In situ growth rate measurements and length control during chemical vapor deposition of vertically aligned multiwall carbon nanotubes

Time-resolved reflectivity is employed as an in situ diagnostic in thermal chemical vapor deposition of vertically aligned arrays of multiwall carbon nanotubes (VAA–MWNT). Fabry–Ṕerot interference fringes and attenuation of a reflected HeNe laser beam are used to measure the length of VAA–MWNT throughout the first 3–8 μm of growth yielding in situ measurements of growth rates and kinetics and the capability to observe the onset and termination of growth. VAA–MWNT growth is characterized between 565 and $\text{750\hspace{0.05em}°}\mathrm{C}$ on Si substrates with evaporated Al/Fe/Mo multilayer catalysts and acetylene feedstock. Nanotube lengths were controlled by rapid evacuation of the chamber at predetermined reflectivities, and it was demonstrated that growth can be restarted at later times. The extinction coefficients of the VAA–MWNT were studied and correlated with nanotube wall structure. Growth rates for VAA–MWNT are found to vary depending on the catalyst preparation, temperature, and time. Both the highest growth rates (0.3 μm/s) and the tallest VAA–MWNT (0.75 mm long) were achieved at $\text{730\hspace{0.05em}°}\mathrm{C}.$