Chemistry Reference
In-Depth Information
metal phosphonate LB films discussed in Chapter 3, built from layers of a va-
riety of metal ions sandwiched within bilayers of the organophosphonate (Seip
et al.
, 1997), and LB films of polyoxometallates (Clemente-Leon
et al.
, 1997) and
of single-molecule nanomagnets (Clemente-Leon
et al.
, 1998). Organic/inorganic
superlattices of polyoxometallates using the LB technique have been prepared using
the adsorption of Keggin heteropolyanions on positively charged MLs of dimethyl-
dioctadecylammonium. Chemically stable Keggin polyoxometallates with general
formula [X
+
n
W
12
O
40
]
−
(8
−
n
)
, with X
+
n
=2H
+
,P
+
5
,Si
+
4
,B
+
3
and Co
+
2
, have been
used. These anions have the same structure but, depending on the nature of X, their
charge varies between
6.
In the latter example of LB-based organic/inorganic superlattices acetate and
benzoate derivatives of Mn
12
were used. Upon spreading a pure sample at the
gas/water interface, the clusters do not form stable Langmuir films. Therefore, a
mixture of these complexes and a lipid is needed to obtain a ML. A homogeneous
ML is formed upon use of behenic acid as matrix. The Langmuir film is stable
over time if the lipid-cluster ratio is 5 or higher. Furthermore, the transfer of the
ML onto a solid hydrophobic substrate is easily achieved. In these examples the
periodicity is OIOOIOOIO
−
3 and
−
...
.
Semiconductor/metal interfaces
Electronic structure
Kroemer's Lemma of Proven Ignorance states that
if, in discussing a semiconductor
problem, you cannot draw an energy band diagram, this shows that you don't know
what you are talking about
. The associated corollary is also quite clear:
if you can
draw one, but you don't, then your audience won't know what are you talking about
(Kroemer, 2001). With this advice in mind let us discuss the energy band diagram
of semiconductor/metal interfaces starting from their energy diagrams, shown in
Fig. 4.22.
Figure 4.22(a) represents the case where the interface dipole
, the energy
difference between the vacuum levels of both organic and metal samples, is zero,
thus indicating that they are aligned. Such a condition is known as the Schottky-
Mott limit and is expected to be fulfilled for weakly interacting interfaces. However,
this is not always the case as will be discussed later. The terms ionization energy
I
E
,
electron affinity
E
A
and metal work function
M
were defined in Section 1.5. The
HOMO and LUMO energies referred to
E
F
are represented by
E
HOMO
and
E
LUMO
,
respectively.
The case of non-vanishing
φ
is shown in Fig. 4.22(b). From this figure we
=
φ
+
−
observe that
I
E
. Here we have assumed a common energy
level,
E
F
, for both metal and semiconductor. This is not quite exact since the metal
induces electronic states in the semiconductor gap that in turn may induce CT and
E
HOMO
M
