Chemistry Reference
In-Depth Information
the increase of the DOS near the Fermi level. The explanation has been demon-
strated by using theoretical calculations in the past [
16
]. The appearance of STM
image also depends on the tip conditions. For instance, if a molecule adsorbed on
the tip apex, the STM image is significantly changed. Therefore we have to discuss
an STM image very carefully and a combination of first principle calculations is
quite beneficial.
2.2.3 Inelastic Electron Tunneling
In the electron tunneling described above, we considered the situation in which
electrons are transferred between the electrodes without energy loss. This process
is called elastic process. However, when oscillators, i.e. molecules, exist in the
tunneling gap, tunneling electrons can interact with them. If the tunneling electron
has enough energy to excite the vibration, it gives a part of energy to the vibra-
tional excitation during the tunneling. In this process the energy of the electron is
different between before and after the tunneling, which is called inelastic electron
tunneling (IET) process. In the IET process a vibrational excitation opens an
additional channel for tunneling electrons (Fig.
2.7
a), giving rise to a substantial
increase in the total current. The IET process was originally observed in metal-
oxide-metal junctions where the molecules are included in the oxide layer [
17
,
18
]
as illustrated in Fig.
2.7
b. We now turn to a bias voltage dependence of tunneling
current in the tunnel junction including an oscillator (molecule) (Fig.
2.7
c-e).
Below a vibrational threshold (eV\ hx), the tunneling current increases almost
linearly with ramping of the bias voltage, which is attributed to the elastic com-
ponent. On the other hand, above the threshold (eV = hx), electrons also tunnel
via an inelastic channel. The probability passing the inelastic channel increases
with the increase of bias voltage. Accordingly, the total current, including both
elastic and inelastic components, shows a kink at eV = hx (Fig.
2.7
c). The kink
becomes the step and the peak in the conductance (dI/dV) (Fig.
2.7
d) and the
differential conductance (d
2
I/dV
2
) spectra (Fig.
2.7
e), respectively. In general,
since the fraction of the inelastic component is very small, like the conductance
change is a few % (Fig.
2.7
d). The peak in d
2
I/dV
2
spectra indicates the vibra-
tional energy and the intrinsic width, which is called IET spectra. The peak
position, that is vibrational energy, corresponds to the signature of molecules and
also represents the interaction with a substrate and surrounding conditions.
Therefore we can chemically identify the species and speculate the structure of
molecules in a tunnel junction. The peak width arises from three different con-
tributions; the intrinsic width C of the vibration, the thermal broadening k
B
T and
the additional broadening depending on experimental conditions. The intrinsic
width of vibration modes for adsorbates is expected to be the order of 1 meV or
greater in surface species, with the peak profile described by a Lorentzian function.
On the other hand, the thermal broadening is described by the Gaussian profile.
Since the measurement of IETS is based on the lock-in detection, we also have to
