Surface Modification of Porous Semiconductors to Improve Gas-Sensing Characteristics

1 Surface Modification of Porous Semiconductors to Improve Gas-Sensing Characteristics 

As shown in the previous section, to achieve the essential parameters of gas sensors, it is necessary to use porous layers with optimal thickness and porosity. However, it should not be forgotten that the surface chemistry of the inner walls of the pores controls the adsorption of gases as well as the capillarity condensation. Therefore, in designing sensors based on porous materials, the opportunity to control these processes, using various treatments for surface functionalizing and stabilization, should not be ignored. As demonstrated earlier for metal oxide-based sensors, such an approach makes it possible to optimize better the parameters of gas sensors. The results of numerous research projects, which can be found in Table 26.2, have shown that such an approach for the design of gas sensors based on porous semiconductors is effective as well. 

For example, Sharma et al. (2007b) demonstrated that texturizing a silicon surface before porosification is a simple and effective way to form highly porous, highly luminescent, thick films of PSi with reduced stress, improved stability, and superior mechanical properties. Good results may also be obtained by partial oxidation of porous silicon. This approach was discussed in Chap. 22 (Vol.2). Fiirjes et al. (2003) found that, after partial oxidation, the sensitivity of both the surface 

Most of the above-mentioned approaches are based on PSi surface modification with Si-C bonds. Finally we obtain derivatized PSi surface, in which the Si-Hx bonds were replaced by Si-C bonds due to hydrosilylation of alkenes or alkynes on the PSi surface (Salonen and Lehto 2008). The change of FTIR spectra of porons silicon after derivatization is shown in Fig. 26.6. The absorption intensity of si-Hx si-H2 decreases substantially after the reaction, indicating that most of the hydrogen has reacted with the unsaturated C=C and C=0 double bonds (Fig. 26.6d). This result is consistent with a hydrosilylation reaction that preferentially consumes the more reactive SiHj and SiH species (negative bands for the SiHx stretch modes around 2,117 cm and the Si-H scissors mode at 915 cm ). 

The group of Buriak (Stewart and Buriak 1998,2001 Buriak and Allen 1998b Buriak 1999 Robins et al. 1999 Buriak et al. 1999 Holland et al. 1999) introduced three different approaches to obtain the chemically derivatized PSi surface Lewis acid-mediated hydrosilylation, white light-promoted hydrosilylation, and cathodic electrografting. The highest treatment efficiency (the proportion of replaced Si-Hjc) of 28 % was obtained with one pentene using Lewis acid-mediated hydrosilylation (Stewart et al. 2000). Boukherroub et al. (2001) extended the usable techniques for hydrosilylation by introducing 

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