Thermal Transport Properties Of Single Molecular Junctions From Semi-classical Langevin Molecular Dynamics Method

The physical size of conventional semiconductor transistors currently reaches nanometer scale.Difficulties such as the increase of device power consumption,non-negligible quantum effects and heat dissipation have emerged in nano-semiconductor devices.Consequently,silicon-based electronic devices with conventional fabrication methods cannot continue to be miniaturized.Meanwhile,in the past several decades,along with the progress of experimental techniques,nanotechnology,which is centered on the manipulation of individual atoms and molecules,has flourished,giving rise to the research field of single molecule electronics.Single molecule electronics is an emerging research direction,aiming at constructing single-molecular electronic,optoelectronic,and thermoelectric devices.It is a new interdisciplinary field born from microelectronics and nanoelectronics,and is now of interest to many researchers.Precise nano-experimental methods allow researchers to connect individual molecules to macroscopic electrodes to form single-molecule junctions.Single molecular junctions exhibit many interesting physical properties,laying the foundation for the exploration of multifunctional molecular devices.The transport and statistical properties at the single molecule scale are key to understanding the microscopic mechanisms of many physical and chemical processes.Experimental measurements of thermal conductance at single molecule and single atom scales have been successfully carried out in recent years,which is an important research progress in the field of single molecule electronics.The successful measurement of single moleculescale thermal conductance enriches the means of characterizing single molecule junctions and is a powerful complement to the measurement of their electrical,thermoelectric,and photoelectric transport properties,and also opens the way to the study of non-equilibrium thermodynamics at the single molecule scale.The thermal conductance of single molecule junctions can originate from the contributions of different degrees of freedom.Among them,the collective excitation of atoms,i.e.,phonons,makes the largest contribution.When studying the phonon thermal conductance of single molecule junctions,we need to consider three important features:nonequilibrium,anharmonicity and quantum effects.However,considering these three features simultaneously is a challenge for theoreticians.The only few rigorous theoretical approaches available are only applicable to minimal models containing a single or a few degrees of freedom and are difficult to be applied to realistic single molecule structures.Here,we focus on theoretical approaches that include atomic-scale structural information and thus can be used for the study of realistic molecular structures,while taking into account the above three important features to some extent.We adopt a semi-classical Langevin molecular dynamics approach to study the thermal transport of single molecule junctions.The semi-classical Langevin molecular dynamics is equivalent to the nonequilibrium Green’s function approach in the low-temperature limit where anharmonic interactions can be neglected,and to the conventional classical molecular dynamics approach in the hightemperature limit where quantum effects can be neglected.There is no numerical instability in the transition region between the two.Therefore,it can simultaneously give the phonon thermal conductance of the actual single molecule junction over the whole temperature range considering both anharmonic and quantum effects to some extent.First,we study the phonon thermal transport properties of the alkane chain between gold and graphene electrodes,respectively.We find that at room temperature the quantum statistics of graphene electrodes are very important and have a greater influence on their thermal transport compared to the single molecule junctions made from gold electrodes.Meanwhile,the anharmonic interactions cannot be neglected in the single molecule junction from gold electrodes.This study illustrates that quantum statistics and anharmonic interactions play different roles in these two types of junctions.Second,we consider current-induced heat generation and transport in single molecular junctions,due to electron-phonon interactions.Using deep learning potential trained from ab initio molecular dynamics,the efficiency of numerics can be boosted by one order of magnitude.The results show that the energy accumulation due to current in single molecule junctions of graphene electrodes is one order of magnitude lower than that injunctions of gold electrodes.This is because the graphene electrode phonon spectrum has a better frequency match with the molecular vibrations,and heat exchange between the two can be carried out through efficient harmonic phonon thermal transport.In gold molecular junctions,heat transport from the molecule to the surrounding electrodes requires redistribution of energy among different vibrational modes through anharmonic interactions,whose efficiency is much lower.Better matching of the phonon spectrum enhances the efficiency of heat transport in single molecule junctions.Our results demonstrate the superiority of graphene as electrodes in building stable single molecular junctions.The numerical method developed in this thesis can treat quantum statistics and anharmonic interaction under the unified theoretical framework.It paves the way for the theoretical study of phonon thermal transport in single molecule junctions in the full temperature range and hybrid heat transport between electrons and phonons due to applied voltage bias.