Multi-mode excited upconversion and downshifting of Ho3+/Yb3+ doped lead-free double perovskite Cs2NaBiCl6 for anti-counterfeiting and thermometry

Rear earth(RE)-doped halide perovskite has generated as an amazing optical material for multicolor luminescence. Free-lead halide double perovskites have attracted interesting because of their non-toxicity and higher instability. Here, RE 3+ doped chloride-based double perovskite phosphors are reported. A series of heavy concentration Ho 3+ /Yb 3+ doped Cs 2 NaBiCl 6 (CNBC) phosphors are prepared with a solvothermal route. The band structure of CNBC is calculated through density functional theory (DFT). The luminescence properties of CNBC:Ho 3+ and CNBC:Ho 3+ /Yb 3+ are investigated through upconversion(UC) and downshifting(DS) processes. From CNBC:Ho 3+ , the brightness orange, deep red and pale green lights are achieved under excitation at 365, 452 and 980 nm. The multicolor emissions of CNBC:Ho 3+ are used to multimode fluorescent anti-counterfeiting. Comparing with CNBC:Ho 3+ samples, the UC emission intensity of CNBC:Ho 3+ /Yb 3+ are dramatically enhanced. The brightness green light can be seen by naked eyes under super lower power pumping(∼10 mW). The temperature sensing properties of CNBC:Ho 3+ /Yb 3+ phosphor are investigated systematically based on the luminescence intensity ratio(LIR) of thermally coupled levels(TCLs)( 5 F 1 / 5 G 6 , 5 F 2,3 / 3 K 8 ) and non-TCLs( 5 F 4 / 5 S 2 , 5 F 5 ). The maximum relative sensitivity(S R ) reaches 5.089% K −1 at 293 K according to the LIR of non-TCLs. The work will broaden the application field of RE 3+ -doped free-lead perovskite such as anti-counterfeiting and thermometry.