Chemistry Reference
In-Depth Information
UHV at 63 K, thus above the Peierls transition (Wang
et al.
, 2003). This beautiful
image reveals that the surface consists of two kinds of linear chains along the
crystallographic
b
-direction. One type of chain, the most prominent, is characterized
by a brighter central feature accompanied by two custodian weaker features. The
second type of chain consists of two weak features. The image is compared to a
simulated image obtained with DFT calculations as a height map of an isosurface
of the LDOS at
E
F
, shown in Fig. 6.18(bottom).
The triplet and the single features of the simulated image are ascribed to the
TCNQ and TTF chains, respectively. In the experimental STM image the distance
between equivalent chains is 1.22 nm, corresponding to the
a
lattice constant
(
a
23 nm), while the periodicity within the chains is 0.38 nm, coincident with
the
b
lattice constant. The comparison of experimental and simulated images reveals
two points of disagreement: (i) the single vs. double spots associated with TTF and
(ii) the intensity and size of the outer spots of the triplet compared to the central
one associated with TCNQ. The simulated images agree with simulations based on
the extended Huckel TB method, also within the Tersoff-Hamann approximation,
with a tip to surface distance of 0.05 nm (Magonov & Whangbo, 1996; p. 200). The
simulated images simply reveal that according to the TTF-TCNQ bulk crystal struc-
ture, the hydrogen atoms of TTF and the nitrogen atoms of TCNQ protrude most on
the
ab
-plane and that the nitrogen atoms have a higher electron density. However,
experimental STM images taken in air show the TCNQ associated triplets with
both intensity distributions, that is, chains with a more pronounced central feature
together with chains with more pronounced external features (Ara
et al.
, 1995).
These discrepancies suggest that perhaps the TTF and TCNQ molecules undergo
a certain geometric deformation due to the intense applied electrical fields induced
by the tip and/or that the Tersoff-Hamann approximation is a very good but insuf-
ficient starting point and therefore that the electronic structure of the tip should be
included.
The real-space characterization of the CDW-induced modulation of a 2D surface
lattice can be ideally performed with variable temperature STMs. The temperature-
dependent modulation can be classified according to the HFW model introduced
in Section 4.2 taking the ideal 1
=
1
.
1 surface structure as the reference lattice (
a
s
and
b
s
) and the projected CDW-modified structure as the overlayer system (
a
o
and
b
o
). In the case of TTF-TCNQ
a
s
=
×
b
and for the images taken at 63 K
(see Fig. 6.18(top)) the transformation matrix corresponds to the identity. However,
below 54Ka2Dsuperstructure appears in the image, as depicted in Fig. 6.19(a),
a
and
b
s
=
which is characterized by the matrix
20
03
.
.
3
The real-space modulation can be obtained by Fourier transforming the image,
as shown in Fig. 6.19(b). Observe the 1
2 periodicity along the
a
∗
-direction and the
/
