## 2 Ecofriendly Strategies to Synthesize High-Quality PNCs
### Synthetic Procedures for PNCs Preparation via Green Solvents  
The conventional synthesis procedure involving the use of organic ligands and solvents has been established to produce PNCs with different morphologies and sizes, tunable optical features, and the possibility to inhibit halide migration into the perovskite lattice itself [69; 70]. However, in addition to the harmful features of these compounds on people's health, some of them show long carbon chain structures that can hinder the carrier's transport into a device, decreasing its final performance [71; 72]. On the other hand, the use of bulky organic ligands makes that the PNCs growth is conducted in organic media (for instance, 1-octadecene or analogous compounds) [73; 74; 75], avoiding the use of more economically and ecofriendly polar/ionic systems where the structural integrity of PNCs is rapidly deteriorated. To face these drawbacks, [PERSON] and coworkers [76; 77] have developed a solvent-free synthetic method where conventional precursors such as CsBr and PbBr\({}_{2}\) were combined with stoichiometric amounts of molten salts NaNO\({}_{3}\), KNO\({}_{3}\), and KBr at high temperatures -350 \({}^{\circ}\)C, under ambient conditions. Furthermore, a mesoporous silica layer (m-SiO\({}_{2}\)) with thermal, chemically stable, and nontoxic characteristics was added to prepare PNCs@m-SiO\({}_{2}\) composites, promoting the perovskites nanoconfinement and preserving their optical properties in polar environments. Here, PNCs@m-SiO\({}_{2}\) materials delivered PLQY \(\sim\)89 \(\pm\) 10%, keeping their PL properties for a time up to 3 h at 180 \({}^{\circ}\)C, \(\sim\)28 days in saline water at 90 \({}^{\circ}\)C and 1 month in both water and aqua regia systems (Figure 4a-c). At this stage, ionic species from the molten salts such as K\({}^{+}\) and Br\({}^{-}\) favor an efficient defect passivation of CsPbBr\({}_{3}\) PNCs surface by filling A- and X-sites [78; 79]. The above-mentioned properties make these PNCs@m-SiO\({}_{2}\) composites suitable for green phosphors for the down-conversion process into liquid crystal displays. On the other hand, [PERSON] and coworkers have proposed for the first time the use of natural deep eutectic solvents (NADESs) during the CsPbBr\({}_{3}\) PNCs synthesis via ligand-assisted reprecipitation (LARP) approach [80]. These NADESs are used as solvents for the mixture reaction, and simultaneously as capping agents stabilizing the final PNCs product. Interestingly, NADESs contain a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) and present some advantages, such as facile preparation, low cost, and biodegradable [81]. By mixing two kinds of HBA such as DL-menthol and thymol, with eight different HBD species such as L-lactic acid, n-octanoic acid, decanoic acid, lauric acid, ethanol, n-butanol, n-hexanol, and n-octanol, several NADESs solvents were obtained. The authors realized that the green solvent composed of thymol-lactic acid and thymol-n-octanol were pivotal to dissolving CsBr and PbBr\({}_{2}\), respectively, while a NADES based on thymol-decanoic acid was used as an antisolvent. By combining this NADES with the corresponding perovskite precursors, high-quality PNCs were obtained with PLQY \(\sim\)97%, narrow full width at half-maximum \(\sim\)19 nm, and long-term stability of the PL properties for 70 days. Modified PNCs are employed as green phosphorous in pc-LEDs, with a maximum power efficiency of \(\sim\)55.61 LmW\({}^{-1}\) at 20 mA. The explanation of the improvement in the material stability arises from the co-existence of -COOH/-OH moieties from carboxylic acid/alcohol, which bind to Pb\({}^{2+}\), reducing the likelihood of having surface defects. Furthermore, [PERSON] and coworkers [82] have shown a novel method for PNCs synthesis via green ionic liquids based on protic methylammonium carboxylates, obtained using the Bronsted acid-base reaction between methylamine and three different carboxylic acids: formic acid, propionic acid, and butyric acid.