Theoretical Research On The Piezoelectricity And Thermal Conductivity Of New Two-dimensional Piezoelectric Materials

Two-dimensional piezoelectric materials have recently received extensive attention from researchers.However,many current researches focus on two-dimensional piezoelectric nanomaterials that have been synthesized experimentally or have a relatively simple crystal structure.In this work,we use the MD method and the DFT method to study two new two-dimensional piezoelectric materials:one is a single-layer h-BAs that has not been synthesized in experiments but has potential application value,and the other is MoS₂-WSe₂ lateral heterojunction which has a relatively complex crystal structure.According to theoretical predictions,cubic BAs single crystals have ultra-high thermal conductivity second only to diamonds.Recently,scientists have verified their ultra-high thermal conductivity through experiments.This work has aroused our great research interest in two-dimensional h-BAs.In this work,we used the tersoff potential function to study the piezoelectric properties and thermal conductivity of single-layer h-BAs through the MD method.Our calculation results show that the piezoelectric coefficient of single-layer h-BAs has obvious anisotropy,and its value along the armchair direction is 8n N/m,while the value along the zigzag direction is 12n N/m.This edge-related anisotropy has been experimentally confirmed.In addition,we used the DFT method to calculate the Born effective charge of the single-layer h-BAs as 0.68 e,and the direct band gap under unstrained conditions is 0.76 e V.Then,we simulated the thermal conductivity of single-layer h-BAs using the non-equilibrium molecular dynamics method.We calculated the thermal conductivity of single-layer h-BAs between 200K and 800K.The results show that the thermal conductivity of h-BAs at room temperature 300K is 186W/m K,and it converges to 95W/m K with increasing temperature.All these results indicate that single-layer h-BAs are expected to be used in many fields such as piezoelectric,semiconductor,and thermal management.Since the lateral heterojunction was first synthesized experimentally in 2014,it has been extensively studied because of its intrinsic strain and built-in electric field at the one-dimensional interface.However,there are very few studies on piezoelectric properties.In this work,we used the DFT method based on modern polarization theory to calculate the piezoelectric coefficients of two different TMD materials.Among them,the piezoelectric coefficient of MoS₂ is 3.7,the piezoelectric coefficient of WSe₂ is 2.74,and the unit is 10^-10N/m.Our calculation results are in good agreement with the previous DFT results.Then,we used the SW potential function and MD simulation method to calculate the piezoelectric coefficient of the MoS₂-WSe₂ lateral heterojunction along the armchair direction.Its value is3.1?10^-10N/m,and the value is between the values of the two different TMD materials that make up the lateral heterojunction.