Current-induced atomic motion, structural instabilities, and negative temperatures on molecule-electrode interfaces in electronic junctions

Abstract

Molecule-electrode interfaces in molecular electronic junctions are prone to chemical reactions, structural changes, and localized heating effects caused by electric current. These can be exploited for device functionality or may be degrading processes that limit performance and device lifetime. We develop a nonequilibrium Green's function based transport theory in which the central region atoms and, more importantly, atoms on molecule-electrode interfaces are allowed to move. The separation of time-scales of slow nuclear motion and fast electronic dynamics enables the algebraic solution of the Kadanoff-Baym equations in the Wigner space. As a result, analytical expressions for dynamical corrections to the adiabatically computed Green's functions are produced. These dynamical corrections depend not only on the instantaneous molecular geometry but also on the nuclear velocities. To make the theoretical approach fully self-consistent, the same time-separation approach is used to develop expressions for the adiabatic, dissipative, and stochastic components of current-induced forces in terms of adiabatic Green's functions. Using these current induced forces, the equation of motion for the nuclear degrees of freedom is cast in the form of a Langevin equation. The theory is applied to model molecular electronic junctions. We observe that the interplay between the value of the spring constant for the molecule-electrode chemical bond and electronic coupling strength to the corresponding electrode is critical for the appearance of structural instabilities and, consequently, telegraphic switching in the electric current. The range of model parameters is identified to observe structurally stable molecular junctions as well as various different kinds of current-induced telegraphic switching. The interfacial structural instabilities are also quantified based on current noise calculations.

First-passage time theory of activated rate chemical processes in electronic molecular junctions

“…In alignment with our previous work [3,4,32,[56][57][58], we assume that the classical motion along the reaction coordinate within the system occurs over long time-scales relative to the characteristic electron tunnelling time. This provides us with the required small parameter to be able to perturbatively solve ( 5) and ( 6) up to the first order in expansion of the exponents with derivatives.…”

“…An applied voltage allows for the flow of electronic current across the system through the valence states of the molecule. Large current densities and power dissipation give way to strong current-induced forces and bondselective heating which act to destabilize the molecular configuration within the junction, resulting in molecular conformational changes including telegraphic switching [1][2][3][4], along with providing the necessary energy for total bond rupture [5][6][7][8][9][10][11]. This is obviously an undesirable feature for promoting molecular electronics as a possible avenue for moving beyond the traditional silicon semiconductor technology into a regime of highly efficient and tailorable molecular-scale devices, and so a thorough theoretical understanding is required for further progress.…”

“…This enables the consideration of highly non-trivial behaviour on the nuclear dynamics at the cost of a fully quantum description. Nevertheless, the method has proven successful in a range of circumstances [1,3,4,6,11,[26][27][28][29][30][31][32].…”

Unraveling current-induced dissociation mechanisms in single-molecule junctions

“…11,43 Similarly, the study of mechanical instabilities of molecular junctions under the influence of nonconservative current-induced forces has so far been based on classical treatments of the nuclei and/or used the harmonic approximation for the description of the nuclear potentials. [44][45][46][47][48] It is also noted that the theoretical framework to study the related process of dissociative electron attachment in the gas phase is well established, [49][50][51][52] but this problem is conceptually simpler because only a single electron that is scattered from the molecule has to be considered. Moreover, the processes of light-induced dissociation or desorption of molecules at surfaces has also been studied in great detail theoretically.…”

Current-induced one-dimensional diffusion of Co ad-atoms on graphene nanoribbons

“…Since we do not detect any preferential direction of motion it is reasonable to assume that the motion is activated by current-induced heating of the GNR. [29][30][31][32][33][34] Adopting a theoretical value of about E GNR D ≈ 0.5 eV for the diffusion barrier for Co on single layer graphene from theory (see Ref. 35, and references therein), we can use the observed hopping rate ν during a pulse, ν ∼ 25 s −1 , for estimating the effective temperature on the GNR.…”