Octylammonium iodide-mediated size control of CsPbI₃ nanocrystals in microfluidic continuous-flow synthesis

Alkylammonium halides are well-established growth regulators for CsPbBr₃ nanocrystals (NCs), yet their role in the growth of CsPbI₃ NCs remains largely unexplored. In this work, the effect of octylammonium iodide (OAI) on the growth of CsPbI₃ NCs is systematically investigated using a microfluidic platform. Increasing the OAI concentration reduces the NC size and blueshifts the emission peak. At a lower reaction temperature, this effect gives NCs as small as 8.9 nm with a PL peak at 661.9 nm. This size-regulation effect is attributed to the additional iodide ions supplied by OAI, which likely promote a nucleation burst that rapidly depletes the monomers, thereby inhibiting subsequent crystal growth. This mechanism is supported by a control experiment, where replacing OAI with octylamine results in a pronounced increase in NC size (>15.4 nm). Furthermore, oleylammonium iodide, an analogue of OAI, can hardly replicate its role. Although it induces some small NCs, the average particle size remains above 15.1 nm, and it tends to cause the coexistence of multiple emissive species.