O O

The (alkoxy)siloxides are also interesting precursors for depositing biphasic thin films. Narula et al. have used [(BtfO)3Si-O-Ti(OBu')3] as a SSP for obtaining antireflection TiO2-SiO2 coatings by atmospheric pressure CVD process [312]. Recently, Schubert et al. have prepared titanium alkoxides modified with a ^-diketonate group

Figure 67. TEM photographs of xerogel derived from Zr[OSi(OBu')3]4 after calcination under O2 at 1200 and 1400 0 C. Reprinted with permission from [301], K. W. Terry et al., J. Am. Chem. Soc. 119, 9745 (1997). © 1997, American Chemical Society.

containing a hydrolyzable alkoxysilane moiety [609]. The hydrolytic conversion of these compounds produced dispersed silica-titania powders. The amount of silica in the final material could be tuned by increasing the number of alkoxy silane groups in the precursors. The presence of silica matrix inhibits the growth of titania nanoparticles; as a result, in the TiO2:2SiO2 system the anatase phase was observed, whereas the formation of rutile was favored in the TiO2:SiO2 system [609]. For anatase-rutile transformation, the titania particles must exceed a certain size, and particle growth is favored in the system containing lower silica content.

Recent reports on nanocomposite systems have shown that the matrix materials not only act as passive hosts in sustaining and protecting the nanocrystallites, but also provide a special environment the dispersoids [610-612]. For example, the change in the emission peak observed in the CdSe/BaTiO3composite system has been attributed to the dielectric environment effects of the matrix on the electronic structure of the embedded CdSe quantum dots [613]. In the context of optical materials, composites containing lanthanide elements or their phases dispersed in an isotropic matrix are of immense technological value and current interest. The neodymium-doped materials are useful as solid-state laser media, phosphors, and optical amplifiers [600-602]. A homogeneous dispersion of high concentrations of Nd3+ ions in a fully connected host matrix like SiO2 is difficult because of the absence of nonbridging oxygen atoms. As a result, the coordinatively unsaturated Nd3+ ions form interaction clusters, resulting in quenching of the luminescence quantum yield. The addition of alumina as a codopant can control the clustering of rare-earth ions by forming Nd-O-Al bonds, which improves the ill-coordinated state of lanthanide centers [614]. Mathur et al. have put forward a new concept of controlling the quenching problem by restricting the movement of lanthanide ions in the host matrix. Since the Nd-Nd separation plays a decisive role in the quenching process, adding the dopant phase in the form of nanocrystals containing Nd-O-Al units is expected to offer positional control with respect to the minimum Nd-Nd separation. A higher amount of alumina can serve as an active matrix by interacting with the surface Nd atoms of the NdAlO3 nanocrystals to form further Nd-O-Al bonds. Moreover, alumina is a suitable matrix because it allows the incorporation of a high concentration of Nd3+ ions, and, in comparison to silica, alumina possesses a higher refractive index and optical transparency. To avoid short Nd-Nd distances, heterometallic precursors

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