Polysilanes, decomposition

Figure 5.3. TDS of solution-processed a-Si films. Three samples were prepared by the thermal decomposition of polysilane under the following conditions sample a, 300 °C for 10 min sample b, 300 °C for 120 min and sample c, 540 °C for 120 min. Desorbed gases from the samples were analyzed using mass spectroscopy while the samples were heated in a vacuum. [Reproduced with permission from Ref. 10. Copyright 2006 Nature Publishing Group.]...

Doubtless, in these reactions unstable intermediate silylmercury compounds such as [H2(CH3)Si]2Hg and [H(CH3)2Si]2Hg are first produced which then undergo decomposition. The formation of the trisilane and higher polysilanes from the reaction of (CH3)2SiHBr with Na/Hg may be rationalized by the insertion into [H(CH3)2Si]2Hg of dimethylsilylene, (CH3)2Si , which is probably produced during the reaction through decomposition of an unstable intermediate species (CH3)2SiHg, followed by decomposition of the adducts. The generation of dimethylsilylene and its insertion into the... 

The Si—Si bond is relatively weak, so homolytic decomposition readily occurs on application of thermal or electrical energy or radiation (light, neutrons, X-ray. .. )a. As expected from the coordinative unsaturated character of the Si atoms, the polysilanes are very reactive with nucleophilic agents as shown... 

Polysilane derivatives constitute a new class of radiation-sensitive materials with interesting physical and electronic properties. The photochemical decomposition of polymers containing disilanyl units seems adequately explained by silicon-silicon bond homolysis and subsequent radical reactions. The solution photochemistry of longer silicon catenates results in the extrusion of substituted monomeric silylenes, as well as the formation of silyl radicals produced by chain homolysis. Recent studies indicating that the... 

Polysilanes are used in these reactions. Preparation of Me2Si from dodecamethylcyclo-hexasilane appears to be the preferred entree into this chemistry for both synthetic and mechanistic studies. In the absence of trapping reagents, the silylene species form oligomers. If irradiation continus more than 20 h, products from radical reactions begin to appear. The thermal decomposition of sterically hindered siliranes also offers a convenient route to silylene intermediates . This is true, for instance, with a base-stabilized... 

The thermal treatment of vinylic polysilane SiC precursor polymers involves <300 °C cross-linking and loss of low molecular weight oligomers. Polymer degradation, which is characterized by significant decomposition of the side groups and chain scission, along with conversion of the polysilane skeleton to poly(carbosilane), takes place between 300 and 750 °C [124],... 

The decomposition chemistry of most polysilanes above 300 °C is quite complex and involves restructuring of the polymer backbone as well as substantial cleavage of the side groups. Generdly a poly(alkyl)silane must go through a carbosilane intermediate prior to formation of the SiC network (Kumada rearrangement) [125] ... 

Fig. 1. Favored radical species related to the thermal decomposition of polysilanes or polycarbosilanes (E = Si, C)...

There are indeed some other methods for producing silicon-based nanocomposites that appeared in the literature. Considerable work has been performed by J. Dahn s group and the decomposition of silane or polysilane pitch within carbonaceous matrices has been widely explored. Other methods based on HEMM of electrochemically inactive phases such as iron, nickel, titanium-nickel, titanium-carbon, silicon-carbon, and so on also have been examined using silicon powder with an initial particle grain size within either micrometric or nanometric ranges. The common problem in all these cases is that it is difficult to predict theoretically the appropriate matrix/silicon particles combination, i.e., there is no simple guidance rule in order to make reasonable predictions. An overview of all the above-described methods is summarized in Table 11.3. 

As an alternative to thermal decomposition, CVD, and laser-assisted H elimination, the flash pyrolysis of soluble alkylpolysilyne in a vacuum is reported recently. Unless they are under vacuum conditions, polysilanes and polysilyne may convert to silicon carbide (SiC). The flash process, which is a rapid removal of volatile organic substances and H2 gas, enables the control of the dimension of Si structure from 2D to 3D. Flash pyrolysis of alkylpolysilyne in a vacuum above 500 °C leads to the formation of poly- Si with a minimal amount of SiH termini, although the size of the crystals is limited to the range between several pm and several nm. 

Earlier studies" " on the thermal decomposition of permethylated polysilanes, (SiMe2) , at 300°C showed that the yields of SiMej are generally low. Later Ishikawa and Kumada " initiated a series of studies on the... 

The breeding of a silylene from such polysilanes lends more support to the possible existence of a silicon-bridged transition state as shown in equation (50). The decomposition of the transition state shown in that equation is essentially the breeding of a SiH2 unit from a cyclic silicon intermediate. 

It has recently been reported that doping of the polysilanes with antimony pentafluoride results in decomposition of the polymers [12]. In fact, mass spectrometric analysis of gaseous products obtained Irom the doping of poly(methylphenylsilane) with antimony pentafluoride vapor shows the presence of tetrafluorosilane and trifluoromethylsilane. Similar experiments using poly(dimethylsilane) indicate the formation of tetrafluorosilane, trifluoromethylsilane and difluorodimethylsilane. These results clearly show that scission of a silicon-silicon bond occurs during doping, as well as a silicon-phenyl and silicon-methyl bond. 

Hydrogenated amorphous silicon was formed by plasma decomposition of monosilane gas. The network has the dimension of close to 3. Polysilane alloy was formed by plasma decomposition of disilane gas.33 The network consists of a mixture of 1-dimensional polysilane and 3-dimensional silicon micro clusters.34 The effective network dimension is lower than that of amorphous silicon. Photoluminescence observations for various silicon-based materials are shown in Figure 14. The peak energy values of the photoluminescence spectra for amorphous silicon, polysilane alloy, hexyl-silicon network polymer and dihexylpolysilane are 0.8, 1.2, 2.8 and 3.3 eV, respectively. This result confrrms that a wide continuous spectra range from ultraviolet to infrared can be covered by the luminescence spectra of silicon based polymers. 

Compared to the C-C bond, the Si-Si bond has a more electropositive nature and heterolytic dissociation occurs more easily due to the low energy o-o excitation [85]. The absorption bands of polysilanes in the near UV range between 290 and 410 nm and they are temperature and structure dependant [86], Since the o-o transition is allowed, the absorption coefficients of their optical absorption spectra are large [42]. Although the Si-Si bond has a similar bond strength to the C-C bond [87], cleavage of the Si-Si bond by UV excitation can easily occur in the solid state as well as in solution. Both may lead to decomposition of the polymeric species [3, 88, 89]. 

Linear alkyl di- and polysilanes with small organic groups and few Si atoms are liquid the other di- and polysilanes are solids and melt, for the most part, without decomposition. For example ... 

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