"The Kelvin Probe is a
yet it is extremely
sensitive to changes
in the top-most atomic layers, such as those
absorption, deposition, wear,
corrosion and atomic displacement."
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
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.