Molecular Dynamics Simulation Of Thermal Energy Transport Across Material Interfaces

Both ab-initio and classical molecular dynamics (MD) were used to study the thermal energy transport phenomena across nano-scale material interfaces. Thermal equilibration in semiconductor ultra-thin layered superlattices was simulated by ab-initio MD with density functional theory (OFT). Equilibrium MD (EMD) and Non-equilibrium MD (NEMD) simulations were performed on Au-SAM (self-assembly monolayer)-Au junctions with alkanedithiols being the SAM molecules. The in-plane thermal conductivities were calculated using EMD with Green-Kubo method. The out-of-plane thermal conductances were calculated in both EMD and NEMD simulations. Au substrate thickness effect, temperature effect, simulated normal pressure effect, molecular chain length effect, molecule coverage effect and molecule-substrate bonding strength effect on thermal conductivity/conductance were studied. Vibration density of states (VDOS) was calculated, and the mechanism of thermal energy transport across the material junctions was analyzed. The calculated thermal conductance at high temperatures agrees well with available experimental data. The temperature dependence of thermal conductance has a similar trend to experimental observations. SAM molecular coverage was found to be important on the interfacial thermal conductance. Analysis of the junction response to a heat pulse showed that the Au-SAM interface resistance was much larger than the substrate and SAM resistances. The results showed that the Au-SAM interface resistance dominated thermal energy transport across the junction The DFT ab-initio method was used to study the bondings of thiols on As-terminated GaAs (001) surfaces. As-S interactions were simulated by the Morse potential, and the parameters were fitted to an energy hypersurface obtained from DFT calculations. NEMD simulations were then performed on GaAs-SAM-GaAs junctions to study thermal energy transport across thiol-GaAs interfaces. NEMD simulations were also carried out to study thermal energy transport across different graphene-polymer interfaces. The results of this study will be useful for the current molecular electronics industry in which thermal dissipation is a critical problem to be resolved. It is concluded that the interfacial resistance is the barrier for thermal transport across molecule-solid junctions. As a result, methods to facilitate thermal transport across the interfaces, such as depositing denser SAM, forming stronger molecule-solid bonds, choosing materials with better vibration coupling, are to be considered in the emerging technology of the manufacturing of molecular electronics.