Chemistry Reference
In-Depth Information
Fig. 3.2 STM image of the
Cu(110) clean surface. The
atomic rows of Cu running
along
½
110
direction are
observed
I conducted most of the experiments at extremely low coverages, where water
molecules exist mainly as an isolated monomer or dimer.
3.3 STM Measurement
All STM measurements were carried out at 6 K. A bias voltage is applied to a
sample, which is denoted as V
s
. Tunneling current denoted as I
t
, is detected after
conversion to voltage by a current amplifier.
I used a tungsten (W) tip as a probe for all STM measurements. W is commonly
used material for STM tip under UHV conditions because W has a mechanical
stiffness and a relatively flat electron density of states (DOS) near the E
F
that
makes it easy to interpret an obtained STM image. W tips were fabricated by
electrochemical etching of W wires in NaOH solution. STM tips fabricated in
atmosphere are usually covered by oxide layers. To remove the oxide layers and
impurities, I performed in situ tip cleaning by applying a high bias voltage which
induces a field evaporation of atoms on the tip apex [
10
] and restructuring of the
tip itself due to a non-uniform electric field [
11
].
All STM images presented in this thesis are acquired in constant current mode.
The STM tip is moved from the top to bottom (Raster scan) while keeping the tip
height (z) to give a constant tunneling current (at set current, I
t
) via the feedback
loop as illustrated in Fig.
3.3
. The variation in surface LDOS causes a change in
tunneling current and then z is adjusted with a given feedback frequency. Conse-
quently the record of dz represents surface corrugations. Due to its straightforward
