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Dense Arrays of Well-Aligned Carbon Nanotube Thin Films Y. Gao, J. Liu, M. Shi, S. H. Elder, and J. W. Virden Supported by OBER/BES-Materials Science, PNNL Level-V Laboratory Directed Research and Development. Carbon nanotubes constitute a new class of materials with a broad range of potential applications because of the combination of their high mechanical strength, tailorable electronic properties, very high surface area, light weight, and excellent chemical and thermal stability. Of particular relevance to most of the applications is the synthesis of aligned carbon nanotubes. Another fascinating research area is carbon nanotubes filled with various elements and compounds, which promise unique physical, chemical, and electronic properties. We synthesized for the first time dense and uniform arrays of well-aligned carbon nanotubes on titanium substrates over large areas, in which all the tubes are simultaneously and completely filled with single crystals of titanium carbide. The dense arrays of the filled carbon nanotubes were fabricated by chemical vapor deposition (CVD) on titanium substrates. Figure 3.5a reveals the dense, well-aligned morphology of the carbon nanotube films. Note that all SEM images were recorded using 70% secondary electron signals and 30% back scattering electron signals, thus the intensity is proportional to the atomic number of the elements that comprise the material. The tube tips in this top-view image appear brighter at the center of the carbon nanotubes, indicating that the cores of the carbon nanotubes are filled with a material having elements of higher atomic number than carbon. Most of the carbon nanowires in Figure 3.5a have similar lengths and are approximately perpendicular to the substrate surface, although a few nanowires are tilted and extend above the film surface. An SEM image (Figure 3.5b) of a tilted nanotube reveals a structure of a nanowire enclosed by carbon. The filled material has a large head near the tube tip, and its diameter is about one-quarter of the carbon nanotube diameter. Further evidence of filled cores and carbon caps is provided by cross-sectional TEM and energy-dispersive x-ray spectroscopy. Electron diffraction patterns obtained from the cores revealed that the cores are cubic TiC. The formation of the filled nanotubes are found to depend on two factors: the solubility of the iron (the catalyst) in the substrate and the free energy of formation for the respective carbide phase. For example, only curved hollow carbon nanotubes were formed on silicon substrates. No carbon nanotubes were observed on molybdenum substrates. The high solubility of catalytic Fe in Mo depleted the catalytic material required to grow the carbon nanotubes. For silicon substrates, the driving force to form SiC is much lower than that for the formation of either TiC or TaC. The free energy of formation of these carbides are in the order of TiC (-43 kcal/mol), TaC (-35 kcal/mol), and SiC (-15 kcal/mol). Although carbon nanotubes are formed on silicon, the growth rate of SiC will be very low compared to that of TiC or TaC, resulting in hollow carbon nanotubes. These tubes tend to be tilted or curved. Therefore it is concluded that the growth of oriented and filled carbon nanotubes requires stable catalytic particles and low free energy of formation of a reaction product in the core. We have also synthesized aligned carbon nanotube thin films using a mesoporous layer containing catalyst nanoparticles on metallic substrates. The mesoporous layer with controlled and uniform pore diameters was prepared using surfactant micelles or liquid-crystal arrays of micelles as structure-templating agents via the assembly and subsequent polymerization of inorganic precursors. Growth of aligned carbon nanotube thin films was carried out by catalytic thermal decomposition of ethylene on the mesoporous layer. Figure 3.6a shows an SEM image of a crack in the carbon nanotube thin films to reveal the alignment. The formation of the cracks in the nanotube films was caused by cracking in the mesoporous layer due to large thermal mismatch between the mesoporous layer and the metallic substrates. Details of the aligned carbon nanotube thin films are shown in Figure 3.6b.   Figure 3.5. (a [left]) SEM image (top view) clearly reveals dense arrays of well-aligned carbon nanotubes grown on a titanium substrate. The high-magnification image (inset) shows brighter contrast at the center of the carbon nanotubes, indicating that the cores of the carbon nanotubes are filled with a material having the atomic number higher than carbon. (b [right]) High-magnification SEM image of a tilted nanowire reveals that the tubes are enclosed by carbon and the filled core has a large head near the tip.   Figure 3.6. (a [left]) SEM image of aligned carbon nanotube thin films on metallic substrates, and (b [right]) high-magnification SEM image. William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147