Publications

GW for transition metal oxide perovskites

No abstract is provided for this article.

Efficient periodic density functional theory calculations of charged molecules and surfaces using Coulomb kernel truncation

Density functional theory (DFT) calculations of charged molecules and surfaces are critical to applications in electrocatalysis, energy materials, and related fields of materials science. DFT implementations such as the Vienna simulation package () compute the electrostatic potential under three-dimensional (3D) periodic boundary conditions, necessitating charge neutrality. In this work, we implement 0D and 2D periodic boundary conditions to facilitate DFT calculations of charged molecules and surfaces, respectively. We implement these boundary conditions using the Coulomb kernel truncation method. Our implementation computes the potential under 0D and 2D boundary conditions by selectively subtracting unwanted long-range interactions in the potential computed under 3D boundary conditions. By combining the Coulomb kernel truncation method with a computationally efficient padding approach, we remove nonphysical potentials from vacuum in 0D and 2D systems. To illustrate the computational efficiency of our method, we perform large supercell calculations of the formation energy of a charged chlorine defect on a sodium chloride (001) surface and perform long-timescale molecular dynamics simulations on a stepped gold (211) water electrode-electrolyte interface.

Ab initio molecular dynamics applied to the dynamics of liquid metals and to the metal-non-metal transition

In recent years, ab initio molecular dynamics (MD) techniques have made a profound impact on the investigation of the structural, electronic and dynamic properties of liquid and amorphous materials. In this paper the authors' approach is described, which is based on a plane-wave basis set and an exact calculation of the electronic groundstate at each MD time step. The description of the electron-ion interaction by optimized ultrasoft pseudopotentials allows transition metals and first-row elements to be treated efficiently. The method has been applied to several systems, including liquid transition metals, amorphous semiconductors and simple metals, with great success. Here it is demonstrated that the technique is powerful enough to allow the investigation of dynamic properties of 1-Li and of the metal-non-metal transition in expanded Hg.

Phys. Rev. B

Ab-initio calculations of the 6D potential energy surfaces for the dissociative adsorption of H2 on the (100) surfaces of Rh, Pd and Ag

Detailed investigations of the six-dimensional potential energy surface (PES) for the dissociative adsorption of a hydrogen molecule on the (100) surface of Rh, Pd and Ag are presented. The calculations are based on local density functional theory with generalized gradient corrections to the exchange-correlation functional, and have been performed using the Vienna ab-initio simulation package vasp. vasp works in a plane-wave basis and uses ultrasoft pseudopotentials. We show that adsorption on Rh(100) and Pd(100) is in general non-activated, but barriers exist along certain reaction channels. The “adiabatic” minimum-energy channel has been determined by a five-dimensional minimization of the total energy at a fixed height of the molecule. The variation of the covalent hydrogen-metal bond along this channel is studied using crystal orbital overlap populations and the electron localization function. On Ag(100), H2 adsorption is strongly activated, with a pronounced variation of the barrier over the surface cell.

V2O3(0001) surface terminations: from oxygen- to vanadium-rich

Well-ordered epitaxial V2O3(0001) films (thickness>80 Å) have been prepared on the clean Rh(111) surface and investigated with scanning tunnelling microscopy, low-energy electron diffraction, high-resolution electron energy loss spectroscopy and high-resolution X-ray photoelectron spectroscopy with synchrotron radiation. Atomically flat V2O3(0001) surfaces, terminated by vanadyl (VO) groups, have been obtained after the reactive oxide deposition and subsequent annealing in vacuum to 600 °C. Annealing the VO termination in oxygen atmosphere (500 °C, 5×10−6 mbar) results in the partial removal of the vanadyl groups and in the formation of an oxygen-richer surface termination, exhibiting a (√3×√3)R30° structure. Structure models for this oxygen-rich termination are proposed, which are characterised by (OV)0.66–O3–V–V⋯ and (OV)0.33–O3–V–V⋯ stacking sequences. Conversely, deposition of submonolayer V coverages onto the VO surface leads to the formation of a metal-rich V2O3(0001) surface, terminated by a close-packed V3 layer on top of the bulk-type O3 plane. Such a VO(111)-layer is only metastable and oxidises back readily upon annealing in vacuum to the vanadyl-terminated V2O3(0001) surface.

Shape and Edge Sites Modifications of MoS2 Catalytic Nanoparticles Induced by Working Conditions: A Theoretical Study

Determination of the morphology and electronic properties of single-layer nanosize MoS2 particles is of considerable interest for a better understanding of the active phase of hydrotreating catalysts. We propose an original approach, based on density functional calculations applied to various types of MoS2 clusters containing up to 200 atoms, to evaluate accurately the surface energies of the Mo-edge and S-edge terminated surfaces. The results are expressed as a function of the chemical potential of sulfur, which is in turn controlled by the temperature and the ratio of partial pressures of H2S and H2. Gibbs–Curie–Wulff equilibrium morphologies reveal that a high chemical potential of sulfur leads to triangular-shaped particles terminated by the Mo-edge surface, giving an interpretation of the observations made recently by scanning tunneling microscopy (STM). Simulations of the STM images for the most stable clusters reveal a qualitatively good agreement with experiments. Moreover, our approach predicts that varying the potential of sulfur modifies the local Mo-edge structure and the shape of the nanoparticles. Finally, the thermodynamic diagram for a MoS2 particle with a realistic size shows that the creation of one sulfur vacancy at the corner and on the edge is possible under HDS working conditions.

Assessing Density Functionals Using Many Body Theory for Hybrid Perovskites

Which density functional is the ``best'' for structure simulations of a particular material? A concise, first principles, approach to answer this question is presented. The random phase approximation (RPA)---an accurate many body theory---is used to evaluate various density functionals. To demonstrate and verify the method, we apply it to the hybrid perovskite ${\mathrm{MAPbI}}_{3}$, a promising new solar cell material. The evaluation is done by first creating finite temperature ensembles for small supercells using RPA molecular dynamics, and then evaluating the variance between the RPA and various approximate density functionals for these ensembles. We find that, contrary to recent suggestions, van der Waals functionals do not improve the description of the material, whereas hybrid functionals and the strongly constrained appropriately normed (SCAN) density functional yield very good agreement with the RPA. Finally, our study shows that in the room temperature tetragonal phase of ${\mathrm{MAPbI}}_{3}$, the molecules are preferentially parallel to the shorter lattice vectors but reorientation on ps time scales is still possible.

Theoretical Analysis of Oxygen Diffusion in monoclinic HfO₂

In this report, we present the detailed analysis of the interstitial oxygen (O2+, O0, O2-) diffusion in monoclinic HfO2 (hafnia) using the first principles calculations. The interstitial oxygen atom kicks out the oxygen atom at the 3-fold-site and occupies the 3-fold-site. And then the newly kicked-out interstitial oxygen atom jumps to the nearest neighbor site and couples again with the atoms at the crystal sites. This kick-out- mechanism is valid for all charge states of the interstitial oxygen in monoclinic HfO2. In hafnia, the interstitial oxygen atom can take 3 charge states (+2, 0, -2) depending on the chemical potential (Ef), whereas the oxygen-vacancy in hafnia can get +2 or 0 charge state being dependent on Ef. In the lower range of Ef, O2+ and O0 might contribute. In the middle range of Ef, the O2- does not contribute to the diffusion process in hafnia because of the pair annihilation process between O2- and oxygen vacancy (V2+) defect pair. We can simulate such a pair annihilation process in hafnia. In the higher range, O2- might contribute the diffusion process.

<i>Ab initio</i>reflectance difference spectra of the bare and adsorbate covered Cu(110) surfaces

The reflectance difference spectra of the bare Cu(110) surface, of the oxygen induced $(2\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ reconstruction, and of the carbon monoxide covered Cu(110) surface are calculated using density functional theory and the independent particle approximation. Generally, good agreement with experiment is found at energies below $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, where a resonance between two surface states dominates the spectrum for the bare surface. The resonance is progressively quenched from the bare surface, over the oxygen covered surface, to the CO covered surface. At higher energies, a rather deep minimum in the reflectance difference spectrum is calculated for all three surfaces, which agrees qualitatively with experiment, but details such as the shape and depth of the minimum are not in good agreement with experiment. We argue that a treatment beyond standard density functional theory is required at higher energies, in particular, for adsorbate covered surfaces. The calculated spectra and the surface band structures are carefully analyzed with respect to transitions between specific electronic states, confirming previous experimental and theoretical assignments. An essential result of the study is that the anisotropy of the intraband plasma frequency tensor (Drude-like term) is important and must be calculated on the same level of theory as the interband contributions to the dielectric function. Only then the results converge with increasing layer numbers.

Defect energetics in ZnO: A hybrid Hartree-Fock density functional study

First-principles calculations based on hybrid Hartree-Fock density functionals provide a clear picture of the defect energetics and electronic structure in ZnO. Among the donorlike defects, the oxygen vacancy and hydrogen impurity, which are deep and shallow donors, respectively, are likely to form with a substantial concentration in $n$-type ZnO. The zinc interstitial and zinc antisite, which are both shallow donors, are energetically much less favorable. A strong preference for the oxygen vacancy and hydrogen impurity over the acceptorlike zinc vacancy is found under oxygen-poor conditions, suggesting that the oxygen vacancy contributes to nonstoichiometry and that hydrogen acts as a donor, both of which are without significant compensation by the zinc vacancy. The present results show consistency with the relevant experimental observations.

Reproducibility in density functional theory calculations of solids

A comparison of DFT methods Density functional theory (DFT) is now routinely used for simulating material properties. Many software packages are available, which makes it challenging to know which are the best to use for a specific calculation. Lejaeghere et al. compared the calculated values for the equation of states for 71 elemental crystals from 15 different widely used DFT codes employing 40 different potentials (see the Perspective by Skylaris). Although there were variations in the calculated values, most recent codes and methods converged toward a single value, with errors comparable to those of experiment. Science , this issue p. 10.1126/science.aad3000 ; see also p. 1394

Core-hole excitations using the projector augmented-wave method and the Bethe-Salpeter equation

We present an implementation of the Bethe-Salpeter equation (BSE) for core-conduction band pairs within the framework of the projector augmented-wave method. For validation, the method is applied to the $K$-edges of diamond, graphite, hexagonal boron-nitride, as well as four lithium-halides (LiF, LiCl, LiI, LiBr). We compare our results with experiment, previous theoretical BSE results, and the density functional theory-based supercell core-hole method. In all considered cases, the agreement with experiment is excellent, in particular for the position of the peaks as well as the fine structure. Comparing BSE to supercell core-hole spectra we find that the latter often qualitatively reproduces the experimental spectrum, however, it sometimes lacks important details. This is shown for the $K$-edges of diamond and nitrogen in hexagonal boron-nitride, where we are capable to resolve within the BSE experimental features that are lacking in the core-hole method. Additionally, we show that in certain systems the supercell core-hole method performs better if the excited electron is added to the background charge. We attribute this improved performance to a reduced self-interaction.

Optical Response of an Interacting Polaron Gas in Strongly Polar Crystals

Optical conductivity of an interacting polaron gas is calculated within an extended random phase approximation which takes into account mixing of collective excitations of the electron gas with longitudinal optical (LO) phonons. This mixing is important for the optical response of strongly polar crystals where the static dielectric constant is rather high, as in the case of strontium titanate. The present calculation sheds light on unexplained features of experimentally observed optical conductivity spectra in n-doped SrTiO 3 . These features appear to be due to dynamic screening of the electron–electron interaction by polar optical phonons and hence do not require additional mechanisms for their explanation.

Towards efficient band structure and effective mass calculations for III-V direct band-gap semiconductors

The band structures and effective masses of III-V semiconductors (InP, InAs, InSb, GaAs, and GaSb) are calculated using the $GW$ method, the Heyd, Scuseria, and Ernzerhof hybrid functional, and modified Becke-Johnson combined with the local-density approximation (MBJLDA)---a local potential optimized for the description of the fundamental band gaps [F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)]. We find that MBJLDA yields an excellent description of the band gaps at high-symmetry points, on par with the hybrid functional and $GW$. However, the effective masses are generally overestimated by $20--30\text{ }\mathrm{%}$ using the MBJLDA local multiplicative potential. We believe this to be related to incorrect nearest-neighbor hopping elements, which are little affected by the choice of the local potential. Despite these shortcomings, the MBJLDA method might be a suitable approach for predicting or interpolating the full band dispersion, if only limited experimental data are available. Furthermore, the method is applicable to systems containing several thousand atoms where accurate quasiparticle methods are not applicable.

Oxygen-Deficient Line Defects in an Ultrathin Aluminum Oxide Film

A model for the straight antiphase domain boundary of the ultrathin aluminum oxide film on the NiAl(110) substrate is derived from scanning tunneling microscopy measurements and density-functional theory calculations. Although the local bonding environment of the perfect film is maintained, the structure is oxygen deficient and possesses a favorable adsorption site. The domain boundary exhibits a downwards band bending and three characteristic unoccupied electronic states, in excellent agreement with scanning tunneling spectroscopy measurements.

Surface reconstruction and electronic properties of clean and hydrogenated diamond (111) surfaces

We present ab initio local-density-functional calculations of the structural and electronic properties of clean and hydrogenated diamond (111) surfaces with one and three dangling bonds (1db, 3db), respectively. For the clean 1db-surface we predict a (2 × 1) reconstruction with symmetric, very slightly buckled dimers at the surface, and a stronger buckling in the deeper layers. Hydrogenation leads to a complete de-reconstruction of the (111)-1db surface. For the 3db-surfaces we have performed a comparative study of the (2 × 1) and ( 3 × 3 ) reconstructions and find the (2 × 1) structure to have a slightly lower energy for both the clean and the hydrogenated surfaces.

Room-temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An <i>ab initio</i> molecular dynamics perspective

The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to ``freely'' spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite-temperature ab initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long-range antiferroelectric ordering of the molecular dipoles is observed. The dynamical correlation between neighboring molecules shows a maximum around room temperature in the mid-temperature phase. In this phase, the rotations are slow enough to (partially) couple to neighbors via the Pb-I cage. This results in a collective motion of neighboring molecules in which the cage acts as the mediator. At lower and higher temperatures, the motions are less correlated.