## 6 Functional Properties and Applications of Nanocomposites
### Optical properties  
The modification of the refractive index with coeval preservation of the transmittance is one of the challenges for particle/matrix nanocomposites, and therefore reported quite frequently in literature. Most of the research deals with TiO\({}_{2}\) nanoparticles, embedded in an organic matrix. This is because all TiO\({}_{2}\) modifications exhibit an inherent high refractive index, as already explained in section 2.1. In some cases, research groups also use semiconducting nanoparticles as ZnS or PbS with inherent high refractive indices. Table 5 summarizes the various and sometimes very differing results. Due to the extreme specific surface area values of nanoparticles with sizes below 10 nm, high nanoparticle contents cannot be expected. Hence, some of the results published in literature and listed in Table 5 are quite questionable. Additionally, the results cannot always be compared among each other, as:  
* especially in the case of TiO\({}_{2}\) not all authors indicate the phase they use
* different units as wt % or vol % are used for the filler
* surface modified nanoparticles as well as pristine nanoparticles are used
* different particle sizes are used
* different processes for the synthesis of the composites are used
* the influence of remaining precursor residuals is unclear. As shown in formula (1), the composite's sum refractive index changes linearly with the filler concentration (all concentrations in vol %), starting with the initial value for the pure polymer [130]. In case of small filler concentrations and almost identical density values for the polymer and the filler or dopant, as described in [130], the relation simplifies to formula (2) (concentrations in wt %). \[n_{composite} =n_{matrix}*C_{matrix}+n_{filter}*C_{filter} \tag{1}\] \[n_{composite} =n_{matrix}+n_{filter}*C_{filter} \tag{2}\]  
For applications in microoptics, the resulting optical transmittance in the visible, e.g., for consumer electronics, and NIR-range, for optical data transmission using the standard telecommunication wavelength at 1310 and 1550 nm, is also of particular interest. At larger sample thickness the transmittance drops significantly due to primary particle agglomeration and prevents the use of the composites in optical devices. Therefore, the published data has to be carefully controlled with respect to the measured sample thickness, a thin layer of some 100 nm thickness can exhibit excellent optical transmittance values in the visible, which drops almost to zero considering technical dimensions of some mm used in the ASTM standard for transmittance measurements applying Lambert-Beers-law. [PERSON] _et al._[94] showed the influence of particle treatment on optical properties: an _ex situ_ silanisation of nanosized Al\({}_{2}\)O\({}_{3}\), dispersion in a MMA resin and subsequent polymerization yielded better optical transmittance values in the NIR than untreated Al\({}_{2}\)O\({}_{3}\) applying the high speed stirrer as well as the high pressure homogenizer. Photoluminescence of nanocomposites is another interesting property reported for several classical nanocomposite systems as well as for nanocomposite particles. Excitation and emission of the composite differ significantly from the pure polymer. This was shown for GaN/polymer nanocomposites [80], and for Al\({}_{2}\)O\({}_{3}\), Y\({}_{2}\)O\({}_{3}\), ZnO and SnO\({}_{2}\)/Sb\({}_{2}\)O\({}_{3}\)/Sb\({}_{2}\)O\({}_{5}\) /polymer thin film nanocomposites [66], respectively. In the latter case the spectra broadened, loose vibronic structure and shifted towards higher photon energies (red-shift). The intensity of the effects is influenced by the polymer used. Polyphenylenevinylene (PPV)-based nanocomposites exhibit a stronger influence than poly[2-(6-cyano-6'-methylheptyloxy)-1,4-phenylene] (CN-PPP)-based nanocomposites. A green photoluminescence was observed for CdSe/polybutylacrylate (PBA) nanocomposites [83]. The emission of the composite also differed significantly from the emission of the polymer.