Chemistry Reference
In-Depth Information
Figure 1.10. (a) (202) plane of TMTTF and (b) (100) plane of TMTSF. TMTTF:
C
2
974
◦
. TMTSF:
/
c
,
a
=
1
.
614 nm,
b
=
0
.
606 nm,
c
=
1
.
428 nm,
β
=
119
.
P
1,
a
51
◦
,
39
◦
,
=
0
.
693 nm,
b
=
0
.
809 nm,
c
=
0
.
631 nm,
α
=
105
.
β
=
95
.
90
◦
. Crystallographic data from Kistenmacher
et al
., 1979.
γ
=
108
.
such temperature, or can exhibit electronic transitions as a function of
P
. It is thus
mandatory to know the phase diagram of a given material when discussing its phys-
ical properties. In general, phase diagrams of MOMs are extremely rich but seldom
completely explored.
Before introducing some reference materials from the myriad of possibilities,
let us first define important energy parameters that are schematized in Fig. 1.11.
Case (a) corresponds to a metal, where the valence band is filled with electrons up
to the Fermi level
E
F
. This ideal situation corresponds to the
T
0 K limit and the
electronic density distribution around
E
F
at finite temperatures will be discussed in
Section 1.7.
The minimum energy required to promote an electron at
E
F
to the vacuum level
E
vac
is the work function
=
φ
M
.
E
vac
can be defined as the energy of an electron at
rest (zero kinetic energy) just outside the surface of the solid. Figure 1.11(b) repre-
sents a semiconductor, characterized by a band gap
E
g
between the valence band
maximum (VBM) and the conduction band minimum (CBM). Hence
E
g
represents
the threshold energy barrier to transitions from occupied to unoccupied states. The
electron affinity
E
A
is defined as the energy required to excite an electron from
the CBM to the local
E
vac
and the ionization energy
I
E
is defined as the energy
needed to excite an electron from the VBM to the local
E
vac
. For a metal
I
E
=
φ
M
.
Finally Fig. 1.11(c) describes the energy band diagram of an isolated molecule,
