Crystal phase evolution in quantum confined ZnO domains on particles via atomic layer deposition

The evolution of the crystal phase of quantum confined polycrystalline ZnO films fabricated by atomic layer deposition (ALD) was studied on spherical particle surfaces. A statistical design of experiments was performed to quantify the crystallite size of the primary peaks associated with polycrystalline ZnO, as well as the change in bandgap associated with the small crystal domains. The factors of interest were the number of ALD cycles, the core particle size, and the use of postdeposition annealing temperatures up to 550 °C. The crystallite size of each peak increased almost linearly with the number of cycles, and was further increased via thermal annealing steps. The < 100> dimension also increased more rapidly with temperature on smaller radius particles, signifying that grain boundary diffusion along the particle surfaces was facilitated with increased curvature. The shift in the optical bandgap of ZnO nanoshells was correlated to the domain size within the films at each point in the experimental matrix. The blue shift of 0.3 eV dissipated beyond crystallite sizes exceeding ∼10 nm, which was indicative of the successful deposition of quantum confined nanostructures. The precision control afforded by ALD can be used to deposit quantum confined materials on substrates, independent of geometry and morphology.