Surface-Energy-Driven Growth of ZnO Hexagonal Microtube Optical Resonators

Abstract

A distinct "split and recoalescence" growth mechanism of tubular ZnO micro/nanostructures is observed for the first time. On the basis of experimental growth studies and first-principles calculations, it is proposed that H2/H2O vapor, added to the traditional carbothermal reduction process, changes the intrinsic surface energy of ZnO crystal, which affects the ZnO crystal growth mode and subsequently controls the geometry of ZnO micro/nanotubes. It is shown that these tubular ZnO micro/nanostructures exhibit regular hexagonal cross sections and smooth outer and inner surfaces. Optical studies of ZnO micro/nanotubes reveal characteristic coherent intensity modulations of their emission spectra which reflect the excitation of either whispering gallery modes or wave-guided modes. The specific type of mode can be selected by controlling the microtube geometry, specifically by its wall thickness to diameter ratio, as demonstrated both experimentally by photoluminescence spectroscopy, and theoretically by finite-element-method simulations. Tubular ZnO microcavities with different wall thickness-radius ratios are synthesized by a novel surface-energy-driven micro/nanotube growth model, which is well interpreted by first-principles calculations. Emission spectra of such tubes reveal distinct whispering gallery and wave-guided interference fringes, which distinctly depend on the wall thickness-radius ratio, as verified by finite-element-method simulations.