CVD Growth of Nanotube Tips

All of the problems associated with mechanical assembly potentially can be solved by direct growth of nanotubes onto AFM tips by catalytic Chemical Vapor Deposition (CVD). In the CVD synthesis of carbon nanotubes, metal catalyst particles are heated in a gas mixture containing hydrocarbon or CO. The gas molecules dissociate on the metal surface and carbon is adsorbed into the catalyst particle. When this carbon precipitates, it nucleates a nanotube of similar diameter to the catalyst particle. Hence, CVD allows control over nanotube size and structure including the production of SWNTs [80], with radii as low as 4 A [81].

The central issues in the growth of nanotube AFM tips by CVD are (1) how to align the nanotubes at the tip such that they are well positioned for imaging and (2) how to ensure there is only one nanotube or nanotube bundle at the tip apex. Two approaches [68,69] for CVD nanotube tip growth have been developed (Fig. 20). First, Hafner et al. [68] grew nanotube tips by CVD from pores created in silicon tips. Electron microscopy revealed that MWNTs grew from pores in the optimal orientation for imaging in these initial studies. These "pore-growth" CVD nanotube tips were shown to exhibit the favorable mechanical and adhesion properties found earlier with manually assembled nanotube tips. In addition, the ability to produce thin, individual nanotube tips has enabled improved resolution imaging [68] of isolated proteins. More recent studies of the pore-growth nanotube tips by Cheung et al. [82] have focused on well-defined iron oxide nanocrystals as catalysts. This effort has enabled the controlled growth of thin SWNT bundles

Fig. 20. CVD nanotube tip growth methods. The left panel illustrates pore growth. (top) The SEM image shows a flattened, porous AFM tip with a MWNT protruding from the flattened area. Scale bar is l|i.m. (bottom) The TEM image demonstrates that the tip consists of a thin, single MWNT. Scale bar is 20 nm. The right panel illustrates the surface growth technique. (top) The SEM image demonstrates that nanotubes are steered towards the tip. Scale bar is l00nm. (bottom) The TEM image reveals that there is an individual SWNT at the tip. Scale bar is l0nm [68,69]

Fig. 20. CVD nanotube tip growth methods. The left panel illustrates pore growth. (top) The SEM image shows a flattened, porous AFM tip with a MWNT protruding from the flattened area. Scale bar is l|i.m. (bottom) The TEM image demonstrates that the tip consists of a thin, single MWNT. Scale bar is 20 nm. The right panel illustrates the surface growth technique. (top) The SEM image demonstrates that nanotubes are steered towards the tip. Scale bar is l00nm. (bottom) The TEM image reveals that there is an individual SWNT at the tip. Scale bar is l0nm [68,69]

1-3 nm in diameter from pores made at the silicon tip ends. The pore-growth method has demonstrated the great potential of CVD to produce controlled diameter nanotube tips, although it still has some limitations. In particular, the preparation of a porous layer can be time consuming and may not place individual SWNTs at the optimal location on the flattened apex.

In a second approach, Hafner et al. [69] grew CVD SWNTs directly from the pyramids of silicon cantilever-tip assemblies. The "surface growth" approach has exploited the trade-off between the energy gain of the nanotube-surface interaction and energy cost to bend nanotubes to grow SWNTs from the silicon pyramid apex in the ideal orientation for high resolution imaging. Specifically, when a growing nanotube reaches an edge of a pyramid, it can either bend to align with the edge or protrude from the surface. The pathway followed by the nanotube is determined by a trade-off in the energetic terms introduced above: if the energy required to bend the tube and follow the edge is less than the attractive nanotube-surface energy, then the nanotube will follow the pyramid edge to the apex; that is, nanotubes are steered towards the tip apex by the pyramid edges. At the apex, the nanotube must protrude along the tip axis since the energetic cost of bending is too high. This steering of nanotubes to the pyramid apex has been demonstrated experimentally [69]. For example, SEM investigations of nanotube tips produced by the surface growth method show that a very high yield of tips contain nanotubes only at the apex, with very few protruding elsewhere from the pyramid. TEM analysis has demonstrated that tips consist of individual SWNTs and small SWNT bundles. In the case of the small SWNT bundles, the TEM images show that the bundles are formed by nanotubes coming together from different edges of the pyramid to join at the apex, thus confirming the surface growth model described above [69]. The surface-growth approach is also important since it provides a conceptual and practical framework for preparing individual SWNT tips by lowering the catalyst density on the surface such that only 1 nanotube reaches the apex [67].

The synthesis of carbon nanotube AFM tips by CVD clearly resolves the major limitations of nanotube tips that arise from the manual assembly method. Rather than requiring tedious mircomanipulation for each tip, it is possible to envision production of an entire wafer of SWNT tips. In addition to ease of production, CVD yields thin, individual SWNT tips that cannot be made by other techniques and represent perhaps the ultimate AFM probe for high-resolution, high aspect ratio imaging.

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