ABTGLES Large-area blown bubble films of aligned nanowires and carbon nanotubes GUIHUA YU* ANYUAN CAO2* AND CHARLES M. LIEBER,3t Department of Chemistry and Chemical Biology, Harvard University Cambridge, Massachusetts 02138, USA department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii 96822, US Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA "These authors contributed equally to this work. e-mail: anyuan @hawaii. edu; cml@cmliris harvard edu Published online: 27 May 2007; doi: 10. 1038/nano. 2007. 150 Many of the applications proposed for nanowires and carbon nanotubes require these components to be organized over large areas with controlled orientation and density. Although progress has been made with directed assembly and Langmuir- Blodgett approaches, it is unclear whether these techniques can be scaled to large wafers and non-rigid substrates. Here, we describe a general and scalable approach for large-area, uniformly aligned and controlled-density nanowire and nanotube films, which involves expanding a bubble from a homogeneous suspension of these materials. The blown-bubble films were transferred to single-crystal wafers of at least 200 mm in diameter, flexible plastics sheets of dimensions of at least 225 X 300 mm and highly curved surfaces, and were also suspended across open frames. In addition, electrical neasurements show that large arrays of nanowire field-effect transistors can be efficiently fabricated on the wafer scale the potential of blown film extrusion to produce continuous films with widths exceeding 1 m, we believe that our approac allow the unique properties of nanowires and nanotubes to be exploited in applications requiring large areas and modest device densities Semiconductor nanowires (NWs)and carbon nanotubes(NTs) BLOWN BUBBLE FILMS exhibit physical properties that make them attractive building blocks for many electronic and optical applications-. To realize To characterize key features of this method, we first focused on such applications, researchers have directed considerable effort to silicon Nw blown-bubble films(BBFs), because Si NWs can be the development of methods of assembly that might ultimately produced in high yield with uniform diameters and electronic lead to integrated systems. For example, there have been studies properties& 12. Si NWs were covalently modified using of individual or small numbers of NW and n devices prepared 5,6-epoxyhexyltriethoxysilane, and then combined with an epoxy y random deposition, electric field directed assembly, flow- solution to yield stable and well-dispersed suspensions from assisted alignment, and selective chemical and biological 0.01-0 22 wt%(see Methods). Once the Si Nw-epoxy patterning,, and up to centimetre-scale assembly of NWs using suspension viscosity increased to 15-25 Pa s during the Langmuir-Blodgett technique. However, it is still unclear polymerization, 0.5-1 g of the suspension was transferred whether these methods can be extended to niformly onto the top surface of the die, and blown into a of NWs and NTs on both rigid and flexible substrates with le bubble using a nitrogen flow (P= 150-200 kPa)that controlled alignment and densi directed the expansion vertically at a rate of 10-15 cm min -I Blown film extrusion is a well-developed process for the(Fig. 1b, c). Stable vertical expansion was facilitated by an upward manufacture of plastic films in large quantities, which involves moving ring that kept the bubble centred over the die. We extruding a molten polymer and inflating it to obtain a balloon, routinely produce bubbles with diameters greater than 25 cm and which can be collapsed and slit to form continuous flat films heights greater than 50 cm using this semi-automated with widths exceeding I m at rates in the order of 500 kg h(see Methods). Larger dies and greater control of the expa (refs. 9-11). We have applied this basic idea for the first time process should enable much larger diameter bubbles to the formation of nanocomposite films where the density produced, in analogy to the bubbles of 1-2 m diameter and orientation of the NWs and NTs are controlled within produced during automated processing of homogeneous the film. The basic steps in our approach(Fig. la) consist of polymers (i) preparation of a homogeneous, stable and controlled he BBFs could be transferred to both rigid and flexible concentration polymer suspension of NWs or NTs, (i)expansion rates during the expansion process. For example, of the polymer suspension using a circular die to form a bubble 150-mm silicon wafers were fixed in positions close to the central at controlled pressure, P, and expansion rate, where stable vertical axis of the die/ bubble(Fig. 1b), and the bubble expansion was expansion is achieved using an external vertical force, F and then continued until it covered the entire wafer surfaces( Fig. Ic) (iii)transfer of the bubble film to substrates or open frame structures. Optical inspection of the Si Nw-BBF transferred to a 150-mn ologyVol2june2007www.nature.com/naturenanotechnology @2007 Nature Publishing Group
Black plate (372,1) Large-area blown bubble films of aligned nanowires and carbon nanotubes GUIHUA YU1 *, ANYUAN CAO2 *† AND CHARLES M. LIEBER1,3† 1Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA 2Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA 3Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA *These authors contributed equally to this work. † e-mail: anyuan@hawaii.edu; cml@cmliris.harvard.edu Published online: 27 May 2007; doi:10.1038/nnano.2007.150 Many of the applications proposed for nanowires and carbon nanotubes require these components to be organized over large areas with controlled orientation and density. Although progress has been made with directed assembly and Langmuir– Blodgett approaches, it is unclear whether these techniques can be scaled to large wafers and non-rigid substrates. Here, we describe a general and scalable approach for large-area, uniformly aligned and controlled-density nanowire and nanotube films, which involves expanding a bubble from a homogeneous suspension of these materials. The blown-bubble films were transferred to single-crystal wafers of at least 200 mm in diameter, flexible plastics sheets of dimensions of at least 225 3 300 mm2 and highly curved surfaces, and were also suspended across open frames. In addition, electrical measurements show that large arrays of nanowire field-effect transistors can be efficiently fabricated on the wafer scale. Given the potential of blown film extrusion to produce continuous films with widths exceeding 1 m, we believe that our approach could allow the unique properties of nanowires and nanotubes to be exploited in applications requiring large areas and relatively modest device densities. Semiconductor nanowires (NWs) and carbon nanotubes (NTs) exhibit physical properties that make them attractive building blocks for many electronic and optical applications1–3. To realize such applications, researchers have directed considerable effort to the development of methods of assembly that might ultimately lead to integrated systems. For example, there have been studies of individual or small numbers of NW and NT devices prepared by random deposition, electric field directed assembly4 , flowassisted alignment5 , and selective chemical and biological patterning6,7, and up to centimetre-scale assembly of NWs using the Langmuir–Blodgett technique8 . However, it is still unclear whether these methods can be extended to large-scale assembly of NWs and NTs on both rigid and flexible substrates with controlled alignment and density. Blown film extrusion is a well-developed process for the manufacture of plastic films in large quantities, which involves extruding a molten polymer and inflating it to obtain a balloon, which can be collapsed and slit to form continuous flat films with widths exceeding 1 m at rates in the order of 500 kg h21 (refs. 9–11). We have applied this basic idea for the first time to the formation of nanocomposite films where the density and orientation of the NWs and NTs are controlled within the film. The basic steps in our approach (Fig. 1a) consist of (i) preparation of a homogeneous, stable and controlled concentration polymer suspension of NWs or NTs, (ii) expansion of the polymer suspension using a circular die to form a bubble at controlled pressure, P, and expansion rate, where stable vertical expansion is achieved using an external vertical force, F, and (iii) transfer of the bubble film to substrates or open frame structures. BLOWN BUBBLE FILMS To characterize key features of this method, we first focused on silicon NW blown-bubble films (BBFs), because Si NWs can be produced in high yield with uniform diameters and electronic properties8,12. Si NWs were covalently modified using 5,6-epoxyhexyltriethoxysilane, and then combined with an epoxy solution to yield stable and well-dispersed suspensions from 0.01–0.22 wt% (see Methods). Once the Si NW–epoxy suspension viscosity increased to 15–25 Pa s during polymerization, 0.5–1 g of the suspension was transferred uniformly onto the top surface of the die, and blown into a single bubble using a nitrogen flow (P ¼ 150–200 kPa) that directed the expansion vertically at a rate of 10–15 cm min21 (Fig. 1b,c). Stable vertical expansion was facilitated by an upward moving ring that kept the bubble centred over the die. We routinely produce bubbles with diameters greater than 25 cm and heights greater than 50 cm using this semi-automated process (see Methods). Larger dies and greater control of the expansion process should enable much larger diameter bubbles to be produced, in analogy to the bubbles of 1–2 m diameter produced during automated processing of homogeneous polymers11. The BBFs could be transferred to both rigid and flexible substrates during the expansion process. For example, two 150-mm silicon wafers were fixed in positions close to the central axis of the die/bubble (Fig. 1b), and the bubble expansion was then continued until it covered the entire wafer surfaces (Fig. 1c). Optical inspection of the Si NW-BBF transferred to a 150-mm ARTICLES 372 nature nanotechnology | VOL 2 | JUNE 2007 | www.nature.com/naturenanotechnology
ARTCLES diameter wafer(Fig. Id)showed that the transferred film is a uniform over the entire substrate, and more generally, our studies show that 80-90% of the transferred films are defect free although we observe small defects in some samples due to trapped gas during film transfer. Higher magnification dark-field ical images(insets, Fig. ld), which resolve individual Si NWs within the transferred bbf show that the si nws recorded from different areas of this large substrate are well aligned along the upward expansion direction of the bubble. Indeed, the angular deviation of the si nws is less than 10 over the entire 150-mm- diameter wafer and represents a very substantial advance over BBEs with different NW densities prepared from 0.01-0.22 wt% Excellent orientational alignment of Si NWs was observed for Si NW-epoxy suspensions(Fig. 2a-d). The alignment and relatively uniform on reproducibly observed in our experiments are still not fully understood Qualitatively, the shear stress associated with the suspension passing through the circumferential edge of the die could align the high aspect ratio nanowires in a polymer fluid along the principal direction of strain. This explanation is consistent with previous observations of shear-induced alignment of rod-like micro/nanostructures in fluid systems. 3. 4. As the bubble expands primarily in the vertical direction, with a continuous supply of NW suspension from the top surface of the die, the orientation of the Nws direction, which is consistent with optical images. Expansion d along a defined direction, as achieved in our approach, is crucial to obtain consistent alignment of NWs over large areas, and enables the overall orientation to be fixed in an absolute sense independent of high resolution imaging. There is a clear decrease in Si NW separation(centre-to-centre acing)and increase in density in the transferred BBFs as the starting concentration increases from 0.01-0 22 wt%(Fig. 2e) We find that the Nw separation can be varied over at least an order of magnitude from 50+8 to 3.0+0.6 um as concentration ncreases from 0.01-0 22 wt%. Correspondingly, Nw dens increases from 4.0+0.6x 10 to 4.0+0.5x 106cm-2 for these same samples. The separations/densities of Si NWs produced so far are relatively modest, but these values are still useful for some applications, such as nanoelectronic transistor arrays for biological/ chemical sensing and displays. The plot of spacing Figure 1 Blown bubble film(BBF) process. a, llustration of () a NW/NT versus wt%(Fig. 2e)shows saturation approaching a micrometre polymer suspension, (i) bubble expansion over a circular die and (il)films suggest that this may be in part attributed to observed Nw ( ), nitrogen gas at pressure Pflows through the die and expands a bubble from aggregation, although one also expects similar behaviour based the NW/NT-epoxy suspension (dark-blue colour) on the top of the die while a particles should rocess(that is, a two-dimensional layer of stable vertical force, f, is applied by means of a wire-ring connected to a on this ive a(particle concentration)-1/ functional controlled speed motor. Black straight lines represent aligned NWs/NTs o.ependence). Hence, it will be interesting in the future to better embedded in the bubble film. b, c, Photographs of directed bubble expansion optimize the surface chemistry to enable uniform higher wt% process at early and final stages, respectively. The ring visible at the top of the suspensions to be prepared. This would allow for data extending bubble moves upwards at a constant speed during expansion. In c, the BBF the separation(density) versus wt% curves, and thus test the (bubble diameter 35 cm; height 50 cm) has coated the surface of two 150-mm separation limits achievable with this approach and also develop silicon wafers. The range of the ruler behind bubble is 0-23 inches. d, Image of a firm understanding of these limits. 0. 10 wt% Si Nw-BBF transferred to a 150-mm si wafer. Insets dark-field In addition, scanning electron microscopy (SEM)and optical images showing Si NWs in the film. The arrows point to the locations transmission electron microscopy (TEM) images (see where the images were recorded. The orientation of the Si NWs corresponds to uniform thickness, typically 200-500 nm. This observed all insets variation is primarily due to differences in suspension volume transferred to the die prior to expansion and the final size of the bubble, and should be reduced by further process optimization The SEM and TEM images of transferred films (see close to the substrate. We attribute the observed Nw distribution Supplementary Information, Fig SIc,d) also show that most of to a drift of the aligned NWs in the polymer fluid to the outer the Si NWs are at the outer surface (of the bubble)and thus surface due to the pressure gradient between the inner and outer naturenanotechnologyivol2JuNe2007www.natur @2007 Nature Publishing Group
Black plate (373,1) diameter wafer (Fig. 1d) showed that the transferred film is uniform over the entire substrate, and more generally, our studies show that 80–90% of the transferred films are defect free, although we observe small defects in some samples due to trapped gas during film transfer. Higher magnification dark-field optical images (insets, Fig. 1d), which resolve individual Si NWs within the transferred BBF, show that the Si NWs recorded from different areas of this large substrate are well aligned along the upward expansion direction of the bubble. Indeed, the angular deviation of the Si NWs is less than 108 over the entire 150-mmdiameter wafer and represents a very substantial advance over previous studies4–8. Excellent orientational alignment of Si NWs was observed for BBFs with different NW densities prepared from 0.01–0.22 wt% Si NW–epoxy suspensions (Fig. 2a–d). The alignment and relatively uniform separation reproducibly observed in our experiments are still not fully understood. Qualitatively, the shear stress associated with the suspension passing through the circumferential edge of the die could align the high aspect ratio nanowires in a polymer fluid along the principal direction of strain. This explanation is consistent with previous observations of shear-induced alignment of rod-like micro/nanostructures in fluid systems5,13,14. As the bubble expands primarily in the vertical direction, with a continuous supply of NW suspension from the top surface of the die, the orientation of the NWs in the BBFs should always follow the upward (longitude) direction, which is consistent with optical images. Expansion along a defined direction, as achieved in our approach, is crucial to obtain consistent alignment of NWs over large areas, and enables the overall orientation to be fixed in an absolute sense during transfer to a substrate, independent of highresolution imaging. There is a clear decrease in Si NW separation (centre-to-centre spacing) and increase in density in the transferred BBFs as the starting concentration increases from 0.01–0.22 wt% (Fig. 2e). We find that the NW separation can be varied over at least an order of magnitude from 50+8 to 3.0+0.6 mm as concentration increases from 0.01–0.22 wt%. Correspondingly, NW density increases from 4.0+0.6 104 to 4.0+0.5 106 cm22 for these same samples. The separations/densities of Si NWs produced so far are relatively modest, but these values are still useful for some applications, such as nanoelectronic transistor arrays for biological/chemical sensing and displays. The plot of spacing versus wt% (Fig. 2e) shows saturation approaching a micrometre separation at higher Si NW concentrations. Our experiments suggest that this may be in part attributed to observed NW aggregation, although one also expects similar behaviour based on this physical process (that is, a two-dimensional layer of particles should have a (particle concentration)21/2 functional dependence). Hence, it will be interesting in the future to better optimize the surface chemistry to enable uniform higher wt% suspensions to be prepared. This would allow for data extending the separation (density) versus wt% curves, and thus test the separation limits achievable with this approach and also develop a firm understanding of these limits. In addition, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (see Supplementary Information, Fig. S1a,b) show that BBFs have a uniform thickness, typically 200–500 nm. This observed variation is primarily due to differences in suspension volume transferred to the die prior to expansion and the final size of the bubble, and should be reduced by further process optimization. The SEM and TEM images of transferred films (see Supplementary Information, Fig. S1c,d) also show that most of the Si NWs are at the outer surface (of the bubble) and thus close to the substrate. We attribute the observed NW distribution to a drift of the aligned NWs in the polymer fluid to the outer surface due to the pressure gradient between the inner and outer (i) (iii) (ii) F P Figure 1 Blown bubble film (BBF) process. a, Illustration of (i) a NW/NT polymer suspension, (ii) bubble expansion over a circular die and (iii) films transferred to crystalline wafers, plastics, curved surfaces and open frames. In (ii), nitrogen gas at pressure P flows through the die and expands a bubble from the NW/NT-epoxy suspension (dark-blue colour) on the top of the die while a stable vertical force, F, is applied by means of a wire-ring connected to a controlled speed motor. Black straight lines represent aligned NWs/NTs embedded in the bubble film. b,c, Photographs of directed bubble expansion process at early and final stages, respectively. The ring visible at the top of the bubble moves upwards at a constant speed during expansion. In c, the BBF (bubble diameter 35 cm; height 50 cm) has coated the surface of two 150-mm silicon wafers. The range of the ruler behind bubble is 0–23 inches. d, Image of a 0.10 wt% Si NW-BBF transferred to a 150-mm Si wafer. Insets, dark-field optical images showing Si NWs in the film. The arrows point to the locations where the images were recorded. The orientation of the Si NWs corresponds to the upward expansion direction. The scale bar is 10 mm, and is the same in all insets. ARTICLES nature nanotechnology |VOL 2 | JUNE 2007 | www.nature.com/naturenanotechnology 373
ABTGLES given the distinct chemical and functional properties available from Nw building blocks,I6. In addition, we modified single walled NTS (SWNTs)and multiwalled NTs (MWNTs) with n-octadecylamine and used the resulting suspensions to prepare BBFs(see Methods). The transferred SWNT- and MWNT-BBFs Fig. 3a and b)show good alignment and uniformity over the 75-mm-diameter substrates used in the experiments, but can be transferred to much larger ones. The SWNTs exhibit an averag separation of 1.5+0.4 um, and 90% are aligned to within 5 of the average orientation. The good alignment of the SWNTs within BBFs is notable in that their lengths, 1-2 approximately ten times shorter than MWNTs and Si Nw(10-15 um) materials used in our find that longer MWNTs, which are somewhat appear straightened in the BBFs. The high degree of alignment for nanostructures with hi ect ratio can be understood within the framework of microhydrodynamics, in which the Peclet number, Pe, is estimated to be in the range of 10 to 106 (≥1) or the lengths of nanostructures studied here 5, 18 e Sequential etching and SEM imaging also show that the NTs are, like the Nws, located at the outer 60 nm of the bbfs, close to a two-dimensional layer, contrasting with NT composites made by solution casting or spin coating -2, which usually contain randomly oriented NTs through the thickness of the films. BBES ON LARGE-AREA AND NON-PLANAR SUBSTRATES The BBF approach was also used to transfer aligned NW and NT films to a broad range of substrates. For example, a Si NW-BBF was transferred to a half cylinder(Fig 3c), and subsequent dark field optical images confirm that the Nws within the film are well aligned. We also note that NW- and NT-BBFs can be 050.100.15 transferred to flexible plastic substrates that are subsequently bent Loading percentage into curved structures. In addition to planar and curved substrates, NW- and NT-BBRs have been transferred to open frames with good orientational alignment of the Nw and NT materials(Fig. 3d), thus demonstrating the great flexibility of Figure 2 Control of aligned NW density in BBFs. a, Photograph of 0.01, 0.03 this approach. and 0. 15 wt% (left to right) epoxy su ns of si Nws. b-d. Dark-field Importantly, our approach has the potential to be scaled to optical images recorded from 0.01 (b), 0.03(c)and 0. 15(d)wt% Si NW-BBFs, structures of very large area -ll. A representative image of an respectively. The scale bars in b, c and d are 50, 20 and 10 um, respectively. SWNT-BBF transferred to a 200-mm wafer(Fig. 3e) shows that e, Plot of the average Si Nw spacing and density as a function of the si Nw the film is remarkably uniform given the unsophisticated transfer loading. Points were obtained from analysis of 3-8 independently prepared process. Moreover, dark-field optical images(insets, Fig. 3e) BBFS per wt% suspension demonstrate that the SwNts have the same orientation and uniform separation across the diameter of this large substrate. Si NW-BBFs were also transferred uniformly, with good control of the Si Nw alignment and density, to a large rectangula walls. The drift velocity of NWs(toward the outer surface and along 225 mm x 300 mm plastic sheet substrate ( Fig. 3f; see also the normal of bubble curvature)and the distance travelled during Supplementary Information, Fig. S3). A histogram of angle bubble expansion can be estimated using the Faxen Laws'5, and are distribution of over 400 Si NWs taken from different locations consistent with our observation of NWs at the outer bubble surface over the entire plastic substrate shows that more than 85% (see Supplementary Methods) of the NWs are aligned within +6 of the primary expansion/ OTHER NANOWIRE AND CARBON NANOTUBE BRES LARGE-AREA TRANSISTOR ARRAYS We have explored the generality of this approach in terms of nanowire and nanotube materials, substrate structures and size The high degree of alignment, controlled density and large area scaling. For example, stable and homogeneous suspensions of the coverage possible with NW- and NT-BBFs could be enabling for direct band-gap Nw Cds were made and then used to prepare a number of integrated electronics applications of these Cds Nw-BBFs(see Methods). Optical images of transferred nanomaterials. To illustrate this potential, we have fabricated BBFs demonstrate uniform, well-aligned and controlled-density arrays of independently addressable NW-FETs from Si NW-BBFs Cds Nw with strong green emission (see Supplementary transferred directly to 75-mm-diameter plastic substrates(see Information, Fig. S2). The ability to prepare aligned and Methods). Figure 4a shows a 3 x 3 repeating transistor array, controlled density arrays from different NWs should open up a where each of the nine elements of the overall array consists of 20×20 @2007 Nature Publishing Group
Black plate (374,1) walls. The drift velocity of NWs (toward the outer surface and along the normal of bubble curvature) and the distance travelled during bubble expansion can be estimated using the Faxen Laws15, and are consistent with our observation of NWs at the outer bubble surface (see Supplementary Methods). OTHER NANOWIRE AND CARBON NANOTUBE BBFs We have explored the generality of this approach in terms of nanowire and nanotube materials, substrate structures and size scaling. For example, stable and homogeneous suspensions of the direct band-gap NW CdS were made and then used to prepare CdS NW-BBFs (see Methods). Optical images of transferred BBFs demonstrate uniform, well-aligned and controlled-density CdS NW with strong green emission (see Supplementary Information, Fig. S2). The ability to prepare aligned and controlled density arrays from different NWs should open up a number of opportunities for basic research and applications, given the distinct chemical and functional properties available from NW building blocks1,16. In addition, we modified singlewalled NTs (SWNTs) and multiwalled NTs (MWNTs) with n-octadecylamine17 and used the resulting suspensions to prepare BBFs (see Methods). The transferred SWNT- and MWNT-BBFs (Fig. 3a and b) show good alignment and uniformity over the 75-mm-diameter substrates used in the experiments, but can be transferred to much larger ones. The SWNTs exhibit an average separation of 1.5+0.4 mm, and 90% are aligned to within 58 of the average orientation. The good alignment of the SWNTs within BBFs is notable in that their lengths, 1–2 mm, are approximately ten times shorter than MWNTs (20–25 mm) and Si NW (10–15 mm) materials used in our studies. We also find that longer MWNTs, which are somewhat curled initially, appear straightened in the BBFs. The high degree of alignment for nanostructures with high aspect ratio can be understood within the framework of microhydrodynamics, in which the Pe´clet number, Pe, is estimated to be in the range of 103 to 106 (1) for the lengths of nanostructures studied here15,18. Sequential etching and SEM imaging also show that the NTs are, like the NWs, located at the outer 60 nm of the BBFs, close to a two-dimensional layer, contrasting with NT composites made by solution casting or spin coating19–21, which usually contain randomly oriented NTs through the thickness of the films. BBFs ON LARGE-AREA AND NON-PLANAR SUBSTRATES The BBF approach was also used to transfer aligned NW and NT films to a broad range of substrates. For example, a Si NW-BBF was transferred to a half cylinder (Fig. 3c), and subsequent dark- field optical images confirm that the NWs within the film are well aligned. We also note that NW- and NT-BBFs can be transferred to flexible plastic substrates that are subsequently bent into curved structures. In addition to planar and curved substrates, NW- and NT-BBFs have been transferred to open frames with good orientational alignment of the NW and NT materials (Fig. 3d), thus demonstrating the great flexibility of this approach. Importantly, our approach has the potential to be scaled to structures of very large area9–11. A representative image of an SWNT-BBF transferred to a 200-mm wafer (Fig. 3e) shows that the film is remarkably uniform given the unsophisticated transfer process. Moreover, dark-field optical images (insets, Fig. 3e) demonstrate that the SWNTs have the same orientation and uniform separation across the diameter of this large substrate. Si NW-BBFs were also transferred uniformly, with good control of the Si NW alignment and density, to a large rectangular 225 mm 300 mm plastic sheet substrate (Fig. 3f; see also Supplementary Information, Fig. S3). A histogram of angle distribution of over 400 Si NWs taken from different locations over the entire plastic substrate shows that more than 85% of the NWs are aligned within +68 of the primary expansion/ alignment direction. LARGE-AREA TRANSISTOR ARRAYS The high degree of alignment, controlled density and large area coverage possible with NW- and NT-BBFs could be enabling for a number of integrated electronics applications of these nanomaterials. To illustrate this potential, we have fabricated arrays of independently addressable NW-FETs from Si NW-BBFs transferred directly to 75-mm-diameter plastic substrates (see Methods). Figure 4a shows a 3 3 repeating transistor array, where each of the nine elements of the overall array consists of 400 independently addressable multi-NW transistors in a 20 20 NW spacing (µm) Loading percentage (wt %) Density (cm–2) 0.00 0.05 0.10 0.15 0.20 0.25 0 20 40 60 104 105 106 107 Figure 2 Control of aligned NW density in BBFs. a, Photograph of 0.01, 0.03 and 0.15 wt% (left to right) epoxy suspensions of Si NWs. b–d, Dark-field optical images recorded from 0.01 (b), 0.03 (c) and 0.15 (d) wt% Si NW-BBFs, respectively. The scale bars in b, c and d are 50, 20 and 10 mm, respectively. e, Plot of the average Si NW spacing and density as a function of the Si NW loading. Points were obtained from analysis of 3–8 independently prepared BBFs per wt% suspension. ARTICLES 374 nature nanotechnology | VOL 2 | JUNE 2007 | www.nature.com/naturenanotechnology
ARTCLES a b d 麟 Figure 3 Versatility of BBFs. a, Dark-field optical image of a SWNT-BBF prepared from 0.07 wt% solution, the film was transferred to a silicon wafer for imaging The scale bar represents 10 um Inset, high-resolution dark-field image highlighting the alignment of individual SWNTs: the scale bar is 2 um. b, Dark-field optical nage of an MWNT-BBF prepared from 0. 15 wt% solution; the scale bar represents 25 um Inset, high-resolution image showing aligned, individual MWNTs; the cale bar is 5 um. c, Image of a 0. 10 wt% Si NW-BBF transferred to a curved surface(a half cylinder with diameter 2.5 cm and length 6 cm). Inset, dark-field tical image showing Si NWs in the film. The red rectangle in the main panel highlights the examined location and the scale bar is 10 um. d, Image of a 0. 10 wt% Si NW-BBF transferred to an open frame with diameter 6 cm. Inset, dark-field optical image showing Si NWs in the film. The red rectangle highlights the examined location and the scale bar is 10 um. e, Image of a 0.07 wt% SWNT-BBF transferred to a 200-mm Si wafer and its line-scanning alignment analysis Insets: high- resolution dark-field images highlighting the alignment of SWNTs in the marked locations (triangles indicate recorded locations). The scale bar represents 2 um, and the same for all three insets. f, Image of a 0.10 wt% si NW-BBF transferred to a 225 mm x 300 mm plastic substrate. Inset, histogram of angular distribution of >400 NWs from the regions marked with numbers 1-6. More than 85% of the NWs are aligned within +6 of the upward expansion direction array(inset, Fig 4a). The FETs(Fig. 4a, right)contain,on Ge/Si core/shell NWs2. Importantly, histograms of V, and I 12 Si NWs per device, although this number can be varied Fig. 4c and inset of Fig. 4b), show that these properties, hanges in the wt% solution used for the BBF and the siz critical to integrated systems, are well constrained, with values of ontact electrodes 0.81±0.32vand15.1±3.7μA, respectively. The good L presentative drain-source current, Ias, versus gate voltage, reproducibility of the Si NW FETs can be attributed to the Ve, data(Fig. 4b; see also Supplementary Information, Fig. S4) uniform density, good alig and preferential distribution of yield a peak transconductance, 8m =dIa/dv,, of 6 uS with an on the NWs at a single surface of the BBFs(see Supplemental current, Ion, of 16 HA, an on/ off ratio >10 and a threshold Information, Fig. S1),as this allows for the fabrication of oltage, V of 0.55 V. These values compare well with, or exceed, repeatable device structures. The straightforward transfer of previous multi-Si NW FETs prepared using Langmuir-Blodgett aligned Si Nw-BBRs to large substrates makes this proces assembly, and moreover, significant improvements should be considerably fficient than previous fluid-directed and possible, for example, by substituting much higher performance Langmuir-Blodgett assembly methods naturenanotechnologyivol2JuNe2007www.natur @2007 Nature Publishing Group
Black plate (375,1) array (inset, Fig. 4a). The FETs (Fig. 4a, right) contain, on average, 12 Si NWs per device, although this number can be varied through changes in the wt% solution used for the BBF and the sizes of the contact electrodes. Representative drain–source current, Ids, versus gate voltage, Vg, data (Fig. 4b; see also Supplementary Information, Fig. S4) yield a peak transconductance, gm ¼ dId/dVg, of 6 mS with an on current, Ion, of 16 mA, an on/off ratio .105 and a threshold voltage, Vt , of 0.55 V. These values compare well with, or exceed, previous multi-Si NW FETs prepared using Langmuir–Blodgett assembly8 , and moreover, significant improvements should be possible, for example, by substituting much higher performance Ge/Si core/shell NWs22. Importantly, histograms of Vt and Ion (Fig. 4c and inset of Fig. 4b), show that these properties, critical to integrated systems, are well constrained, with values of 0.81+0.32 V and 15.1+3.7 mA, respectively. The good reproducibility of the Si NW FETs can be attributed to the uniform density, good alignment and preferential distribution of the NWs at a single surface of the BBFs (see Supplementary Information, Fig. S1), as this allows for the fabrication of repeatable device structures. The straightforward transfer of aligned Si NW-BBFs to large substrates makes this process considerably more efficient than previous fluid-directed5 and Langmuir–Blodgett8 assembly methods. 1 2 3 4 5 6 1 2 3 1 2 3 –20 –10 0 10 20 0 20 40 60 80 100 NW counts Angle (°) Figure 3 Versatility of BBFs. a, Dark-field optical image of a SWNT-BBF prepared from 0.07 wt% solution; the film was transferred to a silicon wafer for imaging. The scale bar represents 10 mm. Inset, high-resolution dark-field image highlighting the alignment of individual SWNTs; the scale bar is 2 mm. b, Dark-field optical image of an MWNT-BBF prepared from 0.15 wt% solution; the scale bar represents 25 mm. Inset, high-resolution image showing aligned, individual MWNTs; the scale bar is 5 mm. c, Image of a 0.10 wt% Si NW-BBF transferred to a curved surface (a half cylinder with diameter 2.5 cm and length 6 cm). Inset, dark-field optical image showing Si NWs in the film. The red rectangle in the main panel highlights the examined location and the scale bar is 10 mm. d, Image of a 0.10 wt% Si NW-BBF transferred to an open frame with diameter 6 cm. Inset, dark-field optical image showing Si NWs in the film. The red rectangle highlights the examined location and the scale bar is 10 mm. e, Image of a 0.07 wt% SWNT-BBF transferred to a 200-mm Si wafer and its line-scanning alignment analysis. Insets: highresolution dark-field images highlighting the alignment of SWNTs in the marked locations (triangles indicate recorded locations). The scale bar represents 2 mm, and is the same for all three insets. f, Image of a 0.10 wt% Si NW-BBF transferred to a 225 mm 300 mm plastic substrate. Inset, histogram of angular distribution of .400 NWs from the regions marked with numbers 1–6. More than 85% of the NWs are aligned within +68 of the upward expansion direction. ARTICLES nature nanotechnology |VOL 2 | JUNE 2007 | www.nature.com/naturenanotechnology 375
ABTGLES a A 0 Threshold voltage Figure 4 Si NW FET arrays on plastic substrates. a, Left panel, photograph of a plastic substrate containing nine Si Nw-FETs device arrays. The device arrays were prepared by standard processing(see Methods) following transfer of the Si NW-BBF to the plastic Inset, optical image of one device array from the centre of the substrate. Right panel, dark-field optical image of one typical top-gated Si Nw-FET device; the scale bar represents 50 um. Inset, optical image of a 4 x 4 Si NW FET subarray. The blue rectangle highlights a single device. b, Typical las-Vg characteristics of a 12-Si NW-FET device recorded with Vas=-1V. Inset, histogram of Ion showing the uniform device characteristics, where the blue curve is a gaussian fit: 15.1+3.7 A. C, Histogram of threshold voltage determined from analysis of more than 60 randomly chosen devices in the larger array; the blue curve is a gaussian fit: 0.81 +0.32 V. CONCLUSIONS produced by layering BBFs containing the same or distinct unctional NWs and/ or NTs, and by scrolling or folding the In summary, we have shown that bubble expansion of NW- and NT-BBRs. There are still challenges that must be homogeneous NW and NT suspensions is a general approach for addressed for our approach to reach the metre scale using preparing well-aligned and controlled-density NW and NT films established blown film extrusion techniques-l, including the over large areas In our work, we demonstrated these key features development of a better understanding of the physics underlying with NW-and NT-BBFs conformally transferred to single-crystal alignment and spacing, and an understanding of the limits of wafers up to 200 mm in diameter, flexible plastic sheets up to matrix materials and properties 225 mm x 300 mm, highly curved surfaces, and also suspended across open frames. We believe that the transfer of aligned and METHODS ontrolled-density NWs and NTs to both large crystalline wafers and flexible plastic substrates represents one of the most critical advances necessary for realizing many applications and /or NW AND NT FUNCTIONALIZATION efficient processing of these materials in several areas of The 20-nm-diameter p-type Si NWs were synthesized by chemical vapour deposition of silane and diborane with a Si: B ratio of 4,000: I using gold approach rields devices with reproducible properties necessary tube with subsequent NW grow downstream edge of afurnace hea ohad REAl to date makes them most applicable to areas such as in 30 s.c. c.m. of H2 at a total pressure of 30 torr. Si and CdS Nw biological/chemical sensors and display substrates were modified with 1%(v/v)5, 6-epoxyhexyltriethoxysilane(( It remains a challenge for the future to see how far the density in tetrahydrofuran (THF) for 2 h, rinsed with THF and cured at l1o"C for of aligned NWs and NTs can be pushed by our approach. More generally, we believe that these NW- and NT-BBES should be sonication (60 w, 10 s). The mass of NWs used in suspension was determined scalable to much larger metre-scale sizes by controlling the steps SwNTs(P3SWNT, purified SWNTs with high carboxylic acid content, of bubble formation and expansion as is currently achieved with Carbon Solutions) and MWNTs (produced by chemical vapour deposition, as homogeneous polymers, and moreover, that our approach described previously 2) were modified with n-octadecylamine(ODA, Sigma could be extended to enable three-dimensional structures24, 25 Aldrich). Briefly, 300 mg SWNTs were mixed with 2 gODA and heated at @2007 Nature Publishing Group
Black plate (376,1) CONCLUSIONS In summary, we have shown that bubble expansion of homogeneous NW and NT suspensions is a general approach for preparing well-aligned and controlled-density NW and NT films over large areas. In our work, we demonstrated these key features with NW- and NT-BBFs conformally transferred to single-crystal wafers up to 200 mm in diameter, flexible plastic sheets up to 225 mm 300 mm, highly curved surfaces, and also suspended across open frames. We believe that the transfer of aligned and controlled-density NWs and NTs to both large crystalline wafers and flexible plastic substrates represents one of the most critical advances necessary for realizing many applications and/or efficient processing of these materials in several areas of electronics. In addition, our studies of large NW FET arrays fabricated from transferred Si NW-BBFs demonstrate that this approach yields devices with reproducible properties necessary for integrated electronics, although the modest density achieved to date makes them most applicable to areas such as biological/chemical sensors23 and displays. It remains a challenge for the future to see how far the density of aligned NWs and NTs can be pushed by our approach. More generally, we believe that these NW- and NT-BBFs should be scalable to much larger metre-scale sizes by controlling the steps of bubble formation and expansion as is currently achieved with homogeneous polymers11, and moreover, that our approach could be extended to enable three-dimensional structures24,25 produced by layering BBFs containing the same or distinct functional NWs and/or NTs, and by scrolling or folding the NW- and NT-BBFs. There are still challenges that must be addressed for our approach to reach the metre scale using established blown film extrusion techniques9–11, including the development of a better understanding of the physics underlying alignment and spacing, and an understanding of the limits of matrix materials and properties. METHODS NW AND NT FUNCTIONALIZATION The 20-nm-diameter p-type Si NWs were synthesized by chemical vapour deposition of silane and diborane with a Si:B ratio of 4,000:1 using gold nanoparticles (Ted Pella) as catalysts8,12. CdS NWs were prepared by thermal evaporation of CdS powder (Sigma-Aldrich) at the centre of a quartz reaction tube with subsequent NW growth on a substrate containing dispersed 30-nm gold nanoparticles located at the downstream edge of a furnace heated to 600 8C in 30 s.c.c.m. of H2 at a total pressure of 30 torr. Si and CdS NW growth substrates were modified with 1% (v/v) 5,6-epoxyhexyltriethoxysilane (Gelest) in tetrahydrofuran (THF) for 2 h, rinsed with THF and cured at 110 8C for 10 min. The functionalized NWs were removed from growth substrates by sonication (60 W, 10 s). The mass of NWs used in suspension was determined from the difference in weight of the substrate before and after sonication. SWNTs (P3-SWNT, purified SWNTs with high carboxylic acid content, Carbon Solutions) and MWNTs (produced by chemical vapour deposition, as described previously26) were modified with n-octadecylamine (ODA, SigmaAldrich). Briefly, 300 mg SWNTs were mixed with 2 g ODA and heated at 120 8C 0.0 0.5 1.0 1.5 2.0 0 5 10 15 20 25 No. of devices Threshold voltage (V) 5 10 15 20 25 0 5 10 15 20 No. of devices Ion (µA) –3 –2 –1 0 1 2 3 0 5 10 15 –Ids (µA) Vg (V) S D G Figure 4 Si NW FET arrays on plastic substrates. a, Left panel, photograph of a plastic substrate containing nine Si NW-FETs device arrays. The device arrays were prepared by standard processing (see Methods) following transfer of the Si NW-BBF to the plastic. Inset, optical image of one device array from the centre of the substrate. Right panel, dark-field optical image of one typical top-gated Si NW-FET device; the scale bar represents 50 mm. Inset, optical image of a 44 Si NW FET subarray. The blue rectangle highlights a single device. b, Typical Ids–Vg characteristics of a 12-Si NW-FET device recorded with Vds ¼ 21V. Inset, histogram of Ion showing the uniform device characteristics, where the blue curve is a gaussian fit: 15.1+3.7 mA. c, Histogram of threshold voltage determined from analysis of more than 60 randomly chosen devices in the larger array; the blue curve is a gaussian fit: 0.81+0.32 V. ARTICLES 376 nature nanotechnology | VOL 2 | JUNE 2007 | www.nature.com/naturenanotechnology
ARTCLES for four days. The ODA-functionalized SWNTs were washed with ethanol and 3. Awouris, P. Molecular electronics with carbon nanotubes. Acc Chem. Res. 35, 1026-1034(2002). dried in air. SEM images show small bundles of ODA-SWNTs with lengths of 4 Smith, P. A, Nordquist, C. D, Jackson, T.N.& Mayer, TSElectric-field asisted assembly and 2 um. MWNTs(40 nm average diameter)were dispersed in 5 M nitric acid, 5. Huang Y Duan, x,Wei,Q&Lieber, C.M. assembly of one-dimensional nanostructures heated at 110C to introduce carboxylic acid groups, and then isolated by into functional networks. Science 291, 630-633(20 filtration and washed with distilled water. Then 500 mg MWNTs were mixed 6. Auvray, S ct al Chemical optimization of self-assembled carbon nanotube transistors. Nano Lett. 5. vith ODA(2 g) and heated at 120C for 3-4 days. The ODA-functionalized 7. Keren, K Berman, R S, Buchstab, E, Sivan, U& Braun, E DNA-templated carbon nanotube field. MWNTs were washed with ethanol and dried in air terconnection and integration of nanowire devices without registration. Nits PREPARATION OF NW AND NT SUSPENSIONS Briston. ]. H. Plastics Films 71-86(Longman Scientific Technical, UK, 1989). Functionalized NWs(1-15 mg)or 5-20 mg ODA-SWNTs were suspended in 10. Quanbeck, 1. Plastics Machinery& 3-4 ml THF to obtain different wt% solutions. Then 5 g epoxy part A(FHI 3: 1 11. Cantor K Blown Film Extriesioe: An introduction.(Hanser Gardner Publications, Cincinnati, 2006). HT Resin, Fiberglass Hawaii) was added to the THF solution, mechanical 12. Cui, Y, Zhong, Z, wang sisters. Namo Left. 3. 149-152(2003). mixed(M37615 Mixer, Barnstead International) for 5 min, and 1.7 g epoxy part 13.Lin, M.Y. et aL Shear-induced behavior in a solution of cylindrical micelles. Phys. Rev. E 53. B(hardener)was added and the solution mixed for an additional 5 min. the resulting suspension was capped to prevent THF evaporation, and allowed to 14. Hobbie, E. K. Wang H Kim, H, Lin-Gibson, . Giruke, E A. Orientation of carbon nanotubes in Pa s(AR-2 rheometer, TA Instruments) 15. Kim, S& Karrila, S 1. Microhydracry namics: Principles and Selected Applications, Ist edn. 76-128 hereafter. Typically, 20-30 h were required before the suspensions reached the 16.Cui, Y Duan, X, Huang. Y. Lieber, C. M. Nanowires and Nanobelts (ed. Wang, ZL)3-68 desired viscosity range. 17. Chen, I et aL Dissolution of full-length single walled carbon nanotubes. J. Phys. Chem. B 105, SEMKAUTOMATED PROCESS FOR PRODUCING BUBBLES Ben-Shaul A Fow-induced gelation of living(micellar) polymers. mddw由1(63mab1n减 outlet(6.35 mm) at the top and a ring centred over the gas outlet, which was moved upward at a controlled speed. Rigid wafers and / or flexible substrates were 20. Ounaies, Z, Park, C. wise, K E. Siochi, E I& Harrison, J S. Electrical properties of single wall set at fixed distances around the expansion axis. NW/NT suspensions(.5-1g) 21. ar,, c. ar a bi ernionof ngl ai arbon nato fbe b ins tu polymeri t ion ande so(ation. were transferred to the polished top surface of the covering the gas outlet, and then N, gas(P=150-200 kPa)was introduced to 22. Xiang, L et aL Ge/Si nanowire heterostructures as high-performance field-effect transistors. Nature initiate bubble expansion. The ring catches the top portion of the bubble and 23. Patolsky, E, Timk, B. P, Zheng, G& Lieber, C M. Nanowire-based nanoelectronic devices in the directs stable vertical expansion at an average speed of 10-15 cm min life sciences, MRS BulL. 32, 142-149(2007). Bubble films were transferred to wafer substrates during the expansion process. 24. Ahn, I et al. Heterogeneous three-dimensional electronics by use of printed semiconductor NW-FET ARRAYS 5. lavey, A, Nam, S. w Friedman, R S, Yan, H. Lieber, C M. Layer-by-layer assembly of nanowires Si NW-BBFs were transferred to 3-inch-diameter Kapton wafers(Kapton FPC26.Andrews,Ret al.Continuous polyimide film, DuPont)that were first coated with a wl-Hm-thick layer of realization. Chem. Phys. Left. 303, 467-474(1999). cured photoresist(SU-8, MicroChem). Following oxygen reactive ion etching (Cirrus 150, Nexx Systems)to remove excess epoxy matrix, photolithography Acknowledgements and metal deposition(50-nm Ni) were used to define source and drai We thank M. Ghasemi-Neihad and R.H. Knapp for helpful discussions. A.C. acknowledges start-up electrodes. A A20-nm HfO, dielectric was deposited over the wafer b low-temperature(110"C)atomic layer deposition, and then gate electrodes M L acknowledges support of this work by Air Force Office of Scientific Research and Deft were defined in a second photolithography and metal deposition (5-nm Cr; Correspondence and requests for materials should be addressed to A.C. and CML 50-nm Au) step. Devices were characterized at room temperature with a Supplementaryinformationaccompaniesthispaperonwww.nat probe station( Summit 12561, Cascade Microtech) and semiconductor parameter analyser(4156 C, Agilent Technologies) Author contributions G.Y. and A C- performed the experiments. G.Y., A C and C ML designed the experiments, discussed the Received 13 February 2007; accepted 30 April 2007: published 27 May 2007. interpretation of results and co-wrote the paper References Competing financial interests L. Lieber, C M. Nanoscale science and technology: building a big future from small things. MRS Biall. The authors declare no competing financial interests. 2. McEuen, P L Single-wall carbon nanotubes. Pin. World 13, 31-36(June 2000) Reprintsandpermissioninformationisavailableonlineathttpd//npgnaturecom/reprintsandpermissions/ naturenanotechnologyivol2JuNe2007www.natur @2007 Nature Publishing Group
Black plate (377,1) for four days. The ODA-functionalized SWNTs were washed with ethanol and dried in air. SEM images show small bundles of ODA-SWNTs with lengths of 2 mm. MWNTs (40 nm average diameter) were dispersed in 5 M nitric acid, heated at 110 8C to introduce carboxylic acid groups, and then isolated by filtration and washed with distilled water. Then 500 mg MWNTs were mixed with ODA (2 g) and heated at 120 8C for 3–4 days. The ODA-functionalized MWNTs were washed with ethanol and dried in air. PREPARATION OF NW AND NT SUSPENSIONS Functionalized NWs (1–15 mg) or 5–20 mg ODA-SWNTs were suspended in 3–4 ml THF to obtain different wt% solutions. Then 5 g epoxy part A (FHI 3:1 HT Resin, Fiberglass Hawaii) was added to the THF solution, mechanically mixed (M37615 Mixer, Barnstead International) for 5 min, and 1.7 g epoxy part B (hardener) was added and the solution mixed for an additional 5 min. The resulting suspension was capped to prevent THF evaporation, and allowed to cure until the viscosity reached 15–25 Pa s (AR-2 rheometer, TA Instruments). During the first 10 h, suspensions were shaken every 2 h, and allowed to sit thereafter. Typically, 20–30 h were required before the suspensions reached the desired viscosity range. SEMI-AUTOMATED PROCESS FOR PRODUCING BUBBLES Controlled bubble expansion was carried out using an apparatus consisting of a 50-mm-diameter stainless steel die with a gas inlet (6.35 mm) at the bottom, an outlet (6.35 mm) at the top and a ring centred over the gas outlet, which was moved upward at a controlled speed. Rigid wafers and/or flexible substrates were set at fixed distances around the expansion axis. NW/NT suspensions (0.5–1 g) were transferred to the polished top surface of the die, forming a membrane covering the gas outlet, and then N2 gas (P ¼ 150–200 kPa) was introduced to initiate bubble expansion. The ring catches the top portion of the bubble and directs stable vertical expansion at an average speed of 10–15 cm min21 . Bubble films were transferred to wafer substrates during the expansion process. NW-FET ARRAYS Si NW-BBFs were transferred to 3-inch-diameter Kapton wafers (Kapton FPC polyimide film, DuPont) that were first coated with a 1-mm-thick layer of cured photoresist (SU-8, MicroChem). Following oxygen reactive ion etching (Cirrus 150, Nexx Systems) to remove excess epoxy matrix, photolithography and metal deposition (50-nm Ni) were used to define source and drain electrodes. A 20-nm HfO2 dielectric was deposited over the wafer by low-temperature (110 8C) atomic layer deposition, and then gate electrodes were defined in a second photolithography and metal deposition (5-nm Cr; 50-nm Au) step. Devices were characterized at room temperature with a probe station (Summit 12561, Cascade Microtech) and semiconductor parameter analyser (4156 C, Agilent Technologies). Received 13 February 2007; accepted 30 April 2007; published 27 May 2007. References 1. Lieber, C. M. Nanoscale science and technology: building a big future from small things. MRS Bull. 28, 486–491 (2003). 2. McEuen, P. L. Single-wall carbon nanotubes. Phys. World 13, 31–36 (June 2000). 3. Avouris, P. Molecular electronics with carbon nanotubes. Acc. Chem. Res. 35, 1026–1034 (2002). 4. Smith, P. A., Nordquist, C. D., Jackson, T. N. & Mayer, T. S. Electric-field assisted assembly and alignment of metallic nanowires. Appl. Phys. Lett. 77, 1399–1401 (2000). 5. Huang, Y., Duan, X., Wei, Q. & Lieber, C. M. Directed assembly of one-dimensional nanostructures into functional networks. Science 291, 630–633 (2001). 6. Auvray, S. et al. Chemical optimization of self-assembled carbon nanotube transistors. Nano Lett. 5, 451–455 (2005). 7. Keren, K., Berman, R. S., Buchstab, E., Sivan, U. & Braun, E. DNA-templated carbon nanotube fieldeffect transistor. Science 302, 1380–1382 (2003). 8. Jin, S. et al. Scalable interconnection and integration of nanowire devices without registration. Nano Lett. 4, 915–919 (2004). 9. Briston, J. H. Plastics Films 71–86 (Longman Scientific & Technical, UK, 1989). 10. Quanbeck, J. Plastics Machinery & Auxiliaries 18 (May/June 2004). 11. Cantor, K. Blown Film Extrusion: An Introduction. (Hanser Gardner Publications, Cincinnati, 2006). 12. Cui, Y., Zhong, Z., Wang, D., Wang, W. U. & Lieber, C. M. High performance silicon nanowire field effect transistors. Nano Lett. 3, 149–152 (2003). 13. Lin, M. Y. et al. Shear-induced behavior in a solution of cylindrical micelles. Phys. Rev. E 53, R4302–R4305 (1996). 14. Hobbie, E. K., Wang, H., Kim, H., Lin-Gibson, S. & Grulke, E. A. Orientation of carbon nanotubes in a sheared polymer melt. Phys. Fluids 15, 1196–1202 (2003). 15. Kim, S. & Karrila, S. J. Microhydrodynamics: Principles and Selected Applications, 1st edn, 76–128 (Butterworth–Heinemann, Boston, 1991). 16. 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Ge/Si nanowire heterostructures as high-performance field-effect transistors. Nature 441, 489–493 (2006). 23. Patolsky, F., Timko, B. P., Zheng, G. & Lieber, C. M. Nanowire-based nanoelectronic devices in the life sciences. MRS Bull. 32, 142–149 (2007). 24. Ahn, J. et al. Heterogeneous three-dimensional electronics by use of printed semiconductor nanomaterials. Science 314, 1754–1757 (2006). 25. Javey, A., Nam, S. W., Friedman, R. S., Yan, H. & Lieber, C. M. Layer-by-layer assembly of nanowires for three-dimensional, multifunctional electronics. Nano Lett. 7, 773–777 (2007). 26. Andrews, R. et al. Continuous production of aligned carbon nanotubes: a step closer to commercial realization. Chem. Phys. Lett. 303, 467–474 (1999). Acknowledgements We thank M. Ghasemi-Nejhad and R.H. Knapp for helpful discussions. A.C. acknowledges start-up funding from Department of Mechanical Engineering and College of Engineering of University of Hawaii; C.M.L. acknowledges support of this work by Air Force Office of Scientific Research and Defense Advanced Research Projects Agency. Correspondence and requests for materials should be addressed to A.C. and C.M.L. Supplementary information accompanies this paper on www.nature.com/naturenanotechnology. Author contributions G.Y. and A.C. performed the experiments. G.Y., A.C. and C.M.L. designed the experiments, discussed the interpretation of results and co-wrote the paper. Competing financial interests The authors declare no competing financial interests. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/ ARTICLES nature nanotechnology |VOL 2 | JUNE 2007 | www.nature.com/naturenanotechnology 377