Structural and electronic property study of (ZnO)_n, n ≤ 168: Transition from zinc oxide molecular clusters to ultrasmall nanoparticles

Global minimum energy structures of (ZnO)_n, n ≤ 168, were determined by using a hybrid genetic algorithm followed by a local geometry optimization at the density functional theory level. New "magic number" structures were found for the (ZnO)_n clusters and ultrasmall nanoparticles. Particles with morphologies of single-, double-, and triple-layered octahedral cages exhibit higher stability than particles with other morphologies. (ZnO)₁₃₂ and (ZnO)₁₆₈ are found to be triple-layer cages with a diameter of ∼2 nm in each dimension. The normalized clustering energies (average cohesive energies) of the multilayered zinc oxide cages can be extrapolated to provide an estimate of the bulk limit. The surface energy densities of the ultrasmall nanoparticles are almost constant. The relatively high stability of the multilayered particles is attributed to the lack of terminal surface atoms and the effective interlayer stacking of hexagonal cells, as the layers in the particles are intact and hexagonally tiled. The epitaxial structural evolution pattern for zinc oxide ultrasmall nanoparticles can be used to predict the most energetically favorable structural evolution pathways connecting clusters and nanoparticles with different morphologies. It is hypothesized that zinc oxide nanoparticles with different morphologies (such as cylinder and octahedron) can be synthesized by using different seeds to initialize the epitaxial growth. The zinc oxide nanoparticles synthesized following our proposed growth mechanism will be predominately terminated by the wurtzite 0001 surface. The band gap of a spherical zinc oxide nanoparticle is predicted to be a linear function of the inverse of the nanoparticle diameter.