Increase In Exciton Binding Energy Research Articles

Investigations on the photoelectric properties of the Mn doped inorganic CsSnCl3 perovskite

Mn dopant significantly improves the photoluminescent performance of CsSnCl3. To find the exact interactions induced by Mn dopant into the CsSnCl3 crystal, we studied the geometric, electronic and excitonic properties for these systems. We found an increased formation energy of the VSn defect, which promoting the stability and the luminescence properties of CsSnCl3 accordingly. Via analyses from the crystal field theory and the COHP calculations, we revealed that the transition of the 3d orbital of Mn2+ ion from 4T1→6A1 plays a crucial role in enhancing the luminescence of CsSnCl3. Also, we show that with the introduction of Mn dopant, the exciton binding energy increases, generating the more tight bounding interactions between the electron and hole pairs which results in the higher luminescence efficiency. Therefore, our results show that Mn doping indeed enhances the photoluminescence properties of CsSnCl3 and facilitates its application in the field of light-emitting devices.

The effect of particle size on the optical and electronic properties of hydrogenated silicon nanoparticles.

We use a combination of many-body perturbation theory and time-dependent density functional theory to study the optical and electronic properties of hydrogen terminated silicon nanoparticles. We predict that the lowest excited states of these silicon nanoparticles are excitonic in character and that the corresponding excitons are completely delocalised over the volume of the particle. The size of the excitons is predicted to increase proportionally with the particle size. Conversely, we predict that the fundamental gap, the optical gap, and the exciton binding energy increase with decreasing particle size. The exciton binding energy is predicted to counter-act the variation in the fundamental gap and hence to reduce the variation of the optical gap with particle size. The variation in the exciton binding energy itself is probably caused by a reduction in the dielectric screening with decreasing particle size. The intensity of the excited state corresponding to the optical gap and other low energy excitations are predicted to increase with decreasing particle size. We explain this increase in terms of the 'band structure' becoming smeared out in reciprocal space with decreasing particle size, increasing the 'overlap' between the occupied and unoccupied quasiparticle states and thus, the oscillator strength. Fourier transforms of the lowest excitons show that they inherit the periodicity of the frontier quasiparticle states. This, combined with the delocalisation of the exciton and the large exciton binding energy, means that the excitons in silicon nanoparticles combine aspects of Wannier-Mott, delocalisation and effect of periodicity of the underlying structure, and Frenkel, large exciton binding energy, excitons.

Effects of strain and hydrostatic pressure on exciton properties in asymmetric zinc-blende (In,Ga)N/GaN coupled double quantum wells

Within the effective mass approximation, we studied the light-hole and heavy-hole exciton states and their optical properties, such as oscillator strength and radiative decay time, in asymmetric zinc-blende GaN/InxGa1−xN/GaN/InyGa1−yN/GaN coupled double quantum wells. The variational method was performed under strain and hydrostatic pressure. The light-hole and heavy-hole exciton binding energies as a function of the left-well width and/or right-well width, middle-barrier thickness, and indium concentration x(y) of (In,Ga)N material are analyzed in detail. The light-hole and heavy-hole exciton binding energies show an increasing trend with increase of hydrostatic pressure. The hydrostatic pressure not only results in an increase in exciton binding energy, interband transition energy, and oscillator strength but also a decrease in the emission wavelength and radiative decay time. In addition, the biaxial strain dependence of the emission wavelength, radiative decay time, and interband transition energy of the exciton is very remarkable.

Quantum Confinement Effects on Excitonic Properties in the 2D vdW quantum system: The ZnO/WSe2 Case

Quantum confinement effects play important roles in determining the electronic structures and optical properties of 2D quantum systems. Herein, the 2D ZnO/WSe2 van der Waals (vdW) system to study the influence of quantum confinement on excitonic optical properties, considering vdW heterobilayers (HBs), sandwiched trilayers (STs), and superlattice (SL), is taken. First‐principles calculations show that the quasiparticle (QP) bandgap, exciton binding energy, and band alignment depend obviously on quantum confinement. The QP bandgap and exciton binding energy decrease from 2.61 and 0.98 eV (HB) to 2.30 and 0.61 eV (ST), then to 2.02 and 0.28 eV (SL). Moreover, the conduction band offsets can be tuned from 0.87 eV (HB) to 0.79 eV (ST), then to 2.27 eV (SL). In addition, in ZnO/WSe2 vdW HB, increasing interlayer distance from 2.09 to 4.29 Å can induce exciton binding energy increase from 0.77 to 1.02 eV and QP bandgap increase from 2.49 to 2.62 eV. These results may be useful to tune excitonic properties and design optoelectronic devices by forming 2D vdW quantum systems.

Observation of excitons at room temperature in ZnSxSe1-x nanostructures embedded in a porous Al2O3 template

This work presents the phenomenon of dielectric confinement of the excitons of quasi-zero-dimensional nanostructures of a ZnSxSe1-x solid solution in a dielectric template of porous anodic aluminum oxide (AАО), which are synthesized by a simple method of vacuum thermal evaporation. The structures obtained possess stable excitons at room temperature with exciton binding energies ranging from 90 to 250 meV depending on composition and size of semiconductor nanostructures. The phenomenon of dielectric confinement of excitons is directly demonstrated by varying the dielectric constant of the semiconductor material. The quantum confinement in the structures under study is small since the dimensions of the nanoparticles exceed considerably the Bohr radius of exciton. The increase in exciton binding energy with decreasing dimensions of nanoparticles is discussed. This increase can be explained by an increase in the dielectric surroundings.

One-Step Vapor-Phase Synthesis and Quantum-Confined Exciton in Single-Crystal Platelets of Hybrid Halide Perovskites.

To investigate the quantum confinement effect on excitons in hybrid perovskites, single-crystal platelets of CH3NH3PbBr3 are grown on mica substrates using one-step chemical vapor deposition. Photoluminescence measurements reveal a monotonous blue shift with a decreasing platelet thickness, which is accompanied by a significant increase in exciton binding energy from approximately 70 to 150 meV. Those phenomena can be attributed to the one-dimensional (1D) quantum confinement effect in the two-dimensional platelets. Furthermore, we develop an analytical model to quantitatively elucidate the 1D confinement effect in such quantum wells with asymmetric barriers. Our analysis indicates that the exciton Bohr radius of single-crystal CH3NH3PbBr3 is compressed from 4.0 nm for the thick (26.2 nm) platelets to 2.2 nm for the thin (5.9 nm) ones. The critical understanding of the 1D quantum confinement effect and the development of a general model to elucidate the exciton properties of asymmetric semiconductor quantum wells pave the way toward developing high-performance optoelectronic heterostructures.

Tunable Excitons in Bilayer Graphene with Opened Energy Gap

In this work we study excitonic absorption in bilayer graphene where an energy gap is opened by a static perpendicular electric field. On the basis of the tight-binding approach and second quantized Hamiltonian the linearized quantum kinetic equations for the density matrix are obtained. It is shown that the exciton binding energy and intensity of the excitonic absorption can be tuned by changing the value of the gap and tight-binding parameters. It is found that the account of circular asymmetry of the energy bands in bilayer graphene leads to a strong increase of exciton binding energy and excitonic absorption.

Pressure-Induced Emission Enhancement, Band-Gap Narrowing, and Metallization of Halide Perovskite Cs3 Bi2 I9.

Low-toxicity, air-stable bismuth-based perovskite materials are attractive substitutes for lead halide perovskites in photovoltaic and optoelectronic devices. The structural, optical, and electrical property changes of zero-dimensional perovskite Cs3 Bi2 I9 resulting from lattice compression is presented. An emission enhancement under mild pressure is attributed to the increase in exciton binding energy. Unprecedented band gap narrowing originated from Bi-I bond contraction, and the decrease in bridging Bi-I-Bi angle enhances metal halide orbital overlap, thereby breaking through the Shockley-Queisser limit under relatively low pressure. Pressure-induced structural evolutions correlate well with changes in optical properties, and the changes are reversible upon decompression. Considerable resistance reduction implies a semiconductor-to-conductor transition at ca. 28 GPa, and the final confirmed metallic character by electrical experiments indicates a wholly new electronic property.

Abnormal blueshift of the absorption edge in graphene nanodots.

In a conventional semiconductor, when the dielectric screening effect is suppressed, the exciton binding energy increases and the corresponding excitonic transition would exhibit a redshift in the spectrum. In this work, I study the optical properties of hexagonal graphene nanodots by using a configuration interaction approach and reveal that the edge of the absorption spectrum shows an abnormal blueshift as the environmental dielectric constant ϵr decreases. The two dominant many-body effects in the nanodot: the quasiparticle and excitonic effects are both found to scale almost linearly with ϵr-1. The former is shown to have a larger proportionality constant and thus accounts for the blueshift of the absorption edge. In contrast to the long-range Coulomb interaction, the on-site Coulomb energy is found to have a negative impact on the bright excitonic states. In the presence of a strong dielectric screening effect, a strong short-range Coulomb interaction is revealed to be responsible for the disintegration of the bright exciton.

Determination of strain relaxation in InGaN/GaN nanowalls from quantum confinement and exciton binding energy dependent photoluminescence peak

GaN based nanostructures are being increasingly used to improve the performance of various devices including light emitting diodes and lasers. It is important to determine the strain relaxation in these structures for device design and better prediction of device characteristics and performance. We have determined the strain relaxation in InGaN/GaN nanowalls from quantum confinement and exciton binding energy dependent photoluminescence peak. We have further determined the strain relaxation as a function of nanowall dimension. With a decrease in nanowall dimension, the lateral quantum confinement and exciton binding energy increase and the InGaN layer becomes partially strain relaxed which decreases the piezoelectric polarization field. The reduced polarization field decreases quantum confined Stark effect along the c-axis and increases electron-hole wave-function overlap which further increases the exciton binding energy. The strong dependency of the exciton binding energy on strain is used to determine the strain relaxation in these nanostructures. An analytical model based on fractional dimension for GaN/InGaN/GaN heterostructures along with self-consistent simulation of Schrodinger and Poisson equations are used to theoretically correlate them. The larger effective mass of GaN along with smaller perturbation allows the fractional dimensional model to accurately describe our system without requiring first principle calculations.

Fractal dimension method for exciton in cylindrical GaAs/AlxGa1−xAs core-shell-cap nanowires

By use of the fractal dimension method, the binding energies of heavy-hole exciton and light-hole exciton in cylindrical GaAs/Al[Formula: see text]Ga[Formula: see text]As core-shell-cap nanowire are explored. In this study, the exciton is confined in GaAs shell of the GaAs/Al[Formula: see text]Ga[Formula: see text]As core-shell-cap nanowire for a given aluminum concentration of [Formula: see text][Formula: see text]=[Formula: see text]0.3. The numerical results of heavy-hole exciton binding energy, light-hole exciton binding energy and fractal dimension parameter are worked out as functions of shell width and core radius. It has been shown by the calculated results that heavy-hole exciton binding energy and light-hole exciton binding energy firstly increase and then decrease as the shell width increases. When the core radius increases, both the heavy-hole exciton binding energy and light-hole exciton binding energy increase gradually. Exciton problems in GaAs shell of the cylindrical GaAs/Al[Formula: see text]Ga[Formula: see text]As core-shell-cap nanowire are solved in a simple manner to avoid complex and lengthy calculations by using the fractal dimension method.

EXCITONIC ABSORPTION IN GAPPED GRAPHENE SYSTEMS

Excitonic absorption in monolayer and bilayer graphene systems with opened energy gap in the field of laser radiation is investigated. The obtained value of excitonic binding energy in monolayer is in good agreement with the exact analytical solution. It is shown that the account of all tight binding parameters in bilayer graphene leads to an increase of exciton binding energy.

Influence of halide composition on the structural, electronic, and optical properties of mixedCH3NH3Pb(I1−xBrx)3perovskites calculated using the virtual crystal approximation method

Extensive studies have demonstrated the promising capability of the organic-inorganic hybrid halide perovskite ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ in solar cells with a high power conversion efficiency exceeding 20%. However, the intrinsic as well as extrinsic instabilities of this material remain the major challenge to the commercialization of perovskite-based solar cells. Mixing halides is expected to resolve this problem. Here, we investigate the effect of chemical substitution in the position of the halogen atom on the structural, electronic, and optical properties of mixed halide perovskites ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}{({\mathrm{I}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{3}$ with a pseudocubic phase using the virtual crystal approximation method within density functional theory. With an increase of Br content $x$ from 0.0 to 1.0, the lattice constant decreases in proportion to $x$ with the function of $a(x)=6.420\ensuremath{-}0.333x$ (\AA{}), while the band gap and the exciton binding energy increase with the quadratic function of ${E}_{g}(x)=1.542+0.374x+0.185{x}^{2}$ (eV) and the linear function of ${E}_{b}(x)=0.045+0.057x$ (eV), respectively. The photoabsorption coefficients are also calculated, showing a blueshift of the absorption onsets for higher Br contents. We calculate the phase decomposition energy of these materials and analyze the electronic charge density difference to estimate the material stability. Based on the calculated results, we suggest that the best match between efficiency and stability can be achieved at $x\ensuremath{\approx}0.2$ in ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}{({\mathrm{I}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{3}$ perovskites.

Design principles for block polymer organic double heterojunction solar cells

An organic double heterojunction photovoltaic device is described and the limits of its power conversion efficiency are investigated via numerical calculations. In the absence of exciton binding energy, fully conjugated block polymer devices exhibit power conversion efficiencies slightly exceeding the Shockley–Queisser limit. As exciton binding energy increases, a decrease in efficiency occurs, but remains over 20% for binding energies less than 0.5 eV. Further calculations show that devices require a high degree of phase purity to leverage the full benefit of the double heterojunction structure. Synthetic targets are identified and their maximum efficiency is calculated based on experimentally measured energy levels, leading to a generalized structural motif.

Superhalogens as building blocks of two-dimensional organic-inorganic hybrid perovskites for optoelectronics applications.

Organic-inorganic hybrid perovskites, well known for their potential as the next generation solar cells, have found another niche application in optoelectronics. This was demonstrated in a recent experiment (L. Dou, et al., Science, 2015, 349, 1518) on atomically thin (C4H9NH3)2PbBr4, where, due to quantum confinement, the bandgap and the exciton binding energy are enhanced over their corresponding values in the three-dimensional bulk phase. Using density functional theory we show that when halogen atoms (e.g. I) are sequentially replaced with superhalogen molecules (e.g. BH4) the bandgap and exciton binding energy increase monotonically with the superhalogen content with the exciton binding energy of (C4H9NH3)2Pb(BH4)4 approaching the value in monolayer black phosphorus. Lead-free admixtures (C4H9NH3)2MI4-x(BH4)x (M = Sn and Ge; x = 0-4) also show a similar trend. Thus, a combination of quantum confinement and compositional change can be used as an effective strategy to tailor the bandgap and the exciton binding energy of two-dimensional hybrid perovskites, making them promising candidates for optoelectronic applications.

Tuning the Luminescence Properties of Organometal Halide Perovskites for Light-Emitting Applications

Organometal halide perovskites are inexpensive materials with desirable characteristics of color-tunable and narrow-band emissions for lighting and display technology, but suffering from the low photoluminescence quantum yields at low excitation fluencies.2 We developed a ligand-assisted reprecipitation (LARP) strategy to fabricate CH3NH3PbX3 (X=Br, I, Cl) nanocrystals.3 By controlling their size and shape, we demonstrated the size- and shape- dependent photoluminescence properties. Especially, we obtain brightly-luminescent and color-tunable colloidal quantum dots with absolute quantum yield up to 70% at room temperature and low excitation fluencies. The comparisons between small sized CH3NH3PbBr3 quantum dots (average diameter 3.3 nm) and corresponding micrometer-sized bulk particles (2-8 μm) suggest that the intense increased photoluminescence quantum yield originates from the increase of exciton binding energy due to size reduction as well as proper chemical passivations of the Br rich surface. We further demonstrated the fabrication of prototype light-emitting devices, providing enhanced color quality for display technology. [1] Shirasaki, Y.; Supran, G. J.; Bawendi, M. G.; Bulovic, V. Nat. Photon. 2013, 7, 13-23 [2] Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H. J. Phys. Rev. Appl. 2014, 2, 034007. [3] Zhang, F.; Zhong, H. Z.; Chen, C.; Wu, X. G.; Hu, X. M.; Huang, H. L.; Han, J. B.; Zou, B. S.; Dong, Y. P. ACS Nano 2015, DOI: 10.1021/acsnano.5b01154

Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology

Organometal halide perovskites are inexpensive materials with desirable characteristics of color-tunable and narrow-band emissions for lighting and display technology, but they suffer from low photoluminescence quantum yields at low excitation fluencies. Here we developed a ligand-assisted reprecipitation strategy to fabricate brightly luminescent and color-tunable colloidal CH3NH3PbX3 (X = Br, I, Cl) quantum dots with absolute quantum yield up to 70% at room temperature and low excitation fluencies. To illustrate the photoluminescence enhancements in these quantum dots, we conducted comprehensive composition and surface characterizations and determined the time- and temperature-dependent photoluminescence spectra. Comparisons between small-sized CH3NH3PbBr3 quantum dots (average diameter 3.3 nm) and corresponding micrometer-sized bulk particles (2-8 μm) suggest that the intense increased photoluminescence quantum yield originates from the increase of exciton binding energy due to size reduction as well as proper chemical passivations of the Br-rich surface. We further demonstrated wide-color gamut white-light-emitting diodes using green emissive CH3NH3PbBr3 quantum dots and red emissive K2SiF6:Mn(4+) as color converters, providing enhanced color quality for display technology. Moreover, colloidal CH3NH3PbX3 quantum dots are expected to exhibit interesting nanoscale excitonic properties and also have other potential applications in lasers, electroluminescence devices, and optical sensors.

DONOR BOUND EXCITONS CONFINED IN WURTZITE InGaN/GaN QUANTUM DOT NANOWIRE HETEROSTRUCTURES

Within the framework of the effective-mass approximation, the donor bound exciton states and interband optical transitions in InGaN / GaN strained quantum dot (QD) nanowire heterostructures (NWHETs) are investigated using a variational method, in which the built-in electric field (BEF) effect due to the spontaneous and piezoelectric polarizations and the three-dimensional (3D) confinement of the electron and hole in the QD are considered. Our results show that the position of the ionized donor, the strong BEF, the In-composition and the QD structural parameters have a significant influence on the donor bound exciton binding energy, the electron interband optical transitions and the exciton oscillator strength. The donor bound exciton binding energy increases obviously if the donor position changes from the left-interface to the right-interface. The BEF reduces the bound exciton binding energy and the exciton oscillator strength. The donor bound exciton binding energy increases, if the In-fraction increases. The emission wavelength monotonically increases if the QD height and radius increase.

Proposal for ZnS/MgxBeyZn1-x-yS Quantum Wells

We propose the use of MgxBeyZn1-x-yS barriers combined with ZnS wells because MgxBeyZn1-x-yS barriers can be lattice-matched to ZnS wells by adjusting magnesium (Mg) (x) and beryllium (Be) ( y) contents. With increasing x and y, conduction and valence band offsets in ZnS/MgxBeyZn1-x-yS single quantum wells (SQWs) increase and thus the effective band gap energy can be controlled by adjusting x and y. In addition, the exciton binding energy increases. We indicate that the use of MgxBeyZn1-x-yS barriers can reduce the induced strain in the ZnS wells and is useful for designing ultraviolet applications using ZnS wells.

Reducing exciton–longitudinal optical phonon coupling with increasing Mg incorporation in MgZnO powders

The coupling between exciton and longitudinal optical (LO) phonon was investigated in the use of temperature-dependent photoluminescence from MgxZn1−xO powders prepared by a sol-gel method in the range of 0≤x≤0.05. The exciton binding energy increases to 73 meV for 5 at. % Mg incorporated powders. The strength of exciton–LO phonon coupling was deduced from the energy shift of exciton emission with temperature variation. The increase of the exciton binding energy results from a decrease of the exciton Bohr radius that is responsible for reducing the coupling strength of exciton–LO phonon as increasing Mg content.