Can molecular projected density of states (PDOS) be systematically used in electronic conductance analysis?

Abstract

SummaryUsing benzenediamine and benzenedithiol molecular junctions as benchmarks, we investigate the widespread analysis of the quantum transport conductance in terms of the projected density of states (PDOS) onto molecular orbitals (MOs). We first consider two different methods for identifying the relevant MOs: (1) diagonalization of the Hamiltonian of the isolated molecule and (2) diagonalization of a submatrix of the junction Hamiltonian constructed by considering only basis elements localized on the molecule. We find that these two methods can lead to substantially different MOs and hence PDOS. Furthermore, within Method 1, the PDOS can differ depending on the isolated molecule chosen to represent the molecular junction (e.g., with or without dangling bonds); within Method 2, the PDOS depends on the chosen basis set. We show that these differences can be critical when the PDOS is used to provide a physical interpretation of the conductance (especially when its value is small, as it happens typically at zero bias). In this work, we propose a new approach in an attempt to reconcile the two traditional methods. Although some improvements were achieved, the main problems remain unsolved. Our results raise more general questions and doubts on a PDOS-based analysis of the conductance.

Improvements on non-equilibrium and transport Green function techniques: The next-generation transiesta

“…How such a projector is chosen is outside the scope of this article, but we refer to Ref. [95] for additional projectors. In tbtrans we save all scalar quantities M j |Γ L |M j for an extra level of information.…”

Energy level alignment at molecule-metal interfaces from an optimally tuned range-separated hybrid functional

“…Examples include systems for which molecular levels are pinned at E F 71,72 , or where significant covalent interactions lead to strong hybridization between molecular and substrate orbitals 73 , such as the Au-benzenedithiol-Au junction 74,75 . As mentioned above, this situation can lead to the molecular resonances being energetically close to the Fermi level, which implies a significant charge transfer.…”

“…Examples include systems for which molecular levels are pinned at E F 71,72 or where significant covalent interactions lead to strong hybridization between molecular and substrate orbitals, 73 such as the Aubenzenedithiol-Au junction. 74,75 As mentioned above, this situation can lead to the molecular resonances being energetically close to the Fermi level, which implies a significant charge transfer. Clearly, the amount of charge transfer will determine the distribution of the charge density of the interface system, which means that a correction scheme for level alignment requires self-consistency.…”

First-principles molecular transport calculation for the benzenedithiolate molecule

“…Due to the fact that only the HOMO level is close to the Fermi energy it is the level mainly contributing to transport which is in general the case in systems with thiol-anchoring groups [58]. The HOMO orbital is an unsaturated sulphur p-orbital and can also be compared to [31,59]. Passivating the unsaturated sulphur p-orbitals would shift the HOMO level away from the Fermi energy and produce lower conductances [16].…”