Surface Effects on the Electrostatic Potential Generated in a Bent Gallium Nitride Nanowire

The aim of this paper is to conduct the first study of the surface effects on the voltage output of bent gallium nitride (GaN) nanowires (NWs), which are promising for nanogenerators. To reach this goal, a 3-D composite beam model was developed and the corresponding theoretical framework was established for the structural responses of piezoelectric NWs. In this study molecular dynamics simulations (MDS) were first carried out to determine the exact material properties for several small NW samples. The MDS-derived size-dependence of parameters provide fitting points for the 3-D composite beam with a core-shell geometry. With the aid of the finite element techniques the equivalent material properties obtained from above fitting procedure enable the use of the core-shell model for larger structures where MDS were not feasible. The obtained results showed that the influence of the surface layer greatly modifies the potential distribution on the cross section and raises the voltage output of bent GaN NWs by up to 120%. In particular, the contribution from the surface piezoelectricity to the surface effect is found to be predominant over that of the surface elasticity and surface stresses.