Self-trapped Exciton Luminescence Research Articles

Pressure-controlled luminescence in fast-response barium fluoride crystals

Cross-luminescence (CL) in a barium fluoride (BaF2) scintillator arising from the recombination of a valence band electron and a core band hole results in a fast picosecond decay time. However, the CL emission wavelength in the vacuum ultraviolet region is difficult to detect, and intrinsically intense and slow nanosecond self-trapped exciton (STE) luminescence occurs. Herein, we report a redshift in the CL emission wavelength with high-pressure application. The wavelength of the CL emission shifted from 221 nm to 240 nm when 5.0 GPa was applied via a sapphire anvil cell. Increasing the pressure decreases the core-valence bandgap due to the downward expansion of the valence band, resulting in a decrease in the valence band minimum. The onset of a phase transition from a cubic crystal structure to an orthorhombic crystal structure at 3.7 GPa inhibited the recombination of conduction band electrons and self-trapped holes, leading to the disappearance of the STE emission. Manipulating the band structure of BaF2 by high-pressure application enables control of its luminescence emission, providing a pathway toward solving the problems inherent in this leading fast-response scintillator.

The origin of broadband blue emission in zero-dimensional organic lead iodine perovskites: A first-principles study.

Broadband blue emission in zero-dimensional perovskites has received considerable attention, which is very important for the realization of stable blue-light emitters; however, the underlying formation mechanism remains unclear. Based on first-principles calculations, we have systematically studied the self-trapped excitons (STEs) behavior and luminescence properties in 0D-(DMA)4PbI6 perovskite. Our calculations show that there is a significant difference between the intrinsic STE luminescence mechanism (∼2.51eV) and experimental observations (∼2.70eV). In contrast, we found that the iodine vacancy (VI) is energetically accessible and exhibits a shallow charge transition level at ∼2.69eV (0/+1) above the valence band maximum, which provides the initial local well for the STEs formation. Moreover, the low electronic dimension synergistic Jahn-Teller distortion facilitates the formation of extrinsic excitons self-trapping. Further excited state electronic structure analysis and configuration coordinate diagram calculations confirmed that the broadband blue emission in 0D-(DMA)4PbI6 is the origin of VI-induced extrinsic STEs instead of intrinsic STEs. Therefore, our simulation results rationalize the experimental phenomena and provide important insights into the formation mechanism of STEs in low-dimensional perovskite systems.

Sb3+/Sm3+ Codoped Cs2NaScCl6 All-Inorganic Double Perovskite: Blue Emission of Self-Trapped Excitons and Red-Emission via Energy Transfer.

The lead-free halide perovskites possess nontoxicity and excellent chemical stability, whereas relatively weak luminescence intensity limits their potential in practical applications. Therefore, strengthening the luminescence intensity and expanding application fields are urgent tasks for the development of lead-free halide perovskites. In this paper, antimony-doped Cs2NaScCl6 crystals synthesized by a solvothermal method emit bright, deep blue photoluminescence at 447 nm. The photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectra demonstrate that Sb3+ doping effectively activate the intrinsic "dark self-trapped exciton (STE)," leading to an impressive photoluminescence quantum yield (PLQY) value of 78.31% for 1% Sb3+ doping. Furthermore, the luminescence intensity remains above 92% compared with the fresh sample without secondary phases detected even after 90 days under environmental conditions. To expand the emission spectra, rare-earth Sm3+ is further incorporated into Cs2NaScCl6:1% Sb3+ crystals. The results show that Sb ions not only enhance intrinsic STE luminescence but also serve as sensitizers to boost the red-light emission of Sm3+, leading to a significant 500-fold increase in red emission intensity. Finally, the PLQY reaches a stunning 86.78%. These findings provide valuable insights in the design of Sb ion-doped lead-free double perovskites, broadening the application fields in various optoelectronic devices.

Luminescence of CsLa1-хCeхSiS4 thiosilicate solid solution: From STE to Ce3+ emission

A series of single crystals of CsLa1-хСехSiS4 monophasic solid solution (x = 0–1) has been obtained for the first time by high-temperature flux synthesis. Methods of XRD and chemical analysis, absorption and low-temperature (from T = 5 K) luminescent spectroscopy were used. The results of the band scheme calculating using density functional theory correlate with spectroscopy data. One non-elementary d → f emission band of Ce3+ ions is observed in the region of 520 nm in the luminescence spectra at room temperature at any value of the x parameter. The luminescence decay kinetics of Ce3+ ions upon excitation by a pulsed electron beam, X-ray synchrotron radiation or intracenter photoexcitation is characterized by a nanosecond component. As the parameter x increases, the decay time is reduced from 132 ns (x = 0.005) to 0.88 ns (x = 1). The luminescence decay kinetics upon photoexcitation at x = 0.005–0.12 is characterized by monoexponential decay with τ = 31.4 ± 0.2 ns. Concentration quenching of the Ce3+ ion photoluminescence is not observed up to the value of the parameter x = 0.12; it only appears at x = 1. The anomalously short decay time of the Ce3+ ions luminescence in CsCeSiS4 upon both X-ray excitation and photoexcitation is associated with concentration quenching.At temperature 5 K, new intense bands at 408 and 688 nm in addition to the Ce3+ emission band observed in the photoluminescence spectra of nominally pure CsLaSiS4 or at the lowest parameter value x = 0.005. These bands correspond to the luminescence of self-trapped excitons (STE) and defects. With increasing x parameter, the STE emission band is reabsorbed by the absorption of Ce3+ ions and is quenched due to the resonance energy transfer STE → Ce3+ center. Thermoluminescence glow curves of CsLa1-xCexSiS4 irradiated with X-ray at T = 90 K are characterized by several low temperature intense peaks, which indicates a high concentration of charge carrier traps.

HCOO- Doping-Induced Multiexciton Emissions in Cs3Cu2I5 Crystals for Efficient X-Ray Scintillation.

Self-trapped exciton (STE) luminescence, typically associated with structural deformation of excited states, has attracted significant attention in metal halide materials recently. However, the mechanism of multiexciton STE emissions in certain metal halide crystals remains largely unexplored. This study investigates dual luminescence emissions in HCOO- doped Cs3Cu2I5 single crystals using transient and steady-state spectroscopy. The dual emissions are attributed to intrinsic STE luminescence originating from the host lattice and extrinsic STE luminescence induced by external dopants, respectively, each of which can be triggered independently at distinct energy levels. Theoretical calculations reveal that multiexciton emission originates from structural distortion of the host and dopant STEs within the 0D lattice in their respective excited states. By meticulously tuning the excitation wavelength and selectively exciting different STEs, the dynamic alteration of color change in Cs3Cu2I5:HCOO- crystals is demonstrated. Ultimately, owing to an extraordinarily high photoluminescence quantum yield (99.01%) and a diminished degree of self-absorption in Cs3Cu2I5:HCOO- crystals, they exhibit remarkable X-ray scintillation characteristics with light yield being improved by 5.4 times as compared to that of pristine Cs3Cu2I5 crystals, opening up exciting avenues for achieving low-dose X-ray detection and imaging.

Antimony doping in A2ZnCl4 (A= Rb, Cs) metal halides enabling tunable near-infrared emission

Lead-free metal halides with stable and tunable emission have shown great promise for broad application prospects in the visible light range. However, it remains a fundamental challenge to realize tunable near-infrared (NIR) luminescence in these metal halide materials. Herein, a cation regulation strategy in zero-dimension organic metal halides A2ZnCl4:Sb3+ (A = Rb, Cs) to achieve tunable NIR luminescence is reported. The prepared A2ZnCl4:Sb3+ (A = Rb, Cs) halides exhibited efficient broadband NIR emission, which originates from self-trapped excitons of the [SbCl4]− polyhedron. Based on the regulation of cation, the [SbCl4]− tetrahedral distortion capacity changes, exciton–phonon coupling is enhanced, leading to a red shift of the emission wavelength. The results provide inspiration for the regulation of the self-trapped excitons luminescence of halides and demonstrate potential as a non-visible light source for night vision.

Excitonic feature in CsAg2I3 crystals prepared by Bridgman method

This study investigated the optical spectra of the CsAg2I3 crystals at low temperatures. In the reflection spectrum, remarkable reflection peaks owing to band-edge exciton transitions were observed at approximately 3.8 eV above the fundamental absorption edge at 3.6 eV. Under excitation in the energy region of exciton transitions, an intense luminescence band attributed to a self-trapped exciton (STE) was observed at 3.37 eV. In addition to STE luminescence, a weak luminescence line was observed at 3.77 eV. Because the value of 3.77 eV is practically equal to the lowest exciton transition energy estimated from the reflection spectrum, the luminescence line at 3.77 eV comes from a free exciton (FE). The intensities of the FE and STE luminescence peaks decrease with increasing temperature. The activation energies of the FE and STE were estimated from the quenching of luminescence intensities. The features of the exciton states in the CsAg2I3 crystals are presented.

Highly efficient self-trapped exciton luminescence of Sb3+-doped (CH6N3)3BiCl6 for ethanol detection

Zero-dimensional (0D) organic–inorganic metal halide hybrids (MHHs) are considered promising luminescent materials due to their unique “host–guest” structure and tunable emission spectrum.

Highly Efficient STEs NIR‐II Broadband Emission in a Perovskite System and its Spectroscopy Applications

AbstractEfficient NIR emitting materials have important application value in phosphor‐converted light‐emitting diodes (pc‐LEDs), due to their high sensitivity of tissue composition responsiveness, and penetration depth (spatial resolution). In this work, a perovskite structure material CaSnO3 that can emit strong broad NIR light at the range of 850–1300 nm peaking at 1010 nm second biological window with a super large Stokes shift of 24463 cm−1 and a sensitive broad UV light response (λmax = 291 nm) by traditional high‐temperature solid‐state reaction that does not require additional doping ions is innovatively developed. Spectroscopic and theoretical calculations reveal that the NIR emission originates from self‐trapped excitons (STEs) enhanced by Jahn–Teller effect when Sn4+ is spontaneously reduced to Sn2+ and occupies the position of Ca2+ 8‐coordinate lattice. A prototype NIR‐II emitting LED device is successfully fabricated combining this perovskite CaSnO3 phosphor with UV 285 nm chip to evaluate its application value. This work provides a novel NIR‐II luminescent material, which is different from other doping types of systems, and STEs luminescence in perovskite has been achieved in the NIR‐II region for the first time, providing a new approach and choice for the development of NIR luminescent materials.

Self‐Trapped Excitons‐Based Warm‐White Afterglow by Room‐Temperature Engineering toward Intelligent Multi‐Channel Information System

AbstractIn the era of intelligence, the output colors of foundational phosphors are expected to be controlled by programs, while current activators‐determined afterglow candidates with fixed spectral channel have limitations in creating customized colors. Here, a long‐lived warm‐white emission is successfully demonstrated originating from self‐trapped excitons (STEs) in non‐toxic Cs2NaInCl6:Ag,Bi, ranging from 400 to 850 nm, of which the afterglow color can be easily customized using filters. This investigation indicates that 3% of Ag alloying breaks the dark STEs and introduces traps for efficient afterglow, while 3% Bi doping further improves the quantum yield to ≈100% and greatly enhances afterglow by ≈100‐fold compared with the initial intensity, allowing for an impressive afterglow persistence of over 20,000 s. Intriguingly, the self‐trapped defect bands from Jahn‐Teller distortions are prolonged from hundreds of femtoseconds to several hours, and they are first detected in the steady‐state absorption spectra after the cessation of excitation sources, contributing to the concluded dynamic afterglow model for STEs. And its intelligent application is corroborated by designed multi‐channel information system. These findings offer a novel scheme for understanding dynamic luminescence of STEs and supply an exemplification of designing white afterglow phosphors.

Impurity, host and defect-related luminescence of CsLaSiS4 thiosilicate crystals doped with Ce3+ ions

Ce+3 doped CsLaSiS4 single crystals were synthesized using high-temperature flux synthesis. XRD analysis showed that the products are isostructural to CsLaSiS4 and crystallize in orthorhombic space group Pnma. Absorption spectra were studied at room temperature, and photoluminescence properties were examined in the temperature range of 5–310 K. The energy of interband transitions Eg = 3.75 eV was determined at room temperature in the Tauc model. At room temperature, only a non-elementary d → f emission band of Ce+3 ions was observed in the 520 nm region. The luminescence kinetics upon excitation by a pulsed cathode beam or X-ray synchrotron radiation exhibited a dominant nanosecond component. Decay time, as well as build-up time, decreased with increasing concentration of Ce+3 ions. Concentration quenching of Ce+3 emission was not observed up to 11.9 mol% of Ce+3 ions. At a low temperature of 5 K, new wide emission bands at 422 and 688 nm appeared in the photoluminescence spectrum. It is shown that the 422 nm band in the photoluminescence spectrum corresponds to host emission, specifically the luminescence of self-trapped excitons (STE). The 688 nm emission band corresponds to defect-related luminescence. STE emission is quenched according to the Mott law at temperatures above 26 K with an activation energy of 20 meV. An efficient energy transfer channel from the STE to the Ce+3 ion via the radiative resonance mechanism is observed. The emission of Ce+3 ions and defect-related luminescence can be excited by the intracenter way, due to electron-hole recombination, or by the creation of defect bound excitons. Based on the obtained spectroscopic data, a band scheme illustrating the processes of relaxation of electronic excitations at T = 5 K in Ce+3 doped CsLaSiS4 crystals is proposed.

Toward first-principles approaches for mechanistic study of self-trapped exciton luminescence

In recent years, broadband photo-luminescence phenomena arising from self-trapped exciton (STE) in metal halides, including perovskites and various low-dimensional derivatives and variants, have attracted increasing attention for their potential diverse optoelectronic applications like lighting, display, radiation detection, and sensing. Despite great success in experimental discovery of many efficient STE emitters, the current understanding of the STE emission mechanism in metal halides is still immature, and often controversial, which calls for help urgently from predictive first-principles theoretical calculation. Although density-functional theory (DFT) based calculations are routinely used to provide electronic band structure of materials and have contributed greatly to qualitative analysis of luminescence mechanism, more in-depth and quantitative information is highly needed to provide guidelines for rational design of new luminescent materials with desirable features. However, due to the complicated nature of STE emission, involving in particular electron–phonon coupling in both ground and excited states, the usage of DFT is no longer a routine job as for ground state properties. While more sophisticated methods formulated in the framework of many-body perturbation theory like GW-Bethe–Salpeter equation are available and provide theoretically rigorous and accurate description of electronic transitions in extended systems, their application to real STE systems is still severely limited due to highly demanding computational cost. In practice, approximated DFT methods are employed, which have their own strengths and limitations. In this review, we focus on the theoretical approaches that have been heavily used in interpreting STE luminescence mechanism, with a particular emphasis on theoretical methods for exciton self-trapping structural optimization. It is hoped that this review, by summarizing the current status and limitations of theoretical research in the STE emission, will motivate more methodological development efforts in this important field, and push forward the frontiers of excited state electronic structure theory of materials in general.

Enhancement of the spectral broadening efficiency for circular polarization states in the high absorption regime for Gaussian and doughnut-shaped beams in fused silica.

We experimentally investigate the spectral broadening in fused silica in the multiphoton absorption regime. Under standard conditions of laser irradiation, linear polarization of laser pulses is more advantageous for supercontinuum generation. However, with high non-linear absorption, we observe more efficient spectral broadening for circular polarizations for both Gaussian and doughnut-shaped beams. The multiphoton absorption in fused silica is studied by measuring the total transmission of laser pulses and by the intensity dependence of the self-trapped exciton luminescence observation. The strong polarization dependence of multiphoton transitions fundamentally affects the broadening of the spectrum in solids.

Seven-photon absorption from Na+/Bi3+-alloyed Cs2AgInCl6 perovskites.

Nonlinear multi-phonon (2-7) absorption in the Na+/Bi3+-alloyed Cs2AgInCl6 lead-free double perovskites with ∼100% photoluminescence quantum yield and superior stability is observed for the first time, which can be pumped by a femtosecond laser in a wide spectral range (800-2600 nm). First-principles calculations verify that the parity-forbidden transition from the valence band maximum and conduction band minimum (at the Γ point) is not broken by Na+/Bi3+ doping, and strong optical band-to-band absorption occurs at the L&X points. Time-resolved emission spectra evidence that single-photon and multi-photon pumping leads to the same self-trapped exciton transition and high-order nonlinear absorption will not induce a remarkable thermal effect. Finally, we demonstrate that the Cs2Na0.4Ag0.6In0.99Bi0.01Cl6 DP shows great potential for next-generation wavelength-selective and highly sensitive multiphoton imaging applications.

Effect of point defects on the STE luminescence of CaF2 single crystals

The study is devoted to the radiation defects created in the calcium fluoride single crystals with reactor irradiation to the fluence of 3.7 × 1017 thermal and 4.6 × 1017 fast (En > 1 MeV) neutrons per cm2, and their effect on the luminescence of the crystals. UV–VIS absorption spectroscopy revealed the formation of point defects, mainly F and F2 centers. From the comparison of the annealing behavior of the absorption and the ion-beam-induced luminescence (IBIL), we conclude that the color centers are responsible for a non-radiation decay of self-trapped excitons (STEs) and thus cause the luminescence quenching. In addition, the Raman analysis revealed a disorder of about 44% and a stress formation in the material after the neutron irradiation. Annealing at 100 °C reduces the disorder to 13% and eliminates the stress.

Thermal quenching of self-trapped exciton luminescence in nanostructured hafnia

The intrinsic optical properties and peculiarities of the energy structure of hafnium dioxide largely determine the prospects for applying the latter in new generation devices of optoelectronics and nanoelectronics. In this work, we have studied the diffuse reflectance spectra at room temperature for a nanostructured powder of nominally pure HfO2 with a monoclinic crystal structure and, as well its photoluminescence in the temperature range of 40–300 K. We have also estimated the bandgap Eg under the assumption made for indirect (5.31 eV) and direct (5.61 eV) allowed transitions. We have detected emission with a 4.2 eV maximum at T < 200 K and conducted an analysis of the experimental dependencies to evaluate the activation energies of thermal quenching (140 meV) and enhancement (3 meV) processes. Accounting for both the temperature behavior of the spectral characteristics and the estimation of the Huang-Rhys factor S » 1 has shown that radiative decay of self-trapped excitons forms the mechanism of the indicated emission. In this case, the localization is mainly due to the interaction of holes with active vibrational modes of oxygen atoms in non-equivalent (O3f and O4f) crystal positions. Thorough study of the discussed excitonic effects can advance development of hafnia-based structures with a controlled optical response.

Self-trapped Exciton Luminescence and Light-emitting-diodes Based on Zero-dimensional Organic-inorganic Hybrid Antimony Chloride

Self-trapped Exciton Luminescence and Light-emitting-diodes Based on Zero-dimensional Organic-inorganic Hybrid Antimony Chloride

The features of deformation-stimulated RbI luminescence

This paper studies deformation-stimulated features of radiative relaxation of self-trapped excitons and recombination assembly of exciton-like luminescence in RbI crystal. Methods of research were luminescence and thermal activation spectroscopy. The identity of the mechanism of manifestation of the X-ray luminescence, tunnel luminescence and thermally stimulated luminescence spectra were found in the elastically deformed RbI crystal, interpreted by the luminescence of self-trapped exciton, tunnel recharge of F′, VK -pairs and thermally stimulated recombination of e−, VK -centres, respectively.The temperatures of the maximum destruction peaks of thermally stimulated luminescence, their spectral composition and activation energies were determined experimentally, on the basis of which the mechanisms of recombination assembly of exciton-like luminescences in a RbI crystal were interpreted. Uniaxial elastic deformation leads to the effective formation of point radiation defects ( F′, HA, VK -centers) in comparison with an unbroken lattice, where the predominant mechanism is the association of interstitial atoms ( H -centres) with the formation of I3−-centres.

Effect of Low-Temperature Deformation on the Ex Luminescence of KI Single Crystals

Simultaneous enhancement of the π- and σ-emissions of self-trapped excitons as well as the so-called E x-luminescence (maxima at ∼3.30, 4.15 and 3.04 eV, respectively) has been revealed in the X-ray luminescence spectra of KI single crystals additionally exposed to elastic uniaxial stress at 90 K. The intensity of all these luminescence bands linearly increases with relative degree of deformation up to ε = 1.0%, while the emissions undergo saturation at higher ε values (above elasticity limit). The similarity of the I = f(ε) dependences for intrinsic luminescence of self-trapped excitons and the E x emission allows to consider the latter also as the luminescence of intrinsic nature, tentatively connected with the radiative decay of self-trapping anion excitons with rather different configuration (with respect to these responsible for the π and σ components).

Water-induced crystal phase transformation of stable lead-free Cu-based perovskite nanocrystals prepared by one-pot method

Cu-based perovskite Nanocrystals (NCs) have a bright future in the field of optoelectronics due to their low toxicity, remarkable optical properties and good environmental stability. However, the industrial application is hindered by complex preparation methods. Herein, a facile one-pot method for synthesizing Cs3Cu2X5 NCs is proposed. The target Cu-based perovskite NCs were qualified with preferable optical performance, including near-unity PLQY (X = Cl and I), adjustable spectrum from 441 to 556 nm and remarkable environmental stability (retained more than 50% Photoluminescence (PL) intensity for 2 months in air). Density functional theory (DFT) calculations, Time-resolved PL (TRPL) and temperature-dependent PL spectrum were implemented to explore the Self-trapped excitons (STEs) luminescence mechanism and the roots for remarkable optical performance. In addition, crystal phase transformation was observed in water post-treatment from blue light-emitting Cs3Cu2I5 NCs to yellow light-emitting CsCu2I3 NCs, relative characterizations were implemented and a plausible reaction mechanism was proposed. Resultantly, a White light-emitting diode (W-LED) constructed by mixing these two phases was endowed with pure white light, which possessed the Commission Internationale de L'Eclairage (CIE) coordinate of (0.32, 0.34), color temperature of 6053 K, color Rendering index (Ra) 0f 91 and maximum luminance of 1558 cd/m2. The contribution indicates that Cu-based perovskite NCs prepared by one-pot method have great potential for luminescent applications.