 SKP370 Scanning Kelvin Probe System
The Scanning Kelvin Probe (SKP) is a non-contact, non-destructive instrument designed to measure the surface work function difference between conducting, coated, or semi-conducting materials and a metallic probe. The technique operates using a vibrating capacitance probe, and through a swept backing potential, the work function difference is measured between the scanning probe reference tip and sample surface. The work function can be directly correlated to the surface condition. A unique aspect of the SKP is its ability to make measurements in a humid or gaseous environment.
The principle of operation of the Kelvin Probe is illustrated in Figure 1.
The figure represents three energy level diagrams depicting two metals having a constant separation, work functions and Fermi levels of (φ1, E1) and (φ2, E2), respectively. The first diagram shows the metals with no electrical contact and differing Fermi  levels. When electrical contact is made (middle diagram) the flow of charge allows the Fermi levels to equalize and gives rise to a surface charge. The potential difference, VC, is related to the difference in work function:
-eVC = φ1 - φ2 (1)
where e is the electron charge. The inclusion of an external backing potential, VB (final diagram), allows for a nulling of the surface charge at a unique point where VB = - VC. This point represents the work function difference between the two materials.
When capacitance is at a minimum, the sample potential, VB is varied automatically to ensure that the null point is always maintained and thus the potential at which nulling occurs equates to the work function difference between the probe and sample. Therefore, the values presented in the analyses are opposite to the actual work function being measured.
Work function can then be used to determine the corrosion potential (Ecorr) at a specific point using the relationship:
Ecorr = Constant + (φ1 - φ2) (2)
where (φ1 - φ2) is the measured work function between the probe and the sample. The constant can be determined by measuring the corrosion potential using a conventional reference electrode in an electrolyte. Once the constant is known for a particular sample, Ecorr can be calculated
directly from the SKP data without an electrolyte.
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