Polarization Properties And Mechanical Behaviour Of Novel Low-dimensional Group Ⅳ-Ⅵ Semiconductors

With the advancement of manufacturing technology and the rapid development of microelectronics integration technology,the miniaturisation,integration and functionalisation of electronic components has become a development trend,and the study of nanoscale materials has become one of the most important aspects in the field of microelectronics.Since the successful exfoliation of graphene from graphite in 2004,its unique properties have stimulated extensive research interest in two-dimensional materials other than graphene.However,unlike graphene,some 2D materials have a certain band gap width and a large number of structural diversity,and therefore have excellent properties in electrical,optical and magnetic applications.One of these is the group IV-VI semiconductor（MX）2D materials,which have been found to have excellent ferroelectric,piezoelectric and optical properties,and which are abundant and environmentally friendly on Earth,and therefore have great potential for research and application in the field of micro devices.Based on first principles calculations,this paper focuses on three aspects of low-dimensional functional materials:（i）exploring new two-dimensional MX,studying its geometry,stability,electronic structure,optical properties and piezoelectric coefficients,predicting its rich functionality and providing guidance for experimental synthesis and nanodevice development;（ii）designing one-dimensional MX ferroelectrics,studying its ferroelectricity,stability,electronic structure,transition modes and synthesis in nanotubes,in line with the miniaturization trend of smart components;（iii）Design a variety of two-dimensional hinged tensile expansion structures of phosphene,MX,and graphene to investigate their mechanical properties such as Young’s modulus,Poisson’s ratio,and deformation mechanisms,to investigate the micromechanisms and enrich the two-dimensional material family.The first part is based on the existing two-dimensional MX prediction,expanding four new MXs with three atomic sublayers and naming them tri-MX（i.e.,trilayer MX,M=Ge,Sn,X=Se,S）.The geometric structure,stability,electronic structure,optical properties,piezoelectric coefficients,and their conversion modes of tri-MX have been systematically studied.The results show that tri-MX has an asymmetric structure,with the piezoelectric coefficients of tri-Sn Se and tri-Sn S reaching 182.31 pm/V and 179.44 pm/V,which exceed those of the highest piezoelectric coefficients in the experimentα-Sn S,which is expected to further improve the efficiency of nano generators.Further analysis shows that differences in the electronegativity of the elements in tri-MX determine differences in structure and deformation patterns under stress.In addition,tri-MX are transmissive in the visible range and can block ultraviolet（UV）light.Combined with their excellent piezoelectricity and special optical properties,they will shine in areas such as electronic screens,energy harvesting and UV protection.Finally,their transition potential from alpha-MXs to tri-MXs is between 0.105 and 0.208 e V/atom,which suggests that the synthesis of tri-MX is feasible.In the second part,a series of one-dimensional IV-VI nanowires（MX,M=Ge,Sn;X=S,Se）with ferroelectric properties were designed for the depolarization field of low dimensional ferroelectrics,and their stability,ferroelectricity,electronic structure,transition modes,and their induced synthesis in nanotubes were systematically studied.The results show that they are intrinsically one-dimensional ferroelectrics with Ps of 6.16×10^-10 C/m,5.88×10^-10 C/m,4.39×10^-10 C/m and 3.27×10^-10 C/m for Ge S,Ge Se,Sn S and Sn Se nanowires,respectively,and can exist stably without dangling bonds.Furthermore,their nanotube-induced synthesis results show that MX nanowires are easier to synthesise in（8,8）boron nitride nanotubes than in conventional carbon nanotubes.In the third part,based on the combinatorial properties and electronic structural similarities of MX structures,a new class of auxetic monolayers with perfectly ordered hexagonal and quadrangular arrangements was developed,including MX（Ge S）,phosphorene（P）,and graphane（G）.Their stability,mechanical properties,negative Poisson’s properties,and deformation mechanisms were systematically studied.The results of energy difference,phonon dispersion,ab initio molecular dynamics（AIMD）simulation,and elastic constant calculations indicate that they possess energy,dynamic,thermal,and mechanical stability.They exhibit intrinsic in-plane auxeticity,mainly due to the combination of electronic effects and special geometric structures.We attribute this expansion behavior to the interaction of basic annular structural elements and unique square hinge structures.Our first principle calculations reveal the bond configuration changes behind the abnormal stress induced lateral strain response.These results provide key insights into the fundamental atomic mechanism of two-dimensional material auxeticity.In this paper,we have achieved results in exploring three aspects of new two-dimensional piezoelectric materials,one-dimensional ferroelectric materials and new two-dimensional negative Poisson materials,which contribute to the design of low-dimensional materials for applications in energy conversion,storage and medicine,as well as new materials.