I want to calibrate the work-function of our Kelvin probe using HOPG as a reference sample.
For the commonly used SPI HOPG, there are three grades, SPI-1, SPI-2 and SPI-3. Is there any difference between their work-function values?
In the literature, the work-function of HOPG is reported to be in a large range, from 4.4 eV to 4.8 eV, though most of them claim it to be 4.6 eV. Is this because they were using different grades of HOPG?
Dear Zhanhao,
Following your previous question, the value of work function for HOPG as a standard reference refers to the intensive study below
https://www.sciencedirect.com/science/article/pii/S0039602801010366
You should use the best grade of HOPG as a reference. You will find the different grade of HOPGs have various surface quality.
I found that the surface roughness of HOPG definitely affects the value of its work function.
Hope it helps.
Xuhua
The work function depends significantly on the microscopic structure of the surface, i.e. step density, defects, and so on. Since the step density depends on the crystallographic orientation, especially for an anisotropic material like HOPG, you will get different values for different grades. Always use the best grade available as reference.
Furthermore, when looking in tables with work-function values, there should always be given the crystallographic orientation. This leads to the fact, that, on a microscopic scale you may get a specific value for the work function for a specific oriented surface, but when averaging over many different surface orientations (e.g. powder), you will get an average value.
Dear Zhanhao Hu,
The work-function is the minimum thermodynamic work (energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. In air, it is difficult to obtain an absolute work-function measurement with KP since contamination can change the work-function of the reference. You should use highly oriented pyrolytic graphite (HOPG), which after cleavage has a work-function of about 4.6 eV. As HOPG usually does not form interface dipoles, even the presence of typical ambient contamination does not change the work-function significantly. This is basically different from metal surfaces where contamination can change the work-function from 0.5 eV to 1 eV.
I hope I have answered your question.
With Best Wishes,
Samir G. Pandya
Thank you very much for all of your answers.
In my experience, and also as reported in some papers, the work function of HOPG changes significantly in air after cleave, although it will not change as much if cleaved and measured all in N2.
There are quite some papers doing work function measurement in air, using KPFM or macroscopic Kelvin probe. However rarely have they provided information of how the calibration was done.
Whereas I see the point of Samir, I'm not that sure that HOPG is always better than metals. Frankly, I would recommend using gold, rather. It seemed to me less variable, in the literature, as compared to HOPG (see also my concerns in
doi:10.3390/ma11060951)
take care
Gold is not stale as HOPG in air and in the nitrogen filled glovebox too. We did it many times. It is right that the freshly cleaved HOPG is not stable, but after while it is very stable. The paper I referred in the above answer has answered this question, Zhanhao. They did thtoughly measurements with HOPG using different techniques. We already made many many testing in the past. That is what we came through to use HOPG as a reference.
Dear Xuhua Wang
Thanks for your following of the question.
You said 'It is right that the freshly cleaved HOPG is not stable, but after while it is very stable. '
How can you know the absolute work-function of HOPG after it becomes stable?
in principle, the 'true' WF of HOPG would be that one decading soon after new peeling-off, before all the ambient contamination is adsorbed (O2, hydrocarbons, etc). Actually, I don't know if this is evan feasible (what is the time for measurement? 10 min will be ok? Or 1 min will already bee too late? Diffusion in air is fast ar RT.
Dear Marco Salerno
In my case, I peeled off the surface to expose the new, then start the measurement within 1 min. The WF obtained decays linearly within at least 3 min, so I can interpolate the curve to get the 'initial' WF of the HOPG.
However, I am not sure if this interpolated 'initial' WF is indeed the true WF of the clean HOPG, because if there is any swift change of the WF immediately after peeling off the HOPG surface, that will make the WF decay curve not exactly linearly from the start, and then the interpolation will not be valid.
@Zhanhao
What you need to do is to control the system using a computer if you don't have an automatic data recording Kelvin measurment. It is a straightforward that you can write a simple program using either Labview or other coding techniques.
Ideally you should take measurements in an inert gas environment to overcome the surface adsorption. However it is possible to take valid measurements if the temperature and humidity in your lab is well controlled.
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