6.3 Gold
103
6.3 Gold
6.3.1 STM/S-DFT: End and Edge Polarization
STM/S profiles in Fig. 6.1a, b revealed that the chain-end Au atoms on Si(553)
substrate are topographically higher than those in the chain interior associated with a
new DOS feature at −0.5 eV below E F [20]. This observation indicates the presence
of edge atom polarization by the densely entrapped electrons of bond between the
edge atom and its nearest neighbors. The polarization increases the atomic volume
and raises the energy of the local valence states. The dI/dV profiles for wires of
different thicknesses in Fig. 6.1c, d further evidence the size trends of polarization.
The wire of narrower (red) cross-section exhibits stronger polarization [21].
Using electron cohesive diffraction, Huang et al. [22] uncovered that the Au–
Au bond contracts only in the outermost two atomic layers of a gold nanosolid in
a radial way, which is further supported by molecular dynamics (MD) simulations
[23]. The Au–Au bond of a 3.5 nm size Au crystal contracting by 7% (Fig. 6.1b)
[22] is below BOLS expectation. The equilibrium Au–Au bond in the monatomic
chain contracts by 30% from the bulk value of 0.29 to 0.21 nm at 4 K, according
to the thermally-resolved strain limits of the chain [24]. The Au–Au bond contracts
by 12% in the fcc(100) skin. The CN-resolved bond contraction is insensitive to the
types of substrate support [25] or to the structural phase or particular elements or the
nature of the bond [26, 27].
DFT calculations of Au clusters with 13–147 atoms have confirmed the BOLSNEP predications (Fig. 6.1e, f) [10]:
(1) The Au–Au bond contracts by up to 30%;
(2) The valence charge transfers from the inner to the outer atomic shells of the
crystal;
(3) The valence charge transits from lower to higher binding energies and the extent
of polarization is more significant for smaller clusters than for larger ones;
(4) Such polarization should be responsible for the enhanced catalytic ability of Au
adatoms.
Further DFT calculations for the binding mechanism between gold nanoparticles
and DNA bases confirmed that [28] negative charges transfer from the inner volume
to the skin of Au nanoparticle as a result of the local quantum entrapment and the
valence states shift up toward the Fermi level due to polarization. Thereby Au dipoles
participate more actively in the binding to guanine. These effects are more prominent
in a smaller nanoparticle.
103
6.3 Gold
6.3.1 STM/S-DFT: End and Edge Polarization
STM/S profiles in Fig. 6.1a, b revealed that the chain-end Au atoms on Si(553)
substrate are topographically higher than those in the chain interior associated with a
new DOS feature at −0.5 eV below E F [20]. This observation indicates the presence
of edge atom polarization by the densely entrapped electrons of bond between the
edge atom and its nearest neighbors. The polarization increases the atomic volume
and raises the energy of the local valence states. The dI/dV profiles for wires of
different thicknesses in Fig. 6.1c, d further evidence the size trends of polarization.
The wire of narrower (red) cross-section exhibits stronger polarization [21].
Using electron cohesive diffraction, Huang et al. [22] uncovered that the Au–
Au bond contracts only in the outermost two atomic layers of a gold nanosolid in
a radial way, which is further supported by molecular dynamics (MD) simulations
[23]. The Au–Au bond of a 3.5 nm size Au crystal contracting by 7% (Fig. 6.1b)
[22] is below BOLS expectation. The equilibrium Au–Au bond in the monatomic
chain contracts by 30% from the bulk value of 0.29 to 0.21 nm at 4 K, according
to the thermally-resolved strain limits of the chain [24]. The Au–Au bond contracts
by 12% in the fcc(100) skin. The CN-resolved bond contraction is insensitive to the
types of substrate support [25] or to the structural phase or particular elements or the
nature of the bond [26, 27].
DFT calculations of Au clusters with 13–147 atoms have confirmed the BOLSNEP predications (Fig. 6.1e, f) [10]:
(1) The Au–Au bond contracts by up to 30%;
(2) The valence charge transfers from the inner to the outer atomic shells of the
crystal;
(3) The valence charge transits from lower to higher binding energies and the extent
of polarization is more significant for smaller clusters than for larger ones;
(4) Such polarization should be responsible for the enhanced catalytic ability of Au
adatoms.
Further DFT calculations for the binding mechanism between gold nanoparticles
and DNA bases confirmed that [28] negative charges transfer from the inner volume
to the skin of Au nanoparticle as a result of the local quantum entrapment and the
valence states shift up toward the Fermi level due to polarization. Thereby Au dipoles
participate more actively in the binding to guanine. These effects are more prominent
in a smaller nanoparticle.