Chemistry Reference
In-Depth Information
R
hc
shows a
certain degree of modulation as the distance between them increases. The observed
step shapes are due to thermal fluctuations, where molecules flow along the steps
(periphery diffusion), move from steps to terraces and then reattach to the step at
different sites (terrace diffusion), and move between the steps and terraces with
no correlation between motion at different sites (attachment-detachment). Steps
can in fact be viewed as 1D interfaces or massless strings that can vibrate with
any wavelength larger than the molecular scale. The restoring force that keeps step
fluctuations from growing indefinitely is the free energy cost of increasing the step
length, which is governed by the step stiffness. Several examples of steps with node-
like shape are observed, such as those given in Fig. 5.13. A detailed discussion of
the properties of steps in thermal equilibrium (for inorganic materials) can be found
in Jeong and Williams (1999).
Figures 5.11, 5.12 and 5.13 were acquired by reducing the AFM cantilever os-
cillation amplitude to a minimum in order to avoid tip-induced step movement
while maintaining good image contrast. If the cantilever oscillation amplitude is
intentionally increased the surface becomes slightly perturbed. Under these con-
ditions tip-induced step movement is observed as manifested by the unwinding of
the spirals, as illustrated in Fig. 5.14. Here is shown the time evolution of TMAFM
images of the surface of a thin film of
In general, the shape of the step connecting two hollow cores at
R
α
-
p
-NPNN taken at ambient conditions.
Tip-induced unwinding of screw dislocations has been reported for spirals in the
layered inorganic material
-HgI
2
(Lang
et al.
, 1996). In this case etching depends
on the mechanical contact between the tip and the
α
α
-HgI
2
step edges leading to
vaporization of the material.
The unwinding of screw dislocations induces the exchange of spiral centres.
Spiral centres necessarily exchange among each other at every turn because of their
close proximity. The tip-induced etching observed in Fig. 5.14 can be viewed as
a time-reversed growth. The tip removes material from the steps, so that the area
of the terraces defined by two pairs of coupled spirals is reduced and as a conse-
quence 2D menisci are formed, as illustrated in Fig. 5.14(a). All menisci shown in
Fig. 5.14 have a height of 1.2 nm. The thinning of a meniscus down to its col-
lapse can be clearly followed in the sequence Fig. 5.14(a-c). The neck dimension
decreases down to
c
. 10-15 nm, where the meniscus breaks. This value is possibly
limited by the tip diameter.
Figure 5.14(c) shows the case of the rupture of the meniscus during scanning.
After rupture the remaining steps are highly unstable and relax by reducing their
curvature (Fig. 5.14(d)). The relaxation is an intrinsic effect and involves subli-
mation of material, simply triggered by the tip, and is a consequence of the re-
duction of the line tension
γ
l
. The example of a 2D island of a given area can
help in understanding this effect. The associated line tension has an energy
γ
l
L
b
,
