Direct Exciton Research Articles

Exciton optical absorption in asymmetric ZnO/ZnMgO double quantum wells with mixed phases**Project supported by the National Natural Science Foundation of China (Grant No. 61764012).

The optical absorption of exciton interstate transition in Zn1 − xlMgxlO/ZnO/Zn1 − xcMgxcO/ZnO/Zn1 − xrMgxrO asymmetric double quantum wells (ADQWs) with mixed phases of zinc-blende and wurtzite in Zn1 − xMgxO for 0.37 < x < 0.62 is discussed. The mixed phases are taken into account by our weight model of fitting. The states of excitons are obtained by a finite difference method and a variational procedure in consideration of built-in electric fields (BEFs) and the Hartree potential. The optical absorption coefficients (OACs) of exciton interstate transition are obtained by the density matrix method. The results show that Hartree potential bends the conduction and valence bands, whereas a BEF tilts the bands and the combined effect enforces electrons and holes to approach the opposite interfaces to decrease the Coulomb interaction effects between electrons and holes. Furthermore, the OACs indicate a transformation between direct and indirect excitons in zinc-blende ADQWs due to the quantum confinement effects. There are two kinds of peaks corresponding to wurtzite and zinc-blende structures respectively, and the OACs merge together under some special conditions. The computed result of exciton interband emission energy agrees well with a previous experiment. Our conclusions are helpful for further relative theoretical studies, experiments, and design of devices consisting of these quantum well structures.

Observation of double indirect interlayer exciton in WSe2/WS2 heterostructure.

Interlayer excitons (IX) are produced by the spatially separated electron-hole pairs due to the robust Coulomb interactions in van der Waals transition metal dichalcogenide (TMDC) heterostructures (HSS). IX is characterized by a larger binding energy, and its lifetime is orders of magnitude longer than that of the direct excitons, providing a significant platform for the manufacture of long-lived exciton devices and the exploration of exciton quantum gas. However, the studies are restricted to the single interlayer exciton, and the simultaneous capture and study of double IX remain challenging in the WSe2/WS2 HS. Here, we demonstrate the existence of double indirect IX in the WSe2/WS2 HS with the emission centers at 1.4585eV (∼25.9meV wide) and 1.4885 eV (∼14.4 meV wide) at cryogenic temperature. Interestingly, the intensities of the double IX emission peaks are almost equal, and the energy difference between them is in a good agreement with the cleavage value of the WS2 conduction band (CB). Additionally, diverse types of excitons in the individual materials were successfully observed in the PL spectra at 8 K. Such unique double IX features, in combination with excellent exciton identification, open up new opportunities for further investigations for new physical properties of TMDCs and explorations for the technological innovation of exciton devices.

Transfer of Direct to Indirect Bound Excitons by Electron Intervalley Scattering in Cs2AgBiBr6 Double Perovskite Nanocrystals

Lead-free halide double perovskites have emerged as a non-toxic alternative to the heavily researched lead-based halide perovskites. However, their optical properties and the initial charge carrier relaxation processes are under debate. In this study, we apply time-resolved photoluminescence and differential transmission spectroscopy to investigate the photo-excited charge carrier dynamics within the indirect band structure of Cs2AgBiBr6 nanocrystals. Interestingly, we observe a high energetic emission stemming from the direct band gap, besides the previously reported emission from the indirect band gap transition. We attribute this emission to the radiative recombination of direct bound excitons. This emission maximum redshifts nearly 1 eV within 10 ps due to electron intervalley scattering, which leads to a transfer of direct to indirect bound excitons. We conclude that these direct bound excitons possess a giant oscillator strength causing not only a pronounced absorption peak at the optical band gap energy but also luminescence to occur at the direct band gap transition in spite of the prevailing intervalley scattering process. These results expand the understanding of the optical properties and the charge carrier relaxation in double perovskites, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites.

Temperature activation of indirect exciton in nanostructures based on MoS2

Micro-photoluminescence studies of MoS2 nanotubes have shown that both direct and indirect excitonic transitions contribute to the emission spectra despite the fact that dozens of monolayers are inside their walls and, therefore, only indirect exciton emission would be expected. In addition, the intensity of the indirect exciton band increases with temperature in contrast to the direct exciton band, the intensity of which decreases. The same effects are observed for MoS2 flakes. To describe these phenomena, we propose a theoretical model that considers the balance between exciton transitions taking into account the spin-forbidden exciton states.

Hydrostatic pressure dependence of indirect and direct excitons in InGaN/GaN quantum wells

We analyze the evolution of the exciton recombination energy ${E}_{\mathrm{PL}}$ and its pressure coefficient $d{E}_{\mathrm{PL}}/dp$, with the laser power density (LPD) exciting excitonic photoluminescence. Two $\mathrm{I}{\mathrm{n}}_{0.17}\mathrm{G}{\mathrm{a}}_{0.83}\mathrm{N}/\mathrm{GaN}$ quantum well (QW) structures are used: sample A consisting of a 5.2-nm-wide single QW and sample B consisting of two 2.6-nm-wide QWs separated by a 0.78-nm-thick barrier. Changes of ${E}_{\mathrm{PL}}$ and $d{E}_{\mathrm{PL}}/dp$ with LPD in both structures cover a wide range of magnitude. They switch character in the vicinity of the indirect to direct exciton induced by increasing LPD. In sample B, at low excitation, a negative pressure coefficient of ${E}_{\mathrm{PL}}$ has been found. Moreover, pressure dependences of ${E}_{\mathrm{PL}}$ are different for the two samples under study. We examined pressure dependence of the threshold LPD for indirect-direct exciton switching in both samples. The reported observations originate to a high extent from screening of a built-in electric field by carriers externally introduced into the quantum wells. We also note that the universal relation between transition energies and their pressure coefficients (observed previously in InGaN/GaN quantum wells) does not hold for samples modified by screening.

Layer-dependent signatures for exciton dynamics in monolayer and multilayer WSe2 revealed by fluorescence lifetime imaging measurement

Two-dimensional (2D) transition-metal dichalcogenide (TMD) materials have aroused noticeable interest due to their distinguished electronic and optical properties. However, little is known about their complex exciton properties together with the exciton dynamics process which have been expected to influence the performance of optoelectronic devices. The process of fluorescence can well reveal the process of exciton transition after excitation. In this work, the room-temperature layer-dependent exciton dynamics properties in layered WSe2 are investigated by the fluorescence lifetime imaging microscopy (FLIM) for the first time. This paper focuses on two mainly kinds of excitons including the direct transition neutral excitons and trions. Compared with the lifetime of neutral excitons (< 0.3 ns within four-layer), trions possess a longer lifetime (~ 6.6 ns within four-layer) which increases with the number of layers. We attribute the longer-lived lifetime to the increasing number of trions as well as the varieties of trion configurations in thicker WSe2. Besides, the whole average lifetime increases over 10% when WSe2 flakes added up from monolayer to four-layer. This paper provides a novel tuneable layer-dependent method to control the exciton dynamics process and finds a relatively longer transition lifetime of trions at room temperature, enabling to investigate in the charge transport in TMD-based optoelectronics devices in the future.

Hartree-Fock approximation for Exciton Mott transition in double quantum well: Direct and Indirect exciton diamagnetism

Semiconductor to Metal transition due to Excitons i.e., Mott Transition (EMT) in GaAs/Al x Ga 1-x As double quantum well has been demonstrated theoretically by observing the abrupt change in diamagnetic susceptibility (χ Dia ) of direct (indirect) excitons at critical concentration. Variation technique combined with Monte-Carlo method is used to calculate Binding Energy (BE) and exciton diamagnetic susceptibility (χ Dia ) by optimizing <H min > by varying the exciton density. To account for many body interaction, Hartree-Fock dielectric screening is used which manipulates the exchange-correlation effects in our observations. The contribution of phonon interaction in the formation of indirect exciton is involved through size dependent screening. Through this work we demonstrate that Exciton Mott Transition, especially semiconductor to metal transition is a type two phase transition which is attributed by the abrupt fall in the diamagnetic susceptibility of excitons. The critical concentration at T = 4 K is estimated for the direct and indirect excitons for various dimensions of the coupled and decoupled wells. • Exciton Mott Transition due to direct and indirect excitons in double quantum well. • Hartree-Fock approximation is adopted for Exciton-Exciton Interaction. • Critical exciton concentration (N ex C ) is estimated for coupled and decoupled wells. • Abrupt fall in Exciton Diamagnetism (χ Dia ) is observed at N ex C . • Transition occurs at lower N ex C in narrow wells due to indirect excitons.

Reduced Binding Energy and Layer-Dependent Exciton Dynamics in Monolayer and Multilayer WS2.

The exciton dynamics in WS2 from monolayer to four-layer was investigated by using fluorescence lifetime imaging measurement (FLIM). The transition process of negatively charged trions is measured and detected using a fluorescence detection method. Compared with neutral excitons, negatively charged trions have a longer fluorescence lifetime. Further exploration illustrated that the fluorescence lifetime of both neutral excitons and trions get longer when the thickness increased. When WS2 was added from monolayer to four-layer, lifetimes of direct transition excitons and trions tended to increase over 10 and 2.5 times, separately, whereas the lifetime of indirect transition excitons tended to be reduced by nearly 2.5 times. This layer-dependent signature is ascribed to the reduced binding energy in thicker WS2 at room temperature, which is verified by density theory functional calculation. Although the direct transition exciton dominates the whole fluorescence decay process, it is influenced by trions and dark excitons. Based on the FLIM results, we proposed four main exciton transition channels during the fluorescence luminescence process. Such layer-dependent transition channel conception helps to control the fluorescence lifetime, which determines the efficiency of the carriers' separation.

Quantum Noise and Squeezed Light by Dipolaritons in the Nonlinear Regime

AbstractThe quantum noise and the squeezing effect, in the transmitted light, by a dipolariton system formed by a double quantum wells microcavity in the strong coupling regime are investigated. It is shown that the indirect exciton nonlinearity generates stronger squeezing than the direct exciton nonlinearity, nevertheless produces very high fluctuations for some particular detunings and nonlinearities. The system shows a considerable sensitivity to the thermal excitations where the coupling to the direct and indirect excitonic thermal baths progressively destroys the nonclassical effect. Despite that, indirect exciton squeezing manifests a higher resistance against the thermal bath temperature. As a result, the emission field is strongly governed by the indirect excitons, varying between robust squeezing and excess noise.

Influence of electric field on direct and indirect exciton in a concentrically coupled quantum ring heterostructure embedded in SiO2 matrix

Abstract 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.

Estimation of dimensional criticality for direct to indirect exciton transition in a double Quantum Well: Through exciton diamagnetism

Exciton diamagnetism is calculated to estimate theoretically the critical well dimension for the transition of direct exciton (DX) to indirect (IDX) in a GaAs/Al0.3Ga0.7As symmetric Double Quantum Well (DQW) by variation ansatz. By tuning the confinement layer widths, the critical well size at which the constituent carriers of DX breaks up and transforms to indirect is observed through the sudden decrease of diamagnetic susceptibility (χDia) of DX. The structural influence on the binding of DX and IDX with (i) light hole (LH) and (ii) heavy hole (HH) has been numerically estimated in the present work. The intricate role of LO phonons in the formation of exciton is accounted through the size-dependent dielectric screening.

Optical properties of anisotropic excitons in phosphorene

We study the eigenenergies and optical properties of both direct excitons in a phosphorene monolayer in different dielectric environments, and indirect excitons in heterostructures of phosphorene with hexagonal boron nitride. For these systems, we solve the 2D Schr\"{o}dinger equation using the Rytova-Keldysh (RK) potential for direct, and both the RK and Coulomb potentials for indirect excitons. The results show that excitons formed from charge carriers with anisotropic effective mass exhibit enhanced (suppressed) optical absorption, compared to their 2D isotropic counterparts, under linearly polarized excitations along the crystal axis with relatively smaller (larger) effective carrier masses. This anisotropy leads to dramatically different excited states than the isotropic exciton. The direct exciton binding energy depends strongly on the dielectric environment, and shows good agreement with previously published data. For indirect excitons, the oscillator strength and absorption coefficient increase as the interlayer separation increases. The choice of RK or Coulomb potential does not significantly change the indirect exciton optical properties, but leads to significant differences in the binding energy for small interlayer separation.

Robust circular polarization of indirect Q-K transitions in bilayer 3R−WS2

Valley-contrasting Berry curvature and orbital magnetic moment have led to highly selective circular polarization of direct excitons at the K valleys in transition-metal dichalcogenides. In addition to K valleys, Q valleys, another critical point in the conduction band, also possess well-defined but distinct magnetic moment. Being akin to the direct excitons at K valleys, indirect excitons associated with Q (K) valleys in the conduction (valence) band could allow circular polarization in principle. Here, we report an experimental observation of the circular polarization of indirect Q-K transitions in noncentrosymmetric bilayer $3R\ensuremath{-}\mathrm{W}{\mathrm{S}}_{2}$. In stark contrast to the circular polarization of direct excitons which depolarizes with increasing lattice temperature, the circular polarization of indirect Q-K excitons is extremely robust and independent on the temperature. Such robust circular polarization can be understood as follows: the spin-orbit coupling in the Q valley is much stronger than that in the K point of the conduction band, significantly suppressing the temperature induced valley depolarization. Our results open up opportunities for exotic valleytronics and quantum information processing applications.

Indirect to direct exciton transition by laser irradiance in a type II core/ shell quantum dot

Effect of laser irradiance on indirect exciton in Type II CdTe/CdSe Core/Shell Quantum Dot (CSQD) is investigated theoretically through diamagnetic susceptibility (χdia). CO2 laser source of wavelength 10 μm with 30 THz frequency is used as an external perturbation. Using variational method, χdia of exciton is computed for different laser intensity (α0) under different shell thickness. Drastic variation in χdia is observed on enhancing α0. This is due to the wide spatial extent of the carriers by the high screening effect of LASER. Transition of localized exciton from core to the shell regime is pictured via the probability density function of exciton. Further, the ground state energy of hole and the electron of this CSQD is determined for various shell radius (Rs). It is inferred that the incident of LASER strongly alters the ground state energy of the carriers. The evolution of direct exciton from indirect exciton by laser irradiance is realized. To conclude, the external laser perturbation plays a crucial role in improving the device performance by stabilizing excitonic states.

High performance two-color hybrid TADF-phosphorescent WOLEDs with bimodal Förster and Dexter-type exciton distribution

Abstract Emitter molecules based on thermally activated delayed fluorescence (TADF) show great potential for highly efficient and stable organic light-emitting diodes (OLEDs). The charge-transfer nature of the emitting excitons often leads to broad emission spectra. This allows to rethink the design of white OLEDs (WOLEDs). While with fluorescent or phosphorescent emitters, three differently emitting molecules are needed to realize WOLEDs with high spectral quality, broadband TADF emitters may allow to reduce the number of emitters to two. Here, we discuss highly efficient blue OLEDs based on a blue TADF emitter 9-[2,3,4,5-tetra(carbazol-9-yl)-6-(trifluoromethyl)phenyl]carbazole (5CzCF3Ph) and utilize a very similar device architecture to realize high performance WOLEDs comprising the red phosphorescent emitter bis(2-methyldibenzo [f,h] quinoxaline) (acetylacetonate) iridium(III) (Ir(MDQ)2(acac)). The OLEDs investigated reach high external quantum efficiencies (EQEs) up to 18.3% for the blue and 17.3% for the white devices. The emission spectra of the two-color WOLEDs achieve color rendering indices (CRIs) higher than 80 along with CIE color coordinates close to ideal warm white emission. Time- and spectrally resolved photoluminescence (PL) measurements allow to identify that both Forster- and Dexter-type energy transfer processes are responsible for the exciton distribution in the corresponding hybrid TADF-phosphorescence emission layer. Furthermore, significant direct exciton formation on the Ir(MDQ)2(acac) emitter, even at the low concentrations used in this work, contributes to the overall white electroluminescence (EL).

Rydberg spectroscopy of indirect excitons.

Measuring the 1s–2p splitting of direct and indirect excitons in van der Waals heterostructures allows their binding energy and dynamics to be determined.

English

The effect of applied crossed electric and magnetic fields on the heterostructure semiconductor is used in the scientific investigation on electronic and optical exciton properties. The aim of this work is to study the magneto-Stark effect for confined excitons in single GaAs-a€–Gaa€—_(0.7) a€–Ala€—_(0.3) As QWs. The magnetic field B is taken as perpendicular to the z-growth direction of the heterostructure, whereas the applied electric field E is along the z-growth direction. The data we used includes intrinsic parameters of the systems and manipulated external magnetic and electric fields. In the model equation, we utilized variational non-degenerate parabolic band approximations using 1 s hydrogen like ion ground state to calculate the position at which spatial distance b/n electron and hole (∆=0), that is, overlap e-h occurred where B→∞ and E→0; we also used Matlab version R2017a to simulate our result as depicted in graphs. As electric field (E) increases along growth z-direction, the spatial distance (∆) increases due to a reduction of Coulomb interaction b/n e–h, whereas increasing the magnetic field (B) perpendicular to the growth z-direction has the reverse effect and shrinks the wave function in the QW plane. This shrinkage enhances the e–h interaction, which in turn, more likely localizes the electron within the same QW as the whole and thus keeps the ground state in a direct exciton which is efficient in photonics. Key words: Spatial distance, exciton, growth direction, non-correlation, magneto-Stark.

Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $\Gamma$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations.

Recent Applications of Interfacial Exciplex as Ideal Host of Power-Efficient OLEDs.

Currently, exploring the applications of intermolecular donor-acceptor exciplex couple as host of OLEDs with phosphorescence, thermally activated delayed fluorescence (TADF) or fluorescence emitter as dopant is a hot topic. Compared to other host strategies, interfacial exciplex has the advantage in various aspects, such as barrier-free charge injection, unimpeded charge transport, and the energy-saving direct exciton formation process at the “Well”-like heterojunction interface region. Most importantly, due to a very fast and efficient reverse intersystem-crossing (RISC) process, such a host is capable of regulating singlet/triplet exciton populations in itself as well as in the dopant emitters both under photoluminescent (PL) and electroluminescent (EL) driving conditions. In this mini-review, we briefly summarize and comment on recent applications of this ideal host in OLEDs (including both thermal-evaporation OLEDs and solution-processed OLEDs) with diverse emitters, e.g., fluorescence, phosphorescence, delayed fluorescence, or others. Special attention is given to illustrate the peculiar achievement of high overall EL performance with superiorities of low driving voltages, slow roll-off rate, high power efficiencies and satisfied device lifetime using this host strategy, which is then concluded by personal perspectives on the relevant next-step in this field.

Density functional theory of two-dimensional electron–hole complexes

The energy of the two-dimensional electron–hole complex has been calculated in the framework of the density functional theory. We show that the energy of a direct two-dimensional exciton, without taking into consideration the exchange–correlation interaction, is very different from the exact value. We find that the number of particles in the indirect electron–hole complexes decreases with increasing interlayer distance in a strong magnetic field.