Development of Lead-free Perovskite Passivated Core/Shell Perovskite Nanocrystals with Enhanced Optoelectronic Properties and Environmental Stability

All-inorganic lead halide perovskite nanocrystals (NCs) have great optoelectronic properties with promising applications in light-emitting diodes (LEDs), lasers, photodetectors, solar cells, and photocatalysis. However, the applications of perovskite NCs face many challenges, including environmental issues and poor endurability of the perovskite-based devices caused by the intrinsic toxicity of Pb and the poor stabilities of perovskites against moisture, heat and light. To address those issues of the lead halide perovskite NCs, we utilized stable lead-free perovskite as a shell to protect the lead halide perovskite NCs, as well as developed lanthanide ions (Ln3+) doped/based-lead-free perovskite NCs to enhance their optical properties, making them compatible substitutions of the lead halide perovskite. In the first work of this dissertation, lead-free vacancy-ordered double perovskite Cs2SnX6 (X = Cl, Br, I) was grown epitaxially on the surface of CsPbX3 NCs by a hot-injection method. The effectiveness of the non-toxic shell protection is demonstrated by the enhanced environmental and phase stability against UV illumination and polar solvents such as water and ethanol. The photoluminescence quantum yields (PL QYs) increase for the CsPbCl3 and CsPbBr3 NCs after shelling because of the type I band alignment of the core/shell materials, while enhanced charge transport properties obtained from CsPbI3/Cs2SnI6 core/shell NCs due to the efficient charge separation in the type II core/shell band alignment. To understand the interfacial properties of core/shell NCs, we developed a facile method to grow a lead-free CsMnCl3 shell on the surface of CsPbCl3 NCs to form CsPbCl3/CsMnCl3 core/shell NCs with enhanced environmental stability and improved PLQYs. Interestingly, the Mn2+ from CsMnCl3 shell can diffuse into the lattice of CsPbCl3 core by thermal annealing of the resulting core/shell NCs, activating the orange PL from Mn2+ dopants. The Mn PL to host CsPbCl3 PL ratio can be precisely tuned by adjusting the thermal annealing time of the core/shell NCs. Incorporation of rare earth lanthanide ions into lead-free halide perovskite nanocrystals (NCs) is an effective and promising way to expand their optical, magnetic, and electrochemical properties. We developed various Ln3+ (including Yb3+, Er3+, and Nd3+), doped Sb3+- or Bi3+-based and Sb3+/Bi3+ alloyed lead-free perovskite NCs, including vacancy-induced perovskite (A3B(III)2X9), double perovskite (A2B(I)B(III)X6), and layered-double perovskite (A4B(II)B(III)2X12) NCs in this work. Interestingly, it is found that the lanthanide ions, even Ln3+ ions with large ionic radii, such as Nd3+ ions with their 112 pm ionic radius, in an octahedral (Oh) coordination environment, prefer smaller isovalent Sb(III) Oh cation sites (90 pm) instead of Bi(III) Oh sites (117 pm) in the lead-free perovskite NCs due to the relatively high polarizabilities of lanthanide ions and the more intense second-order Jahn-Teller in [SbCl6]3- compared to that in [BiCl6]3-. The efficient Ln3+doping in Sb3+-rich alloyed perovskite NCs was confirmed by elemental analysis and leads to enhanced Ln3+ ion near-infrared (NIR) photoluminescence (PL) of the doped NCs. Furthermore, we first synthesized Cs2KLnCl6 NCs with strong blue emissions through bandgap radiative excitonic recombination, and the characteristic NIR emissions from Ln3+ can be turned on by doping Sb3+. This study provides a fundamental understanding of Ln3+ doping behavior in lead-free perovskite NCs and new opportunities for designing efficient Ln3+-based functional materials.