Browsing by Subject "Molecular Electronics"
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Item Charge Transport and Contact Effects in Nanoscopic Conjugated Molecular Junctions Characterized by Conducting Probe Atomic Force Microscopy(2008-10) Kim, Bong SooThis thesis describes electrical characterization of nanoscale molecular junctions based on a small assembly of molecules. Gaining rigorous knowledge about nanoscopic molecular junctions is essential to the field of molecular electronics, a field that is driven by the potential of utilizing molecules as active elements in electronic circuits. Further advancement requires detailed understanding of factors that influence charge transport through molecules. Critical aspects include molecular length, molecular structure, contact effects, and energy level alignment. For example, the precise dependence of resistance (or conductance) on molecular length is subject to the electronic structure of the molecule and to the charge transport mechanisms. In addition, contact effects can be dominant in current-voltage characteristics due to the inherently small dimensions of these junctions. To address these issues, my research focused on understanding how currents flow through molecular assemblies in metal-molecule-metal junctions using conducting probe atomic force microscopy (CP-AFM). The CP-AFM technique allows us to form a molecular junction conveniently by contacting metal-coated AFM tips with self-assembled monolayers (SAMs) on metal substrates, and the current-voltage characteristics can then be recorded. Electrical measurements on several series of conjugated molecules revealed the length dependent tunneling efficiency of each molecular structure. In addition, spectroscopic measurements on the metal/molecule interfaces revealed a direct correlation between contact resistance and energy level alignment. In terms of transport mechanisms, a mechanistic transition from nonresonant tunneling to field emission was observed under high bias.Item Electrical characterization of long conjugated molecular wires.(2011-04) Choi, Seong HoThe establishment of structure-property relationships is central to molecular science and this principle applies equally well to the developing field of molecular and organic/polymer electronics. My dissertation research centers on studying the structure-property relationship of charge transport in molecular wires. In particular it involves demonstrating a long-standing prediction of a mechanistic transition from tunneling to hopping as the lengths of molecular wires are varied. In analogous conceptual and technical frameworks, my research has continued to examine hopping conduction as a function of precisely controlled chain architectures. These experiments directly address variable rates of intrachain hopping conduction upon a change of structural entities, which will provide guidance in designing highly conductive molecular wires as well as understanding transport in bulk organic devices. In this work, we provided direct evidence for a change in transport mechanism from tunneling to hopping in molecular junctions based on three conjugated systems; oligophenyleneimine (OPI) wires ranging in length from 1.5-7.3 nm, oligonaphthalene-fluoreneimine (ONI) wires from 2.3-10.1 nm, and oligotetrafulvalenepyromellitimide-imine (OTPI) wires from 2.5-20.2 nm. Our experimental approach involved contacting molecular wires that were grown from one electrode using controlled aryl imine addition chemistry; a metal-coated atomic force microscope tip is used to make the second contact. We showed that near 4 or 5 nm in length the mechanism of transport in the wires changes abruptly, as evidenced by striking changes in length, temperature and electric field dependence of the current-voltage (I-V) characteristics. For longer wires, we have been able to analyze the bias dependence to establish at least three different regimes of transport, and we were also able to estimate the single wire conductivity and the hopping energy for each wires, which shows the great sensitivity of hopping transport to the precisely controlled wire architectures in solid-states for the first time. Overall, these experiments open significant opportunities to probe the physical organic chemistry of molecular conduction, e.g. the roles of specific functional groups and bonding architectures on hopping transport in molecular wires.