REP。RT5 cross points in a very straightforward, low- LEDs and electronically more complex nano- 7. A K. Boal et al, Nature 404, 746(2000) cost, fast, and scalable process. Although the devices. 8. R. C. Hayward, D. A. Sayille, I. A Aksay, Nature 404, separations between individual NWs are not These studies provide a general and ratio- 9. M. L, H. Schnablegger, s. Mann, Nature 402,393 completely uniform, a periodic array can be nal approach for hierarchical assembly of ll asily envisioned with a patterned surface as nanomaterials into well-defined functional 10. J. Liu et al, Chem. Phys. Lett. 303, 125 11. M. Burghard et al an yield functional devices(see below). We believe that our approach for directed shown that NWs can be assembled into par- 15.T Rueckes et al. Science 289, 94(2000) assembly of multiple crossed NW arrays offers allel arrays with control of the average sepa- 16.X. Duan et al. Nature 409,66(2001) substantial advantages over current efforts, ration and, by combining fluidic alignment 17. S Noda et al, Science 289,604 direct manipulation of individual NWs and NTs also possible to control periodicity. In addi- 19. Y Cu X Duan Hu, C M Lieber, J. Phys. Chem. B (15), and electric fields(12, 16, 27)to make tion, we have demonstrated the possibility of 20. D. C Duffy et al., Anal. Chem. 70, 4974(1998) single crossed structures. With random deposi- layer-by-layer assembly of crossed and more 21. C A Stover, D L Koch, C. Cohen, J. Fluid Mech. 238 tion and manipulation, it is difficult to obtain complex structures by varying the flow direc- 22 D L Koch,ESG Shaqfeh, Phys. Fluids A 2, 2093(1990). ultiple crossbars required for integrated nano- tion in sequential steps and have obtained 23. We have shown that the Nws with well-defined an evices. Although electric fields enable more preliminary results suggesting that this ap- controllable lengths can be prepared using gold nano control over assembly, this method is also lim- proach can be extended to lD nanostructures, ited by O electrostatic interference between such as carbon NTs(28), We believe that 24. Y Hua ng x uan. C M, Lieber, unpublished data nearby electrodes as separations are scaled be- flow assembly represents a general strategy 25. C De Rosa et al, Macromolecules 33, 4871(2000) nent of extensive lithography to fabricate the blocks into structures needed for wiring, in- z7. heCtic felds can be used to align suspensions of emiconductor Nws into parallel Nw arrays ar structures. Our fluidic approach is intrinsically could enable a bottom-up manufacturing par- very parallel and scalable and, moreover, it adigm for future nanotechnologies. trode arrays are used to create a field pa allows for the directed assembly of geometri- tial complications in the assembly of multiple cross References and notes at the submicrometer scale the angles between flow directions in sequential 1.J Hu, T w. Odom, C M Lieber, Acc. Chem. Res 32, 28. Additional studies show that suspensions of single- assembly steps. For example, an equilateral tri- 2. C. Dekker, Phys. Today 52(no 5), 22( 1999) gle(Fig. 4C)was assembled in a three- layer 3. J. R Heath et al. Science 280, 1716(1998) kin, inorg. Chem. 39 deposition sequence with 60 angles between 5. C.B. Murray. C.R. Kagan, M.G.Bawendi, Science 270, acknowledges support of this wor the three flow directions. The method of flow by the Office of Naval Research and Defense vanced Projects Research Agency alignment thus provides a flexible way to meet 6. C P Collier et al, Annu. Rev. Phys. Chem. 49, 371 October 2000: accepted 13 December 2000 tions, including those requiring assembly of multiple"layers"of NWs. An important feature of this layer-by-layer Fast Drop Movements Resulting dent of the others, and thus a variety of homo- and heterojunction configurations can be ob- from the Phase Change on ained at each crossed point by simply changing the composition of the NW suspension used fo Gradient Surface each step. For example, it should be possible to Susan DanieL, Manoj John C che dividual nanoscale devices using our approach with n-type and p-type NWs(16, 19)and NTs, The movement of liquid drops on a surface with a radial surface tension gradient in which the nws and nts act as both the is described here. When saturated steam passes over a colder hydrophobic wiring and active device elements(15). A typ substrate, numerous water droplets nucleate and grow by coalesc ical 2 by 2 crossbar array made of n-type InP surrounding drops. The merging droplets exhibit two-dimensional random mo NWs. in which all eight ends of the Nws tion somewhat like the Brownian movements of colloidal particles. when a urface tension gradient is designed into the substrate surface, the random this point(Fig. 4D). Transport measurements movements of droplets are biased toward the more wettable side of the surface ( Fig. 4E) show that current can flow through Powered by the energies of coalescence and collimated by the forces of the any two of the eight ends and enable the elec- chemical gradient, small drops(0.1 to 0. 3 millimeter) display speeds that are trical characteristics of individual Nws and the hundreds to thousands of times faster than those of typical Marangoni flows NW-NW junctions to be assessed. The current- This effect has implications for passively enhancing heat transfer in heat voltage(1-n data recorded for each of the four changers and heat pipes. cross points exhibit linear or nearly linear be- havior(red curves) and are consistent with ex- The movements of liquids resulting fror analysis devices(2). Although the usual Ma- pectations for n-n type junctions. Because sin- anced surface tension forces constitute rangoni motions are triggered by variations in e Nw-NW p-n junctions formed by random portant surface phenomenon, known as temperature or composition on a liquid surface, deposition exhibit behavior characteristic of rangoni effect (). When regulated properly, a surface tension heterogeneity(5-7)on a solid light-emitting diodes (LEDs)(16), we believe these types of flows are of value in several substrate can also induce such motion. The typ- that our approach could be used to assemble industrial applications, such as the design and ical speeds of these flows(speeds ranging from high-density and individually addressable nano- operation of microfluidic and integrated dNa micrometers to millimeters per second)or encemag org SCIENCE VOL 291 26 JANUARY 2001 633cross points in a very straightforward, low￾cost, fast, and scalable process. Although the separations between individual NWs are not completely uniform, a periodic array can be easily envisioned with a patterned surface as described above. These crossbar structures can yield functional devices (see below). We believe that our approach for directed assembly of multiple crossed NW arrays offers substantial advantages over current efforts, which have used random deposition (14, 16), direct manipulation of individual NWs and NTs (15), and electric fields (12, 16, 27) to make single crossed structures. With random deposi￾tion and manipulation, it is difficult to obtain multiple crossbars required for integrated nano￾devices. Although electric fields enable more control over assembly, this method is also lim￾ited by (i) electrostatic interference between nearby electrodes as separations are scaled be￾low the micrometer level and (ii) the require￾ment of extensive lithography to fabricate the electrodes for assembly of multiple NW device structures. Our fluidic approach is intrinsically very parallel and scalable and, moreover, it allows for the directed assembly of geometri￾cally complex structures by simply controlling the angles between flow directions in sequential assembly steps. For example, an equilateral tri￾angle (Fig. 4C) was assembled in a three-layer deposition sequence with 60° angles between the three flow directions. The method of flow alignment thus provides a flexible way to meet the requirements of many device configura￾tions, including those requiring assembly of multiple “layers” of NWs. An important feature of this layer-by-layer assembly scheme is that each layer is indepen￾dent of the others, and thus a variety of homo￾and heterojunction configurations can be ob￾tained at each crossed point by simply changing the composition of the NW suspension used for each step. For example, it should be possible to directly assemble and subsequently address in￾dividual nanoscale devices using our approach with n-type and p-type NWs (16, 19) and NTs, in which the NWs and NTs act as both the wiring and active device elements (15). A typ￾ical 2 by 2 crossbar array made of n-type InP NWs, in which all eight ends of the NWs are connected by metal electrodes, demonstrates this point (Fig. 4D). Transport measurements (Fig. 4E) show that current can flow through any two of the eight ends and enable the elec￾trical characteristics of individual NWs and the NW-NW junctions to be assessed. The current￾voltage (I-V) data recorded for each of the four cross points exhibit linear or nearly linear be￾havior (red curves) and are consistent with ex￾pectations for n-n type junctions. Because sin￾gle NW-NW p-n junctions formed by random deposition exhibit behavior characteristic of light-emitting diodes (LEDs) (16), we believe that our approach could be used to assemble high-density and individually addressable nano￾LEDs and electronically more complex nano￾devices. These studies provide a general and ratio￾nal approach for hierarchical assembly of 1D nanomaterials into well-defined functional networks that can bridge the nanometer through millimeter size regimes. We have shown that NWs can be assembled into par￾allel arrays with control of the average sepa￾ration and, by combining fluidic alignment with surface-patterning techniques, that it is also possible to control periodicity. In addi￾tion, we have demonstrated the possibility of layer-by-layer assembly of crossed and more complex structures by varying the flow direc￾tion in sequential steps and have obtained preliminary results suggesting that this ap￾proach can be extended to 1D nanostructures, such as carbon NTs (28). We believe that flow assembly represents a general strategy for organization of NW and NT building blocks into structures needed for wiring, in￾terconnects, and functional devices and thus could enable a bottom-up manufacturing par￾adigm for future nanotechnologies. References and Notes 1. J. Hu, T. W. Odom, C. M. Lieber, Acc. Chem. Res. 32, 435 (1999). 2. C. Dekker, Phys. Today 52 (no. 5), 22 (1999). 3. J. R. Heath et al., Science 280, 1716 (1998). 4. C. A. Mirkin, Inorg. Chem. 39, 2258 (2000). 5. C. B. Murray, C. R. Kagan, M. G. Bawendi, Science 270, 1335 (1995). 6. C. P. Collier et al., Annu. Rev. Phys. Chem. 49, 371 (1998). 7. A. K. Boal et al., Nature 404, 746 (2000). 8. R. C. Hayward, D. A. Sayille, I. A. Aksay, Nature 404, 56 (2000). 9. M. Li, H. Schnablegger, S. Mann, Nature 402, 393 (1999). 10. J. Liu et al., Chem. Phys. Lett. 303, 125 (1999). 11. M. Burghard et al., Adv. Mater. 10, 584 (1998). 12. P. A. Smith et al., Appl. Phys. Lett. 77, 1399 (2000). 13. S. J. Tan et al., Nature 393, 49 (1998). 14. M. S. Fuher et al., Science 288, 494 (2000). 15. T. Rueckes et al., Science 289, 94 (2000). 16. X. Duan et al., Nature 409, 66 (2001). 17. S. Noda et al., Science 289, 604 (2000). 18. X. Duan, C. M. Lieber, Adv. Mater. 12, 298 (2000). 19. Y. Cui, X. Duan, J. Hu, C. M. Lieber, J. Phys. Chem. B 104, 5213 (2000). 20. D. C. Duffy et al., Anal. Chem. 70, 4974 (1998). 21. C. A. Stover, D. L. Koch, C. Cohen, J. Fluid Mech. 238, 277 (1992). 22. D. L. Koch, E. S. G. Shaqfeh, Phys. Fluids A 2, 2093 (1990). 23. We have shown that the NWs with well-defined and controllable lengths can be prepared using gold nano￾cluster catalysts (M. S. Gudiksen, J. Wang, C. M. Lieber, in preparation). 24. Y. Huang, X. Duan, C. M. Lieber, unpublished data. 25. C. De Rosa et al., Macromolecules 33, 4871 (2000). 26. M. Gleiche, L. F. Chi, H. Fuchs, Nature 403, 173 (2000). 27. Electric fields can be used to align suspensions of semiconductor NWs into parallel NW arrays and single NW crosses (16), where patterned microelec￾trode arrays are used to create a field pattern. Fring￾ing fields and charging can, however, lead to substan￾tial complications in the assembly of multiple crosses at the submicrometer scale. 28. Additional studies show that suspensions of single￾walled carbon nanotubes and duplex DNA can be aligned in parallel arrays with the fluidic approach. 29. We thank H. Stone, J. Ng, and T. Rueckes for helpful discussions. C.M.L. acknowledges support of this work by the Office of Naval Research and Defense Ad￾vanced Projects Research Agency. 10 October 2000; accepted 13 December 2000 Fast Drop Movements Resulting from the Phase Change on a Gradient Surface Susan Daniel, Manoj K. Chaudhury,* John C. Chen The movement of liquid drops on a surface with a radial surface tension gradient is described here. When saturated steam passes over a colder hydrophobic substrate, numerous water droplets nucleate and grow by coalescence with the surrounding drops. The merging droplets exhibit two-dimensional random mo￾tion somewhat like the Brownian movements of colloidal particles. When a surface tension gradient is designed into the substrate surface, the random movements of droplets are biased toward the more wettable side of the surface. Powered by the energies of coalescence and collimated by the forces of the chemical gradient, small drops (0.1 to 0.3 millimeter) display speeds that are hundreds to thousands of times faster than those of typical Marangoni flows. This effect has implications for passively enhancing heat transfer in heat ex￾changers and heat pipes. The movements of liquids resulting from unbal￾anced surface tension forces constitute an im￾portant surface phenomenon, known as the Ma￾rangoni effect (1). When regulated properly, these types of flows are of value in several industrial applications, such as the design and operation of microfluidic and integrated DNA analysis devices (2–4). Although the usual Ma￾rangoni motions are triggered by variations in temperature or composition on a liquid surface, a surface tension heterogeneity (5–7) on a solid substrate can also induce such motion. The typ￾ical speeds of these flows (speeds ranging from micrometers to millimeters per second) on a R EPORTS www.sciencemag.org SCIENCE VOL 291 26 JANUARY 2001 633