Numerical simulation and emission diagnostics of the gas -phase growth environment during carbon nanotube synthesis by plasma-enhanced chemical vapor deposition
The unique electrical, thermal, and mechanical properties of carbon nanotubes (CNTs) have elicited many concepts for important applications in areas such as nanoelectronics, communications, thermal transport, and composite materials. Yet the efficacy of their use in these applications often depends on precise control of the synthesis process. However CNT growth mechanism is not fundamentally understood due to their complex growth environment. Knowledge of chemical precursors plays a vital role in the understanding of growth mechanisms, but the precise composition of precursor concentrations for CNT growth has not been well established.
The objective of the present study is to simulate and diagnose the gas-phase environment for CNT synthesis in a plasma-enhanced chemical vapor deposition (PECVD) reactor. Simulations seek to compare the role of different gas-phase reactions to identify the indicator species for CNT formation. The temperature of the gas-phase CNT growth environment was diagnosed via in situ optical emission spectroscopy. Gas temperatures in the plasma have been measured to develop, and optimize a new heat loss model, and to study gas-phase chemistry for CNT growth using detailed chemical kinetics.
Simulations show that C2H2, CH3, and H are the major species formed in the plasma at CNT growth conditions. C 2H2 is the main gas-phase precursor for CNT growth, and H atom is responsible to etch the undesired carbon on the substrate. The gas-phase chemistry in the plasma is dominated by the neutral species. The present work confirms that key parameter [H]/[C2H2] ratio differentiates between the diamond and the non-diamond growth regime. Conduction is the only relevant mode of the heat transfer in the reactor. The simulated gas temperatures based on a new heat transfer model follow the trends, and are nearly within error bars of the measured gas temperatures. The present gas-phase simulations contribute to an explanation of the experimental observations of CNT growth at different input plasma powers and different feed gas compositions.