First of all: since the physics behind UPS and XPS is the same, you can use both. However, UPS is usually done with smaller slits and therefore better detector resolution. But in principle you can measure the secondary electron edge of an (ideally biased) sample and calculate the work function from XPS like from UPS. You just need to set your detector to a smaller pass energy than usual.

you can make use of these links

Article Investigation of work function and chemical composition of t...

Article An XPS Study of Amorphous Thin Films of Mixed Oxides In2O3-S...

and the pdf attached

First of all: since the physics behind UPS and XPS is the same, you can use both. However, UPS is usually done with smaller slits and therefore better detector resolution. But in principle you can measure the secondary electron edge of an (ideally biased) sample and calculate the work function from XPS like from UPS. You just need to set your detector to a smaller pass energy than usual.

Dear Dr. Rajagopalan Pandey,

I suggest you to have a look at the following recent papers:

-Investigation of work function and chemical composition of thin films of borides and nitrides

A. Mezzi, P. Soltani, S. Kaciulis, A. Bellucci, M. Girolami, M. Mastellone, D. M. Trucchi

Surface and Interface Analysis, Volume50, Issue11, Pages 1138-1144 (2018)

Available at: https://onlinelibrary.wiley.com/doi/full/10.1002/sia.6442

-Investigation on energy bandgap states of amorphous SiZnSnO thin films

Lee, B.H., Cho, K., Lee, D. et al.

Scientific Report 9, Art. n.19246 (2019)

Available at: https://www.nature.com/articles/s41598-019-55807-2

-X-ray photoelectron spectroscopy: Towards reliable binding energy referencing

G.Greczynski, L.Hultman

Progress in Materials Science, Volume 107 (2020)

Available at: https://www.sciencedirect.com/science/article/pii/S0079642519300738

Best regards, Pierluigi Traverso

Hi Rajagopalan,

Added to Jürgen comment, I would say that this depends on what you are looking for. The information depth of the two techniques is not the same. They are roughly 5 nm and 1 nm for XPS and UPS, respectively. This means that if you would like to investigate the work function of the very near surface of your sample, I would suggest the UPS.

However, I would also highly recommend Kelvin probe force microscopy for work function study.

See the paper attached, please.

Best wish

Christian

Yes, you can use XPS for calculating the work function of amorphous materials. Because XPS and UPS are two different techniques. From XPS we collected the signal from the top 5 to 10 nm of the surface, however in case of UPS, we obtained the information from the very top surface i.e from 0.5 to 1.5 nm . You please see the below links

https://www.nature.com/articles/s41598-020-58592-5.pdf?origin=ppub

https://avs.scitation.org/doi/abs/10.1116/6.0000031?journalCode=jva

I hope these work for you.

Just two remarks regarding the probing depth:

1) The depths given here are values for primary electrons, i.e. electrons that have not experienced any inelastic effects after the emission from their state. The determination of the work function is done by analyzing secondary electron emission, i.e. electrons which have been slowed down by interactions/scattering in the crystal before leaving. These may also come from deeper regions of the material.

2) The work function is not a bulk property, it is characteristic of the surface orientation, so either way you are determining a surface property, not a bulk crystal property, see e.g.

Article Orientation dependence of the work function for metal nanocrystals

Since you are analyzing amorphous samples you may get a broader secondary electron edge and the value you determine will either be a mean value or maybe there is a preferred surface orientation despite the overall amorphous character of your phase.

Dear Jurgen,

The thin film surface analyses can be carried out by using a monochromatic Al Kα source for XPS, an ultraviolet He (I and II lines) lamp for UPS. The samples must fixed on the standard sample stage by using metallic clips, guaranteeing the electrical contact between the films and the apparatus ground. Work functions of the samples can be performed and investigated by XPS at optimum pressure in analysis chamber and UPS. Work function ϕ of the samples can be calculated by measuring the characteristic cut‐off energy in the He I&II obtained spectrum.

https://onlinelibrary.wiley.com/doi/full/10.1002/sia.6442

Ashish

I know all those things. My comment was focused on the fact that the penetration depths given above are for primary electrons while the work function is determined from the secondary electrons that are not described by the universal curve.

Dear Jurgen,

Yes that is true. It is an onset of secondary electron cutoff.

As I know the work function (Φ) is the energy required to remove an electron from the Fermi energy to the vacuum level (EVAC - EF). Work function changed in one of two ways either by shifting the Fermi level position (via doping) with a constant ionization potential or by changing the ionization potential (via changes in the surface dipoles) with a fixed Fermi energy. The work function of a material can be measured using several different methods, including ultraviolet photoelectron spectroscopy (UPS) and photoelectron spectroscopy. As we know work function of a material can be extremely sensitive of the surrounding environment (e.g. gases, adsorbed species, humidity, etc.), the most commonly device UPS/XPS which can work under ultra-high vacuum. By isolating the material from the environment, UPS is able to determine an “intrinsic” work function by measuring the kinetic energies (EKE) of photoelectrons excited by incident ultraviolet light (hν). Take the graph of intensity vs. binding energy, the binding energy of the sample can be calculated from the energy of the incident light, kinetic energy, and the work function of the detector is plotted versus intensity. Finally, the work function of the sample can be found by taking the difference between the excitation energy and the secondary electron cutoff (the energy after which no secondary electrons are being excited). The work functions of various amorphous and crystalline transparent conducting oxides can be measured using XPS/UPS; it can directly measure barrier heights of amorphous material. But, the films, which is made by deposition generally exhibits varying work functions dependent on the composition and deposition parameters.

Ashish

dear Dimitris,

Its possible.

Ashish

Hello Ashish Thakur , I think you are missing my point. What you write is correct and I have done (and also taught) all of these things plenty of times myself.

The point I was trying to make with my posts here is that the work function is always a surface property and you always detect this surface feature, no matter whether it's XPS or UPS. You can bias a sample in both methods and receive an isolated signal, it's just better resolved with UPS because you can operate the spectrometer at a higher resolution. That's all I wanted to say here, at no point did I exhibit a doubt of the basic, well-established method.

Dimitris Tsikritzis : as I often post in XPS discussions, be careful with the C1s charge correction, see this project by G. Greczynski:

https://www.researchgate.net/project/Why-you-should-not-use-the-C-1s-peak-for-referencing-XPS-spectra

Dear Jurgen,

I am agree with you in a totality. No question about it. I have used instrument for my research 20 years ago.

XPS and UPS both can be used. But, depth of the technique is not the same for two of device. I know very well UPS is better resolved device. Basically we need a chamber, a molecular beam epitaxy chamber equipped with reflection high-energy electron diffraction RHEED is interconnected with the x-ray and ultraviolet photoelectron spectroscopy (XPS-UPS) analysis chamber via a transfer chamber. If I talk about energy resolution, they have different in values. The samples can be biased at minimum electron volt to make the measurements of the very low-energy region of the spectrum reliable. For finding high binding-energy cutoff of the secondary electrons and the valence-band VB region of the substrate for various thicknesses of overlayer on nanolevel UPS is considered a better device.

Ashish

Thank you Rajagopalan.

The Surface Photovoltage Spectroscopy (SPS) is more accurate in measuring work function, though XPS is also used, but the analysis is not that straight forward, you need to have read extensively on how to do the data analysis, while SPS is straight forward as in not great skills are required to extract the work function value