Calculate Exciton Binding Energies Research Articles

Impact of Polythiophene ((C4H4S)n; n = 3, 5, 7, 9) Units on the Adsorption, Reactivity, and Photodegradation Mechanism of Tetracycline by Ti-Doped Graphene/Boron Nitride (Ti@GP_BN) Nanocomposite Materials: Insights from Computational Study.

This study addresses the formidable persistence of tetracycline (TC) in the environment and its adverse impact on soil, water, and microbial ecosystems. To combat this issue, an innovative approach by varying polythiophene ((C4H4S)n; n = 3, 5, 7, 9) units and the subsequent interaction with Ti-doped graphene/boron nitride (Ti@GP_BN) nanocomposites was applied as catalysts for investigating the molecular structure, adsorption, excitation analysis, and photodegradation mechanism of tetracycline within the framework of density functional theory (DFT) at the B3LYP-gd3bj/def2svp method. This study reveals a compelling correlation between the adsorption potential of the nanocomposites and their corresponding excitation behaviors, particularly notable in the fifth and seventh units of the polythiophene configuration. These units exhibit distinct excitation patterns, characterized by energy levels of 1.3406 and 924.81 nm wavelengths for the fifth unit and 1.3391 and 925.88 nm wavelengths for the seventh unit. Through exploring deeper, the examination of the exciton binding energy emerges as a pivotal factor, bolstering the outcomes derived from both UV-vis transition analysis and adsorption exploration. Notably, the calculated exciton binding energies of 0.120 and 0.103 eV for polythiophene units containing 5 and 7 segments, respectively, provide compelling confirmation of our findings. This convergence of data reinforces the integrity of our earlier analyses, enhancing our understanding of the intricate electronic and energetic interplay within these intricate systems. This study sheds light on the promising potential of the polythiophene/Ti-doped graphene/boron nitride nanocomposite as an efficient candidate for TC photodegradation, contributing to the advancement of sustainable environmental remediation strategies. This study was conducted theoretically; hence, experimental studies are needed to authenticate the use of the studied nanocomposites for degrading TC.

Ab-initio investigation of the structural stability, electronic and optical properties of the LiBO2 compound by using the G0W0+BSE approach

This article investigates the structural stability, and electronic and optical properties of the LiBO2 compound using density functional theory (DFT) and solving the many-body Bethe-Salpeter equation within the G0W0+BSE approach. The structural properties showed that the monoclinic structure of LiBO2 is thermodynamically and dynamically stable within both LDA and GGA approximations. The band structure results indicated that the direct band gap of LiBO2 at the Γ point is 6.05 eV, which increased to 8.95 eV by considering the quasi-particle effects in G0W0@LDA calculations. The maximum reflectance in the x, y, and z directions is 0.48, 0.86, and 0.54, respectively, with the corresponding static refractive index values of 1.52, 1.61, and 1.54. The direct optical band gap obtained from the imaginary part of the dielectric function is 5.95 eV, 5.95 eV, and 6.10 eV for the various directions x, y, and z respectively. Considering the electron-hole effects in the G0W0+BSE approach, the optical gap showed more accurate results, and the value of 7.54 eV was obtained in the x and z-directions and 8.10 eV in the y-direction. Therefore, excitonic effects within the G0W0+BSE approximation, in comparison to G0W0+RPA, leads to the redshift of the optical absorption spectrum. The calculated exciton binding energy is 1.41 eV for the x and z-directions and 0.85 eV for the y-direction. Finally, it can be stated that LiBO2 is a wide-gap insulator with a gap greater than 7 eV in the monoclinic structure. This indicates the high transparency of the LiBO2 crystal, with the prospect of application in optical instruments.

A difference method with semi-analytical approach for achieving accuracy in optical gaps of 2D materials using exciton model in fractional space

A semi-analytical approach for the difference method using numerically calculated G0W0 band gaps and analytically calculated exciton binding energies based on the fractional Coulomb potential model is proposed to calculate optical gaps of 46 2D materials ranging from ultra-violet to infrared region. The suggested methodology is compared with difference methods of a similar hybrid approach, utilizing conventional exciton models based on Wannier–Mott theory to achieve a significant reduction in the average relative mean square error of optical gaps, up to one-third, benchmarked with a fully numerical approach, employing G0W0 band gaps and the state-of-the-art Bethe–Salpeter equation for binding energy calculation.

Metal to semiconductor transition of two-dimensional NbSe2 through hydrogen adsorption: A first-principles study

Based on first-principles calculations, we predict that the recently synthesized two-dimensional (2D) NbSe2 can be changed from the metallic to the semiconducting phase upon the adsorption of H with an indirect bandgap of 2.99 eV. The bandgap opening of the 2D NbSe2 only occurs when the hydrogen coverage is high, and it is sensitive to mechanical strain. The hydrogenated 2D NbSe2 is dynamically stable under a tensile strain of up to 9%, whereas a compressive strain leads to instability of the system. The optical spectra obtained from the GW-Bethe–Salpeter equation calculations suggest that 2D NbSe2 is highly isotropic, and it will not affect the polarization of light along the x- or y-direction. The optical bandgap, describing the transition energy of the exciton, is sensitive to the mechanical strain with the calculated exciton binding energy of ∼0.42 eV. These intriguing properties suggest that H functionalized 2D NbSe2, grown on a substrate with a larger lattice parameter, can be used to modulate the bandgap of NbSe2. This is beneficial in developing a nanoscale field effect and optoelectronic devices.

Binding energy of the hybrid exciton in heterostructures of colloidal CdSe-ZnS quantum dots and two-dimensional transition metal dichalcogenides

Using continuum model calculations within the framework of the $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ approach and effective mass approximation, we theoretically study the physical properties of the hybrid exciton state in type-II heterostructures composed of two-dimensional monolayer transition metal dichalcogenides (2D TMDs) and zero-dimensional (0D) CdSe-ZnS core-shell quantum dots (QDs). Interactions between an electron located in the lowest unoccupied molecule orbital (LUMO) state of the QD and a hole in the valence band of the 2D TMD along with their effects on charge distribution and exciton binding energy are self-consistently calculated via solving the Schr\odinger and Poisson equations. We show that (i) the hole wave function of the 2D TMD is tightly bounded within several unit cells by an electron located in the LUMO state of the QD while the distribution of the electron wave function is marginally changed by the Coulomb attraction from the hole in the 2D TMD; (ii) the binding energies of the hybrid exciton decrease from 110 to 25 meV with the radius of the CdSe core increasing from 2 to 8 nm, which is several times lower than that of the exciton of 2D TMDs; and (iii) the exciton binding energies of heterostructures composed by different 2D TMD materials are similar. We explain the size dependence of exciton binding energy as the result of the delocalization of the electron wave function with QD size and attribute the similar binding energy obtained in different 2D/0D heterostructures to the weak dielectric screening effect of different 2D TMDs on electron-hole interaction. These results indicate that the physical properties of a hybrid exciton in 2D/0D heterostructures is dominated by the properties of the QD instead of the 2D TMDs. We further compare our calculated exciton binding energy with the corresponding experimental result; good agreement between experiment and theory provides evidence for the validity of our theoretical approach.

A theoretical study of the absorption spectra of electron-deficient pentacene derivatives using DFT and TDDFT.

Non-fullerene acceptor based organic bulk heterojunction solar cells have been a hot topic because their power conversion efficiencies have been up to 16.35%. Functionalized 6,13‑bis (trimethylsilyl alkynyl) pentacenes with strong electron withdrawing groups, which can be easily modified to improve charge transport properties and film morphology, seem to be promising soluble non-fullerene pentacene-based organic acceptors. But how the substitutions of electron withdrawing groups influence their electronic structures, then change the absorption spectra and power conversion efficiencies, is still not clear. In this paper, we utilize density functional theory and time-dependent density functional theory to study the effects of substitutions of different electron withdrawing groups (CN, CF3, NO2) and different positions of these groups in 6,13‑bis (trimethylsilyl alkyl) pentacene molecule on their physical and optical properties. We find that the experimental power conversion efficiencies are positively/negatively correlated with calculated dipole moments/exciton binding energies of these functionalized molecules. The computed results indicate that the molecules substituted with CN group have much larger dipole moment than the others. For the same electron withdrawing group, the dipole moment at the R2 position is generally larger than that at the R1 position. Furthermore, we find that the calculated exciton binding energy of these molecules functionalized at the R2 position is lower than that at the R1 position. In addition, the result of absorption spectra confirm that these functionalized 6,13‑bis (trimethylsilyl alkynyl) pentacenes have stronger absorption strength than C60 in the both visible and the ultraviolet regions.

Tellurium Doping and the Structural, Electronic, and Optical Properties of NaYS2(1-x)Te2x Alloys.

New ternary and quaternary NaYS2(1–x)Te2x alloys (with x = 0, 0.33, 0.67, and 1) are proposed as promising candidates for photon energy conversion in photovoltaic applications. The effects of Te doping on crystal, spectral, and optical properties are studied within the framework of periodic density functional theory. Increasing Te content decreases the band gap (Eg) considerably (from 3.96 (x = 0) to 1.62 eV (x = 0.67)) and fits a quadratic model (Eg(x) = 3.96–6.78x + 4.70x2, (r2 = 0.96, n = 4)). The band gap of 1.62 eV makes the NaYS0.67Te1.33 alloy ideal for photovoltaic applications for their ability to absorb in the visible segment of the sunlight spectrum. The calculated exciton binding energies are 9.78 meV for NaYS1.33Te0.67 and 6.06 meV for NaYS0.67Te1.33. These values of the order of the thermal energy at room temperature suggest an easily dissociable hole–electron pair. The family of NaYS2(1–x)Te2x alloys are, therefore, promising candidates for visible photocatalytic devices and worthy of further experimental and theoretical investigations.

Excitonic effects in two-dimensional TiSe2 from hybrid density functional theory

Transition metal dichalcogenides (TMDs), whether in bulk or in monolayer form, exhibit a rich variety of charge-density-wave (CDW) phases and stronger periodic lattice distortions. While the actual role of nesting has been under debate, it is well understood that the microscopic interaction responsible for the CDWs is the electron-phonon coupling. The case of TiSe$_2$ is however unique in this family in that the normal state above the critical temperature $T_\mathrm{CDW}$ is characterized by a small quasiparticle bandgap as measured by ARPES, so that no nesting-derived enhancement of the susceptibility is present. It has therefore been argued that the mechanism responsible for this CDW should be different and that this material realizes the excitonic insulator phase proposed by Walter Kohn. On the other hand, it has also been suggested that the whole phase diagram can be explained by a sufficiently strong electron-phonon coupling. In this work, in order to estimate how close this material is to the pure excitonic insulator instability, we quantify the strength of electron-hole interactions by computing the exciton band structure at the level of hybrid density functional theory, focusing on the monolayer. We find that in a certain range of parameters the indirect gap at $q_{\mathrm{CDW}}$ is significantly reduced by excitonic effects. We discuss the consequences of those results regarding the debate on the physical mechanism responsible for this CDW. Based on the dependence of the calculated exciton binding energies as a function of the mixing parameter of hybrid DFT, we conjecture that a necessary condition for a pure excitonic insulator is that its noninteracting electronic structure is metallic.

Exciton Binding Energy in Molecular Triads

The power conversion efficiency of state of the art organic photovoltaics is predicted to be limited at about 15%. This limit can be increased by an improved charge carrier mobility and by a lower exciton binding energy. In order to achieve this, we suggest a concept based on organic triads, comprising a donor, spacer, and acceptor submolecule. On the basis of time dependent density functional theory (TD-DFT) simulations we investigate the lowest excited state of the well-known Carotenoid-Porphyrin-C60 triad and obtain a calculated exciton binding energy of 25 meV, justifying the experimentally observed and reported separation of photo generated charges. Further, we introduce a new triad with the ability to not only improve and control the separation process, but also to improve the charge carrier transport properties. We used molecular dynamics (MD) simulations to optimize the geometry of a cluster of 25 triads. From this organic cluster, we picked one triad with its four neighbors, again calculated the ...

Excitons in van der Waals heterostructures: The important role of dielectric screening

The existence of strongly bound excitons is one of the hallmarks of the newly discovered atomically thin semi-conductors. While it is understood that the large binding energy is mainly due to the weak dielectric screening in two dimensions (2D), a systematic investigation of the role of screening on 2D excitons is still lacking. Here we provide a critical assessment of a widely used 2D hydrogenic exciton model which assumes a dielectric function of the form {\epsilon}(q) = 1 + 2{\pi}{\alpha}q, and we develop a quasi-2D model with a much broader applicability. Within the quasi-2D picture, electrons and holes are described as in-plane point charges with a finite extension in the perpendicular direction and their interaction is screened by a dielectric function with a non-linear q-dependence which is computed ab-initio. The screened interaction is used in a generalized Mott-Wannier model to calculate exciton binding energies in both isolated and supported 2D materials. For isolated 2D materials, the quasi-2D treatment yields results almost identical to those of the strict 2D model and both are in good agreement with ab-initio many-body calculations. On the other hand, for more complex structures such as supported layers or layers embedded in a van der Waals heterostructure, the size of the exciton in reciprocal space extends well beyond the linear regime of the dielectric function and a quasi-2D description has to replace the 2D one. Our methodology has the merit of providing a seamless connection between the strict 2D limit of isolated monolayer materials and the more bulk-like screening characteristics of supported 2D materials or van der Waals heterostructures.

Electronic States of Lead-Salt Nanosheets

The electronic states of lead-salt nanosheets are calculated with an effective mass model. In recognition of the anisotropic shape of the structure, the electron wave functions are separated into two parts. Motion along the narrow dimension is dominated by the quantum confinement, while the lateral motion is dominated by the Coulomb interaction. Strong contrast of the dielectric constant of the host environment further strengthens the Coulomb interaction. The calculated exciton binding energy for lead-salt nanosheets is smaller than that of nanowires of similar size but still larger than the thermal energy at room temperature, so it should be possible to observe excitons at room temperature. The calculation results are compared with existing data, and show good quantitative agreement.

Excitons in Time-Dependent Density-Functional Theory.

This chapter gives an overview of the description of the optical and dielectric properties of bulk insulators and semiconductors in time-dependent density-functional theory (TDDFT), with an emphasis on excitons. We review the linear-response formalism for periodic solids, discuss excitonic exchange-correlation kernels, calculate exciton binding energies for various materials, and compare the treatment of excitons with TDDFT and with the Bethe-Salpeter equation.

Effect of Dot Size on Exciton Binding Energy and Electron-Hole Recombination Probability in CdSe Quantum Dots.

Exciton binding energy and electron–hole recombination probability are presented as two important metrics for investigating effect of dot size on electron–hole interaction in CdSe quantum dots. Direct computation of electron–hole recombination probability is challenging because it requires an accurate mathematical description of the electron–hole wave function in the neighborhood of the electron–hole coalescence point. In this work, we address this challenge by solving the electron–hole Schrodinger equation using the electron–hole explicitly correlated Hartree–Fock (eh-XCHF) method. The calculations were performed for a series of CdSe clusters ranging from Cd20Se19 to Cd74608Se74837 that correspond to dot diameter range 1–20 nm. The calculated exciton binding energies and electron–hole recombination probabilities were found to decrease with increasing dot size. Both of these quantities were found to scale as Ddot–n with respect to the dot diameter D. One of the key insights from this study is that the electron–hole recombination probability decreases at a much faster rate than the exciton binding energy as a function of dot size. It was found that an increase in the dot size by a factor of 16.1, resulted in a decrease in the exciton binding energy and electron–hole recombination probability by a factor of 12.9 and 4.55 × 105, respectively.

Theory of neutral and charged excitons in monolayer transition metal dichalcogenides

We present a microscopic theory of neutral excitons and charged excitons (trions) in monolayers of transition metal dichalcogenides, including molybdenum disulfide. Our theory is based on an effective mass model of excitons and trions, parametrized by ab initio calculations and incorporating a proper treatment of screening in two dimensions. The calculated exciton binding energies are in good agreement with high-level many-body computations based on the Bethe-Salpeter equation. Furthermore, our calculations for the more complex trion species compare very favorably with recent experimental measurements, and provide atomistic insight into the microscopic features which determine the trion binding energy.

Direct calculation of exciton binding energies with time-dependent density-functional theory

Excitons are electron-hole pairs appearing below the band gap in insulators and semiconductors. They are vital to photovoltaics, but are hard to obtain with time-dependent density-functional theory (TDDFT), since most standard exchange-correlation (xc) functionals lack the proper long-range behavior. Furthermore, optical spectra of bulk solids calculated with TDDFT often lack the required resolution to distinguish discrete, weakly bound excitons from the continuum. We adapt the Casida equation formalism for molecular excitations to periodic solids, which allows us to obtain exciton binding energies directly. We calculate exciton binding energies for both small- and large-gap semiconductors and insulators, study the recently proposed bootstrap xc kernel [S. Sharma et al., Phys. Rev. Lett. 107, 186401 (2011)], and extend the formalism to triplet excitons.

Theoretical investigation of InAs/GaSb type-II pin superlattice infrared detector in the mid wavelength infrared range

In this study, we present the theoretical investigation of type-II InAs/GaSb superlattice p-i-n detector. Kronig-Penney and envelope function approximation is used to calculate band gap energy and superlattice minibands. Variational method is also used to calculate exciton binding energies. Our results show that carriers overlap increases at GaSb/InAs interface on the higher energy side while it decreases at InAs/GaSb interface on the lower energy side with increasing reverse bias due to shifting the hole wavefunction toward to the GaSb/InAs interface decisively. Binding energies increase with increasing electric field due to overall overlap of electron and hole wave functions at the both interfaces in contrast with type I superlattices. This predicts that optical absorption is enhanced with increasing electric field.

Electron–electron interaction effects on the photophysics of metallic single-walled carbonnanotubes

Single-walled carbon nanotubes are strongly correlated systems withlarge Coulomb repulsion between two electrons occupying the samepz orbital. Within a molecular Hamiltonian appropriate for correlatedπ-electron systems, we show that optical excitations polarized parallel to thenanotube axes in the so-called metallic single-walled carbon nanotubes areexcitons. Our calculated absolute exciton energies in twelve different metallicsingle-walled carbon nanotubes, with diameters in the range 0.8–1.4 nm, are innearly quantitative agreement with experimental results. We have also calculatedthe absorption spectrum for the (21, 21) single-walled carbon nanotube in theE22 region. Our calculated spectrum gives an excellent fit to the experimental absorptionspectrum. In all cases our calculated exciton binding energies are only slightly smaller thanthose of semiconducting nanotubes with comparable diameters, in contradiction to resultsobtained within the ab initio approach, which predicts much smaller binding energies. Weascribe this difference to the difficulty of determining the behavior of systems withstrong on-site Coulomb interactions within theories based on the density functionalapproach. As in the semiconducting nanotubes we predict in the metallic nanotubes atwo-photon exciton above the lowest longitudinally polarized exciton that canbe detected by ultrafast pump-probe spectroscopy. We also predict a subgapabsorption polarized perpendicular to the nanotube axes below the lowest longitudinalexciton, blueshifted from the exact midgap by electron–electron interactions.

Carrier dynamics in stacked InP∕GaAs quantum dots

We investigated two stacked layers of InP∕GaAs type-II quantum dots by transmission electron microscopy and optical spectroscopy. The results reveal that InP quantum dots formed in two quantum dot layers are more uniform than those from a single layer structure. The thermal activation energies as well as the photoluminescence decays are rather independent of the separation between quantum dot layers and the presence of the second layer. The quantum dot optical emission persists for thermal activation energy larger than the calculated exciton binding energy. The photoluminescence decay is relatively fast for type-II alignment.

Proposed strategy to sort semiconducting nanotubes by band-gap

We propose a strategy that uses a tunable infra-red source and an alternating non-linear potential defined by an electrode to sort a suspension of assorted semiconducting nanotubes. The band-gap scales with the inverse of the nanotube diameter, hence the infra-red frequency can be tuned to create excitons in some of the nanotubes; these excitons will be polarized by the potential. Since, a polarized exciton is a dipole, the excited nanotubes will experience a net force and may then diffuse towards the electrode, unlike the other nanotubes. We discuss experimental parameters such as IR intensity, electrode design, and potential frequency for a pilot experiment to sort nanotubes with lengths ≈0.5μm. The basic physics of the system has been illustrated using a Hartree model applied to nanotubes with nanoscale lengths. The calculated exciton binding energy suddenly drops to zero and the force on the nanotube increases dramatically when the exciton disassociates as the nanotube moves towards the electrode. The quantum adiabatic theorem shows that excitons will be adiabatically polarized for potential frequencies typical for experiments ≈1–10MHz. The analysis indicates that the manipulation of nanotubes with nanometer lengths requires nanoscale electrodes.

Huge Excitonic Effects in Layered Hexagonal Boron Nitride

The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor.