Hybrid Density Functional Study Research Articles

Hybrid Density Functional Study on the p-Type Conductivity Mechanism in Intrinsic Point Defects and Group V Element-Doped 2D β-TeO2.

The newly discovered two-dimensional (2D) β-TeO2 possesses extraordinarily high p-type carrier mobility and demonstrates immense potential in the electronics field. However, current research on its p-type conductivity mechanisms and the modifications of element doping remains relatively insufficient. In this study, the intrinsic point defects and extrinsic element doping in monolayer β-TeO2 are comprehensively analyzed to probe the potential sources of the intrinsic p-type conductivity and the extrinsic p-type doping possibility in 2D β-TeO2 through hybrid density functional calculations. Our results reveal that the vacancy defects with low formation energies have deep transition levels and thus cannot be used as sources of unintentional p-type conductivity in 2D β-TeO2. The investigations and discussions via Group V element doping modifications in 2D β-TeO2 indicate that bismuth (Bi) doping can easily and significantly enhance the p-type conductivity of 2D β-TeO2 under the presence of O-rich, which can be achieved experimentally. Furthermore, Bi doping can significantly increase carrier mobility without seriously affecting the electronic structure. The finding shows that the Bi element is an ideal dopant candidate for a p-type modification in 2D β-TeO2. Our calculations pave an alternative strategy to achieve the realization of superior p-type conductivity in 2D β-TeO2.

Underlying Effects of Temperature and Pressure on the Concentration of Carriers and Intrinsic Defects in the Bi2MoO6 Photocatalyst: A Hybrid Density Functional Study

Underlying Effects of Temperature and Pressure on the Concentration of Carriers and Intrinsic Defects in the Bi₂MoO₆ Photocatalyst: A Hybrid Density Functional Study

A hybrid density functional study of tensile-induced changes in phonon dispersion, electronic structure and optical absorption of bilayer BN for optoelectronic applications

The electronic, phonon dispersion and optical properties of bilayer BN have been investigated with density functional theory (DFT) calculations. Interestingly, applying the biaxial strain of 4% converts the indirect band gap of bilayer BN to a direct band gap. We show that the interaction between the BN layers results in the removal of two-fold degeneracy and separation of ZA and ZO phonon modes near the Γ point. Across the entire range of applied biaxial tensile strains, the bilayer BN exhibits a dynamically stable behavior. The phonon bandgap of bilayer BN decreases under tensile strain. The bilayer BN has strong adsorption in the UV region and the optical absorption peaks are red-shifted by applying the tensile strain. These findings are essential for the strain engineering of bilayer BN and future fabrication of flexible electronic and optoelectronic devices.

Indium defect complexes in (In x Ga1−x )2O3: a combined experimental and hybrid density functional theory study

Indium defects in small concentration (In x Ga1−x )2O3 were studied using a combination of spectroscopic and magnetic measurements on thin films varying the indium concentration, coupled with hybrid density functional theory simulations using the supercell method. X-ray diffraction spectra along with Tauc plots and density of states plots reveal a decrease (increase) in the electronic band gap (interlayer lattice spacing) due to the inclusion of indium in monoclinic Ga2O3, while room-temperature Hall measurements show an increase in n-type conductivity. Formation energy calculations reveal that the defect complex of substitutional indium at the octahedrally coordinated cation site (InGa) coupled with an indium interstitial (Ini) in the largest Ga–O cavity in the bulk (ia ), where the two impurities are a maximal distance away in the unit cell, results in the lowest formation energy across much of the electronic band gap; near the conduction band edge the single InGa defect becomes the lowest energy defect, though. These calculations help shed light on the impurity band enhanced, n-type conductivity increase due to small concentration indium doping in Ga2O3 as seen in the spectroscopic/magnetic measurements.

Nitrogen-doped graphene quantum dots for extensive production of hydrogen: A hybrid density functional study

Photocatalytic water splitting has emerged as a promising approach for storing solar energy in the form of chemical energy, which can solve the energy crisis and environmental issues. Herein we have explored the potentiality of nitrogen-doped graphene quantum dots for solar-driven hydrogen evolution. The motivation behind applying quantum dots as a photocatalyst is to utilize a major portion of the solar energy spectrum to enhance hydrogen yield. The alignment of frontier molecular orbitals (FMOs) in comparison to the redox potential of water reveals that all the studied quantum dots are eligible for photocatalytic hydrogen evolution. The energy gap between FMOs varies with the size of quantum dots from 3.01 to 2.32 eV, showing absorption of light in the visible range. The Gibbs free energy calculation shows that the formation of intermediate species is energetically unfavorable without exposure to light at pH=7, except for hydrogen adsorption. However, the photogenerated electron and hole drive the steps of oxygen and hydrogen evolution by making those a downhill process. The optical absorption spectra show that all the studied quantum dots absorb visible light.

Role of BaTiO3 crystal surfaces on the electronic properties, charge separation and visible light–response of the most active (001) surface of LaAlO3: A hybrid density functional study

Surface engineering has been proved as an efficient technique to boost charge separation and visible light absorption of semiconductor photocatalyst materials. Herein, we offer a first insight into the photocatalytic properties and electronic structure of cubic LaAlO3(001) modified with cubic BaTiO3 (001), (011) and (111) surfaces. Compared to the bulk LaAlO3, the BaTiO3/LaAlO3(001) heterostructures showed superior redshift of absorption, work functions and a suitable staggered type–II band alignment to separate the photoinduced carriers. The BaTiO3(001) surface coupled with LaAlO3(001) surface show improved visible light photoactivity owing to the more surface complex and terminal barium atoms forming the main active adsorption sites on its surface. These findings offer new insight into the understanding of preferential exposure of photocatalytic active surfaces, which will aid in designing active heterostructures for photocatalytic reactions, as well as understanding the mechanism of photocatalysis.

Magnetic and Electronic Properties of Sr Doped Infinite-Layer NdNiO2 Supercell: A Screened Hybrid Density Functional Study.

To understand the influence of doping Sr atoms on the structural, magnetic, and electronic properties of the infinite-layer NdSrNiO2, we carried out the screened hybrid density functional study on the Nd9-nSrnNi9O18 (n = 0-2) unit cells. Geometries, substitution energies, magnetic moments, spin densities, atom- and lm-projected partial density of states (PDOS), spin-polarized band structures, and the average Bader charges were studied. It showed that the total magnetic moments of the Nd9Ni9O18 and Nd8SrNi9O18 unit cells are 37.4 and 24.9 emu g-1, respectively. They are decreased to 12.6 and 4.2 emu g-1 for the Nd7Sr2Ni9O18-Dia and Nd7Sr2Ni9O18-Par unit cells. The spin density distributions demonstrated that magnetic disordering of the Ni atoms results in the magnetism decrease. The spin-polarized band structures indicated that the symmetry of the spin-up and spin-down energy bands around the Fermi levels also influence the total magnetic moments. Atom- and lm-projected PDOS as well as the band structures revealed that Ni(dx2-y2) is the main orbital intersecting the Fermi level. As a whole, electrons of Sr atoms tend to locate locally and hybridize weakly with the O atoms. They primarily help to build the infinite-layer structures, and influence the electronic structure near the Fermi level indirectly.

The Origin of the Chiral gauche-Conformation Preferences in Chloromethyl Methyl Ether and Analogues Containing S and Se Atoms: A Hybrid-Density Functional Study and Natural Bond Orbital Interpretation

The Origin of the Chiral gauche-Conformation Preferences in Chloromethyl Methyl Ether and Analogues Containing S and Se Atoms: A Hybrid-Density Functional Study and Natural Bond Orbital Interpretation

The role of native point defects and donor impurities in the electrical properties of ZnSb2O4: a hybrid density-functional study.

The ceramic material zinc antimony oxide ZnSb2O4 has promising electrical and magnetic properties, making it suitable for various applications such as electrochemical and energy storage. However, the effects of point defects and impurities on its electrical properties have never been revealed. Here, we employ hybrid density-functional calculations to investigate the energetics and electronic properties of native point defects and donor impurities in ZnSb2O4. The energetically favorable configurations of native point defects under selected growth conditions (O-rich and O-poor) are identified based on the calculated formation energies. The study finds no shallow donor and shallow acceptor defects with low formation energies. Still, the oxygen vacancy (VO) has the lowest formation energy among the donor-type defects under O-rich and O-poor conditions. However, it acts as a very deep acceptor, making it unlikely to provide free electron carriers to the conduction band. Moreover, electron carriers are likely to be compensated by the formation of zinc vacancies (VZn) and the zinc substituted for antimony (ZnSb), which behave as dominant acceptors. Our analysis of the charge neutrality conditions estimates that the Fermi level of undoped ZnSb2O4 would be pinned in a range that is 2.60 eV to 3.12 eV above the VBM for O-rich to O-poor growth conditions, respectively, suggesting that this material is semi-insulating. The possibility of enhancing free electron carriers by doping with Al, Ga, In, and F impurities is also investigated. Our results, however, indicate that high n-type conductivity is hindered by self-compensation in which the impurities additionally act as electron killers. Our results suggest that other impurity candidates and approaches may need to be considered to efficiently dope this material into n-type. Overall, this work paves the way for point defect engineering in this class of ternary oxides.

Strain Engineering of 2D-C3N5 Monolayer and Its Application in Overall Water-Splitting: a Hybrid Density Functional Study

The recent experimental synthesis of 2D graphitic C3N5 has attracted lot of interests in its electronic and optical properties and its comparison with other graphitic C3N4 and C3N3. To this end, we performed DFT calculations using more accurate HSE06 functional and estimated the corresponding electronic properties. From a comparative study of the band structures of C3N3, C3N4, and C3N5, we found that, the electronic band-gap decreases as 3.24 eV (C3N3) > 2.81 eV (C3N4) > 2.19 eV (C3N5) with increase in the number of nitrogen atoms in the unit cell of these graphitic carbon nitrides. Further, the strain dependency of the band structure of 2D g-C3N5 under uniaxial and biaxial strain is performed using the same HSE- 06 functional. We found a systematic decrease of band-gap as strain increases. Out of the two types of strain, the biaxial strain has been found to be more efficient in modulating the band-gap. The effect of strain on the structure is also explored by analyzing the bond lengths and bond-angles as well as the charge density plots. Furthermore, we found that at a biaxial strain of 20% strain an interesting structural rearrangement occurs in 2D g-C3N5, which reults in a finite magnetic moment arising from the loss of spin-degeneracy of electronic levels. Finally, by studying the evolution of band-gap, band-alignments and optical absorption as a function of strain we are able to predict that biaxially compressed C3N5 with strain in the range 12-14% can be a promising photocatalyst in overall water-splitting with an excellent optical absorption in the visible light spectrum.

Electronic Properties and Lattice Dynamics of LixCoO2 and NaxCoO2 (x = 0, 0.5, 1) Studied by Hybrid Density Functional Theory

A systematic hybrid density functional theory study on the electronic and vibrational properties of MxCoO2 compounds with M = Li, Na and is reported. The used DFT‐PBE0 method describes the structural parameters of the studied compounds well in comparison to experimental data. All studied magnetic species are treated as ferromagnets and the Co(IV) atoms possess a magnetic moment of 1.2 μB. At 0 K, favors a monoclinic structure very close to trigonal symmetry and behaves as a Mott insulator. The electronic bandgap increases as x increases from 0 to 1. The simulated infrared and Raman spectra together with full phonon dispersion relations show that the intercalation of Li and Na ions affects the lattice dynamics of CoO2 in a different way.

Hybrid density functional theory study on zinc blende GaN and diamond surfaces and interfaces: Effects of size, hydrogen passivation, and dipole corrections

GaN-diamond heterojunctions show promise in high electron mobility transistor applications. This prompts the need for accurate models of structural and electronic properties. We detail here a hybrid density functional theory study of zinc blende (ZB) GaN and diamond heterostructures using slab supercell models. The dependence of electronic properties on supercell size, pseudo-hydrogen passivation, and dipole corrections is detailed. The larger bulk modulus of diamond is shown to provide a templating structure for GaN to grow upon, where the large lattice mismatch is accounted for through the inclusion of a cationic Ga adlayer. Looking at both type I and II surfaces and interfaces of GaN shows the instability of zb GaN without an adlayer (type II), where increased size, pseudo-hydrogen passivation and dipole corrections do not remove the spurious interaction between the top and bottom layers in type II GaN. Layer dependent density of states, local potential differences, and charge density differences show that the type I interface (with a Ga adlayer) is stable with an adhesion energy of 0.704 eV/Å2 (4.346 J/m2); interestingly, the diamond charge density intercalates into the first layer of GaN, which was seen experimentally for wurtzite GaN grown over diamond. The type II interface is shown to be unstable which implies that, to form a stable, thin-film zb interface between GaN and diamond, the partial pressure of trimethylgallium must be controlled to ensure a Ga layer exists both on the top and bottom layer of the GaN thin film atop the diamond. We believe our results lead to a better understanding of the GaN/diamond multifaceted interface present in the GaN overgrowth on diamond heterojunction used in electronic device applications.

Hybrid density functional study on band structure engineering of ZnS(110) surface by anion–cation codoping for overall water splitting

The (Ru + C)-codoped ZnS(110) surface is predicted to be a potential candidate for solar-driven water splitting.

G-C3N4/TiO2-B{100} heterostructures used as promising photocatalysts for water splitting from a hybrid density functional study.

Fabrication of heterostructures has been shown to be a good strategy to improve photocatalytic performance. By using first-principles calculation based on hybrid density functionals, the photocatalytic mechanism of g-C3N4/TiO2-B{100} heterostructures is investigated to understand the process of water decomposition. We find that the reduction of the band gap of g-C3N4/TiO2-B{100} heterostructures enhances the visible light response range. g-C3N4/TiO2-B{100} heterostructures have direct band gaps, staggered band alignment, electron flow from g-C3N4 to TiO2-B{100} surfaces and straddling water decomposition potential, and are potential Z-scheme photocatalysts. Photoinduced carriers can be effectively separated using the Z-scheme photocatalytic mechanism. Our results demonstrate that g-C3N4/TiO2-B{100} heterostructures can enhance light absorption, prolong the life of photoinduced carriers, and further improve the photocatalytic activity. We believe that our findings can provide a reference for explaining the enhancement mechanism of the g-C3N4/TiO2 photocatalyst as observed in the experiment.

Computational study of group III-V semiconductors and their interaction with oxide thin films

We present a hybrid density-functional study of indium phosphide coated with titania subnanometric films. We modelled the InP phase with a 2.4 nm thick (110) slab, and studied the junctions formed with thin films of (101) and (001) anatase TiO2. In both cases, the interface is stabilized by In-O and Ti-P chemical bonds. By analysing the band edges alignment, one can see that in both cases a staggered type-II heterojunction is formed, with important implications for photocatalysis.

Defects and Strain Engineering of Structural, Elastic, and Electronic Properties of Boron-Phosphide Monolayer: A Hybrid Density Functional Theory Study.

Defects and in-plane strain have significant effects on the electronic properties of two-dimensional nanostructures. However, due to the influence of substrate and environmental conditions, defects and strain are inevitable during the growth or processing. In this study, hybrid density functional theory was employed to systematically investigate the electronic properties of boron-phosphide monolayers tuned by the in-plane biaxial strain and defects. Four types of defects were considered: B-vacancy (B_v), P-vacancy (P_v), double vacancy (D_v), and Stone–Wales (S-W). Charge density difference and Bader charge analysis were performed to characterize the structural properties of defective monolayers. All of these defects could result in the boron-phosphide monolayer being much softer with anisotropic in-plane Young’s modulus, which is different from the isotropic modulus of the pure layer. The calculated electronic structures show that the P_v, D_v, and S-W defective monolayers are indirect band gap semiconductors, while the B_v defective system is metallic, which is different from the direct band gap of the pure boron-phosphide monolayer. In addition, the in-plane biaxial strain can monotonically tune the band gap of the boron-phosphide monolayer. The band gap increases with the increasing tension strain, while it decreases as the compression strain increases. Our results suggest that the defects and in-plane strain are effective for tuning the electronic properties of the boron-phosphide monolayer, which could motivate further studies to exploit the promising application in electronics and optoelectronics based on the boron-phosphide monolayer.

Effect of in content on thermoelectric performance of InxGa1−xN alloys: Hybrid density functional theory study based on realistic models

In this study, first-principles periodic calculations with localized Gaussian type orbitals within a hybrid density functional theory approach were performed to study the electronic, structural, thermodynamic, and transport properties of InxGa1−xN nitride alloys. The compositions were modeled in the range of 0.06<x<0.87 using a 32-atom supercell. According to previous studies, significant composition-dependent gap bowing occurs for both the lattice parameters and energy band gap. Our study of the thermodynamic stability indicated a notable trend toward spinodal decomposition. Infrared spectra are used as fingerprints to uniquely determine the composition and structure of alloys. Based on the transport properties calculated in the present study (Seebeck coefficient, power factor, and figure of merit), the ternary structure with ≈ 20% In is the optimal thermoelectric material and preliminary investigations confirmed that the transport properties can be altered by point defects.

Hybrid Density Functional Theory Study of Native Defects and Nonmetal (C, N, S, and P) Doping in a Bi2WO6 Photocatalyst.

Native defects and nonmetal doping have been shown to be an effective way to optimize the photocatalytic properties of Bi2WO6. However, a detailed understanding of defect physics in Bi2WO6 has been lacking. Here, using the Heyd–Scuseria–Ernzerhof hybrid functional defect calculations, we study the formation energies, electronic structures, and optical properties of native defects and nonmetal element (C, N, S, and P) doping into Bi2WO6. We find that the Bi vacancy (Bivac), O vacancy (Ovac), S doping on the O site (SO), and N doping on the O site (NO) defects in the Bi2WO6 can be stable depending on the Fermi level and chemical potentials. By contrast, the substitution of an O atom by a C or P atom (CO, PO) has high formation energy and is unlikely to form. The calculated electronic structures of the Bivac, Ovac, SO, and NO defects indicate that the band-gap reduction of Ovac2+, Bivac3–, and SO defects is mainly due to forming shallow impurity levels within the band gap. The calculated absorption coefficients of Ovac2+, Bivac3–, and SO show strong absorption in the visible light region, which is in good agreement with the experimental results. Hence, Ovac2+, Bivac3–, and SO defects can improve the adsorption capacity of Bi2WO6, which helps enhance its photocatalytic performance. Our results provide insights into how to enhance the photocatalytic activity of Bi2WO6 for energy and environmental applications through the rational design of defect-controlled synthesis conditions.

The Origin of the Chiral gauche-Conformation Preferences in Halomethyl Methyl Ethers. A Hybrid-Density Functional Study and Natural Bond Orbital Interpretation

The effects of the stereoelectronic interaction on the structural and conformational properties of fluoromethyl methyl ether (1) and its analogues containing Cl (2), Br (3), and I (4) atoms were investigated by means of hybrid-density functional based method (B3LYP/Def2-TZVPP) and natural bond orbital (NBO) interpretation. Consistent with the reported experimental data, the chiral gauche conformations of compounds 1–4 are more stable than their corresponding anti conformations. The results showed that the energy barriers of the racemization processes of the chiral gauche-conformations of compounds 1–4 via the plane symmetrical anti conformations are smaller than the plane symmetrical eclipsed transition structures. According to the NBO analysis, the hyperconjugative generalized anomeric effect (HCGAE) increased from compound 1 to compound 4, which explains the increase of the energy discrepancy between their corresponding chiral gauche- and plane symmetrical anti-conformations. In contrast to the trend for the HCGAE, the determined dipole moment discrepancy between the chiral gauche- and plane symmetrical anti-conformations (µanti–µgauche) reduced from compound 1 to compound 4. This fact reveals that there is no correlation between the variations of the conformational preferences in compounds 1–4 and the electrostatic effects associated with the dipole–dipole interactions. There is an excellent agreement between the racemization energy barriers and the proportion between the GAE for compounds 2–4 and compound 1 (i.e., GAEx/GAE1) as well as the difference between the Wiberg Bond Indexes of C1–O2 bonds (Δ[WBIanti-gauche(C1–O2)]). The associations between the HCGAE, pairwise steric exchange energies (PSEE), WBI, dipole moments, orbital integrals, structural parameters, and conformational behaviors of compounds 1–4 have been studied.

Native point defects and low p-doping efficiency in Mg2(Si,Sn) solid solutions: A hybrid-density functional study

We perform hybrid-density functional calculations to investigate the charged defect formation energy of native point defects in Mg2Si, Mg2Sn, and their solid solutions. The band gap correction by hybrid-density functional is found to be critical to determine the charged defect density in these materials. For Mg2Si, Mg interstitials are dominant and provide unintentional n-type conductivity. Additionally, as the Mg vacancies can dominate in Mg-poor Mg2Sn, p-type conductivity is possible for Mg2Sn. However, the existence of low formation energy defects such as MgSn1+ and IMg2+ in Mg2Sn and their diffusion can cause severe charge compensation of hole carriers resulting in low p-type doping efficiency and thermal degradation. Our results indicate that, in addition to the extrinsic doping strategy, alloying of Mg2Sn with Mg2Si under Mg-poor conditions would be necessary to enhance the p-type conductivity with less charge compensation.