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The last decade has seen incredible growth in the quality of experiments being done on single molecule junctions. Contemporary experimental measurements have expanded far beyond simple electron transport. Measurement of vibronic effects, quantum interference and coherence effects, molecular optical response (Raman spectroscopy), and molecular spintronics are just some of the continuing areas of research in single molecule junctions. Experimental advancements demand advanced theoretical treatments, which can be used accurately within appropriate physical regimes, in order to understand measured phenomena and predict interesting directions for future study. In this dissertation we will study systems with strong intra-system interactions using a many-body states based approach. We will be focused on three related processes in molecular junctions: electron transport, electronic energy transfer, and molecular excitation. Inelastic electron transport in the regime of strong and nonlinear electron-vibration coupling within and outside of the Born-Oppenheimer regime will be investigated. To understand their appropriateness, we will compare simple semi-classical approximations in molecular redox junctions and electron-counting devices to fully quantum calculations based on many-body system states. The role of coherence and quantum interference in energy and electron transfer in molecular junctions is explored. Experiments that simultaneously measure surface enhanced Raman scattering and electron conduction have revealed a strong interaction between conducting electrons and molecular excitation. We investigate the role of the molecular response to a classical surface plasmon enhanced electric field considering the back action of the oscillating molecular dipole. Raman scattering is quantum mechanical by nature and involves strong interaction between surface plasmons in the contacts and the molecular excitation. We develop a scheme for treating strong plasmon-molecular excitation interactions quantum mechanically within nonequilibrium molecular junctions. Finally we perform preliminary calculations of the Raman spectrum of a three-ring oligophenylene vinylene terminating in amine functional groups molecule in a molecular junction and compare our results to experimental measurements. This work is the first steps towards full calculations of the optical response of current-carrying molecular junction, which should combine classical calculations of the plasmon enhanced electric field with quantum calculations for the plasmon-molecular exciton interaction and nonequilibrium Raman scattering.
A comprehensive overview of the physical mechanisms that control electron transport and the characteristics of metal-molecule-metal (MMM) junctions. As far as possible, methods and formalisms presented elsewhere to analyze electron transport through molecules are avoided. This title introduces basic concepts--a description of the electron transport through molecular junctions—and briefly describes relevant experimental methods. Theoretical methods commonly used to analyze the electron transport through molecules are presented. Various effects that manifest in the electron transport through MMMs, as well as the basics of density-functional theory and its applications to electronic structure calculations in molecules are presented. Nanoelectronic applications of molecular junctions and similar systems are discussed as well. Molecular electronics is a diverse and rapidly growing field. Transport Properties of Molecular Junctions presents an up-to-date survey of the field suitable for researchers and professionals.
In recent years, ever more electronic devices have started to exploit the advantages of organic semiconductors. The work reported in this thesis focuses on analyzing theoretically the energy level alignment of different metal/organic interfaces, necessary to tailor devices with good performance. Traditional methods based on density functional theory (DFT), are not appropriate for analyzing them because they underestimate the organic energy gap and fail to correctly describe the van der Waals forces. Since the size of these systems prohibits the use of more accurate methods, corrections to those DFT drawbacks are desirable. In this work a combination of a standard DFT calculation with the inclusion of the charging energy (U) of the molecule, calculated from first principles, is presented. Regarding the dispersion forces, incorrect long range interaction is substituted by a van der Waals potential. With these corrections, the C60, benzene, pentacene, TTF and TCNQ/Au(111) interfaces are analyzed, both for single molecules and for a monolayer. The results validate the induced density of interface states model.
The Molecular Basis of Electron Transport presents the proceedings of the Miami Winter Symposia, held in Miami, Florida, on January 13–14, 1972. This book focuses on the development of the mitochondrial electron transport system by a symbiotic relationship of some bacteria with the cell. Comprised of 15 chapters, this volume starts with an overview of the structure and function of mitochondria. This book then explains all of the major categories of mitochondrial phenomena and provides the detailed molecular mechanism for mitochondrial energy coupling. Other chapters discuss the five postulates of the electromechanochemical model, including the super molecule concept, the principle of electromechanochemical energy transduction, conformon coupling, field-induced generation of the linkage system, and the de facto unit of mitochondrial control. Finally, the reader is introduced to the liver microsomal enzyme system, which catalyzes the hydroxylation of a variety of drugs, hydrocarbons, and fatty acids. Biologists, molecular biologists, and biochemists will find this book extremely useful.
This technique has the potential for application in nano-scale systems beyond single-molecule junctions. These results constitute another step toward the development of single-molecule devices with commercial applications. Finally, the methods presented in this thesis offer further insights into the electronic structure of molecular junctions. We show that we can assess energy-level alignment at metal molecule interfaces– this alignment is a crucial parameter controlling the proper- ties of the interface. We also demonstrate that we can probe large regions ( 2eV) of the transmission function which governs charge transport through the junction. By being able to control level alignment, we are also able to offer preliminary studies on single-molecule junctions in the resonant transport regime. Combined, the results presented in this thesis grant new insights into electron transport at the nanoscale and provide new routes for the development of functional single-molecule devices.
This review volume is based primarily on the balance equation approach developed since 1984. It provides a simple and analytical description about hot electron transport, particularly, in semiconductors with higher carrier density where the carrier-carrier collision is much stronger than the single particle scattering. The steady state and time-dependent hot electron transport, thermal noise, hot phonon effect, the memory effect, and other related subjects of charge carriers under strong electric fields are reviewed. The application of Zubarev's nonequilibrium statistical operator to hot electron transport and its equivalence to the balance equation method are also presented. For semiconductors with very low carrier density, the problem can be regarded as a single carrier transport which will be treated non-perturbatively by the nonequilibrium Green's function technique and the path integral theory. The last part of this book consists of a chapter on the dynamic conductivity and the shot noise suppression of a double-carrier resonant tunneling system.
This textbook is aimed at second-year graduate students in Physics, Electrical Engineering, or Materials Science. It presents a rigorous introduction to electronic transport in solids, especially at the nanometer scale.Understanding electronic transport in solids requires some basic knowledge of Hamiltonian Classical Mechanics, Quantum Mechanics, Condensed Matter Theory, and Statistical Mechanics. Hence, this book discusses those sub-topics which are required to deal with electronic transport in a single, self-contained course. This will be useful for students who intend to work in academia or the nano/ micro-electronics industry.Further topics covered include: the theory of energy bands in crystals, of second quantization and elementary excitations in solids, of the dielectric properties of semiconductors with an emphasis on dielectric screening and coupled interfacial modes, of electron scattering with phonons, plasmons, electrons and photons, of the derivation of transport equations in semiconductors and semiconductor nanostructures somewhat at the quantum level, but mainly at the semi-classical level. The text presents examples relevant to current research, thus not only about Si, but also about III-V compound semiconductors, nanowires, graphene and graphene nanoribbons. In particular, the text gives major emphasis to plane-wave methods applied to the electronic structure of solids, both DFT and empirical pseudopotentials, always paying attention to their effects on electronic transport and its numerical treatment. The core of the text is electronic transport, with ample discussions of the transport equations derived both in the quantum picture (the Liouville-von Neumann equation) and semi-classically (the Boltzmann transport equation, BTE). An advanced chapter, Chapter 18, is strictly related to the ‘tricky’ transition from the time-reversible Liouville-von Neumann equation to the time-irreversible Green’s functions, to the density-matrix formalism and, classically, to the Boltzmann transport equation. Finally, several methods for solving the BTE are also reviewed, including the method of moments, iterative methods, direct matrix inversion, Cellular Automata and Monte Carlo. Four appendices complete the text.