Coverage porous silicon

An alternate route to formation of alkyl monolayers is via Lewis acid catalyzed reactions of alkenes with the hydrogen terminated surface. In this approach, a catalyst such as ethyl aluminum dichloride is used to mediate the hydrosilylation reaction of an alkene (or alkyne), resulting in the same type of product as in the case of the photochemical or thermal reactions. This type of reaction is well known based on molecular organosilane chemistry and has also been used successfully to alkylate porous silicon [31]. Although this route has been shown to work on H/Si(lll), the resulting monolayers are found to have lower coverages than those achieved using the photochemical or thermal approach [29], Another concern with this approach is the possibility of trace metal residues from the catalyst that could adversely affect the electronic properties of these surfaces (even when present at levels below the detection limit of most common surface analysis techniques). 

Gupta et al. [20] found second-order kinetics for desorption from the dihydride phase on porous silicon. The corresponding activation energy from isothermal measurements is 43 kcal/mol. The TPD experiments of Flowers et al. [40] on Si(100)-2 X 1 also showed that desorption from the dihydride was second order, with an activation energy of 47 kcal/mol. This result is derived from a model that estimates the equilibrium density of dihydrides as a function of coverage and temperature. This model has been criticized... 

FIGURE 1.238 Diffusivities of n-heptane in Vyeor porous glass (open symbols) and in porous silicon (solid symbols) as a function of surface coverage measured at different temperatures. (Adapted from Micropor. Mesopor. Mater., 125, Dvoyashkin, M., Khokhlov, A., Naumov, S., and Valiullin, R., Pulsed field gradient NMR study of surface diffusion in mesoporous adsorbents, 58-62, 2009. Copyright 2009, with permission from Elsevier.)... 

Fig, 1 Arrhenius plot of surface diffusivities for -heptane in porous silicon with an average pore diameter of 5.6 nm for different surface coverages c (Dvoyashkin et al. 2009)... 

Robinson MB, Dillon AC, Haynes DR, George SM (1992) Effect of thermal annealing and surface coverage on porous silicon photoluminescence. Appl Phys Lett 61 1414-1416... 

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