CONTROLLING THE PHOTOPHYSICAL PROPERTIES OF FLUORESCENT PEROVSKITE NANOCRYSTALS BY SHAPE, SIZE AND DIMENSIONALITY

Abstract

In the past few years, all lead halide perovskite nanomaterials have attracted great attention for the potential applications of photovoltaics and optoelectronics. Their high photoluminescence quantum yields (PLQY), tremendous power conversion efficiency (PCE), narrow emission bands, and tunability of band gaps by size, shape, dimensionality, and composition make them ideal aspirants for optoelectronic devices, e.g., solar cells, LEDs, lasers, specific photodetectors, and visible-light communication. Nonetheless, the main weaknesses of lead halide perovskite are the poor chemical stability under ambient conditions and lead toxicity. These are very sensitive in moisture, air, light, and temperature due to their low formation energy. These drawbacks limit their commercial applications, which are urgently needed to be solved. To address these issues, I have focused my research on innovative ways that increase the optical performance and stability of lead halide perovskite nanocrystals. The toxicity of halide PNCs has also been eliminated, and lead-free, very stable 2D perovskite NCs with controlled optical characteristics have been created. The thesis' title is “Controlling the Photophysical Properties of Fluorescent Perovskite Nanocrystals by Shape, Size, and Dimensionality”. There are six chapters in this thesis. Chapter 1 Introduces the basic introduction about the background and current state of nanoscience and nanotechnology, nanomaterials, fluorescent naomaterials, and halide perovskite nanocrystals. The introduction then focuses on understanding what causes the remarkable optical properties of lead halide perovskites. Further, it covers the various classes of perovskites that are possible combinations in current research and the methods of synthesis are also explained including their advantages and limitations. I described the different applications for perovskite NCs, including their use in LEDs, memory devices, photodetectors, security systems, photocatalysis, solar cells, lasers, and X-ray detectors. The literature review provides a detailed overview of the evolution of halide perovskite NCs. Chapter 2 addresses that the addition of a minor amount of water into the precursor solution can improve the stability and photoluminescence quantum yield of CsPbBr3 nanocrystals through a ligand-assisted re-precipitation (LARP) method. In this way, the shape and phase transformation from CsPbBr3 nanoplates to CsPbBr3/Cs4PbBr6 nanorods and Cs4PbBr6 nanowires can be controlled with increasing water content in the precursor solution. Upon adding water up to an ideal amount, CsPbBr3 maintains its phase and nanoplate morphology. The key role of water amount for tuning the crystallinity, stability, morphology optical properties, and phase transformation of cesium lead halide perovskite nanocrystals will be beneficial in the future commercialization of optoelectronics Chapter 3 addresses the development of lead free halide 2D PNCs with high stability. In this chapter,we have demonstrated the formation of Zn based all inorganic mixed halide 2D perovskite Cs2ZnX4 (X = Cl4, Cl2Br2 and Cl2I2) nanocrystals (PNCs) first time with high photoluminescence quantumyield of 51.93%. Perovskite NCs were synthesized by using ligand-assisted re-precipitation (LARP) technique at 80oC.I have observed tunability of their optical band gap from 3.68to 3.54 eV. These blue fluorescent NCs displayed morphological change from hexagonal to quasi spherical and maintained their excellent stability over six months. The as prepared NCs were characterized by using UV-Vis., PL, XRD, TEM, FE-SEM, EDS, TCSPC, Raman, FT-IR, XPS and TGA analyticaltechniques. Chapter 4 describes the synthesis of lead-free Cs2CuCl4, Cs2CuBr4, and Cs2CuBr2Cl2 perovskite nanostructures with non-toxiccopper (Cu). In this work, I adopted a ligand assisted re-precipitation (LARP) synthesis route formaterial preparation at room temperature. Water is used as a polar solvent to dissolve the metal halideprecursors rapidly. For the first time, I achieved a high PLQY of 38% in mixed halide perovskiteCs2CuBr2Cl2NCs with blue luminescence in an aqueous medium. Cu-based perovskites Cs2CuCl4,Cs2CuBr4, and Cs2CuBr2Cl2 demonstrated their morphology as square nanoplates, nanorods, andrectangular nanoplates, respectively. The long-term photoluminescence stability of these storedperovskites was also examined at different intervals for 45 days.Chapter 5 focuses on the stability and photophysical properties improvement of cesium lead halide PNCs by Cu-BTC MOF. Here, I engaged perovskite NCs in Cu2+ ion based metal– organic framework (MOF) Cu‑BTC (BTC = 1,3,5‑ benzene tricarboxylate) by physical mixing of MOF with CsPbBr3 NCs in toluene solution. MOF‑ protected perovskite NCs achievedhigh photoluminescence quantum yield 96.51% than pristine state CsPbBr3 NCs (51.66%).Along withthe improvement in optical properties, the long-term stability of CsPbBr3 NCs in the solution phase also increases considerably upon loading in Cu‑BTC MOF. Moreover, the changes in the luminescentintensity of the samples have been observed for 3 months in the solution. After 1 month, pristine CsPbBr3 NCs lose their emission intensity 68% from the initial, while the MOF‑ protected CsPbBr3 NCsshow only a 10% reduction from the initial. These results indicate that the effective passivation of Cu‑BTC MOF inhibits the aggregation of NCs, protecting them from the defective atmosphere. The excellent photoluminescence findings provide a new pathway for future optoelectronic applications. Chapter 6 concentrates on producing organolead bromide perovskites quantum dots inside the lead-based metal-organic frameworks (Pb-MOFs) adopting a simple route at room temperature. Here, methyl ammonium bromide (MABr) was added into Pb-MOFs to develop MAPbBr3 QDs within the Pb-MOFs. The as-synthesized MAPbBr3@Pb-MOFs composite exhibits superior environmental and chemical stability in different solvents including water. The photoluminescent property of QDs was maintained even up to 6 months when immersed in water. The highly chemical stable QDs show bright green luminescent in various solvents even sonicated in a sonication bath for 4 min and kept up to 2 months. With the same aim, the obtained composite was used as a luminescent probe for the detection of metal ions from water. These findings suggest that the excellent chemical stability of MAPbBr3@Pb-MOFs composite terminates the hurdle of commercialization as well as explores the potential application of perovskites as a fluorescent sensor in water.