Transport phenomena in molecular-scale devices
Doctor of Philosophy
The physics of atomic-scale systems is a subject of considerable interest, from both a basic-science and an engineering standpoint. We discuss three sets of experiments, each designed to elucidate a particular aspect of nanoscale physics. The first of these aspects is spin-dependent transport in atomic-scale ferromagnetic wires. Early reports of large magnetoresistive effects in this type of device led to speculation about possible mechanisms for enhancing spin polarization in ferromagnetic constrictions, as well as excitement about the potential applications for such an effect. An experiment carefully designed to exclude other mechanisms for conductance changes, however, leads us to conclude that there is no evidence for a large magnetoresistive effect per se in constricted ferromagnetic wires. A second area of interest is hysteretic conductance switching in single-molecule transistors incorporating bipyridyl dinitro oligo(phenylene ethynylene) dithiol (BPDN-DT). An early hypothesis to explain the observed hysteresis involved strong electron-vibration coupling leading to shifts in molecular energy levels. A change in the charge state of the molecule could both lead to a change in the conductance across the molecule and tend to stabilize the charge on the molecule, leading to hysteretic switching behavior. To examine this hypothesis, we fabricated and measured three-terminal devices allowing us to control the charge on the molecule independent of the source-drain bias. We find that the evidence argues against a charge-transfer-based mechanism for the conductance switching; instead, it is more likely that a change in the molecule-electrode coupling is responsible for this behavior. The final area addressed in this dissertation is that of current-dependent electronic noise in single molecules. In many nanoscale devices, the discrete nature of the carriers of electric current leads to fluctuations about the average current; these fluctuations are known as shot noise. Correlations between the charge carriers can change the dependence of the magnitude of this shot noise on the value of the average current. One of the best-known examples of a correlated-electron effect in single-molecule conductance is the Kondo effect. Theorists have predicted that a single-molecule transistor in the Kondo state would exhibit significantly enhanced shot noise. We discuss the experimental challenges in making single molecule noise measurements, as well as possible techniques to address these challenges.