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Iron oxide magnetic nanoparticles (NPs) are stimuli-responsive materials at the forefront of nanomedicine. Their realistic finite temperature simulations are a formidable challenge for first-principles methods. Here, we use density functional tight binding to open up the required time and length scales and obtain global minimum structures of Fe3O4 NPs of realistic size (1400 atoms, 2.5 nm) and of different shapes, which we then refine with hybrid density functional theory methods to accomplish proper electronic and magnetic properties, which have never been accurately described in simulations. On this basis, we develop a general empirical formula and prove its predictive power for the evaluation of the total magnetic moment of Fe3O4 NPs. By converting the total magnetic moment into the macroscopic saturation magnetization, we rationalize the experimentally observed dependence with shape. We also reveal interesting reconstruction mechanisms and unexpected patterns of charge ordering.

We investigate band gaps, equilibrium structures, and phase stabilities of several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO2,ZrO2, and WO3. We are particularly concerned with assessing the performance of hybrid functionals built with the fraction of Hartree-Fock exact exchange obtained from the computed electronic dielectric constant of the material. We provide comparison with more standard density-functional theory and GW methods. We finally analyze the chemical reduction of TiO2 into Ti2O3, involving a change in oxide stoichiometry. We show that the dielectric-dependent hybrid functional is generally good at reproducing both ground-state (lattice constants, phase stability sequences, and reaction energies) and excited-state (photoemission gaps) properties within a single, fully ab initio framework.

A combined experimental and first principles study of Cu defects in bulk ZnO is presented. Cu particles are epitaxially deposited on the polar O-ZnOð000 1Þ surface at room temperature. Upon heating, a broadening of the quasielastic peak in high resolution electron energy loss spectra is observed, corresponding to an electronic doping effect of Cu atoms in bulk ZnO with an ionization energy of 88 meV. Cu impurities in ZnO, although commonly acting as acceptors, are presently observed to induce shallow donor states. We assign these to interstitial Cu species on the basis of a hybrid density functional study.

Hybrid functional density functional theory calculations of the oxygen vacancy (VO) in monoclinic bulk WO(3) provide a coherent rationalization for the strong dependence of WO(3-x) electronic properties on the VO concentration and of the semiconductor-to-metal transition, a phenomenon intimately connected to the electrochromic effect. Different VO centers containing W(4+), W(5+), and W(6+) species are expected to coexist. Optical transition levels are 0.7-1.0 eV below the conduction band minimum, in agreement with experiments. The complex nature of V(O) in WO(3) can only be detected with methods that properly describe band gaps and polaronic distortions at defect sites.

The electronic structure and spectral properties of self-trapped holes (STH's) in SiO(2) are revisited using pure and hybrid density functional theory (DFT) approaches and cluster models. The structure and electron spin resonance (ESR) parameters [hyperfine coupling constants (hfcc's) and g factors] have been determined for the two classical variants of STH's: STH(1), consisting of a hole trapped at the 2p nonbonding orbital of an O atom bridging two Si atoms, and STH(2), a metastable defect where the hole is delocalized over the 2p orbitals of two bridging O atoms. The computed ESR parameters fully support the experimental assignments based on the analysis of the ESR spectra. However, a nearly quantitative agreement between measured and computed hfcc's and g factors is found only with a hybrid functional where 50% of the Hartree-Fock exchange is mixed in with the DFT exchange. Standard hybrid functionals, such as B3LYP, or pure DFT functionals, such as BLYP, fail completely in describing the two defects. According to these results, STH(2) can form only in correspondence of specific O-Si-O angles of about 80 degrees-90 degrees, which classifies the defect center as a molecular polaron.

In order to assign the defect responsible for the experimental electron paramagnetic resonance (EPR) signal with trigonal symmetry (T center), we have studied the properties of different paramagnetic centers in yttria-stabilized cubic zirconia by computing the EPR g -tensor from density functional perturbation theory. We have considered reduced vacancy-zirconium complexes and reduced Ti impurities. These first-principles calculations allow us to discard the experimental assignment of the T center to an extrinsic Ti3+ ion nearest neighbor to a single vacancy. Instead, the calculated EPR g tensors of both a Zr3+ or a Ti3+ ion at the center of a divacancy aligned along the 111 directions are compatible with the experimental EPR signal. However, since the EPR signal of the T center is correlated experimentally with an optical absorption band at 370 nm, calculated optical excitations allow us to decide in favor of the Ti3+ divacancy complex.

We have investigated the origin of the experimentally observed change in photoactivity of anatase and rutile TiO2 induced by substitutional N-doping using state-of-the-art density functional theory calculations. Our results show that in both polymorphs N 2p localized states just above the top of the O 2p valence are present. In anatase these states cause a redshift of the absorption band edge towards the visible region. In rutile, instead, this effect is offset by the concomitant N-induced contraction of the O 2p band, resulting in an overall increase of the optical transition energy. Experimental trends are well described by these results.

Point defects on oxide surfaces are presumed to be preferential nucleation sites for supported metal clusters. Under typical growth conditions, dimers constitute the first step in island nucleation. Firstprinciples calculations on the formation of Pd dimers on regular and defect sites of the MgO(100) surface show that nucleation occurs with large dimer binding energies at divacancies and charged oxygen vacancies (F+ centers), while it is less favorable on steps and neutral F centers. The extensive database of defect trapping/attachment properties gives a firm basis to rationalize recent atomic-force microscopy findings and provides guidelines valid, in general, for ionic substrates