Fig. 4a shows the to-
pographic STM im-
age of sputtered gold
islands on a flat
graphite surface. By 4
: lowering the tip-
sample distance ma-
; ; terial from the Pt-Ir-
tip has been striped-
a; ©) off before imaging,
creating a Pt-Ir-dot
Figure 3: air-STM of gold islands on graphite with PtIr-dot on top of gold island on top of a gold is-
imaged by STM: (a) topography, (b) barrier height image. land in the upper
right corner of the
image. Fig. 4b gives
the corresponding BH-image taken simultaneously with the topographic image, showing a clear
material contrast between the three different materials. Also the lowering of the BH at the edges of
islands due to the cos(o)-term is evident. The absolute value of the BH in general does not corre-
spond to the work function of the concerned materials, since (a) at small tunnel distances z the tun-
nel barrier gradually collapse down to zero, i.e. the BH is not constant but a function of z, (b) the
mean tunnel distance may change due to attractive or repulsive forces between tip and sample and
(c) when working in air surface contaminants strongly influence the effective BH, leading in most
cases to reduced apparent work functions.
Examples for BH-imaging of Me-DLC coatings can be found in (12).
Material Contrast by Friction Force Microscopy
Friction force microscopy (FFM), also called lateral force microscopy (LFM), is a method which
may provide contrasts between different materials in atomic force microscopy (AFM). Due to the
friction between the scanning tip and the sample the AFM cantilever makes a torsional twist whose
amplitude can be detected simultaneously with the vertical movement of the tip (due to sample to-
pography) by reflecting a laser beam on the cantilever back side and conducting it into a quadrant
photo detector. The twisting of cantilever is not only influenced by the local friction coefficient, but
also by the local tilt of the surface. To minimise this effect, forward and backward scanned FFM
images have to be subtracted
from one another (friction-
20 %P Joop) which cancels out most
topographic cross-talk if the
surface slopes are not too
steep (13,14).
Figure 5 shows AFM topogra-
phy and lateral force image of
sputtered gold islands on
graphite. Obviously, gold has
= Is a significant lower friction
= against the silicon tip com-
Figure 4: Gold islands on graphite imaged by AFM/FFM. Left: topogra- pared to the graphite substrate.
phy, right: lateral force image The increased lateral forces at
6
