Usually HOPG is used for calibrating Kelvin probe tip work-function.
However I found if operated in the air, HOPG surface will quickly adsorb air species and the measured contact potential drifts dramatically over a period of several hours after cleave.
Also, the work-function values for HOPG in the literature vary a lot from 4.5 to 4.8 eV. Which value should I trust?
How are you calibrating your Kelvin probe?
Dear Zhanhao,
According to our experience, the work function of a freshly cleaved HOPG is 4.48 eV. We found this value is acceptable for all our experiments to test a large quantities of different thin fim electrodes. You are right that HOPG is relatively stable. It does change in the air from hours to hours if the room temperature and humidity are not controlled well. Ideally Kelvin probe measurements should be done in vacuum.
For your reference, i include one of our published papers which has the reference where the work function of HOPG 4.48eV comes from, which we think the measurements done reliably.
https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.200700614
Hope it helps
Xuhua
Hello Zhanhao,
I would carry out the measurements in a glove box in N2 atmosphere or still better in Ar-gas or Ar/He gas. It is important to avoid oxygen and humidity contact of samples. Furthermore, I would calibrate HOPG in reference to a gold sample within this clean atmosphere. This could help to improve the results.
Generally, Kelvin probe measurements are very surface sensitive. So, all results are valid only with respect to the special conditions for the reference sample.
Best regards
R. Mitdank
Further Rüdiger's suggestion, i wouldn't do any measurements in an inert gas environment due to the gas flow influencing the vibration of the Kelvin tip. We did try this before. The results are not reliable. Normally we do the measurements either in the air or in vacuum. Hope it helps.
Thank you all for the useful advice.
I have experience doing the measurement in N2 before, and didn't found significant data variation, probably because I has switched off circulation when operating.
I agree that it is best to operate KP in inert environment, since tip surface may also adsorb something causing data variation.
In any case, I want to study sample WF in air. So I am trying to measure HOPG immediately after cleave, observe the trend of CPD shift in air in a long time scale, and then deduce back to what the intrinsic value should be for a clean HOPG before any air adsorption. Hope there is no big error in this logic.
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