Piezoelectricity And Applications Of Two-dimensional Films Along The Direction Perpendicular To The Film Plane

It is challenging to realize the piezoelectricity of 2D films by exerting the stress along the direction perpendicular to the film plane.The new piezoelectric films can transfer the mechanical stress perpendicular to the films into electric energy and possess high conversion efficiency,so they have more broad application prospects,such as medical device?sphygmomanometer?and ultrathin nano-sensor devices?tactile sensor as the bionic skin of robot?.Then the out-of-plane piezoelectric coefficients e₃₃ and d₃₃ of film have the special importance.We investigate the elastic properties and piezoelectricity properties of group V-IV-III-VI films?SnN-InO,GeN-GaO,and SiN-AlO?using first-principles calculations.It is found that the SiN-AlO and SnN-InO films have the large piezoelectric coefficients which perpendicular to the film plane due to their broken inversion symmetry along the direction perpendicular to the film plane.The e₃₃(38.69×10^-10C m^-1)of SiN-AlO is the largest,which is larger than that(5.20×10^-10 C m^-1)of gallium nitride nanowires'surface layers by 7.4 times,while the out-of-plane piezoelectric strain coefficient d₃₃(5.29×10³ pm V^-1)of SnN-InO is the largest,which is one order of magnitude larger than that(0.42×10³ pm V^-1)of the Y-doped ZnO nano sheets.The special direction's piezoelectricity of the SiN-AlO and SnN-InO films indicates that they have the new applications in nanoscale energy harvesting devices,nano-sensor?the bionic skin of robot?and other micro-electromechanical devices,which can transfer the mechanical stress perpendicular to the films into electric energy.