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.

Structural Transformation and Photoluminescent Property of Manganese-Doped Bismuth-Based Perovskites.

Here, we synthesized pure Cs3Bi2Cl9 (CBC) and manganese (Mn)-doped crystals with different feeding ratios, leading to changes in structure and luminescence. The crystals Cs3Bi2Cl9-Mn (CBCM) formed by doping a minor amount of Mn2+ (Bi/Mn = 8:1) maintain the orthorhombic phase structure of the host, but when Bi/Mn = 2:1, the crystal structure is more inclined to form Cs4MnBi2Cl12 (CMBC) of a trigonal phase. Combined with density functional theory (DFT) calculation, the results demonstrate that a moderate amount of Mn2+ doping can create impurity energy levels in the forbidden band. However, as the structure transitions, the type of energy band structure changes from indirect to direct, with completely different electronic orbital features. Temperature-dependent time-resolved and steady-state photoluminescence spectroscopies are used to explore the structure-related thermal properties and transitional process. Differences energy transfer routes are revealed, with CBCM relying on intersystem energy transfer and CMBC mainly depending on direct excitation of Mn2+ to produce d-d transitions. Furthermore, since CMBC is temperature-sensitive, we perform the first photoluminescent (PL) lifetime temperature measurement using CBMC and obtain a maximum relative sensitivity of 1.7 %K-1 and an absolute sensitivity of 0.0099 K-1. Our work provides insight into the mechanism of Mn2+ doping-induced luminescence and offers a potentially effective doping strategy for improving the PL properties of lead-free metal halide perovskites.

The luminescent properties of metal halides are determined by the inorganic framework and solvent molecules.

The luminescent properties of metal halides are usually considered to be determined by the inorganic framework. In this work, we propose that the luminescent properties of metal halides are determined by both the inorganic framework and the solvent [Denoted as (inorganic framework + n·solvent molecules), n = 0, 1, 2…] through the abundant solvatochromic or thermochromic effect of tetrabutylammonium lead bromides [TPB, T = TBA (tetrabutylammonium), P = Pb (lead), B = Br (bromide)] containing water (H2O) and ethanol (EtOH). One-dimensional (1D) TPB can form ligands of [[Pb5Br18]8- + 2H2O(H)], [[Pb5Br18]8- + 2H2O(H) + 2H2O] and [[Pb5Br18]8- + 2EtOH] by solvent or heat treatment has completely different luminescent properties resulting from different solvents. They exhibit broad spectral emission due to strong electron-phonon coupling, as do other 1D metal halides. However, the 1D TPB containing only [[Pb5Br18]8- achieves extremely rare narrow-band green emission, with full width at half maximum (FWHM) of 21 nm at room temperature and 8 nm at low temperature, color gamut covers 95 % of the International Telecommunication Union recommendation 2020 standard. This work provides new guidance for the modulation of photophysical properties of metal halides, as well as new materials for the display and smart materials fields.

Photophysical properties of tetrabutylammonium metal chlorides with different inorganic frameworks.

The luminescence of single crystals of (TBA)PbCl3, (TBA)2Sb2Cl8, (TBA)3Bi2Cl9 and (TBA)SnCl5·2EtOH (TBA = tetrabutylammonium, EtOH = Ethanol) synthsized were assigned distinctively to the centres of self-trapped excitons (STEs), TBA+, TBA+ and co-emission of STEs and TBA+. This work demonstrates that organic cations without benzene or aromatic rings can also be used as the sole luminescence centres for metal halides.

Highly Sensitive Dual-Mode Optical Thermometry of Er3+/Yb3+ Codoped Lead-Free Double Perovskite Microcrystal.

In this Letter, erbium (Er3+) and ytterbium (Yb3+) codoped perovskite Cs2Ag0.6Na0.4In0.9Bi0.1Cl6 microcrystal (MC) is synthesized and demonstrated systematically to the most prospective optical temperature sensing materials. A dual-mode thermometry based on fluorescence intensity ratio and fluorescence lifetime provides a self-reference and highly sensitive temperature measurement under dual wavelength excitation at a temperature from 300 to 470 K. Combined with the white-light emission derived from self-trapped excitons (STEs), the characteristic emission peak of Er3+ ions can be observed under 405 nm laser excitation. The fluorescence intensity ratio (FIR) between perovskite and Er3+ is used as temperature-dependent probe signal, of which maximum value for relative and absolute sensitivities reaches to 1.40% K-1 and 8.20 × 10-2 K-1. Moreover, Er3+ luminescence becomes stronger with the feeding Yb3+ increasing under 980 nm laser excitation. The energy transfer of Er3+ and Yb3+ is revealed by power-dependent photoluminescence (PL) spectroscopy, and the involved upconversion mechanism pertains to the two-photon excitation process. The results reveal that the Er3+/Yb3+ codoped lead-free double perovskite MC is a good candidate for a thermometric material for the novel dual-mode design.