Comparisons on properties and growth mechanisms of carbon nanotubes fabricated by high-pressure and low-pressure plasma-enhanced chemical vapor deposition

Abstract

Effects of plasma pressure and the presence of nitrogen on growth of carbon nanotubes (CNTs) and their properties were studied by using microwave plasma chemical vapor deposition (MPCVD) (pressure=600–3300 Pa) and electron cyclotron resonance chemical vapor deposition (ECR-CVD) (pressure=0.3–0.6 Pa) systems. CH 4/H 2 and CH 4/N 2 were used as source gases, and Co as the catalyst. The structures and properties of CNTs were characterized by using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Raman spectra, and field emission I– V measurements. The results show that CNTs made by higher plasma pressure system have a higher growth rate (typically 1–3 μm/min), smaller tube diameter, better field emission properties, and better tube quality. The growth rate is related to the availability of carbon source. The morphology change from spaghetti-like to well-aligned CNTs is discussed in terms of directed ions. The change in field emission properties is reasoned in terms of geometric enhancement factor and screening effect for different tube morphologies. The presence of nitrogen plasma can have the following effects: increasing tube diameter, increasing straightness of CNTs, forming of bamboo-like CNTs, deterioration of field emission properties, and shifting of Raman peak toward lower-frequency side (or increasing residual tensile stress).