Ultra-high-yield growth of vertical single-walled carbon nanotubes: Hidden roles of hydrogen and oxygen

Abstract

An oxygen-assisted hydrocarbon chemical vapor deposition method is developed to afford large-scale, highly reproducible, ultra-high-yield growth of vertical single-walled carbon nanotubes (V-SWNTs). It is revealed that reactive hydrogen species, inevitable in hydrocarbon-based growth, are damaging to the formation of sp 2 -like SWNTs in a diameter-dependent manner. The addition of oxygen scavenges H species and provides a powerful control over the C͞H ratio to favor SWNT growth. The revelation of the roles played by hydrogen and oxygen leads to a unified and universal optimum-growth condition for SWNTs. Further, a versatile method is developed to form V-SWNT films on any substrate, lifting a major substrate-type limitation for aligned SWNTs.nanomaterials ͉ hydrocarbon chemistry ͉ plasma ͉ sp 2 -sp 3 carbon ͉ catalysis C arbon nanotubes in aligned forms (1-6) are interesting for various scientific and practical applications ranging from electronics to biological devices. Although several chemical vapor deposition (CVD) methods have been developed to grow vertically aligned multiwalled carbon nanotubes, growth of vertical single-walled carbon nanotubes (V-SWNTs) is still in an early stage (7,8). The formation of nanotube vertical films is indicative of high-yield growth from densely packed catalytic particles. That is, almost every particle produces a nanotube, and bundling of neighboring tubes leads to collective vertical growth. A key element in a recent ethylene CVD method that has obtained V-SWNT growth is the addition of Ϸ150 ppm water vapor (8). Another method that has obtained V-SWNT growth is an alcohol CVD method using ethanol as the carbon feedstock. In both cases, the key role played by the oxygen-containing species is suggested to be oxidation of amorphous carbon to facilitate SWNT growth.Although we have made an effort to use both the waterassisted and alcohol CVD growth methods, we have been unable to obtain vertically aligned SWNTs, which indicates the narrow parameter space and growth window of these methods. Also, the specific roles played by the oxygen-containing species remain speculative and require further investigation. A more detailed understanding could lead to improved and more reproducible growth conditions. Here, we present a molecular oxygen-assisted plasma-enhanced CVD (PECVD) growth of V-SWNTs at the full 4-inch wafer scale. We find that adding oxygen (Ϸ1%) to methane in the PECVD affords highly reproducible growth of densely packed SWNTs. Control experiments and knowledge from previous diamond PECVD work suggest that the key role played by oxygen in the 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 sp 2 -like graphitic SWNT structures. We reveal that reactive hydrogen species are generally unfavorable to SWNT formation and growth and can etch preformed SWNTs. The negative role of hydrogen and the positive role of oxygen have important implications to SWNT growth in general by various me...

Chirality-controlled synthesis of single-wall carbon nanotubes using vapour-phase epitaxy

“…Too high concentration of hydrogen is unfavourable, probably because it leads to hydrogenation and etching of nanotubes. On the other hand, too low concentration of hydrogen will likely produce less ordered carbon structures, such as amorphous carbon, which may hamper the nanotube growth, as suggested by literature reports [39][40][41] .…”

Photophysics of Individual Single-Walled Carbon Nanotubes

“…27,28 The catalyst lifetime can be extended by using carbon sources containing oxygen 18,24,28 or by introducing water, oxygen, or hydrogen gas during the synthesis. 29,30 Chemical vapor deposition offers a number of advantages over the arc discharge and laser vaporization methods for SWNT synthesis. For example, although both arc discharge and laser vaporization convert >70% of the carbon source to SWNTs, they unfortunately produce nanotubes as tangled bundles in a powdery soot.…”

“…The probe ultrasonicator was usually fitted with a ¼" microtip, but could also accommodate a ½" tip that delivered half the Finally, SWNTs were dispersed in DNA following a procedure similar to that outlined for the surfactant systems. Slight modifications were incorporated according to the protocol suggested by Zheng et al 121,132 Two custom oligonucleotides, singlestranded DNA (GT) 30 and DNA (GT) 15 with a 5'-labeled Biotin linker, were ordered with standard desalting purification and used as-received (IDT DNA Technologies, Inc.).…”

Resonance R aman Spectroscopy

“…After the first success of VA-SWNT synthesis, different methods have been also reported by other groups in producing this kind of forest-like structures or thick VA-SWNT, like microwave plasma CVD [35], O 2 -assisted CVD [36], and etc [37,38,39,40], including water-assisted super-growth [41]. In the case of vertical growth of SWNTs, the alcohol pressures inside the growth chamber during SWNT synthesis were kept to be approximately 10 Torr (1.3 kPa) for our system, which yield an average diameter of as-grown SWNTs around 1.9-2.0 nm with several micrometers in length [42].…”