Abstract
20-nm nanoparticles of zinc oxide (ZnO) were used to make high-density ZnO discs by uniaxial pressing at 4 ton/cm(2) pressure and sintering at 1200 degrees C for 1 hour. High-oxygen thermal annealing performed on the ZnO discs was found to have a profound impact especially enhanced grain growth even at a low annealing temperature of only 400 degrees C. Moreover, we observed a unique secondary growth of ZnO nanoparticles and growth of multilayer grains that have not been reported elsewhere. The strong solid state reaction during annealing was probably attributed to the high surface area of the 20-nm ZnO nanoparticles that promoted strong surface reaction even at low annealing temperatures. The ZnO discs have been found to contain a very high concentration of structural defects (oxygen vacancies and zinc/oxygen interstitials) that was indicated by the dominant and broad visible photoluminescence (PL) emission in the green band with peaks at (519 - 533) nm, and it was found that this visible emission was greatly increased after annealing treatment especially at 800 degrees C. Annealing treatment also was found to improved the grain crystallinity as illustrated by the lowering of intrinsic compressive stress based on the XRD lattice constant and full-wave half-maximum (FWHM) data. The electrical properties of the ZnO discs were also greatly influenced by the annealing treatment especially a big drop in the breakdown voltage from 362 V (as-grown sample) to 170 V (800 degrees C sample). The resistivity also experienced a dramatic drop from 267 k Omega.cm (as-grown sample) to 74.6 k Omega.cm (800 degrees C sample). High-oxygen thermal annealing can be employed as a new technique in controlling the breakdown voltage of ZnO discs made from ZnO nanoparticles with improved structural properties.