Chemistry Reference
In-Depth Information
In MOMs dimensionality is a major issue. As discussed back in Chapter 1,
although all materials are structurally 3D, some of them exhibit physical properties
with lower dimensionality, 1D or 2D, mainly due to the pseudo-planar conformation
of the molecules. In fact for bulk materials one cannot strictly use the terms 1D or
2D because intermolecular interactions build anisotropic but indeed 3D networks.
Hence, one is led to using the prefixes pseudo or quasi when referring to 1D or 2D
systems. However, ideal 1D and 2D systems can be artificially prepared exhibiting
real 1D and 2D properties, respectively, and we will find some examples of this in
the next sections.
6.1 1D molecular metals
Electronic structure of 1D metals
In order to study the electronic structure of 1D metals and compare it to the simple
model discussed in Section 1.7, we start with the reference system Au/Si(111)-
(5
1), an ideal artificial inorganic 1D metal patterned by a vicinal surface. We
shall see here how the simple Fermi liquid scenario breaks down (Segovia
et al
.,
1999). In this system gold atoms build linear chains along the substrate [110] direc-
tion, with a gold-gold spacing of 0.383 nm within the chains, and with an interchain
spacing of about 2.0 nm, determined by the width of the substrate terraces. This
long spacing guarantees negligible interchain interactions and that the system is
truly 1D. The preparation of this and similar systems is extremely tedious because
the experimental parameters are critical. These samples necessarily have to be pre-
pared and measured
in situ
in UHV, in order to avoid contaminant-induced artifacts.
Figure 6.1 shows the ARUPS spectra of Au/Si(111)-(5
×
×
1) taken at different emis-
sion angles
θ
e
at
T
sub
12 K and with linearly polarized monochromatic 21.2 eV
photons.
Since the gold chains are located at the surface of the sample, the wave vectors of
the electronic states linked to the chains lie in the surface plane. This wave vector,
k
=
2
m
e
E
K
¯h
2
sin
e
, is strictly conserved in the photoemission measurements.
The spectra from Fig. 6.1(a) show a band approaching
E
F
and finally crossing it,
demonstrating the metallic character. It is important to notice that no emission from
the substrate can be observed at
E
F
because the substrate is semiconducting.
Figure 6.1(b) displays the 2D band structure
E
(
k
), fitted to the free-electron
parabola
E
/
θ
2
m
e
, following the physicists' most simplified approach. Al-
though the bands clearly cross
E
F
, the intensity of the spectra at
E
F
,
N
(
E
F
), is
rather low, a property predicted within the Tomonaga-Luttinger scenario and in-
compatible with the Fermi liquid model (see discussion in Section 1.7). A closer
examination of the ARUPS spectra near
E
F
reveals the dispersion of two separated
¯
h
2
k
2
=
||
/
