Studies On The Thermal Transport Properties Of Low-Dimensional Semiconductor Nanostructures:An Atomic-Bond-Relaxation Approach

Low-dimensional semiconductor nanostructures,i.e.,one-dimensional Si nanowires(nanotubes)and two-dimensional black phosphorus(BP)membranes,possess enormous potential applications in the fields of new-type nanoelectronics/optoelectronic devices due to their excellent physcial and chemical properties.Among these properites,the thermal transport properties have attracted intensive attention.While minimizing the devices,the unique transport properites in low-dimensional nanostructures are much different from the bulk materials.Besides,heat dissipations in nanoelectronics/optoelectronic devices limit the stable performances when the size of these devices decrease.Therefore,it is key to explore new-type nanoelectronic devices with high thermal conduction properties for breaking the limit of Moore s law and improving performances.At the nanoscale,the materials can be approached core-shell configurations.Due to termination of lattice periodicity,the atoms in the surface shell have low-coordination numbers.With decreasing the size,the surface-to-volume ratio of surface shell rises;the bond of the edge atoms become shorter and stronger spontaneously.As a result,the low-dimensional nanostructures keep at the self-equilibrium state,and lead to new thermal transport properties.Although some achievements from experiments and calculations have been obtained to explore the thermal transport properties of low-dimensional nanostructures,there are several fundamental problems have not been solved,i.e.,the size and temperature effects on phonon scatterings in one-dimensional semiconductor nanostructures,the interface effect on phonon transmissvity and thermal boundary resistance as well as the relationship between geometry and interface thermal conductance.In our work,based on surface atomic-bond-relaxation(ABR)consideration and phonon dynamics model,we explored thermal transport properties of low-dimensional nanostructures,including the thermal conductivities of one-dimensional Si nanowires and nanotubes,thermal boundary resistance and thermal donductivity in Si/Ge core-shell nanowires,the effect of geometry parameters on thermal conductivity in two-dimensional silicon films with periodic nanopores arrys as well as the interface effect on interface thermal conductance in BP/SiO2 heterojunctions.Also,we invesitgated Raman frequency shifts of monlayer BP under unaxial strain based on combination of mass-spring model and Lagrangian dynamics.The major research findings are summarized as follows:(1)We explored the size-and temperature effects on the thermal conductivities of one-dimensional Si nanowires(nanotubes).It was found that the decrease of size increases phonon scatterings,leading to the decrease of phonon relaxation time and thermal conductivity.More phonons are excited when the temperature rises,which lead to the increase of thermal conductivity in low temperature zone;at the mean time,phonon-phonon scatterings increase and contribute to the reduction of thermal conductivity in high temperature zone.In addition,the thermal boundary resistence induced by the interface of one-dimensional Si/Ge core/shell nanowires decreases the interface phonon transmissivity and results in much lower thermal conductivity than that in pristine Si or Ge nanowires.(2)We investigated the geometry effect on thermal conductivity in periodic nanoporous Si films(np-SiFs).We found that both surface effect induced by the coordination defects of surface atoms and interactions among pores in np-SiFs play the vital role for the thermal properties.Moreover,the inner surfaces of pores with negative curvature and interaction among nanopores in the film can lower the total energy and modify thermal transport properties in periodic np-SiFs.Furthermore,the decrease of thickness and periodicity length of np-SiFs depress the mean phonon velocity,which lead to the decrease of thermal conductivity.(3)We put forward a model for size-dependent interface thermal conductance in BP/SiO2 heterojunction.It was found that the mismatch strain in heterojunctions changes the phonon coupling strength at interface.The reduction of BP thickness decreases the mismatch strain in heterojunction and enhances interface coupling interactions,resulting in the improvement of interface thermal conductance.In addition,more phonons in BP/SiO2 will be excited,leading to the increase of phonon flux and interface thermal conductance when the temperature becomes large.(4)Based on combination of mass-spring model and Lagrangian dynamics,we investigated the Raman frequency shifts of monolayer BP under unaxial strain.It was found that the orientation-dependent bond lengths and bond angles lead to the different trends and rates of Ag2,B2g and Ag1 modes when BP membrane is under armchair unaxial strain or zigzag unaxial strain.Also,we found that Ag2 and B2g show blue(red)shifts and Ag1 mode has a red(blue)shift under the approach of armchair(zigzag)strain.