## 6 Functional Properties and Applications of Nanocomposites
### Optical properties
For ZnO/vinylester resins photoluminescence spectra also showed a dependence of nanoparticle loading. As expected, the pristine polymer possessed no luminescence. Interestingly already a particle loading of 1 wt % ZnO showed a significant luminescence. With increasing particle concentration an increase in intensity was observed, but no influence on the emission maximum [131]. ZnO/polybutanediolmonoacrylate (PBDMA) nanocomposites showed an increase of the excitation and the emission wavelengths with increasing particle sizes [132]. A similar relationship was observed for ZnO quantum dots dispersed in PMMA [133]. [PERSON] _et al._[134] investigated also the photoluminescence of ZnO nanoparticles in a PMMA matrix. These authors attributed the UV emission located at 334 nm to the quantum size effects of the nanoparticles, and the photoluminescence peak located at 346 nm to the presence of bound excitons in R-(COO)-ZnO complexes. This is the same mechanism as described for luminescent oxide/PMMA nanoparticles [89, 135, 136]. Similar effects are observed at nanocomposite particles with a morphology as described in section 3.2. Photoluminescence was found in TiO\({}_{2}\)/PMMA nanocomposite particles with an emission maximum at 420 nm [137]. A strong luminescence was found in core/shell nanoparticles made of HfO\({}_{2}\), ZrO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), or ZnO cores and PMMA-shell, respectively [89, 136]. The luminescence of nonconducting oxide/polymer nanoparticles was mainly attributed to the presence of carboxylate groups at the interface ceramic/PMMA [135] whereas ZnO as a semiconductor exhibited an inherent luminescence. These authors also found a strong influence of the coating polymer on luminescence. The variation of the coating organic compounds lead to significant changes in the emission spectrum. Furthermore an influence of the particle size on emission maxima and on width of emission lines was found for ZrO\({}_{2}\) and ZnO. In Figure 8 the size dependency of the emission maxima of photoluminescence for several nanocomposite systems is shown. Data for the three totally different ZnO/Polymer nanocomposite types complement each other. A further development was multifunctional nanocomposite particles where magnetic properties and luminescent properties have been combined in one particle [89]. The particles consisted of a superparamagnetic Fe\({}_{2}\)O\({}_{3}\) core, coated with an organic dye, and finally with a protective polymer layer. Depending on the organic dye used, photoluminescence could be adjusted. There is a high application potential for photoluminescence nanocomposites. Examples are epoxy nanocomposites containing ZnO nanoparticles for solid state lightning [138], and ZnO/polymer core/shell nanoparticles for _in vitro_ cell imaging [139]. A different approach towards composite with luminescent or lasing properties is the solution or dispersion of rare earth metal complexes in polymer matrices. PMMA, containing a 2.5 wt % Eu-complex was investigated with respect to the 613 nm fluorescence pumped by a 457 nm Ar\({}^{+}\) laser [140]. Another Eu-complex with particle sizes between 30 and 100 nm was dispersed in a commercial available Araldite GY 251 and the photoluminescence as function of the solid load (up to 5 wt %) was measured at 618 nm [141]. \begin{table}
\begin{tabular}{|l|l|l|l|l|} \hline
**Nano-Filler** & **Diameter [nm]** & **Matrix** & **An, Refractive index increase** & **Reference** \\ \hline ZnS/PMAA + acetic acid; & ZnS: 3 nm & DMAA/St/DVB & & [78] \\
50 vol \% & & & 0.023 & \\ \hline ZrO\({}_{2}\) 50 wt \% & 5 nm & PC & 0.067 (at 589 nm) & [142] \\ TiO\({}_{2}\) 42 wt \% & 7 nm & PC & 0.135 (at 589 nm) & \\ \hline TiO\({}_{2}\), 60 wt \% & Amorphous & Epoxy & 0.221 & [143] \\ \hline ME-capped ZnS 30 wt \% & ZnS: 3 nm & DMAA/St/DVB & 0.048 & [87] \\ \hline TiO\({}_{2}\) 27.3 vol \% & \(<\)10 nm & PS & 0.22 & [144] \\ TiO\({}_{2}\) 90 vol \% & & & 0.41 & \\ \hline TiO\({}_{2}\) acetic acid mod.