Molecular Electronic Junction Transport: Some Pathways and Some Ideas

Abstract

When a single molecule, or a collection of molecules, is placed between two electrodes and voltage is applied, one has a molecular transport junction. We discuss such junctions, their properties, their description, and some of their applications. The discussion is qualitative rather than quantitative, and focuses on mechanism, structure/function relations, regimes and mechanisms of transport, some molecular regularities, and some substantial challenges facing the field. Because there are many regimes and mechanisms in transport junctions, we will discuss time scales, geometries, and inelastic scattering methods for trying to determine the properties of molecules within these junctions. Finally, we discuss some device applications, some outstanding problems, and some future directions.

Using coherence to enhance function in chemical and biophysical systems

“…For example, when a molecule spans two electrodes 20,21 transport junctions form, revealing striking differences in current according to the interference of the pathways by which electrons can be routed through a molecule 22,23 . As the electron tunnels through the molecule it traverses physical, structural pathways according to the amplitudes and energies of molecular orbitals [24][25][26] . Quantum interference between pathways through the π -system can prevail, despite competing pathways through the σ -bonds.…”

Toward a First-Principles Evaluation of Transport Mechanisms in Molecular Wires

“…charge-localized deformations on specific molecular subregions. 1,20,22 As a consequence, the charge migrates through the molecule by subsequently moving through so-called hopping sites. Experimental data also point to an intermediate regime, in which hopping sites extend over large portions of the system, e.g.…”

“…The methodology presented in this study relies on a connection between the predominating charge transport mechanism and charge localization properties: One could link tunneling transport to a completely delocalized charge, while increasing charge localization could be associated with hopping transport. 20,22 The charge is then predominantly located on specific subregions of the molecule, where polarons are formed. 15,55,56 The molecular wires under study are considered in their singly oxidized radical-cationic state, as holes rather than electrons dominate the transport in the wires under study here.…”

The Green’s function for the Hückel (tight binding) model

“…In a field that has attracted much attention both experimentally and theoretically in the last decade, the transmission of current across molecules, a striking phenomenon, quite nonclassical, is observed. This is quantum interference, zero or low conductance when electrodes are attached to specific sites across a molecule [5,21]. Quantum interference occurs when the Green's function, whose absolute value squared is related to the current transmitted, vanishes.…”