Hybrid Density Functional Theory Calculations Research Articles

Theoretical investigation of catalytic n-butane isomerization over H-SSZ-13

Hybrid density functional theory calculations are used to investigate different mechanisms of the isomerization of n-butane to isobutane via intermediate formation of olefins. The monomolecular mechanism for isomerization of butene and isobutene is found to be prevalent, with a Gibbs free energy barrier of 155 kJ/mol at 400°C, compared to the bimolecular mechanism (190 kJ/mol) due to less favorable entropy for the latter. Hydrogen transfer reactions that convert olefins into alkanes (and vice versa) are also included in the investigations, and show a free energy barrier of 203 kJ/mol for conversion of isobutene to isobutane. Additionally, a methyl transfer mechanism is discussed as a possible pathway for formation of C3 and C5 side products, in comparison to the bimolecular mechanism; the highest barrier of the initial methyl transfer is calculated to be 227 kJ/mol. We discuss the influence of entropy and anharmonicity on all mechanisms, stating that through the uncertainties in computational methods when calculating these systems, the calculated reaction barriers are likely to be overestimated here.

Density Functional Theory Calculations for Multiple Conformers Explaining the Regio- and Stereoselectivity of Ti-Catalyzed Hydroaminoalkylation Reactions.

Hybrid Density Functional Theory (DFT) calculations for multiple conformers of the insertion reactions of a methylenecyclopropane into the Ti-C bond of two differently α-substituted titanaaziridines explain the experimentally observed differences in regioselectivity between catalytic hydroaminoalkylation reactions of methylenecyclopropanes with α-phenyl-substituted secondary amines and corresponding stoichiometric reactions of a methylenecyclopropane with titanaaziridines, which can only be achieved with α-unsubstituted titanaaziridines. In addition, the lack of reactivity of α-phenyl-substituted titanaaziridines as well as the diastereoselectivity of the catalytic and stoichiometric reactions can be understood.

On the Symmetry, Electronic Properties, and Possible Metallic States in NASICON-Structured A4V2(PO4)3 (A = Li, Na, K) Phosphates.

In this work, the electronic structure and properties of NASICON-structured A4V2(PO4)3, where A = Li, Na, K were studied using hybrid density functional theory calculations. The symmetries were analyzed using a group theoretical approach, and the band structures were examined by the atom and orbital projected density of states analyses. Li4V2(PO4)3 and Na4V2(PO4)3 adopted monoclinic structures with the C2 space group and averaged vanadium oxidation states of V+2.5 in the ground state, whereas K4V2(PO4)3 adopted a monoclinic structure with the C2 space group and mixed vanadium oxidation states V+2/V+3 in the ground state. The mixed oxidation state is the least stable state in Na4V2(PO4)3 and Li4V2(PO4)3. Symmetry increases in Li4V2(PO4)3 and Na4V2(PO4)3 led to the appearance of a metallic state that was independent of the vanadium oxidation states (except for the averaged oxidation state R32 Na4V2(PO4)3). On the other hand, K4V2(PO4)3 retained a small band gap in all studied configurations. These results might provide valuable guidance for crystallography and electronic structure investigations for this important class of materials.

First-principles calculations of electronic and optical properties of Ce3+-activated phosphors toward evaluating valence stability of Eu ions

The valence stability of activator ions with multiple stable valence states has always been a major concern in the field of luminescent materials. Here we present a computational protocol that is able to predict the valence stability of the europium (Eu) ion in its activated phosphors, in terms of the energy difference between the ground-state level of Eu2+ and the Fermi level of the host. The protocol is based on hybrid density functional theory (DFT) and wavefunction-based multiconfigurational ab initio calculations of the dopant Ce3+, in conjunction with the established relationship between energy levels of Ce3+ and Eu2+. The predictions are in good agreements with experimental findings. It is demonstrated that across the study set consisting of five borate-containing phosphors, the valence stability of the dopant Eu2+ is positively correlated with the host band gap, and this positive correlation is more significant than the negative correlation with the energy level of Eu2+ within the host band gap. Microscopic analysis reveals that a strong bonding of electrons within the anion groups together with a large dopant site is beneficial for the reduction of Eu3+ into Eu2+. The influence of hybrid DFT functional on the predicted valence stability of a given compound was found be negligible due to cancellation of changes in the host band gap and the Eu2+ ground level. This study clarifies the reasons for the stabilization of Eu2+ in phosphors synthesized in air at high temperature and is expected to help to screen out the host compounds that could stabilize Eu2+ activators for diverse optical applications.

G-B3C2N3: A Potential Two Dimensional Metal-free Photocatalyst for Overall Water Splitting.

Based on hybrid density functional theory (DFT) calculations, we propose a new two-dimensional (2D) B-C-N material, graphitic-B3C2N3(g-B3C2N3), with the promising prospect of metal-free photocatalysis. We find it to be a near ultraviolet (UV) absorbing direct band gap (3.69 eV) semiconductor with robust dynamical and mechanical stability. Estimating the band positions with respect to water oxidation and hydrogen reduction potential levels and through a detailed analysis of reaction mechanism of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), we observe that g-B3C2N3monolayer can be efficiently used for hydrogen fuel generation over entire pH range and for spontaneous water splitting at basic pH range. Upon biaxial strain application, band positions get realigned along with the free energy change that is involved in HER and OER. Consequently, operational range of pH for OER gets broadened and the proposed material exhibits the ability to perform spontaneous and simultaneous oxidation and reduction even in neutral pH. The combination of variation in pH and applied strain can be used as a key to control the reducing or oxidizing abilities precisely for diverse photocatalytic reactions to attain environmental sustainability.

Complexity reduction in density functional theory: Locality in space and energy.

We present recent developments of the NTChem program for performing large scale hybrid density functional theory calculations on the supercomputer Fugaku. We combine these developments with our recently proposed complexity reduction framework to assess the impact of basis set and functional choice on its measures of fragment quality and interaction. We further exploit the all electron representation to study system fragmentation in various energy envelopes. Building off this analysis, we propose two algorithms for computing the orbital energies of the Kohn-Sham Hamiltonian. We demonstrate that these algorithms can efficiently be applied to systems composed of thousands of atoms and as an analysis tool that reveals the origin of spectral properties.

Millimeter wave spectroscopy of propynal isotopologues and structure determination

Rotational transitions of propynal (HCCCHO) have been measured in the 150–900 GHz region by millimeter wave spectroscopy and in the far infrared region by high resolution FTIR spectroscopy using a synchrotron source. For the parent isotopologue, assignment of MMW transitions up to very high quantum numbers (J = 100, Ka = 25) reveals evidence of extensive perturbations in the ground vibrational state due to Fermi-asymmetry resonance with an excited vibrational state. A fit to nearly 3000 ground state transitions yields effective constants that are suitable for describing relatively unperturbed rotational levels up to Ka = 13. Over 1000 transitions were assigned and fitted for each singly substituted 13C species and nearly 400 transitions for the 18O variant. Re-analysis of literature data on deuterated species, aided by centrifugal distortion constants from hybrid density functional theory calculations at the B2PLYP/aug-cc-pVTZ level, provides a further set of rotational constants. This allows determination of a Rm(2) structure for propynal with the following geometry: r(C≡C) = 1.2066(15), r(CC) = 1.4486(14), r(CO) = 1.2087(10), r(C­H ald) = 1.1069(8), r(C­Hacet) = 1.0578(13), θ(CCC) = 176.71(22), θ(OCC) = 123.23(7), θ(HCCald) = 114.43(31), θ(HCCacet) = 178.45(16). A new Rs structure was also derived.

Copper(II)‐TEMPO Interaction

AbstractCopper(II) complexes with N‐oxyl reactants such as TEMPO can selectively oxidize alcohols to aldehydes or ketones. The proposed copper intermediates of the oxidation reaction were extensively theoretically studied, but they were never experimentally detected. Here, we present an analysis of “frozen” intermediates that contain alcohols without α‐hydrogen atoms, thus preventing oxidation. The copper(II)‐TEMPO complexes with a bipyridine‐type ancillary ligand were isolated by electrospray ionization mass spectrometry and investigated spectroscopically by cryogenic photodissociation spectroscopy. The vibrational characteristics of the complexes suggest that TEMPO retains its unpaired electron even upon coordination with copper(II). In agreement, the electronic photodissociation spectra of the TEMPO‐copper(II) complexes show a characteristic band for gaseous copper(II) complexes. These results contradict interpretations of some previous density functional theory (DFT) analyses of possible reaction intermediates. The experimental data were confronted with theoretical results obtained by pure DFT (OPBE, TPSS) and hybrid DFT (TPSSH, B3LYP) calculations. The methods favor different ground states and capture various aspects of the experimental results demonstrating a multiconfigurational character of the copper(II)‐TEMPO complexes.

A non-self-consistent tight-binding electronic structure potential in a polarized double-ζ basis set for all spd-block elements up to Z = 86.

Existing semiempirical molecular orbital methods suffer from the usually minimal atomic-orbital (AO) basis set used to simplify the calculations. Here, a completely new and consistently parameterized tight-binding electronic structure Hamiltonian evaluated in a deeply contracted, properly polarized valence double-zeta basis set (vDZP) is described. The inner-shell electrons are accounted for by standard, large-core effective potentials and approximations to them. The primary target of this so-called density matrix tight-binding method is to reproduce the one-particle density matrix P of a molecular ωB97X-V range-separated hybrid density functional theory (DFT) calculation in exactly the same basis set. Additional properties considered are orbital energies, dipole polarizabilities and dipole moments, and dipole polarizability derivatives. The key features of the method are as follows: (a) it is non-self-consistent with an overall fixed number of only three required matrix diagonalizations; (b) only AO overlap integrals are needed to construct the effective Hamiltonian matrix; (c) new P-dependent terms emulating non-local exchange are included; and (d) only element-specific empirical parameters (about 50 per element) need to be determined. The method globally achieves a high accuracy for the target properties at a speedup compared to the ωB97X-V/vDZP reference of about 3-4 orders of magnitude. It performs robustly for difficult transition metal complexes, for highly charged or zwitterionic systems, and for chemically unusual bonding situations, indicating a generally robust approximation of the (self-consistent) Kohn-Sham potential. As an example application, the vibrational Raman spectrum of an entire protein with 327 atoms with respect to the DFT reference calculation is shown. This method may be used out-of-the-box to generate molecular/atomic features for machine learning applications or as the basis for accurate high-speed DFT methods.

Modeling the Electronic and Optical Properties of Lead-Based Perovskite Materials: Insights from Density Functional Theory and Electrostatic Embedding

We present an investigation of the geometric, electronic, and optical properties of various bulk MAPbX3 (X = Cl, Br, and I) perovskite systems in different phases and of different heterointerface models built between MAPbI3 (hereafter MAPI) and TiO2, linked by a bifunctional para-benzoic acid derivative, in view of their importance as key components of perovskite solar cells. To this end, we combine periodic hybrid density functional theory (DFT) calculations, to model the geometric and electronic properties of these systems, with time-dependent DFT calculations (TD-DFT) carried out on non-periodic clusters, extracted from the periodic models, embedded in an array of point charges reproducing the periodic electrostatic environment, thus enabling the modeling of their optical properties in condensed phases. We found that all systems investigated present favorable key features for their use as building blocks for solar devices. Furthermore, the proposed computational approach, combining periodic and non-periodic calculations, appears as a reliable and effective tool to model both the electronic and the optical properties of various key components of materials related to photovoltaic applications at low computational cost.

The electronic structure of β-TeO2 as wide bandgap p-type oxide semiconductor

Wide bandgap oxide semiconductors have gained significant attention in the fields from flat panel displays to solar cells, but their uses have been limited by the lack of high mobility p-type oxide semiconductors. Recently, β-phase TeO2 has been identified as a promising p-type oxide semiconductor with exceptional device performance. In this Letter, we report on the electronic structure of β-TeO2 studied by a combination of high-resolution x-ray spectroscopy and hybrid density functional theory calculations. The bulk bandgap of β-TeO2 is determined to be 3.7 eV. Direct comparisons between experimental and computational results demonstrate that the top of a valence band (VB) of β-TeO2 is composed of the hybridized Te 5s, Te 5p, and O 2p states, whereas a conduction band (CB) is dominated by unoccupied Te 5p states. The hybridization between spatially dispersive Te 5s2 states and O 2p orbitals helps us to alleviate the strong localization in the VB, leading to small hole effective mass and high hole mobility in β-TeO2. The Te 5p states provide stabilizing effect to the hybridized Te 5s-O 2p states, which is enabled by structural distortions of a β-TeO2 lattice. The multiple advantages of large bandgap, high hole mobility, two-dimensional structure, and excellent stability make β-TeO2 a highly competitive material for next-generation opto-electronic devices.

Strongly constrained and appropriately normed density functional theory exchange-correlation functional applied to rare earth oxides

Density functional theory (DFT) is one of the main tools for studying the electronic structure of solids and molecules. Nevertheless, one of the main drawbacks of the implementation of DFT is the so-called self-interaction error (SIE) that can yield undesired delocalization errors and ultimately results in the prediction of metals instead of experimentally observed insulators. These SIEs can be amended by using more evolved exchange-correlation functionals than standard local density approximation such as the recent meta-generalized gradient approximation strongly constrained and appropriately normalized (SCAN) functional that is successful in describing electronic properties of $3d$ transition metal oxides. Nevertheless, the ability of such a functional to describe electronic properties of materials involving more localized states such as $4f$ orbitals is rather elusive. Here, we show that, even though SCAN can sometimes predict the insulating character of some compounds, it often fails in predicting the correct band edge orbital character of insulators. By comparing our SCAN results with benchmark simulations obtained with more accurate hybrid DFT calculations, we ascribe this failure to insufficiently amended SIEs by SCAN that results in an underestimation of Hund's splitting associated with $4f$ states. Thus, although appropriate for $3d$ transition metal elements, the SCAN functional is not yet a sufficient platform for studying electronic properties of materials involving rare-earth elements where $4f$ states play a key role in the properties.

Photocatalytic and Piezocatalytic Properties of h-NbP and h-NbN Monolayers for Green Hydrogen Production: Insight from Density Functional Theory Calculations

Two-dimensional (2D) materials with intrinsic piezo- and ferro-electricity are highly prospective in designing novel catalysts for green hydrogen generation. This work systematically investigates the photocatalytic and piezocatalytic capabilities of buckled hexagonal (h-) NbP and NbN ferroelectric monolayers utilizing hybrid density functional theory (DFT) calculations. First, we delve into monolayers’ structural, electronic, ferroelectric, and optical properties. This reveals that NbP and NbN can drive the photocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, in the visible region of the solar spectrum and can also be used in designing a z-scheme photocatalyst for efficient overall water splitting. Particularly, strong photon absorption of 4 × 105 cm–1 with 89% absorption efficiency makes these monolayers ideal candidates to explore. It further reveals that the presence of robust out-of-plane ferroelectricity induces an intrinsic electric field which helps in enhancing the separation of photoinduced charge carriers. We also explore the elastic and piezoelectric properties of these monolayers to understand their piezocatalytic behavior. The low Young’s modulus of 51.76 N m–1 for NbP and 104.65 N m–1 for NbN monolayers demonstrates the higher mechanical flexibility. A sizable vertical piezoelectricity d31 of 0.81 pm V–1 and 0.56 pm V–1 for NbP and NbN, respectively, is highest among the similar type buckled honeycomb group III–V monolayers. In addition, the in-plane piezoelectricity d11 of 36.02 pm V–1 and 17.61 pm V–1 for NbP and NbN monolayers, respectively, results in the generation of sufficient piezopotentials to demonstrate their piezocatalytic water splitting ability. These properties reveal the potentiality of NbP and NbN for designing efficient piezo- and photo-catalysts for green hydrogen generation.

Reduction of Nitric Oxide to Nitrous Oxide in Flavodiiron Proteins: Catalytic Mechanism and Plausible Intermediates

The flavin dependent nonheme diiron proteins comprise a family of enzymes, which can act as scavengers for both molecular oxygen and nitric oxide. The reduction of nitric oxide to nitrous oxide and water in flavodiiron proteins (FDPs) has been studied both experimentally and computationally, but the reaction mechanism is far from well understood. From experiments, it is known that two NO molecules can bind to the reduced active site, forming an observable diferrous dinitrosyl complex. A main question has been whether nitrous oxide can be formed directly from the diferrous dinitrosyl complex or if further reduction and/or protonation is needed to make this step feasible. Experiments have shown that nitrous oxide can be formed in a deflavinated form of the enzyme, indicating that further reduction is not needed. In the present study, hybrid density functional theory calculations are performed on a cluster model of the Thermotoga maritima FDP active site. We show that nitric oxide can be reduced to nitrous oxide and water using a direct coupling mechanism, i.e., without further additions to the reduced active site. The diferrous dinitrosyl complex can form an unstable N–N bridging hyponitrite intermediate, which can rotate into an N–O bond bridging hyponitrite with a low barrier. From this intermediate, the N–O bond cleavage leading to release of nitrous oxide is energetically feasible. An energy profile for the entire catalytic cycle of such a direct coupling mechanism is presented, and it is shown that the suggested mechanism agrees with data on FDP variants. Finally, an energy profile for the entire process starting with the fully reduced enzyme turning over four NO equivalents is constructed. This energy profile suggests explanations to experimentally observed states, such as the dihydroxyl form of the fully oxidized diferric state, and the difference with respect to returning to the original oxidized state after NO reduction between the flavinated and the deflavinated form of the enzyme.

In2Se3, In2Te3, and In2(Se,Te)3 Alloys as Photovoltaic Materials.

In its lowest-energy three-dimensional (3D) hexagonal crystal structure (γ phase), In2Se3 has a direct band gap of ∼1.8 eV and displays high absorption coefficient, making it a promising semiconductor material for optoelectronics. Incorporation of Te allows for tuning the band gap, adding flexibility to device design and extending the application range. Here we report results of hybrid density functional theory calculations to assess the electronic and optical properties of γ-In2Se3, γ-In2Te3, and γ-In2(Se1-xTex)3 alloys, and initial experiments on the growth and characterization of γ-In2Se3 thin films. The predicted band gap of 1.84 eV for γ-In2Se3 is in good agreement with the absorption onset derived from transmission and reflection spectra of thin films. We show that incorporation of Te gives γ-In2(Se1-xTex)3 alloys with a band gap ranging from 1.84 eV down to 1.23 eV, thus covering the optimal band gap range for single-junction solar cells. In addition, the γ-In2Se3/γ-In2(Se1-xTex)3 bilayer could be employed in tandem solar-cell architectures absorbing at Eg ≈ 1.8 eV and at Eg ≤ 1.4 eV, toward overcoming the ∼33% efficiency set by the Shockley-Queisser limit for single junction solar cells. We also discuss band gap bowing and mixing enthalpies, aiming at adding γ-In2Se3, γ-In2Te3, and γ-In2(Se1-xTex)3 alloys to the available toolbox of materials for solar cells and other optoelectronic applications.

Intrinsic Defects and Their Role in the Phase Transition of Na-Ion Anode Na2Ti3O7.

The development of high-power anode materials for Na-ion batteries is one of the primary obstacles due to the growing demands for their use in the smart grid. Despite the appealingly low cost and non-toxicity, Na2Ti3O7 suffers from low electrical conductivity and poor structural stability, which restricts its use in high-power applications. Viable approaches for overcoming these drawbacks reported to date are aliovalent doping and hydrogenation/hydrothermal treatments, both of which are closely intertwined with native defects. There is still a lack of knowledge, however, of the intrinsic defect chemistry of Na2Ti3O7, which impairs the rational design of high-power titanate anodes. Here, we report hybrid density functional theory calculations of the native defect chemistry of Na2Ti3O7. The defect calculations show that the insulating properties of Na2Ti3O7 arise from the Na and O Schottky disorder that act as major charge compensators. Under high-temperature hydrogenation treatment, these Schottky pairs of Na and O vacancies become dominant defects in Na2Ti3O7, triggering the spontaneous partial phase transition to Na2Ti6O13 and improving the electrical conductivity of the composite anode. Our findings provide an explanation on the interplay between intrinsic defects, structural phase transitions, and electrical conductivity, which can aid understanding of the properties of composite materials obtained from phase transitions.

Mind the Interface Gap: Exposing Hidden Interface Defects at the Epitaxial Heterostructure between CuO and Cu2O.

Well designed and optimized epitaxial heterostructures lie at the foundation of materials development for photovoltaic, photocatalytic, and photoelectrochemistry applications. Heterostructure materials offer tunable control over charge separation and transport at the same time preventing recombination of photogenerated excitations at the interface. Thus, it is of paramount importance that a detailed understanding is developed as the basis for further optimization strategies and design. Oxides of copper are nontoxic, low cost, abundant materials with a straightforward and stable manufacturing process. However, in individual applications, they suffer from inefficient charge transport of photogenerated carriers. Hence, in this work, we investigate the role of the interface between epitaxially aligned CuO and Cu2O to explore the potential benefits of such an architecture for more efficient electron and hole transfer. The CuO/Cu2O heterojunction nature, stability, bonding mechanism, interface dipole, electronic structure, and band bending were rationalized using hybrid density functional theory calculations. New electronic states are identified at the interface itself, which are originating neither from lattice mismatch nor strained Cu-O bonds. They form as a result of a change in coordination environment of CuO surface Cu2+ cations and an electron transfer across the interface Cu1+-O bond. The first process creates occupied defect-like electronic states above the valence band, while the second leaves hole states below the conduction band. These are constitutional to the interface and are highly likely to contribute to recombination effects competing with the improved charged separation from the suitable band bending and alignment and thus would limit the expected output photocurrent and photovoltage. Finally, a favorable effect of interstitial oxygen defects has been shown to allow for band gap tunability at the interface but only to the point of the integral geometrical contact limit of the heterostructure itself.

How graphenic are graphynes? Evidence for low-lying correlated gapped states in graphynes.

Graphynes can be structurally envisioned as 2D extensions to graphene, whereby linearly bonded carbon linkages increase the distance between trigonal carbon nodes. Many graphynes have been predicted to exhibit a Dirac-like semimetallic (SEM) graphenic electronic structure, which could potentially make them competitive with graphene for applications. Currently, most graphynes remain as attractive synthetic targets, and their properties are still unconfirmed. Here, we demonstrate that the electronic structure of hexagonal α-graphyne is analogous to that of biaxially strained graphene. By comparison with accurate quantum Monte Carlo results on strained graphene, we show that the relative energetic stability of electronic states in this correlated 2D system can be captured by density functional theory (DFT) calculations using carefully tailored hybrid functionals. Our tuned hybrid DFT approach confirms that α-graphyne has a low energy correlated Mott-like antiferromagnetic insulating (AFI) state, which competes with the SEM state. Our work shows that the AFI-SEM crossover in α-graphyne could be tunable by in-plane biaxial strain. Applying our approach to other graphynes shows that they should also exhibit correlated AFI states, which could be dominant even at zero strain. Calculations using an onsite Coulombic repulsive term (i.e., DFT + U) also confirm the predictions of our hybrid DFT calculations. Overall, our work strongly suggests that graphynes are not as graphenic (i.e., Dirac-like) as often previously predicted by DFT calculations using standard generalized gradient approximation functionals. However, due to the greater electronic versatility (e.g., tunable semiconducting bandgaps and accessible spin polarized states) implied by our study, graphynes could have novel device applications that are complementary to those of graphene.

Direct determination of band-gap renormalization in degenerately doped ultrawide band gap β- Ga2O3 semiconductor

${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ is emerging as a promising wide band-gap semiconductor for high-power electronics and deep ultraviolet optoelectronics. It is highly desirable to dope it with controllable carrier concentrations for different device applications. This work reports a combined photoemission spectroscopy and theoretical calculation study on the electronic structure of Si doped ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ films with carrier concentration varying from $4.6\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ to $2.6\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. Hard x-ray photoelectron spectroscopy was used to directly measure the widening of the band gap as a result of occupation of conduction band and band-gap renormalization associated with many-body interactions. A large band-gap renormalization of 0.3 eV was directly observed in heavily doped ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. Supplemented with hybrid density functional theory calculations, we demonstrated that the band-gap renormalization results from the decrease in energy of the conduction band edge driven by the mutual electrostatic interaction between added electrons. Moreover, our work reveals that Si is a superior dopant over Ge and Sn, because $\mathrm{Si}\phantom{\rule{0.28em}{0ex}}3s$ forms a resonant donor state above the conduction band minimum, leaving the host conduction band mostly unperturbed and a high mobility is maintained though the doping level is high. Insights of the present work have significant implications in doping optimization of ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and realization of optoelectronic devices.

Effect of halogen doping on the electronic, electrical, and optical properties of anatase TiO2

Titanium dioxide (TiO2) is one of the most used oxides in renewable energy applications, such as hydrogen production, photovoltaics, and light-emitting diodes. To further improve the efficiency of the devices, doping strategies are used to modify their fundamental properties. Here, we used density functional theory (DFT) simulations to explore the effect of all the halogen dopants on the structural, electronic, and optical properties of TiO2. We investigated both the interstitial and the oxygen substitutional positions, and for the optimized structures, we used hybrid DFT calculations to predict the electronic and optical properties. In all cases, we found that halogen dopants reduce the bandgap of the pristine TiO2 while gap states also arise. The halogen dopants constitute a single acceptor when they occupy interstitial sites, while when they are inserted in oxygen sites, they act as donors. This can be established by the states that form above the valence band. It is proposed that these states contribute to the significant changes in the optical and electronic properties of TiO2 and can be beneficial to the photovoltaic and photocatalytic applications of TiO2. Importantly, the iodine doping of TiO2 significantly reduces the bandgap of TiO2 while increasing its dielectric constant, making it suitable for light-harvesting applications.