An analysis of exciton transport, electron tunneling, and electromigration in nanotube and nanowire systems

Abstract

Herein three systems - electromigration in metal nanowires, electron tunneling in single-molecules, and carbon nanotube photovoltaics - are investigated. In the first area, electromigrative failure of metal nanowires has been shown to form single-molecule tunnel junctions, but the process has remained unpredictable, limiting the yield of devices under current methods. Electromigration in micron diameter and larger wires is well understood as the migration of vacancies in the bulk crystal, but both the quantitative predictions and qualitative features of that mechanism break down at the nanometer scale. We propose, and validate against experimental data, that as the wire diameter falls below a micron,- the increased surface-area-to-volume ratio and the low barrier to surface atom translation shift the dominant mechanism of electromigration from bulk transport to surface transport. We then apply the model to design a process controller to guide gradual electromigration. We then turn to investigating the tunnel junctions themselves. Diverse physical insights have been gained from electron tunneling measurements of single molecules, but to date all observations have been static i.e. subject to long integration times. We performed temporally resolved measurements, revealing underlying molecule dynamics. In particular we find that molecules can stochastically switch between discrete inelastic transport states, suggesting discretized molecule reconfiguration consistent with the body of literature from Scanning Tunneling Microscopy. Finally, we investigate carbon nanotube (CNT) network solar cells. The large parametric space associated with the nanometer-scale heterogeneous material, including the mixture of nanotube length, chirality, orientation, etc., has prevented proof-of-concept devices from revealing a research pathway to practical efficiencies. To address this empirical limitation, we derived a model of CNT photovoltaic steady-state operation from the light absorption and exciton transport behaviors of single and aggregate nanotubes. To do so, we treated single nanotube properties as random variables, describing the nanotube network as distributions of those properties. Applying the model to different solar cell architectures, we predict that efficiencies will be dramatically higher in high density films of verticallyaligned nanotubes. We also show that the film thickness must be at an optimum, and that as a rule of thumb the film thickness should be approximately the exciton diffusion length.