Three-dimensional self-assembly of complex, millimeter-scale structures through capillary bonding.

In this communication we describe the organization of millimeter-sized subunits into complex, three-dimensional (3-D) arrays using mesoscale self-assembly (MESA)sself-assembly of components with lateral dimensions comparable to the distance over which the attractive forces operate.1,2 Previous reports have demonstrated the self-assembly of small objects into ordered arrays displaying simple translational symmetry in two or three dimensions; examples include mesoscale assemblies1-3 and crystalline aggregates of spherical4 or non-spherical5 colloids. For possible application in the fabrication of functional constructs such as densely interconnected 3-D electronic2b or optical devices, we wished to extend the range of structures accessible through MESA. Here, we apply concepts derived from the study of molecules6sincluding shape and surface complementarity,1 helicity, and enantioselective recognition1,3sto the self-assembly of mesoscale structures that display symmetries more complex than those resulting from simple extended 2and 3-D crystalline arrays. In the present study, as in our previous investigations of MESA, the force responsible for aggregation is capillarity, that is, the minimization of interfacial free energy.1,2 Capillary interactions may be considered roughly analogous to chemical bonds; this analogy, though convenient, has limitationsswe note that obvious differences exist between these two classes of bonds.6 Here, capillary bonding occurs between films of a liquid metalsa lowmelting (47 °C) bismuth alloy7spatterned on the surface of mmsized polyhedral subunits (pieces).2 When heated above its melting point, the alloy forms capillary bonds that are strong enough to support open lattice structures and, when cooled below its melting point, locks the structures in place.2,8,9 In addition, the resulting metal-metal contacts can serve as a starting point for the design of systems that form electrical connections through selfassembly.2b We fabricated the polyurethane pieces using a replica molding procedure, patterned them with adhesive-backed copper tape, and selectively coated their exposed copper surfaces by dipping the pieces into molten alloy.2,10 For the self-assembly experiments, we placed the pieces in an indented 500-mL round-bottomed flask, filled the flask with an approximately isodense KBr solution, and rotated it at 10-20 rpm while heating in a 60 °C water bath.2,10 The alloy melted within a few minutes, and collisions between regions bearing molten alloy enabled the pieces to assemble. Upon completion, we stopped the agitation and allowed the solution to cool to room temperature, causing the alloy to solidify and furnishing aggregates sturdy enough to be removed and examined. In the first system studied, we used shape complementarity between indented regions on square rods to direct the formation of an open-square array (Figure 1). The four indentations bearing alloy-coated copper tape forced adjacent rods to lie at right angles with respect to one another (Figure 1a). A well-ordered, defectfree “Lincoln log” aggregate 1, defining an approximately orthorhombic open space, formed after ∼1 h of agitation (Figure 1b,c). Malformed aggregatessfor example, three pieces defining an L-shape or a section with one or more missing piecesbroke apart and reformed without defects under the agitation conditions used. We reproduced array 1 in each of four separate repetitions of the experiment and therefore believe it to be the most stable structure in this system. This stability is reasonable since 1 maximizes the number of capillary bonds and thus minimizes the interfacial free energy of the system. In addition, 1 is the most compact structure possible and should therefore be less susceptible than other aggregates to the mechanical shearing caused by rotary agitation. Next, we designed a system to mimic the ubiquitous helical conformations of linear polymers (Figure 2).11 Figure 2a depicts rectangular slabs designed to form helices. Self-assembly gener(1) (a) Wu, H.; Bowden, N.; Whitesides, G. M. Appl. Phys. Lett. 1999, 75, 3222-3224. (b) Bowden, N. B.; Weck, M.; Choi, I. S.; Whitesides, G. M. Acc. Chem. Res. 2001, 34, 231-238. (2) (a) Breen, T. L.; Tien, J.; Oliver, S. R. J.; Hadzic, T.; Whitesides, G. M. Science 1999, 284, 948-951. (b) Gracias, D. H.; Tien, J.; Breen, T. L.; Hsu, C.; Whitesides, G. M. Science 2000, 289, 1170-1172. (3) Rothemund, P. W. K. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 984989. (4) Recent reviews: (a) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693-713. (b) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545-610. (5) Selected examples: (a) Wang, Z. L.; Harfenist, S. A.; Vezmar, I.; Whetten, R. L.; Bentley, J.; Evans, N. D.; Alexander, K. B. AdV. Mater. 1998, 10, 808-812. (b) Mann, S.; Davis, S. A.; Hall, S. R.; Li, M.; Rhodes, K. H.; Shenton, W.; Vaucher, S.; Zhang, B. J. J. Chem. Soc. Dalton Trans. 2000, 3753-3763. (6) For a discussion of the analogies between MESA and molecular selfassembly, see ref 1b. (7) The composition of the alloy (Small Parts, Inc.) is as follows (wt %): 44.7% Bi; 22.6% Pb; 8.3% Sn; 5.3% Cd; 19.1% In. (a) Smith, W. C. Min. Metall. 1945, 26, 561-562. (b) Maribo, D.; Sondergaard, N. A. IEEE Trans. Compon. Hybrids Manuf. Technol. 1987, 10, 452-455. (8) This strategy is reminiscent of self-alignment of electonic components during flip chip bonding: Lau, J. H. Flip Chip Technologies; McGraw Hill: New York, 1996. (9) For studies of rotational self-assembly using the surface tension of molten solids, see: Syms, R. R. A.; Gormley, C.; Blackstone, S. Sens. Actuators, A 2001, 88, 273-283 and references therein. (10) See Supporting Information for details. (11) Kirshenbaum, K.; Zuckermann, R. N.; Dill, K. A. Curr. Opin. Struct. Biol. 1999, 9, 530-535. Figure 1. (a) Schematic diagram of polyurethane notched square rods, where l ) 25 mm, w ) h ) 4.8 mm, a ) 3.0 mm, and b ) 2.5 mm. Clear regions represent uncoated polymer, while shaded regions correspond to areas coated by copper tape and alloy. Notches in the rods forced the pieces to connect at right angles; the final product was opensquare array 1. (b) End view of 1. (c) Oblique view of 1. 8119 J. Am. Chem. Soc. 2001, 123, 8119-8120