Rational control of hydrothermal nanowire synthesis and its applications

Abstract

Hydrothermal nanowire synthesis is a rapidly emerging nanowire discipline that enables low temperature growth and batch process. It has a major impact on the development of novel energy conversion devices, high density electronics, and optical devices. However, detailed growth mechanism is still in early stage of its development. This thesis presents the fundamental understanding of controlled zinc oxide nanowire synthesis in a hydrothermal system based on thermodynamic / kinetic analysis of heterogeneous chemical reactions. Governing parameters of hydrothermal growth were evaluated with experimental growth rates and calculated solubility plots. Supersaturation was shown to be a key parameter for the hydrothermal nanowire synthesis. Morphology control of the nanowire synthesis was tested with various additional cations during synthesis. Changes in morphology and aspect ratio with different cations were explained by electrostatic competing ion model. Based on experimental results and complex ion charge distribution, the growth direction was biased via electrostatic competition from cation-complexes that adsorb to the crystal in a face-specific manner, thereby reducing zinc ion-complex adsorption and suppressing growth along that face. Dynamic control of nanowire synthesis was investigated under microfluidic environment with continuous flow. Microfluidic growth conditions were analyzed with the parametric experiments and finite element modeling. Nanowire growth under complex geometry was also evaluated. This rational control of hydrothermal nanowire synthesis was applied to fabricate high efficiency alternative current electroluminescent devices, in-situ fabricated light emitting diodes, photovoltaic devices, and field emission devices.