Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature

Despite significant progress in carbon nanotube (CNT) synthesis by thermal chemical vapor deposition (CVD), the factors determining the structure of the resulting carbon filaments and other graphitic nanocarbons are not well understood. Here, we demonstrate that gas chemistry influences the crystal structure of carbon filaments grown at low temperatures (500 °C). Using thermal CVD, we decoupled the thermal treatment of the gaseous precursors (C2H4/H2/Ar) and the substrate-supported catalyst. Varying the preheating temperature of the feedstock gas, we observed a striking transition between amorphous carbon nanofibers (CNFs) and crystalline CNTs. These results were confirmed using both a hot-wall CVD system and a cold-wall CVD reactor. Analysis of the exhaust gases (by ex situ gas chromatography) showed increasing concentrations of specific volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) that correlated with the structural transition observed (characterized using high-resolution transmission electron microscopy). This suggests that the crystallinity of carbon filaments may be controlled by the presence of specific gas phase precursor molecules (e.g., VOCs and PAHs). Thus, direct delivery of these molecules in the CVD process may enable selective CNF or CNT formation at low substrate temperatures. The inherent scalability of this approach could impact many promising applications, especially in the electronics industry.

A transition from nanofibers to crystalline carbon nanotubes grown at a fixed substrate temperature occurred as the temperature of feedstock gas preheating increased. This structural transition was found to correlate with the formation of specific volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs).Figure optionsDownload as PowerPoint slideResearch highlights
► Gas preheating temperature (Tp) affects structure of carbon nanostructures grown.
► Transition between amorphous CNFs and crystalline CNTs for small increase in Tp.
► Specific VOCs and PAHs correlated to structural transition.
► Transition was observed with both hot-wall and cold-wall reactors.