Fermi level alignment in molecular nanojunctions and its relation to charge transfer

TL;DR

This paper investigates how Fermi level alignment in molecular junctions influences charge transfer, using density functional theory to analyze bipyridine and BPDT molecules attached to gold, revealing the relation between charge distribution and energy level positioning.

Contribution

It provides a quantitative analysis linking Fermi level alignment to charge transfer in molecular junctions, highlighting the roles of Pauli repulsion and orbital filling.

Findings

Fermi level aligns near LUMO for bipyridine

Fermi level aligns near HOMO for BPDT

Charge transfer is governed by Pauli repulsion and orbital filling

Abstract

The alignment of the Fermi level of a metal electrode within the gap of the hi ghest occupied (HOMO) and lowest unoccupied orbital (LUMO) of a molecule is a key quantity in molecular electronics, which can vary the electron transparency of a single molecule junction by orders of magnitude. We present a quantitative analysis of the relation between this level alignment (which can be estimated from charging free molecules) and charge transfer for bipyridine and biphenyl dithiolate (BPDT) molecules attached to gold leads based on density functional theory calculations. For both systems the charge distribution is defined by a balance between Pauli repulsion with subsequent electrostatic screening and the filling of the LUMO, where bipyridine loses electrons to the leads and BPDT gains electrons. As a direct consequence the Fermi level of the metal is found close to the LUMO for bipyridine and close to the HOMO for BPDT.