Carbon nanotube morphology has been engineered via simple control of applied voltage during dc plasma chemical vapor deposition growth. Below a critical applied voltage, a nanotube configuration of vertically aligned tubes with a constant diameter is obtained. Above the critical voltage, a nanocone-type configuration is obtained. The strongly field-dependent transition in morphology is attributed primarily to the plasma etching and decrease in the size of nanotube-nucleating catalyst particles. A two-step control of applied voltage allows a creation of dual-structured nanotube morphology consisting of a broad base nanocone (ϳ200 nm dia.) with a small diameter nanotube ͑ϳ7 nm͒ vertically emanating from the apex of the nanocone, which may be useful for atomic force microscopy.

It is shown that unidirectionally aligned carbon nanotubes can be grown on electrically conductive network of carbon microfibers via control of buffer layer material and applied electric field during dc plasma chemical vapor deposition growth. Ni catalyst deposition on carbon microfiber produces relatively poorly aligned nanotubes with significantly varying diameters and lengths obtained. The insertion of Ti 5 nm thick underlayer between Ni catalyst layer and C microfiber substrate significantly alters the morphology of nanotubes, resulting in much better aligned, finer diameter, and longer array of nanotubes. This beneficial effect is attributed to the reduced reaction between Ni and carbon paper, as well as prevention of plasma etching of carbon paper by inserting a Ti buffer layer. Such a unidirectionally aligned nanotube structure on an open-pore conductive substrate structure may conveniently be utilized as a high-surface-area base electrodes for fuel cells, batteries, and other electrochemical and catalytic reactions. Carbon nanotubes ͑CNTs͒ have emerged as a new and attractive class of materials with unique electrical, mechanical, and various physical and chemical properties. [1][2][3] Potential applications include nanoelectronic devices, 4-7 catalyst supports, 8 storage materials for hydrogen and other gases, 9,10 and probe tips for atomic force microscope ͑AFM͒. 2,11 While there have been a very large number of publications on nanotube growth and their applications, engineering the shape of CNTs is an important issue for successful applications of nanotubes. Aligned and well separated CNT morphology is important for many potential applications, where a high electric-field concentration is needed as in field emission applications, and where a large surface area is desirable as in catalytic reactions, such as fuel cell or battery applications.The growth of CNTs by dc plasma-enhanced chemical vapor deposition ͑CVD͒ involves many processing parameters, such as bias field, plasma power, temperature, chamber pressure, and feed gas composition. In general, the diameter and distribution of CNTs synthesized by CVD processing are dependent on the morphology of catalyst particles or layer. The shape of CNTs can be controlled by the applied bias voltage, the nature of catalyst and buffer layer deposition as well as the composition of plasma in a dc plasma-enhanced CVD process. 12,13 The growth direction of the nanotubes can be controlled by the electrical field related to either applied bias or plasma induced bias, which is often perpendicular to the substrate surface. More recently, multiple sharp bending of CNTs to produce a zig-zag morphology has also been demonstrated by repeatedly altering the applied field directions. 14 Such an alignment of CNTs has been demonstrated mostly on flat surfaces, and there have been few reports on aligned CNT growth on a fiber-configured substrate.It is well known that CNT nucleation and growth occurs mostly on semiconductive Si substrate as well as electrically insulating ceram...