| Since graphene has been exfoliated successfully,two-dimensional（2D）materials attract a large number of people to study widely.Compared with bulk materials,two-dimensional materials display unique characteristics,such as high mechanical strength,high photocatalytic activity,excellent optical and magnetic properties.Meanwhile,the properties of 2D materials can be facilely altered by applying external strains,controlling defects,or stacking multiple layers of 2D structures.These unique advantages suggest that 2D materials could pave a novel avenue for many innovative devices,and 2D piezoelectric materials is one group of them.The piezoelectric materials can apply to energy harvesting,sensors actuators and so on,is the indispensable part of a variety of fields.Recently,both experimental and theoretical studies develop rapidly,demonstrating that the piezoelectricity of 2D materials has been a research focus in the ever-increasing energy conversion area.In the discovery of 2D materials,theoretical calculation has been proved to be an effective approach to evaluate their piezoelectric responses.The calculated piezoelectric coefficient of Mo S2 monolayer has been verified exactly by the subsequent experiments,which strongly suggests the conclusion.Although the piezoelectric properties of a lot of 2D materials have been systematically investigated and those with very large piezoelectric coefficients have been reported,it is still quite difficult to accurately determine the origin of piezoelectricity in the 2D materials.For device applications,2D materials are generally utilized in the form of multilayered nanosheets.Unfortunately,the piezoelectricity of most reported 2D materials,such as 2H-Mo S2,exists only in monolayer or multilayer materials and decreases gradually with increasing layer number.More seriously,since the alternating layers of those nanosheets are stacked alternatively with an opposite orientation,multilayered nanosheets with an even number of layers are centrosymmetric and can lose their piezoelectric response,while only the odd-layer nanosheets with a broken inversion symmetry can exhibit piezoelectricity.Such odd-even effect of piezoelectricity in 2D materials increases the difficulty in materials preparation and limits their practical applications.Thus,a layer-dependent piezoelectric behavior of 2D materials has to be considered in the devices.For the above reasons,the main research works are as follows:1.Inspired by the typical monolayers with black-phosphorene-type structures,the structural stability,electronic structure,and piezoelectricity of the 2D ternary Ga XY（X=Se and Te;Y=Cl,Br,and I）monolayers are systematically studied by the first-principles density functional theory（DFT）.The calculations show that the ternary monolayer compounds exhibit more desirable stability and electronic properties as compared with those of binary compounds.Remarkably,the ternary Ga XY monolayers have a unique anisotropic piezoelectricity.The calculated piezoelectric coefficients d11 and d12 are as large as 15.57pm V-1 for Ga Te F and 3.78 pm V-1 for Ga Se I,respectively.To determine the periodic trend of piezoelectric response,a linear correlation between the coefficients d11 and d12 and the difference in anionic polarizabilitiesαX、αY is proposed.It is found that d11 or d12 of Ga XY monolayers is directly proportional to（αX-αY）or（αY-αX）,respectively,and such anisotropic correlation between the piezoelectricity and polarizability could be applied to account for the piezoelectricity in other ternary and even multicomponent 2D systems.2.Based on first-principles calculations,we report an unexplored 2D Ga In S3 material that can be exfoliated from its bulk crystal,and 2D Ga In S3 nanosheets have good thermal and structural stabilities and oxidation resistance,as well as tunable wide bandgap.Due to the strain sensitivity of Ga In S3 nanosheets,the indirect-to-direct bandgap transition can be achieved under a small strain.Moreover,the particular stacking structure with C2v symmetry endows 2D Ga In S3 nanosheets with promising piezoelectricity without the odd-even effect,which is much different with those of previously reported 2D piezoelectric materials.With the increasing number of layers,the piezoelectric stress and strain coefficients0)0)113and d11 of 2D Ga In S3nanosheets are sustained at a consistent level with those of monolayer(around 0.23 C m-2 and 2.08 pm V-2,respectively).The stability in piezoelectricity accompanying with high carrier mobility endows 2D Ga In S3nanosheets with promising application prospects in nanosized sensors,piezotronics,and energy-harvesting devices.Those electronic and piezoelectric properties of Ga In S3 nanosheets as demonstrated in this work call for further practical applications and experimental verifications. |
