Electron transport through chemically-derived nanostructures

Electrical devices that combine the functionality of lithographically patterned semiconductor circuits with the flexibility inherent in chemical systems present an opportunity to explore fundamental physical processes and simultaneously, to address more practical applications. This dissertation describes efforts to make such chemically-derived structures, by incorporating individual nanometre-sized molecules (C60 and semiconductor nanocrystals/nanorods) into a single electron transistor geometry. Electrical transport measurements are used to characterise these devices.; A significant barrier to building these devices is the difficulty in connecting electrodes from the external metre-sized world where measurements are made to a nanometre-sized object. One solution is to place the molecule in a junction formed by the electromigration-induced failure of a metal nanowire, a technique that produces electrically stable transistors in surprisingly high yields. Understanding the process of junction formation is not only of intrinsic interest, but it has also proven to be a highly reliable diagnostic tool to differentiate between successful devices and tunnel junctions not containing the sample molecule.; The fabrication of C60-based single electron transistors was one of the first applications of this method. Extensive transport spectroscopy was performed on these devices by tuning parameters such as the transistor bias voltage, the applied electric and magnetic fields and the temperature. The electronic and vibrational degrees of freedom of the cluster appear to couple during electron transport and a model for this behaviour was proposed, based on molecular oscillations within a transistor junction. On several occasions, individual C60 molecules were well connected to the junction electrodes, permitting observation of the Kondo effect in these devices.; The break-junction technique was also used to create single electron transistors based upon colloidal semiconductor nanocrystals, facilitating spectroscopy on a semiconductor material that is quantum confined in three dimensions. The techniques to produce these nanocrystals are chemically quite flexible and recent syntheses have produced elongated nanorods where confinement is lost in one direction (resulting in a one-dimensional quantum wire). Preliminary experiments on these objects exhibit an intriguing dependence on the applied magnetic field.