Since the discovery of carbon nanotubes (CNTs), many studies have been carried out on their fabrication. The synthesis of CNTs can be divided into non-catalytic and catalytic methods. In the latter, for growing CNTs, various catalysts such as Ni, Pd, Fe, Cu, Co, Mg-Ni, Ni-Pd, Ni-Fe, Ni-Co and Ni-Au have been used by physical or chemical methods in recent years. In this work, we describe experiments that systematically explore the effect of NH3 gas on the Ni nano-particle formation and the role of Ni catalyst thickness on diameter, morphology and growth rate of CNTs synthesized by thermal chemical vapor deposition (TCVD). The Ni thin films with thicknesses of 2.5, 5, 10, 20 and 30 nm as a catalyst deposited on the SiO2(60nm)/Si (100) system by using DC magnetron sputtering technique. The deposited samples were annealed at the 900° C in ammonia environment for 50 min.
The shape and average Size of the synthesized Ni nano-particles were studied by employing atomic forces microscopy (AFM). The mean diameter of Ni nano-particles in the Ni films with thicknesses of 2.5, 5, 10, 20 and 30 nm were measured about 30, 35, 45, 55 and 70 nm, respectively. Then, the growth behavior of multi wall CNTs (MWCNTs) on the Ni nano-particles, synthesized from our carbon source (CH4) has been investigated. Surface morphology and structure of the grown MWCNTs was determined by scanning electron microscopy (SEM). The results revealed that the average diameter of MWCNTs was about 60 nm for all the Ni catalyst thicknesses. SEM observation indicated that by increasing Ni film thickness from 2.5 to 10 nm, surface density and growth rate of the grown MWCNTs increased, but with increas of Ni film thickness from 10 to 30 nm surface density and the growth rate of the MWCNTs decreased. In addition, based on our SEM observations, we have found that some of Ni nano-particles were encapsulated at the tip of CNTs for the all Ni thicknesses under study.
|