Silicon carbide whiskers with superlattice structure: A precursor for a new type of nanoreactor
aV. I. Vernadski Institute of General and Inorganic Chemistry, Ukrainian National Academy of Sciences, 32/34 Prospect Palladina, 03680 Kyiv, Ukraine
Available online 7 March 2008.
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Abstract
Silicon carbide whiskers exhibit growth predominantly in the 
1 1 1
direction. The high level of impurities, stacking faults and nanosized twins govern the formation of homojunctions and heterojunctions in crystals. The structure of the whiskers comprises a hybrid superlattice, i.e. contains elements of doped and composite superlattices. An individual SiC whisker can contain hundreds of quantum wells with anomalous chemical properties. This paper shows that it is possible to selectively etch quantum wells and to construct whiskers with quasi-regularly distributed slit-like nanopores (nanoreactors), which are bordered by polar planes {1 1 1}, {0 0 0 1} or a combination of them, and also to produce flat SiC nanocrystals bordered by polar planes.
Keywords: Silicon carbide (SiC) whiskers; Nanocrystalline nanostructure; Superlattice; Porous materials; Nanoreactor
Fig. 1. Original SiC whiskers: (a) SEM image of whisker; (b) HRTEM image of whisker with twin interlayers and stacking faults (the inset shows selected-area electron diffraction picture of whisker, which was indexed as two superimposed diffraction gratings rotated by 110° (or 70°) relative to each other (zone axis [1 1 0]); (c) magnified image of small section of SiC whisker in photo (b) with inset 1 showing parallel contrast that corresponds to projections of (1 1 1) planes with lattice parameter 0.25 nm onto the image plane, and inset 2 showing a magnified image of parallel contrast with microtwins.
Fig. 2. IR spectrum of gas phase formed as a result of reaction SiC whiskers with boiling HF + HNO3.
Fig. 3. Change in crystal mass loss and concentration of impurity elements upon etching in a HF + HNO3: (a) dependence of mass change for SiC whiskers on etching time; specific surface area of SiC whiskers: (1) 4.6 ± 0.2, (2) 5 ± 0.5, (3) 5 ± 0.5, (4) 11.3 ± 1.5, (5) 15 ± 1.0, (6) 7 ± 0.8, (7) 22 ± 3.0 and (8) 35 ± 3.0 m2 g−1; (b) decrease in concentration of acceptor impurities in SiC whiskers at different stages of the etching process.
Fig. 4. Diffractograms of SiC whiskers produced by pyrolysis of rice husk [16]: (1) original whiskers; (2) whiskers after etching in HF + HNO3 for 15 h. Mass loss 58 mass%.
Fig. 5. Morphological features of SiC whiskers after etching in HF + HNO3, and models for the nanoreactor: (a) SEM images; (b) TEM image; (c) HRTEM image; (d) models of single nanoreactor: (1) nanoreactor bordered by polar planes of 
-SiC polytypes; (2) nanoreactor bordered by polar planes of 3C- and -SiC polytypes; (3) nanoreactor bordered by polar planes of 3C polytype; (4) model of crystal fragment with numerous nanoreactors located perpendicular to the crystal axis; (5) model of crystal with nanoreactors formed upon etching SiC whisker, which has twin joints along the crystal axis.
