Abstract Submitted to the    NANOTUBE'04 Conference:

We studied carbon nanotube growth by atmospheric pressure thermal CVD of methane, from Al₂O₃/Fe/Mo and Fe/Mo catalysts deposited by e-beam evaporation. In prior work, the Al₂O₃ underlayer is often deposited using a different process than the catalysts [1], or formed by oxidizing an aluminum layer [2,3]. We sequentially evaporated 20/1.5/3.0 nm of Al₂O₃/Fe/Mo onto polished silicon wafers and onto silicon microstructures (1-20 mm width and depth), and did not clean or oxidize the substrates before growth.

Experiments with long samples (100 mm long, 15 mm wide) centered in a tube furnace show sharp transitions from 200 nm diameter carbon clusters, to "spider"-like MWNT agglomerates growing from the clusters, to a dense web of SWNTs and thin MWNTs. This occurs as the gas 300/100 sccm CH₄/Ar) temperature changes along the sample, from approximately 800^oC to 925^oC. HRTEM with in-situ EDX analysis indicates that the clusters have layered graphitic walls with sharp corners, and are impregnated with Fe. Tangled films grow from vertical sidewalls of RIE-etched features, and isolated NTs stretch across gaps of several microns between structures. When Fe/Mo is deposited without Al₂O₃, only sparse kinked MWNTs grow from horizontal surfaces, yet isolated straight NTs as long as 200 mm grow across fields of silicon "grass" structures.

References:
[1] J.W. Ward, B.Q. Wei, and P.M. Ajayan. Substrate effects on the growth of carbon nanotubes by thermal decomposition of methane. Chem. Phys. Lett., 376:717-725, 2003.
[2] L. Delzeit, B. Chen, A. Cassell, R. Stevens, C. Nguyen, and M. Meyyapan. Multilayered metal catalysts for controlling the density of single-walled carbon nanotube growth. Chem. Phys. Lett., 348:368-374, 2001.
[3] T. de los Arcos, Z.M. Whu, and P. Oelhafen. Is aluminum a suitable buffer layer for carbon nanotube growth? Chem. Phys. Lett., 380:419-423, 2003.