Hi!
I have an Asylum Research MFP-3D and it is written in the manual that SKPM is 'semi-quantitative', meaning that the Po channel during the second (lift) pass measures the 'surface potential' in mV, however this can be the resulting potential after several 3 contributions: workfunction (WF) difference between tip and sample, so-called contact potential difference (CPD), say Vcpd, such that WFt-WFs=eVcpd, where indices t and s are for tip and sample, respectively. (BTW: the left hand side being WFt-WFs is due to the point that the tip potential is assumed higher than the sample one; opposite than, e.g., Park AFM setup).
However, the total 'surface potential', call it Vsp, has other contributions: Vsp=Vcpd+Vq+Vext, where Vext may be applied externally (think it you look at a semiconductor device like a FET during operation, or interdigitated electrodes); and by Vq I mean V due to static charge q that may be present on the sample surface. So, the most widespread use of SKPM is indeed to measure the WFs, after known WFt, and the hypothesis that both Vext and Vq are 0. Now, if one wants to use it the other way around, with, ideally, Vcpd=0 (e.g. by using a tip and sample - or substrate - of same material, say both Au or both Ti); and measuring Vq, (we still assume that Vext, what is set in the software as Vsample offset, for instance, is also 0).
The point is mainly, in my opinion, how the sample is contacted. I think that it should not be contacted at all: you contact it, when want to measure WF. But if you contact to measure Vq, not only you share the original Vq on the sample with the contact pads; but even probably fully discharge it, if the sample is additionally connected to the ground plate (as recommended in the manual itself, once again, for conductive samples: but only to measure WF!) So my suggestion is: do not contact the sample at all. But would like to learn what other people think about that!
Additionalluy, about measuring Vq: can one think of a battery as a 'standard' for charge (say potential)? For example, a 1.5 V thin battery for watch or similar devices? Shall we read +1.5 V, if we set the + side up, and -1.5 V if we set the - side up? In this case, shall we contact the poles to the thin wire coming from the head? I think not, but am not sure.
Please help.
From experience:
1. You don't get stable KPFM signal if you don't contact the sample.
2. Using the battery is rather reliable, AFM machine can also allow you to apply specific bias. Whether you read +1.5 or -1.5 V will depend on which potential difference your AFM measures. Sometimes it is measured with respect to the sample, and sometimes with respect to the tip.
Thank you, Evgeniya.
About 2., sure one can also apply bias from the AFM-control PC, (there are always edit fields like 'sample' or 'tip voltage'). However, because this runs internally, I wanted to have an 'external', say, 'sample contained' voltage. Alternatively, one could also unse an external DC power supply; but, again, it wouldn'pt be as compact and 'sample-like' as a battery.
About 1: well, so you mean also that it is not impossible (or doesn not make sense) to measure dielectrics? Because in dielectrics, even if one contacts both the bottom of the sample and the sides (say by painting with Ag paste all around), still, as soon as the tip is a little bit inside the sample, say just 1 mm away from the Ag paste on the edge, for sure there is no electrical continuity. If your are right, so it is not possible to measure static charge at all. Is thsi the case? Whay the manuals advise against the contribution of static charge, then?
Just doubting of everything, nothin personal, of course!
Take care.
Hi there. I see that Amer Al-Taee (University of Mosul) recommended my question: thank you, Amer; I would like to learn from you what is your opinion on the raised points!
all the best.
Nothing personal, just science :P
Actually, chances are you won't even be able to land the tip if you sample accumulates too much static charge.
Hi again, Evgeniya.
It is an exciting discussion, for me :)
Simply, I wouldn't be that sure as you are, about some points.
For example, FYI, just few minutes ago
I succeeded in imaging a 1.5 V battery
(stable image, reasonable potential levels read)
on my Park XE100 in SKPM mode.
When the + electrode was face up, I read +1.8 V;
while, on just turning the battery upside down,
I measured -1.2 V.
So, sign as expected, stable imaging,
and also reasonable absolute levels:
consistent with an offset at the sample stub of +0.3 V.
In both cases (directions), the 'sample' contact was on the bottom side,
so actually at the electrode opposite to the true 'sample'
(top surface, where the tip 'lands').
Just by chance? I can hardly think so.
BTW: In the Asylum Research AFM,
which measures the potential based on a different mode (double pass),
indeed the test with the + up gave (despite the reasonable level read: +1.5 V)
some instabilities at both potential and height map,
which we could not cancel out easily.
Actually, this AFM is based on contact (Tapping) on the first pass,
while the Park one 'should be' working in Non-Contact.
Also, if I remember correct (have to check),
I contacted the top of the battery (ie the true 'sample' surface), in that case.
Will be happy to hear more from your side,
if you have time and will!
Ciao
The comment about static charge refers to a charged dielectric, not to the battery :D You should have no problems with stable signal on a battery. Let me know when you get stable KPFM on dielectric ;) I think I even saw such claims but was never able to do it.
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