Simulations of Heat Transport in Single-Molecule Junctions: Investigations of the Thermal Diode Effect

TL;DR

This study uses molecular dynamics simulations to analyze heat transport in single-molecule junctions, compares methods for calculating thermal conductance, and investigates the potential for thermal diode effects, finding limited diode behavior in simple alkane systems.

Contribution

It introduces and compares two MD-based techniques for thermal conductance calculation and assesses the feasibility of molecular junctions as thermal diodes with structural modifications.

Findings

Thermal conductance is approximately length-independent for alkane chains.

RNEMD and AEMD methods provide complementary insights into thermal transport.

Thermal diode effect is negligible in simple alkane junctions unless very large thermal biases are applied.

Abstract

With the objective to understand microscopic principles governing thermal energy flow in nanojunctions, we study phononic heat transport through metal-molecule-metal junctions using classical molecular dynamics (MD) simulations. Considering a single-molecule gold-alkanedithiol-gold junction, we first focus on aspects of method development and compare two techniques for calculating thermal conductance: (i) The Reverse Nonequilibrium MD (RNEMD) method, where heat is inputted and extracted at a constant rate from opposite metals. In this case, the thermal conductance is calculated from the nonequilibrium temperature profile that is created on the junction. (ii) The Approach-to-Equilibrium MD (AEMD) method, with the thermal conductance of the junction obtained from the equilibration dynamics of the metals. In both methods, simulations of alkane chains of growing size display an approximate length-independence of the thermal conductance, with calculated values matching computational and experimental studies. The RNEMD and AEMD methods offer different insights on thermal transport, and we discuss their relative benefits and shortcomings. Assessing the potential application of molecular junctions as thermal diodes, the alkane junctions are made spatially asymmetric by modifying their contact regions with the bulk, either by using distinct endgroups or by replacing one of the Au contacts by Ag. Anharmonicity is built into the system within the molecular force-field. Using the RNEMD method, we show that, while the temperature profile strongly varies (compared to the gold-alkanedithiol-gold junctions) due to these structural modifications, the thermal diode effect is inconsequential in these systems -- unless one goes to very large thermal biases. This finding suggests that one should seek molecules with considerable internal anharmonic effects for developing nonlinear thermal devices.