Self-trapped Exciton Emission Research Articles

Electron–phonon coupling mediated self-trapped exciton emission in organic–inorganic hybrid manganese halides

Electron–phonon coupling mediated self-trapped exciton emission in organic–inorganic hybrid manganese halides

Green synthesis of Mn doped Cs3Cu2X5 (X=Cl, Br) powders with excellent photoluminescence and stability for anti-counterfeiting

Lead halide perovskite have drawn enormous attention recently for their excellent optoelectronic properties in many fields. However, the toxicity of Pb element and poor stability extremely hinder their practical applications. Herein, 0 D orthorhombic Cs3Cu2X5 powders (X = Cl, Br) are simply synthesized by a novel method, which using H2O as green solvent, CuX2 as the substitute of CuX. They exhibit broadband blue-green photoluminescence (PL) emissions of 468–530 nm with large Stokes shifts, which are attributed to their intrinsic self-trapped exciton (STE) emission characteristics. Moreover, Mn doped Cs3Cu2Cl5 powder present higher PLQY (78%) and longer average lifetime (108 μs), which also have an improved stability for moist air, UV light and high temperature. The obtained Cs3Cu2Br5 microrods show a low PLQY ∼20% and bad stability, but the faded photoluminescence can be recovered by further thermal treatment. Finally, a kind of green economic anti-counterfeiting ink is obtained, which would be activated by high temperature and deep-ultraviolet. The CIE coordinate of fabricated green emitting LED is (0.30, 0.53), which indicates they have a potential for future lighting application.

Boosting the Self-Trapped Exciton Emission in Cs4SnBr6 Zero-Dimensional Perovskite via Rapid Heat Treatment.

Zero-dimensional (0D) tin halide perovskites feature extraordinary properties, such as broadband emission, high photoluminescence quantum yield, and self-absorption-free characteristics. The innovation of synthesis approaches for high-quality 0D tin halide perovskites has facilitated the flourishing development of perovskite-based optoelectronic devices in recent years. However, discovering an effective strategy to further enhance their emission efficiency remains a considerable challenge. Herein, we report a unique strategy employing rapid heat treatment to attain efficient self-trapped exciton (STE) emission in Cs4SnBr6 zero-dimensional perovskite. Compared to the pristine Cs4SnBr6, rapid thermal treatment (RTT) at 200 °C for a duration of 120 s results in an augmented STE emission with the photoluminescence (PL) quantum yield rising from an initial 50.1% to a substantial 64.7%. Temperature-dependent PL spectra analysis, Raman spectra, and PL decay traces reveal that the PL improvement is attributed to the appropriate electron-phonon coupling as well as the increased binding energies of STEs induced by the RTT. Our findings open up a new avenue for efficient luminescent 0D tin-halide perovskites toward the development of efficient optoelectronic devices based on 0D perovskites.

Achieving Highly Efficient Warm-White Light Emission in All-Inorganic Copper-Silver Halides via Structural Regulation.

Single-component metal halides with white light emission are highly attractive for solid-state lighting applications, but it is still challenging to develop all-inorganic lead-free metal halides with high white-light emission efficiency. Herein, by rationally introducing silver (Ag) into zero-dimensional (0D) Cs3 Cu2 Br5 as new structural building unit, a one-dimensional (1D) bimetallic halide Cs6 Cu3 AgBr10 is designed that emits strong warm-white light with an impressive photoluminescence quantum yield (PLQY) of 94.5% and excellent stability. This structural transformation lowers the conduction band minimum while maintaining the localized nature of the valence band maximum, which is crucial in expanding the excitation spectrum and obtaining efficient self-trapped excitons (STEs) emission simultaneously. Detailed spectroscopy studies reveal that the white-light originates from triplet STEs emission, which can be remarkably improved by weakening the strong electron-phonon coupling and thus suppressing phonon-induced non-radiative processes. Moreover, the interesting temperature-dependent emission behavior, together with self-absorption-free property, make Cs6 Cu3 AgBr10 as sensitive luminescent thermometer and high-performance X-ray scintillator, respectively. These findings demonstrate a general approach to achieving effective single-component white-light emitters based on lead-free, all-inorganic metal halides, thereby opening up a new avenue to explore their versatile applications such as lighting, temperature detection and X-ray imaging.

A High-Rigidity Organic-Inorganic Metal Halide Hybrid Enabling Reversible and Enhanced Self-Trapped Exciton Emission under High Pressure.

Zero-dimensional organic-inorganic metal halide hybrids provide ideal bulk-crystal platforms for exploring the pressure engineering of electron-phonon coupling (EPC) and self-trapped exciton (STE) emission at the molecular level. However, the low stiffness of inorganic clusters hinders the reversible tuning of these physical properties. Herein, we designed a Sb3+-doped metal halide with a high emission yield (89.4%) and high bulk modulus (35 GPa) that enables reversible and enhanced STE emission (20-fold) under pressure. The high lattice rigidity originates from the corner-shared cage-structured inorganic tetramers and ring-shaped organic ligands. Further, we reveal that the pressure-enhanced emission regime below 4.5 GPa is owing to the lattice hardening and preferably EPC strength reducing, while the pressure-insensitive emission regime within 4.5-8.5 GPa results from the enhanced intercluster Coulombic attraction force that resists intracluster compression. These results provide insights into the structure-property relation and molecular engineering of zero-dimensional metal halides toward wide-band and pressure-sensitive light sources.

Doping-induced Ti3+ state and oxygen vacancies in TiO2: A single-chip combinatorial investigation

TiO2 is widely recognized as a high performance photocatalyst for photocatalytic applications. Oxygen vacancies (VO) and titanium defects are known to make a significant contribution to the enhanced photocatalytic efficiency of TiO2, accordingly control of these two centers is pivotal to optimizing the surface photochemical activity. In this work, the formation of VO and Ti3+ ions induced by n-type doping of TiO2 with Nb is demonstrated using a combinatorial investigation approach. Chemical Beam Vapour Deposition (CBVD) is used to fabricate Nb concentration-graded (NbxTi1-x)O2 anatase films with 0.009 ≤ x ≤ 0.033 over a 5 mm length on a single chip. Cathodoluminescence (CL) microanalysis shows the luminescence intensity decreases with increasing Nb concentration; however, the presence of a self-trapped exciton (STE) emission, characteristic of anatase TiO2, indicates that the Nb incorporation does not affect the structural properties of TiO2. Notably, the introduction of Nb donors induces the formation of the signature luminescence bands of VO and Ti3+ at 2.05 and 2.60 eV at 80 K, respectively, and their intensities rise by up to 28% with increasing Nb content. The formation of VO and Ti3+ is attributed to the effect of excess electrons locally produced by Nb donor doping. The thermal activation energies for the STE and defect-related emissions are found to be identical at 32 ± 4 meV, suggesting that the recombination kinetics is mediated by Nb donors. Significantly, we demonstrate that the near-surface VO density can be further increased by two different post-growth treatment approaches: (i) remote hydrogen plasma and (ii) low-energy electron beam irradiation (LEEBI), without effecting the valence state of Ti. Our results open opportunities to optimize the photocatalytic properties of TiO2 by defect engineering.

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.

White light emitting diodes based on lanthanide ions doped Cs2NaInCl6 double perovskites

Lead-free halide double perovskites are widely used for phosphors converted light emitting diode (pc-LED) devices. However, it remains a challenge to modulate their spectra to meet different application requirements. Herein, Cs2NaInCl6:Sb3+ double perovskite crystals with excellent blue emission of self-trapped excitons (STEs) were prepared and a lanthanide ions (Ln3+) doping strategy was employed to enable multicolor emissions in this system. The emission colors can be continuously adjusted in a wide range by altering the Ln3+ ions doping levels. Especially, white light emissions with tunable correlated color temperature were obtained in Tb3+, Ho3+ ions codoped samples. Outstanding photoluminescence quantum yields of the samples are ascribed to the sensitization effect of Sb3+ ions and the efficient energy-transfer from STEs to Ln3+ ions. LED devices were further fabricated to demonstrate the great application potentials of the prepared phosphors. These findings provide deep insights for the designing of multicolor double perovskite phosphors.

Excellent Switchable Properties, Broad-Band Emission, Ferroelectricity, and High Tc in a Two-Dimensional Hybrid Perovskite: (4,4-DCA)2PbBr4 Exploited by H/F Substitution.

Switchable materials have gained significant attention due to their potential applications in data storage, sensors, and switching devices. Two-dimensional (2D) hybrid perovskites have demonstrated promising prospects for designing switchable materials, where the dynamic motion of the organic components coupled with the distortion of the inorganic framework provides the driving force for triggering multifunctional switchable properties. Herein, through the H/F substitution strategy, we report a polar 2D hybrid lead-based perovskite, (4,4-DCA)2PbBr4 (4,4-DCA = 4,4-difluorocyclohexylammonium) (1), which exhibits dual-stable behavior in a dielectric and second harmonic generation (SHG) response during the reversible phase transition process near the high Curie temperature Tc ∼ 409 K. The phase transition temperature is significantly increased by 41 K compared to the corresponding non-fluorinated (CHA)2PbBr4 (CHA = cyclohexylammonium). Remarkably, the material shows rare broad-band yellow emission under UV excitation, attributed to the induction of self-trapped exciton emission by the distortion of the [PbBr6]4- octahedra, as confirmed by the first-principles analysis. 1 also exhibited ferroelectricity with a saturation polarization value and a small coercive field. This study provides a new insight into the modification of multifunctional switchable materials through the H/F substitution strategy.

A‐site coordinating cation engineering in zero‐dimensional antimony halide perovskites for strong self‐trapped exciton emission

AbstractLow‐dimensional hybrid halide perovskites represent a promising class of materials in optoelectronic applications because of strong broad self‐trapped exciton (STE) emissions. However, there exists a limitation in designing the ideal A‐site cation that makes the material satisfy the structure tolerance and exhibit STE emission raised by the appropriate electron–phonon coupling effect. To overcome this dilemma, we developed an inorganic metal‐organic dimethyl sulfoxide (DMSO) coordinating strategy to synthesize a series of zero‐dimensional (0D) Sb‐based halide perovskites including Na3SbBr6·DMSO6 (1), AlSbBr6·DMSO6 (2), AlSbCl6·DMSO6 (3), GaSbCl6·DMSO6 (4), Mn2Sb2Br10·DMSO13 (5) and MgSbBr5·DMSO7 (6), in which the distinctive coordinating A‐site cation [Am‐DMSO6]n+ efficiently separate the [SbXz] polyhedrons. Advantageously, these materials all exhibit broadband‐emissions with full widths at half maxima (FWHM) of 95–184 nm, and the highest photoluminescent quantum yield (PLQY) of 3 reaches 92%. Notably, compounds 2–4 are able to remain stable after storage of more than 120 d. First‐principles calculations indicate that the origin of the efficient STE emission can be attributed to the localized distortion in [SbXz] polyhedron upon optical excitation. Experimental and calculational results demonstrate that the proposed coordinating strategy provides a way to efficiently expand the variety of novel high‐performance STE emitters and continuously regulate their emission behaviors.

A ratiometric optical thermometer with dual-color emission based on Eu2+-doped CsCu2I3 microcrystals

With the increasing demand for non-contact fluorescence intensity ratio-based optical thermometry, novel phosphor materials with high-efficiency, dual-emitting centers, and differentiable temperature sensitivity are highly desired. In this work, rare earth Eu2+ ions were incorporated into CsCu2I3 microcrystals by solid-state reaction. Under a single UV excitation, the as-synthesized samples exhibit two emissions: 452 nm blue emission from the 5d→4f transition of Eu2+ and 582 nm yellow emission from self-trapped exciton emission of CsCu2I3. The photoluminescence quantum yield reaches to 50%. The dual-band emission of Eu2+-doped CsCu2I3 shows different temperature responses in the range of 260–360 K. Based on fluorescence intensity ratio technology, the maximum absolute sensitivity and relative sensitivity are 0.091 K–1 (at 360 K) and 2.60%/K (at 260 K), respectively. These results suggest that Eu2+-doped CsCu2I3 could be a good candidate for highly sensitive optical thermometer.

Designing multi-mode optical thermometers via Sb3+/Er3+ co-doped Cs2NaInCl6 lead-free double perovskite microcrystals

Contactless optical thermometers are widely studied in scientific research and technology because of their apparent advantages. The combination of multi-mode thermometric techniques can realize self-referenced temperature sensing, making temperature monitoring signals more accurate. Herein, a series of Cs2NaInCl6: Sb3+, Er3+ microcrystals (MCs) were prepared via the ligand-assisted reprecipitation method. Under 320 nm excitation, the energy transfer from the self-trapped excitons (STEs) to Er3+ is established, which leads to the blue emission of STEs and intrinsic emissions of Er3+. Additionally, the MCs can also produce up-conversion green emissions under the excitation of NIR laser. Based on this, a multi-mode thermometer is developed, including the luminescence intensity ratio of two thermally coupled levels of Er3+ (2H11/2/4S3/2), the luminescence intensity ratio of a non-thermally coupled system of STEs and Er3+(4S3/2), and fluorescence lifetime of Er3+(4S3/2). Moreover, the MCs exhibit thermochromic properties, the luminescent color changes from blue to green as the temperature increases. To monitor the temperature more objectively by color variation, a novel sensing mode is constructed via the relationship between chromatic coordinates and temperature. The favorable thermometry performances for four-mode temperature sensing reveal that Cs2NaInCl6: Sb3+/Er3+ MCs are good candidates for thermometric materials for the novel multi-mode design.

Self-Trapped Exciton Emission in Highly Polar Oxonium-Based Perovskites Triggered by Antimony Doping

Self-Trapped Exciton Emission in Highly Polar Oxonium-Based Perovskites Triggered by Antimony Doping

Zr4+ and Bi3+ codoped Cs2Ag0.3Na0.7InCl6 double perovskite for single-composition white-light emitting phosphors and multimodal optical anti-counterfeiting

Single-composition white-light emitter with excitation-dependent emissions is highly desired for phosphor-converted white-light emitting diodes (WLEDs) and optical anti-counterfeiting, in terms of the optical self-absorption and degeneration induced by the different components. However, the rational regulation of excitation-dependent multi-emissions in single perovskite materials is still a grand challenge. Herein, Bi3+ and Zr4+ codoped Cs2Ag0.3Na0.7InCl6 double perovskite microcrystals (DPMCs) were reported for single-composition white-light emitting phosphors and multimodal optical anti-counterfeiting. As a result, the photoluminescent quantum yield (PL QY) and color rendering index (CRI) were recorded up to 80.3% and 95.8, respectively. Specifically, under ultraviolet (UV) excitation at 365 nm, the Zr4+-doped DPMCs exhibited distinct emission profiles with single broad band centered at ∼570 nm for various Zr4+ doping concentrations, which could be attributed to the self-trapped exciton (STE) emission of the host. While the DPMCs show dual emission bands centered at ∼470 and ∼570 nm excited at 254 nm, which originates from the [ZrCl6]2- octahedral and STE, respectively, and their integral PL intensity ratio can be gradually adjusted by the content of Zr4+. The broad multicolor emissions with color kinetic features of the single-composition DPMCs offer exciting opportunities for constructing WLEDs and multimodal optical anti-counterfeiting.

Up and down-conversion photoluminescence energy demonstration of Ho3+/Yb3+ co-doped double perovskites microcrystals

Recently, lanthanide-doped fluorescence materials with perovskite structures have attracted great attention due to their abundant 4f energy levels, of which up- and down-conversion photoluminescence (PL) usually can be achieved by energy-level transition of single or multiple rare-earth ions (REI). Here, we synthesized for the first time a series of holmium (Ho) and ytterbium (Yb) co-doped Cs2Ag0.4Na0.6In0.9Bi0.1Cl6 (CANIBC) microcrystals (MCs) by hydrothermal method and realized the synergistic effect of efficient dual-mode PL of host and REI. The CANIBC MCs show yellow-white light emission stimulated by a 405 nm laser, of which the down-conversion (DC) PL is superimposed by the self-trapped excitons (STEs) emission of [AgCl6]5- octahedral and the energy level leap emission of REI. While the green emission of Ho3+ can be observed on the up-conversion (UC) PL stimulated by a 980 nm laser. We systematically analyzed the effect of CANIBC MCs in the dual-mode PL by time-resolved spectroscopy and found the existence of energy transfer between the STEs to REI in the DC PL process. Whereas in UC PL there is only energy transfer between REI. The variable-temperature dual-mode PL, reflects good temperature measurement performance. Simultaneously, it is effectively processed into a powder and mixed with 415 nm chips to create warm white light-emitting diodes with a CIE of (0.34, 0.36).

Elucidating the Role of Antimony Dopant in Optical Properties of Brightly Luminescent Zero-Dimensional Organic–Inorganic Metal Halides

Doping strategies are regarded as an effective means to improve the optical properties of organic–inorganic metal halides and have attracted widespread attention. Here, we synthesized undoped and Sb3+-doped MA4InX7 (X = Cl, Br; MA+ = CH3NH3+) single crystals. Sb3+-doped samples exhibited bright broadband emissions of extrinsic self-trapped excitons (STEs). The photoluminescence quantum yields of Sb3+-doped MA4InX7 (X = Cl, Br) are increased by more than 38 and 15 times compared with undoped samples, respectively. While a similar behavior in the optical characteristics was observed before and after introducing Sb3+, explained well by the non-ignorable trace Sb3+ ions in the precursors, which often confused researchers’ judgement about the origin of photoluminescence (PL) emission in previous studies. In addition, an apparent concentration quenching effect was found when excessive dopants were fed. The close connection between extrinsic STE emissions and the vibrational mode of [SbX6]3– octahedrons was revealed by temperature-dependent PL spectra and Raman spectra. Our work provides a deep understanding of extrinsic STE emission and thus facilitates the design and development of eco-friendly light-emitting metal halides.

Sequential thermochromic switching in zero-dimensional Cs2ZnCl4 metal halides

All-inorganic zero-dimensional (0D) metal halides are highly attractive for optoelectronic applications, but rational control over the temperature dependence of luminescence in these materials remains an unfulfilled task. This study presents an investigation of thermo-responsive luminescence in doped Cs2ZnCl4 crystals. Density functional theory calculations reveal a thermal elevation of the orbital state of the dopant ions originating from distortion of the local coordination polyhedron, leading to a blueshift of the self-trapped exciton (STE) emissions. By examining a series of Cs2ZnCl4 crystals doped with Sb3+, Te4+, Cu+, and Mn2+, we quantitatively address the polyhedral distortion and its effect on the luminescence properties. Owing to the 0D nature of the host material, we further synthesize Sb3+/Mn2+ co-doped Cs2ZnCl4 with dual emissions that display distinct luminescence responses to thermal stimuli. The advances in these optical materials enable a new tactic to construct tunable thermochromic luminescence for applications such as thermal sensing and optical encryption.

Low-Temperature Solution Synthesis of Stable Cs3Cu2Br5 Single Crystals for Visible Light Communications

Inorganic perovskites CsPbX3 (X = Cl, Br, I) have shown great potential as luminescent materials for a wide range of photoelectric devices. However, the practical use of these materials is limited due to the toxicity of lead and poor stability. Here, we present a facile low-temperature, solution-based method to synthesize lead-free and highly stable Cs3Cu2Br5 single crystals (SCs) without the use of organic solvents. Owing to the self-trapped exciton emissions, Cs3Cu2Br5 SCs exhibit a strong broadband blue emission with a high photoluminescence quantum yield (PLQY) upon 254 nm ultraviolet light excitation. In addition, the Cs3Cu2Br5 SCs show a high stability against heat, humidity, and UV light. Therefore, the Cs3Cu2Br5 SCs are utilized as emitters in white light emitting diodes (WLEDs), demonstrating a high color rendering index of 81 and a decent commission internationale de l'Eclairage coordinate of (0.30, 0.34). Furthermore, the prepared WLEDs are used in wireless visible light communications, showing a -3 dB bandwidth of 6.7 MHz and an achievable data rate of 45 Mbps. Our study provides a novel organic-solvent-free, low-temperature method to synthesize Cs3Cu2Br5 SCs and could promote the development of Cu-based metal halides in visible light communications.

Boosting self-trapped exciton emission from Cs3Cu2I5 nanocrystals by doping-enhanced exciton-phonon coupling

Boosting self-trapped exciton emission from Cs3Cu2I5 nanocrystals by doping-enhanced exciton-phonon coupling

First-Principles Investigation on the Stability, Electronic Structure, and Exciton Self-Trapping Mechanism of 0D and 1D Cs3Cu2Cl5

Copper(I) halides with broadband emission from self-trapped excitons (STEs) are promising luminescent materials due to their structural diversities and intriguing photophysical properties. Despite great efforts invested in these materials, an in-depth understanding of their structural variations and the corresponding STE emission mechanisms is still lacking. Here, first-principles calculations reveal two distinct STEs in 0D and 1D polymorphs of Cs3Cu2Cl5, which feature the same chemical composition but different crystal structures. The electronic band structure analysis reveals that the strong Cu–Cl 4s–3p coupling along the 1D [Cu2Cl5]3– chain brings a large band dispersion, resulting in an increase of optical anisotropy in the 1D structure. The STE in 0D Cs3Cu2Cl5 involves the formation of one Cu–Cu bond and the breakage of two Cu–Cl bonds within an isolated [Cu2Cl5]3– unit. In contrast, the exciton self-trapping in the 1D structure is found to be associated with the formation of one Cu–Cu bond accompanied by two Cu–Cl bonds in the [Cu2Cl5]3– chain. Despite significant structural differences in 0D and 1D Cs3Cu2Cl5, quantitative characterizations of the configuration coordinate diagrams in the two polymorphs lead to similar STE emission energies (2.38 and 2.48 eV for 0D and 1D, respectively), which theoretically explains similar green-light emission experimentally observed in both compounds. This study provides important insights for understanding complex relations between the structural dimensionality, building units, and emission properties in low-dimensional metal halides.