=0 cellpadding=0 align=left
+ style='border-collapse:collapse;border:none;margin-left:6.75pt;margin-right:
+.75pt'>
+ <tr>
+  <td width=133 style='width:99.9pt;border:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><b><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>System</span></b></p>
+  </td>
+  <td width=96 style='width:1.0in;border:solid black 1.0pt;border-left:none;
+  padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><b><i><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Wingdings 3";position:relative;top:2.0pt'>r</span></i></b><b><i><span
+  style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif";
+  position:relative;top:2.0pt'>&#934;<sub>m</sub></span></i></b><b><span
+  style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'> [eV]</span></b></p>
+  </td>
+  <td width=108 style='width:81.0pt;border:solid black 1.0pt;border-left:none;
+  padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><b><i><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>&#956; / A </span></i></b><b><span
+  style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>[D/nm<sup>2</sup>]</span></b></p>
+  </td>
+ </tr>
+ <tr>
+  <td width=133 style='width:99.9pt;border:solid black 1.0pt;border-top:none;
+  padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>Kr on Au(111)</span></p>
+  </td>
+  <td width=96 style='width:1.0in;border-top:none;border-left:none;border-bottom:
+  solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>-0.42</span></p>
+  </td>
+  <td width=108 style='width:81.0pt;border-top:none;border-left:none;
+  border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>1.1</span></p>
+  </td>
+ </tr>
+ <tr>
+  <td width=133 style='width:99.9pt;border:solid black 1.0pt;border-top:none;
+  padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>Xe on Au(111)</span></p>
+  </td>
+  <td width=96 style='width:1.0in;border-top:none;border-left:none;border-bottom:
+  solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>-0.53</span></p>
+  </td>
+  <td width=108 style='width:81.0pt;border-top:none;border-left:none;
+  border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>1.4</span></p>
+  </td>
+ </tr>
+ <tr>
+  <td width=133 style='width:99.9pt;border:solid black 1.0pt;border-top:none;
+  padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>Kr on Ag(111)</span></p>
+  </td>
+  <td width=96 style='width:1.0in;border-top:none;border-left:none;border-bottom:
+  solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>-0.46</span></p>
+  </td>
+  <td width=108 style='width:81.0pt;border-top:none;border-left:none;
+  border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>1.2</span></p>
+  </td>
+ </tr>
+ <tr>
+  <td width=133 style='width:99.9pt;border:solid black 1.0pt;border-top:none;
+  padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>Xe on Ag(111)</span></p>
+  </td>
+  <td width=96 style='width:1.0in;border-top:none;border-left:none;border-bottom:
+  solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>-0.59</span></p>
+  </td>
+  <td width=108 style='width:81.0pt;border-top:none;border-left:none;
+  border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>1.6</span></p>
+  </td>
+ </tr>
+ <tr>
+  <td width=133 style='width:99.9pt;border:solid black 1.0pt;border-top:none;
+  padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>Kr on Cu(111)</span></p>
+  </td>
+  <td width=96 style='width:1.0in;border-top:none;border-left:none;border-bottom:
+  solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>-0.49</span></p>
+  </td>
+  <td width=108 style='width:81.0pt;border-top:none;border-left:none;
+  border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>1.3</span></p>
+  </td>
+ </tr>
+ <tr>
+  <td width=133 style='width:99.9pt;border:solid black 1.0pt;border-top:none;
+  padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>Xe on Cu(111)</span></p>
+  </td>
+  <td width=96 style='width:1.0in;border-top:none;border-left:none;border-bottom:
+  solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>-0.62</span></p>
+  </td>
+  <td width=108 style='width:81.0pt;border-top:none;border-left:none;
+  border-bottom:solid black 1.0pt;border-right:solid black 1.0pt;padding:0in 5.4pt 0in 5.4pt'>
+  <p class=MsoNormal align=center style='text-align:center;line-height:200%;
+  text-autospace:none'><span style='font-size:12.0pt;line-height:200%;
+  font-family:"Times New Roman","serif"'>1.6</span></p>
+  </td>
+ </tr>
+</table>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoFootnoteText style='margin-right:-2.2pt;line-height:200%'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><br
+clear=all>
+</span><sup><span style='font-family:"Times New Roman","serif"'>*</span></sup><span
+style='font-family:"Times New Roman","serif"'> The change in the work function,
+<a name="OLE_LINK7"></a><a name="OLE_LINK6"><i>&#916;&#934;<sub>m</sub></i>,</a><i>
+</i>is defined as the difference in the work function of the clean substrate
+and the work function of the surface after the adsorption of inert gas atoms.</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">As discussed earlier, the work function of metals is affected by adsorbates present on the surface. The knowledge of adsorption characteristics of organic thiols on metals and the synthetic accessibility of a variety of structures makes these SAMs excellent systems to be employed in the systematic study of the influence of adsorbates on the work function of the metallic substrates. </font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-weight:normal'>            </span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoHeader style='margin-left:.5in;text-indent:-.5in;line-height:200%'><i><span
+style='line-height:200%;font-weight:normal'>1.5.<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+</span></span></i><i><span style='font-weight:normal'>Self-Assembled Monolayers
+of Organic Thiols on Metals</span></i></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>Self-Assembled
+Monolayers of organic thiols on metals have received considerable attention
+over the last two decades.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>16-19</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+From a purely scientific standpoint these structurally ordered systems offer a
+great opportunity for studying structure–property relationships of
+organic-inorganic interfaces. Studies of the influence of the molecular
+structure on the packing of thiols on noble-metal surfaces have led to
+important insights into the molecular scale morphology of the monolayers. It is
+now understood that the thiol group binds to gold and often long-range order is
+observed in the resulting organic adlayer.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>17-23</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+This understanding has further led to studies of the influence of synthetically
+accessible adsorbate structures on a variety of surface characteristics
+including wetting properties,</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>20,24</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+electronic characteristics,</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>25-27</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+and chemical reactivity.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>28-30</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+Figure 4 shows a schematic of a structure of an organic thiolate SAM on a metal
+surface, together with design motifs that can be used to tune the properties of
+the surface.</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'><img width=557
+height=249 id="Picture 3" src="for%20STC%20Wiki_SAMs_files/image006.jpg"></span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 4.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Schematic diagram of a SAM of organic thiolates supported
+on a metal surface. The anatomy and characteristics of the SAM are highlighted.
+Based on ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>19</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>.</span></p>
-''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; mso-fareast-font-family: &quot;Times New Roman&quot;; font-weight: normal"><span style="mso-list: Ignore">1.6.<span style="font: 7.0pt &quot;Times New Roman&quot;">            </span></span></span>''''<span style="font-weight: normal">Self-Assembled Monolayers of Alkanethiols and their Influence on the Work Function of Metals</span>''
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">The first study addressing the effects of organic thiol SAMs on the work function of gold was published by Evans and Ulman.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>31</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> The authors performed ellipsometry and Kelvin-probe measurements on a series of alkanethiol SAMs with varying alkyl spacer lengths on a gold surface. Ellipsometry revealed that the thickness of the monolayers systematically increased with the alkyl chain length, which supported the formation of dense thiolate-bound monolayers on gold. The studied alkanethiol SAMs showed a linear decrease of the work function of the metal with the number of methylene units in the alkyl chain length on the monolayer constituent, with a slope of -9.3 meV / methylene group. This has been interpreted using a model involving a dipole layer residing on top of the metal, as depicted in Figure 5. The net dipole moment of the organic SAMs was found to have the positive end at the organic / air interface, thus effectively decreasing the work function as compared to clean gold (see Equation 3). <span style="mso-spacerun: yes"> </span>The addition of methylene units in the alkyl chain increased the magnitude of the dipole moment showing the abovementioned trend in the work-function change with alkyl chain length. Extrapolating the measured changes of the work function to a hypothetical monolayer without methylene groups revealed that Au – S layer decreases the work function of the substrate by as much as ca. 0.5 eV, which is qualitatively consistent with charge rearrangement due to the formation of a chemical bond. Additionally, the dielectric constant of the alkyl chain layer, ''ε<sub>2 </sub>''in Figure 5, was suggested to change with the alkyl chain length thus highlighting that both the SAM dipole moment and the dielectric constant influence the metal work function, the latter parameter because it effectively depolarizes the dipole layer. The decrease of the gold work function caused by the alkanethiols studied by Evans and Ulman was as large as 0.70 eV. However, it should be stressed that the method applied to measure the work function did not take into account any adsorbates that might have been present on the reference (in this case a “clean” gold surface); thus, the absolute value of the work function depression with respect to truly clean gold surface was most likely underestimated.<span style="mso-tab-count: 1">          </span></font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>As
+discussed earlier, the work function of metals is affected by adsorbates present
+on the surface. The knowledge of adsorption characteristics of organic thiols
+on metals and the synthetic accessibility of a variety of structures makes these
+SAMs excellent systems to be employed in the systematic study of the influence
+of adsorbates on the work function of the metallic substrates. </span></p>
-<span style="line-height: 200%; mso-no-proof: yes"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">  </font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 5.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Schematic diagram of an alkanethiol SAM on gold. The organic adlayer can be envisaged as two layers of dipoles with dipole moments ''μ<sub>1</sub>'' and ''μ<sub>2</sub>'' and the corresponding dielectric constants ''ε<sub>1</sub>'' and ''ε<sub>2</sub>''. The net dipole moment, ''μ<sub>net</sub>'' is also shown. Based on ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">31</font></span><span style="font-weight: normal"></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Further investigations of alkyl thiol monolayers containing a polar aromatic group showed that both the direction and the magnitude of the dipole moment of the molecules forming the monolayer are important factors determining the work function of the underlying metal. Evans et al. used Kelvin probe measurements to show that the structure of the organic adsorbate had a critical effect on the magnitude and the sign of the work-function change.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>32</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> In particular, the substitution of the terminal alkyl chain in one of the studied SAMs to a fluoroalkyl chain resulted in a net dipole moment of opposite direction to that of the SAM with the terminal alkyl chain (see Figure 6). In effect, while the terminal-alkyl-chain SAM causes a ''depression'' of the work function of gold by ca. 0.45 eV, the SAM with terminal fluoroalkyl chain ''increases'' the work function by as much as 0.75 eV. Thus, the authors clearly showed that the structure of the organic SAM, and in particular the effective net dipole moment of the organic structure, has a dramatic effect on the work-function change of the underlying metal.</font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%; mso-no-proof: yes"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">  </font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoHeader style='margin-left:.5in;text-indent:-.5in;line-height:200%'><i><span
+style='line-height:200%;font-weight:normal'>1.6.<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+</span></span></i><i><span style='font-weight:normal'>Self-Assembled Monolayers
+of Alkanethiols and their Influence on the Work Function of Metals</span></i></p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 6.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Schematic diagram of gold coated with alkanethiol SAMs containing polar aromatic groups studied by Evans et al. in ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">32</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">. The differences in the structures of SAMs are highlighted as well as the direction of the effective dipole moments of the organic adlayers.</font></span><span style="font-weight: normal"></span>
+<p class=MsoNormal style='text-indent:.5in'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'>The first study
+addressing the effects of organic thiol SAMs on the work function of gold was published
+by Evans and Ulman.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>31</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+The authors performed ellipsometry and Kelvin-probe measurements on a series of
+alkanethiol SAMs with varying alkyl spacer lengths on a gold surface. Ellipsometry
+revealed that the thickness of the monolayers systematically increased with the
+alkyl chain length, which supported the formation of dense thiolate-bound
+monolayers on gold. The studied alkanethiol SAMs showed a linear decrease of
+the work function of the metal with the number of methylene units in the alkyl
+chain length on the monolayer constituent, with a slope of -9.3 meV / methylene
+group. This has been interpreted using a model involving a dipole layer
+residing on top of the metal, as depicted in Figure 5. The net dipole moment of
+the organic SAMs was found to have the positive end at the organic / air
+interface, thus effectively decreasing the work function as compared to clean
+gold (see Equation 3).  The addition of methylene units in the alkyl chain
+increased the magnitude of the dipole moment showing the abovementioned trend
+in the work-function change with alkyl chain length. Extrapolating the measured
+changes of the work function to a hypothetical monolayer without methylene
+groups revealed that Au – S layer decreases the work function of the substrate
+by as much as ca. 0.5 eV, which is qualitatively consistent with charge
+rearrangement due to the formation of a chemical bond. Additionally, the
+dielectric constant of the alkyl chain layer, <i>&#949;<sub>2 </sub></i>in
+Figure 5, was suggested to change with the alkyl chain length thus highlighting
+that both the SAM dipole moment and the dielectric constant influence the metal
+work function, the latter parameter because it effectively depolarizes the
+dipole layer. The decrease of the gold work function caused by the alkanethiols
+studied by Evans and Ulman was as large as 0.70 eV. However, it should be
+stressed that the method applied to measure the work function did not take into
+account any adsorbates that might have been present on the reference (in this
+case a “clean” gold surface); thus, the absolute value of the work function
+depression with respect to truly clean gold surface was most likely
+underestimated.          </span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><span style="mso-spacerun: yes"> </span></font></font></span>
+<p class=MsoNormal align=left style='text-align:left;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><img
+width=576 height=270 id="Picture 9"
+src="for%20STC%20Wiki_SAMs_files/image007.jpg"></span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Lu et al. successfully used Kelvin-probe force microscopy (KPFM) to image a gold surface patterned with a mixed alkylthiolate monolayer.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>33</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> Alkylthiol terminated with a methyl group and another alkylthiol terminated with a carboxylic acid group were patterned via the microcontact printing technique.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>19</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> The observed contrast in the KPFM images was an effect of the difference in the work function of the gold-surface regions coated with the two adsorbates possessing different dipole moments. In the case of the two different alkyl thiolates studied by Lü the contrast was as large as 0.4 eV. Furthermore, a gold surface patterned with methyl-group-terminated alkylthiols containing different numbers of methylene units showed a trend in the measured surface potential, corresponding to a linear decrease of the work function with the slope of ca. -14 meV / methylene group, a behavior qualitatively similar to that reported by Evans.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>31</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 5.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Schematic diagram of an alkanethiol SAM on gold. The
+organic adlayer can be envisaged as two layers of dipoles with dipole moments <i>&#956;<sub>1</sub></i>
+and <i>&#956;<sub>2</sub></i> and the corresponding dielectric constants <i>&#949;<sub>1</sub></i>
+and <i>&#949;<sub>2</sub></i>. The net dipole moment, <i>&#956;<sub>net</sub></i>
+is also shown. Based on ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>31</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">In an elegant and comprehensive study Alloway and coworkers investigated the influence of alkanethiol and partially-fluorinated-alkanethiol SAMs on the work function of gold.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>34</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> In contrast to the reports Evans and Lu, the work of Alloway et al. was based on ultraviolet photoelectron spectroscopy (UPS), which is an absolute method performed under ultrahigh vacuum conditions, and thus it is anticipated to reflect the true values of the work function changes caused by thiolate SAMs on gold substrates.[#_ftn6 <span class="MsoFootnoteReference"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="line-height: 200%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">[f]</font></font></span></span></span></span>] All of the samples of alkylthiolate SAMs on gold showed a work function more than 1 eV ''lower'' than the work function of clean gold. Similarly to previously mentioned reports,</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>31,33</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> the authors showed that a change in the number of methylene units results in a change in the work function. Fitting the data points with a linear function yielded the slope of -19 meV / methylene unit, an absolute value larger than the values reported by both Evans et al. and Lu et al. Partially fluorinated alkyl thiolate SAMs showed an ''increase'' in the work function of the underlying substrate by as much as ca. 0.5 eV, consistent with the different direction of the dipole moment of the surface-attached molecules in the case of fluorinated and non-fluorinated chains. Analysis of the measured changes in the work function of gold as a function of the calculated projection of the molecular dipole moment onto the surface normal in the alkyl- and perfluoroalkylthiolate SAMs revealed that the Au – sulfur interaction depresses the work function by ca. 0.5 eV, which is the same value as that found by Evans et al. for similar systems.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>31</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> An interesting behavior was observed in a series of SAMs formed with alkylthiolates of different length terminated with a trifluoromethyl group. Figure 7 shows a schematic representation of these systems. </font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%; mso-no-proof: yes"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">  </font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoNormal style='text-indent:.5in'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'>Further investigations
+of alkyl thiol monolayers containing a polar aromatic group showed that both the
+direction and the magnitude of the dipole moment of the molecules forming the
+monolayer are important factors determining the work function of the underlying
+metal. Evans et al. used Kelvin probe measurements to show that the structure
+of the organic adsorbate had a critical effect on the magnitude and the sign of
+the work-function change.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>32</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+In particular, the substitution of the terminal alkyl chain in one of the
+studied SAMs to a fluoroalkyl chain resulted in a net dipole moment of opposite
+direction to that of the SAM with the terminal alkyl chain (see Figure 6). In
+effect, while the terminal-alkyl-chain SAM causes a <i>depression</i> of the
+work function of gold by ca. 0.45 eV, the SAM with terminal fluoroalkyl chain <i>increases</i>
+the work function by as much as 0.75 eV. Thus, the authors clearly showed that
+the structure of the organic SAM, and in particular the effective net dipole
+moment of the organic structure, has a dramatic effect on the work-function
+change of the underlying metal.</span></p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 7.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Schematic diagram of SAMs on gold studied by Alloway et al. The projections of the dipole moment of polar trifluoromethyl group onto the surface normal (dashed line) are shown by the vertical arrows. Based on ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">34</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">. </font></span><span style="font-weight: normal"></span>
+<p class=MsoNormal><span style='font-size:12.0pt;line-height:200%;font-family:
+"Times New Roman","serif"'><img width=576 height=319 id="Picture 12"
+src="for%20STC%20Wiki_SAMs_files/image008.jpg"></span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 6.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Schematic diagram of gold coated with alkanethiol SAMs
+containing polar aromatic groups studied by Evans et al. in ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>32</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>. The differences
+in the structures of SAMs are highlighted as well as the direction of the
+effective dipole moments of the organic adlayers.</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Due to the different orientation of the polar CF<sub>3</sub> groups with respect to the surface normal in odd- and even-numbered alkylthiolate SAMs, the magnitude of the effective-dipole-moment projection onto the surface normal, shown in Figure 7 with blue and red arrows, is different and manifests itself with an odd-even effect on the measured work function of the underlying gold substrate.</font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'> </span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><span style="mso-tab-count: 1">            </span>De Boer et al. studied the effects of alkylthiolate and perfluoroalkylthiolate SAMs on the work function of gold and silver.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>10</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> Using the Kelvin probe technique the researchers found qualitatively similar changes of the work function, caused by the studied SAMs, for both metals. In particular, the perfluorinated alkylthiolate SAMs ''increased'' the work function by 0.6 eV and 1.1 eV for gold and silver, respectively. The alkylthiolate SAMs, on the other hand, ''decreased'' the work function for both metals respectively by 0.8 eV, and 0.6 eV. Again, the changes in the work function caused by the organic monolayers were rationalized in terms of the SAM dipole layer and the resulting surface potential difference for different molecular structures. De Boer et al. took one step further and demonstrated the effect of the SAMs on the performance of organic diodes employing silver electrodes. These devices were built as models for organic light emitting diodes (OLEDs) based on the use of a semitransparent silver anode. The ionization potential of the active organic layer used in these OLEDs – poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) – is not matched with the untreated anode’s work function, resulting in a hole injection barrier of ca. 0.9 eV, which is a rather large value requiring operation of the device at high voltage. The devices described by the authors were prepared in a few configurations: a) Ag anode / MEH-PPV / Ag, b) Ag anode / perfluoroalkylthiolate SAM / MEH-PPV / Ag, and c) Ag anode / alkylthiolate SAM / MEH-PPV / Ag. Figure 8 shows the energy diagram of the Ag anode / organic layer interface for configurations a) and b). Due to the dipole layer of the perfluoroalkylthiolate SAM the work function of the silver anode surface was increased by 1.1 eV, thus effectively eliminating the hole-injection barrier, Δ<sub>h</sub>, from the electrode to the organic layer and forming an ohmic contact. Indeed, the current – voltage characteristics for the investigated devices showed ohmic behavior in the device incorporating perfluoroalkylthiolate SAM, in contrast to the device without the SAM on top of the silver anode, which showed an onset voltage for the conduction. Interestingly, the presence of alkylthiolate SAM on the silver anode (configuration c) of the device) increased the onset voltage of the diode even further when compared to the device without any SAM. This was rationalized in terms of the reduction of the work function of silver by the alkylthiolate SAM, which resulted in the increase of the hole-injection barrier with respect to case a).</font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'>Lu et al. successfully
+used Kelvin-probe force microscopy (KPFM) to image a gold surface patterned
+with a mixed alkylthiolate monolayer.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>33</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+Alkylthiol terminated with a methyl group and another alkylthiol terminated
+with a carboxylic acid group were patterned via the microcontact printing
+technique.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>19</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+The observed contrast in the KPFM images was an effect of the difference in the
+work function of the gold-surface regions coated with the two adsorbates
+possessing different dipole moments. In the case of the two different alkyl
+thiolates studied by Lü the contrast was as large as 0.4 eV. Furthermore, a gold
+surface patterned with methyl-group-terminated alkylthiols containing different
+numbers of methylene units showed a trend in the measured surface potential,
+corresponding to a linear decrease of the work function with the slope of ca. -14
+meV / methylene group, a behavior qualitatively similar to that reported by
+Evans.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>31</sup></span></p>
-<span style="line-height: 200%; mso-no-proof: yes"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">  </font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'>In an elegant and
+comprehensive study Alloway and coworkers investigated the influence of alkanethiol
+and partially-fluorinated-alkanethiol SAMs on the work function of gold.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>34</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+In contrast to the reports Evans and Lu, the work of Alloway et al. was based
+on ultraviolet photoelectron spectroscopy (UPS), which is an absolute method
+performed under ultrahigh vacuum conditions, and thus it is anticipated to
+reflect the true values of the work function changes caused by thiolate SAMs on
+gold substrates.<a href="#_ftn6" name="_ftnref6" title=""><span
+class=MsoFootnoteReference><span class=MsoFootnoteReference><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>[f]</span></span></span></a>
+All of the samples of alkylthiolate SAMs on gold showed a work function more
+than 1 eV <i>lower</i> than the work function of clean gold. Similarly to previously
+mentioned reports,</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>31,33</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+the authors showed that a change in the number of methylene units results in a
+change in the work function. Fitting the data points with a linear function
+yielded the slope of -19 meV / methylene unit, an absolute value larger than the
+values reported by both Evans et al. and Lu et al. Partially fluorinated alkyl
+thiolate SAMs showed an <i>increase</i> in the work function of the underlying
+substrate by as much as ca. 0.5 eV, consistent with the different direction of
+the dipole moment of the surface-attached molecules in the case of fluorinated
+and non-fluorinated chains. Analysis of the measured changes in the work
+function of gold as a function of the calculated projection of the molecular
+dipole moment onto the surface normal in the alkyl- and perfluoroalkylthiolate SAMs
+revealed that the Au – sulfur interaction depresses the work function by ca.
+.5 eV, which is the same value as that found by Evans et al. for similar
+systems.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>31</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+An interesting behavior was observed in a series of SAMs formed with
+alkylthiolates of different length terminated with a trifluoromethyl group.
+Figure 7 shows a schematic representation of these systems. </span></p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 8.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Schematic diagram of a) Ag anode / MEH-PPV interface and b) Ag anode / perfluoroalkylthiol SAM / MEH-PPV interface. Vacuum levels close to the surface, E<sub>vac</sub>(s), SAM induced change of the work function, ΔΦ<sub>m</sub>, and hole-injection barriers, Δ<sub>h</sub>, for both devices are shown. Based on ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">10</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">. </font></span><span style="font-weight: normal"></span>
+<p class=MsoNormal><span style='font-size:12.0pt;line-height:200%;font-family:
+"Times New Roman","serif"'><img width=576 height=287 id="Picture 11"
+src="for%20STC%20Wiki_SAMs_files/image009.jpg"></span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 7.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Schematic diagram of SAMs on gold studied by Alloway
+et al. The projections of the dipole moment of polar trifluoromethyl group onto
+the surface normal (dashed line) are shown by the vertical arrows. Based on
+ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>34</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>. </span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">In summary, alkylthiolate-based monolayers on coinage metals have been shown to reproducibly modify the work function of the underlying substrates. Additionally, the molecular structure of constituents of SAMs, together with simple electrostatic considerations, has been shown to be useful in terms of designing surfaces with a desired work-function change. It is important to stress that the modification of the work function of metal electrodes with alkylthiolate SAMs has been demonstrated independently by different research groups, and that the ability to tune the work function has been successfully applied to solve specific technological problems.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>9,10</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'>Due to the different
+orientation of the polar CF<sub>3</sub> groups with respect to the surface
+normal in odd- and even-numbered alkylthiolate SAMs, the magnitude of the
+effective-dipole-moment projection onto the surface normal, shown in Figure 7
+with blue and red arrows, is different and manifests itself with an odd-even
+effect on the measured work function of the underlying gold substrate.</span></p>
-''<span style="mso-bidi-font-size: 12.0pt; line-height: 200%; mso-fareast-font-family: &quot;Times New Roman&quot;; font-weight: normal"><span style="mso-list: Ignore">1.7.<span style="font: 7.0pt &quot;Times New Roman&quot;">            </span></span></span>''''<span style="font-weight: normal">Self-Assembled Monolayers of Conjugated Thiols and their Influence on the Work Function of Metals</span>''
+<p class=MsoNormal><span style='font-size:12.0pt;line-height:200%;font-family:
+"Times New Roman","serif"'>            De Boer et al. studied the effects of
+alkylthiolate and perfluoroalkylthiolate SAMs on the work function of gold and
+silver.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>10</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+Using the Kelvin probe technique the researchers found qualitatively similar
+changes of the work function, caused by the studied SAMs, for both metals. In
+particular, the perfluorinated alkylthiolate SAMs <i>increased</i> the work
+function by 0.6 eV and 1.1 eV for gold and silver, respectively. The
+alkylthiolate SAMs, on the other hand, <i>decreased</i> the work function for
+both metals respectively by 0.8 eV, and 0.6 eV. Again, the changes in the work
+function caused by the organic monolayers were rationalized in terms of the SAM
+dipole layer and the resulting surface potential difference for different molecular
+structures. De Boer et al. took one step further and demonstrated the effect of
+the SAMs on the performance of organic diodes employing silver electrodes. These
+devices were built as models for organic light emitting diodes (OLEDs) based on
+the use of a semitransparent silver anode. The ionization potential of the
+active organic layer used in these OLEDs – poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene
+vinylene] (MEH-PPV) – is not matched with the untreated anode’s work function,
+resulting in a hole injection barrier of ca. 0.9 eV, which is a rather large
+value requiring operation of the device at high voltage. The devices described
+by the authors were prepared in a few configurations: a) Ag anode / MEH-PPV /
+Ag, b) Ag anode / perfluoroalkylthiolate SAM / MEH-PPV / Ag, and c) Ag anode /
+alkylthiolate SAM / MEH-PPV / Ag. Figure 8 shows the energy diagram of the Ag
+anode / organic layer interface for configurations a) and b). Due to the dipole
+layer of the perfluoroalkylthiolate SAM the work function of the silver anode
+surface was increased by 1.1 eV, thus effectively eliminating the
+hole-injection barrier, &#916;<sub>h</sub>, from the electrode to the organic
+layer and forming an ohmic contact. Indeed, the current – voltage characteristics
+for the investigated devices showed ohmic behavior in the device incorporating
+perfluoroalkylthiolate SAM, in contrast to the device without the SAM on top of
+the silver anode, which showed an onset voltage for the conduction.
+Interestingly, the presence of alkylthiolate SAM on the silver anode (configuration
+c) of the device) increased the onset voltage of the diode even further when
+compared to the device without any SAM. This was rationalized in terms of the
+reduction of the work function of silver by the alkylthiolate SAM, which
+resulted in the increase of the hole-injection barrier with respect to case a).</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">A limitation on the use of monolayers consisting of long-chain alkyl thiols for modifying injection barriers between metals and organics is the intrinsic electrically insulating characteristics of the alkyl chains. Monolayers consisting of π-conjugated thiols, which have been shown to exhibit considerably enhanced conductivity,</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>26,35</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> may be more promising for this type of application. Monolayers composed of π-conjugated systems based on oligo(phenylethynyl)benzenethiols and oligo(phenyl)benzenethiols have been shown to form self-assembled domains on gold.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>21,22,36-38</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> A variety of data from different measurement techniques supports the formation of these SAMs on gold and silver with the thiol group binding to the underlying metal and the molecular backbone orienting itself nearly perpendicularly to the surface plane of the substrate.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>21,22,36,39-41</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> Even in the case of molecular structures based on biphenyl thiols with very polar substituents this structural arrangement seems to hold, as demonstrated by Kang et al. in their comprehensive report.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>22</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> As with alkylthiolate SAMs, the understanding of the structure of π-conjugated thiolate SAMs on metal substrates opened the possibility to study the influence of the molecular structure on the electronic properties of the underlying substrates.</font></font></span>
+<p class=MsoNormal><span style='font-size:12.0pt;line-height:200%;font-family:
+"Times New Roman","serif"'><img width=575 height=335 id="Picture 10"
+src="for%20STC%20Wiki_SAMs_files/image010.jpg"></span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Campbell et al. studied the effects of oligo(phenylethynyl)benzenethiolate SAMs on copper on the performance of organic diodes.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>42</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> Two molecular systems were compared, with the substitution of a terminal hydrogen atom for a fluorine atom as the only difference between the SAM constituents. The researchers compared the current – voltage characteristics of organic diodes with different structures – Cu anode / MEH-PPV / Ca, and Cu anode / SAM / MEH-PPV / Ca. The onset voltage for the conduction in the studied devices showed the expected trend, with the lowest onset recorded for the SAM composed of the fluorine-substituted molecules and the highest onset voltage for the corresponding SAM without fluorine substitution. The rationalization of the observed effect invoked the difference in the hole-injection barrier from the electrode to the organic layer due to the change in the work function caused by the formation of SAMs. Kelvin probe measurements indeed showed that the work function of the copper electrodes coated with the different SAMs qualitatively follows the prediction based on the well known effect of the sheet of dipole moments on top of the metal.</font></font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 8.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Schematic diagram of a) Ag anode / MEH-PPV interface
+and b) Ag anode / perfluoroalkylthiol SAM / MEH-PPV interface. Vacuum levels
+close to the surface, E<sub>vac</sub>(s), SAM induced change of the work
+function, &#916;&#934;<sub>m</sub>, and hole-injection barriers, &#916;<sub>h</sub>,
+for both devices are shown. Based on ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>10</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>. </span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Zehner et al. were the first to systematically study the work function modification of gold by conjugated thiols.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>43</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> The authors applied Kelvin-probe measurements to a series of oligo(phenylethynyl)benzenethiolate SAMs on gold. The rather impressive number of studied systems included molecular structures with different lengths of the π-conjugated backbones as well as different polar end-substituents. The trends of the work-function change reported in this work are actually not consistent with the predictions based on the molecular dipole moment. In particular, the molecular systems with dipole moments that should theoretically lower the work function actually increase it. Nevertheless, the authors attributed the observed work-function changes to the different dipole moment values of the studied molecular systems, and further conclusions followed.</font></font></span>
+<p class=MsoNormal><span style='font-size:12.0pt;line-height:200%;font-family:
+"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Chen et al. investigated work function of gold substrates coated with a series of differently substituted terphenyl thiolate SAMs.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>44</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> All of the studied systems showed a depression of the work function when compared with clean gold. A gold surface with an overlayer of a SAM of terphenylthiolate, characterized by only a small dipole moment, showed a work-function value 0.90 eV lower than that measured for the clean gold. Because the dipole moment of the molecule is small, this change has to be attributed to the charge redistribution caused by the formation of Au – S bond. Substitution of hydrogen on phenyl rings in the terphenylbackbone with fluorine atoms showed an increase of the work function of the underlying substrate, in accordance with qualitative predictions based on the dipole moment considerations. The authors showed their ability to tune the work function from 4.30 eV to 4.95 eV by simply using different SAMs on top of the gold electrodes. The structures of the SAMs and the corresponding values of the measured work function are shown in Figure 9. Further, the authors showed changes in the hole-injection barrier, Δ<sub>h</sub>, into a copper-phthalocyanine (CuPc) layer deposited on top of the different SAM-coated gold substrates. This was done by measuring the position of the highest molecular energy level of CuPc (HOMO) with respect to the Fermi level for the different samples. The changes in the ''measured'' hole-injection barrier roughly follow the dependence: ''Δ<sub>h </sub>''</font></font></span>''<span style="line-height: 200%; mso-ascii-font-family: &quot;Times New Roman&quot;; mso-hansi-font-family: &quot;Times New Roman&quot;; mso-bidi-font-family: &quot;Times New Roman&quot;; mso-char-type: symbol; mso-symbol-font-family: Symbol"><font face="Symbol"><font size="12.0pt"><span style="mso-char-type: symbol; mso-symbol-font-family: Symbol">µ</span></font></font></span>''''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> Φ<sub>m</sub></font></font></span>''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">, where ''Φ<sub>m</sub>'' is the work function ''measured'' for the SAM-coated gold substrates. Thus, the use of the different π-conjugated-thiolate SAMs allowed for tuning the hole-injection barrier from the gold electrode into CuPc organic layer. The values of the hole-injection barrier, </font></font></span>''<span style="line-height: 200%; mso-ascii-font-family: &quot;Times New Roman&quot;; mso-hansi-font-family: &quot;Times New Roman&quot;; mso-bidi-font-family: &quot;Times New Roman&quot;; mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;"><font face="&quot;Wingdings 3&quot;"><font size="12.0pt"><span style="mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;">r</span></font></font></span>''''<sub><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">h</font></font></span></sub>''<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">, are also shown in Figure 9.</font></font></span>
+<p class=MsoNormal style='text-indent:.5in'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'>In summary,
+alkylthiolate-based monolayers on coinage metals have been shown to
+reproducibly modify the work function of the underlying substrates. Additionally,
+the molecular structure of constituents of SAMs, together with simple
+electrostatic considerations, has been shown to be useful in terms of designing
+surfaces with a desired work-function change. It is important to stress that
+the modification of the work function of metal electrodes with alkylthiolate
+SAMs has been demonstrated independently by different research groups, and that
+the ability to tune the work function has been successfully applied to solve
+specific technological problems.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>9,10</sup></span></p>
-<span style="line-height: 200%; mso-no-proof: yes"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">  </font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoNormal><span style='font-size:12.0pt;line-height:200%;font-family:
+"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 9.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Schematic representation of SAMs on gold studied by Chen et al. The measured work functions for the SAM-coated substrates, Φ<sub>m</sub>, as well as the hole-injection barriers measured for Au / SAM / CuPc systems, </font></span><span style="line-height: 200%; mso-ascii-font-family: &quot;Times New Roman&quot;; mso-hansi-font-family: &quot;Times New Roman&quot;; mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;; font-weight: normal"><font face="&quot;Wingdings 3&quot;"><font size="10.0pt"><span style="mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;">r</span></font></font></span><sub><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">h</font></span></sub><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">, are shown. Based on ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">44</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">. </font></span><span style="font-weight: normal"></span>
+<p class=MsoHeader style='margin-left:.5in;text-indent:-.5in;line-height:200%'><i><span
+style='line-height:200%;font-weight:normal'>1.7.<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+</span></span></i><i><span style='font-weight:normal'>Self-Assembled Monolayers
+of Conjugated Thiols and their Influence on the Work Function of Metals</span></i></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>A
+limitation on the use of monolayers consisting of long-chain alkyl thiols for
+modifying injection barriers between metals and organics is the intrinsic
+electrically insulating characteristics of the alkyl chains. Monolayers
+consisting of &#960;-conjugated thiols, which have been shown to exhibit
+considerably enhanced conductivity,</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>26,35</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+may be more promising for this type of application. Monolayers composed of &#960;-conjugated
+systems based on oligo(phenylethynyl)benzenethiols and
+oligo(phenyl)benzenethiols have been shown to form self-assembled domains on
+gold.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>21,22,36-38</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+A variety of data from different measurement techniques supports the formation
+of these SAMs on gold and silver with the thiol group binding to the underlying
+metal and the molecular backbone orienting itself nearly perpendicularly to the
+surface plane of the substrate.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>21,22,36,39-41</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+Even in the case of molecular structures based on biphenyl thiols with very
+polar substituents this structural arrangement seems to hold, as demonstrated
+by Kang et al. in their comprehensive report.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>22</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+As with alkylthiolate SAMs, the understanding of the structure of
+&#960;-conjugated thiolate SAMs on metal substrates opened the possibility to
+study the influence of the molecular structure on the electronic properties of
+the underlying substrates.</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Apart from other experimental reports on the metal-work-function changes induced by π-conjugated SAMs,</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>45,46</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> particular attention should be brought to theoretical work dealing with the energy level alignment of organic overlayers on gold. Among those, the reports by Heimel et al.,</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>3,47</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> and Romaner et al.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>15,48</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> are perhaps of the highest interest in the context of this thesis. These studies focus on theoretical description of biphenylthiolate-based systems as there is a vast amount of data showing these SAMs possess two dimensional order and their structure has been studied extensively, thus making them good model systems for theoretical investigations.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>19,22</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> Employing density functional theory (DFT), Heimel et al. showed that biphenylthiolate SAMs terminated with three different end-groups (a weak π-donor –SH, a strong π-donor –NH<sub>2</sub>, and a strong π-acceptor –CN; see Figure 10) can result in large changes of the work function of the underlying gold substrate.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>3,47</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span><span style="mso-bidi-font-size: 12.0pt; line-height: 200%"></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>Campbell
+et al. studied the effects of oligo(phenylethynyl)benzenethiolate SAMs on
+copper on the performance of organic diodes.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>42</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+Two molecular systems were compared, with the substitution of a terminal
+hydrogen atom for a fluorine atom as the only difference between the SAM
+constituents. The researchers compared the current – voltage characteristics of
+organic diodes with different structures – Cu anode / MEH-PPV / Ca, and Cu
+anode / SAM / MEH-PPV / Ca. The onset voltage for the conduction in the studied
+devices showed the expected trend, with the lowest onset recorded for the SAM
+composed of the fluorine-substituted molecules and the highest onset voltage
+for the corresponding SAM without fluorine substitution. The rationalization of
+the observed effect invoked the difference in the hole-injection barrier from
+the electrode to the organic layer due to the change in the work function
+caused by the formation of SAMs. Kelvin probe measurements indeed showed that
+the work function of the copper electrodes coated with the different SAMs
+qualitatively follows the prediction based on the well known effect of the
+sheet of dipole moments on top of the metal.</span></p>
-<span style="line-height: 200%; mso-no-proof: yes"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">  </font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"></font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>Zehner
+et al. were the first to systematically study the work function modification of
+gold by conjugated thiols.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>43</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+The authors applied Kelvin-probe measurements to a series of oligo(phenylethynyl)benzenethiolate
+SAMs on gold. The rather impressive number of studied systems included
+molecular structures with different lengths of the &#960;-conjugated backbones
+as well as different polar end-substituents. The trends of the work-function
+change reported in this work are actually not consistent with the predictions
+based on the molecular dipole moment. In particular, the molecular systems with
+dipole moments that should theoretically lower the work function actually
+increase it. Nevertheless, the authors attributed the observed work-function
+changes to the different dipole moment values of the studied molecular systems,
+and further conclusions followed.</span></p>
-<span style="line-height: 200%"><font size="10.0pt">Figure 10.</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt"> Schematic representation of biphenylthiolate SAMs on gold studied theoretically by Heimel et al. The calculated changes of the work function caused by the SAMs, </font></span><span style="line-height: 200%; mso-ascii-font-family: &quot;Times New Roman&quot;; mso-hansi-font-family: &quot;Times New Roman&quot;; mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;; font-weight: normal"><font face="&quot;Wingdings 3&quot;"><font size="10.0pt"><span style="mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;">r</span></font></font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">Φ<sub>m</sub>, and the offsets between the HOMO and the Fermi level, </font></span><span style="line-height: 200%; mso-ascii-font-family: &quot;Times New Roman&quot;; mso-hansi-font-family: &quot;Times New Roman&quot;; mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;; font-weight: normal"><font face="&quot;Wingdings 3&quot;"><font size="10.0pt"><span style="mso-char-type: symbol; mso-symbol-font-family: &quot;Wingdings 3&quot;">r</span></font></font></span><sub><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">HOMO</font></span></sub><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">, are shown. Results from ref. </font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">47</font></span><span style="line-height: 200%; font-weight: normal"><font size="10.0pt">. </font></span><span style="font-weight: normal"></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>Chen
+et al. investigated work function of gold substrates coated with a series of
+differently substituted terphenyl thiolate SAMs.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>44</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+All of the studied systems showed a depression of the work function when
+compared with clean gold. A gold surface with an overlayer of a SAM of
+terphenylthiolate, characterized by only a small dipole moment, showed a
+work-function value 0.90 eV lower than that measured for the clean gold.
+Because the dipole moment of the molecule is small, this change has to be
+attributed to the charge redistribution caused by the formation of Au – S bond.
+Substitution of hydrogen on phenyl rings in the terphenylbackbone with fluorine
+atoms showed an increase of the work function of the underlying substrate, in
+accordance with qualitative predictions based on the dipole moment considerations.
+The authors showed their ability to tune the work function from 4.30 eV to 4.95
+eV by simply using different SAMs on top of the gold electrodes. The structures
+of the SAMs and the corresponding values of the measured work function are
+shown in Figure 9. Further, the authors showed changes in the hole-injection
+barrier, &#916;<sub>h</sub>, into a copper-phthalocyanine (CuPc) layer
+deposited on top of the different SAM-coated gold substrates. This was done by
+measuring the position of the highest molecular energy level of CuPc (HOMO) with
+respect to the Fermi level for the different samples. The changes in the <i>measured</i>
+hole-injection barrier roughly follow the dependence: <i>&#916;<sub>h </sub></i></span><i><span
+style='font-size:12.0pt;line-height:200%;font-family:Symbol'>µ</span></i><i><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+&#934;<sub>m</sub></span></i><span style='font-size:12.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>, where <i>&#934;<sub>m</sub></i> is the
+work function <i>measured</i> for the SAM-coated gold substrates. Thus, the use
+of the different &#960;-conjugated-thiolate SAMs allowed for tuning the
+hole-injection barrier from the gold electrode into CuPc organic layer. The
+values of the hole-injection barrier, </span><i><span style='font-size:12.0pt;
+line-height:200%;font-family:"Wingdings 3"'>r</span></i><i><sub><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>h</span></sub></i><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>,
+are also shown in Figure 9.</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'><img width=575
+height=285 id="Picture 5" src="for%20STC%20Wiki_SAMs_files/image011.jpg"></span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Because of the electron-donating ability of the amino group to the π-conjugated biphenyl backbone, this system exhibits a dipole moment with the positive end at the organic / vacuum interface, thus ''decreasing'' the work function of the substrate by 2.7 eV. On the other hand, a strongly electron-withdrawing substituent, the cyano group, forms a dipole with its positive end at the metal / organic interface. This results in an ''increased'' work function of the substrate by as much as 2.7 eV. Interestingly, the mercapto-substituted biphenylthiolate SAM on gold (the structure in the middle of Figure 10), a system with essentially no dipole moment along the long molecular axis, showed the work function to be ''decreased'' by 1.0 eV when compared to that of clean gold. This has been attributed to the dipole layer formed by the charge rearrangement due to the Au – S bond formation, and labeled a ''bond dipole''. Analysis of the charge-density distribution led the authors to conclude that the bond dipole in all three studied systems is the same and, thus, that the charge rearrangement due to Au – S binding does not depend on the end-substituent of the biphenylthiolate backbone. Surprisingly, the offsets of the highest molecular orbital and the Fermi level for all three systems were calculated to be the same, despite the large differences in the gas-phase molecular ionization potentials. This has serious consequences for tuning of the hole-injection barrier from the gold electrode to the organic monolayer, as the authors clearly showed that the use of SAM constituents with clearly different molecular ionization potentials does not necessarily translate into a different offset between the HOMO and the Fermi level. However, it would be prudent not to draw general conclusions from these findings, as the results were obtained for biphenyl-based systems only and it is not clear if the used DFT methodology describes the studied systems in sufficient detail and accuracy. </font></font></span>''''''
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 9.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Schematic representation of SAMs on gold studied by
+Chen et al. The measured work functions for the SAM-coated substrates, &#934;<sub>m</sub>,
+as well as the hole-injection barriers measured for Au / SAM / CuPc systems, </span><span
+style='font-size:10.0pt;line-height:200%;font-family:"Wingdings 3";font-weight:
+normal'>r</span><sub><span style='font-size:10.0pt;line-height:200%;font-weight:
+normal'>h</span></sub><span style='font-size:10.0pt;line-height:200%;
+font-weight:normal'>, are shown. Based on ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>44</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>. </span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">Using the same DFT approach as in Ref. </font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>47</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt">, Romaner et al. studied the influence of the surface coverage on the work-function changes induced by substituted biphenylthiolate SAMs on gold.</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>48</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> Though some of the theoretically studied systems are not likely to be synthetically accessible, rather important conclusions were reached. Due to the polarizable nature of the biphenyl backbone of the SAM constituents the molecule – molecule distance on the gold surface has a large effect on the effective dipole-moment-layer depolarization. It was found that the increase of the surface coverage of the biphenylthiolate moieties not only increases the surface density of the dipole moments, but also leads to an increase in molecule – molecule depolarization effects, which effectively decreases the contribution of the additional dipoles to the change of the work function. Also, while the high-coverage systems showed the same behavior as those studied by Heimel et al.,</font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"><sup>47</sup></font></font></span><span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> at lower coverages there seemed to be a dependence of the energy offset between the HOMO and the Fermi level on the molecular ionization potential of the SAM constituents.</font></font></span>
+<p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<span style="line-height: 200%"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="12.0pt"> </font></font></span>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>Apart
+from other experimental reports on the metal-work-function changes induced by &#960;-conjugated
+SAMs,</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>45,46</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+particular attention should be brought to theoretical work dealing with the
+energy level alignment of organic overlayers on gold. Among those, the reports
+by Heimel et al.,</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>3,47</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+and Romaner et al.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>15,48</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+are perhaps of the highest interest in the context of this thesis. These
+studies focus on theoretical description of biphenylthiolate-based systems as
+there is a vast amount of data showing these SAMs possess two dimensional order
+and their structure has been studied extensively, thus making them good model
+systems for theoretical investigations.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>19,22</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+Employing density functional theory (DFT), Heimel et al. showed that
+biphenylthiolate SAMs terminated with three different end-groups (a weak &#960;-donor
+–SH, a strong &#960;-donor –NH<sub>2</sub>, and a strong &#960;-acceptor –CN;
+see Figure 10) can result in large changes of the work function of the
+underlying gold substrate.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>3,47</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+</span></p>
-''<span style="line-height: 200%; mso-fareast-font-family: &quot;Times New Roman&quot;"><font size="14.0pt"><span style="mso-list: Ignore">2.<span style="font: 7.0pt &quot;Times New Roman&quot;">                 </span></span></font></span>''''<span style="line-height: 200%"><font size="14.0pt">References</font></span>''
+<p class=MsoNormal style='text-autospace:none'><span style='font-size:12.0pt;
+line-height:200%;font-family:"Times New Roman","serif"'><img width=462
+height=278 id="Picture 6" src="for%20STC%20Wiki_SAMs_files/image012.jpg"></span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(1)<span style="mso-tab-count: 1">        </span>Cahen, D.; Kahn, A. ''Adv. ''</font>''<span lang="PL" style="mso-ansi-language: PL"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;">Mater.</font></span>''<span lang="PL" style="mso-ansi-language: PL"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> '''2003''', ''15'', 271-277.</font></span>
+<p class=MsoHeader style='line-height:200%'><span style='font-size:10.0pt;
+line-height:200%'>Figure 10.</span><span style='font-size:10.0pt;line-height:
+%;font-weight:normal'> Schematic representation of biphenylthiolate SAMs on
+gold studied theoretically by Heimel et al. The calculated changes of the work
+function caused by the SAMs, </span><span style='font-size:10.0pt;line-height:
+%;font-family:"Wingdings 3";font-weight:normal'>r</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>&#934;<sub>m</sub>,
+and the offsets between the HOMO and the Fermi level, </span><span
+style='font-size:10.0pt;line-height:200%;font-family:"Wingdings 3";font-weight:
+normal'>r</span><sub><span style='font-size:10.0pt;line-height:200%;font-weight:
+normal'>HOMO</span></sub><span style='font-size:10.0pt;line-height:200%;
+font-weight:normal'>, are shown. Results from ref. </span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>47</span><span
+style='font-size:10.0pt;line-height:200%;font-weight:normal'>. </span></p>
-<span lang="PL" style="mso-ansi-language: PL"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(2)<span style="mso-tab-count: 1">        </span>Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. ''Adv. ''</font></span>''<font face="&quot;Times New Roman&quot;,&quot;serif&quot;">Mater.</font>''<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> '''1999''', ''11'', 605-625.</font>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(3)<span style="mso-tab-count: 1">        </span>Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.-L. ''Acc. Chem. Res.'' '''2008''', ''41'', 721-729.</font>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>Because
+of the electron-donating ability of the amino group to the &#960;-conjugated
+biphenyl backbone, this system exhibits a dipole moment with the positive end
+at the organic / vacuum interface, thus <i>decreasing</i> the work function of
+the substrate by 2.7 eV. On the other hand, a strongly electron-withdrawing
+substituent, the cyano group, forms a dipole with its positive end at the metal
+/ organic interface. This results in an <i>increased</i> work function of the
+substrate by as much as 2.7 eV. Interestingly, the mercapto-substituted
+biphenylthiolate SAM on gold (the structure in the middle of Figure 10), a
+system with essentially no dipole moment along the long molecular axis, showed
+the work function to be <i>decreased</i> by 1.0 eV when compared to that of
+clean gold. This has been attributed to the dipole layer formed by the charge
+rearrangement due to the Au – S bond formation, and labeled a <i>bond dipole</i>.
+Analysis of the charge-density distribution led the authors to conclude that
+the bond dipole in all three studied systems is the same and, thus, that the
+charge rearrangement due to Au – S binding does not depend on the
+end-substituent of the biphenylthiolate backbone. Surprisingly, the offsets of
+the highest molecular orbital and the Fermi level for all three systems were
+calculated to be the same, despite the large differences in the gas-phase molecular
+ionization potentials. This has serious consequences for tuning of the
+hole-injection barrier from the gold electrode to the organic monolayer, as the
+authors clearly showed that the use of SAM constituents with clearly different
+molecular ionization potentials does not necessarily translate into a different
+offset between the HOMO and the Fermi level. However, it would be prudent not
+to draw general conclusions from these findings, as the results were obtained
+for biphenyl-based systems only and it is not clear if the used DFT methodology
+describes the studied systems in sufficient detail and accuracy. </span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(4)<span style="mso-tab-count: 1">        </span>Murphy, E. L.; Good, R. H. ''Phys. Rev.'' '''1956''', ''102'', 1464.</font>
+<p class=MsoNormal style='text-indent:35.4pt;text-autospace:none'><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>Using
+the same DFT approach as in Ref. </span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>47</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>,
+Romaner et al. studied the influence of the surface coverage on the work-function
+changes induced by substituted biphenylthiolate SAMs on gold.</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>48</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+Though some of the theoretically studied systems are not likely to be
+synthetically accessible, rather important conclusions were reached. Due to the
+polarizable nature of the biphenyl backbone of the SAM constituents the molecule
+– molecule distance on the gold surface has a large effect on the effective
+dipole-moment-layer depolarization. It was found that the increase of the
+surface coverage of the biphenylthiolate moieties not only increases the
+surface density of the dipole moments, but also leads to an increase in molecule
+– molecule depolarization effects, which effectively decreases the contribution
+of the additional dipoles to the change of the work function. Also, while the
+high-coverage systems showed the same behavior as those studied by Heimel et
+al.,</span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'><sup>47</sup></span><span
+style='font-size:12.0pt;line-height:200%;font-family:"Times New Roman","serif"'>
+at lower coverages there seemed to be a dependence of the energy offset between
+the HOMO and the Fermi level on the molecular ionization potential of the SAM
+constituents.</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(5)<span style="mso-tab-count: 1">        </span>Hölzl, J.; Schulte, F. K.; Wagner, H. ''Solid surface physics''<nowiki>; Springer-Verlag: Berlin, 1979; Vol. 85.</nowiki></font>
+<p class=MsoNormal><span style='font-size:12.0pt;line-height:200%;font-family:
+"Times New Roman","serif"'>&nbsp;</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(6)<span style="mso-tab-count: 1">        </span>Eastman, D. E. ''Phys. Rev. B: Condens. Matter'' '''1970''', ''2'', 1.</font>
+<p class=MsoHeader style='margin-left:.5in;text-indent:-.5in;line-height:200%'><i><span
+style='font-size:14.0pt;line-height:200%'>2.<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
+</span></span></i><i><span style='font-size:14.0pt;line-height:200%'>References</span></i></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(7)<span style="mso-tab-count: 1">        </span>Bock, C.; Pham, D. V.; Kunze, U.; Kafer, D.; Witte, G.; Woll, C. ''J. Appl. Phys.'' '''2006''', ''100'', 114517/1-114517/7.</font>
+<p class=MsoNormal style='line-height:normal'><span
+style='font-family:"Times New Roman","serif"'>            (1)        Cahen, D.;
+Kahn, A. <i>Adv. </i></span><i><span lang=PL style='font-family:"Times New Roman","serif"'>Mater.</span></i><span
+lang=PL style='font-family:"Times New Roman","serif"'> <b>2003</b>, <i>15</i>,
+-277.</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(8)<span style="mso-tab-count: 1">        </span>Santato, C.; Cicoira, F.; Cosseddu, P.; Bonfiglio, A.; Bellutti, P.; Muccini, M.; Zamboni, R.; Rosei, F.; Mantoux, A.; Doppelt, P. ''Appl. Phys. Lett.'' '''2006''', ''88'', 163511/1-163511/3.</font>
+<p class=MsoNormal style='line-height:normal'><span lang=PL style='font-family:
+"Times New Roman","serif"'>            (2)        Ishii, H.; Sugiyama, K.; Ito,
+E.; Seki, K. <i>Adv. </i></span><i><span style='font-family:"Times New Roman","serif"'>Mater.</span></i><span
+style='font-family:"Times New Roman","serif"'> <b>1999</b>, <i>11</i>, 605-625.</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(9)<span style="mso-tab-count: 1">        </span>Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. ''Phys. Rev. B: Condens. Matter'' '''1996''', ''54'', R14321.</font>
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (3)        Heimel,
+G.; Romaner, L.; Zojer, E.; Bredas, J.-L. <i>Acc. Chem. Res.</i> <b>2008</b>, <i>41</i>,
+-729.</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(10)<span style="mso-tab-count: 1">      </span>De Boer, B.; Hadipour, A.; Mandoc, M. M.; Van Woudenbergh, T.; Blom, P. W. M. ''Advanced Materials (Weinheim, Germany)'' '''2005''', ''17'', 621-625.</font>
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (4)        Murphy,
+E. L.; Good, R. H. <i>Phys. Rev.</i> <b>1956</b>, <i>102</i>, 1464.</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(11)<span style="mso-tab-count: 1">      </span>Nieuwenhuys, B. E.; Bouwman, R.; Sachtler, W. M. H. ''Thin Solid Films'' '''1974''', ''21'', 51-8.</font>
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (5)        Hölzl,
+J.; Schulte, F. K.; Wagner, H. <i>Solid surface physics</i>; Springer-Verlag:
+Berlin, 1979; Vol. 85.</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(12)<span style="mso-tab-count: 1">      </span>Nieuwenhuys, B. E.; Van Aardenne, O. G.; Sachtler, W. M. H. ''Chem. Phys.'' '''1974''', ''5'', 418-28.</font>
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+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (41)      Frey,
+S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.;
+Terfort, A. <i>Langmuir</i> <b>2001</b>, <i>17</i>, 2408-2415.</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-tab-count: 1">            </span>(48)<span style="mso-tab-count: 1">      </span>Romaner, L.; Heimel, G.; Zojer, E. ''Phys. Rev. B: Condens. Matter'' '''2008''', ''77'', 045113-9.</font>
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (42)      Campbell,
+I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris,
+J. P. <i>Appl. Phys. Lett.</i> <b>1997</b>, <i>71</i>, 3528-3530.</span></p>
-<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> </font>
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (43)      Zehner,
+R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. <i>Langmuir</i> <b>1999</b>, <i>15</i>,
+-1127.</span></p>
-</div><div style="mso-element: footnote-list"><br clear="all" />
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (44)      Chen,
-----
+W.; Huang, C.; Gao, X. Y.; Wang, L.; Zhen, C. G.; Qi, D.; Chen, S.; Zhang, H.
-<div id="ftn1" style="mso-element: footnote">
+L.; Loh, K. P.; Chen, Z. K.; Wee, A. T. S. <i>J. Phys. Chem. B</i> <b>2006</b>,
+<i>110</i>, 26075-26080.</span></p>
-[#_ftnref1 <span class="MsoFootnoteReference"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="line-height: 200%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">[a]</font></font></span></span></span></font></span>]<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> In terms of Fermi – Dirac statistics the Fermi level is defined as the energy at which the probability of finding an electron is 0.5 for a system at thermal equilibrium. For metals at room temperature the probability of finding an electron below the Fermi level is close to unity.</font>
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (45)      Zangmeister,
+C. D.; Picraux, L. B.; van Zee, R. D.; Yao, Y.; Tour, J. M. <i>Chem. Phys.
+Lett.</i> <b>2007</b>, <i>442</i>, 390-393.</span></p>
-</div><div id="ftn2" style="mso-element: footnote">
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (46)      Risko,
+C.; Zangmeister, C. D.; Yao, Y.; Marks, T. J.; Tour, J. M.; Ratner, M. A.; van
+Zee, R. D. <i>Journal of Physical Chemistry C</i> <b>2008</b>, <i>112</i>,
+-13225.</span></p>
-[#_ftnref2 <span class="MsoFootnoteReference"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="line-height: 200%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">[b]</font></font></span></span></span></font></span>]<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> In a real experiment the photoelectron-kinetic-energy analyzer causes an additional drop in the potential, which adds a rigid offset to the kinetic energy of the photoelectrons. More on this can be found in Ref. 12. </font>
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (47)      Heimel,
+G.; Romaner, L.; Bredas, J.-L.; Zojer, E. <i>Phys. Rev. Lett.</i> <b>2006</b>, <i>96</i>,
+/1-196806/4.</span></p>
-</div><div id="ftn3" style="mso-element: footnote">
+<p class=MsoNormal style='line-height:normal'><span style='font-family:"Times New Roman","serif"'>            (48)      Romaner,
+L.; Heimel, G.; Zojer, E. <i>Phys. Rev. B: Condens. Matter</i> <b>2008</b>, <i>77</i>,
+-9.</span></p>
-[#_ftnref3 <span class="MsoFootnoteReference"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="line-height: 200%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">[c]</font></font></span></span></span></font></span>]<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> In practice the photoelectrons are additionally accelerated by an external electric field, which has to be taken account in the calculations. </font>
+<p class=MsoNormal style='margin-left:.5in;text-indent:-.5in;line-height:normal'><span
+style='font-family:"Times New Roman","serif"'>&nbsp;</span></p>
-</div><div id="ftn4" style="mso-element: footnote">
+<p class=MsoNormal style='margin-left:.5in;text-indent:-.5in'>&nbsp;</p>
-[#_ftnref4 <span class="MsoFootnoteReference"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="line-height: 200%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">[d]</font></font></span></span></span></font></span>]<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> The solid-state electron affinity and solid-state ionization potential discussed here are not the same as the corresponding values measured for bulk organic layer. The contact with the metal electrode causes so-called “band bending”, which influences the values of both electron affinity and ionization potential. This is discussed in more detail in Ref. 13.</font>
+</div>
-</div><div id="ftn5" style="mso-element: footnote">
+<div><br clear=all>
-[#_ftnref5 <span class="MsoFootnoteReference"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="line-height: 200%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">[e]</font></font></span></span></span></font></span>]<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> The two described mechanisms provide an oversimplified picture of the charge-redistribution process. Contribution from orbital interactions is also invoked as another mechanism of charge redistribution. See ref. 24.</font>
+<hr align=left size=1 width="33%">
-</div><div id="ftn6" style="mso-element: footnote">
+<div id=ftn1>
-[#_ftnref6 <span class="MsoFootnoteReference"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="mso-special-character: footnote"><span class="MsoFootnoteReference"><span style="line-height: 200%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA"><font face="&quot;Times New Roman&quot;,&quot;serif&quot;"><font size="10.0pt">[f]</font></font></span></span></span></font></span>]<font face="&quot;Times New Roman&quot;,&quot;serif&quot;"> As mentioned in section 1.4 any adsorbate will change the work function of a metallic surface. Thus, only working under ultrahigh vacuum conditions with properly cleaned metal surfaces can yield true work function values of the metal surface.</font>
+<p class=MsoFootnoteText><a href="#_ftnref1" name="_ftn1" title=""><span
+class=MsoFootnoteReference><span style='font-family:"Times New Roman","serif"'><span
+class=MsoFootnoteReference><span style='font-size:10.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>[a]</span></span></span></span></a><span
+style='font-family:"Times New Roman","serif"'> In terms of Fermi – Dirac
+statistics the Fermi level is defined as the energy at which the probability of
+finding an electron is 0.5 for a system at thermal equilibrium. For metals at
+room temperature the probability of finding an electron below the Fermi level
+is close to unity.</span></p>
-</div></div>
+</div>
+<div id=ftn2>
+<p class=MsoFootnoteText><a href="#_ftnref2" name="_ftn2" title=""><span
+class=MsoFootnoteReference><span style='font-family:"Times New Roman","serif"'><span
+class=MsoFootnoteReference><span style='font-size:10.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>[b]</span></span></span></span></a><span
+style='font-family:"Times New Roman","serif"'> In a real experiment the
+photoelectron-kinetic-energy analyzer causes an additional drop in the
+potential, which adds a rigid offset to the kinetic energy of the photoelectrons.
+More on this can be found in Ref. 12. </span></p>
+</div>
+<div id=ftn3>
+<p class=MsoFootnoteText><a href="#_ftnref3" name="_ftn3" title=""><span
+class=MsoFootnoteReference><span style='font-family:"Times New Roman","serif"'><span
+class=MsoFootnoteReference><span style='font-size:10.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>[c]</span></span></span></span></a><span
+style='font-family:"Times New Roman","serif"'> In practice the photoelectrons
+are additionally accelerated by an external electric field, which has to be taken
+account in the calculations. </span></p>
+</div>
+<div id=ftn4>
+<p class=MsoFootnoteText><a href="#_ftnref4" name="_ftn4" title=""><span
+class=MsoFootnoteReference><span style='font-family:"Times New Roman","serif"'><span
+class=MsoFootnoteReference><span style='font-size:10.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>[d]</span></span></span></span></a><span
+style='font-family:"Times New Roman","serif"'> The solid-state electron
+affinity and solid-state ionization potential discussed here are not the same
+as the corresponding values measured for bulk organic layer. The contact with
+the metal electrode causes so-called “band bending”, which influences the
+values of both electron affinity and ionization potential. This is discussed in
+more detail in Ref. 13.</span></p>
+</div>
+<div id=ftn5>
+<p class=MsoFootnoteText><a href="#_ftnref5" name="_ftn5" title=""><span
+class=MsoFootnoteReference><span style='font-family:"Times New Roman","serif"'><span
+class=MsoFootnoteReference><span style='font-size:10.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>[e]</span></span></span></span></a><span
+style='font-family:"Times New Roman","serif"'> The two described mechanisms provide
+an oversimplified picture of the charge-redistribution process. Contribution
+from orbital interactions is also invoked as another mechanism of charge
+redistribution. See ref. 24.</span></p>
+</div>
+<div id=ftn6>
+<p class=MsoFootnoteText><a href="#_ftnref6" name="_ftn6" title=""><span
+class=MsoFootnoteReference><span style='font-family:"Times New Roman","serif"'><span
+class=MsoFootnoteReference><span style='font-size:10.0pt;line-height:200%;
+font-family:"Times New Roman","serif"'>[f]</span></span></span></span></a><span
+style='font-family:"Times New Roman","serif"'> As mentioned in section 1.4 any
+adsorbate will change the work function of a metallic surface. Thus, only
+working under ultrahigh vacuum conditions with properly cleaned metal surfaces
+can yield true work function values of the metal surface.</span></p>
+</div>
+</div>
+</body>
+</html>

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