13892 DOI: 10.1021/la1004787 Langmuir 2010, 26(17), 13892–13896 Published on Web 08/02/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Directing the Formation of Nanostructured Rings via Local Oxidation Andrew Stannard,* Haya Alhummiany, Emmanuelle Pauliac-Vaujour, James S. Sharp, and Philip Moriarty School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom Uwe Thiele School of Mathematics, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom Received February 1, 2010. Revised Manuscript Received July 15, 2010 We provide compelling evidence that ring formation in solutions of thiol-passivated Au nanoparticles is driven by breath figure dynamics. A method for the controlled placement of rings of nanoparticles on a solid substrate, which exploits variations in substrate wettability to fix the positions of the submicrometer water droplets formed in the breath figure process, has been developed. This is achieved by heterogeneously patterning hydrogen-terminated silicon substrates with oxide regions that act as adsorption sites for the droplets. The droplets in turn template the formation of thiol-passivated Au nanoparticle rings during spin-casting from volatile solvents. I. Introduction Motivated in part by their unique optoelectronic properties, 1 the fabrication of nanorings has been pursued via many different routes including droplet molecular-beam epitaxy, 2 masked deposition, 3 and templating-based techniques. 4 The rich physics and physical chemistry involved in nanofluid dewetting offers a number of alternative routes toward generating nanoparticle rings on solid substrates, and a rather wide variety of mechanisms have been put forward to explain ring formation during the evaporation of solvent from nanoparticle- or nanorod-containing solutions/ suspensions. These span nucleation and expansion of holes in the solvent film, 5-9 the Marangoni effect, 10 periodic contact line pinning and depinning, 11 the formation of gas bubbles in a very thin solvent film, 12 magnetic dipolar interactions, 6 and breath figure formation. 13,14 The latter phenomenon, which exploits the droplets of water that condense on a substrate due to evaporative cooling as templates for nanoring formation, has been used successfully for a range of materials other than nanoparticles/nanorods including single-molecule magnets, 15 polymers (see ref 16 for a review), and hybrid polymer-nanoparticle systems. 17 When a volume of nanofluid solution is deposited onto a solid substrate, the resulting drying-mediated self-organized morpho- logy of nanoparticles will depend on a range of parameters such as solvent volatility, thin film stability, and interparticle interact- ions. 18,19 The formation of nanoparticle rings via the creation of holes in a partially wetting volatile thin film has typically been thought of as a three-stage process: nucleation, growth, and arrest. For thin films, the first stage can occur via two mechanisms. Holes can appear homogeneously via thermally driven nucleation with- out spatial and temporal correlations of the nucleation centers. Alternatively, holes can appear heterogeneously via defect-driven nucleation, resulting in temporal correlations and, thus, rings of similar sizes. Once a hole in the thin volatile film is nucleated, evaporation of solvent from the rim results in a retreat of the substrate-solvent-air contact line; i.e., the hole expands. As the nanoparticles remain in solution, they are collected at the rim. Eventually, growth is arrested and a nanoparticle ring is formed by the deposition of the accumulated nanoparticles. The origin of the arrest is a slightly contested issue. A continuum model approach by Ohara and Gelbart 5 suggests that the accumulation of particles at the contact line produces an increased frictional force which stops the hole expansion. When this force becomes large enough, hole growth is arrested. This would result in an upper limit to ring size for a given concentration of nanoparticles in solution. A numerical simulation approach by Yosef and Rabani, 7 however, showed that the increased nano- particle concentration at the rim does not retard the expansion, and hole growth is only arrested via global solvent evaporation. In their model the nanofluid film thins globally via evaporation *To whom correspondence should be addressed. E-mail: andrew.stannard@ nottingham.ac.uk. (1) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; K€ all, M.; Bryant, G. W.; de Abajo, F. J. G. Phys. Rev. Lett. 2003, 90, 057401. (2) Gong, Z.; Niu, Z. C.; Huang, S. S.; Fang, Z. D.; Sun, B. Q.; Xia, J. B. Appl. Phys. Lett. 2005, 87, 093116. (3) Larsson, E. M.; Alegret, J.; K€ all, M.; Sutherland, D. S. Nano Lett. 2007, 7, 1256–1263. (4) Sun, Z.; Li, Y.; Zhang, J.; Li, Y.; Zhao, Z.; Zhang, K.; Zhang, G.; Guo, J.; Yang, B. Adv. Funct. 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