Molecular Simulation Study On The Stress And Heat Transfer Anisotropy Of Low-Dimensional Black Phosphorus

The research of materials at the nanoscale is to explore from the origin of the properties of materials.However,the important research objects of mechanical analysis research at the nanometer scale are low-dimensional material.The monolayer black phosphorene(monolayer phosphorene)discovered in 2014 has become a two-dimensional material that is currently the focus of attention due to its excellent photoelectric conversion characteristics,high carrier mobility,and adjustable band gap.Considering the important role of monolayer phosphorene and phosphorus nanotubes(an annularly closed one-dimensional material composed of monolayer phosphorene curled in a certain direction,PNT)in nano optoelectronic devices,it is very important to study the physical properties of monolayer phosphorene and phosphorus nanotubes.However,it is known that monolayer phosphorene exhibits significant anisotropy in terms of physical properties and most studies on anisotropy of phosphorene in the literature have focused on the armchair(AC)and zigzag(ZZ)directions.In addition,there is a large difference in the theoretical calculation results of the research on the thermal conductivity of single-layer phosphorene.Therefore,this paper uses molecular dynamics(MD)simulation methods to study the anisotropy of the mechanical and thermal properties of monolayer phosphorene,and preliminary predicts the mechanical and thermal properties of phosphorus nanotubes:This paper analyzes the tensile strength and ultimate strain of a single layer of phosphorene in the general direction(amorphous lattice direction)under uniaxial tension,and discusses the relationship between the elastic modulus and the rising angle of the single layer of phosphorene in the general direction.The thermal stability and mechanical properties of different chiral phosphorus nanotubes composed of these different directions of phosphorene during uniaxial compression.The results show that the elastic modulus of the monolayer phosphorene under different crystal orientations gradually increases with the increase of the rising angle,but the elastic modulus of the monolayer phosphorene in the [110] direction(ST)and its perpendicular direction(PS)increases and decreases abnormally.Phosphorus nanotubes with different chirality composed of monolayer phosphorene with different crystal orientations,the elastic modulus gradually decreases with the increase of the rising angle,which is consistent with the trend of their thermal stability.The main reason is that when curled in the AC direction,the out-of-plane bending stiffness is small,and the phosphorus atoms across the plane are easily bent,and the bond length and angle are changed at the same time.Therefore,the internal stress of the AC-PNT formed by bending smallest and most stable structure.Phosphorus nanotubes with a small rise angle have a large elastic modulus and high thermal stability.This is because when a single-layer phosphorene is rolled in these directions,out-of-plane phosphorus atoms are bent,and when the rise angle continues to increase,When the single-layer phosphorene is curled in a direction with a large ascending angle,the proportion of in-plane atoms bending becomes larger,and the bending stiffness gradually increases until the ascending angle reaches 90 °,forming a ZZ-type phosphorus nanotube(ZZ-PNT),maximum bending stiffness.Thermal conductivities along the armchair(AC)and zigzag(ZZ)direction,ST direction and PS direction are systematically studied.Thermal transport tendency from large to small is in the order of ZZ,PS,ST,and AC.The accuracy of MD in describing anisotropy is proved by acoustic phonon dispersion,and the discrepancy of specific thermal conductivity from the first-principle calculation is mainly due to the mismatch of low-frequency optical phonons.There is an enhancement of thermal conductivity with uniaxial tensile strain along ST and PS direction.Potential energy distribution through atom vibration is chosen to describe strain effect from different calculation methods.Nanotubes rolling from phosphorene have a gentle decrease of thermal conductivity with finite length along the same direction.These results shed light on direction-dependent thermal transport properties of phosphorene along with the applicability in MD simulation.The in-plane heat transfer properties of the multi-layered phosphorene heterostructure were studied by combining the monolayered phosphorene with different crystal orientations to form a multi-layered phosphorene heterostructure.It was found that changing the crystal orientation of the phosphorene in the multilayered phosphorene heterostructure can more accurately regulate the thermal conductivity of the multilayered phosphorene structure.At the same time,the effect of the number of layers on the thermal conductivity of phosphorene was studied.The calculation results showed that the thermal conductivity tended to increase from monolayer phosphorene to double-layer phosphorene.The interaction enhances out-of-plane phonon coupling and reduces the band gap,which has a positive effect on the thermal transport of doublelayer phosphorene.