UPSI Digital Repository (UDRep)
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UPSI Digital Repository (UDRep)
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| Abstract : Perpustakaan Tuanku Bainun |
| Self-assembled monolayers (SAMs) of thiols, dithiols, or other bonding moieties are attractive
molecular systems with expected applications in novel areas such as molecular electronics,
biotechnology and chemical and bio-sensing. With two thiol or two dicarboxylic acid moieties, with
aromatic and/or aliphatic backbone structures, these molecules have the ability to connect two
metal electrodes, and have been chosen for this fundamental work on molecular wires.
Our work was first concerned with a study of the morphology and structure of self-assembled
monolayers of such molecules on Au(l 11), at low and at high molecular coverage. We used scanning
tunneling microscopy (STM) to investigate the self-assembly of two prototypic symmetric dithiols
(1,6-hexanedithiol and biphenyl-4,4'-dimethanethiol) from dilute aqueous solutions and were able to
correlate their growth with the deconstruction of the Au(111) herringbone pattern known to produce
adatoms. For both molecules, we observed the formation of an initial low-density monolayer
where the molecules are lying down and paired by 0.45 A tall protrusions, assigned to
Au adatoms. The other thiol terminal group is imaged differently, revealing a strong asymmetry in
the dithiol bonding. The formation of vacancy islands and, thus, the extraction of additional
adatoms from terraces were detected only after substantial molecular rearrangement and loss of
bonding asymmetry. It is a first important result of our work to highlight the involvement of Au
adatoms in the interfacial structure of dithiols on Au(l 11).
The self-assembly of dithiols is complex and for the sake of refining preparation
methods of dithiol monolayers, we pursued by studying the interfacial implication of the solvent on
the growth. More specifically, our work address the development of 1,4-benzenedimethanethiol SAMs
on Au(l 11) in water and in hexane, which correspond to polar and non-polar solvent, respectively.
Our investigations revealed that complete and ordered SAMs of lying-down dithiols can form on clean
Au(l 11) in water within a few seconds, and that in hexane the adsorption is initially impeded by
the rapid growth of an ordered hexane film that is gradually replaced by disordered domains of
dithiol until completion of a saturated monolayer of standing-up dithiols. In the study, the STM
data were complemented by electrochemical desorption (EC) and x-ray photoelectron spectroscopy
(XPS) measurements. Our work has resolved the progression of the self-assembly in both these polar
and non-polar solvents, g1vmg a new and clearer understanding on their
implication on the interface evolution. The work further stresses the need for considering the
whole trio solvent-dithiol-substrate when describing the self-assembly process.
In the third part of our work, we report our study of the evolution of the metal-molecule
interfaces during the formation and measurement of metal-molecule-metal break-junctions prepared by
STM. The latest are templates of nanoscale molecular electronic devices. Statistically relevant
samples of current-distance curves were recorded using a Python script written for this purpose and
conductance histograms were built from the data. Our work focused on dithiol and dicarboxylic acid
BJT made when using tip and sample electrodes made of different metal or allied: the substrates
were Au(l 11) surfaces unmodified or modified with a Cu monolayer prepared by underpotential
electrochemical deposition (UPD) or modified with a Cu multilayer prepared by overpotential
deposition, and the tip was made of Au or Cu. An important result of this section is to show that,
even for small amount of Cu, Cu-molecule-Cu BJT are always preferred. Even at the very low voltage
conditions (l mV) of our study, metal transfer is thus important. An important corollary of our
study is that using ambient-stable Cu UPD-modified Au(l 11), it is possible to reproduce
Cu-metal-Cu molecular nano-junctions, which are otherwise difficult to measure due to the
reactivity of Cu electrodes.
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