Chemistry Reference
In-Depth Information
Fig. 5 STM images of Au monomers, dimers, and different chains on 2 ML MgO/Ag(001)
(
19 nm
2
). For the dimer, an upright (Au
2
0.4 V, 19
"
) and a flat-lying isomer is observed
(Au
2
!
)[
48
]
configuration is thermodynamically preferred by 0.34 eV, a finding that explains its
abundance in STM images of the MgO film (Fig.
5
)[
47
]. The preference of charged
over neutral Au clusters becomes even more pronounced for larger aggregates
[
48
]. A Au
3
chain for example (Bader charge
1|e|) has a higher binding energy
than various neutral isomers (Fig.
6
). This finding holds for Au tetramers, where a
doubly charged Au
4
chain (Bader value:
1.6|e|) is energetically preferred over
different rhomboidal structures that can be found on bulk MgO(001). Note that the
two extra electrons in the Au
4
chains are localized at the terminal atoms, empha-
sizing the shape-determining role of the Coulomb repulsion. With increasing atom
count, 2D Au aggregates gain stability with respect to 1D configurations, as the
formation of additional Au-Au bonds outweighs the energy surplus due to charge
delocalization [
48
]. Whereas, a Au
2
5
chain - having two negative charges - is still
iso-energetic with a flat-lying Au
5
sheet on MgO/Ag(001), a 2D Au
6
island has
already a lower energy than the corresponding linear structure (Fig.
6
). The critical
atom number at which the 1D
2D dimensionality crossover occurs has been
determined to five/six, both in experiment and theory.
So far, the amount of charge transfer into oxide-supported Au clusters has
proven mainly by DFT calculations. However, this quantity can be accessed also
by experiment, if the electron filling of specific quantum-well states (QWS), in
particular, of the highest-occupied (HOMO) and the lowest-unoccupied (LUMO)
state, can be measured with STM conductance spectroscopy. How this technique is
exploited to quantify the charge transfer into Au chains on alumina thin films is
demonstrated in the following (Fig.
7
)[
37
,
40
].
Also on alumina films, gold spontaneously forms linear clusters that are 8-22
!
Å
long and contain between two and seven atoms [
37
,
40
]. These Au chains exhibit a
particularly simple electronic structure that arises from the overlap of Au 6s and 6p
