Lead-free metal halide perovskite crystal enables excitation-mode-selective multiexcitonic emissions
Smart luminescent crystals that show switchable light emissions in response to different forms of excitations, such as ultraviolet (UV) and near-infrared photons, X-ray irradiation, and mechanical force, have attracted pervasive attention from scientists and industry professionals. In particular, the flexibility and diversity afforded by ion luminescence endow impurity-doped inorganic luminescent materials with enormous superiorities in realizing excitation-selective luminescence. The dynamical control of optical features by leveraging multiple excitation modes has been widely established in various oxide matrixes co- or tri-doped with lanthanide or transition metal ions. This exciting ability to “switch” light emissions opens up a wide array of forefront applications such as sensing, optoelectronics, anti-counterfeiting, and data storage.
Nevertheless, there are challenges with these oxide powder systems, such as complicated dopant compositions and closely packed dopant polyhedrons. They typically display low efficiency and imbalanced performance due to unwanted crosstalk and severe random scattering. As a promising alternative to ion luminescence, self-trapped exciton (STE) in low-dimensional metal halide perovskite transparent crystals has been recently proven useful for generating excitation-mode-selective luminescence. The vacancy-ordered structure results in mutually independent STEs with minimal energy exchange interactions, making them ideal for selective STE activation by a given excitation. Despite substantial advances in tunable photoluminescence, controlling STE emission behaviors using X-ray and mechanic excitations has not been established in these perovskite crystals. Therefore, there is a growing demand for novel perovskite crystals with enhanced optical capabilities to control light emissions under multiple excitation modes.
This study presents an experimental investigation into highly variable and multimodally excitable STE luminescence in a single component of all-inorganic halide crystal. We focused on a series of rare-earth halide double perovskites, known for their high defect tolerances, excellent stability, and high doping capacity. The corner-sharing [NaCl6]5- and [RECl6]3- octahedral units are alternately arranged to form the highly ordered three-dimensional framework (Figure 1a). This unique arrangement effectively reduces the electronic dimensionality by decoupling electronic orbitals of nearest [RECl6]3-octahedrons, favoring STE formation. By solely doping trivalent antimony into the crystal, we identified three mutually independent STEs related to the host and dopant that can be selectively populated through different excitation pathways.
Photoluminescence characterizations confirm the excitation-wavelength-controlled PL switching behavior of the crystal when exposed to UV light. Notably, the crystal displayed single or dual STE bands under 335 or 302 nm excitation, rendering intense blue or white emission with quantum yields close to 100% (Figure 1b). Our control experiments validate that two STE states populated by UV light are related to [SbCl6]3- unit. It is also noteworthy that the blue emission observed in the host crystal is more likely attributed to antimony impurities, rather than the perovskite itself as suggested in previous reports (Figure 1c). Interestingly, we detected an additional STE emission in UV range from Sc3+-related polyhedron upon high-energy excitation of X-ray, enabling an intense violet-blue radioluminescence with a high light yield and low detection limit. After ceasing X-ray excitation, the crystal continued to emit bright persistent luminescence from the blue STE state, lasting more than 9 hours (Figure 1d).
An interesting observation in our study was the bright luminescence emitted by the crystals during grinding, which was visible to the naked eye even in ambient light (Figure 1e). As the crystal was fragmented into powder, the mechanoluminescence (ML) gradually disappeared and could not be recovered by UV charging. Nevertheless, we found that highly reproducible ML could be achieved by recrystallizing the grounded powder through a simple hydrothermal process. Our results suggest a fractoluminescence mechanism of this ML behavior. Remarkably, the high doping capacity of the Sc3+ site allowed unprecedented ML tuning from UV-C to NIR within Cs2NaScCl6 crystals using different dopant ions. Finally, this ability to manipulate switchable multiexcitonic emissions using different excitation modes has been leveraged for multi-level all-in-one authentication and logic encryption technology (Figure 1f).
Figure 1. Excitation-mode-selective multiexcitonic emissions. a, Schematic illustration of Cs2NaScCl6 crystal structure. b, PLE and PL spectra of Cs2NaScCl6:Sb3+ crystal, and insets show the optical images. Scale bar = 1 mm. c, The integral intensity of Cs2NaScCl6 crystals under 335 nm excitation as a function of Sb3+ content detected by ICP-MS analysis. d, PersL decay profile and photographs of the crystal after X-ray charging for 15 minutes. Inset shows the initial PersL intensity as a function of charging time. e, ML and cathodoluminescence (CL) spectra of the crystal. Insets show the ML photographs and the proposed ML mechanism. f, Application of logic encryption through individually addressable emission features of the as-prepared crystals.
In closing, our research has demonstrated extended control over the multiexcitonic emissions in all-inorganic halide double perovskite crystals, which are ideal as excitation-mode-selective luminescence materials. We believe these findings will inspire further innovations in the design principles of advanced metal halide perovskite crystals with a broadened range of applications.
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