Considerable theoretical effort has been devoted in recent years to the problem of molecular-scale electronics, fuelled by the interesting physics of conductance at the molecular level, by the anticipation for technological applications, and by laboratory measurements of transport through a variety of imaginative molecular heterojunctions. Our own research in this area has been primarily focused on two much less explored but equally fascinating problems, namely current-driven dynamics in molecular-scale electronics and light-controlled current in nanojunctions. In the former framework, we have successfully applied current to devise and explore new forms of molecular machines, to drive single-molecule surface reactions, and to develop nano-junctions whose conductance properties oscillate in time in a controllable manner. In the latter we designed and studied a light-triggered, plasmon-enhanced switch based on extension of the alignment problem to surface-adsorbed molecules, developed a dressed states formalism to optically-control transport through semiconductor-based nanojunctions, explored the case of graphene electrodes, and introduced a hybrid quantum-classical numerical method to model electron transport between an optically excited and a dark plasmonic nanospheres. For an introduction to the problem of current-driven phenomena in nanoelectronics please see the review by Jorn and Seideman,Seideman and Guo, and Seideman. For more information please see the following book .
Current-driven desorption of hydrogen from corrugated hydrogenated graphene
The electronic properties of graphene can be manipulated by adsorbing atomic hydrogen onto its surface, and locally desorbing the hydrogen by applying an electric field via an STM tip. We collaborate with an Argonne National Laboratory experimental group to understand and hence control and optimize the desorption mechanism. To that end we combine a theory of current-driven dynamics in molecular electronics developed in our group’s earlier research with ab-initio calculations of the underlying potential surfaces, quantum dynamical simulations of the desorption dynamics and nonequilibrium Green function studies of the electronic lifetimes involved. Our model combines with the experimental observations to determine the reaction mechanism. Calculations clarify also the origin and implications of the observed marked dependence of the desorption yield on the surface curvature.
Ultrafast electron-phonon dynamics in a junction
Although the vast majority of studies of transport via molecular-scale heterojunctions have been conducted in the (static) energy domain, experiments are currently beginning to apply time domain approaches to the nanoscale transport problem, combining spatial with temporal resolution. We study the interaction of a molecular phonon with an electronic wavepacket transmitted via a conductance junction within a time-domain model that treats the electron and phonon on equal footing and spans the weak to strong electron-phonon coupling strengths. Interference between two coherent energy pathways in the electronic subspace is observed, thus complementing previous studies of coherent phenomena in conduction junctions, where the stationary framework was used to study interference between spatial pathways.
publication
Electron transfer between discrete and continuum systems
In applications of charge transfer, for instance solar energy conversion and catalysis, the desired forward charge transfer competes with undesired back-transfer and recombination. Here we enhance the desired, forward charge transfer while suppressing the undesired back transfer and recombination events using torsional alignment. Our approach is simple – back transfer is a slow process, much slower than the forward charge transfer because it involves an initial relaxation of the excited acceptor state. The duration of torsional alignment can be controlled and we tune it to be longer than the forward but much shorter than the back transfer. Before the transfer is initiated by a laser pulse the molecule is torsionally aligned such that the electron can comfortably zip through. By the time it tries to return the torsion has long since relaxed to the original, twisted configuration and recombination is suppressed.