On the existence of Si–C double bonded graphene-like layers Muhammad N. Huda * , Yanfa Yan, Mowafak M. Al-Jassim National Renewable Energy Laboratory, Golden, CO 80401, USA article info Article history: Received 13 June 2009 In final form 12 August 2009 Available online 15 August 2009 abstract Upon analyzing an earlier experimental study by density-functional theory we have shown that graphene-like SiC layers can exist. We found that, for a particular stacking sequence, Si@C double bond was responsible for the much larger interlayer distances observed in synthesized multi-walled SiC nanotubes. The Si/C ratios in SiC layers determine the extent of interlayer distances and bonding nature. It has been also shown that for some intermediate ratios of Si:C and/or with other stacking sequences, a collapse of SiC layers to tetrahedrally bonded system is possible. We have argued that these synthesized Si@C double-bonded multi-wall silicon-carbide nanotubes may provide a pathway for future realization of SiC graphene-like materials. Ó 2009 Published by Elsevier B.V. 1. Introduction Silicon carbide is one of the hardest materials and is very suit- able for electronic devices made for operations in extreme condi- tions, such as in high-power, high-frequency and high- temperature environments [1]. There is a wide spread interest in SiC materials for electronic and opto-electronic applications. Inter- estingly, though carbon and silicon have the same valence struc- tures and diamond-type bulk SiC materials are available, graphene-like SiC has not been reported so far. In fact a key feature of graphene-like SiC materials, the Si@C double bonds, are yet to be found in extended or periodic SiC structures, whereas the C@C double bonds can be found easily [2]. This is mainly due to the large energy differences between their sp2 and sp3 bonding [3]. The large covalent radii for Si (as compared to C) prohibit the for- mation of p-bonding involving Si atoms. So far, Si@Si and Si@C double bonds have only been found in molecular systems, such as bis-silene [4]. Recently, there have been great efforts to study SiC nanotubes both theoretically and experimentally. Density- functional theory (DFT) calculations with local density approxima- tion (LDA) have been used to deal with the extended systems such as SiC graphitic monolayer and single wall SiC nanotubes (SiCNTs). A major postulation behind these studies is the speculative p- bonding between Si and C. So far, SiC graphitic sheets and single- walled SiCNTs have not yet been synthesized experimentally. In contrast, like graphite, layered hexagonal phase exists for boron ni- trides (h-BN) with B@N bonds, and hence graphene like structure of BN or nanotubes can easily be justified [5]. So to achieve SiC nanotubes which are similar to carbon nanotubes, justification of Si@C double bonds in two dimensions is needed. Recently, multi-walled SiCNTs have been successfully synthe- sized by different groups (for example, Refs. [6–11]). For example, using alumina membranes as templates, well aligned SiC tubular structures were prepared by Wang et al. [10] with tailored diame- ter and wall thickness. These tubes showed semiconducting behav- ior, as expected. In fact, SiCNTs were first synthesized by Sun et al. [6] via the disproportionate reaction of SiO with multi-walled CNTs. Multi-walled SiCNTs with no oxygen content were demon- strated by transmission electron microscopy. Interestingly, these SiCNTs exhibited much larger inter-layer distances (3.8 Å, 4.2 Å and 4.5 Å, etc.) than multi-walled CNTs (typically 3.4 Å). By analyz- ing their data from electron-energy-loss-spectroscopy (EELS), the authors also noted the possible p-like bonding between Si and C from these SiCNTs. If these results are theoretically justified, the difficult-to-make Si@C double bonds in an extended system may have indeed been demonstrated. Thus, this multi-wall SiCNTs may provide a pathway for future realization of SiC graphene-like materials. In this Letter, we present DFT calculations to justify the exis- tence of Si@C double bonds in the multi-walled SiCNTs, by under- standing the origins of the peculiar large inter-layer distance found in the multi-walled SiCNTs. Our DFT–LDA calculations reveal that to exhibit the large interlayer distances, the walls of the nanotubes must be SiC monolayers. Such SiC monolayers are graphene-like and exhibit Si@C double bonds. They must also be arranged in a way so that Si (C) atoms in one layer are on the top of Si (C) atoms in the next layer. Otherwise, the SiC monolayers would buckle with strong interlayer interaction, leading to much smaller interlayer distances. We further found that the actual interlayer distances are determined by the Si/C ratios in the monolayers. Our calculated interlayer distances with various Si/C ratios agree well with the experimentally observed distances. Thus, our results provide strong justifications for the p-bonding feature observed by EELS 0009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.cplett.2009.08.028 * Corresponding author. E-mail address: Muhammad.Huda@nrel.gov (M.N. Huda). Chemical Physics Letters 479 (2009) 255–258 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett