## 1 Introduction
Therefore, some attempts have been made to replace the surface ligands of the CsPbX\({}_{3}\) PNCs with organic compounds such as dodecylbenzene sulfonic acid to provide chemical stability to the perovskites, allowing multiple purifications in polar solvents as well as stability against environmental conditions. Unfortunately, the external quantum efficiency in some devices is decreased, attributed to poor conductivity in the photomaterial [31]. To solve these drawbacks, novel synthetic procedures for PNCs growth have been highlighted by the use of capping agents with higher binding capability than conventional OA and OLA, achieving high-quality perovskites with PL quantum yield (PLQY) >90% [14; 32; 33]. This is the case of prominent capping ligands such as the diddode-cyldimethylammonium halide family [34], DDAX (X = Cl, Br), and zwitterionic compounds as the case of 3-N,N-(dimethyloctadecylammonio)propanesulfonate [14]. Even DDABr (also known as DDAB) generates partial exfoliation of the PNCs surface, removing part of the [PbBr\({}_{6}\)]\({}^{4-}\) octahedra and conducing a decrease in the particle size (strong nanoparticle confinement) [35], this ligand shows a dual role: DDA\({}^{+}\) cations also favor a ligand exchange with the native oleylammonium species, while Br\({}^{-}\) anions fill/replace the halide defect sites, decreasing the density of electron traps [22]. Simultaneously, the above zwitterionic species depict positively and negatively charged functional organic groups which can be anchored to X- anions, and A-site cations, respectively [8]. Therefore, DDAB and zwitterionic ligands hinder the loss of capping agents and the removal of halide domains, promoting the emergence of PNCs with PLQY values up to 100% and longer stability (Figure 1a-c) [14; 36], even to harsh polar environments such as alcohols without any type of encapsulation (Figure 1d) [37]. On the other hand, bidentate ligands such as 2,2\({}^{\prime}\)-iminodibenzoic acid (IDA) have been studied to restrain the fast \(\alpha\)-to-\(\delta\) phase transformation in CsPbI\({}_{3}\) PNCs, through a direct coordination interaction between surface Pb atoms and the double -COOH moiety from the capping agent [38]. In this way, the lattice distortion into the nanocrystal is restricted, enhancing the long-term stability of the photoactive phase in the photomaterial with a PLQY \(\mathtt{\
eg 95\%}\) (Figure 1e). Concerning promising synthetic routes, guanidinium (GA\({}^{+}\)) cations have been added during the preparation of FAPbBr\({}_{3}\) PNCs, achieving a maximum suppression of defect sites into the perovskite to obtain a PLQY \(\mathtt{\
eg 93\%}\)[39]. Even GA\({}^{+}\) species depict a bigger size than FA\({}^{+}\), going beyond the tolerance factor limit; low GAcontents \(\sim\)10 mol% can be incorporated into the nanocrystal due to the entropy stabilization. Here, GA\({}^{+}\) can compensate both bulk and surface defect sites by introducing more hydrogen bonds given by the extra-amino group from the cation, interacting with surface Br domains. As a result of the formation of mixed cation FA\({}_{1-x}\)GA\({}_{x}\)PbBr\({}_{3}\) PNCs with enhanced optical and electronic features, efficient green LEDs with EQEs = 23.4% have been fabricated. Following this, the other approaches highlighting the use of thiocyanate and tetrafluoroborate salts [8; 40] and novel purification methods to remove both undercoordinated Pb and unreacted species [26], have been proposed to produce highly emissive PNCs with improved optical features and color quality. Nevertheless, the washing process of the PNCs using low polar solvents also favors the loss of a fraction of ligands, hindering the control of the particle size and emerging a mixture of bulk and nanoscale materials with low stability [41].