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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.

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.

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]

ABSTRACTIn this review, we discuss the rich ultrafast response at terahertz (THz) frequencies of two-dimensional (2D) materials. Thanks to their unique optoelectronic properties and exceptional tunability, van der Waals organic and inorganic 2D materials, such as graphene, transition metal dichalcogenides (TMDs), and 2D perovskites, are emerging as promising platforms for the development of nano-electronic and nano-photonic devices in the THz range. The investigation of the ultrafast charge carriers dynamics resulting from their reduced dimensionality is crucial for guiding the engineering route towards novel nanotechnologies. Here, we first give a brief overview of the state-of-the-art experimental schemes for inspecting the ultrafast response of 2D materials in the THz range, including the generation and the detection of THz light and the prototypical optical pump [Formula: see text] THz probe setup. Then, we present and discuss the most relevant results, reviewing the THz ultrafast signatures of charge carriers and excitons dynamics in graphene, TMDs, and 2D perovskites. Finally, we provide a vision of the emerging tools for characterizing the ultrafast THz dynamics at the nanoscale.

We present a method of calculating the energy gap of a charge-neutral excitation using only ground-state calculations. We report Quantum Monte Carlo calculations of Γ→ Γ and Γ → X particle-hole excitation energies in diamond carbon. We analyze the finite-size effect and find the same 1/L decay rate as that in a charged excitation, where L is the linear extension of the supercell. This slow decay is attributed to the delocalized nature of the excitation in supercells too small to accommodate excitonic binding effects. At larger system sizes, the apparent 1/L decay crosses over to a 1/L^3 behavior. Estimation of the scale of exciton binding can be used to correct finite-size effects of neutral gaps.

In this Perspective article, the reader is introduced to several theoretical methods of determining the exciton wave functions and the corresponding eigenenergies. The methods covered are either analytical, semianalytical, or numeric. All the details associated with the different methods are made explicit, thus allowing newcomers to do research on their own, without experiencing a steep learning curve. The Perspective starts with a variational method and ends with a simple semianalytical approach to solve the Bethe–Salpeter equation (BSE) in gapped 2D materials. For the first methods addressed in this Perspective, the authors focus on a single layer of hexagonal boron nitride (hBN) and of transition metal dichalcogenide (TMD), as these are exemplary materials in the field of 2D excitons. For explaining the Bethe–Salpeter method, the biased bilayer graphene, which presents a tunable bandgap is chosen. The system has the right amount of complexity (without being excessive). This allows the presentation of the solution in a context that can be easily generalized to more complex systems or to apply it to simpler models. [ABSTRACT FROM AUTHOR]

Circular Bragg gratings compose a very appealing photonic platform and nanophotonic interface for the controlled light-matter coupling of emitters in nanomaterials. Here, we discuss the integration of exfoliated monolayers of WSe$_2$ with GaInP Bragg gratings. We apply hyperspectral imaging to our coupled system, and explore the spatio-spectral characteristics of our coupled monolayer-cavity system. Our work represents a valuable step towards the integration of atomically thin quantum emitters in semiconductor nanophotonic cavities.

Heterostructures made from van der Waals (vdW) materials provide a template to investigate a wealth of proximity effects at atomically sharp two-dimensional (2D) heterointerfaces. In particular, near-field charge and energy transfer in vdW heterostructures made from semiconducting transition metal dichalcogenides (TMD) have recently attracted interest to design model 2D “donor–acceptor” systems and new optoelectronic components. Here, using Raman scattering and photoluminescence spectroscopies, we report a comprehensive characterization of a molybedenum diselenide ($\mathrm{MoSe}_2$) monolayer deposited onto hexagonal boron nitride (hBN) and capped by mono- and bilayer graphene. Along with the atomically flat hBN susbstrate, a single graphene epilayer is sufficient to passivate the $\mathrm{MoSe}_2$ layer and provides a homogenous environment without the need for an extra capping layer. As a result, we do not observe photo-induced doping in our heterostructure and the $\mathrm{MoSe}_2$ excitonic linewidth gets as narrow as 1.6 meV, approaching the homogeneous limit. The semi-metallic graphene layer neutralizes the 2D semiconductor and enables picosecond non-radiative energy transfer that quenches radiative recombination from long-lived states. Hence, emission from the neutral band edge exciton largely dominates the photoluminescence spectrum of the $\mathrm{MoSe}_2$/graphene heterostructure. Since this exciton has a picosecond radiative lifetime at low temperature, comparable with the non-radiative transfer time, its low-temperature photoluminescence is only quenched by a factor of $3.3 \pm 1$ and $4.4 \pm 1$ in the presence of mono- and bilayer graphene, respectively. Finally, while our bare $\mathrm{MoSe}_2$ on hBN exhibits negligible valley polarization at low temperature and under near-resonant excitation, we show that interfacing $\mathrm{MoSe}_2$ with graphene yields a single-line emitter with degrees of valley polarization and coherence up to ${\sim }$ 15 %.

The optical anisotropy of the Sb2Se3 crystals was investigated at 300 and 11 K. Excitonic features of four excitons (A, B, C, and D) were observed in the optical spectra of the Sb2Se3 single crystals and in the photoelectric spectra of the Me–Sb2Se3 structures. The exciton parameters, such as the ground (n = 1) and excited (n = 2) state positions and the binding energy (Ry), were determined. The effective mass of the electrons at the bottom of the conduction band (mc* = 0.67m0) as well as the holes at the four top valence bands (mv1* = 3.32m0, mv2* = 3.83m0, mv3* = 3.23m0 and mv4* = 3.23m0) were calculated in the Г-point of the Brillouin zone. The magnitude of the valence band splitting V1–V2 due to the spin–orbit interaction (Δso = 35 meV) and the crystal field (Δcf = 13 meV) were estimated in the Brillouin zone center. The energy splitting between the bands V3–V4 was 191 meV. The identified features were discussed based on both the theoretically calculated energy band structure and the excitonic band symmetry in the Brillouin zone (k = 0) for crystals with an orthorhombic symmetry (Рnma). The photoelectric properties of the Me–Sb2S3 structures were investigated in the spectral range 1–1.8 eV under E||c and E⟂c polarization conditions and at different applied voltages.

Transition metal dichalcogenides are two-dimensional semiconductors with strong in-plane covalent and weak out-of-plane interactions, resulting in exfoliation into monolayers with atomically thin thickness. This creates a new era for the exploration of two-dimensional physics and device applications. Among them, MoS2 is stable in air and easily available from molybdenite, showing tunable band-gaps in the visible and near-infrared waveband and strong light-matter interactions due to the planar exciton confinement effect. In the single-layer limit, monolayer MoS2 exhibits direct band-gaps and bound excitons, which are fundamentally intriguing for achieving the nanophotonic and optoelectronic applications. In this review, we start from the characterization of monolayer MoS2 in our group and understand the exciton modes, then explore thermal excitons and band renormalization in monolayer MoS2. For nanophotonic applications, the recent progress of nanoscale laser source, exciton-plasmon coupling, photoluminescence manipulation, and the MoS2 integration with nanowires or metasurfaces are overviewed. Because of the benefits brought by the unique electronic and mechanical properties, we also introduce the state of the art of the optoelectronic applications, including photoelectric memory, excitonic transistor, flexible photodetector, and solar cell. The critical applications focused on in this review indicate that MoS2 is a promising material for nanophotonics and optoelectronics.

Abstract Optical excitation of quasi‐2D perovskites leads to excited‐state populations of excitons, free charge carriers, or a mixture of both, depending on the type and amount of 2D spacer used. The fluence dependence of three quantities: 1) the time‐resolved photoluminescence decay, 2) the photoluminescence quantum yield (PLQY) after pulsed excitation, and 3) the initial rate of photon emission, allow the mixture of excited states present to be determined. These can be described by a simple model considering noninteracting populations of excitons and charge carriers in separate subvolumes of the film. The model reproduces all unique features of the data, such as the anomalous peak of the PLQY at intermediate fluences, due to bimolecular free carrier emission gaining efficiency before exciton–exciton annihilation reduces the exciton emission efficiency. The excited state population varies from 100% excitons in films made from high concentrations of butylamine spacers to ≈7% excitons and 93% free carriers for low concentrations of 1‐naphthylmethylamine spacers. The effective rates of free carrier recombination and exciton–exciton annihilation are high, often on the order of 1 × 10−9 cm3 s−1. The implications for the different excited‐state populations and their dynamics in terms of device engineering are discussed.

The photoluminescence peculiarities and the excited state absorption of the tetrapod-shaped CdTe/CdSe nanocrystals are studied in the single-photon excitation regime. A giant blue shift (≈ 129 meV) of the charge-transfer photoluminescence spectra is revealed for the first time in the molecular nanoscale donor–acceptor system with the pump intensity growth. The photoluminescence lines of the CdTe and CdSe domains, originating from direct electron-hole transitions were observed in addition to the tunable photoluminescence across the indirect gap. Differential transmission spectra reveal the superposition of the exciton’s transitions bleaching originating from different structural domains of the CdTe/CdSe nanocrystals, which confirms the observed separated excitonic emission bands in the photoluminescence spectra. The observed features of the photoluminescence and differential transmission spectra allow to determine and to analyze possible exciton’s relaxation channels in the CdTe/CdSe nanocrystals. Obtained results make nanocrystals in the form of nano-tetrapods promising for implementation in optoelectronic nanodevices.

By means of time-resolved photoluminescence (TR-PL) spectroscopy, we present a detailed investigation of the carrier relaxation dynamics in a CsPbBr3 bulk sample and microcrystal ensemble at cryogenic temperature on a picosecond time scale. We provide evidence of a long temperature-dependent cooling rate for the excitons and free carriers population, with an initial cooling time constant of a few tens of picoseconds. A relaxation bottleneck in the thermalization process was found that cannot be explained by the Auger effect or hot phonon population, since we address a very low excitation regime, not commonly investigated in literature, where such processes are not effective. Adding a continuous wave optical bias to the picosecond excitation, we probed the photoinduced PL decrease of the localized states and the photoinduced PL increase of the population in the high energy states. A long recovery time from the photoinduced PL decrease was found for localized states and quite significant differences were detected, depending on the resonance/off resonance bias used in the experiment.

Magneto-optical spectroscopy based on the transverse magneto-optical Kerr effect (TMOKE) is a sensitive method for investigating magnetically-ordered media. Previous studies were limited to the weak coupling regime where the spectral width of optical transitions considerably exceeded the Zeeman splitting in magnetic field. Here, we investigate experimentally and theoretically the transverse Kerr effect in the vicinity of comparatively narrow optical resonances in confined quantum systems. For experimental demonstration we studied the ground-state exciton resonance in a (Cd,Mn)Te diluted magnetic semiconductor quantum well, for which the strong exchange interaction with magnetic ions leads to giant Zeeman splitting of exciton spin states. For low magnetic fields in the weak coupling regime, the Kerr effect magnitude grows linearly with increasing Zeeman splitting showing a dispersive S-shaped spectrum, which remains almost unchanged in this range. For large magnetic fields in the strong coupling regime, the magnitude saturates, whereas the spectrum becomes strongly modified by the appearance of two separate peaks. TMOKE is sensitive not only to the sample surface but can also be used to probe in detail the confined electronic states in buried nanostructures if their capping layer is sufficiently transparent.

Solar energy conversion to electricity using organic semiconductor materials is a complex process due to the underlying transport physics of electrons and photons. Experimental characterizations of the 3-D morphology of bulk heterojunction organic photovoltaics are challenging; the poor contrast of the reconstructed morphology due to weak electronic scattering of organic molecules is a major impediment for microstructural imaging by electron microscopy. Thus, enhancing the power-conversion efficiency (PCE) of organic solar cells requires predictive design of both material and processing parameters. To this end, large-scale coarse-grained molecular simulations are needed to probe the morphology of the nanostructures, to develop process–structure–performance correlations that assist in fundamental understanding of the physical mechanisms, and to simultaneously aid in selection and optimization of design parameters. Here, we summarize the outcomes from high-performance coarse-grained molecular dynamics simulations that are employed to mimic solvent evaporation and thermal annealing of typical bulk heterojunction solar cell active layers. The latter consist of a blend of electron-donor and electron-acceptor materials. We extensively explore the dependence of PCE and the thermo-mechanical stability of the blend morphology on different solution processing conditions, and correlate the identified parameters with the dominant design variables and the microstructure. The simulations reveal that the composition of constituent donor and acceptor materials, respective molecular weights, polydispersity of donor polymer chains, and the thermal annealing temperature are the major parameters that significantly impact the morphology, thermo-mechanical stability, and subsequently the PCE of organic photovoltaics. [ABSTRACT FROM AUTHOR]

Excitons, or coupled electron-hole excitations, are important both for fundamental optical properties of materials as well as and for the functionality of materials in opto-electronic devices. Depending on the material they are created in, excitons can come in many forms, from Wannier–Mott excitons in inorganic semiconductors, to molecular Frenkel or bi-molecular charge-transfer excitons in disordered organic or biological heterostructures. This multitude of materials and exciton types poses tremendous challenges for ab initio modeling. Following a brief overview of typical ab initio techniques, we summarize our recent work based on Many-Body Green’s Functions Theory in the GW approximation and Bethe–Salpeter Equation (BSE) as a method applicable to a wide range of material classes from perfect crystals to disordered materials. In particular, we emphasize the current challenges of embedding this GW-BSE method into multi-method, mixed quantum-classical (QM/MM) models for organic materials and illustrate them with examples from organic photovoltaics and fluorescence spectroscopy. Our perspectives on future studies are also presented.

In this Letter, we report the second harmonic generation (SHG) in a colloidal solution of CdSe/CdS nanoplatelets (NPLs) in the case of two-photon excitation in the region of exciton transition from the light holes sub-band to the conduction sub-band (549 nm, 1lh-1e) by femtosecond laser pulses (1064 nm, 100 kHz). Discovery of the SHG in colloidal solutions of NPLs opens up new perspectives for their use as biomarkers or for optical imaging, since in contrast to two-photon photoluminescence the SHG can increase both spectral and temporal resolution by several orders of magnitude. The SHG is also extremely important to consider when designing two-photon pumping lasers.

Exciton-surface plasmon polariton (exciton-SPP) interactions in semiconductor-metal hybrid nanostructures connect two fundamentally different quantum mechanical excitations with strikingly different dispersion relations and optical response. The main focus in investigating these light–matter interactions and nanostructures is to control the light via light on the nanometric length scale and ultrafast timescale, not achievable by present photonic or electronic technologies. Here, we provide a concise description of the relevant background physics and an overview of the manifestations, challenges and applications of weak and strong exciton-SPP interactions. It is now well established that semiconductor-metal hybrid nanostructures offer exciting opportunities to investigate intriguing quantum phenomena and many-body interactions in condensed matter systems even at room temperature. We also review how exciton-SPP interactions have been influencing various research directions. It is amply evident that a comprehensive understanding of these interactions may play an important role in several research areas. The recent progress made in this field is expected to give rise to novel applications of active plasmonic structures. Synopsis The ability of surface plasmon polaritons to localize light on the nanoscale dimensions results in a strong field enhancement, enabling efficient intensification of light–matter interaction with excitons in semiconductors. Two interaction regimes, weak and strong coupling are identified. Both these regimes exhibit intriguing quantum mechanical physical effects and are promising for diverse applications. Interestingly, unlike in pure atomic systems, in semiconductor-metal hybrid structures, these quantum-optical effects are observable even at room temperature.

As a platform for optoelectronic devices based on exciton dynamics, monolayer transition metal dichalcogenides (TMDCs) are often placed near metal interfaces or inside planar cavities. While the radiative properties of point dipoles at metal interfaces has been studied extensively, those of excitons, which are delocalized and exhibit a temperature-dependent momentum distribution, lack a thorough treatment. Here, we analyze the emission properties of excitons in TMDCs near planar metal interfaces and explore their dependence on exciton center-of-mass momentum, transition dipole orientation, and temperature. Defining a characteristic energy scale k _B T _c = ( ℏk ) ^2 /2 m ( k being the radiative wavevector and m the exciton mass), we find that at temperatures T ≫ T _c and low densities where the momentum distribution can be characterized by Maxwell–Boltzmann statistics, the modified emission rates (normalized to free space) behave similarly to point dipoles. This similarity in behavior arises due to the broad nature of wavevector components making up the exciton and point dipole emission. On the other hand, the narrow momentum distribution of excitons for T < T _c can result in significantly different emission behavior as compared to point dipoles. These differences can be further amplified by considering excitons with a Bose Einstein distribution at high phase space densities, such as in a condensate phase. We find suppression or enhancement of emission relative to the point dipole case by several orders of magnitude. These insights can help optimize the performance of optoelectronic devices that incorporate 2D semiconductors near metal electrodes and can inform future studies of exciton radiative dynamics at low temperatures. Additionally, these studies show that nanoscale optical cavities are a viable pathway to generating long-lifetime exciton states in TMDCs.

Microcavity-polaritons in lattice geometries have been used to study a wide range of interesting physics. Meanwhile, organic materials have shown great promise on the road towards polaritonic devices, as the strong binding energy of their Frenkel excitons permits room temperature condensation and lasing. Whilst there are theoretical treatments of the condensation processes in planar organic microcavities, models of lattice geometries are lacking. Here, we introduce a model for describing the dynamics of lattices of zero-dimensional organic microcavities, where the dominant condensation mechanism involves the emission of a vibrational phonon. We also provide an open source software module that can be easily modified for any lattice geometry or dimension. The vibronic transition provides a tool for targeted condensation in a particular eigenmode of the system, which we highlight by examining a dimer and a dimerised chain. For the dimer, we observe a double resonance in the condensation efficiency that arises from tuning the condensate-reservoir detuning into resonance with either the symmetric or antisymmetric mode. This mechanism was also exploited in the dimerised chain, to selectively condense the system in to either the bulk states or the topological edge states, under homogeneous pumping of all cavities. We also showed an interesting signature of chiral transport when pumping a single cavity in the chain, where the direction of propagation depends on the sublattice being pumped.

In photonics and condensed-matter physics, optical control of many-body phenomena is significant for fundamental studies and optoelectronic applications. Herein, pronounced saturable excitonic behaviors in the nontransient optical response of monolayer MoS2 pumped by continuous-wave laser are observed. These behaviors include bleaching of the excitonic absorption and a blueshift of the exciton resonance at increasing power of pump light, which are attributed to the band-filling nonlinearity of excitons and repulsion-dominated interaction between them, respectively. Moreover, it is also shown that the substrate has a strong impact on these phenomena, which can be ascribed to the changes in the dielectric screening effect and the trion spectral weight. In nontransient regime, the results not only demonstrate the analogy between excitons and atoms with respect to photon--particle and interparticle interactions, but also show the optical control of this many-body effect. These findings can also push forward the research of photonic devices based on 2D materials. [ABSTRACT FROM AUTHOR]

In photonics and condensed-matter physics, optical control of many-body phenomena is significant for fundamental studies and optoelectronic applications. Herein, pronounced saturable excitonic behaviors in the nontransient optical response of monolayer MoS2 pumped by continuous-wave laser are observed. These behaviors include bleaching of the excitonic absorption and a blueshift of the exciton resonance at increasing power of pump light, which are attributed to the band-filling nonlinearity of excitons and repulsion-dominated interaction between them, respectively. Moreover, it is also shown that the substrate has a strong impact on these phenomena, which can be ascribed to the changes in the dielectric screening effect and the trion spectral weight. In nontransient regime, the results not only demonstrate the analogy between excitons and atoms with respect to photon--particle and interparticle interactions, but also show the optical control of this many-body effect. These findings can also push forward the research of photonic devices based on 2D materials. [ABSTRACT FROM AUTHOR]

The research on emerging layered two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS2), reveals unique optical properties generating significant interest. Experimentally, these materials were observed to host extremely strong light-matter interactions as a result of the enhanced excitonic effect in two dimensions. Thus, understanding and manipulating the excitons are crucial to unlocking the potential of 2D materials for future photonic and optoelectronic devices. In this review, we unravel the physical origin of the strong excitonic effect and unique optical selection rules in 2D semiconductors. In addition, control of these excitons by optical, electrical, as well as mechanical means is examined. Finally, the resultant devices such as excitonic light emitting diodes, lasers, optical modulators, and coupling in an optical cavity are overviewed, demonstrating how excitons can shape future 2D optoelectronics.

Multi-dimensional coherent spectroscopy (MDCS) has become an extremely versatile and sensitive technique for elucidating the structure, composition, and dynamics of condensed matter, atomic, and molecular systems. The appeal of MDCS lies in its ability to resolve both individual-emitter and ensemble-averaged dynamics of optically created excitations in disordered systems. When applied to semiconductors, MDCS enables unambiguous separation of homogeneous and inhomogeneous contributions to the optical linewidth, pinpoints the nature of coupling between resonances, and reveals signatures of many-body interactions. In this review, we discuss the implementation of MDCS to measure the nonlinear optical response of excitonic transitions in semiconductor nanostructures. Capabilities of the technique are illustrated with recent experimental studies that advance our understanding of optical decoherence and dissipation, energy transfer, and many-body phenomena in quantum dots and quantum wells, semiconductor microcavities, layered semiconductors, and photovoltaic materials.

The localized states in ZnO nanowires (NWs) through the growth of ZnS shell have been introduced in this paper. Morphology and optical properties of the ZnO/ZnS core-shell heterostructured NWs after different rapid thermal annealing (RTA) treatments are investigated. Transmission electron microscopy measurements show the gradual disappearing of the jagged boundary between ZnO and ZnS with the increase of RTA temperature, while a decrease of interfacial composition fluctuation and a formation of ZnOS phase can be found after a RTA treatment of 300°C. Temperature-dependent photoluminescence exhibits the features of “S-shape” peak positions and a “valley shape” for the emission width, implying the existence of localized excitons in the core-shell NWs. Moreover, it is noted that the RTA treatments can lower the localized degree which is confirmed by optical measurement. The results indicate that the optical behavior of excitons in ZnO/ZnS core-shell heterostructured NWs can be manipulated by appropriate thermal treatments, which is very important for their practical device applications.

In this paper we present an alternative G _0 W _0 -BSE procedure, suitable for calculation of the quasi-particle and optical properties in 2D semiconductors. The method completely excludes the spurious Coulomb interaction with 2D crystal replicas. The calculated band gap energies of hexagonal boron nitride (hBN), MoS _2 and MoTe _2 monolayers are in good agreement with other theoretical results. The 2D Bethe–Salpeter equation is derived and reduced to a 2D-hydrogen Schrödinger equation in which enter the G _0 W _0 band gap, DFT effective masses, and RPA screened Coulomb interaction. This formulation is applied to the problems of determining exciton binding energies and estimating the quasiparticle band gap in hBN, as well as in some transition-metal dichalcogenides. A semiclassical procedure is used in the limit of high polarizability λ in order to obtain the analytical expression for exciton binding energies.

Time-dependent density functional theory (TDDFT) simulations of transient core-level spectroscopies require a balanced treatment of both valence- and core-electron excitations. To this end, tuned range-separated hybrid exchange–correlation functionals within the generalized Kohn–Sham scheme offer a computationally efficient means of simultaneously improving the accuracy of valence and core excitation energies in TDDFT by mitigating delocalization errors across multiple length-scales. In this work range-separated hybrid functionals are employed in conjunction with the velocity-gauge formulation of real-time TDDFT to simulate static as well as transient soft x-ray near-edge absorption spectra in a prototypical solid-state system, monolayer hexagonal boron nitride, where excitonic effects are important. In the static case, computed soft x-ray absorption edge energies and line shapes are seen to be in good agreement with experiment. Following laser excitation by a pump pulse, soft x-ray probe spectra are shown to exhibit characteristic features of population induced bleaching and transient energy shifts of exciton peaks. The methods outlined in this work therefore illustrate a practical means for simulating attosecond time-resolved core-level spectra in solids within a TDDFT framework.

Junction temperature (Tj) and current have important effects on light-emitting diode (LED) properties. Therefore, the electroluminescence (EL) spectra of blue and green LEDs were investigated in a Tj range of 120−373 K and in a current range of 80−240 mA based on accurate real-time measurements of Tj using an LED with a built-in sensor unit. Two maxima of the emission peak energy with changing Tj were observed for the green LED, while the blue LED showed one maximum. This was explained by the transition between the donor-bound excitons (DX) and free excitons A (FXA) in the green LED. At low temperatures, the emission peak energy, full width at half maximum (FWHM), and radiation power of the green LED increase rapidly with increasing current, while those of the blue LED increase slightly. This is because when the strong spatial potential fluctuation and low exciton mobility in the green LED is exhibited, with the current increasing, more bonded excitons are found in different potential valleys. With a shallower potential valley and higher exciton mobility, excitons are mostly bound around the potential minima. The higher threshold voltage of the LEDs at low temperatures may be due to the combined effects of the band gap, dynamic resistance, piezoelectric polarization, and electron-blocking layer (EBL).

The electronic and optical properties of semiconducting silicon nanotubes (SiNTs) are studied by means of the many-body Green’s function method, i.e., GW approximation and Bethe–Salpeter equation. In these studied structures, i.e., (4,4), (6,6) and (10,0) SiNTs, self-energy effects are enhanced giving rise to large quasi-particle (QP) band gaps due to the confinement effect. The strong electron−electron (e−e) correlations broaden the band gaps of the studied SiNTs from 0.65, 0.28 and 0.05 eV at DFT level to 1.9, 1.22 and 0.79 eV at GW level. The Coulomb electron−hole (e−h) interactions significantly modify optical absorption properties obtained at noninteracting-particle level with the formation of bound excitons with considerable binding energies (of the order of 1 eV) assigned: the binding energies of the armchair (4,4), (6,6) and zigzag (10,0) SiNTs are 0.92, 1.1 and 0.6 eV, respectively. Results in this work are useful for understanding the physics and applications in silicon-based nanoscale device components.

We study spatially trapped ensembles of dipolar excitons in coupled quantum wells by means of photoluminescence and photocurrent spectroscopy. The photogenerated excitons are confined in very clean GaAs double quantum well structures and electrostatically trapped by local gate electrodes. We find that the common approach of electrostatic trap geometries can give rise to an in-plane imbalance of charge carriers especially when an over-barrier excitation is utilized. The excess charge carriers can give rise to an effective parabolic confinement potential for the excitons. In photoluminescence spectra, we identify the emission of both neutral indirect excitons and states influenced by the excess charge carrier density. We find that the charge imbalance in the excitonic ensemble strongly influences the radiative lifetimes of both. Our findings shine a new light on the properties of trapped dipolar exciton ensembles. This is of significant relevance to common interpretations of experimental results in terms of signatures for the formation of ‘dark’ and ‘gray’ excitonic condensates.

A novel beyond-CMOS device concept, the Bilayer pseudoSpin Junction Transistor (BiSJT), is proposed. Like the previously proposed Bilayer pseudoSpin FET (BiSFET), the BiSJT is motivated by the possibility of interlayer electron-hole exciton condensation in bilayer 2-D material systems, and could provide switching energies of a few 10 s of zJ, orders of magnitude below even end-of-the-roadmap CMOS. The BiSJT is, however, current-controlled, and may allow for simpler device design, smaller device area, and more flexible gate design. [ABSTRACT FROM AUTHOR]

Theoretical study of excitons spectra is offered in this report as for Zn3P2 crystals. Spectra are got in the zero approach of the theory of perturbations with consideration of both the anisotropy of the dispersion law and the selection rules. The existence of two exciton series was found, which corresponds to two valence bands (hh, lh) and the conductivity band (c). It is noteworthy that anisotropy of the dispersion law plus the existence of crystalline packets (layers) normal to the main optical axis, both will permit the consideration of two-dimensional excitons too. The high temperature displaying of these 2D-exciton effects is not eliminated even into bulk crystals. The calculated values of the binding energies as well as the oscillator's strength for the optical transitions are given for a volume (3D) and for two-dimensional (2D) excitons. The model of energy exciton transitions and four-level scheme of stimulated exciton radiation for receiving laser effect are offered.

In the present work we combine spectroscopic, DSC and computational approaches to examine the multiple extracellular Glu mutants E204Q/E194Q, E204Q/E194Q/E9Q and E204Q/E194Q/E9Q/E74Q of bacteriorhodopsin by varying solvent ionic strength and composition. Absorption spectroscopy data reveal that the absorption maxima of multiple EC Glu mutants can be tuned by the chloride concentration in the solution. Visible Circular dichroism spectra imply that the specific binding of Cl- can modulate weakened exciton chromophore coupling and reestablish wild type-like bilobe spectral features of the mutants. The DSC data display reappearance of the reversible thermal transition, higher Tm of denaturation and an increase in the enthalpy of unfolding of the mutants in 1 M KCl solutions. Molecular dynamics simulations indicate high affinity binding of Cl- to Arg82 and to Gln204 and Gln194 residues in the mutants. Analysis of the experimental data suggests that simultaneous elimination of the negatively charged side chain of Glu194 and Glu204 is the major cause for mutants’ alterations. Specific Cl- binding efficiently coordinates distorted hydrogen bonding interactions of the EC region and reconstitutes the conformation and structure stability of mutated bR in WT-like fashion. [ABSTRACT FROM AUTHOR]