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Showing papers on "Electronic structure published in 2014"


Journal ArticleDOI
Jingsi Qiao1, Xianghua Kong1, Zhixin Hu1, Feng Yang1, Wei Ji1 
TL;DR: A detailed theoretical investigation of the atomic and electronic structure of few-layer black phosphorus (BP) is presented to predict its electrical and optical properties, finding that the mobilities are hole-dominated, rather high and highly anisotropic.
Abstract: Two-dimensional crystals are emerging materials for nanoelectronics. Development of the field requires candidate systems with both a high carrier mobility and, in contrast to graphene, a sufficiently large electronic bandgap. Here we present a detailed theoretical investigation of the atomic and electronic structure of few-layer black phosphorus (BP) to predict its electrical and optical properties. This system has a direct bandgap, tunable from 1.51 eV for a monolayer to 0.59 eV for a five-layer sample. We predict that the mobilities are hole-dominated, rather high and highly anisotropic. The monolayer is exceptional in having an extremely high hole mobility (of order 10,000 cm(2) V(-1) s(-1)) and anomalous elastic properties which reverse the anisotropy. Light absorption spectra indicate linear dichroism between perpendicular in-plane directions, which allows optical determination of the crystalline orientation and optical activation of the anisotropic transport properties. These results make few-layer BP a promising candidate for future electronics.

3,622 citations


Journal ArticleDOI
TL;DR: It is found that carrier energy distributions are sensitive to the electronic band structure of the metal: gold and copper produce holes hotter than electrons by 1–2 eV, while silver and aluminium distribute energies more equitably between electrons and holes.
Abstract: Decay of surface plasmons to hot carriers finds a wide variety of applications in energy conversion, photocatalysis and photodetection. However, a detailed theoretical description of plasmonic hot-carrier generation in real materials has remained incomplete. Here we report predictions for the prompt distributions of excited ‘hot’ electrons and holes generated by plasmon decay, before inelastic relaxation, using a quantized plasmon model with detailed electronic structure. We find that carrier energy distributions are sensitive to the electronic band structure of the metal: ​gold and ​copper produce holes hotter than electrons by 1–2 eV, while ​silver and ​aluminium distribute energies more equitably between electrons and holes. Momentum-direction distributions for hot carriers are anisotropic, dominated by the plasmon polarization for ​aluminium and by the crystal orientation for noble metals. We show that in thin metallic films intraband transitions can alter the carrier distributions, producing hotter electrons in ​gold, but interband transitions remain dominant.

631 citations


Journal ArticleDOI
TL;DR: In this paper, a model describing the molecular orientation disorder in CH3NH3PbI3, solving a classical Hamiltonian parametrised with electronic structure calculations, with the nature of the motions informed by ab initio molecular dynamics.
Abstract: We report a model describing the molecular orientation disorder in CH3NH3PbI3, solving a classical Hamiltonian parametrised with electronic structure calculations, with the nature of the motions informed by ab initio molecular dynamics. We investigate the temperature and static electric field dependence of the equilibrium ferroelectric (molecular) domain structure and resulting polarisability. A rich domain structure of twinned molecular dipoles is observed, strongly varying as a function of temperature and applied electric field. We propose that the internal electrical fields associated with microscopic polarisation domains contribute to hysteretic anomalies in the current-voltage response of hybrid organic-inorganic perovskite solar cells due to variations in electron-hole recombination in the bulk.

512 citations


Journal ArticleDOI
TL;DR: In this paper, a band unfolding technique was used to recover an effective primitive cell picture of the band structure of graphene under the influence of different types of perturbations, which involves intrinsic p...
Abstract: We use a band unfolding technique to recover an effective primitive cell picture of the band structure of graphene under the influence of different types of perturbations. This involves intrinsic p ...

434 citations


Journal ArticleDOI
TL;DR: The electronic structure and chemical composition of efficient CH3NH3PbI3 perovskite solar cell materials deposited onto mesoporous TiO2 were studied using photoelectron spectroscopy with hard X-rays and results indicating similar electronic structures are shown.
Abstract: The electronic structure and chemical composition of efficient CH3NH3PbI3 perovskite solar cell materials deposited onto mesoporous TiO2 were studied using photoelectron spectroscopy with hard X-rays. With this technique, it is possible to directly measure the occupied energy levels of the perovskite as well as the TiO2 buried beneath and thereby determine the energy level matching of the interface. The measurements of the valence levels were in good agreement with simulated density of states, and the investigation gives information on the character of the valence levels. We also show that two different deposition techniques give results indicating similar electronic structures.

428 citations


Journal ArticleDOI
04 Dec 2014-Nature
TL;DR: The crystal structure of this exotic non-equilibrium state of YBa2Cu3O6+x is reported, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations, and the enhancement in the character of the in-plane electronic structure is likely to favour superconductivity.
Abstract: Femtosecond X-ray diffraction and ab initio density functional theory calculations are used to determine the crystal structure of YBa2Cu3O6.5 undergoing optically driven, nonlinear lattice excitation above the transition temperature of 52 kelvin, under which conditions the electronic structure of the material changes in such a way as to favour superconductivity. Andrea Cavalleri and colleagues use femtosecond X-ray diffraction measurements and ab initio density functional theory calculations to determine the crystal structure of YBa2Cu3O6+x undergoing optically driven, nonlinear lattice excitation at 100 kelvin. In this exotic non-equilibrium state, the electronic structure of the material changes in such a way as to favour superconductivity. The results reveal that in the driven state the superconducting planes are displaced closer and away from one another in a staggered manner, explaining how superconducting coupling can be enhanced or reduced, inside and between the bilayers. Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures1,2,3. In complex oxides, this method has been used to melt electronic order4,5,6, drive insulator-to-metal transitions7 and induce superconductivity8. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in YBa2Cu3O6+x (refs 9, 10). Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations. We find that nonlinear lattice excitation in normal-state YBa2Cu3O6+x at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu–O2 intra-bilayer and, respectively, inter-bilayer distances, accompanied by anisotropic changes in the in-plane O–Cu–O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the character of the in-plane electronic structure is likely to favour superconductivity.

426 citations


Journal ArticleDOI
15 Aug 2014-Science
TL;DR: Frequency, multidimensional magnetic resonance spectroscopy reveals that all four manganese ions of the catalyst are structurally and electronically similar immediately before the final oxygen evolution step; they all exhibit a 4+ formal oxidation state and octahedral local geometry.
Abstract: The photosynthetic protein complex photosystem II oxidizes water to molecular oxygen at an embedded tetramanganese-calcium cluster. Resolving the geometric and electronic structure of this cluster in its highest metastable catalytic state (designated S3) is a prerequisite for understanding the mechanism of O-O bond formation. Here, multifrequency, multidimensional magnetic resonance spectroscopy reveals that all four manganese ions of the catalyst are structurally and electronically similar immediately before the final oxygen evolution step; they all exhibit a 4+ formal oxidation state and octahedral local geometry. Only one structural model derived from quantum chemical modeling is consistent with all magnetic resonance data; its formation requires the binding of an additional water molecule. O-O bond formation would then proceed by the coupling of two proximal manganese-bound oxygens in the transition state of the cofactor.

399 citations


Journal ArticleDOI
TL;DR: The similar responses of single centers and a SiV ensemble in a low strain reference sample prove the ability to fabricate almost perfect single SiVs, revealing the true nature of the defect's electronic properties.
Abstract: The negatively charged silicon vacancy (SiV) color center in diamond has recently proven its suitability for bright and stable single photon emission. However, its electronic structure so far has remained elusive. We here explore the electronic structure by exposing single SiV defects to a magnetic field where the Zeeman effect lifts the degeneracy of magnetic sublevels. The similar responses of single centers and a SiV ensemble in a low strain reference sample prove our ability to fabricate almost perfect single SiVs, revealing the true nature of the defect's electronic properties. We model the electronic states using a group-theoretical approach yielding a good agreement with the experimental observations. Furthermore, the model correctly predicts polarization measurements on single SiV centers and explains recently discovered spin selective excitation of SiV defects.

376 citations


Journal ArticleDOI
TL;DR: In this article, the effect of the $d$-band shape on the local electronic structure of adsorbates was investigated, e.g., energy and filling of adorbate-metal antibonding states.
Abstract: The $d$-band shape of a metal site, governed by the local geometry and composition of materials, plays an important role in determining trends of the surface reactivity of transition-metal alloys. We discuss this phenomenon using the chemisorption of various adsorbates such as C, N, O, and their hydrogenated species on Pd bimetallic alloys as an example. For many alloys, the $d$-band center, even with consideration of the $d$-band width and $\mathit{sp}$ electrons, can not describe variations in reactivity from one surface to another. We investigate the effect of the $d$-band shape, represented by higher moments of the $d$ band, on the local electronic structure of adsorbates, e.g., energy and filling of adsorbate-metal antibonding states. The upper $d$-band edge ${\ensuremath{\varepsilon}}_{u}$, defined as the highest peak position of the Hilbert transform of the density of states projected onto $d$ orbitals of an active metal site, is identified as an electronic descriptor for the surface reactivity of transition metals and their alloys, regardless of variations in the $d$-band shape. The utilization of the upper $d$-band edge with scaling relations enables a considerable reduction of the parameter space in search of improved alloy catalysts and further extends our understanding of the relationship between the electronic structure and chemical reactivity of metal surfaces.

346 citations


Journal ArticleDOI
TL;DR: In this paper, a comprehensive approach to understand the electronic structure of monoclinic scheelite bismuth vanadate (ms-BiVO4), including both valence band (VB) and conduction band (CB) orbital character, is presented.
Abstract: A comprehensive approach to understanding the electronic structure of monoclinic scheelite bismuth vanadate (ms-BiVO4), including both valence band (VB) and conduction band (CB) orbital character, is presented Density functional theory (DFT) calculations are directly compared to experimental data obtained via X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, resonant inelastic X-ray spectroscopy (RIXS), and X-ray photoelectron spectroscopy to provide a complete portrait of the total and partial density of states (DOS) near the bandgap DFT calculations are presented to confirm the VB maximum and CB minimum are comprised primarily of O 2p and V 3d orbitals, respectively Predicted triplet d-manifold splitting of V 3d CB states, arising from lone pair-induced lattice distortions, is quantified by V L- and O K-edge XAS Furthermore, the partial contributions to the total DOS within both the CB and VB, determined by RIXS, are found to be in excellent agreement with DFT calculations Energy le

336 citations


Journal ArticleDOI
01 Dec 2014-Carbon
TL;DR: In this article, 1-, 2-and 4-layer graphitic carbon nitride (g-C3N4) nanosheets were synthesized in a well-crystallized form by controlling the intercalation time in a simple inter-calation-exfoliation process.

Journal ArticleDOI
TL;DR: In this paper, the authors report on ab initio electronic structure and Car-Parrinello molecular dynamics simulations for several structural models of the prototype MAPbI3 perovskite for solar cells applications.
Abstract: We report on ab initio electronic structure and Car–Parrinello molecular dynamics simulations for several structural models of the prototype MAPbI3 perovskite for solar cells applications. We considered both configurations having a preferred orientation of the MA cations, giving rise to a net dipole alignment, and configurations with an isotropic distribution of the MA cations, respectively representative of polar (ferroelectric) and apolar (antiferroelectric) structures. Our calculations demonstrate the preferred stability of a set of polar structures over apolar ones, with an energy difference within 0.1 eV and a conversion barrier within 0.2 eV per unit cell (four MAPbI3), thus possibly accessible at room temperature. Ferroelectric-like orientations lead to a quasi I4cm structure for the inorganic component, characterized by lack of inversion symmetry, while the antiferroelectric-like orientations are associated to a quasi I4/mcm structure. Ab initio molecular dynamics simulations on the polar structur...

Journal ArticleDOI
TL;DR: Electronic structure calculations show the interlayer separation changes with twist due to repulsion between sulfur atoms, resulting in shifts of the indirect optical transition energies.
Abstract: Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacking single-crystal monolayers. Varying interlayer twist angle results in strong tuning of the indirect optical transition energy and second-harmonic generation and weak tuning of direct optical transition energies and Raman mode frequencies. Electronic structure calculations show the interlayer separation changes with twist due to repulsion between sulfur atoms, resulting in shifts of the indirect optical transition energies. These results show that interlayer alignment is a crucial variable in tailoring the properties of two-dimensional heterostructures.

Journal ArticleDOI
TL;DR: This work examines possible defect structures and their impact on the MoS2 monolayer electronic properties, using density functional theory in combination with scanning tunneling microscopy to identify the nature of the most likely defects.
Abstract: Monolayer MoS2 is a direct band gap semiconductor which has been recently investigated for low-power field effect transistors. The initial studies have shown promising performance, including a high on/off current ratio and carrier mobility with a high-κ gate dielectric. However, the performance of these devices strongly depends on the crystalline quality and defect morphology of the monolayers. In order to obtain a detailed understanding of the MoS2 electronic device properties, we examine possible defect structures and their impact on the MoS2 monolayer electronic properties, using density functional theory in combination with scanning tunneling microscopy to identify the nature of the most likely defects. Quantitative understanding based on a detailed knowledge of the atomic and electronic structures will facilitate the search of suitable defect passivation techniques. Our results show that S adatoms are the most energetically favorable type of defect and that S vacancies are energetically more favorable than Mo vacancies. This approach may be extended to other transition-metal dichalcogenides (TMDs), thus providing useful insights to optimize TMD-based electronic devices.

Journal ArticleDOI
TL;DR: The first ever quantum calculation of the electronic levels of [2Fe-2S] and [4Fe-4S] clusters free from any model assumptions is presented, demonstrating that the widely used Heisenberg double exchange model underestimates the number of states by one to two orders of magnitude.
Abstract: Iron–sulfur clusters are a universal biological motif. They carry out electron transfer, redox chemistry and even ​oxygen sensing, in diverse processes including ​nitrogen fixation, respiration and photosynthesis. Their low-lying electronic states are key to their remarkable reactivity, but they cannot be directly observed. Here, we present the first ever quantum calculation of the electronic levels of [2Fe–2S] and [4Fe–4S] clusters free from any model assumptions. Our results highlight the limitations of long-standing models of their electronic structure. In particular, we demonstrate that the widely used Heisenberg double exchange model underestimates the number of states by one to two orders of magnitude, which can conclusively be traced to the absence of Fe d -> d excitations, thought to be important in these clusters. Furthermore, the electronic energy levels of even the same spin are dense on the scale of vibrational fluctuations and this provides a natural explanation for the ubiquity of these clusters in catalysis in nature.

Journal ArticleDOI
TL;DR: Absorption and photoluminescence investigations show that inter-band crossings are responsible for the observed multiple emission peaks and the incorporation of ∼1 at% of S into the quantum dots can effectively modify the electronic structure of the S-GQDs by introducing S-related energy levels between π and ρ of C.
Abstract: Sulphur-doped carbon-based materials have attracted a great deal of interest because of their important applications in the fields of oxygen reduction reactions, hydrogen storage, supercapacitors, photocatalysts and lithium ion batteries. Here, we report a new member of sulphur-doped carbon-based materials, i.e. sulphur doped graphene quantum dots (S-GQDs). The S-GQDs were prepared by a hydrothermal method using fructose and sulphuric acid as source materials. Absorption and photoluminescence investigations show that inter-band crossings are responsible for the observed multiple emission peaks. The incorporation of ∼1 at% of S into the quantum dots can effectively modify the electronic structure of the S-GQDs by introducing S-related energy levels between π and π* of C. The additional energy levels in the S-GQDs lead to efficient and multiple emission peaks.

Journal ArticleDOI
18 Dec 2014-Nature
TL;DR: It is shown that a correlated two-electron wave packet can be reconstructed from a 1.2-femtosecond quantum beat among low-lying doubly excited states in helium, and multidimensional spectroscopy experiments of the type reported here will provide benchmark data for testing fundamental few-body quantum dynamics theory in more complex systems.
Abstract: The concerted motion of two or more bound electrons governs atomic and molecular non-equilibrium processes including chemical reactions, and hence there is much interest in developing a detailed understanding of such electron dynamics in the quantum regime. However, there is no exact solution for the quantum three-body problem, and as a result even the minimal system of two active electrons and a nucleus is analytically intractable. This makes experimental measurements of the dynamics of two bound and correlated electrons, as found in the helium atom, an attractive prospect. However, although the motion of single active electrons and holes has been observed with attosecond time resolution, comparable experiments on two-electron motion have so far remained out of reach. Here we show that a correlated two-electron wave packet can be reconstructed from a 1.2-femtosecond quantum beat among low-lying doubly excited states in helium. The beat appears in attosecond transient-absorption spectra measured with unprecedentedly high spectral resolution and in the presence of an intensity-tunable visible laser field. We tune the coupling between the two low-lying quantum states by adjusting the visible laser intensity, and use the Fano resonance as a phase-sensitive quantum interferometer to achieve coherent control of the two correlated electrons. Given the excellent agreement with large-scale quantum-mechanical calculations for the helium atom, we anticipate that multidimensional spectroscopy experiments of the type we report here will provide benchmark data for testing fundamental few-body quantum dynamics theory in more complex systems. They might also provide a route to the site-specific measurement and control of metastable electronic transition states that are at the heart of fundamental chemical reactions.

Journal ArticleDOI
TL;DR: It is shown that the resonant coupling of photogenerated singlet excitons to a high-energy manifold of fullerene electronic states that enables efficient charge generation, bypassing localized charge-transfer states, and suggest that fullerenes cluster size, concentration, and dimensionality control charge generation efficiency, independent of exciton delocalization.
Abstract: Natural photosynthetic complexes accomplish the rapid conversion of photoexcitations into spatially separated electrons and holes through precise hierarchical ordering of chromophores and redox centers. In contrast, organic photovoltaic (OPV) cells are poorly ordered, utilize only two different chemical potentials, and the same materials that absorb light must also transport charge; yet, some OPV blends achieve near-perfect quantum efficiency. Here we perform electronic structure calculations on large clusters of functionalized fullerenes of different size and ordering, predicting several features of the charge generation process, outside the framework of conventional theories but clearly observed in ultrafast electro-optical experiments described herein. We show that it is the resonant coupling of photogenerated singlet excitons to a high-energy manifold of fullerene electronic states that enables efficient charge generation, bypassing localized charge-transfer states. In contrast to conventional views, ...

Journal ArticleDOI
TL;DR: The electronic structure and optical property calculations on a surface-functionalized hexagonal boron-nitride bilayer confirm the existence of such photocatalysts and verify the reaction mechanism.
Abstract: Highly efficient solar energy utilization is very desirable in photocatalytic water splitting. However, until now, the infrared part of the solar spectrum, which constitutes almost half of the solar energy, has not been used, resulting in significant loss in the efficiency of solar energy utilization. Here, we propose a new mechanism for water splitting in which near-infrared light can be used to produce hydrogen. This ability is a result of the unique electronic structure of the photocatalyst, in which the valence band and conduction band are distributed on two opposite surfaces with a large electrostatic potential difference produced by the intrinsic dipole of the photocatalyst. This surface potential difference, acting as an auxiliary booster for photoexcited electrons, can effectively reduce the photocatalyst's band gap required for water splitting in the infrared region. Our electronic structure and optical property calculations on a surface-functionalized hexagonal boron-nitride bilayer confirm the existence of such photocatalysts and verify the reaction mechanism.

Journal ArticleDOI
TL;DR: The X-ray structure of a charge-neutral Au24(SCH2Ph-(t)Bu)20 nanocluster is reported, which features a bi-tetrahedral Au8 kernel protected by four tetrameric staple-like motifs.
Abstract: Solving the total structures of gold nanoclusters is of critical importance for understanding their electronic, optical and catalytic properties. Herein, we report the X-ray structure of a charge-neutral Au24(SCH2Ph-tBu)20 nanocluster. This structure features a bi-tetrahedral Au8 kernel protected by four tetrameric staple-like motifs. Electronic structure analysis is further carried out and the optical absorption spectrum is interpreted. The Au24(SCH2Ph-tBu)20, Au23(S-c-C6H11)16 and Au25(SCH2CH2Ph)18 nanoclusters constitute the first crystallographically characterized “trio”.

Journal ArticleDOI
TL;DR: In this paper, the structural, electronic, and magnetic properties of 3$d$ transition metal (TM) atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) are addressed using density functional theory.
Abstract: The structural, electronic, and magnetic properties of 3$d$ transition metal (TM) atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed germanene are addressed using density functional theory. Based on the adsorption energy, TM atoms prefer to occupy at the hollow site for all the cases. The obtained values of the total magnetic moment vary from 0.97 $\mu_B$ to 4.95 $\mu_B$ in case of Sc to Mn-adsorption, respectively. A gap of 74 meV with a strongly enhanced splitting of 67 meV is obtained in case of Sc-adsorption, whereas metallic states are obtained in case of Ti, Cr, Mn, Fe, and Co. Non-magnetic states are realized for Ni, Cu, and Zn-adsorption. Moreover, semiconducting nature is obtained for non-magnetic cases with a gap of 26 to 28 meV. Importantly, it is found that V-adsorbed germanene can host the quantum anomalous Hall effect. The obtained results demonstrate that TM atoms and nearest neighbour Ge atoms are ferro-magnetically ordered in the cases of V, Mn, Fe, Co, Ni, Cu, and Zn, while anti-ferromagnetic ordering is obtained for Sc, Ti, and Cr. In addition, the effects of the coverage of all TM atoms on the electronic structure and the ferro-magnetic and anti-ferro-magnetic coupling in case of Mn are examined. The results could help to understand the effect of TM atoms in a new class of two-dimensional materials beyond graphene and silicene.

Journal ArticleDOI
TL;DR: In this paper, the density matrix renormalization group (DMRG) theory and its symmetrization scheme for quantum chemistry applied to calculate the excited states structure was used to understand the nature of the low-lying excited state structure.

Journal ArticleDOI
TL;DR: In this article, the vibronic structure of the spin-triplet optical transition in diamond nitrogen-vacancy (NV) centers is described using accurate first-principles methods based on hybrid functionals.
Abstract: In this work we present theoretical calculations and analysis of the vibronic structure of the spin-triplet optical transition in diamond nitrogen-vacancy (NV) centres. The electronic structure of the defect is described using accurate first-principles methods based on hybrid functionals. We devise a computational methodology to determine the coupling between electrons and phonons during an optical transition in the dilute limit. As a result, our approach yields a smooth spectral function of electron–phonon coupling and includes both quasi-localized and bulk phonons on equal footings. The luminescence lineshape is determined via the generating function approach. We obtain a highly accurate description of the luminescence band, including all key parameters such as the Huang–Rhys factor, the Debye–Waller factor, and the frequency of the dominant phonon mode. More importantly, our work provides insight into the vibrational structure of NV centres, in particular the role of local modes and vibrational resonances. In particular, we find that the pronounced mode at 65 meV is a vibrational resonance, and we quantify localization properties of this mode. These excellent results for the benchmark diamond (NV) centre provide confidence that the procedure can be applied to other defects, including alternative systems that are being considered for applications in quantum information processing.

Journal ArticleDOI
TL;DR: Under O-rich growth condition, the preparation of Co-, Cr-, and Ni-doped TiO2 becomes relatively easy in the experiment due to their negative impurity formation energies, which suggests that these doping systems are easy to obtain and with good stability.
Abstract: The electronic structures, formation energies, and band edge positions of anatase TiO2 doped with transition metals have been analyzed by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The model structures of transition metal-doped TiO2 were constructed by using the 24-atom 2 × 1 × 1 supercell of anatase TiO2 with one Ti atom replaced by a transition metal atom. The results indicate that most transition metal doping can narrow the band gap of TiO2, lead to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. Under O-rich growth condition, the preparation of Co-, Cr-, and Ni-doped TiO2 becomes relatively easy in the experiment due to their negative impurity formation energies, which suggests that these doping systems are easy to obtain and with good stability. The theoretical calculations could provide meaningful guides to develop more active photocatalysts with visible light response.

Journal ArticleDOI
TL;DR: In this article, the relative stability of nine TiO2 polymorphs under different pressures is investigated, including the dispersion correction for van der Waals interaction and the Hubbard U term for the Ti 3d orbital in the DFT-GGA calculation.
Abstract: Enthalpies of nine TiO2 polymorphs under different pressures are presented to study the relative stability of the TiO2 polymorphs. It is important to include the dispersion correction for van der Waals interaction and the Hubbard U term for the Ti 3d orbital in the DFT-GGA calculation to correctly reproduce the relative stability of rutile, brookite, and anatase. Band structures for the TiO2 polymorphs are calculated by density functional theory with generalized gradient approximation, and the band energies at high symmetry k-points are corrected using the GW method to accurately determine the band gap. The differences between direct band gap energies and indirect band gap energies are very small for rutile and columbite-structured TiO2, indicating a quasi-direct band gap character. For optical response calculations, two-particle effects have been included by solving the Bethe–Salpeter equation for Coulomb correlated electron–hole pairs. TiO2 with baddeleyite, pyrite, and fluorite structures has optical t...

Journal ArticleDOI
TL;DR: Electronic structure calculations performed using the experimentally determined centrosymmetric structure are similar to those performed with the inversion symmetry absent but with the important implication that Cd3As2 is a three-dimensional (3D)-Dirac semimetal with no spin splitting; all bands are spin degenerate and there is a 4-fold degenerate bulk Dirac point at the Fermi energy along Γ-Z in the Brillouin zone.
Abstract: The structure of Cd3As2, a high-mobility semimetal reported to host electrons that act as Dirac particles, is reinvestigated by single-crystal X-ray diffraction. It is found to be centrosymmetric rather than noncentrosymmetric as previously reported. It has a distorted superstructure of the antifluorite (M2X) structure type with a tetragonal unit cell of a = 12.633(3) and c = 25.427(7) A in the centrosymmetric I41/acd space group. The antifluorite superstructure can be envisioned as consisting of distorted Cd6□2 cubes (where □ = an empty cube vertex) in parallel columns, stacked with opposing chirality. Electronic structure calculations performed using the experimentally determined centrosymmetric structure are similar to those performed with the inversion symmetry absent but with the important implication that Cd3As2 is a three-dimensional (3D)-Dirac semimetal with no spin splitting; all bands are spin degenerate and there is a 4-fold degenerate bulk Dirac point at the Fermi energy along Γ–Z in the Brill...

Journal ArticleDOI
TL;DR: In this article, structural, mechanical and electronic properties of Laves phases Al2Zr and Al2Hf with C14-type structure were investigated by performing the first-principle calculations.

Journal ArticleDOI
TL;DR: In this article, a method combining density functional theory based molecular dynamics (DFTMD) and free energy perturbation (FEP) methods is presented for reversible insertion of a proton or electron in a periodic DFTMD model system.
Abstract: ConspectusAll-atom methods treat solute and solvent at the same level of electronic structure theory and statistical mechanics. All-atom computation of acidity constants (pKa) and redox potentials is still a challenge. In this Account, we review such a method combining density functional theory based molecular dynamics (DFTMD) and free energy perturbation (FEP) methods. The key computational tool is a FEP based method for reversible insertion of a proton or electron in a periodic DFTMD model system. The free energy of insertion (work function) is computed by thermodynamic integration of vertical energy gaps obtained from total energy differences. The problem of the loss of a physical reference for ionization energies under periodic boundary conditions is solved by comparing with the proton work function computed for the same supercell. The scheme acts as a computational hydrogen electrode, and the DFTMD redox energies can be directly compared with experimental redox potentials.Consistent with the closed s...

Journal ArticleDOI
TL;DR: In this article, van der Waals density functional theory calculations (DFT-D) was used to investigate the adsorption of eight organic molecules (acetone, acetonitrile, ammonia, benzene, methane, methanol, ethanol, and toluene) onto silicene.
Abstract: Adsorption of eight organic molecules (acetone, acetonitrile, ammonia, benzene, methane, methanol, ethanol, and toluene) onto silicene has been investigated using van der Waals density functional theory calculations (DFT-D). The calculated values of the adsorption energies vary from −0.11 to −0.95 eV. Quantitatively, these values are higher than the corresponding adsorption energies of the molecules adsorbed on graphene. In addition, electronic structure calculations have been performed. The obtained values of the band gap range from 0.006 to 0.35 eV for acetonitrile to acetone, respectively. Furthermore, the effective mass of the electron is estimated and found to be comparatively small, which is expected to result in high electron mobility. In addition, we study the effect of Li atoms doped in pristine and acetone adsorbed silicene. In particular, we focus on the variation of the adsorption energy with respect to the number of Li atoms in the systems. Our results suggest new approaches for the use of si...

Journal ArticleDOI
TL;DR: In this paper, the vibronic structure of the spin-triplet optical transition in diamond nitrogen-vacancy centres is described using accurate first-principles methods based on hybrid functionals.
Abstract: In this work we present theoretical calculations and analysis of the vibronic structure of the spin-triplet optical transition in diamond nitrogen-vacancy centres. The electronic structure of the defect is described using accurate first-principles methods based on hybrid functionals. We devise a computational methodology to determine the coupling between electrons and phonons during an optical transition in the dilute limit. As a result, our approach yields a smooth spectral function of electron-phonon coupling and includes both quasi-localized and bulk phonons on equal footings. The luminescence lineshape is determined via the generating function approach. We obtain a highly accurate description of the luminescence band, including all key parameters such as the Huang-Rhys factor, the Debye-Waller factor, and the frequency of the dominant phonon mode. More importantly, our work provides insight into the vibrational structure of nitrogen vacancy centres, in particular the role of local modes and vibrational resonances. In particular, we find that the pronounced mode at 65 meV is a vibrational resonance, and we quantify localization properties of this mode. These excellent results for the benchmark diamond nitrogen-vacancy centre provide confidence that the procedure can be applied to other defects, including alternative systems that are being considered for applications in quantum information processing.