## 2 Nanocomposite Materials
### Synthetic approaches to nanocomposite materials  
Preparative methods aiming to synthesize hybrid NP-polymer based nanocomposites can be broadly classified as _in situ_ and _ex situ_ techniques. The former are based on the direct formation of inorganic nanostructures in the polymer, while the latter involve the incorporation of pre-synthesized NPs in monomers before the polymerization or alternatively directly in the polymer matrix. Figure 3: Schematic representation of engineered NCs incorporated in polymer matrices. (a) Playing with capping ligand alkyl chain length and steric hindrance; (b) exploiting capping ligand bearing a reactive moiety (c) bi-functional ligand modified NCs (d) NCs modified with polymer ligands. In the picture: fluorescence view of a flat imprinted structure on CdSe(c) ZnS NC/ PMMA nanocomposite. In particular, in the _in situ_ method inorganic precursors of NPs can be introduced in monomers before the polymerization, or after the polymerization. NP formation can occur either by chemically reduction [93], thermal decomposition [94] or induced by light (UV) irradiation [95]. In the sol-gel method, solid precursors and low temperature processes are used for covalent bond formation in solutions [7]. Low temperature processes allow the preservation of the organic groups from their decomposition, but at the same time often provide kinetic rather than thermodynamically most stable structures, thus leading to the formation of amorphous structures. However, sol-gel processes exploit mild reaction conditions and show broad solvent compatibility that make them ideal candidates either for the formation of inorganic networks in the presence of a premade organic polymer or the polymerization before, during, or after the sol-gel process. The resulting hybrid materials will be characterized by the combination of both component properties, as well as by the phase morphology and interface features. Therefore, nanocomposite properties and the final performance of the derived devices will strongly depend on the process parameters and their optimization. The main drawback of the _in situ_ approach lies in the limited control over the preparative and processing conditions, which strongly influence the properties of the final nanocomposite material, combined to the relatively weak interactions between the formed NPs and the host matrix, which reduce the polymer ability to coordinate the NC surface [96, 97]. On the other hand, the _ex situ_ methods allow the direct transfer of the original size dependent properties of inorganic NPs in the host matrix and enables their effective combination with the well defined characteristics of the polymer. Moreover, appropriately tuning the interaction between NC surface and the specific polymer chemistry can provide a sort of passivation of nanostructures that are sensitive towards environmental conditions (oxygen, humidity, _etc._). _Ex situ_ methods enable the right and gradual dosage of the inorganic nanostructures within the polymeric matrix and offer the possibility to tune the ordering capability, obtaining nanocomposites ranging from highly ordered superlattices, _i.e.,_ in suitable block copolymer host systems, to the so-called stochastic mixtures, where the particles are randomly dispersed. Different approaches for the _ex situ_ preparation of nanocomposites can be classified in three main classes, based on the NC encapsulation strategy, as illustrated in Figure 4. Indeed the incorporation of NPs in the polymer matrix can be supported by the use of a common solvent (Figure 4.1), or can be promoted by the presence of initiator groups attached on NC surface that can be effectively linked with the organic host (Figure 4.2). Homogeneous dispersion can be also obtained by coordinating the NC surface with chemical functional groups of the polymeric chains (Figure 4.3). The first approach has been used to prepare nanocomposite materials based on high quality blue emitting CdS NCs.