Solution-Assisted Assembly of Organic Semiconducting Single

7428

Langmuir 2007, 23, 7428-7432

Solution-Assisted Assembly of Organic Semiconducting Single Crystals on Surfaces with Patterned Wettability Shuhong Liu,† Wechung Maria Wang,† Stefan C. B. Mannsfeld,† Jason Locklin,† Peter Erk,‡ Marcos Gomez,‡ Frauke Richter,‡ and Zhenan Bao*,† Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305, and BASF AG, 67056 Ludwigshafen, Germany ReceiVed February 19, 2007. In Final Form: May 20, 2007 Two efficient approaches to assembling organic semiconducting single crystals are described. The methods rely on solvent wetting and dewetting on substrates with patterned wettability to selectively direct the deposition or removal of organic crystals. Substrates were functionalized with different self-assembled monolayers (SAMs) to achieve the desired wettabilities. The assembly of different organic crystals over centimeter-squared areas on Au, SiO2, and flexible plastic substrates was demonstrated. By designing line features on the substrate, the alignment of crystals, such as CuPc needles, was also achieved. As a demonstration of the potential application of this assembly approach, arrays of single-crystal organic field-effect transistors were fabricated by patterning organic single crystals directly onto and between transistor source and drain electrodes.

Introduction The search for low-cost, large-area, flexible devices has led to a remarkable increase in the research and technical development of organic semiconductors.1-4 Single-crystal organic field-effect transistors are ideal device structures for studying fundamental science associated with charge transport in organic materials and have demonstrated high mobility and outstanding electrical characteristics.5-10 For example, an exceptionally high carrier mobility of 20 cm2/Vs has been demonstrated for rubrene singlecrystal field-effect transistors.7,10,11 However, despite their high performance, integrating single crystals into practical electronic device applications remains a technical challenge.5 A key difficulty is that organic single-crystal devices are usually fabricated one device at a time by handpicking a single crystal and placing it onto the device substrate. Thus, they are difficult to mass produce with high throughput. Furthermore, it is almost impossible to fabricate devices using very small organic crystals that are submicrometer in size using this handpicking process.12 To overcome this problem, several groups have recently investigated strategies to grow single crystals directly on the substrate surface through vapor-processing techniques.4,12,13 * Corresponding author. E-mail: [email protected]. † Stanford University. ‡ BASF AG. (1) Newman, C. R.; Frisbie, C. D.; da Silva, D. A.; Bredas, J. L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436-4451. (2) Ling, M. M.; Bao, Z. N. Chem. Mater. 2004, 16, 4824-4840. (3) Stingelin-Stutzmann, N.; Smits, E.; Wondergem, H.; Tanase, C.; Blom, P.; Smith, P.; De Leeuw, D. Nat. Mater. 2005, 4, 601-606. (4) Mbenkum, B. N.; Barrena, E.; Zhang, X.; Kelsch, M.; Dosch, H. Nano Lett. 2006, 6, 2852-2855. (5) de Boer, R. W. I.; Gershenson, M. E.; Morpurgo, A. F.; Podzorov, V. Phys. Status Solidi A 2004, 201, 1302-1331. (6) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 16441646. (7) Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin, V.; Rogers, J. A.; Gershenson, M. E. Phys. ReV. Lett. 2004, 93, 086602. (8) Briseno, A. L.; Tseng, R. J.; Ling, M. M.; Talcao, E. H. L.; Yang, Y.; Wudl, F.; Bao, Z. AdV. Mater. 2006, 18, 2320-2324. (9) Reese, C.; Bao, Z. J. Mater. Chem. 2006, 16, 329-333. (10) Reese, C.; Chung, W.; Ling, M.; Roberts, M.; Bao, Z. Appl. Phys. Lett. 2006, 89, 202108. (11) Zeis, R.; Besnard, C.; Siegrist, T.; Schlockermann, C.; Chi, X. L.; Kloc, C. Chem. Mater. 2006, 18, 244-248. (12) Tang, Q.; Li, H.; Song, Y.; Xu, W.; Hu, W.; Jiang, L.; Liu, Y.; Wang, X.; Zhu, D. AdV. Mater. 2006, 18, 3010-3014.

However, to fully realize the low-cost advantage of organic electronics, solution-processing approaches have to be developed for the assembly of organic semiconducting crystals directly onto device structures.14,15 Solvent wetting and dewetting processes on surfaces with patterned wettability have shown great promise for assembling micro- and nano-objects, such as colloidal crystals, carbon nanotubes, and biomolecules.16-20 Recently, our group reported using this approach to assemble and align metallic nanorods with dimensions of ∼250 nm diameter and ∼6 µm length.21 To explore this concept further, we investigated the assembly of organic semiconducting single crystals, such as copper phthalocyanine (CuPc) with different shapes (sheets or needles) and dimensions (5-100 µm) and 5,5′-bis(4-tert-butylphenyl)-2,2′bithiophene (tPTTPt), using two strategies. In the first strategy, an organic crystal suspension was first cast on a substrate with patterned wettability and then removed. Upon receding of the three-phase contact line of the suspension, organic crystals were transported to the wetting regions and deposited. In the second strategy, crystals were patterned into solvent dewetting regions instead of wetting regions. In this case, organic single crystals were first deposited uniformly on surfaces with patterned wettability and then rinsed with a solvent. Crystals on the wetting regions were selectively lifted off, resulting in patterned crystals on the dewetting regions. Both strategies are capable of patterning crystals on a large scale. Finally, we demonstrate the application of this patterning method by patterning crystals directly onto transistor source-drain electrode structures, thereby forming arrays of field-effect transistors. (13) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S.; Tseng, R. J.; Reese, C.; Robert, M. E.; Yang, Y.; Wudl, F.; Bao, Z. Nature 2006, 444, 913917. (14) Briseno, A. L.; Aizenberg, J.; Han, Y. J.; Penkala, R. A.; Moon, H.; Lovinger, A. J.; Kloc, C.; Bao, Z. J. Am. Chem. Soc. 2005, 127, 12164-12165. (15) Mannsfeld, S. C. B.; Locklin, J.; Reese, C.; Roberts, M. E.; Lovinger, A. J.; Bao, Z. AdV. Func. Mater., in press, 2007. (16) Fan, F.; Stebe, K. J. Langmuir 2004, 20, 3062-3067. (17) Fan, F.; Stebe, K. J. Langmuir 2005, 21, 1149-1152. (18) Wang, Y.; Maspoch, D.; Zou, S.; Schatz, G. C.; Smalley, R. E.; Mirkin, C. A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 2026-2031. (19) Zhao, Y.; Fang, Y. Langmuir 2006, 22, 1891-1895. (20) Akamatsu, K.; Samitsu, S.; Tsuruoka, T.; Hasegawa, J.; Nawafune, H. Small 2006, 2, 1130-1133. (21) Liu, S.; Tok, J. B.-H.; Locklin, J.; Bao, Z. Small 2006, 2, 14481453.

10.1021/la700493p CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007

Letters

Langmuir, Vol. 23, No. 14, 2007 7429

Figure 1. Schematic of procedures for organic single-crystal assembly. (Scheme 1A) Strategy one: (1) Drop-casting of a crystal suspension on a substrate with hydrophilic and hydrophobic patterns. (2) Settling of crystals onto the substrate. (3) Formation of discrete droplets during the removal of the crystal suspension. (4) Patterned crystals on the substrate. (Scheme 1B) Bright-field optical micrographs showing the evaporation process of the CuPc crystal suspension. The sizes of CuPc crystals are around 5-20 µm. The substrate patterns contain 100 × 100 µm2 hydrophilic features (unmodified Au) spaced 250 µm apart. The hydrophobic regions are HDT-functionalized Au surfaces. (Scheme 2) Strategy two: (1) Preparation of a uniform film containing organic single crystals. (2) Lift-off of crystals on hydrophilic regions by rinsing with a hydrophilic solvent. (3) Lift-off of crystals on hydrophobic regions by rinsing with a hydrophobic solvent.

Experimental Section Copper phthalocyanine (CuPc) crystals of three different sizes were provided by BASF Chemical Co.: (1) CuPc sheets ∼5-20 µm in length and width, (2) CuPc sheets ∼25-50 µm in length and width, and (3) CuPc needles ∼50-100 µm in length, with a length to width aspect ratio of ∼10. 5,5′-Bis(4-tert-butylphenyl)-2,2′bithiophene (tPTTPt) molecules were synthesized in our laboratory,22 with the corresponding crystals prepared by first dissolving ∼6 mg/mL tPTTPt in bromobenzene at ∼150 C° and then allowing the solution to cool to room temperature. The precipitated crystals were filtered and then rinsed several times with methanol and ethanol. CuPc and tPTTPt crystals were both dispersed in water to form a suspension with a concentration of ∼2 mg/mL. Microcontact printing (µCP)23,24 was used to pattern surfaces into hydrophilic and hydrophobic regions by stamping self-assembled monolayers (SAMs). For Au surfaces, the hydrophobic regions were composed of 1-hexadecanethiol (HDT) (static water-contact angle ≈ 98°), and the hydrophilic or less hydrophobic regions were either treated with 3-mercapto-1-propanesulfonic acid SAM (MPSA; sodium salt; hydrophilic, static water-contact angle of ∼17°) or were not treated (static water-contact angle of ∼64°). The SAMfunctionalized Au surfaces were rinsed with ethanol and dried with N2 before use. For both SiO2 and indium tin oxide (ITO) surfaces on flexible plastic substrates, the hydrophobic regions were modified with n-octadecyltrichlorosilane (OTS) (static water-contact angle of (22) Sung, A.; Ling, M.-M.; Tang, M. L.; Bao, Z.; Locklin, J. Chem. Mater. 2007, 19, 2342-2351. (23) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274-3275. (24) Jeon, N. L.; Lin, W.; Erhardt, M. K.; Girolami, G. S.; Nuzzo, R. G. Langmuir 1997, 13, 3833-3838.

∼90°). The hydrophilic or less hydrophobic regions were either modified with phenyltrimethoxysilane (soaked in 5 mM toluene solution at 150 C° for 5 min; static water-contact angle of ∼64°) or were not treated (water completely wets). Both the SAMfunctionalized SiO2 and indium tin oxide (ITO) surfaces were rinsed with toluene and dried with N2 before use. SEM images were taken on an FEI XL30 Sirion scanning electron microscope operated at an acceleration voltage of 5 kV.

Results and Discussion The schematics of the two strategies for organic single-crystal assembly are shown in Figure 1. In the first strategy (Scheme 1A), similar to procedures that we previously reported for the self-assembly of metallic nanorods,21 a crystal suspension (∼1 to 2 mL) was drop-cast onto a substrate with patterned hydrophilic and hydrophobic regions. Crystals were then allowed to settle in the water suspension, resulting in a high concentration of crystals close to the substrate surface. After ∼10 min, a pipet was used to remove the excess suspension from the substrate (the crystals left in the removed suspension could be reused), leaving behind small, discrete droplets of crystal suspension covering the hydrophilic regions. After the water evaporated, the patterned aggregates of organic single crystals as shown in part 4 of Scheme 1A are obtained. As an illustration of this process, a series of optical micrographs showing the evaporation process of CuPc crystal suspensions (crystal size is ∼5-20 µm in length and width) are shown in Scheme 1B. The hydrophilic patterns are 100 × 100 µm2 features on the Au surface whereas

7430 Langmuir, Vol. 23, No. 14, 2007

Letters

Figure 2. (A, B) Bright-field optical micrographs of CuPc crystals (∼5-20 µm) deposited on Au surfaces with 100 × 100 µm2 (A) and 50 µm line (B) hydrophilic features. The inset SEM image in A shows a close-up of the marked region. (C, D) Bright-field optical micrographs of patterned Au surfaces with exposed Au regions etched away (darker regions) (C) and CuPc crystals (∼5-20 µm) deposited on exposed Au surfaces (D). The inset SEM image in D shows a close-up of the marked region. (E) Bright-field optical micrographs of CuPc crystals (∼5-20 µm) deposited on plastic substrates patterned with 100 × 100 µm2 hydrophilic squares. The inset shows the image of the flexible plastic substrate. The CuPc crystals in A-E are from the same batch. (F, G) Bright-field optical micrographs of tPTTPt crystals (∼2-5 µm) (F) and CuPc crystals (∼25-50 µm) (G) deposited on Au surfaces with 100 × 100 µm2 hydrophilic squares. (H) Bright-field optical micrographs of CuPc crystals (∼50-100 µm in length) aligned on Au surfaces with hydrophilic lines that are 20 µm in width. In images A, B, D, F, and G, the hydrophilic and hydrophobic regions are unmodified Au and HDT-modified Au, respectively. In image E, the hydrophilic and hydrophobic regions are phenyltrimethoxysilane- and OTS-modified ITO, respectively. In image H, the hydrophilic and hydrophobic regions are MPSA- and HDT-modified Au, respectively.

the hydrophobic regions are functionalized with HDT SAMs. In the second strategy, a methanol suspension of organic crystals was first cast on a patterned substrate and then dried by methanol evaporation. The obtained uniform film containing organic crystals was then rinsed with pure solvents. During the rinsing process, crystals on the solvent wetting regions were lifted off as a result of the solvent creeping into the crystal-substrate interface,20 leading to patterned crystals in the solvent-dewetting regions. We first investigated whether the crystals could be assembled onto the desired surface regions using strategy one. We used CuPc crystals (∼5-20 µm in length and width) as a model system for this study because they are readily available and possess interesting semiconductor properties.25 Figure 2A,B shows optical micrographs of CuPc crystals patterned into squares (100 × 100 µm2) and lines (50 µm width) in the hydrophilic regions (unmodified Au surfaces); barely any crystals are found in the HDT-functionalized hydrophobic regions. Such crystal patterns could be achieved over centimeter length scales. To test the resolution of this patterning method, we prepared hydrophilic line patterns with different widths (120, 80, 40, 20, 10, and 5 µm). Figure 2C shows a sample in which the designed features were made visible by etching the Au regions (darker regions) away using a standard Au etchant.26 The aggregates of crystals patterned on the corresponding substrates are shown in Figure 2D. It can be seen that the crystal patterns with the 5 and 10 µm line widths are not as well resolved as with the wider lines. We also designed patterned dots (∼10 µm in diameter) on substrates, but no obvious patterning of crystals was observed (images not shown). These results suggest that the feature sizes on substrates need to be larger than the crystals for well-defined patterning. (25) Yamada, K.; Takeya, J.; Shigeto, K.; Tsukagoshi, K.; Aoyagi, Y.; Iwasa, Y. Appl. Phys. Lett. 2006, 88, 122110. (26) Wilbur, J. L.; Kumar, A.; Biebuyck, H. A.; Kim, E.; Whitesides, G. M. Nanotechnology 1996, 7, 452-457.

The SEM cross-sectional images of the samples showed that the patterned regions are composed of stacked crystals with an approximate thickness of several micrometers. This patterning strategy can be extended to assemble different kinds of crystals with a range of sizes and shapes on different substrates. Besides using Au surfaces as shown above, patterning of crystals on SiO2 surfaces and plastic substrates was also achieved when they were patterned into regions with appropriate water-contact angles. One example of the CuPc crystals (∼5-20 µm in length and width) patterned onto organic flexible substrates is shown in Figure 2E. Furthermore, this patterning process could also be used to assemble other crystals, such as 5,5′-bis(4-tert-butylphenyl)-2,2′-bithiophene (tPTTPt) (∼2-5 µm in length and width, Figure 2F), rubrene (∼2-10 µm in length and width, not shown), and larger CuPc crystals (∼25-50 µm in length and width, Figure 2G). With large CuPc crystals, only a few crystals (in some regions only one crystal) were deposited in each hydrophilic region (100 × 100 µm2) as shown in Figure 2G, indicating that patterning one single-crystal per domain could potentially be realized by further optimizing the surface energy and assembly conditions. Besides patterning crystals in designated regions, this method could also be used to control the orientation of needlelike crystals. Our previous study on the alignment of metallic nanorods showed that alignment could be attained if the width of the hydrophilic lines was smaller than the length of the nanorods.21 A similar trend is observed for these crystals. As shown in Figure 2H, needle-shaped CuPc crystals (∼50-100 µm in length) were aligned onto surfaces with patterned 20 µm alternating hydrophilic (MPSA-functionalized Au) and hydrophobic (HDT-functionalized Au) line features. To better understand the key factors governing the assembly process of crystals, we investigated the impact of the time for crystal settling (second step in Scheme 1A) on the number of

Letters

Langmuir, Vol. 23, No. 14, 2007 7431

Figure 3. (A-C) Images of CuPc crystals (∼5-20 µm) deposited on SiO2 surfaces with different hydrophilic features using water as the rinsing solvent. (A, C) Bright-field optical micrograph. (B) Cross-polarized optical micrograph. The inset SEM image in A shows a close-up of the marked region. (D-G) Bright-field optical micrographs of CuPc crystals (∼5-20 µm) deposited on SiO2 surfaces with 100 × 100 µm2 hydrophilic squares using different rinsing solvents. (H) Bright-field optical micrographs of a CuPc crystal (∼5-20 µm) suspension on a plastic substrate with 100 × 100 µm2 hydrophilic squares. The white arrow points to some of the crystals floating on top of the liquid suspension. In images A-G, the hydrophilic and hydrophobic regions are SiO2 and OTS-modified SiO2, respectively. In image H, the hydrophilic and hydrophobic regions are ITO and OTS-modified ITO, respectively.

crystals deposited on the hydrophilic regions using CuPc crystals (∼5-20 µm in length and width) as a model system. The time for crystal settling is defined as the period of time between placing a crystal suspension on a substrate and removing the crystal suspension from the substrate. Experiments with crystal settling times of 0 min (no settling time), 10 min, 1.5 h, and 3.5 h were performed. We observed that very few crystals were deposited after 0 min whereas a much larger number of crystals was deposited when the crystal settling time was longer than 10 min (image not shown). The number of crystals did not increase significantly when the crystal settling time was prolonged from 10 min to 3.5 h. This result suggests that the number of crystals deposited on the surface depends on the crystal concentration near the substrate surface. It takes less than ∼10 min for most of the CuPc crystals to precipitate in this case. Additionally, we found that instead of removing the crystal suspension from the substrates using a pipet, dip-coating or spincoating under appropriate conditions could also result in the selective patterning of crystals, which suggests that large-scale assembly and alignment may be realized using this method. The adhesion between the assembled crystals and the substrate surface was also tested. The assembled crystals could be easily rinsed off using water or methanol after the patterning process. They can also be transferred to a flat polydimethylsiloxane (PDMS) stamp by pressing a stamp gently on the substrate and then detaching it. No obvious damage to crystals was observed by rinsing or detachment. Therefore, the assembled crystals may be transferred to other surfaces or directly onto device structures. The second strategy is designed to pattern crystals selectively onto the solvent dewetting regions. Figure 3A-C shows the optical micrographs of CuPc crystals (∼5-20 µm in length and width) patterned selectively onto the hydrophobic regions using water as the rinsing solvent. In this experiment, the hydrophilic features are unmodified SiO2 surfaces, and the hydrophobic regions are OTS-functionalized SiO2 surfaces. Most of the crystals on the hydrophilic regions were removed after water rinsing, resulting in patterned crystals only on the hydrophobic regions. Using this method, crystals were patterned into a variety of

structures, including letters of the alphabet and reversed squares, which are difficult to achieve using the first strategy. Furthermore, this type of crystal patterning could be achieved over larger than centimeter scales. Besides water, other solvents were also used to rinse the films containing CuPc crystals. Figure 3D-G shows patterned CuPc crystals (∼5-20 µm in length and width) after rinsing with hydrophilic solvents such as ethylene glycol and hydrophobic solvents such as chloroform, hexane, and toluene. The 100 × 100 µm2 features are patterned hydrophilic regions. It can be seen that the patterns formed after rinsing with hydrophilic and hydrophobic solvents are reversed. Among the crystal patterns formed by rinsing with different hydrophobic solvents, the sample rinsed with chloroform shows the best selectivity, with almost no residue left on the hydrophilic regions (Figure 3 E). To better understand the mechanism for crystal patterning that strategy two is based on, we designed the following process that could be observed under an optical microscope. A water suspension of CuPc crystals (∼5-20 µm in length and width, 2 mg/mL) was placed on a pattered substrate and then allowed to precipitate for several minutes. No patterning was observed after the crystals settled. However, after the substrates were gently shaken several times back and forth, CuPc crystals formed patterns on the hydrophobic regions of the substrate as seen in Figure 3H. During the whole shaking process, substrates were covered with crystal suspensions, as indicated by an arrow in Figure 3 H. This result further supports the hypothesis that the formation of patterns by this strategy is a result of the selective removal of crystals in wetting regions by solvents. In addition, we were also able to apply the same patterning method on Au and plastic substrates with water contact angles similar to those for the SiO2 surfaces used above. In this case, HDT- and MPSA-modified Au and OTS-modified ITO and ITO were used, respectively. These results indicate that the selective patterning most probably depended on different wetting properties of the surfaces. One potential application of this study is the patterning of organic semiconductors for large-area field-effect transistors. The advantage of this approach is that it is applicable to any

7432 Langmuir, Vol. 23, No. 14, 2007

Letters

Figure 4. Bright-field optical micrograph of arrays of tPTTPt single-crystal transistors produced by the first patterning strategy (A) and electrical characteristics of the corresponding single-crystal transistors (B). (A) Au electrodes were functionalized with MPSA, and SiO2 surfaces were functionalized with OTS. (B) Transfer characteristics were measured at a fixed source-drain voltage, VDS ) -100 V. IDS: source-drain current. VG: gate voltage.

organic semiconductor crystals, both soluble and insoluble in organic solvents. Furthermore, organic single crystals typically show higher charge carrier mobilities than their thin film counterparts.8 The dispersion liquid used for this process is typically water, which is environmentally friendly. As a demonstration of concept, organic single crystals were directly patterned onto transistor source-drain electrodes. A highly doped Si wafer was used as the gate electrode whereas a thermally grown oxide (300 nm) was used as the dielectric layer. Bottomcontact devices with a channel length of 4 µm were defined by photolithography (metal thickness: Ti ) 1.5 nm and Au ) 40 nm). Au electrodes were functionalized to be hydrophilic with MPSA, and the SiO2 surface was rendered hydrophobic by OTS modification. Both CuPc and tPTTPt crystals were patterned directly onto transistor source-drain electrodes using the first strategy described previously (image for tPTTPt shown in Figure 4A). The crystals used are much larger than the electrode gap so that they can contact both electrodes. Field-effect transistor behavior was observed only for tPTTPt crystals as shown in Figure 4B. CuPc devices did not show any transistor behavior possibly because of the rough crystal surface (SEM image in Figure 2A inset) preventing good lamination of the crystal to the dielectric interface. Presently, the transistor performance observed for tPTTPt is lower than values reported in the literature because the transistor fabrication process has not been optimized so far. We have tried to bake the devices at ∼100 °C for 1 h inside the glovebox as well as dry the devices in vacuum overnight, but the device performance was not improved. For this assembly method to be useful for transistor device fabrication, it is crucial to prepare high-quality single crystals with clean, flat surfaces. Also, we found previously that the crystals need to be very thin (less than 1 µm) to form good conformal contact with the dielectric surface as well as the electrodes.8,15 Therefore, further work is needed to improve crystal quality and optimize assembly conditions and contact between the dielectric/semiconductor and electrode/semiconductor. Furthermore, as another future direction, the influence of SAMs on electrical transport properties should also be taken into consideration when SAMs are used to functionalize both of the electrodes and the dielectric surface. It has been reported that SAMs may

affect the charge injection at the metal-organic semiconductor interface as well as reduce the density of charge-trapping states at the semiconductor-dielectric interface.27,28

Conclusion We have reported two simple approaches to the self-assembly of organic single crystals. Both methods are based on solvent wetting and dewetting on substrates with patterned wettability. The advantage of these methods is their simplicity, involving only surface patterning, rinsing, and drop-casting, dip-coating, or spin-coating. Using the first strategy, we demonstrated the assembly of organic crystals such as CuPc with two different sizes and tPTTPt over centimeter-squared areas on Au, SiO2, and flexible plastic substrates. By designing appropriate line features on the substrate, the alignment of CuPc needles was achieved. Using the second strategy, the assembly of CuPc crystals on different substrate patterns was demonstrated. As a demonstration of concept, we used these methods to successfully pattern organic single-crystal transistor arrays. Future research will be directed to optimize the assembly process and improve the performance of the organic single-crystal transistors. Acknowledgment. S.L. acknowledges financial support from the Stanford Graduate Fellowship. W.M.W. acknowledges financial support from a Stanford Graduate Fellowship and a National Science Foundation Graduate Research Fellowship. S.C.B.M. acknowledges postdoctoral fellowship support from the Deutsche Forschungsgemeinschaft (DFG) through grant MA 3342/1-1. J.L. acknowledges financial support from an Intelligence Community Postdoctoral Fellowship. Z.B. acknowledges partial financial support from BASF Future Business, the Finmeccanica Faculty Scholar Award, a Sloan Research Fellowship, and the Center for Polymeric Interfaces and Macromolecular Assemblies (NSF-Center MRSEC under award no. DMR-0213618). LA700493P (27) Kim, S. H.; Lee, J. H.; Lim, S. C.; Yang, Y. S.; Zyung, T. Jpn. J. Appl. Phys. 2004, 43, 60-62. (28) McDowell, M.; Hill, I. G.; McDermott, J. E.; Bernasek, S. L.; Schwartz, J. Appl. Phys. Lett. 2006, 88, 073505.