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Position-selective solution phase growth of fullerene crystals
Carbon 145 (2019) 31e37
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Carbon journal homepage: www.elsevier.com/locate/carbon
Position-selective solution phase growth of fullerene crystals Chibeom Park a, 1, Jinho Lee a, b, 1, Minkyung Lee b, Taekyung Yoon a, b, Hee Cheul Choi a, b, * a b
Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-ro, Nam-Gu, Pohang, 37673, South Korea Department of Chemistry, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-Gu, Pohang, 37673, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2018 Received in revised form 25 December 2018 Accepted 3 January 2019 Available online 4 January 2019
An efficient method for the formation of hexagonal disk shape C60 crystals at desired positions on a solid substrate has been achieved. The detailed growth behavior was studied through in-situ observation and series of control experiments. Our position selective growth technique enabled a fabrication of bottom electrode type C60 single crystal devices which exhibited n-type field-effect transistor behavior and photo response. Moreover, the C60 single crystal devices were further exploited to study alkali metal doped C60 crystal and its superconductivity. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction The crystallization of highly conjugated organic molecules has been one of the most important research subjects for over hundred years not only for fundamental scientific interests in understanding intermolecular interactions involved in crystallization into specific morphologies, but also for high potential in optoelectronic device applications [1,2]. In most cases of organic crystal devices, the crystals grown in solution or vapor phase are manually transferred on a solid substrate, and their optical and electrical properties are characterized as a form of proto-type device [3e5]. However, the inefficiency and difficulty of transfer process limits fundamental and accurate investigation on their properties as well as the extension to further applications such as large area electronic devices with high throughput and efficient integration. Therefore, it is necessary to develop methods to directly crystallize organic molecules into crystals at desired positions on a solid substrate, which has been demonstrated in some reports [6e10]. Most of these attempts involve a substrate surface modification to overcome the strong tendency of organic molecules to self-organize in random locations. Representatively, Briseno et al. fabricated large array of organic single crystal field effect transistor (FET) by selective crystallization of vaporized organic molecules on a printed octadecyltriethoxysilane (OTS) film [6]. Similar approach has been attempted
* Corresponding author. Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-ro, Nam-Gu, Pohang, 37673, South Korea. E-mail address: [email protected] (H.C. Choi). 1 These authors equally contributed to this work. https://doi.org/10.1016/j.carbon.2019.01.012 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
for solution phase crystallization by Goto et al. by employing micro patterned holes on a substrate for nucleation region [7]. However, the substrate modification process for position-selective crystallization used in both cases potentially possesses undesired or extrinsic effect on device performance. Therefore, it is necessary to develop a new method that grants crystallization of target molecules with a position selectivity. Among diverse target molecules, fullerene was chosen to demonstrate above-mentioned approach. The spherical molecular geometry and high symmetry of fullerene provide large void space in its crystal, which can be occupied by various chemical species such as alkali metal atoms and small organic molecules [11,12]. It is well known that the alkali metal atoms intercalated between C60 molecules provide electrons and make them superconducting when doped properly [13e15]. Another interesting phenomenon is that the inclusion of solvent molecules during the solution phase crystallization results in peculiar self-assembly behavior, by which fullerene molecules are spontaneously guided to form into specific morphologies depending on the type of solvent [16e20], solvent ratio [21] and the composition ratio in the case of C60 - C70 hetero crystals [22]. We previously showed that hexagonal disks and wire structures could be selectively obtained by solution phase crystallization using specific solvents [19], and their growth could be further controlled by changing the direction of solvent drying force of solution droplet [23] when the crystallization occurs especially by the drop-drying process. These results imply the possibility that the fullerene crystals can be grown at the desired positions of substrate under controlled crystallization environment, which will enable in-depth analysis of crystallization process as well as further studies on individual fullerene crystals, especially focusing on the
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transport and superconducting properties in forms of electrical devices. Herein we report a simple but efficient way to form the hexagonal disk shape C60 single crystals at desired positions on a solid substrate. The C60 crystals were directly grown on a thermally oxidized silicon or fused quartz substrate by drop-drying method, and the position selectivity was provided by pre-patterned C60 thin films which acted as crystallization seeds. Such position controlled C60 crystal growth enabled in-situ monitoring of the crystallization process to unveil the growth behavior. The position-selectivity and the shape of C60 crystals were further controlled by changing the dimension of seed patterns. By applying the same method on a substrate with pre-defined metal electrodes, we fabricated bottom electrode type C60 single crystal electronic devices which showed clear n-type semiconducting behavior and electrical photo responses. The C60 single crystal device was further utilized to study the optical and electrical property change upon potassium doping process. The implementation of these devices demonstrates that our position-selective crystal growth can be extended to other large area device applications. 2. Experimental procedure 2.1. Position-selective growth of C60 single crystal C60 powder (MTR Ltd., purity > 99.95%) was used as a source without further purification. A patterned mask to deposit the C60 seed thin film pattern was fabricated by standard photolithography or e-beam lithography technique (spacing between patterns: 50 mm). The C60 seed thin film was prepared by thermal evaporation on a SiO2/Si or fused silica substrate under 108 torr at 510 C followed by the lift-off process less than 30 s (The detailed process is described in ESM) The crystallization solution was prepared by filtrating the saturated C60 solution dissolved in CCl4 using a syringe filter (Whatman International Ltd., pore size > 20 nm) to remove undissolved C60 source. 50 ml of filtered solution was dropped onto the patterned C60 thin film on a substrate. To provide enough time for crystal growth, we performed the drop drying process on the prepared substrate placed in a vial filled with 3 ml of additional CCl4.
2.4. Image processing for C60 crystal growth analysis Each optical microscope image was first divided into each seed pattern and then cropped to square images (124 x 124 pixels, corresponding size of 25 25 mm). The RGB images were converted to 8-bit grayscale after an adjustment of the contrast and brightness, then the grayscale was inverted to make the crystal area bright. The resulting images were again converted to black and white image with a threshold value that represents the initial RGB images the best. The number of white pixels was converted to the area of individual C60 crystal, and the square root of the area was used for lateral dimension. The morphology of C60 seeds was examined by tapping mode atomic force microscopy (AFM, Nano scope IIIa, Digital Instrument Inc.) 2.5. Fabrication and measurement of C60 single crystal devices The metal electrodes (5 nm Ti/30 nm Au) were pre-defined by a conventional photolithography and lift-off process on a SiO2/Si (300 nm thick SiO2 on a p-type Si) substrate. The seed patterns were formed to be located between metal electrodes and position selective C60 crystal growth was performed. The field-effect transistor behavior (Ar environment) and photo response (ambient condition) were measured using a semiconductor analyzer (Keithley 4200s) at room temperature. 2.6. Potassium doping of C60 single crystal devices and in-situ measurements of Raman spectra and electrical resistance change Potassium doping was performed at high vacuum (P ~ 106 Torr) using a home-built chamber which can provide potassium vapor by heating elemental potassium. Before the doping, C60 single crystal was annealed at 120 C to remove the intercalated solvent. Twoprobe electrical resistance was measured using a semiconductor analyzer (Keithley 4200s) and Raman spectrum was measured using a WITEC Alpha 300R Raman spectroscope with a 532 nm laser. The power of laser was maintained below 100 mW to prevent thermal damage of the crystal. The detailed experimental setup and process are described elsewhere [24]. 2.7. Measurement of superconductivity
2.2. TEM sample preparation 50 nm thick Au film was deposited on top of the C60 crystal arrays grown on a SiO2/Si substrate using e-beam evaporator. The Au film containing C60 crystal arrays was separated from the substrate by etching SiO2 with 1M KOH aqueous solution, then transferred onto a 20 nm-thick silicon nitride membrane TEM grid. The Au film was then removed by placing the Au side of the TEM grid down on the surface of commercial Au etchant solution. The HRTEM image and electron diffraction patterns were acquired using a transmission electron microscope (TEM) in National Institute for Nanomaterials Technology (NINT) at POSTECH. 2.3. In-situ observation of C60 crystal array growth A transmission mode inverted optical microscope (Nikon, Eclipse TS100) equipped with a long working distance (18 mm) objective (Olympus, SLMPLN50x) was used to observe the position selective C60 crystal growth. The drop drying process condition was identical to the above-mentioned position selective growth of C60 crystal except that the SiO2/Si substrate was replaced with a transparent fused silica. The optical microscope images were consecutively obtained using a CMOS camera (Lumenera Corp., INFINITY1-1M) at every second right after the C60/CCl4 solution was dropped.
The C60 single crystal devices properly doped with potassium was mounted in a closed-cycle cryostat (Sungwoo Instrument, Korea) which was connected to a glove box to prevent sample exposure to air. Four-probe electrical resistance was measured using a semiconductor analyzer (Keithley 4200-scs model) and the temperature was measured using a diode temperature sensor directly in contact to the sample substrate. 3. Results and discussion 3.1. Position-selective growth of C60 single crystal Fig. 1a schematically depicts our approach to grow hexagonal shape C60 single crystals at specific positions on a solid substrate. Saturated C60 solution dissolved in CCl4 was drop-dried in the presence of CCl4 vapor on top of a SiO2/Si substrate where the arrays of C60 thin film patterns were pre-defined as crystallization seeds. (See Methods for detailed process) These seed arrays are composed of circular C60 thin film patterns evenly distributed along the substrate (1 cm 1 cm) with a regular spacing of 50 mm (Fig. 1b). The scanning electron microscope (SEM) and lowmagnification optical microscope images show that the resulting crystals are located only at the positions of C60 thin film seed
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Fig. 1. Position-selective growth of C60 single crystal on solid substrate. (a) Schematic illustration of experiments. (b) Scanning electron microscope (SEM) image of C60 thin film seed patterns. (c) SEM image of C60 single crystal arrays selectively grown at seed positions. (Inset: Low-magnification optical microscope image) (d) Zoomed in SEM image of (c). (e) X-ray diffraction (XRD) of C60 crystal arrays. (f) Transmission electron microscopy (TEM) image of hexagonal-shaped C60 single crystal and (g) corresponding electron diffraction. (A colour version of this figure can be viewed online.)
patterns, and such a position-selectivity is observed over the entire substrate (Fig. 1c). It should be noted that similar position-selective growth of C60 crystal is also possible with other solvent systems (Fig. S1 in ESM). Most of the crystals show overall hexagonal shape in the top view SEM image, which implies that the lateral crystal growth is preferred. As shown in the zoomed-in SEM image in Fig. 1d, some of the crystals are single hexagonal disks while others have aggregated form which is composed of multiple crystals. We previously reported that C60 molecules crystallize together with CCl4 into hexagonal shape single crystals upon drop-drying process [19]. To investigate crystalline feature of the C60 crystal directly grown on the seed pattern in detail, we performed X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments. XRD data from as-grown sample showed one dominant peak at 8.23 of two theta which is well matched to (0001) plane of reported simple hexagonal structure (a ¼ 10.10 Å, c ¼ 10.75 Å) [25] (Fig. 1e). This result indicates that the wide top surface of the hexagonal disk crystal is parallel to the substrate, as also confirmed by SEM images. Note that the small peak at 10.84 ο comes from (111) plane of face-centered cubic pure C60 crystal structure (a ¼ 14.11 Å), which might be generated from natural escape of CCl4. For further crystal structure study in TEM, the as-grown crystals were transferred onto a TEM grid using e-beam evaporated gold thin film (See Methods and Fig. S2 in ESM for the detailed sample preparation).
The bright-field TEM image shows two sharp edges of hexagon crossing at 120 ο (Fig. 1f). The corresponding selected area electron diffraction (SAED) pattern exhibited clear hexagonal symmetry, and the d-spacing value of each spot was well matched to the known crystal structure [25] (Fig. 1g). Same SAED patterns were obtained at multiple locations of each crystal, which indicates that the C60 crystal grown on the seed pattern is also single crystalline similar to the one grown from a regular drop-drying process. From numerous experiments, we found that the introduction of CCl4 vapor environment during drop-drying process was a very important factor for the successful position-selective growth of C60 crystal arrays. In ambient condition, the solvent completely dried in less than 2 min, and unwanted small aggregates were obtained at off-seed positions (Fig. S3 in ESM). When the solvent drying time was increased to 20 min in a sealed vial condition, the position selectivity was improved but the resulting crystals are lack of welldefined morphologies. On the other hand, under CCl4 environment achieved by putting additional CCl4 on the bottom of vial, solvent drying time was further increased, and hexagonal disk shape C60 crystals were grown only at seed positions. This result implies that slow crystallization is essential for position-selectivity as well as well-defined morphologies, which is in accordance with common phenomenon in single crystal growth [26,27]. However, such a significant improvement of crystal growth with additional CCl4 is
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not only attributed to the increase in solvent drying time, but it is also related to the solvent annealing effect before the drop-drying process started. As shown in the atomic force microscope (AFM) images, the as-prepared C60 thin film seed pattern is greatly deformed just by an exposure to CCl4 vapor for a few minutes. A few bumpy structures were formed, and the rms surface roughness of flat area was also increased from 1.22 nm to 2.15 nm (Fig. S4 in ESM). The deformation of seed pattern occurs through a repetitive dissolution and condensation of C60 molecules by adsorbed solvent molecules, which naturally happens in the drop-drying with additional CCl4. Indeed, the position-selective growth was more successful when the seed patterns were stayed in the CCl4 vapor for a few more minutes before the solution drop, instead of immediate solution drop (Fig. S5 in ESM). Therefore, we performed all the drop-drying process in an uncapped vial with partially filled CCl4 at the bottom, and the solution was dropped a few minutes after the substrate was placed. 3.2. In-situ observation of position-selective growth of C60 single crystals In situ observation of crystal growth is generally very difficult, especially for the solution phase crystallization since the crystals are mobile in the solution during the crystallization process, which makes it almost impossible to track individual ones for clear understanding the crystal growth mechanism. However, in our case, since the crystal growth occurs only at specific positions, we could directly monitor and analyze the entire crystallization process. For this, we replaced the SiO2/Si substrate with a transparent fused silica and continuously monitored the growth of C60 crystals from the bottom of the vial using a transmission mode inverted optical microscope (See the Methods and Fig. S6 in ESM for detailed experimental setup). After applying a droplet of C60/CCl4 solution on the target substrate, we obtained series of images from total 20 seed patterns at every second, then combined them into a movie clip (Movie S1 in ESM). Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.01.012. By processing each image, we also calculated the lateral dimension of crystal and growth rate. (See the Methods for detailed image processing) Fig. 2a and b show a representative result (third row and fourth column in Movie S1 from ESM) among 20 seed positions. The seed pattern is clearly shown in the first image taken at 1 min, and no significant change was observed until 10 min. However, the boundary of seed pattern becomes blurry implying that the seed patterns were deformed by the solvent. Upon further drying of solvent, a small black dot appears at bottom left of the seed at 10.5 min, and it gradually grows but its lateral dimension saturates at 1.4 mm. From 12 min, however, the lateral dimension of crystal abruptly increased with a rate of 2.9 mm/min and forms hexagonal shape at 14 min. The hexagonal shape crystal further grows to about 15 mm until the solvent completely dries off at 22 min. Note that the spikes appeared at 22 min in both graphs are the result of the agglomerated solvent before complete drying, as shown in the image. One interesting observation is that the color intensity of the crystal remains same after 14 min. This implies that the crystal growth dominantly occurs lateral direction since the images were taken under transmission mode and the color intensity is closely related to the thickness of the crystal. All other crystals grown at different positions also showed similar growth behavior with final lateral dimensions of 14.7 ± 1.67 mm except one missed crystal (Fig. S7a in ESM). As also observed in Fig. 1d, some crystals are stacked or aggregated form, which is originated from multiple nucleation at one seed pattern. The number of nucleation at each seed pattern varies from 0 to 4 over 20 seed positions of our
observation area (Fig. S7b in ESM). All nucleation sites on each circular seed pattern are represented in Fig. S7c in ESM. Although the center of seed pattern seems to be preferred for nucleation site, the nucleation also occurs at other areas. According to all the results above, the crystallization process can be summarized as follows (Fig. 2c). When the thin film C60 seed pattern is exposed to CCl4 vapor for a few minutes before solution drop, solvated nucleates are generated on the seed pattern by the solvent annealing effect. (Stage 0) After the C60/CCl4 solution is dropped, the pre-formed nucleates collect C60 and CCl4 molecules from the solution. (Stage I) At this time, the solution becomes locally supersaturated near the seed pattern because C60 molecules are also provided from the seed pattern, which minimize new nucleation at other location. As solvent dries, continued collection of C60 and CCl4 molecules makes nucleates big enough to be shown by an optical microscope. (Stage II) Once the nucleate reaches to a certain size, the crystal grows laterally resulting in hexagonal disk shape, (Stage III) and the crystallization process is ceased with complete drying of solvent. (Stage IV). 3.3. Position-selective C60 crystal growth behavior depending on the geometry of seed patterns As shown in in-situ observation of crystal growth, our positionselective C60 crystal growth is originated by the nucleation formed at seed pattern, and the final crystal shape is determined by the number of nucleation. To evaluate the influence of dimension of seed patterns on the C60 crystal growth, we performed control experiments by systematically changing 1) the diameter of seed patterns from 1 to 10 mm and 2) the thickness of seed patterns from 10 nm to 120 nm. The thickness of seed patterns was 60 nm for the variation of diameter, and the diameter was 5 mm for the variation of thickness. After the drop-drying process was completed, optical microscope images were taken from randomly selected 6 spots of 3 different substrates (2 spots per substrate, 266 seed patterns per each spot), and the position-selectivity and crystal shape were analyzed (Figs. S8a and b in ESM). Under all conditions that we examined, most of crystals were grown at the seed pattern position. Specifically, the C60 crystals were grown at 96% of seed patterns when the diameter of seed pattern is 1 mm, and 100% position selectivity was obtained with the diameter larger than 5 mm. At the same time, 20 C60 crystals were found at off-position with 1 mm diameter of seed pattern, and the number of displaced crystals was gradually decreased as the diameter of seed patterns increased (Fig. 3a). Meanwhile, no clear relation was observed between the position-selectivity and the thickness of seed patterns (Fig. 3b). When the thickness of seed patterns was varied from 10 nm to 120 nm, the position selectivity was over 99%, and the number of displaced crystals were 10e20 in all cases. This result indicates that the larger seed pattern provides more probability of nucleation at seed pattern, considering that the displaced crystals were grown at the solution droplet surface and landed on the substrate (Supporting Movie 1). Unlike the positionselectivity, the crystal shape is affected by both diameter and thickness of seed patterns (Fig. 3c and d). We first categorized the C60 crystals grown at seed pattern sites into half hexagonal, full hexagonal, aggregated and unspecified shape based on the presence of clear facets shown in optical microscope images. When the diameter of seed patterns is less than 5 mm or the thickness of seed patterns is less than 60 nm, 50e60% of crystals have half or full hexagonal shape with clear facets. In this case, the C60 crystals become thicker as the diameter or thickness of seed pattern increases (Fig. 3e,f, Fig. S9 in ESM). However, when the seed pattern has 10 mm diameter or 120 nm of thickness, such faceted crystals were not obtained and almost 100% of crystals have aggregated
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Fig. 2. In-situ observation of position-selective growth of C60 single crystals. (a) Selected optical microscope images of a representative crystal at specific times marked by arrows in the graph. Full images were shown in Movie 1. (b) Calculated lateral dimension of the crystal (blue curve) and its growth rate (red curve). (c) The proposed crystallization mechanism based on in-situ observation of position-selective growth. (A colour version of this figure can be viewed online.)
Fig. 3. Position selective C60 crystal growth behavior depending on the diameter (a, c, e) and thickness (b, d, f) of seed pattern. (a, b) The percentage of the crystals grown at seed position (black) and the number of crystals found at off-seed position (blue). Total 266 seed positions were investigated. (c, d) Relative ratio between half hexagonal (yellow), full hexagonal (orange) and aggregated (blue) shape crystals. (e, f) Thickness of half or full hexagonal shape crystals from selected area. (A colour version of this figure can be viewed online.)
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form. That is, the number of nucleation significantly increases when the total volume of each seed pattern exceeds a threshold value. 3.4. Application of position-selective growth with fullerene crystals An important advantage of our method is that it is a facile onepot method to grow target crystals at specific positions of solid substrate, which enabled us to fabricate a bottom electrode type C60 crystal device with pre-patterned metal electrodes. (See Methods for detailed process) Since the crystal growth was the last step in our device, the C60 crystal did not experience any damage or contamination that is often encountered in the fabrication process of organic crystal devices. Moreover, the preferential lateral growth of our C60 crystals provided intimate physical contacts not only to the substrate but also to the metal electrodes (Fig. 4a, inset). We first characterized electrical properties of the C60 crystals at room temperature in argon environment, after the incorporated solvent molecules were removed with thermal annealing at 120 C. The device showed clear n-type transport behavior with a gradual modulation of back gate voltage, and the field-effect mobility of the C60 crystal was calculated to be 1.01 103 cm2/V∙s (Fig. 4a). It also exhibited prompt photo responses under UV illumination (lex ¼ 365 nm, 0.01 mW/cm2) (Fig. 4b). More importantly, our bottom electrode type C60 crystal device is an ideal system to study chemical doping process due to the large area of crystal surface open to the environment which does not only provide easy access of dopant species but also enables in-situ optical characterizations together with electrical measurements. We conducted well-known potassium doping process, and the Raman
spectra and electrical resistance simultaneously measured during the potassium doping exhibited clear changes as the doping proceeds, which is similar to previous reports [28,29] (Fig. 4 c). The superconducting K3C60 phase was obtained by stopping the supply of potassium vapor when the electrical resistance is minimum and Ag (2) Raman peak is shifted to 1451 cm1 [13,30]. Note that the successful K3C60 phase transformation was also evidenced by newly appeared (111) XRD peak of K3C60 system (Fig. S10 in ESM). To confirm the superconductivity, we measured temperaturedependent resistance at cryo-temperature (Fig. 4d). The abrupt drop of resistance was observed at 17 K, which is similar to the known Tc of K3C60 [13]. However, when the same potassium doping experiment was conducted with the C60 thin film sample for comparison, Tc was about 13 K. Moreover, single crystal sample showed clearly metallic behavior at above Tc while polycrystalline one did not. These results imply that the quality of our single C60 crystal obtained by position-selective growth is high enough to be used for further fundamental studies. 4. Conclusion In summary, we successfully grew hexagonal disk shape C60 crystals selectively at the C60 thin film type seed patterns by dropdrying of C60/CCl4 solution in CCl4 vapor environment. The entire growth process was monitored by transmission mode optical microscopy, and the detailed growth behavior was analyzed by image processing technique. The systematic control experiments with different seed pattern dimension reveals that the position selectivity was critically dependent on the diameter of seed pattern, while the crystal shape is affected by both of diameter and
Fig. 4. (a) Transfer curve measured from the C60 single crystal device. (Inset: representative SEM image of the device. (b) Photo response upon light illumination in vacuum (lex ¼ 365 nm, 0.01 mW/cm2, VDS ¼ 40 V). (c) Changes of electrical resistance (black line) and Raman spectra depending on potassium doping time. (d) Temperature dependent normalized dc electrical resistance for K3C60 single crystal (red curve) and K3C60 thin film (blue curve). (A colour version of this figure can be viewed online.)
C. Park et al. / Carbon 145 (2019) 31e37
thickness of seed pattern. The position selective growth was further utilized to fabricate bottom electrode type C60 single crystal device which showed n-type field effect characteristics and photo response. Potassium doping into C60 crystal was monitored by insitu measurements of Raman and electrical resistance, and properly doped (K3C60) single crystal exhibited enhanced superconductivity compared with thin film device. As demonstrated in this work, our position selective growth method can be a powerful tool for fundamental studies on the crystal growth and chemical doping as well as organic crystal-based device applications. Acknowledgement The HRTEM image and electron diffraction patterns were obtained at national institute for nanomaterials technology (NINT) in Pohang, Korea. We thank Dr. Jin-Woo Kim at Pohang Accelerator Laboratory for XRD measurement. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.01.012. References [1] C. Park, J.E. Park, H.C. Choi, Crystallization-induced properties from morphology-controlled organic crystals, Acc. Chem. Res. 47 (2014) 2353e2364. [2] J. Mei, Y. Diao, A.L. Appleton, L. Fang, Z. Bao, Integrated materials design of organic semiconductors for field-effect transistors, J. Am. Chem. Soc. 135 (2013) 6724e6746. [3] R.W.I. de Boer, M.E. Gershenson, A.F. Morpurgo, V. Podzorov, Organic singlecrystal field-effect transistors, Phys. Status Solidi A. 201 (2004) 1302e1331. [4] C. Reese, Z. Bao, Organic single-crystal field-effect transistors, Mater. Today 10 (2007) 20e27. [5] V.C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R.L. Willett, T. Someya, et al., Elastomeric transistor stamps: reversible probing of charge transport in organic crystals, Science 303 (2004) 1644e1646. [6] A.L. Briseno, S.C.B. Mannsfeld, M.M. Ling, S. Liu, R.J. Tseng, C. Reese, et al., Patterning organic single-crystal transistor arrays, Nature 444 (2006) 913e917. [7] O. Goto, S. Tomiya, Y. Murakami, A. Shinozaki, A. Toda, J. Kasahara, et al., Organic single-crystal arrays from solution-phase growth using micropattern with nucleation control region, Adv. Mater. 24 (2012) 1117e1122. [8] H. Li, B.C.K. Tee, J.J. Cha, Y. Cui, J.W. Chung, S. Y, et al., High-mobility field-effect transistors from large-area solution-grown aligned C60 single crystals, J. Am. Chem. Soc. 134 (2012) 2760e2765. [9] K. Nakayama, Y. Hirose, J. Soeda, M. Yoshizumi, T. Uemura, M. Uno, et al., Patternable solution-crystallized organic transistors with high charge carrier
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Position-selective solution phase growth of fullerene crystals Chibeom Park a, 1, Jinho Lee a, b, 1, Minkyung Lee b, Taekyung Yoon a, b, Hee Cheul Choi a, b, * a b
Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-ro, Nam-Gu, Pohang, 37673, South Korea Department of Chemistry, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-Gu, Pohang, 37673, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2018 Received in revised form 25 December 2018 Accepted 3 January 2019 Available online 4 January 2019
An efficient method for the formation of hexagonal disk shape C60 crystals at desired positions on a solid substrate has been achieved. The detailed growth behavior was studied through in-situ observation and series of control experiments. Our position selective growth technique enabled a fabrication of bottom electrode type C60 single crystal devices which exhibited n-type field-effect transistor behavior and photo response. Moreover, the C60 single crystal devices were further exploited to study alkali metal doped C60 crystal and its superconductivity. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction The crystallization of highly conjugated organic molecules has been one of the most important research subjects for over hundred years not only for fundamental scientific interests in understanding intermolecular interactions involved in crystallization into specific morphologies, but also for high potential in optoelectronic device applications [1,2]. In most cases of organic crystal devices, the crystals grown in solution or vapor phase are manually transferred on a solid substrate, and their optical and electrical properties are characterized as a form of proto-type device [3e5]. However, the inefficiency and difficulty of transfer process limits fundamental and accurate investigation on their properties as well as the extension to further applications such as large area electronic devices with high throughput and efficient integration. Therefore, it is necessary to develop methods to directly crystallize organic molecules into crystals at desired positions on a solid substrate, which has been demonstrated in some reports [6e10]. Most of these attempts involve a substrate surface modification to overcome the strong tendency of organic molecules to self-organize in random locations. Representatively, Briseno et al. fabricated large array of organic single crystal field effect transistor (FET) by selective crystallization of vaporized organic molecules on a printed octadecyltriethoxysilane (OTS) film [6]. Similar approach has been attempted
* Corresponding author. Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-ro, Nam-Gu, Pohang, 37673, South Korea. E-mail address: [email protected] (H.C. Choi). 1 These authors equally contributed to this work. https://doi.org/10.1016/j.carbon.2019.01.012 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
for solution phase crystallization by Goto et al. by employing micro patterned holes on a substrate for nucleation region [7]. However, the substrate modification process for position-selective crystallization used in both cases potentially possesses undesired or extrinsic effect on device performance. Therefore, it is necessary to develop a new method that grants crystallization of target molecules with a position selectivity. Among diverse target molecules, fullerene was chosen to demonstrate above-mentioned approach. The spherical molecular geometry and high symmetry of fullerene provide large void space in its crystal, which can be occupied by various chemical species such as alkali metal atoms and small organic molecules [11,12]. It is well known that the alkali metal atoms intercalated between C60 molecules provide electrons and make them superconducting when doped properly [13e15]. Another interesting phenomenon is that the inclusion of solvent molecules during the solution phase crystallization results in peculiar self-assembly behavior, by which fullerene molecules are spontaneously guided to form into specific morphologies depending on the type of solvent [16e20], solvent ratio [21] and the composition ratio in the case of C60 - C70 hetero crystals [22]. We previously showed that hexagonal disks and wire structures could be selectively obtained by solution phase crystallization using specific solvents [19], and their growth could be further controlled by changing the direction of solvent drying force of solution droplet [23] when the crystallization occurs especially by the drop-drying process. These results imply the possibility that the fullerene crystals can be grown at the desired positions of substrate under controlled crystallization environment, which will enable in-depth analysis of crystallization process as well as further studies on individual fullerene crystals, especially focusing on the
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transport and superconducting properties in forms of electrical devices. Herein we report a simple but efficient way to form the hexagonal disk shape C60 single crystals at desired positions on a solid substrate. The C60 crystals were directly grown on a thermally oxidized silicon or fused quartz substrate by drop-drying method, and the position selectivity was provided by pre-patterned C60 thin films which acted as crystallization seeds. Such position controlled C60 crystal growth enabled in-situ monitoring of the crystallization process to unveil the growth behavior. The position-selectivity and the shape of C60 crystals were further controlled by changing the dimension of seed patterns. By applying the same method on a substrate with pre-defined metal electrodes, we fabricated bottom electrode type C60 single crystal electronic devices which showed clear n-type semiconducting behavior and electrical photo responses. The C60 single crystal device was further utilized to study the optical and electrical property change upon potassium doping process. The implementation of these devices demonstrates that our position-selective crystal growth can be extended to other large area device applications. 2. Experimental procedure 2.1. Position-selective growth of C60 single crystal C60 powder (MTR Ltd., purity > 99.95%) was used as a source without further purification. A patterned mask to deposit the C60 seed thin film pattern was fabricated by standard photolithography or e-beam lithography technique (spacing between patterns: 50 mm). The C60 seed thin film was prepared by thermal evaporation on a SiO2/Si or fused silica substrate under 108 torr at 510 C followed by the lift-off process less than 30 s (The detailed process is described in ESM) The crystallization solution was prepared by filtrating the saturated C60 solution dissolved in CCl4 using a syringe filter (Whatman International Ltd., pore size > 20 nm) to remove undissolved C60 source. 50 ml of filtered solution was dropped onto the patterned C60 thin film on a substrate. To provide enough time for crystal growth, we performed the drop drying process on the prepared substrate placed in a vial filled with 3 ml of additional CCl4.
2.4. Image processing for C60 crystal growth analysis Each optical microscope image was first divided into each seed pattern and then cropped to square images (124 x 124 pixels, corresponding size of 25 25 mm). The RGB images were converted to 8-bit grayscale after an adjustment of the contrast and brightness, then the grayscale was inverted to make the crystal area bright. The resulting images were again converted to black and white image with a threshold value that represents the initial RGB images the best. The number of white pixels was converted to the area of individual C60 crystal, and the square root of the area was used for lateral dimension. The morphology of C60 seeds was examined by tapping mode atomic force microscopy (AFM, Nano scope IIIa, Digital Instrument Inc.) 2.5. Fabrication and measurement of C60 single crystal devices The metal electrodes (5 nm Ti/30 nm Au) were pre-defined by a conventional photolithography and lift-off process on a SiO2/Si (300 nm thick SiO2 on a p-type Si) substrate. The seed patterns were formed to be located between metal electrodes and position selective C60 crystal growth was performed. The field-effect transistor behavior (Ar environment) and photo response (ambient condition) were measured using a semiconductor analyzer (Keithley 4200s) at room temperature. 2.6. Potassium doping of C60 single crystal devices and in-situ measurements of Raman spectra and electrical resistance change Potassium doping was performed at high vacuum (P ~ 106 Torr) using a home-built chamber which can provide potassium vapor by heating elemental potassium. Before the doping, C60 single crystal was annealed at 120 C to remove the intercalated solvent. Twoprobe electrical resistance was measured using a semiconductor analyzer (Keithley 4200s) and Raman spectrum was measured using a WITEC Alpha 300R Raman spectroscope with a 532 nm laser. The power of laser was maintained below 100 mW to prevent thermal damage of the crystal. The detailed experimental setup and process are described elsewhere [24]. 2.7. Measurement of superconductivity
2.2. TEM sample preparation 50 nm thick Au film was deposited on top of the C60 crystal arrays grown on a SiO2/Si substrate using e-beam evaporator. The Au film containing C60 crystal arrays was separated from the substrate by etching SiO2 with 1M KOH aqueous solution, then transferred onto a 20 nm-thick silicon nitride membrane TEM grid. The Au film was then removed by placing the Au side of the TEM grid down on the surface of commercial Au etchant solution. The HRTEM image and electron diffraction patterns were acquired using a transmission electron microscope (TEM) in National Institute for Nanomaterials Technology (NINT) at POSTECH. 2.3. In-situ observation of C60 crystal array growth A transmission mode inverted optical microscope (Nikon, Eclipse TS100) equipped with a long working distance (18 mm) objective (Olympus, SLMPLN50x) was used to observe the position selective C60 crystal growth. The drop drying process condition was identical to the above-mentioned position selective growth of C60 crystal except that the SiO2/Si substrate was replaced with a transparent fused silica. The optical microscope images were consecutively obtained using a CMOS camera (Lumenera Corp., INFINITY1-1M) at every second right after the C60/CCl4 solution was dropped.
The C60 single crystal devices properly doped with potassium was mounted in a closed-cycle cryostat (Sungwoo Instrument, Korea) which was connected to a glove box to prevent sample exposure to air. Four-probe electrical resistance was measured using a semiconductor analyzer (Keithley 4200-scs model) and the temperature was measured using a diode temperature sensor directly in contact to the sample substrate. 3. Results and discussion 3.1. Position-selective growth of C60 single crystal Fig. 1a schematically depicts our approach to grow hexagonal shape C60 single crystals at specific positions on a solid substrate. Saturated C60 solution dissolved in CCl4 was drop-dried in the presence of CCl4 vapor on top of a SiO2/Si substrate where the arrays of C60 thin film patterns were pre-defined as crystallization seeds. (See Methods for detailed process) These seed arrays are composed of circular C60 thin film patterns evenly distributed along the substrate (1 cm 1 cm) with a regular spacing of 50 mm (Fig. 1b). The scanning electron microscope (SEM) and lowmagnification optical microscope images show that the resulting crystals are located only at the positions of C60 thin film seed
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Fig. 1. Position-selective growth of C60 single crystal on solid substrate. (a) Schematic illustration of experiments. (b) Scanning electron microscope (SEM) image of C60 thin film seed patterns. (c) SEM image of C60 single crystal arrays selectively grown at seed positions. (Inset: Low-magnification optical microscope image) (d) Zoomed in SEM image of (c). (e) X-ray diffraction (XRD) of C60 crystal arrays. (f) Transmission electron microscopy (TEM) image of hexagonal-shaped C60 single crystal and (g) corresponding electron diffraction. (A colour version of this figure can be viewed online.)
patterns, and such a position-selectivity is observed over the entire substrate (Fig. 1c). It should be noted that similar position-selective growth of C60 crystal is also possible with other solvent systems (Fig. S1 in ESM). Most of the crystals show overall hexagonal shape in the top view SEM image, which implies that the lateral crystal growth is preferred. As shown in the zoomed-in SEM image in Fig. 1d, some of the crystals are single hexagonal disks while others have aggregated form which is composed of multiple crystals. We previously reported that C60 molecules crystallize together with CCl4 into hexagonal shape single crystals upon drop-drying process [19]. To investigate crystalline feature of the C60 crystal directly grown on the seed pattern in detail, we performed X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments. XRD data from as-grown sample showed one dominant peak at 8.23 of two theta which is well matched to (0001) plane of reported simple hexagonal structure (a ¼ 10.10 Å, c ¼ 10.75 Å) [25] (Fig. 1e). This result indicates that the wide top surface of the hexagonal disk crystal is parallel to the substrate, as also confirmed by SEM images. Note that the small peak at 10.84 ο comes from (111) plane of face-centered cubic pure C60 crystal structure (a ¼ 14.11 Å), which might be generated from natural escape of CCl4. For further crystal structure study in TEM, the as-grown crystals were transferred onto a TEM grid using e-beam evaporated gold thin film (See Methods and Fig. S2 in ESM for the detailed sample preparation).
The bright-field TEM image shows two sharp edges of hexagon crossing at 120 ο (Fig. 1f). The corresponding selected area electron diffraction (SAED) pattern exhibited clear hexagonal symmetry, and the d-spacing value of each spot was well matched to the known crystal structure [25] (Fig. 1g). Same SAED patterns were obtained at multiple locations of each crystal, which indicates that the C60 crystal grown on the seed pattern is also single crystalline similar to the one grown from a regular drop-drying process. From numerous experiments, we found that the introduction of CCl4 vapor environment during drop-drying process was a very important factor for the successful position-selective growth of C60 crystal arrays. In ambient condition, the solvent completely dried in less than 2 min, and unwanted small aggregates were obtained at off-seed positions (Fig. S3 in ESM). When the solvent drying time was increased to 20 min in a sealed vial condition, the position selectivity was improved but the resulting crystals are lack of welldefined morphologies. On the other hand, under CCl4 environment achieved by putting additional CCl4 on the bottom of vial, solvent drying time was further increased, and hexagonal disk shape C60 crystals were grown only at seed positions. This result implies that slow crystallization is essential for position-selectivity as well as well-defined morphologies, which is in accordance with common phenomenon in single crystal growth [26,27]. However, such a significant improvement of crystal growth with additional CCl4 is
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not only attributed to the increase in solvent drying time, but it is also related to the solvent annealing effect before the drop-drying process started. As shown in the atomic force microscope (AFM) images, the as-prepared C60 thin film seed pattern is greatly deformed just by an exposure to CCl4 vapor for a few minutes. A few bumpy structures were formed, and the rms surface roughness of flat area was also increased from 1.22 nm to 2.15 nm (Fig. S4 in ESM). The deformation of seed pattern occurs through a repetitive dissolution and condensation of C60 molecules by adsorbed solvent molecules, which naturally happens in the drop-drying with additional CCl4. Indeed, the position-selective growth was more successful when the seed patterns were stayed in the CCl4 vapor for a few more minutes before the solution drop, instead of immediate solution drop (Fig. S5 in ESM). Therefore, we performed all the drop-drying process in an uncapped vial with partially filled CCl4 at the bottom, and the solution was dropped a few minutes after the substrate was placed. 3.2. In-situ observation of position-selective growth of C60 single crystals In situ observation of crystal growth is generally very difficult, especially for the solution phase crystallization since the crystals are mobile in the solution during the crystallization process, which makes it almost impossible to track individual ones for clear understanding the crystal growth mechanism. However, in our case, since the crystal growth occurs only at specific positions, we could directly monitor and analyze the entire crystallization process. For this, we replaced the SiO2/Si substrate with a transparent fused silica and continuously monitored the growth of C60 crystals from the bottom of the vial using a transmission mode inverted optical microscope (See the Methods and Fig. S6 in ESM for detailed experimental setup). After applying a droplet of C60/CCl4 solution on the target substrate, we obtained series of images from total 20 seed patterns at every second, then combined them into a movie clip (Movie S1 in ESM). Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.01.012. By processing each image, we also calculated the lateral dimension of crystal and growth rate. (See the Methods for detailed image processing) Fig. 2a and b show a representative result (third row and fourth column in Movie S1 from ESM) among 20 seed positions. The seed pattern is clearly shown in the first image taken at 1 min, and no significant change was observed until 10 min. However, the boundary of seed pattern becomes blurry implying that the seed patterns were deformed by the solvent. Upon further drying of solvent, a small black dot appears at bottom left of the seed at 10.5 min, and it gradually grows but its lateral dimension saturates at 1.4 mm. From 12 min, however, the lateral dimension of crystal abruptly increased with a rate of 2.9 mm/min and forms hexagonal shape at 14 min. The hexagonal shape crystal further grows to about 15 mm until the solvent completely dries off at 22 min. Note that the spikes appeared at 22 min in both graphs are the result of the agglomerated solvent before complete drying, as shown in the image. One interesting observation is that the color intensity of the crystal remains same after 14 min. This implies that the crystal growth dominantly occurs lateral direction since the images were taken under transmission mode and the color intensity is closely related to the thickness of the crystal. All other crystals grown at different positions also showed similar growth behavior with final lateral dimensions of 14.7 ± 1.67 mm except one missed crystal (Fig. S7a in ESM). As also observed in Fig. 1d, some crystals are stacked or aggregated form, which is originated from multiple nucleation at one seed pattern. The number of nucleation at each seed pattern varies from 0 to 4 over 20 seed positions of our
observation area (Fig. S7b in ESM). All nucleation sites on each circular seed pattern are represented in Fig. S7c in ESM. Although the center of seed pattern seems to be preferred for nucleation site, the nucleation also occurs at other areas. According to all the results above, the crystallization process can be summarized as follows (Fig. 2c). When the thin film C60 seed pattern is exposed to CCl4 vapor for a few minutes before solution drop, solvated nucleates are generated on the seed pattern by the solvent annealing effect. (Stage 0) After the C60/CCl4 solution is dropped, the pre-formed nucleates collect C60 and CCl4 molecules from the solution. (Stage I) At this time, the solution becomes locally supersaturated near the seed pattern because C60 molecules are also provided from the seed pattern, which minimize new nucleation at other location. As solvent dries, continued collection of C60 and CCl4 molecules makes nucleates big enough to be shown by an optical microscope. (Stage II) Once the nucleate reaches to a certain size, the crystal grows laterally resulting in hexagonal disk shape, (Stage III) and the crystallization process is ceased with complete drying of solvent. (Stage IV). 3.3. Position-selective C60 crystal growth behavior depending on the geometry of seed patterns As shown in in-situ observation of crystal growth, our positionselective C60 crystal growth is originated by the nucleation formed at seed pattern, and the final crystal shape is determined by the number of nucleation. To evaluate the influence of dimension of seed patterns on the C60 crystal growth, we performed control experiments by systematically changing 1) the diameter of seed patterns from 1 to 10 mm and 2) the thickness of seed patterns from 10 nm to 120 nm. The thickness of seed patterns was 60 nm for the variation of diameter, and the diameter was 5 mm for the variation of thickness. After the drop-drying process was completed, optical microscope images were taken from randomly selected 6 spots of 3 different substrates (2 spots per substrate, 266 seed patterns per each spot), and the position-selectivity and crystal shape were analyzed (Figs. S8a and b in ESM). Under all conditions that we examined, most of crystals were grown at the seed pattern position. Specifically, the C60 crystals were grown at 96% of seed patterns when the diameter of seed pattern is 1 mm, and 100% position selectivity was obtained with the diameter larger than 5 mm. At the same time, 20 C60 crystals were found at off-position with 1 mm diameter of seed pattern, and the number of displaced crystals was gradually decreased as the diameter of seed patterns increased (Fig. 3a). Meanwhile, no clear relation was observed between the position-selectivity and the thickness of seed patterns (Fig. 3b). When the thickness of seed patterns was varied from 10 nm to 120 nm, the position selectivity was over 99%, and the number of displaced crystals were 10e20 in all cases. This result indicates that the larger seed pattern provides more probability of nucleation at seed pattern, considering that the displaced crystals were grown at the solution droplet surface and landed on the substrate (Supporting Movie 1). Unlike the positionselectivity, the crystal shape is affected by both diameter and thickness of seed patterns (Fig. 3c and d). We first categorized the C60 crystals grown at seed pattern sites into half hexagonal, full hexagonal, aggregated and unspecified shape based on the presence of clear facets shown in optical microscope images. When the diameter of seed patterns is less than 5 mm or the thickness of seed patterns is less than 60 nm, 50e60% of crystals have half or full hexagonal shape with clear facets. In this case, the C60 crystals become thicker as the diameter or thickness of seed pattern increases (Fig. 3e,f, Fig. S9 in ESM). However, when the seed pattern has 10 mm diameter or 120 nm of thickness, such faceted crystals were not obtained and almost 100% of crystals have aggregated
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Fig. 2. In-situ observation of position-selective growth of C60 single crystals. (a) Selected optical microscope images of a representative crystal at specific times marked by arrows in the graph. Full images were shown in Movie 1. (b) Calculated lateral dimension of the crystal (blue curve) and its growth rate (red curve). (c) The proposed crystallization mechanism based on in-situ observation of position-selective growth. (A colour version of this figure can be viewed online.)
Fig. 3. Position selective C60 crystal growth behavior depending on the diameter (a, c, e) and thickness (b, d, f) of seed pattern. (a, b) The percentage of the crystals grown at seed position (black) and the number of crystals found at off-seed position (blue). Total 266 seed positions were investigated. (c, d) Relative ratio between half hexagonal (yellow), full hexagonal (orange) and aggregated (blue) shape crystals. (e, f) Thickness of half or full hexagonal shape crystals from selected area. (A colour version of this figure can be viewed online.)
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form. That is, the number of nucleation significantly increases when the total volume of each seed pattern exceeds a threshold value. 3.4. Application of position-selective growth with fullerene crystals An important advantage of our method is that it is a facile onepot method to grow target crystals at specific positions of solid substrate, which enabled us to fabricate a bottom electrode type C60 crystal device with pre-patterned metal electrodes. (See Methods for detailed process) Since the crystal growth was the last step in our device, the C60 crystal did not experience any damage or contamination that is often encountered in the fabrication process of organic crystal devices. Moreover, the preferential lateral growth of our C60 crystals provided intimate physical contacts not only to the substrate but also to the metal electrodes (Fig. 4a, inset). We first characterized electrical properties of the C60 crystals at room temperature in argon environment, after the incorporated solvent molecules were removed with thermal annealing at 120 C. The device showed clear n-type transport behavior with a gradual modulation of back gate voltage, and the field-effect mobility of the C60 crystal was calculated to be 1.01 103 cm2/V∙s (Fig. 4a). It also exhibited prompt photo responses under UV illumination (lex ¼ 365 nm, 0.01 mW/cm2) (Fig. 4b). More importantly, our bottom electrode type C60 crystal device is an ideal system to study chemical doping process due to the large area of crystal surface open to the environment which does not only provide easy access of dopant species but also enables in-situ optical characterizations together with electrical measurements. We conducted well-known potassium doping process, and the Raman
spectra and electrical resistance simultaneously measured during the potassium doping exhibited clear changes as the doping proceeds, which is similar to previous reports [28,29] (Fig. 4 c). The superconducting K3C60 phase was obtained by stopping the supply of potassium vapor when the electrical resistance is minimum and Ag (2) Raman peak is shifted to 1451 cm1 [13,30]. Note that the successful K3C60 phase transformation was also evidenced by newly appeared (111) XRD peak of K3C60 system (Fig. S10 in ESM). To confirm the superconductivity, we measured temperaturedependent resistance at cryo-temperature (Fig. 4d). The abrupt drop of resistance was observed at 17 K, which is similar to the known Tc of K3C60 [13]. However, when the same potassium doping experiment was conducted with the C60 thin film sample for comparison, Tc was about 13 K. Moreover, single crystal sample showed clearly metallic behavior at above Tc while polycrystalline one did not. These results imply that the quality of our single C60 crystal obtained by position-selective growth is high enough to be used for further fundamental studies. 4. Conclusion In summary, we successfully grew hexagonal disk shape C60 crystals selectively at the C60 thin film type seed patterns by dropdrying of C60/CCl4 solution in CCl4 vapor environment. The entire growth process was monitored by transmission mode optical microscopy, and the detailed growth behavior was analyzed by image processing technique. The systematic control experiments with different seed pattern dimension reveals that the position selectivity was critically dependent on the diameter of seed pattern, while the crystal shape is affected by both of diameter and
Fig. 4. (a) Transfer curve measured from the C60 single crystal device. (Inset: representative SEM image of the device. (b) Photo response upon light illumination in vacuum (lex ¼ 365 nm, 0.01 mW/cm2, VDS ¼ 40 V). (c) Changes of electrical resistance (black line) and Raman spectra depending on potassium doping time. (d) Temperature dependent normalized dc electrical resistance for K3C60 single crystal (red curve) and K3C60 thin film (blue curve). (A colour version of this figure can be viewed online.)
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thickness of seed pattern. The position selective growth was further utilized to fabricate bottom electrode type C60 single crystal device which showed n-type field effect characteristics and photo response. Potassium doping into C60 crystal was monitored by insitu measurements of Raman and electrical resistance, and properly doped (K3C60) single crystal exhibited enhanced superconductivity compared with thin film device. As demonstrated in this work, our position selective growth method can be a powerful tool for fundamental studies on the crystal growth and chemical doping as well as organic crystal-based device applications. Acknowledgement The HRTEM image and electron diffraction patterns were obtained at national institute for nanomaterials technology (NINT) in Pohang, Korea. We thank Dr. Jin-Woo Kim at Pohang Accelerator Laboratory for XRD measurement. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.01.012. References [1] C. Park, J.E. Park, H.C. Choi, Crystallization-induced properties from morphology-controlled organic crystals, Acc. Chem. Res. 47 (2014) 2353e2364. [2] J. Mei, Y. Diao, A.L. Appleton, L. Fang, Z. Bao, Integrated materials design of organic semiconductors for field-effect transistors, J. Am. Chem. Soc. 135 (2013) 6724e6746. [3] R.W.I. de Boer, M.E. Gershenson, A.F. Morpurgo, V. Podzorov, Organic singlecrystal field-effect transistors, Phys. Status Solidi A. 201 (2004) 1302e1331. [4] C. Reese, Z. Bao, Organic single-crystal field-effect transistors, Mater. Today 10 (2007) 20e27. [5] V.C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R.L. Willett, T. Someya, et al., Elastomeric transistor stamps: reversible probing of charge transport in organic crystals, Science 303 (2004) 1644e1646. [6] A.L. Briseno, S.C.B. Mannsfeld, M.M. Ling, S. Liu, R.J. Tseng, C. Reese, et al., Patterning organic single-crystal transistor arrays, Nature 444 (2006) 913e917. [7] O. Goto, S. Tomiya, Y. Murakami, A. Shinozaki, A. Toda, J. Kasahara, et al., Organic single-crystal arrays from solution-phase growth using micropattern with nucleation control region, Adv. Mater. 24 (2012) 1117e1122. [8] H. Li, B.C.K. Tee, J.J. Cha, Y. Cui, J.W. Chung, S. Y, et al., High-mobility field-effect transistors from large-area solution-grown aligned C60 single crystals, J. Am. Chem. Soc. 134 (2012) 2760e2765. [9] K. Nakayama, Y. Hirose, J. Soeda, M. Yoshizumi, T. Uemura, M. Uno, et al., Patternable solution-crystallized organic transistors with high charge carrier
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