In Situ Growing and Patterning of Aligned Organic Nanowire Arrays

Dec 17, 2008 - Second, the solutes aggregate on the nucleation centers, and the nanowires parallel to the pulling direction start growing up along the...
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Langmuir 2009, 25, 665-671

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In Situ Growing and Patterning of Aligned Organic Nanowire Arrays via Dip Coating Nanliu Liu,† Yan Zhou,‡ Lei Wang,† Junbiao Peng,† Jian Wang,*,† Jian Pei,*,‡ and Yong Cao† Institute of Polymer Optoelectronic Materials and DeVices, South China UniVersity of Technology, and Key Laboratory of Specially Functional Materials, Ministry of Education, Guangzhou 510640, China, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking UniVersity, Beijing 100871, China ReceiVed NoVember 4, 2008. ReVised Manuscript ReceiVed December 3, 2008 Aligned organic nanowire arrays are grown in situ and patterned via dip coating. By optimizing the stick-slip motion, the solvent evaporation conditions, and the solution concentration, parallel organic nanowire arrays with tunable length and desirable density and periodicity are directly grown and aligned on the substrate. Organic FETs based on the organic nanowire array have been successfully fabricated with a mobility of 1 × 10-4 cm2 · V-1 · s-1.

Introduction Over the past decade, semiconductor and metallic nanowires (NWs) have emerged as building blocks in nanoscale electronic, photonic, and optoelectronic devices such as single-molecule supercondutors,1 nanosensors,2 nanometer-scale FETs,3,4 nanolasers,5 nanoLED,6 and so forth because of their unique electronic and optical properties. It is of great importance to deposit and align the NWs precisely in the desired position in order to fabricate large-scale integrated devices.7 Compared to the industrial standard “top-down” patterning process, which involves lithography techniques and would eventually meet the limitations of yield cost and physical principles for further development, the “bottom-up” process proposed by Lieber8 can directly grow and align NWs on substrates in a high-quality manner at low cost. Several bottom-up methods to pattern 1D nanostructures are continuously being exploited, such as vapor-liquid-solid (VLS) processes,9 the Langmuir-Blodgett (LB) assembly technique,10,11 electric-field-assisted alignment,12-14 dip coating.15 Most of these efforts have been successfully demonstrated to form large, uniform area device arrays from inorganic nanowire suspensions. * To whom correspondence should be addressed. (J.W.) E-mail: [email protected]. Phone: 86-20-8711-4525. Fax: 86-20-8711-0606. (J.P.) E-mail: [email protected]. Phone: 86-10-6275-8745. † South China University of Technology. ‡ Peking University. (1) Hopkins, D. S.; Pekker, D.; Goldbart, P. M.; Bezryadin, A. Science 2005, 308, 1762. (2) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M. Nature 2002, 420, 57. (3) Tang, Q.; Li, H.; Song, Y.; Xu, W.; Hu, W.; Jiang, L.; Liu, Y.; Wang, X.; Zhu, D. AdV. Mater. 2006, 18, 3010. (4) Kim, D. H.; Lee, D. Y.; Lee, H. S.; Lee, W. H.; Kim, Y. H.; Han, J. I.; Cho, K. AdV. Mater. 2007, 19, 678. (5) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (6) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66. (7) Fan, H.; Werner, P.; Zacharias, M. Small 2006, 2, 700. (8) Lieber, C. M. MRS Bull. 2003, 28, 486. (9) Mohammad, S. N. Nano Lett. 2008, 5, 1532. (10) Yang, P. Nature 2003, 425, 243. (11) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255. (12) Sardone, L.; Palermo, V.; Devaux, E.; Credgington, D.; De Loos, M.; Marletta, G.; Cacialli, F.; Van Esch, J.; Samorı`, P. AdV. Mater. 2006, 18, 1276. (13) Li, M.; Bhiladvala, R. B.; Morrow, T. J.; Sioss, J. A.; Lew, K. K.; Redwing, J. M.; Keating, C. D.; Mayer, T. S. Nat. Nanotechnol. 2008, 3, 88. (14) Ranjan, N.; Vinzelberg, H.; Mertig, M. Small 2006, 2, 1490. (15) Huang, J.; Fan, R.; Connor, S.; Yang, P. Angew. Chem., Int. Ed. 2007, 46, 2414.

However, simple, scalable patterning techniques to fabricate aligned arrays and devices in situ are rarely reported for organic nanowires, which has several advantages over their inorganic counterparts such as solution processibility,16 flexibility,17,18 tunable optoelectric properties via chemical modification, and so forth. Herein, we report the position- and size-controllable organic NW arrays made by a simple dip-coating method. By optimizing the slip-stick motions (the slipping distances and the sticking time of the substrate), the concentration of the solution, and the evaporation conditions (the heating temperature and the solvent), parallel organic nanowire arrays with tunable length and desirable density and intervals are grown and aligned directly on the substrate. Furthermore, organic FETs based on the organic nanowire array have been successfully achieved.

Results and Discussion A large planar π-conjugated condensed benzothiophene derivative (compound 1, Figure 1a) with a long alkyl chain and nine fused aryl rings was employed in this work. The π-π stacking between large aromatic planes and van der Waals interactions between the long alkyl chains enable the molecules to self-assemble into 1D nano- or microstructures.19,20 When heated for about 1 h, 1 can dissolve in many common solvents such as chloroform, toluene, and p-xylene. Before being immersed in solution, the substrates are thoroughly cleaned. The substrate movement is provided by a high-precision linear motor stage. By discontinuous slip-stick motions, parallel-aligned NW arrays were grown and aligned on the substrate with desirable density, length, and intervals. As shown in Figure 1b,c, each nanowire is aligned along the direction parallel to the substrate pulling direction. The nanowire arrays were obtained from the 10 µg/ mL p-xylene solution heated to 60 °C via dip coating with a sticking time of 60 s, a slipping distance of 50 µm, and a pulling speed of 50 µm/s. During dip coating, air was blown onto the (16) Zhou, Y.; Liu, W.; Ma, Y.; Wang, H.; Qi, L.; Cao, Y.; Wang, J.; Pei, J. J. Am. Chem. Soc. 2007, 129, 12386. (17) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123. (18) Forrest, S. R. Nature 2004, 428, 911. (19) Niu, Q.; Zhou, Y.; Wang, L.; Peng, J.; Pei, J.; Wang, J.; Cao, Y. AdV. Mater. 2008, 20, 964. (20) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb, U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Mullen, K. AdV. Mater. 2005, 17, 684.

10.1021/la8036633 CCC: $40.75  2009 American Chemical Society Published on Web 12/17/2008

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Figure 1. (a) Chemical structure of 1. (b) Optical microscopy image of the nanowire arrays deposited from the p-xylene solution (10 µg/mL) heated to 60 °C with a sticking time of 60 s. The pulling distance and the pulling speed were set to 50 µm and 50 µm/s, respectively. (c, d) SEM images of magnified NW array and single nanowire from panel b. (Inset of panel d) Selected area electron diffraction (SAED) pattern of the nanowire. The arrow indicates the substrate pulling direction.

Figure 2. Schematic illustrations of the dip-coating process. (a) The initial state after the substrate was immersed in the solution. (b) The meniscus was stretched at the beginning of the stick event right after the substrate was pulled up for a distance. (c) At the end of the stick event, the meniscus was thinned, and the solution surface was lowered as a result of solvent evaporation. (d) The subsequent substrate pulling slipped the meniscus to a new location in which the new meniscus state was as same as that in panel b.

substrate by a small electric fan. The average length of the NWs is about 100 µm, and the nanowire diameter is about 400-500 nm (Figure 1d). The density of the array is around 12/100 µm. The symmetric selected-area electron diffraction (SAED) pattern of the NWs (Figure 1d inset) reveals that the in situ self-assembled nanowires are well crystallized. The simultaneous alignment of the organic NWs during their self-assembling growth via dip coating is essentially an evaporation-induced process15,21-23 that involves competition between the pinning force determined by the nonzero contact angle of the solvent on the substrate and the substrate’s roughness and the dewetting force mainly coming from surface tension. Zhang et al.24 introduced a solvent evaporation process to grow and pattern aligned squaraine nanowires in one step and fabricated photo(21) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (22) Deegan, R. D. Phys. ReV. E 2000, 61, 475. (23) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. ReV. E 2000, 62, 756. (24) Zhang, C.; Zhang, X.; Zhang, X.; Fan, X.; Jie, J.; Chang, J. C.; Lee, C. S.; Zhang, W.; Lee, S. T. AdV. Mater. 2008, 20, 1716.

conductive devices on a substrate with predefined electrode arrays. As demonstrated in their work, the density, the length, and the periodicity of the nanowire arrays can be adjusted by changing the evaporation conditions. However, because the substrate is immobile during the patterning process, it is difficult to realize different densities, lengths, and periodicities in a single solution. Moreover, the natural solvent evaporation-only induced selfassembly process is not suitable for generating multiple arrays from low-concentration solutions because the contact line receded continuously in dilute solutions, resulting in no abrupt ruptures of the nanowire growth.25 In our dip-coating process, the nanowire growth would be broken by the substrate’s motion during the preprogrammed slipping event, leading to multiarray formation. As a result, the method has the ability to tune the length and periodicity of the aligned nanowire arrays independently for various solution concentrations during one processing step. Figure 2 illustrates the schematic diagram of the dip-coating process. As the immersed substrate is pulled up from the solution, (25) Maheshwari, S.; Zhang, L.; Zhu, Y.; Chang, H. C. Phys. ReV. Lett. 2008, 100, 044503.

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Figure 3. (a) Optical image of the NW arrays deposited from a 200 µg/mL p-xylene solution with sticking times of 10, 20, 30, 40, 50, and 60 s from top to bottom, respectively. The slipping distance was set to 50 µm. (b) Optical image of the NW arrays deposited from a 10 µg/mL p-xylene solution with sticking times of 20, 30, 40, 50, and 60 s from top to bottom, respectively. The slipping distance was set to 50 µm. (c) Optical image of the NW arrays deposited from an 80 µg/mL p-xylene solution with sticking times of 30, 40, 50, 60, 70, 80, and 90 s from top to bottom, respectively. The slipping distance was set to 100 µm. (d) Optical image of the NW arrays deposited from an 80 µg/mL p-xylene solution with infinite sticking time. All of the solutions were heated to 60 °C. (e) Dependence of the nanowire length on the sticking time.

the pinned meniscus rises with the substrate and becomes stretched as a result of the viscous drag. Followed by the pulling, the substrate stops (sticks). Different from the dip-coating process from the inorganic nanowires suspension, two things happen during the stick event in a homogeneous organic solution. First, the solutes start to deposit, and the nucleation centers of the nanowires are formed. Second, the solutes aggregate on the nucleation centers, and the nanowires parallel to the pulling direction start growing up along the capillary flow as a result of the anisotropic intermolecular interaction. Early work has shown that increasing the withdrawal speed dragged more solution because of the viscous drag of the moving substrate.26 Therefore, we expected that tuning the pulling speed of the substrate would (26) Yimsiri, P.; Mackley, M. R. Chem. Eng. Sci. 2006, 61, 3496.

change the density of the nanowires. However, under the same evaporation condition (the p-xylene solution with a concentration of 40 µg/mL heated at 60 °C with air blow) and using the same sticking time (20 s) and the same slipping distance (100 µm), we observed no apparent change in either the density of the nanowires or the morphologies of the NW arrays at pulling speeds of 25, 50, 100, and 150 µm/s from the same solution. The reason is probably that the viscous drag is independent of the pulling up speed because of the low viscosity of the solution. The viscosity of the most concentrated p-xylene solution of 200 µg/mL in our experiment is 0.63 cP, whereas the viscosity of other dilute solutions of 80, 40, and 10 µg/mL is 0.61, 0.61, and 0.60 cP, respectively, at room temperature. As a result, we fixed the slipping speed at 50 µm/s unless otherwise specified.

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Figure 4. (a) Optical image of the NW arrays deposited from a 100 µg/mL p-xylene solution with programmed slipping distances of 5, 25, 50, 100, and 200 µm from top to bottom, respectively. The actual intervals are 45, 65, 90, 140, and 240 µm, respectively. (b) Optical image of the NW arrays deposited from a 40 µg/mL p-xylene solution with programmed slipping distances of 50, 100, and 200 µm from top to bottom, respectively. The actual intervals are 80, 130, and 230 µm, respectively. All sticking times were set to 20 s, and all solutions were heated to 60 °C.

Huang et al. found that the density of the NWs patterned on a substrate via dip coating from an inorganic nanowire dispersion depended on the sticking time.15 We discovered that for organic nanowire array growth and patterning via dip coating, the length rather than the density of the NWs was determined by the sticking time (i.e., the longer the sticking time, the longer the nanowires; Figure 3). In the inorganic nanowire suspension, the length of the NWs is fixed, and the capillary flow of the solvent carries the dispersed NWs toward the contact line. As the sticking time increases, more NWs are “sent” to the contact line by the capillary flow, thereby increasing the NW density. In our work, the organic nanowires’ in situ growth and array patterning occur simultaneously on the substrate in the homogeneous organic solution. As explained earlier, during the sticking event, the solutes deposit and form the nucleation centers at the contact line. After the seeds are formed, the capillary flow carries the organic molecules toward the contact line, and they self-assemble into organic nanowires through intermolecular interactions. When the sticking time increases, more organic molecules are carried to the contact line by capillary flow, resulting in longer nanowires. Figure 3e illustrates the dependence of the nanowire length on the sticking time. In a 200 µg/mL p-xylene solution, as the sticking time increased from 10 to 50 s, the length of the nanowires that selfassembled in situ doubled from 30 to 60 µm. Further increasing the sticking time to 60 s did not increase the nanowire length any more. The reason for the high limit on the nanowire length is attributed to the contact line discontinuous stepwise recession. During the sticking event, the meniscus is continuously thinned by solvent evaporation. As soon as the pinning force cannot hold the solution, the contact line will slip to a new position, leading to the rupture of nanowire growth and the formation of a new array,25,27 and the length of the nanowires will reach a maximum. This situation is like the static condition when the growth of the nanowires is induced by solvent evaporation only.24 (27) Xu, J.; Xia, J.; Hong, S.; Lin, Z.; Qiu, F.; Yang, Y. Phys. ReV. Lett. 2006, 96, 066104.

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However, for the p-xylene solutions with low concentrations (less than 100 µg/mL in our experiment), infinite sticking time (static condition) led to neither multiarray formation nor nanowire length saturation as evidenced by Figure 3d. In the dilute solution, the contact line is likely to recede in a continuous manner, and no abrupt ruptures would occur.25 Therefore, the growth of the nanowires in low-concentration p-xylene solutions will be continuous. To form multiarrays, the external force (movement of the substrate) has to be applied to retreat the contact line and split the nanowires as shown in Figure 3b,c. Compared to the solvent evaporation-only process that can be applied only to high-concentration solution, the dip-coating process expands the range of operation by 1 order of magnitude from 100 to 10 µg/mL. Figure 3e also reveals that the nanowires grow faster in high-concentration solution than in low-concentration solution. It is obvious that in high-concentration solution more solutes are carried to the contact line in a fixed time, resulting in a faster growth rate. Only when the nanowire length approaches its maximum value in high-concentration solution does the nanowire growth in low-concentration solution surpass the growth in highconcentration solution. The intervals between neighboring arrays (periodicity) can be easily controlled by the programmed slipping distance (Figure 4). As long as the sticking time is not long enough to induce contact line self-slipping due to solvent evaporation in highconcentration solution, the subsequent substrate pulling up after sticking can break the nanowire growth and slip the contact line to a new position, thereby controlling the periodicity of the arrays. When the slipping distance was set to 5, 25, 50, 100, and 200 µm and the sticking time was set to 20 s, the actual distance between the NW arrays deposited from the p-xylene solution (100 µg/mL) heated to 60 °C was about 45, 65, 90, 140, and 240 µm (Figure 4a), respectively. The 40 µm difference in distance is due to the receding liquid surface caused by solvent evaporation. As illustrated in Figure 2b, at the beginning of the sticking event, right after the substrate was pulled up for a distance, the solution surface remained at the original position, and the meniscus was stretched. At the end of the sticking event, the solution surface dropped to a new position, which was Le lower than the original one because of solvent evaporation (Figure 2c). Subsequent substrate pulling (for a distance of Lp) slipped the meniscus to a new location in which the new meniscus state was as same as that in (Figure 2b) (i.e., the distances between the contact line and the solution surface were the same in parts b and d of Figure 2). As a result, the interval L between the adjacent arrays is equal to the sum of Lp and Le (i.e., L ) Lp + Le; Figure 2d). From the experiment, the descending speed of the solution surface is calculated to be about 2 µm/s. As discussed earlier, substrate movement is essential in splitting the nanowires and forming multiarrays in the low-concentration solutions. Figure 4b shows that by setting a sticking time of 20 s the substrate pulling up helps form the nanowire multiarrays in the 40 µg/mL p-xylene solution heated to 60 °C. By the same arguments, the 30 µm difference between preprogrammed slipping distances of 50, 100, and 200 µm and the actual interval of 80, 130, and 230 µm, respectively, is attributed to the receding solution surface. The decreasing speed of the solution surface is calculated to be 1.5 µm/s. In the self-assembly process, the solvent is crucial28,29 to the quality of the NW arrays grown and aligned on the substrate when the solute molecules precipitate from the solution after the (28) Nguyen, T. Q.; Martel, R.; Avouris, P.; Bushey, M. L.; Brus, L.; Nuckolls, C. J. Am. Chem. Soc. 2004, 126, 5234. (29) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen, M.; Zhao, J.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390.

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Figure 5. Optical images of the self-assembled nanowire array from p-xylene solutions with concentrations of (a) 10, (b) 40, (c) 80, and (d) 200 µg/mL. The sticking time was set to 40 s. (e) Dependence of the nanowire density on the solution concentration.

solvent has evaporated. Several solvents with boiling points from low to high, such as chloroform (61 °C), toluene (111 °C), p-xylene (138 °C), and dichlorobenzene (180 °C), have been tested to optimize the nanowire array fabrication. The contact angle between all of the tested solvents and the substrate is about 10-20°, which favors the formation of a pinned contact line.15,21 Without heating and blowing with air, disordered NWs were grown on the substrate from a 40 µg/mL chloroform solution, whereas no NW was found from the solution with other solvents of the same concentration, suggesting that the nucleation centers of the nanowires cannot form in a solution with solvents evaporating at low speed. When the solution was heated to 60 °C, no nanowires were formed on the substrate from the chloroform and dichlorobenzene solutions, whereas uniformly aligned nanowire arrays were self-assembled in situ on the glass substrate from the toluene and p-xylene solutions via dip coating, in which the nanowire arrays from the p-xylene solution were longer and more uniform. When the dichlorobenzene solution was heated to 100 °C and the substrate was blown with air at the same time, scattered NWs were formed. Under the same heating temperature of 100 °C with blown air, the NWs from the p-xylene solution became accumulated and disordered. The experimental results show that the growth of the nanowire seeds that was the starting point of the nanowire self-assembly process was controlled by the solvent evaporation speed and solvent reflux near the contact line. The seeds can be formed at room temperature in solvents with low boiling point such as chloroform and at relatively high temperature in solvents with high boiling point such as toluene, p-xylene, and dichlorobenzene. To induce the nanowire self-assembly process at the contact line, the

evaporation speed of the solvent has to roughly match the seed growth speed. Moreover, in solvents with low boiling points, the vapor concentration of the solvent near the contact line would increase at high temperature to induce solvent reflux, which redissolves the newly formed nanowire seeds. As a result, heating the solvents and blowing air onto the substrate surface increase the solvent evaporation rate and prevent solvent reflux, thereby controlling the nanowire array fabrication. The density of the nanowires could be tuned by varying the concentration of the solution. Upon increasing the concentration of 1 in p-xylene from 10 to 200 µg/mL, the density of the nanowires (sticking time was 40 s) in one array increased from 12 to 160 per 100 µm (Figure 5e). However, as the concentration increases, the orientation of the self-assembled nanowires becomes less orderly and more and more branches, crossovers, and bundles are formed, as clearly shown in Figure 5a,d. At the highest concentration of 200 µg/mL, the self-assembled nanowire array looks like dense, entangled bushes (Figure 5d). During the nanowire in situ growth process, the initial nucleation state is critical to the nanowire growth direction in the beginning before the capillary force takes effect. Too many nucleation seeds would aggregate and overlap with each other when they were formed at the contact line, leading to cross-linked nanowires.30 As the self-assembly process continued, the capillary force slowly oriented the nanowires along its direction to form orderly, aligned nanowire arrays. When the concentration of the solution was diluted to 10 µg/mL, the self-assembled NWs were completely (30) Rep, D. B. A.; Roelfsema, R.; van Esch, J. H.; Schoonbeek, F. S.; Kellogg, R. M.; Feringa, B. L.; Palstra, T. T. M.; Klapwijk, T. M. AdV. Mater. 2000, 12, 563.

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Figure 6. (a) Optical image of the FET device based on the nanowire array. (b) Output characteristics of the FET. (c) Transfer characteristics of the FET.

separated and well aligned (Figure 5a). The average distance between adjacent nanowires is about 10 µm. If the solution was diluted to less than 10 µg/mL, then no nanowires were formed on the substrate. Additionally, we successfully fabricated large-area nanowire arrays via dip coating. The highly n-doped Si wafer with 300 nm of SiO2 on top as the substrate has an area of 10 × 20 mm2. It was cleaned thoroughly as detailed earlier, followed by 30 min of treatment with ozone plasma. After the cleaning, the substrate was immersed in an 80 µg/mL p-xylene solution of 1 and was heated to 50 °C, and the dip-coating process was carried out with a slipping distance of 100 µm and a sticking time of 40 s. The self-assembled arrays were 10 mm wide, and the edge area was curved because of the bend in the contact line due to the fringe effect.15 To fabricate the FET device, a 20-µm-diameter polyethylene (PE) fiber as the shadow mask was mounted on the wafer above one array, and a layer of 100 nm of gold was deposited onto the array by thermal evaporation under a pressure of 4 × 10-4 Pa. The transistor characteristics of the aligned nanowire array device were measured under the ambient atmosphere. The 300 nm SiO2 wafer has a capacitance per unit area of 11 nF/cm2. Typical p-channel FET characteristics were found in the fieldeffect transistor device with a mobility of 1 × 10-4 cm2 · V-1 · s-1 (Figure 6). There are two factors that may limit the FET mobility. One is the degree of NW crystallization. The other is the interface between the NW and the substrate. Because the symmetric SAED pattern reveals that the NWs are highly crystallized, the relatively low mobility is mainly due to the traps formed at the interface

between the NW and the untreated SiO2 substrate.31 Using OTSor HMDS-treated substrates for dip-coating organic NW arrays is currently underway in our laboratory. In conclusion, our newly developed, low-cost dip-coating method to grow and pattern aligned organic nanowire arrays in situ from homogeneous organic solutions offers more processing parameters than both the evaporation-only induced organic NW array patterning method and the inorganic NW array dip-coating method from an inorganic NW suspension. By tuning the slipping distances, the sticking time of the substrate, the concentration of the solution, the solvent, and the heating temperature, we were able to grow parallel organic nanowire arrays with adjustable length, density and periodicities and align them on the substrate. To induce the nanowire self-assembly process at the contact line, the evaporation speed of the solvent has to roughly match the seed growth speed. Organic FET based on the in situ grown and aligned nanowire array has been successfully achieved without using photolithography.

Experimental Section Materials. 1 was synthesized following published procedures.16 All of the solvents were purchased from Aldrich, Inc. and Acros Organics without further purification. Sample Preparation. Before immersion in the solution, the substrates (glass or SiO2-coated Si) were cleaned in an ultrasonic bath sequentially with acetone, isopropanol, detergent, deionized water, and isopropanol for at least 10 min. (31) Pernstich, K. P.; Goldmann, C.; Krellner, C.; Oberhoff, D.; Gundlach, D. J.; Batlogg, B. Synth. Met. 2004, 146, 325.

Letters Device Characterization. Substrate movement was provided by a high-precision linear motor stage from Zolix Instruments Co. Ltd. (model TSA400-B). The minimum incremental motion is 2.5 µm. The transistor characteristics of the aligned nanowire array device were measured under the ambient atmosphere using an Agilent 4155C semiconductor parameter analyzer connected to a Cascade manual probe station. The viscosity was measured with a digital viscometer from Brookfield Engineering Laboratories, Inc. (model LVDV-I+). The SEM images of the structures were obtained with a field-emission scanning electron microscope (FESEM, LEO 1530 VP) operated at an accelerating voltage of 5 kV. To minimize sample charging, the samples were coated with a thin gold film (∼10 nm) right before SEM examination by thermal evaporation under a pressure of 4 × 10-4 Pa. The optical microscope images were recorded with a Nikon eclipse E600 POL with a DXM1200F digital camera. The SAED

Langmuir, Vol. 25, No. 2, 2009 671 image was obtained with a Jeol JEM100CXII operating at an accelerating voltage of 80 kV. The samples were prepared according to the following steps: first, the nanowires were scraped off of the substrate; second, the nanowires were dispersed in ethanol by sonication for several minutes; and third, the nanowire dispersion was drop cast onto a holey carbon film supported by a copper TEM sample grid.

Acknowledgment. The authors are deeply grateful to the Ministry of Science and Technology (Nos. 2009CB623604, 2009CB930604, and 2006CB921602) and the National Natural Science Foundation of China (Nos. 20574021, 50433030, 20521202, 20425207, and 20632020) for their financial support. LA8036633