The Technique

The Kelvin Probe is an extremely sensitive analytical tool. It measures changes in contact potential difference between a reference material and a sample which depend upon changes in the work function of the material being studied. The work function is defined as the amount of energy need to release electrons from the materials surface- it is a 'barometer' of the samples optical, electrical and mechanical properties.
The Kelvin Probe is a non-invasive technique, yet it is extremely sensitive to changes in the top-most atomic layers, such as those caused by absorption, deposition, wear, corrosion and atomic displacement. In some cases it can detect less that one-thousandth of an absorbed layer, see Baikie, 'Old Principles...New Techniques', PhD Thesis, Univerisity of Twente 1988, ISBN 90-9002444-1.
Although not as well known as some other surface analysis techniques the probe has undergone a dramatic renaissance over the past few years. Advances in hardware design and signal processing technology have improved the resolution of the instrument while also ensuring it can be used in vacuum. Improvements have been made to the spatial and energy resolution of this technique.
Once limited to around 100um, new designs based upon an atomic force microscope can map surface properties in the 20-30 nm range. Several spectroscopic varients have been developed for the analysis of surface, interface and sub-surface, regions of semiconductors.
The paper 'Study of high and low work function surfaces for hyperthermal surface ionization using an absolute Kelvin probe' by Baikie et al, Journal of Vacuum Science and Technology A, Vol. 19, page 1460, 2001, shows work function topographies of surfaces held in vacuum. It also demonstrates the large change in work function of approximately 2 eV as a thin layer of oxygen is incorporated into a Rhenium surface held at 800K.
This paper also reports data on low work function surfaces such as LaB₆, Gd and Ca. High work function surfaces can be use to generate positive ions. Low work function surfaces can be used as electron sources and to generate negative ions.