Chemistry Reference
In-Depth Information
superconducting, research should be continued and perhaps in a few years super-
conductivity will be conclusively found in OFETs.
Let us end this section with the interesting case of the controlled doping of
thin C
60
films with potassium (Cepek
et al.
, 2000). Figure 6.25 shows the evolution
measured
in situ
of the valence band spectra of 1 ML C
60
grown on a Ag(100) surface
as a function of potassium exposure. The C
60
molecules form a closed-packed
compact layer, with an overlayer-substrate commensurable register represented
by a
c
(6
4) reconstruction, as evidenced by LEED patterns. The LUMO band
filling is represented by
n
LUMO
=
×
N
at
. At the bottom of the figure, the spectra of
a clean Ag(100) surface and a thick C
60
multilayer film (
5 ML) are shown for
comparison. The C
60
HOMO has a binding energy of
c
. 2 eV and disperses towards
higher binding energies as a function of potassium coverage. The peak that emerges
close to
E
F
for increasing
n
LUMO
corresponds to the partially filled LUMO up to
the
n
LUMO
>
6 case, where the LUMO becomes completely filled. In this case we
can no longer talk about LUMO, since it is fully occupied, but instead of the new
HOMO. This beautiful example complements the discussion from Section 1.7 on
electronic structure and band filling, because it clearly shows that a partially filled
band, e.g.,
n
LUMO
0) while a completely
filled band leads to an insulating or semiconducting character (
N
(
E
F
)
3, leads to metallic character (
N
(
E
F
)
=
0). Solid
K
3
C
60
and K
6
C
60
exhibit metallic and insulating character, respectively. In addition,
the fact that for
n
LUMO
≤
=
3 the HOMO and LUMO bands shift differently indicates
that the C
60
molecules become distorted in this doping regime.
6.4 Structure-property relationships
The relationships between structures and properties can be classified as intrinsic and
extrinsic owing to the molecular arrangement and morphology, respectively. The
term intrinsic refers to the 3D packing of molecules, which depends on the geometry
and chemical nature of the molecules, and thus on intermolecular interactions.
Extrinsic structure-property relationships are related to the formation of interfaces,
e.g., grain boundaries, and the presence of defects. In both cases the role of external
variables such as
T
,
P
,
B
as well as of internal variables such as the type of guest
molecules is essential.
Intrinsic structure-property relationships
The role of solvents
Organic solvents not only participate in the synthesis and crystallization of MOMs
but they can also form part of new crystallographic phases as guest molecules. In
