Lead-free Cs₂NaInCl₆:Bi³⁺/Mn2⁺ double perovskite nanocrystals to nanosheets with improved photoluminescence quantum yield for anti-counterfeit marks and LED applications

Lead-free double perovskites (DPs) in the form of two-dimensional (2D) nanosheets (NSs) have emerged as exceptional candidates compared to zero-dimensional (0D) nanocrystals (NCs). This preference is attributed to their intriguing fundamental properties and promising applications in optoelectronics. Notably, NSs offer superior performance due to their enhanced surface passivation capabilities, facilitating more efficient treatment of surface defects and thereby enhancing optoelectronic properties. However, the realization of Cs2NaInCl6 as lead-free DPs NSs is yet to be achieved. In this study, we successfully synthesized Cs2NaInCl6 co-doped with Bi3+/Mn2+ NSs. The two-step solution process synthesis enables us to achieve dimensionality control, progressing from 0D NCs to 2D NSs. Comprehensive optical spectroscopy analyses reveal that the NSs exhibit a suppressed non-radiative decay rate (Knr) of 5.03 × 10−3 ns−1 in contrast to NCs with a Knr of 3.21 × 10−2 ns−1. This observation is indicative of a well-passivated surface in NSs. In contrast, NSs with identical composition and surface morphology significantly enhance the photoluminescence quantum yields (PLQYs) up to 73%, which represents the highest PLQYs recorded in Cs2NaInCl6 DPs to the best of our knowledge. Beyond fundamental investigations, the study explores the practical applications of these NSs as anti-counterfeit marks created on a flexible substrate, demonstrating robust emission under UV light. Additionally, the fabrication and testing of a light-emitting diode (LED) showcase the versatility of these NSs in optoelectronic devices. This comprehensive exploration of the optical, morphological, and application aspects of Cs₂NaInCl₆ doped with Bi³⁺/Mn2⁺ NSs contributes valuable insights to the evolving landscape of lead-free perovskite materials, highlighting their potential for diverse technological applications.