Stress-Induced Shift of Band Gap in ZnO Nanowires from Finite-Element Modeling

Attractive mechanical, optical, and electronic properties of semiconducting ZnO nanowires make them prime candidates for a variety on energy-harvesting technologies, including photovoltaics and piezoelectric nanogenerators. In order to enhance the efficiency and versatility of such devices, it is paramount to elucidate the connections between the different property realms, i.e., to establish how mechanical distortions can affect the electronic and optical response of the nanowires, depending on their size, shape, and morphology. For example, it was recently demonstrated that band-gap downshifts of up to -0.1 eV can be induced in monolithic ZnO nanowires by an application of tensile strain [see Wei et al., Nano Lett. 12, 4595 ( 2012)]. Here, we conduct mesoscale-level, finite-element-method-based modeling of the coupled elastic and electronic properties of both already-synthesized monolithic ZnO nanowires and yet-to-befabricated Zn-ZnO core-shell structures with diameters ranging from 100 to 800 nm. Our investigation suggests that, after an optimization of the size, shape, and mutual crystallographic orientations of the core and shell regions, core-shell nanowires can exhibit downward band-gap shifts of up to -0.3 eV ( i.e., approximately 10% of the stress-free ZnO band-gap value) under tensile distortions, which can greatly expand the utility of such nanostructures for optoelectronic applications.