Self-trapped Excitons Research Articles

Enhanced UV Light Responsivity in <110>‐Oriented 2D Perovskites Realized by Pressure‐Induced Ultrafast Exciton Transport

AbstractTwo‐dimensional (2D) <100>‐oriented perovskites exhibit superior optoelectronic properties, offering significant potential in photovoltaic, light‐emitting, and photodetection applications. Nevertheless, their enlarged interlayer spacing restricts longitudinal carrier transport, thereby limiting its potential applications. While <110>‐oriented 2D perovskites provide a prospective solution with their compact interlayer spacing, their inherent structure, characterized by octahedra tilting, indirectly hinders carrier transport due to the generation of self‐trapped excitons (STEs) caused by strong electron‐phonon coupling. Here, we adeptly regulate the photoluminescence (PL) from STEs to free excitons (FEs) emission within the 2D <110>‐oriented (API)PbBr4 single crystal through structure optimization under pressure treatment. Besides, we observed anisotropic FE transport with an anisotropy ratio of 4.97. The exciton mobility reaches a peak of 93.6 cm2 V−1 s−1 at 2.7 GPa, a value comparable to those of their three‐dimensional (3D) counterparts, which is attributed to a reduction in electron‐phonon coupling and exciton reduced mass. Additionally, this ultrafast exciton transport significantly enhances UV light responsivity, exhibiting an increase of approximately 5000 times at 2.7 GPa in comparison to ambient conditions. These findings highlight the prospective application of 2D <110>‐oriented perovskites in high‐performance optoelectronic devices through intrinsic structural modulation.

Regulating Second‐Harmonic Generation in 2D Chiral Perovskites Through Achiral Organic Spacer Cation Alloying Strategy

AbstractThe inherent structural flexibility and chiroptical activity of 2D chiral perovskites make them promising for the nonlinear optical (NLO) application. A comprehensive understanding of the second‐harmonic generation (SHG) mechanism in 2D chiral perovskites is essential for developing NLO devices. However, the rational design of 2D chiral perovskite structures to regulate SHG properties remains challenging. Herein, to regulate SHG response, an achiral organic spacer cation alloying strategy is employed to construct a series of 2D chiral perovskites. Through the measurement of temperature‐dependent photoluminescence (PL) spectra, it is revealed that the material design strategy can effectively modulating self‐trapped exciton (STE) emission. More importantly, it is confirmed that there is a competitive relationship between STE emission and SHG in 2D chiral perovskites. Meanwhile, the microscopic imaging of circularly polarized‐SHG is demonstrated in chiral perovskites. This work will not only advance the understanding of the SHG mechanism in 2D chiral perovskites but also provide inspiration for the rational design and synthesis of perovskites for NLO devices.

Enhanced UV Light Responsivity in -Oriented 2D Perovskites Realized by Pressure-Induced Ultrafast Exciton Transport.

Two-dimensional (2D) <100>-oriented perovskites exhibit superior optoelectronic properties, offering significant potential in photovoltaic, light-emitting, and photodetection applications. Nevertheless, their enlarged interlayer spacing restricts longitudinal carrier transport, thereby limiting its potential applications. While <110>-oriented 2D perovskites provide a prospective solution with their compact interlayer spacing, their inherent structure, characterized by octahedra tilting, indirectly hinders carrier transport due to the generation of self-trapped excitons (STEs) caused by strong electron-phonon coupling. Here, we adeptly regulate the photoluminescence (PL) from STEs to free excitons (FEs) emission within the 2D <110>-oriented (API)PbBr4 single crystal through structure optimization under pressure treatment. Besides, we observed anisotropic FE transport with an anisotropy ratio of 4.97. The exciton mobility reaches a peak of 93.6 cm2 V-1 s-1 at 2.7 GPa, a value comparable to those of their three-dimensional (3D) counterparts, which is attributed to a reduction in electron-phonon coupling and exciton reduced mass. Additionally, this ultrafast exciton transport significantly enhances UV light responsivity, exhibiting an increase of approximately 5000 times at 2.7 GPa in comparison to ambient conditions. These findings highlight the prospective application of 2D <110>-oriented perovskites in high-performance optoelectronic devices through intrinsic structural modulation.

Unlocking Full-Spectrum Brilliance: Dimensional Regulation in Lead-Free Metal Halides for Superior Photoluminescence.

Lead-free metal halides with tunable structures have emerged as a new class of optoelectronic materials. The arrangement of metal halide polyhedra defines their structural dimensionality and serves as a key factor influencing their optical properties. To investigate this, we synthesized four different antimony (Sb)-doped indium (In)-based metal halides, all of which possess zero-dimensional (0D) electronic structures but exhibit 3D, 2D, 1D, and 0D structural dimensionality at the molecular level. With a decreasing of structural dimensionality, their self-trapped exciton (STE) emission shows a red shift with peak position from 496 to 663 nm. We revealed that the red shift is caused by increased distortion of [SbCl6]3- octahedra as the structural dimensionality decreases, leading to lowered energy levels of STE and a corresponding red shift. The tunable STE emission makes these metal halides promising for anticounterfeiting and white LED applications. These findings provide a new strategy for tuning STE emission in lead-free metal halides.

Self-Trapped Excitons in 3R ZnIn2S4 with Broken Inversion Symmetry for High-Performance Photodetection.

Exploring novel materials with intrinsic self-trapped excitons (STEs) is crucial for advancing optoelectronic technologies. In this study, 2D 3R-phase ZnIn2S4, featuring broken inversion symmetry, is introduced to investigate intrinsic STEs. This material exhibits a broadband photoluminescence (PL) emission with a full width at half maximum of 164nm and a large Stokes shift of ≈0.6eV, which arises from the distortion of [ZnS4]6- tetrahedral unit induced by the symmetry breaking and strong electron-phonon coupling. The photophysical properties of the STEs exhibit a high Huang-Rhys factor (15.0), rapid STEs formation time (166fs), and extended STEs lifetime (1039ps), as demonstrated by experimental evidence from temperature-dependent PL, Raman spectroscopy, and ultrafast absorption spectroscopy. Additionally, STE-induced photoconductiveeffect is elucidated, indicating that intrinsic STEs in 3R-ZnIn2S4 can provide a synergistic effect that enhances absorption capacity, localization, and lifetime by capturing the self-trapped hole state. Consequently, the 2D 3R-ZnIn2S4 photodetector exhibits remarkable broad-spectrum photosensitivity, including a photo-switching ratio of 11286, response times of less than 0.6ms, responsivity of 15.2AW-1, detectivity of 1.02×10¹¹ Jones, and external quantum efficiency of 5032% under 375nm light. These findings provide new ideas for exploring materials with intrinsic STEs to achieve novel high-performance photodetector applications.

Fine-Tuning Self-Trapped Excitons via External Coordination of Perovskite Octahedra: Simultaneous Achievement of Significant Stokes Shifts and Enhanced Emission

Fine-Tuning Self-Trapped Excitons via External Coordination of Perovskite Octahedra: Simultaneous Achievement of Significant Stokes Shifts and Enhanced Emission

Achieving Luminous Efficiency Enhancement and Near‐Infrared Emission in Moisture‐Stable 0D Zinc‐Based Halides

AbstractZero‐dimensional (0D) metal halides are attractive due to their structure‐dependent and tunable photoluminescence properties. Herein, a new 0D organic–inorganic hybrid Zn‐based halide, (C4H12N2)ZnBr4, featuring a long‐term stable crystal structure and moisture‐stable PL emission under various extreme conditions is reported. A strong electron–phonon coupling effect enables the Zn‐based halide to display highly efficient blue light at 472 nm with a large Stokes shift of 7385 cm−1. Intriguingly, heterovalent substitution of Cu+‐Zn2+ further enhances the photoluminescence quantum efficiency to 60% as the introduction of Cu+ effectively suppresses the nonradiative recombination process. Besides, the formation of twisted [SbBr4]− tetrahedra via Sb3+‐Zn2+ substitution help to achieve a broadband near‐infrared (NIR) emission (760 nm) with full width at half maxima (FWHM) of 203 nm, enabling the potential applications in night‐vision and nondestructive fruit damage inspection. Detailed structural and optical analyses are used to investigate the photophysical processes of different self‐trapped exciton (STE) emission for pristine and Cu+/Sb3+‐doped 0D metal halides. These findings advance the understanding of spectral regulation mechanism via heterovalent substitution and initiate more exploitation of luminescent metal halides for emerging applications.

On-Off Switching of Singlet Self-Trapped Exciton Emission Endows Antimony-Doped Indium Halides with Excitation-Wavelength-Dependent Luminescence.

Excitation-wavelength-dependent (Ex-De) emitters are a fascinating category of luminescent materials whose emission properties vary with the wavelength of the light used for excitation. Antimony (Sb3+)-doped indium (In)-based metal halides are efficient light emitters; however, the peak fluorescence emission of most Sb3+-activated In-halide remains independent of the excitation wavelength. Here, the study introduces a new Sb3+-doped In-halide cluster, (BDPA)2InCl5:Sb (BDPA+ = C15H18N+, benzyldimethylphenylammonium), which demonstrates efficient Ex-De emission originating from the on-off switchable fluorescence behavior of singlet self-trapped exciton (STE) in 5-coordinate Sb3+ dopant. Interestingly, when excited within the range of 240-370nm, photoluminescence (PL) spectra of (BDPA)2InCl5:Sb show both singlet and triplet STE emission. However, under excitation wavelengths of 370 to 420nm, the singlet STE emission is absent, resulting in a noticeable correlated color temperature change from 1700 to 3800 K. The study provides a new approach to designing color-tunable Sb3+-based luminophores, and also presents a novel application scenario for the widely recognized Sb3+ doping strategy.

Intense white-light emission of amorphous lead chloride trimers at high pressure.

Developing efficient, single-phase white-light phosphors remains a formidable challenge in optoelectronics. Herein, high pressure initially induces trimeric contraction and distortion in (C9NH20)9(ZnCl4)2[Pb3Cl11], regulating the transition equilibrium of self-trapped excitons (STEs) with varied emission colors. Then, considerable structural distortion and destruction lead to intense white-light emission of an amorphous phase. A narrowed bandgap with promoted excitation efficiency, as well as strengthened electron-phonon coupling effect with increased binding energy of STEs, together result in the significant emission enhancement. This work provides valuable insights into white-light luminescent materials and offers new strategies for designing white-light-emitting devices.

Controllable Multi-Exciton Zero-Dimensional Antimony-Based Metal Halides for White-light Emission and β-Ray Detection.

Self-trapped exciton (STE) emission, typified by antimony (Sb), with broadband characteristics, represents the next generation of materials for solid-state lighting and radiation detection. However, little is known about the multiexciton behavior of the Sb emission center. Here, we proposed a general approach for designing antimony-centered multi-exciton emitting materials through self-assembly. Benefitting from controllable multiexciton behavior, dual-band white light emission spanning the entire visible spectrum was achieved. Relying on the reduction of an effective atomic number brought by self-assembly, excellent scintillation response to β-rays was attained. This study offers unprecedented insight into hybrid single/triple STE emission and unveils new avenues for single-emitter white-light emission, as well as radiographic testing using low-risk β-rays as sources.

Warm-White Light Emission of Lead-free CsAg2I3 Single Crystal Scintillator with a One-Dimensional Electronic Structure.

Presently, the exploration of novel inorganic lead-free perovskite scintillators has emerged as a prominent topic in the field of perovskite materials. Extensive attention has been garnered by materials such as Cs3Cu2I5 due to their notable advantage in scintillation intensity, but the response time constants in the microsecond or even millisecond range severely constrain their potential applications in scintillators. In this study, large-sized (5-6 mm) CsAg2I3 single crystals with an ultrafast warm-white light emission on a nanosecond time scale are presented. Specifically, upon X-ray excitation, the single crystal demonstrates a broad-spectrum white light emission with a color temperature as high as 5129 K, attributed to its self-trapped exciton emission. The 137Cs energy spectrum reveals that CsAg2I3 possesses an ultrafast response for γ rays with a time constant of 15 ns, which is significantly faster than that of Cs3Cu2I5. Furthermore, time-resolved photoluminescence unveils a subnanosecond component with a response time of 0.9 ns. The characteristics of ultrafast warm-white light emission exhibit the significant potential of CsAg2I3 in radiation scintillation detection and its probability of playing a pivotal role in future radiation detection technology.

Lattice distortion enhanced self-trapped excitons emission in antimony halide crystalline clusters

Lattice distortion enhanced self-trapped excitons emission in antimony halide crystalline clusters

Colloidal Synthesis of Blue-Emitting Cs3TmCl6 Nanocrystals via Localized Excitonic Recombination for Down-Conversion White Light-Emitting Diodes.

Lead-halide perovskite nanocrystals (NCs) have gained significant attention for their promising applications in lighting and display technologies. However, blue-emitting NCs have struggled to match the high efficiency of their red and green counterparts. Moreover, many reported blue-emitting perovskite NCs contain heavy metal lead (Pb), which poses risks to human health and the environment. In this study, we synthesized rare-earth-based Cs3TmCl6 NCs via the hot injection method, which exhibit a broadband blue emission at 440 nm. Combined experimental and theoretical studies indicate that the broadband emission in Cs3TmCl6 arises from self-trapped excitons due to the excited-state structural distortion of the [TmCl6]3- cluster. Furthermore, the ultrafast dynamics of charge carriers were analyzed using time-resolved photoluminescence and transient absorption measurements. Encouraged by the remarkable thermal, light, and water stabilities of Cs3TmCl6 NCs, as evidenced by experimental and theoretical results, a white light-emitting diode was further designed and fabricated using the Cs3TmCl6 NCs as the color converter. The device exhibits outstanding performance, achieving a long half-lifetime of 336 h and a large color-rendering index of 87.0. Combining eco-friendly features and a facile synthesis method, the rare-earth-based Cs3TmCl6 NCs mark a significant breakthrough as a reliable blue emitter, showcasing their future potential in lighting and display applications.

Simultaneous achieving color‐tuning long persistent luminescence and phosphorescent quantum yield of 81.05% in 2D organic metal halide perovskite

AbstractIonically bonded organic metal halide perovskite‐like luminescent materials, which incorporate organic cations and metal halides, have emerged as a versatile multicomponent material system. However, these materials still face challenges in terms of low phosphorescence quantum yields and limited long persistent luminescence (LPL) colors. Herein, we present the design and synthesis of an intraligand charge‐transfer organic‐based metal halide perovskite‐like material, in which organic cations form a compact supramolecular hydrogen‐bonded organic framework (HOF) structure, exhibiting crystallization‐induced phosphorescence emission of ligand, while metal halides form a unique two‐dimensional (2D) structure that displays intrinsic self‐trapped excitons (STE) emission under the radiation of UV light. Notably, the metal halide hybrid is found to exhibit enhanced phosphorescent photoluminescence efficiency of up to 81.05% and tunable LPL from cyan to orange compared to the pristine organic phosphor, due to the structural distortion and scaffolding effects of 2D metal halides as well as a well‐packed HOF structure. Optical characterizations and theoretical calculations reveal that charge transfer from organic cations and halogen to ligand as well as STE from inorganic layers are responsible for the tunable LPL. Meanwhile, the high‐efficiency phosphorescent quantum yield is attributed to stronger hydrogen bond stacking as well as structural distortion of metal halogen bands. Thus, the obtained LPL provides potentials in anti‐counterfeiting, security systems, and so on.

Realizing Near-Unity Photoluminescence Efficiency in Antimony-Doped Indium-Based Halides Induced by Strong Electron-Phonon Coupling.

Exploring a zero-dimensional (0D) hybrid halide with a large Stokes shift and efficient broad-band emission is highly desirable due to its enormous potential for solid-state lighting (SSL) application. However, it is still challenging to develop a highly emissive 0D hybrid halide with low toxicity and remarkable stability. Herein, we developed a novel indium-based metal halide A5In2Cl16·4H2O (A = doubly protonated 1,4-diaminobutane) whose inorganic octahedrons are completely isolated by the organic cations to form the 0D structure. Experimental and theoretical studies confirmed that Sb-doped A5In2Cl16·4H2O exhibits broad yellow emission with a photoluminescence quantum yield (PLQY) of up to 98%. The intense yellow emission can be attributed to the radiative recombination of triplet self-trapped excitons (STEs) in [SbCl6]3- octahedrons caused by the strong electron-phonon coupling. Benefiting from the excellent stability and photoluminescence performance, A5In2Cl16·4H2O:15%Sb was used as the yellow phosphor to prepare a white-light-emitting diode (WLED) device with a color rendering index of 87.8 and a luminous efficiency of up to 36.18 lm/W, demonstrating its potential in SSL applications. This work provides a guidance for developing environmentally friendly, efficient, and stable ultraviolet (UV)-excited broad-band emission materials.

Dynamic Phosphorescence/Fluorescence Switching in Hybrid Metal Halides Toward Time‐Resolved Multi‐Level Anti‐Counterfeiting

AbstractHybrid metal halides (HMHs) with time‐resolved luminescence behavior promise to be a breakthrough in multi‐level anti‐counterfeiting, but controlling the dynamic switching between phosphorescence and fluorescence is extremely challenging. Herein, an array of 0D HMHs is constructed by screening the π‐conjugated ligand with room‐temperature phosphorescence (RTP). Compared to the organic chromophore, (ETPP)2ZrCl6 possesses a misaligned stacking and rigid structure, contributing to an improved phosphorescence quantum yield (ΦP = 27.50%) and an extended phosphorescence lifetime (τ = 0.6234 s), as the intervening of inorganic unit [ZrCl6]2− suppresses the energy losses caused by nonradiative relaxation and prompts the intersystem crossover (ISC) process. Not only that, the interplay of phosphorescence‐fluorescence dual‐mode emission can be intelligently controlled by doping the active metal Te4+, resulting in a dynamic switching between RTP phosphorescence and self‐trapped exciton (STE) fluorescence. DFT calculations reveal the governing origins of RTP‐STE from the intermolecular ISC channels and spin‐orbit coupling (SOC) coefficients. These precise images into periodic pixelated arrays enable the multi‐level anti‐counterfeiting and information encryption. This work proposes a fluorescence‐phosphorescence co‐modulating strategy under the premise of dissecting the structural origins for optimizing RTP phosphorescence, which paves the way for designing high‐security‐level anti‐counterfeiting materials.

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.

A 0D Ge(II)-Halide-Based Perovskite with Enhanced Semiconducting Behavior for Electronic Capacitors.

Perovskite materials have surged to the forefront of materials science, captivating researchers worldwide with their distinctive crystal lattice arrangement and remarkable optical, electric and dielectric attributes. The current study focuses on the development of a novel zero-dimensional (0D) Ge(II)-based hybrid perovskite, formulated as NH3(CH2)2NH3GeF6, and synthesized through a gradual evaporation process conducted at room temperature. The crystal structure is characterized by an arrangement of organic cations and isolated octahedral [GeF6]2- groups. This configuration is stabilized by relatively weak intermolecular bonds. A comprehensive analysis of the material's thermal properties using differential scanning calorimetry (DSC) revealed a distinct phase transition occurring at approximately 323 K, which was further confirmed through electrical measurements. The studied compound provided a broad absorption range across the visible spectrum and an optical band gap of 3.30 eV, indicating its potential for semiconducting applications in optoelectronic devices. Photoluminescence PL analysis displays a blueish broad-band emission with a high color rendering index CRI value of 91, when excited at 325 nm. This emission primarily originates from the self-trapped excitons (STEs) recombination in the inorganic [GeF6]2-. Herein, the temperature-dependent behavior of grain conductivity exhibited an Arrhenius-type pattern, with an activation energy (E a) of 0.46 eV, confirming the semiconductor nature of the investigated compound. In addition, a deep investigation of the alternating current conductivity, analyzed using Jonscher's law, demonstrates that the conduction mechanism is effectively described by the correlated barrier hopping (CBH) model. The dielectric performances show a significant dielectric constant (ε' ∼ 103). Thus, all these interesting physical properties of this hybrid perovskite have paved the way for advancements in various technological applications, particularly in the field of electronic capacitors.

White Light Emission in Zero-Dimensional Indium Hybrid with Hydrogen Bond.

Low-dimensional organic-inorganic metal halides (OIMHs) have been explored as single-component white light emitters for applications in solid-state lighting. Herein, we report a zero-dimensional (0D) In-based OIMH (TMPDA)[InCl5(H2O)] (TMPDA = N,N,N',N'-tetramethyl-1,4-phenylenediamine), which crystallizes in the noncentrosymmetric P212121 space group and contains hydrogen bonds between the adjacent [InCl5(H2O)]2- octahedra in structure. It exhibits a large optical band gap (4.10 eV) and dual-band emission under UV light. Spectroscopic analysis and theoretical calculation indicate that the high (404 nm)- and low (513 nm)-energy emissions are attributed to the bound excitons in organic ligands and self-trapped excitons in [InCl5(H2O)]2- units, respectively. It is found that Sb doping in this 0D hybrid provides additional orange (590 nm) emission assigned to the 3P1 → 1S0 triplet radiative recombination. By adjusting the doping level, the emission color can be turned from turquoise to orange, and interestingly, a single-component white-light emission is realized by balancing the high-energy emission from organic ligand, the turquoise emission from [InCl5(H2O)]2-, and the orange one from [SbCl5(H2O)]2-. This work not only provides a new OIMH showing the single-component white light emission but also demonstrates the potential of In-based hybrids with hydrogen bonds for solid-state luminescence.

Synthesis of β-Ga2O3:Mg Thin Films by Electron Beam Evaporation and Postannealing

Doping divalent metal cations into Ga2O3 films plays a key role in adjusting the conductive behavior of the film. N-type high-resistivity β-Ga2O3:Mg films were prepared using electron beam evaporation and subsequent postannealing processing. Various characterization methods (X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence, etc.) revealed that the Mg content plays an important role in affecting the film quality. Specifically, when the Mg content in the film is 3.6%, the S2 film’s resistivity, carrier content, and carrier mobility are 59655.5 Ω·cm, 1.95 × 1014 cm3/C, and 0.53682 cm2/Vs. Also, the film exhibits a smoother surface, more refined grains, and higher self-trapped exciton emission efficiency. The Mg cation mainly substitutes the Ga+ cation at a tetrahedral site, acting as a trap for self-trapped holes.