Method of Selective Trigger-Immobilization for Nanostructure Formation Gyoujin Cho,* ,† Imsun Seo, † Sunggi Jung, † EungJu Oh, ‡ and Bing M. Fung § Department of Chemical Engineering and Nanotechnology Center, Sunchon National University, 315 Maegok Sunchon, Chonnam, Korea 540-742, Department of Chemistry, Myongji University, San 38-2, Nam Dong, Yong In, Kyonggi Do, Korea 449-728, and Department of Chemistry and Biochemistry, The University of Oklahoma, Norman, Oklahoma 73019 Received May 3, 2003 We report a new method to control both the nucleation and growth of polypyrrole (Ppy) and gold for the formation of nanometer-sized patterns using regioselectively immobilized oxidant and reductant as nucleation sites, respectively, for Ppy and gold. Poly(styrene-b-ethylene oxide) was used as a nanometer scale template. The height of Ppy and gold can be selectively grown only on PEO domains from 5 to 10 nm. As the demand for miniaturization of electronic devices is largely consumer-driven, factors such as low cost and massive market applications are important. 1 Conse- quently, researchers in many disciplines are looking for simple and inexpensive ways for the production of nanoscopic-sized, sophisticated structures that play a central role in microelectronics. Up to the present, the top-down approach, which includes lithography and pattern transfer, has been used in general. However, as the top-down method has reached cost and technical limits at the level of about 100 nm, bottom-up, cost-effective strategies are employed as an alternative way, since they allow nature to do the assembly work and the control of molecules with a length scale down to about 10 nm. 2 As a typical bottom-up method, the microphase-separated block copolymers as templates have been used to control the regioselective nucleation on a designated surface and growth along one domain of block copolymers. 3-5 Here, we develop a new method which mimics natural systems in which structurally organized organic surfaces catalyti- cally or epitaxially induce the regioselective nucleation and growth of specifically oriented inorganic and organic structures. 6 The process used involves the selective immobilization of a trigger onto one domain of a block copolymer and the subsequent controlled release of the trigger for the regioselective nucleation and growth of inorganic or organic materials (Figure 1). This approach will provide a simple way for the construction of 3-di- mensional nanostructures of metals and polymers. Our approach is quite simple. Initially, a trigger is selectively immobilized on a designated region of the substrate surface; following that, a solution containing a precursor for forming the desired nanosized material is added to the system; finally, the metal or polymer nucleates and grows on top of the trigger-immobilized regions to form a 3-dimensional nanostructure because the trigger is gradually released at the interface between the solution and the surface. In our experiments, we selected polypyrrole (Ppy) and gold as standard materials for a metal and a polymer, respectively. Since gold can be formed by the reduction of HAuCl 4 with NaBH 4 , and Ppy can be prepared by the oxidative polymerization of pyrrole with FeCl 3 , NaBH 4 and FeCl 3 were chosen as the corre- sponding triggers. The experimental procedure is de- scribed in the following. To form a template, 0.1 mg of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) (M n 58600-block-31000, with 1.03 polydispersity) was dissolved in 11 mL of a mixed solvent of chloroform and acetonitrile with a volume ratio of 10:1; 0.03 mg of the trigger (NaBH 4 for gold and FeCl 3 for Ppy) was then added to the block copolymer solution, and a thin film was prepared by dipping of the substrate (carbon-coated mica) in the solution. In this process, a condensed brush of PS-b-PEO chains was formed with the PS blocks anchoring at the carbon-coated mica surface and the PEO blocks extending into the solution. After annealing the film at 150 °C under an inert atmosphere, the solvent evaporated, and an ordered monolayer of PS-b-PEO “precipitated” onto the surface and collapsed. 7 Under noncontact AFM (Park Scientific Autoprobe CP) with silicon cantilevers (ultra- levers, 2 μm thick, resonant frequency ∼ 320 kHz; Park Scientific), the resulting pattern could be observed as protrusions (bright spots in the AFM image of Figure 2). Similar structures are found in a certain concentration range for amphiphilic diblock copolymers. 8 Figure 2b shows the protruded structures with a typical height of about 2 nm and a mean diameter of about 110 nm. The trigger, NaBH 4 or FeCl 3 , is solubilized preferentially in the PEO domain of the block copolymer, where it is coordinated and stabilized by the ether units. 9 In other words, the trigger in the thin film is exclusively im- mobilized in the nanometer-sized structures of PEO. Figure 2c shows the TEM image for the trigger-im- mobilized thin film. Since the TEM image was taken without any staining, the PS-b-PEO film is not visible † Sunchon National University. ‡ Myongji University. § The University of Oklahoma. (1) Ratner, M. Nature 2000, 404, 137-138. (2) Ozin, G. A. Chem. Commun. 2002, 419-432. (3) Cho, G.; Jang, J.; Jung, S.; Moon, I.; Lee, J.; Cho, Y.; Fung, B. M.; Yuan, W.; O’Rear, E. A. Langmuir 2002, 18, 3430-3433. (4) Cho, G.; Park, K.; Jang, J.; Jung, S.; Moon, J.; Kim, T. Electrochem. Commun. 2002, 4, 336-339. (5) Seo, I.; Pyo, M.; Cho, G. Langmuir 2002, 18, 7253-7257. (6) Mann, S.; Douglas, D. A.; Jon, M. D.; Trevor, D.; Brigid, R. H.; Fiona, C. M. J.; Nicholas, R. Science 1993, 261, 1286-1292. (7) Meiners, J. C.; Elbs, H.; Ritzi, A.; Mlynek, J.; Krausch, G. J. J. Appl. Phys. 1996, 80, 2224-2227. (8) Boontongkong, Y.; Cohen, R. E. Macromolecules 2002, 35, 3647- 3652. (9) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024-6036. 6576 Langmuir 2003, 19, 6576-6578 10.1021/la034756y CCC: $25.00 © 2003 American Chemical Society Published on Web 06/26/2003