Synergistic optimization of optical properties and stability for CsEuCl3 nanocrystals by Sr2+ doping and TPU encapsulation☆

Rare earth-based perovskite nanocrystals CsEuX 3 (X = Br, Cl, or I) have emerged as an ideal alternative system to toxic lead-based halide perovskites due to the lead-like characteristics of Eu 2+ ions (similar ionic radius, identical valence state and coordination number) and their unique luminescent properties: narrow-band blue emission with a full width at half maximum (FWHM) ≤ 25 nm from 5d→4f transitions. However, the relatively low photoluminescence quantum yield (PLQY) and poor environmental stability especially for CsEuCl 3 nanocrystals have seriously hindered their applications. To address these issues, a dual strategy based on strontium ion (Sr 2+ ) doping and thermoplastic polyurethane (TPU) coating is proposed to synergistically modify CsEuCl 3 nanocrystals in this paper. TPU@CsEuCl 3 :Sr 2+ nanocrystals films with significantly improved environmental stability and PL properties were successfully prepared. The partial substitution of Sr 2+ for Eu 2+ reduces the surface defects of nanocrystals and mitigates the oxidation of Eu 2+ . As a result, the ratio of Eu 2+ /Eu 3+ increases from 3.9% of undoped sample to 6.0% for CsEuCl 3 :Sr 2+ and PLQY correspondingly enhances from 5.04% to 13.88%. Further encapsulation with flexible TPU introduces a hydrophobic surface that effectively blocks the erosion of water and oxygen in the environment on nanocrystals. The ratio of Eu 2+ /Eu 3+ in the TPU@CsEuCl 3 :Sr 2+ nanocrystals film markedly increases from 6.0% to 39.9%, and PLQY slightly improves to 14.02%. Most importantly, the environmental stability of the film is greatly enhanced. After ultraviolet irradiation in air for 150 min, PL intensity of nanocrystal film still retains 69.5% of its initial value, whereas the undoped and uncoated sample completely loses its luminescent properties after only 60 s.