Friday, February 08, 2013

Kelvin Probe

Hey, I'm an experimentalist. It may be a revelation to some because I see this blog linked to many different sites, and some of them referred to it as ".. written by a high energy theorist..."! Me? High energy theorist? :)

Anyhow, I think it is always good to highlight certain experimental techniques, especially the ones that I'm quite familiar with. So here's one.

Often, for most of us, our first encounter to measuring the work function of something is via the photoelectric effect. Later on, if some of us go into photoemission spectroscopy, we deal with the work function there as well. However, that is not the only means to measure the work function. A technique that does not require the use of any light source to perform such measurement is called the Kelvin probe.

If you understand the physics behind a pn junction in semiconductors, then you've already understood the physics used in a Kelvin probe. You have a metal (the probe) with a particular Fermi energy, in close proximity (or in contact) with another material that has a different Fermi energy. This creates a contact potential difference. The Kelvin probe measures this contact potential difference, which is the difference between the two Fermi energies.

The link that I gave above gives you more details on such a measurement. If you know the value of the Fermi energy of the probe, you get the value of the work function of the material being studied. The only caveat here is that if the sample you are looking at is a semiconductor, you do not get the "work function", but rather, the value of the Fermi energy with respect to the vacuum state. In many cases, this is called the work function (energy between the Fermi level and the vacuum state) of the semiconductor, but one should not confuse this with the photoemission threshold, because the Fermi level resides in the gap. The photoemission threshold is the energy between the top of the valence band and the vacuum level.

Confused? Let's go on.

A significant improvement in the Kelvin probe technique was introduced with the invention of the Atomic Force Microscopy (AFM). It turns out that one can adapt an AFM system to work in the Kelvin probe mode. This technique is now called Kelvin Probe Force Microscopy (KPFM). I've linked to a very nice review paper on the physics and capabilities of KPFM technique. This technique allows for the mapping of the surface potential (or work function if you know the work function of the tip) of a surface with spatial resolution of the order of 100 nm.

So why is this useful. Obviously, knowledge of the surface potential is crucial in the understanding of the behavior of many materials (read some of the references in the KPFM review paper). It is another parameter that is part of the characteristics of the material. It also provides another check to the value of the work function that are often obtained from photoemission experiments. In fact, some material actually react to the exposure of light (especially UV), and the value of the work function obtained may not be as accurate as it should be from photoemission measurement.

There. So now, if you haven't heard about this before, you've learned something new! :)


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