Size-Dependent Phase Transition in Perovskite Nanocrystals.

Complex structure of halide and oxide perovskites strongly affect their physical properties. Here the effect of reduced dimensions to the nanoscale has been investigated by a combination of single-dot optical experiments with a phase transition theory. Methylammonium lead bromide (CH3NH3PbBr3) nanocrystals with two average particle sizes of ~2 and ~4 nm with blue- and green-photoluminescence, respectively, were spectrally and temporally probed on a single particle level from 5 K to 295 K. The results show that the abrupt blue-shift of the photoluminescence spectra and lifetimes at ~150 K can be attributed to the cubic-to-tetragonal phase transition in the large 4 nm nanocrystals, while this phase transition is completely absent for the small 2 nm particles in the investigated temperature range. Theoretical calculations based on Landau theory reveal strong size-dependent effect on temperature-induced phase transitions in individual CH3NH3PbBr3 nanocrystals, corroborating experimental observations. This effect should be considered in structure-property analysis of ultra-small perovskite crystals.