Chemistry Reference
In-Depth Information
ARUPS spectra measured in an independent set of experiments on TTF-TCNQ
single crystals along the
b
-axis are shown in Fig. 6.8 (Claessen
et al.
, 2002). Again,
clean surfaces parallel to the
ab
-plane were obtained by
in situ
cleavage of the
crystals at a base pressure of 10
−
10
mbar. As can be seen, the data are in excellent
agreement with those from Zwick
et al.
(1998). At the
-point two peaks at 0.19
(a in Fig. 6.8) and 0.54 eV (b in Fig. 6.8) below
E
F
are clearly identified. For in-
creasing wave vectors the a feature becomes more prominent up to
k
4nm
−
1
,
while the b feature seems to split contributing to both the a and d features. For even
higher momenta a weak structure (c in Fig. 6.8) moves back again from
E
F
and
displays a dispersion symmetric about
k
=
=
2
.
7nm
−
1
, corresponding to the Z-point.
Simultaneously, structure d in Fig. 6.8 disperses to higher binding energy and even-
tually becomes obscured by peak c. In addition, a dispersionless feature at about
1.5 eV binding energy is also observed. From Fig. 6.8 we obtain
k
F
=
8
.
4nm
−
1
,
in perfect agreement with the results from Zwick
et al.
(1998) and with the calcu-
lations performed using Eq. (1.34).
Astonishingly, the same ARUPS spectra have been observed for
ex situ
grown
TTF-TCNQ thin films (Rojas
et al.
, 2001). The films were obtained by thermal
sublimation in HV (
2
.
10
−
6
mbar) on cleaved KCl(100) substrates and consisted of
highly oriented and strongly textured rectangular-shaped microcrystals as shown
in the TMAFM image of Fig. 6.9. The molecular
ab
-planes are parallel to the
substrate surface and the microcrystals are oriented with their
a
- and
b
-axis par-
allel to the [110] and [110] substrate directions, respectively, due to the cubic
symmetry of the substrates. ARUPS spectra taken on the as-received films, mea-
sured along the substrate equivalent [100] directions at
T
∼
100 K, are shown in
Fig. 6.10.
Due to the morphology of the films the bands are mapped along the directions
bisecting both
a
- and
b
-directions, which does not correspond to any high symmetry
direction in reciprocal space. In spite of this, band dispersion is clearly observed.
The resemblance of the spectra to those obtained from single crystals is striking
and a consequence of the high surface stability of the TTF-TCNQ films in air.
Band dispersion is also observed, even with the overlap of both
a
- and
b
-directions
in the films, because of the shape of the band surface in reciprocal space, which
may be approximated by a sheet in
E
vs.
-X,
-Y space with
∂
E
/∂
k
a
∗
=
0
and
0, where
E
represents the band energy and
k
a
∗
and
k
b
∗
stand for
the wave vectors along the
a
∗
and
b
∗
directions, respectively. In addition to the
pseudogap already observed in the metallic phase in the single crystal samples an
extra shift of about 50 meV is observed due to contamination and non-stoichiometry
at grain boundaries. Taking into account this 50 meV shift,
k
F
∼
∂
E
/∂
k
b
∗
=
2.5 nm
−
1
is
obtained.
In spite of the fact that (001) surfaces are stable in air, one has to worry
about surface contamination, stoichiometry and potential damage induced by VUV
