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Quantum Dots - Theory and Applications || Theory of Excitons and Excitonic Quasimolecules Formed from Spatially Separated Electrons and Holes in Quasi-Zero- Dimensional Nanostructures
摘要： The theory of an exciton formed from a spatially separated electron and a hole is developed within the framework of the modified effective mass method. The effect of significantly increasing the exciton binding energy in quantum dots of zinc selenide, synthesized in a borosilicate glass matrix and relative to that in a zinc selenide single crystal is revealed. It is shown that the short-wavelength shift of the peak of the low-temperature luminescence spectrum of samples containing zinc selenide quantum dots, observed under experimental conditions, is caused by quantum confinement of the ground-state energy of the exciton with a spatially separated electron and hole. A review devoted to the theory of excitonic quasimolecules (biexcitons) (made up of spatially separated electrons and holes) in a nanosystem that consists of ZnSe quantum dots synthesized in a borosilicate glass matrix is developed within the context of the modified effective mass approximation. It is shown that biexciton (exciton quasimolecule) formation has a threshold character and is possible in a nanosystem, where the spacing between quantum dots' surfaces is larger than a certain critical arrangement. An analogy of the spectroscopy of the electronic states of superatoms (or artificial atoms) and individual alkali metal atoms theoretically predicted a new artificial atom that was similar to the new alkali metal atom.
关键词： excitonic quasimolecules,Excitons,superatoms,spatially separated electrons and holes,exciton binding energy,quantum dots
更新于2025-09-23 15:19:57
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Understanding the excitation wavelength dependent spectral shift and large exciton binding energy of tungsten disulfide quantum dots and its interaction with single-walled carbon nanotubes
摘要： Herein, we investigate the origin of excitation wavelength dependent spectral features and high fluorescence quantum yield in fluorescent 2D tungsten disulfide (WS2) quantum dots (QDs) of average size 2.4 nm. The as-prepared WS2 QDs possess high optical bandgap and reasonably high fluorescence quantum yield ~15.4% in the green region without any functionalization. The broad photoluminescence (PL) spectrum consists of multiple peaks owing to emissions from excitonic transitions and surface defect-related transitions. The excitation wavelength-dependent spectral redshift and narrowing of line shape in the PL peak are analyzed carefully, and it is attributed to the selective excitation/recombination of carriers from different energy levels. The temperature-dependent PL analysis yields an exciton binding energy of ~301 meV in the QDs. Furthermore, we study the interaction between fluorescent WS2 QDs and single-walled carbon nanotubes (SWCNTs) and explore the mechanism of systematic quenching of PL of QDs by SWCNTs. The nature of the Stern–Volmer plot is found to be linear, and the time-resolved fluorescence measurements reveal that the quenching follows primarily the static behavior. Our study further reveals that defect sites in SWCNTs primarily act as the binding sites for WS2 QDs and form non-fluorescent complexes for effective quenching of the PL. The strong interaction between the WS2 QDs and the SWCNTs is evidenced from the spectral shift in the X-ray photoelectron spectroscopy and Raman peaks. Our study reveals the origin of excitation wavelength dependent PL emission from WS2 QDs and the nature of the interaction between WS2 QDs and SWCNTs, which are important for their applications in biomedical imaging and sensing, such as surface-enhanced Raman scattering, etc.
关键词： WS2 quantum dots,Fluorescence quenching,Defects,Carbon nanotubes,Exciton binding energy
更新于2025-09-16 10:30:52
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Exciton states in InGaAsP/InP core–shell quantum dots under an external electric field
摘要： The effect of an external electric field on the exciton states of InGaAsP/InP core–shell quantum dots is investigated through the variational method. The effect of the shell thickness, core radius, electric field strength, and material components on the exciton states are analyzed in detail. The numerical results show that the electron and hole energies decrease as the shell thickness or core radius is increased. The Bohr radius is a nonmonotonic function of the shell thickness or core radius, and the change of the exciton binding energy is nonlinear as the shell thickness or core radius is increased. With increasing electric field strength, the Bohr radius increases while the exciton binding energy decreases. The exciton binding energy decreases (increases) as the Ga (As) component is increased.
关键词： Electric field,Core–shell quantum dot,Exciton binding energy
更新于2025-09-16 10:30:52
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Influence of electric field on direct and indirect exciton in a concentrically coupled quantum ring heterostructure embedded in SiO2 matrix
摘要： Excitonic states of Concentric Double Quantum Ring (CDQR) heterostructures embedded in SiO2 matrix under the influence of electric field has been calculated theoretically using variational technique. CDQR has been confined in a quantum well of suitable width which serve as the height of the quantum ring along which the electric field is applied. Central barrier width, outer ring width and carrier location has been tuned to understand the localization of both direct and indirect excitons. Effect of ring dimension and electric field on the energy states of Inner Ring Exciton (IRE) and Outer Ring Exciton (ORE) has been determined. Tilting of conduction and valence bands due to the application of electric field invokes a triangular confinement to the carriers and the same has been discussed. Diamagnetic susceptibility which is merely an indicator of electron hole separation and intersubband transition energy has also been calculated for both symmetric and asymmetric quantum ring. Height of the QR has also been changed to study its effect on the diamagnetic susceptibility of an exciton.
关键词： Diamagnetic Susceptibility,Quantum Ring,Exciton,Binding energy,Electric Field
更新于2025-09-12 10:27:22
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Surface Modification for Improving the Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene over Inorganic Lead Halide Perovskite Quantum Dots
摘要： Inorganic lead halide perovskite quantum dots (iLHP-QDs) have recently been used in the photocatalytic reaction. However, the factors that influence the photocatalytic performance of the iLHP-QDs has not been fully investigated. Herein, we synthesised a series of iLHP-QDs with varied halide ratio (CsPbX3, X=I, I0.67Br0.33, I0.5Br0.5, I0.33Br0.67, Br) and studied its influence on the photocatalytic performance by monitoring the polymerization of TerEDOT. The CsPbI3 QDs showed the best performance owing to its narrow bandgap and low exciton binding energy. Moreover, the photocatalytic performance of the iLHP-QDs could be simply improved by being treated with methyl acetate, which can be attributed to the replacement of the oleic acid by the short acetate acid, and the introduction of the traps on the surface of QDs in the post-treatment. These results could help design more efficient photocatalytic system and further promote the application of iLHP-QDs.
关键词： photocatalytic performance,quantum dots,exciton binding energy,charge transport efficiency,halide perovskite
更新于2025-09-11 14:15:04
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Reference Module in Materials Science and Materials Engineering || Excitonic Model versus Band Gap Model in Organic Materials
摘要： The binding energy Eb of an optical excitation is a key parameter for the understanding of the opto-electronic properties of organic solids in general and of conjugated polymers in particular. It controls the dissociation of an electronic excitation of either singlet or triplet character into a pair of free charges as well as the reverse process, i.e., the recombination of an electron–hole (eh) pair yielding an excitation that can decay radiatively or nonradiatively. If Eb is large, photogeneration of charge carriers is an endothermic, inefficient process. It is obvious that in an organic solar cell (OSC) one would like Eb to be as small as possible, while in a light emitting diode (LED) it is the opposite because charge recombination requires a driving force. In this article an outline of the problem of defining the exciton binding in a molecular solid will be presented followed by experimental and theoretical advances and a critical review of relevant conceptual frameworks with particular emphasis on conjugated polymers.
关键词： Organic Materials,Conjugated Polymers,Band Gap Model,Opto-electronic Properties,Exciton Binding Energy
更新于2025-09-11 14:15:04
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Extremely reduced dielectric confinement in two-dimensional hybrid perovskites with large polar organics
摘要： Two dimensional inorganic–organic hybrid perovskites (2D perovskites) suffer from not only quantum confinement, but also dielectric confinement, hindering their application perspective in devices involving the conversion of an optical input into current. In this report, we theoretically predict that an extremely low exciton binding energy can be achieved in 2D perovskites by using high dielectric-constant organic components. We demonstrate that in (HOCH2CH2NH3)2PbI4, whose organic material has a high dielectric constant of 37, the dielectric confinement is largely reduced, and the exciton binding energy is 20-times smaller than that in conventional 2D perovskites. As a result, the photo-induced excitons can be thermally dissociated efficiently at room temperature, as clearly indicated from femtosecond transient absorption measurements. In addition, the mobility is largely improved due to the strong screening effect on charge impurities. Such low dielectric-confined 2D perovskites show excellent carrier extraction efficiency, and outstanding humidity resistance compared to conventional 2D perovskites.
关键词： dielectric confinement,humidity resistance,carrier mobility,exciton binding energy,2D perovskites
更新于2025-09-10 09:29:36
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Enhanced Luminescence from InGaN/GaN Nano-disk in a wire Array Caused by Surface Potential Modulation during Wet Treatment
摘要： Here we have demonstrated the profound impact of surface potential on the luminescence of an array of InGaN/GaN nano-disk in a wire heterostructure. The change in surface potential is brought about by a combination of dry and successive wet-processing treatments. The photoluminescence (PL) properties are determined as a function of size and varying surface potential. The change in hole bound state energy due to parabolic potential well near the side-wall is found to be the dominating factor. The PL peak position, full width at half-maximum (FWHM), strain relaxation and integrated PL intensity are studied as a function of incident power and temperature. The devices demonstrate higher integrated PL intensity and slope efficiency.
关键词： exciton binding energy,photoluminescence,QCSE,nano-disk,surface potential,strain relaxation,InGaN/GaN,quantum confinement
更新于2025-09-04 15:30:14
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Superbound Excitons in 2D Phosphorene Oxides
摘要： The optical excitations in layered phosphorene oxides are studied via ab initio calculation together with GW approximation for the self-energy and solving the Bethe–Salpeter equation (BSE) for the excitations. It is found that the electronic structure of phosphorene oxides closely depend on the oxygen concentration, for the high oxygen coverage structure P4O10, it shows a strong localized molecular like electronic structure with exciton binding (Eb) energy up to 3.0 eV, which is several times larger than the ordinary Eb value in various low dimensional materials. This study may provide an alternative way to design functional layered materials with large exciton binding energy by controlling the oxidation level in phosphorene oxides.
关键词： GW approximation,exciton binding energy,Bethe–Salpeter equation,ab initio calculation,phosphorene oxides
更新于2025-09-04 15:30:14