Efficient energy transfer from self-trapped excitons to Mn^(2+) dopants in CsCdCl_(3):Mn^(2+) perovskite nanocrystals

Mn^(2+)doping has been adopted as an efficient approach to regulating the luminescence properties of halide perovskite nano-crystals(NCs).However,it is still difficult to understand the interplay of Mn^(2+)luminescence and the matrix self-trapped exciton(STE)emission therein.In this study,Mn^(2+)-doped CsCdCl_(3) NCs are prepared by hot injection,in which CsCdCl_(3) is selected because of its unique crystal structure suitable for STE emission.The blue emission at 441 nm of undoped CsCdCl_(3) NCs originates from the defect states in the NCs.Mn^(2+)doping promotes lattice distortion of CsCdCl_(3) and generates bright orange-red light emission at 656 nm.The en-ergy transfer from the STEs of CsCdCl_(3) to the excited levels of the Mn^(2+)ion is confirmed to be a significant factor in achieving efficient luminescence in CsCdCl_(3):Mn^(2+)NCs.This work highlights the crucial role of energy transfer from STEs to Mn^(2+)dopants in Mn^(2+)-doped halide NCs and lays the groundwork for modifying the luminescence of other metal halide perovskite NCs.

Interaction of moiréexcitons with cavity photons in two-dimensional semiconductor hetero-bilayers

Moirématerials,composed of two single-layer two-dimensional semiconductors,are important because they are good platforms for studying strongly correlated physics.Among them,moirématerials based on transition metal dichalcogenides(TMDs)have been intensively studied.The hetero-bilayer can support moiréinterlayer excitons if there is a small twist angle or small lattice constant difference between the TMDs in the hetero-bilayer and form a type-Ⅱ band alignment.The coupling of moiréinterlayer excitons to cavity modes can induce exotic phenomena.Here,we review recent advances in the coupling of moiréinterlayer excitons to cavities,and comment on the current difficulties and possible future research directions in this field.

Efficient transfer of metallophosphor excitons via confined polaritons in organic nanocrystals

We investigate the transfer of phosphorescent energy between co-assembled metallophosphors in crystalline nanostructures [Angew. Chem. Int. Ed. 57 7820(2018) and J. Am. Chem. Soc. 140 4269(2018)]. Neither Dexter's nor Forster's mechanism of resonance energy transfer(RET) could account fully for the observed rates, which exceed 85% with significant temperature dependence. But there exists an alternative pathway on RET mediated by intermediate states of resonantly confined exciton–polaritons. Such a mechanism was used to analyze artificial photosynthesis in organic fluorescents [Phys.Rev. Lett. 122 257402(2019)]. For metallophosphors, the confined modes act as extended states lying between the molecular S_(1) and T_(1) states, offering a bridge for the long-lived T_(1) excitons to migrate from donors to acceptors. Population dynamics with parameters taken entirely based on experiments fits the observed lifetimes of phosphorescence across a broad range of doping and temperature.

Dynamic recombination of triplet excitons in polymer heterojunctions

The dynamic recombination of two triplet excitons with opposite spins in the heterojunction structure has been investigated using a nonadiabatic evolution method.We demonstrate that luminous composite states including the excited polaron and the biexciton can be formed efficiently via the triplet exciton-triplet exciton reaction in the heterojunction and therefore this reaction can enhance the electroluminescence efficiency considerably,which is consistent qualitatively with experimental observations.Meanwhile,we find that,although the heterojunctions are beneficial to the generation of luminescent particles,large band offset caused by the heterojunction structure is not helpful to improve the electroluminescence efficiency.In addition,the mechanism of the triplet exciton-triplet exciton reaction in heterojunction is different from that of two similar coupling chains.Our results may deepen the understanding of the electroluminescence mechanism in polymer light-emitting devices.

The Role of Radiative Surface Modes and Longitudinal Excitons in the Formation of Exciton-Polariton Luminescence Spectra of CdS-Type Crystals

The low-temperature (T = 2 K) exciton-polariton luminescence (EPL) spectra in the vicinity of the exciton-resonance frequency An=1 for CdS-type crystals have been theoretically and experimentally investigated with allowance for the mechanical exciton decay . The results of the numerical calculations of the partial and interference contributions of the bulk and radiative surface spectral modes to the EPL in the geometry of additional s- and p-polarized waves emitted into vacuum are analyzed. It is shown that the contributions of purely longitudinal excitons and their interference with polaritons of the upper dispersion branch near the longitudinal frequency ωL to the EPL are small (∼10% - 30%);nevertheless, they must be taken into account to obtain quantitative agreement with experimental data. Specifically these contributions are responsible for the formation of an additional line (along with the fundamental AT line) in the case of oblique incidence of radiation.

Dynamic, Charge Photogeneration and Excitons Distribution Function in Organic Bulk Heterojunction Solar Cells

In this work, the excitons distribution function in organic bulk hetero junction solar cells, at a depth z has been determined from solving the charge continuity equation, by exploiting the Laplace transform with appropriate boundary conditions. Next, the influence of the electron-hole pair separation distance on the excitons dissociation probability, the internal quantum efficiency and the binding energy, has been studied. The simulated results show that the probability density of the carriers photo generated depends on the generation rate, excitons dissociation and the charge carriers in the cells. The potential improvement of the internal quantum efficiency of charge generation depends on electron-hole pair separation distance, the excitons dissociation probability into free charges and depends strongly on the optical absorption of the photons in the active layer.

Light-emitting diodes based on all-inorganic copper halide perovskite with self-trapped excitons

Light-emitting diodes based on lead halide perovskite have attracted great attention due to their outstanding performance.However,their application is plagued by the toxicity of Pb and the poor stability.Herein novel copper-based all inorganic perovskite CsCu2I3 with much enhanced stability has been reported with a potential photoluminescence quantum yield(PLQY)over 20%and self-trapped excitons(STE).By taking advantage of its extraordinary thermal stability,we successfully fabricate high-quality CsCu2I3 film through direct vacuum-based deposition(VBD)of CsCu2I3 powder.The resulting film shows almost the same PLQY with the synthesized powder,as well as excellent uniformity and stability.The perovskite light-emitting diodes(Pe-LED)based on the evaporated CsCu2I3 emitting layer achieve a luminescence of 10 cd/m2 and an external quantum efficiency(EQE)of 0.02%.To the best of our knowledge,this is the first CsCu2I3 Pe-LED fabricated by VBD with STE property,which offers a new avenue for lead-free Pe-LED.

Lasing behaviour from the condensation of polaronic excitons in a ZnO nanowire

Atoms under optical and magnetic trapping in a limited space at a very low temperature can lead to Bose-Einstein condensation （BEC）, even in a one-dimensional （1D） optical lattice. However, can the confinment of dense excitons in a 1D semiconductor microstructure easily reach the excitonic BEC？ A lightly Mn（II）-doped ZnO nanowire under a femtosecond laser pulse pump at room temperature produces single-mode lasing from coherent bipolaronic excitons, which is much like a macroscopic quantum state due to the condensation of the bipoaronic excitons if not real BEC. In this process, longitudinal biphonon binding with the exciton plays an important role. We revisit this system and propose possibility of bipolaronic exciton condensation. More studies are needed for this condensation phenomenon in 1D microcavity systems.

On the binding energies of excitons in polar quantum well structures in a weak electric field

The binding energies of excitons in quantum well structures subjected to an applied uniform electric field by taking into account the exciton longitudinal optical phonon interaction is calculated. The binding energies and corresponding Stark shifts for Ⅲ-Ⅴ and Ⅱ-Ⅵ compound semiconductor quantum well structures have been numerically computed. The results for GaAs/A1GaAs and ZnCdSe/ZnSe quantum wells are given and discussed. Theoretical results show that the exciton-phonon coupling reduces both the exciton binding energies and the Stark shifts by screening the Coulomb interaction. This effect is observable experimentally and cannot be neglected.

Ground state energy of excitons in quantum dot treated variationally via Hylleraas-like wavefunction

In this work, the effects of quantum confinement on the ground state energy of a correlated electron-hole pair in a spherical and in a disc-like quantum dot have been investigated as a function of quantum dot size. Under parabolic confinement potential and within effective mass approximation Ritz＇s variational method is applied to Hylleraas-like trial wavefunction. An efficient method for reducing the main effort of the calculation of terms like τekh exp （-λτeh） is introduced. The main contribution of the present work is the introduction of integral transforms which provide the calculation of expectation value of energy and the related matrix elements to be done analytically over single-particle coordinates instead of Hvlleraas coordinates.

INTERPLAY BETWEEN FRENKEL EXCITONS AND PHOTONS

We investigate the effect of photons and phonons on Frenkel exciton spectrum. The Hamilto nian for a coupled system of the Frenkel excitons. the photons and the phonons is derived. The phonon-exciton interaction makes the energy band of the Frenkel excitons narrow. The photons and the Frenkel excitons make up Frenkel exciton polaritons which are quasi-particles. The Frenkel exciton polariton spectrum consists of two branches. It is found that the photon-exciton interaction makes the ground energy of the Frenkel excitons increase.

Localized self-trapping in two-dimensional molecular lattice with interaction between Wannier-Mott excitons and phonon lattice

We investigate the interactions of lattice pbonons with Wannier-Mott exciton, the exciton that has a large radius in two-dimensional molecular lattice, by the method of continuum limit approximation, and obtain that the self-trapping can also appear in two-dimensional molecular lattice with a harmonic and nonlinear potential. The exciton effect on molecular lattice does not distort the molecular lattice but only makes it localized and the localization can also react, again through phonon coupling, to trap the energy and prevents its dispersion.

Spinless Excitons Implicit in SO(5) Theory

A pair of up-down operators are constructed explicitly for S.C.Zhang's SO(5) theory of high Tc superconductivity.From them two good quantum numbers are derived.The up-down operators are related to the spin-independent excitons which are not considered before.

Factors influencing entanglement of two excitons in a coupled exciton-photon system

The relation between the excitonic purity and the concurrence in a system of two coupled large semiconduction quantum dots mediated by a single-mode cavity field is investigated by using linear entropy theory. The results show the difference in describing two modes of excitonic entanglement between linear entropy and concurrence. The relation between nonclassical property of cavity field and the entanglement degree of excitons is also discussed. The results show that two modes of exciton can reach maximal entanglement when the cavity exhibits an antibunching effect.

Localized self-trapping in the two-dimensional discrete molecular lattice with the interaction between Frenkel excitons and phonons

We investigate the interactions of lattice phonons with Frenkel exciton, which has a small radius in a twodimensional discrete molecular lattice, by the virtue of the quasi-discreteness approximation and the method of multiplescale, and obtain that the self-trapping can also appear in the two-dimensional discrete molecular lattice with harmonic and nonlinear potential. The excitons＇ effect on the molecular lattice does not distort it but only causes it to localize which enables it to react again through phonon coupling to trap the energy and prevent its dispersion.

Fractional-dimensional approach for excitons in Ga As films on AlxGa(1-x)As substrates

Binding energies of excitons in GaAs films on AlxGal-xAs substrates are studied theoretically with the fractional- dimensional approach. In this approach, the real anisotropic ＂exciton ＋ film＂ semiconductor system is mapped into an effective fractional-dimensional isotropic space. For different aluminum concentrations and substrate thicknesses, the exci- ton binding energies are obtained as a function of the film thickness. The numerical results show that, for different aluminum concentrations and substrate thicknesses, the exciton binding energies in GaAs films on AlxGal_xAs substrates all exhibit their maxima with increasing film thickness. It is also shown that the binding energies of heavy-hole and light-hole excitons both have their maxima with increasing film thickness.

Coherent excitons at different orientation arrangements of local transition dipole moments in circular light-harvesting complexes

The coherent exciton plays an important role in the photosynthetic primary process, and its functions are deeply dependent on the orientation arrangements of local transition dipole moments （TDMs）. We theoretically and systematically study the physical property of the coherent exciton at different orientation arrangements of the local TDMs in circular light-harvesting （LH） complexes. Especially, if the orientation arrangements are different, the delocalized TDMs of the coherent excitons and the energy locations of the optically active coherent excitons （OACEs） can be obviously different, and then there are more manners to capture, store and transfer light energy in and between LH complexes. Similarly, if the orientation arrangements are altered, light absorption and radiative intensities can be converted fully between the OACEs in the upper and lower coherent exciton bands, and then the blue and red shifts of the absorption and radiative bands of the pigment molecules can occur simultaneously at some orientation arrangements. If the systems are in the vicinities of the critical orientation arrangements, the weak static disorder or small thermal excitation can destroy the coherent electronic excitations, and then the coherent exciton cannot exist any more.

Voids in the Hall Effect;Excitons in HiT_c

The outstanding difference between high temperature superconductors and low temperature superconductors is the sign of the Hall coefficient, properly understood. Since the Lorentz force acts on particles, not voids nor immobile ions, we propose that the experimental positive coefficient is due to dispersion dynamics in valence bands, i.e. on electrons with positive charge/mass ratio, but with negative charge and negative effective mass. In HiT ccompounds, anionic and cationic doping creates holes that substitute for the lattice distortions that bind Cooper pairs in metallic superconductors such as Nb. In both types of superconductor, the conventional notion of antiparallel spins S = 0, with paired wave vectors k and -k, is maintained;but in the ceramics “holes” h, produced by chemical doping and measured in the normal state, are available to bond super-conducting Boson pairs via h−or h02?excitons.

Numerical Verification of Transition’s Energies of Excitons in Quantum Well of ZnO with the Finite Difference Method

This paper shows that the experimental results of quantum well energy transitions can be found numerically. The cases of several ZnO-ZnMgO wells are considered and their excitonic transition energies were calculated using the finite difference method. In that way, the one-dimensional Schrödinger equation has been solved by using the BLAS and LAPACK libraries. The numerical results are in good agreement with the experimental ones.

THE EVOLUTION OF WANNIER-MOTT EXCITONS IN NAMI-SEMICONDUCTOR DRIVEN BY A STRONG ELECTRICAL FIELD

We present here a formulation of subdynamics to calculatee Wannier-Mott excitons in a Nami-semiconductor driven by a strong electrical field. The formula of the evolution of density operator for the time-dependent Liouvillian is given. In terms of this formula rue can calculate the nonlinear response of the absorbing coefficient of light for nami-semicouducton. The results are helpful to study either the nonlinear behavior of the Wannier-Mott excitons in a nami-semicouducton driven by a strong harmonic field or the optic properties of these Kinds of materials.