High-density single-walled carbon nanotubes forming self-directed networks on nanoscale patterned substrates are produced by chemical vapor deposition. The roles of catalyst particles for the growth of single-walled carbon nanotubes are investigated. Transition-metal particles of 3-7 nm size, obtained by depositing thin catalyst films (0.5-1 nm), remain as active catalysts for the growth of high-density single-walled carbon nanotubes on select catalyst supports (SiO2). Highly organized single-walled carbon nanotube architectures with controlled density can be fabricated following the predefined submicrometer-sized substrate patterns, suggesting the possibility of building nanotube-based molecular-scale electronic devices.