A coaxial structure of multiwall carbon nanotubes on vertically aligned Si nanorods and its intrinsic characteristics

We present a unique process for fabricating silicon nanorods wrapped with a graphitic material on a silicon substrate by the chemical vapor deposition method with no metal catalyst, as well as characterization of their intrinsic properties. First, well-ordered silicon nanorod axes were grown via an electroless metal deposition method, followed by chemical vapor deposition to wrap the axes with a carbon nanotube. Interestingly, the use of ethanol treatment before chemical vapor deposition prevents the formation of SiO _x layers, which may be necessary as seed layers for carbon nanotube growth. Since this method for carbon nanotube growth does not involve a metal catalyst, the intrinsic properties of the Si NRs were well characterized. A few characterization methods (XPS, Raman spectroscopy, and EELS) were carried out to prove that the Si NRs were completely surrounded by CNTs. In addition, the conductance results (by terahertz time-domain spectroscopy) show that the charge carrier transport characteristics of the multiwall carbon nanotubes are found mainly in the outermost shell, and that the Si NR surface was well passivated by the multiwall carbon nanotube structure. This coaxial structure, which does not require a metal catalyst, represents a significant step forward for realizing applications for carbon nanotube devices.