NANO LETTERS
Patterning Microsphere Surfaces by Templating Colloidal Crystals
2005 Vol. 5, No. 1 143-146
Gang Zhang, Dayang Wang,* and Helmuth Mo1 hwald Max Planck Institute of Colloids and Interfaces, D-14424, Potsdam, Germany Received November 12, 2004
ABSTRACT By using the upper single or double layers in colloidal crystals as masks during Au vapor deposition, various Au patterns have been successfully constructed on the surfaces of the lower spheres. The dimension and geometry of the Au patterns obtained are dependent on the orientation of the colloidal crystal templates. Our patterning procedure is independent of the curvature and chemical composition of the surfaces, which definitely pave a promising way to pattern highly curved surfaces.
We have developed a facile and generally applicable technique to pattern the surfaces of colloidal spheres. Using the upper single or double layers of colloidal crystals as masks during Au vapor deposition, various Au patterns were successfully fabricated on the surfaces of the lower spheres with positional precision of better than 50 nm. To link chemical processes in or on spheres one has to fabricate designed patterns on them.1 While the recent development of lithography technology enables one to fabricate patterns with feature dimensions less than 100 nm and even on nonplanar surfaces, it remains a great challenge to create patterns on the surfaces of objects with curvatures less than 50 µm.2 Although half a Au shell has been constructed on colloidal spheres by controlled Au vapor deposition,3 the creation of patterns on their surfaces is not achieved yet due to lack of proper masks. Herein we show that high precision patterns can be realized by newly developed methods of colloid ordering. Two-dimensional (2D) colloidal crystals provide wellordered templates. By using them as masks during metal vapor deposition, metallic patterns with defined dimension and geometry have been formed on planar substrates.4 Ordered arrays of cavities within 2D colloidal crystals of larger spheres have recently been utilized as traps to define the location of smaller colloidal spheres to form binary crystal structures.5 Yang and co-workers have utilized the top layer of colloidal crystals as masks for reactive ion etching to create ordered arrays of nonspherical particles.6 In the current work, we employ the top monolayers or bilayers in colloidal crystals as masks to pattern the surfaces of the spheres underneath. Figure 1 schematically represents * Corresponding author. Fax: +49 331 567 9202. Email:
[email protected]. 10.1021/nl048121a CCC: $30.25 Published on Web 12/04/2004
© 2005 American Chemical Society
our procedure. At first, colloidal crystals with 2 or 3 layers of spheres were constructed on silicon wafers. During Au vapor deposition, ordered arrays of interstices between the upper spheres allow Au vapor to reach the lower spheres. After peeling off the upper layers, colloidal spheres with Au patterned surfaces were achieved (Supporting Information). The features of surface patterns are dependent on the packing symmetries of the upper layers. When the preferential orientation of colloidal crystals is (111) parallel to the substrates, the spheres are hexagonally close-packed. Underneath the triangle interstices within the top layer, not only the spheres in the underlying layer but also the interstices between them are positioned in a defined way, represented in Figure 1. Figure 2a shows a typical SEM image of 925 nm polystyrene spheres covered with Au patterns formed by templating with the upper monolayer with (111) crystallization orientation parallel to the substrate. Since the samples for SEM measurements in our work were not sputter-coated by Au, bright dots on dark spheres in Figure 2a represent Au domains. High magnification SEM image shows that after Au vapor deposition, triangular Au domains are found on the top of each sphere in the underlying layers and three small Au dots at its edge (Figure 2b). As shown in Figure 2a, packing faults resulted in variation of Au patterns on spheres. When the preferential orientation of colloidal crystals is (100) parallel to the substrates, the square interstices within the top layer are exclusively blocked by the spheres in the underlying layer (inset in Figure 2c). Using this crystal facet as a template, we constructed the square Au domains on the top of the spheres (Figure 2c). When the preferential orientation of colloidal crystals is (110) parallel to the substrates, two triangular interstices within the top layer are exclusively blocked by one sphere in the underlying
Figure 1. Schematic illustration of the procedure to create colloidal spheres with Au-patterned surfaces by the combination of Au vapor deposition and using the top mono- or bilayers of colloidal crystals with (111) facets parallel to the substrates as masks. The orange areas represent Au domains remained on spheres after removing the first and second sphere layer.
layer (inset in Figure 2d). By templating (110) crystal planes, two triangular Au dots were obtained on spheres, as shown in Figure 2d. Figure 2a testifies that this enables patterning a great deal of spheres. The patterns shown in Figure 2 are well reproducible, exhibiting positional accuracy better than 50 nm. Nevertheless, we currently have difficulty to precisely determine the cross-section profiles of Au domains deposited on spheres, particularly created by templating (111) and (110) crystal planes. The out-of-plane heights of the Au domains on PS spheres formed by templating (100) crystal planes were roughly determined by atom force microscopy, which 144
may be varied by the Au mass thickness. As suggested in the literature, the precision of patterns constructed by nanosphere lithography should be comparable to that of electron beam lithography.4 To determine the precise geometries of Au patterns formed on colloidal spheres is currently at exploration in our lab. In the current work, we were also successfully able to fabricate Au patterns on 270 nm polystyrene spheres by utilizing the upper layers of their colloidal crystals as templates. Figure 3a shows a typical low magnification SEM image of the 270 nm polystyrene spheres coated with Au patterns formed by templating the top monolayer with (111) crystallization orientation. The high magnification SEM image (Figure 3b) clearly reveals the same character of Au patterns as observed in Figure 2b; a big triangular Au domain being at the middle and three small ones at the edge on the top surface of spheres. This testifies that our patterning process is independent of the curvature of spheres. Furthermore, within the ABCABC stacking crystals with preferential orientation facets of (111), half of the triangular interstices between the spheres in the first layer were blocked by the triangle interstices between the spheres in the second layer, enabling Au vapor to reach the spheres in the third layer. After consecutively peeling off the first and second layers, small Au dots were found atop the spheres of the third layer, shown in Figure 4. Comparing with those formed by using the upper monolayer of ordered packed spheres as templates, Au patterns on spheres derived from templating of the upper bilayer of ordered spheres have only singletriangle Au domains on the top of the sphere, consistent with previous studies on nanosphere lithography.4 Likely due to the nonuniform diffusion of Au vapor during deposition, the size of the Au domain observed in Figure 4 is not uniform as compared with those observed in Figure 3b. In colloidal crystals, ABAB stacking structures usually coexist with ABCABC stacking ones. It is worthwhile to note that we may not create triangular Au domains in all spheres in the third layer. As the sphere of the third layer is located just beneath that of the first layer in these structures, no Au patterns were formed on the third layer spheres in ABAB stacking structures with (100) facets parallel to substrates (Figure S1, Supporting Information). In ABCABC structures, the interstices between spheres for Au vapor penetration is completely blocked by the third layer spheres, so no patterns are observed on spheres of the fourth layer. In addition to working with latex spheres, by using our patterning procedure, Au patterns can be formed on silica spheres as well, as shown in Figure 5, demonstrating the independence of our procedure on the chemical nature of the sphere surface. In comparison with those on polystyrene spheres, the Au patterns formed on the silica spheres have nonuniform shapes. This is due to the low ordering degree of silica colloidal crystals caused by the broad size distribution of silica spheres used in the current work. In summary, we have demonstrated a facile and versatile method to create Au patterns on submicron-sized spheres by using monolayers or bilayers of highly ordered packed spheres as masks during Au vapor deposition. The dimension Nano Lett., Vol. 5, No. 1, 2005
Figure 2. Low (a) and high (b) magnification scanning electron microscope (SEM) picture of 925 nm polystyrene spheres with Aupatterned surfaces generated by templating the top monolayers of colloidal crystals with (111) facets parallel to the substrates. Bright spots correspond to Au domains on spheres. SEM pictures of 925 nm polystyrene spheres with Au-patterned surfaces generated by using the top monolayers of colloidal crystals with preferential crystal orientation: (100) (c) and (110) (d) parallel to substrates, as masks. The insets in c and d show the schematic depiction of formation of Au patterns by templating with (100) and (110) facets.
Figure 4. SEM pictures of 270 nm polystyrene spheres with Aupatterned surfaces generated by templating the top bilayer of colloidal crystals with (111) facets parallel to the substrates. The arrows indicate the residual spheres of the second layers.
Figure 3. Low (a) and high (b) magnification scanning electron microscope (SEM) picture of 270 nm polystyrene spheres with Aupatterned surfaces generated by templating the top monolayers of colloidal crystals with (111) facets parallel to the substrates. The arrows indicate a residual sphere of the top layer and the deformed Au patterns formed due to packing faults. Nano Lett., Vol. 5, No. 1, 2005
and geometry of the Au patterns obtained are dependent on the orientation of the colloidal crystal templates. Our patterning procedure has proved independent of the curvature and chemical composition of the surfaces, which definitely pave a promising way to pattern highly curved surfaces, which is hard to achieve by routine lithographic techniques. Nelson theoretically predicted that the patterned surfaces of microspheres might give the spheres a valence similar to the sp3 chemical bonds associated with carbon, creating a tetravalent chemistry of spheres.7 Thus, Au patterns deposited 145
to realize “colloidal covalent chemistry” to create micronor nanosized hierarchical structures. Acknowledgment. We thank Max Planck Society for the financial support. Supporting Information Available: Details of materials and experimental method. This material is available free of charge via the Internet at http://pubs.acs.org. References
Figure 5. SEM picture of 520 nm silica spheres with Au-patterned surfaces generated by templating the top monolayers of colloidal crystals with preferential crystal orientation: (111) parallel to substrates as masks.
on PS or silica spheres should therefore be recruited as specific functional sites for directional bonding, allowing one
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(1) Wang, D.; Mo¨hwald, H. J. Mater. Chem. 2004, 14, 459. (2) For a review, see: Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (3) Lu, Y.; Xiong, H.; Jiang, X.; Xia, Y.; Prentiss, M.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12724. (b) Love, J. C.; Gates, B. D.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Nano Lett. 2002, 2, 891. (c) Bao, Z.; Chen, L.; Weldon, M.; Chandross, E.; Cherniavskaya, O.; Dai, Y.; Tok, J. Chem Mater. 2002, 14, 24. (4) For a review, see: Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. (5) Wang, D.; Mo¨hwald, H. AdV. Mater. 2004, 16, 244. (6) Choi, D.; Yu, H. K.; Jang, S. G.; Yang, S. M. J. Am. Chem. Soc. 2004, 126, 7019. (7) Nelson, D. R. Nano Lett. 2002, 2, 1125.
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Nano Lett., Vol. 5, No. 1, 2005
