Structural and optical properties of carbon nanowalls by plasma-enhanced chemical vapor deposition method

Abstract

Vertically aligned carbon nanowalls (CNWs) were synthesized by catalyst-free 150 MHz high frequency plasma enhanced chemical vapor deposition (PECVD) system, where methane and hydrogen gases were used as the reactive gases. Parametric studies were done to determine the optimum parameters required to obtain tuneable growth of CNWs with favourable properties at high quality film. The parameters included are the optimized temperature, rf power and methane flow-rate. The effects of the growth conditions on the optical properties of the samples were investigated by UV-Vis-NIR spectroscopy and photoluminescence (PL) spectroscopy, while the morphologies and structural properties were characterized by Micro-Raman scattering, Fourier transform infrared (FTIR) spectroscopy and field emission scanning electron microscopy. Shape, density and quality of CNWs were extensively studied and found to be significantly affected by the synthesis parameters. Based on the investigation of the morphologies of catalyst-free CNWs deposited on silicon substrate, it was observed that the optimized parameters have specific range of values. The synthesis temperature, rf power and methane flow-rate are in range of 750-800oC, 10-40 watt and 14-18 sccm, respectively. The morphologies of CNWs produced in different growth conditions looked quite similar but significant thinning of the walls was observed. Wall thickness and height of CNWs were strongly dependent on growth conditions while the lengths were not greatly affected. In Raman spectrum of CNWs, there were four peaks which consist of a low intensity D band, a sharp and strong G band, and low intensity 2D band and D + G band. The intensity ratio of the D band to G band was about 0.15 which indicates a lower number of defects in the CNW structures. FTIR results showed that the absorption band can be associated to the structural vibration of CNWs. From UV-Vis-NIR measurement, the minimum reflectance was found at 0.011 % which indicates that this material is a good candidate for use in a blackbody-like coating. The PL result of the CNW films produced one peak which signifies a narrow opening band gap indicating the presence of an epitaxial bilayer graphene. The energy gap of the CNWs was 1.92 eV which can be regarded as a semiconductor-like characteristic. Tuneable CNWs synthesized using 150 MHz PECVD method can be considered as a good candidate for materials used in optical and optoelectronic devices.