Molecular diffusion and self-organization on metal surfaces : sub-phthalocyanine on Ag(111)

The adsorption and layering of chloro[subphthalocyaninato]boron(III) (SubPc) on Ag(111)
has been studied in detail by means of Scanning Tunneling Microscopy (STM) and photoelectron
spectroscopy at room temperature. SubPc is a polar molecule with an aromatic
14-�-electron system. An interesting phase behavior is observed for the �rst molecular layer
of SubPc growing on Ag(111). At a coverage below � 0:2 monolayer (ML), only a twodimensional
(2D) lattice gas is present, whereas at a coverage in the order of 0.2{0.5 ML
two-dimensional condensed molecular islands are in coexistence with the 2D lattice gas. In
these condensed islands, the molecules self-organize into well ordered overlayers with a honeycomb
pattern which has a low packing density. At a coverage of 0.6{0.9 ML the molecules
self-organize into a 2D hexagonal close packed (hcp) pattern. This hcp pattern is observed in
coexistence with a dense 2D gas phase. For the honeycomb and the hcp pattern, individual
molecules are imaged with sub-molecular resolution. Typically the molecules are imaged as
characteristic three lobes. The coexistence of the honeycomb pattern with the 2D lattice gas
is studied in detail. A dynamic equilibrium of the condensed honeycomb phase with its gas
phase by exchange of molecules is observed. The energy barrier for surface di�usion as well
as the condensation energy to form 2D islands is estimated and discussed.
Interesting and unusual features of the ordered molecular overlayers are observed. For
each pattern, the honeycomb and the hcp, only two di�erent orientations with respect to
the Ag(111) substrate exist. For the honeycomb pattern, the two di�erent orientations have
di�erent chirality, although the SubPc molecule itself is achiral. Not only single SubPc
molecules show di�usion but also whole ordered islands exhibit mobility. This becomes evident
in the observed
ipping process of overlayers, where whole condensed islands change
their orientation between the two stable orientations of the pattern. In the case of the honeycomb
pattern, this constitutes a transition between the two chiral layers. Phase transitions
from the gas phase to the condensed hcp phase and vice versa of whole overlayer islands
could be controlled by the STM tip. In vacancy islands of the Ag(111) surface produced
by sputter defects, it is possible to reversible change the phase of the con�ned molecular
overlayer by tip induced experiments. The two phases, the hcp phase and the gas phase,
exist at the same conditions at room temperature. Therefore one of these two phases is
thermodynamically not favored and in a metastable state. A microscopic model based on
the electric �eld between tip and sample is proposed for this tip-controlled phase transition.
In addition to the STM experiments the adsorption geometry and the electronic structure
of SubPc on Ag(111) is studied by means of photoelectron spectroscopy. It is observed that
the intact SubPc molecule is mainly physisorbed on the Ag(111) with the Cl towards the
Ag surface. The �-electron system of the molecule undergoes only slight changes upon
adsorption on the Ag surface. In general, the observed features of the molecular overlayers
are attributed to the charge distribution of SubPc and to its resulting dipole moment.