N

1200 oC

in catalytic, magnetic, electronic, and optical applications [596-602].

Scheme 29.

The combination of two discrete phases with different properties, at the molecular level, provides an avenue to the design of new hybrid materials as well as the possibility of modulating the properties of one or more of the components. It is also a unique way to generate materials that have special properties that are unknown in the individual components or new compositions (materials) not known before. The concept of dispersing an active phase into a supporting phase (matrix) is not new, and a large number of examples are available where functional nanoparticles have been embedded in polymer, glass, or ceramic matrices to modify the properties [289, 296, 299, 301, 303, 306, 308, 309]. Since the properties of nanocomposite materials depend not only on the properties of their individual parents but also on their degree of mixing and interfacial characteristics, considerable efforts are focused on the development of new chemical routes for producing these systems. The phase segregation poses a challenge in the synthesis of compositionally different nanocomposites because a uniform intermingling of two different phases cannot be obtained by mechanical blending. The above problem nevertheless can be addressed by using single-molecule precursors in which the elements required to form the two phases (dispersoid and matrix) are assembled in a molecular framework.

Composites of a metal and metal oxide phase could be interesting in view of their special material properties, like hardness, plasticity, luminescence, conductivity, photoelec-tronic properties etc. The strategy to obtain such biphasic nanocomposites in a single-step approach is based on the use of molecular precursors containing either a zerovalent or a meta-stable (unusual oxidation state) metallic center [410]. On thermal treatments, the meta-stable oxidation state dis-proportionates via a redox reaction to form a more stable higher oxidation state and the elemental form. For example, Cu(I) species have been used in sol-gel and CVD processes to obtain the Cu/Cu2O system.

A particularly interesting example is the formation of biphasic composites from the CVD of tert-butoxides of bivalent germanium, tin, and lead with the general formula

M(O'Bu)2 (M(II) = Ge, Sn, Pb) [143]. Veith et al. have studied the gas-phase pyrolysis (450-550 °C) of Mn(O'Bu)2 compounds to obtain composite films where metal particles are wrapped in oxide shells (Eq. (22)).

The molecular structure of the Ge precursor is shown in Figure 62. The formation of biphasic particles is chemically driven because the electron transfer necessary for the observed redox process (2M11 ^ M0 + MIV) occurs in a molecular entity, which enforces a narrow size distribution and self-organization of the two phases in the CVD deposit. The SEM images of the films show a regular distribution of spherical particles. The globular morphology of metalmetal oxide composite particles was investigated by nano-indentation and X-ray photoelectron spectroscopy (XPS) studies [603].

The Ge 3d XPS spectrum (Fig. 63) of the Ge/GeO2films exhibits two peaks, where the signal located at the lower binding energy is due to the germanium metal. Pure GeO2 films (obtained by the CVD of Ge(OPr')4 under identical conditions) showed a peak corresponding only to the Ge-O phase (Fig. 63), which confirmed the formation of a composite material.

The Ge content in the composite film was found to increase upon argon sputtering. This suggested that the metal particles are possibly wrapped in an oxide matrix that was peeled off during the sputtering process. To confirm that the sputter process does not lead to the reduction of germanium oxide to elemental germanium, the GeO2 sample was also sputtered with Ar+ ions, but no Ge(0) was detected, even after a long sputtering time (Fig. 63). The nanomechanical characterization [603] of the Ge/GeO2 films with an atomic force microscope-coupled nanoindenter (NI-AFM) supported a core-shell-type structure. Figure 64 shows the AFM images of the composite film before and after nano-indentation. The triangular impressions are due to indentations with different load forces. The applied force versus displacement (penetration depth) gave the hardness profile across the film. The hardness increased with increasing indented depth, reached a maximum value (7.2 GPa), and

Figure 62. Molecular structure of [Ge(O'Bu)2]2.

Ge°2 J

t Ge 3d \ Ge(II) NL Ar+

Ge/GeO J

X V Ar+

0 0

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