Photo-Induced Crawling Motion of Azobenzene Crystals on Modified Gold Surfaces

Photo-induced crawling motion of a crystal of 3,3'-dimethylazobenzene (DMAB) on gold surfaces having different surface properties and various patterns was studied. DMAB crystals crawl continuously when exposed to UV and visible lights simultaneously from different directions. On a gold surface functionalized by a thiol having a hydroxyl group at the terminal (16-hydroxy-1-hexadecanethiol (HOC₁₆SH)), the crystals crawled with a relatively high velocity (ca. 4 μm min^-1), and they changed the crystal shape while keeping a distinct crystal face. On a gold surface functionalized by a thiol having an alkyl chain terminal (1-hexadecanethiol (C₁₆SH)), the crawling was observed with a slower velocity (ca. 1.5 μm min^-1). However, the shape of the crystals became a droplet-like shape soon after the irradiation started, and the shape persisted during the motion. Light intensity dependence of the crawling velocity of the droplet-like crystal on this surface showed that UV light has stronger dependence for the motion than the visible light. On a substrate with a stripe pattern of alternating C₁₆SH-modified gold and hexadecyltrimethylsilane (HDTMS)-modified glass, crystals crawled only on the surface of the C₁₆SH-modified gold, which may be due to the wettability hysteresis at the surface. On a substrate with a stripe pattern of HOC₁₆SH-modified gold and HDTMS-modified glass, crystals were attracted to the gold side. On a gold substrate with a periodic pattern of different height (ca. 50 nm) but having a uniform treatment with C₁₆SH, crystals crawled up and down the steps without significant disturbance at the boundary of the step. Therefore, wettability of the surface has a greater impact on controlling the motion of the crystal than the surface structure. The present results not only unveil the crawling behavior on various surfaces but also offer a guide to controlling the motion toward applications for novel carriage vehicles to transport molecules/objects on a surface.