Exciton Binding Research Articles

Direct Linearly Polarized Emission in van der Waals LEDs via Flexoelectric Effect

AbstractFollowing the rapid development of information technology, modern polarized light, which is a critical component for display and data transmission, has been in demand for miniaturization and high efficiency, rendering two‐dimensional (2D) semiconductors potential candidates. The traditional polarized light is usually generated by external optical structures or polarizers that influence the scaling and bring up losses. Previous works have reported polarized light emission from inversion‐asymmetric 2D semiconductors such as black phosphorus (BP), black arsenide phosphorus (AsP), and rhenium disulfide (ReS2), however, their emission wavelengths are not in the visible range. Here, a direct emission of linearly polarized light is demonstrated from van der Waals light–emitting diodes (vdWLEDs) via the flexoelectric effects by inducing the non‐uniform strain in monolayer (ML) transition metal dichalcogenides (TMDCs). In this work, the effects of strain including excitonic binding energy and exciton dipole moment distribution is analyzed by the density functional theory (DFT) then we show that linearly polarized photoluminescence (PL) with a degree of linear polarization (DOLP) of ≈17% can be realized at room temperature (RT), and the polarization angle is perpendicular to the direction of the strain‐gradient. By incorporating the strained ML TMDCs into vdWLEDs, electroluminescence (EL) with DOLP of ≈19% can be observed at RT. This work puts forward a direct and universal strategy for fabricating polarized LEDs based on inversion‐symmetric semiconductors.

Effects of magnetic, electric, and non-resonant intense laser fields on exciton binding energy and exciton absorption in a symmetric Gaussian single quantum well

Effects of magnetic, electric, and non-resonant intense laser fields on exciton binding energy and exciton absorption in a symmetric Gaussian single quantum well

The Wannier-Mott Exciton, Bound Exciton, and Optical Phonon Replicas of Single-Crystal GaSe

We report the absorption and photoluminescence spectra of GaSe single crystals in the near-edge region. The temperatures explored the range from 17 to 300 K. Specifically, at a temperature of 17 K, the photoluminescence spectrum reveals an interesting phenomenon: the Wannier-Mott exciton separates into two states. These states are a triplet state with an energy of 2.103 eV and a singlet state with an energy of 2.109 eV. The energy difference between these two states is 6 meV. Furthermore, the bound exciton (BX) can be localized at an energy of 2.093 eV. It is worth noting that its phonon replicas (BX-nLO) can be clearly distinguished up to the fourth order. Interestingly, the energy gaps between these replicas exhibit a consistent spacing of 7 ± 0.5 meV. This intriguing finding suggests a high-quality crystalline structure as well as a strong coupling between the phonon and BX-nLO. Additionally, at low temperatures, both the ground state (n = 1) at 2.11 eV and the excited state (n = 2) at 2.127 eV of free excitons can be observed.

A Bound Exciton Resonance Modulated by Bulk and Localized Coherent Phonons in Double Perovskites.

Optically excited electronic excitations are coupled to the soft and polar halide perovskite lattice, generating coherent phonons after subpicosecond interband laser-excitation. In Ag-based halide double perovskites, Ag-vacancies can bind free excitons, resulting in a pronounced bound exciton resonance. Here, we report the detection of three modulation frequencies corresponding to coherent phonons in Ag-based double perovskite nanocrystals at distinct spectral positions at the bound exciton resonance. Two of them are found in oscillatory spectral shifts of the bound exciton resonance and are identified as Cs- and Br-related bulk phonons. Surprisingly, a third frequency is observed as an intensity modulation. We argue that this amplitude oscillation is a consequence of an optically generated vibronic wave packet localized at a Ag-vacancy. Consequently, the localized coherent phonon modulates the giant oscillator strength of the bound exciton. This optically induced and spatially localized lattice shaking could potentially be useful for initiating photochemical reactions with atomic precision.

Determination of Gallium Concentration in Silicon from Low‐Temperature Photoluminescence Analysis

Increasing the understanding of the electronic properties of gallium (Ga) in silicon (Si) used nowadays to manufacture p‐type Si solar cells is of key technological importance. In this contribution, the results of the effect of Ga concentration on the low‐temperature photoluminescence (PL) spectra in crystalline Si are reported. The Ga‐doped Si samples studied have negligible boron concentrations, which can complicate spectral analysis of the bound exciton (BE) lines. The split Ga BE ground state at T = 10 K is analyzed and the PL intensity ratios for the BE to free exciton peaks are compared. By comparing these to known Ga concentrations, derived from capacitance–voltage measurements, an all‐optical (PL) calibration curve for the quantification of Ga concentration in Si is established. The effects of both temperature and excitation power on the PL intensity ratios are also studied. By combining the temperature‐induced changes in the PL intensity ratios with the calibration curve at 10 K, a calibration function has been determined. It is found that the decay rates of the PL intensity ratios as a function of excitation power are independent of the chosen (split) BE peak. The major benefits of this method and its limitations are discussed.

Optical Orientation of Free and Bound Excitons in Hexagonal Crystals

Optical Orientation of Free and Bound Excitons in Hexagonal Crystals

Determination of lateral strain in InGaAsSb alloys and its effect on structural and optical properties

In_xGa_{1-x}As_ySb_{1-y} epilayers with a fixed In content of textit{x} = 0.145 were grown on GaSb(100) substrates using liquid-phase epitaxy (LPE). The lattice mismatch between the quaternary epilayers and substrates was analyzed for different As contents (textit{y}) by X-ray omega-2theta. Epilayers with As content between textit{y} = 0.120 and textit{y} = 0.124 exhibited a positive lattice mismatch, leading to compressive strain. These samples showed a high crystalline quality and flat surfaces, as confirmed by high-resolution X-ray diffraction (HR-XRD) and atomic force microscopy (AFM). Quaternary alloys with As content between textit{y} = 0.133 and textit{y} = 0.141 displayed a negative lattice mismatch, resulting in tensile strain. Structural defects in these samples were evidenced by HR-XRD and on AFM micrographs. Raman measurements also revealed that lateral strain has a direct impact on the intensities of the LO-like, phonon–plasmon and disorder-activated longitudinal acoustic modes. For all In_xGa_{1-x}As_ySb_{1-y} films, photoluminescence (PL) spectra showed a bound exciton (BE) transition, with additional features observed in samples under tensile strain, indicating the involvement of native defect centers and donor–acceptor pairs. This study provides new insights into the effect of lateral strain on the crystalline and surface quality, and optical properties of quaternary alloys, relevant for novel optoelectronic applications.

Gate-Tunable Bound Exciton Manifolds in Monolayer MoSe2

Two-dimensional (2D) semiconductors with point defects are predicted to host a variety of bound exciton complexes analogous to trions and biexcitons due to strong many-body effects. However, despite the common observation of defect-mediated subgap emission, the existence of such complexes remains elusive. Here, we report the observation of bound exciton (BX) complex manifolds in monolayer MoSe2 with intentionally created monoselenium vacancies (VSe) using proton beam irradiation. The emission intensity of different BX peaks is found to exhibit contrasting dependence on electrostatic doping near the onset of free electron injection. The observed trend is consistent with the model in which free excitons exist in equilibrium with excitons bound to neutral and charged VSe defects, which act as deep acceptors. These complexes are more strongly bound than trions and biexcitons, surviving up to around 180 K, and exhibit moderate valley polarization memory, indicating partial free exciton character.

Free exciton and bound excitons on Pb and I vacancies and O and I substituting defects in PbI2: Photoluminescence and DFT calculations

A complete study of structure, chemical composition, morphology and photoluminescence (PL) is performed for multilayer PbI2 flakes. The reduction of thickness results in uneven spatial mapping of PL and Raman intensities, while the positions and shapes of the spectra do not undergo notable changes. The most intense peaks in the PL spectra are the direct free exciton (FX) peak at 2.44 eV (508 nm) and the bound exciton (BX) at 2.41 eV (516 nm). The variation of above/below-bandgap excitation has allowed to find three more defect-related peaks at 2.35, 2.19 and 2.04 eV. So far, these three BXs have not been studied. Our DFT calculations combined with the experimental results allow to clarify the origins of BX peaks as the recombination from the optical conduction band edge to the defect levels located above the valence band: 2.41 eV to O substituting I (OI), 2.35 eV to Pb vacancy (VPb), and 2.19/2.04 eV to IPb with split level. We have defined that the known absorption at 1.55 eV (800 nm) is related to VI nonradiative recombination center. The temperature behaviors allow to determine the thermal dissociation energy of FX of 0.22 eV. The dependences also show a co-interaction between defect levels.

DFT and TD-DFT study of Optical and Electronic Properties of new donor–acceptor–donor (D–A–D′) monomers for polymer solar cells

AbstractThe Density Functional Theory (DFT) and time-dependent-DFT method with Becke’s three-parameter Lee-Yang–Parr functional approach at a basis set of 6-311G was used to analyze the ground state and excited state properties of newly designed donor–acceptor–donor (D–A–D′) donor molecules based on triphenylamine and carbazole as donor units and benzothiadiazole and its derivatives as acceptor units to make a total of nine potential monomers. The energies associated with highest occupied molecular orbital, lowest occupied molecular orbital, energy gap (Eg), electron excitation (Eopt), exciton binding (Eb) and open-circuit voltage (Voc) were calculated, and the simulated absorption spectra in both gas and chlorobenzene solvent were plotted. The outcomes of replacing the acceptor building unit and substituting the donor units to tailor the optoelectronic properties of the designed monomers were discussed. The monomer molecules A7, A8 and A9 are suitable for [6,6]-phenyl-C61-butyric acid methyl ester because of their small Eg, Eopt, Eb and, more importantly, large Voc values. Suggesting changing the acceptor unit and substituting the donor units of the D–A–D′ seem to be an excellent approach to tailoring the optoelectronic properties of the molecules.

Theoretical Study of the Exciton Binding Energy and Exciton Absorption in Different Hyperbolic-Type Quantum Wells under Applied Electric, Magnetic, and Intense Laser Fields.

In this study, we investigated the exciton binding energy and interband transition between the electron and heavy-hole for the single and double quantum wells which have different hyperbolic-type potential functions subject to electric, magnetic, and non-resonant intense laser fields. The results obtained show that the geometric shapes of the structure and the applied external fields are very effective on the electronic and optical properties. In the absence of the external fields, the exciton binding energy is a decreasing function of increasing well sizes except for the strong confinement regime. Therefore, for all applied external fields, the increase in the well widths produces a red-shift at the absorption peak positions. The magnetic field causes an increase in the exciton binding energy and provides a blue-shift of the absorption peak positions corresponding to interband transitions. The effect of the electric field is quite pronounced in the weak confinement regime, it causes localization in opposite directions of the quantum wells of the electron and hole, thereby weakening the Coulomb interaction between them, causing a decrease in exciton binding energy, and a red-shift of the peak positions corresponding to the interband transitions. Generally, an intense laser field causes a decrease in the exciton binding energy and produces a red-shift of the peak positions corresponding to interband transitions.

Using an Atomically Thin Layer of Hexagonal Boron Nitride to Separate Bound Charge-Transfer Excitons at Organic Interfaces

Although there has been extensive interest in combining organic and two-dimensional (2D) materials to produce hybrid heterostructures for optoelectronic applications (including photovoltaics), very few studies have demonstrated how such hybrid structures might outperform structures formed by either component alone. This work presents a scalable method to produce centimeter-sized organic-2D multilayer structures, and shows that the photon-to-free-carrier conversion yield is significantly enhanced when monolayer h-BN is inserted at an organic donor-acceptor interface. This result should inspire future studies on incorporating 2D materials into organic devices to improve efficiency.

Libwfa: Wavefunction analysis tools for excited and open‐shell electronic states

AbstractAn open‐source software library for wavefunction analysis, libwfa, provides a comprehensive and flexible toolbox for post‐processing excited‐state calculations, featuring a hierarchy of interconnected visual and quantitative analysis methods. These tools afford compact graphical representations of various excited‐state processes, provide detailed insight into electronic structure, and are suitable for automated processing of large data sets. The analysis is based on reduced quantities, such as state and transition density matrices (DMs), and allows one to distill simple molecular orbital pictures of physical phenomena from intricate correlated wavefunctions. The implemented descriptors provide a rigorous link between many‐body wavefunctions and intuitive physical and chemical models, for example, exciton binding, double excitations, orbital relaxation, and polyradical character. A broad range of quantum‐chemical methods is interfaced with libwfa via a uniform interface layer in the form of DMs. This contribution reviews the structure of libwfa and highlights its capabilities by several representative use cases.This article is categorized under: Software > Quantum Chemistry Theoretical and Physical Chemistry > Spectroscopy

Identification of free and bound exciton emission of MgO single crystal in vacuum ultraviolet spectral range

Temperature dependencies of optical reflectance and cathodoluminescence (CL) spectra were measured for the MgO single crystal using a custom-built vacuum ultraviolet (VUV) spectroscopic system. Simultaneous observation enabled us to identify free exciton (FE) and bound exciton (BE) emissions by comparing the CL emission with the exciton resonance structures. The results indicated that the BE emission dominates the near-band edge emission, and the FE emission was observed as a shoulder at 300 K. The results ensure strong excitonic nature and potential of a rock salt-structured MgO-based material system for an active element in the VUV light emitter.

Exciton Modulation in Perylene-Based Molecular Crystals Upon Formation of a Metal-Organic Interface From Many-Body Perturbation Theory.

Excited-state processes at organic-inorganic interfaces consisting of molecular crystals are essential in energy conversion applications. While advances in experimental methods allow direct observation and detection of exciton transfer across such junctions, a detailed understanding of the underlying excitonic properties due to crystal packing and interface structure is still largely lacking. In this work, we use many-body perturbation theory to study structure-property relations of excitons in molecular crystals upon adsorption on a gold surface. We explore the case of the experimentally-studied octyl perylene diimide (C8-PDI) as a prototypical system, and use the GW and Bethe-Salpeter equation (BSE) approach to quantify the change in quasiparticle and exciton properties due to intermolecular and substrate screening. Our findings provide a close inspection of both local and environmental structural effects dominating the excitation energies and the exciton binding and nature, as well as their modulation upon the metal-organic interface composition.

High-Temperature Excitonic Bose–Einstein Condensate in Centrosymmetric Two-Dimensional Semiconductors

The realization of high-temperature excitonic Bose-Einstein condensation (BEC) in practical materials poses great challenges, because of strict constraints in symmetry, exciton binding, lifetime, and interaction. Here, using first-principles methods and symmetry analysis, we propose a new route to realize high-temperature excitonic BEC in centrosymmetric 2D materials, exploiting the parity symmetry of band edges and reduced Coulomb screening. We demonstrate it by taking monolayer TiS3 as an example, whose lowest-energy exciton shows small exciton mass, small Bohr radius, large binding, and long lifetime simultaneously. The phase diagram of electron-hole systems is further constructed, showing that both BEC and superfluidity can be realized at high temperature and in a broad range of exciton density. Importantly, we reveal that the high-temperature character of excitonic BEC is robust against thickness, beneficial for its experimental observation. By application of this general strategy to 2D materials in the database, monolayer AuBr and BiS2 are identified as promising candidates for high-temperature excitonic BEC.

How free exciton–exciton annihilation lets bound exciton emission dominate the photoluminescence of 2D-perovskites under high-fluence pulsed excitation at cryogenic temperatures

Photoluminescence (PL) spectra of atomically thin 2D lead iodide perovskite films are shown to depend on excited-state density, especially at cryogenic temperatures. At high excited-state densities and low temperatures, free exciton (FE) emission is so suppressed by exciton–exciton annihilation (EEA) that other—normally much weaker—emissions dominate the PL spectrum, such as emission from bound excitons (BEs) or PbI2 inclusions. In the Ruddlesden–Popper perovskite with phenethylammonium (PEA) ligands (PEA2PbI4, PEPI), FE emission dominates at all temperatures at the excited-state densities reached with continuous wave excitation. At higher excited state densities reached with femtosecond pulsed excitation, the PL at temperatures under 100 K is dominated by BE emission redshifted from that of FE by 40.3 meV. Weak emission from PbI2 inclusions 170 meV higher in energy than FE PL is also observable under these conditions. Equilibrium between BE and FE states explains why FE emission first increases with decreasing temperature from 290 until 140 K and then decreases with decreasing temperature as the BEs become stable. A Dion–Jacobson (DJ) material based on 1,4-phenyl-enedimethanammonium (PDMA) supports the reduction of FE emission by EEA at cryogenic temperatures. However, in the PDMA-based DJ material, BE emission is never as pronounced. At low temperatures and high-excited state densities caused by pulsed excitation, a broad emission redshifted by 390 meV from the FE dominates. Based on comparison with temperature-dependent measurements of PbI2 films, this emission is suggested to arise from PbI2 inclusions in the material. Possible avenues for improving PL at room temperature are discussed concerning these findings.

Tunable interlayer excitons in two-dimensional SiC/MoSSe van der Waals heterostructures

In two-dimensional (2D) van der Waals heterostructures, the excitonic effects are significant enhanced due to the reduction of screening. Accurate description of light-matter interaction of 2D heterostructures is essential to understand their excitonic optical response and radiative lifetime. In this paper, the quasiparticle band structures and excitonic optical properties of SiC/MoSSe heterostructures are investigated by the GW + Bethe-Salpeter equation (GW + BSE) approach. Here the four stable structures 1-AA, 1-A'B, 2-AA and 2-A'B of SiC/MoSSe heterostructures are all type-II band alignment semiconductors, which are favorable for producing interlayer excitons. We find that low energy interlayer excitons of all the four structures posess high binding energies, and the optical dipole oscillator strength and radiative lifetimes can be modulated by several orders of magnitude as a result of the different intrinsic dipole moments of MoSSe. More interestingly, we also find that in 1-A'B and 2-A'B, the lowest-energy exciton is bright interlayer exciton with a strong absorption peak. The detailed analysis of the binding energies, transition contributions and exciton lifetimes of these interlayer excitons will be helpful for SiC/MoSSe heterostructures applications in the optoelectronics.

Optical properties of two-dimensional black phosphorus

Recently, black phosphorus (BP), an emerging layered two-dimensional (2D) material, has aroused much research interest. Distinguished from most of other 2D materials, BP is always a direct bandgap semiconductor regardless of the thickness, with the bandgap ranging from 0.3 eV (bulk) to 1.7 eV (monolayer), which is just fill the gap in the bandgap between graphene and transition metal dichalcogenides (TMDCs) in this frequency range. Besides, the BP exhibits many intriguing properties, such as high carrier mobility, highly tunable and anisotropic physical properties, which render the BP another star 2D material following graphene and TMDCs. In this review, we mainly focus on the advances in the optical properties of 2D BP, with the content covering the intrinsic optical properties and external perturbation effects on optical properties. Regarding the intrinsic optical properties, we introduce the anisotropic and layer-dependent optical absorption from interband transitions, the layer-dependent exciton binding energy and exciton absorption, visible-to-infrared photoluminescence, and stability of absorption and photoluminescence. As for external perturbation effects on optical properties, we introduce in-plane uniaxial and biaxial strain effects, gate-induced quantum confined Franz-Keldysh effect and Burstein-Moss effect. And finally we give a brief summary and outlook, pointing out some several interesting and important issues of BP, which need further studying urgently such as hyperbolic plasmons, intersubband transitions, optical properties in heterostructures and twist angle moiré superlattice and so on. This review gives an overview of the optical properties of BP and is expected to arouse the interest in further studying the BP.

Photovoltaic and spectroscopic properties of bacteriochlorin-based photosensitizer: molecular approach

The biophotovoltaic features of the porphyrin- and bacteriochlorin-based solar cells in different media were investigated through quantum chemistry calculations. The results show that a decrease in the chemical potential and electrophilicity facilitates the charge transfer process. Also, the bacteriochlorins have a lower energy barrier of electron injection than that of porphyrin, because of a longer charge transfer distance and less electron–hole overlap. Moreover, nonpolar solvents accelerate the free charge production, due to the favorable changes in the exciton radius and exciton binding. Improved spectroscopic properties are observed for the dyes in the solvent medium, which may be related to an increment in the probability of the electronic transition and reduced band gaps. Based on the simulated absorption spectra, bacteriochlorins are the preferred light-harvesting materials than porphyrin in the visible region. Moreover, the development of the push–pull model in the bacteriochlorin structure increases their energy conversion ability, due to advanced quantum chemistry reactivity properties. According to the final efficiency, bacteriochlorins are the preferred candidates in comparison with porphyrin to be applied in the dye-sensitized solar cells, in agreement with the experimental results.