Negative Differential Resistance in Spin-Crossover Molecular Devices

TL;DR

This study demonstrates robust negative differential resistance in spin-crossover molecular devices using STM, supported by theoretical models, and attributes the effect to tip density of states and Coulomb blockade phenomena.

Contribution

It provides the first detailed experimental observation of NDR in SCO molecules and links it to electronic structure and tip effects through combined experimental and theoretical analysis.

Findings

NDR observed in SCO molecules is robust across substrates and temperature.

DFT+NEGF calculations show NDR depends on tip density of states.

Coulomb blockade model explains NDR via molecular orbitals.

Abstract

We demonstrate, based on low-temperature scanning tunneling microscopy (STM) and spectroscopy, a pronounced negative differential resistance (NDR) in spin-crossover (SCO) molecular devices, where a Fe$^{\text{II}}$ SCO molecule is deposited on surfaces. The STM measurements reveal that the NDR is robust with respect to substrate materials, temperature, and the number of SCO layers. This indicates that the NDR is intrinsically related to the electronic structure of the SCO molecule. Experimental results are supported by density functional theory (DFT) with non-equilibrium Green's functions (NEGF) calculations and a generic theoretical model. While the DFT+NEGF calculations reproduce NDR for a special atomically-sharp STM tip, the effect is attributed to the energy-dependent tip density of states rather than the molecule itself. We, therefore, propose a Coulomb blockade model involving three molecular orbitals with very different spatial localization as suggested by the molecular electronic structure.