**2.2 Ligand assisted reprecipitation (LARP) technique**

In this method, a polar solvent such as dimethylformamide (DMF) which dissolve all starting materials and capping ligands have been used to prepare the precursor solution [25, 26]. This solution is added dropwise into vigorously stirred toluene which is not a good solvent for starting materials and perovskite crystal. Zhang et al. has demonstrated the synthesis of colour- tuneable PNSs (average particle size of 3.3 ± 0.7 nm) with a PLQYs of 50–70% (**Figure 2**) [25]. It was claimed that a slight descending of PLQY was observed by the ascending of the size of the perovskite crystal (2–8 μm, PLQY < 0.1%). In the nanometer scales, surface defects of the crystals can easily be passivated by ligands. Thus, most of the photo-generated charges can recombine before there are trapped by defects on the surface. However, due to the less ligand passivation, the number of defects on the bulk perovskite structure`s surface, which increased the number of trapped charges, are significantly high, resulting very low PLQY. In another study, well-defined cubic and thermally stable FAPbX3 nanocrystals (about 10 m) has been prepared by LARP method. The reported PLQY for the NPS was 75% [27]. A new procedure, which was used to obtain a core-shell shape by using the LARP approach has been recently demonstrated. With this proposed method, a solutionprocessed, stable core–shell-type Methyl ammonium (MA<sup>+</sup> ) + Octyl ammonium (OA<sup>+</sup> ) lead bromide perovskite NPs (≈5–12 nm) with good PLQY was prepared. In addition to this, Core–shell-type NPs was accomplished by systematically changing the molar ratio of capping ligands, OABr, and MABr without altering total amount of alkylammonium bromide and synthesis conditions. The color tunability of NPs in the blue to green spectral region (438–521 nm), high PLQY, and reasonable stability under ambient condition are credited to the quantum confinement imparted by the crystal engineering associated with core–shell NP formation [28–30].
