High-Efficiency and Stable Long-Persistent Luminescence from Undoped Cesium Cadmium Chlorine Crystals Induced by Intrinsic Point Defects

Achieving Excitation Wavelength Dependence of Cesium Cadmium Halogen Quantum Dots with Multi-Excitonic Emission Center

Excitation wavelength-dependent emission or multiexcitonic emission in metal halide perovskite crystal is observed and has demonstrated broad application in the fields of imaging and lighting. However, these two interesting luminescence phenomena in all-inorganic lead-free halide perovskite quantum dots (QDs) are largely unexplored. Here, we have successfully synthesized CsCdCl3-xBrx (0 ≤ x ≤ 1.5) QDs with a uniform size distribution that present excitation wavelength-dependent emission caused by surface defect states and two other different emissions including the intrinsic host self-trapped excitons and Br-induced extrinsic self-trapped excitons, respectively. Structural characterizations and the calculated distortion index confirm that Br- ions partially occupy the sites of Cl- ions of the [CdCl6]4- octahedron with both C3v and D3d symmetry, which induces the local lattice distortion of CsCdCl3 QDs and promotes the formation of multiexcitonic emission. Meanwhile, the crystal structures of pure and Br-activated CsCdCl3 QDs are demonstrated by element mapping and surface states. Combined with the theory calculations, temperature-dependent photoluminescence measurements are performed to clarify the multiexcitonic emission mechanism and further verify the broad green emission comes from [CdCl6-nBrn]4- in the D3d and C3v symmetries. These findings put forward an effective strategy to design the novel excitation wavelength-dependent or multiexcitonic emissive perovskite and provide exciting opportunities for the application in X-ray images.

Time-Dependent Room-Temperature Afterglow of Carbon Dots Constructed by Trap-Induced Multiemission Centers

Traps, due to the ability to capture, store, and release charge carriers, have attracted significant attention in the construction of long afterglow materials. In this study, a one-step in situ calcination strategy was employed to fabricate carbon dot (CD)-based composites, and the traps were designed as one of the emission centers within the composite system. Upon removal of ultraviolet light, the materials showed a time-dependent afterglow color (TDAC), with the luminescent color gradually changing from orange to green. The study indicates that the dynamic afterglow results from the energy transfer from traps to the surface triplet state of the CDs. In addition, CDs generated during the in situ calcination process serve as dopants, increasing the number of original traps and facilitating the formation of new ones. Based on the TDAC characteristics, we demonstrate the applications in anti-counterfeiting and information encryption. This strategy offers new insights into the development of multicolor afterglow materials.

Efficient energy transfer from organic triplet states to Mn2+ dopants for dynamic tunable multicolor afterglow in 1D hybrid cadmium chloride

Metal ion-doped organic-inorganic hybrid metal halides have emerged as promising room-temperature phosphorescence (RTP) materials owing to their tunable afterglow properties and significant potential in information security applications. However, optimizing RTP performance and achieving dynamic control over afterglow colors remain challenging in 1D hybrid systems, primarily because of the inefficient energy transfer from RTP-active organic components to external emissive sites. Herein, we report a novel 1D hybrid metal halide benchmark material, [(NBP)Cd2Cl5H2O] (NBP-Cd, NBP = N-benzylpiperidone), and a series of Mn2+-doped derivatives, NBP-Cd:xMn2+ (where x represents doping levels from 1% to 50%). The undoped compound exhibits blue-white fluorescence and exceptional long-lasting yellow-green organic RTP with a duration of up to 2 s. Upon Mn2+ doping, the afterglow color transitions progressively from yellow-green (1-5%) to yellow (10%), orange (20%), and finally red (50%), accompanied by a reduction in afterglow duration. This dynamic multicolor afterglow behavior is attributed to efficient energy transfer from the stable triplet states within the organic component to the 4T1 level of the Mn2+ dopants. Remarkably, the NBP-Cd:10% Mn2+ crystal demonstrates exceptional excitation-dependent dual-mode photoluminescence properties. These distinctive features underscore the significant potential of this model system for advanced applications in anti-counterfeiting technologies and high-level information encryption systems.

Abnormal Slow Phonon Dynamics Toward Prolonging Excited States Dynamics Enabled by Crystalline-Assembling Donor-Acceptor Molecules

Phonon dynamics are a critical factor to control the optical properties of excited states in light-emitting materials. Here, we report an extremely slow relaxation of photoexcited lattice vibrations enabled by assembling the donor-acceptor (D-A) molecules [2-(9,9-dimethylacridin-10(9H)-yl)-9,9-dimethyl-9H-thioxanthene 10,10-dioxide], namely AC molecules, into dipolar crystal. By using photoexcitation-modulated Raman spectroscopy, we find that the crystalline-lattice vibrations monitored by Raman-scattering laser beam of 785 nm demonstrate an un-usual slow relaxation in the time scale of seconds after ceasing photoexcitation beam of 343 nm in such dipolar crystal. This presents extremely slow phonon dynamics enabled by crystalline-assembling the D-A molecules into a dipolar crystal. Simultaneously, the photoluminescence (PL) exhibits a prolonged behavior, lasting 10 ms after ceasing photoexcitation in dipolar AC crystal. This phenomenon provides an experimental hypothesis that the slow phonon dynamics function as an important mechanism to unusually prolong excited states dynamics upon crystalline-assembling the D-A molecules into dipolar crystal. This hypothesis can be verified by directly suppressing the phonon dynamics through freezing D-A molecular liquid into dipolar crystalline solid at 77 K to largely prolong the PL to 1 s- after removing photoexcitation. Clearly, crystalline-assembling D-A molecules provide the necessary conditions to enable slow phonon dynamics toward prolonging excited states dynamics.

Highly efficient tunable white emission with ultralong afterglow in Sb3+/Mn2+-codoped CsCdCl3 crystals for multifunctional applications

Recently, metal halides have attracted much attention due to their fascinating optical properties. However, achieving efficient white emission with ultralong afterglow remains a great challenge. Herein, we report Sb3+/Mn2+-codoped CsCdCl3 and multiple emission bands can be observed, which are derived from the self-trapped exciton emission of the Sb-Cl moiety and the d-d transition of Mn2+. Thus, tunable emission from cyan to orange light can be obtained. Moreover, efficient white emission with a luminous efficiency of 74% is observed when the energy-transfer efficiency from Sb3+ to Mn2+ is 34.5%. In particular, Sb3+/Mn2+-codoped CsCdCl3 shows bright orange afterglow emission, and the afterglow intensity is 1000 times that of CsCdCl3 and 20 times that of CsCdCl3:Mn2+. Upon combining this with thermoluminescence spectra, it is found that Mn2+/Sb3+ codoping can effectively regulate the depth and density distribution of trap defects, resulting in the ultralong afterglow duration exceeding 12 h at room temperature. Surprisingly, white light stimulation can provide additional photonic energy for Sb3+/Mn2+-codoped CsCdCl3, which enables the rapid release of trapped carriers to the emission center and rejuvenates afterglow emission after 12 h pre-delay. Finally, we demonstrated the applications of the as-synthesized compounds in single-component white light illumination, multiple optical anti-counterfeiting and information encryption.

Simultaneous achieving negative photoconductivity response and volatile resistive switching in Cs2CoCl4 single crystals towards artificial optoelectronic synapse

The development of negative photoconductivity (NPC)-related devices is of great significance for numerous applications, such as optoelectronic detection, neuromorphic computing, and optoelectronic synapses. Here, an unusual but interesting NPC phenomenon in the novel cesium cobalt chlorine (Cs2CoCl4) single crystal-based optoelectronic devices is reported, which simultaneously possess volatile resistive switching (RS) memory behavior. Joint experiment-theory characterizations reveal that the NPC behavior is derived from the intrinsic vacancy defects of Cs2CoCl4, which could trap photogenerated charge carriers and produce an internal electric field opposite to the applied electric field. Such NPC effect enables an abnormal photodetection performance with a decrease in electrical conductivity to illumination. Also, a large specific detectivity of 2.7 × 1012 Jones and broadband NPC detection wavelength from 265 to 780 nm were achieved. In addition to the NPC response, the resulting devices demonstrate a volatile RS performance with a record-low electric field of 5 × 104 V m-1. By integrating the characteristics of electric-pulse enhancement from RS and light-pulse depression from NPC, an artificial optoelectronic synapse was successfully demonstrated, and based on the simulation of artificial neural network algorithm, the recognition application of handwritten digital images was realized. These pioneer findings are anticipated to contribute significantly to the practical advancement of metal halides in the fields of in-memory technologies and artificial intelligence.

Stable Long-Persistent Luminescence from Self-Activated CaSb2O6 Induced by Intrinsic Defects

Long-persistent luminescence (LPL) materials have attracted intensive attention due to their fascinating emission after excitation. However, current LPL materials typically depend on external doping to introduce traps or emitting centers, resulting in a complex synthesis and controllability. For the first time, we develop another category of undoped LPL materials based on antimonate CaSb2O6, which exhibits blue LPL for over 8000 s. Both experimental and theoretical evidence indicate that excitons are trapped by intrinsic oxygen vacancies. Then, they are detrapped and recombine through singlet and triplet emission of Sb3+ to form LPL. Moreover, CaSb2O6 maintains approximately 100% of its initial LPL performance and structural integrity even after being treated under 1000 °C, UV irradiation, and extreme conditions (pH = 1 or 13). This study highlights the significant potential of antimonates as robust and versatile luminescent materials.

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.

Ultralong Room-Temperature Phosphorescence in Ca(II) Metal-Organic Frameworks Based on Nicotinic Acid Ligands

In recent years, metal-organic framework (MOF) materials with long persistent luminescence (LPL) have inspired extensive attention and presented various applications in security systems, information anticounterfeiting, and biological imaging fields. However, obtaining LPL materials with ultralong lifetime remains challenging. Halogen atoms, as nonmetallic elements existing in the frameworks, can not only induce the heavy-atom effect, effectively enhancing spin-orbit coupling and promoting intersystem crossing (ISC) processes, but also suppress non-radiative transition of the triplet states through the intra- and intermolecular interactions. Specifically, fluorine atoms with the strongest electronegativity may form intermolecular aggregate interlockings through halogen-bonding interactions that restrict molecular motions and vibrations, thereby improving phosphorescent lifetime. With the aforementioned considerations, two distinct types of MOFs with/without fluorine atoms (namely, Ca-MOF and 5FCa-MOF) were synthesized. Notably, by introducing fluorine atoms into MOFs, fluorine-induced intermolecular aggregate interlockings effectively enhanced the phosphorescent lifetime of 5FCa-MOF exceeding 264 ms compared to that of Ca-MOF (103.94 ms). Remarkably, both MOFs displayed bright LPL to the naked eye after removal of the irradiation source, especially 5FCa-MOF which can last for about 2 s. By introducing fluorine atoms, 5FCa-MOF exhibits greatly enhanced ISC with a rate constant up to 4.1 × 106 s-1 and suppressed non-radiative decay down to 3.73 s-1, thereby extending the LPL time. The thus obtained LPL provides potential in information encryption, security systems, optical anticounterfeiting, and so on.

Enabling Multimodal Luminescence in a Single Nanoparticle for X-ray Imaging Encryption and Anticounterfeiting

Multimodal luminescent materials hold great promise in a diversity of frontier applications. However, achieving the multimodal responsive luminescence at the single nanoparticle level, especially besides light stimuli, has remained a challenge. Here, we report a conceptual model to realize multimodal luminescence by constructing both mechanoluminescence and photoluminescence in a single nanoparticle. We show that the lanthanide-doped fluoride nanoparticles are able to produce excellent mechanoluminescence through X-ray irradiation, and color-tunable mechanoluminescence becomes available by selecting suitable lanthanide emitters in a core-shell-shell structure. Furthermore, the design of a multilayer core-shell nanostructure enables multimodal emissions including radioluminescence, persistent luminescence, mechanoluminescence, upconversion, downshifting, and thermal-stimulated luminescence simultaneously in a single nanoparticle under multichannel excitation and stimuli. These results provide new insights into the mechanism of X-ray induced mechanoluminescence in nanocrystals and contribute to the development of smart luminescent materials toward X-ray imaging encryption, stress sensing, and anticounterfeiting.

2-xx

Traditional information encryption materials that rely on fluorescent/phosphorescent molecules are facing an increasing risk of counterfeiting or tampering due to their static reading mode and advances in counterfeiting technology. In this study, a series of Mg2-xZnxSnO4 (x = 0.55, 0.6, 0.65, 0.7 0.75, 0.8) that realizes the writing, reading, and erasing of dynamic information is developed. When heated to 90 °C, the materials exhibit a variety of dynamic emission changes with the concentration of Zn2+ ions. As the doping concentration increased, the ratio of the shallow trap to deep trap changed from 7.77 to 20.86. When x = 0.55, the proportion of deep traps is relatively large, resulting in a higher temperature and longer time required to read the information. When x = 0.80, the proportion of shallow traps is larger and the encrypted information is easier to read. Based on the above features, encryption binary codes device was designed, displaying dynamic writing, reading, and erasing of information under daylight and heating conditions. Accordingly, this work provides reliable guidance on advanced dynamic information encryption.

Efficient Deep-Blue Light-Emitting Diodes from Highly Luminescent Eu2+-Doped Alkali Metal Halide Nanocrystals via Lattice Field Modulation

Lead-halide perovskite nanocrystals (NCs) are promising for fabricating deep-blue (<460 nm) light-emitting diodes (LEDs), but their development is plagued by low electroluminescent performance and lead toxicity. Herein, the synthesis of 12 kinds of highly luminescent and eco-friendly deep-blue europium (Eu2+)-doped alkali-metal halides (AX:Eu2+; A = Na+, K+, Rb+, Cs+; X = Cl-, Br-, I-) NCs is reported. Through adjustment of the coordination environment, efficient deep-blue emission from Eu-5d → Eu-4f transitions is realized. The representative CsBr:Eu2+ NCs exhibit a high photoluminescence quantum yield of 91.1% at 441 nm with a color coordinate at (0.158, 0.023) matching with the Rec. 2020 blue specification. Electrically driven deep-blue LEDs from CsBr:Eu2+ NCs are demonstrated, achieving a record external quantum efficiency of 3.15% and half-lifetime of ∼1 h, surpassing the reported metal-halide deep-blue NCs-based LEDs. Importantly, large-area LEDs with an emitting area of 12.25 cm2 are realized with uniform emission, representing a milestone toward commercial display applications.

Temperature-Dependent Color-Tunable Afterglow in Zirconium-Doped CsCdCl3 Perovskite for Advanced Anti-Counterfeiting and Thermal Distribution Detection

Persistent luminescence (PersL) materials exhibit thermal-favored optical behavior, enabling their unique applications in security night vision signage, in vivo bioimaging, and optical anti-counterfeiting. Therefore, developing efficient and color-tunable PersL materials is significantly crucial in promoting advanced practical use. In this study, hexagonal Zr4+ -doped CsCdCl3 perovskite is synthesized via a hydrothermal reaction with a tunable photoluminescent (PL) behavior through heterovalent substitution. Moreover, the incorporation of Zr4+ ions result in an extra blue emission band, originating from the enhanced excitonic recombination in D3d octahedrons. Furthermore, the afterglow performances of the samples are dramatically improved, along with the noticeable temperature-dependent PersL as well as the thermo-luminescence with tunable color output. Detailed analysis reveals that the unique temperature-dependent PersL and thermo-luminescence color change are attributed to the presence of multiple luminous centers and abundant traps. Overall, this work facilitates the development of optical intelligence platforms and novel thermal distribution probes with the as-developed halides perovskite for its superior explored PersL characteristic.

Unraveling the Photoluminescent Properties of Sb-Doped Cd-Based Inorganic Halides: A First-Principles Study

Sb-doped Cd-based inorganic halides, with varying connections of CdCl6 octahedra ranging from 0D to 3D, exhibit a variety of photoluminescent properties. Single-band emission is observed in Sb-doped Rb4CdCl6 (0D) and Cs2CdCl4 (2D), while dual-band emission is seen in Sb-doped RbCdCl3 (1D) and CsCdCl3 (3D). Density-functional-based first-principles calculations were conducted. The results reveal that cation vacancies, acting as charge compensators, influence the luminescence properties of dopant centers. In CsCdCl3, the local cation vacancy VCd″ for Sb3+ at the Cd2+ site ([Sb□Cl9]6-) significantly modifies the photoluminescence property, accounting for the observed dual-band emission alongside the nonlocal compensation case. This effect is insignificant in Sb-doped Rb4CdCl6, RbCdCl3, and Cs2CdCl4, due to the large distances or high formation energies of Cd vacancies in these hosts. However, in Sb-doped RbCdCl3, two different potential energy minima, one that involves typical structure relaxation and the other that is off-center, lead to the observed dual-band emission. Furthermore, the shift of the charge transition level illustrates the different temperature dependences of the dual-band emission caused by the charge-compensating point defects. These insights not only enhance our understanding of luminescent materials based on halides containing ns2 dopants but also provide valuable guidance for the design and optimization of luminescent materials.

Enhanced Photoluminescence Quantum Yield of Metal Halide Perovskite Microcrystals for Multiple Optoelectronic Applications

Recently, metal 1halide perovskites have shown compelling optoelectronic properties for both light-emitting devices and scintillation of ionizing radiation. However, conventional lead-based metal halide perovskites are still suffering from poor material stability and relatively low X-ray light yield. This work reports cadmium-based all-inorganic metal halides and systematically investigates the influence of the metal ion incorporation on the optoelectronic properties. This work introduces the bi-metal ion incorporation strategy and successfully enhances the photoluminescence quantum yield (98.9%), improves thermal stability, and extends the photoluminescence spectra, which show great potential for white light emission. In addition, the photoluminescent decay is also modulated with single metal ion incorporation, the charge carrier lifetime is successfully reduced to less than 1 µs, and the high luminescent efficiency and X-ray light yield (41 000 photons MeV-1 ) are maintained. Then, these fast scintillators are demonstrated for high-speed light communication and sensitive X-ray detection and imaging.

Recent advances in the design of afterglow materials: mechanisms, structural regulation strategies and applications

Afterglow materials are attracting widespread attention owing to their distinctive and long-lived optical emission properties which create exciting opportunities in various fields. Recent research has led to the discovery of many new afterglow materials featuring high photoluminescence quantum yields (PLQY) and lifetimes of up to several hours under ambient conditions. Afterglow materials are typically categorized according to their luminescence mechanism, such as long-persistent luminescence (LPL), room temperature phosphorescence (RTP), or thermally activated delayed fluorescence (TADF). Through rational design and novel synthetic strategies to modulate spin-orbit coupling (SOC) and populate triplet exciton states (T1), luminophores with long lifetimes and bright afterglow characteristics can be realized. Initial research towards afterglow materials focused mainly on pure inorganic materials, many of which possessed inherent disadvantages such as metal toxicity or low energy emissions. In recent years, organic-inorganic hybrid afterglow materials (OIHAMs) have been developed with high PLQY and long lifetimes. These hybrid materials exploit the tunable structure and easy processing of organic molecules, as well as enhanced SOC and intersystem crossing (ISC) processes involving heavy atom dopants, to achieve excellent afterglow performance. In this review, we begin by briefly discussing the structure and composition of inorganic and organic-inorganic hybrid afterglow materials, including strategies for regulating their lifetime, PLQY and luminescence wavelength. The specific advantages of organic-inorganic hybrid afterglow materials, including low manufacturing costs, diverse molecular/electronic structures, tunable structures and optical properties, and compatibility with a variety of substrates, are emphasized. Subsequently, we discuss in detail the fundamental mechanisms used by afterglow materials, their classification, design principles, and end applications (including sensing, anticounterfeiting, and photoelectric devices, among others). Finally, existing challenges and promising future directions are discussed, laying a platform for the design of afterglow materials for specific applications.

Afterglow Phosphor Goes Transparent

Recently, transparent afterglow phosphors have attracted increasing interest due to the mitigated self-absorption and the ensuing improved light output, which have inspired many advanced applications, including volumetric display and three-dimensional optical encryption. To date, the most successful afterglow phosphors remain those traditional oxide, nitride, or sulfide powders which are not transparent due to a severe scattering effect. By reduction of the number of interfaces and engineering the refractive index, the scattering effect could be circumvented effectively. To this end, four material systems, including transparent afterglow single crystals, transparent phosphorescent organics, transparent afterglow glass, and luminescent nanocomposites, were reviewed in this Perspective. We started with the discussion of the nontransparency origin. Through a careful inspection of Rayleigh scattering theory, a general solution involving both refractive index and particle size was proposed to reduce the scattering effect. Many representative works on transparent afterglow phosphors were systematically reviewed, where the typical synthesis methods and the advantages and disadvantages of each system were critically presented. In the last part, bottlenecks, prospects, and future development directions based on transparent afterglow phosphors are proposed.