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Insulating films with a thickness of only a few atomic layers
residing on a conducting substrate provide sufficient electronic
decoupling to allow us to study the inherent electronic properties
of individual molecules. As electrons can tunnel through these
ultrathin films, adsorbates can stilll be imaged with a scanning
tunneling microscope. In the example shown here of
individual pentacene molecules on one, two, and three layers of
NaCl on copper surfaces, we used this technique to image molecular
orbitals directly. STM images (see Fig. 1) acquired at bias
voltages corresponding to the negative (NIR) and positive ion resonance
(PIR) perfectly resemble the structures of the lowest unoccupied
(LUMO) and highest occupied (HOMO) molecular orbital of the free
molecule, that is, the bare molecule in vacuum. This opens up the
fascinating and unprecedented possibility to obtain direct images
of native molecular orbitals, completely disentangled from
the electronic structure of the substrate.
Molecular orbitals depend critically on the detailed molecular
structure. Single-molecule chemistry by scanning probe manipulation
allows the controlled breaking and formation of individual bonds.
Molecular orbital imaging in conjunction with single-molecule
chemistry introduces a new way to follow bond formation with utmost
detail. We have studied the bond formation between a single gold
atom and a pentacene molecule employing molecular imaging. Of particular
interest is the buildup of metal molecule complexes, which are
also the first step towards controlled electronic contacting of
single molecules. Figure 2 shows the bond formation between a
pentacene molecule and a gold atom on a bilayer of NaCl. In Fig. 2A,
the reactants are already located close to each other. The bond
was formed by inelasting tunneling (IET), and the resulting complex
(Fig. 2B) has a mirror plane that is perpendicular to the
long axis of the molecule, indicating that the gold atom is attached
to the central ring of pentacene (6-gold-pentacene) (Figs. 2C
and D). The bond can be broken again by IET-induced excitation
of the entire complex. Different Au-pentacene isomers were
formed with this technique. The reversibility of the complex formation
suggests that it is an additional reaction of the gold atom to
one of the pentacene's aromatic rings and involves neither the
substitution of a hydrogen atom nor the creation of a defect
in the substrate.
The gold-pentacene complexes exhibit two peaks in their dI/dV
spectrum, as in case of the individual pentacene molecule, but
with a much smaller gap. Orbital images taken at occupied and unoccupied
states are very similar, indicating that tunneling in
and out involves the same orbital (Fig. 3). Indeed, detailed
STM experiments indicate that Au-pentacence is neutral, therefore
electron counting shows directly that Au-pentacene
has one orbital that is only singly occupied. (SOMO: singly occupied
molecular orbital).
These experiments have been supported by theoretical calculations
performed by the groups of C. Joachim (CEMES Toulouse) and M. Persson
(University of Liverpool).
References
J. Repp, G. Meyer, S. Stojkovic, A. Gourdon, C. Joachim, Physical
Review Letters 94, 026803 (2005).
J. Repp, G. Meyer, S. Paavilainen, F. Olsson, M. Persson, Science
312, 1197 (2006).
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IBM Research
