Chemistry Reference
In-Depth Information
Fig. 6.5
Cluster/substrate lattice misfit as a function of the number
N
of atoms in the cluster at
400 K (data from [
32
]) and 500 K. The cluster lattice constant attains the bulk value at
N
>
10
while at a given critical coverage of 56.3% compressed Pb islands are formed
with lattice constant reduced by 2.65% with respect to that of the bulk Pb phase
[
16
,
38
]. This effect reveals dramatic change in the magnitude of lateral inter-
actions from low- to high-coverage range and reflects the environment-dependent
character of the interactions at the interface layer [
10
,
13
,
15
]. The observed size-
dependent cluster/substrate misfit variation affects the general diffusion behavior.
This is demonstrated for three-atomic island by variation of the lateral Pb-Pb inter-
actions through the value of the hopping integral
,(
6.6
). Hence, keeping constant
the cluster/substrate normal interaction, we generate variable misfit in the system,
Fig.
6.6
. Since the number of atoms in the cluster is fixed, the shape effect is elim-
inated and therefore the oscillations of the diffusion coefficient are caused exclu-
sively by the misfit variation. In all cases for islands with specific number of four,
five, six, and seven atoms,
D
varies non-monotonically, oscillating with the misfit
value, Fig.
6.6
. It is essential to note that if the misfit does not affect the cluster
diffusion, we would observe monotonic dependence of
D
with increasing the lateral
bonding. The simulation results also suggest that the contribution of cluster misfit
is more pronounced at small
N
up to 10 atoms. Above this value the misfit is nearly
constant with cluster size and the shape effect dominates the diffusion behavior.
For three-atomic clusters
D
changes between 0.2 and 0.7, while for seven-atomic
clusters this variation is smaller, being between 0.04 and 0.2. All values of
D
are
normalized to the diffusion coefficient of single atom. The above results stand for
essential contribution of the lattice misfit to the island diffusion behavior.
Because of the quite general physical origin of the misfit variation with cluster
size (broken crystal symmetry and periodicity, small number of interacting atoms,
ΞΎ
