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Recently, geometry-induced quantum effects in a new quasi-1D system, or nanograting (NG) layers, were introduced and investigated. Dramatic changes in band structure and unconventional photoluminescence effects were found in silicon quantum wells with high-energy barriers. Nanograting metal–semiconductor junctions were fabricated and investigated. Here, we report the latest results on a special type of p-n junction in which the charge confinement of the NG is enhanced. The reverse bias dark current is increased in contrast to the metal–semiconductor junctions. When such a junction works as a photovoltaic cell, NG significantly increases short-circuit current and conversion efficiency without affecting open-circuit voltage. These effects are explained by the formation of geometry-induced excitons. To distinguish exciton formation from G-doping effects, we fabricated NGs in both n-type and p-type top layers and obtained qualitatively the same results. To further verify the excitonic mechanism, we analyzed photoluminescence spectrums previously obtained from NG and other NG-like periodic structures. The collected experimental results and previous findings are well explained by the formation of geometry-induced excitons and corresponding quasi-flat bands. Geometry-induced quantum effects can be used to significantly increase the conversion efficiency of photovoltaic cells and enhance the characteristics of other optoelectronic devices. [ABSTRACT FROM AUTHOR]

Technology for fabricating red organic light-emitting diodes (OLEDs) based on a multiple quantum well (MQW) structure is considered. The study aimed to develop an OLED using the MQW structure to prevent the quantum efficiency roll-off at high operating voltages, a common issue in traditional OLEDs that use doped host-guest systems. Two OLED types were fabricated to achieve this: one with a doped host-guest system and another with an MQW structure. The electrical characteristics of both devices were compared in this research, focusing on quantum efficiency. A narrow-band orange organic emitter with thermally activated delayed fluorescence emission (TADF) 2,3,5,6-tetrakis(3,6-diphenyl-9H-carbazol-9-yl)- 1,4-benzenedicarbonitrile (4CzTPN-Ph) was used for the new structure, sandwiched between two layers of wide-band semiconductor mCBP, creating a quasi-two-dimensional (2D) heterostructure. Implementing the MQW structure significantly reduced the quantum efficiency roll-off at higher voltages, offering a major advantage over traditional OLEDs, where such roll-off often leads to performance reduction and shortened device lifetimes. The results showed that the OLED with the MQW structure demonstrated a brightness exceeding 600 cd/m², and stable quantum efficiency across the entire operating voltage range. Unlike traditional OLEDs, where doped systems often face challenges with uneven charge and energy distribution, the MQW structure enables better exciton confinement and more efficient utilization, thus improving device stability. Furthermore, using the MQW structure allowed for enhancement of the colour characteristics of OLEDs, making them more saturated and accurate, which is important for commercial applications such as displays and lighting systems. [ABSTRACT FROM AUTHOR]

The first part of this work evaluates Al-doped ZnO (AZO) as an optically transparent top-gate material for studies on semiconductor quantum dots. In comparison with conventional Ti gates, samples with AZO gates demonstrate a more than three times higher intensity in the quantum dot emission under comparable excitation conditions. On the other hand, charges inside a process-induced oxide layer at the interface to the semiconductor cause artifacts at gate voltages above  U ≈  1 V. The second part describes an optical and simulation study of a vertical electric-field (F)-induced switching from a strong to an asymmetric strong–weak confinement in GaAs cone-shell quantum dots (CSQDs), where the charge carrier probability densities are localized on the surface of a cone. These experiments are performed at low U and show no indications of an influence of interface charges. For a large F, the measured radiative lifetimes are substantially shorter compared with simulation results. We attribute this discrepancy to an F-induced transformation of the shape of the hole probability density. In detail, an increasing F pushes the hole into the wing part of a CSQD, where it forms a quantum ring. Accordingly, the confinement of the hole is changed from strong, which is assumed in the simulations, to weak, where the local radius is larger than the bulk exciton Bohr radius. In contrast to the hole, an increasing F pushes the electron into the CSQD tip, where it remains in a strong confinement. This means the radiative lifetime for large F is given by an asymmetric confinement with a strongly confined electron and a hole in a weak confinement. To our knowledge, this asymmetric strong–weak confinement represents a novel kind of quantum mechanical confinement and has not been observed so far. Furthermore, the observed weak confinement for the hole represents a confirmation of the theoretically predicted transformation of the hole probability density from a quantum dot into a quantum ring. For such quantum rings, application as storage for photo-excited charge carriers is predicted, which can be interesting for future quantum photonic integrated circuits. [ABSTRACT FROM AUTHOR]

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]

We studied the interaction between the neutral and charged excitons of monolayer MoSe2 at various temperatures via quantum beat spectroscopy in the time domain. We introduced temperature as an efficient control knob to regulate the relative photoluminescence intensities of the neutral and charged excitons to obtain maximum quantum beat resolution. Furthermore, our quantum beat measurements under different temperatures indicate that the decoherence time of the coupled exciton-trion state slightly decreases from 530 fs at 3.5 K to 420 fs at 63 K with increased temperature due to the low-energy acoustic phonon-induced dephasing. [ABSTRACT FROM AUTHOR]

We studied the interaction between the neutral and charged excitons of monolayer MoSe2 at various temperatures via quantum beat spectroscopy in the time domain. We introduced temperature as an efficient control knob to regulate the relative photoluminescence intensities of the neutral and charged excitons to obtain maximum quantum beat resolution. Furthermore, our quantum beat measurements under different temperatures indicate that the decoherence time of the coupled exciton-trion state slightly decreases from 530 fs at 3.5 K to 420 fs at 63 K with increased temperature due to the low-energy acoustic phonon-induced dephasing. [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.

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.

Abstract Recent progress in high-technology equipment enables the fabrication of quantum dots such as GaAs, and GaAlAs confining a finite number of excitons and allowing for control of the properties of quantum dots. Biexciton quantum dots are the simplest example that can be used to upgrade optoelectronics technologies. This theoretical research investigates a model of the biexciton state in the strongly oblate ellipsoidal quantum dot with the relativistic corrections of mass and Hamiltonian in the framework of the quantum field theory due to the importance of the relativistic effect for this type of quantum dot shapes. The Sturmian function transformation and Wick ordering method to calculate the vacuum state energy eigenvalue of the biexciton system are utilized. Based on the relativistic behavior of interactions, the mass corrections to the Hamiltonian are defined. Dependence of the relativistic mass on the distances between electrons and the constituent mass to the coupling constant is obtained. The results show that as increasing quantum dot size, the relativistic mass and Hamiltonian corrections terms decrease.

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]

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]

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.

Two-dimensional (2D) layered materials have received much attention due to the unique properties stemming from their van der Waals (vdW) interactions, quantum confinement, and many-body interactions of quasi-particles, which drive their exotic optical and electronic properties, making them critical in many applications. Here, we review our past years’ findings, focusing on many-body interactions in 2D layered materials, including phonon anharmonicity, electron–phonon coupling (e-ph), exciton dynamics, and phonon anisotropy based on temperature (polarization)-dependent Raman spectroscopy and Photoluminescence (PL). Our review sheds light on the role of quasi-particles in tuning the material properties, which could help optimize 2D materials for future applications in electronic and optoelectronic devices.

We report the complex dielectric function ε = ε1 + iε2 of MoS2/WS2 and WS2/MoS2 heterostructures and their constituent monolayers MoS2 and WS2 for an energy range from 1.5 to 6.0 eV and temperatures from 39 to 300 K. Comparisons between the optical properties of the heterostructures and their monolayers were conducted. Critical-point (CP) energies of the heterostructures were traced back to their origins in the monolayers. Low-temperature measurements confirmed the existence of only three excitonic CPs from 1.5 to 2.5 eV due to the overlap of trion B− of the MoS2 monolayer and exciton A0 of the WS2 monolayer. Due to the dielectric screening effect, most CPs exhibit red shifts in the heterostructures compared to their monolayer counterparts.

The band energy structures of CeF2Cl and CeFCl2 crystals have been calculated using the projector augmented-wave (PAW) method and the hybrid exchange-correlation functional PBE0. The valence band top consists of 2p states of F and 3p states of Cl. An energy gap is observed between the 5d states of Ce in the bottom part of the conduction band of both crystals, forming two subbands, 5d1 and 5d2, with very different effective electron masses (2.49 m0 and 0.19 m0 for CeF2Cl and 5.95 m0 and 0.84 m0 for CeFCl2, respectively). The 4f states of Ce are placed within the forbidden band. The obtained values for the band gap of CeF2Cl and CeFCl2 crystals are 6 eV and 4.6 eV, respectively. [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]

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.

Radiative relaxations of electronic excitations - self-trapped excitons (STE) in regular lattice sites (intrinsic luminescence) and exciton-like formations (ELF) in the field of homologous cations (exciton-like luminescence). It has been found that the maximum effect of luminescence enhancement occurs upon uniaxial deformation along the crystallographic direction (compared to ), which coincides with the direction of the self-trapped anion exciton () in the KBr crystal matrix. The exciton mechanism was estimated from the increase in the intensity of the intrinsic σ(4, 42eV) - and π(2, 3eV) - luminescences of STE, and the enhancement of luminescence intensity of near-single Na+ (2.85 eV), pair ions Na+ , Na+ (3.1 eV) and Na+ Pb++ (3.4 eV)- centers - recombination mechanism of radiative relaxation of electronic excitations.

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]

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]

The electronic structure and dynamics of 2D transition metal dichalcogenide (TMD) monolayers provide important underpinnings both for understanding the many-body physics of electronic quasi-particles and for applications in advanced optoelectronic devices. However, extensive experimental investigations of semiconducting monolayer TMDs have yielded inconsistent results for a key parameter, the quasi-particle band gap (QBG), even for measurements carried out on the same layer and substrate combination. Here, we employ sensitive time- and angle-resolved photoelectron spectroscopy (trARPES) for a high-quality large-area MoS2 monolayer to capture its momentum-resolved equilibrium and excited-state electronic structure in the weak-excitation limit. For monolayer MoS2 on graphite, we obtain QBG values of ≈2.10 eV at 80 K and of ≈2.03 eV at 300 K, results well-corroborated by the scanning tunneling spectroscopy (STS) measurements on the same material.

Exciton resonances in monolayer transition-metal dichalcogenides (TMDs) provide exceptionally strong light–matter interaction at room temperature. Their spectral line shape is critical in the design of a myriad of optoelectronic devices, ranging from solar cells to quantum information processing. However, disorder resulting from static inhomogeneities and dynamical fluctuations can significantly impact the line shape. Many recent works experimentally evaluate the optical properties of TMD monolayers placed on a substrate and the line shape is typically linked directly to the material’s quality. Here, we highlight that the interference of the substrate and TMD reflections can strongly influence the line shape. We further show how basic, room-temperature reflection measurements allow investigation of the quantum mechanical exciton dynamics by systematically controlling the substrate reflection with index-matching oils. By removing the substrate contribution with properly chosen oil, we can extract the excitonic decay rates including the quantum mechanical dephasing rate. The results provide valuable guidance for the engineering of exciton line shapes in layered nanophotonic systems.

By solving the Liouville equation, third-order nonlinear terms of the optical absorptive two quantum-level system are obtained through iterative density matrix calculations. In the improved model, the full frequency range is taken into account instead of just the weak absorptive limit. he process investigated in this study can be likened to saturation fitting for heavy-hole excitons in the InAs quantum well. Additionally, we demonstrate that the iterative method used for analyzing band-to-band excitonic spectra enables easy retrieval of the static dielectric constant.

In this study, polyvinylidene fluoride (PVDF) fibers doped with hydrated calcium nitrate were prepared using electrospinning. The samples were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), optical spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), Raman, and photoluminescence (PL) spectroscopy. The results are complementary and confirm the presence of chemical hydrogen bonding between the polymer and the dopant. Additionally, there was a significant increase in the proportion of the electroactive polar beta phase from 72 to 86%. It was shown that hydrogen bonds acted as a transport pathway for electron capture by the conjugated salt, leading to more than a three-fold quenching of photoluminescence. Furthermore, the optical bandgap of the composite material narrowed to the range of visible light energies. For the first time, it the addition of the salt reduced the energy of the PVDF exciton by a factor of 17.3, initiating photocatalytic activity. The calcium nitrate-doped PVDF exhibited high photocatalytic activity in the degradation of methylene blue (MB) under both UV and visible light (89 and 44%, respectively). The reaction rate increased by a factor of 2.4 under UV and 3.3 under visible light during piezophotocatalysis. The catalysis experiments proved the efficiency of the membrane design and mechanisms of catalysis are suggested. This study offers insight into the nature of chemical bonds in piezopolymer composites and potential opportunities for their use. [ABSTRACT FROM AUTHOR]

Nitride cylindrical quantum dots (NCQDs) are commonly used materials in optical chemical sensors and biosensors. We investigated the exciton-related optical properties of wurtzite ZnSnN2/InxGa1−xN cylindrical quantum dots (CQDs) with finite potential barriers. The energy states in a CQD system were computed by a variational scheme using the single-band effective mass approximation, which considers the effect of the built-in electric field (BEF). The results are presented in the form of a specific function of CQD structural parameters including height, radius, and In concentration in the barrier. Our major findings are as follows. First, the excitonic transition energy decreases with increasing CQD size and increases with increasing In concentration. Second, the oscillator strength rapidly decreases with increasing CQD height and slowly increases with increasing CQD radius. Furthermore, we obtained the absorption coefficient (AC) of the exciton as a function of the incident photon energy for different CQD structural parameters. We concluded that, in general, increasing any of the aforementioned structural parameters causes either a blueshift or redshift of the AC peak. A similar phenomenon occurs for the resonant peak intensity. In particular, a strong BEF leads to a redshift of the resonant peak and a decrease in its intensity. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides (2D TMDs) have been demonstrated as one of the most outstanding materials not only for fundamental science but also for a wide range of photonic applications. However, an efficient way to control their excitonic properties is still needed for advanced applications with superior device performance. Here, we show that the exciton lifetime of WSe2 monolayer can be prolonged using metamaterials. We observe a ∼100% reduction in the electron-hole recombination rate of WSe2 monolayer placed on a hyperbolic metamaterial substrate and demonstrate that such a remarkable change results from the destructive image dipole interaction with the in-plane exciton transition dipole. Furthermore, this substantial increase in exciton lifetime leads to order-of-magnitude (10-fold) enhancement of photocurrent in the 2D WSe2-based hybrid photodetector with metamaterials. Tailoring the optical transition properties of 2D TMD materials with specially designed metamaterials, demonstrated here, will pave the way for developing 2D material-based optoelectronics.

In the model of effective masses and rectangular potentials for an electron and a hole, the influence of a uniform electric field on the energy spectrum and wave functions of the exciton and the oscillator strengths of interband quantum transitions in the semiconductor (GaAs/AlxGa1-xAs) quantum dot-quantum ring nanostructure is theoretically investigated. The stationary Schrödinger equations for noninteracting quasiparticles in the presence of an electric field cannot be solved analytically. For their approximate solution, the unknown wave functions are sought in the form of an expansion over the complete set of cylindrically symmetric wave functions, and the electron energy is found by solving the corresponding secular equation. The exciton binding energy is found using perturbation theory. The dependences of the energy spectra, the wave functions of an electron, hole, and exciton, and the intensity of interband optical quantum transitions on the magnitude of the electric field strength are analyzed.

This study aims to determine the optimum composition of the CeBr1−xIx compound to achieve the maximum light output. It is based on calculations of the band energy structure of crystals, specifically taking into account the characteristics of the mutual location of local and band 5d states of the Ce3+ ions. The band energy structures for CeBr2I and CeBrI2 crystals were calculated using the projector augmented wave method. The valence band was found to be formed by the hybridized states of 4p Br and 5p I. The 4f states of Ce3+ are located in the energy forbidden band gap. The conduction band is formed by the localized 5d1 states, which are created by the interaction between the 5d states of Ce3+ and the 4f0 hole of the cerium ion. The higher-lying delocalized 5d2 states of Ce3+ correspond to the energy levels of the 5d states of Ce3+ in the field of the halide Cl0 (Br0) hole. The relative location of 5d1 and 5d2 bands determines the intensity of 5d–4f luminescence. The bottom of the conduction band is formed by localized 5d1 states in the CeBr2I crystal. The local character of the bottom of the conduction band in the CeBr2I crystal favors the formation of self-trapped Frenkel excitons. Transitions between the 5d1 and 4f states are responsible for 5d–4f exciton luminescence. In the CeBrI2 crystal, the conduction band is formed by mixing the localized 5d1 and delocalized 5d2 states, which leads to quenching the 5d–4f luminescence and a decrease in the light output despite the decrease in the forbidden band gap. CsBr2I is the optimum composition of the system to achieve the maximum light output. [ABSTRACT FROM AUTHOR]

Plasmon resonances can greatly enhance light–matter interactions of two-dimensional van der Waals materials. However, the quality factor of plasmonic resonances is limited. Here, we demonstrate a plasmonic quasi-bound state in the continuum (quasi-BIC), which is composed of gold nanorod pairs. Through controlling the rotation angle of the nanorods, the quality factor of the plasmonic BIC mode can be tuned. Simulation results show that the plasmonic BIC combines the advantages of high-quality factor from the BIC effect and small mode volume from plasmonic resonance. Experiment results show that the designed plasmonic BIC mode exhibits a quality factor higher than 15 at the wavelength of around 1250 nm. Through integrating the plasmonic bound state structure with monolayer molybdenum ditelluride (MoTe2), the exciton emission of MoTe2 in the PL spectrum split into two exciton-polariton modes, which is attributed to the high Q factor and strong interaction between the BIC mode and excitons of MoTe2. [ABSTRACT FROM AUTHOR]

The formation of structures of different sizes on the surface of a single sample and the study of the mechanisms of their excitations and energy transitions is a promising area of research. ZnO/SiO2/Si heterostructures were formed by two-stage electrochemical etching of silicon wafers and synthesizing zinc oxide nanoparticles using sol-gel technology. By scanning electron and atomic force microscopies, various pores and clusters were identified. The presence of five levels of surface hierarchy provided the synthesis of light-emitting particles with different properties. Light emission mechanisms are associated with the recombination of excitons and the hyperfine structure of charged particles trapped on oxygen vacancy. [ABSTRACT FROM AUTHOR]

We present an ab initio study on quasiparticle (QP) excitations and excitonic effects in two-dimensional (2D) GaN based on density-functional theory and many-body perturbation theory. We calculate the QP band structure using  G W  approximation, which generates an indirect band gap of  4.83  eV ( K → Γ ) for 2D GaN, opening up  1.24  eV with respect to its bulk counterpart. It is shown that the success of plasmon-pole approximation in treating the 2D material benefits considerably from error cancellation. On the other hand, much better gaps, comparable to  G W  ones, could be obtained by correcting the Kohn–Sham gap with a derivative discontinuity of the exchange–correlation functional at much lower computational cost. To evaluate excitonic effects, we solve the Bethe–Salpeter equation (BSE) starting from Kohn–Sham eigenvalues with a scissors operator to open the single-particle gap. This approach yields an exciton binding energy of  1.23  eV in 2D GaN, which is in good agreement with the highly demanding  G W -BSE results. The enhanced excitonic effects due to reduced dimensionality are discussed by comparing the optical spectra from BSE calculations with that by random-phase approximation (RPA) for both the monolayer and bulk GaN in wurtzite phase. Additionally, we find that the spin–orbit splitting of excitonic peaks is noticeable in 2D GaN but buried in the bulk crystal. [ABSTRACT FROM AUTHOR]

Perovskite nanocrystals (PeNCs) are known for their use in numerous optoelectronic applications. Surface ligands are critical for passivating surface defects to enhance the charge transport and photoluminescence quantum yields of the PeNCs. Herein, we investigated the dual functional abilities of bulky cyclic organic ammonium cations as surface-passivating agents and charge scavengers to overcome the lability and insulating nature of conventional long-chain type oleyl amine and oleic acid ligands. Here, red-emitting hybrid PeNCs of the composition CsxFA(1−x)PbBryI(3−y) are chosen as the standard (Std) sample, where cyclohexylammonium (CHA), phenylethylammonium (PEA) and (trifuluoromethyl)benzylamonium (TFB) cations were chosen as the bifunctional surface-passivating ligands. Photoluminescence decay dynamics showed that the chosen cyclic ligands could successfully eliminate the shallow defect-mediated decay process. Further, femtosecond transient absorption spectral (TAS) studies uncovered the rapidly decaying non-radiative pathways; i.e., charge extraction (trapping) by the surface ligands. The charge extraction rates of the bulky cyclic organic ammonium cations were shown to depend on their acid dissociation constant (pKa) values and actinic excitation energies. Excitation wavelength-dependent TAS studies indicate that the exciton trapping rate is slower than the carrier trapping rate of these surface ligands. [ABSTRACT FROM AUTHOR]

Excitonic condensation and superfluidity have recently received a renewed attention, due to the fabrication of bilayer systems in which electrons and holes are spatially separated and form stable pairs known as indirect excitons. Dichalcogenides- and graphene-based bilayers are nowadays built and investigated, giving access to systems with (i) only spin degeneracy and (ii) spin and valley degeneracy. Simulation studies performed in the last decades at T = 0 for simple, model electron–hole bilayers, as function of the interlayer distance and in-layer carrier density, have revealed in case (i) the formation of biexcitons in a tiny region of the parameter space and in case (ii) the formation of stable compounds made of four electrons and four holes (quadriexcitons) in a sizable region of the parameter space. Of some interest is the relation of the properties of isolated biexcitons (quadriexcitons) and those of their finite-density counterpart. In fact, the isolated biexciton has been repeatedly studied in the last years with simulations and other techniques. No simulations, instead, are available to our knowledge for the isolated quadriexciton, for which we present here results of the first quantum Monte Carlo (QMC) study. Stability with respect to the dissociation into biexcitons and the pair correlations while varying the interlayer distance d are discussed. [ABSTRACT FROM AUTHOR]

In the current work, we study the intense laser pulse influences on the behaviors of the first excitonic transition in a core/shell quantum dot submitted to an electric field. Therefore, the exciton binding energy and the mean distance between the correlated electron–hole pair are discussed, considering the electric field and laser strength. Our calculations show that both external fields play significant repulsive effects. Through their effects, they oppose the attractive nature of the Coulomb potential between the correlated pair, which decreases the excitonic binding energy. We also analyze the dissociation process by determining the photo-ionization cross-section (PICS). Our findings show that the peaks of the PICS redshift when the shell thickness b − a increases. For a given core radius, the laser and electric field induce a shift toward the low-energy region for the PICS; this displacement is more pronounced for the laser case. Our study also compares simple quantum dots and core/shell quantum dots to show the effect of the inner radius on the obtained results. Our theoretical results can lead to promising applications of exciton-based devices controlled by sizes and external fields. [ABSTRACT FROM AUTHOR]

In quantum dots, where confinement is strong, interactions between charge carriers play an essential role in the performance of semiconductor materials for optical gain. Therefore, understanding this phenomenon is critical for achieving new devices with enhanced features. In this context, the current study examines the optical properties of an exciton confined in a spherical InAs quantum dot under the influence of magnetic and intense laser fields. We investigate the oscillator strength, exciton lifetime, and optical gain, considering the effects of both external fields. We also pay particular attention to the influence of quantum dot size on the results. Our calculations show that the two external fields have opposite effects on our findings. Specifically, the applied magnetic field increases the oscillator strength while the intense laser reduces it. In addition, the optical gain peaks are redshifted under the application of the intense laser, whereas the magnetic field causes a blueshift of the peak threshold. We also find that both external perturbations significantly influence the exciton lifetime. Our study considers the outcomes of both the exciton's ground ( 1 s ) and first excited ( 1 p ) states. The theoretical results obtained in this study have promising implications for optoelectronic devices in the ∼3–4 μ m wavelength range only through the control of quantum dot sizes and external perturbations. [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.

Changing the polycondensation reaction route, positively and negatively charged TiO2 nanoparticles have been synthesized. On the base of the synthesized TiO2 nanoparticles, porous TiO2 films have been formed with different surface charges. J-aggregates of two anionic dyes were formed on positively charged TiO2 films, while J-aggregates of cationic dye was formed on negatively charged one. Combining J-aggregates with different J-band spectral positions, we have obtained two composites consisting of J-aggregates pairs with an effective excitation energy transfer between them. In both cases, simple mixing of J-aggregate in solution did not result in the composite formation and only sequential deposition of J-aggregate from corresponding solutions was successful. Interesting, not only charge alternating composite of cationic pseudoisocyanine J-aggregates and anionic thiacarbocyanine J-aggregates was realized, but also the same was done in the case of the composite of two anionic carbocyanine J-aggregates.