Orthogonal growth of horizontally aligned single-walled carbon nanotube arrays

Direction-controlled growth of horizontally aligned single-walled carbon nanotubes (SWNTs) on r-plane sapphire substrates and their alignment mechanisms are demonstrated. On a flat r-plane substrate, anisotropic nanotube-substrate interaction is known to align SWNTs parallel to the [11̄01̄] direction of the sapphire. We find that the introduction of a slight miscut (-1° inclined to the [11̄01̄] direction) on the substrate changed the SWNT growth direction by 90°, aligning perpendicular to the [11̄01̄] direction. This dramatic change of the growth direction is explained by the contribution of newly proposed one-dimensional surface atomic rows and/or atomic steps appeared on the r-plane. Annealing the substrate in hydrogen atmosphere prior to SWNT growth recovers the original nanotube growth direction, while annealing in air deteriorates the alignment. The direct growth of an orthogonally aligned SWNT array is achieved through optimized surface treatment. Site-selective directional control of aligned SWNTs is also demonstrated by applying hydrogen annealing to the miscut substrate whose surface is partially covered with SiO₂. Our study gives insights into the alignment mechanism on single crystal substrates and offers a new means to assemble SWNTs for advanced integrated structures.