Time-Dependent Density-Functional Theory and Excitons in Bulk and Two-Dimensional Semiconductors

Turkowski, Volodymyr; Din, Naseem Ud; Rahman, Talat S.

doi:10.3390/computation5030039

Cited by 24 publications

(17 citation statements)

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“…The excitation or optical absorption is an even more demanding task. Available theoretical approaches are time-dependent DFT (TD-DFT) [ 28 , 29 ] or the Bethe-Salpeter equation (BSE) [ 30 , 31 ]. The latter includes excitonic effects which are crucial for these types of materials.…”

Section: Introductionmentioning

confidence: 99%

Nature of Excitons in Bidimensional WSe2 by Hybrid Density Functional Theory Calculations

2018

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2D tungsten diselenide (2D-WSe2) is one of the most successful bidimensional materials for optoelectronic and photonic applications, thanks to its strong photoluminescence properties and to a characteristic large exciton binding energy. Although these optical properties are widely recognized by the scientific community, there is no general understanding of the atomistic details of the excitonic species giving rise to them. In this work, we present a density functional theory investigation of excitons in 2D-WSe2, where we compare results obtained by standard generalized gradient approximation (GGA) methods (including spin-orbit coupling) with those by hybrid density functionals. Our study provides information on the size of the self-trapped exciton, the number and type of atoms involved, the structural reorganization, the self-trapping energy, and the photoluminescence energy, whose computed value is in good agreement with experimental measurements in the literature. Moreover, based on the comparative analysis of the self-trapping energy for the exciton with that for isolated charge carriers (unbound electrons and holes), we also suggest a simplified approach for the theoretical estimation of the excitonic binding energy, which can be compared with previous estimates from different approaches or from experimental data.

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Section: Introductionmentioning

confidence: 99%

Nature of Excitons in Bidimensional WSe2 by Hybrid Density Functional Theory Calculations

2018

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“…A direct exciton energy calculation requires the calculation of the two-particle spectrum by solving the Bethe–Salpeter equation. The development of time-dependent DFT has paved the road [68]. Thus, albeit the discussion concerning the suitability of TD-DFT for a proper description of excitonic matter is still alive, we have followed this approach as a pragmatic choice.…”

Section: Discussionmentioning

confidence: 99%

Transport and Optical Gaps in Amorphous Organic Molecular Materials

et al. 2019

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The standard procedure to identify the hole- or electron-acceptor character of amorphous organic materials used in OLEDs is to look at the values of a pair of basic parameters, namely, the ionization potential (IP) and the electron affinity (EA). Recently, using published experimental data, the present authors showed that only IP matters, i.e., materials with IP > 5.7 (<5.7) showing electron (hole) acceptor character. Only three materials fail to obey this rule. This work reports ab initio calculations of IP and EA of those materials plus two materials that behave according to that rule, following a route which describes the organic material by means of a single molecule embedded in a polarizable continuum medium (PCM) characterized by a dielectric constant ε. PCM allows to approximately describe the extended character of the system. This “compound” system was treated within density functional theory (DFT) using several combinations of the functional/basis set. In the preset work ε was derived by assuming Koopmans’ theorem to hold. Optimal ε values are in the range 4.4–5.0, close to what is expected for this material family. It was assumed that the optical gap corresponds to the excited state with a large oscillator strength among those with the lowest energies, calculated with time-dependent DFT. Calculated exciton energies were in the range 0.76–1.06 eV, and optical gaps varied from 3.37 up to 4.50 eV. The results are compared with experimental data.

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“…If DFT single particle theory is not sufficient, one can use the DFT orbitals as input for many-body perturbation theories, such as the GW approximation [13] for better quasiparticle energies, in which the self-energy ∑ is expanded in terms of the single particle Green's function G and the screened Coulomb interaction W. Another scheme is the Bethe-Salpeter equation (BSE) approach [14,15] to account for excitonic effects. Recently, the combination of TDDFT and excitons was presented in [16]. For highly correlated systems, which need a good description of the localized 3d or 4f states, inclusion of a Hubbard U into a generalized gradient approximation (GGA) may be sufficient for a proper description of the electronic structure (called GGA+U).…”

Section: Discussion and New Challengesmentioning

confidence: 99%

Challenges for Theory and Computation

Schwarz

2017

Computation

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Abstract:The routinely made assumptions for simulating solid materials are briefly summarized, since they need to be critically assessed when new aspects become important, such as excited states, finite temperature, time-dependence, etc. The significantly higher computer power combined with improved experimental data open new areas for interdisciplinary research, for which new ideas and concepts are needed.

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Time-Dependent Density-Functional Theory and Excitons in Bulk and Two-Dimensional Semiconductors

Cited by 24 publications

References 50 publications

Nature of Excitons in Bidimensional WSe2 by Hybrid Density Functional Theory Calculations

Nature of Excitons in Bidimensional WSe2 by Hybrid Density Functional Theory Calculations

Transport and Optical Gaps in Amorphous Organic Molecular Materials

Challenges for Theory and Computation

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