Chemistry Reference
In-Depth Information
in the image are the result of the interference between the remaining molecular
lobes.
As mentioned at the beginning of this chapter, a STM is able to probe the real-
space distribution of theMOs and this can be achieved if the molecules are electron-
ically decoupled from the substrate, or in other words if the MOs are unperturbed.
In fact, when the substrates are metals or semiconductors the electronic structure of
the molecules can become strongly perturbed. Hence the influence of the substrate
has to be reduced, if the inherent electronic properties of individual molecules are
to be studied, by interposing a thin insulating layer between the molecule and the
conducting substrate. The thickness of this insulating layer has to be only a few
atomic layers to permit tunnelling across it. This has been successfully shown with
individual pentacene molecules on layers of NaCl on copper surfaces. In this case
the STM images perfectly resemble the structures of the HOMO and LUMO of the
free molecule (Repp
et al.
, 2005).
In the case of individual C
60
molecules adsorbed on clean Si(111)-(7
7) sur-
faces the experimental intramolecular structure compares well to
ab initio
calcula-
tions based on density functional theory (DFT) in spite of the covalent character of
the substrate (Pascual
et al.
, 2000). In this case a certain degree of uniaxial strain
has to be considered to simulate the electronic influence of the substrate.
For C
2
H
2
adsorbed on a Cu(100) surface the tunnelling conductance as measured
with a STM, defined as d
I
t
/
×
d
V
t
, is shown to increase at
V
t
=
358 mV, resulting from
the excitation of the internal
CH
stretching mode. The inelastic scanning tunnelling
spectroscopy (ISTS) technique makes such determination possible and has opened
up the extraordinary possibility of performing vibrational spectroscopy with sin-
gle molecules on surfaces, representing a major achievement, since the vibrational
fingerprint of an adsorbate allows its chemical identification (Stipe
et al.
, 1998).
Earlier high-resolution electron energy loss spectroscopy (HREELS) experiments
performed for the determination of the vibrational modes of chemisorbed MLs of
C
2
H
2
on (111) surfaces of nickel, palladium and platinum gave
ν
370 meV
(Gates & Kesmodel, 1982). The electron energy loss spectroscopy (EELS) tech-
nique consists of irradiating a sample with monochromatic electrons with ener-
gies lower than typically 500 eV and measuring the energy of the inelastically
back-scattered electrons. Details in the meV regime can only be achieved in the
high-resolution (HR) mode.
The example of ISTS of a single C
6
H
6
molecule chemisorbed on a Ag(110)
surface is illustrated in Fig. 4.6(a). The isolated C
6
H
6
molecules exhibit inelas-
tic peaks at
ν
CH
±
4 and
±
19 mV, while fully C
6
H
6
covered Ag(110) (Fig. 4.6(b))
exhibits peaks at
44 mV, where C
6
H
6
molecules are in a very weakly
adsorbed state. These differences in the spectra between isolated molecules and
MLs, where lateral molecule-molecule interactions are present in addition to the
±
7 and
±
