REP。RTs In these experiments, a flow duration of 30 order of 100 nm or less. We note that the rapidly on NH, terminated monolayers. min produced a density of about 250 Nws deposition rate and hence average separation which have a partial positive charge, than on per 100 um or an average NW-NW separa- versus time depend strongly on the surface either methy l-terminated monolayers or bare tion of -400 nm. Extended deposition time chemical functionality. Specifically, we show SiO, surfaces. It is also important to red can produce NW arrays with spacings on the that the GaP, InP, and Si NWs deposit more nize that the minimum separation of align.i NWs that can be achieved without NW-Nw Assem bly of A contacts will depend on the lengths of the array NWs used in the assembly process. Recent (A)Schematic view of progress demonstrating control of Nw lengths from the 100-nm to tens-of-microme nto a chemically pat- ter scale (23)should increase the terned substrate. The accessible spacings without contact ight gray areas corre- Our results demonstrate ordering of Nw SiO, /Si substrate structure over multiple length scalesorga as the dark gray areas nization of nanometer diameter wires with correspond to either 100-nm to micrometer-scale separations over ethyl-terminated or C millimeter-scale areas. This hierarchical or NWs der can readily bridge the microscopic and terminated 2 macroscopic worlds, although to enable as- sembly with greatest control requires that the atial position also be defined. We achieved Parallel arrays of GaP this important goal by using complementary NWs aligned on poly chemical interactions between chemically patterned substrates and NWs(Fig. 3A) Scanning electron microscopy(SEM) images paration. The dark regions in the of representative experiments( Fig 3, B to D) rrespond to the NH -terminated Sio, /Si surface. The NWs are preferentially attracted to NH ated regions. The PMMA was patterned with standard electron beam(E-beam)lithography, and the same as those of the surface patterns. the resulting SiOz surface was functionalized by immersion in a solution of 0.5% APTES in ethanol for These data demonstrate that the Nws are 10 min, followed by 10 min at 100 C. The scale bars correspond to 5 um and 2 um in(B)and (C). preferentially assembled at positions de pectively.(D)Parallel arrays of GaP NWs with 500-nm separation obtained with a patterned SAM by the chemical patten and, moreover, surface. The sio / Si surface was first functionalized with methy -terminate 0c immersion in pure that the periodic patterns can organize the by functionalization with APTEs y to form an array of parallel features with a 500-nm period, followed NWs into a regular superstructure. It is im termed by e-beam lithe ) The sc ant to recognize that the patterned surface alone does not provide good control of the ID nanostructure organization. Assembly of NTs Fig. 4, Layer-by-layer A (10, I1) and NWs(24) on patterned sub- port measurements of strates shows 1D nanostructures aligned with crossed NW arrays (A bridging, and looping around patterned areas and B)Typical SEM with little directional control. Our use of fluid flows avoids these substantial problems and ays of InP NWs ob- nables controlled assembly in one or more assembly process with directions. By combining this approach with orthogonal flow direc ng method nanoscale domain formation in diblock c tial steps Flow direc-B a polymers(25) and spontaneous ordering of tions are highlighted molecules(26 ), it should by erate well-ordered Nw arrays beyond the ingle of GaP NWs limitations of conventional lithography obtained in a three- Our general approach can be used to or ganize NWs into with60° angles be structures. which are critical for building dense nanodevice arrays, with the use of the which are indicated by layer-by-layer scheme illustrated in Fig. 1B abered arrows The scale bars correspond to 500 nm in E (A),(B), and(C). (D)SEM image of a typical 2 by 2 cross The formation of crossed and more complex ial assembly of n-typ NWs with uctures requires that the nanostructure-sub- rthogonal flows. Ni/in/Au contact electrodes, which we strate interaction is sufficiently strong that posited by thermal evaporation, were patterned by E sequential flow steps do not affect preceding beam lithography. The NWs were briefly(3 to 5 s)etc ones: We find that this condition can be in oxid achieved. For example, alternating the flow layer before electrode deposition. The scale bar corre- sponds to 2 um.(E)Representative l-V curves from two- O in orthogonal directions in a two-step assem- rminal measurements on a 2 by 2 crossed array. the bly process yields crossbar structures(Fig. 4, reen curves represent the i-V of four individual NWs(ad -0.8. 4 0.0 0.4 0.8 A and B). Both figures show that multiple bg, cf, eh), and the red curves represent /-V across the four Voltage(V) crossbars can be obtained with only hundreds n-n crossed junctions(ab, cd, ef, gh) of nanometer separations between individual 632 26JanUary2001Vol291ScieNcewww.sciencemag.orgIn these experiments, a flow duration of 30 min produced a density of about 250 NWs per 100 mm or an average NW-NW separa￾tion of ;400 nm. Extended deposition time can produce NW arrays with spacings on the order of 100 nm or less. We note that the deposition rate and hence average separation versus time depend strongly on the surface chemical functionality. Specifically, we show that the GaP, InP, and Si NWs deposit more rapidly on NH2-terminated monolayers, which have a partial positive charge, than on either methyl-terminated monolayers or bare SiO2 surfaces. It is also important to recog￾nize that the minimum separation of aligned NWs that can be achieved without NW-NW contacts will depend on the lengths of the NWs used in the assembly process. Recent progress demonstrating control of NW lengths from the 100-nm to tens-of-microme￾ter scale (23) should increase the range of accessible spacings without contact. Our results demonstrate ordering of NW structure over multiple length scales—orga￾nization of nanometer diameter wires with 100-nm to micrometer-scale separations over millimeter-scale areas. This hierarchical or￾der can readily bridge the microscopic and macroscopic worlds, although to enable as￾sembly with greatest control requires that the spatial position also be defined. We achieved this important goal by using complementary chemical interactions between chemically patterned substrates and NWs (Fig. 3A). Scanning electron microscopy (SEM) images of representative experiments (Fig. 3, B to D) show parallel NW arrays with lateral periods the same as those of the surface patterns. These data demonstrate that the NWs are preferentially assembled at positions defined by the chemical pattern and, moreover, show that the periodic patterns can organize the NWs into a regular superstructure. It is im￾portant to recognize that the patterned surface alone does not provide good control of the 1D nanostructure organization. Assembly of NTs (10, 11) and NWs (24) on patterned sub￾strates shows 1D nanostructures aligned with, bridging, and looping around patterned areas with little directional control. Our use of fluid flows avoids these substantial problems and enables controlled assembly in one or more directions. By combining this approach with other surface-patterning methods, such as nanoscale domain formation in diblock co￾polymers (25) and spontaneous ordering of molecules (26), it should be possible to gen￾erate well-ordered NW arrays beyond the limitations of conventional lithography. Our general approach can be used to or￾ganize NWs into more complex crossed structures, which are critical for building dense nanodevice arrays, with the use of the layer-by-layer scheme illustrated in Fig. 1B. The formation of crossed and more complex structures requires that the nanostructure-sub￾strate interaction is sufficiently strong that sequential flow steps do not affect preceding ones: We find that this condition can be achieved. For example, alternating the flow in orthogonal directions in a two-step assem￾bly process yields crossbar structures (Fig. 4, A and B). Both figures show that multiple crossbars can be obtained with only hundreds of nanometer separations between individual Fig. 3. Assembly of periodic NW arrays. (A) Schematic view of the assembly of NWs onto a chemically pat￾terned substrate. The light gray areas corre￾spond to NH2-termi￾nated surfaces, where￾as the dark gray areas correspond to either methyl-terminated or bare surfaces. NWs are preferentially at￾tracted to the NH2- terminated regions of the surface. (B and C) Parallel arrays of GaP NWs aligned on poly- (methyl methacrylate) (PMMA) patterned sur￾face with 5- and 2-mm separation. The dark regions in the image correspond to residual PMMA, whereas the bright regions correspond to the NH2-terminated SiO2/Si surface. The NWs are preferentially attracted to NH2- terminated regions. The PMMA was patterned with standard electron beam (E-beam) lithography, and the resulting SiO2 surface was functionalized by immersion in a solution of 0.5% APTES in ethanol for 10 min, followed by 10 min at 100°C. The scale bars correspond to 5 mm and 2 mm in (B) and (C), respectively. (D) Parallel arrays of GaP NWs with 500-nm separation obtained with a patterned SAM surface. The SiO2/Si surface was first functionalized with methyl-terminated SAM by immersion in pure hexamethyldisilazane (HMDS) for 15 min at 50°C, followed by 10 min at 110°C. This surface was patterned by E-beam lithography to form an array of parallel features with a 500-nm period, followed by functionalization with APTES (10). The scale bar corresponds to 500 nm. Fig. 4. Layer-by-layer assembly and trans￾port measurements of crossed NW arrays. (A and B) Typical SEM images of crossed ar￾rays of InP NWs ob￾tained in a two-step assembly process with orthogonal flow direc￾tions for the sequen￾tial steps. Flow direc￾tions are highlighted by arrows in the imag￾es. (C) An equilateral triangle of GaP NWs obtained in a three￾step assembly process, with 60° angles be￾tween flow directions, which are indicated by numbered arrows. The scale bars correspond to 500 nm in (A), (B), and (C). (D) SEM image of a typical 2 by 2 cross array made by sequential assembly of n-type InP NWs with orthogonal flows. Ni/In/Au contact electrodes, which were deposited by thermal evaporation, were patterned by E￾beam lithography. The NWs were briefly (3 to 5 s) etched in 6% HF solution to remove the amorphous oxide outer layer before electrode deposition. The scale bar corre￾sponds to 2 mm. (E) Representative I-V curves from two￾terminal measurements on a 2 by 2 crossed array. The green curves represent the I-V of four individual NWs (ad, bg, cf, eh), and the red curves represent I-V across the four n-n crossed junctions (ab, cd, ef, gh). R EPORTS 632 26 JANUARY 2001 VOL 291 SCIENCE www.sciencemag.org