Surface porous silicon

Low reflective surface Porous silicon itself Diamond-Uke carbon Optical (79, 80, 81, 82) and optoelectrical devices (32) Solar cells (93) Sehirone et al. (1997), Lipinski et al. (2003), Panek (2004), Aroutiounian et al. (2007)... 

Depending mostly on the degree of quantum confinement and on the chemical state of its surface, porous silicon could luminesce from the near-infrared ( 1.5 pm) to the near-UV as a result of distinct emission bands having different origins (Table 2 Cullis et al. 1997 Bisi et al. 2000). The near-infrared band has not been as extensively studied as the visible bands. It was observed in both partially oxidized porous silicon and oxygen-lfee samples. It has been related to both quantum-size effect and surface states (Koch et al. 1993). The UV band has been observed only in oxidized porous silicon. It has been related to oxide luminescence, with the silicon nanocrystals playing a potential role in the photoexcitation process(Qin et al. 1996). 

Using a Si-C bond forming approach, it is possible to attach a wide variety of organic functional groups to the porSi surface. Porous silicon can be rapidly analyzed by transmission infrared spectroscopy (see Fig. 1 and chapter Characterization of Porous Silicon by Infrared Spectroscopy ), thus making it possible to easily track the progress of surface reactions. As will be shown below, reactions often consume the surface Si-H bonds, so monitoring the decrease in area of v(SiH c) is a convenient method that permits the yield of the reaction to be calculated. Often this is... 

Increasing the surface-to-bulk ratio of the sample to be studied. This is easily done in the case of highly porous materials, and has been exploited for the characterization of supported catalysts, zeolites, sol-gels and porous silicon, to mention a few. 

Gupta P, Dillon A C, Bracker A S and George S M 1991 FTIR studies of H2O and D2O decomposition on porous silicon surfaces Surf. Sc/. 245 360-72... 

The concepts and basic approach used in studies of electrical fluctuations in corrosion processes proved to be very successful as well in mechanistic studies of electrode reactions taking place at materials covered by passivating films. A typical example is the electrochemical dissolution of silicon. From an analysis of the noise characteristics of this process, it has been possible to identify many features as well as the conductivity of the nanostructures of porous silicon being formed on the original silicon surface. 

Fig. 3.1.7 The surface diffusion coefficient / surface of cyclohexane (squares) and acetone (circles) in porous silicon with 3.6-nm mean...

Electropolishing is well established as a simple, in situ method to separate porous silicon layers from the silicon electrode. By switching the anodic current density from values below JPS to a value above JPS, the PS film is separated at its interface to the bulk electrode. The flatness of a PS surface separated by electropolishing is sufficient for optical applications, as shown in Fig. 10.10. 

Another way to use silicon wafers as DLs was presented by Meyers and Maynard [77]. They developed a micro-PEMFC based on a bilayer design in which both the anode and the cathode current collectors were made out of conductive silicon wafers. Each of fhese componenfs had a series of microchannels formed on one of their surfaces, allowing fhe hydrogen and oxygen to flow through them. Before the charmels were machined, a layer of porous silicon was formed on top of the Si wafers and fhen fhe silicon material beneath the porous layer was electropolished away to form fhe channels. After the wafers were machined, the CEs were added to the surfaces. In this cell, the actual diffusion layers were the porous silicon layers located on top of the channels because they let the gases diffuse fhrough fhem toward the active sites near the membrane. 

Figure 8.3 Hydrogen-terminated Si(l 11), Si(lOO) and porous silicon surfaces.

It is worth mentioning that the photooxidation of porous silicon behaves differently [49]. Indeed, ETIR spectra show that there is a tremendous increase in vsi o, without a correspondingly large loss of vsi H peak intensity. The decrease of the vsi H band is offset by an increase in the vosi—h band, resulting in no net loss of hydride species on the surface during the course of the photooxidation reaction. These data apparently suggest that oxidation does not result in the removal of H atoms, implying that Si—Si bonds are attacked directly. 

Fig. 8.6 Schematic set-up of a DIOS-TOF-MS system. Initially, the sample is deposited on the porous silicon surface. Subsequently, a laser pulse is directed to the silicon surface, and the analytes are desorbed. Ions that are generated are transferred into a time-of-flight mass spectrometer.

The surface of the porous silicon chip offers multiple possibilities for the covalent coupling of functional groups. Thus, functionalization of a DIOS target by immobilization of trypsin was described by Xu et al. [21]. The enzyme was immobilized using cyanuric chloride as coupling agent following an amino-functionalization (Fig. 8.11). 

Porous silicon is under extensive study, largely due to its luminescence properties. For electroluminescence, however, some form of contact has to be made with the Si, and this necessitates deposition of another phase inside the pores of the Si in order to contact as much as possible of the internal area of the high-surface-area Si. With this in mind, CdS has been deposited inside the pores of porous silicon via a two-stage method [73]. Cd(OH)2 was deposited from an ammoniacal bath at pH 8, followed by conversion of the Cd(OH)2 to CdS by treatment with thioac-etamide at pH 8. This was repeated several times until the pores were essentially filled with CdS. The reason that this two-stage process was needed is that either the Si was unstable at the temperatures and pH values needed to deposit CdS from a thiourea solution, or CdS was formed in solution rather than on the Si surface using thioacetamide. 

Buriak, J. M Stewart, M. P Geders, T. W Allen, M. J., Choi, H. C Smith, J., Raftery, D. and Canham, L. T. Lewis acid mediated hydrosilylation on porous silicon surfaces. Journal of the American Chemical Society 121, 11491 (1999). 

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