Capturing Transient Growth Windows for Narrow-Emitting Perovskite Nanoplatelets

Colloidal perovskite nanoplatelets offer thickness-tunable, directional emission, but maintaining narrow ensemble photoluminescence linewidths at greater thicknesses remains challenging. Here, we show that under selected reaction conditions, distinct CsPbBr3 nanoplatelet thickness populations emerge in succession during synthesis, each offering a transient window for isolation before further morphological evolution. By tuning growth kinetics through precursor composition and overall dilution under ambient conditions, we make these windows experimentally accessible. In situ photoluminescence monitoring tracks the successive populations and guides the time-selective isolation of 4- and 5-monolayer nanoplatelets. The resulting ensembles exhibit narrow cyan emission with linewidths down to 70 and 80 meV, respectively. Post-synthetic surface treatment increases the photoluminescence quantum yield to 70 % while preserving narrow emission, and halide exchange extends the approach to red-emitting iodide nanoplatelets. These findings demonstrate that favorable nanoplatelet morphologies can be accessed as transient populations along a growth trajectory, establishing control over growth kinetics and isolation time as a strategy for obtaining narrow-emitting colloidal nanostructures.