Chemistry Reference
In-Depth Information
Fig. 6.1 Water monomer
and small clusters adsorbed
on a Pt(111) surface.
Reprinted with permission
from Ref. [
20
]. Copyright
2004, American Physical
Society
when water molecules are adsorbed onto Cu(110) at *80 K, they spontaneously
aggregate into the 1-D structure with its width of 10 Å, which is aligned along
[001] axis. Theoretical calculations predicted that the 1-D structure consists of
interlinked water pentagon units [
23
]. The water growth along the Pt step edges
also shows a zigzag H-bond chain [
24
]. This class of 1-D structure is of particular
interest, because such a 1-D configuration is known to exist in biological systems,
mediating proton translocation across the transmembrane proteins [
3
]. Water
chains confined in carbon nanotubes were intensively studied as a model system to
investigate the structure and dynamical properties [
4
,
5
].
As described above, several types of water clusters and the growth of extended
structures have been revealed, but the structural characterization of intermediate
size clusters (3-5 molecules) still remains lacking.
6.2 Results and Discussions
6.2.1 Water Trimer on Cu(110)
A water trimer is produced by the reaction between a water monomer and dimer.
Figure
6.3
shows the production of a water trimer. A monomer and dimer are
characterized as a round and bi-stable fluctuating images, respectively, and they
diffuse on the surface even at 6 K. A trimer is formed spontaneously when they
sufficiently come close to each other. A trimer is characterized by an asymmetric
triangle protrusion and never dissociated into a monomer and dimer spontaneously
at the conditions of its production, indicating it is energetically favorable. It is
found that four orientations exist for a trimer, which can be interconverted with a
voltage pulse of STM (Fig.
6.4
a-d). The threshold voltage was *0.12 V.
The structure of trimer is optimized by DFT calculations
1
[shown in the bottom
of the STM images in Fig.
6.4
]. In the optimized structure water molecules aligns
1
DFT calculations were performed using the STATE code [Y. Morikawa et al. Phys. Rev. B 69,
041403 (2004).], within the Perdew-Burke-Ernzerhof (PBE) [J. P. Perdew et al. Phys. Rev. Lett.
77, 3865 (1996).] generalized gradient approximation (GGA). Wave functions and argumentation
charge were expanded in a plane-wave basis set, and electron-ion interactions were described by
