Role of Oxygen in PECVD Carbon Nanotubes Growth: Experiments and Modeling

We investigate by modeling supported by experiments the role of oxygen in carbon nanotubes (CNT) growth by plasma enhanced chemical vapor deposition (PECVD) process. We found that a suitable content of molecular oxygen in a H 2 /CH 4 mixture can significantly increase the vertical CNT growth rate. We take the advantage of such system in order to better understand the mechanisms responsible for nanotube growth in a chemical environment represented by a ternary C–H–O system. In our approach, gas phase and surface chemistries are considered through two quantitative modeling approaches. As a first approximation, the plasma is described by spatially averaged bulk properties, and the species molar fractions are determined using transient zero-dimensional (0D) thermo-chemical model. Then two-dimensional model (2D) is proposed with reducing gas chemistry and considering surface reactions through site fraction and surface molar concentrations. The nanotube growth rate is then obtained by compiling the effects of gas and surface reactions at the gas/substrate interface via surface CHEMKIN software. To get an accurate estimation of the density of sites for bulk CNT growth, SEM and HRTEM analysis were used. The influence of the oxygen content, substrate temperature and microwave power on CNT growth rate are obtained from the model and compared to the experiments. There was good agreement between experimental and the modeling results, providing an insight into optimizing PECVD CNT growth. The specific role of generated oxygenated species such as H 2 O, OH, CO and O is found to be decisive.