As mentioned above, the site-selectivity of the floating catalyst CVD method is a powerful tool to design structures used in three-dimensional structures such as MEMs and NEMs. To demonstrate this feat, we have shown simultaneous multidirectional growth and multilayered growth of ordered nanotubes. To provide depth for horizontal growth, thick silica layers (up to ~8.5 |im) were deposited by plasma-enhanced chemical vapor deposition (PECVD) to create high aspect ratio silica features. Similarly to the thin silica layers we used for planar substrates, patterns of Si/SiO2 of various shapes were generated by photolithography followed by a combination of wet and/or dry etching. CVD growth of nanotubes is stimulated by exposing the substrate to xylene/ferrocene vapor mixture at 800°C, as described above.
Simultaneous multidirectional growth of highly oriented nanotubes can be harnessed to simultaneously grown nanotubes in several predetermined directions. For instance, we can realize nanotube growth in mutually orthogonal directions by using templates consisting of deep etched trenches separating at least several-|im-tall and -wide SiO2towers or lines. Figure 8.3). displays vertically and horizontally aligned nanotube arrays as well as more complex multidirectional growth of the MWNT layers. In the case of trenched substrate, MWNT layers are adjacent to each other, providing an excellent example that demonstrates the concept.
We are also able to create structures where the nanotubes have oblique inclinations (i.e., neither orthogonal nor planar with respect to the substrate plane) by using deep-trench templates with inclined silica surfaces into the displayed "daisy" shape or other illustrative examples, where nanotubes grow normal to the walls of such circular trenches, resulting in membranelike iris-shaped structures. These examples also show the flexibility of our approach to obtain radially oriented nanotubes with the entire spectrum of in-plane orientations on the substrate plane. Multilayer growth of ordered nanotubes may be carried out by selective nanotube growth on SiO2 in a direction normal to the surface on partially freestanding silica architectures. Figure 8.3c shows an example of this type of growth, where nanotubes grow in two opposite directions (up and down) from a suspended SiO2 layer. This suspended transparent layer of silicon oxide (thickness is about 8 |im) on a silicon base pillar is generated by photolithography and deep etching (40-50 |im) of Si wafer.
FiouRe 8.3. Extended three-dimensional patterns made of ordered multiwalled carbon nanotube films provide insight into multidirectional growth on a complex surface. (a) "Daisies" grown from truncated cones of SiO2 on a silicon wafer. (b) Simultaneous vertical and horizontal aligned nanotube arrays of nanotubes grown on a template with deep etched trenches, where the length of nanotubes in both vertical and horizontal growth is about 60 micrometers, and the thickness of the SiO2 layer is 8.5 micrometers. (c) Multilayer growth of carbon nanotubes into three dimensions on the edge of a free-standing plate of an 8.5-micron thick silica disk (up, down, and out) identified by arrows.
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