Hydrogen-terminated porous silicon

Boukherroub R, Morin S, Wayner DDM, Bensebaa F, Sproule Gl, Baribeau JM, Lockwood DJ (2001) Ideal passivation of luminescent porous silicon by thermal, noncatalytic reaction with alkenes and aldehydes. Chem Mater 13 2002-2011 Boukherroub R, Wojtyk JTC, Wayner DDM, Lockwood DJ (2002) Thermal hydrosilylation of undecylenic acid with porous silicon. J Electrochem Soc 149 59-63 Boukherroub R, Petit A, Loupy A, Chazalviel JN, Ozanam F (2003) Microwave-assisted chemical functionalization of hydrogen-terminated porous silicon surfaces. J Phys Chem B 107 13459-13462... 

Petit A, Dehnotte M, Loupy A, Chazalviel J-N, Ozanam F, Boukherroub R (2008) Microwave effects on chemical functionalization of hydrogen-terminated porous silicon nanostmctures. J Phys Chem C 112 16622-16628... 

Omae S, Minemato T, Murozono M et al (2006) Crystal growth mechanism of spherical silicon fabricated by dropping method. Jpn J Appl Phys 45 3577-3580 Park JH, Gu L, von Maltzhan G, Ruoslahti E, Bhatia SN, Sailor MJ (2009) Biodegradable luminescent porous silicon nanoparticles for in-vivo applications. Nat Mater 8 331-336 Petit A, Delmotte M, Loupy A, Chazalviel JN, Ozanam F, Boukherroub R (2008) Microwave effects on chemical functionalization of hydrogen terminated porous silicon nanostructurer. J Phys Chem 112(42) 16622-16628... 

Boukherroub R, Petit A, Loupy A, Chazalviel JN, Ozanam F (2003) Microwave-assisted chemical functionalization of hydrogen-terminated porous silicon surfaces. J Phys Chem B 107 13459 Buriak JM (1999) Silicon-carbon bonds on porous silicon surfaces. Adv Mater 11 265-267... 

The radical-based functionalization of silicon surfaces is a growing area because of the potential practical applications. Although further knowledge is needed, the scope, limitations, and mechanism of these reachons are sufficiently well understood that they can be used predictably and reliably in the modification of hydrogen-terminated silicon surfaces. The radical chemistry of (TMSlsSiH has frequently served as a model in reactions of both hydrogen-terminated porous and flat silicon surfaces. We trust that the survey presented here will serve as a platform to expand silicon radical chemistry with new and exciting discoveries. 

This chapter will focus on organic/silicon interfaces formed via solution phase reactions using hydrogen-terminated crystalline silicon surfaces as a starting point. While some of the surface chemistry issues have been reviewed previously [7,8], more recent developments will be emphasized here. We will not discuss the considerable literature of reactions with porous silicon [8], or studies of molecules reacting with clean silicon surfaces under ultrahigh vacuum (UHV) conditions [9-11] which have been reviewed elsewhere. 

In aqueous environments, Si-C bonded species show greater kinetic stability than Si-0 bonded species. The excellent stability of the Si-C chemistry on silicon surfaces was first demonstrated by Chidsey in the early 1990s (Linford and Chidsey 1993). Despite the lower bond strength of Si-C vs. Si-0 (Table 1), Si-C bonds are kinetically more stable. This is due to the lower electronegativity of C relative to O, which lowers the susceptibility of silicon to associative attack by nucleophiles. The most ubiquitous reaction used to form a Si-C bond to hydrogen-terminated porous Si is hydrosilylation (Eq. 19), a reaction first demonstrated by Buriak (1999, 2002 Buriak and Allen 1998) and developed by Boukherroub, Chazalviel, Lockwood, and many others (Canham... 

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

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). 

FTIR has been used to study the sihcon/electrolyte interface. The formation of porous silicon on n-Si during photoetching in fluoride media can readily be followed since the hydrogen-terminated surface is identified by the Si-H stretch bands centred around 2100 cm (Peter et ah, 1989 Peter et ah, 1990a). Similarly, the transition from a hydrogen-terminated to an oxide-covered surface during electropolishing has been followed by in-situ infrared spectroscopy (da Fonseca et al, 1996 and 1997). 

Scheme 1 Proposed mechanism for porous silicon degradation in aqueous solutions, adapted from Allongue et al. (1993). (a) A Si-H-terminated surface immersed in H2O. (b) The Si-H bond undergoes hydrolytic attack and is converted to Si-OH and produces a hydrogen molecule, (c) The Si-OH at the surface polarizes and weakens the Si-Si backbonds, which are then attacked by H2O, producing HSi(OH)3. (d) In solution, the HSi(OH)3 molecule is quickly converted to Si(OH)4 releasing a second hydrogen molecule...

Linford MR, Fenter P, Eisenberger PM, Chidsey CED (1995) Alkyl monolayers on silicon prepared from 1-alkenes and hydrogen-terminated silicon. J Am Chem Soc 117 3145-3155 Liu YM, Chen BL, Cao F, Chan HLW, Zhao XZ, Yuan JK (2011) One-pot synthesis of three-dimensional silver-embedded porous silicon micronparticles for lithium-ion batteries. J Mater Chem 21 17083-17086... 

Fig. 1 Comparison of the magnetization curves of bare n silicon (full line), as-etched porous silicon prepared from the same wafer (dotted line), and aged porous silicon offering a native oxide layer (dashed line). All three samples show a diamagnetic behavior, whereas the estimated susceptibility varies between —1.2 10 (Si), —7.5 10 (as-etched), and —2.6 10 (aged). The measurements have been performed at T = 4.2 K. The diamagnetic behavior of porous silicon decreases if the sample is oxidized (in contrast to hydrogen terminated) which might be caused by the occurrence of dangling bonds...

Another specificity of porous silicon, particularly in its as-anodized state (hydrogen-terminated), is its poor stability in air (Wolkin et al. 1999) or other chemically active environments. Thus, the PL characteristics (e.g., peak wavelength, intensity) may change very rapidly during the measurement itself. When precision is crucial, such as in measurement of degree of polarization, it is essential to ensure good stability during the acquisition time of all spectra. 

Although silicon nanocrystals are now more commonly prepared by a variety of means which are easier to scale up, e.g., pyrolysis of silanes (Xuegeng et al. 2004), thermal treatment of silsesquioxanes (Hessel et al. 2006, 2010), and from reactions of molecular silicon compounds (Wilcoxon et al. 1999 Bley and Kauzlarich 1996), this review will concentrate on routes which proceed via the formation of porous silicon. More general reviews of silicon nanocrystals Irom physics and chemistry perspectives are available (Shirahata 2011 Kang et al. 2011 Heitmann et al. 2005). Derivatization of porous silicon and SiNCs usually relies on the chemistry of the hydrogen-terminated silicon surface, which shares some of the organic reactivity of hydrosilanes (Buriak 2002). Reaction with alcohols results in Si-O-C bonded monolayers (Sweryda-Krawiec et al. 1999), but these are suseeptible to hydrolysis under ambient conditions. Alternately, addition of surface Si-H aeross a C = C double bond produces Si-C bonded monolayers, which are very stable. 

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