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supramolecules: they can be rotated around the surface normal or displaced laterally.
When two decamers are pushed towards each other they separate spontaneously
to a distance of 0.9 nm. The interaction between NN molecules on Au(111) is re-
lated to the asymmetry of the molecular charge distribution induced by the NO
2
group. Below a critical number of molecules, open linear chains with unsaturated
bonds at their ends energetically prefer to close into rings as observed for the de-
camers. The
C
2
symmetry of the decamers in the experimental images requires
the number of molecules of each type of chirality,
n
1
and
n
2
, in the cluster to be
even. Thus, the decamers have a well-defined overall chirality and each cluster
has a mirror-symmetric isomer with the same energy in which
n
1
and
n
2
are in-
terchanged. Its structure allows the hydrogen bonding of all oxygen atoms in the
cluster.
We end this section with two examples of the substrate's surface reorganiza-
tion induced by the incoming molecules: CuPc/Ag(110) (Bohringer
et al.
, 1997)
and HtBDC/Cu(110) (Schunack
et al.
, 2001). In the CuPc/Ag(110) case, CuPc
molecules interact with steps in a rather surprising way, inducing their reorganiza-
tion as a function of coverage. After adsorption of 0.6 ML and annealing, a drastic
change in the morphology of the clean Ag(110) surface occurs. The authors define
1 ML as one molecule per 15.8 substrate surface atoms, corresponding to a dense
molecular superstructure. Extended areas of substrate (110) terraces are separated
by bunches of steps aligned along the substrate [110] direction. The surface thus
becomes 3D faceted. However, after deposition of 1 ML CuPc 2D step faceting
occurs.
At low coverages HtBDC molecules deposited on Cu(110) surfaces decorate the
substrate steps, when held at RT, indicating that the diffusion barrier for an individ-
ual molecule on the flat Cu(110) surface is low enough to allow the molecules to
be mobile. However, at low
T
sub
STM images show individual immobile molecules
away from steps with a planar geometry with six lobes arranged in a distorted
hexagon with threefold rotational symmetry (see Fig. 4.9(A)). Each lobe can be
assigned to one of the tertbutyl appendages. At
T
sub
160 K, a 1D diffusion of the
single molecules along the [110] substrate direction sets in.
When a controlled manipulation of the molecules is performed by reducing the
tunnelling resistance by changing
I
t
or
V
t
or both, the HtBDCmolecules are pushed
outside the scanned area. The resulting
cleaned
surface area reveals the existence
of local disruption of the topmost copper surface layer. About 14 copper atoms
are expelled from the surface in two adjacent [110] rows, forming a trench-like
base for anchoring of the molecules as shown in Fig. 4.9(B). The spontaneous
surface disruption formed underneath the molecules during the adsorption process
is a generic way to reduce the mobility of the molecules and bind them to the surface
at even low coverages.
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