Chemistry Reference
In-Depth Information
Fig. 6.7 a-d STM images of the four orientations of a chain-like tetramer. The conversion is
induced with a voltage pulse of the STM. The schematic models are illustrated in the images. The
images were obtained at V
s
= 23 mV and I
t
= 0.5 nA. Scan area is 37 9 22 Å
2
formed. A distortion in the image appearance, which is also observed for a [110]-
trimer, is discernible and there are four orientations as shown in Fig.
6.7
a-d. In
addition to the chain configuration, several structural isomers emerge due to the
increased degrees of freedom for a tetramer. The conversion into other isomers can be
induced with a voltage pulse of STM. The voltage pulse over a chain-like tetramer
induces the conversion of a parallelogram into a triangle- or round-shaped tetramer
(Fig.
6.8
b, c). These three structural isomers can be mutually converted. The
threshold voltages of the conversions are estimated to be *120, *140 and
*180 mV for parallelogram, triangle, and round-shaped tetramers, respectively,
with the tip fixed at V
s
= 23 mV and I
t
= 0.5 nA over a tetramer.
Several optimized structures of tetramer were determined by DFT calculations.
2
The optimized structures are shown below the STM images in Fig.
6.8
. Similar to
trimer, the H-bond acceptor molecule located at the end is higher than the others in
the chain configuration, which contributes to the brightest protrusion in the image.
The vertical displacement between the H-bond acceptor and donor located at the
opposite end is 0.74 Å. Two more stable structures are determined; one has a
cyclic configuration in which each molecule is linked via H bond. A round-shaped
tetramer is assigned to the cyclic configuration. The other forms a ''tetrahedral''
configuration in which the center molecule simultaneously acts as a double donor
and a single acceptor of H bond. The similar configuration was proposed for a
tetramer
on
Cu(111)
[
12
].
A
triangle-shaped
tetramer
is
assigned
to
this
2
The water tetramers were put on one side of five-layer slabs with 4 9 6 periodicities and
Brillouin zone sampling was done using 2 9 2 Monkhorst-Pack k-point sets for the cell.
