Study of the catalyst evolution during annealing preceding the growth of carbon nanotubesby microwave plasma-enhanced chemical vapour deposition

Abstract

A two-step catalyst annealing process is developed in order to control thediameter of nickel catalyst particles for the growth of carbon nanotubes(CNTs) by microwave plasma-enhanced chemical vapour deposition (MWPECVD). Thermal annealing of a continuous nickel film in a hydrogen(H2) environment in a first step is found to be insufficient for the formationof nanometre-size, high-density catalyst particles. In a second step, aH2 MW plasma treatment decreases the catalyst diameter by a factor of two andincreases the particle density by a factor of five. An x-ray photoelectron spectroscopystudy of the catalyst after each step in the annealing process is presented. Itis found that the catalyst particles interact with the substrate during thermalannealing, thereby forming a silicate, even if a buffer layer in between the catalyst andthe substrate is intended to prevent silicate formation. The silicate formationand reduction is shown to be directly related to the CNT growth mechanism,determining whether the catalyst particles reside at the base or the tip of the growingCNTs. The catalyst particles are used for the growth of a high-density CNTcoating by MW PECVD. CNTs are analysed with electron microscopy and Ramanspectroscopy.