Growth of carbon nanotubes characterized by field emission measurements during chemical vapor deposition

The growth of multiwall carbon nanotubes is characterized in situ in a chemical vapor deposition reactor by measuring the current extracted by field emission from the growing nanostructures. The lengthening of the nanotubes provokes an increase of the emitted current at constant applied voltage, and the use of a phosphor screen allows to observe the individual emitters during the growth. A simple model permits furthermore to estimate the growth rate. The nanotubes grow with a closed cap under ${10}^{-2}-{10}^{-4}\mathrm{mbar}$ of ${\mathrm{C}}_2{\mathrm{H}}_2$ at $700°\mathrm{C}$ with a growth rate over $1\mu \mathrm{m}/\mathrm{s}$ that increases with the ${\mathrm{C}}_2{\mathrm{H}}_2$ pressure. The growth of the nanotubes is neither simultaneous nor homogeneous over the cathode and involves a different activation time for every emitter.