ZnO nanowires are excellent candidates for energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. The key parameters governing the general performance of the integrated devices include the dimensions of the ZnO nanowires used, their doping level, and surface trap density. However, although the method used to grow these nanowires has a strong impact on these parameters, its influence on the performance of the devices has been neither elucidated nor optimized yet. In this paper, we implement numerical simulations based on the finite element method combining the mechanical, piezoelectric, and semiconducting characteristic of the devices to reveal the influence of the growth method of ZnO nanowires. The electrical response of vertically integrated piezoelectric nanogenerators (VING) based on ZnO nanowire arrays operating in compression mode is investigated in detail. The properties of ZnO nanowires grown by the most widely used methods are taken into account on the basis of a thorough and comprehensive analysis of the experimental data found in the literature. Our results show that the performance of VING devices should be drastically affected by growth method. Important optimization guidelines are found. In particular, the optimal nanowire radius that would lead to best device performance is deduced for each growth method.

ZnO nanowires are excellent candidates for energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. These nanowires are usually non-intentionally n-doped during their growth. The essential role of doping, surface traps and surface Fermi level pinning in the actual response of piezoelectric semiconductors has already been demonstrated. In order to go further, this paper investigates the influence of the density and of the dynamics of surface traps on such important parameters as the output generated potential and the effective piezoelectric coefficient. We implemented numerical simulations based on the finite element method by combining the mechanical, piezoelectric, and semiconducting characteristic of ZnO nanowires array based nanocomposites (the so-called vertically integrated nanogenerator configuration) operated in compression. It was found that a certain amount of surface traps was required to obtain a usable generated output potential from the studied devices in the range of dimensions and doping level reported in most experimental results. Moreover, the surface traps influence was strongly dependent on their dynamics. As a first step towards the analysis of traps dynamics, we compared the two extreme cases of ultra-slow and ultra-fast traps. The symmetry and asymmetry of the piezoelectric response and a comparison to thin film was also discussed. This study demonstrates that the realistic modelling of the piezoelectric response of semiconductor based transducers should account for traps dynamics effects.

The question of the length dependence of the electromechanical response of semiconducting (SC) piezoelectric nanowires (NWs) was explored. We identified a new physical mechanism of piezoresponse saturation, which originates from the combination of the influence of interface traps and piezoelectric polarization in the depleted NW. Our results are in better qualitative agreement with experimental observations than presently existing theories. To do so, we used the finite element method to simulate the coupled set of equations describing mechanical, piezoelectric and SC properties. In order to reduce the number of parameters, simulations focused on the case of uniform ZnO NWs grown along the c-axis. Saturation was explained by the incapacity of surface traps to maintain depletion along the whole NW beyond a certain length, as a result of the electric potential shift induced by piezoelectric polarization. An analytical model was developed to support this analysis. It provided the dependence trends of saturation length and piezoresponse as a function of NW dimensions, doping level, surface traps density and crystal polarity, as well as with external pressure, in fair agreement with simulation results. Moreover, we discovered that one consequence of this mechanism was that crystal polarity had an impact on the smoothness of the radius-dependent transition between high and low piezoresponse under axial stress. These results have important implications for the optimization of electromechanical sensors and nanogenerators based on piezoelectric SC NWs and related composite materials.