Raman scattering from molecular conduction junctions: Charge transfer mechanism

Abstract

We present a model for the charge transfer contribution to surface enhanced Raman spectroscopy (SERS) in a molecular junction. The model is a generalization of the equilibrium scheme for SERS of a molecule adsorbed on a metal surface [B.N.J.Persson. Chem. Phys. Lett. 82, 561 (1981)]. We extend the same physical consideration to non-equilibrium situation in a biased molecular junction and to non-zero temperatures. Two approaches are considered and compared: A semiclassical approach appropriate for non-resonance Raman scattering, and a quantum approach based on the non-equilibrium Green function method. Non equilibrium effects on this contribution to SERS are demonstrated with numerical examples. It is shown that the semiclassical approach provides an excellent approximation to the full quantum calculation as long as the molecular electronic state is outside the Fermi window, that is, as long as the field induced charge transfer is small.

Current-induced atomic dynamics, instabilities, and Raman signals: Quasiclassical Langevin equation approach

“…When coupling with the electrode, a k could change due to interaction between the molecule and the electrode. 92 We will take it as a parameter and focus on the displacement correlation function instead. Now, x k (t) is an operator, and the average in Eq.…”

Single molecular shuttle-junction: Shot noise and decoherence

“…Surface-enhanced Raman scattering is used to study the mechanisms in molecular junctions both experimentally [51][52][53] and theoretically [54][55][56]. The approaches based on surfaceenhanced Raman scattering have been widely applied in the study of equilibrium materials over the last 30 years [57][58][59][60][61][62][63].…”

“…Recently, these methods have been applied to the nonequilibrium charge transport process in molecular devices. Theoretically, the electron population at the molecular level and changes in the heat can be detected with this approach [54][55][56]. Under a magnetic field, the spin degree of freedom can be exploited.…”

Coherence in charge and energy transfer in molecular junctions

“…[42][43][44][45][46] In molecular optoelectronics 8 it is customary to distinguish between charge and energy transfer processes between the molecule and contacts (as well as inside the molecular complex). For example, elastic electron transport (single charges moving through the system) is at the heart of charge transfersurface enhanced Raman spectroscopy (CT-SERS), 47,48 while pure energy transfer (transfer of excitation without charge transfer) accounts for coupling between molecular excitations and excitations of the leads (excitonlike or neutral pairings of electronlike and holelike excitations). 17,[49][50][51][52][53] Considering nonequilibrium transport through molecular junctions, charge and energy transport processes happen simultaneously, and a rigorous description must therefore account for this.…”