Cryochemical synthesis films

Simultaneous evaporation of metal with organic and inorganic substances followed by vapor deposition on a substrate allows the production of composite films containing M nanoparticles stabilized in various dielectric matrices [2, 28]. The use of monomer molecules in this process polymerizing during deposition or as a result of the subsequent reactions yields polymeric nanocomposite films with metal inclusions [2, 3, 28, 37]. The new low-temperature synthesis of polymeric nanocomposite films has been elaborated recently. This synthesis is based on the deposition of M/SC and monomers vapors at temperature 80 K followed by low-temperature solid-state polymerization of obtained films in conditions of frozen thermal movement of molecules (cryochemical synthesis) [2], This synthesis has important features, which will be considered further. 

As mentioned above, the new method of cryochemical synthesis of polymer nanocomposite films has been developed based on co-deposition of M/ SC and monomer vapors at temperature 80K and subsequent low-temperature solid-state polymerization of monomer matrix ([2] and works cited herein). It has been established that a number of monomers (acrylonitrile, formaldehyde, /i-xylylene and its derivatives) polymerize in solid state in absence of thermal movement of molecules owing to own specific supra-molecular structure. When reaction is initiated by y- or UV-radiation the formation of a polymer matrix occurs even at the temperatures close to temperature of liquid helium [66-69]. 

Similar histograms were determined by TEM for Pb-, Zn-, and Cd-containing nanocomposite PPX films prepared by vapor deposition cryochemical synthesis [85]. The value d of metal nanocrystals in these films is also 5nm. The same approximately size d ( 4.5nm) has been evaluated from Ai/2 of X-ray diffraction peak for semiconductor PbS nanocrystals in PbS-PPX nanocomposite [71]. It should be particularly emphasized that d value of M/SC nanocrystals embedded by cryosynthesis in PPX and C1PPX matrices does not depend on M/SC content as for low loading (0.2-2 vol.% for Ag in PPX and C1PPX [75, 80] and 0.01-1 vol.% for Pb in PPX [85]) and for high loading (5-11 vol.% for PbS in PPX [3, 71, 86]) systems. 

The more detailed study of the crystallite size distribution and crystallite arrangement in PPX matrix was carried out for cryochemically synthesized nanocomposite PbS-PPX films in the range of PbS content from 4.7 to 10.2 vol.%. The distribution curves obtained from analysis of wide-angle X-ray scattering (WAXS) data is presented in Figure 10.5 [71, 86]. The curve for nanocomposite with 4.7 vol.% of PbS differs a little from histograms of the crystallite size distribution determined by TEM for nanocomposite films Ag-PPX and Pb-PPX with low metal content [75, 80]. It means that, as the average size of crystals, distribution in the sizes almost does not depend on nature of M/SC incorporated in polymer as a result of cryochemical synthesis. 

Composite PPX PbO films produced by PVD cryochemical synthesis are sensitive sensors on humidity. The conductivity of the PPX film containing 10 vol.% of PbO nanoparticles sharply increases with the rise of air humidity. The influence of water vapors on the film conductivity is reversible at replacement of humid air on dry one the conductivity comes back quickly to an initial value for dry air and direct and reverse response times are 10 15 sec [89]. 

PPX films containing PbO nanocrystals are sensitive sensors on humidity. The conductivity of the PPX film with about 10 vol. % of PbO nanocrystals versus the air humidity is presented in Figure 2.12. PbO nanocrystals in a PPX matrix are formed by oxidation of Pb nanocrystals in Pb-PPX nanocomposite films produced by cryochemical synthesis (see above). 

Films of pure PPX and PPX composites with nanoparticles of various metals resulted from cryochemical solid-state synthesis were studied by the dielectric spectroscopy method [104], Dielectric spectroscopy has proven very useful for studying the structure and dynamics of polymer materials as well as the transport mechanism of charge carriers. To study features of the polymer structure dielectric test methods were used due to their high sensitivity to morphological changes. 

The cryochemical vapor deposition synthesis of metal-polymer films (from the gaseous state to the solid polymer one, bypassing the liquid phase) allows the production of both new organometallic structures and new valuable composites with high concentrations of nano-sized metal or semiconductor particles. 

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