Chemistry Reference
In-Depth Information
of the clean substrate and
E
α
-
NPD
HOMO
of a thin film (4.5 nm). For
α
-NPD/InP(110) it
turns out that
-
NPD/InP(110) and CuPc/InP(110) represent examples of straddling and staggered
band lineups, respectively.
In general the clean surfaces of inorganic semiconductors are reactive due to
the presence of dangling bonds in the UHV environment (large
0
.
4 eV and for CuPc/InP(110)
0
.
6 eV (see Fig. 4.29).
α
γ
s
values). It is
thus expected that
0. Furthermore, when the molecule-substrate interaction
is strong enough, drastic changes in the electronic structure of the molecule or of
the substrate's surface can be achieved. For PbPc thin films grown on InAs(100)
surfaces the oxidation state of Pb changes at the interface, Pb
+
2
=
Pb
0
, as has been
→
shown with UPS/XPS (Papageorgiou
et al.
, 2001).
In the case of C
60
molecules on Ge(111) surfaces we saw in Fig. 4.7 that indi-
vidual molecules can strongly perturb the surface reconstruction by inducing shifts
of the adatom rows. In the ML coverage regime C
60
exhibits different reconstruc-
tions as a function of
T
ann
intimately related to the perturbation
i
nduce
d
at the
underlying surface. For 520
770 K, C
60
o
rde
rs wi
th
a (3
√
3
3
√
3)
R
30
◦
<
T
ann
<
×
97
0 K
the (
√
13
×
√
13)
R
14
◦
superstructure
reconstruction and for 770
<
T
a
nn
<
is generated. A view of the (
√
13
×
√
13)
R
14
◦
reconstruction as deduced from
grazing incidence X-ray diffraction (GIXRD) measurements is shown in Fig. 4.30.
Because X-rays have a large penetration depth, the surface to bulk signal ratio
can be increased by illuminating the sample at very small (grazing) angles, thus
reducing the probed depth.
The crystallographic structure of this interface reveals the formation of periodic
defects in the unit cell created by the removal of six germanium atoms, large enough
to accommodate individual C
60
molecules. In fact, such an induced defect prioritizes
the orientation of the C
60
molecules with one of their hexagonal rings parallel to
the substrate's surface plane. It appears that this reconstruction is achieved by the
generation of surface holes rather than by strong rehybridization.
Reconstructed surfaces of metals are also perturbed by the C
60
molecules, as has
been shown for bare Au(110)-
p
(1
5) superstructure
is induced (Pedio
et al.
, 2000). Again, the adsorption of C
60
is accompanied by
important displacements of underlying gold atoms.
This kind of study is gaining interest among the scientific community and will
provide deeper insight into the understanding of the generation of interfaces in-
volving organic materials.
×
2) where a Au(110)-
p
(6
×
Semiconductor/insulator interfaces
The most representative element belonging to such interfaces is the OFET. A sim-
plified scheme of an OFET based on an organic thin film is given in Fig. 4.31.
