Piezoelectric Properties Of Two-dimensional Janus Structure Novel Materials

The research of nano-generators and other flexible piezoelectric devices has attracted wide attention in recent years,and the research of two-dimensional（2D）piezoelectric materials is also on the rise.Finding and exploring 2D piezoelectric materials with large piezoelectric coefficient has become the focus of research.However,it is difficult to find2D materials with piezoelectric properties directly,mainly because the piezoelectric materials are restricted by strict symmetry.Because of the lack of mirror symmetry in Janus structure,all 2D semiconductor materials with Janus structure have piezoelectric properties in theory.Therefore,it is undoubtedly a shortcut to find 2D piezoelectric materials with large piezoelectric coefficient from 2D semiconductor materials with Janus structure.Based on the first-principles,starting from the 2D material with Janus structure,this paper explores the 2D piezoelectric material with large piezoelectric coefficient and stability.（1）Referring to the structures in Ge Se monolayer and Sn Se monolayer,we study the electronic structure and piezoelectric properties of the Janus M₂Se X（M=Ge,Sn;X=S,Te）monolayer.Due to the lack of inversion symmetry and mirror symmetry,as well as flexible mechanical properties,the 2D Janus M₂Se X monolayer has piezoelectric properties.The calculation shows that the in-plane piezoelectric coefficient d₁₁ reaches345.08 pm/V and the out-of-plane piezoelectric coefficient d₃₁ reaches 3.83 pm/V.Compared with most of the 2D piezoelectric materials that have been found,the in-plane piezoelectric coefficient of Janus M₂Se X monolayer is tens or even hundreds of times larger.The calculation based on the deformation potential（DP）theory shows that the 2D Janus M₂Se X monolayer has high carrier mobility.Large piezoelectric coefficient and high carrier mobility show the application potential of 2D Janus M₂Se X monolayer in flexible electronic devices and piezoelectric devices.（2）There are few reports on 2D Janus polyatomic thickness piezoelectric materials.Next,we study the piezoelectric properties of Janus Bi₂X₂Y（X=S,Se,Te;Y=S,Se,Te;X≠Y）monolayer with polyatomic thickness.These structures all contain large in-plane piezoelectric coefficient d₁₁,in which the d₁₁ of the 2D Bi₂Te₂S monolayer is as high as 34.96 pm/V,which is one order of magnitude larger than that of the previously reported Mo S₂.（3）We study the piezoelectric properties of 2D Janus Bi Te X（X=Cl,Br,I）monolayer by using the first principle combined with non-equilibrium Green’s function.The magnitude law of Janus Bi Te X monolayer piezoelectric properties is revealed,which is of guiding significance for the discovery of other 2D piezoelectric materials.The piezoelectric response of these materials in the devices is analyzed,and the current regulation effect of 2D piezoelectric materials in micro-nano piezoelectric devices is clarified.（4）Based on the consideration of practical application,we study the effects of strain and multi-layer structure on the piezoelectric properties of 2D Li Al Te₂.The results show that 2D Li Al Te₂ has large in-plane and out-of-plane piezoelectric coefficients,and the piezoelectric properties of 2D Li Al Te₂ can be greatly enhanced by applying small strain and multi-layer stacking.