Study On The Mechanical-electronic Coupling Effects Of Bending ZnO Nanowires:an Atomic-bond-relaxation Approach

ZnO nanowires(NWs)have attracted increasing interest owing to the excellent performance in optoelectronics(e.g.ultraviolet laser,strain sensors,solar cell,field effect transistors and so on)and biological components(glucose sensors)because of its wide and direct bandgap(3.37 eV),large exciton binding energy(60 meV)and biosafety compatibility properties.Currently,the piezoelectric nanogenerator based on ZnO nanowire array has been successfully synthesized in experiment,for the first time to achieve the conversion from mechanical energy to electrical energy in nanoscale.However,in theoretical,the underlying mechanism regarding energy conversion and modulation should be further pursued.It is generally known that the nanosystem will be in self-equilibrium state as the consequence of high surface-to-volume ratio(SVR)and coordination imperfection,and the lattice strain will taken place and further affect the related characteristic parameters(e.g.bandgap,piezoelectric constant and so on).In addition,the external stimuli(e.g.tensile,compressive and bending)would also effectively tune the coupling properties from mechanics and electronics.In this paper,in terms of the atomic bond relaxation(ABR)consideration and continuum mechanics,we establish a core-shell configuration model to investigate the size dependence of bond length,band gap shift and piezoelectric constant,the bending strain dependence of bandgap shift and the piezoelectric potential.Firstly,we systematically studied the size and bending effect on bond length and the bandgap shift in different radius of curvature.Secondly,we investigate the size dependence of piezoelectric constant and polarization,the piezoelectric potential in different radius of curvature,moreover,the influence of length and diameter on the piezoelectric potential.The achievements are shown as follows:(1)Based on the ABR consideration,we develop a quantitative analytical method that describes the effects original from surface relaxation and bending strain,also investigate the relationship between bandgap and the radius of curvature.It is concluded that the under-coordinated atoms in the surface result in the bond of edge atoms shorter and stronger.The interatomic potential in a nanostructure will be perturbed by the self-equilibrium strain and the single-body Hamiltonian lead to the blue-shift of bandgap;Moreover,the average bond length of ZnO NWs increase under bending strain,also the bond energy will be decreased and as the consequence of the red-shift of bandgap;the self-equilibrium strain increase with decreasing the diameter of ZnO NWs.(2)We further explore size dependence of piezoelectric constant and polarization,the mechanism with regard to the variation of piezoelectric potential under bending strain.It is found that under-coordinated atoms on the surface result in the bond of edge atoms shrink and the nanowire will be in the self-equilibrium state,the self-equilibrium strain will result in the center of gravity of the negative charges will no longer coincide with the position of the positive central atom and an electric dipole is generated.For the same number of atoms,the nanowire had the smaller volume than that of bulk ZnO,which lead to the increase of the polarization with increasing self-equilibrium strain,so the piezoelectric constant increases with decreasing the size of ZnO NWs;Polarization will be increased with nonlinear relation when the strain increases.When the system is in a small strain,the influence of the strain of higher order quantity can be neglected,the polarization will be increased with linear relation as the strain increases;In addition,the effects of diameter,length and radius of curvature on the piezoelectric potential are discussed in detail based on the developed method.