Negative differential resistance in molecular junctions: application to graphene ribbon junctions

TL;DR

This paper investigates the origin of negative differential resistance in molecular and graphene ribbon junctions using NEGF calculations, revealing how asymmetry and localization effects lead to NDR behavior.

Contribution

It provides a detailed theoretical analysis of NDR in molecular and graphene-based junctions, highlighting the role of asymmetry and charge localization.

Findings

Asymmetry in electrode coupling causes NDR at high bias.

Localization of molecular orbitals reduces transmission and current.

Even-odd effects observed in graphene chain junctions.

Abstract

Using self-consistent calculations based on Non-Equilibrium Green's Function (NEGF) formalism, the origin of negative differential resistance (NDR) in molecular junctions and quantum wires is investigated. Coupling of the molecule to electrodes becomes asymmetric at high bias due to asymmetry between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. This causes appearance of an asymmetric potential profile due to a depletion of charge and reduction of screening near the source electrode. With increasing bias, this sharp potential drop leads to an enhanced localization of the HOMO and LUMO states in different parts of the system. The reduction in overlap, caused by localization, results in a significant reduction in the transmission coefficient and current with increasing bias. An atomic chain connected to two Graphene ribbons was investigated to illustrate these effects. For a chain substituting a molecule, an even-odd effect is also observed in the NDR characteristics.