An atomic force microcopy study of the mechanical and electricalproperties of monolayer films of molecules with aromatic end groups Page: 2 of 11
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Molecular electronics is an emerging field that is drawing considerable attention due to the
potential applications offered by inexpensive plastic electronics devices based on
organic/polymeric materials instead of traditional silicon based solid-state semiconductors -1.
Molecules with good conductive
Force properties contain groups with their
highest occupied molecular orbital
(HOMO) and lowest unoccupied
current molecular orbital (LUMO) levels
current separated by only a small energy gap.
Au (111) Among these, double and triple C-
b) Cbonds and aromatic rings are widely
used. However, semiconductors made
of 'r-conjugated molecules suffer
I I MPAA I MPPA from intrinsically low charge carrier
mobility. Unlike single crystal
semiconductor materials, electrons or
H H holes "hop" between molecules in
Figure 1. (a) schematic of AFM measurement of current and order to carry charges '' . Among the
friction on self-assembled monolayer organic molecules / Au
(111) surface. (b) schematic illustration of MPAA, and (c) MPPA. factors that control the "hopping" rate,
intermolecular ;r-'r stacking is one of the most important 4'''10
Although many conjugated molecules form ;r-/7 stacked molecular superstructures,
phenylacetylene compounds are promising due to their stability in air and against ultraviolet light,
rigid framework and the fact that they can be attached to gold strongly (40 kcal/mol) via thiol-
terminated groups. Previous studies have shown that controlled self-assemblies of 'r-conjugated
molecules can result in a 2-D superlattice on Au(111) surfaces at room temperature 1. Here we
study the electric transport properties of two such systems and the effects of mechanical
Two conjugated molecules were studied: (4-mercaptophenyl)-phenylacetylene (MPPA) 12-
15 and (4-mercaptophenyl) anthrylacetylene (MPAA) 11 16 The presence of three fused aromatic
rings in MPAA is expected to result in a stronger 7-/ intermolecular stacking than in the case of
MPPA that contains one benzene ring only 1 . By applying mechanical load to self-assembled
films of these molecules we intend to intentionally disturb the 7-r intermolecular stacking and to
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Fang, Liang; Park, J.Y.; Ma, H.; Jen, A.K.-Y. & Salmeron, M. An atomic force microcopy study of the mechanical and electricalproperties of monolayer films of molecules with aromatic end groups, article, September 6, 2007; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc895404/m1/2/: accessed October 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.