**2.5 Emulsion method**

In order to control the crystallization of perovskite and obtain uniform NPs, emulsion synthesis method was modified [40]. By using this method, it is possible to tune the size of PNSs under 10 nm with an PLQY up to 92% [41]. In this method, an emulsion is prepared with two immiscible solvents. After that, a demulsifier is added into this for initiating solvent mixing and start crystallization (**Figure 5**). For this procedure, DMF and n-hexane are very good candidates as immiscible solvents while tert-butanol or acetone is used as demulsifier solvent [41]. This method is suitable to obtain PNSs in solid state which can be used in another solvent matrix for an application later. Furthermore, long alkyl ammonium halides are used as capping agent.

#### **Figure 5.**

*(a) Schematic illustration of preparation of nano-crystals by emulsion method; (b) ternary phase diagram (DMF/OA/n-hexane); (c) image of a typical CH3NH3PbBr3 emulsion, colloidal NP solution and solid-state powder of CH3NH3PbBr3 NPs (reproduced with permission of Ref. [41]).*
