Angewandte
Chemie
measured height of the alkane chains at negative bias is
(0.05 ꢀ 0.02) nm lower than that at the positive bias, while the
measured height of the head groups at negative bias is nearly
identical to that at the positive bias. Such bias-polarity-
dependent contrast variation associated with alkane linking
units in Ch-14 was reproducible in repeated experiments
under different imaging conditions (see Supporting Informa-
tion), and has never been reported for simple alkanes and
alkane derivatives.
The same bias-dependent conformational changes can be
observed at different imaging conditions, with higher contrast
at positive bias and lower contrast at negative bias for alkane
linking chains (see Supporting Information). The consistency
in the contrast variation of the linking units is supportive of
the results presented in Figure 1. Note that systematic
variations can be identified in association with the cholesteric
groups, as revealed by the sectional profiles at different
tunneling conditions (see Figure 1e and the Supporting
Information). Therefore, the contrast variations from the
localized swaying of the mesogen parts schematically illus-
trated in Figure 1g could be considered insignificant. In
addition, variations of mesogen parts among independent
images could be attributed to different image conditions and
different STM tips.
In the case of alkane derivatives, the vertical contrast in
the STM image of the methylene regions is dominated by the
hydrogen atom positions predicted by perturbation theory
and extended Hückel calculations, and is therefore dominated
by topography effects and does not have any discernible
dependence on the magnitude and polarity of the bias
voltage.[9–11] The variation in vertical contrast of the functional
groups of various alkane derivatives relative to the methylene
units was affected by both topographic and electronic factors,
with ionization potential (IP) accounting for the observed
contrast. A lower IP leads to higher contrast in the STM
images. This correlation could originate from the contribution
of the HOMO to the tunneling current in STM. A lower IP
corresponds to closer energy levels between the HOMO and
the Fermi level of the tip, and more diffuse molecular orbital
structures. Both effects are beneficial to the enhancement of
the coupling between the adsorbate and the tip.
It has been concluded that no dependence on bias polarity
can be observed for either methylene units or terminal
functional groups.[9–11] These studies are complementary to
our observations, which show little observable dependence on
bias polarity of the mesogen unit in both specimens examined,
while the contrast variation is solely associated with the
methylene portion. It is therefore reasonable to conclude that
the contrast of the mesogen unit does not contribute to the
observed contrast reversal of the methylene units.
According to the above results and analysis, it is plausible
to attribute the image contrast variations to the field-
dependent bistable conformational change of the dimesogen
ChLCs with flexible chains. The proposed conformational
change induced by an electric field is illustrated in Figure 1g
with a side view of the molecular structure. The long
connecting lines stand for the C14 flexible linking chains and
the spindle-shaped structures for the stereoscopic structure of
Ch groups. The short tails attached to the spindles represent
the isohexyl tails on the Ch groups. This scheme is a simplified
description that ignores the asymmetric conformation
observed in STM images.
It has been reported that large dipole moments exist in
cholesteric molecules.[4] We calculated the charge populations
of the Ch-14 molecules (see Supporting Information) and
found that the linker alkane chains are almost neutrally
charged, while the dipole moments are mainly contributed by
the mesogen part (the dipole moment vectors of the mesogen
part are presented in the Supporting Information). It is clear
that the dominant dipole moment vector is along the y and
z axes, that is, approximately perpendicular to the direction of
the linking unit. As demonstrated in Figure 1g, when a
negative bias is applied the alkyl chains tend to position
themselves close to the graphite surface (illustrated as a solid-
line structure). When the bias polarity was abruptly switched
to positive, the dipoles of the Ch head group tend to rotate
accordingly, which results in upward swinging of the flexible
linking chains (illustrated as a dashed-line structure). The
height difference of the alkane chains (Dh) is determined
from the measured cross-sectional profile. The value of Dh
measured from different images is (0.05 ꢀ 0.02) nm (see
Figure 1d and the Supporting Information). This molecular
chord structure of the Ch-14 molecule can be switched
between two bistable states using an electric field.
In a parallel study, large-scale assembly structures of CBC
molecules containing a rigid linker unit can be observed on a
HOPG surface. The assembly structure of CBC is also in the
form of a stripe pattern, with bright bands composed of
stereoscopic cholesteric moieties and relatively low-contrast
bands corresponding to the rigid linking chains of bipheny-
lene groups (Figure 2a). The contrast of the biphenyl groups
is reduced in comparison with that of the Ch groups because
of their lower topographical position. Identical structures and
contrast of the CBC monolayer can be observed independent
of bias polarities and bias values (Figure 2b), which is
indicated in the sectional profiles in Figure 2c and d. No
clear alternative arrangement of Ch groups was observed
because of the flat conformation of biphenylene groups
parallel to the HOPG surface. Different images for CBC
assemblies at different locations using different tips were
obtained for verification (see Supporting Information).
According to the packing model of CBC illustrated in
Figure 2e, the isohexyl groups are aligned nearly perpendic-
ular to the molecular axis. The two isohexyl groups show
higher contrast as brighter dots located at both sides of Ch
head groups. This observation is illustrative of the appreciable
effects of linker units on the details of the assembling
characteristics as a result of varied intermolecular interac-
tions.
A control STM experiment was conducted with alkanes
and Ch-14 molecules in the same image (see Supporting
Information). An apparent contrast variation for the C14
linking chains in Ch-14 molecules can be observed, while no
observable contrast changes can be found for the alkane
molecules. Therefore, the observed contrast variation of
alkane linking units at alternate bias polarities can only be
attributed to the positional variation, rather than the elec-
tronic structures of the HOMO and LUMO of the alkanes.
Angew. Chem. Int. Ed. 2006, 45, 6889 –6893
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6891