Electronic and optical properties of bismuth oxyhalides from ab initio calculations (original) (raw)
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Physical Review B, 2021
In order to realize the significant potential of optical materials such as metal halides, computational techniques which give accurate optical properties are needed, which can work hand-in-hand with experiments to generate high efficiency devices. In this work a computationally efficient technique based on semiconductor Bloch equations (SBEs) is developed and applied to the material BiSBr. This approach gives excellent agreement with the experimental optical gap, and also agrees closely with the excitonic stabilisation energy and the absorption spectrum computed using the far more computationally demanding ab initio Bethe-Salpeter approach. The SBE method is a good candidate for theoretical spectroscopy on large-or low dimensional systems which are too computationally expensive for an ab initio treatment.
Excitonic absorption and Urbach's tail in bismuth sulfide single crystals
Applied Physics A: Solids and Surface, 1988
The absorption coefficient of bismuth sulfide single crystals has been measured through more than four orders of magnitude and in the range of energies from 1.25 to 1.70 eV. A detailed study as a function of temperature has been carried out from 29 to 300 K. An Urbach tail for low values of absorption has been found. This tail and its temperature evolution fit the expression for ionic materials. An excitonic region appears at low temperature and the shape of the exciton peak is Gaussian, which corresponds to a strong exciton-phonon coupling. The exciton binding energy is estimated (28 ___ 3 meV) and then the energy gap at 29 K is obtained (E0=1.523__0.003 eV). The fundamental electronic transition has been found to be a strongly anisotropic allowed direct transition. From reflectivity measurements a localized level at 1.361 eV at 29 K has been found. The change of the gap with temperature is interpreted through an electron-phonon mechanism.
Chemistry-a European Journal, 2000
The electronic excitations of the low-valence bismuth cluster cations Bi 5 3 , Bi 8 2 , and Bi 9 5 have been studied with experimental and theoretical techniques. The UV-visible spectra of the bismuth ions were measured in acidic chloroaluminate melts (mixture of 1-methyl-3-benzyl imidazolium chloride and AlCl 3 ). The spectra of the Bi 5 3 and Bi 8 2 ions agree fairly well with previous reports, but also revealed additional low-energy absorptions. Ab initio methods were employed to assign the experimentally observed electronic transitions of these homopolyatomic bismuth cations. Structures were optimized at the RHF, MP2, and B3LYP levels of theory by using split-valence LANL2DZ basis sets that were augmented with one and two sets of pure d functions. The computed structures agree well with the results of neutron diffraction analyses of melts. Electronically excited states of the three clusters were treated by using the CI-Singles theory. The results of these calculations were used to explain the observed UV-visible spectra. The observed electronic excitations in the UV-visible range are all found to result from transitions involving the molecular orbitals formed by 6p-atomic-orbital overlap. This leads to the necessity of using basis sets that include d-type functions, which allow for an adequate description of the bonding that results from such p-orbital overlap. Spin-orbit coupling becomes increasingly important with increasing atomic number and its consideration is necessary when describing the electronic transitions in clusters of heavy atoms. The calculations show that singlet ± triplet transitions, which are made accessible by strong spin-orbit coupling, are responsible for some of the observed absorptions.
Ab initio calculations of electronic excitations: Collapsing spectral sums
We present a method for the evaluation of electronic excitations of advanced materials by reformulating spectral sum-over-states expressions such that only occupied states appear. All empty states are accounted for by one effective energy. Thus we keep the simplicity and precision of the sum-over-states approach while speeding up calculations by more than an order of magnitude. We demonstrate its power by applying it to the GW method, where a huge summation over empty states appears twice ͑screening and self-energy͒. The precision is shown for bulk Si and solid and atomic Ar. We then use it to determine the band gap of the technologically important oxide SnO 2 .
An Ab Initio Exciton Model Including Charge-Transfer Excited States
Journal of chemical theory and computation, 2017
The Frenkel exciton model is a useful tool for theoretical studies of multi- chromophore systems. We recently showed that the exciton model could be used to coarse-grain electronic structure in multichromophoric systems, focusing on singly-excited exciton states [Acc. Chem. Res. 2014, 47, 2857-2866]. However, our previous implementation excluded charge-transfer excited states, which can play an important role in light-harvesting systems and near-infrared optoelectronic materials. Recent studies have also emphasized the significance of charge-transfer in singlet fission, which mediates the coupling between the locally excited states and the multi-excitonic states. In this work, we report on an ab initio exciton model that incorporates charge-transfer excited states, and demonstrate that the model provides correct charge-transfer excitation energies and asymptotic behavior. Comparison with TDDFT and EOM-CC2 calculations shows that our exciton model is robust with respect to system siz...
Ab Initio Calculation of Excitonic Effects in the Optical Spectra of Semiconductors
Physical Review Letters, 1998
An ab initio approach to the calculation of excitonic effects in the optical absorption spectra of semiconductors and insulators is formulated. It starts from a quasiparticle band structure calculation and is based on the relevant Bethe-Salpeter equation. An application to bulk silicon shows a substantial improvement with respect to previous calculations in the description of the experimental spectrum, for both peak positions and line shape. [S0031-9007(98)
The Journal of Chemical Physics, 1971
A method for the determination of complete exciton band structures in molecular crystals is given. Pairwise exciton interactions are derived from resonance-pair data using an exciton "superexchange" approach. Koster and Slater's impurity cluster formulation is found to be applicable to nontrivial interchange symmetry systems, within the "restricted Frenkel-Davydov" theory. The derivation starts from the recent general formulation for isotopically mixed crystals of arbitrary concentrations. The resonance pair states are given by the second-order self-energy of the mixed crystal Green's function. General symmetry arguments and moment sum rules have been worked out for resonance pairs. It is demonstrated for naphthalene-Its resonance pairs in naphthalene-ds that superexchange corrections are not only inevitable for the 'E2• pair states but that they can also be utilized to assign experimental pairwise interactions to definite crystal directions, i.e., specific pairs. The naphthalene first singlet excited state 0-0 vibronic exciton band is successfully described by the "restricted Frenkel-Da vydov" dispersion relation:
Excitation energies in density functional theory: An evaluation and a diagnostic test
2008
Electronic excitation energies are determined using the CAM-B3LYP Coulomb-attenuated functional ͓T. Yanai et al. Chem. Phys. Lett. 393, 51 ͑2004͔͒, together with a standard generalized gradient approximation ͑GGA͒ and hybrid functional. The degree of spatial overlap between the occupied and virtual orbitals involved in an excitation is measured using a quantity ⌳, and the extent to which excitation energy errors correlate with ⌳ is quantified. For a set of 59 excitations of local, Rydberg, and intramolecular charge-transfer character in 18 theoretically challenging main-group molecules, CAM-B3LYP provides by far the best overall performance; no correlation is observed between excitation energy errors and ⌳, reflecting the good quality, balanced description of all three categories of excitation. By contrast, a clear correlation is observed for the GGA and, to a lesser extent, the hybrid functional, allowing a simple diagnostic test to be proposed for judging the reliability of a general excitation from these functionals-when ⌳ falls below a prescribed threshold, excitations are likely to be in very significant error. The study highlights the ambiguous nature of the term "charge transfer," providing insight into the observation that while many charge-transfer excitations are poorly described by GGA and hybrid functionals, others are accurately reproduced.
High Level ab Initio Quantum Mechanical Predictions of Infrared Intensities
The Journal of Physical Chemistry A, 2002
Vibrational intensities associated with the infrared spectra of H 2 O, C 2 H 2 , HCN, H 2 CO, CH 4 , and SiH 4 were theoretically predicted by applying ab initio quantum mechanical methods at seven different levels of theory. The self-consistent field, second-order Møller-Plesset perturbation method, configuration interaction with single and double excitations, double excitations coupled-cluster, quadratic configuration interaction including single and double excitations (QCISD), coupled-cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory were applied. The atomic orbital basis sets employed included frozen-core double-(DZ), triple-(TZ), DZ plus single polarization (DZP), TZ plus double polarization (TZ2P), TZ plus triple polarization (TZ3P), TZ2P augmented with one set of higher angular momentum functions [TZ2P(f,d)], TZ2P(f,d) with one set of diffuse functions [TZ2P(f,d)+diff], TZ3P augmented with two sets of higher angular momentum functions [TZ3P(2f,2d)], [TZ3P(2f,2d)] with two sets of diffuse functions [TZ3P(2f,2d)+2diff], split valence plus polarization [6-311G(d,p) and 6-311G(3d,3p)], split valence with added polarization and diffuse functions [6-311++G(d,p) and 6-311++G(3d,3p)], Dunning's correlation consistent polarized valence [cc-pVXZ (X ) 2-5)] basis sets, as well as augmented correlation consistent polarized valence [aug-cc-pVXZ (X ) 2-5)] basis sets. The theoretical infrared intensities predicted at the different levels of theory for the studied molecules were compared with available experimental data.
Ab-initio study of excitonic effects in conventional and organic semiconductors
physica status solidi (b), 2005
The excitonic effects on the optical absorption properties of organic semiconductors as well as gallium nitride are studied from first-principles. The Coulomb interaction between the electron and the hole is accounted for by solving the two-particle Bethe-Salpeter Equation. In the organic semiconductors the exciton binding energies strongly depend on the molecular size, the crystalline packing, as well as the polarization direction of the incoming light. We show that the electron-hole interaction can lead to strongly bound excitons with binding energies of the order of 1 eV or to a mere redistribution of oscillator strength. In several cases, the screening is efficient enough such that free charge carriers govern the optical absorption process. In the inorganic counterparts the sensitivity of the exciton binding energy is tested against the structural parameters and the screening of the electron-hole Coulomb interaction.