Simulations of heat transport in single-molecule junctions: Investigations of the thermal diode effect

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, and we discuss their benefits and shortcomings. Assessing the potential application of molecular junctions as thermal diodes, 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. We find 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.

Significant thermal rectification induced by phonon mismatch of functional groups in a single-molecule junction

“…The velocities are correlated every 0.2 fs in a total integration time of 0.05 ns. Besides, the interfacial thermal resistance can be attributed to the vibrational matching across the interface [35], and the differences of interfacial coupling strength in two modes can lead to thermal rectification [15,24]. In order to quantify the vibrational overlap between DOS of atoms or components at the interface, the overlap level (S) of the phonon bands of two lattices was proposed by Li et al [36] and in the form of…”

“…However, there is no such configuration in our system. We speculate that the mismatched lattice vibrations of asymmetric materials at both ends of the SMJ, or the mismatched vibrational modes of asymmetric functional groups in the SMJ can lead to the thermal diode effect [15]. In order to test this hypothesis, we calculate the vibrational DOS of the different components in the systems containing diamond lattice, CNT, and biphenyl group, amino group, and carbonyl group of SMJ under the forward and reverse modes.…”

“…However, no thermal diode effects were found in these systems built with SMJs. Wang et al predicted that asymmetric structures and nonlinear thermal behaviors in SMJs are required to produce significant thermal rectification [15].…”

Thermal transport in fullerene-based molecular junctions: molecular dynamics simulations

“…The AEMD method was described in several studies, for example, [27,39,40]. In this method, the thermal conductance is calculated from the equilibration dynamics by monitoring the temperatures of the two bulk solids.…”

“…These molecules were examined in both single-molecule [1,2] and self-assembled [14][15][16][17][18][19] experiments. Moreoever, alkane chains have been extensively studied both theoretically and computationally [20][21][22][23][24][25][26][27][28] with the objective to elucidate mechanisms of thermal transport, devise methodologies, and benchmark theoretical-computational predictions.…”