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are four high-symmetry adsorption sites; on-top(a), short-bridge(b), long-bridge(c)
and hollow sites(d). An unambiguous way to determine the adsorption site with STM
is observing an atomic arrangement of the substrate and molecules at the same time.
It is, however, extremely difficult due to the small corrugation of Cu surface.
Therefore I employed an alternative way as follows; a maker molecule whose
adsorption site is well-established is co-adsorbed with water, and the water location
can be determined by comparing the relative position between them. A carbon
monoxide (CO) molecule is well-established to be adsorbed on a top site [
16
]
on Cu(110). In addition, CO molecules can be easily identified by STM-IETS where
the hindered rotation is observed. According to the relative position of water mol-
ecules to CO, the adsorption site is determined to be a top site (inset of Fig.
4.1
a). The
adsorption structure of water was determined by DFT calculations.
1
Figure
4.1
b
shows the optimized structure and the on-top configuration is very consistent with
the experiment. The adsorption energy is 0.34 eV and the Cu-O bond length is
2.21 Å. The water adsorption is accompanied by a charge transfer from the occupied
lone pair orbital of the molecule (1b
1
orbital) to unoccupied metal d-orbitals, con-
sequently, a water molecule prefers to occupy the electron deficient on-top
adsorption sites. It is noted that the DFT calculations indicated the water molecule
shows a slight displacement from the exact top site and the displacement from the
center of a substrate atom is *0.5 Å. Ren and Meng also predicted the similar
displacement by 0.5 Å [
11
], and Tang and chen reported the displacement of 0.88 Å
[
10
]. The center of protrusion in the STM image, however, is exactly positioned on
the on-top site. The calculated barrier to the azimuthal rotation is less than 0.02 eV,
implying almost free rotation of a water molecule around the on-top site, resulting in
the featureless round protrusion. It might be suggested that the facile rotation of a
water monomer arises from its zero-point motion.
4.2.2 Diffusion of a Water Monomer on Cu(110)
It was found that water molecules thermally diffuse along the atomic row even at
6 K on Cu(110). The fractional image of the upper right monomer in Fig.
4.1
a
indicates that the molecule hops to the adjacent on-top site when the STM tip
scanned over it. Not only the thermal activation but also the tip-induced diffusion
must be taken into account in STM measurement. The contribution of the latter
depends on tunneling conditions, i.e., the gap distance and the applied bias voltage.
To elucidate the diffusion mechanism, I investigated the diffusion rate using the
time-resolved measurement of the STM. Two different techniques were employed
to determine the diffusion rate. At low bias voltages, the atom tracking technique
1
DFT calculations were performed using the STATE code [Y. Morikawa et al. Phys. Rev. B 69,
041403 (2004).]. In the calculation the water molecules is put on one side of five-layer Cu slabs
and 2 9 3 surface unit cell with 16 k point sampling.
