DOI: 10.1021/la9033723 13311 Langmuir 2009, 25(23), 13311–13314 Published on Web 10/21/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society Particulate Templates and Ordered Liquid Bridge Networks in Evaporative Lithography Ivan U. Vakarelski,* ,† Jin W. Kwek, † Xiaosong Tang, ‡ Sean J. O’Shea, ‡ and Derek Y. C. Chan §, ) † Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, 627833 Singapore, ‡ Institute of Materials Research and Engineering, 3 Research Link, 117602 Singapore, § Particulate Fluids Processing Centre, Department of Mathematics & Statistics, The University of Melbourne, Parkville VIC 3010, Australia, and ) Department of Mathematics, National University of Singapore, 117543 Singapore Received September 8, 2009. Revised Manuscript Received October 15, 2009 We investigate the properties of latex particle templates required to optimize the development of ordered liquid bridge networks in evaporative lithography. These networks are key precursors in the assembly of solutions of conducting nanoparticles into large, optically transparent, and conducting microwire networks on substrates (Vakarelski, I. U.; Chan, D. Y. C.; Nonoguchi, T.; Shinto, H.; Higashitani, K. Phys. Rev. Lett., 2009, 102, 058303). An appropriate combination of heat treatment and oxygen plasma etching of a close-packed latex particle monolayer is shown to create open-spaced particle templates which facilitates the formation of ordered fully connected liquid bridge networks that are critical to the formation of ordered microwire networks. Similar results can also be achieved if non-close-packed latex particle templates with square or honeycomb geometries are used. The present results have important implications for the development of the particulate templates to control the morphology of functional microwire networks by evaporative lithography. Evaporative lithography provides a simple, low cost, and energy efficient method of creating large ordered network by the assembly of constituents that are originally in particulate suspensions. 1-4 An important potential application is in the manufacture of large transparent electrodes using a network of fine conducting micro- wires assembled from conducting nanoparticles on a glass substrate, for example, in photovoltaic cells. 5 Recently, using ideas based on the familiar coffee ring phenomenon, 6 Vakarelski et al. 1 demon- strated a simple method of creating such conducting gold microwire networks by first assembling a monolayer of polystyrene latex particles (50-100 μm in size) onto a glass substrate followed by the deposition of an aqueous suspension of 20 nm gold nanoparticles to cover the latex particles. As the solvent evaporates, a liquid bridge network first develops on the substrate around the base of the 2D lattice of latex particles. Further evaporation of the solvent then leaves behind a conducting network of microwires (1-3 μm thick) formed by the self-assembly of the gold nanoparticles that can span up to few square centimeters in size. This method of wire lithography uses the slow evaporation of liquid bridge networks that have been formed around a particu- late template to assemble conducting nanoparticles in suspensions into connected wire networks on the substrate. This approach avoids the need to fabricate complex physical masks to regulate the spatial variation in evaporation rates to create the desired network topology. 2,3 However, using hexagonal close-packed arrays of monodisperse latex particle crystals without further treatment as templates (Figure 1f), in the final network the wires tend to adopt a random topology (Figure 1h) which is inefficient in terms of fabricating high conductivity network coatings. Here, we demonstrate that, using an appropriately spaced template, instead of a close-packed particle template, it is possible to achieve a variety of fully connected, symmetrical network patterns. In our earlier work, 1 the gold nanoparticle suspension also contained copolymers whose surfactant properties help stabilize the liquid bridge network. However, we observed that nano- particle-free aqueous solutions of sodium dodecyl sulfate (SDS) also form the same liquid bridge network. As our goal here is to investigate the relationship between the morphology of the latex particulate template and the liquid bridge pattern, we simply use SDS solutions in the present work. A schematic drawing of the simplified experimental procedure used here is given in Figure 1a and b. The liquid bridge network formed at the late stage of the evaporation process (see, for example, Figure 1e and h) is indicative of the respective microwires network that would be formed if surfactant stabilized nanoparticle suspensions of appro- priate concentrations are used. In Figure 2, we demonstrate the basic unit in the formation of a precursor liquid bridge where two latex particles on a substrate are covered initially by the evaporating solution. To demonstrate the key stages of the process, we use a droplet of 0.2 mM SDS aqueous solution. As the water evaporates and the meniscus falls below the equator of the particles, the pendular rings around the base of each particle remain connected by a thin liquid bridge lying on the substrate (Figure 2b-e). As evaporation proceeds, the shrinking pendular rings form two nodes at the base of the latex particles joined by a thin “liquid thread” lying in contact with the substrate (Figure 2e). According to the theory of capillarity, such a liquid thread should be unstable when its length exceeds its width by about three times. 7,8 The quantitative *Corresponding author. Telephone: þ 65 6796 3880. Fax: þ 65 6316 6183. E-mail: ivakarelski@gmail.com. (1) Vakarelski, I. U.; Chan, D. Y. C.; Nonoguchi, T.; Shinto, H.; Higashitani, K. Phys. Rev. Lett. 2009, 102, 058303. (2) Harris, D. J.; Hu, H.; Conrad, J. C.; Lewis, J. A. Phys. Rev. Lett. 2007, 98, 148301. (3) Harris, D. J.; Lewis, J. A. Langmuir 2008, 24, 3681. (4) Hong, S. W.; Byun, M.; Lin, Z. Angew. Chem., Int. Ed. 2009, 48, 512. (5) Kang, M.-G.; Kim, M.-S.; Kim, J.; Guo, L. J. Adv. Mater. 2008, 20, 4408. (6) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 6653. (7) Langbein, D. J. Fluid. Mech. 1990, 213, 251. (8) Roy, R. V.; Schwartz, L. W. J. Fluid. Mech. 1999, 391, 293.