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Despite defect‐mediated states in monolayer semiconductors have been considered as efficient emitters in cryogenic conditions, they are severely quenched at room temperature (RT) because of multiple thermal‐induced non‐radiative channels, hindering their practical use. Here, robust and tunable defect‐mediated emissions at RT, designated as D1 (1.82 eV) and D2 (1.62 eV), are discovered in monolayer WS2 through argon plasma treatment, which, via multiple corroborative experiments, are attributed to distinct physical processes: bound excitons associated with sulfur vacancies (Vs) and the band‐to‐acceptor recombination, respectively. Remarkably, the defective sample exhibits over ten‐fold increase in both photoluminescence quantum yield and full‐width at half‐maximum (FWHM) compared to its intrinsic counterpart. Leveraging these pronounced edges, light‐emitting diodes (LEDs) functioning at RT achieve broadband (FWHM: 490 meV) and continuously tunable emission from 1.6 to 2.0 eV, as well as the highest electroluminescence (EL) external quantum efficiency (EQE) of ≈1.39% among transient LEDs based on monolayer semiconductors to date, thereby unveiling a new strategy for emission tailoring at the atomic‐scale limit. [ABSTRACT FROM AUTHOR]

All-inorganic perovskite films have emerged as promising candidates for laser gain materials owing to their outstanding optoelectronic properties and straightforward solution processing. However, the performance of blue perovskite lasing still lags far behind due to the inevitable high density of defects. Herein, we demonstrate that defects can be utilized instead of passivated/removed to form bound excitons to achieve excellent blue stimulated emission in perovskite films. Such a strategy emphasizes defect engineering by introducing a deep-level defect in mixed-Rb/Cs perovskite films through octylammonium bromide (OABr) additives. Consequently, the OA-Rb/Cs perovskite films exhibit blue amplified spontaneous emission (ASE) from defect-related bound excitons with a low threshold (13.5 μJ/cm 2 ) and a high optical gain (744.7 cm -1 ), which contribute to a vertical-cavity surface-emitting laser with single-mode blue emission at 482 nm. This work not only presents a facile method for creating blue laser gain materials but also provides valuable insights for further exploration of high-performance blue lasing in perovskite films.

Structure of heterogeneous photocatalyst with adequate coupling interface has an important effect on energy transfer and photoinduced electron-hole pairs separation. In this work, Ag-embedded ZnO heterogeneous porous nanosheets (PNSs) have been synthesized by thermal pyrolyzing of Ag 2 CO 3 -modified Zn 5 (CO 3) 2 (OH) 6 nanosheets at 400 °C. Photoelectron spectra and X-ray diffraction measurements demonstrate that there exists an adequate coupling between Ag and ZnO phase. Ag-embedded ZnO PNSs, in contrast to bare ZnO PNSs, performs 6-fold enhancement in natural-daylight-driven degradation of pyridine which requires participation of ·OH radicals. This result demonstrates that the adequate coupling interface between Ag and ZnO for Ag-embedded ZnO PNSs promotes an energy transfer from local surface plasma resonance of Ag nanoparticles to bound excitons in ZnO region, activating them to dissociate into free photoinduced carriers. In addition, external control experiments indicate that the best photocatalytic degradation is obtained in the 7–10 pH range of 10 mg/L pyridine solution in presence of 0.5 g/L photocatalyst dosage. The optimal Ag-embedded ZnO PNSs under outdoor daylight irradiation also performs an efficiently photocatalytic degradability for pyridine present in ground water. Our work provides a new approach to fabricate ZnO-based photocatalyst with efficient energy transfer for treating stubborn organics present in polluted water. • Ag-embedded ZnO porous nanosheets (AEZPNSs) with strong coupling interface were fabricated. • Bound excitons in the ZnO region of AEZPNSs irradiated by daylight could dissociate. • AEZPNSs performed daylight-driven catalytic removal of pyridine in ground water. [ABSTRACT FROM AUTHOR]

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The present work reports on the tunable optical emission from oxygen vacancies in ZnO nanorods. The common green luminescence at room temperature originated from oxygen vacancy defects can be mediated by simply controlling pH values of alkaline solutions without the introduction of any intentional dopants. The peak energies of the green band are found to be monotonously tuned in the range from 2.17 to 2.47 eV, which could promote the applications of tunable wavelength ZnO optoelectronic devices. The temperature dependent behavior of exciton transitions and oxygen vacancy recombinations in ZnO nanorods is investigated in detail. It is suggested that the localized bound exciton (LBX) attached near the lateral facets of ZnO nanorods could survive up to nearly room temperature due to the large binding energy. [ABSTRACT FROM AUTHOR]

Since the research on diamond doping remains an issue for electronic applications, the luminescence spectroscopy of bound excitons appears essential to evaluate the properties of shallow dopants in diamond. In this paper, the formation and recombination processes of bound excitons in diamond are reviewed for boron acceptors, phosphorus, and arsenic donors. Pulsed and continuous excitations are considered, the latter being currently used for the measurement of donor and acceptor concentrations in diamond. [ABSTRACT FROM AUTHOR]

Despite their outstanding photovoltaic performance, organic-inorganic perovskite solar cells still face severe stability issues and limitations in their device dimension. Further development of perovskite solar cells therefore requires a deeper understanding of loss mechanisms, in particular, concerning the origin and impact of trap states. Here, different surface properties of submicrometer sized CH3NH3PbI3 particles are studied as a model system by photoluminescence spectroscopy to investigate the impact of the perovskite crystal surface on photoexcited states. Comparison of single crystals with either isolating or electron-rich surface passivation indicates the presence of positively charged surface trap states that can be passivated in case of the latter. These surface trap states cause enhanced nonradiative recombination at room temperature, which is a loss mechanism for solar cell performance. In the orthorhombic phase, the origin of multiple emission peaks is identified as the recombination of free and bound excitons, whose population ratio critically depends on trap state properties. The dynamics of exciton trapping at 50 K are observed on a time-scale of tens of picoseconds by a simultaneous population decrease and increase of free and bound excitons, respectively. These results emphasize the potential of surface passivation to further improve the performance of perovskite solar cells. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) semiconductors with point defects are predicted to host a variety of bound exciton complexes analogous to trions and biexcitons due to strong many-body effects. However, despite the common observation of defect-mediated subgap emission, the existence of such complexes remains elusive. Here, we report the observation of bound exciton (BX) complex manifolds in monolayer MoSe 2 with intentionally created monoselenium vacancies (V Se ) using proton beam irradiation. The emission intensity of different BX peaks is found to exhibit contrasting dependence on electrostatic doping near the onset of free electron injection. The observed trend is consistent with the model in which free excitons exist in equilibrium with excitons bound to neutral and charged V Se defects, which act as deep acceptors. These complexes are more strongly bound than trions and biexcitons, surviving up to around 180 K, and exhibit moderate valley polarization memory, indicating partial free exciton character.

Photoluminescence and X-ray diffraction are used to investigate 2H and 3R polytypes of MoS 2 layered crystals intercalated with Cl 2 molecules. The spectral structure of the observed bound excitons emission is individual for each polytype, which can be used for unambiguous identification of the 2H and 3R polytypes. The DFT band structure calculations have been done for three molecular layers of 2H-MoS 2 and 3R-MoS 2 polytypes with and without intercalation of Cl 2 molecules. The spectroscopic experimental results confirmed theoretical calculations performed for both cases and it has been demonstrated that halogen intercalation completely changes the local band structure of both 2H- and 3R-MoS 2 polytypes. [ABSTRACT FROM AUTHOR]

Abstract: Numerical calculations of the excitonic absorption spectra in a strained Cd x Zn1−x O/ZnO quantum dot are investigated for various Cd contents. We calculate the quantized energies of the exciton as a function of dot radius for various confinement potentials and thereby the interband emission energy is computed considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption as a function of photon energy for different dot radii is discussed. Decrease of exciton binding energy and the corresponding optical band gap with the Cd concentration imply that the confinement of carriers decreases with composition x. The main results show that the confined energies and the transition energies between the excited levels are significant for smaller dots. Non-linearity band gap with the increase in Cd content is observed for smaller dots in the strong confinement region and the magnitude of the absorption spectra increases for the transitions between the higher excited levels. [Copyright &y& Elsevier]

We study exciton states in Zn(Cd)Se/ZnMgSSe quantum wells (QWs) with various degrees of diffusion blurring in the interfaces by the methods of optical spectroscopy. We show that at low temperatures the QW emission spectra are determined by free and neutral donor-bound excitons. Blurring of the heterointerfaces leads to the increase in the energy shift between the emission line maxima of free and bound excitons. We explain the nonlinear dependence of the steady-state photoluminescence intensity on the excitation-power density in terms of the neutralization of charged donors at the photoexcitation of heterostructures. We observed a complex long-time dynamics of the reflection coefficient, evoked by the charge-redistribution processes in the heterostructure, near the QW exciton resonances under the irradiation. [ABSTRACT FROM AUTHOR]

The results of luminescence measurements in magnetic fields up to 23T of strain free GaN bulk crystals doped with gadolinium are presented. The magnetic field splitting pattern observed for the Gd-related transitions in the "red" spectral range is consistent with its identification as due to intracenter transitions between Gd3+ ion levels. The Gd-related emission structures observed close to the GaN bandgap show magnetic field behavior which could be understood in terms bound exciton or donor-acceptor pair recombination involving Gd ions. Disappointingly, no giant Zeeman splitting, which otherwise could confirm ferromagnetism in this material, was observed. [ABSTRACT FROM AUTHOR]

Abstract: Freestanding GaN layers of various thicknesses grown by HVPE have been studied by time-resolved spectroscopy combined with structural and electrical measurements. We have observed an increase of the PL lifetime with increasing layer thickness; however, a saturation of the recombination times has been detected for the GaN layers thicker than 400 μm. We explain the observed thickness-dependent behavior of the decay times by competition of two nonradiative mechanisms; namely, for layers with thickness less than 400 μm the main nonradiative channel is related to the structural defects, while in thicker layers the recombination decay time is limited by impurities and/or vacancies. [Copyright &y& Elsevier]

Abstract: Different emission channels, due to recombination of neutral donor-bound excitons (D0X) in GaN, are discussed from photoluminescence experiments performed in magnetic fields up to 28T for a freestanding sample and a heteropepitaxial layer doped with silicon. For these samples, in addition to the principal recombination channel of D0X, in which donors are left in their ground states, the well-resolved two-electron satellites (TES) involving different donor excitations were observed. A detailed analysis of the magneto-optical data provides important knowledge about the energy structure of the oxygen and silicon donors in wurtzite GaN. The application of high magnetic fields allowed us to tune the intra-donor excitations across the LO-phonon energy. A strong enhancement of TES intensity and the appearance of several anti-crossing processes in the vicinity of the LO-phonon replica of the principal D0X transition have been observed. These effects are discussed in terms of resonant interaction between LO phonons and donor-bound electrons (magneto-polaron effect). [Copyright &y& Elsevier]

Luminescence experiments in magnetic fields up to 28 T on a freestanding gallium nitride (GaN) sample and a heteroepitaxial GaN layer lightly doped with silicon are presented. In these samples, the principal D0X recombination channel is accompanied by two electron satellites (TES), which involve the excited donor states as well as by the longitudinal-optic (LO) phonon replica of the principal transitions. When the internal donor excitations are magnetically tuned into resonance with the LO-phonon excitation, the intensity of TES is strongly enhanced and clear avoided-crossings between the LO-phonon replica of the principal D0X transition and TES involving highly excited states of oxygen and silicon are resolved. The observed behavior is explained in terms of resonant interaction between LO-phonons and donor-bound electrons. It is found that the resonant magnetopolaron interaction is stronger for the oxygen as compared to the silicon donor. [ABSTRACT FROM AUTHOR]

Abstract: Two phosphorus-doped CVD diamond layers with nominal concentrations of 3×1018 cm−3 and 1×1019 cm−3 were studied by low-temperature cathodoluminescence. The measurements show that the effective doping concentration varies systematically across the sample area. Exciting individual spots with increasing local donor densities we observe novel bound exciton lines to appear and finally to dominate the spectra. The new lines are down-shifted from the conventional P-bound exciton line by up to ≈80 meV. They were studied as a function of excitation level and temperature. The data lead us to conclude that excitons can bind to phosphorus pairs with different spacings giving evidence for exciton–donor pair interactions. [Copyright &y& Elsevier]

Abstract: The stability of the charged excitons X+ and X− as a function of the spherical quantum dot radius R and also as a function of the effective mass ratio σ is studied with a formulation of the Hartree–Fock approximation using a calculation method which is based on the quantum adiabatic theorem. Our results indicate that the binding energies of X+ and X− are different except for σ =1, the difference reduces as the ratio σ increases, the binding energy of X+ is higher than that of X−, the binding energy of both charged excitons decreases as the mass ratio σ increases and there is a critical mass ratio σ c, such that for σ > σ c the complex is unstable. Furthermore, studying the behavior of both charged excitons as a function of the dot radius for σ =0.8, we found that both charged excitons are unstable. [Copyright &y& Elsevier]

Abstract: The comparative study of the excitonic radiative recombination in 2H-MoS2:Cl2 and 2H-WS2:Br2 layered crystals is presented. The steady-state emission spectra of both the investigated materials include two distinct regions. The short-wavelength region consisting of sharp lines is produced by bound excitons, related to the halogen molecules intercalated in the layered semiconductors during the growth process. The second spectral region—a vibronic broad band identical for both types of crystals, is attributed to the deep radiative centres, caused by the intrinsic defects of the host lattices. The atypical temperature increase of the excitonic radiative decay time observed for 2H-WS2:Br2 in the range is related to the presence of a long lifetime sublevel, placed 0.3meV above the lowest excitonic state. [Copyright &y& Elsevier]

Abstract: Experiments and theoretical modeling are presented on the origin of efficient electroluminescence from boron implanted Si-LEDs. At low lattice temperatures two bound exciton traps created by high dose boron implantation were observed in the most efficient LEDs with external power efficiency above 0.12%. The temperature dependence of the correlation between the EL intensity from free and bound excitons is analyzed by a rate equation model. This analysis reveals that the bound excitons have a unique characteristic of a low recombination rate. The enhancement of EL from free electron-hole pairs with increasing temperature is due to the thermal activation of carriers from bound exciton traps. [Copyright &y& Elsevier]

We study phosphorus-doped diamond films grown by microwave-assisted CVD on high-pressure synthetic type Ib(111) substrates by Hall effect and low-temperature cathodoluminescence measurements. In the electrical measurements, the electron concentration is thermally activated with an energy Eact≈600 meV for various donor concentrations Nd while the data indicate compensation of the donors by an acceptor with concentration Na lower than Nd by one order of magnitude or more. The optical data demonstrate the incorporation of phosphorus donors and boron acceptors by line-resolved donor-acceptor pair (DAP) spectra, by boron-related broad luminescence bands at 2.3, 3.8 and 4.6 eV, and by a phosphorus-related extremely sharp and intense bound exciton spectrum detected in this perfect appearance for the first time. Details of this spectrum are discussed. A novel feature is a triplet emission at ≈5.38 eV whose properties hint to an isoelectronic trap incorporating P donors and B acceptors on nearest-neighbor or close lattice sites. [Copyright &y& Elsevier]

We present results of an ESR study of tetra-interstitial agglomerates (I4) in silicon. The defects were formed upon annealing of electron-irradiated silicon samples pre-doped with hydrogen and reveal two ESR spectra, i.e., the B3 which is related to positive charge state of the defect and the NL51 which is related to the neutral defect in an excitonic triplet state. Temperature dependence of the intensity of the NL51 spectra allowed determining parameters of the excitonic state. From photo-ESR measurements, the energy level of the excitonic state was determined to lie at ∼1.03 eV above the valence band. [Copyright &y& Elsevier]

We have studied by low-temperature cathodoluminescence phosphorus-doped CVD diamond films grown on synthetic high-pressure (1 1 1) diamond substrates. A perfect phosphorus-bound exciton spectrum is observed at very high intensity and exorbitantly narrow line widths down to 2.0 meV showing no-phonon (NP)- and wave vector-conserving TA-, TO-, LO-, and (TO+OΓ)-phonon transitions. All lines are split into up to five components, which are well resolved in the sharp NP-transition. Temperature-controlled measurements demonstrate that the line splittings are in the upper excitonic state. At 5.38 eV, only 26 meV below the excitonic gap, we detect a novel, very shallow line spectrum with spectral features characteristic of an isoelectronically bound exciton. Our data are consistent with the isoelectronic trap being a phosphorus donor–boron acceptor pair on close lattice sites. We discuss this model in the light of previous data from GaP and Si isoelectronic centers. [Copyright &y& Elsevier]

The optical properties of excitonic recombinations in bulk ZnO are investigated by photoluminescence (PL) measurements. At liquid helium temperature the neutral donor bound excitons are positioned at 3.364, 3.362 and 3.361 eV, the line at 3.364 eV dominates the PL spectra. Annealing of the crystals demonstrates that the 3.364 eV vanishes, it is most likely caused by the hydrogen related donor. Two-electron satellite transitions of the donor bound excitons allow to determine the donor binding energies to 43, 52 and 55 meV. These results are in line with the temperature dependent Hall effect measurements. In the as-grown crystals two donors with binding energies of 30 and 50 meV control the conductivity, whereas after annealing only one donor with a binding energy of about 50 meV is necessary to fit the data perfectly. In addition at 3.335 eV an excitonic recombination is observed, which supported by spatially resolved cathodoluminescence measurements, is attributed to excitons bound to structural defects. [Copyright &y& Elsevier]

Total energy, binding energy, recombination rate (of the electron–hole pair) for an exciton (X) bound in a parabolic two-dimensional quantum dot by a donor impurity located on the z-axis at a distance d from the dot plane, are calculated by using the Hartree formalism with a recently developed numerical method (PMM) for the solution of the Schro¨dinger equation. As our analysis indicates there is a critical dot radius Rc such that for R the complex is unstable and with an increase of the impurity distance this critical radius increases. Furthermore, there is a critical value of the mass ratio σ=m&ast;e/m&ast;h such that for σ<σc the complex is stable. The appearance of this stability condition depends both on the impurity distance and the dot radius, in a way that with an increase of the impurity distance we have an increase in the maximum dot radius where this stability condition appears. For dot radii greater than this maximum dot radius (for fixed impurity distance) the complex is always stable. [Copyright &y& Elsevier]

We discuss the stability of an exciton bound to an ionized donor impurity in a GaAs/Ga1−xAlxAs semiconductor quantum well subjected to an external magnetic field for different values of the impurity location. The binding energy is calculated in the effective mass approximation by means of variational method. At zero magnetic field, the complex becomes unstable when the impurity is far away from the center of the well. When the magnetic field increases, the stability holds in all cases for an impurity located at the center of the well. [Copyright &y& Elsevier]

Impurity doping is a viable route toward achieving desired subgap optical response in semiconductors. In strongly excitonic two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs), impurities are expected to result in bound-exciton emission. However, doped TMDs often exhibit a broad Stokes-shifted emission without characteristic features, hampering strategic materials engineering. Here we report observation of a well-defined impurity-induced emission in monolayer WS 2 substitutionally doped with rhenium (Re), which is an electron donor. The emission exhibits characteristics of localized states and dominates the spectrum up to 200 K. Gate dependence reveals that neutral impurity centers are responsible for the observed emission. Using GW-Bethe-Salpeter equation (GW-BSE) calculations, we attribute the emission to transitions between spin-split upper Re band and valence band edge.

Abstract: Due to their layered structure, 2H layered transition metal dichalcogenide indirect bandgap compounds MoS2, WS2 can be intercalated with foreign atoms and molecules. The 2H-MoS2, 2H-WS2 and 2H-WSe2 crystals intercalated with halogen molecules at low temperatures exhibit a strong luminescence due to the bound excitons recombination. In this work, the results of comparative study of the excitonic luminescence kinetics for 2H-WS2:Br2 and 2H-MoS2:Cl2 single crystals are presented. A simple kinetic model describing the temperature dependencies of the steady-state luminescence intensity and of the radiative decay time is proposed. The lifetimes of the excitonic levels and the activation energy of the excitonic luminescence quenching caused by an extrinsic self-trapping process for both studied compounds were determined. [Copyright &y& Elsevier]