Pore Diameter and Interpore Spacing

Pore Diameter and Interpore Spacing. Pore diameter and interpore spacing are the prominent morphological parameters which as discussed above are determined by two large groups of factors those that affect the dimension of the space charge layer and those that affect the distribution of the reactions on the pore bottoms. The first group of 

FIGURE 8.72. Schematic illustration of the effect of a given amount of local current perturbation on the profile of a small pore relative to a larger pore. The profile of the small pore is altered more than that of the larger pore. 

FIGURE 8.73. Band structures of different materials during PS formation under anodic polarization, (a) Lowly doped p-Si (b) moderately doped p-Si (c) heavily doped p-Si (d) lowly doped n-Si (e) moderately doped n-Si (f) heavily doped n-Si R is the resistance of the substrate. 

On the other hand, increasing the concentration of HF increases the dissolution rate of oxide, which in turn increases the sharpness of the pore bottom. As a result, the pores become smaller and the walls thicker. 

It is important to note that although surface defect sites are associated with the initiation of pores, they do not determine the density and dimension of the pores in the bulk PS. The bulk morphology of PS is determined by the property of semiconductors and anodization conditions. However, under certain conditions such as those for the formation of macropores on lowly doped materials, control of the initiation sites by surface patterning can to some extent change the PS morphology. 

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The AAO substrates with uniform and parallel nanoporous structure were prepared in a two-step electrochemical anodization of aluminum foils in 4 % oxalic acid, at the constant current density of 3 A/dm, during 60 min. The pore diameter and average interpore spacing were 40 5 nm and 120 20 nm, correspondingly. The arrays of Ag nanoparticles on AAO were formed by thermal evaporation of silver onto a AAO substrate at room temperature. The mass thickness of silver was varied by varying the deposition time. Two series of samples with thickness 30, 60, 90, 120, 150 and 180 nm were prepared. 

Examples of Pore Diameter, Interpore Spacing and Pore Density of the PS Formed under Different Conditions (a More Extensive List is Given in Ref.1) ... 

The variation of interpore spacing or wall thickness is more complex than pore diameter. There is not much information on the relationship between wall thickness and formation conditions. In general, the walls are on the same order as or smaller than pore diameter.24 In particular, interpore spacing depends on potential it increases with potential at small currents but at certain current it starts to decrease with increasing potential. When interpore spacing is reduced to zero, which occurs in the transition region, pores no longer form, and instead, shallow pits form. 

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