Gold Nanoparticles Provide Catalytic Platform for Uninhibited Single Wall Carbon Nanotube Conductor Growth from Post to Post in Modified CVD Process

A modified chemical vapor deposition (CVD) process that uses nanoparticles of gold as catalyst materials has been used to grow ultra long single wall carbon nanotubes (SWNT)  to form electrical conductors for use in microelectronic devices at the University of California Irvine 

University of California Irvine (UCI) Integrated Nanosystems Research Facility Nanotechnology Group led by Professor Peter J. Burke with  Zhen Yu and Shengdong Li have patented systems and methods for the fabrication of ultra long single-walled carbon nanotubes as electrical conductors. Ultra long carbon nanotubes may be used to make carbon nanotube field emission transistors (CNTFET). For the growth of single-walled nanotubes, experimental results indicate that the concentration of nanoparticle catalysts influence the growth rate of the nanotube

The ultra long nanotubes are grown on a metal underlayer or gold or gold and chromium using a modified chemical vapor deposition (CVD) furnace.  The Regents of the University of California (Oakland, CA) earned U.S. Patent 7,645,482 for their work, which was supported by a DARPA grant.

The metal underlayer is patterned photolithographically using a lift-off techniques. Although a variety of metals can be used to form the metal underlayer such as nickel, aluminum, iridium, chromium, gold, or a transition metal, gold or a chromium-gold bilayer are preferred.  

The metal underlayer has a thickness of about 100 nm to 300 nm. The metal underlayer creates a platform for nanotube growth. The metal underlayer platform prevents steric forces created by the substrate from inhibiting the growth of the nanotubes. Therefore, the metal underlayer facilitates production of ultra long carbon nanotubes. 

Arrays of long, straight nanotubes can be grown via the UCI methods using a single furnace system, without the need for rapid heating. The single furnace system comprises a modified CVD reaction chamber which reduces the turbulence of the gas flow of the hydrocarbon source provided during the growth phase. 

The reduced turbulence creates an enhanced environment for ultra-long nanotube formation. In addition, a raised platform, comprising an underlayer of metal, is deposited onto a substrate. The raised platform allows the nanotube to grow freely suspended from the substrate in the low turbulence gas flow. This reduces any steric force impedance caused by the substrate and enables the nanotube to be grown to lengths on the order of centimeters.

FIG. 4 is a schematic drawing of nanotube growth from an elevated catalyst site.

The metal underlayer is preferably comprised of a conductive metal such as gold. The nanotube is able to grow from one metal underlayer platform to another such platform. The nanotube is thereby connected at both ends to a conductive material and forms a nanotube electrode without the need for a post nanotube formation processing step. The metal underlayer tends to melt during the CVD growth process and flows over the nanotube just after growth during the CVD run.

FIG. 5 is an SEM image showing an array of nanotubes grown using the carbon nanotube synthesis methods disclosed herein. 

FIG. 6 is a mosaic of SEM images showing an array of 1.5 mm long SWNTs grown using the carbon nanotube synthesis methods   

FIG. 7 is a high magnification SEM image of initial and terminal points of the long nanotubes shown in FIG. 6. 

FIG. 11 is a SEM image of nanotubes grown without a metal underlayer.
 

FIG. 12 includes a schematic (A) and a SEM image (B) showing two nanotubes bridging the gap.

Carbon nanotubes are wires of pure carbon with nanoscale dimensions. The diameter of a single-walled carbon nanotube (SWNT) is typically in the range of approximately 1-5 nm. SWNTs generally consist of a single atomic layer thick sheet of graphite configured into a cylinder. Multi-walled carbon nanotubes (MWNT) generally consist of a plurality of concentric nanotube shells and have a diameter generally on the order of about 50 nm. Nanotubes have potential applications in a wide variety of formats including electronics, materials, biotechnology and the like.