Wednesday, April 28, 2010

Kelvin probe in-situ TEM

First some background:
When a nanowire is put into resonance with the in-situ TEM-SPM method by applying a alternating electric field with the resonance frequency of the nanowire between the two tips a offset voltage can be used to reduce the netcharges between the nanowire and the opposing tip. This offset voltage should be dependent on the work-function of the tip material and the nanowire itself, when the netcharges are removed by applying the offset voltage it is visualized in TEM as a reduction of the resonance amplitude of the wire. So by plotting the resonance amplitude as function of offset voltage one can in turn measure the work function of the nonmaterial. This is called the kelvin probe method.

The results:
I made some test on this, and the first result was as can bee seen below, a plot with a nice linear behavior with a minimum, indicating that the difference in work-function of the Mo6S3I6 nanomaterial and the gold-tip is 0.76 eV, -reasonable?
I tried to repeat the experiment but got inconsistent results it appeared that the minimum amplitude due to the offset voltage was dependent on the distance between the nanowire and the opposing tip. Therefore a made a experimental serie on distance between nanowire and tip, as can be seen below. The length of the nanowire is about 6 um and the tested range of distances were 0.4 um to 9.9 um. As seen in the plot only short distances produces a minimum amplitude dependence on offset voltage, and the minimum shifts in offset voltage for different nanowire tip distances. This is strange if the minimum in resonance amplitude is said to only be dependent on the work-function of the nanowire and opposing tip. There also exist a non-linear term ter in the Kelvin probe equation, perhaps this is influencing the results? Short range effects due to electrostatic forces might also have an influence, but if this was the case one should expect the oposite effect but here at longer distances the effect of a minimum amplitude dependence on offset voltage disappear.

Electrostatic charging is a common problem in TEM, in which the electron beam introduces charges in the sample. I also tested this dependecy in the Kelvin probe experiment. As can be seen in the last graph above, several beam intensities in the range from 0.18mA/cm2 to 2.5mA/cm2 were tested and indead it do have a influence on the resonance amplitude minimum. The minimum shifts roughly 1 Volt at the lowest beam setting relative to the highest. The intensity ranges tested here are considered to be in the normal intensity range, no extreme setting that is.

So as for now, no good result were achieved, proper tip distance setting and how to reduce beam influence needs to be solved.

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