Charge transport in single molecules

The ability to measure, control and understand charge transport in single molecules connected with external electrodes is a basic requirement in molecular electronics. The charge transport depends on the electronic states of the molecules, as well as on other factors, such as the details of the molecule-electrode contact, and the local environment of the molecules. A reliable technique allowing one to measure and control the charge transport in single molecules is highly desired. This dissertation describes charge transport studies in various molecular junctions fabricated by using nanoelectrodes on silicon chip, an adjustable scanning tunneling microscope (STM) break junction, and a combination of mechanical break junction and electrodeposition techniques.; The dissertation first describes molecular junctions fabricated by combining electrodeposition with electron beam lithography (EBL). This approach allows one to study electron transport at low temperature, but the success rate is low, and it is difficult to determine exactly how many molecules are in the junctions and how the molecules are attached to the two electrodes. Second, it discusses an STM break junction method. Using this method, the electron transport mechanism of single alkanedithiols and the effects of molecule-electrode contacts on single molecule junctions have been studied. As an effort to control charge transport in single molecules, an electrochemical gate has been used to control the redox state of redox active molecules. And finally, it describes a combined mechanically controlled break junction and electrochemical deposition/etching technique. The last approach provides both stability and flexibility, and is suitable for studying charge transport properties in single molecule junctions or atomic point contacts in aqueous solutions and under electrochemical potential control.