In situ optical investigation of carbon nanotube growth in hot-filament chemical vapor deposition

In situ optical reflectivity was measured to examine the growth of vertically aligned carbon nanotubes (CNTs) by the hot-filament chemical vapor deposition (HFCVD) method using Fe catalysts pre-treated with argon plasma. The structural characteristics, such as the thickness and surface roughness of CNT films, deduced from the analysis of the optical reflectivity while taking the scattering due to surface roughness into account were consistent with those estimated from ex situ SEM images. Moreover, the linear regression analysis of the temporal variation of reflectivity revealed that an incubation period of ∼50 s preceded the noticeable growth of CNTs, that the growth of CNTs was not linearly dependent on time, that the growth rate increased monotonically to the maximum value before dropping rapidly, and that the active growth of CNTs up to tens of micrometer occurred in the short span of a few tens of seconds. Thus, in situ reflectivity measurement can be used to investigate CNT growth, and may shed some light on the details of the CNT growth mechanism in the HFCVD method.