Chemistry Reference
In-Depth Information
Fig 20 Connection
between Au charge scale
and the stretching frequency
of adsorbed CO [
92
]
spectrum, which has been identified with the help of a DFT calculation. The large
shift of 285 cm
1
with respect to the frequency of the gas-phase CO arises because
the Au 6s valence electron is strongly polarized away from the MgO surface due to
Pauli repulsion with the oxygen ions to which the Au preferentially binds. The
concomitant transfer of electrons into the C-O antibonding 2
*
orbital leads to this
dramatic weakening of the intermolecular bond, hence a red shift of the stretching
frequency. This shift is even larger than for an Au atom residing on a color center
holding one electron, i.e., there the shift is 220 cm
1
[
93
].
π
3.1 CO Adsorption on Au on Thin MgO/Ag Films
Molecular species can also be identified directly on oxide-supported gold
nanostructures, using low-temperature STM. For example, single CO molecules
are imaged as circular depressions on the MgO surface (Fig.
21a
).
A similar imaging contrast is observed on most metal surfaces, where it origi-
nates from the absence of CO orbitals for tunneling transport close to the Fermi
level (
E
F
)[
94
]. CO attachment to the tip gives rise to a distinct contrast change of
the adsorbates, which now appear as dark rings of
diameter (Fig.
21b
). This
imaging mode enables CO identification even on corrugated surfaces, e.g., next to
an Au deposit, where the tiny CO-induced depressions obtained with metallic tips
are no longer detectable. Figure
21c
shows a corresponding STM image of an Au
island saturated with CO. Apparently, the CO-induced features are exclusively
localized along the island perimeter, while the center remains adsorbate-free. An
alternative method to identify CO molecules on the oxide surface is inelastic-
electron-tunneling spectroscopy (IETS) with the STM [
95
-
97
]. The peak/dip
positions observed in the second-derivative d
2
l/dV
2
spectra correspond to the
vibrational energy modes of the molecules. The CO vibrational mode with the
highest excitation cross section in STM-IETS is the frustrated rotation [
95
]. Conse-
quently, bias voltages of 45 mV have been chosen to identify CO molecules next to
the 2D Au islands via their vibrational signature (Fig.
22a-d
).
8
Å
