90
4 Measurement of Work Function
space between the reference and the specimen, ε 0 is the vacuum permittivity, and A
is the area of the capacitor.
i = −εε 0 A ·
x 1 ω cos(ωt + α)
x
2
0
·
(4.15)
If a bias voltage, sometimes called a backing potential, is applied to the reference
so that the potential of the reference is equal to that of the specimen, = 0 and
no induced current flows. This is the principle of the Kelvin probe method. In other
words, the Kelvin probe method detects a CPD by detecting the current induced
by changing the distance between the reference and the specimen. In practice, the
induced current is measured as a function of the backing potential (Fig. 4.23). When
no current is induced, should be zero. From the equation = CPD + BP,
the backing potential for the zero-induced current corresponds to the CPD value but
with the opposite sign.
The typical distance x used in the measurement is about 0.5 mm. In principle,
the distance between the reference and the sample does not affect the measured
CPD. However, it often does in experiments, possibly owing to a nonideal capacitor
arrangement. Therefore, once the absolute work function of the reference is determined using a sample whose absolute work function is known, a series of work
function measurements should be conducted with the same distance. However, it
Fig. 4.23 Schematic illustration of measuring contact potential difference by Kelvin method (see
text for explanation)