Excellent green but less impressive blue luminescence from CsPbBr3 perovskite nanocubes and nanoplatelets

Green photoluminescence (PL) from CsPbBr3 nanocubes (∼11 nm edge-length) exhibits a high quantum yield (>80%), narrow spectral width (∼85 meV), and high reproducibility, along with a high molar extinction coefficient (3.5 × 10(6) M(-1) cm(-1)) for lowest energy excitonic absorption. In order to obtain these combinations of excellent properties for blue (PL peak maximum, λ max < 500 nm) emitting samples, CsPbBr3 nanocubes and nanoplatelets with various dimensions were prepared. Systematic increases in both the optical gap and transition probability for radiative excitonic recombination (PL lifetime 3-7 ns), have been achieved with the decreasing size of nanocubes. A high quantum yield (>80%) was also maintained, but the spectral width increased and became asymmetric for blue emitting CsPbBr3 nanocubes. Furthermore, PL was unstable and irreproducible for samples with λ max ∼ 460 nm, exhibiting multiple features in the PL. These problems arise because smaller (<7 nm) CsPbBr3 nanocubes have a tendency to form nanoplatelets and nanorods, eventually yielding inhomogeneity in the shape and size of blue-emitting nanocrystals. Reaction conditions were then modified achieving nanoplatelets, with strong quantum confinement along the thickness of the platelets, yielding blue emission. But inhomogeneity in the thickness of the nanoplatelets again broadens the PL compared to green-emitting CsPbBr3 nanocubes. Therefore, unlike high quality green emitting CsPbBr3 nanocubes, blue emitting CsPbBr3 nanocrystals of any shape need to be improved further.