Vertical Crystallization of C₆₀ Nanowires by Solvent Vapor Annealing Process

We report that C60 molecules are spontaneously crystallized into vertical nanowires by the solvent vapor annealing (SVA) process. C60 molecules have been known to be assembled into wire-like crystals by simply dropping and drying C60 solutions in m-xylene on a solid substrate. By the drop-drying process, C60 nanowires have been mostly grown laterally on a solid substrate, as the major force applied to the droplet during the drying process is parallel to the substrate. On the other hand, the SVA process seems to provide an ideal environment under which the direction of the dominant drying force of a droplet becomes vertical. When a thermally evaporated C60 film is exposed to m-xylene solvent vapor under controlled SVA environments at room temperature, C60 molecules are found to be crystallized into vertical nanowires. The effect of solvent vapor pressure on the vertical growth of C60 nanowire is examined by comparative studies using mesitylene and 1,3-dichlorobenzene. The versatility of the SVA process for the growth of vertical organic nanostructures is further demonstrated by the successful formations of vertically grown C60 2D disks and 5,7,12,14-pentacenetetrone anisotropic crystals by employing carbon tetrachloride and toluene solvent vapors, respectively.