Ryan Jorn

Proton conduction in exchange membranes across multiple length scales.

Concerns over global climate change associated with fossil-fuel consumption continue to drive the development of electrochemical alternatives for energy technology. Proton exchange fuel cells are a particularly promising technology for stationary power generation, mobile electronics, and hybrid engines in automobiles. For these devices to work efficiently, direct electrical contacts between the anode and cathode must be avoided; hence, the separator material must be electronically insulating but highly proton conductive. As a result, researchers have examined a variety of polymer electrolyte materials for use as membranes in these systems. In the optimization of the membrane, researchers are seeking high proton conductivity, low electronic conduction, and mechanical stability with the inclusion of water in the polymer matrix. A considerable number of potential polymer backbone and side chain combinations have been synthesized to meet these requirements, and computational studies can assist in the challenge of designing the next generation of technologically relevant membranes. Such studies can also be integrated in a feedback loop with experiment to improve fuel cell performance. However, to accurately simulate the currently favored class of membranes, perfluorosulfonic acid containing moieties, several difficulties must be addressed including a proper treatment of the proton-hopping mechanism through the membrane and the formation of nanophase-separated water networks. We discuss our recent efforts to address these difficulties using methods that push the limits of computer simulation and expand on previous theoretical developments. We describe recent advances in the multistate empirical valence bond (MS-EVB) method that can probe proton diffusion at the nanometer-length scale and accurately model the so-called Grotthuss shuttling mechanism for proton diffusion in water. Using both classical molecular dynamics and coarse-grained descriptions that replace atomistic representations with collective coordinates, we investigated the proton conductivity of polymer membrane structure as a function of hydration level. Nanometer-sized water channels form torturous pathways that are traversed by the charges during fuel cell operation. Using a combination of coarse-grained membrane structure and novel multiscale methods, we demonstrate emerging approaches to treat proton motion at the mesoscale in these complex materials.

Competition between current-induced excitation and bath-induced decoherence in molecular junctions

A general framework is presented to describe a resonant inelastic current inducing dynamics in the nuclear degrees of freedom of a molecule embedded between two electrodes. This approach makes use of the scattering theory of density matrices to account for the interaction between the scattering charge and the molecular modes to all orders and reduces in appropriate limits to both the standard master equation treatment for vibrational heating and the Landauer formalism for purely elastic transport. While the method presented here is equivalent to these approaches in limiting cases, it also goes well beyond their restrictions by incorporating the full quantum dynamics in the vibrational subspace in the presence of tunneling current. By application to the Au-C(60)-Au junction, it is shown that inclusion of vibrational coherences, which were previously neglected, is crucial to accurately predict the dynamics induced by current in molecular devices. Interaction with a bath of phonon modes is incorporated within the Bloch model and the competition between the bath-induced relaxation processes and the current-induced excitation is studied in detail over a range of temperatures.

Current-driven dynamics in molecular junctions: endohedral fullerenes.

We introduce a new paradigm for single molecule devices based on electronic actuation of the internal atom/cluster motion within a fullerene cage. By combining electronic structure calculations with dynamical simulations, we explore current-triggered dynamics in endohedrally doped fullerene molecular junctions. Inelastic electron tunneling through a Li atom localized resonance in the Au-Li@C(60)-Au junction initiates fascinating, strongly coupled 2D dynamics, wherein the Li atom exhibits large amplitude oscillation with respect to the fullerene wall and the fullerene cage bounces between the gold electrodes, slightly perturbed by the embedded atom motion. Implications to the fields of single molecule electronics and nanoelectromechanical systems are discussed.

Implications and Applications of Current-Induced Dynamics in Molecular Junctions

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.

Theory of current-induced dynamics in molecular-scale devices

We develop a theoretical framework for the study of inelastic resonant transport and current-driven dynamics in molecular nanodevices. Our approach combines a Born-Oppenheimer solution of the coordinate-, energy-, and voltage-dependent self-energy with a time-dependent scattering solution of the vibrational dynamics. The formalism is applied to two classic problems in current-triggered dynamics. As a simple example of bound-bound events in the nuclear subspace we study the problem of current-induced oscillations in Au-C60-Au heterojunctions. As a well-studied example of bound-free events in the nuclear subspace we revisit the problem of scanning-tunneling-microscopy-triggered H-atom desorption from a Si(100) surface. Our numerical results are supported by a simple analytically soluble model.

Dissipation in molecular junctions

A recently developed theory that formulates the phenomena of inelastic transport and current-driven dynamics in molecular-scale electronics within a time-dependent scattering approach is extended to account for dissipation of the current-induced excitation through coupling to electrode phonons and electron-hole pairs. Our approach treats the electronic transport, the nuclear dynamics, and the energy and phase exchange between the electronic and the vibrational subspaces in the course of the inelastic scattering event within the Schrodinger picture, whereas the dissipation of the energy deposited in the nuclear modes is accounted for within a density matrix approach. Subsequent to formulation of the theory in terms of population relaxation and phase decoherence rates, we develop approaches for computing these rates, treating on equal footing the dissipation due to excitation of electron-hole pairs and that due to the interaction with phonons. Finally, we test the derived rates by application to the model problem of CO adsorbed on metal surfaces, an example that has been extensively studied previously and for which several experimental results are available for comparison.

The role of charge localization in current-driven dynamics

We explore the role of charge localization in current-triggered, resonance-mediated, dynamical events in molecular junctions. To that end we use a simple model for a molecular rattle, a Li+C9H−9 zwitterion attached between two metal clusters. By varying the size of the metal clusters we systematically vary the degree of delocalization of the electronic orbitals underlying the resonant current, and thus can draw general conclusions regarding the effect of delocalization on dynamical processes induced by resonance inelastic current in molecular electronics. In the small cluster limit, we find interesting quantum dynamics in the nuclear subspace, corresponding to coherent tunneling of the wave packet through the barrier of an asymmetric double-well potential. These dynamics are rapidly damped with increasing charge delocalization in extended systems.