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MRS BULLETIN/NOVEMBER 2002 833 CNT Packing Density in Anodic Aluminum Oxide Templates Depends on Pore Aspect Ratios Potential applications for carbon nanotubes (CNTs) include electron-field emitters, quantum wires, molecular filters, and artificial muscles. Fundamental to many applications is the packing density of aligned CNTs standing normal to a substrate. Densely packed, aligned CNTs have recently been fabricated using anodic aluminum oxide nanotemplates (AAOTs) that contain pores and are embedded with Co catalysts. The length and packing density of the resulting CNTs match the pore length and number density of the pores in the AAOTs. Researchers at the Departments of Chemical Engineering and Materials Science and Engineering, Pohang University of Science and Technology, Korea, have demonstrated a method to control the packing density of CNTs fabricated from AAOTs. As reported in the October issue of Chemistry of Materials, the research team led by K.-H. Lee exploited a competition between two processes to control the packing density: CNT growth in AAOTs from Co-catalyzed C2H2 pyrolysis and amorphous-carbon deposition on the pore walls of the AAOTs. The degree of competition was controlled by varying the pore aspect ratio in the AAOTs, which were formed using a previously published two-step anodization process. A clean, highly pure, electropolished Al sheet was anodized at 40 V in 0.3 M oxalic acid solution at 15°C for 12 h and then etched using a mixture of phosphoric and chromic acids. A second anodization, performed under the same conditions as the first, but for only 10 min or 20 min, resulted in highly ordered AAOTs with pore depths of 1 μm and 2 μm, respectively. In order to facilitate uniform Co electrodeposition, the voltage was dropped from 40 V to 14 V in 1-V increments, which decreases the thickness of the alumina barrier at the pore bottoms. Subsequent treatment with 0.1 M phosphoric acid widened the pore diameters to 40 nm or 80 nm and also acted to decrease the barrier layer. Co catalyst was electrochemically deposited with uniform height at the bottom of the pores. Three distinct AAOTs were fabricated, all with the same pore density (~1010 cm-2) but differing in pore dimensions: (1) 1-μm pore depth, 80 nm diameter; (2) 2-μm pore depth, 80 nm diameter; and (3) 2-μm pore depth, 40 nm diameter. After reducing the Co particles, the CNTs were grown by catalytic pyrolysis in a tube reactor using 10% C2H2 and 20% H2 in an Ar carrier gas for 20 min at 650°C. After the flow of C2H2 and H2 was stopped, the samples were cooled to room temperature in an Ar atmosphere. Lee and co-workers found that the CNT packing density depends on the pore aspect ratios; estimates of packing density from scanning electron microscope (SEM) images are ~109 cm-2 for (1), 107 cm-2 for (2), and 105 cm-2 for (3). The SEM images also reveal that the CNT diameters nearly match the pore diameters. Lee and co-workers found that packing density did not change with time even as the CNTs continued to grow and become curved. High-resolution transmission electron microscope images reveal that the CNTs consist of ~40–50 wellordered graphitic layers surrounding a hollow core. Lee and the research team believe that in the initial stages of their experiments, CNT growth and carbon deposition on the pore walls occur simultaneously. Furthermore, the deposited carbon inhibits the growth of CNTs by Co catalysis, which results in short CNTs inside the pores. Carbon is deposited to a greater degree in smaller, deeper pores because, as the researchers point out, C2H2 is transported by Knudsen diffusion. The num-