Near-infrared Cr³⁺-doped lead-free halide perovskite microcrystals for information encryption and temperature thermometry

Lead-free double perovskite materials have attracted lots of interest because they can be doped with luminescence activators to modify their optical characteristics and improve photoluminescence performances. Herein, a solvothermal method was utilized to synthesize single Cr³⁺-doped Cs₂NaInCl₆ phosphors. Under 292 nm excitation, Cr³⁺-doped Cs₂NaInCl₆ microparticles exhibited a strong broad near-infrared emission with a good photoluminescence quantum yield of 42.9%. The prepared sample exhibited a prominent peak at 960 nm and a full width at half maximum of 138 nm, which can be attributed to the spin-allowed transition (⁴T₂ → ⁴A₂) of Cr³⁺ ions. Additionally, the emission band of Cr³⁺ displayed a noticeable red shift as temperature increased owing to the variations in the strength of the crystal field. The thermometry based on the bandwidth and band-shift modes showed good sensitivities of 0.337 nm K⁻¹ and 0.272 nm K⁻¹, respectively. Finally, the security ink was made for information encryption.