PS and silicon

The growth front of a PS layer (not individual pores) is always perpendicular to the surface of the substrate with back electrical contact. The growth front is planar, independent of the orientation of the silicon samples, because the growth of pores depends on the supply of carriers which are transported from the back of the sample. The interface between the PS layer and silicon substrate is essentially flat with a difference between high and low points within a few micrometers. 

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Many theories on the formation mechanisms of PS emerged since then. Beale et al.12 proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. Smith et al.13-15 described the morphology of PS based on the hypothesis that the rate of pore growth is limited by diffusion of holes to the growing pore tip. Unagami16 postulated that the formation of PS is promoted by the deposition of a passive silicic acid on the pore walls resulting in the preferential dissolution at the pore tips. Alternatively, Parkhutik et al.17 suggested that a passive film composed of silicon fluoride and silicon oxide is between PS and silicon substrate and that the formation of PS is similar to that of porous alumina. 

It is known that poly(methyl methacrylate) (PMMA), polystyrene (PS), and silicon oxide (Si02) produce monodispersed spheres. They are easily produced using common experimental equipment and many of them are commercially... 

Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements (i.e., O, N, and Si). Furthermore, they have very large molecular structures, often chainlike in nature, that often have a backbone of carbon atoms. Some common and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. These materials typically have low densities (Figure 1.4), whereas their mechanical characteristics are generally dissimilar to those of the metallic and ceramic materials—they are not as stiff or strong as these... 

The dissolution of PS during PS formation may occur in the dark or under illumination. Both are essentially corrosion processes, by which the silicon in the PS is oxidized and dissolved with simultaneous reduction of the oxidizing species in the solution. The material in the PS, which is distant from the growing front is little affected by the external bias due to the high resistivity of PS and is essentially at the open circuit potential (OCP). Such corrosion process is responsible for the formation of micro PS of certain thickness (stain film) in HF solutions containing oxidants under an unbiased condition. 

Microcrystallites of direct semiconductors usually show a simple exponential decay of the PL intensity P with time, with time constants r in the ps and ns range at RT. A similar simple exponential decay (r = 20ms at 2 K) is observed for PL from mesoporous silicon of high porosity, which shows a weak confinement effect... 

PS formed chemically without an applied potential in a mixture of HF and an oxidizing agent, e.g. HN03, is called a stain film. The similar nature of electrochemi-cally formed micro PS and stain films was pointed out in 1960 [Arl]. Stain films are usually microporous. They are of predominantly monocrystalline character and show visible PL [Scl6, Jil], If metal films are present on the silicon surface, mesopore formation may also be observed [LilO]. 

During the last 5 years, there have been several reports of multiblock copolymer brushes by the grafting-from method. The most common substrates are gold and silicon oxide layers but there have been reports of diblock brush formation on clay surfaces [37] and silicon-hydride surfaces [38]. Most of the newer reports have utilized ATRP [34,38-43] but there have been a couple of reports that utilized anionic polymerization [44, 45]. Zhao and co-workers [21,22] have used a combination of ATRP and nitroxide-mediated polymerization to prepare mixed poly(methyl methacrylate) (PMMA)Zpolystyrene (PS) brushes from a difunctional initiator. These Y-shaped brushes could be considered block copolymers that are surface immobilized at the block junction. 

Figure 6.9 A schematic representation of orthogonal process for nanoparticles self-assembly (a) a patterned silicon wafer with Thy-PS and PVMP polymers fabricated through photolithography and (b) orthogonal surface functionalization through Thy-PS/DP-PS recognition and PVMP/acid-nanoparticle electrostatic interaction. Reprinted with permission from Xu et al. (2006). Copyright 2006 American Chemical Society.

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