Accuracy of dielectric-dependent hybrid functionals in the prediction of optoelectronic properties of metal oxide semiconductors: a comprehensive comparison with many-body GW and experiments

Understanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G 0 W 0 as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS)

Cerium-doped zirconium dioxide, a visible-light-sensitive photoactive material of third generation

The dispersion of small amounts of Ce4+ ions in the bulk of ZrO2 leads to a photoactive material sensitive to visible light. This is shown by monitoring with EPR the formation and the reactivity of photogenerated (lambda > 420 nm) charge carriers. The effect, as confirmed by DFT calculations, is due to the presence in the solid of empty 4f Ce states at the mid gap, which act as intermediate levels in a double excitation mechanism. This solid can be considered an example of a third-generation photoactive material

Paramagnetic defects in polycrystalline zirconia: An EPR and DFT study

The paramagnetic defects present in pristine zirconium dioxide (ZrO2) and those formed upon reductive treatments (either annealing or UV irradiation in H-2) are described and rationalized by the joint use of electron paramagnetic resonance (EPR) and DFT supercell calculations. Three types of Zr3+ reduced sites have been examined both in the bulk of the solid (one center) and at the surface (two centers). Trapping electron centers different from reduced Zr ions are also present, whose concentration increases upon annealing. A fraction of these sites are paramagnetic showing a symmetric signal at g = 2.0023, but the majority of them are EPR silent and are revealed by analysis of electron transfer from the reduced solid to oxygen. The presence of classic F-type centers (electrons in bulk oxygen vacancies) is disregarded on the basis of the g-tensor symmetry. This is expected, on the basis of theoretical calculations, to be anisotropic and thus incompatible with the observed signal. In general terms, ZrO2 has Some properties similar to typical reducible oxides, such as TiO2 and CeO2 (excess electrons stabilized at cationic sites), but it is much more resistant to reduction than this class of materials. While point defects in doped (Y3+, Ca2+) ZrO2 materials have been widely investigated for their role as ionic conductors, the defectivity of pristine ZrO2 is much less known; this paper presents a thorough analysis of this phenomenon

Communication : Hole localization in Al-doped quartz SiO2 within ab initio hybrid-functional DFT

We investigate the long-standing problem of hole localization at the Al impurity in quartz SiO2, using a relatively recent DFT hybrid-functional method in which the exchange fraction is obtained ab initio, based on an analogy with the static many-body COHSEX approximation to the electron self-energy. As the amount of the admixed exact exchange in hybrid functionals has been shown to be determinant for properly capturing the hole localization, this problem constitutes a prototypical benchmark for the accuracy of the method, allowing one to assess to what extent self-interaction effects are avoided. We obtain good results in terms of description of the charge localization and structural distortion around the Al center, improving with respect to the more popular B3LYP hybrid-functional approach. We also discuss the accuracy of computed hyperfine parameters, by comparison with previous calculations based on other self-interaction-free methods, as well as experimental values. We discuss and rationalize the limitations of our approach in computing defect-related excitation energies in low-dielectric-constant insulators

Defect calculations in semiconductors through a dielectric-dependent hybrid DFT functional : the case of oxygen vacancies in metal oxides

We investigate the behavior of oxygen vacancies in three different metal-oxide semiconductors (rutile and anatase TiO2, monoclinic WO3, and tetragonal ZrO2) using a recently proposed hybrid density-functional method in which the fraction of exact exchange is material-dependent but obtained ab initio in a self-consistent scheme. In particular, we calculate charge-transition levels relative to the oxygen-vacancy defect and compare computed optical and thermal excitation/emission energies with the available experimental results, shedding light on the underlying excitation mechanisms and related materials properties. We find that this novel approach is able to reproduce not only ground-state properties and band structures of perfect bulk oxide materials but also provides results consistent with the optical and electrical behavior observed in the corresponding substoichiometric defective systems

Quasiparticle interfacial level alignment of highly hybridized frontier levels: H2_2O on TiO2_2(110)

Knowledge of the frontier levels' alignment prior to photo-irradiation is necessary to achieve a complete quantitative description of H$_2$O photocatalysis on TiO$_2$(110). Although H$_2$O on rutile TiO$_2$(110) has been thoroughly studied both experimentally and theoretically, a quantitative value for the energy of the highest H$_2$O occupied levels is still lacking. For experiment, this is due to the H$_2$O levels being obscured by hybridization with TiO$_2$(110) levels in the difference spectra obtained via ultraviolet photoemission spectroscopy (UPS). For theory, this is due to inherent difficulties in properly describing many-body effects at the H$_2$O-TiO$_2$(110) interface. Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) $G_0W_0$, we disentangle the adsorbate and surface contributions to the complex UPS spectra of H$_2$O on TiO$_2$(110). We perform this separation as a function of H$_2$O coverage and dissociation on stoichiometric and reduced surfaces. Due to hybridization with the TiO$_2$(110) surface, the H$_2$O 3a$_1$ and 1b$_1$ levels are broadened into several peaks between 5 and 1 eV below the TiO$_2$(110) valence band maximum (VBM). These peaks have both intermolecular and interfacial bonding and antibonding character. We find the highest occupied levels of H$_2$O adsorbed intact and dissociated on stoichiometric TiO$_2$(110) are 1.1 and 0.9 eV below the VBM. We also find a similar energy of 1.1 eV for the highest occupied levels of H$_2$O when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than those estimated from UPS difference spectra, which are inconclusive in this energy region. Finally, we apply self-consistent QP$GW$ (scQP$GW$1) to obtain the ionization potential of the H$_2$O-TiO$_2$(110) interface.Comment: 12 pages, 12 figures, 1 tabl

EPR properties of Au atoms adsorbed on various sites of the MgO(100) surface from relativistic DFT calculations

Using all electron fully relativistic DFT calculations we have computed the EPR properties of Au atoms bound to various sites of the MgO surface. Changes in g-tensor and hyperfine coupling constants provide a way to identify the gold adsorption site and to map the surface morphology by comparison of measured and calculated EPR spectra. We found a strong reduction of the isotropic hyperfine coupling constant, aiso(Au), for adsorbed gold compared to the free atom; this reduction, which is about 45% for terrace sites, is more pronounced when Au interacts with low-coordinated sites like steps, edges and corners where it is about 60%. The reduction of aiso(Au) is accompanied by a corresponding increase of the superhyperfine interaction with the surface oxygen sites, as measured by aiso(17O). Large anisotropies in the g-tensor are computed for all sites

Characterization of O^--Centers on Single Crystalline MgO(001)-Films

Defects play an important role for understanding the properties of oxide surfaces. However, a detailed atomistic characterization of the properties of defects in particular of point defects is still a challenging task. On polycrystalline material it is the large variety of different species in terms of local environment, electronic properties as well as the metastable nature of most of these species, which complicates matters. EPR spectroscopy has proven to be a versatile tool to characterize electronic properties as well as local environment of paramagnetic point defects on oxide surfaces. In this study we elucidate the properties of O--centers on MgO surfaces under ultrahigh vacuum (UHV) conditions using a single-crystalline MgO(001) surface as a well-defined model system. The O--centers were produced by reaction of N2O with previously prepared F+-centers, which were shown to be located at step edges of the MgO islands in a previous study. The experimental efforts are combined with ab-inito quantum chemistry calculations to gain a more detailed understanding of the electronic properties of the defects under consideration. The experimental and theoretical values of the g-tensor components are almost in quantitative agreement. In addition to the discussion of the properties of O--centers the paper will shed some light on the impact that doping of the surface in this case with Mo(V)-species present on the pristine surface has. In particular, we are able to provide evidence for the fact that there is redox chemistry between the O--centers and Mo-centers. The crosstalk between different redox active sites on the surface is an important phenomenon that is not limited to model systems as discussed here, but should also be taken into consideration if discussing the properties of high performance catalysts