We investigated the growth of metal-free carbon nanotubes (CNTs) on glass substrates with a microwave plasma enhanced chemical-vapor deposition (MPECVD) method. An amorphous carbon (a-C) layer was used as a catalyst layer to grow metal-free CNTs. The a-C layer was deposited on Corning glass substrates with a RF magnetron sputtering method using a carbon target at room temperature. The CNTs were prepared with the use of a MPECVD method by using methane (CH(4)) and hydrogen (H(2)) gas. We have pretreated a-C catalyst layer using H(2) plasma at 600 degrees C for 3 min, and the CNTs were grown with the different thickness of a-C catalyst layer for 30 min at 600 degrees C. Also, we investigated the growth trend of the CNTs against the growth time with the substrate of a-C catalyst layer that has optimized thickness. Field-emission scanning electron microscopy (FE-SEM) images showed the growth trend of the CNTs against the thickness of a-C catalyst layer and growth time. Raman spectroscopy of CNTs shows that the grown CNTs were multi-walled. Energy dispersive spectroscopy (EDS) measurements confirmed that the CNTs consisted solely of carbon. (C) 2010 Published by Elsevier B. V.