## 2 Nanocomposite Materials
### NC functionalization  
While NCs can be easily dissolved in various solvents, they need to be homogeneously dispersed in an inert or functional host matrix in order to effectively exploit their specific functionalities in composite materials to be used in practical applications. Basic requirements is then the good dispersibility of the NCs in such organic materials. Colloidal NCs offer the notable benefit of a prompt and easy processability of their surface, which can be functionalized by changing capping ligands to make them compatible with several host environments. In particular, most particles will need to suitable tailor their surface to achieve favourable interactions with the host polymer. Indeed, ligand conformation, ligand chain packing on a faceted surface and polymer chain diffusion into the ligand layer are critical parameters interplaying a delicate equilibrium [81]. Therefore, a judicious choice of a suitable ligand for NCs can ensure a good dispersion and maximize the interactions between NCs and host matrix. The capping agent used in the synthesis can be replaced by post-synthesis treatments with another one having higher compatibility with the polymer matrix. The capping exchange with the new ligand can occur either by mass action or by using functional groups presenting stronger affinity for the NC surface than the pristine one. Electron-donor groups (thiols [82], phosphines [83], amines [84], and nitrogen containing aromatics like pyridine and its derivative [85]) can be used to displace the original ligand exploiting their strong affinity for the NC surface (Figure 3). The new ligand can be a bifunctional agent, carrying, on one end, a functionality with strong affinity for NC surface and, on the other end, a specific chemical group useful for further NC processing. Alternatively, the new capping agent can simply differs from the original one simply for the length of the alkyl chain. Oleic acid (OLEA) capped CdS NCs can be functionalized with alkylamine differing in the alkyl chain length (C\({}_{8}\)-C\({}_{16}\)) and dispersed in poly(methyl methacrylate) (PMMA) or polystyrene (PS) matrices exploiting the mass action principle [86,87]. The dispersion of hydrophilic metal NPs in hydrophobic polymer matrices can be promoted by exploiting electrostatic interactions to drive the phase transfer of metal NPs into an organic solvent. [88] For instance, a bifunctional ligand such as mercaptosuccinic acid (MSA) can anchor Au NP surface through the thiol moiety whereas the two free carboxylic groups can experience, in suitable pH conditions, electrostatic interactions with organic soluble cationic surfactants (for instance tetraoctylammonium bromide dissolved in toluene). Such electrostatic interactions allow the phase transfer of Au NPs from water to organic solvent (toluene), retaining the original optical properties, and the resulting NPs can be incorporated in PMMA-based thermoplastic polymers [89]. Capping ligand carrying reactive functionalities can be exploited to improve NC dispersion in the polymer host matrix involving the capping layer in the polymerization process. As an example, OLEA coated TiO\({}_{2}\) nanorods and Fe\({}_{2}\)O\({}_{3}\) NCs can be easily functionalized with double bond terminated ligands [90] in order to exploit the new moiety in UV induced polymerization processes. Alternatively, a reactive group can be designed and introduced on the pristine capping ligand by chemical reaction. [PERSON] _et al._ demonstrated the insertion of a 2-bromoproponyl ester group (an initiator of the atom transfer radical polymerization process; ATRP) on OLEA coated Fe\({}_{2}\)O\({}_{3}\) NCs. A styrene shell was then polymerized directly on NC surface through an ATRP approach [91]. [PERSON] _et al._ proposed a ligand consisting of a chain of reactive esters, which can be substituted with different molecules containing amino-functionalities.