Abstract

Lead halide perovskite nanocrystals are being heavily studied due to their excellent optical properties but lead toxicity is still a major issue. Recently, double perovskite nanocrystals have emerged as a promising alternative. In this work, lead-free double perovskite nanocrystals were synthesized using the ligand-assisted reprecipitation (LARP) method. The synthesis of Cs2AgIn(0.9)Bi(0.1)Cl6 nanocrystals (NCs) was performed at 800 C and the resulting NCs have a bright orange emission when excited at 375 nm. They feature two emission peaks, a violet emission that is due to free excitons and self-trapped emissions (STEs) that lead to orange emission. The orange emission peak has a maximum of 620 nm and a photoluminescence quantum yield (PLQY) of 33%. 

The surface of the NCs was functionalized with polystyrene (PS), poly (2-hydroxyethyl methacrylate) (Poly 2-HEMA), poly (methacrylic acid methyl ester) (PMMA), and poly (vinylidene fluoride) (PVDF). The polymer encapsulations enhanced the thermal stability of the NCs along with their optical properties. Polymers provide an additional layer of protection and prevent the degradation of NCs due to external factors such as moisture and heat. The polymer encapsulations with PS and PMMA, respectively, increased the PLQY of the NCs up to 63% and 61% respectively. 

Composite thin films of lead-free NCs with polymer encapsulations were fabricated by spin coating with a thickness of approx. 100 nm and were employed as luminescent temperature sensing layers. They showed a strong and reversible sensitivity of their luminescence intensity towards temperature in the range of 293 K and 353 K. NCs with PS and Poly-2-HEMA encapsulations exhibited temperature sensitivity of 1.13% K–1 and 1.49% K–1, respectively. The thin films showed good thermal stability for at least 5 heating–cooling cycles. 

The temperature stability and sensitivity of these composite thin films allow them to be used as sensor layers. Real-time monitoring of various chemical and biological reactions can be performed with the help of the photothermal effect. We have integrated these temperature sensing layers into dedicated microfluidic chips and are currently studying their use for the photothermal detection of analytes in microchannels.