Difference between revisions of "Photoelectron Spectrometer XPS and UPS"

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X-ray Photoelectron Spectroscopy and UV Photoelectron Spectroscopy are techniques for studying the surface characteristic of materials.
X-ray Photoelectron Spectroscopy and UV Photoelectron Spectroscopy are techniques for studying surface characteristics of materials.


===Overview Physics of XPS and UPS===
===Overview Physics of XPS and UPS===
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[[Image:PS-surfaceanalysis.jpg|thumb|right|300px |The small sampling depth of XPS and UPS arises because most of the photoelectrons generated do NOT make it out of the solid – they are scattered below the surface and not detected.  Only those within 1-10 nm of the surface get out and can be analyzed.]]
[[Image:PS-surfaceanalysis.jpg|thumb|right|300px |The small sampling depth of XPS and UPS arises because most of the photoelectrons generated do NOT make it out of the solid – they are scattered below the surface and not detected.  Only those within 1-10 nm of the surface get out and can be analyzed.]]


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Other XPS examples
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XPS Analysis
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Image:Angle resolved data.jpg|Spectra a,c and e are “normal takeoff” angle spectra of the O(1s) region from indium tin oxide surfaces that have been subjected to various pretreatments.  Spectra b,d and f are from the same samples, with high takeoff angles of analysis, giving more near-surface information.  In this case we see new peaks from hydroxyl (-OH) species located right at the surface of the oxide.
Image:Angle resolved data.jpg|Spectra a,c and e are “normal takeoff” angle spectra of the O(1s) region from indium tin oxide surfaces that have been subjected to various pretreatments.  Spectra b,d and f are from the same samples, with high takeoff angles of analysis, giving more near-surface information.  In this case we see new peaks from hydroxyl (-OH) species located right at the surface of the oxide.


Image:Xps_spectra.jpg|These high resolution XPS spectra show the power of the technique for the characterization of ITO surfaces with different PA modifiers.  Note especially the C(1s) spectra region in (b) where we observe several different peaks due to carbon in its various forms within each molecule.  The peak with a binding energy (BE) of ca. 285 eV is due to carbon in the backbone of the molecule, whereas higher BE peaks are seen for carbon bonded to fluorine (see the peaks at 291 and 293 eV for FHOPA).  Similar molecular differences are seen in the O(1s) (a) P(2p) and In(4s) © and F(1s) spectral regions.
Image:Xpsspectra.jpg|These high resolution XPS spectra show the power of the technique for the characterization of ITO surfaces with different PA modifiers.  Note especially the C(1s) spectra region in (b) where we observe several different peaks due to carbon in its various forms within each molecule.  The peak with a binding energy (BE) of ca. 285 eV is due to carbon in the backbone of the molecule, whereas higher BE peaks are seen for carbon bonded to fluorine (see the peaks at 291 and 293 eV for FHOPA).  Similar molecular differences are seen in the O(1s) (a) P(2p) and In(4s) © and F(1s) spectral regions.


Image: Ups_au_graph.jpg|Using UV photons as the excitation source we probe photoelectrons emitted from the valence band region of our materials of interest.  Here for example is a UPS spectrum (He(I) excitation) for a clean gold surface.  The regions of most interest are the high kinetic energy photoelectrons, and the energy where we begin to detect them (red line) and the low kinetic energy electrons and the lowest energy where they can be detected (blue line).  The energy difference between these two threshold energies (w), subtracted from the source energy (21.2 eV) gives us an estimate of the effective surface work function of this material – in this case the work function for clean Au is about 5.1 eV.
Image: Ups_au_graph.jpg|Using UV photons as the excitation source we probe photoelectrons emitted from the valence band region of our materials of interest.  Here for example is a UPS spectrum (He(I) excitation) for a clean gold surface.  The regions of most interest are the high kinetic energy photoelectrons, and the energy where we begin to detect them (red line) and the low kinetic energy electrons and the lowest energy where they can be detected (blue line).  The energy difference between these two threshold energies (w), subtracted from the source energy (21.2 eV) gives us an estimate of the effective surface work function of this material – in this case the work function for clean Au is about 5.1 eV.
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Latest revision as of 11:32, 18 January 2012

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X-ray Photoelectron Spectroscopy and UV Photoelectron Spectroscopy are techniques for studying surface characteristics of materials.

Overview Physics of XPS and UPS

OLEDs and OPVs consist of thin films of organic materials, sandwiched between contacting electrodes. We need analytical tools which tell us:

  • Elemental composition of metal, metal oxide and organic surfaces (top 1-10 nm)
  • The molecular state of those elements in that same region
  • The frontier orbital energies which control rates of charge transfer, photopotentials, onset voltages, etc. see Work Function of Metals


XPS uses high energy X-ray photons to excite “core” electrons in the near-surface region UPS uses lower energy photons in the deep UV region to excite valence electrons.

We use high-vacuum surface electron spectroscopies: X-ray photoelectron spectroscopy (XPS)and UV-photoelectron spectroscopy (UPS) to provide the elemental, molecular and energetic information we require about these materials. Surface analysis is carried out in high vacuum spectrometers, with sophisticated sample handling capabilities. The sample is prepared in a chamber to which a variety of devices can be attached. The idea is to keep the surface as clean as possible, and to selectively add monolayers of organic materials to these surfaces, without the need to break vacuum between analyses. The sample is located at the center of the analytical chamber, and positioned so that we can excite it with either X-rays or UV photons. Once the photoelectrons are ejected from the sample, they are collected by a series of focusing lenses, and then separated according to their kinetic energy in a “hemispherical” analyzer. We use either a single “channeltron” detector (UPS) or a multi-channel detector(XPS)

Technique

The small sampling depth of XPS and UPS arises because most of the photoelectrons generated do NOT make it out of the solid – they are scattered below the surface and not detected. Only those within 1-10 nm of the surface get out and can be analyzed.

Other XPS examples

XPS Analysis


Significance