Thermal transport in molecular junctions: Design principles and simulations

With the objective of understanding 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 develop techniques for calculating thermal conductance: (i) Langevin Nonequilibrium MD (LNEMD) method, which generates nonequilibrium steady state with Langevin thermostats. (ii) Reverse Nonequilibrium MD (RNEMD) method, where heat is inputted and extracted at a constant rate from opposite metals to form a temperature bias at steady state. (iii) Approach-to-Equilibrium MD (AEMD) method, with the thermal conductance of the junction obtained from the equilibration dynamics of the metals. Simulations with all methods show consistent magnitudes of thermal conductance, and chains of a growing size display an approximate length-independence of the thermal conductance, with calculated values matching literature. In addition, heat exchange statistics was examined for method (i), where it was shown that while the average heat current across the junction behaves physically, the probability distribution of the current violates fundamental fluctuation symmetry. We apply our methods by (1) characterizing gold-fullerene-gold junctions to investigate thermal transport behavior of stacking fullerenes and (2) attempting to induce thermal diode effect in the gold-alkanedithiol-gold junction.