Single-Molecule Electrical Components and Intramolecular Circuits: Control of Electron Transport through a Molecular Junction

Abstract

Molecular electronics is a promising field with many potential applications in nanotechnology and electrical engineering. During the past two decades, a remarkable progress has been made in single-molecule conductance measurements due to advances in scanning tunneling microscopy, self-assembly techniques, and quantum transport theories. However, for realization of practical molecular devices and circuits, investigation on single-molecule conductance measurements is insufficient, and it is required to understand control of electron transport in molecular junctions through interactions with external fields and environment. This dissertation explores control of electron transport in molecular junctions by (i) applying a laser field (coherent light), (ii) exerting a gate electric field, and (iii) altering temperature.

Coherent light is a potential tool for operating ultrafast electronic devices due to a variety of control options, e.g., field strength, frequency, and polarization. In order to simulate light-driven electron transport in molecular junctions, we develop a formulation which enables the modeling of electron transport through multi-terminal and multi-orbital systems in time-periodic driving fields in terms of transmission probabilities. Based on this methodology, a new type of driven quantum tunneling, referred to as coherent revival of tunneling, is presented. Moreover, by utilizing photon-assisted tunneling and destructive quantum interference, numerical simulations show large on-off switch ratios and weak-field operation conditions in phenyl-acetylene macrocycle and cross-conjugated systems.

Inspired by examples in molecular machinery, a new type of single-molecule transistor, the single-molecule electric revolving door (S-MERD), is proposed. The analysis of electron transport through S-MERDs uses the Landauer formalism together with density functional theory in the zero-bias limit. The simulations show that door states can be operated by a practical external electric field, and the large on-off conductance ratio is less sensitive to the molecule-electrode coupling.

Electron transport through intramolecular circuits such as molecular units in series is a fundamental subject in molecular electronics. The modified Simmons equation, temperature and length dependence of conductance, the equivalent conductance of series and parallel intramolecular circuits, and Arrhenius behavior are discussed in this dissertation. The activation energy - gate characteristics are first introduced to study the conduction mechanism transition from tunneling to thermally activated hopping.