Uniform diameter, horizontally aligned single-walled carbon nanotubes (SWNTs) are grown on sapphire substrate through the control of Fe nanoparticle size by annealing in ultrahigh vacuum (UHV). We find that thermal annealing in UHV reduces the Fe nanoparticle size and significantly narrows their size distribution. These size-controlled Fe nanoparticles enable the catalytic growth of uniformdiameter SWNTs while maintaining their horizontal alignment. Systematic analyses of Raman radial breathing modes with three laser wavelengths indicate that ∼76% of the horizontally aligned SWNTs have diameters in the range of 1.3-1.4 nm under optimized annealing conditions. Longer UHV annealing induces the increase in the Fe nanoparticle size, giving large-diameter SWNTs. Precise correlation between the metal nanoparticle size and SWNT diameter is demonstrated. Effects of UHV annealing on the nanoparticle size are discussed in terms of evaporation, subsurface diffusion, and surface precipitation of catalyst metals.
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