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The properties of excitons, or correlated electron-hole pairs, are of paramount importance to optoelectronic applications of materials. A central component of exciton physics is the electron-hole interaction, which is commonly treated as screened solely by electrons within a material. However, nuclear motion can screen this Coulomb interaction as well, with several recent studies developing model approaches for approximating the phonon screening of excitonic properties. While these model approaches tend to improve agreement with experiment, they rely on several approximations that restrict their applicability to a wide range of materials, and thus far they have neglected the effect of finite temperatures. Here, we develop a fully first-principles, parameter-free approach to compute the temperature-dependent effects of phonon screening within the ab initio [Formula: see text]-Bethe-Salpeter equation framework. We recover previously proposed models of phonon screening as well-defined limits of our general framework, and discuss their validity by comparing them against our first-principles results. We develop an efficient computational workflow and apply it to a diverse set of semiconductors, specifically AlN, CdS, GaN, MgO, and [Formula: see text]. We demonstrate under different physical scenarios how excitons may be screened by multiple polar optical or acoustic phonons, how their binding energies can exhibit strong temperature dependence, and the ultrafast timescales on which they dissociate into free electron-hole pairs.

By using ab initio simulations based on density functional theory and many‐body perturbation theory, a comprehensive analysis of the distinct optical signatures of various tellurene polymorphs and their associated unique anisotropic excitonic characteristics is presented. Despite the atomic thickness of these materials, these findings reveal that their optical absorbance reaches as high as 50% in the near‐infrared and visible range. This investigation highlights the exceptional potential of these 2D semiconducting materials in the development of ultra‐thin and flexible homo‐ and hetero‐junctions for solar light harvesting, achieving photoconversion efficiencies up to 19%, a performance level comparable to current silicon technologies. [ABSTRACT FROM AUTHOR]

Triplet–triplet annihilation (TTA), or triplet fusion, is a biexcitonic process in which two triplet‐excited molecules can combine their energy to promote one into an excited singlet state. To alleviate the dependence of the TTA rate and yield on triplet diffusion in both solid and solution environments, intramolecular TTA (intra‐TTA) has been recently proposed in conjugated molecular systems able to hold multiple triplet excitons simultaneously. Developing from the previous demonstration of TTA performance enhancement in sensitized upconversion solutions, here similar improvements in triplet harvesting in solid‐state films are reported under electrical excitation in organic light emitting diodes (OLEDs). At low dye concentration and low current densities, the intra‐TTA active OLED shows a +40% improved external quantum efficiency with respect to the reference device, and a TTA spin‐statistical factor f 4DPA of 0.4, close to that determined in fluid solution for the individual chromophore (0.45). These results therefore indicate the utility of this molecular design strategy across a wider range of TTA applications, and with particular utility in the further development of low‐power TTA‐enhanced OLEDs. [ABSTRACT FROM AUTHOR]

Excitons are expected to be a high‐efficiency emission source in UV light‐emitting devices. However, the damping of the excitonic laser oscillation has been reported under conditions where the excitonic states are expected to be populated in the conventional theory. In order to understand the exciton dynamics under the thermal nonequilibrium state, a theoretical model including various energy species in semiconductors such as electrons, phonons, and photons is required. Herein, a 2D phononic–excitonic–radiative model is constructed to analyze the exciton dynamics in a 2D system. 2D excitons with four principal quantum number states and the continuum in the lowest energy level of the AlN/GaN/AlN quantum wells are considered. It is found that the 2D phonon significantly augments the excitation transition rate. When the high recombination rate corresponding to stimulated emission is considered, the exciton binding energy of 108 meV is not enough to reduce the population in the high‐order discreet states and the continuum states, while the binding energy of 215 meV corresponding to the one monolayer GaN has an advantage of reducing these populations. The analysis of population flux has an advantage in discussing the increase in the kinetic energy transfer to the 1S exciton. [ABSTRACT FROM AUTHOR]

Stimulated emission properties of AlGaN‐based multiple quantum wells with an emission wavelength of about 280 nm are studied by photoluminescence excitation (PLE) spectroscopy. The excitation power density dependence of the PLE spectra is evaluated at excitation energies from 4.4788 to 4.6293 eV, which allows the spontaneous and stimulated emission PLE spectra to be obtained. A clear peak is observed in the PLE spectrum for spontaneous emission, indicating that the spontaneous emission is due to the excitonic optical transition. A clear peak is also observed in the PLE spectrum for stimulated emission, suggesting that excitons are involved in the stimulated emission process. Furthermore, the threshold excitation power density for stimulated emission is analyzed as a function of the excitation energy and the threshold excitation power density depends on the excitation energy. The excitation energy at which the threshold excitation power density is minimized agrees with the peak of the stimulated emission PLE spectrum. This observation directly indicates that excitons are important in optical gain formation. [ABSTRACT FROM AUTHOR]

High‐resolution, low‐excitation photoluminescence (PL) spectroscopy is performed for unintentionally doped, silicon‐doped, and magnesium‐doped homoepitaxial aluminum nitride (AlN) films, using a wavelength‐tunable high‐repetition‐rate laser. The wavelength‐tunable laser is used to distinguish between the luminescence and scattering signals from AlN. Providing the high‐resolution, low‐excitation PL spectra, the current understanding of the deep‐ultraviolet luminescence properties of AlN is reviewed and potential assignments for the unknown luminescence lines and bands are discussed. Although previous studies have led to a consensus on the origins of some emission peaks and bands such as the neutral silicon donor‐bound exciton transition and free exciton transitions involving longitudinal optical phonons, it is shown that many of the emission peaks are still unidentified. The origins of all the emission peaks should be elucidated to enable control of the electronic and optoelectronic properties of AlN. [ABSTRACT FROM AUTHOR]

A layered Na2SnO3 with honeycomb structure was synthesized by solid state route using sodium carbonate and tin oxide. The prepared sample was characterized by different physicochemical characterization techniques like X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X‐ray spectroscopy (EDS), X‐ray photoelectron spectroscopy (XPS), ultraviolet visible spectroscopy (UV–vis), photoluminescence spectra (PL), N2 adsorption technique (Brunauer–Emmett–Teller, electrochemical impedance spectroscopy (EIS). The photodegradation of rose bengal dye under UV light was studied. Due to honeycomb structure and high surface area of Na2SnO3, the complete degradation of rose Bengal was observed in 90 min under UV light irradiation. Results of photocatalytic dye degradation reaction demonstrated that Na2SnO3 can be employed as photocatalyst under UV light irradiation. [ABSTRACT FROM AUTHOR]

Anisotropic optical 2D materials are crucial for achieving multiple‐quanta functions within quantum materials, which enables the fabrication of axially polarized electronic and optoelectronic devices. In this work, multiple excitonic emissions owning polarization‐sensitive orientations are clearly detected in a multilayered quasi‐1D ZrS3 nanoribbon with respect to the nanostripe edge. Four excitons denoted as AS1, AS2, AS, and A2 with E ⊥ b polarized direction and one prominent A1 exciton with E || b polarized emission are simultaneously detected in the polarized micro‐photoluminescence (µPL) measurement of 1.9–2.2 eV at 10 K. In contrast to light emission, polarized micro‐thermoreflectance (µTR) measurements are performed to identify the polarization dependence and verify the excitons in the multilayered ZrS3 nanoribbon from the perspective of light absorption. At 10 K, a prominent and broadened peak on the lower‐energy side, containing an indirect resonant emission (DI) observed by µPL and an indirect defect‐bound exciton peak (AInd) observed by both µPL and µTR, is simultaneously detected, confirming the existence of a quasi‐direct band edge in ZrS3. A van der Waals stacked p‐GaSe/n‐ZrS3 heterojunction solar cell is fabricated, which demonstrates a maximum axially‐polarized conversion efficiency up to 0.412% as the E || b polarized light incident onto the device. [ABSTRACT FROM AUTHOR]

Since dissipative processes are ubiquitous in semiconductors, characterizing how electronic and thermal energy transduce and transport at the nanoscale is vital for understanding and leveraging their fundamental properties. For example, in low-dimensional transition metal dichalcogenides (TMDCs), excess heat generation upon photoexcitation is difficult to avoid since even with modest injected exciton densities exciton-exciton annihilation still occurs. Both heat and photoexcited electronic species imprint transient changes in the optical response of a semiconductor, yet the distinct signatures of each are difficult to disentangle in typical spectra due to overlapping resonances. In response, we employ stroboscopic optical scattering microscopy (stroboSCAT) to simultaneously map both heat and exciton populations in few-layer MoS2 on relevant nanometer and picosecond length- and time scales and with 100-mK temperature sensitivity. We discern excitonic contributions to the signal from heat by combining observations close to and far from exciton resonances, characterizing the photoinduced dynamics for each. Our approach is general and can be applied to any electronic material, including thermoelectrics, where heat and electronic observables spatially interplay, and it will enable direct and quantitative discernment of different types of coexisting energy without recourse to complex models or underlying assumptions.

We present a comprehensive first-principles study on the optoelectronic properties of the single-layer nickel diazenide (penta-NiN2), a pentagon-based 2D semiconductor with ideal Cairo tessellation, whose bulk counterpart has been recently synthesized. To address its quasiparticle band structure and excitonic effects on its optical absorption spectrum, we carry out ab initio calculations based on many-body perturbation theory within the GW-Bethe–Salpeter equation (BSE) framework. Our results reveal a quasiparticle band gap of 1.05 eV by employing the eigenvalue self-consistent GW approach, corroborating its potential in optoelectronics. The band gap exhibits an anomalous negative dependence on temperature, verified through the band gap pressure coefficient. Acoustic phonon-limited scattering analyses indicate an ultrahigh hole mobility of ∼87 × 104 cm2 V–1 s–1 along the [010] direction. The most prominent absorption peak of monolayer penta-NiN2 is associated with resonant excitons, corresponding to transitions from the valence band maximum to conduction band minimum + 2, which is explained by analyzing the state symmetry of the band edges. Hence, this pentagonal 2D semiconductor exhibits compelling and promising properties deserving deeper exploration in infrared optoelectronics and high-speed devices. [ABSTRACT FROM AUTHOR]

The ring-like peripheral light-harvesting complex 2 (LH2) expressed by many phototrophic purple bacteria is a popular model system in biological light-harvesting research due to its robustness, small size, and known crystal structure. Furthermore, the availability of structural variants with distinct electronic structures and optical properties has made this group of light harvesters an attractive testing ground for studies of structure–function relationships in biological systems. LH2 is one of several pigment-protein complexes for which a link between functionality and effects such as excitonic coherence and vibronic coupling has been proposed. While a direct connection has not yet been demonstrated, many such interactions are highly sensitive to resonance conditions, and a dependence of intra-complex dynamics on detailed electronic structure might be expected. To gauge the sensitivity of energy-level structure and relaxation dynamics to naturally occurring structural changes, we compare the photo-induced dynamics in two structurally distinct LH2 variants. Using polarization-controlled 2D electronic spectroscopy at cryogenic temperatures, we directly access information on dynamic and static disorder in the complexes. The simultaneous optimal spectral and temporal resolution of these experiments further allows us to characterize the ultrafast energy relaxation, including exciton transport within the complexes. Despite the variations in PPC molecular structure manifesting as clear differences in electronic structure and disorder, the energy-transport and—relaxation dynamics remain remarkably similar. This indicates that the light-harvesting functionality of purple bacteria within a single LH2 complex is highly robust to structural perturbations and likely does not rely on finely tuned electronic- or electron-vibrational resonance conditions. [ABSTRACT FROM AUTHOR]

Excitons are the neutral quasiparticles that form when Coulomb interactions create bound states between electrons and holes. Due to their bosonic nature, excitons are expected to condense and exhibit superfluidity at sufficiently low temperatures. In interacting Chern insulators, excitons may inherit the nontrivial topology and quantum geometry from the underlying electron wavefunctions. We theoretically investigate the excitonic bound states and superfluidity in flat-band insulators pumped with light. We find that the exciton wavefunctions exhibit vortex structures in momentum space, with the total vorticity being equal to the difference of Chern numbers between the conduction and valence bands. Moreover, both the exciton binding energy and the exciton superfluid density are proportional to the Brillouin-zone average of the quantum metric and the Coulomb potential energy per unit cell. Spontaneous emission of circularly polarized light from radiative decay is a detectable signature of the exciton vorticity. We propose that the vorticity can also be experimentally measured via the nonlinear anomalous Hall effect, whereas the exciton superfluidity can be detected by voltage-drop quantization through a combination of quantum geometry and Aharonov-Casher effect. Topological excitons and their superfluid phase could be realized in flat bands of twisted Van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

Quantum technologic and spintronic applications require reliable material platforms that enable significant and long‐living spin polarization of excitations, the ability to manipulate it optically in external fields, and the possibility to implement quantum correlations between spins, i.e., entanglement. Here it is demonstrated that these conditions are met in bulk crystals of lead halide perovskites. A giant optical orientation of 85% of excitons, approaching the ultimate limit of unity, in FA0.9Cs0.1PbI2.8Br0.2 crystals is reported. The exciton spin orientation is maintained during the exciton lifetime of 55 ps resulting in high circular polarization of the exciton emission. The optical orientation is robust to detuning of the excitation energy up to 0.3 eV above the exciton resonance and remains larger than 20% up to detunings of 0.9 eV. It evidences pure chiral selection rules and suppressed spin relaxation of electrons and holes, even with large kinetic energies. The exciton and electron–hole recombinations are distinguished by means of the spin dynamics detected via coherent spin quantum beats in magnetic field. Further, electron–hole spin correlations are demonstrated through linear polarization beats after circularly polarized excitation. These findings are supported by atomistic calculations. All‐in‐all, the results establish lead halide perovskite semiconductors as suitable platform for quantum technologies. [ABSTRACT FROM AUTHOR]

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light‐harvesting, it was clearly outpaced by TiO2 in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time‐resolved photoelectron spectroscopy and many‐body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. It is demonstrated that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 µs. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large time frame for counter‐measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems. [ABSTRACT FROM AUTHOR]

Integrating free‐standing complex oxides with two‐dimensional (2D) materials has recently attracted great interest, due to the rich physics evolving from such structures. Enhancing and tuning the opto–electronic properties of these systems is of high importance for a multitude of applications, such as sensors, memory devices or optical communications. The electrostatic control of photoluminescence of monolayer MoS2 at room temperature via integration of free‐standing BaTiO3 (BTO), a ferroelectric perovskite oxide is presented. It is shown that the use of BTO leads to highly tunable exciton emission of MoS2 in a minimal range of gate voltages. Due to BTO's ferroelectric polarization‐induced doping, large peak emission shifts as well as a large and tunable A trion binding energy in the range of 40–100 meV are observed. These measurements are compared with those carried out when the BTO is replaced by a hexagonal boron nitride (hBN) dielectric layer, confirming BTO's superior gating properties and thus lower power consumption. Additionally, advantage of the ferroelectric switching of BTO is taken by fabricating devices where the BTO layer is decoupled from the gate electrode with a SiO2 layer. Choosing to isolate the BTO allows to induce large remanent behavior of MoS2’s excitonic features. [ABSTRACT FROM AUTHOR]

The measurement of the electronic bandgap and exciton binding energy in quasi-one-dimensional materials such as carbon nanotubes is challenging due to many-body effects and strong electron–electron interactions. Unlike bulk semiconductors, where the electronic bandgap is well known, the optical resonance in low-dimensional semiconductors is dominated by excitons, making their electronic bandgap more difficult to measure. In this work, we measure the electronic bandgap of networks of polymer-wrapped semiconducting single-walled carbon nanotubes (s-SWCNTs) using non-ideal p-n diodes. We show that our s-SWCNT networks have a short minority carrier lifetime due to the presence of interface trap states, making the diodes non-ideal. We use the generation and recombination leakage currents from these non-ideal diodes to measure the electronic bandgap and excitonic levels of different polymer-wrapped s-SWCNTs with varying diameters: arc discharge (~1.55 nm), (7,5) (0.83 nm), and (6,5) (0.76 nm). Our values are consistent with theoretical predictions, providing insight into the fundamental properties of networks of s-SWCNTs. The techniques outlined here demonstrate a robust strategy that can be applied to measuring the electronic bandgaps and exciton binding energies of a broad variety of nanoscale and quantum-confined semiconductors, including the most modern nanoscale transistors that rely on nanowire geometries. [ABSTRACT FROM AUTHOR]

The boundary slip condition is pivotal for nanoscale fluid motion. Recent research has primarily focused on simulating the interaction mechanism between the electronic structure of two-dimensional materials and slip of water at the nanoscale, raising the possibility for ultralow friction flow of water at the nanoscale. However, experimentally elucidating electronic interactions at the dynamic solid–liquid interface to control boundary slip poses a significant challenge. In this study, the crucial role of electron structures at the dynamic solid–liquid interface in regulating slip length was revealed. Notably, the slip length of water on the molybdenum disulfide/graphene (MoS2/G) heterostructure (100.9 ± 3.6 nm) significantly exceeded that of either graphene (27.7 ± 2.2 nm) or MoS2 (5.7 ± 3.1 nm) alone. It was also analyzed how electron transfer significantly affected interface interactions. Excess electrons played a crucial role in determining the type and proportion of excitons at both MoS2–water and MoS2/G–water interfaces. Additionally, by applying voltage, distinct photoluminescence (PL) responses at static and dynamic interfaces were discovered, achieving a 5-fold modulation in PL intensity and a 2-fold modulation in the trion to exciton intensity ratio. More electrons transfer from the top graphene to the bottom MoS2 at the MoS2/G–water interface, reducing surface charge density. Thus, the reduction of electrostatic interactions between the solid and water leads to an increased slip length of water on the MoS2/G heterostructure. The process aids in comprehending the origin of frictional resistance at the subatomic scale. This work establishes a foundation for actively controlling and designing of fluid transport at the nanoscale. [ABSTRACT FROM AUTHOR]

Resonances of free excitons at energies A ~ 1.0409 eV, B ~ 1.0445 eV, and C ~ 1.2690 eV were detected in reflection spectra of single crystals of the direct-gap compound CuInSe2 at a temperature of 8.6 K. The appearance of the three reflection resonances A, B, and C could be explained by removal of degeneracy from valence-band energy levels due to the influence of crystal field and spin–orbit interaction on the tetragonal lattice of CuInSe2 with ΔCF ~ 5.4 meV and ΔSO ~ 224 meV. Measurements of the temperature dependence of the reflection spectra in the range 8.6–90 K showed the energies of free-exciton resonances A and B increased because of deformation of the unit cell (tetragonal stretching) of the CuInSe2 lattice with the chalcopyrite structure. The binding energies of free excitons A and B were determined as 10.7 and 152 meV based on temperature quenching of exciton resonances A ~ 1.0409 eV and B ~ 1.0445 eV [ABSTRACT FROM AUTHOR]

Herein, time‐integrated and time‐resolved photoluminescence (PL) measurement experiments are performed on coupled GaAs quantum dot (QD) molecules with larger laser spot size and a single‐coupled QD molecule to investigate the dynamics of exciton states. A linearly polarized laser along the coupled direction of a laterally‐coupled GaAs QD molecule [11¯$\overset{\cdot}{1}$0] is used to determine the coupling effects of the different exciton states. For the ensemble of coupled QD molecules, the exciton PL peak shifts to lower energy (≈70 meV) and shows a linear increase in the PL intensity, demonstrating the properties of exciton states as a function of the laser excitation power. In addition, a power‐dependent redshift is observed in the single‐coupled GaAs QD molecule as a function of the excitation power; however, the two distinguished exciton states become resolved and show different power factors when the laser excitation power is linearly increased. This result clearly indicates the presence of energy transfer between the two different exciton states, X1 and X2. Furthermore, when the polarized laser excitation power is increased, a coupled biexciton consisting of two different localized exciton states (X1 and X2) is clearly observed, and a power‐dependent spectral redshift is detected in the exciton and biexciton states. At a high excitation power (15 I0), the decay times of the two different localized exciton states clearly show different lifetimes; and the decay time difference due to the size difference can be ignored at a low excitation power. Theoretically, the decay times of the two different exciton states, wherein the size deviation can be ignored, should be the same. Based on the results of the time‐integrated and time‐resolved PL measurements and the comparison between the ensemble and single exciton PL peaks, it is concluded that the coupling process in laterally‐coupled GaAs QD molecules is strongly supported against long‐range separation. [ABSTRACT FROM AUTHOR]

Light–matter interactions between plasmonic and excitonic modes have attracted considerable interest in recent years. A major challenge in achieving strong coupling is the identification of suitable metallic nanostructures that combine tight field confinement with sufficiently low losses. Here, we report on a room-temperature study on the interaction of tungsten disulfide (WS2) monolayer excitons with a hybrid plasmon polariton (HPP) mode supported by nanogroove grating structures milled into single-crystalline silver flakes. By engineering the depth of the nanogroove grating, we can change the character of the HPP mode from propagating surface plasmon polariton-like (SPP-like) to localized surface plasmon resonance-like (LSPR-like). Using reflection spectroscopy, we demonstrate strong coupling with a Rabi splitting of 68 meV between the WS2 monolayer excitons and the lower HPP branch for an optimized nanograting configuration with 60 nm deep nanogrooves. In contrast, only weak coupling between the constituents is observed for shallower and deeper nanogratings since either the field confinement provided by the HPP is not sufficient or the damping is too large. The possibility to balance the field confinement and losses render nanogroove grating structures an attractive platform for future applications. [ABSTRACT FROM AUTHOR]

Excitons – two-particle correlated electron-hole pairs – are the dominant low-energy optical excitation in the broad class of semiconductor materials, which range from classical silicon to perovskites, and from two-dimensional to organic materials. The study of excitons has been brought on a new level of detail by the application of photoemission momentum microscopy – a technique that has dramatically extended the capabilities of time- and angle resolved photoemission spectroscopy. Here, we review how the photoelectron detection scheme enables direct access to the energy landscape of bright and dark excitons, and, more generally, to the momentum-coordinate of the exciton wavefunction. Focusing on two-dimensional materials and organic semiconductors, we first discuss the typical photoemission fingerprint of excitons in momentum microscopy and highlight that it is possible to obtain information not only on the electron- but also hole-component. Second, we focus on the recent application of photoemission orbital tomography to such excitons, and discuss how this provides a unique access to the real-space properties of the exciton wavefunction. We detail how studies performed on two-dimensional transition metal dichalcogenides and organic semiconductors lead to very similar conclusions, and, in this manner, highlight the strength of momentum microscopy for the study of optical excitations in semiconductors.

Abstract Anisotropic optical 2D materials are crucial for achieving multiple‐quanta functions within quantum materials, which enables the fabrication of axially polarized electronic and optoelectronic devices. In this work, multiple excitonic emissions owning polarization‐sensitive orientations are clearly detected in a multilayered quasi‐1D ZrS3 nanoribbon with respect to the nanostripe edge. Four excitons denoted as AS1, AS2, AS, and A2 with E ⊥ b polarized direction and one prominent A1 exciton with E || b polarized emission are simultaneously detected in the polarized micro‐photoluminescence (µPL) measurement of 1.9–2.2 eV at 10 K. In contrast to light emission, polarized micro‐thermoreflectance (µTR) measurements are performed to identify the polarization dependence and verify the excitons in the multilayered ZrS3 nanoribbon from the perspective of light absorption. At 10 K, a prominent and broadened peak on the lower‐energy side, containing an indirect resonant emission (DI) observed by µPL and an indirect defect‐bound exciton peak (AInd) observed by both µPL and µTR, is simultaneously detected, confirming the existence of a quasi‐direct band edge in ZrS3. A van der Waals stacked p‐GaSe/n‐ZrS3 heterojunction solar cell is fabricated, which demonstrates a maximum axially‐polarized conversion efficiency up to 0.412% as the E || b polarized light incident onto the device.

The order of bright and dark excitonic states in lead-halide perovskite nanocrystals is debated. It has been proposed that the Rashba effect, driven by lattice-induced symmetry breaking, causes a bright excitonic ground state. Direct measurements of excitonic spectra, however, show the signatures of a dark ground state, bringing the role of the Rashba effect into question. We use an atomistic theory to model the exciton fine structure of perovskite nanocrystals, accounting for realistic lattice distortions. We calculate optical gaps and excitonic features that compare favorably with experimental works. The exciton fine structure splittings show a nonmonotonic size dependence due to a structural transition between cubic and orthorhombic phases. Additionally, the excitonic ground state is found to be dark with spin triplet character, exhibiting a small Rashba coupling. We additionally explore the effects of nanocrystal shape on the fine structure, clarifying observations on polydisperse nanocrystals.

Abstract In this paper, we investigate the optical, electronic, vibrational, and excitonic properties of four two-dimensional $$\beta$$ β -pnictogen materials—nitrogenene, phosphorene, arsenene, and antimonene—via density functional theory calculations and the Bethe–Salpeter equation. These materials possess indirect gaps with significant exciton binding energies, demonstrating isotropic behavior under circular light polarization and anisotropic behavior under linear polarization by absorbing light within the visible solar spectrum (except for nitrogenene). Furthermore, we observed that Raman frequencies red-shift in heavier pnictogen atoms aligning with experimental observations; simultaneously, quasi-particle effects notably influence the linear optical response intensively. These monolayers’ excitonic effects lead to optical band gaps optimized for solar energy harvesting, positioning them as promising candidates for advanced optoelectronic device applications.

Light–matter interactions between plasmonic and excitonic modes have attracted considerable interest in recent years. A major challenge in achieving strong coupling is the identification of suitable metallic nanostructures that combine tight field confinement with sufficiently low losses. Here, we report on a room-temperature study on the interaction of tungsten disulfide (WS2) monolayer excitons with a hybrid plasmon polariton (HPP) mode supported by nanogroove grating structures milled into single-crystalline silver flakes. By engineering the depth of the nanogroove grating, we can change the character of the HPP mode from propagating surface plasmon polariton-like (SPP-like) to localized surface plasmon resonance-like (LSPR-like). Using reflection spectroscopy, we demonstrate strong coupling with a Rabi splitting of 68 meV between the WS2 monolayer excitons and the lower HPP branch for an optimized nanograting configuration with 60 nm deep nanogrooves. In contrast, only weak coupling between the constituents is observed for shallower and deeper nanogratings since either the field confinement provided by the HPP is not sufficient or the damping is too large. The possibility to balance the field confinement and losses render nanogroove grating structures an attractive platform for future applications.

Layered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin‐orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher‐energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low‐temperature magneto‐optical measurements with sophisticated many‐particle theory, the origin of the bright exciton emission in perovskites is elucidated by tracking the thermalization of dark and bright excitons under a magnetic field. The unexpectedly high emission is clearly attributed to a pronounced phonon‐bottleneck effect, considerably slowing down the relaxation toward the energetically lowest dark states. It is demonstrated that this bottleneck can be tuned by manipulating the bright‐dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications. [ABSTRACT FROM AUTHOR]

The 2D semiconductors are an ideal platform for exploration of bosonic fluids composed of coupled photons and collective excitations of atoms or excitons, primarily due to large excitonic binding energies and strong light‐matter interaction. Based on first‐principles calculations, it is demonstrated that the phonon polaritons formed by two infrared‐active phonon modes in monolayer MoSi2N4 and WSi2N4 possess ultra‐high confinement factors of around ≈105 and 103, surpassing those of conventional polaritonic thin‐film materials by two orders of magnitude. It is observed that the first bright exciton possesses a substantial binding energies of 750 and 740 meV in these two monolayers, with the radiative recombination lifetimes as long as 25 and 188 ns, and the Rabi splitting of the formed cavity‐exciton polaritons reaching 373 and 321 meV, respectively. The effective masses of the cavity exciton polaritons are approximately 10−5me, providing the potential for high‐temperature quantum condensation. The ultra‐confined and ultra‐low‐loss phonon polaritons, as well as strongly‐coupled cavity exciton polaritons with ultra‐small polaritonic effective masses in these two monolayers, offering the flexible control of light at the nanoscale, probably leading to practical applications in nanophotonics, meta‐optics, and quantum materials. [ABSTRACT FROM AUTHOR]

The 2D semiconductors are an ideal platform for exploration of bosonic fluids composed of coupled photons and collective excitations of atoms or excitons, primarily due to large excitonic binding energies and strong light‐matter interaction. Based on first‐principles calculations, it is demonstrated that the phonon polaritons formed by two infrared‐active phonon modes in monolayer MoSi2N4 and WSi2N4 possess ultra‐high confinement factors of around ≈105 and 103, surpassing those of conventional polaritonic thin‐film materials by two orders of magnitude. It is observed that the first bright exciton possesses a substantial binding energies of 750 and 740 meV in these two monolayers, with the radiative recombination lifetimes as long as 25 and 188 ns, and the Rabi splitting of the formed cavity‐exciton polaritons reaching 373 and 321 meV, respectively. The effective masses of the cavity exciton polaritons are approximately 10−5me, providing the potential for high‐temperature quantum condensation. The ultra‐confined and ultra‐low‐loss phonon polaritons, as well as strongly‐coupled cavity exciton polaritons with ultra‐small polaritonic effective masses in these two monolayers, offering the flexible control of light at the nanoscale, probably leading to practical applications in nanophotonics, meta‐optics, and quantum materials. [ABSTRACT FROM AUTHOR]

Metaoptics formed by ultrathin and planar building blocks enable compact and efficient optical devices that manipulate light at the nanoscale. The development of tunable metaoptics holds the promise of miniaturized and efficient optical systems that can dynamically adapt to changing conditions or requirements, propelling innovations in fields ranging from telecommunication and imaging to quantum computing and sensing. Two-dimensional (2D) materials show strong promise in enabling tunable metaoptics due to their exceptional electronic and optical properties from the quantum confinement within the atomically thin layers. In this review, we discuss the recent advancements and challenges of 2D material-based tunable metaoptics in both linear and nonlinear regimes and provide an outlook for prospects in this rapidly advancing area. [ABSTRACT FROM AUTHOR]

Transparent photovoltaic (TPV) devices offer the potential to generate power without being visible to the human eye, making them ideal for use in building integrated photovoltaic applications. TPV devices based on inorganic materials offer eco‐friendly frameworks with stable performances. However, their low power conversion efficiency and incapacity to produce onsite power for real‐time practical applications limit their widespread deployment. Mitigation of exciton quenching by reducing the interface and bulk recombination can significantly improve the performance of TPVs. To address this, the present study investigates the effect of passivation layers (PLs) in all‐metal‐oxide TiO2/Cu2O TPV. The TiO2/Cu2O TPV interface is passivated by depositing thin Al2O3 and Ga2O3 films. The study comprehensively analyzes how passivation controls the interfacial and bulk defects. Electrochemical impedance spectroscopy and X‐ray photoelectron spectroscopy are used to elucidate exciton quenching mechanisms. The results show that the insertion of a thin PL on top of TiO2 effectively mitigates exciton quenching by suppressing hydroxyl ions, Ti2+, and Ti3+ bulk states. Ga2O3 PL, in particular, leads to a substantial enhancement in short‐circuit current density (12.4 mA cm−2) and open‐circuit voltage (669 mV). The study also demonstrates the real‐time onsite energy production and its utilization through the operation of an electric fan. [ABSTRACT FROM AUTHOR]

Abstract Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light‐harvesting, it was clearly outpaced by TiO2 in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time‐resolved photoelectron spectroscopy and many‐body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. It is demonstrated that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 µs. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large time frame for counter‐measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems.

Abstract Quantum technologic and spintronic applications require reliable material platforms that enable significant and long‐living spin polarization of excitations, the ability to manipulate it optically in external fields, and the possibility to implement quantum correlations between spins, i.e., entanglement. Here it is demonstrated that these conditions are met in bulk crystals of lead halide perovskites. A giant optical orientation of 85% of excitons, approaching the ultimate limit of unity, in FA0.9Cs0.1PbI2.8Br0.2 crystals is reported. The exciton spin orientation is maintained during the exciton lifetime of 55 ps resulting in high circular polarization of the exciton emission. The optical orientation is robust to detuning of the excitation energy up to 0.3 eV above the exciton resonance and remains larger than 20% up to detunings of 0.9 eV. It evidences pure chiral selection rules and suppressed spin relaxation of electrons and holes, even with large kinetic energies. The exciton and electron–hole recombinations are distinguished by means of the spin dynamics detected via coherent spin quantum beats in magnetic field. Further, electron–hole spin correlations are demonstrated through linear polarization beats after circularly polarized excitation. These findings are supported by atomistic calculations. All‐in‐all, the results establish lead halide perovskite semiconductors as suitable platform for quantum technologies.

Colloidal nanoplatelets of HgSe and HgTe prepared indirectly through cation exchange reactions can transfer many of the advantageous properties of atomically precise, 2D cadmium chalcogenides to the near‐infrared (NIR) spectral window. In this work, HgSe and HgTe nanoplatelets are studied to understand their fundamental photophysical properties, particularly those areas of similarity and difference from cadmium‐based NPLs, and to examine their potential as optical gain media. Similar to cadmium chalcogenide NPLs, low‐temperature photoluminescence of HgTe NPLs displays two‐color emission that depends on temperature, sample, fluence, excitation frequency, and irradiation time. Both HgTe and HgSe show nanosecond emission dynamics at temperatures as low as 2.5 K, with no indication that bright‐dark excitonic splitting governs the low‐temperature photoluminescence. Collectively, experimental data is most consistent with emission from a negative trion state at low temperature. Although the mercury chalcogenide nanoplatelets are shown to have broadened optical resonances compared to the cadmium chalcogenides from which they are derived, they retain slow Auger recombination and can display low‐threshold amplified spontaneous emission in the NIR spectral window. Optical pumping thresholds for HgTe NPLs are observed as low as 4.4 µJ cm−2 and highlight the potential 2D nanoplatelets as gain medium in the near‐infrared. [ABSTRACT FROM AUTHOR]

2D Ruddlesden‐Popper metal‐halide perovskites feature particularly strong excitonic effects, making them a fascinating playground for studying exciton physics. A complete understanding of the properties of this quasi‐particle is crucial to fully exploit the tremendous potential of 2D perovskites (2DP) in light emission applications. Despite intense investigations, some of the exciton properties remain elusive to date, for example, the energy‐ordering of the exciton states within the so‐called fine structure manifold. Using optical spectroscopy, it demonstrates that in the archetypical 2DP (PEA)2PbI4, in contradiction to theoretical predictions, the energy of the bright out‐of‐plane exciton state is higher than that of two in‐plane states. Having elucidated the order of exciton fine structure, it determines the g‐factor of the dark exciton transition, together with the values of the electron and hole g‐factors in the direction parallel to the c‐axis of the crystal. In this way, it provides for the first time, a complete picture of the exciton fine structure in (PEA)2PbI4 2DP. [ABSTRACT FROM AUTHOR]

Layered halide perovskites are solution‐processed natural heterostructures where quantum and dielectric confinement down to the nanoscale strongly influence the optical properties, leading to stabilization of bound excitons. Detailed understanding of the exciton properties is crucial to boost the exploitation of these materials in energy conversion and light emission applications, with on‐going debate related to the energy order of the four components of the most stable exciton. To provide theoretical feedback and solve among contrasting literature reports, this work performs ab initio solution of the Bethe–Salpeter equation (BSE) for symmetrized reference Cs2PbX4 (X = I and Br) models, with detailed interpretation of the spectroscopic observables based on group‐theory analysis. Simulations predict the following Edark < Ein‐plane < Eout‐of‐plane fine‐structure assignment, consistent with recent magneto‐absorption experiments and obtain similar increase in dark/bright splitting when going from lead‐iodide to a lead‐bromide composition as found experimentally. The authors further suggest that polar distortions may lead to stabilization of the in‐plane component and end‐up in a bright lowest exciton component, discuss exciton landscape over a broad energy range and clarify the exciton spin‐character, when large spin‐orbit coupling is in play, to rationalize the potential of halide perovskites as triplet sensitizers in combination with organic dyes. [ABSTRACT FROM AUTHOR]

The Random Walk on Spheres (RWS) algorithm for solving anisotropic transient diffusion problems based on a stochastic learning procedure for calculation of the exit position of the anisotropic diffusion process on a sphere is developed. Direct generalization of the Random Walk on Spheres method to anisotropic diffusion equations is not possible, therefore, we have numerically calculated the probability density for the exit position on a sphere. The first passage time is then represented explicitly. The method can easily be implemented to solve diffusion problems with spatially varying diffusion coefficients for complicated three-dimensional domains. Particle tracking algorithm is highly efficient for calculation of fluxes to boundaries. We apply the developed algorithm for solving an exciton transport in a semiconductor material with a threading dislocation where the measured functions are the exciton fluxes to the semiconductor's substrate and on the dislocation surface. [ABSTRACT FROM AUTHOR]

The optical response of 2D layered perovskites is composed of multiple equally‐spaced spectral features, often interpreted as phonon replicas, separated by an energy Δ ≃ 12 − 40 meV, depending upon the compound. Here the authors show that the characteristic energy spacing, seen in both absorption and emission, is correlated with a substantial scattering response above ≃ 200 cm−1 (≃ 25 meV) observed in resonant Raman. This peculiar high‐frequency signal, which dominates both Stokes and anti‐Stokes regions of the scattering spectra, possesses the characteristic spectral fingerprints of polarons. Notably, its spectral position is shifted away from the Rayleigh line, with a tail on the high energy side. The internal structure of the polaron consists of a series of equidistant signals separated by 25–32 cm−1 (3–4 meV), depending upon the compound, forming a polaron vibronic progression. The observed progression is characterized by a large Huang‐Rhys factor (S > 6) for all of the 2D layered perovskites investigated here, indicative of a strong charge carrier – lattice coupling. The polaron binding energy spans a range ≃ 20–35 meV, which is corroborated by the temperature‐dependent Raman scattering data. The investigation provides a complete understanding of the optical response of 2D layered perovskites via the direct observation of polaron vibronic progression. The understanding of polaronic effects in perovskites is essential, as it directly influences the suitability of these materials for future opto‐electronic applications. [ABSTRACT FROM AUTHOR]

Over the past couple of decades, immense research has been carried out to understand the photo‐physics of an organic solar cell (OSC) that is important to enhance its efficiency and stability. Since OSCs undergoes complex photophysical phenomenon, studying these factors has led to designing new materials and implementing new strategies to improve efficiency in OSCs. In this regard, the invention of the non‐fullerene acceptorshas greatly revolutionized the understanding of the fundamental processes occurring in OSCs. However, such vital fundamental research from device physics perspectives is carried out on glovebox (GB) processed OSCs and there is a scarcity of research on air‐processed (AP) OSCs. This review will focus on charge carrier dynamics such as exciton diffusion, exciton dissociation, charge‐transfer states, significance of highest occupied molecular orbital‐offsets, and hole‐transfer efficiencies of GB‐OSCs and compare them with the available data from the AP‐OSCs. Finally, key requirements for the fabrication of efficient AP‐OSCs will be presented from a charge‐carrier dynamics perspective. The key aspects from the charge‐carrier dynamics view to fabricate efficient OSCs either from GB or air are provided. [ABSTRACT FROM AUTHOR]

We have observed a new optical transition in the photoreflectance spectra of indirect-gap BiOCl thin films, which were grown on SrTiO3 substrates. The position of this transition is close in energy to its bulk critical point (CP) energy. Moreover, these are significantly lower than a higher-lying direct-type CP from an energetic point of view. The spectral line shape analysis for our observed signal suggests the presence of an excitonic effect of this compound. We determined its dependence of the optical anomaly on temperature ranging from 80 K to RT. We adopted the Varshni model for this analysis. At last, we compared photonic properties of BiOCl with those of an element and binary semiconductors. [ABSTRACT FROM AUTHOR]

The non-Drude-like behavior of the real part of the photoinduced permittivity Re εP of GaAs and Si samples in the gigahertz range is detected by direct cavity measurements under conditions of fiber-optic irradiation at a wavelength of λ = 0.97 microns with changing power P in the range of 0–1 W. It is shown that, in accordance with the hypothesis of the exciton mechanism of photoinduced microwave dielectric permittivity, Re εP increases with increasing P (approaching saturation above P = 200 mW) instead of decreasing within the framework of free-charge carriers according to Drude. The generality of the behavior of the real parts of the photoinduced permittivity observed in semiconductors of different types (direct-gap GaAs and indirect-gap Si) in different electrodynamic systems (waveguides, cavities, metastructures) testifying to the universality of the exciton mechanism is demonstrated. Optically controlled metastructures in the GHz band containing resonant electrically conductive elements filled with GaAs and Si samples are proposed for the first time: a metastructure based on linear dipoles and a half-wave electric dipole based on a multipass spiral. The gigahertz responses of the metastructures and transformation of the responses associated with changes in the dielectric permittivity of Si and GaAs during photoexcitation are measured for the first time. Based on the hypothesis put forward about the effect of excitons on photoexcitation, the observed saturation effect of gigahertz photoinduced permittivity is discussed. [ABSTRACT FROM AUTHOR]

Chiral semiconductor nanostructures and nanoparticles are promising materials for applications in biological sensing, enantioselective separation, photonics, and spin-polarized devices. Here, we studied the induction of chirality in atomically thin only two-monolayer-thick CdSe nanoplatelets (NPLs) grown using a colloidal method and exchanged with L-alanine and L-phenylalanine as model thiol-free chiral ligands. We have developed a novel two-step approach to completely exchange the native oleic acid ligands for chiral amino acids at the basal planes of NPLs. We performed an analysis of the optical and chiroptical properties of the chiral CdSe nanoplatelets with amino acids, which was supplemented by an analysis of the composition and coordination of ligands. After the exchange, the nanoplatelets retained heavy-hole, light-hole, and spin-orbit split-off exciton absorbance and bright heavy-hole exciton luminescence. Capping with thiol-free enantiomer amino acid ligands induced the pronounced chirality of excitons in the nanoplatelets, as proven by circular dichroism spectroscopy, with a high dissymmetry g-factor of up to 3.4 × 10−3 achieved for heavy-hole excitons in the case of L-phenylalanine. [ABSTRACT FROM AUTHOR]

In this study, we report on the dynamics of symmetry‐breaking charge transfer (SB‐CT) in a specific derivative of bianthracene, 9,9',10,10'‐tetraphenyl‐2,2'‐bianthracene (TPBA). Unlike the conventional 9,9'‐bianthracene, TPBA exhibits a notably planar structure due to reduced steric hindrance between anthracene chromophores. This structural characteristic results in a pronounced short‐range charge transfer coupling, driven by substantial overlap integrals of hole and electron. This is in contrast to the 9,9'‐bianthracene case, where long‐range Coulombic coupling dominates while minimizing short‐range interactions. Consequently, TPBA reveals an adiabatic mixture between locally‐excited and charge‐transfer diabats, where the extent of their contributions is modulated by the dielectric properties of the surrounding environment. Furthermore, our investigation reveals that the SB‐CT reaction in TPBA is mainly influenced by solvation dynamics, akin to other molecules displaying SB‐CT. Additionally, the dynamics of triplet formation in TPBA vary depending on the solvent polarity. These findings underscore the significance of chromophore connectivity in efficiently manipulating excited‐state potential energy landscapes. [ABSTRACT FROM AUTHOR]

The strong coupling between electronic transitions and resonant cavity modes, facilitated by coherent energy transfer, presents unprecedented opportunities for tailoring the photoelectronic properties of constituent components. Here, the concept of Kerker effect is leveraged to demonstrate the dynamic control of scattering directionality in dielectric nanostructures by tuning the exciton‐photon coupling. First, theoretical evidence for a significant modification of the scattering directionality of a dielectric metastructure engineered by excitonic polaritons is provided. As a proof of concept, self‐coupled metasurfaces composed of bulk MoS2, which exhibit a forward/backward scattering ratio up to 20, are constructed. Importantly, tunable directionality is achieved by thermally controlling the excitonic coupling to the Mie modes. The simulated results are in good agreement with the experimental measurements, and the subsequent multipole decompositions effectively elucidate the underlying mechanism, attributed to the interplay between electric and magnetic dipole modes that are modified by excitons. The findings shed light on the control of light flow in the far field through coherent light–matter interactions, thereby opening up numerous possibilities for active optical antennas and quantum emitters on a nanoscale. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) spectroscopy has proven to provide deep insights into the optoelectronic properties of monolayer MoS2$\left(\text{MoS}\right)_{2}$. Herein, a corresponding study is conducted on the excitonic properties of mechanically exfoliated monolayer MoS2$\left(\text{MoS}\right)_{2}$ under multivariate physical and chemical stimuli. Specifically, midgap exciton states that originate from lattice defects are characterized and they are compared to existing models. Through statistical data analyses of substrate‐, temperature‐, and laser‐power‐dependent measurements, a PL enhancement is revealed through physisorption of water molecules of the controversially discussed excited‐state A biexciton (AXX*$A^{\left(\text{XX}\right)^{\star}}$). In addition, analyses of monolayer MoS2$\left(\text{MoS}\right)_{2}$ on gold substrates show that surface roughness does not account for changes in doping level within the material. Also, a shift in the electron–phonon coupling properties that arises from thin films of water that are physisorbed on top of the samples is reported. [ABSTRACT FROM AUTHOR]