Silicon nanocrystals hydrogenated

Structural Properties of Hydrogenated Silicon Nanocrystals and Nanoclusters... 

Table 6.2 Performance of the CheFSI methods for large hydrogenated silicon nanocrystals. AH...

Zhou ZY, Brus L, Friesner R (2003) Electronic structure and luminescence of 1.1- and 1.4-nm silicon nanocrystals oxide shell versus hydrogen passivation. Nano Lett 3 163-167... 

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. 

Apart from the thin films of amorphous semiconductors, nanocrystalline form of these materials has also attracted much attention due to its higher efficiency and stability. While an amorphous semiconductor, for example, a-Si H is a single phase material, its nanocrystalline form (nc-Si H, also very often, but less correctly, called pc-Si H) can be described as a bi-phasic material consisting of a dispersion of silicon nanocrystals embedded in silicon or other silicon-based hydrogenated amorphous matrices, whose volume fraction could be varied by selecting the proper plasma deposition conditions. The nc-Si H films have been recently demonstrated to be an interesting alternative to a-Si H films (Conibeer et al., 2006). 

Porous silicon (por-Si) formed by electrochemical etching of (llO)-oriented p-type Si wafers is an example of novel nanostructured medium with controllable optical properties. It was found to exhibit the strong in-plane birefringence (up to 18 %) and free-carrier dichroism [1-5]. Both phenomena originate from the form anisotropy of Si nanocrystals and voids assembling the material [3-5]. Below, we report the analysis of the dichroism in por-Si on the basis of the generalized effective-medium approximation (EMA) [7] as well as prominent anisotropy of absorption by silicon-hydrogen surface bonds. 

The dispersion and assembly behavior of silicon (Si) nanoclusters are controlled in organic and aqueous suspensions. Hydrogen-terminated Si nanoclusters are stably dispersed in non-polar solvents, but assemble on aqueous suspension surfaces. The nanoclusters spontaneously pack into lattice arrangements when a slow assembly rate is maintained. In addition, converting the nanocrystal surface to carboxylic acid termination stably disperses the nanocrystals in aqueous suspensions. They show size-dependent photoluminescence (PL) from yellow to green in the suspensions, while partly oxidizing the surface causes blue PL. 

As the temperature is raised to domain 3, hydrogen corresponding to the residual C-H bonds from the Si-CHrSi backbone is progressively released with a continuous but slow density increase. Above about 1000°C, clusters of free carbon and nanocrystals of p-SiC are formed. At the end of domain 3, ceramic grade Nicalon Si-C-0 fibers consist of a dispersion of free carbon clusters and SiC nanocrystals in an amorphous Si-C-0 matrix [14]. As the temperature is increased to domain 4, growth of p-SiC nanocrystals and decomposition of ternary silicon oxycarbide occurs in the pyrolytic residue, and evolution of CO and SiO is accompanied by further weight loss. 

Bollero A, Gutfleisch O, Kubis M, Muller K-H, Schultz L (2000) Hydrogen disproportionation by reactive milling and recombination of Nd2(Fel-xCox)14B alloys. Acta Mater 48 4929—4934 Bychto L, Balaguer M, Pastor E, Chirvony V, Matveeva E (2008) Influence of preparation and storage conditions on photoluminescence of porous silicon powder with embedded Si nanocrystals. J Nanopart Res 10 1241-1249... 

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