Chemistry Reference
In-Depth Information
Fig. 4.14
Schematic drawing of energies at a film surface. The
solid curve
represents the electro-
static potential across the film surface. The oscillations in the potential are caused by the Friedel
oscillations of the surface electron density. Its value deep in the vacuum side is usually called the
vacuum energy
Fig. 4.15
The work function of Pb films as a function of thickness on Ge(111)
√
3
√
3 substrate
×
Experimentally, WF oscillations have been measured with scanning tunneling
spectroscopy. Qi et al. [
51
] have investigated the thickness dependence of the WF
on flat-top-wedged Pb islands using the STM and STS measurements. Figure
4.16
a
shows the STM image of a flat-top Pb island with thickness increasing from left
(11ML) to right (15ML). The inset shows the atomically resolved STM image of
the top (111) plane. Fig.
4.16
b shows the cross-sectional profile along the line
Figure
4.16
a, c, d shows the topographic and WF images of the island taken
simultaneously. From the WF image, we can clearly see that even layers (12 and
14ML) have larger work functions compared to odd layers (11, 13, and 15ML).
The tunneling current strongly depends on the tip-sample distance because of its
dependence on the image potential. However, it can be shown that the quantity
dln
I
d
s
is a constant to first approximation [
52
]. Hence, the work function can
be determined from
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