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Artificial light harvesting, a process that involves converting sunlight into chemical potential energy, is considered to be a promising part of the overall solution to address urgent global energy challenges. Conjugated polyelectrolyte complexes (CPECs) are particularly attractive for this purpose due to their extended electronic states, tunable assembly thermodynamics, and sensitivity to their local environment. Importantly, ionically assembled complexes of conjugated polyelectrolytes can act as efficient donor-acceptor pairs for electronic energy transfer (EET). However, to be of use in material applications, we must understand how modifying the chemical structure of the CPE backbone alters the EET rate beyond spectral overlap considerations. In this report we investigate the dependence of the EET efficiency and rate on the electronic structure and excitonic wave function of the CPE backbone. To do so, we synthesized a series of alternating copolymers where the electronic states are systematically altered by introducing comonomers with electron withdrawing and electron-rich character while keeping the linear ionic charge density nearly fixed. We find evidence that the excitonic coupling may be significantly affected by the exciton delocalization radius, in accordance with analytical models based on the line-dipole approximation and quantum chemistry calculations. Our results imply that care should be taken when selecting CPE components for optimal CPEC EET. These results have implications for using CPECs as key components in water-based light-harvesting materials, either as standalone assemblies or as adsorbates on nanoparticles and thin films.

Based on density functional theory (DFT), the GW approximation and Bethe–Salpeter equation (BSE), we performed a theoretical calculation to study the electronic and optical properties of penta-graphene (PG) monolayers. Our findings reveal that PG behaves as a semiconductor with an indirect band gap of 2.27 eV at the DFT-GGA level. By incorporating the GW approximation based on many-body perturbation theory, we observed an increase in the band gap, resulting in a quasi-direct band gap of 4.53 eV. Furthermore, we employed the G0W0-RPA and G0W0-BSE approximations to compute the optical spectra of the monolayer in the absence and in the presence of electron–hole interaction, respectively. The results indicate that the inclusion of electron–hole interactions leads to a red-shift of the absorption spectrum towards lower energies compared to the spectrum obtained from the G0W0-RPA approximation. Notably, the optical absorption spectra are predominantly governed by the first bound exciton, characterized by a significant binding energy of 2.07 eV. Our results suggest that the PG monolayer, with its wider band gap and enhanced excitonic effects, is potentially a suitable candidate for the design and fabrication of optoelectronic components. [ABSTRACT FROM AUTHOR]

Electronic and optical properties of an exciton in the ZnSe/CdSe/ZnSe quantum well are investigated using the Razavy and Pöschl -Teller confined potentials. The theoretical investigations on exciton binding energy, oscillator strength, radiative lifetime, absorption coefficient and changes of refractive index are focused on the structural parameters of these two potentials. They are carried out using variational method and the matrix density approach. Out of two structural parameters (A and M) in the Razavy potential, M is found to be much more effective than A, similarly, λ parameter is more effective than the other parameter χ in Pöschl-Teller potential. The single quantum well becomes double quantum well when the value of A is less than the value of M. The potential asymmetry is developed if the structural parameters are altered. Further, the optical properties are strongly affected by the geometrical size, changes in the structure and the associated structural parameters. It leads to alter the optical properties drastically in the quantum well. The results are compared with the available literature and we hope that they can be used for the design of potentials for the future opto-electronic devices. [ABSTRACT FROM AUTHOR]

Electrical modulation of luminescence is significant to modern light-emitting devices. Monolayer transition metal dichalcogenides are emerging direct-bandgap luminescent materials with unique excitonic properties, and the multiple exciton complexes provide new opportunities to modulate the property of luminescence in atomically thin semiconductors. Here, we report an electrical control of exciton emission in the oscillator strength and spatial distribution of excitons in a monolayer WS2. Effective modulation of excitonic emission intensity with a degree of modulation of ~ 92% has been demonstrated by an electric field at room temperature. The spatial carrier redistribution tuned by a lateral electric field results in distinct excitonic emission patterns by design. The modulation approach to exciton oscillator strength and distribution provides an efficient way to investigate the exciton diffusion dynamics and to construct electrically tunable optoelectronic devices. [ABSTRACT FROM AUTHOR]

We theoretically studied the exciton geometric structure in layered semiconducting transition metal dichalcogenides. Based on a three-orbital tight-binding model for Bloch electrons which incorporates their geometric structures, an effective exciton Hamiltonian is constructed and solved perturbatively to reveal the relation between the exciton and its electron/hole constituent. We show that the electron—hole Coulomb interaction gives rise to a non-trivial inheritance of the exciton geometric structure from Bloch electrons, which manifests as a valley-dependent center-of-mass anomalous Hall velocity of the exciton when two external fields are applied on the electron and hole constituents, respectively. The obtained center-of-mass anomalous velocity is found to exhibit a non-trivial dependence on the fields, as well as the wave function and valley index of the exciton. These findings can serve as a general guide for the field-control of the valley-dependent exciton transport, enabling the design of novel quantum optoelectronic and valleytronic devices. [ABSTRACT FROM AUTHOR]

Strong coupling between resonantly matched surface plasmons of metals and excitons of quantum emitters results in the formation of new plasmon-exciton hybridized energy states. In plasmon-exciton strong coupling, plasmonic nanocavities play a significant role due to their ability to confine light in an ultrasmall volume. Additionally, two-dimensional transition metal dichalcogenides (TMDCs) have a significant exciton binding energy and remain stable at ambient conditions, making them an excellent alternative for investigating light-matter interactions. As a result, strong plasmon-exciton coupling has been reported by introducing a single metallic cavity. However, single nanoparticles have lower spatial confinement of electromagnetic fields and limited tunability to match the excitonic resonance. Here, we introduce the concept of catenary-shaped optical fields induced by plasmonic metamaterial cavities to scale the strength of plasmon-exciton coupling. The demonstrated plasmon modes of metallic metamaterial cavities offer high confinement and tunability and can match with the excitons of TMDCs to exhibit a strong coupling regime by tuning either the size of the cavity gap or thickness. The calculated Rabi splitting of Au-MoSe2 and Au-WSe2 heterostructures strongly depends on the catenary-like field enhancement induced by the Au cavity, resulting in room-temperature Rabi splitting ranging between 77.86 and 320 meV. These plasmonic metamaterial cavities can pave the way for manipulating excitons in TMDCs and operating active nanophotonic devices at ambient temperature.

Temperature field engineering provides a robust framework for investigating the behavior of phonons and excitons within two-dimensional (2D) layered van der Waals materials. Herein, we measured the photoluminescence (PL) spectroscopy of few-layer WSe2 and WSe2/graphene van der Waals heterostructures at various temperatures (from 10 to 300 K). The results indicate that the PL intensity of the heterostructure is lower than that of few-layer WSe2, which is attributed to charge transfer at the interface under photoexcitation conditions. The PL spectroscopy spectrum of the heterostructure is primarily composed of four distinct spectral features, arising from distinct phonon-assisted processes, direct recombination, and excitonic contributions. Furthermore, these physical processes can be modulated by varying the temperature. For instance, the phonon-assisted PL process gradually diminishes with the decrease in temperature, whereas the excitonic PL process emerges more distinctly with the reduction in temperature. These findings can provide support for understanding the photophysical processes in 2D heterostructure materials and lay the groundwork for optoelectronic devices based on nanomaterials. [ABSTRACT FROM AUTHOR]

We report a comprehensive study of Sperrylite (PtAs2), the main platinum source in natural minerals, as a function of applied pressures up to 150 GPa. While no structural phase transition is detected from pressure-dependent X-ray measurements, the unit cell volume shrinks monotonically with pressure following the third-order Birch–Murnaghan equation of state. The mildly semiconducting behavior found in pure synthesized crystals at ambient pressures becomes more insulating upon increasing the applied pressure before metalizing at higher pressures, giving way to the appearance of an abrupt decrease in resistance near 3 K at pressures above 92 GPa consistent with the onset of a superconducing phase. The pressure evolution of the calculated electronic band structure reveals the same physical trend as our transport measurements, with a non-monotonic evolution explained by a hole band that is pushed below the Fermi energy and an electron band that approaches it as a function of pressure, both reaching a touching point suggestive of an excitonic state. A Lifshitz transition of the electronic structure and an increase in the density of states may naturally explain the onset of superconductivity in this material. [ABSTRACT FROM AUTHOR]

Strain-free GaAs cone-shell quantum dots have a unique shape, which allows a wide tunability of the charge-carrier probability densities by external electric and magnetic fields. Here, the influence of a lateral electric field on the optical emission is studied experimentally using simulations. The simulations predict that the electron and hole form a lateral dipole when subjected to a lateral electric field. To evaluate this prediction experimentally, we integrate the dots in a lateral gate geometry and measure the Stark-shift of the exciton energy, the exciton intensity, the radiative lifetime, and the fine-structure splitting (FSS) using single-dot photoluminescence spectroscopy. The respective gate voltage dependencies show nontrivial trends with three pronounced regimes. We assume that the respective dominant processes are charge-carrier deformation at a low gate voltage U, a vertical charge-carrier shift at medium U, and a lateral charge-carrier polarization at high U. The lateral polarization forms a dipole, which can either enhance or compensate the intrinsic FSS induced by the QD shape anisotropy, dependent on the in-plane orientation of the electric field. Furthermore, the data show that the biexciton peak can be suppressed by a lateral gate voltage, and we assume the presence of an additional vertical electric field induced by surface charges. [ABSTRACT FROM AUTHOR]

Nanolayers of MoS2 can be grown to be used as active elements in nano-optoelectronic devices such as two-dimensional (2D) light emitters and optical detectors. The growth of 2D flakes might result in the formation of not only isolated triangles but also complex polycrystal flakes when different flakes interact during their in-plane expansion. In this paper, we investigate how monolayer MoS2 flakes of different shapes are affected by the biaxial strain resulting from the cooling process after chemical vapor deposition growth. The single- and polycrystal flakes are characterized at the nanoscale level by correlating morphological, electrical, and optical measurements and imaging. The main focus is given to the analysis of the exciton/trion photoluminescence (PL) components extracted from the spectra at different areas of the flake surface. According to the Raman imaging, the whole flake has built-in heterogeneous tensile strain, with the perimeter and the grain boundaries between the single-crystal parts of the poly flakes exhibiting a lower strain level (0.2–0.3%) and the central area being more strained (∼0.4%). At the perimeter and grain boundaries, the PL undergoes the strain-related blueshift accompanied by a weakening of the contribution of the long-wave trion to the spectrum and the trion binding energy. The trion formation is known to be proportional to a local electron concentration. The trion PL imaging compared to the surface potential mapping confirms a decrease in n-doping at the perimeter and grain boundaries, leading to the trion weakening. To confirm the results of electron drift to strained areas of the flake, creating the trion, the band bending at the tensile-strained flake has been theoretically calculated and modeled. The effect of edge defects at the perimeter and grain boundaries on the doping, which leads to the enhancement or inhibition of the trion formation depending on the edge and grain boundary interface type, is also discussed. [ABSTRACT FROM AUTHOR]

All‐inorganic CsPbI3 perovskite is a promising emitter for deep‐red light‐emitting diodes (LEDs). However, presently fabricated CsPbI3 polycrystalline films are composed of island‐like polycrystals, encountering the problems of serious interface current leakage and low‐efficiency carrier radiative recombination. Here, a CsPbI3‐xBrx quantum dots (QDs) matrix encapsulated CsPbI3 polycrystal film is reported to address the low‐efficiency issue of island‐like CsPbI3 polycrystalline film applied in deep‐red LEDs. The developed QDs matrix encapsulation strategy has two benefits. One is the filling of void space in the island‐like CsPbI3 polycrystal film to suppress the interface current leakage. The other more important benefit is utilizing the strong carrier confinement effect of QDs to strengthen the formation of excitons in the composite film and thus improve the electroluminescence efficiency. Moreover, interesting grain growth is found between CsPbI3‐xBrx QDs and CsPbI3 polycrystals, which further enhances the exciton transfer effect brought by the QDs matrix and optoelectronic properties of the fabricated composite films. Based on the obtained high‐quality QDs/polycrystal composite films, efficient and bright deep‐red LEDs are achieved with a peak external quantum efficiency of 15.24% and a maximum luminance of 3691 cd m−2. [ABSTRACT FROM AUTHOR]

In the effective mass approximation, single-particle, excitonic states, and interband and intraband transitions in CdSe nanoplatelet are considered in the presence of an external axial uniform electrostatic field. It has been demonstrated that under the influence of the field, the binding energy between the electron and the hole in the nanoplatelet decreases compared to the case in the absence of the field. It is shown that with increasing electric field strength, the resonant frequencies of interband electroabsorption undergo a red shift. On the other hand, the resonance frequencies of intraband absorption shift to the region of high energies. A similar shift of photoluminescence spectrum peaks under the influence of the field has been observed. The results of theoretical calculations of the luminescence threshold frequency values are in good agreement with the corresponding experimental data. It is shown that the dependence of the optical rectification peaks on the external field is non-monotonic with pronounced maxima. [ABSTRACT FROM AUTHOR]

In this paper, we introduce the pulsed laser ablation in liquid technique for the synthesis of tungsten oxide nanoparticle colloid in water. Tungsten trioxide nanoparticles produced at different laser fluence in DD water at room temperature by pulsed laser ablation of the W target have been studied. The effect of increasing laser fluence on the structural, morphological and optical properties of WO3 NPs was investigated by using X-ray diffraction (XRD), optical properties, photoluminescence (PL), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy and transmission electron microscopy (TEM). The XRD data demonstrated that all of the produced WO3 NPs are polycrystalline and confirm producing WO3 NPs in all samples at water environments. The absorption spectra of all samples contained the excitonic/plasmonic absorption peak of WO3 NPs. Direct optical energy gap of WO3 NPs prepared at different laser fluence was in the range of 3.02–3.1 eV at room temperature. The PL measurement indicates a peak emission centered at 404 nm, and increasing the laser fluence results in a redshift. The PL spectra showed band-to-band transitions and WO3 oxygen vacancies. The surface morphology of WO3 NPs investigated by FESEM revealed the formation of spherical NPs morphologies, and the TEM result shows the particle size and concentration of wo3 increased from 6 to 33 nm with increasing laser fluence. The goal of this experiment was to demonstrate how the laser fluence might be used to regulate the concentering of WO3 nanoparticles. In other words, laser fluence are a helpful tool for managing the concentering and particle size of created WO3 nanoparticles. [ABSTRACT FROM AUTHOR]

Porphyrins are excellent light‐harvesting complexes. Presently they are unsuitable for photovoltaic applications, as their excellent light absorbance is compensated to a large extent by their poor transport properties, where most excitons are lost by recombination. Arranging porphyrins in regular, strongly bound, lattices of surface‐anchored metal‐organic frameworks (PP‐SURMOFs) may facilitate charge carrier dissociation, but does not significantly enhance the conductive properties. In most cases, photogenerated excitons traverse undirected, Brownian motion through a hopping process, resulting in a substantial diffusion length to reach electrodes, leading to significant exciton loss through recombination. Here, we propose to guide exciton diffusion indirectly by an external electric field. We show that electric fields, even as strong as 1 V nm−1, do not affect the HOMO‐LUMO gap of the porphyrins. However, fields of 0.1 V nm−1 and even less demonstrate a notable Stark effect, with slight band gap reductions, for some PP‐SURMOFs. When applied as an electric field gradient, for instance, via the substrate, it creates a unidirectional hopping pathway for the excitons. Consequently, we expect a significant reduction of exciton diffusion length leading to increased utilization of photogenerated excitons as they reach the electrodes. This strategy holds promise for integrating photoactive molecules in photovoltaic and photocatalytic applications. [ABSTRACT FROM AUTHOR]

In the recent years, the thermal Hall transport has risen as an important diagnosis of the physical properties of the elementary excitations in various quantum materials, especially among the Mott insulating systems where the electronic transports are often featureless. Here we review the recent development of thermal Hall effects in quantum magnets where all the relevant excitations are charge-neutral. In addition to summarizing the existing experiments, we pay a special attention to the underlying mechanisms of the thermal Hall effects in various magnetic systems, and clarify the connection between the microscopic physical variables and the emergent degrees of freedom in different quantum phases. The external magnetic field is shown to modify the intrinsic Berry curvature properties of various emergent and/or exotic quasiparticle excitations in distinct fashions for different quantum systems and quantum phases, contributing to the thermal Hall transports. These include, for example, the conventional ones like the magnons in ordered magnets, the triplons in dimerized magnets, the exotic and fractionalized quasiparticles such as the spinons and the magnetic monopoles in quantum spin liquids. We review their contribution and discuss their presence in the thermal Hall conductivity in different physical contexts. We expect this review to provide a useful guidance for the physical mechanism of the thermal Hall transports in quantum magnets. [ABSTRACT FROM AUTHOR]

In this conceptual review, we present the optical and electronic characteristics of quantum dots for the designing of organic synthesis experiments through the photocatalysis. We discuss the control parameters associated to the charge recombination processes on the semiconductor interface. The positioning of bands and control of intragap states, such as charge donors and acceptors, can define the performance of faradaic processes. Electrochemical strategies, such as voltammetry, can be used as a powerful tool for the determining of the electronic band edges with good precision. [ABSTRACT FROM AUTHOR]

We theoretically study biexcitons and quadrons in quantum dots with parabolic confinement and give a complete comparison between the two excitations. The calculation of quadron and biexciton binding energies as functions of electron-to-hole confinement potentials and mass ratios, using the unrestricted Hartree–Fock method, shows the essential differences between biexcitons and quadrons. The crossover between the negative and positive binding energies is indicated. The effect of an external magnetic field on the quadron and biexciton binding energies has also been investigated. In addition, the crossover between anti-binding and binding of both excited quadron and biexciton states in a certain range of the electron-to-hole oscillator length ratios has been found. [ABSTRACT FROM AUTHOR]

Direct bandgap semiconductors possess a unique attribute: the presence of a free exciton state with a high total oscillator strength. This property makes them highly promising for applications in ultrafast optical signal processing and optical computing. One such protocol for optical computing is based on four‐wave mixing (FWM). In this study, the nonlinear optical effect in polycrystalline thin films of halide perovskite MAPbI3 (MA+ = CH3NH3+$_3^+$) at low temperatures is demonstrated. Through analyzing the spectroscopy of the FWM signal, studying the photoluminescence excitation spectra, and comparing the findings with results from MAPbI3 single crystals, it has been discovered that the strongest nonlinear response is observed at the free exciton resonance and in the region of shallow defect states. Surprisingly, the presence of FWM in cross‐linear excitation geometry has been observed, indicating potential involvement of other nonlinear or many‐body effects. The observations of FWM with free excitons even in highly defective MAPbI3 thin films demonstrate the robustness of the exciton resonance and highlight the practical prospects for utilizing this material in optical computing. [ABSTRACT FROM AUTHOR]

Perylenetetracarboxylic diimide (PTCDI) is an n-type organic semiconductor molecule that has been widely utilized in numerous applications such as photocatalysis and field-effect transistors. Polarizability and dipole moment, which are inherent properties of molecules, are important parameters that determine their responses to external electric and optical fields, physical properties, and reactivity. These parameters are fundamentally important for the design of innovative materials. In this study, the effects of external electric fields on absorption and fluorescence spectra were investigated to obtain the PTCDI parameters. The PTCDI substituted by an octyl group (N,N′-Dioctyl-3,4,9,10-perylenedicarboximide) dispersed in a polymethyl methacrylate (PMMA) matrix was studied in this work. The features of vibronic progression in the absorption spectrum were analogous to those observed in solution. The red shift of the absorption band caused by the Stark effect was mainly observed in the presence of an external electric field. Changes in parameters such as the dipole moment and polarizability between the ground and the Franck–Condon excited states of the PTCDI monomer were determined. The fluorescence spectrum shows a contribution from a broad fluorescence band at wavelengths longer than the monomer fluorescence band. This broad fluorescence is ascribed to the excimer-like fluorescence of PTCDI. The effects of the electric field on the fluorescence spectrum, known as the Stark fluorescence or electrofluorescence spectrum, were measured. Fluorescence quenching is observed in the presence of an external electric field. The change in the polarizability of the monomer fluorescence band is in good agreement with that of the electroabsorption spectrum. A larger change in the polarizability was observed for the excimer-like fluorescence band than that for the monomer band. This result is consistent with exciton delocalization between PTCDI molecules in the excimer-like state. [ABSTRACT FROM AUTHOR]

We report the temperature dependences of the dielectric function ε = ε1 + iε2 and critical point (CP) energies of the uniaxial crystal GaSe in the spectral energy region from 0.74 to 6.42 eV and at temperatures from 27 to 300 K using spectroscopic ellipsometry. The fundamental bandgap and strong exciton effect near 2.1 eV are detected only in the c-direction, which is perpendicular to the cleavage plane of the crystal. The temperature dependences of the CP energies were determined by fitting the data to the phenomenological expression that incorporates the Bose–Einstein statistical factor and the temperature coefficient to describe the electron–phonon interaction. To determine the origin of this anisotropy, we perform first-principles calculations using the mBJ method for bandgap correction. The results clearly demonstrate that the anisotropic dielectric characteristics can be directly attributed to the inherent anisotropy of p orbitals. More specifically, this prominent excitonic feature and fundamental bandgap are derived from the band-to-band transition between s and pz orbitals at the Γ-point. [ABSTRACT FROM AUTHOR]

2D materials, particularly 2D semiconductors, and layered antiferromagnetic (AFM) compounds, exhibit rich light‐matter coupling phenomena in low‐dimensional systems. In this work, the occurrence of interlayer exciton–magnon coupling (EMC) is observed in Néel‐type AFM of MnPX3 (X = S or Se) and monolayer (1L) WS2 heterostructures. The energy of neutral exciton in 1L WS2 can be tuned by the adjacent AFM order, resulting in an additional energy shift of 10–14 meV. Furthermore, by filtering the photoluminescence spectrum with graphene and conducting magnetic measurements, the correlation is elucidated between the interlayer EMC and AFM transition process. In MnPS3, this coupling behavior exhibits sensitivity to the changes of magnetic structures, manifesting with a correlated length ξ of 8.34 Å at temperatures higher than Néel temperature (TN). The decoupling process in WS2/MnPS3 heterostructure below 50 K is originated from a weak out‐of‐plane ferromagnetic order, confirming the presence of an XY‐type magnetic phase transition in MnPS3 at sub‐TN temperature. This study provides a fundamental understanding of EMC and its potential applications in the integration of optical and magnetic devices. [ABSTRACT FROM AUTHOR]

Reflection spectra of a heterostructure with a GaAs/AlGaAs quantum well (QW) of 30 nm wide have been studied under additional optical excitation in an external electric field. The influence of the electric field on all parameters of light-hole and heavy-hole exciton resonances, was studied upon selective excitation of various optical transitions. The effect of compensating for the Stark shift upon excitation to the ground exciton state of the QW is found. A sharp increase in the Stark shift of excitons in QW was found upon optical creation of charge carriers in the GaAs buffer layer. A microscopic calculation of exciton states in various electric fields has been performed. A comparison of the calculated and measured Stark shift of the heavy-hole exciton is used to obtain the dependence of the electric field strength in the QW on the applied voltage. [ABSTRACT FROM AUTHOR]

We investigate the quantification of entanglement between the photonic and excitonic modes in a semiconductor microcavity injected with squeezed light. By deriving and subsequently establishing the solutions to the quantum Langevin equations, we quantify the transient entanglement and the steady-state entanglement between the photonic and excitonic modes in the low-excitation regime. It turns out that the cavity mode and the exciton mode are entangled in both the weak and strong coupling regimes, and there is the entanglement between the cavity mode and the exciton mode even in the absence of direct coupling between them. Furthermore, though the transit entanglement increases with the squeeze parameter, it decreases with the initial average intensity of the cavity mode. Also, we demonstrate that, in the strong coupling regime, the steady-state entanglement grows with coupling strength while, in the weak coupling regime, it decreases. [ABSTRACT FROM AUTHOR]

The intrinsic weak and highly nonlocal dielectric screening of two-dimensional materials is well-known to lead to high sensitivity of their optoelectronic properties to environment. Less studied theoretically is the role of free carriers in those properties. Here, we use ab initio GW and Bethe-Salpeter equation calculations, with a rigorous treatment of dynamical screening and local-field effects, to study the doping dependence of the quasiparticle and optical properties of a monolayer transition-metal dichalcogenide, 2H MoTe2. We predict a quasiparticle band gap renormalization of several hundreds of meV for experimentally attainable carrier densities and a similarly sizable decrease in the exciton binding energy. This results in an almost constant excitation energy for the lowest-energy exciton resonance with an increasing doping density. Using a newly developed and generally applicable plasmon-pole model and a self-consistent solution of the Bethe-Salpeter equation, we reveal the importance of accurately capturing both dynamical and local-field effects to understand detailed photoluminescence measurements.

The presence of the effect of thermally activated delayed fluorescence (TADF) in organic light-emitting materials (emitters), manifested in the "collecting" of triplet excitons in organic semiconductor complexes that do not contain noble metals, creates excellent prerequisites for the application of TADF materials in the technology of manufacturing organic light-emitting diodes (OLED). The significant progress in solving theoretical and technical problems, achieved in the process of development of highly efficient TADF materials, paves the way for the formation of the future of organic electronics. This review presents the analyses of the nature of the long-term fluorescence generation mechanism at the molecular level and the up-to-date strategies for designing TADF donor-acceptor materials, as well as exciplex intermolecular complexes. Special attention is focused on the analysis of TADF emitter ambipolar materials with a highly twisted, rigid molecular structure, which reveal a tendency towards the multi-channel emission mechanisms and their implementation in a variety of OLED structure architectures.

We present femtosecond pump-probe measurements of neutral and charged exciton optical response in monolayer MoSe2 to resonant photoexcitation of a given exciton state in the presence of 2D electron gas. We show that creation of charged exciton (X−) population in a given K+, K− valley requires the capture of available free carriers in the opposite valley and reduces the interaction of neutral exciton (X) with the electron Fermi sea. We also observe spectral broadening of the X transition line with the increasing X− population caused by efficient scattering and excitation induced dephasing. From the valley-resolved analysis of the observed effects we are able to extract the spin-valley relaxation times of free carriers as a function of carrier density. Moreover, we analyze the oscillator strength and energy shift of X in the regime of interaction with electron Fermi sea under resonant excitation. From this we can observe the process of X decay by radiative recombination paired with trion formation. We demonstrate an increase of neutral exciton relaxation rate with the introduction of Fermi sea of electrons. We ascribe the observed effect to the increased efficiency of the trion formation, as well as the radiative decay caused by the screening of disorder by the free carriers.

By virtue of adiabatic dynamics method, the response of excitons upon an external electric field is studied in a conjugated polymer bimolecular structure. As the electric field strength is over the critical value, the exciton is dissociated into a couple of free electron and hole. For the excitons in a bimolecular structure, the critical dissociation field is affected by the interaction of the molecules, as well as by the electron-electron interaction and the electron-phonon interaction. Due to the dynamical evolution calculations, the critical electric field strength of exciton dissociation decreases nonlinearly as the interchain interaction increases. The critical dissociation field strength also decreases nonlinearly with the increase of electron-electron interaction strength. However, the critical dissociation field strength increases linearly as electron-phonon interaction strength increases. [ABSTRACT FROM AUTHOR]

Semiconductor nanocrystals can replace conventional bulk materials completely in displays and light‐emitting diodes. Exciton transport dominates over charge carrier transport for materials with high exciton binding energies and long ligands, such as halide perovskite nanocrystal films. Here, how beneficial superlattices – nearly perfect 3D assemblies of nanocrystals ‐ are to exciton transport is investigated. Surprisingly, the high degree of order is not as crucial as the individual nanocrystal size, which strongly influences the splitting of the excitonic manifold into bright and dark states. At very low temperatures, the energetic splitting is larger for the smallest nanocrystals, and dark levels with low oscillator strength effectively trap excitons inside individual nanocrystals, suppressing diffusion. The effect is reversed at elevated temperatures, and the larger nanocrystal size becomes detrimental to exciton transport due to enhanced exciton trapping and dissociation. The results reveal that the nanocrystal size must be strongly accounted for in design strategies of future optoelectronic applications. [ABSTRACT FROM AUTHOR]

In this study, we explore the exciton dynamics in a WS2/MoS2 van der Waals (vdW) heterostructure under varying pressures by integrating a laser-confocal photoluminescence (PL) spectroscope and an atomic force microscope (AFM). For the WS2/MoS2 heterostructure, the exciton emission belonging to MoS2 is too weak to be distinguished from the PL spectra. However, upon contact with a Si probe, the emission intensity of WS2 excitons significantly decreases from 34,234 to 6560, thereby matching the intensity level of MoS2. This alteration substantially facilitates the exploration of interlayer excitonic properties within the heterostructures using PL spectroscopy. Furthermore, the Si probe can apply out-of-plane localized pressure to the heterostructure. With increasing pressure, the emission intensity of the WS2 trions decreases at a rate twice that of other excitons, and the exciton energy increases at a rate of 0.1 meV nN−1. These results elucidate that the WS2 trions are particularly sensitive to the out-of-plane pressure within a WS2/MoS2 vdW heterostructure. [ABSTRACT FROM AUTHOR]

Abstract—We have considered the interaction of an exciton mode with photonic modes in a GaN structure several pm in size. A technique for obtaining spectra from such structures has been demonstrated. The cavity shape and dimensions most optimal for efficient light/matter interaction have been selected. The theoretical spectrum for the selected cavity has been obtained and analyzed. [ABSTRACT FROM AUTHOR]

Nanocrystalline zinc oxide (ZnO) with crystallite size in the range 85 -89 nm are prepared by the thermal decomposition of carbonate precursor under three different atmospheres, viz., in air ambiance, under nitrogen flow, and in vacuum. O 1s X-ray photoelectron spectra indicate that the atomic percentage of oxygen vacancies is more in the sample prepared under vacuum while adsorbed oxygen is more in the sample prepared in air ambience. Optical band gaps for the samples are in the range 3.17–3.20 eV. Room temperature photoluminescence spectra of the samples reveal band edge emission in the UV range (⁓3.18 eV) and broad defect mediated visible emission (⁓1.60–2.70 eV). Emission profile of excitonic emission at 80 K can be resolved into two free excitonic peaks at 3.42 eV (FX-A) and 3.46 eV (FX-B), donor bound excitonic peaks (D0X), peak due to transition of free electrons to acceptor levels (FA) donor-acceptor pair (DAP) emission and phonon replicas. D0X peak at ⁓3.35 eV has contribution from oxygen vacancies (Vo) and Zn interstitials (Zni). The most intense peak in the case of all the three samples is the FA peak at ⁓ 3.31 eV which has contribution from zinc vacancies (VZn). Temperature dependent variation of the dominant D0X and FA peaks in the range 80–400 K are analyzed using modified Varshni model which considers contributions from electron-phonon scattering and lattice thermal expansion. Defect dependent emission intensity is maximum at 80 K due to freeze-out of phonons and decreases with increase in temperature. The evolution of the colour of luminescence with temperature is studied using CIE plot method.

The exciton binding energy (Eb) is a key parameter that governs the physics of many optoelectronic devices. At their best, trustworthy and precise measurements of Eb challenge theoreticians to refine models, are a driving force in advancing the understanding of a material system, and lead to efficient device design. At their worst, inaccurate Eb measurements lead theoreticians astray, sow confusion within the research community, and hinder device improvements by leading to poor designs. This review article seeks to highlight the pros and cons of different measurement techniques used to determine Eb, namely, temperature‐dependent photoluminescence, resolving Rydberg states, electroabsorption, magnetoabsorption, scanning tunneling spectroscopy, and fitting the optical absorption. Due to numerous conflicting Eb values reported for halide perovskites (HP) and transition metal dichalcogenides (TMDC) monolayers, an emphasis is placed on highlighting these measurements in an attempt to reconcile the variance between different measurement techniques. It is argued that the experiments with the clearest indicators are in agreement on the following values: ≈350–450 meV for TMDC monolayers between SiO2 and vacuum, ≈150–200 meV for hBN‐encapsulated TMDC monolayers, ≈200–300 meV for common lead‐iodide 2D HPs, and ≈10 meV for methylammonium lead iodide. [ABSTRACT FROM AUTHOR]

The exciton properties in cubical quantum dot (CQD), with parabolic confining potential, are theoretically investigated. We have used the two-band model, the effective mass approximation, and the variational method. The analytical expressions of the binding energy, the normalized photoluminescence (PL) energy transition, the spatial extension, and the oscillator strength of the exciton, in the ground state, have been obtained. The numerical calculations for the typical GaAs/AlxGa1−xAs CQD are presented. The effects of the CQD length and the Al concentration on the exciton properties are discussed. The results of the calculation illuminate that Al concentration and the CQD length can make an important impact on the exciton binding energy and the PL peak energy. [ABSTRACT FROM AUTHOR]

Plexcitonic systems based on metal nanostructures and molecular J-aggregates offer an excellent opportunity to explore the intriguing interplay between plasmonic excitations and excitons, offering unique insights into light–matter interactions at the nanoscale. Their potential applications in photocatalysis have prompted a growing interest in both their synthesis and the analysis of their properties. However, in order to construct a high-performing system, it is essential to ensure chemical and spectral compatibility between both components. We present the results of a study into a hybrid system, achieved through the coupling of gold nanobipyramids with organic molecules, and demonstrate the strengthened photochemical properties of such a system in comparison with purely J-aggregates. Our analysis includes the absorbance and photoluminescence characterization of the system, revealing the remarkable plexcitonic interaction and pronounced coupling effect. The absorbance spectroscopy of the hybrid systems enabled the investigation of the coupling strength (g). Additionally, the photoluminescence response of the J-aggregates and coupled systems reveals the impact of the coupling regime. Utilizing fluorescence lifetime imaging microscopy, we established how the photoluminescence lifetime components of the J-aggregates are affected within the plexcitonic system. Finally, to assess the photodegradation of J-aggregates and plexcitonic systems, we conducted a comparative analysis. Our findings reveal that plasmon-enhanced interactions lead to improved photostability in hybrid systems. [ABSTRACT FROM AUTHOR]

The presence of the effect of thermally activated delayed fluorescence (TADF) in organic light-emitting materials (emitters), manifested in the "collecting" of triplet excitons in organic semiconductor complexes that do not contain noble metals, creates excellent prerequisites for the application of TADF materials in the technology of manufacturing organic light-emitting diodes (OLED). The significant progress in solving theoretical and technical problems, achieved in the process of development of highly efficient TADF materials, paves the way for the formation of the future of organic electronics. This review presents the analyses of the nature of the long-term fluorescence generation mechanism at the molecular level and the up-to-date strategies for designing TADF donor-acceptor materials, as well as exciplex intermolecular complexes. Special attention is focused on the analysis of TADF emitter ambipolar materials with a highly twisted, rigid molecular structure, which reveal a tendency towards the multi-channel emission mechanisms and their implementation in a variety of OLED structure architectures. [ABSTRACT FROM AUTHOR]

We study theoretically fundamental Coulomb-correlated complexes: neutral and charged excitons, also known as trions, in transition metal dichalcogenides monolayers. We focus on the situation where one of the electrons occupies an excited, high-lying, conduction band characterized by a negative effective mass. We develop the theory of such high-lying excitons and trions with negative effective mass and demonstrate the key role of the non-parabolicity of the high-lying conduction band dispersion in the formation of the bound exciton and trion states. We present simple, accurate and physically justified trial wavefunctions for calculating the binding energies of Coulomb-bound complexes and compare the results of variational calculations with those of a fully numerical approach. Within the developed model, we discuss recent experimental results on the observation of high-lying negative effective mass trions. [ABSTRACT FROM AUTHOR]

In this study, all-inorganic perovskite quantum dots (QDs) for pure blue emission are explored for full-color displays. We prepared CsPbBr3 and Cs3NdCl6 QDs via hot injection methods and mixed in various ratios at room temperature for color blending. Nd-doped CsPb(Cl/Br)3 QDs showed a blueshift in emission, and the photoluminescence quantum yields (PLQY, ΦPL) were lower in the 460–470 nm range due to surface halogen and Cs vacancies. To address this, we introduced a silane molecule, APTMS, via a ligand exchange process, effectively repairing these vacancies and enhancing Nd doping into the lattice. This modification promotes the PLQY to 94% at 466 nm. Furthermore, combining these QDs with [1]Benzothieno[3,2-b][1]benzothiophene (BTBT), a conjugated small-molecule semiconductor, in a composite film reduced PLQY loss caused by FRET in solid-state QD films. This approach achieved a wide color gamut of 124% National Television System Committee (NTSC), using a UV LED backlight and RGB perovskite QDs in a BTBT-based organic matrix as the color conversion layer. Significantly, the photostability of this composite was enhanced when used as a color conversion layer (CCL) under blue-LED excitation. [ABSTRACT FROM AUTHOR]

The temperature-dependent intensities of the exciton (X) and biexciton (XX) peaks from single GaAs cone–shell quantum dots (QDs) are studied with micro photoluminescence (PL) at varied excitation power and QD size. The QDs are fabricated by filling self-assembled nanoholes, which are drilled in an AlGaAs barrier by local droplet etching (LDE) during molecular beam epitaxy (MBE). This method allows the fabrication of strain-free QDs with sizes precisely controlled by the amount of material deposited for hole filling. Starting from the base temperature T = 3.2 K of the cryostat, single-dot PL measurements demonstrate a strong enhancement of the exciton emission up to a factor of five with increasing T. Both the maximum exciton intensity and the temperature T x , m a x of the maximum intensity depend on excitation power and dot size. At an elevated excitation power, T x , m a x becomes larger than 30 K. This allows an operation using an inexpensive and compact Stirling cryocooler. Above T x , m a x , the exciton intensity decreases strongly until it disappears. The experimental data are quantitatively reproduced by a model which considers the competing processes of exciton generation, annihilation, and recombination. Exciton generation in the QDs is achieved by the sum of direct excitation in the dot, plus additional bulk excitons diffusing from the barrier layers into the dot. The thermally driven bulk-exciton diffusion from the barriers causes the temperature enhancement of the exciton emission. Above T x , m a x , the intensity decreases due to exciton annihilation processes. In comparison to the exciton, the biexciton intensity shows only very weak enhancement, which is attributed to more efficient annihilation processes. [ABSTRACT FROM AUTHOR]

At present, the available experimental data on the optical properties of layered III–VI monochalcogenide compounds are scattered and somewhat contradictory, although they are the parent materials for promising two-dimensional (2D) structures. This work is devoted to optical studies of bulk GaSe crystals, whose perfect structural properties are confirmed by Raman studies, observation of singlet-triplet splitting of 1.5 meV, and the polarized photoluminescence measurements from the sample edge. We analyze the band structure of GaSe, namely the sequence and energies of direct and indirect exciton transitions, using cw and time-resolved micro-photoluminescence measurements with variation of temperature. It turns out that the direct band gap in bulk GaSe is at 2.13 eV, close to calculated values. The indirect exciton transition is located ∼15 meV below the direct exciton (2.11 eV). Its intensity quickly quenches and characteristic decay time strongly shortens with increasing temperature, while the contribution of the direct exciton is relatively enhanced. [ABSTRACT FROM AUTHOR]

A model of the photocurrent generation of charge carriers in blends of donor (D) and acceptor (A) materials structured on the nanoscale is considered. The absorption of a quantum of light in one of these materials creates a molecular exciton, which can reach the interface between the D and A phases and form an interfacial charge transfer (CT) exciton on this interface, which dissociates into an electron-hole pair. The probabilities of the dissociation of CT excitons into free current carriers are calculated as a function of the electric field and the thermalization length of the electron-hole pair. [ABSTRACT FROM AUTHOR]

Currently, one of the priority tasks is the development of intelligent materials that are sensitive to stimuli and can be used in catalytic applications for efficient decomposition of organic pollutants. Particularly relevant in this regard is the development of inexpensive non‐metallic hybrid polymer photocatalysts, and the explanation of the mechanism of using light energy in such systems is still the subject of open discussions. In this work, hybrid polymer fiber membranes based on polyvinylidene fluoride (PVDF) with graphite flakes were obtained by electrospinning. The morphology of the obtained samples was investigated using SEM microscopy methods, and their crystalline structure was analysed using XRD, Raman, and FTIR methods. Using RFE spectroscopy, it was shown that the addition of graphite leads to ion‐dipole interaction between the carbon surface and the polymer dipoles, changing the ratio of covalent and semi‐ionic bonds on the surface. The catalytic properties investigated during the decomposition of the organic dye methylene blue (MB) demonstrated high piezocatalytic activity of doped membranes and high photocatalytic activity of PVDF/C0.5. It was established that the photocatalytic activity is due to excitonic excitation. [ABSTRACT FROM AUTHOR]