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Electronic properties of several two dimensional halides from ab initio calculations
Beilstein Journal of Nanotechnology
Using density functional theory, we study the electronic properties of several halide monolayers. We show that their electronic bandgaps, as obtained with the HSE hybrid functional, range between 3.0 and 7.5 eV and that their phonon spectra are dynamically stable. Additionally, we show that under an external electric field some of these systems exhibit a semiconductor-to-metal transition.
The Journal of Physical Chemistry C, 2018
Thin layers of inorganic halide perovskites A n+1 M n X 3n+1 (n = 1-6, A= Cs, M = Pb and Sn, and X = Cl, Br, and I) have been studied in orthorhombic and cubic phases along with layers of monoclinic CsSnCl 3. It is found that one unit-cell-thick layers have low stability except monoclinic phase of CsSnCl 3 where formation energy is slighly less than bulk value. However Cs 2 PbI 4 is unstable in both cubic and orthorhombic phases. The formation energy for n > 3 becomes comparable to bulk but the inclusion of spin-orbit coupling is found to be important for the stabilization particularly for layers with Pb. Importantly, layers of environment friendly Sn based systems have similar values of the formation energy in orthorhombic and cubic phases as well as similar band gaps which make them good materials for solar cell applications as temperture range changes during their opration. The studied 66 cubic and orthorhombic nanosystems have direct band gap (0.6-2.9 eV) using generalized gradient approximation for the exchange-correlation functional but the use of HSE06 method increases the band gap. The reduced dimensionality leads to elongation (contraction) of MX 6 octahedra perpendicular (parallel) to the plane of the layers and an increase in the band gap.
The Journal of Physical Chemistry A, 2008
All electron ab initio calculations for the interaction of H 2 O with Cl 2 and Br 2 are reported for the ground state and the lowest triplet and singlet Π excited states as a function of both the X-X and OX bond lengths (X) Cl or Br). For the ground state and lowest triplet state, the calculations are performed with the coupled cluster singles, doubles, and perturbative triple excitation level of correlation using an augmented triplebasis set. For the 1 Π state the multireference average quadratic coupled cluster technique was employed. For several points on the potential, the calculations were repeated with the augmented quadruple-basis set. The ground-state well depths were found to be 917 and 1183 cm-1 for Cl 2 and Br 2 , respectively, with the triplebasis set, and they increased to 982 and 1273 cm-1 for the quadruple-basis set. At the geometry of the ground-state minimum, the lowest energy state corresponding to the unperturbed 1 Π states of the halogens increases in energy by 637 and 733 cm-1 , respectively, relative to the ground-state dissociation limit of the H 2 OX 2 complex. Adding the attractive ground-state interaction energy to that of the repulsive excited state predicts a blue-shift, relative to that of the free halogen molecules, of ≈1600 cm-1 for H 2 O-Cl 2 and ≈2000 cm-1 for H 2 O-Br 2. These vertical blue-shifts for the dimers are greater than the shift of the band maximum upon solvation of either halogen in liquid water.
arXiv (Cornell University), 2022
Mono-and bi-alkali antimonides, X2YSb (X and Y from Group I), are promising for nextgeneration electron emitters due to their capability of producing high-quality electron beams. However, these materials are not yet well understood, in part due to the technical challenges in growing pure, ordered alkali antimonides. For example, in the current literature there is a lack of complete understanding of the mechanically stable crystal structures of these materials. As a first step towards understanding this issue, this paper presents an ab initio study of stability of single-crystal monoand bi-alkali antimonides in the D03 structure, the structure generally assumed in the literature for these materials. Finding that many of these materials actually are unstable in the D03 structure, we formulate a new set of Goldschmidt-like tolerance factors that accurately predict D03 stability using a procedure analogous to machine-learning perceptron-based analysis. Next, we consider possible stable structures for materials that we predict to be unstable in the D03 structure. Taking as examples the mono-and bi-alkali antimonides Cs3Sb and Cs2KSb, which also are technologically interesting for photoemission and photoabsorption applications, respectively, we note that the most unstable phonon displacements are consistent with the cubic structure, and we therefore perform extensive ab initio searches to identify potential ground-state structures in a cubic lattice. Our X-ray diffraction experiments confirm that indeed these two materials are not stable in the D03 structure and show scattering that is consistent with our new, proposed stable structures. Finally, we explore ab initio the implications of the breaking of the D03 symmetry on the electronic structure, showing significant impact on the location of the optical absorption edge.
The Journal of Chemical Physics, 1993
Results are presented from ab initio calculations on the symmetrical alkali halide dimers made up of Li, Na, K, F, and Cl. We examine the sensitivity of representative monomer and dimer geometries to the variation of the basis set with and without polarization and diffuse functions. The geometries are then compared with available experimental results. We have also calculated vibrational frequencies at the restricted Hartree–Fock level and examined the changes in geometry brought about by correlation using second-order Mo/ller–Plesset perturbation theory. It is found that Hartree–Fock theory in a modest basis set with diffuse and polarization functions yields results comparable to much larger sp basis sets and that the theoretical results are in good agreement with the experimental results for the Li and Na dimers. Our best results for the K-containing species tend to have bond lengths that are too long for the monomers and this error is carried over for the dimers. We also find a ne...
Electronic and optical properties of bismuth oxyhalides from ab initio calculations
Materials Science and Engineering: B, 2021
So far, semiconductors materials have gained huge attention from researchers because of their superior electrocatalytic activity and employment in different areas. The bismuth oxyhalides BiOXs bulks have great electrocatalytic properties and stability. Astonishingly, few of them have synthesized and there is still a lack of theoretical and experimental data on their bandgap energy, which limits the chance of producing new physical properties. Using density functional theory and beyond with GW and GW BSE approaches, we investigate the electronic and optical properties such as absorption coefficient of bismuth oxyhalides BiOXs (where, X = F, Cl, Br, and I). Our results show that the GW bandgap nature of BiOF is direct, whereas the GW bandgaps of BiOCl, BiOBr, and BiOI are indirect, which is a range from 2.33 eV (BiOI) to 4.04 eV (BiOF). Here, we suggest that can be taken the GW bandgap calculations of BiOCl as a reference to the place of experimental values. Under an external electric field, we succeed to reduce easily and quickly the bandgaps energy of our compounds. Besides, we show that under an external electric field a semiconductor can be changed to a metal with a strong field of more than 0.5 V/Å. In this framework, we have shown that this strategy is better than the incorporation method for engineering in order to find material with a bandgap that we want. We found that the dramatic redshift of the peaks at the BSE level as compared to DFT and RPA indicates strong excitonic effects.
Chemical Physics, 2008
We report the calculation of a six-dimensional CCSD(T)/aug-cc-pVQZ potential energy surface for the electronic ground state of NH þ 3 together with the corresponding CCSD(T)/aug-cc-pVTZ dipole moment and polarizability surface of 14 NH þ 3 . These electronic properties have been computed on a large grid of molecular geometries. A number of newly calculated band centers are presented along with the associated electric-dipole transition moments. We further report the first calculation of vibrational matrix elements of the polarizability tensor components for 14 NH þ 3 ; these matrix elements determine the intensities of Raman transitions. In addition, the rovibrational absorption spectra of the m 2 , m 3 , m 4 , 2m 2 À m 2 , and m 2 þ m 3 À m 2 bands have been simulated.
Molecular Modeling and Electronic Structure Calculations
2017
This laboratory is designed to use the program GAMESS (General Atomic Molecular Electronic Structure System, developed in Gordon research group at Iowa State) through a website called nanoHUB (www.nanoHUB.org) to determine the geometric and electronic properties of numerous small molecules. GAMESS uses ab initio and semi-empirical calculations to determine these properties. Ab initio (“from first principles”) calculations solve the Schrödinger equation using the exact computational expression for the energy of the electrons. The particular ab initio method that we will use for this lab is called HartreeFock (HF). HF uses an approximate wavefunction to solve Schrödinger, so the resulting molecular properties are approximate, but for many applications the accuracy is adequate for interpreting experiments. Semi-empirical calculations use an approximate energy expression for the electrons, but solve for the exact wavefunction associated with this expression. Usually the energy expressio...