Charge Transport and Contact Effects in Nanoscopic Conjugated Molecular Junctions Characterized by Conducting Probe Atomic Force Microscopy

Abstract

This 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.