H-termination, silicon

A recent theoretical study by Takeuchi et al. [140] has examined the mechanism for the reaction of both alkenes and alkynes with the H-terminated silicon surface using periodic DFT calculations, and the results are in good agreement with the proposed radical-based mechanism [137]. In particular, the calculations show that the reaction occurs through a carbon-based radical intermediate which must be sufficiently stabilized to proceed by abstraction of a surface hydrogen (as in the case of styrene) if the intermediate is not stable enough, it will preferentially desorb (as in the case of ethylene). The calculations also show that reaction with terminal alkynes should proceed faster and lead to more stable products than with terminal alkenes [140]. 

Linford and coworkers have shown that the attachment of alkenes to H-terminated silicon surfaces can also be initiated by direct mechanical scribing, in a process termed chemomechanical functionalization [145-147]. The reaction of 1-alkenes (as well as 1-alkynes) leads to attachment of the molecule to the surface through two new Si—C bonds. The proposed mechanism is the mechanical cleavage of Si—H and Si—Si bonds, leading to silicon radicals that then react with the reactive liquid. Interestingly, Linford and coworkers have also extended this work to show that chemomechanical functionalization can be carried out not only on H-terminated Si, but also on sihcon covered with oxide, and have shown that the process works with a variety of halides, alcohols, and epoxides in both the liquid and gas phase [146]. 

Methods for the direct organic functionalization of silicon using wet chemical methods usually begin with H-terminated silicon surfaces. This is because the H-terminated surface is reasonably stable and thus can be handled in various solvents, facilitating the attachment of a variety of molecules via solution phase chemical reactions. While both H-terminated Si(lll) and Si(100) surfaces have been used as starting points for organic... 

Fig. 2. STM images of molecular nanostructures of styrene on H-terminated silicon surfaces resulting from reaction at single dangling bonds via the radical chain mechanism depicted in Fig. 1. This process leads to the growth of molecular lines on H/Si(100) and irregularly shaped islands on H/Si(lll).

Direct Si-C bonding to the H-terminated silicon surface is expected to lead to a modified surface that has a much better stability against oxidation than that of the H-terminated surface. It has been reported that the (111) Si surface can be completely methylated by anodization in a Grignard reagent. " ... 

It is well known that C60 neither reacts with nor is adsorbed by bare silica, but it reacts with Si-H bonds on H-terminated silicon wafers forming covalently bonded monolayers [94]. The introduction of C60 dissolved in toluene should occur in the interfacial region between the PVFA-co-PVAm chains which are adsorbed on to silica and transform the reversibly adsorbed and flexible polyelectrolyte layer into an irreversibly adsorbed and more rigid structure. However, amino groups inside the adsorbed PVFA-co-PVAm coils are not available for surface functionalization with C60 (Scheme 5). Never-... 

Bressers PMMC, Plakman M, Kelly JJ (1996) Etching and electrochemistry of silicon in acidic bromine solutions. J Electroanal Chem 406 131-137 Carraro C, Maboudian R, Magagnin L (2007) Metallization and nanostructuring of semiconductor surfaces by galvanic displacement processes. Surf Sci Rep 62 499-525 Chabal YJ, Harris AL, Raghavachari K, Tully JC (1993) Inlfared spectroscopy of H-terminated silicon surfaces. Int J Mod Phys B 7 1031-1078... 

Chabal YJ, Harris AL, Raghavachari K, Tully JC (1993) Infrared spectroscopy of H-terminated silicon surfaces. Int J Mod Phys B 7 1031-1078... 

To date, numerous radical-induced hydrosilylations of terminal olefins or acetylenes have been reported for the H-terminated Si(l 11) surfaces. These reactions are mainly performed by using thermal conditions, UV irradiation, or electrochemistry. More recently, a very mild method was developed for the attachment of high-quality organic monolayers on crystalline silicon surfaces. 

Robertson has summarized the three recent classes of models of a-Si H deposition [439]. In the first one, proposed by Ganguly and Matsuda [399, 440], the adsorbed SiHa radical reacts with the hydrogen-terminated silicon surface by abstraction or addition, which creates and removes dangling bonds. They further argue that these reactions determine the bulk dangling bond density, as the surface dangling bonds are buried by deposition of subsequent layers to become bulk defects. 

The first models for the electrochemical dissolution process of silicon in HF assumed a fluoride-terminated silicon surface to be present in electrolytes containing HF [Ge6, Du3[. However, by IR spectroscopy it was found that virtually the whole surface is covered by hydride (Si-H) [Ni3[. No evidence of Si-F groups is found in IR spectra independent of HF concentration used [Ch9[. This is surprising insofar as the Si-F (6 eV) bond is much stronger than the Si-H (3.5 eV) bond, and so it cannot be assumed that Si-F is replaced by Si-H during the electrochemical dissolution. This led to the conclusion that if a silicon atom at the surface establishes a bond to a fluorine atom it is immediately removed from the surface. 

It is also possible to produce covalently bonded alkyl MLs on Si(l 11) surfaces using a variety of chemical reactions with passivated H-terminated Si(l 11), but the preparation methods are more complex than the immersion strategy in part due to the higher reactivity of silicon. This is a major achievement because it allows direct coupling between organic and bio-organic materials and silicon-based semiconductors. Both pyrolysis of diacyl peroxides (Linford Chidsey, 1993) and Lewis acid-catalyzed hydrosilylation of alkenes and direct reaction of alkylmagnesium bromide (Boukherroub et al, 1999) on freshly prepared Si(lll)-H produce surfaces with similar characterishcs. These surfaces are chemically stable and can be stored for several weeks without measurable deterioration. Thienyl MLs covalently bonded to Si(l 11) surfaces have also been obtained, in which a Si(l 11)-H surface becomes brominated, Si(lll)-Br, and is further reacted with lithiated thiophenes (He etal, 1998). 

In addition to Grignard chemistry and hydrogermylation, for which analogs also exist for silicon, there is another type of functionalization chemistry that has been carried out on germanium surfaces that of alkanethiol attachment to H-terminated Ge. Alkanethiols are well known for their use in self-assembled monolayers (SAMs) on gold surfaces. 

While the detection of the Si-H and Si-C modes indicates HREELS can probe the buried molecule/silicon interface, in general this method will be most sensitive to the terminal groups at the vacuum/monolayer interface. This is illustrated in Fig. 9 where spectra for several modified surfaces with different terminal functionalities are shown. In each case this terminal group is tethered to the surface via a Cio alkyl linker yet the spectra are significantly different. This is particularly evident in the spectra for the thienyl terminated surface in which the aromatic C-H stretch is clearly observed. In contrast this mode is quite small in the FTIR spectra, which are dominated by the contributions of the alkyl linker chain [51]. The observation of strong terminal group modes in the HREELS spectra indicates that these functional groups are likely present at the surface of the film and not buried back towards the H-terminated surface. This is consistent with their availability for sequential reactions as discussed in the previous section. 

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