Controlled Nanotube Growth by Chemical Vapor Deposition

Recent interest in CVD nanotube growth is also due to the idea that aligned and ordered nanotube structures can be grown on surfaces with control that is not possible with arc-discharge or laser ablation techniques [23,32].

2.1 Aligned Multi-Walled Nanotube Structures

Methods that have been developed to obtain aligned multi-walled nanotubes include CVD growth of nanotubes in the pores of mesoporous silica, an approach developed by Xie's group at the Chinese Academy of Science [33,34]. The catalyst used in this case is iron oxide particles created in the pores of silica, the carbon feedstock is 9% acetylene in nitrogen at an overall 180 torr pressure, and the growth temperature is 600° C. Remarkably, nanotubes with lengths up to millimeters are made (Fig. 7a) [34]. Ren has grown relatively large-diameter MWNTs forming oriented 'forests' (Fig. 7b) on glass substrates using a plasma assisted CVD method with nickel as the catalyst and acetylene as the carbon feedstock around 660° C [35].

Our group has been devising growth strategies for ordered multi-walled and single-walled nanotube architectures by CVD on catalytically patterned substrates [23,32]. We have found that multi-walled nanotubes can self-assemble into aligned structures as they grow, and the driving force for self-alignment is the Van der Waals interactions between nanotubes [36]. The growth approach involves catalyst patterning and rational design of the substrate to enhance catalyst-substrate interactions and control the catalyst particle

Fig. 7. Aligned multi-walled nanotubes grown by CVD methods. (a) An ultra-long aligned nanotube bundle (courtesy of S. Xie). (b) An oriented MWNT forest grown on glass substrate (courtesy of Z. Ren)

size. Porous silicon is found to be an ideal substrate for this approach and can be obtained by electrochemical etching of n-type silicon wafers in hydrofluoric acid/methanol solutions. The resulting substrate consisted of a thin nanoporous layer (pore size ~ 3 nm) on top of a macroporous layer (with submicron pores). Squared catalyst patterns on the porous silicon substrate are obtained by evaporating a 5nm thick iron film through a shadow mask containing square openings. CVD growth with the substrate is carried out at 700°C under an ethylene flow of 1000 mL/min for 15 to 60min. Figure 8 shows SEM images of regularly positioned arrays of nanotube towers grown from patterned iron squares on a porous silicon substrates. The nanotube towers exhibit very sharp edges and corners with no nanotubes branching away from the blocks. The high resolution SEM image (Fig. 8c) reveals that the MWNTs (Fig. 8c inset) within each block are well aligned along the direction perpendicular to the substrate surface. The length of the nanotubes and thus the height of the towers can be controlled in the range of 10-240|j.m by varying the CVD reaction time. The width of the towers is controlled by the size of the openings in the shallow mask. The smallest self-oriented nanotube towers synthesized by this method are 2|j.m x 2|j.m.

The mechanism of nanotube self-orientation involves the nanotube base-growth mode [36]. Since the nanoporous layer on the porous silicon substrate serves as an excellent catalyst support, the iron catalyst nanoparticles formed on the nanoporous layer interact strongly with the substrate and remain pinned on the surface. During CVD growth, the outermost walls of nano-tubes interact with their neighbors via van der Waals forces to form a rigid bundle, which allows the nanotubes to grow perpendicular to the substrate (Fig. 8d). The porous silicon substrates exhibit important advantages over plain silicon substrates in the synthesis of self-aligned nanotubes. Growth on substrates containing both porous silicon and plain silicon portions finds that nanotubes grow at a higher rate (in terms of length/min) on porous silicon than on plain silicon. This suggests that ethylene molecules can permeate through the macroporous silicon layer (Fig. 8d) and thus efficiently feed the growth of nanotubes within the towers. The nanotubes grown on porous silicon substrates have diameters in a relatively narrow range since catalyst nanoparticles with a narrow size distribution are formed on the porous supporting surface, and the metal-support interactions prevent the catalytic metal particles from sintering at elevated temperatures during CVD.

2.2 Directed Growth of Single-Walled Nanotubes

Ordered, single-walled nanotube structures can be directly grown by methane CVD on catalytically patterned substrates. A method has been devised to grow suspended SWNT networks with directionality on substrates containing lithographically patterned silicon pillars [25,37]. Contact printing is used to transfer catalyst materials onto the tops of pillars selectively. CVD of rirvcvi

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Fig. 8. Self-oriented MWNT arrays grown by CVD on a catalytically patterned porous silicon substrate. (a) SEM image of tower structures consisted of aligned nanotubes. (b) SEM image of the side view of the towers. (c) A high magnification SEM image showing aligned nanotubes in a tower. Inset: TEM image showing two MWNTs bundling together. (d) Schematic diagram of the growth process

Fig. 8. Self-oriented MWNT arrays grown by CVD on a catalytically patterned porous silicon substrate. (a) SEM image of tower structures consisted of aligned nanotubes. (b) SEM image of the side view of the towers. (c) A high magnification SEM image showing aligned nanotubes in a tower. Inset: TEM image showing two MWNTs bundling together. (d) Schematic diagram of the growth process methane using the substrates leads to suspended SWNTs forming nearly ordered networks with the nanotube orientations directed by the pattern of the pillars (Fig. 9).

The growth approach starts with developing a liquid-phase catalyst precursor material that has the advantage over solid-state supported catalysts in allowing the formation of uniform catalyst layers and for large-scale catalytic patterning on surfaces [25,37]. The precursor material consists of a triblock copolymer, aluminum, iron and molybdenum chlorides in mixed ethanol and butanol solvents. The aluminum chloride provides an oxide framework when oxidized by hydrolysis and calcination in air. The triblock copolymer directs the structure of the oxide framework and leads to a porous catalyst structure upon calcination. The iron chloride can lead to catalytic particles needed for the growth of SWNTs in methane CVD. The catalyst precursor material is first spun into a thin film on a poly-dimethyl siloxane (PDMS) stamp, followed by contact-printing to transfer the catalyst precursor selectively onto

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