Crystal Phase Evolution in Quantum Confined ZnO Domains on Particles via Atomic Layer Deposition

Abstract

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.

Particle atomic layer deposition

“…A blue-shift in absorbance corresponds to an increase in bandgap relative to the bulk, E g, bulk, which is a measure of the separation between the valence and conduction band energy levels of a material. The evolution of the crystal phase of quantum confined polycrystalline ZnO films fabricated by atomic layer deposition (ALD) was studied on spherical particle surfaces (King et al 2009b ) (Fig. 34 ).…”

“…

Fig. 34 Deposited ALD ZnO nanoshells exhibiting 3D quantum confinement: ( a ) STEM image of a ZnO-coated 550 nm SiO 2 sphere; ( b ) STEM image of a ZnO-coated 100 nm SiO 2 sphere; ( c ) TEM image of a ZnO-coated 250 nm SiO 2 sphere (King et al 2009b )

Fig. 35 Quantum-confined bandgap shift across all ZnO ALD films in the experimental matrices with respect to the experimentally measured crystallite size of each.

…”

“…35 Quantum-confined bandgap shift across all ZnO ALD films in the experimental matrices with respect to the experimentally measured crystallite size of each. Inset image represents measurement geometry; the solid line is the predicted shift based on the Brus model for ZnO (King et al 2009b ) …”

“… Deposited ALD ZnO nanoshells exhibiting 3D quantum confinement: ( a ) STEM image of a ZnO-coated 550 nm SiO 2 sphere; ( b ) STEM image of a ZnO-coated 100 nm SiO 2 sphere; ( c ) TEM image of a ZnO-coated 250 nm SiO 2 sphere (King et al 2009b ) …”

Atomic layer deposition: an enabling technology for the growth of functional nanoscale semiconductors

“…Core-shell structures are formed when a 0D or 1D nanostructure, such as a nanoparticle or nanorod is coated with a thin layer of another material. Such core-shell nanostructures might show enhanced material properties when compared to the core-only nanostructure, which provide significant potential for numerous applications ALD of ZnO has been implemented to coat particles of Si [358], SiO 2 [359][360][361][362], TiO 2 [363], and Al 2 O 3 [364] for applications in photocatalysis. A viscous flow or fluidized bed reactor configuration is suitable for coating nanoparticles using ALD.…”