Recently developed lead-free double perovskite nanocrystals (NCs) have been proposed for the possible application in solution- processed optoelectronic devices. However, the optoelectronic applications of double perovskite NCs have been hampered due to the structural and chemical instability in the presence of polar molecules. Here, we report a facile strategy for the synthesis and purification of Cs₂AgBiBr₆ double perovskite NCs that remained stable even after washing with polar solvent. This is realized with our efficient colloidal route to synthesize Cs₂AgBiBr₆ NCs that involve stable and strongly coordinated precursor such as silver- trioctylphosphine complex together with bismuth neodecanoate, which leads to a significantly improved chemical and colloidal stability. Using layer-by-layer solid-state ligand exchange technique, a compact and crack-free thin film of Cs₂AgBiBr₆ NCs were fabricated. Finally, perovskite solar cells consisting of Cs₂AgBiBr₆ as an absorber layer were fabricated and tested.

Producing environmentally stable monolayers and few-layers of hafnium disulphide (HfS₂) with a high yield to reveal its unlocked electronic and optoelectronic applications is still a challenge. HfS₂ is a layered two-dimensional material of group-IV transition metal dichalcogenides. For the first time, we demonstrate a simple and cost-effective method to grow layered belt-like nanocrystals of HfS₂ with a notably large interlayer spacing followed by their chemical exfoliation. Various microscopic and spectroscopic techniques confirm that these as-grown crystals exfoliate into single or multiple layers in a few minutes using solvent assisted ultrasonification method in N-cyclohexyl-2-pyrrolidone. The exfoliated nanosheets of HfS₂ exhibit an indirect bandgap of 1.3 eV with high stability against surface degradation. Furthermore, we demonstrate that these nanosheets hold potential for electronic applications by fabricating a field-effect transistor based on few-layered HfS₂, exhibiting a field-effect mobility of 0.95 cm²/(V·s) with a high on/off current modulation ratio of 10, 000 in ambient conditions. The method is scalable and has a potential significance for both academic and industrial purposes.