Theoretical Calculation On The Thermal Properties Of Silicon/Carbon Based Two-dimensional Nanomaterials

Thermal property is one of the most important intrinsic properties of materials.In this thesis,two kinds of mostly used theoretical calculation method,which is,molecular dynamic simulation and the first principle calculation,are used to investigate the silicon/carbon based two dimensional nanomaterials focusing on their thermal properties.Firstly,Thermal boundary conductance?TBC?of open carbon nanotube?CNT?and crystal silicon was investigated by the method of molecular dynamics?MD?simulation.Van der Waals interaction was used to form the interface between the vertically mounted CNT and the silicon surface.The interfacial TBC was extracted from the thermal relaxation between CNT and Si with different initial temperatures.An enhancement of TBC was spotted with the increase of the external pressure.At the interfacial region,the phonon densities of states of CNT and Si were altered by the external pressure,especially at the frequency between 2 THz and 15 THz,which could be associated with the enhancement of TBC.Secondly,We investigate with molecular dynamics simulations the dependences of thermal conductivity???of polycrystalline graphene on grain boundary?GB?energy and grain size.Hexagonal grains and grains with random shapes and sizes are explored,and their thermal properties and phonon densities of states are characterized.It is found that?decreases exponentially with increasing GB energy,and decreasing grain size reduces?.GB-induced phonon softening and scattering,as well as reduction in the number of heat conducting phonons,contribute to the decrease in thermal conductivity.Two-dimensional?2D?carbon allotrope called penta-graphene was recently proposed from first-principles calculations and various similar penta-structures emerged.We performed a comparative study of thermal transport properties of three representative pentagonal structures,namely penta-graphene,penta-SiC₂,and penta-SiN₂,by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations.Unexpectedly,the thermal conductivity of the three penta-structures exhibits diverse strain dependence,despite their very similar geometry structures.While the thermal conductivity of penta-graphene exhibits standard monotonic reduction by stretching,penta-SiC₂ possesses an unusual nonmonotonic up-and-down behavior.More interestingly,the thermal conductivity of penta-SiN2 has 1 order of magnitude enhancement due to the strain induced buckled to planar structure transition.The mechanism governing the diverse strain dependence is identified as the competition between the change of phonon group velocity and phonon lifetime of acoustic phonon modes with combined effect from the unique structure transition for penta-SiN2.The disparate thermal transport behavior is further correlated to the fundamentally different bonding nature in the atomic structures with solid evidence from the distribution of deformation charge density and more in-depth molecular orbital analysis.The reported giant and robust tunability of thermal conductivity may inspire intensive research on other derivatives of penta-structures as potential materials for emerging nanoelectronic devices.The fundamental physics understood from this study also solidifies the strategy to engineer thermal transport properties of broad 2D materials by simple mechanical.Finally,as the the differences between the silicon based pentagonal and hexagonal structures is barely researched.Based on first-principles calculations,we studied the strain modulated phonon transport behavior of two 2D pentagonal?penta-SiH and bilayer penta-silicene?and one hexagonal silicene structures?H-silicene?,of which the penta-SiH and H-silicene mean the structures are hydrogenated for the purpose of thermodynamical stability.Even with the similar strain dependent thermal transport behavior in penta-SiH and bilayer penta-Silicene,we find that the governing mechanism is still different.For both pentagonal silicene structures,the thermal conductivity presents a large improvement at first as tensile strain increases from 0 to 10%and then stabilizes with strain larger than 10%.Detailed analysis shows that,the in-plane modes contributed the most part to the group velocity enhancement under strains in penta-SiH which is opposite from the bilayer penta-graphene.Although the phonon group velocity and phonon lifetime of both structures increase with applied strain.On the other hand,similarity was found in pentagonal silicene and hexagonal silicene despite the differences in geometry structures.Furthermore,based on the detailed analysis between the pentagonal?penta-SiH?and hexagonal silicene structures?H-silicene?,the difference in out-of-plane phonon scattering can not be ignored:different major scattering channels of the out-of-plane flexual modes result in different thermal conductivity sensitivity to strains.And the disparity in anharmonicity leads to the different thermal conductivity under no strain.