Room-temperature synthesis of highly luminescent methylammonium lead bromide nanocubes encapsulated in block copolymer micelles: impact of solvent choice on crystallization and stability

Abstract

Herein, we demonstrate a facile, room-temperature method for synthesizing highly luminescent methylammonium lead bromide (MAPbBr₃) nanocubes encapsulated within colloidal polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) micelles in a non-polar 1,3,5-trimethylbenzene (TMB) solvent. The synthesis involves five distinct stages: micellization of the PS-b-P2VP block copolymer, dissolution and complexation of PbBr₂ precursors, coordination between bromoplumbate complexes and P2VP segments within the micelle core, multiple emulsion, and confined crystallization of perovskite nanocubes. Unlike conventional ligand-assisted methods, the colloidal micelles act as soft nanoreactors by controlling the nucleation and growth of the nanocubes through multiple emulsion. Moreover, the micelles serve as colloidal templates, preserving a significant concentration of [PbBr₃]⁻ ions formed during the complexation stage. We also demonstrate that the choice of solvent for transporting MA⁺ and Br⁻ ions from the TMB phase to the micelle core significantly influences the dimensions of the resulting MAPbBr₃ perovskite. Utilizing methanol as the transport medium yields encapsulated cubic MAPbBr₃ nanocubes with a high photoluminescence quantum yield (PLQY) of ∼77%, and these encapsulated nanocubes exhibit long-term stability with a superior PLQY of ∼88%. In contrast, employing dimethylformamide (DMF) to solvate MA⁺ cations mainly yields MAPbBr₃ microcrystals with reduced PLQY, as the formation of encapsulated MAPbBr₃ nanocubes is hindered. This hindrance is attributed to the high entropic penalty associated with the diffusion and penetration of DMF-solvated MA⁺ cations through the PS shells. We believe that this versatile approach can be extended to synthesize a wide range of nanomaterials with well-defined morphologies, monodisperse sizes and long-term stability.