Halide Double Perovskite Nanocrystals Doped with Rare‐Earth Ions for Multifunctional Applications

Abstract Most lead‐free halide double perovskite materials display low photoluminescence quantum yield (PLQY) due to the indirect bandgap or forbidden transition. Doping is an effective strategy to tailor the optical properties of materials. Herein, efficient blue‐emitting Sb3+‐doped Cs2NaInCl6 nanocrystals (NCs) are selected as host, rare‐earth (RE) ions (Sm3+, Eu3+, Tb3+, and Dy3+) are incorporated into the host, and excellent PLQY of 80.1% is obtained. Femtosecond transient absorption measurement found that RE ions not only served as the activator ions but also filled the deep vacancy defects. Anti‐counterfeiting, optical thermometry, and white‐light‐emitting diodes (WLEDs) are exhibited using these RE ions‐doped halide double perovskite NCs. For the optical thermometry based on Sm3+‐doped Cs2NaInCl6:Sb3+ NCs, the maximum relative sensitivity is 0.753% K−1, which is higher than those of most temperature‐sensing materials. Moreover, the WLED fabricated by Sm3+‐doped Cs2NaInCl6:Sb3+ NCs@PMMA displays CIE color coordinates of (0.30, 0.28), a luminous efficiency of 37.5 lm W−1, a CCT of 8035 K, and a CRI over 80, which indicate that Sm3+‐doped Cs2NaInCl6:Sb3+ NCs are promising single‐component white‐light‐emitting phosphors for next‐generation lighting and display technologies.

(0.4 mL) was swiftly injected at 165 °C. When the reaction mixture's temperature increased to 175 °C, the above solution was immediately cooled with an ice-water bath. The Cs2NaInCl6 NCs were obtained via three centrifugation process. The NCs in the mixture solution were centrifuged at 9000 rpm for 20 min, then the supernatant was discarded. The precipitate was dispersed in toluene, and centrifuged at 10000 rpm for 15 min, and the supernatant was discarded again, and the as-prepared samples were freeze-dried. Then, the obtained precipitate was dissolved to n-hexane, and centrifuged at 6000 rpm for 15 min. The supernatant containing Cs2NaInCl6 NCs were obtained. As for Cs2NaInCl6: Sb 3+ NCs, the synthesis process was identical to the synthesis of Cs2NaInCl6 NCs, but replaced 0.5 mmol In(OAc)3 (0.144 g) with 0.25 mmol Sb(OAc)3 (0.0747 g) and 0.25 mmol In(OAc)3 (0.072 g). The optimal ratio of Sb(OAc)3 and In(OAc)3 was ascertained via the concentrations-dependent on PL spectra, wherein the ratio of Sb(OAc)3 and In(OAc)3 as 1:1, the PLQY was the highest (Figure S1).

Synthesis of RE ions doped Cs2NaInCl6: Sb 3+ NCs
The RE ions doped Cs2NaInCl6: Sb 3+ NCs were synthesized using the same method employed for the undoped and Sb 3+ -doped Cs2NaInCl6 NCs with minor modifications.
Additional lanthanide acetate (0.95 mmol) was casted to the raw materials, and all the other reaction parameters were the same. For different amounts Tb 3+ -doped Cs2NaInCl6: Sb 3+ NCs, 0.25, 0.5, 0.75, 1, and 1.25 mmol Tb(OAc)3·H2O were casted to the raw materials, respectively, and all the other reaction parameters were the same.

LED fabrication
The powder of Sm 3+ -doped Cs2NaInCl6: Sb 3+ NCs were dispersed in a transparent PMMA/toluene solution and stirred for 2 h and then coated onto a 310 nm UV LED chip. The LEDs from Sm 3+ -doped Cs2NaInCl6: Sb 3+ NCs were thus fabricated after drying of the color conversion layer.

Characterizations
For TEM measurement, JEM-2100F at an accelerating voltage of 200 kV was used. XRD measurements were performed using a Bruker D8 Advance X diffractometer (Cu Kα, λ = 1.5406 Å). High-resolution TEM (HRTEM) images and EDS elemental mapping images were collected with a FEI Tecnai F20. X-ray photoelectron spectroscopy (XPS) measurements were carried on a Thermo ESCALAB 250 spectrometer equipped with a monochromatic Al Kr radiation source (1486.6 eV).
Absorption spectra were collected using a Shimadzu UV-2550 spectrophotometer.