Chemistry Reference
In-Depth Information
depicted in Fig.
2.4
b. An alternative description about H atom transfer in liquid was
proposed by Borgis and Hynes [
9
,
10
]. As shown in Fig.
2.4
c the asymmetric single
or the double well potential, where the wave packet of hydrogen is localized in one
well, is continuously converted into symmetric potential by surrounding solvents.
But the strong interaction between the molecules results in a significantly reduced
barrier, where the ZPE overcomes the barrier. In this situation H transfer takes place
adiabatically and hydrogen motion is no longer governed by tunneling.
2.2 Scanning Tunneling Microscope
2.2.1 Overview
STM is a powerful tool to image conductive surfaces with an atomic resolution. In
principle, an optical microscope cannot realize such a high resolution due to its
diffraction limit. STM was developed in 1982 by Binnig and Rohrer at IBM Zürich
Research Laboratory [
11
]. They were awarded the Nobel Prize in 1986 for the
development of the STM. In STM, the highest resolution is considered to be
0.1 nm laterally and 0.01 nm vertically. STM set a new direction for nanoscience
and nanotechnology and has continued to evolve as a fascinating tool to investi-
gate physical, chemical, and biological processes at the spatial limit.
STM is based on the concept of quantum tunneling of electron. When a con-
ducting tip having an atomically sharp apex, is brought very close (few Å) to the
conductive surface while a bias voltage is applied between the tip and the surface,
electrons can tunnel through the barrier between them. The tunneling current is
described as a function of the relative distance between the tip and surface, the
applied voltage, and the local density of states (LDOS) of the tip and surface. Since
the tunneling current depends exponentially onto the tip-surface distance, the
current has a considerably sensitivity to the surface corrugation. If the tip apex
consists of a single or few atoms, it has an atomic level sensitivity against the
surface corrugation. The STM image is acquired by scanning the surface with
controlling the distance between the tip and surface. Figure
2.5
shows a schematic
illustration of the basic STM setup. The STM measurement is carried out by fol-
lowing manner; first, bias voltage is applied between a sharp tip and a conductive
substrate, and then the tip is brought close to the substrate using a coarse piezo
control system of the tip along z direction (surface normal). This coarse motion is
turned off when the tip and the surface become sufficiently close (few Å) and the
tunneling current is detected. At the tunneling region, the fine control of the tip in all
three dimensions is required to maintain the tip-sample distance d (3 \ d \ 10Å).
Once the tunneling junction is established, the bias voltage and the tip position with
respect to the surface can be varied. The usual bias voltage to image molecules
chemisorbed on surfaces is in a range of ±2 V. Two different modes to obtain STM
image exist. First, the tip is scanned with constant z height (constant height mode).
