6576
Langmuir 2003, 19, 6576-6578
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 Consequently, 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 catalytically 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-dimensional 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 †
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.
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 HAuCl4 with NaBH4, and Ppy can be prepared by the oxidative polymerization of pyrrole with FeCl3, NaBH4 and FeCl3 were chosen as the corresponding triggers. The experimental procedure is described in the following. To form a template, 0.1 mg of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) (Mn 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 (NaBH4 for gold and FeCl3 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 (ultralevers, 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, NaBH4 or FeCl3, 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 immobilized in the nanometer-sized structures of PEO. Figure 2c shows the TEM image for the trigger-immobilized thin film. Since the TEM image was taken without any staining, the PS-b-PEO film is not visible (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, 36473652. (9) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024-6036.
10.1021/la034756y CCC: $25.00 © 2003 American Chemical Society Published on Web 06/26/2003
Letters
Langmuir, Vol. 19, No. 17, 2003 6577
Figure 1. Descriptive illustration of the trigger-immobilization method for the construction of 3-dimensional nanostructures of polymers or metals.
Figure 2. (a) Topological AFM image, (b) 3-dimensional AFM image, and (c) TEM image of the trigger-immobilized nanostructures formed by PS-b-PEO deposited on a carbon-coated mica substrate.
Figure 3. (a) Topological AFM image and (b) 3-dimensional AFM image of the spinodal structures formed by the polymerization of pyrrole on top of the PS-b-PEO film. (c) A superposition of cross-sectional AFM images of the spinodal structures before (blue line) and after (red line) the growth of polypyrrole.
because of its small electron density. However, the immobilized trigger (FeCl3 or NaBH4) was observed as dark spots. The sizes of the dark regions (Figure 2c) are almost the same as the sizes of the protrusions in Figure 2a. The immobilized trigger can be slowly released by dipping the film into a selected medium. For example, using diethyl ether as a medium, the trigger (FeCl3 or NaBH4) can be released due to the swelling of the PEO units in the medium, and the release of trigger was proved by using atomic absorption spectroscopy by detecting the traces of elements (Fe or Na) present in the medium. For the Ppy system, the film containing immobilized FeCl3 was dipped into the pyrrole solution for 2-12 h. After the fim was gently removed from the solution, it was washed with diethyl ether and dried under vacuum at room temperature for 24 h. The dried film was characterized using AFM with the noncontact mode. Figure 3a and b shows the AFM images obtained for a sample with a polymerization time of 2 h. The protruded surface structures in Figure 3b have heights of 10 nm and diameters of 110 nm, compared with heights of 2 nm and diameters of 110 nm for the original film. This contrast is shown as superimposed height profiles of the two kinds
Figure 4. (a) Topological AFM image and (b) 3-dimensional AFM image of the spinodal structures formed by the formation of metallic gold on top of the PS-b-PEO film. (c) A superposition of cross-sectional AFM images of the spinodal structures before (blue line) and after (red line) the growth of gold. (d) TEM image and (e) energy-dispersive X-ray analysis spectra of a film with the growth of gold for 10 s.
of films in Figure 3c. Therefore, the extended structures can be identified as Ppy grown on top of the PEO domains originally present. After the regioselective nucleation of Ppy, its further growth occurred exclusively along the vertical direction for a period of about 2 h, but the growth along horizontal directions is favored for a longer reaction time (>2 h), because it is known that the growth of Ppy is kinetically preferred on hydrophobic PS surfaces.10-12 Therefore, well-spaced Ppy domains with nanometer-sized (10) Cho, G.; Glatzhofre, D. T.; Fung, B. M.; Yuan, W.-L.; O’Rear, E. A. Langmuir 2000, 16, 4424-4429. (11) Huang, Z.; Wang, P. C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480-6484. (12) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115-7120.
6578
Langmuir, Vol. 19, No. 17, 2003
structures were apparent only up to a reaction time of 2 h under 1 mM pyrrole solution; longer reaction times (>2 h) lead to the merge of the Ppy posts. Three-dimensional gold nanostructures were constructed using NaBH4 as a trigger as well. A PS-b-PEO film containing immobilized NaBH4 was dipped into a 1 mM diethyl ether solution of HAuCl4 for 10 s to 30 min. The resulting film was treated as mentioned above, and the topographic image of the dried film was investigated using AFM with the noncontact mode. For samples with a reaction time of 10 s, extended structures of protrusions were observed, as shown in Figure 4a and b. The height profile of the film with the extended structures and that of an original trigger-immobilized film are superimposed and shown in Figure 4c. The heights of the posts are estimated to be about 12 nm, with diameters of 250 nm. To ascertain that the protruded structures observed in Figure 4a and b were indeed gold, TEM and EDX analyses were carried out, and the results are shown in parts d and e of Figure 4, respectively. Figure 4d shows a representative morphology of gold grown on a thin film for 10 s. The center-to-center spacing (∼50 nm) and irregular arrangement of circular features seen in the AFM image (Figure 4a and b) correlate very well with the features observed in the top-view TEM image. These results confirm that
Letters
we have successfully constructed 3-dimensional gold nanostructures following the trigger-immobilized domains. Since the reaction of HAuCl4 with NaBH4 is very fast, and gold can grow along both lateral and vertical directions, the protruded structures in the film can quickly merge with each other and disappear as the reaction time increases (>30 s). In summary, we have made use of two fundamental principlessmicrophase separation of an amphiphilic block copolymer and selective immobilization of a trigger by the ethylene oxide unitssto immobilize the trigger into nanostructured domains. The nanometer-sized structures formed by the reaction of a precursor with the released trigger replicate the substrate pattern very well. We emphasize that the structures reported here are replicated from the trigger immobilized in prepatterned films on the substrate. With this approach, the immobilized trigger can be used to produce 3-dimensional structures of widely varying morphologies and sizes for a number of polymers and metals. Acknowledgment. This work was supported by KOSEF (R05-2001-000-00166-0), for which we are grateful. LA034756Y
