Scintillators, which are capable of converting ionizing radiation into visible photons, are an integral part of medical, security, and commercial diagnostic technologies such as X-ray imaging, nuclear cameras, and computed tomography. Conventional scintillator fabrication typically involves high-temperature sintering, generating agglomerated powders or large bulk crystals, which pose major challenges for device integration and processability. On the other hand, colloidal quantum dot scintillators cannot be cast into compact solid films with the necessary thickness required for most X-ray applications. Here, we report the room-temperature synthesis of a colloidal scintillator comprising CsPbBr 3 nanosheets of large concentration (up to 150 mg/mL). The CsPbBr 3 colloid exhibits a light yield (∼21000 photons/MeV) higher than that of the commercially available Ce:LuAG single-crystal scintillator (∼18000 photons/MeV). Scintillators based on these nanosheets display both strong radioluminescence (RL) and long-term stability under X-ray illumination. Importantly, the colloidal scintillator can be readily cast into a uniform crack-free largearea film (8.5 × 8.5 cm 2 in area) with the requisite thickness for high-resolution X-ray imaging applications. We showcase prototype applications of these high-quality scintillating films as X-ray imaging screens for a cellphone panel and a standard central processing unit chip. Our radiography prototype combines large-area processability with high resolution and a strong penetration ability to sheath materials, such as resin and silicon. We reveal an energy transfer process inside those stacked nanosheet solids that is responsible for their superb scintillation performance. Our findings demonstrate a large-area solution-processed scintillator of stable and efficient RL as a promising approach for low-cost radiography and X-ray imaging applications.

Luminescent solar concentrators (LSCs) are able to efficiently harvest solar energy through large-area photovoltaic windows, where fluorophores are delicately embedded. Among various types of fluorophores, all-inorganic perovskite nanocrystals (NCs) are emerging candidates as absorbers/emitters in LSCs due to their size/composition/dimensionality tunable optical properties and high photoluminescence quantum yield (PL QY). However, due to the large overlap between absorption and emission spectra, it is still challenging to fabricate high-efficiency LSCs. Intriguingly, zero-dimensional (0D) perovskites provide a number of features that meet the requirements for a potential LSC absorber, including i) small absorption/emission spectral overlap (Stokes shift up to 1.5 eV); ii) high PL QY (>95% for bulk crystal); iii) robust stability as a result of its large exciton binding energy; and iv) ease of synthesis. In this work, as a proof-of-concept experiment, Cs 4 PbBr 6 perovskite NCs are used to fabricate semi-transparent large-area LSCs. Cs 4 PbBr 6 perovskite film exhibits green emission with a high PL QY of ≈58% and a small absorption/emission spectral overlap. The optimized LSCs exhibit an external optical efficiency of as high as 2.4% and a power conversion efficiency of 1.8% (100 cm 2 ). These results indicate that 0D perovskite NCs are excellent candidates for high-efficiency LSCs compared to 3D perovskite NCs.

HIGHLIGHTS • A gram-scale CsPbBr 3 perovskite nanosheet colloid was synthesized by a green method at room temperature. • The perovskite nanosheet colloid shows uncompromised photoluminescence quantum yield upon storage for over 8 months. • A self-assembly crack-free thin film of the colloidal nanosheets demonstrated an efficient X-ray imaging screen.

Near‐infrared (NIR) afterglow is keenly sought in emerging areas including deep‐tissue imaging and night‐vision surveillance, while only few successes in powder phosphors are achieved through solid‐state calcination. In this work, a perovskite single crystal, namely Cs2Na0.2Ag0.8InCl6:Yb3+, is grown in solution via a simple hydrothermal reaction. Through a co‐doping strategy involving both Na+ and Yb3+, dual‐band emission in the visible and NIR region is activated by self‐trapped excitons (STE) and lanthanide ions, respectively. Importantly, the total photoluminescence quantum yield (PL QY) of both bands is boosted to ≈82%. Intriguingly, a long‐lasting afterglow at the NIR band (≈7200 s) and a simultaneous photochromism is observed after ceasing the excitation. Importantly, the transparency of crystals exhibit a pronounced contrast in the decoloration process, enabling a quantitative analysis of photochromism at varied temperatures. On the other hand, the transparent crystals enable the design of a light‐storage battery free of reabsorption, featuring a linear power output with crystal loading. This work proposes a new paradigm to quantitatively correlate the afterglow traps to photochromism, opening many possibilities to practical applications of NIR‐afterglow transparent crystals.

Despite their low toxicity and phase stability, lead-free double perovskite nanocrystals, Cs2AgInCl6 in specific, have suffered from low quantum yield of photoluminescence. This is mainly due to two reasons, including (i) the quenching effect from metal silver which was usually formed at high temperature from Ag+ reduction in the presence of organic amines and (ii) the parity-forbidden transition of pristine double perovskites. Here, we reported a room-temperature synthesis of Cs2AgInCl6 nanocrystals in an inverse microemulsion system, where Ag+ reduction was largely suppressed. By codoping Bi and Na ions, dark self-trapping excitons (STEs) were converted into bright ones, enabling a bright phosphor of photoluminescence quantum yield up to 56%. Importantly, the doping approach at room temperature relaxed the parity-forbidden transition (1S0 → 3P2) of Bi-6s2 orbitals, revealing a fine structure of a triband excitation profile. Such spin-rule relaxation was ascribed to symmetry breaking of the doped lattice, which was evidenced by Raman spectroscopy. In a proof-of-concept experiment, the bright nanocrystals were used as a color-converting ink, which enabled a stable white light light-emitting diode to operate in various environments, even under water, for long-term service.

The red emission of Cs4PbI6 zero-dimensional perovskite is found heterogeneous between individual particles, yet exhibits an enhanced stability towards both anion exchange reaction and photo radiation than CsPbI3.

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