Dear colleagues,
I'm calculating local potential (LOCPOT) data in VASP for the semiconductor/metal interface in order to extract work functions (WF), VBM, CBM, etc. It's known that in the case of two different WFs a step will appear when dipole correction is included (e.g. see https://www.researchgate.net/post/Who-can-help-me-solve-the-problem-about-work-function)
Indeed, I easily got such a step for the case of Ag/h-BN interface, for example (see Fig. 1) when activate LDIPOL = .TRUE., IDIPOL = 3 (i.e. Z direction).
However, for some other semiconductor/metal interfaces when using the same dipole correction I have very strange curve at the vacuum positions that looks like apple cores or oscillations.
Nevertheless, in Fig. 2 one can still see a step despite strange oscillations.
However, in Fig. 3 no step is presented at all.
So, my two questions are:
1) Why do these strange curves may appear at the vacuum level? (instead of a simple step as in Fig.1)
2) What should I change/control to fix it and find a step
P.S.
I have already played with vacuum size. In Fig.2 and 3 it's twice higher than the structure thickness. No evidences that further increase of vacuum size can help me.
Also, I was tested a bit DIPOL flag (center mass position for dipole correction) and found that VASP can set it automatically without any problem when the vacuum is high enough. But maybe I'm wrong and I have to set up manually DIPOL flag in some trouble cases.
I will highly appreciate any ideas and advices! Thank you.
It might be a wrap-around error. If so, then you can fix it with setting PREC=High.
Dear Konstantin,
I think, if your length scale is very less, then you will not get a sharp change in surface potential. This is due to contribution from surface electrons from other sites. But, if you increase the length scale , you will not get the fluctuations and a smooth change in potential may be obtained.
Thanks
N Das
Dear Nityananda Das ,
Thank you a lot for the answer!
By "length scale" do you mean the scale of Y-axis? If that, I've started to think the same honestly... So, maybe in Fig. 1 I simply have a big difference of WFs, so I don't need to zoom-in my scale and that's why I see a smooth line. While, in contrast, when WFs difference it too small and when I enlarge Y-axis too much, no smooth line is observed (Fig. 3). Your idea that it's caused by some small (but visible at some scale) contribution of surface electrons sound reasonable for me.
So, finally, do you propose that there's no any error and I just don't need to pay attention to these "strange curves" visible only at the small scale?
Dear Konstantin,
My idea is that the charge distribution inside the material has a periodic structure. This is not constant throughout the whole material.
So, it is expected that potential at any point due to this periodic charge distribution must be periodic ,if we look very minutely ( I mean the variation within A⁰ order. ) . This order of fluctuation will reduce as we move away from the surface.
So, I think the fluctuations will be there and the average fluctuation will be reduced as we proceed away from the surface.
If we don't want to look at these fluctuations, we need to get data for the points separated more than atomic distance and the scale division of y axis needs to be larger least count.
MY MAIN POINT IS THAT THE FLUCTUATIONS SHOULD BE THERE.
Thanks
N Das
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