In U.S. Patent Application 20100098904, Stanford University Professor of Chemistry Hongjie Dai, David Mann and Guangyu Zhang disclose a method of molecular oxygen-assisted plasma-enhanced CVD growth for high yield of vertically aligned single wall carbon nanotubes (SWNTs) at the full 4-inch wafer scale. Various control experiments revealed the negative effect of hydrogen species to the formation and growth of SWNTs as well as etching effects of hydrogen plasma to pre-formed SWNTs. A 4-inch CVD system was used for the SWNT synthesis.
The key role played by oxygen in the present high yield SWNT growth is to balance C and H radicals, and specifically to provide a C-rich and H-deficient condition to favor the formation of sp2-like graphitic structures. In addition to molecular oxygen, other oxygen sources, such as oxygen-containing organic and inorganic compounds, can also produce high yields of vertically aligned SWNTs. With the addition of suitable amount of an oxygen-containing source to suppress H species, various types of PECVD setups can produce SWNTs at ultra-high yield and efficiency.
Further, a method to form V-SWNT films for the first time on any desirable substrate (including metals and plastics) with strong interfacial adhesion is presented.
Further, a method to form V-SWNT films for the first time on any desirable substrate (including metals and plastics) with strong interfacial adhesion is presented.
FIG. 2a depicts a SEM image showing SWNT towers with various widths (20 .mu.m, 5 .mu.m, 1 .mu.m, 500 nm, 300 nm from left to right of the front region of the image) and vertical SWNT sheets (20 .mu.m, 5 .mu.m, 1 .mu.m, 500 nm, 300 nm, 100 nm thick from top to bottom of the upper part of the image) after 30 min growth.
FIG. 2b depicts an AFM image of the patterned catalyst strips (bright 300 nm and 100 nm wide regions respectively) comprised of densely packed Fe nanoparticles used for the growth of the 300 nm and 100 nm thick vertical SWNT sheets (pointed by arrows) in FIG. 2a.
FIG. 2b depicts an AFM image of the patterned catalyst strips (bright 300 nm and 100 nm wide regions respectively) comprised of densely packed Fe nanoparticles used for the growth of the 300 nm and 100 nm thick vertical SWNT sheets (pointed by arrows) in FIG. 2a.
FIG. 2d depicts a SEM image of square and circular towers of V-SWNTs from different growths than the sample in FIG. 2a illustrating the reproducibility of the synthesis. FIG. 2e depicts a SEM images of lines of V-SWNTs from different growths than the sample in FIG. 2a illustrating the reproducibility of the synthesis.
Carbon nanotubes (also referred to as carbon fibrils) are seamless tubes of graphite sheets, first discovered as multi-layer concentric tubes or multi-walled carbon nanotubes and subsequently as single-walled carbon nanotubes. Carbon nanotubes have shown promising applications including nanoscale electronic devices, high strength materials, electron field emission, tips for scanning probe microscopy, and gas storage.
Generally, single-walled carbon nanotubes (SWNTs) have advantages over multi-walled carbon nanotubes for use in these applications because they have fewer defects and are therefore stronger and more conductive than multi-walled carbon nanotubes of similar diameter. Moreover, single-walled carbon nanotubes with substantially uniform alignment have been shown to have further advantages over non-aligned nanotubes. For example, vertical single-walled carbon nanotubes (V-SWNTs), have attracted particular interest for some of the above applications.
In addition to their multiplicity and alignment, other physical parameters of carbon nanotubes also have important implications in their utility. For example, the level of purity is often vital to the applicability of carbon nanotubes in electronic devices. The control of physical dimensions of carbon nanotubes, such as diameter, length and chirality, is also of benefit, for example, in hydrogen storage applications. Nevertheless, current methods of preparation often suffer from narrow parameter windows and/or low reproducibility.
Thus, the availability of single-walled carbon nanotubes, particularly vertical single-walled carbon nanotubes, in quantities and with attributes necessary for practical technology is still problematic. As a result, processes for the production of high quality single-walled carbon nanotubes are still needed.
The Stanford team found that the formation of dense and relatively uniform particles was beneficial for V-SWNT growth. During nanotube growth, the compositions of gases in the tube-furnace were methane (.about.66%), hydrogen (.about.12%), oxygen (.about.1%) and Ar (.about.21% as carrier gas) with a total pressure of 0.3-0.4 Torr. The gas flow rates were CH4/H2/O2=160 sccm/30 sccm/2.4 sccm (standard cubic centimeter per minute). Ar was used as carrier gas for CH4/H2/O2. The percentage of partial pressures of various gases followed CH4/H2/O2=66% :12%:1% (The rest is Ar). The RF plasma was generated at a power of 60-70 W for 10 to 30 min for nanotube growth. This condition was highly reproducible in growing vertical SWNTs from run to run and day to day.
Without oxygen, the yield of SWNTs of any appreciable length is low, demonstrating the importance of oxygen in the initial nanotube nucleation and formation stage and not just during the sustained growth stage. Increased H2 presence often leads to systematic decrease in SWNT yield, with or without oxygen presence.
It was observed that increasing the H2 concentration while keeping the alcohol vapor pressure constant systematically reduces the yield of nanotubes. This provides evidence that hydrogen rich environments are also undesirable and have negative effects to the yield of SWNTs in standard thermal CVD. Further, a connection can be made with non-hydrocarbon based SWNT synthesis methods such as carbon monoxide CVD. CO-CVD without the involvement of hydrogen indeed produces high yield of SWNTs especially under high temperature and pressure when sufficient C-feedstock is obtained. Other H-free high-yield growth of SWNTs includes laser ablation and arc-discharge that vaporizes solid carbon without involving any hydrogen.
No comments:
Post a Comment