Implications and Applications of Current-Induced Dynamics in Molecular Junctions

Abstract

Instances of strongly nonadiabatic electronic-vibrational energy transfer have been studied since the early days of quantum mechanics and remain a topic of fundamental interest. Often such transfers are associated with electronic resonances, temporary states where transient localization of charge on the molecule provides a mechanism for channeling electronic energy into vibrational excitation. Extensively studied in the gas phase, electron resonance scattering also occurs with surface adsorbed molecules, where it manifests itself in broadened cross sections and desorption of adsorbates from metal surfaces. In this Account, we focus on a related topic: the implications of nonadiabatic, resonance-mediated scattering to the exciting field of molecular electronics. In this context, researchers can induce directed nuclear dynamics and control these processes in single molecules in contact with metallic and semiconducting electrodes. We discuss a variety of consequences and applications of current-driven nuclear excitation in molecular devices, ranging from the design of new forms of molecular machines to surface chemistry at the single-molecule level and atom-resolved lithography. We highlight two specific examples of molecular nanomachines. In the first, a Au-C(60)-Au transistor, the current induces the oscillatory motion of the center-of-mass coordinate of the C(60). The second, a zwitterion-based rattle, demonstrates excitation of intramolecular motion as the positively charged moiety is threaded back and forth through the negatively charged carbon ring. Finally, we discuss the current-induced desorption of organic molecules from Si(100) both to suggest the potential for controlled surface nanochemistry and to develop guidelines for the design of stable molecular junctions. Modeling the exchange of energy between tunneling electrons and the vibrational degrees of freedom of a target molecule subject to bias voltage, open boundary conditions in the electronic subspace, and the dissipative effects of the electrodes poses a fascinating challenge to contemporary theories of inelastic electron transport. A scattering theory of density matrices is motivated by the need to address large amplitude, chemically relevant dynamics in tandem with an appropriate treatment of the electronic scattering problem. We provide a qualitative discussion of the theory and note the limits in which it reduces to well-known approaches.

Chemistry at molecular junctions: Rotation and dissociation of O2 on the Ag(110) surface induced by a scanning tunneling microscope

“…While Saalfrank and co-workers 19, 21 studied the contributions of both mechanisms to STM-induced switching of molecules on surfaces, in most theoretical studies of STM-induced chemical processes the "resonance" mechanism is considered to be the dominant mechanism, 11 motivating sophisticated models and dynamics simulations involving multiple electronic states. 6,14,16,18,22 However, based on our results, we propose that the "dipole" mechanism might be the dominant mechanism of IET in the STM-induced dissociation of O 2 on Ag(110) due to short lifetimes of relevant excited electronic states.…”

“…11,14,16,22 This resonance corresponds to the transient filling of the lowest unoccupied molecular orbital (LUMO) of the chemisorbed molecule. For the opposite direction of tunneling current, a hole can become transiently trapped in the molecular orbital.…”

“…6,[10][11][12][13][14][15][16][17][18][19][20][21][22] The primary goal of these theoretical models and computational approaches is to describe the underlying process: inelastic electron tunneling (IET). While most of the tunneling current passing from the STM tip to a molecule-surface sample, or vice versa, is elastic, a few electrons can tunnel inelastically, particularly under certain system-specific conditions of voltage and current.…”

An ignition key for atomic-scale engines

“…The bias is a sensitive control parameter for the current-driven excitation of atomic motion in resonant systems [30,31]. In this letter we show how resonances can be exploited to turn the NC force on and off, akin to turning the engine of a car on and off.…”

A non-equilibrium equation-of-motion approach to quantum transport utilizing projection operators

“…19 Examples of such formulations include scattering theory methods, 13,[20][21][22][23] quantum master equation approaches, 24-31 pseudoparticle 14,16,[32][33][34][35][36] and Hubbard [37][38][39][40][41] non-equilibrium Green function techniques.…”