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Although photocatalytic decomposition of NO (deNO) into N

The electrochemical reduction of nitrate (eNO

NOx emission heavily affects our environment and human health. Photocatalytic denitrification (deNOx) attracted much attention because it is low-cost and nonpolluting, but undesired nitrite and nitrate were produced in reality, instead of harmless N2. Unveiling the active sites and the photocatalytic mechanism is very important to improve the process. Herein, we have employed a combinational scenario to investigate the reaction mechanism of NO2 and H2O on anatase TiO2(101). On the one hand, a polaron-corrected GGA functional (GGA + Lany-Zunger) was applied to improve the description of electronic states in photoassisted processes. On the other hand, a reaction phase diagram (RPD) was established to understand the (quasi) activity trend over both perfect and defective surfaces. It was found that a perfect surface is more active via the Eley-Rideal mechanism without NO2 adsorption, while the activity on defective surfaces is limited by the sluggish recombinative desorption. A photogenerated hole can weaken the OH* adsorption energies and circumvents the scaling relation of the dark reaction, eventually enhancing the deNOx activity significantly. The insights gained from our work indicate that tuning the reactivity by illumination-induced localized charge and diverse reaction pathways are two methods for improving adsorption, dissociation, and desorption processes to go beyond the conventional activity volcano plot limit of dark conditions.

Nitrogen doping has been suggested to increase the photocatalytic activity of titanium dioxide in the visible. For this purpose, narrowing of the mobility gap (the gap between conductive states) wo...

Copper-indium-gallium-selenide (CIGS) alloys are successfully applied in thin-film solar cells. For a better use of the solar spectrum, they also offer the possibility of multijunction devices by tuning the composition in the different layers. As-grown CIGS is intrinsically p-type due to copper vacancies (${V}_{\mathrm{Cu}}$), but n-type doping is also useful for applications. While ${\mathrm{Cu}\mathrm{In}\mathrm{Se}}_{2}$ can be easily turned n-type, ${\mathrm{Cu}\mathrm{Ga}\mathrm{Se}}_{2}$ cannot, and this represents a problem, because increasing the band gap of CIGS requires a high $\mathrm{Ga}/\mathrm{In}$ ratio. Investigating the effect of hydrogen on ${\mathrm{Cu}\mathrm{Ga}\mathrm{Se}}_{2}$ by an optimized hybrid functional, we show that hydrogenation from an atomic source as, e.g., by a hydrogen plasma treatment, can turn the material n-type due to the formation of shallow donor ${V}_{\mathrm{Cu}}+2\mathrm{H}$ complexes, while ${\mathrm{H}}_{2}$ implantation, producing an internal hydrogen reservoir, can be used to produce semi-insulating material. We also show that under normal process conditions, unintentional hydrogen incorporation does not have a significant effect on ${\mathrm{Cu}\mathrm{Ga}\mathrm{Se}}_{2}$.

Supercell models are often used to calculate the electronic structure of local perturbations from the ideal periodicity in the bulk or on the surface of a crystal or in wires. When the defect or adsorbent is charged, a jellium counter charge is applied to maintain overall neutrality, but the interaction of the artificially repeated charges has to be corrected, both in the total energy and in the one-electron eigenvalues and eigenstates. This becomes paramount in slab or wire calculations, where the jellium counter charge may induce spurious states in the vacuum. We present here a self-consistent potential correction scheme and provide successful tests of it for bulk and slab calculations.

Layered materials are presently under intense study, and most applications require knowledge about their defects. It has been shown earlier that the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE), with parameters tuned to reproduce the relative position of the band edges and to satisfy the generalized Koopmans' theorem in the given material, is capable of providing defect properties very accurately in traditional bulk semiconductors. This success is thought to be connected to the proper description of electronic screening. In this paper we investigate whether such a functional can be optimized for layered compounds, such as GaSe and hexagonal BN (hBN). While we find this to be possible in the bulk materials, as expected, the optimal parameters change significantly in the monolayers (ML), due to the effect of the surface on the electronic screening. For ML GaSe (thickness $\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\AA{}$), where the dielectric constant does not change much within the layer, a reoptimization is possible, while for the atomically thin ML hBN the optimization criteria can only be met approximately. In other words, the accuracy which can be achieved for the electronic structure of defects by a HSE functional will always be less in atomically thin monolayers than in the bulk built from such layers. We also show that the accuracy of HSE functionals, with parameters optimized for the bulk in traditional (nonlayered) bulk semiconductors, also diminishes in surface calculations, even for thick slabs, if the dielectric constant $({\ensuremath{\varepsilon}}^{\ensuremath{\infty}})$ is low. Generally, the accuracy of bulk and surface calculation by any functional can be very different.

Deubiquitylating (DUB) enzymes free covalently linked ubiquitin moieties from ubiquitin-ubiquitin and ubiquitin-protein conjugates, and thereby maintain the equilibrium between free and conjugated ubiquitin moieties and regulate ubiquitin-mediated cellular processes. Here, we performed genetic analyses of mutant phenotypes in

GaN is of great importance for optoelectronic and power electronic devices, and carbon is one of its most common impurities, used also actively for preparing semi-insulating buffer layers. Experimental investigations reveal at least five carbon-related centers, however, theory could provide definite assignment so far for only two of those: in terms of simple carbon substitutionals on nitrogen $({\mathrm{C}}_{\mathrm{N}})$ and gallium $({\mathrm{C}}_{\mathrm{Ga}})$ sites. We apply here an optimized hybrid functional (which reproduces the band gap and is Koopmans' compliant) to investigate small carbon complexes. We find that an increasing carbon concentration, beyond the compensation of the $n$-type doping by the acceptor ${\mathrm{C}}_{\mathrm{N}}$, makes its complexes with ${\mathrm{C}}_{\mathrm{Ga}}$ and ${(\mathrm{C}\ensuremath{-}\mathrm{N})}_{\mathrm{N}}$ split interstitials competitive in formation energy. Except for $p$-doped samples, these complexes are electrically passive, so the Fermi level is pinned a little below midgap, ensuring the semi-isolating behavior even for high carbon concentrations. The charge transition levels, calculated by the optimized functional, allow the interpretation of all carbon-related deep-level spectra as well as of their relation to the carbon-to-donor ratio.

The performance of density functional theory depends largely on the approximation applied for the exchange functional. We propose here a novel screened exchange potential for semiconductors, with parameters based on the physical properties of the underlying microscopic screening and obeying the requirements for proper asymptotic behavior. We demonstrate that this functional is Koopmans-compliant and reproduces a wide range of band gaps. We also show, that the only tunable parameter of the functional can be kept constant upon changing the cation or the anion isovalently, making the approach suitable for treating alloys., Comment: 6 pages, 4 figures

Nitrogen interstitials (N$_\mathrm{i}$) have the lowest formation energy among intrinsic defects of hexagonal boron nitride (hBN) under n-type and N-rich conditions. Using an optimized hybrid functional, which reproduces the gap and satisfies the generalized Koopmans condition, an N$_\mathrm{i}$ configuration is found which is lower in energy than the ones reported so far. The (0/-) charge transition level is also much deeper, so N$_\mathrm{i}$ acts as a very efficient compensating center in n-type samples. Its calculated photoluminescence (PL) at 3.0 eV agrees well with the position of an N-sensitive band measured at 3.1 eV. It has been also found that the nitrogen vacancy (V$_\mathrm{N}$) cannot be the origin of the three boron electron (TBC) electron paramagnetic resonance (EPR) center and in thermal equilibrium it cannot even exist in n-type samples., Comment: 7 pager, 3 figures

The possibility of splitting water by UV-light on a TiO 2 electrode has created great interest in the material, however, it has been later questioned whether rutile can do the job at all without external bias. Anatase was suggested instead, but its efficiency is still a subject of debate. The problem is related to the position of the band edges, that is, of the Fermi-level ( E F ), with respect to the redox potentials of water. Here we present hybrid-functional calculations to align the band structures with the vacuum level, assuming the rutile (110) and anatase (101) surface being exposed to water. Our results show that both are capable of water splitting if no adsorbates other than molecular water are present. On a fully hydroxylated surface (i.e., H + and OH − adsorption on undercoordinated surface oxygen and titanium atoms, respectively), E F is only ∼0.5 eV above the H + /H 2 potential in case of anatase, and – depending on the level of reduction – roughly at, or below it for rutile. We also show, that the band edges (and E F ) shift up if OH + groups dominate the surface, increasing the driving force for water splitting. This is in line with the experience on titania reduced in hydrogen. Our results are further confirmed by calculating E F without the presence of water, and comparing it to work function measurements by photoelectron spectroscopy.

Unintentionally doped n-type beta -Ga2O3 becomes highly resistive after annealing at high temperatures in oxygen ambient. The annealing process also induces an electron paramagnetic resonance (EPR) center, labeled IR1, with an electron spin of S=1/2 and principal g-values of g(xx)=2.0160, g(yy)=2.0386, and g(zz)=2.0029 with the principal axis of g(zz) being 60 degrees from the [001](*) direction and g(yy) along the b-axis. A hyperfine (hf) structure due to the hf interaction between the electron spin and nuclear spins of two equivalent Ga atoms with a hf splitting of similar to 29G (for Ga-69) has been observed. The center can also be created by electron irradiation. Comparing the Ga hf constants determined by EPR with corresponding values calculated for different Ga vacancy-related defects, the IR1 defect is assigned to the double negative charge state of either the isolated Ga vacancy at the tetrahedral site (V-Ga(I)(2-)) or the V-Ga(I)-Ga-ib-V-Ga(I) complex. Funding Agencies|DFGGerman Research Foundation (DFG) [FR2833/63-1]; HLRN [hbc00027]; Institute of Global Innovation Research, Tokyo; University of Agriculture and Technology, Japan

GaSe is a layered semiconductor with an optical band gap tunable by the number of layers in a thin film. This is promising for application in micro/optoelectronics and photovoltaics. However, for that, knowledge about the intrinsic defects are needed, since they may influence device behavior. Here we present a comprehensive study of intrinsic point defects in both bulk and monolayer (ML) GaSe, using an optimized hybrid functional which reproduces the band gap and is Koopmans' compliant. Formation energies and charge transition levels are calculated, the latter in good agreement with available experimental data. We find that the only intrinsic donor is the interlayer gallium interstitial, which is absent in the case of the ML. The vacancies are acceptors, the selenium interstitial is electrically inactive, and small intrinsic defect complexes have formation energies too high to play a role in the electronic properties of samples grown under quasi-equilibrium conditions. Bulk GaSe is well compensated by the intrinsic defects, and is an ideal substrate. The ML is intrinsically p-type, and p-type doping cannot be compensated either. The opening of the band gap changes the defect physics considerably with respect to the bulk.

𝛽-Ga2O3 has recently received attention as an ultra-wide-band-gap material, promising as transparent conductor and for high-power device applications. In this talk, we review some recent computational work on the electronic band structure and it defects. The electronic band structure was calculated using the quasiparticle self-consistent GW method including a correction of the screened Coulomb interaction W for electron-hole interaction and lattice-polarization effect. The selection rules of the optical transition help understand the anisotropy of the optical absorption.1 Among the point defects, which have already been discussed in various papers,2 we here focus on the two types of Ga–vacancies, which can occur on both the tetrahedral and octahedral Ga site. Electron paramagnetic resonance (EPR) related to the Ga-vacancy has been reported.3,4 Here we present first-principles calculations of the g-factor and hyperfine structure, which allow us to identify the EPR spectrum observed after high energy particle irradiation, with the tetrahedral Ga vacancy in the 2- charge state. Modification of the EPR spectrum under low temperature photoexcitation are explained by the following model. The EPR spectrum of the tetrahedral Ga vacancy shows superhyperfine interaction with two Ga atoms which are nearest neighbors to the oxygen on which the defect localizes in the q=2- charge state. The g-tensor has its largest deviation from the free-electron value for the crystallographic b-direction in which the defect Ga-O-Ga complex is oriented. According to Ref. 2, the tetrahedral Ga vacancy has its 2-/3- transition level closer to the conduction band minimum than the octahedral one. Upon illumination, this defect may become inactivated while the octahedral one is activated. The latter has a different orientation of its main largest g-tensor component but similar superhyperfine splitting with two Ga atoms. These computational findings explain the corresponding experimental observations. 1 Amol Ratnaparkhe and Walter R. L. Lambrecht, Appl. Phys. Lett. 110, 132103 (2017) 2 Peter Deak, Quoc Duy Ho, Florian Seemann, Balint Aradi, Michael Lorke, and Thomas Frauenheim, Phys. Rev. B 95, 075208 (2017) and references therein. 3 B. E. Kananen , L. E. Halliburton , K. T. Stevens , G. K. Foundos , and N. C. Giles, Appl. Phys. Lett. 110, 202104 (2017) 4 H.Jurgen von Bardeleben, unpublished

The synergic effect of mixing rutile (R) and anatase (A) crystals in photocatalysis is often attributed to a staggered alignment of the band structures, but it is widely disputed whether the conduction band edge of rutile or that of anatase is higher. Photoelectron spectroscopy (PES) supports the former, and flat-band potential (FBP) measurements the latter. Theoretical alignment of the bulk band structures as well as calculated offsets across actual interfaces support the PES data. The theoretical study presented here shows that the FBP data can be explained by taking into account the adsorption of OH– and H+ ions in the electrolyte solution. We conclude that, in the case of nanopowders applied in photocatalysis, the dipole layer created by surface adsorption and the one at the R/A interface both have an influence on the alignment, and the end result may change from experiment to experiment.

Despite the efforts to implement the idea of a deep level impurity intermediate band (IB) into bulk solar cell materials, a breakthrough in efficiency increase has not yet been achieved. Taking Sn-doped ${\mathrm{CuGaS}}_{2}$ as an example, we investigate the problem here from the perspective of defect physics, considering all possible charge states of the dopant and its interaction with native defects. Using an optimized hybrid functional, we find that ${\mathrm{Sn}}_{\mathrm{Ga}}$ has not only a donor-type (+/0), but also an acceptor-type ($0/\ensuremath{-}$) charge transition level. We estimate the probability of the optical transition of an electron from/to the neutral defect to/from the conduction-band edge to be about equal, therefore, the lifetimes of the excited carriers are probably quite short, limiting the enhancement of the photocurrent. In addition, we find that doping with ${\mathrm{Sn}}_{\mathrm{Ga}}$ leads to the spontaneous formation of the intrinsic acceptor ${\mathrm{Cu}}_{\mathrm{Ga}}$ defects which passivate the donor ${\mathrm{Sn}}_{\mathrm{Ga}}$ and pin the Fermi level to a position (1.4 eV above the valence-band edge) where both defects are ionized. As a result, the possibility of absorption in the middle of the visible range gets lost. These two recombination and passivation mechanisms appear to be quite likely the case for other donors and other similar host materials as well, explaining some of the experimental bottlenecks with IB solar cells based on deep level impurities.

Due to its wide band gap and availability as a single crystal, $\ensuremath{\beta}\ensuremath{-}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$ has potential for applications in many areas of micro/optoelectronics and photovoltaics. Still, little is as yet known about its intrinsic defects, which may influence carrier concentrations and act as recombination centers. From a theoretical point of view, the problem is that standard (semi)local approximations of density functional theory usually cannot handle wide band-gap oxides, while results of tuned hybrid functional calculations so far have shown little quantitative coincidence with experimental data on $\ensuremath{\beta}\ensuremath{-}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$. Here, we show a method for selecting the optimal hybrid, which reproduces not only the band gap, but also satisfies the generalized Koopmans' theorem. Unless the screening is strongly orbital/direction dependent in the given material, such an optimal hybrid can reproduce the whole $GW$ band structure quite accurately. With the optimized functional, and introducing a modification into the charge correction process, we are able to give a consistent description of observed carrier trapping by intrinsic defects in $\ensuremath{\beta}\ensuremath{-}\mathrm{G}{\mathrm{a}}_{2}{\mathrm{O}}_{3}$. With the exception of gallium interstitials, which can act as shallow donors, all other intrinsic defects are deep. Gallium vacancies are the main compensating acceptors in $n$-type samples, while both oxygen interstitials and vacancies act as hole traps, in addition to small hole polarons. Considering the limitations imposed by a medium-sized (160-atom) supercell in an ionic solid, the calculated adiabatic and vertical transitions are in good agreement with available experimental data.

Dirac materials are characterized by exceptional mobility, orders of magnitude higher than any semiconductor, due to the massless pseudorelativistic nature of the Dirac fermions. These systems being semimetallic, the lack of a genuine band-gap poses a serious limitation to their possible applications in electronics. We recently demonstrated that thin TiO2 nanowires can exhibit 1D Dirac states similar to metallic carbon nanotubes, with the crucial difference that these states lie inside the conduction band in proximity of a wide band gap. We analyze the robustness of the Dirac states respect to an Anderson disorder model and substitutional impurity and compare to different one dimensional systems. The results suggest that thin anatase TiO2 nanowires can be a promising candidate material for switching devices.

Defects influence the electronic and optical properties of crystals, so their identification is crucial to develop device technology for materials of micro-/optoelectronics and photovoltaics. The identification requires the accurate calculation of the electronic transitions and the paramagnetic properties of defects. The achievable accuracy is strongly limited in the case of the (semi)local approximations to density functional theory, because of the underestimation of the gap and of the degree of localization. In the past two decades, hybrid functionals, mixing semilocal and nonlocal exchange semiempirically, have emerged as an alternative. Very often, however, the parameters of such hybrids have to be tuned from material to material. In this paper, we describe the theoretical foundations for the proper tuning and show that if the relative positions of the band edge states are well reproduced, and the generalized Koopmans's theorem is fulfilled by the given parameterization, the calculated defect levels and localizations can be very accurate. As demonstrated here, this can be achieved with the two-parameter Heydt-Scuseria-Ernzerhof hybrid, HSE(α,μ) for diamond, Si, Ge, TiO2, GaAs, CuGaS(Se)2, GaSe, GaN, and Ga2O3. The paper describes details of the parameterization process and discusses the limitations of optimizing HSE functionals. Based on the gained experience, future directions for improving exchange functionals are also provided.

A combined experimental/theoretical study of the EPR in irradiated $\beta$-Ga$_2$O$_3$ is presented. Four EPR spectra, two $S=1/2$ and two $S=1$, are observed after high-energy proton or electron irradiation. One of the S=1/2 spectra (EPR1) can be observed at room temperature and below and is characterized by the spin Hamiltonian parameters $g_b=2.0313$, $g_c=2.0079$, $g_{a*}=2.0025$ and a quasi isotropic hyperfine interaction with two equivalent Ga neighbors of $~\sim$14 G on $^{69}$Ga. The second (EPR2) is observed after photoexcitation (with threshold 2.8 eV) at low temperature and is characterized by $g_b=2.0064$, $g_c=2.0464$, $g_{a*}=2.0024$ and a quasi isotropic hyperfine interaction with two equivalent Ga neighbors of 10 G. A spin $S=1$ spectrum with a similar g-tensor and a 50\% reduced hyperfine splitting accompanies each of these, which is indicative of a defect of two weakly coupled $S=1/2$ centers. DFT calculations of the magnetic resonance fingerprint of a wide variety of native defect models are carried out to identify these EPR centers in terms of specific defect configurations. The EPR1 center is proposed to correspond to a complex of two tetrahedral $V_\mathrm{Ga1}$ with an interstitial Ga in between them. This model was previously shown to have lower energy than the simple tetrahedral Ga vacancy and has a $2-/3-$ transition level higher than other $V_\mathrm{Ga}$ related models, which would explain why the other ones are already in their diamagnetic $3-$ state and are thus not observed if the Fermi level is pinned approximately at this level. The EPR2 spectra are proposed to correspond to the octahedral $V_\mathrm{Ga2}$. Models based on self-trapped holes and oxygen interstitials are ruled out because they would have hyperfine interaction with more than two Ga nuclei and because they can not support a corresponding $S=1$ center.

Proton irradiation of both n-type and semi-insulating bulk samples of β-Ga2O3 leads to the formation of two paramagnetic defects with spin S = 1/2 and monoclinic point symmetry. Their high introduction rates indicate them to be primary irradiation induced defects. The first electron spin resonance (EPR1) has a g-tensor with principal values of gb = 2.0313, gc = 2.0079, and ga* = 2.0025 and quasi-isotropic superhyperfine interaction of 13G with two equivalent Ga neighbors. Under low temperature photoexcitation, this defect is quenched and replaced by a different metastable spin S = 1/2 center of comparable intensity. This second defect (EPR2) has similar principal g-values of gb = 2.0064, gc = 2.0464, and ga* = 2.0024 and shows equally superhyperfine interaction with two equivalent Ga atoms. This EPR2 defect is stable up to T = 100 K, whereas for T > 100 K the initial defect is recovered. Density functional theory calculations of the spin Hamiltonian parameters of various intrinsic defects are carried out using the gauge including projector augmented wave method in order to determine the microscopic structure of these defects. The intuitive models of undistorted gallium monovacancies or self-trapped hole centers are not compatible with neither of these two defects.

How small nanodiamonds can be is a crucial question for biomedical applications. To answer this question, we present here molecular dynamic simulations of the annealing of very small diamond clusters (diameter between 0.3 and 1.3 nm) of various shape in vacuum and in the presence of oxygen. Isothermal cycles of 30 ps were carried out at 500, 1000, 1500, and 2000 K with 10 ps ramps between them. Predominantly {100} faceted diamond clusters as small as 1 nm (~ 250 atoms) survive these short anneals up to 1500 K. Longer anneals at 1500 K, as well “accelerated” MD at very high temperatures, indicate that the diamond core is still preserved when thermal equilibration is reached. The primary effect of oxygen seems to be the saturation of threefold-coordinated surface carbon atoms and the etching of lower coordinated ones. Oxygen accelerates the graphitization somewhat but does not affect the critical size. Our result means that nanodiamonds with a core of only 0.8 nm can be kinetically stable up to 1500 K. This is significantly less than the lower limit of the thermodynamic stability (~ 1.9 nm).

First-principles spin-polarized density functional theory computations in generalized gradient approximation to investigate the adsorption of N 2 O on the neutral and negative surfaces of the anatase TiO 2 (1 0 1) and on the neutral and positive surfaces of CO pre-adsorbed TiO 2 were carried out. The adsorption energies for N 2 O adsorbed on the TiO 2 and the CO pre-adsorbed surfaces were obtained. Mechanisms for redox reaction of CO + N 2 O conversion to CO 2 + N 2 on the neutral and positive TiO 2 surfaces have been proposed and their photocatalytic involvement is described.

The stability and electronic structure of bare [001] nanowires of anatase and rutile have been investigated using ab initio density functional calculations. It was found that symmetry plays an important role in both properties below a critical diameter. Up to 2.1 nm in anatase and 3.7 nm in rutile, those {100}-walled anatase and {110}-walled rutile wires are most stable, which retain the nonsymmorphic character of the bulk space group. Our results explain the observed properties of atomic size anatase nanowires. The wires with screw symmetry also show a consistently larger gap in their electronic structures compared to similarly walled wires without it. Additionally, in rutile the indirect or direct character of the band structure is coupled to the presence or absence of the screw axis.

The functionality of semiconductors and insulators depends mainly on defects which modify the electronic, optical, and magnetic spectra through their gap levels. Accurate calculation of the latter is not only important for the experimental identification of the defect, but influences also the accuracy of other calculated defect properties, and is the most difficult challenge for defect theory. The electron self-interaction error in the standard implementations of ab initio density functional theory causes a severe underestimation of the band gap, leading to a corresponding uncertainty in the defect level positions in it. This is a widely known problem which is usually dealt with by a posteriori corrections. A wide range of corrections schemes are used, ranging from ad hoc scaling or shifting, through procedures of limited validity (like the scissor operator or various alignment schemes), to more rigorous quasiparticle corrections based on many-body perturbation theory. We will demonstrate in this paper that consequences of the gap error must to be taken into account in the total energy, and simply correcting the band energy with the gap level shifts is of limited applicability. Therefore, the self-consistent determination of the total energy, free of the gap-error, is preferred. We will show that semi-empirical screened hybrid functionals can successfully be used for this purpose.

We have investigated the absorption of 0.9, 1.4 nm silicon carbide nanoparticles (SiC NPs) by time-dependent density functional calculations, focusing on the effect of different oxygen adsorbates of the surface. We have found that negatively charged Si-O−, Si-COO− defects dramatically lower the optical gap of SiC NPs. Our findings can help interpret recent controversary experiments on colloidal SiC NPs.

Calculations by the HSE06 functional for rutile and anatase crystals reproduce the observed width of the valence and conduction bands, as well as of the gap. From the band structures the branching point energies are obtained as reference for both phases, allowing us to deduce a generic band alignment scheme without a specific interface model. The results show that both bands of rutile lie higher than those of anatase, so in mixed systems the holes are accumulated in the former while the electrons in the latter. Consequences for photocatalytic and photoelectrochemical applications are discussed.

β-Ga2O3 has recently been considered for power electronics applications but p-type doping is an issue, especially since small hole polarons were predicted and observed in this material. Recently, by using a gap-optimized, Koopmans-compliant hybrid functional, we were able to reproduce the observed charge transition levels of defects, including the hole polaron in the intrinsic material. In this study, the same hybrid functional is used to confirm the assignment of the electron paramagnetic resonance (EPR) spectrum observed in Mg-doped β-Ga2O3 samples. The Mg impurity introduces a deep acceptor level in β-Ga2O3 corresponding to a small polaron, localized at an oxygen site adjacent to the substitutional Mg. The so-called O1 site is energetically favored and the superhyperfine interactions with the neighboring tetrahedrally coordinated Ga atom are 1.9 times stronger than with the neighboring octahedrally coordinated Ga. The calculated hyperfine tensor agrees well with the EPR measurement. These results confirm that Mg cannot act as a shallow acceptor in β-Ga2O3.

We use the Density Functional based Tight-Binding (SCC-DFTB) method in a Quantum Mechanics/Molecular Mechanics embedding scheme to simulate single SiC quantum dots of different shapes and with diameters of up to 1 nm, embedded in a block of α -Quartz with cell vectors of a = 15 nm and c = 13 nm. First results show that during the annealing process three recurring motives appear at the SiC/SiO2 interface of the embedded nanocrystals: Si-Si bonds often in Si interstitial-like configurations, C-C bonds involving all possible combinations of sp3 and sp2 hybridized carbon and threefold coordinated oxygen atoms. All of these defects, alone or in complexes, may play a crucial role in quenching the luminescence of these embedded nanocrystals. Detailed studies of the spectroscopic properties if the point defects identified in this work is indicated (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Supercell calculations using density functional theory with local or semi-local exchange functionals have proved to be a very successful tool in defect engineering, apparently leading to some degree of overconfidence lately. With a case study on selected defects, we demonstrate that the approximations involved in these functionals lead not only to an underestimation of the gap but also to related errors in the total energy difference of two defect configurations (be these in the same or different charge states). Sometimes the error is so serious, that even the common expectation: “DFT provides the correct ground state” is refuted. We also demonstrate, that semi-empirical hybrid exchange functionals, which reproduce the correct band gap, give good total energy differences even in these cases. In such calculations, the position of Kohn-Sham gap levels with respect to the band edges gives a good estimation of the vertical ionization energy. Based on that, we propose a simple correction scheme for checking the necessity of extended total energy calculations with a hybrid functional.