REPORTS structures, such as NWs and carbon nano- alignment and average separation of the direction (inset, Fig. 2C). Our observed bes (nts), has met with much less success NWs. First, we find that the degree of align- results can be explained within the frame- (10-12), although these materials offer great ment can be controlled by the flow rate. With work of shear flow(21, 22). Specifically, the potential as building blocks for applications increasing flow rates, the width of the Nw channel flow near the substrate surface re- in nanoelectronics (1-3, 13-16)and photon- angular distribution with respect to the flow sembles a shear flow and aligns the Nws in direction(e.g, inset in Fig. 2C) substantially the flow direction before they are immobi- To achieve the substantial potential of narrows. Comparison of the distribution lized on the substrate. Higher flow rates pro- NWs and NTs in these and other areas of widths measured over a range of conditions duce larger shear forces and hence lead to nanotechnology will require the controlled shows that the width decreases quickly from better alignment. and predictable assembly of well-ordered our lowest flow rate, 4 mm/s, and ap. In addition, the average Nw surface cov- structures. We report an approach for the proaches a nearly constant value at 10 erage can be controlled by the flow duration hierarchical assembly of ID nanostructures mm/s(Fig. 2C). At the highest flow rates(Fig 2D). Experiments carried out at a con- whereby NWs are aligned in fluid flows with examined in our studies, more than 80% of stant flow rate show that the Nw density the separation and spatial location readily the NWs are aligned within +5 of the flow increases systematically with flow duration. ontrolled. Crossed Nw arrays were also pre. pared with layer-by-layer assembly with dif- Fig. 1. Schematic of fluidic A ferent flow directions for sequential steps. nnel structures for flow as- o-s=Q Transport studies show that the crossed Nw sembly. (A)A channel formed arrays form electrically conducting networks, when the PDMS mold was PDMS mold with individually addressable device function substrate. Nw assembly was at each NW-NW cross point. This approach carried out by flowing an Nw ubstrate can be potentially used for organizing other care 9 olled flow rate for a de the channel B arallel array ID nanostructures into highly integrated de- with vice arrays and thus offers a general pathway set duration. Parallel arrays of for bottom-up assembly of new electronic NWs are observed in the flow and photonic nanosystems. direction on the substrate The gallium phosphide (GaP), indium when the PDMS mold is re- phosphide(InP), and silicon(Si)NWs used Nw arrays can be obtained in these studies were synthesized by laser- by changing the flow direc- ssisted catalytic growth(1, 18, 19)and sub- tion sec tially in a laye first laver crossed array quently suspended in ethanol solution. In by-layer assembly process general, we assembled arrays of NWs by passing suspensions of the NWs through flu idic channel structures formed between a B poly(dimethylsiloxane)(PDMS) mold(20) and a flat substrate(Fig. 1). Parallel and crossed arrays of NWs can be readily achieved with single(Fig. 1A)and sequentia crossed(Fig. 1B)flows, respectively, for the assembly process as described below. a typical example of parallel assembly of NWs(Fig. 2A) shows that virtually all the NWs are aligned along one direction, i.e., the flow direction. There are also some small 16Fc deviations with respect to the flow direction, 250}D which we will discuss below Examination of 三200 the assembled (Fig. 2B) shows that the alignment readily ≥150 10 extends over hundreds of micrometers. In d, alignment of the Nws has been found 目 ▲Ange(deg) to extend up to millimeter length scales and seems to be limited by the size of the fluidic channels, on the basis of experiments carried out with channels with widths ranging from Flow rate(mm/s) 010203040 50 to 500 um and lengths from 6 to 20 mm. We carried out several types of experi- Fig. 2. Parallel assembly of NW arrays (A and B)SEM images of parallel arrays of InP NWs aligned ments to understand factors controlling the by channel fiow. The scale bars correspond to z m and 5o tim in (A and (B), respectively. The bled monolayer(SAM) by immersion in a 1 mM chloro triethoxysilane(APTES)for 30 min, followed by heating at 110 C for 10 min(10). All of the substrates used in the following experiment were functionalized in a similar way unless otherwise pecified. (C)NW angular spread with respect to the flow direction versus flow rate. Each data hould be addressed. E. The average density of NWs versus flow time. The average density was calculated by dividing the qually to this work. average number of NWs at any cross section of the channel by the width of the channeL All of the mail cml@cmliris. harvard. edu periments were carried out with a flow rate of 6.40 mm/s www.sciencemagorgSciEnceVol29126jAnuAry2001 631structures, such as NWs and carbon nano￾tubes (NTs), has met with much less success (10–12), although these materials offer great potential as building blocks for applications in nanoelectronics (1–3, 13–16) and photon￾ics (17). To achieve the substantial potential of NWs and NTs in these and other areas of nanotechnology will require the controlled and predictable assembly of well-ordered structures. We report an approach for the hierarchical assembly of 1D nanostructures whereby NWs are aligned in fluid flows with the separation and spatial location readily controlled. Crossed NW arrays were also pre￾pared with layer-by-layer assembly with dif￾ferent flow directions for sequential steps. Transport studies show that the crossed NW arrays form electrically conducting networks, with individually addressable device function at each NW-NW cross point. This approach can be potentially used for organizing other 1D nanostructures into highly integrated de￾vice arrays and thus offers a general pathway for bottom-up assembly of new electronic and photonic nanosystems. The gallium phosphide (GaP), indium phosphide (InP), and silicon (Si) NWs used in these studies were synthesized by laser￾assisted catalytic growth (1, 18, 19) and sub￾sequently suspended in ethanol solution. In general, we assembled arrays of NWs by passing suspensions of the NWs through flu￾idic channel structures formed between a poly(dimethylsiloxane) (PDMS) mold (20) and a flat substrate (Fig. 1). Parallel and crossed arrays of NWs can be readily achieved with single (Fig. 1A) and sequential crossed (Fig. 1B) flows, respectively, for the assembly process as described below. A typical example of parallel assembly of NWs (Fig. 2A) shows that virtually all the NWs are aligned along one direction, i.e., the flow direction. There are also some small deviations with respect to the flow direction, which we will discuss below. Examination of the assembled NWs on larger length scales (Fig. 2B) shows that the alignment readily extends over hundreds of micrometers. In￾deed, alignment of the NWs has been found to extend up to millimeter length scales and seems to be limited by the size of the fluidic channels, on the basis of experiments carried out with channels with widths ranging from 50 to 500 mm and lengths from 6 to 20 mm. We carried out several types of experi￾ments to understand factors controlling the alignment and average separation of the NWs. First, we find that the degree of align￾ment can be controlled by the flow rate. With increasing flow rates, the width of the NW angular distribution with respect to the flow direction (e.g., inset in Fig. 2C) substantially narrows. Comparison of the distribution widths measured over a range of conditions shows that the width decreases quickly from our lowest flow rate, ;4 mm/s, and ap￾proaches a nearly constant value at ;10 mm/s (Fig. 2C). At the highest flow rates examined in our studies, more than 80% of the NWs are aligned within 65° of the flow direction (inset, Fig. 2C). Our observed results can be explained within the frame￾work of shear flow (21, 22). Specifically, the channel flow near the substrate surface re￾sembles a shear flow and aligns the NWs in the flow direction before they are immobi￾lized on the substrate. Higher flow rates pro￾duce larger shear forces and hence lead to better alignment. In addition, the average NW surface cov￾erage can be controlled by the flow duration (Fig. 2D). Experiments carried out at a con￾stant flow rate show that the NW density increases systematically with flow duration. 1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA. 2 Di￾vision of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA. *These authors contributed equally to this work. †To whom correspondence should be addressed. E￾mail: cml@cmliris.harvard.edu Fig. 1. Schematic of fluidic channel structures for flow as￾sembly. (A) A channel formed when the PDMS mold was brought in contact with a flat substrate. NW assembly was carried out by flowing an NW suspension inside the channel with a controlled flow rate for a set duration. Parallel arrays of NWs are observed in the flow direction on the substrate when the PDMS mold is re￾moved. (B) Multiple crossed NW arrays can be obtained by changing the flow direc￾tion sequentially in a layer￾by-layer assembly process. Fig. 2. Parallel assembly of NW arrays. (A and B) SEM images of parallel arrays of InP NWs aligned by channel flow. The scale bars correspond to 2 mm and 50 mm in (A) and (B), respectively. The silicon (SiO2/Si) substrate used in flow assembly was functionalized with an NH2-terminated self-assembled monolayer (SAM) by immersion in a 1 mM chloroform solution of 3-aminopropyl￾triethoxysilane (APTES) for 30 min, followed by heating at 110°C for 10 min (10). All of the substrates used in the following experiment were functionalized in a similar way unless otherwise specified. (C) NW angular spread with respect to the flow direction versus flow rate. Each data point in the figure was obtained by statistical analysis of angular distribution of ;200 NWs (e.g., see inset). The inset shows a histogram of NW angular distribution at a flow rate of 9.40 mm/s. (D) The average density of NWs versus flow time. The average density was calculated by dividing the average number of NWs at any cross section of the channel by the width of the channel. All of the experiments were carried out with a flow rate of 6.40 mm/s. R EPORTS www.sciencemag.org SCIENCE VOL 291 26 JANUARY 2001 631