Hexagonal-phase NaYF₄ (β-NaYF₄) has been acknowledged to be one of the most efficient doping hosts to prepare bright lanthanide-doped luminescent nano-bioprobes for various biomedical applications. However, to date, it remains a great challenge to synthesize ultra-bright lanthanide-doped β-NaYF₄ nano-bioprobes under a low reaction temperature by using conventional synthetic methods. Herein, we first develop an acetic acid (HAc)-mediated coprecipitation method for the preparation of ultra-bright lanthanide-doped β-NaYF₄ nanoprobes under a low reaction temperature at 200 °C. Based on a series of comparative spectroscopic investigations, we show that the use of HAc in the reaction environment can not only promote the rapid α–β phase transformation of NaYF₄ host at 200 °C within 1 h but also boost the absolute photoluminescence quantum yield (PLQY) of NaYF₄ nanocrystals to 30.68% for near-infrared emission and to 3.79% for upconversion luminescence, both of which are amongst the highest values for diverse lanthanide-doped luminescent nanocrystals ever reported. By virtue of their superior near-infrared luminescence, we achieve optical-guided dynamic vasculature imaging in vivo of the whole body at a high spatial resolution (23.8 µm) under 980 nm excitation, indicating its potential for the diagnosis and treatment evaluation of vasculature-related diseases.

Self-trapped excitons (STEs) emission from halide perovskites with strong exciton-phonon coupling has attracted considerable attention due to the widespread application in optoelectronic devices. Nevertheless, the in-depth understanding of the relationship between exciton-phonon coupling and luminescence intensity remains incomplete. Herein, a doping-enhanced exciton-phonon coupling effect is observed in Cs₃Cu₂I₅ nanocrystals (NCs), which leads to a remarkable increasement of their STEs emission efficiency. Mechanism study shows that the hetero-valent substitution of Cu⁺ with alkaline-earth metal ions (AE²⁺) causes a greater degree of Jahn–Teller distortion between the ground state and excited state structures of [Cu₂I₅]³⁻ clusters as evidenced by our spectral analysis and first-principles calculations. As a consequence, an X-ray detector based on these Cs₃Cu₂I₅:AE NCs delivers an X-ray imaging resolution of up to 10 lp·mm⁻¹ and a low detection limit of 0.37 μGy_air·s⁻¹, disclosing the potential of doping-enhanced exciton-phonon coupling effect in improving STEs-emission and practical application for X-ray imaging.

The practical application of all-inorganic semiconductor lead halide perovskite nanocrystals (LHP NCs) has been limited by their poor stability. Recently, a lot of research on core–shell structure has been done to improve the stability of perovskite NCs, but the effect was far from the application requirements. Herein, we, for the first time, report a convenient approach to synthesize organic–inorganic double shell CsPbBr₃@SiO₂@polystyrene (PS) NCs with an inter-core of CsPbBr₃, the intermediate layer of SiO₂ shell, and outmost PS shell. Particularly, the CsPbBr₃@SiO₂@PS NCs maintained more than 90% of their initial photoluminescence (PL) intensity under one month's ultraviolet lamp irradiation or in 85 °C and 85% relative humidity (RH) condition. The white-light-emitting-diodes (WLEDs) were fabricated by encapsulating commercial InGaN chip with CsPbBr₃@SiO₂@PS NCs and K₂SiF₆:Mn⁴⁺ (KSF:Mn⁴⁺) phosphor with a luminous efficacy of ~ 100 lm/W at 20 mA current and a color gamut of 128% of the National Television Standards Committee (NTSC) standard. In addition, these WLEDs still maintain 91% of the initial luminous efficacy after 1200 h of continuous lighting. These results demonstrated that double shell-protected CsPbBr₃ perovskite NCs have great potential in the field of WLEDs.