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A comprehensive study of excitonic properties of 2D layered perovskites is provided, with an emphasis on understanding and controlling the exciton fine structure. First, an overview of the optical properties is presented, discussing the challenges in determining the bandgap and exciton binding energies. Through magneto‐optical spectroscopic measurements (up to B = 140 T), scaling laws are established for exciton binding energy as a function of the band gap and the diamagnetic coefficient. Using an in‐plane magnetic field, the exciton fine structure for various 2D perovskites is examined to measure the energy splitting between the excitonic levels. The exciton fine structure and exchange interaction are correlated with structural parameters, employing an effective mass model, to highlight the role of steric effect on the exchange interaction. These findings reveal that lattice distortions, introduced by organic spacers, significantly influence the exchange interaction, driving a tunable energy spacing between dark and bright excitons. This unique feature of 2D perovskites, not present in other semiconductors, offers a novel tuning mechanism for exciton control, making these materials highly promising for efficient light emitters and advanced quantum technologies. [ABSTRACT FROM AUTHOR]

Low‐temperature condensation thermodynamics is a fundamental research in the field of condensed matter physics. Prior studies have extensively explored the exciton condensation of indirect‐bandgap semiconductors like silicon (Si) and germanium (Ge), which predominantly focus on temperature effects but neglect the relationship between the initial condensed state and the external excitation power density. Here, based on pure diamond, the impact of excitation power density on condensed‐state thermodynamics is analyzed. With power density as a key variable, an inter‐dependency among exciton, electron–hole plasma, and electron–hole droplets can be observed in diamond. For instance, as the average excitation power density increases (6.15 to 246 mW cm−2), the exciton emission quenching temperature rises from 60 to 120 K. This is because the variation in initial states of the excitonic condensed phase under different excitation power densities leads to the changes in quenching energy, which subsequently affects the temperature dependence of the exciton quenching. This study pioneers a novel approach to explore luminescent thermodynamics in indirect‐bandgap semiconductors with both excitation power density and temperature considered as dimensions simultaneously. [ABSTRACT FROM AUTHOR]

Triplet pairs (TT) in crystalline molecular semiconductors have unique spin properties of interest for quantum information or enhancing solar photoconversion. The population and diffusion dynamics of TT have been the subject of recent studies, both in covalent dimers and in crystalline systems. Here, we monitor the triplet population in neat polycrystalline and amorphous films of a heteroacene with known TT spectral properties and tunable spin polarization depending on the intermolecular geometry. Transient measurements reveal an anomalous power dependence in polycrystalline films that we attribute to the fast diffusion and interaction of dissociated triplet pairs confined to one‐dimensional stacks of strongly coupled molecules. The nongeminate triplet interaction after dephasing facilitates conversion to the triplet 3TT and eventually T1+S0. Amorphous films have no power dependence and proceed directly from 1TT to 3TT and subsequently T1+S0 via state mixing facilitated by nonparallel geometries and weak exchange coupling. [ABSTRACT FROM AUTHOR]

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]

A widely discussed explanation for the prevalence of pairs or clusters of closely spaced electronic chromophores in photosynthetic light-harvesting proteins is the presence of ultrafast and highly directional excitation energy transfer pathways mediated by vibronic excitons, the delocalized optical excitations derived from mixing of the electronic and vibrational states of the chromophores. We discuss herein the hypothesis that internal conversion processes between exciton states on the <100 fs timescale are possible when the excitonic potential energy surfaces are controlled by the vibrational modes that induce charge transfer character in a strongly coupled system of chromophores. We discuss two examples, the peridinin–chlorophyll protein from marine dinoflagellates and the intact phycobilisome from cyanobacteria, in which the intramolecular charge-transfer (ICT) character arising from out-of-plane distortion of the conjugation of carotenoid or bilin chromophores also results in localization of the initially delocalized optical excitation on the vibrational timescale. Tuning of the ground state conformations of the chromophores to manipulate their ICT character provides a natural photoregulatory mechanism, which would control the overall quantum yield of excitation energy transfer by turning on and off the delocalized character of the optical excitations. [ABSTRACT FROM AUTHOR]

Here I provide a summary of my group's research on the photophysics of nanocrystal quantum dots and briefly discuss multipeak luminescence structures related to excitons, trions, and biexcitons in halide perovskite nanocrystals. [ABSTRACT FROM AUTHOR]

The development of nonfullerene acceptors (NFAs), represented by ITIC, has contributed to improving the power conversion efficiency (PCE) of organic solar cells (OSCs). Although tuning the electronic structures to reduce the exciton binding energy (Eb) is considered to promote photocharge generation, a rational molecular design for NFAs has not been established. In this study, we designed and developed two ITIC‐based NFAs bearing spiro‐substituted bithiophene or biphenyl units (named SpiroT‐DCI and SpiroF‐DCI) to tune the frontier molecular orbital (FMO) distribution of NFAs. While the highest occupied molecular orbitals (HOMOs) of SpiroF‐DCI and ITIC are delocalized in the main π‐conjugated framework, the HOMO of SpiroT‐DCI is distributed on the bithiophene unit. Reflecting this difference, SpiroT‐DCI exhibits a smaller Eb than either SpiroF‐DCI or ITIC, and exhibits greater external quantum efficiency in single‐component OSCs. Furthermore, SpiroT‐DCI shows improved PCEs for bulk‐heterojunction OSCs with a donor of PBDB‐T, compared with that of either SpiroT‐DCI or ITIC. Time‐resolved spectroscopy measurements show that the photo‐induced intermolecular charge separation is effective even in pristine SpiroT‐DCI films. This study highlights the introduction of spiro‐substituted bithiophene units that are effective in tuning the FMOs of ITIC, which is desirable for reducing the Eb and improving the PCE in OSCs. [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.

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]

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]

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]

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]

A brief overview is presented of results obtained at the Rzhanov Institute of Semiconductor Physics in the development of photon detectors and emitters promising for use in quantum cryptography systems, along with miniature quantum frequency standards based on the effect of coherent population trapping. [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]

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.

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]

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]

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]

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]

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]