Energy-dependent resonance broadening in symmetric and asymmetric molecular junctions from an ab initio non-equilibrium Green's function approach

TL;DR

This paper introduces an energy-dependent resonance broadening function for molecular junctions, computed via ab initio NEGF, providing a more accurate description of resonance features affecting transport properties.

Contribution

It develops a novel energy-dependent resonance broadening function using non-Hermitian matrix diagonalization, improving upon the traditional energy-independent Lorentzian approximation.

Findings

The new $(E)$ accurately reproduces transmission spectra from DFT-NEGF.

Application to symmetric and asymmetric junctions demonstrates improved understanding of transport.

Decomposition into lead components offers insights into molecular orbital contributions.

Abstract

The electronic structure of organic-inorganic interfaces often feature resonances originating from discrete molecular orbitals coupled to continuum lead states. An example are molecular junctions, individual molecules bridging electrodes, where the shape and peak energy of such resonances dictate junction conductance, thermopower, I-V characteristics and related transport properties. In molecular junctions where off-resonance coherent tunneling dominates transport, resonance peaks in the transmission function are often assumed to be Lorentzian functions with an energy-independent broadening parameter $\Gamma$. Here we define a new energy-dependent resonance broadening function, $\Gamma(E)$, based on diagonalization of non-Hermitian matrices, which can describe resonances of a more complex, non-Lorentzian nature and can be decomposed into components associated with the left and right lead, respectively. We compute this quantity via an \emph{ab initio} non-equilibrium Green's function approach based on density functional theory for both symmetric and asymmetric molecular junctions, and show that our definition of $\Gamma(E)$, when combined with Breit-Wigner formula, reproduces the transmission calculated from DFT-NEGF. Through a series of examples, we illustrate how this approach can shed new light on experiments and understanding of junction transport properties in terms of molecular orbitals.