Small-scale effects on the piezopotential properties of tapered gallium nitride nanowires: The synergy between surface and flexoelectric effects

Apart from the conventional wurtzite nanowires (NWs) with constant cross-sections, tapered wurtzite NWs with truncated conical shapes are also widely employed in the design of piezotronic nanodevices. In this paper, the piezopotential properties of tapered gallium nitride (GaN) NWs are comprehensively studied by using a multiscale modelling technique. Our atomic-scale simulations show that due to the surface effect, the material properties of tapered NWs strongly rely on the position in NWs. Analytic models based on the core-shell theory and the thermodynamic theory are developed for the position-dependent material properties of tapered NWs. Besides the surface effect, the flexoelectric effect is another factor significantly affecting the piezopotential properties of tapered NWs by inducing additional polarization. After employing the analytic models for position-dependent material properties and introducing a modified electromechanical theory, both the surface and flexoelectric effects are considered in the continuum mechanics modelling of the piezopotential of tapered NWs. Our results show that the small-scale effects can increase the potential difference generated in tapered NWs, which will become more significant as the tip radius decreases and the half cone angle increases. Moreover, in the tapered NW with a small half cone angle the small-scale effects are found to majorly originate from the surface effect, while the flexoelectric effect becomes the dominant factor when the tapered NW possesses a relatively large half cone angle.