Controlled Growth of GaN Single-Crystal Nanowires Demonstrated
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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 ordering, said the researchers. The researchers employed a general theory of light diffraction to demonstrate that their honeycomb arrays of CNTs should lead to a diffraction pattern with triangular symmetry. Triangular symmetry was observed in the diffraction patterns obtained by shining green and blue laser light perpendicular to the plane of the lattice. The research team said that the rotational symmetry of the patterns shows that the scattering nanotubes are circularly symmetric in the plane and that the small hexatic pattern distortion points to the possibility of misaligned crystalline macro regions. The researchers also demonstrated that the specific dependency of the spot intensity on their order is consistent with theory. Citing previous work demonstrating 2D hexagonal bandgap structures, the researchers reasoned that they could employ previously published theoretical MRS BULLETIN/FEBRUARY 2003
results after proper dielectric-constant scaling, which implied that their honeycomb array of nanotubes should act as a 2D photonic-bandgap crystal with a bandgap of about 0.5 µm. While demonstration of the photonic bandgap in their nanotube arrays is in progress, the researchers believe they can obtain photonic bandgaps in the visible range. They also said that the nanotubes can be coated for better control of the photonic-crystal parameters or
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