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Trapped charge in KPFM measurement

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  • Trapped charge in KPFM measurement

    Hi everybody,

    I am a PhD student (2 years) and I am measuring the potential of different metal samples such as titanium with the KPFM technique. We have made a surface treatment to the titanium that can produce a residual charge (or trapped charge) on its surface. My doubt is, when I connect the sample with the tip, this charge is maintained and can be measured? or when the contact is made, the residual charge, being a metal, is cancelled?

    Thanks in advance,

  • #2
    Dear Danièl, this is a very interesting question! In fact, it is written in the manual by Asylum 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 contac 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, you want to use 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, yes, 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: yes, do not contact the sample at all.
    I have another question, 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.