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Vacancies in wurtzite GaN and AlN are studied using a computational method which is based on the density functional theory (DFT) and takes into account the errors arising from use of finite-sized supercells and the DFT band gap underestimation. Negatively charged N vacancies in GaN and AlN are found to be stable, with formation energies similar to and higher than those of Ga and Al vacancies in n-type material under Ga-and Al-rich growth conditions, respectively. The localization and energies of the defect levels close to the computational conduction band edge are considered in detail.

Spectral transmittance and reflectance in the 300 to 2500 nm solar-optical wavelength range were calculated for nanoparticles of titanium dioxide and vanadium dioxide with radii between 5 and 100 nm embedded in transparent dielectric media. Both of the materials are of large importance in green nanotechnologies: thus TiO 2 is a photocatalyst that can be applied as a porous film or a nanoparticle composite on indoor or outdoor surfaces for environmental remediation, and VO 2 is a thermochromic material with applications to energy-efficient fenestration. The optical properties, including scattering, of the nanoparticle composites were computed from the Maxwell-Garnett effective-medium theory as well as from a four-flux radiative transfer model. Predictions from these theories approach one another in the limit of small particles and in the absence of optical interference. Effects of light scattering can be modeled only by the four-flux theory, though. We found that nanoparticle radii should be less than ~20 nm in order to avoid pronounced light scattering.