Self-trapped Exciton Emission Research Articles

Structural and photophysical properties of two-dimensional lead bromide hybrid perovskite (C8H17NH3)2PbBr4

The structural versatility of organic compounds imparts manifold properties to hybrid organic−inorganic perovskites, where two-dimensional (2D) halide perovskites are potential future platforms for light-emitting devices because of their unique quantum well configuration and dielectric confinement. Notwithstanding the numerous studies on the organic−inorganic perovskites carried out to date, precisely charactering the structure and photoluminescence mechanism remain considerable challenges. Herein, by applying Rietveld refinement, we have obtained crystal structure and atomic sites of 2D layered OA2PbBr4 perovskite, which undergoes a phase transition at 190 K with activation energies of 29.6 and 3.8 meV for the two phases, respectively. The theoretical analysis based on the crystal structure were performed to understand the nature of vibrations and band structure. Steady-state PL shows a narrow emission and a broad emission located at 409 and 487 nm, respectively. With temperature cooling down to 118 K, narrow emission gradually narrowed and enhanced. Meanwhile, intensity of broad emission first weakens and then increases. The turning point occurs near 198 K. By applying time-resolved PL (TRPL) and transient absorption (TA) spectra experiment, we observed a rapid carrier transfer from the FE to the STE state, and demonstrated the narrow and broad bands originate from free exciton and self-trapped exciton emission, respectively. And the self-trapping depth is proved to be 26.3 meV by low-temperature PL. Our findings indicate the synthesized 2D perovskite has potential applications in optoelectronic and display devices.

Glass Disorder Modulated Luminescence in Zero-Dimensional Antimony-Chloride Coplanar Dimers for Optical Anti-counterfeiting.

Zero-dimensional metal halides have received wide attention due to their structural diversity, strong quantum confinement, and associated excellent photoluminescence properties. A reversible and tunable luminescence would be desirable for applications such as anti-counterfeiting, information encryption, and artificial intelligence. Yet, these materials are underexplored, with little known about their luminescence tuning mechanisms. Here we report a pyramidal coplanar dimer, (TBA)Sb2Cl7 (TBA = tetrabutylammonium), showing broadband emission wavelength tuning (585-650 nm) by simple thermal treatment. We attribute the broad color change to structural disorder induced by varying the heat treatment temperatures. Increasing the heating temperature transitions the material from long-range ordered crystalline phase to highly disordered glassy phase. The latter exhibits stronger electron-phonon coupling, enhancing the self-trapped exciton emission efficiency. The work provides a new material platform for manifold optical anti-counterfeiting applications and sheds light on the emission color tuning mechanisms for further design of stimuli-responsive materials.

Ultrafast Dynamics of Self-Trapped Excitons in Cs2AgInCl6:Al3+ Double Perovskite Nanocrystals.

It is a huge challenge to increase the photoluminescence (PL) of lead-free halide perovskites, and understanding the mechanism behind exciton dynamics can provide a valuable solution. Herein, we achieved enhanced broad-band emission at ambient conditions in Cs2AgInCl6 by tuning self-trapped excitons (STEs) through Al3+ doping. Cryogenic measurements showed an inhomogeneous nature of STE emission due to the presence of defect states and is subject to thermal quenching. An increased Huang-Rhys factor (S-factor) resulted in better electron-phonon coupling and high-density STE states post Al3+ doping. Femtosecond transient absorption (fs-TA) results provided insights into the distribution dynamics of excitons, which occurs through gradient energy levels from free excitons (FE) to STEs, where each STE state potentially possesses higher quantized energy states. Overall, this study aims to comprehend the origins of self-trapping and decay of STEs in Cs2AgInCl6:Al3+ and emphasizes the potential of compositional engineering to mitigate self-trapping in this material.

Self-Trapped Exciton Emission in Highly Polar 0D Hybrid Ammonium/Hydronium-Based Perovskites Triggered by Antimony Doping.

Various monovalent cations are employed to construct metal halide perovskites with various structures and functionalities. However, perovskites based on highly polar A-site cations have seldom been reported. Here, a novel hybrid 0D (NH4)x(OH3)3-xInCl6 perovskite with highly polar hydronium OH3+ cations is introduced in this study. Upon doping with Sb3+, hybrid 0D (NH4)x(OH3)3-xInCl6 single crystals exhibited highly efficient broadband yellowish-green (550 nm) and red (630 nm) dual emissions with a PLQY of 86%. The dual emission arises due to Sb3+ occupying two sites within the crystal lattice that possess different polarization environments, leading to distinct Stokes shift energies. The study revealed that lattice polarity plays a significant role in the self-trapped exciton emission of Sb3+-doped perovskites, contributing up to 25% of the Stokes shift energy for hybrid 0D (NH4)x(OH3)3-xInCl6:Sb3+ as a secondary source, in addition to the Jahn-Teller deformation. These findings highlight the potential of Sb3+-doped perovskites for achieving tunable broadband emission and underscore the importance of lattice polarity in determining the emission properties of perovskite materials.

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.

Temperature-dependent self-trapped exciton emission in Cu(I) doped zinc-based metal halides from well-resolved excited state structures

Temperature-dependent self-trapped exciton emission in Cu(I) doped zinc-based metal halides from well-resolved excited state structures

Tunable emission color from lead-free zero-dimensional Mn2+-doped Cs3InCl6 Single Crystals

In recent years, the magneto-optical properties of zero-dimensional metal halides have attracted widespread interest. In this study, we employed a vacuum solid-state reaction method to synthesize zero-dimensional, lead-free, and all-inorganic Cs3InCl6 single crystals (SCs). In addition to the blue emission of self-trapped excitons (STEs) inherent to pristine Cs3InCl6, the strategic doping with Mn2+ ions not only facilitates multicolor emissions but also significantly boosts the photoluminescence quantum yield (PLQY) from a modest 1.21% to an impressive 32.18%. We carried out an exhaustive investigation into the fundamental mechanisms responsible for these multicolor emissions and the intricate compositional transitions that ensue. Furthermore, the pronounced correlation between temperature-dependent photoluminescence (PL) and magnetic characteristics unveiled the genesis of ferromagnetism within both the pristine and Cs3InCl6: xMn2+ samples. The doping-induced modifications in magnetic behavior furnish a coherent explanation for the anomalous inverse temperature dependence emission wavelength shift. Our research highlights the adjustable PL and magnetic attributes of Mn2+ doped Cs3InCl6, yielding invaluable insights that are poised to advance its application in the next wave of optoelectronic and magnetic technologies.

Effects of rare earth codoping on the optical and scintillation properties of Cs3Cu2I5:Tl single crystals

In this work, the effects of rare earth ion codoping on the optical and scintillation performance of Cs3Cu2I5:Tl single crystals were studied. Cs3Cu2I5:Tl single crystals codoped with different concentrations of Sc3+, Y3+, and La3+ ions were grown by the Bridgman method. It was found that rare earth ion codoping reduces the optical transmittance of the as-grown crystals, in particular the coloring caused by Sc3+codoping. The luminescence mechanism associated with self-trapped exciton (STE) and Tl-related emissions has not been affected by the introduction of rare earth ion. The gamma scintillation yield and energy resolution are degraded after rare earth ion codoping.

Considerable Piezochromism in All-Inorganic Zero-Dimensional Perovskite Nanocrystals via Pressure-Modulated Self-Trapped Exciton Emission.

Piezochromic materials refer to a class of matters that alter their photoluminescence (PL) colors in response to the external stimuli, which exhibit promising smart applications in anti-counterfeiting, optoelectronic memory and pressure-sensing. However, so far, most reported piezochromic materials have been confined to organic materials or hybrid materials containing organic moieties with limited piezochromic range of less than 100 nm in visible region. Here, we achieved an intriguing piezochromism in all-inorganic zero-dimensional (0D) Cs3Cu2Cl5 nanocrystals (NCs) with a considerable piezochromic range of 232 nm because of their unique inorganic rigid structure. The PL energy shifted from the lowest-energy red fluorescence (1.85 eV) to the highest-energy blue fluorescence (2.83 eV), covering almost the entire visible wavelength range. Pressure-modulated self-trapped exciton emission between different energy levels of self-trapped states within Cs3Cu2Cl5 NCs was the main reason for this piezochromism property. Note that the quenched emission, which is over five times more intense than that in the initial state, is retained under ambient conditions upon decompression. This work provides a promising pressure indicating material, particularly used in pressure stability monitoring for equipment working at extreme environments.

Efficient tunable white emission and multiple reversible photoluminescence switching in organic Tin(IV) chlorides via regulating the host lattice environment of antimony ions for multifunctional applications

The host lattice environments of Sb3+ has a great influence on its photophysical properties. Here, we synthesized three zero-dimensional organic metal halides of (TPA)2SbCl5 (1), Sb3+-doped (TPA)SnCl5(H2O)·2H2O (Sb3+-2), and Sb3+-doped (TPA)2SnCl6 (Sb3+-3). Compared with the intense orange emission of 1, Sb3+-3 has smaller lattice distortion, thus effectively suppressing the exciton transformation from singlet to triplet self-trapped exciton (STE) states, which makes Sb3+-3 has stronger singlet STE emission and further bring a white emission with a photoluminescence quantum efficiency (PLQE) of 93.4%. Conversely, the non-emission can be observed in Sb3+-2 even though it has a similar [SbCl5]2- structure to 1, which should be due to its indirect bandgap characteristics and the effective non-radiative relaxation caused by H2O in the lattice. Interestingly, the non-emission of Sb3+-2 can convert into the bright emission of Sb3+-3 under TPACl DMF solution treatment. Meanwhile, the white emission under 315 nm excitation of Sb3+-3 can change into orange emission upon 365 nm irradiation, and the luminescence can be further quenched by the treatment of HCl. Therefore, a triple-mode reversible luminescence switch of off-onI-onII-off can be achieved. Finally, we demonstrated the applications of Sb3+-doped compounds in single-component white light illumination, latent fingerprint detection, fluorescent anti-counterfeiting, and information encryption.

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.

Cs2NaGdCl6:Tb3+─A Highly Luminescent Rare-Earth Double Perovskite Scintillator for Low-Dose X-ray Detection and Imaging.

Rare-earth-based double perovskite (DP) X-ray scintillators have gained significant importance with low detection limits in medical imaging and radiation detection owing to their high light yield (LY) and remarkable spatial resolution. Herein, we report the synthesis of 3D double perovskite (DP) crystals, namely, Cs2NaGdCl6 and Tb3+-Cs2NaGdCl6 using hydrothermal reaction. Cs2NaGdCl6 DP single crystals exhibited a blue self-trapped exciton (STE) emission at 470 nm under ultraviolet (265 nm) excitation with a photoluminescence quantum yield (PLQY) of 8.4%. Introducing Tb3+ ions into Cs2NaGdCl6 has resulted in quenching of STE emission and enhancing green emission at 549 nm attributed to the 5D4 → 7F5 transition of Tb3+, suggesting efficient energy transfer (ET) from STE to Tb3+. This ET process is evidenced by the appearance of Tb3+ bands in the excitation spectra of the host, the shortening of the STE lifetimes in the presence of Tb3+ ions, and the enhancement of PLQY (72.6%). Furthermore, Cs2NaGdCl6:5%Tb3+ films of various thicknesses (0.1-0.6 mm) were synthesized and their X-ray scintillating performance has been examined. The Cs2NaGdCl6:5%Tb3+ film with 0.4 mm thickness has exhibited an excellent linear response to the X-ray dose rate with a low detection limit of 41.32 nGyair s-1, an LY of 39,100 photons MeV-1, and excellent radiation stability. Benefiting from the strong X-ray excited luminescence (XEL) of Cs2NaGdCl6:5%Tb3+, we developed a Cs2NaGdCl6:5%Tb3+ X-ray scintillator screen with a least thickness (0.1 mm), exhibiting remarkable imaging ability with a spatial resolution of 10.75 lp mm-1. These results suggest that Cs2NaGdCl6:Tb3+ can be a potential candidate for low-dose and X-ray imaging applications.

Boosting Blue Self-Trapped Exciton Emission in All-Inorganic Zero-Dimensional Metal Halide Cs2ZnCl4 via Zirconium (IV) Doping.

Low-dimensional metal halides with efficient luminescence properties have received widespread attention recently. However, nontoxic and stable low-dimensional metal halides with efficient blue emission are rarely reported. We used a solvothermal synthesis method to synthesize tetravalent zirconium ion-doped all-inorganic zero-dimensional Cs2ZnCl4 for the first time. Bright blue emission in the range of 370 nm-700 nm with a emission maximum at 456 nm was observed in Zr4+:Cs2ZnCl4 accompanied by a large Stokes shift, which was due to self-trapped excitons (STEs) caused by the lattice vibrations of the twisted structure. Simultaneously, the PLQY of Zr4+:Cs2ZnCl4 achieve an impressive 89.67%, positioning it as a compelling contender for future applications in blue-light technology.

High‐Performance White Emission from Cu‐Based Perovskite Compound

AbstractThe white emissions based on lead‐free perovskites with broadband emission due to self‐trapped excitons (STEs) are considered as an ideal candidate for next‐generation white light‐emitting diodes (WLEDs). Recently, copper‐based halide perovskites Cs3Cu2I5@CsCu2I3 composites films with a high color‐rendering index (CRI) have been developed for WLEDs. However, the Cs3Cu2I5@CsCu2I3 composites generally exhibit a relatively low photoluminescence quantum yield (PLQY). Herein, a high PLQY up to 86.75% copper‐based perovskite Gua3Cu2I5 (Gua = guanidine) films with a broadband strong yellow STEs emission centered at 544 nm is synthesized. By combining with efficient blue STEs emission Cs3Cu2I5 component via composition engineering of Cs3Cu2I5@Gua3Cu2I5, a high PLQY of up to 96.1% is achieved on the white emission composites films. Moreover, a high CRI of 93 WLEDs is achieved by integrating Cs3Cu2I5@Gua3Cu2I5 composites film onto a UV LED chip. These findings provide a simple and effective strategy for designing highly efficient white light‐emitting diodes.

Tuneable Efficient White Emission of Sb3+/Mn2+ Co-Doped Lead-Free Perovskites for Single-Component White Light-Emitting Diodes.

Inorganic lead-free perovskite nanocrystals (NCs) with broadband self-trapped exciton (STEs) emission and low toxicity have shown enormous application prospects in the field of display and lighting. However, white light-emitting diodes (WLEDs) based on a single-component material with high photoluminescence quantum yield (PLQY) remain challenging. Here, we demonstrate a novel codoping strategy by introducing Sb3+/Mn2+ ions to achieve the tuneable dual emission in lead-free perovskite Cs3InCl6 NCs. The PLQY increases to 59.64% after doping with Sb3+. The codoped Cs3InCl6 NCs exhibit efficient white light emission due to the energy transfer channel from STEs to Mn2+ ions with PLQY of 51.38%. Density functional theory (DFT) calculations have been used to verify deeply the effects of Sb3+/Mn2+ doping. WLEDs based on Sb3+/Mn2+-codoped Cs3InCl6 NCs are explored with color rendering index of 85.5 and color coordinate of (0.398, 0.445), which have been successfully applied as photodetector lighting sources. This work provides a new perspective for designing novel lead-free perovskites to achieve single-component WLEDs.

Efficient and tunable white light emitting from Sb3+, Tb3+, and Sm3+ Co-doped Cs2NaInCl6 double perovskite via multiple energy transfer processes

Inorganic lead-free double perovskites have the advantages of low toxicity, broadband emission, and good stability, which make them promising luminescent materials for lighting applications. However, due to the limited regulation of their self-trapped exciton emission, it is still greatly challenging to achieve white light emitting from a single double perovskite host. Herein, efficient and tunable white light is realized in Cs2NaInCl6: Sb3+, Tb3+, Sm3+ double perovskite by controlling the ratios of the doped three ions with blue, green, and red emissions, respectively. The steady-state and transient fluorescence spectra of singly- and doubly-doped double perovskites reveal the existence of multiple energy transfer channels in the triply-doped phosphors, including from Sb3+ to Tb3+, Sb3+ to Sm3+, and Tb3+ to Sm3+. Benefiting from these channels, the color coordinates of the triply-doped phosphors can cross the whole white light area of the CIE chromaticity diagram by adjusting the ratios of the three dopants, and the maximum internal quantum yield of the white light phosphors is 66.61%. The white emission phosphors show the characteristic of being independent of excitation wavelength within 310–360 nm. Furthermore, the emission intensity at 430 K of the white light phosphor Cs2NaInCl6: 0.01Sb3+, 0.65Tb3+, 0.20Sm3+ remains 50% of that at room temperature. A WLED device fabricated with the phosphor and a 365 nm LED chip exhibits a high color rendering index of 90.9, correlated color temperature of 5469 K, and CIE coordinates of (0.333 and 0.328). The results indicate that the as-prepared double perovskite materials are promising candidates in the solid-state lighting field.

Pressure-triggered intense natural white emission via controllable distortions of antimony chloride dimers

Zero-dimensional (0D) hybrid metal halides (HMHs), featured with unique self-trapped exciton (STE) emissions, are emerging photoluminescence (PL) materials for photoelectronic applications. Compared with metal-halogen monomers, dimeric units in 0D HMHs possess more structural flexibility and less spatial confinement of excitons, creating new opportunities for PL engineering. Herein, high pressure is employed on 0D (C3H12N2)2Sb2Cl10 to controllably distort inter- and intra-octahedral structures within [Sb2Cl10]4− dimers. Intense natural white emission is achieved with dramatically increased photoluminescence quantum yield from less than ≈1% to 74.2 %. The high-pressure emission of (C3H12N2)2Sb2Cl10 is proven to originate from the STE recombination from triplet states with varied emission equilibrium, as well as the promoted excitonic trapping with restrained nonradiative recombination. This work offers valuable insights into the structure effects on STE emission and the transition mechanisms of metal-halogen dimers, which are crucial for developing novel metal halides for illumination.

Self-Trapped Excitons in Metal-Halide Perovskites Investigated by Time-Dependent Density Functional Theory.

We present a theoretical study of the formation of self-trapped excitons (STEs) and the associated broadband emission in metal-halide perovskites Cs4SnBr6 and Cs2AgInCl6, using time-dependent density functional theory (TDDFT) with the dielectric-dependent hybrid (DDH) functional. Our approach allows for an accurate description of the excitonic effect and geometry relaxation in the electronic excited states and yields optical gap, STE emission energy, and emission spectra in reasonable agreement with experiments. We point out the significance of considering geometry relaxations in the electronic excited state by showing that the exciton-phonon coupling computed in the ground-state atomic geometry is insufficient to describe the physical properties of STEs. Overall, we find that TDDFT with the DDH hybrid functional is a suitable approach for the study of the formation of STEs in perovskite and provides insights for designing metal-halide perovskites with tailored emission properties.

The effect of solvent on the formation of low-dimensional metal halides and their self-trapped exciton emission

Low-dimensional organic–inorganic hybrid metal halides, with broadband luminescence, have attracted much attention for optoelectronic applications due to their rich in structural diversity and solution processibility. However, it is still unclear about how the solvent molecules influence the optoelectronic properties of the solution processed low-dimensional metal halides. Here, we prepared five different antimony-based crystal structures, [SbCl6]3- as the metal halide octahedron and 4, 4-difluoropiperidine (DFPD+) as organic cation, by using different solvents: hydrochloric acid (HCl) aqueous solution and four organic solvents (dimethylformamide (DMF), methanol (MeOH), acetonitrile (ACN) and dimethylacetamide (DMAC)). We revealed the relation between their crystal structures and optical properties, and we found the participation of organic molecules in the crystal structure causes significant lattice distortions, which is beneficial for achieving self-trapped exciton (STE) emission. Among them, (DFPD)6SbCl9·2DMAC exhibits a remarkable photoluminescence (PL) quantum yield of approximately 90 %. The STE dynamics in (DFPD)6SbCl9·2DMAC were characterized by femtosecond transient absorption and time-resolved PL spectroscopies. Simultaneously, this study also provides new directions for expanding the application of low-dimensional luminescent metal halides: in addition to UV-LEDs, efficient and rapid detection of methanol or acetonitrile can be achieved using the raw materials, while also realizing the potential application of multi-level optical anti-counterfeiting.

Highly emissive 0D Tin(II) hybrid for white LED

The pursuit of environmentally friendly and highly luminescent phosphors for down-conversion white light-emitting diodes (WLEDs) remains a significant focus of research. Here, we present a novel zero-dimensional Sn(II) hybrid exhibiting vibrant orange emission peaking at 598 nm and an outstanding photoluminescence quantum yield of 69%. This hybrid, designated as (C7H8N3)4SnBr6, was synthesized via the reaction of 2-aminobenzimidazole with SnBr2, revealing distinctive self-trapped exciton emission characteristics, contributing to its broad-band emission and prolonged lifetime. Subsequently, a down-conversion WLED was constructed by integrating the Sn(II) hybrid with a green phosphor ((Sr, Ba)2SiO4:Eu2+) and a blue phosphor (BaMgAl10O17:Eu2+), resulting in CIE coordinates of (0.313, 0.331), a correlated color temperature of 6466 K, and an impressive color rendering index of 90.6. This work enriches the family of lead-free hybrid phosphors, showcasing their potential for highly efficient white LED applications.