"A major improvement
of this new method
is that the surfaces
do not need to touch
each other.
It only requires very
weak electric fields,
which are not likely
to influence the
electrical or
chemical structure
of the material."


































The Kelvin Probe Principles

When two conducting materials with different work functions are brought together, for example via an external wire contact, electrons in the material with the lower work function flow to the one with the higher work function. If these materials are made into a parallel plate capacitor, equal and opposite surface charges form. Measuring the contact potential is then exquisitely simple: an external potential is applied to the capacitor until the surface charges disappear, and at this point the external potential equals the contact potential. Kelvin realized this experimentally measured using two large flat polished disc's of Zinc and Copper and a gold-leaf electroscope to measure the charge transfer upon electrical contact.
In 1932 William Zisman of Harvard University introduced a new method to measure the contact potential. He mounted a vibrating reference surface, or tip, just above a sample electrode. The output voltage varies periodically as the tip vibrates, and the peak-to-peak voltage depends upon the difference between the contact potential and the external voltage. Changes in contact potential can then be detected by determining the external potential that yields a minimum or "null" output voltage. This technique lead to development of systems that automatically track shifts in the contact potential due to changes in the work function of the sample.
A major improvement of this new method is that the surfaces do not need to touch each other. It only requires very weak electric fields, which are not likely to influence the electrical or chemical structure of the material. Biasing of the sample rather than the tip will reduce noise, and the use of a low impedance current sensitive amplifier rather than a high impedance voltage sensitive amplifier minimizes the interference from parasitic capacities. However this isn't the whole story: real-world commercial systems inherently produce talkover from the driver (the device used to vibrate the tip).
This can substantially influence work function measurements and consequently an "off-null" technique has been developed by Baikie, see for instance, Baikie et al, 'Noise and the Kelvin Method, Review of Scientific Instruments, Vol. 62, page 1326, 1991. This system has the added advantage of allowing accurate measurements of mean capacity which can be used to perform scanning measurements at a constant height allowing the user to automatically perform measurements under identical experimental parameters. Without this feature the apparent work function difference will be different each time the user starts an experiment.
It is important to recognize that the Kelvin Probe is a relative technique capable of approximately 1 mV relative resolution. The work function of the tip must therefore be known in order to obtain the absolute work function of the sample. This problem has been addressed by illuminating a low work function reference sample with monochromatic ultraviolet light, and then measuring the energy at which current starts to flow. This technique can detect changes in absolute work function of 30-50 meV, see Baikie et al, 'Work Function study of rhenium oxidation using an ultra-high vacuum scanning Kelvin probe, Journal of Applied Physics, Vol. 88, page 4371, 2000.
 

Single Electron Transistor Device results scan
Kelvin Probe 50micron tip measuring a Single Electron Transistor Device
Scanning Kelvin Probe
UHV Kelvin Probe
Solar Panels
Scanning Kelvin Probe