Synthesis and Characterization of Double Perovskite Nanocrystals with Controlled Electronic Structure

Abstract

Lead halide perovskites (LHPs) are potent optoelectronic materials such as light emitting diodes, photovoltaic cells, scintillator and etc., due to high quantum yield and charge carrier mobility which originate from their defect tolerant nature. However, LHPs suffer from the potential toxicity concerns and structural lability against heat, moisture and light. Thus, the development of LHPs of non-toxic composition, having structural stability has become highly active research area nowadays. As an alternative to lead halide perovskite, various types of lead-free perovskites have been recently studied for optoelectronic materials. Especially, double perovskite (DP), showing elpasolite perovskite structure (A2M+M3+X6 where A = Cs, CH3NH3: M+ = Ag, Au, Tl: M3+ = In, Au, Bi: X = Cl, Br, I.), is rising. This thesis reports a synthesis of highly phase-pure Cs2NaBiX6 (X = Cl, Br) double perovskite (DP) nanocrystals (NCs). They show characteristic absorption features of sharp and discrete peaks mostly originated from the Bi3+ s–p transition (6s2 → 6s1p1) in [BiX6]3− units within the crystal lattice of elpasolite structure. Such unique optical properties are attributed to the non-bonding character of electropositive sodium and electronically-isolated [BiX6]3− units in crystals. The shape of Cs2NaBiX6 NCs could be quantitatively controlled by adjusting the reaction temperature. Reaction temperatures above 180 °C favors development of cuboctahedral (CO) shape, whereas development of cuboidal (CB) shape is favored below 170 °C. CB NCs can be subsequently converted to CO NCs by heating to 200 °C. The CO NCs promote growth of heterostructure adducts on the (111) facets: these adducts could be post-eliminated by etching. Mn-doped Cs2NaBiCl6 NCs are synthesized: they show efficient energy transfers from the NC host to the dopants. The synthesis and shape control of Cs2NaBiX6 NCs and Mn-doped Cs2NaBiCl6 NCs could expand a new type of lead-free DP NCs applicable to optoelectronic applications. Also, we conduct the research to utilize Na based double perovskite materials having a discrete electronic structure as a scintillator. To improve scintillation efficient, we focused on thermalization of hot carriers. A more quantized electronic structure causes the photoelectrons to slowly lose energy and induces the excited photoelectrons to create many excitons, resulting in a more efficient scintillation effect. Here, we reports Cs2MTbCl6 (M = Na, K) double perovskite nanocrystals. As we hypothesized, Cs2MTbCl6 NCs shows enhanced scintillation effect compare to CsPbBr3 NCs which are conventionally used as scintillator candidates. We expect that Cs2MTbCl6 NCs could potentially be applied to X-ray scintillation/detection and fluorophores.