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upsi-136682025-11-28 Self-assembly of molecular wires on Au and Cu for molecular electronics 2013 Aisyah Mohamad Sharif <p>Self-assembled monolayers (SAMs) of thiols, dithiols, or other bonding moieties are attractive</p><p>molecular systems with expected applications in novel areas such as molecular electronics,</p><p>biotechnology and chemical and bio-sensing. With two thiol or two dicarboxylic acid moieties, with</p><p>aromatic and/or aliphatic backbone structures, these molecules have the ability to connect two</p><p>metal electrodes, and have been chosen for this fundamental work on molecular wires.</p><p>Our work was first concerned with a study of the morphology and structure of self-assembled</p><p>monolayers of such molecules on Au(l 11), at low and at high molecular coverage. We used scanning</p><p>tunneling microscopy (STM) to investigate the self-assembly of two prototypic symmetric dithiols</p><p>(1,6-hexanedithiol and biphenyl-4,4'-dimethanethiol) from dilute aqueous solutions and were able to</p><p>correlate their growth with the deconstruction of the Au(111) herringbone pattern known to produce</p><p>adatoms. For both molecules, we observed the formation of an initial low-density monolayer</p><p>where the molecules are lying down and paired by 0.45 A tall protrusions, assigned to</p><p>Au adatoms. The other thiol terminal group is imaged differently, revealing a strong asymmetry in</p><p>the dithiol bonding. The formation of vacancy islands and, thus, the extraction of additional</p><p>adatoms from terraces were detected only after substantial molecular rearrangement and loss of</p><p>bonding asymmetry. It is a first important result of our work to highlight the involvement of Au</p><p>adatoms in the interfacial structure of dithiols on Au(l 11).</p><p>The self-assembly of dithiols is complex and for the sake of refining preparation</p><p>methods of dithiol monolayers, we pursued by studying the interfacial implication of the solvent on</p><p>the growth. More specifically, our work address the development of 1,4-benzenedimethanethiol SAMs</p><p>on Au(l 11) in water and in hexane, which correspond to polar and non-polar solvent, respectively.</p><p>Our investigations revealed that complete and ordered SAMs of lying-down dithiols can form on clean</p><p>Au(l 11) in water within a few seconds, and that in hexane the adsorption is initially impeded by</p><p>the rapid growth of an ordered hexane film that is gradually replaced by disordered domains of</p><p>dithiol until completion of a saturated monolayer of standing-up dithiols. In the study, the STM</p><p>data were complemented by electrochemical desorption (EC) and x-ray photoelectron spectroscopy</p><p>(XPS) measurements. Our work has resolved the progression of the self-assembly in both these polar</p><p>and non-polar solvents, g1vmg a new and clearer understanding on their</p><p>implication on the interface evolution. The work further stresses the need for considering the</p><p>whole trio solvent-dithiol-substrate when describing the self-assembly process.</p><p>In the third part of our work, we report our study of the evolution of the metal-molecule</p><p>interfaces during the formation and measurement of metal-molecule-metal break-junctions prepared by</p><p>STM. The latest are templates of nanoscale molecular electronic devices. Statistically relevant</p><p>samples of current-distance curves were recorded using a Python script written for this purpose and</p><p>conductance histograms were built from the data. Our work focused on dithiol and dicarboxylic acid</p><p>BJT made when using tip and sample electrodes made of different metal or allied: the substrates</p><p>were Au(l 11) surfaces unmodified or modified with a Cu monolayer prepared by underpotential</p><p>electrochemical deposition (UPD) or modified with a Cu multilayer prepared by overpotential</p><p>deposition, and the tip was made of Au or Cu. An important result of this section is to show that,</p><p>even for small amount of Cu, Cu-molecule-Cu BJT are always preferred. Even at the very low voltage</p><p>conditions (l mV) of our study, metal transfer is thus important. An important corollary of our</p><p>study is that using ambient-stable Cu UPD-modified Au(l 11), it is possible to reproduce</p><p>Cu-metal-Cu molecular nano-junctions, which are otherwise difficult to measure due to the</p><p>reactivity of Cu electrodes.</p><p></p> 2013 thesis https://ir.upsi.edu.my/detailsg.php?det=13668 https://ir.upsi.edu.my/detailsg.php?det=13668 text eng N/A openAccess Doctoral Perpustakaan Tuanku Bainun Fakulti Sains dan Matematik N/A
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