Three-Dimensional Mesoscale Self-Assembly

Microfabrication, as it is now practiced, is based on photolithography and is an intrinsically two-dimensional technology.1 Many important objectives in microfabrication, microelectronic devices having new architectures,2 photonic band-gap crystals,3 and biomimetic structures,4 require 3-D fabrication. This communication describes the fabrication of a model 3-D microstructure, an open, spherical, metal lattice composed of hexagonal elements ∼100 μm in size (Figure 1a).5 The procedure used to fabricate this structure combined the following three elements that will, we believe, be widely applicable in 3-D microfabrication: (i) the surface of a drop of liquid to act as a template for the structure, (ii) self-assembly using capillary forces at a liquidliquid interface6,7 to order the components on the surface of the spherical drop, and (iii) microelectrodeposition of metal to form and weld together the small, regular metallic structural components. The components on which this structure is based are open hexagons designed to support the modification of their surface properties. Using capillary forces8 for self-assembly requires careful control of the wettability of the surfaces of these hexagonal rings. Hexagons A are designed to assemble around a drop of water suspended in heptane. All sides and one hexagonal face are hydrophobic; the other hexagonal face is hydrophilic. Hexagons B have all sides plus one hexagonal face hydrophilic and one hexagonal face hydrophobic; these hexagons assemble around a drop of perfluorodecalin (PFD), chloroform, or chlorobenzene suspended in water. Figure 2 outlines the preparation of hexagonal rings. An array of 6-μm deep hexagonal molds was prepared using standard photolithographic techniques. Gold was electroplated into these molds.9 To fabricate hexagons A, a thin layer of Si (50 nm) was evaporated on top of the wafer; this Si formed a hydrophilic layer of native SiO2. The photoresist was removed by rinsing with acetone. The Si layer was removed simultaneously, leaving Si only on top of the hexagons. The wafers were briefly (<15 min) sonicated in ethanol to lift off the hexagons. Addition of hexadecanethiol to the suspension of hexagons rendered the gold surfaces of the hexagons hydrophobic. To fabricate hexagons B, the photoresist was removed prior to evaporation of Si or Ti. The Si and Ti both formed native oxide layers and covered all sides except for the bottom; this process created hexagons with all sides and one hexagonal face hydrophilic. We used a liquid drop as a template to direct the self-assembly of the hexagonal rings. The use of a liquid-liquid interface allowed the hexagons to adjust their positions relative to one another without frictional impediment under the influence of capillary forces.6 The procedure used to assemble the spherical lattice relied on several stages of self-assembly. The hexagons were prepared and suspended in the continuous phase. On addition of a drop of the discontinuous phase, an approximately spherical liquid-liquid interface formed spontaneously. The hexagons segregated and ordered on this interface. Hexagons A were suspended in heptane, and a small drop of water (diameter ∼1-2 mm, 50 μL) was added. The hexagons assembled on the surface of the drop (Figure 1b). In all experiments, an excess of hexagons was used, which remained suspended in heptane. These hexagons did not interfere with the assembled spheres. The inside of the sphere was pink; this pattern of coloration demonstrated that the hydrophilic Si/SiO2 sides faced the water phase. Assembly of hexagons B around a droplet of PFD in water gave similar spherical assemblies but with a pink exterior surface (Figure 1c). Because the hexagons had a low aspect ratio (∼1:16) and a large center hole, some overlapped, giving double layers. Similar effects have been observed in other mesoscale self-assembling systems.10 Other defects that can be seen in Figure 1a-c are voids that are due to poor packing of the hexagons.11 When self-assembly of the hexagons at the surface of the drop was complete, the resulting structure was, of course, held together only by capillary forces. These structures were surprisingly robust; pressing together two drops of hexane, suspended in water and covered with hexagons, * To whom correspondence should be addressed. Tel: (617) 495-9430. Fax: (617) 495-9857. E-mail: gwhitesides@gmwgroup.harvard.edu. (1) Moreau, W. M. Semiconductor Lithography; Plenum Press: New York, 1988. (2) Lent, C. S.; Tougaw, P. D.; Porod, W.; Bernstein, G. H. Nanotechnology 1993, 4, 49. (3) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, 1995. (4) Gopel, W. Biosens. Bioelectron. 1995, 10, 35. (5) Initial experiments showed that the size of the hexagons can be reduced to 10 μm without fundamentally changing the assembly process. Hexagons with sizes smaller than 10 μm could be fabricated, but the agitation of these particles might be difficult. The capillary forces become small (in an absolute sense), and the assemblies will be easily disturbed. (6) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233. (7) Terfort, A.; Bowden, N.; Whitesides, G. M. Nature 1997, 386, 162. (8) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. Simpson, A. W.; Hodkinson, P. H. Nature 1972, 237, 320. (9) We used a commercial gold sulfite plating solution (Technigold 25 E.; Technic, Inc. Cranston, RI), with current densities of 1-5 mA/cm2 and solution temperatures of 45-60 °C. (10) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. Figure 1. (a) Porous sphere composed of self-assembled hexagonal rings welded by electrodeposition of Ag. The rings had approximately 100μm sides and were assembled on a drop of chlorobenzene in an aqueous silver-plating solution. Defects are indicated by arrows: A, void; B, double layer; C, point at which the electrode was attached. (b) Mesoscale selfassembly of hexagons; hexagons A assembled on a drop of water in heptane. (c) Hexagons B assembled on a drop of perfluorodecalin in water. (d) Two spheres pressed against each other do not fuse; the assemblies have substantial stability even without welding to connect the particles. Scale bar ) 100 μm. 8267 J. Am. Chem. Soc. 1998, 120, 8267-8268