Direct Observation of the Exciton Self-Trapping Process in CsCu2I3 Thin Films

Transient Photoinduced Pb2+ Disproportionation for Exciton Self-Trapping and Broadband Emission in Low-Dimensional Lead Halide Perovskites

Low-dimensional lead halide perovskites with broadband emission hold great promise for single-component white-light-emitting (WLE) devices. The origin of their broadband emission has been commonly attributed to self-trapped excitons (STEs) composed of localized electronic polarization with a distorted lattice. Unfortunately, the exact electronic and structural nature of the STE species in these WLE materials remains elusive, hindering the rational design of high-efficiency WLE materials. In this study, by combining ultrafast transient absorption spectroscopy and ab initio calculations, we uncover surprisingly similar STE features in two prototypical low dimensional WLE perovskite single crystals: 1D (DMEDA)PbBr4 and 2D (EDBE)PbBr4, despite of their different dimensionalities. Photoexcited excitons rapidly localize to intrinsic STEs within ∼250 fs, contributing to the white light emission. Crucially, STEs in both systems exhibit characteristic absorption features akin to those of Pb+ and Pb3+. Further atomic level theoretical simulations confirm photoexcited electrons and holes are localized on the Pb2+ site to form Pb+- and Pb3+-like species, resembling transient photoinduced Pb2+ disproportionation. This study provides conclusive evidence on the key excited state species for exciton self-trapping and broadband emission in low dimensional lead halide WLE perovskites and paves the way for the rational design of high-efficiency WLE materials.

Highly stable mixed-phase Cs-Cu-I films with tunable optoelectronic properties for UVB photodetector applications

Ultraviolet (UV) photodetector plays an important role in military, civilian and people's daily life, and is an indispensable part of spectral detection. However, photodetectors target at the UVB region (280-320 nm) are rarely reported, and the devices detected by medium-wave UV light generally have problems such as low detection rate, low sensitivity, and poor stability, which are difficult to meet the market application needs. Herein, Cs-Cu-I films with mixed-phase have been prepared by vacuum thermal evaporation. By adjusting the proportion of evaporation sources (CsI and CuI), the optical bandgaps of mixed-phase Cs-Cu-I films can be tuned between 3.7 eV and 4.1 eV. This absorption cut-off edge is exactly at both ends of the UVB band, which indicating its potential application in the field of UVB detection. Finally, the photodetectors based on Cs-Cu-I/n-Si heterojunction are fabricated. The photodetector shows good spectral selectivity for UVB band, and has a photoresponsivity of 22 mA/W, a specific detectivity of 1.83*1011 Jones, an EQE over 8.7% and an on/off ratio above 20.

Broadband Near-Infrared Emission from 0D Hybrid Copper Halides

Metal halides have attracted increasing attention owing to their outstanding optoelectronic properties and tunable emission characteristics. Among these, low-dimensional metal halides have emerged as a novel and efficient luminescent material, primarily attributed to their broad spectral emission induced by self-trapped excitons (STEs). However, achieving highly efficient deep red and near-infrared (NIR) emission in metal halides remains a challenge. In this study, we report a novel zero-dimensional (0D) copper-based metal halide [Na2(DMSO)8]Cu4Br6 as the NIR light source, which exhibits a full width at half-maximum (FWHM) of 195 nm peaking at 685 nm, an impressive quantum efficiency of 68% and a large Stokes shift of 299 nm. Through comprehensive spectral analysis and meticulous data calculations, we substantiate that the emission originates from STEs formed within the 0D structure. Furthermore, we demonstrate the potential application of [Na2(DMSO)8]Cu4Br6 as an invisible light source in night vision by combining it with a commercially available 365 nm ultraviolet (UV) chip. This work not only enriches the family of low-dimensional metal halide materials but also inspires the potential of low-dimensional metal halides in night vision applications.

Recent Advances and Opportunities of Eco-Friendly Ternary Copper Halides: A New Superstar in Optoelectronic Applications

Recently, the newly-emerging lead-free metal-halide materials with less toxicity and superior optoelectronic properties have received wide attention as the safer and potentially more robust alternatives to lead-based perovskite counterparts. Among them, ternary copper halides (TCHs) have become a vital group due to their unique features, including abundant structural diversity, ease of synthesis, unprecedented optoelectronic properties, high abundance, and low cost. Although the recent efforts in this field have made certain progresses, some scientific and technological issues still remain unresolved. Herein, a comprehensive and up-to-date overview of recent progress on the fundamental characteristics of TCH materials and their versatile applications is presented, which contains topics such as: i) crystal and electronic structure features and synthesis strategies; ii) mechanisms of self-trapped excitons, luminescence regulation, and environmental stability; and iii) their burgeoning optoelectronic devices of phosphor-converted white light-emitting diodes (WLEDs), electroluminescent LEDs, anti-counterfeiting, X-ray scintillators, photodetectors, sensors, and memristors. Finally, the current challenges together with future perspectives on the development of TCH materials and applications are also critically described, which is considered to be critical for accelerating the commercialization of these rapidly evolving technologies.

Nature of Self-Trapped Exciton Emission in Zero-Dimensional Cs2ZrCl6 Perovskite Nanocrystals

Low dimensional perovskite-inspired materials with self-tapped exciton (STE) emission have stimulated a surge of cutting-edge research in optoelectronics. Despite numerous efforts on developing versatile low-dimensional perovskite-inspired materials with efficient STE emissions, there is little emphasis on the intrinsic dynamics of STE-based broad emission in these materials. Here, we investigated the excited state dynamics in zero-dimensional (0D) Cs2ZrCl6 nanocrystals (NCs) with efficient blue STE emission. By using femtosecond transient absorption (fs-TA) spectroscopy, the ultrafast STE formation process within 400 fs is directly observed. Then, the formed STEs relax to an intermediate STE state with a lifetime of ∼180 ps before reaching the emissive STE state with a lifetime of ∼15 μs. Our work offers a comprehensive and precise dynamic picture of STE emission in low-dimensional metal halides and sheds light on extending their potential applications.

One-dimensional scintillator film with benign grain boundaries for high-resolution and fast x-ray imaging

Fast and high-resolution x-ray imaging demands scintillator films with negligible afterglow, high scintillation yield, and minimized cross-talk. However, grain boundaries (GBs) are abundant in polycrystalline scintillator film, and, for current inorganic scintillators, detrimental dangling bonds at GBs inevitably extend radioluminescence lifetime and increase nonradiative recombination loss, deteriorating afterglow and scintillation yield. Here, we demonstrate that scintillators with one-dimensional (1D) crystal structure, Cs₅Cu₃Cl₆I₂ explored here, possess benign GBs without dangling bonds, yielding nearly identical afterglow and scintillation yield for single crystals and polycrystalline films. Because of its 1D crystal structure, Cs₅Cu₃Cl₆I₂ films with desired columnar morphology are easily obtained via close space sublimation, exhibit negligible afterglow (0.1% at 10 ms) and high scintillation yield (1.2 times of CsI:Tl). We have also demonstrated fast x-ray imaging with 27 line pairs mm^-1 resolution and frame rate up to 33 fps, surpassing most existing scintillators. We believe that the 1D scintillators can greatly boost x-ray imaging performance.

One-Dimensional Hybrid Copper(I) Iodide Single Crystal with Renewable Scintillation Properties

Low-dimensional hybrid copper(I) halides attract considerable attention in the field of light emissions. In this work, we obtained the centimeter-sized single crystal of 1,3-propanediamine copper(I) iodide (PDACuI₃) with a solvent evaporation method. The single crystal X-ray diffraction of PDACuI₃ reveals that the [CuI₄] tetrahedra form the corner-connected chains separated by PDAs, forming a one-dimensional structure with an orthorhombic space group of Pbca. The band gap is determined to be 4.03 eV, and the room temperature photoluminescence (PL) quantum yield is determined to be 26.5%. The thermal quenching and negative thermal quenching of emission are observed via temperature-dependent PL spectra, and our study shows that the intermediate nonradiative state below the self-trapped exciton state may get involved in these temperature-dependent behaviors. The X-ray scintillation performance of PDACuI₃ single crystals is also evaluated, and the relative light output renewed to 94.3% of the fresh one after a low-temperature annealing. On the basis of our results, PDACuI₃ single crystals provide nontoxicity and renewable scintillation performance, thus showing potential application in the area of low-cost radiation detectors.

Synthesis of Thermally Stable and Highly Luminescent Cs5 Cu3 Cl6 I2 Nanocrystals with Nonlinear Optical Response

Low-dimensional Cu(I)-based metal halide materials are gaining attention due to their low toxicity, high stability and unique luminescence mechanism, which is mediated by self-trapped excitons (STEs). Among them, Cs₅ Cu₃ Cl₆ I₂ , which emits blue light, is a promising candidate for applications as a next-generation blue-emitting material. In this article, an optimized colloidal process to synthesize uniform Cs₅ Cu₃ Cl₆ I₂ nanocrystals (NCs) with a superior quantum yield (QY) is proposed. In addition, precise control of the synthesis parameters, enabling anisotropic growth and emission wavelength shifting is demonstrated. The synthesized Cs₅ Cu₃ Cl₆ I₂ NCs have an excellent photoluminescence (PL) retention rate, even at high temperature, and exhibit high stability over multiple heating-cooling cycles under ambient conditions. Moreover, under 850-nm femtosecond laser irradiation, the NCs exhibit three-photon absorption (3PA)-induced PL, highlighting the possibility of utilizing their nonlinear optical properties. Such thermally stable and highly luminescent Cs₅ Cu₃ Cl₆ I₂ NCs with nonlinear optical properties overcome the limitations of conventional blue-emitting nanomaterials. These findings provide insights into the mechanism of the colloidal synthesis of Cs₅ Cu₃ Cl₆ I₂ NCs and a foundation for further research.

Charge Transport Dynamics of Quasi-Type II Perovskite Janus Nanocrystals in High-Performance Photoconductors

CsPbBr₃-Pb₄S₃Br₂ Janus nanocrystals (NCs) are the only nanomaterial where the epitaxial structure of perovskite and chalcogenide materials has been realized at the nanoscale, but their exciton dynamics mechanism has not yet been thoroughly investigated or applied in photodetection applications. This work reports an attractive device performance of perovskite photoconductors based on epitaxial CsPbBr₃-Pb₄S₃Br₂ Janus NCs, as well as the carrier relaxation and transfer mechanism of the heterojunction. By a combination of transient optical absorption and quantum dynamics simulation, it is demonstrated that the photogenerated holes on CsPbBr₃ can be successfully extracted by Pb₄S₃Br₂, while the hole transfer proceeds about three times faster than energy loss and remains "hot" for about 300 fs. This feature has favorable effects on long-range charge separation and transport; therefore, the Janus NCs photoconductors exhibit an exceptional responsivity of 34.0 A W^-1 and specific detectivity of 1.26 × 10¹⁴ Jones.

Citric acid tuned negative thermal quenching of all inorganic copper-based perovskites

Copper-based perovskites, with lower electronic dimensions and high photoluminescence quantum yields (PLQY), which are non-toxic and thermally stable, have been reported since 2019 and have immediately attracted great attention. So far, only a few studies have researched the temperature-dependent photoluminescence properties, posing a challenge in ensuring the stability of the material. In this paper, the temperature-dependent photoluminescence properties have been investigated in detail, and a negative thermal quenching of all-inorganic CsCu₂I₃ perovskites has been studied. Moreover, the negative thermal quenching property can be tuned with the assistance of citric acid, which has not been reported before. The Huang-Rhys factors are calculated to be 46.32/38.31, which is higher than for many semiconductors and perovskites.

Highly Efficient White Emission from Semiconductor Ink Based on Copper Iodide Nanoclusters

Recent developments in the perovskite field have aimed at exploring cluster-based organic-inorganic copper(I) halides as novel luminescent materials because of their low toxicity and structural diversity. However, the poor framework stability and low dispersion in solvent constitute the key challenges to their practical applications such as luminescent inks. Herein, we report the preparation of highly luminescent inks via one-pot solution synthesis, which consisted of ionic Cu_mI_n clusters (tetrabutylammonium copper iodide) coupled with polymer polyvinylpyrrolidone (PVP). Benefiting from the high-affinity PVP to stabilize and disperse the Cu-I inorganic units, the obtained hybrid nanocrystals exhibit high structural stabilitiy/photostability and good dispersion in ethanol. The characteristics of bright white light emission from inks were explored by temperature-dependent photoluminescence experiments and theoretical calculations. Attractively, the stable, highly luminescent inks show great potential for practical applications, such as anticounterfeiting and imaging identification. Our study offers a new material designing strategy that may be generalized to many other material classes.

Enhanced Optoelectronic Performance Induced by Ion Migration in Lead-Free CsCu2I3 Single-Crystal Microrods

Lead-free perovskite has attracted great attention in realizing high-performance optoelectronic devices due to their excellent atmospheric stability and nontoxic characteristics. Although a pronounced ion migration effect has been observed in this new class of materials, its potential in enhancing the overall device performance is yet to be fully explored. In this work, we studied the effect of ion migrations on the carrier transport behavior and found that the recoverable migration process can contribute to enhancing the on/off ratio in a lead-free CsCu2I3 single-crystal microrod-based photodetector. In detail, we synthesized CsCu2I3 single-crystal microrods via an in-plane self-assembly supersaturated crystallization approach. These microrods with well-defined morphologies were then used to construct ultraviolet (UV)-band photodetectors, which outperform most reported lead-free perovskite photodetectors based on individual single crystals. Simultaneously, ion migration can result in asymmetric band bending in the two-terminal device, as confirmed by surface potential profiling with Kelvin probe force microscopy (KPFM). Such an effect can be harnessed to increase the on/off ratio by almost an order of magnitude. Furthermore, the lead-free CsCu2I3 single crystal exhibits excellent thermal and air stabilities. These findings demonstrate that the CsCu2I3 single-crystal microrods can be used in stable and efficient photodetection, and the ion migration effect can potentially be utilized for improving the optoelectronic performance of lead-free devices.

Electron-Phonon Coupling Mediated Self-Trapped-Exciton Emission and Internal Quantum Confinement in Highly Luminescent Zero-Dimensional (Guanidinium)6Mn3X12 (X = Cl and Br)

We report a large Stokes shift and broad emission band in a Mn-based organic-inorganic hybrid halide, (guanidinium)₆Mn₃Br₁₂ [GuMBr], consisting of trimeric units of distorted MnBr₆ octahedra representing a zero-dimensional compound with a liquid like crystalline lattice. Analysis of the photoluminescence (PL) line width and Raman spectra reveals the effects of electron-phonon coupling, suggestive of the formation of Frenkel-like bound excitons. These bound excitons, regarded as the self-trapped excitons (STEs), account for the large Stokes shift and broad emission band. The excited-state dynamics was studied using femtosecond transient absorption spectroscopy, which confirms the STE emission. Further, this compound is highly emissive with a PL quantum yield of ∼50%. With chloride ion incorporation, we observe enhancement of the emissive properties and attribute it to the effects of intrinsic quantum confinement. Localized electronic states in flat bands lining the gap and their strong coupling with phonons are confirmed with first-principles calculations.

Unraveling the Phase Transition and Luminescence Tuning of Pb-Free Cs-Cu-I Perovskites Enabled by Reaction Temperature and Polar Solvent

Ternary Pb-free Cs-Cu-I perovskites have attracted widespread attention because of their excellent optical properties and environmentally friendly advantages. Herein, two different Pb-free ternary Cs₃Cu₂I₅ nanocrystals (NCs) and CsCu₂I₃ microrods (MRs) were synthesized via a heating method. The phase and morphology transition from blue emission of Cs₃Cu₂I₅ NCs to yellow emission of CsCu₂I₃ MRs could be tuned effectively by manipulating the reaction temperature, decreasing the maximum photoluminescence quantum yields (PLQYs) from 82.7% to ∼10%. More interestingly, the Cs₃Cu₂I₅ NCs could self-assemble into stacking chains, which exhibited a strong dependence on the polarity of solvents. In addition, it was demonstrated that the rapid phase transition and luminescence tuning between Cs₃Cu₂I₅ and CsCu₂I₃ films took only a few seconds by direct heating or exposure to the polar solvent. This work may deepen the understanding of the phase transition process in Cu-based perovskites and provide a fluorescence material with a short switching time for anticounterfeiting applications.

Self-trapped exciton emission in inorganic copper(I) metal halides

The broad emission and high photoluminescence quantum yield of self-trapped exciton (STE) radiative recombination emitters make them an ideal solution for single-substrate, white, solid-state lighting sources. Unlike impurities and defects in semiconductors, the formation of STEs requires a lattice distortion, along with strong electron-phonon coupling, in low electron-dimensional materials. The photoluminescence of inorganic copper(I) metal halides with low electron-dimensionality has been found to be the result of STEs. These materials were of significant interest because of their lead-free, all-inorganic structures, and high luminous efficiencies. In this paper, we summarize the luminescence characteristics of zero- and one-dimensional inorganic copper(I) metal halides with STEs to provide an overview of future research opportunities.

Strong Second- and Third-Harmonic Generation in 1D Chiral Hybrid Bismuth Halides

Breaking the symmetry of a crystal structure can enable even-order nonlinear activities, including second-harmonic generation (SHG). The emerging chiral hybrid organic-inorganic metal halides feature unique optical and electronic properties and flexible crystal structures, making them a class of promising nonlinear optical materials. However, their nonlinear response performances are currently inferior to traditional nonlinear crystals, because of the lack of research on resonant enhancement and third-harmonic generation (THG). Herein, we designed chiral hybrid bismuth halides with naturally nonsymmetrical structure to enable SHG. Simultaneously, these chiral compounds preserve 1D crystal structures to create strong free exciton, broad self-trapped exciton (STE), and discrete band energy levels, which facilitate the resonant enhancement of SHG and THG susceptibilities. These new chiral films showcase superior effective SHG susceptibility (χ⁽²⁾ ∼ 130.5 pm V^-1 at an interesting wavelength of 1550 nm), exceeding that of the reference, a commercial LiNbO₃ (χ⁽²⁾ ∼ 83.4 pm V^-1) single-crystal film. Furthermore, their THG intensities are even higher than their SHG intensities, with effective THG susceptibility (χ⁽³⁾) being ∼9.0 × 10⁶ pm² V^-2 at 1550 nm (37 times that of the reference monolayer WS₂). Their high SHG and THG performances indicate the promising future of these 1D chiral hybrid bismuth halides toward nonlinear optical applications.

Deep-ultraviolet photodetector based on pulsed-laser-deposited Cs3Cu2I5 films/n-Si heterojunction

All-inorganic lead-free perovskite Cs₃Cu₂I₅ thin films were prepared using pulsed laser deposition. Effects of the substrate temperature, laser energy, and laser frequency on the film structure and optoelectronic properties were studied. A heterojunction photodetector based on Cs₃Cu₂I₅/n-Si was constructed, and the deep-ultraviolet photoresponse was obtained. A high I_light/I_dark ratio of 130 was achieved at -1.3V, and the peak response of the heterojunction photodetector was 70.8 mA/W (280 nm), with the corresponding specific detectivity of 9.44×10¹¹cm⋅Hz^1/2⋅W^-1. Moreover, the device showed good stability after being exposed to air for 30 days.

Highly Luminescent Zero-Dimensional Organic Copper Halides for X-ray Scintillation

The present work reports highly efficient flexible and reabsorption-free scintillators based on two zero-dimensional (0D) organic copper halides (TBA)CuX₂ (TBA = tetrabutylammonium cation; X = Cl, Br). The (TBA)CuX₂ exhibit highly luminescent green and sky-blue emissions peaked at 510 and 498 nm, with large Stokes shifts of 224 and 209 nm and high photoluminescence quantum yields (PLQYs) of 92.8% and 80.5% at room temperature for (TBA)CuCl₂ and (TBA)CuBr₂ single crystals (SCs), respectively. Interestingly, above room temperature, their PLQYs increase with temperature and reach near unity at 320 and 345 K for (TBA)CuCl₂ and (TBA)CuBr₂, respectively. The excellent properties originate from self-trapped excitons (STEs) in individual [CuX₂]^- quantum rods, which is demonstrated by the temperature-dependent PL, ultrafast transient absorption (TA) combined with density functional theory (DFT) calculations. The (TBA)CuX₂ scintillators show bright radioluminescence (RL), impressive linear response to dose rate in a broad range, and high light yields. Their potential application in X-ray imaging is demonstrated by using (TBA)CuX₂ composite scintillation screens. Importantly, flexible scintillators are demonstrated to be superior than flat ones for imaging nonplanar objects by conformally coating, which produce accurate images with negligible distortion.

Photoinduced Dynamics of (CH3NH3)4Cu2Br6 Thin Films Indicating Efficient Triplet Photoluminescence

Hybrid organic-inorganic halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for their efficient, strongly Stokes-shifted emission is still unclear. Here we report the successful preparation of (CH₃NH₃)₄Cu₂Br₆ thin films with a zero-dimensional molecular salt structure featuring "isolated" [Cu₂Br₆]^4- ions. Time-resolved broadband PL measurements provide an excited-state lifetime of 114 μs at 298 K. Results from femto- to microsecond UV-vis-NIR transient absorption experiments combined with DFT/TDDFT calculations suggest the formation of a long-lived structurally relaxed triplet species through intersystem crossing (61 ps), which almost exclusively decays by phosphorescence. In addition, time scales for structural relaxation and cooling processes are extracted from a global kinetic analysis of the transient spectra. Calculations for the isolated [Cu₂Br₆]^4- anion and the (CH₃NH₃)₄Cu₂Br₆ crystal suggest a strong impact of the crystal environment on the structure of the anion.

Highly Efficient Cool-White Photoluminescence of (Gua)3Cu2I5 Single Crystals: Formation and Optical Properties

Zero-dimensional lead-free organic-inorganic hybrid metal halides have drawn attention as a result of their local metal ion confinement structure and photoelectric properties. Herein, a lead-free compound of (Gua)₃Cu₂I₅ (Gua = guanidine) with a different metal ion confinement has been discovered, which possesses a unique [Cu₂I₅]^3- face-sharing tetrahedral dimer structure. First-principles calculation demonstrates the inherent nature of a direct band gap for (Gua)₃Cu₂I₅, and its band gap of ∼2.98 eV was determined by experiments. Worthy of note is that (Gua)₃Cu₂I₅ exhibits a highly efficient cool-white emission peaking at 481 nm, a full-width at half-maximum of 125 nm, a large Stokes shift, and a photoluminescence quantum efficiency of 96%, originating from self-trapped exciton emission. More importantly, (Gua)₃Cu₂I₅ single crystals have a reversible thermoinduced luminescence characteristic due to a structural transition scaled by the electron-phonon coupling coefficients, which can be converted back and forth between cool-white and yellow color emission by heating or cooling treatment within a short time. In brief, as-synthesized (Gua)₃Cu₂I₅ shows great potential for application both in single-component white solid-state lighting and sensitive temperature scaling.

Oriented-Structured CsCu2I3 Film by Close-Space Sublimation and Nanoscale Seed Screening for High-Resolution X-ray Imaging

An all-inorganic lead-free halides Cs-Cu-I system, represented by Cs₃Cu₂I₅ and CsCu₂I₃, has attracted attention for their good photophysical characteristics recently. Successive works had reported their application potential in light-emitting devices. However, there is no report for CsCu₂I₃ in X-ray scintillation detectors so far. We notice that CsCu₂I₃ may be advantageous in such an application due to the one-dimensional crystal structure, the congruent-melting feature, and the high spectral matching to some photosensors. In this work, we explore the scintillation properties and imaging application of CsCu₂I₃ in X-ray scintillator detector. The oriented structure is designed to enhance the imaging performance of a CsCu₂I₃ detector. Close-space sublimation process and nanoscale seed screening strategy are employed to realize this design by producing a large-area (25 cm²) CsCu₂I₃ thick film layer with the oriented nanorod structure. This CsCu₂I₃ detector eventually achieves a high spatial resolution of 7.5 lp mm^-1 in X-ray imaging.

All-inorganic copper(i)-based ternary metal halides: promising materials toward optoelectronics

All-inorganic lead halides, including CsPbX₃ (X = Cl, Br, I), have become important candidate materials in the field of optoelectronics. However, the inherent toxicity of metal lead and poor material stability have hindered further applications of traditional metal halides, CsPbX₃. Therefore, copper(i)-based ternary metal halides are expected to become promising substitutes for traditional metal halides because of their nontoxicity, excellent optical properties and good stability under ambient conditions. This article reviews the recent development of all-inorganic low-dimensional copper(i)-based ternary metal halides by introducing their various synthesis methods, crystal structures, properties and their optoelectronic applications. In addition, the prospects for future challenges and the potential significance of copper(i)-based ternary metal halides in optoelectronic fields are presented.