Lead‐Free Sodium–Indium Double Perovskite Nanocrystals through Doping Silver Cations for Bright Yellow Emission

Direction Dependent Ferroelectricity and Conductivity in a Single Crystal 2D Halide Double Perovskite

Halide ferroelectric materials have garnered a lot of interest because of their distinctive electrical and structural characteristics. In this study, the design and development of a new non-centrosymmetric 2D layered halide double perovskite material, Cl1.14Br2.86PA4AgInBr8 (CPAIn) is reported. This material shows ferroelectric properties above room temperature, with a Curie temperature of 190 °C. This behavior is achieved through the substitution of the halogenated A-site organic linker, 3-chloropropylammonium. CPAIn exhibits anisotropic ferroelectric behavior with higher spontaneous polarization of 6.25 µC cm-2 along the perpendicular direction to the octahedral layers, whereas the value decreases to 0.174 µC cm-2 between sheets. While using bottom contact to study the nature of polarity within a sheet, the P-E loop displays capacitive loop. The nature and value of polarization is highly direction dependent, and to further understand the mechanism of conduction, a combination of temperature-dependent impedance studies and poling dependent conductivity techniques are employed. These directional dependent properties hold immense potential in memory devices, sensors and photovoltaics, piezoelectric devices and energy storage.

Sb3+/Sm3+ Codoped Cs2NaScCl6 All-Inorganic Double Perovskite: Blue Emission of Self-Trapped Excitons and Red-Emission via Energy Transfer

The lead-free halide perovskites possess nontoxicity and excellent chemical stability, whereas relatively weak luminescence intensity limits their potential in practical applications. Therefore, strengthening the luminescence intensity and expanding application fields are urgent tasks for the development of lead-free halide perovskites. In this paper, antimony-doped Cs2NaScCl6 crystals synthesized by a solvothermal method emit bright, deep blue photoluminescence at 447 nm. The photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectra demonstrate that Sb3+ doping effectively activate the intrinsic "dark self-trapped exciton (STE)," leading to an impressive photoluminescence quantum yield (PLQY) value of 78.31% for 1% Sb3+ doping. Furthermore, the luminescence intensity remains above 92% compared with the fresh sample without secondary phases detected even after 90 days under environmental conditions. To expand the emission spectra, rare-earth Sm3+ is further incorporated into Cs2NaScCl6:1% Sb3+ crystals. The results show that Sb ions not only enhance intrinsic STE luminescence but also serve as sensitizers to boost the red-light emission of Sm3+, leading to a significant 500-fold increase in red emission intensity. Finally, the PLQY reaches a stunning 86.78%. These findings provide valuable insights in the design of Sb ion-doped lead-free double perovskites, broadening the application fields in various optoelectronic 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.

Lead-free double perovskite Cs2NaInCl6 nanocrystals doped with Sb3+ and Tb3+ for copper ions detection in lubricating oil

Detecting heavy metal copper ions in lubricating oil holds immense significance for assessing mechanical wear and predicting mechanical failure. While perovskite nanocrystals offer high sensitivity in detecting copper ions, traditional lead halide perovskites suffer from lead toxicity defects. Lead-free perovskites, like Cs2NaInCl6, avoid the issue of lead toxicity but display lower luminescence intensity due to the presence of forbidden optical transitions. To address these issues, this study synthesized Cs2NaInCl6 nanocrystals (NCs) co-doped with Sb3+ and Tb3+ ions for copper ions detection in lubricating oil. The introduction of Sb3+ effectively reduced the band gap of the Cs2NaInCl6 host, creating an energy transfer pathway for Tb3+ emission via self-trapped excitations (STEs). Moreover, the doping of Tb3+ ions resulted in the suppression of STEs emission due to electron transfer from STEs to Tb3+. The emission of Tb3+ increased initially and then decreased with the increasing Tb3+ concentration, peaking at 40 %. Finally, Cs2NaInCl6: 2.5 %Sb3+, 40 %Tb3+ NCs were employed as probes for copper ions detection, exhibiting superior sensitivity and selectivity compared to similar probes. The presence of copper ions introduced competition between copper and Tb3+ for electrons from STEs, consequently leading to the quenching of multiple emission intensities associated with STEs and Tb3+. This method shows promising potential in predicting mechanical failure.

Exploring Highly Efficient Broadband Self-Trapped-Exciton Luminophors: from 0D to 3D Materials

The review summarizes our recent reports on brightly-emitting materials with varied dimensionality (3D, 2D, 0D) synthesized using "green" chemistry and exhibiting highly efficient photoluminescence (PL) originating from self-trapped exciton (STE) states. The discussion starts with 0D emitters, in particular, ternary indium-based colloidal quantum dots, continues with 2D materials, focusing on single-layer polyheptazine carbon nitride, and further evolves to 3D luminophores, the latter exemplified by lead-free double halide perovskites. The review shows the broadband STE PL to be an inherent feature of many materials produced in mild conditions by "green" chemistry, outlining PL features general for these STE emitters and differences in their photophysical properties. The review is concluded with an outlook on the challenges in the field of STE PL emission and the most promising venues for future research.

Sb-Doped Cs3 TbCl6 Nanocrystals for Highly Efficient Narrow-Band Green Emission and X-Ray Imaging

Metal halide nanocrystals (NCs) with high photoluminescence quantum yield (PLQY) are desirable for lighting, display, and X-ray detection. Herein, the novel lanthanide-based halide NCs are committed to designing and optimizing the optical and scintillating properties, so as to unravel the PL origin, exciton dynamics, and optoelectronic applications. Sb-doped zero-dimensional (0D) Cs3 TbCl6 NCs exhibit a green emission with a narrow full width of half maximum of 8.6 nm, and the best PLQY of 48.1% is about three times higher than that of undoped NCs. Experiments and theoretical calculations indicate that 0D crystalline and electronic structures make the exciton highly localized on [TbCl6 ]3- octahedron, which boosts the Cl- -Tb3+ charge transfer process, thus resulting in bright Tb3+ emission. More importantly, the introduction of Sb3+ not only facilitates the photon absorption transition, but also builds an effective thermally boosting energy transfer channel assisted by [SbCl6 ]3- -induced self-trapped state, which is responsible for the PL enhancement. The high luminescence efficiency and negligible self-absorption of the Cs3 TbCl6 : Sb nanoscintillator enable a more sensitive X-ray detection response compared with undoped sample. The study opens a new perspective to deeply understand the excited state dynamics of metal halide NCs, which helps to design high-performance luminescent lanthanide-based nanomaterials.

Broadband Emission Origin in Metal Halide Perovskites: Are Self-Trapped Excitons or Ions?

It has always been a goal to realize high efficiency and broadband emission in single-component materials. The appearance of metal halide perovskites makes it possible. Their soft lattice characteristics and significant electron-phonon coupling synergistically generate self-trapped excitons (STEs), contributing to a broadband emission with a large Stokes shift. Meanwhile, their structural/compositional diversity provides suitable active sites and coordination environments for doping of ns2 ions, allowing 3 Pn ( n =0,1,2) →1 S0 transitions toward broadband emission. The ns2 ions emission is phenomenologically similar to that of STE emission, hindering in-depth understanding of their emission origin, and leading to failure to meet the design requirements for practical applications. In this scenario, herein, the fundamentals and development of such two emission mechanisms are summarized to establish a clear and comprehensive understanding of the broadband emission phenomenon, which may pave the way to an ideal customization of broadband-emission metal halide perovskites.

Pressure Coupled Lanthanide Ion Doping to Enhance Optical Properties in BaTiO3

Self-trapped excitons (STEs) typically give broadband photoluminescence emission with a large Stokes shift, which is important for the enhancement of the optical properties of materials. Here, low-dimensional La-doped BaTiO3 nanocrystals with defects are prepared using supercritical CO2 (SC CO2 ). The generation of the STEs is facilitated by doping La3+ ions and introducing CO2 pressure, which effectively enhance the luminescence intensity of BaTiO3 . This discovery shows that the La ion doping concentration can modulate the photoluminescence of BaTiO3 nanocrystals under pressure. This work deepens the understanding of the influence of rare-earth-doped luminescent materials under pressure and provides insight to improve the capabilities of optical devices.

Manipulating Lone-Pair-Driven Luminescence in 0D Tin Halides by Pressure-Tuned Stereochemical Activity from Static to Dynamic

The excellent luminescence properties and structural dynamics driven by the stereoactivity of the lone pair in a variety of low-dimensional ns2 metal halides have attracted growing investigations for optoelectronic applications. However, the structural and photophysical aspects of the excited state associated with the lone pair expression are currently open questions. Herein, zero-dimensional Sn-based halides with static stereoactive 5 s2 lone pairs are selected as a model system to understand the correlations between the distinctive lone pair expression and the excited-state structural relaxation and charge carrier dynamics by continuous lattice manipulation. Lattice compression drives 5 s2 lone pair active switching and self-trapped exciton (STE) redistribution by suppressing excited-state structural deformation of the isolated SnBr4 2- units. Our results demonstrate that the static expression of the 5 s2 lone pair results in a red broadband triplet STE emission with a large Stokes shift, while its dynamic expression creates a sky-blue narrowband emission dominated by the radiative recombination of singlet STEs. Our findings and the photophysical mechanism proposed highlight the stereochemical effects of lone pair expression in controlling light emission properties and offer constructive guidelines for tuning the optoelectronic properties in diverse ns2 metal halides.

Highly efficient, self-powered UV photodiodes based on leadfree perovskite nanocrystals through interfacial engineering

Double perovskite crystals are promising alternatives for lead-based perovskites that has potential to address toxicity and instability issues. In this study, Cs2AgBiCl6nanocrystals (NCs) with high absorption coefficients were synthesized by hot-injection method. The bandgap engineering was realized by tuning the halide composition in Cs2AgBiCl6to Cs2AgBiBr6. Both NCs were used as light-absorbing layers in lead-free perovskite photodiodes that exhibit wavelength-selectivity for UV-visible light operatable even at a bias voltage of 0 V. Cs2AgBiBr6-based photodiode exhibits a characteristic detection peak at 340 nm with a responsivity of 3.21 mA W-1, a specific detectivity up to 8.91 × 1010Jones and a fast response speed with a rise/fall time of 30/35 ms. The excellent performance of self-driven photodiodes lights up the prospect of lead-free perovskite NCs in highly efficient optoelectronic devices.

Valence conversion and site reconstruction in near-infrared-emitting chromium-activated garnet for simultaneous enhancement of quantum efficiency and thermal stability

Achievement of high photoluminescence quantum efficiency and thermal stability is challenging for near-infrared (NIR)-emitting phosphors. Here, we designed a "kill two birds with one stone" strategy to simultaneously improve quantum efficiency and thermal stability of the NIR-emitting Ca3Y2-2x(ZnZr)xGe3O12:Cr garnet system by chemical unit cosubstitution, and revealed universal structure-property relationship and the luminescence optimization mechanism. The cosubstitution of [Zn2+-Zr4+] for [Y3+-Y3+] played a critical role as reductant to promote the valence transformation from Cr4+ to Cr3+, resulting from the reconstruction of octahedral sites for Cr3+. The introduction of [Zn2+-Zr4+] unit also contributed to a rigid crystal structure. These two aspects together realized the high internal quantum efficiency of 96% and excellent thermal stability of 89%@423 K. Moreover, information encryption with "burning after reading" was achieved based on different chemical resistance of the phosphors to acid. The developed NIR-emitting phosphor-converted light-emitting diode demonstrated promising applications in bio-tissue imaging and night vision. This work provides a new perspective for developing high-performance NIR-emitting phosphor materials.

Metal Halide Single Crystals RbCdCl3:Sn2+ and Rb3SnCl7 with Blue and White Emission Obtained via a Hydrothermal Process

Until now, effective blue light-emitting materials are essentially needed for the creation of white light and precise color renderings in real-world applications, but the efficiency of blue light-emitting materials has lagged far behind. Here, we present a hydrothermal method to synthesize tin-based metal halide single crystals (RbCdCl3:Sn2+ and Rb3SnCl7). Two single crystal materials with different shapes and phases can simultaneously be synthesized in the same stoichiometric ratio. Rb3SnCl7 has a bulk shape, while RbCdCl3:Sn2+ has a needle shape. The deep blue emission (436 nm) of RbCdCl3:Sn2+ can be obtained under the optimal excitation wavelength irradiation. However, pure blue emission (460 nm) to white light can be obtained by changing the excitation wavelength in Rb3SnCl7. The refinement spectra of the electronic structures of RbCdCl3:Sn2+ and Rb3SnCl7 are investigated by density functional theory. It is concluded that the difference in the distribution of Cl energy states leads to the existence of Cl local defect states, which is the reason for the rich luminescence of the two single crystals. These findings provide a path for realizing single-phase broadband white-emitting materials.

Bright Green-Emitting All-Inorganic Terbium Halide Double Perovskite Nanocrystals for Low-Dose X-ray Imaging

Inorganic halide double perovskite (DP) nanocrystals (NCs) have attracted great attention because of their nontoxicity, mild reaction conditions, good stability, and excellent optical and optoelectronic properties. Herein, we prepare the inorganic terbium halide DP Cs2BTbCl6 (B = Na or Ag) NCs with bright green photoluminescence (PL) emission. The Na-Tb-based DP NCs exhibit better PL properties compared with the Ag-Tb-based DP NCs, which is due to Cs2NaTbCl6 NCs having a more localized charge carrier distribution on the [TbCl6]3- octahedron. The incorporation of Sb3+ dopant in Cs2NaTbCl6 NCs can construct a more efficient energy transfer process, resulting in a doubling of PL efficiency. Furthermore, Cs2NaTbCl6: Sb3+ NCs possess excellent X-ray scintillating performance with a low-dose detection limit of 140 nGyair/s, which is nearly 5 times more sensitive than the undoped NCs. The optimized NCs show great application prospects in X-ray imaging. This work helps deepen the understanding of the luminescence mechanism, excited state dynamics, and scintillation property in Tb-based DP NCs.

Manipulation of Shallow-Trap States in Halide Double Perovskite Enables Real-Time Radiation Dosimetry

Storage phosphors displaying defect emissions are indispensable in technologically advanced radiation dosimeters. The current dosimeter is limited to the passive detection mode, where ionizing radiation-induced deep-trap defects must be activated by external stimulation such as light or heat. Herein, we designed a new type of shallow-trap storage phosphor by controlling the dopant amounts of Ag+ and Bi3+ in the host lattice of Cs2NaInCl6. A distinct phenomenon of X-ray-induced emission (XIE) is observed for the first time in an intrinsically nonemissive perovskite. The intensity of XIE exhibits a quantitative relationship with the accumulated dose, enabling a real-time radiation dosimeter. Thermoluminescence and in situ X-ray photoelectron spectroscopy verify that the emission originates from the radiative recombination of electrons and holes associated with X-ray-induced traps. Theoretical calculations reveal the evolution process of Cl-Cl dimers serving as hole trap states. Analysis of temperature-dependent radioluminescence spectra provides evidence that the intrinsic electron-phonon interaction in 0.005 Ag+@ Cs2NaInCl6 is significantly reduced under X-ray irradiation. Moreover, 0.025 Bi3+@ Cs2NaInCl6 shows an elevated sensitivity to the accumulated dose with a broad response range from 0.08 to 45.05 Gy. This work discloses defect manipulation in halide double perovskites, giving rise to distinct shallow-trap storage phosphors that bridge traditional deep-trap storage phosphors and scintillators and enabling a brand-new type of material for real-time radiation dosimetry.

Surface Reconstruction of Lead-Free Perovskite Cs2Ag0.6Na0.4InCl6:Bi by Hydroxylation with Blue-Light-Excited Performance

Molecular surface reconfiguration strategies have been instrumental to performance improvements of halide perovskite photovoltaic applications in recent years. However, research into the optical properties of the lead-free double perovskite Cs₂AgInCl₆ on the complex reconstructed surface is still lacking. Here, blue-light excitation in double perovskite Cs₂Na_0.4Ag_0.6InCl₆ with Bi doping has been successfully achieved by excess KBr coating and ethanol-driven structural reconstruction. Ethanol drives the formation of hydroxylated Cs_2-yK_yAg_0.6Na_0.4In_0.8Bi_0.2Cl_6-yBr_y in the Cs₂Ag_0.6Na_0.4In_0.8Bi_0.2Cl₆@xKBr interface layer. The hydroxyl group adsorbed on the interstitial sites of the double perovskite structure induces a transfer of local space electrons to the [AgCl₆] and [InCl₆] octahedral regions, enabling them to be excited with blue light (467 nm). The passivation of KBr shell reduces the non-radiative transition probability of excitons. Blue-light-excited flexible photoluminescence devices based on hydroxylated Cs₂Ag_0.6Na_0.4In_0.8Bi_0.2Cl₆@16KBr are fabricated. The application of hydroxylated Cs₂Ag_0.6Na_0.4In_0.8Bi_0.2Cl₆@16KBr as down-shift layer in GaAs photovoltaic cell module can increase its power conversion efficiency by 3.34%. The surface reconstruction strategy provides a new way to optimize the performance of lead-free double perovskite.

Facile synthesis of Sb3+-doped (Bmim)2InCl5(H2O) through a grinding method for light-emitting diodes

Organic-inorganic metal hybrid halides (OIMHs) as a new kind of photoelectric material have gained much attention in recent years because of their excellent performance in solid-state lighting applications. However, the preparation of most OIMHs is complex and requires a long preparation time in addition to the solvent providing the reaction environment. This greatly limits their further applications. Here, we synthesized zero-dimensional lead-free OIMH (Bmim)₂InCl₅(H₂O) (Bmim = 1-butyl-3-methylimidazolium) by a facile grinding method at room temperature. Through Sb³⁺ doping, Sb³⁺:(Bmim)₂InCl₅(H₂O) shows a bright broadband emission centered at 618 nm under UV excitation, which could be attributed to the self-trapped exciton (STE) emission of Sb³⁺ ions. To explore their ability in the field of solid-state lighting, a white-light-emitting diode (WLED) device based on Sb³⁺:(Bmim)₂InCl₅(H₂O) with a high color rendering index of 90 was fabricated. This work enriches In³⁺-based OIMHs and provides a new direction for the simple fabrication of OIMHs.

Halide Double Perovskite Nanocrystals Doped with Rare-Earth Ions for Multifunctional Applications

Most lead-free halide double perovskite materials display low photoluminescence quantum yield (PLQY) due to the indirect bandgap or forbidden transition. Doping is an effective strategy to tailor the optical properties of materials. Herein, efficient blue-emitting Sb³⁺ -doped Cs₂ NaInCl₆ nanocrystals (NCs) are selected as host, rare-earth (RE) ions (Sm³⁺ , Eu³⁺ , Tb³⁺ , and Dy³⁺ ) are incorporated into the host, and excellent PLQY of 80.1% is obtained. Femtosecond transient absorption measurement found that RE ions not only served as the activator ions but also filled the deep vacancy defects. Anti-counterfeiting, optical thermometry, and white-light-emitting diodes (WLEDs) are exhibited using these RE ions-doped halide double perovskite NCs. For the optical thermometry based on Sm³⁺ -doped Cs₂ NaInCl₆ :Sb³⁺ NCs, the maximum relative sensitivity is 0.753% K^-1 , which is higher than those of most temperature-sensing materials. Moreover, the WLED fabricated by Sm³⁺ -doped Cs₂ NaInCl₆ :Sb³⁺ NCs@PMMA displays CIE color coordinates of (0.30, 0.28), a luminous efficiency of 37.5 lm W^-1 , a CCT of 8035 K, and a CRI over 80, which indicate that Sm³⁺ -doped Cs₂ NaInCl₆ :Sb³⁺ NCs are promising single-component white-light-emitting phosphors for next-generation lighting and display technologies.

Unveiling the Localized Exciton-Based Photoluminescence of Manganese Doped Cesium Zinc Halide Nanocrystals

Lead-free metal halide nanocrystals (NCs) have aroused increasing attention due to their unique optoelectronic properties based on localized excitons (LEs). However, the vital influencing factors for the LEs based photoluminescence (PL) are still not well-understood due to the coupling of various intrinsic and extrinsic factors of the NCs. Herein, by engineering the phase, size, morphology, and chemical composition, we are able to decouple the intrinsic and extrinsic factors of manganese doped cesium zinc-halide NCs. We found both the intrinsic metal-halide coordination field and the extrinsic crystal defects have significant influences on the LEs' recombination and energy transfer processes, and hence determine the PL efficiency. Unlike for the free excitons (FEs) based PL, the phase as well as the crystal morphology do not play major roles for the LEs based PL. This work provides a new insight for the study of LE dynamics of metal halide NCs.

Revealing the Mechanism of Pressure-Induced Emission in Layered Silver-Bismuth Double Perovskites

Pressure-induced emission (PIE) associated with self-trapping excitons (STEs) in low-dimensional halide perovskites has attracted great attention for better materials-by-design. Here, using 2D layered double perovskite (C₆ H₅ CH₂ CH₂ NH₃ ⁺ )₄ AgBiBr₈ as a model system, we advance a fundamental physicochemical mechanism of the PIE from the perspective of carrier dynamics and excited-state behaviors of local lattice distortion. We observed a pressure-driven STE transformation from dark to bright states, corresponding a strong broadband Stokes-shifted emission. Further theoretical analysis demonstrated that the suppressed lattice distortion and enhanced electronic dimensionality in the excited-state play an important role in the formation of stabilized bright STEs, which could manipulate the self-trapping energy and lattice deformation energy to form an energy barrier between the potential energy curves of ground- and excited-state, and enhance the electron-hole orbital overlap, respectively.

Wide-coverage and Efficient NIR Emission from Single-component Nanophosphors through Shaping Multiple Metal-halide Packages

Wide-coverage near infrared (NIR) phosphor-converted LEDs possess promising potential for practical applications, but little is developed towards the efficient and wide-coverage NIR phosphors. Here, we report the single-component lanthanide (Ln³⁺ ) ions doped Cs₂ M(In_0.95 Sb_0.05 )Cl₆ (M=alkali metal) nanocrystals (NCs), exhibiting emission from 850 to 1650 nm with high photoluminescence quantum yield of 20.3 %, which is accomplished by shaping the multiple metal halide octahedra of double perovskite via the simple alkali metal substitution. From Judd-Ofelt theoretical calculation and spectroscopic investigations, the shaping of metal halide octahedra in Cs₂ M(In_1-x Sb_x )Cl₆ NCs can break the forbidden of f-f transition of Ln³⁺ , thus increasing their radiative transition rates and simultaneously boosting the energy transfer efficiency from host to Ln³⁺ . Finally, the wide-coverage NIR LEDs based on Sm³⁺ , Nd³⁺ , Er³⁺ -tridoped Cs₂ K_0.5 Rb_0.5 (In_0.95 Sb_0.05 )Cl₆ NCs are fabricated and employed in the multiplex gas sensing and night-vision application.

Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

A universal hydrochloric acid-assistant powder-to-powder strategy for quick and mass preparation of lead-free perovskite microcrystals

Lead-free halide perovskite materials possess low toxicity, broadband luminescence and robust stability compared with conventional lead-based perovskites, thus holding great promise for eyes-friendly white light LEDs. However, the traditionally used preparation methods with a long period and limited product yield have curtailed the commercialization of these materials. Here we introduce a universal hydrochloric acid-assistant powder-to-powder strategy which can accomplish the goals of thermal-, pressure-free, eco-friendliness, short time, low cost and high product yield, simultaneously. The obtained Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ microcrystals exhibit bright self-trapped excitons emission with quantum yield of (98.3 ± 3.8)%, which could retain (90.5 ± 1.3)% and (96.8 ± 0.8)% after continuous heating or ultraviolet-irradiation for 1000 h, respectively. The phosphor converted-LED exhibited near-unity conversion efficiency from ultraviolet chip to self-trapped excitons emission at ~200 mA. Various ions doping (such as Cs₂Na_0.9Ag_0.1InCl₆:Ln³⁺) and other derived lead-free perovskite materials (such as Cs₂ZrCl₆ and Cs₄MnBi₂Cl₁₂) with high luminous performance are all realized by our proposed strategy, which has shown excellent availability towards commercialization.

Lead-Free Solid State Mechanochemical Synthesis of Cs2NaBi1-xFexCl6 Double Perovskite: Reduces Band Gap and Enhances Optical Properties

Efficient and stable lead-free halide double perovskites (DPs) have attracted great attention for the future generation of electronic devices. Herein, we have developed a doping approach to incorporate Fe³⁺ ions into the Cs₂NaBiCl₆ crystal unit and reveal a crystallographic and optoelectronic study of the Cs₂NaBi_1-xFe_xCl₆ double perovskite. We report a simple solid-state mechanochemical method that has a solvent-free, one-step, green chemistry approach for the synthesis of Cs₂NaBi_1-xFe_xCl₆ phosphor. The analysis of powder X-ray diffraction (XRD) data determines the contraction of the lattice due to the incorporation of Fe³⁺ cations, and this effect is well supported by X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and solid-state nuclear magnetic resonance spectroscopy (ss-NMR). The band gap is reduced with increasing Fe content owing to the strong overlap of the Fe-3d orbitals with Cl-3p orbitals and shift of the valence band maxima (VBM) toward higher energies, as confirmed by ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) analyses. Photoluminescence (PL) studies of Cs₂NaBi_1-xFe_xCl₆ phosphors exhibit a large Stokes shift, broadband emission, and increased PL intensity more than ten times for 15% of Fe content phosphor with enhancement in the average decay lifetimes (up to 38 ns) compared to pristine Cs₂NaBiCl₆ DP. These results indicate that the transition of dark self-trapping of excitons (STEs) into bright STEs enhances yellow emission. XRD, UV, and thermo-gravimetric analysis (TGA) confirmed that the Cs₂NaB_1-xFe_xCl₆ DPs have good structural and thermal stabilities. Our findings indicate that the doping of Fe³⁺ cations into the Cs₂NaBiCl₆ lattice is a constructive strategy to enhance significantly the optoelectronic properties of these phosphors.

Modulating the afterglow time of Mn2+ doped double perovskites by size tuning and its applications in dynamic information display

Mn²⁺ doped lead-free double perovskites are emerging afterglow materials that can avoid the usage of rare earth ions. However, the regulation of the afterglow time is still a challenge. In this work, the Mn doped Cs₂Na_0.2Ag_0.8InCl₆ crystals with afterglow emission at about 600 nm are synthesized by a solvothermal method. Then, the Mn²⁺ doped double perovskite crystals are crushed into different sizes. As the size decreases from 1.7 mm to 0.075 mm, the afterglow time decreases from 2070 s to 196 s. Steady-state photoluminescence (PL) spectra, time resolved PL, thermoluminescence (TL) reveal the afterglow time monotonously decreases due to the enhanced nonradiative surface trapping. The modulation on afterglow time will greatly promote their applications in various fields, such as bioimaging, sensing, encryption, and anti-counterfeiting. As a proof of concept, dynamic display of information is realized based on different afterglow times.

The Role of Polymers in Halide Perovskite Resistive Switching Devices

Currently, halide perovskites (HPs) are gaining traction in multiple applications, such as photovoltaics and resistive switching (RS) devices. In RS devices, the high electrical conductivity, tunable bandgap, good stability, and low-cost synthesis and processing make HPs promising as active layers. Additionally, the use of polymers in improving the RS properties of lead (Pb) and Pb-free HP devices was described in several recent reports. Thus, this review explored the in-depth role of polymers in optimizing HP RS devices. In this review, the effect of polymers on the ON/OFF ratio, retention, and endurance properties was successfully investigated. The polymers were discovered to be commonly utilized as passivation layers, charge transfer enhancement, and composite materials. Hence, further HP RS improvement integrated with polymers revealed promising approaches to delivering efficient memory devices. Based on the review, detailed insights into the significance of polymers in producing high-performance RS device technology were effectively understood.

Modulation of the Excitation States in All-Inorganic Halide Perovskites via Sb3+ and Bi3+ Codoping

Sb³⁺-doped halide perovskites are promising candidates for solid-state lighting due to their diverse fluorescent colors and high efficiency. However, the mismatched high excitation energy with commercial UV chips is one of the critical issues to be addressed. Herein, a Bi³⁺ codoping strategy was established as a general and efficient approach to modulate the excitation spectrum from the Sb³⁺-doping center in all-inorganic perovskites of Cs₂InCl₅·H₂O, Cs₂NaInCl₆, and Rb₃InCl₆. The incorporated Bi³⁺ greatly enhanced the splitting of the A band (¹S₀-³P₁ transition) and boosts the enormous redshift of the low-energy branch in all these systems. The interactions persist strongly even at extremely low doping concentrations, suggesting a dipole-based long-range interaction. The results provide an in-depth insight into the contribution mechanism of Bi³⁺ to Sb³⁺ in all-inorganic perovskites, which throws light upon tuning the excitation spectrum of broadband emission from the extrinsic self-trapped exciton (STE).

Tb3+ and Bi3+ Co-Doping of Lead-Free Cs2NaInCl6 Double Perovskite Nanocrystals for Tailoring Optical Properties

Lead halide perovskites have achieved remarkable success in various photovoltaic and optoelectronic applications, especially solar cells and light-emitting diodes (LEDs). Despite the significant advances of lead halide perovskites, lead toxicity and insufficient stability limit their commercialization. Lead-free double perovskites (DPs) are potential materials to address these issues because of their non-toxicity and high stability. By doping DP nanocrystals (NCs) with lanthanide ions (Ln³⁺), it is possible to make them more stable and impart their optical properties. In this work, a variable temperature hot injection method is used to synthesize lead-free Tb³⁺-doped Cs₂NaInCl₆ DP NCs, which exhibit a major narrow green photoluminescence (PL) peak at 544 nm derived from the transition of Tb^{3+ 5}D₄→⁷F₅. With further Bi³⁺ co-doping, the Tb³⁺-Bi³⁺-co-doped Cs₂NaInCl₆ DP NCs are not only directly excited at 280 nm but are also excited at 310 nm and 342 nm. The latter have a higher PL intensity because partial Tb³⁺ ions are excited through more efficient energy transfer channels from the Bi³⁺ to the Tb³⁺ ions. The investigation of the underlying mechanism between the intrinsic emission of Cs₂NaInCl₆ NCs and the narrow green PL caused by lanthanide ion doping in this paper will facilitate the development of lead-free halide perovskite NCs.

Doping/Alloying Pathways to Lead-Free Halide Perovskites with Ultimate Photoluminescence Quantum Yields

Tailored modifications of halide lead-free perovskites (LFPs) via doping/alloying with metal cations have been recognized as a promising pathway to highly efficient inorganic phosphors with photoluminescence (PL) quantum yields of up to 100 %. Such materials typically display selective sensitivity to UV light, a broad PL range, and long PL lifetimes as well as a unique compositional variability and stability-an ideal combination for many light-harvesting applications. This Minireview presents the state-of-the-art in doped LFPs, focusing on the reports published mostly in the last two to three years. We discuss the factors determining the efficiency and spectral parameters of the broadband PL of doped LFPs depending on the dopant and host matrix, both in micro- and nanocrystalline states, address the most relevant challenges this rapidly developing research area is facing, and outline the most promising concepts for further progress in this field.

Cubic Halide Double Perovskite Nanocrystals with Anisotropic Free Excitons and Self-Trapped Exciton Photoluminescence

In this work, we first developed Cs₂KBiCl₆ cubic double perovskite nanocrystals and a series of morphologically isotropic double perovskite nanocrystals. Different contributions of different elements to self-trapped states were revealed by density functional theory. Meanwhile, these double perovskite nanocrystals exhibit the coexistence of free and self-trapped exciton dual-color photoluminescence. Femtosecond transient absorption spectroscopy can confirm that the double perovskite nanocrystals produce a relatively obvious structural deformation in the excited state. We infer that this can lead to a large deviation of the excitation and emission transition dipoles, thus causing large photoluminescence anisotropy. Most importantly, we observe for the first time that both free exciton emission and self-trapped exciton emission are highly anisotropic, which are comparable to or even better than that of lead halide perovskites. This research paves the way for exploring more possibilities and practical applications.

Highly Luminescent One-Dimensional Organic-Inorganic Hybrid Double-Perovskite-Inspired Materials for Single-Component Warm White-Light-Emitting Diodes

Double perovskites (DPs) are one of the most promising candidates for developing white light-emitting diodes (WLEDs) owing to their intrinsic broadband emission from self-trapped excitons (STEs). Translation of three-dimensional (3D) DPs to one-dimensional (1D) analogues, which could break the octahedral tolerance factor limit, is so far remaining unexplored. Herein, by employing a fluorinated organic cation, we report a series of highly luminescent 1D DP-inspired materials, (DFPD)₂ M^I InBr₆ (DFPD=4,4-difluoropiperidinium, M^I =K⁺ and Rb⁺ ). Highly efficient warm-white photoluminescence quantum yield of 92 % is achieved by doping 0.3 % Sb³⁺ in (DFPD)₂ KInBr₆ . Furthermore, single-component warm-WLEDs fabricated with (DFPD)₂ KInBr₆ :Sb yield a luminance of 300 cd/m² , which is one of the best-performing lead-free metal-halides WLEDs reported so far. Our study expands the scope of In-based metal-halides from 3D to 1D, which exhibit superior optical performances and broad application prospects.

Excitation-Dependent Tunable White Light of ns2 Ions Doped Rb2SnCl6 Vacancy Ordered Double Perovskite

Single-matrix white light-emitting diodes are still a challenge. Achieving tunable white light emission from lead-free perovskites attracts much attention. Herein, nanoscale Rb₂SnCl₆ (RSC) vacancy ordered double perovskite were synthesized by an optimized precipitation method. Further, using ns² ions (Bi³⁺, Te⁴⁺, and Sb³⁺) doped RSC vacancy ordered double perovskite to obtain broadband blue, yellow-green, and orange-red emission. The color temperature adjustable high-quality white light emission based on the codoping strategy is obtained by controlling the doping ratio of Bi³⁺ and Te⁴⁺. Additionally, the white light-emitting diodes encapsulated by this single matrix white luminescent material exhibit excellent stability. Combined with theoretical calculations, it is elucidated that these high-efficiency emissions are not only derived from the unique electronic transition of ns² ions but also related to the energy band properties and the crystal field. This work shows that ns² ions doped RSC vacancy ordered double perovskite can meet the needs of different luminous colors and color temperature tunable white light emission. Meanwhile, it is a strong competitor to replace the lead-based perovskite.

Photon-Energy-Dependent Reversible Charge Transfer Dynamics of Double Perovskite Nanocrystal-Polymer Nanocomposites

Combining steady-state photoluminescence and transient absorption (TA) spectroscopy, we have investigated the photoinduced charge transfer dynamics between lead-free Mn-doped Cs₂NaIn_0.75Bi_0.25Cl₆ double perovskite (DP) nanocrystals (NCs) and conjugated poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV). Upon ultraviolet-A excitation, the photoinduced absorption feature of DP NCs/MDMO-PPV nanocomposites disappeared, and the stimulated emission weakened in the TA spectrum. This was due to charge transfer from the MDMO-PPV polymers to DP NCs. Upon a higher photon-energy ultraviolet-C excitation, stimulated emission and photoinduced absorption features vanished, indicating there existed a reversible charge transfer from DP NCs to MDMO-PPV polymers. Reversible charge transfer of Mn-doped DP NCs/MDMO-PPV nanocomposites was tuned by varying the excitation photon-energy. The manipulation of reversible charge transfer dynamics in the perovskite-polymer nanocomposites opens a new avenue for optical and optoelectronic applications.

Unveiling Local Electronic Structure of Lanthanide-Doped Cs2 NaInCl6 Double Perovskites for Realizing Efficient Near-Infrared Luminescence

Lanthanide ion (Ln³⁺ )-doped halide double perovskites (DPs) have evoked tremendous interest due to their unique optical properties. However, Ln³⁺ ions in these DPs still suffer from weak emissions due to their parity-forbidden 4f-4f electronic transitions. Herein, the local electronic structure of Ln³⁺ -doped Cs₂ NaInCl₆ DPs is unveiled. Benefiting from the localized electrons of [YbCl₆ ]^3- octahedron in Cs₂ NaInCl₆ DPs, an efficient strategy of Cl^- -Yb³⁺ charge transfer sensitization is proposed to obtain intense near-infrared (NIR) luminescence of Ln³⁺ . NIR photoluminescence (PL) quantum yield (QY) up to 39.4% of Yb³⁺ in Cs₂ NaInCl₆ is achieved, which is more than three orders of magnitude higher than that (0.1%) in the well-established Cs₂ AgInCl₆ via conventional self-trapped excitons sensitization. Density functional theory calculation and Bader charge analysis indicate that the [YbCl₆ ]^3- octahedron is strongly localized in Cs₂ NaInCl₆ :Yb³⁺ , which facilitates the Cl^- -Yb³⁺ charge transfer process. The Cl^- -Yb³⁺ charge transfer sensitization mechanism in Cs₂ NaInCl₆ :Yb³⁺ is further verified by temperature-dependent steady-state and transient PL spectra. Furthermore, efficient NIR emission of Er³⁺ with the NIR PLQY of 7.9% via the Cl^- -Yb³⁺ charge transfer sensitization is realized. These findings provide fundamental insights into the optical manipulation of Ln³⁺ -doped halide DPs, thus laying a foundation for the future design of efficient NIR-emitting DPs.

Studies on the photoelectronic properties of a manganese (Mn)-doped lead-free double perovskite

Taking Cs₂NaBiCl₆, Cs₂AgInCl₆ and Cs₂AgBiCl₆ as examples of lead-free double perovskites (DPs), we study the photoluminescence (PL) properties of Mn-doped DPs. The electron localization function (ELF) reveals the more ionic nature of the Na-Cl bond in Cs₂NaBiCl₆ than that of the Ag-Cl bond in Cs₂AgBiCl₆. Bader charge calculations confirm the nominal +2 valence state of Mn ions in both DPs. Mn²⁺ ions introduce two defect levels in the band gap of the Cs₂NaBiCl₆ host, accounting for the d-d transition (⁴T₁-⁶A₁ transition) of Mn²⁺ and thus the subsequent orange PL. The changes of the crystal field and their influences on the emission energy of Mn²⁺ ions in different DPs are evaluated by calculating the Racah parameters (B and C) and the crystal field strength (Dq) obtained from energies of the terms of d⁵ in the Cs₂NaBiCl₆:Mn²⁺ and Cs₂AgInCl₆:Mn²⁺ systems. The results show that Dq in Cs₂AgInCl₆:Mn²⁺ is stronger than that in Cs₂NaBiCl₆:Mn²⁺. The analyses on bonding interactions of the Mn-Cl bond via ELF and the integrated projected pCOHP also confirm the stronger ionic bonding interactions and thus the boost of the crystal field strength in the Cs₂AgInCl₆:Mn²⁺ system, which results in the blue-shift of the Mn²⁺ introduced PL peak from Cs₂AgInCl₆ to Cs₂NaBiCl₆. Our results provide a new strategy to modulate the emission wavelengths, i.e., tuning the crystal field.

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

Double perovskites with bright emission, low toxicity, and excellent stability have drawn considerable attention. Herein, we report the hydrothermal synthesis of Mn²⁺-doped Cs₂Na_1-xAg_xBiCl₆ double perovskites that exhibit dual emission modes. Introducing Ag⁺ ions to Cs₂NaBiCl₆ samples enables a bright self-trapped exciton (STE) emission in orange-red color, whereas Mn²⁺ dopants induce a yellow-orange emission. Importantly, Mn²⁺ doping into Cs₂Na_1-xAg_xBiCl₆ double perovskites with an indirect bandgap enables a high photoluminescence quantum yield of 49.52 ± 2%. Density functional theory calculations reveal that bringing Ag⁺ ions into Cs₂NaBiCl₆ can localize wave function to the [AgCl₆]^5- octahedron and convert dark transitions to bright STE transitions. Moreover, the 3d orbitals of Mn²⁺ dopants hybridize with Bi-6p and Cl-3p orbitals at the conduction band minimum, resulting in direct electron transfer from the host to Mn²⁺ and a significant increase in photoluminescence efficiency. These results shed light on the optical physical process of Mn²⁺-doped systems, providing useful information for further improvement of the photoluminescence efficiency of double perovskites.

The effect of halogenated spacer cations on structural symmetry-breaking in 2D halide double perovskites

Herein, we present a strategy to introduce above-room temperature non-centrosymmetry into two-dimensional halide double perovskites (A'₄M'M''X₈) using a halogenated A'-site organic linker, 3-chloro/bromo propyl amine. These crystals exhibit anisotropic polarization with three orders of magnitude variation between different crystallographic axes. The non-centrosymmetry is further confirmed by piezo-force microscopy studies and its role in the thermal and optical properties was investigated.

Impact of bismuth-doping on enhanced radiative recombination in lead-free double-perovskite nanocrystals

Lead-free double-perovskite nanocrystals (NCs) have received considerable attention as promising candidates for environmentally friendly optical applications. Furthermore, double-perovskite nanostructures are known to be physically stable compared to most other inorganic halide perovskites, with a generic chemical formula of ABX₃ (e.g., A = Cs⁺; B = Sn²⁺ or Ge²⁺; X = Cl^-, Br^-, I^-, or their combination). However, relevant experimental studies on the photophysical properties are still insufficient for Pb-free double-perovskite NCs. Herein, we synthesized Cs₂Ag_0.65Na_0.35InCl₆ NCs doped with bismuth (Bi³⁺) ions and investigated their photophysical properties to reveal the role of the dopant on the enhanced photoemission properties. Specifically, it was found that the photoluminescence quantum yield (PLQY) increased up to 33.2% by 2% Bi-doping. The optical bandgap of the NCs decreased from 3.47 eV to 3.41 eV as the amount of the dopant increased from 2% to 15%. To find out the effect of Bi-doping, the temperature-dependent PL properties of the undoped and doped NCs were investigated by utilizing steady-state and time-resolved PL spectroscopy. With increasing the temperature from 20 K to 300 K, the PL intensities of the doped NCs decreased slower than the undoped ones. The correlated average PL lifetimes of both the bismuth-doped and undoped NCs decreased with increasing the temperature. The experimental results revealed that all the NC samples showed thermal quenching with the temperature increasing, and the PL quenching was suppressed in bismuth-doped NCs.

All-Inorganic Perovskite Single Crystals for Optoelectronic Detection

Due to their many varieties of excellent optoelectric properties, perovskites have attracted large numbers of researchers in the past few years. For the hybrid perovskites, a long diffusion length, long carrier lifetime, and high μτ product are particularly noticeable. However, some disadvantages, including high toxicity and instability, restrict their further large-scale application. By contrast, all-inorganic perovskites not only have remarkable optoelectric properties but also feature high structure stability due to the lack of organic compositions. Benefiting from these, all-inorganic perovskites have been extensively explored and studied. Compared with the thin film type, all-inorganic perovskite single crystals (PSCs) with fewer grain boundaries and crystalline defects have better optoelectric properties. Nevertheless, it is important to note that only a few reports to date have presented a summary of all-inorganic PSCs. In this review, we firstly make a summary and propose a classification method according to the crystal structure. Then, based on the structure classification, we introduce several representative materials and focus on their corresponding growth methods. Finally, applications for detectors of all-inorganic PSCs are listed and summarized. At the end of the review, based on the current research situation and trends, some perspectives and advice are proposed.

Tailoring Photophysical Dynamics in a Hybrid Gallium-Bismuth Heterometallic Halide by Transferring from an Indirect to a Direct Band Structure

Low-dimensional lead-free metal halides have emerged as novel luminous materials for solid-state lighting, remote thermal imaging, X-ray scintillation, and anticounterfeiting labeling applications. However, the influence of band structure on the intriguing optical property has rarely been explored, especially for low-dimensional hybrid heterometallic halides. In this study, we have developed a lead-free zero-dimensional gallium-bismuth hybrid heterometallic halide, A₈(GaCl₄)₄(BiCl₆)₄ (A = C₈H₂₂N₂), that is photoluminescence (PL)-inert because of its indirect-band-gap character. Upon rational composition engineering, parity-forbidden transitions associated with the indirect band gap have been broken by replacing partial Ga³⁺ with Sb³⁺, which contains an active outer-shell 5s² lone pair, resulting in a transition from an indirect to a direct band gap. As a result, broadband yellow PL centered at 580 nm with a large Stokes shift over 200 nm is recorded. Such an emission is attributed to the radiative recombination of an allowed direct transition from triplet ³P₁ states of Sb³⁺ based on experimental characterizations and theoretical calculations. This study provides not only important insights into the effect of the band structure on the photophysical properties but a guidance for the design of new hybrid heterometallic halides for optoelectronic applications.

Lead-free broadband orange-emitting zero-dimensional Sb3+-doped indium-based organic–inorganic metal halides

An orange-yellow phosphor Sb3+: (CH3NH3)4InCl6·Cl was prepared via a mechanical ball-milling method. Sb3+-doped (CH3NH3)4InCl6·Cl was able to emit orange light (∼607 nm) under UV light excitation, and the PLQY is as high as 67.72%.

Lead-free all-inorganic halide double perovskite materials for optoelectronic applications: progress, performance and design

The geometrical and electronic structures of all-inorganic halide double perovskites and their applications in optoelectronic devices are reviewed. Novel design methods are desirable to develop this type of perovskite with superior performance.

Double perovskite microcrystals-based white light-emitting diodes without reabsorption of multiphase phosphors

Here, Tb³⁺ ions are incorporated into Cs₂Ag_0.6Na_0.4InCl₆:Bi double perovskite microcrystals via a re-crystallization method. Tb³⁺ ions doping not only makes the white light spectrum adjustable, but also maintains the high photoluminescence quantum yield (PLQY). The optimal value of PLQY is 95%. These are comparable to the current highest values. Noteworthy is that, intrinsic emission of Tb³⁺ ions is attributed to the effective energy transfer from the trapped exciton state of the double perovskite host to Tb³⁺ ions. Finally, mixing 30% Tb³⁺ alloyed Cs₂Ag_0.6Na_0.4InCl₆:Bi and Cs₂NaInCl₆:10%Sb phosphors, a series of double-perovskite-based white light-emitting diodes (WLEDs) are prepared. The color coordinates of the best WLEDs are (0.34, 0.32), the lumen efficiency is 42 lm/W, and the color rendering index is 94.3. It is worth mentioning here that there is no blue light loss caused by energy reabsorption between the two phosphors, because the excitation wavelengths of the two phosphors are concentrated in the ultraviolet band. This work provides a new strategy for preparing high-performance WLED.

Recent Advances in All-Inorganic Zero-Dimensional Metal Halides

All-inorganic zero-dimensional (0D) metal halides are composed of isolated metal halide polyhedrons bridged by monovalent alkali metal ions. The unique structure gives rise to molecule-like electronic configuration and consequently highly attractive optical properties. In comparison with their three-dimensional (3D) counterparts, the 0D metal halides exhibit characteristic features such as broadband emission and long-term stability. In addition, 0D metal halides can be constructed from a diverse range of metal ions and permit high-level impurity doping, thereby offering great structural designability and spectral tunability. This Review surveys recent advances in 0D metal halides, including crystal preparation, luminescence modulation, and emerging applications.

Bright Triplet Self-Trapped Excitons to Dopant Energy Transfer in Halide Double-Perovskite Nanocrystals

For inorganic semiconductor nanostructure, excitons in the triplet states are known as the "dark exciton" with poor emitting properties, because of the spin-forbidden transition. Herein, we report a design principle to boost triplet excitons photoluminescence (PL) in all-inorganic lead-free double-perovskite nanocrystals (NCs). Our experimental data reveal that singlet self-trapped excitons (STEs) experience fast intersystem crossing (80 ps) to triplet states. These triplet STEs give bright green color emission with unity PL quantum yield (PLQY). Furthermore, efficient energy transfer from triplet STEs to dopants (Mn²⁺) can be achieved, which leads to white-light emitting with 87% PLQY in both colloidal and solid thin film NCs. These findings illustrate a fundamental principle to design efficient white-light emitting inorganic phosphors, propelling the development of illumination-related applications.

Excitation wavelength tunable white light emission in vacancy-ordered double perovskite

Single matrix white luminescent materials are relatively rare. Here, we report an excitation wavelength-dependent Cs₂HfCl₆:xSb (CHC:xSb) vacancy-ordered double perovskite where, by adjusting the excitation wavelength, different types of white light emission can be obtained.

Pressure-Driven Reverse Intersystem Crossing: New Path toward Bright Deep-Blue Emission of Lead-Free Halide Double Perovskites

Maximizing the regeneration of singlet excitons remains a considerable challenge in deep-blue emission systems to obtain low-cost, high-efficiency fluorescent materials. However, the formation of the long-lifetime triplet excitons generally dominates the radiative process, making it greatly difficult to harvest deep-blue emission with high color purity because of the depression of singlet excitons. Here, a very bright deep-blue emission in double perovskite Cs₂Na_0.4Ag_0.6InCl₆ alloyed with Bi doping (CNAICB) was successfully achieved by pressure-driven reverse intersystem crossing (RISC), an abnormal photophysical process of energy transfer from the excited triplet state back to the singlet. Therein, the inherently broad emission of CNAICB was associated with the self-trapped excitons (STEs) at excited triplet states, whereas the radiative recombination of STEs populated in excited singlet states was responsible for the observed deep-blue emission. Moreover, the deep-blue emission corresponds to Commission Internationale de L'Eclairage (CIE) coordinates (0.16, 0.06) at 5.01 GPa, which meets the requirement of Rec. 2020 display standards. Likewise, pressure was introduced as an efficient tool to rule out the possibility of the recombination of free excitons and clarify the long-standing conventional dispute over the origin of the low-wavelength emission of Cs₂AgInCl₆. Our study not only demonstrates that pressure can be a robust means to boost the deep-blue emission but also provides deep insights into the structure-property relationship of lead-free CNAICB double perovskites.