Difference between revisions of "Photoelectron Spectrometer XPS and UPS"

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Image: Ups_au_plus_alkane.jpg|Now we have added a single monolayer of an alkane thiol modifier, which binds strongly to the Au surface, and changes the UPS spectrum, the effective work function of the Au surface.  This modifier is a 12-carbon chain, terminated with a phenyl group, and we see a new ionization peak (orange) for the ionization of these phenyl groups.  The width of the photoemission spectrum has changed as well, indicating a new work function for this modified Au surface.
Image: Ups_au_plus_alkane.jpg|Now we have added a single monolayer of an alkane thiol modifier, which binds strongly to the Au surface, and changes the UPS spectrum, the effective work function of the Au surface.  This modifier is a 12-carbon chain, terminated with a phenyl group, and we see a new ionization peak (orange) for the ionization of these phenyl groups.  The width of the photoemission spectrum has changed as well, indicating a new work function for this modified Au surface.


Image:Ups_fluoridated_alkanes.jpg|Now we see a series of UPS data for Au surface modified with alkanethiols of different lengths (from C3 to C18) (left panel) and a similar series of alkanethiols which are fluorinated at 1,2,4 and 10 positions along the chain (right panel).  At the bottom of the screen we have schematically shown the orientation of the molecular dipole moments for these modifiers – the normal alkanes point the positive end of the dipole away from the surface, and lower the work function; the fluorinated alkanes point the negative end of the dipole away from the surface and increase the work function.






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Revision as of 12:59, 22 April 2009

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

What is the Problem?

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.

What is our approach? Physics of XPS and UPS

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)


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.
<swf width= "640" height="480">http://depts.washington.edu/cmditr/media/pes.swf</swf>


Data interpretation