Atomistic study of surface effects on the electromechanical coupling of ZnO nanostructures
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Zinc Oxide (ZnO) has been widely studied as a piezoelectric semiconductor. Since ZnO nanowires (NW) have recently been utilized experimentally as nanogenerators to harvest electrical energy resulting from mechanical deformation, its potential use in energy harvesting applications has attracted significant interest. However, theoretical and computational studies of piezoelectricity are typically based on classical continuum mechanics, which does not account for critical nanoscale surface effects. Since surface effects are dominant at the nanoscale, molecular simulations which can capture the surface effect are needed. The focus of this thesis is in applying, for the first time, polarizable core-shell atomistic models to study the bulk and surface-dependent piezoelectric properties of ZnO nanostructures. We first validate the mechanical properties of bulk ZnO, then calculate the bulk piezoelectric coefficients by classical molecular simulation. Using techniques to ensure charge neutralization at the surfaces of ZnO, we examine ZnO thin films and report, for the first time, surface piezoelectric constants for the polar (0001) surface of ZnO. We then examine the utility of using ZnO nanowires for electromechanical energy conversion by studying their piezoelectric properties under axial loading. We find that due to the reduced polarization at the surfaces of ZnO, the piezoelectric constants of ZnO decrease with decreasing size, thus leading to the finding that if enhanced energy generation using ZnO is desired, further miniaturization to the nanometer scale may not be the solution.
Thesis (Ph.D.)--Boston University, 2013.