Patterned Growth of Carbon Nanotubes or Nanofibers

NASA Tech Briefs, Jul 2004

Numerous parameters of deposition conditions, structures, and compositions affect what is grown.

Ames Research Center, Moffett Field, California

A method and apparatus for the growth of carbon nanotubes or nanofibers in a desired pattern has been invented. The essence of the method is to grow the nanotubes or nanofibers by chemical vapor deposition (CVD) onto a patterned catalyst supported by a substrate.

The figure schematically depicts salient aspects of the method and apparatus in a typical application. A substrate is placed in a chamber that contains both ion-beam sputtering and CVD equipment. The substrate can be made of any of a variety of materials that include several forms of silicon or carbon, and selected polymers, metals, ceramics, and even some natural minerals and similar materials. Optionally, the substrate is first coated with a non-catalytic metal layer (which could be a single layer or could comprise multiple different sublayers) by ion-beam sputtering. The choice of metal(s) and thickness(es) of the first layer (if any) and its sublayers (if any) depends on the chemical and electrical properties required for subsequent deposition of the catalyst and the subsequent CVD of the carbon nanotubes.

A typical first-sublayer metal is Pt, Pd, Cr, Mo, Ti, W, or an alloy of two or more of these elements. A typical metal for the second sublayer or for an undivided first layer is Al at a thickness > or =1 nm or Ir at a thickness > or =5 nm. Proper choice of the metal for a second sublayer of a first layer makes it possible to use a catalyst that is chemically incompatible with the substrate.

In the next step, a mask having holes in the desired pattern is placed over the coated substrate. The catalyst is then deposited on the coated substrate by ion-beam sputtering through the mask. Optionally, the catalyst could be deposited by a technique other than sputtering and/or patterned by use of photolithography, electron-beam lithography, or another suitable technique. The catalytic metal can be Fe, Co, Ni, or an alloy of two or more of these elements, deposited to a typical thickness in the range from 0.1 to 20 nm.

Following deposition of the patterned catalyst, a shutter is moved into place to protect the sputtering equipment against CVD of carbon, then a hydrocarbon feed gas (primarily CKU, C^sub 2^H^sub 2^, or C^sub 2^H^sub 4^) heated to a suitable temperature is admitted into the chamber. Optionally, the feed gas can be part of a mixture that includes an inert carrier gas. The heated feed gas decomposes into hydrogen and carbon, with deposition of the carbon on the catalyst. Whether what are grown are single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs), or nanofibers depends primarily on the feed gas and temperature used in CVD and secondarily on the structures and compositions of the first layer and the patterned catalytic second layer. For example, for growing SWNTs, the preferred gas is CH^sub 4^ and the preferred temperature is [asymptotically =]900 °C. For growing MWNTs, the preferred gas is C^sub 2^H^sub 2^ or C^sub 2^H^sub 4^, the preferred temperature is [asymptotically =]750 °C. For growing nanofibers, it is preferable to form a plasma discharge in the chamber and to maintain the temperature between 400 and 700 °C.

This work was done by Lance D. Delzeit of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.

Inquiries concerning rights for the commercial use of this invention should be addressed to the Patent Counsel, Ames Research Center, (650) 604-5104. Refer to ARC-14613.