Electric-Field-Assisted Position and Orientation Control of Organic

Oct 31, 2014 - The present result shows a possibility of controlling the position and orientation of organic single crystals by the use of an electric...
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Electric-Field-Assisted Position and Orientation Control of Organic Single Crystals Kenji Kotsuki,† Seiji Obata,‡ and Koichiro Saiki*,†,‡ †

Department of Chemistry and ‡Department of Complexity Science & Engineering, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan S Supporting Information *

ABSTRACT: We have investigated the motion of growing pentacene single crystals in solution under various electric fields. The pentacene single crystals in 1,2,4-trichlorobenzene responded to the electric field as if they were positively charged. By optimizing the strength and frequency of an alternating electric field, the pentacene crystals automatically bridged the electrodes on SiO2. The pentacene crystal with a large aspect ratio tended to direct the [11̅ 0] orientation parallel to the conduction direction, which will be suitable from a viewpoint of anisotropy in mobility. The present result shows a possibility of controlling the position and orientation of organic single crystals by the use of an electric field, which leads to high throughput and low cost industrial manufacturing of the single crystal array from solution. field was applied to a colloidal suspension of nanowires, which induced a dielectrophoretic (DEP) force in the nanowires. If the dielectric constant of the nanowire is larger (smaller) than that of the solvent, a positive (negative) Clausius−Mossotti factor generates the attractive (repulsive) DEP force between the nanowire and the region with high electric-field gradient.12 The DEP torque, arising from the interaction between the electric field and the polarization in the nanowire, aligns the nanowire with the electric field.12 As a consequence, the nanowire can bridge the two electrodes. In this paper, we have fabricated pentacene SC FETs from solution under the application of an electric field. Pentacene has been long a benchmark material in the development of OFETs, and various growth methods have been carried out so far. Solution growth of pentacene yielded thin films14 and single crystals.15 However, there have been no works to grow the pentacene SCs at the desired position and to control the orientation. We have found that pentacene SCs automatically hovered and landed on the channel between the FET electrodes by optimizing the strength and frequency of the electric field. Furthermore, when the SC had a large aspect ratio (length/width), the crystallographic orientation tended to align with the electric field. The FET measurement revealed that this orientation was suitable for the charge carrier transport in pentacene SCs.

1. INTRODUCTION Organic field effect transistors (OFETs) have attracted much attention as the electronic device of the future due to their flexibility, wide applications, and low environmental impact. One of the most advantageous points of organic substances is the availability of solution processes, which will realize low-cost production of electronic devices. Based on recent developments of soluble organic semiconductor molecules, various kinds of OFETs have been fabricated through a solution process.1−8 Among these devices, the solution-processed single crystal OFETs (SC-OFETs) show excellent high field-effect mobility, probably because they have less grain boundaries and a higher degree of crystallinity.5−8 To fabricate an array of SC-OFETs for practical applications, it is necessary to grow organic SCs precisely at the channel regions.1,2,9 The sites at which the SCs are grown are often formed by the use of photolithography. In this case, there is no reason that the crystal orientation aligns with each other among the SCs. Due to the large anisotropy of electrical properties in organic substances, especially in the SCs, the randomly oriented SCs in the array might result in a wide dispersion of the device performance. In addition, rather a complicated surface treatment process is necessary to fabricate the template for the growth of organic SCs. To solve these problems, we propose a facile top-down fabrication process using an electric field. While we investigated the solution growth of pentacene SCs, we have found that the growing pentacene crystallites in solution responded to the electric field. With respect to the inorganic substances, there have been several works on the electric field assisted assembly of nanowires.10−13 The electric © 2014 American Chemical Society

Received: August 17, 2014 Revised: October 30, 2014 Published: October 31, 2014 14286

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continued to increase approximately for 20 s (Figure1b). Then the electric field was applied (Figure1c). Solvent evaporated gradually and finally disappeared after 300 s (Figure 1d). During the above procedure, the solution was observed by optical microscopy with a high speed camera.

2. EXPERIMENTAL SECTION The present crystallization procedure is schematically illustrated in Figure 1. The FET electrodes of gold or reduced graphene oxide

3. RESULTS AND DISCUSSION 3.1. Application of DC Electric Field. At first, an electrostatic field was applied between the two electrodes (Figure 2). When the electric field strength was 1.0 × 105 V/m, part of the pentacene microcrystals moved toward the cathode and were captured (Figure 2a). The remaining part of the pentacene microcrystals, on the other hand, dispersed over the substrate and were not captured at the channel region. When the electric field was increased to 2.0 × 105 V/m, most of the pentacene microcrystals moved to the cathode and covered the channel in the direction from the cathode to the anode (Figure 2b). When the electric field was further increased to 1.4 × 106 V/m, the similar result was observed (Figure 2c). These results indicate that pentacene crystals are positively charged and electrophoresis occurs in solution. Such a response of organic materials in solvent was observed also in the case of C60 in mxylene or isopropyl alcohol and rubrene in p-xylene15(summarized in Table S1, Supporting Information). In the case of an electrostatic field, however, most of the crystals gathered at the cathode. To place the pentacene crystal just between the two electrodes, application of an alternating voltage was examined. 3.2. Application of AC Electric Field. Figure 3a shows the polarized optical micrographs of a pentacene crystal under an alternating electric field of a square wave (6.0 × 105 V/m, 10 Hz). Pictures (i)−(iv) show the movement of the pentacene crystal within 0.2 s, which corresponds to two cycles of the square wave. Pictures (i) and (iii) show the state at which the moving direction of pentacene crystal was just reversed to the right, while pictures (ii) and (iv) show the state at which the moving direction was just reversed to the left. The color of the crystal looks violet at states (i) and (iii), while it looks brown at states (ii) and (iv). Along with the reversal of moving direction, the pentacene crystal rotated slightly, resulting in the change of color. A similar phenomenon was observed for the other frequency (0.5 Hz), which is shown in Figure S1 in the Supporting Information. The uniform color over the crystal and its sensitive response to the crystal direction indicated single crystallinity of pentacene crystals. The above observation revealed that the pentacene crystal responded to the alternating electric field and moved periodically on the channel. To find the optimum condition of placing the pentacene crystal (a typical size of 100 μm in length) just on the channel (50 μm length), the strength and

Figure 1. Schematic illustration of crystallization process of pentacene single crystals. (RGO) were formed on a SiO2 substrate beforehand. The channel length was 50 μm. In the case of gold electrodes, the SiO2/Si substrate (the thickness of oxide layer 300 nm) was cleaned by exposing it to ozone gas at 50 °C for 10 min using a UV/ozonizer (Samco, UV-1). Then, the electrodes with a thickness of 20 nm were formed by evaporating gold through a shadow mask in vacuum. On the other hand, the RGO electrodes were fabricated in a similar way as described in ref 16. The SiO2/Si substrate with a mask on was exposed to ozone gas at 50 °C for 10 min using a UV/ozonizer. The area exposed to the UV/ozone irradiation turned hydrophilic, while the area covered with the mask remained hydrophobic. The graphene oxide (GO) solution was prepared from graphite powder (supplied from Nippon Graphite Industries and SEC Carbon) through a modified Hummers’ method,17 and it was sonicated for 30 min to crack the GO sheets into small flakes with a size of several micrometers. By spin-coating this solution onto the substrate at a rate of 3000 rpm, the GO flakes were deposited only on the hydrophilic area. The GO film with a thickness of 5−10 nm was exposed to hydrazine-hydrate vapor at 100 °C for 1 h and subsequently annealed in vacuum at 500 °C for 30 min. This process converted the GO film to a highly conductive RGO sheet, the conductivity of which was ∼103 S/m.18 Finally, to turn the channel solvophilic with 1,2,4-trichlorobenzene, this substrate was exposed to ozone gas at 50 °C for 1 min using the UV/ozonizer. To prevent the pentacene molecules from oxidation, a crystallization process was performed under a N2 atmosphere in a glovebox. Pentacene (Sigma-Aldrich and used after purification) was dispersed in 1,2,4-trichlorobenzene (Tokyo Chemical Industry Co.) at the concentration of 0.025 wt % at 200 °C. This solution was drop-cast on the substrate kept at 130 °C (Figure 1a). Immediately after the drop-cast, pentacene microcrystals appeared in the liquid and their size

Figure 2. Pentacene microcrystals captured around the channel under various electrostatic fields: 1.0 × 105 V/m (a), 2.0 × 105 V/m (b), and 1.4 × 106 V/m (c). Dashed lines in (b) and (c) indicate the position of electrode edges. 14287

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Figure 3. (a) Polarized optical microscope images of a pentacene single crystal between gold electrodes under the AC electric field (6.0 × 105 V/m, 10 Hz) of a square wave for 0.2 s. The left electrode is biased, and the right one is grounded. The change of crystal color reflects that of the molecular orientation. (b) Illustration of motion of pentacene crystals for various combinations of frequency and strength of AC electric field observed in the video. Red arrows indicate the amplitude of each crystal, and the rectangles surrounded by dashed lines show the results.

frequency of the electric field was changed variously. At first, the frequency was changed from 1 to 103 Hz at 6.0 × 105 V/m. The amplitude of vibration was found to become smaller with increasing frequency (Video S1, Supporting Information). At a frequency of 103 Hz, the crystal could not follow the alternating electric field probably because of the viscous damping. At a frequency of 1 Hz, on the other hand, the crystal moved over the electrode’s surface. The crystals could move over the electrode because the solvent with finite thickness covers the electrode and the electric field exists over the electrode with the component perpendicular to the surface. At medium frequencies of 10−102 Hz, the crystal tended to vibrate mostly on the channel. Next the strength of an electric field was changed from 2.0 × 104 V/m to 6.0 × 105 V/m at a frequency of 10 Hz (Video S2, Supporting Information). The amplitude of crystal vibration was found to increase with the strength of the electric field. Too low electric field could attract no crystal. For the high electric fields, however, pentacene crystals tended to move over the electrode’s surface and were trapped on the electrode. In the case of medium electric fields, the pentacene crystals vibrate just around the channel. The movement of crystals for various electric fields and frequencies are summarized in Figure 3b. For the channel length of 50 μm, the optimized strength and frequency of electric field were 1.5 × 105 V/m and 100 Hz, respectively. Under this condition, pentacene crystals with submillimeter length and submicrometer thickness could be successfully placed on the channel automatically. In the following, the pentacene crystal grown under the optimum condition was analyzed. The growth time was fixed at 300 s. 3.3. Characterization of Crystal Orientation. The orientation of pentacene crystal placed between the electrodes will be discussed. Figure 4a shows a typical example of the pentacene crystal captured on the channel. The pentacene crystals have two parallel sides, while the apexes of an acute

angle are truncated slightly in most of the crystals as shown in Figure S2a−c (Supporting Information). The acute angle between two sides ranges from 72° to 75°, which is close to that of a bulk phase crystal (75°) rather than that of a single crystal phase (77°).19 Based on the structure of a bulk phase pentacene crystal (Figure S2d), the longer and shorter sides in Figure 4a are ascribed to the (110) and (1̅10) facets of a pentacene crystal, respectively, and the crystal axis is determined as shown in Figure 4b. To analyze the distribution of the crystal orientation with respect to the channel, θ is defined as an angle of the [1̅10] direction measured from the edge of electrode as shown in Figure 4a. θ is 0° when the [1̅10] direction is parallel to the electrode edge, and θ is 104.8° when the [110] direction is parallel to the electrode edge. Further, the aspect ratio A is defined as the ratio of the length along the (110) facet to that along the (1̅10) facet. In the case of the crystal in Figure 4a, θ is 82° and A is 2.2. We evaluated θ and A for various pentacene crystals captured on the channel and the relation between θ and A is shown in Figure 4c. For the pentacene crystal with an aspect ratio lower than 1.5, θ seems to have no relation with the aspect ratio A. With increasing A, the variation of θ decreases and θ becomes close to 90°. This tendency is considered to originate from a DEP torque which has been reported to align the inorganic nanowire10,12 and rubrene crystals.20 In the present case, the DEP torque works more effectively for the pentacene crystals with a large A. 3.4. Characterization of FET Performance. We characterized the FET performance of the pentacene crystals fabricated by the above method (1.5 × 105 V/m, 100 Hz, and 300 s). The FETs with the RGO electrodes showed a higher mobility than those with gold electrodes. This improvement of mobility could be ascribed to the affinity of pentacene to RGO.21 In the following, the mobility was evaluated for the FETs with the RGO electrodes. Figures 5a 14288

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Figure 5. (a) Output and (b) transfer characteristics of the pentacene crystal shown in Figure 4a. (c) Field-effect mobility μ of pentacene FETs as a function of the angle θ.

al. first observed the anisotropy in field effect mobility of pentacene SC although the relation with the crystal orientation was not determined.22 Soeda et al. observed a high mobility in the [010] direction, while the mobility decreases by an order of magnitude in the [100] direction.23 Therefore, the high mobility in the [010] direction in Figure 5c is consistent with the previous result. The increase in mobility was observed also around 81° and 108° in the present study. Recent firstprinciples calculation indicated that the mobility in the pentacene crystal becomes large in the directions centered at [11̅ 0].24 Since the angles 81° and 108° are close to the [11̅ 0] direction, the increase in this region might be explained in terms of highest coupling for holes.24 However, it is too premature to discuss the anisotropy further at the present stage because another factor such as the contact between the SCs and the electrode might affect the apparent anisotropy. Finally, we mention the mobility of pentacene SCs. The maximum mobility of 0.49 cm2/(V s) in the present work is smaller than the values reported before (2−5 cm2/(V s)), which were fabricated in vacuum or ionic liquid.22,23,25 In vacuum, the mixing of impurities was suppressed during vapor growth. Due to the immiscibility of pentacene in fluorine based ionic liquids, pentacene would exclude the remnant during the crystal growth.25 In contrast, 1,2,4-trichlorobenzene which has a high boiling point and good affinity with pentacene, is likely to remain in the crystals. Moreover, in the previous study, pressure

Figure 4. (a) Optical image of a pentacene single crystal automatically placed between the RGO electrodes. (b) Pentacene crystal structure corresponding to that in (a). (c) Crystallographic orientation of pentacene crystals as a function of the aspect ratio A for the samples formed on the channel between the RGO electrodes. θ is the angle between the [1̅10] direction and the electrode edge as shown in (a).

and b shows typical output and transfer characteristics, which were measured on the device made of the pentacene crystal as shown in Figure 4a. The absence of nonlinear nature and high pinch-off voltage are considered to originate from a small contact resistance and a low threshold voltage. Indeed, an average threshold voltage of whole devices was obtained as small as −0.8 ± 4.2 V. The FET mobility evaluated in the saturation region is plotted as a function of θ in Figure 5c. Most of the samples show a mobility higher than 0.2 cm2/(V s), although the FETs have been formed in a fast and simple way. Higher mobility is observed for the samples with θ around 51°, 81°, and 108°. The vertical dashed lines in Figure 5c mean the angles at which the primitive low index directions [010], [1̅10], and [100] are parallel to the conduction direction (perpendicular to the electrode edge). The anisotropy in the transport properties of pentacene SCs have been studied experimentally and theoretically.22−24 Lee et 14289

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Bao, Z. Tuning Charge Transport in Solution-Sheared Organic Semiconductors Using Lattice Strain. Nature 2011, 480, 504−508. (4) Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S. C. B.; Chen, J.; Nordlund, D.; Toney, M. F.; Huang, J.; Bao, Z. UltraHigh Mobility Transparent Organic Thin Film Transistors Grown by an Off-Centre Spin-Coating Method. Nat. Commun. 2014, 5, 3005. (5) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.; Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet Printing of Single-Crystal Films. Nature 2011, 475, 364−367. (6) Liu, C.; Minari, T.; Lu, X.; Kumatani, A.; Takimiya, K.; Tsukagoshi, K. Solution-Processable Organic Single Crystals with Bandlike Transport in Field-Effect Transistors. Adv. Mater. 2011, 23, 523−526. (7) Soeda, J.; Hirose, Y.; Yamagishi, M.; Nakao, A.; Uemura, T.; Nakayama, K.; Uno, M.; Nakazawa, Y.; Takimiya, K.; Takeya, J. Solution-Crystallized Organic Field-Effect Transistors with ChargeAcceptor Layers: High-Mobility and Low-Threshold-Voltage Operation in Air. Adv. Mater. 2011, 23, 3309−3314. (8) Li, H.; Tee, B. C.; Cha, J. J.; Cui, Y.; Chung, J. W.; Lee, S. Y.; Bao, Z. High-Mobility Field-Effect Transistors from Large-Area SolutionGrown Aligned C60 Single Crystals. J. Am. Chem. Soc. 2012, 134, 2760−2765. (9) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. Patterning Organic Single-Crystal Transistor Arrays. Nature 2006, 444, 913−917. (10) Liu, Y.; Chung, J.; Liu, W. K.; Ruoff, R. S. Dielectrophoretic Assembly of Nanowires. J. Phys. Chem. B 2006, 110, 14098−14106. (11) Raychaudhuri, S.; Dayeh, S. A.; Wang, D.; Yu, E. T. Precise Semiconductor Nanowire Placement through Dielectrophoresis. Nano Lett. 2009, 9, 2260−2266. (12) Freer, E. M.; Grachev, O.; Duan, X.; Martin, S.; Stumbo, D. P. High-Yield Self-Limiting Single-Nanowire Assembly with Dielectrophoresis. Nat. Nanotechnol. 2010, 5, 525−530. (13) Shekhar, S.; Stokes, P.; Khondaker, S. I. Ultrahigh Density Alignment of Carbon Nanotube Arrays by Dielectrophoresis. ACS Nano 2011, 5, 1739−1746. (14) Minakata, T.; Natsume, Y. Direct Formation of Pentacene Thin Films by Solution Process. Synth. Met. 2005, 153, 1−4. (15) Kimura, Y.; Niwano, M.; Ikuma, N.; Goushi, K.; Itaya, K. Organic Field Effect Transistor Using Pentacene Single Crystals Grown by a Liquid-Phase Crystallization Process. Langmuir 2009, 25, 4861−4863. (16) Kotsuki, K.; Tanaka, H.; Obata, S.; Stauss, S.; Terashima, K.; Saiki, K. The Importance of Spinning Speed in Fabrication of SpinCoated Organic Thin Film Transistors: Film Morphology and Field Effect Mobility. Appl. Phys. Lett. 2014, 104, 233306. (17) Hirata, M.; Gotou, T.; Horiuchi, S.; Fujiwara, M.; Ohba, M. Thin-Film Particles of Graphite Oxide 1: High-Yield Synthesis and Flexibility of the Particles. Carbon 2004, 42, 2929−2937. (18) Obata, S.; Tanaka, H.; Saiki, K. Electrical and Spectroscopic Investigations on the Reduction Mechanism of Graphene Oxide. Carbon 2013, 55, 126−132. (19) Yoshida, H.; Sato, N. Crystallographic and Electronic Structures of Three Different Polymorphs of Pentacene. Phys. Rev. B 2008, 77, 235205. (20) Matsukawa, T.; Kobayashi, S.; Onodera, T.; Oikawa, H.; Itaya, K. Electric-Field-Induced Orientation Control of Organic Semiconductor Rubrene Crystals. Mater. Chem. Phys. 2013, 137, 947−950. (21) Lee, S.; Jo, G.; Kang, S.; Wang, G.; Choe, M.; Park, W.; Kim, D.; Kahng, Y. H.; Lee, T. Enhanced Charge Injection in Pentacene FieldEffect Transistors with Graphene Electrodes. Adv. Mater. 2011, 23, 100−105. (22) Lee, J. Y.; Roth, S.; Park, Y. W. Anisotropic Field Effect Mobility in Single Crystal Pentacene. Appl. Phys. Lett. 2006, 88, 252106. (23) Soeda, J.; Okamoto, T.; Hamaguchi, A.; Ikeda, Y.; Sato, H.; Yamano, A.; Takeya, J. Two-Dimensional Crystal Growth of Thermally Converted Organic Semiconductors at the Surface of Ionic Liquid and High-Mobility Organic Field-Effect Transistors. Org. Electron. 2013, 14, 1211−1217.

was applied to ensure the contact between the SC and the electrode,22 or the top-contact gold electrode was modified with F4-TCNQ to improve the carrier injection.23 In the present work, we have focused on the position and orientation control of growing pentacene SC by an electric field and thus the FET measurement was done for the as-deposited samples from solution. The remaining solvent and the insufficient contact with the electrode might deteriorate the FET performance. There seems to be room to increase the mobility by optimizing the fabrication process.

4. SUMMARY In summary, we have developed a method to control the position of a growing organic single crystal by an electric field. The real-time observation revealed the pentacene single crystals responded to the electric field as if they were positively charged in solution. The use of the optimized AC electric field could automatically place the pentacene crystals between the electrodes, and the crystals were deposited just bridging the two electrodes probably by a dielectrophoretic force. The crystal with a large aspect ratio tended to direct the [1̅10] orientation parallel to the conduction direction, which will be suitable from a viewpoint of anisotropy in mobility. The present result shows a possibility of controlling the position and orientation of organic single crystals by the use of an electric field, which leads to high throughput and low cost industrial manufacturing of SC-OFETs array from solution.

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ASSOCIATED CONTENT

S Supporting Information *

Video S1 showing the motion of pentacene crystals under the AC electric field of 6.0 × 105 V/m at frequencies from 1 to 1000 Hz. Video S2 showing the comparison of two crystals under the AC electric field of 6.0 × 105 and 1.5 × 105 V/m at a frequency of 10 Hz. Table S1 summarizing the electrophoretic direction of various molecules in solvent. Figures S1−S3 showing the crystal movement at 0.5 Hz, examples of pentacene crystals together with assignment of crystal orientation, and the distribution of FET mobility with respect to the aspect ratio, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Nippon Graphite Industries, Ltd. and SEC Carbon, Ltd. for kindly providing us graphite powder for preparation of graphene oxide.

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REFERENCES

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(24) Stehr, V.; Pfister, J.; Fink, R. F.; Engels, B.; Deibel, C. FirstPrinciples Calculations of Anisotropic Charge-Carrier Mobilities in Organic Semiconductor Crystals. Phys. Rev. B 2011, 83, 155208. (25) Takeyama, Y.; Ono, S.; Matsumoto, Y. Organic Single Crystal Transistor Characteristics of Single-Crystal Phase Pentacene Grown by Ionic Liquid-Assisted Vacuum Deposition. Appl. Phys. Lett. 2012, 101, 083303.

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