I'm studying the properties of nickel oxide thin films for its application in quantum dot led as a hole transport layer. One of the primary parameter is its electron affinity. Can XPS (X-ray photoelectron spectroscopy) be used to measure it?
I agree with Joydeep. XPS uses monochromatic X-rays to eject electrons due to the photoelectric effect. The amount of energy that the electron is ejected with is proportional to the binding energy for that element's electron state.
The work function can be obtained from the secondary cutoff, but you better use UPS for WF determination. As for photoemission on semiconductors the VBmax gives the ionization potential you can add Egap for obtaining the electron affinity. If you have single crystalline samples monoXPS is the better choice to measure VBmax because you integrate over the Brillouin zone due to the high kinetic energy in contrast to UPS where you have k-conservation and JDOS. If you have strong n-type samples the electron affinity is close to EF.
XPS is frequently used for measuring the binding energies and permits also deduce the oxidation state of materials. I think, to calculate the work function (WF) in XPS, you need a reference signal (like Au or Ag deposited on the same surface). As said by Christian, for the WF it is better to use UPS (cut-off) rather then XPS.
As your material is an oxide, it is possible that you observe some satellite pics then you have to be careful.
As far as I remember you can write the Born-Haber cycle who is taking into account the photoionization then you can deduce the ionization potential or someting like that from the Z+1 approximation if you know some parameters. I don't remeber exactly but for more details, you can have a look to the book of Hüfner on photoemission spectrocopy.
Yes, the cut-off in the UPS spectra is much more accurate than monochromatic XPS. It is better to use He I source for this purpose.
The primary peaks allow the determination of the BEs of the electronic states in the sample, while the secondary electrons are essentially used for the measurement of the workfunction of the sample
How can we see the secondary cut-off for insulators or semiconductors using UPS? I mean i apply the bias of about -9 but I cant see. Should i apply higher bias voltages?
Usually you don't need any bias for this measurement. Just use the He I line and collect the spectrum from about -5 till 20 eV in binding energy. You will see the cut-off at high BE. Measure its value and subtract it from the He I photon energy of 21.2 eV.
UPS( He1) is a good technique to measure workfunction (WF). but you get be careful when you work on insulator/semiconductor. because you might not get well defined femi level on those material, it is hard to find a ref point for absolute WF measurement. There might be charging which causes second cut off drift. If Femi level is well defined, you can use it as ref( 0ev) to calibrate UPS spectrum. but if there is no well defined femi level, to use 2nd cut off as WF is very questionable.
It was reported that XPS can be use to measure WF, and you have to apply bias to the sample during measurement, because this helps to distinguish analyzer and sample cutoff, and collect emitted electrons more efficiently into the analyzer. For UPS, you don't need to apply Bias as Saulius mentioned above.
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