Atomistic Modeling of ZnO and GaN Nanowire Elasticity, Fracture and Piezoelectric Size EffectsRavi Agrawal, Ph.D., Northwestern University, 2010.
Major Professor: Dr. Horacio D. Espinosa.
Semiconducting nanowires are envisioned as suitable building blocks for next generation electronic and photonic devices because of their interesting optical, mechanical and electroemechanical properties. Various techniques have been developed to fabricate the nanostructures of semiconducting materials in different morphologies like nanowires, nanobelts etc. However, the optimal use of these nanostructures in future devices requires component level characterization of individual building blocks. Experimental methods, direct and indirect, are under development to characterize the material properties of these individual nanostructures. In parallel with the experimental investigations, theoretical modeling is essential to accurately capture unique phenomenon observed at the nanoscale thereby bridging the gaps between experiments and theory. The theoretical models, validated against experimentation, can thus be used to perform design oriented parametric studies for optimal performance of devices.
In this work, the mechanical properties of semiconducting zinc oxide (ZnO) and gallium nitride (GaN) nanowires under uniaxial tension were investigated using atomistic modeling. The computational findings were compared against the appropriate uniaxial tensile experiments for a direct comparison. A fundamental understanding of size dependent elastic behavior was developed for ZnO and GaN nanowires using semi-empirical methods. Failure modes of these nanowires were also investigated and the observed discrepancies were addressed by performing more accurate first principles-based density functional theory (DFT) calculations. Quantum mechanical calculations also allowed us to probe the electromechanical behavior of these nanowires to explore the size effects on piezoelectric coefficients. In addition to atomistic modeling, atomic-level metrological experiments were performed on individual GaN nanowires using Atom Probe Tomography (APT) with the objective of characterizing dopant concentrations. It was envisioned that such atomic-level information would be relevant to be incorporated in atomistic models for a better comparison with experimentally measured properties, in particular for the electrical and electroemechanical properties. This research work is expected to define pathways to reliably establish structure-property relationships at the nanoscale in semiconducting nanowires.