Porous silicon . See

Figure 4. Formation condition for porous silicon solid line - peak current density, dotted line - current density at the maximum slope (see Figure.2).18...

Because it is believed that carbon nanotubes will play an important role in ultrasmall device preparation in the future (see Chap. 5), fabricating regular supramolecular assembhes of carbon nanotubes represents an important challenge. One example is shown in Fig. 4.56. A layer of porous sihcon is first prepared on a silicon support, and iron is then deposited on the porous silicon using an appropriate mask and an electron beam. When the patterned surface is exposed to ethylene gas, carbon nanotubes grows selectively on the sites covered with the iron catalyst. Again, this technique allows a range of different catalysts and patterns to be used, and various patterns of carbon nanotube arrays are easily fabricated. 

Consider the combustion reaction between a solid reactant and a gas oxidizer present initially in the constant volume of a porous medium (see Section IV,D,1). In this case, thermodynamic calculations for the silicon-nitrogen system have been made for constant volumes (Skibska et al, 1993b). The calculations yield the adiabatic combustion temperature, as well as pressures and concentration, as functions of the silicon conversion. As shown in Fig. 34a, the reactant gas pressure (curve 3) increases even though conversion increases. This occurs because... 

In-situ photoemission seems possible with semiconductors since tunneling out of a Pt-lr tip into the conduction band of certain materials has been reported (see Sec. 3). The large biases used in UHV are in fact not necessary in the liquid since the band edges of the semiconductor are almost fixed with respect to the vacuum level, which is often not the case in vacuum (see Sec. 3). In the case of porous silicon, using liquid interfaces seems promising and easier than in vacuum (see Sec. 5.1.3). In the liquid environment the main problem will certainly be the collection of photons within a difficult geometry due to the electrochemical cell. 

Luminescence may also arise as a consequence of electrode reactions at semiconductors that result in injection of minority carriers under accumulation conditions (see references in Kelly et al., 1999). An interesting example is the efficient red electroluminescence observed during the reduction of persulphate ions at a porous silicon layer on n-Si (Meulenkamp et al, 1995). Here the luminescence arises from electron/hole recombination, and the holes are injected by the persulphate radical ion according to the scheme... 

One of the arguments used in the porous silicon literature to explain the typical dimension of the structures states that the pore formation is governed by the width of the space charge region formed near the semi-conductor/electrolyte junction (see, for example, [17]). The argument seems to be very plausible as it works well for the mesoporous structures (10-100 nm pore size) obtained from n-type Si. The model has also been proposed for porous SiC [18] and elaborated in Shishkin et al. [14],... 

The preparation of a porous silicon carbide has been described by Fox and co-workers (10). The synthesis is based on heating the organosilicon pol3nmer (0 1158iOj 5) at 1600 C under argon (see Fig. 1). The pyrolysis reaction results in an intramolecular carbothermic reduction, i.e. the carbon bonded to silicon is used to remove oxygen and to form the carbide (the commercial manufacture of silicon carbide uses an external source of carbon for example, by mixing quartz sand and petroleum cokes). The product is purified by oxidation to remove excess carbon, followed by treatment with HF to remove silica. 

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