Phase Transition, Doping (Mn2+ and Mo3+) and Luminescence of Cs2NaBiCl6 and CH3NH3PbBr3 Perovskites: Bulk and Nanocrystals

Abstract

Halide perovskites often change their crystal structure based on external condition like temperature and pressure. This structural phase transition may lead to changes in their inherent semiconducting properties like bandgap, luminescence and carrier mobility. Another way of tailoring properties is by lattice doping. For example, dopants like Mn2+ and Mo3+ introduces interesting luminescence properties. The emphasis of this thesis work is to study the correlation between structural phase transition, doping, and luminescence in halide perovskites. The work has been extended for both macroscopic bulk crystals and nanocrystals. We started with Cs2NaBiCl6 double perovskite that is known to undergo a temperature dependent phase change from cubic (Pm-3m) to tetragonal (I4/mmm), below 110 K. Mn2+ and Mo3+ ions are doped as luminescence centers in Cs2NaBiCl6. Doping Mn2+ ions leads to a broad orange-red colored emission owing to its d-d electronic transition. It has been found that neither Mn2+–doping alter the phase transition of the host around 110 K, nor the phase the transition alters the dopant emission. The emission depends on the local structure around the Mn2+ ions, which appears to remain unaffected in spite of the global phase transition in the host. In difference, the Mo3+ dopants show a sharp near-infrared emission (~1095 nm), that show anomalous behavior around phase transition temperature. Furthermore, the structural phase transition might change for nanocrystals, because of the higher contribution of the surface energy. Indeed, our data do not show the signature of the cubic to tetragonal transition of Cs2NaBiCl6 nanocrystals ~110 K, unlike their bulk counterpart. Lastly, we prepared colloidal methylammonium lead bromide (MAPbBr3) nanocrystals with different shapes– nanocubes and nanoplatelets. The size– and shape–dependent optical properties were studied. The nanoplatelets, along with their capping ligands, behave more like pseudo two dimensional perovskites. Overall, the thesis work provides insights into the interplay between dopants, dimensionality, and phase stability of halide perovskites for optoelectronic applications.