Chemistry Reference
In-Depth Information
Overlap between
p
π
orbitals leads to cohesive energies of typically less than
0.4 eV molec
−
1
. The much stronger ionic and covalent bonding have binding ener-
gies of
3 eV atom
−
1
, respectively. Finally, physisorption is the weakest
form of absorption to a solid surface characterized by a lack of a true chemical bond
(chemisorption) between substrate and adsorbate and will be discussed in Chapter
4 (see e.g., Zangwill, 1988).
The effect of intermolecular interactions can be readily observed when compar-
ing the absorption spectrum of a molecule in solution to that in the solid state. In
solution, where the molecules can be considered as isolated, the spectra are char-
acterized by sharp lines corresponding to absorption bands. However, in the solid,
intermolecular interactions cause the formation of exciton bands and splitting of
the levels. This phenomenon is often referred to as Davydov splitting. This splitting
is thus a measure of the strength of the interactions and for MOMs it can amount
to 0.2-0.3 eV.
The binding forces and cohesion energies can be experimentally evaluated with
an atomic force microscope (AFM) in some cases. The AFM was developed by
G. Binnig, C. F. Quate and Ch. Gerber (Binnig
et al
., 1986) and its working princi-
ple is based on the deflection of a microfabricated cantilever due to repulsive and
attractive forces between atoms on the sample surface and atoms at the cantilever
tip. The deflection is measured using a laser beam and a photodiode detector and
scanning in the
x
-,
y
- and
z
-directions is performed by a piezoelectric translator.
For instance, the binding force of an electron donor-acceptor complex has been
evaluated as
c
. 70 pN (Skulason & Frisbie, 2002). These experiments have been
performed by measuring pull-off forces between AFM tips and substrates coated
with complementary self-assembled monolayers (SAMs) capable of specific chem-
ical binding (see Section 3.2 for the preparation technique leading to SAMs). In a
pull-off experiment, typically used to unfold proteins, exposed functional groups on
the two SAMs surfaces bind upon tip-substrate contact. The binding is quantified
by measuring the pull-off force required to rupture the contact (see Fig. 1.6). If the
tip is sufficiently sharp, the contact area is of the order of a few nm
2
, so that pull-off
involves breaking a small integer number of bonds. In the example discussed here
a gold-covered AFM tip has been covered with a SAM consisting of an alkyl linear
chain with a thiol end that binds covalently to the gold film, and a TMPD end.
Additionally, a flat gold surface has been covered with a SAM again with a thiol
end and a TCNQ end.
AFMs have also been used to estimate the cohesive energy of ionic materials
with face-centred-cubic structure (Fraxedas
et al
., 2002a). In these experiments
an ultrasharp AFM tip (tip radius
R
∼
10 and
∼
10 nm) indents a flat surface of a single
crystal and the dynamical mechanical response of the surface during indentation
is transformed into a force plot (applied force vs. penetration). It turns out that the
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