Intense Hydrochromic Photon Upconversion from Lead-Free 0D Metal Halides For Water Detection and Information Encryption

Upconversion Luminescence in a Photostable Ion-Paired Yb-Eu Heteronuclear Complex

Lanthanide-based upconversion molecular complexes have potential application in diverse fields and attracted considerable research interest in recent years. However, the similar coordination reactivity of lanthanide ions has constrained the designability of target molecule with well-defined structure, and many attempts obtained statistical mixtures. Herein, an ion-paired Yb-Eu heteronuclear complex [Eu(TpPy)2][Yb(ND)4] (TpPy=tris[3-(2-pyridyl)pyrazolyl]hydroborate, ND=3-cyano-2-methyl-1,5-naphthyridin-4-olate) was designed and synthesized. Thanks to the radius difference between Eu3+ (1.07 Å) and Yb3+ (0.98 Å) ions, the hexadentate TpPy ligand was selected to coordinate with Eu3+ and the Yb3+ with a smaller radius was chelated by bidentate ND ligand. As a result, the sites of Eu3+ and Yb3+ in the complex can be clarified by high-resolution mass spectrometry and single-crystal structure analysis. Upon the excitation of Yb3+ at 980 nm, the upconversion emission of Eu3+ was realized through a cooperative sensitization process. Furthermore, [Eu(TpPy)2][Yb(ND)4] demonstrated excellent photostability during continuous high-power density 980 nm laser irradiation, with a LT95 (the time to 95 % of the initial emission intensity) of 420 minutes. This work provides the first example of a pure ion-paired Yb-Eu heteronuclear complex upconversion system and may bring insights into rational design of lanthanide-based upconversion molecular complexes.

Enhancement of NIR-II Emission of Er3+ by Doping Fe3+ in Double Perovskites: Multimode Luminescence for Versatile Optoelectronic Applications

Erbium ions are commonly used to extend the photoelectric properties of metal halide perovskites from visible to near-infrared range. However, achieving high-efficiency multimode luminescence in a single system is difficult due to the weak absorption associated with forbidden 4f-4f transitions. In this study, a unique strategy is proposed to adjust multimode luminescence and enhance the second near-infrared region (NIR-II) emission in Cs2NaBiCl6 by incorporating Fe3+ ions. The as-prepared material demonstrates reversible thermochromism, driven by strong electron-phonon coupling effect, and exhibits tunable luminescence that can be adjusted by altering excitation energy and temperature. Notably, benefitting from the charge transfer transition of Fe3+-Cl- along with the influence of Fe3+ doping on the geometrical and electronic structures, the blue-excitable (450 nm) NIR-II emission around 1541 nm from Er3+ is realized for the first time, achieving an intensity 16.7 times higher and a maximum photoluminescence quantum yield (PLQY) of 22.5 %. This enhancement enables innovative applications such as two-dimensional information encryption by the multi-channel cooperative responses and improved NIR imaging. The study highlights the potential of Fe3+ doping in optimizing absorption and multimode luminescence in perovskites, opening avenues for advanced applications in blue-excitable NIR light emitting diodes (LEDs), thermometer, anti-counterfeiting, and NIR imaging.

Stabilizing Dye-Sensitized Upconversion Hybrids by Cyclooctatetraene

Lanthanide-doped upconversion nanoparticles (UCNPs) can convert low-energy near-infrared (NIR) light into high-energy visible light, making them valuable for broad applications. UCNPs often suffer from poor light-harvesting capabilities, which can be significantly improved by incorporating organic dye antennas. However, the dye-sensitized upconversion systems are prone to severe photobleaching in an ambient atmosphere. Here, we present a synergistic approach to mitigate photobleaching by introducing triplet state quencher cyclooctatetraene (COT). COT effectively suppresses the generation of singlet oxygen by quenching the triplet states of the dye and consumes the existing singlet oxygen through oxidant reactions. The inclusion of COT extends the half-life of IR806 by 4.7-times by preventing the oxidation of its poly(methylene) chains. Without significantly affecting emission intensity and dynamics, COT effectively stabilized dye-UCNPs, demonstrating a notable 3.9-fold increase in half-life under continuous laser irradiation. Our findings suggest a new strategy to enhance the photostability of near-infrared dyes and dye-sensitized upconversion nanohybrids.

Highly Emissive Lanthanide-Based 0D Metal Halide Nanocrystals for Efficient Ultraviolet Photodetector

Recently, lanthanide-based 0D metal halides have attracted considerable attention for their applications in X-ray imaging, light-emitting diodes (LEDs), sensors, and photodetectors. Herein, lead-free 0D gadolinium-alloyed cesium cerium chloride (Gd3+-alloyed Cs3CeCl6) nanocrystals (NCs) are introduced as promising materials for optoelectronic application owing to their unique optical properties. The incorporation of Gd3+ in Cs3CeCl6 (CCC) NCs is proposed to increase the photoluminescence quantum yield (PLQY) from 57% to 96%, along with significantly enhanced phase and chemical stability. The structural analysis is performed by density functional theory (DFT) to confirm the effect of Gd3+ in Cs3Ce1- xGdxCl6 (CCGC) alloy system. Moreover, the CCGC NCs are applied as the active layer in UVPDs with different Gd3+ concentration. The excellent device performance is shown at 20% of Gd3+ in CCGC NCs with high detectivity (7.938 × 1011 Jones) and responsivity (0.195 A W-1) at -0.1 V at 310 nm. This study paves the way for the development of lanthanide-based metal halide NCs for next-generation UVPDs and other optoelectronic applications.

Large-Scale Mechanochemical Synthesis of Cesium Lanthanide Chloride for Radioluminescence

Cesium lanthanide chloride (Cs3LnCl6), a recently developed class of lanthanide-based zero-dimensional metal halides, has garnered a significant amount of interest because of its potential applications in scintillators, light-emitting diodes, and photodetectors. Although cesium lanthanide chloride demonstrates exceptional scintillator properties, conventional synthesis methods involving solid-state and solution-phase techniques are complex and limited on the reaction scale. This study presents a facile mechanochemical synthesis method for producing Cs3CeCl6, Cs3TbCl6, and Cs3EuCl6 metal halides on a 5 g scale. These materials exhibit intense blue-violet, green, and red emissions upon ultraviolet excitation, with high photoluminescence quantum yields ranging from 54% to 93%. Furthermore, Cs3CeCl6, Cs3TbCl6, and Cs3EuCl6 metal halides exhibit intense radioluminescence spanning from the ultraviolet to the visible region. This research shows the potential of the scalable mechanochemical synthesis of lanthanide-based metal halides for the advancement of luminescent materials for scintillators.

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.

Advanced Optical Information Encryption Enabled by Polychromatic and Stimuli-Responsive Luminescence of Sb-Doped Double Perovskites

The smart materials with multi-color and stimuli-responsive luminescence are very promising for next generation of optical information encryption and anti-counterfeiting, but these materials are still scarce. Herein, a multi-level information encryption strategy is developed based on the polychromatic emission of Sb-doped double perovskite powders (SDPPs). Cs2NaInCl6:Sb, Cs2KInCl6:Sb, and Cs2AgInCl6:Sb synthesized through coprecipitation methods exhibit broadband emissions with bright blue, cyan, and orange colors, respectively. The information transmitted by specific SDPP is encrypted when different SDPPs are mixed. The confidential information can be decrypted by selecting the corresponding narrowband filter. Then, an encrypted quick response (QR) code with improved security is demonstrated based on this multi-channel selection strategy. Moreover, the three types of SDPPs exhibit three different water-triggered luminescence switching behaviors. The confidential information represented by Cs2NaInCl6:Sb can be erased/recovered through a simple water spray/drying. Whereas, the information collected from the green channel is permanently erased by moisture, which fundamentally avoids information leakage. Therefore, different encryption schemes can be designed to meet a variety of encryption requirements. The multicolor and stimuli-responsive luminescence greatly enrich the flexibility of optical information encryption, which leaps the level of security and confidentiality.

Designing Yolk-Shell Nanostructures for Reversible Water-Vapor-Responsive Dual-Mode Switching of Fluorescence and Structural Color

Metal halide perovskites offer ample opportunities to develop advanced optoelectronic devices. This work showcases that the integration of metal halide perovskites into metal oxide nanoshells with controllable interior cavities can enable water-vapor-responsive dual-mode switching of fluorescence and structural color. Through a ship-in-a-bottle method to introduce a controlled amount of CsPbBr3 into MnO2 nanoshells, we have designed CsPbBr3@MnO2 yolk-shell nanostructures, which can uptake a defined amount of water to exhibit rapid (less than 1 s) and reversible (≥100 cycles) responses in both fluorescence on-off and color change when exposed to dynamic water vapor. These responses originate from the water-triggered phase transformation of CsPbBr3 to CsPb2Br5 and the structural color change of the MnO2 shell. The altered electronic and bonding structure at the oxide-halide interface, rapid water accumulation in the yolk-shell cavity, and protective effect of the oxide shell facilitate the reversible transformations. The response characteristics of the yolk-shell nanostructures have been further demonstrated in fabricating patterned films capable of multiple fluorescence/structural color responses, highlighting their potential for applications in advanced anticounterfeiting and encryption.