Chemistry Reference
In-Depth Information
Figure 1.31. XPS spectra of (TMTTF)
2
PF
6
(black dots) and BEDT-TTF (grey
dots) taken at RT on as-received single crystals.
in Chapter 4, non-centrosymmetric anions order below a given temperature within
the BFS structure and this leads to metal-insulator transitions. This can be easily
understood here because of the doubling of the lattice period.
One way of experimentally exploring the electronic structure of solids is by
means of photoemission spectroscopies such as UPS and X-ray photoelectron
spectroscopy (XPS), where photoexcited electrons are analyzed dispersively as
a function of their kinetic energy. The electronic structure of the reference material
TTF-TCNQ will be extensively discussed in Section 6.1. Figure 1.31 shows the
XPS spectra of the S2
p
core line for (TMTTF)
2
PF
6
(black dots) and BEDT-TTF
(grey dots).
The chemical environment of sulfur atoms is identical for both donor molecules
TMTTF and BEDT-TTF, formed by carbon-sulfur-carbon bonds (see Table 1.1),
so that the XPS spectra should reveal the different charge states involved. Neutral
BEDT-TTF shows a single component with a spin-orbit doublet with binding en-
ergies 163.2 and 164.5 eV for the S2
p
3
/
2
and S2
p
1
/
2
lines, respectively, and with a
branching ratio, defined as the intensity ratio between the S2
p
1
/
2
and S2
p
3
/
2
fea-
tures, of 0.46. In the case of (TMTTF)
2
PF
6
we observe two equally intense main
lines, which readily rules them out as originating from a single spin-orbit doublet,
and a weaker structure at
169 eV, which arises from surface charging effects but
is not relevant for the present discussion.
The (TMTTF)
2
PF
6
spectra can be satisfactorily decomposed into two main lines,
each with their corresponding spin-orbit doublet, the lower energy line coinciding
∼
