Self-Assembly Technique Enables Fabrication of Large-Area, Periodic Arrays of Aligned Carbon Nanotubes
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RESEARCH/RESEARCHERS Self-Assembly Technique Enables Fabrication of Large-Area, Periodic Arrays of Aligned Carbon Nanotubes Heretofore, carbon nanotubes (CNTs) could not be made into photonic crystals because prevailing synthetic techniques did not produce two-dimensional (2D) arrays that were simultaneously well aligned and periodic, and covered a largeenough area. Plasma-enhanced chemical vapor deposition (PECVD) is typically
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employed in conjunction with a catalyst (Ni, Fe, or Co) deposited as either a thin film formed by magnetron sputtering or as dots, which can be made by electron-beam lithography or by electrochemical deposition. Although large areas of aligned CNT arrays can be inexpensively grown from thin films or from electrochemically deposited dots, they are not periodic. CNT arrays grown from e-beam lithography are periodic but are expensive to produce and
are limited to small areas. A team of researchers from Boston College; Natick Soldier Center in Natick, Mass.; the HahnMeitner Institute in Berlin; Nanolab, Inc. in Brighton, Mass.; and the University of Massachusetts at Boston has developed a self-assembly technique that produces large, 2D, periodic arrays of CNTs. In the January issue of Nano Letters, Boston College researchers K. Kempa and Z.F. Ren, Natick Army researchers
MRS BULLETIN/FEBRUARY 2003
RESEARCH/RESEARCHERS
B. Kimball and M. Sennett, and co-workers report on the development of nanosphere lithography, which utilizes commercially available suspensions of monodisperse polystyrene (PS) nanospheres (1 µm, 0.5 µm, 0.25 µm, and 0.125 µm in diameter). The researchers applied several µL of suspension to a clean silicon wafer. After immersion in de-ionized water and modification of the surface tension with a dodecylsodiumsulfate solution, a large, highly ordered monolayer of nanospheres formed on the water surface. Draining the water deposited the monolayer onto the silicon surface—or onto the surface of virtually any sufficiently flat substrate. Both the uniform diffraction color and a fast Fourier transform analysis confirmed that the monolayer was essentially free of defects. The researchers subsequently employed the nanosphere monolayer as a template for catalyst deposition—electron-beam evaporation of Ni—after which the PS nanospheres were chemically removed. Images made by atomic force microscopy and scanning electron microscopy (SEM) showed that the Ni dots formed a honeycomb pattern. The Ni dots can be annealed first or used directly to grow CNT arrays by hot-filament PECVD. In this work, aligned CNTs were grown using acetylene gas as the carbon source and ammonia gas as both the plasma enhancer and growth promoter. The researchers note that SEM images show that the CNTs are not as straight as those synthesized previously, but feel confident that the quality will improve with better nanosphere removal and with more control of the CNT growth. The colorful appearance of CNT arrays due to diffraction demonstrates their high metallicity (low dielectric loss) and the high degree of
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