Photoluminescence and cathodoluminescence ͑CL͒ spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green ͑yellow͒ emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy V o ϩ ͑the single negatively charged interstitial oxygen ion O i Ϫ) center in the particle.

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First-principles calculations for the diffusion of transition metal solutes in nickel challenge the commonly accepted description of solute diffusion rates in metals. The traditional view is that larger atoms move slower than smaller atoms. Our calculation shows the opposite: larger atoms, in fact, can move much faster than smaller atoms. Conventional mechanisms involving the effect of misfit strain or the solute-vacancy binding interactions cannot explain this counterintuitive diffusion trend. Instead, the origin of this behavior stems from the bonding characteristics of the d electrons of solute atoms, suggesting that a similar diffusion trend also occurs in other types of host lattices.