Self-trapped Exciton Emission Research Articles

Direct Electron Transfer Enables Highly Efficient Dual Emission Modes of Mn2+-Doped Cs2Na1-xAgxBiCl6 Double Perovskites.

Double perovskites with bright emission, low toxicity, and excellent stability have drawn considerable attention. Herein, we report the hydrothermal synthesis of Mn2+-doped Cs2Na1-xAgxBiCl6 double perovskites that exhibit dual emission modes. Introducing Ag+ ions to Cs2NaBiCl6 samples enables a bright self-trapped exciton (STE) emission in orange-red color, whereas Mn2+ dopants induce a yellow-orange emission. Importantly, Mn2+ doping into Cs2Na1-xAgxBiCl6 double perovskites with an indirect bandgap enables a high photoluminescence quantum yield of 49.52 ± 2%. Density functional theory calculations reveal that bringing Ag+ ions into Cs2NaBiCl6 can localize wave function to the [AgCl6]5- octahedron and convert dark transitions to bright STE transitions. Moreover, the 3d orbitals of Mn2+ dopants hybridize with Bi-6p and Cl-3p orbitals at the conduction band minimum, resulting in direct electron transfer from the host to Mn2+ and a significant increase in photoluminescence efficiency. These results shed light on the optical physical process of Mn2+-doped systems, providing useful information for further improvement of the photoluminescence efficiency of double perovskites.

Luminescence from Self‐Trapped Excitons and Energy Transfers in Vacancy‐Ordered Hexagonal Halide Perovskite Cs2HfF6 Doped with Rare Earths for Radiation Detection (Advanced Optical Materials 19/2022)

Cs2HfX6 halides show a vacancy-ordered cubic double perovskite structure when X = Cl, Br, and I. As to the first member of the halide family, Zhiping Luo and co-workers (article number 2201374) identify Cs2HfF6 as a new type of vacancy-ordered hexagonal perovskite. It is composed of single layers of building blocks. Tunable emission is obtained based on self-trapped exciton emissions from the host and energy transfers in the rare-earth-doped material. From energy-dependent cathodoluminescence and time-resolved X-ray radioluminescence, the co-doped material is found to be more sensitive to radiation energy than the singly-doped or pristine host.

Zero-dimensional rubidium zinc halide blue-emitting quantum dots for X-ray imaging

Low-dimensional-networked metal halide (MH) materials are emerged as a new class of semiconductors owing to their unique structure and outstanding photoelectric properties. Compared with the extensively studied three-dimensional (3D) MHs, the zero-dimensional (0D) MHs are of great interest to be widely studied. Here we report the successful colloidal synthesis of all-inorganic lead-free 0D MHs, including undoped and Cu-doped Rb2ZnCl4 quantum dots (QDs) with bright broad blue and cyan emissions, respectively. Combining the theoretical and experimental study, the band structures are well characterized and the originations of the blue and cyan emissions are attributed to the singlet and triplet self-trapped exciton (STE) emissions, respectively. Finally, owing to the bright radioluminescence (RL), the Cu-doped Rb2ZnCl4 QDs is prepared into a thin film to demonstrate the feasibility for X-ray imaging application. This work not only provides a cyan-emitting all-inorganic 0D MH QDs, but also exploits it as a scintillator for X-ray imaging.

Efficient and tunable photoluminescence for terbium-doped rare-earth-based Cs2NaYCl6 double perovskite.

Lead-free double perovskite materials with efficient and stable self-trapped exciton (STE) emissions show enormous potential for next-generation solid-state lighting. However, the low-emission efficiency and difficulty of spectral regulation are two major obstacles to their application. Here, all-inorganic rare-earth-based double perovskite Cs2NaYCl6 single crystals with strong blue emissions were reported as effective hosts to accommodate lanthanide ion doping. By controlling the introduction of Tb3 + ions and efficient energy transfer from the STEs to the dopants, the emission color of Cs2NaYCl6 single crystals was flexibly modulated from blue to green. The quantum yields were also significantly improved from 10% to 78.81% by optimizing the Tb3 + ion concentration. Further, stable light-emitting diode prototypes based on Cs2NaYCl6 color conversion materials were fabricated to demonstrate the practical applications of rare-earth-based double perovskite.

High Quantum Efficiency of Stable Sb‐Based Perovskite‐Like Halides toward White Light Emission and Flexible X‐Ray Imaging

AbstractAlthough low‐dimensional hybrid metal halides have emerged as promising light emitters due to their broadband emission originated from self‐trapped exciton (STE), the synthesis of lead‐free halides with high photoluminescence quantum yield remains a challenge. Herein, a series of new luminescent single crystals of antimony‐based metal halides [Na(DMSO)2]3SbBr6−xClx (x = 0, 2, 3, 4, 6) is synthesized. By optimizing the ratio of Br and Cl, [Na(DMSO)2]3SbBr3Cl3 exhibits an ultra‐broadband yellow light emission peaking at 606 nm with a full width at half‐maximum of 160 nm and a high photoluminescence quantum yield of 90.6%. Both experimental characterizations and theoretical calculations indicate that STE emission is responsible for the ultra‐broadband yellow light emission. Furthermore, blue LED chip‐pumped white light‐emitting diodes by using the yellow‐emitting [Na(DMSO)2]3SbBr3Cl3 are demonstrated, showing a correlated color temperature of 3948 K and a color rendering index of 85. Finally, a large‐size and flexible [Na(DMSO)2]3SbBr3Cl3 scintillator used for X‐ray imaging is prepared by a template assembled method, exhibiting a high spatial resolution of 15.5 lp mm−1 and a low detection limit of 285.8 nGyair s−1. This work highlights the potential of Sb‐based halides in solid‐state illumination and X‐ray imaging.

Boosting the Self-Trapped Exciton Emission in Cs2NaYCl6 Double Perovskite Single Crystals and Nanocrystals.

Halide double perovskites have aroused substantial research interest because of their unique optical properties and intriguing flexibility for various compositional adjustments. Herein, we report the synthesis and photophysics of rare-earth element yttrium (Y)-based double perovskite single crystals (SCs) and nanocrystals (NCs). The pristine Cs2NaYCl6 bulk SCs exhibit a weak sky-blue emission with a low photoluminescence quantum yield (PLQY) of 7.68% based on the self-trapped exciton (STE), while no PL emission was observed for NCs. Excitingly, the STE emission of SCs and NCs is greatly enhanced via Sb3+ doping. The optimized Cs2NaYCl6:Sb3+ SCs and NCs exhibit high PLQYs up to 82.5% and 51.8%, respectively. Theoretical calculations and charge-carrier dynamic studies demonstrate that the giant emission enhancement after Sb3+ doping is related with the enhancement of the sensitization of the emissive STE states, the passivating of the nonradiative carrier trapping processes, and the regulation of carrier-phonon coupling.

Organic Cation-Directed Modulation of Emissions in Zero-Dimensional Hybrid Tin Bromides.

Zero-dimensional hybrid metal halides (0D HMHs) are attractive due to their intriguing self-trapped exciton (STE) emission properties. However, the effect of organic cations on the emission of 0D HMHs is relatively underexplored. Herein, we report two types of 0D hybrid tin bromides, (BMe)2SnBr6 (BMe = C8N2H18) and (MeH)2SnBr6 (MeH = C7N2H16), which share similar structural features with different hydrogen bonding (HB) interactions between [SnBr6]4- anions and organic cations. The (BMe)2SnBr6 with weak HB interactions exhibits only STE emission, while the (MeH)2SnBr6 exhibits both STE and charge transfer exciton emissions owing to the strong HB interactions, resulting in an excitation-dependent emission at cryogenic conditions. Detailed structural analyses and Hirshfeld surface calculations confirm that the enhanced HB interactions are essential to obtain the multiple emissions in (MeH)2SnBr6.

Chiral 2D Cu(I) Halide Frameworks

Hybrid multifunctional materials have great potential in a wide variety of applications due to their flexible combination of organic and inorganic components. Introducing chiral organic modules into the metal halide frameworks can effectively generate multifunctional materials, achieving new functionalities with noncentrosymmetric structures. Here, by incorporating (R)- or (S)-piperidine-3-carboxylic acid (R/S-PCA) as the templating cation, we report the synthesis and characterization of three pairs of new 2D chiral hybrid Cu(I) halides, namely, (R/S-PCA)CuBr2, (R/S-PCA)CuBr2·0.5H2O, and (R/S-PCA)CuI2. These chiral Cu(I) halides crystallize in the noncentrosymmetric space group C2 and belong to a new structural type similar to layered silicates. The optical absorption edges of these chiral materials can be tuned by changing the halide or upon the absorption of water and range from 2.70 to 3.66 eV. A dynamic conversion between (R/S-PCA)CuBr2 and (R/S-PCA)CuBr2·0.5H2O occurs through exposure to moisture or vacuum drying along with changes in the reversible bandgap and photoluminescence. Chiroptical properties such as circular dichroism, circular polarized light emission, and second harmonic generation are investigated. Density functional theory calculations (DFT) show the indirect and direct bandgap natures of these Cu(I) halides and reveal the mechanism for the broadband self-trapped exciton emission at the excited state. The fascinating structural type, chiroptical properties, and reversible hydrochromic behavior of these Cu(I)-based halides make them viable candidates for next-generation multifunctional optoelectronic materials.

Efficient and self-healing copper halide Cs3Cu2Cl5 film assisted by microwave treatment for high-resolution X-ray imaging

Lead-free Cs3Cu2Cl5 metal halide exhibits high photoluminescence quantum yield (PLQY), large Stokes shifts and small self-absorption due to self-trapped excitons (STE) emission, which undoubtedly is a potential scintillator material. However, Cs3Cu2Cl5 nanocrystals (NCs) are susceptible to degradation by moisture at high humidity environment, which severely affects its long-term application in X-ray imaging. Herein, we prepared Cs3Cu2Cl5 scintillator films with a PLQY of 72.4% and a high spatial resolution of 20 lp mm−1. Moreover, we firstly propose a self-healing strategy with facile microwave treatment to recover the crystal structure of Cs3Cu2Cl5 film which damaged by moisture, while the photoluminescence (PL) and radioluminescence (RL) intensities of the damaged Cs3Cu2Cl5 film are repaired to more than 95% and the spatial resolution can be restored to 20 lp mm−1. Furtherly, the imaging quality of the self-healing scintillator film can be maintained stably by cyclic microwave treatment. It found that microwave treatment provides a simultaneous and molecular level heating environment for the Cs3Cu2Cl5 film, which drives the hydrolysates to re-crystallize along with the surface of the destroyed crystals. Therefore, microwave treatment opens a new avenue to recover Cs3Cu2Cl5 films degraded by moisture, and provides a solution for long-term application of scintillator films at low cost.

Competitive Dual-Emission-Induced Thermochromic Luminescence in Organic-Metal Halides.

Lead-free Halides, especially Mn-based ones, are preferred as hotspots in the exploration of photoluminescent materials. However, there are few reports on sensitive reversible thermochromism and switchable dual emission originating from self-trapped exciton emission in pure Mn-Based materials. Here, we report a new Mn-based hybrid material [TMPA]2MnI4 (TMPA = trimethylphenylammonium), which shows two emission peaks at 545 and 660 nm benefitting from the d-d orbital transition of Mn2+ and the generation of self-trapped excitons, respectively. Due to the different sensitivity to temperature, the stages of thermal activation and thermal quenching of the two emission types are also inconsistent, showing a certain competition relationship and dominating the emission colors in different temperature ranges, resulting in adjustable green-orange-green thermochromic luminescence from 100 to 403 K (both high and low temperatures correspond to green, and orange is displayed at near room temperature). Therefore, thermochromic luminescence can be easily achieved by controlling the temperature under the guidance of excited states. This work provides new insights into the synthesis and application of thermochromic materials. Therefore, it is certain that regulating temperature while being guided by excited states will achieve thermochromic luminescence. This research offers fresh perspectives on the development and potential of thermochromic materials.

Excitation-Dependent Luminescence of 0D ((CH3)4N)2ZrCl6 across the Full Visible Region.

Herein, we report a novel hybrid organic-inorganic Zr(IV) metal halide ((CH3)4N)2ZrCl6, which demonstrates fascinating excitation-dependent luminescence across the full visible region. The single crystal of ((CH3)4N)2ZrCl6 showed an unexpected high degree of symmetry and formed a unique 0D structure with isolated [ZrCl6]2- octahedrons. Amazingly, three different emission groups emerged under changeable excitation light. The first emission group peaked at 462 nm with a ns lifetime and a μs lifetime, which is assigned to free-exciton fluorescence and thermally activated delayed fluorescence (TADF). The second group of emissions featured elaborate multipeak light-emitting components, which is ascribed to the d-d transitions of Zr(IV). The third emission group centered at 660 nm was attributed to the typical self-trapped exciton (STE) emission. To our best knowledge, this work for the first time reports a 0D organic-inorganic metal halide with distinctive excitation-dependent full-visible-spectrum luminescence via four different emission mechanisms.

Site-selective luminescence of Eu3+ ions in silicate-tungstates with apatite and scheelite structures

The present work studies europium-ion spectroscopic features in solid solutions based on silicate-tungstates Ca2La6.8Eu1.2Si5.6W0.4O26.4 and Ca8Eu2Si3W3O26 microcrystalline powders with the crystal structure of silicate apatite and scheelite respectively. The spectroscopic features were studied by means of photoluminescence spectroscopy and X-ray excited luminescence at temperatures 4.6, 90 and 295 K. In Ca2La6.8Eu1.2Si5.6W0.4O26.4 only intensive luminescence was observed, which was characterized by a set of 5D0 → 7FJ dominant intraconfigurational transitions for Eu3+ ion. In Ca8Eu2Si3W3O26, both 5D0→ 7FJ intraconfigurational transitions for Eu3+ ion and wide 430 nm emission band corresponding to host self-trapped exciton (STE) emission are observed. This STE emission band is reabsorbed by the f – f absorption of Eu3+ ions by means of energy transfer from host to Eu3+. The asymmetry coefficient which characterizes the shape of the emission spectrum of Eu3+ ions strongly depends on the energy of exciting photons. The Eu3+ ion might occupy two nonequivalent crystallographic sites. Some features of this phenomenon were discussed.

Highly efficient blue emissive copper halide Cs5Cu3Cl6I2 scintillators for X-ray detection and imaging

In recent years, copper-based halide scintillators have attracted tremendous attention due to their excellent luminescent properties and low manufacturing costs. Herein, a high-performance and low-cost Cs5Cu3Cl6I2 scintillator is prepared by simple mechanical grinding. The mixed-halide Cs5Cu3Cl6I2 phosphor exhibits a blue emission centered at 475 nm with a large stokes shift, originating from the self-trapped exciton emission of Cu sites. The photoluminescence quantum yield (PLQY) of Cs5Cu3Cl6I2 can be increased to 88.4% from 20.4% by the efficient suppression of the anion vacancy defects. In addition, the Cs5Cu3Cl6I2 scintillator shows a high steady-state X-ray to light conversion efficiency of about 57000 photons/MeV, good X-ray linear response, and low detection limit of 71.9 nGyair/s. The X-ray imaging results based on the Cs5Cu3Cl6I2 scintillator screen display a high spatial resolution of 9.0 line-pairs per millimeter (lp/mm). Furthermore, Cs5Cu3Cl6I2 exhibits good radiation stability under a total dose of 113.58 Gyair. Overall, the Cs5Cu3Cl6I2 scintillator prepared by mechanical grinding method has the advantages of low cost, high efficiency, and good radiation resistance, suggesting its huge potential in X-ray detection and imaging.

Zero-Dimensional (Piperidinium)2MnBr4: Ring Puckering-Induced Isostructural Transition and Strong Electron-Phonon Coupling-Mediated Self-Trapped Exciton Emission.

We report on the synthesis, structure, and photophysical properties of a lead-free organic-inorganic hybrid halide, (Piperidinium)2MnBr4 (PipMBr). It crystallizes in a monoclinic P21/n structure, with isolated MnBr4 tetrahedra representing a zero-dimensional compound. It undergoes a reversible isostructural transition at 422/417 K in the heating/cooling cycle owing to the hydrogen-bonding rearrangement mediated by ring puckering of piperidinium cations. This compound exhibits green emission with a photoluminescence quantum yield of 51%. Interestingly, strong electron-longitudinal optical phonon coupling with γLO of 237 meV is evidenced from the broadening of the temperature-dependent emission linewidth and the Raman spectrum. Such strong electron-phonon coupling and a relatively low Debye temperature (137 K) suggest the self-trapped exciton emission in this compound.

Self‐Trapped Excitons in 2D SnP2S6 Crystal with Intrinsic Structural Distortion

AbstractExploring and utilizing novel materials with self‐trapped excitons (STEs) is highly desired due to their unique physical properties and multiple optoelectronic applications. Here, a 2D SnP2S6 crystal is reported with obvious STEs emission caused by distorted [SnS6]8‐ and [P2S6]4‐ octahedral units and strong electron‐phonon coupling. The STEs feature wide photoluminescence (PL) spectra range from 600 to 850 nm and a large Stokes redshift of ≈0.6 eV. Carrier dynamics measurements including temperature‐dependent PL and transient absorption reveal a large Huang–Rhys factor of ≈18.3 and a self‐trapping process of ≈101–103 ps in the 2D SnP2S6 crystal. Such self‐trapped states enable 2D SnP2S6 crystal a wide spectral response range and excellent photodetection performance. As a result, high responsivity (22.8 A W‐1) and detectivity (3.98 × 1010 Jones) are achieved under 365 nm light illumination. These results provide a deep insight into the photophysical process of STEs, which lays the foundation for developing novel STEs‐based materials and optoelectronic devices.

Efficient Broadband Near-Infrared Emission from Lead-Free Halide Double Perovskite Single Crystal.

Ultra-broadband near-infrared (NIR) luminescent materials are the most important component of NIR light-emitting devices (LED) and are crucial for their performance in sensing applications. A major challenge is to design novel NIR luminescent materials to replace the traditional Cr3+ -doped systems. We report an all-inorganic bismuth halide perovskite Cs2 AgBiCl6 single crystal that achieves efficient broadband NIR emission by introducing Na ions. Experiments and density functional theory (DFT) calculations show that the NIR emission originates from self-trapped excitons (STE) emission, which can be enhanced by weakening the strong coupling between electrons and phonons. The high photoluminescence quantum efficiency (PLQY) of 51 %, the extensive full width at half maximum (FWHM) of 270 nm and the stability provide advantages as a NIR luminescent material. The single-crystal-based NIR LED demonstrated its potential applications in NIR spectral detection as well as night vision.

Efficient Er3+ doped single-component Cs2Ag(Na)In(Bi)Cl6 phosphors for full-spectrum light-emitting diodes

Single-component phosphors with efficient and stable white-light emission are designed to overcome the trouble of various deteriorations and reabsorptions in the conventional mixing multiple-component phosphors for lighting applications. Here, Er3+ is introduced partially into the double perovskite Cs2Ag(Na)In(Bi)Cl6 lattice to tune white-light emission through adjusting its content. The as-synthesized material itself possesses strong tunable white-light emission under irradiation by 405 nm light due to the combination of self-trapped excitons emission and efficient characteristic emission of Er3+. Steady-state photoluminescence (PL) and time-resolved PL spectra are measured to characterize luminescence properties and clarify the luminescence mechanism. The PL quantum yield of undoped Cs2Ag(Na)In(Bi)Cl6 can achieve 64%. After doping 2 mmol Er3+, energy transfer efficiency from perovskite to Er3+ can reach 30.1%, with an improved PL quantum yield of 71%. The full-spectrum white-light emission can be realized by the Er3+ doped Cs2Ag(Na)In(Bi)Cl6 phosphors-converted 410–420 nm light-emitting diode. Commission International de I'Eclairage (CIE) color coordinates of white light can be improved from (0.47, 0.41) to (0.33, 0.35) with the increase of Er3+ content, and the corresponding color temperature is tuned from 2532 to 5760 K. These characteristics demonstrate that Er3+ doped Cs2Ag(Na)In(Bi)Cl6 phosphors are promising single-component white-light phosphors for next-generation lighting.

Crystal growth and scintillation properties of Tm3+ doped LaCl3 single crystal for radiation detection

Single crystal of Tm-doped LaCl3 (LaCl3:Tm3+) with the dimension of diameter 7 mm and length 55 mm was successfully grown using an in-house-developed Bridgman furnace. The grown crystal has the self-trapped exciton (STE) emission in the range of 300–500 nm along with the characteristic emissions corresponding to the 4f-4f transition of Tm3+ ions under X-ray excitation. The scintillation characterizations of the grown crystal were performed by pulse height spectra, scintillation decay time, as well as pulse shape discrimination studies under γ-ray and α-particle excitation sources. The LaCl3:Tm3+ crystal was found to have a high light yield (42,000 ± 4,200 ph/MeV) with 4.2% energy resolution under γ-rays excitation. Moreover, the pulse shape discrimination was studied using 137Cs and 241Am as the radiation sources with a good FOM value. The results conclude the LaCl3:Tm3+ single crystal to be a promising candidate for X-ray and γ-ray detection purposes.

A heterovalent doping strategy induced efficient cyan emission in Sb3+-doped CsCdCl3 perovskite microcrystal for solid state lighting

Metal halides can produce a broadband emission spectrum of self-trapped excitons (STEs) with a large stokes shift because they have strong electron-phonon coupling and soft lattice. In this study, CsCdCl3:xSb3+ perovskite microcrystals were prepared using a simple co-precipitation strategy at room-temperature. Their composition, morphology and phase structure were characterized by EDS, XPS, SEM and XRD. The result showed that the structure of the CsCdCl3:xSb3+ perovskite microcrystals was hexagonal. In terms of luminescence, the un-doped CsCdCl3 sample showed a quite broadband emission from the host STEs with a large Stokes shift of 340 nm. The PLE and PL of the CsCdCl3:0.007Sb3+ sample show two absorptions at the wavelength of 300 and 355 nm, which is because of the 1S0 → 1P1 and 1S0 → 3P1 transitions of Sb3+ ions and the enhanced cyan emission from STEs emission of [SbCl6] octahedra induced by a heterovalent doping strategy, respectively. In addition, in the PL spectra, there are two luminescence centers in the CsCdCl3:xSb3+ samples. The emission peak at 500 nm is higher than that at 600 nm, which can be attributed to the STEs of [SbCl6] and the two face-sharing octahedra [Sb2Cl9], respectively. The formation of STEs can be demonstrated by the large S value. The CsCdCl3:xSb3+ samples undergo two thermal processes, with an activation energy of 127 meV (Ea2) at a high temperature and 101 meV (Ea1) at a low temperature. Finally, they have good thermal and anti-water stability, and thus can be possibly applied in white LED with CIE color coordinates of 0.4119 and 0.3897, the CCT of 3227 K and CRI of 79.3.

Light Emission of Self-Trapped Excitons in Inorganic Metal Halides for Optoelectronic Applications.

Self-trapped excitons (STEs) have recently attracted tremendous interest due to their broadband emission, high photoluminescence quantum yield, and self-absorption-free properties, which enable a large range of optoelectronic applications such as lighting, displays, radiation detection, and special sensors. Unlike free excitons, the formation of STEs requires strong coupling between excited state excitons and the soft lattice in low electronic dimensional materials. The chemical and structural diversity of metal halides provides an ideal platform for developing efficient STE emission materials. Herein, an overview of recent progress on STE emission materials for optoelectronic applications is presented. The relationships between the fundamental emission mechanisms, chemical compositions, and device performances are systematically reviewed. On this basis, currently existing challenges and possible development opportunities in this field are presented.