Oil Interfaces and Bulk Oil Phases for

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Quasi-phase diagrams at air/oil interfaces and bulk oil phases for crystallization of small-molecular semiconductors by adjusting Gibbs adsorption Satoshi WATANABE, Takahisa Ohta, Ryota Urata, Tetsuya Sato, Kazuto Takaishi, Masanobu Uchiyama, Tetsuya Aoyama, and Masashi Kunitake Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01603 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 6, 2017

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Quasi-phase diagrams at air/oil interfaces and bulk oil phases for crystallization of small-molecular semiconductors by adjusting Gibbs adsorption Satoshi Watanabe,1* Takahisa Ohta,1 Ryota Urata,1 Tetsuya Sato,1 Kazuto Takaishi,2 Masanobu Uchiyama,3,4 Tetsuya Aoyama,3 Masashi Kunitake1* 1)

Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1

Kurokami, Chuou-ku, Kumamoto, Japan 860-8555 2)

Graduate School of Natural Science and Technology, Okayama University, Tsushimanaka

3-1-1, kita-ku, Okayama city, Okayama, Japan 700-8530 3)

Elements Chemistry Laboratory, RIKEN, 2-1, wako city, Saitama, Japan 351-0198

4)

Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo 7-3-1,

Bunkyou-ku, Tokyo 113-8656

EMAIL ADDRESS OF CORRESPONDING AUTHOR Dr. S. Watanabe: [email protected], Prof. M. Kunitake: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The temperature and concentration dependencies of the crystallization of two smallmolecular semiconductors were clarified by constructing quasi-phase diagrams at air/oil interfaces and in bulk oil phases. A quinoidal oligothiophene derivative with four alkyl chains (QQT(CN)4) in 1,1,2,2-tetrachroloethane (TCE) and a thienoacene derivative with two alkyl chains (C8-BTBT) in o-dichlorobenzene were used. The apparent crystal nucleation temperature (Tn) and dissolution temperature (Td) of the molecules were determined based on optical microscopy examination in closed glass capillaries and open dishes during slow cooling and heating processes, respectively. Tn and Td were considered estimates of the critical temperatures for nuclear formation and crystal growth, respectively. The Tn values of QQT(CN)4 and C8BTBT at the air/oil interfaces were higher than those in the bulk oil phases, whereas the Td values at the air/oil interfaces were almost the same as those in the bulk oil phases. These Gibbs adsorption phenomena were attributed to the solvophobic effect of the alkyl chain moieties. The temperature range between Tn and Td corresponds to suitable supercooling conditions for ideal crystal growth based on the suppression of nucleation. The Tn values at the water/oil and oil/glass interfaces did not shift compared with those of the bulk phases, indicating that adsorption did not occur at the hydrophilic interfaces. Promotion and inhibition of nuclear formation for crystal growth of the semiconductors were achieved at the air/oil and hydrophilic interfaces, respectively.

KEYWORDS Phase diagrams, small-molecular semiconductors, crystal dissolution, crystal nucleation, supercooling, Gibbs adsorption, air/oil interface


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Introduction Organic semiconductors have attracted considerable attention for application in solar cells1–3, light-emitting diodes1,4,5, and field-effect transistors6–8 in flexible and printed electronics because of their unique properties, which include their mechanical flexibility, light weight, and printable fabrication in solution processes. One of the weaknesses of organic semiconductors is their low carrier mobilities due to hopping transport and carrier traps at large domain boundaries in phaseseparated structures. However, high carrier mobilities comparable to those of inorganic semiconductor devices have been achieved in single crystals of organic semiconductor materials.9–11 Therefore, fine control of the crystal size, shape, orientation, purity, and anisotropic direction of organic semiconductor devices is needed, particularly for the manufacture of single crystals. According to classical theories, crystal growth is generally explained as consisting of two steps: nucleation and growth. To achieve ideal crystal growth such as that in the production of single crystals, well-controlled crystal growth only from single nuclei is required under supercooling (supersaturation) conditions, in which nucleation is suppressed. Even at the lab level, achieving fine control of crystallization is not easy, and the production of single crystals used to be achieved accidentally. Although novel methodologies to prepare organic semiconductors in single crystal form are eagerly awaited because of the expected high performance of the resulting devices, the incorporation of the single crystal growth process is not easy when using real device manufacturing processes. Several pioneering studies on wet process methodologies to control the crystallinity of organic semiconductors on substrates have been reported. In these studies, one-dimensional crystal


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growth was achieved by controlling the direction of evaporation. The Hasegawa group proposed the use of “double shot inkjet printing”12. Both simple pattering on the micron scale and control of the crystal growth on a substrate have been achieved using double shot techniques. In this approach, crystallization in one direction can be achieved by inkjet printing a droplet of organic semiconductor in a good solvent on the edges of narrow pools of poor solvents, which are printed in advance for the introduction of crystallization. The Bao group proposed two methodologies. In the “droplet-pinned crystallization technique”, organic semiconductor inks were cast on patterned silicon blocks fabricated using photolithography. The patterned protrusion blocks enabled regulation of the crystal orientation via pinning of the ink solution and one-directional advancement of the air/liquid/solid ternary interface line by evaporation.13,14 Recently, these researchers proposed an advanced surface crystallization method called “the blading technique”.15 In this method, a glass blade rubs the substrate surface while leaving behind the ink, and then, the evaporation speed and direction can be controlled by the motion of the blade. To achieve fine control of the crystallization process, knowledge of the concentration and temperature dependencies of small-molecular semiconductors in solvents in the form of a concentration–temperature phase diagram is crucial. In this study, we constructed quasi-phase diagrams of small-molecular semiconductors with alkyl chains, 2,7-dioctyl[1]benzothieno[3,2-b] [1] benzothiophene (C8-BTBT) and a quinoidal oligothiophene derivative (QQT(CN)4), based on two critical points, the critical nucleation and crystal dissolution points in the bulk solution phase and at air/oil and water/oil interfaces. Oil/solid and air/oil interfaces play significant roles in the crystallization process on substrates. Therefore, the phase diagrams at the interfaces were expected to differ from those in the


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homogeneous bulk phases. The adsorption of amphiphilic molecules with long alkyl chains at air/water interfaces16–18 is common in Gibbs films, and adsorption at air/oil interfaces has also been reported.19 C8-BTBT is a common high-performance p-type organic semiconductor, which forms platetype single crystals.20,21 QQT(CN)4 is a unique ambipolar semiconductor, which can be converted into n-type and p-type organic semiconductors with thermal annealing, solvent exposure, and light-irradiation treatments.22–25 Recently, we reported the use of soft lithography for the selective fabrication of QQT(CN)4 crystals on patterned regions; the crystals exhibited good carrier mobility as high as that of a single crystal of QQT(CN)4.26


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Experiment

Materials QQT(CN)4 was synthesized according to a previously described method with improvements, as shown in Figure 1a and b.22 C8-BTBT (Sigma-Aldrich, ≥99%) was used without further purification. 1, 1, 2, 2-tetrachroloethane (TCE) and o-dichlorobenzene (DCB) were purchased from Nippon Kasei Chemical Co., Ltd. and Wako Pure Chemical Industries, Ltd., respectively. Acetone and chloroform used for cleaning the substrates were purchased from Wako Pure Chemical Industries, Ltd.

Experimental procedure Observation of crystallization in closed glass capillaries The apparent crystal nucleation temperature (Tn) at the air/oil interfaces was determined by direct optical microscopy observation of the formation of crystals with a defined shape on the micron scale, typically rhombus- or hexagonal-shaped crystals, during cooling at a rate of 1 °C per 5 min. The apparent crystal dissolution temperature (Td) was determined by direct optical microscopy observation of crystal dissolution during heating at the same rate. To determine Tn and Td in the bulk oil phases, the direct observation of the QQT(CN)4 or C8BTBT solution was performed in closed glass capillaries with diameters of 200 µm. The semiconductors were dissolved in the solvents by heating at 50 °C, and the solution was injected into the glass capillaries by capillary force. In some cases, the solutions in the glass capillaries were covered with pure water to remove the air/oil interface (Figure 1c). The glass capillaries


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were sealed at both sides with epoxy resin adhesives to prevent solvent evaporation and were set on Peltier plates before being examined with a digital microscope (Sanko, Dino-Lite Plus) or optical microscope (BX51, Olympus, Japan). The both semiconductor samples were cooled at a cooling rate of 1 °C per 5 min until crystals and/or precipitates appeared and were then heated at the same rate until the crystals and/or precipitates dissolved (Figure 1d).

Observation of crystallization in an open dish The crystallization in open dishes (diameters of 5 cm and depths of 1 cm) filled with 5 mL of the solutions was examined using optical microscopy to compare the crystallization and dissolution temperatures at air/oil interfaces with those in the glass capillaries. The crystals adsorbed on the air/oil interfaces were examined by focusing on the interface (Figure 1c).

Surface pressure measurements To evaluate the adsorption of QQT(CN)4 at the air/TCE interface, the surface pressures and surface tensions were measured using a Wilhelmy-type surface tensiometer (FSD-220, USI Co., Ltd. Japan). First, the open glass dishes containing 5 mL of the QQT(CN)4 solution were placed in a thermostatic bath near Td for 10 min to completely dissolve the QQT(CN)4. The temperature dependences of the surface pressure of QQT(CN)4 at concentrations of 6, 13, and 19 mM in TCE were determined at cooling rates of 0.10–0.20 °C min−1. The surface pressure was defined as the difference in the surface tensions according to the following equation: (1)


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where π(T) is the surface pressure [mN m−1] and

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and

are the surface tensions [mN

m−1] for pure TCE and QQT(CN)4 in TCE at T [°C], respectively. The surface pressure curves were obtained by subtracting the surface tension curve measured with pure solvent from those of the solution.

Preparation and characterization of QQT(CN)4 and C8-BTBT crystals using crossed-Nicol microscopy and X-ray diffraction The crystals were fabricated by slow crystallization of QQT(CN)4 and C8-BTBT based on nuclear formation promoted at the air/TCE interfaces. First, QQT(CN)4 in TCE (at a concentration of 13 mM) and C8-BTBT in DCB (at a concentration of 100 mM), which were placed in dishes, were slowly cooled down from 25 °C to 18 °C (below Tn) and from 15 °C to 8 °C (below Tn), respectively, and held for 2–3 min to achieve nucleation at the air/oil interfaces. Then, the solutions were heated to 21 °C and 12 °C (supercooling states), respectively, and stored overnight to achieve crystal growth from the nuclei. The crystals formed at the air/oil interfaces were collected using a pipette. The solutions with the crystals were added to hexane. The crystal sedimentations were purified by decantation several times. X-ray diffraction (XRD) measurements of the crystal powders were performed using an X-ray diffractometer (Rint 2500 VHF, Rigaku Corporation, Japan) with Cu-Kα1 radiation. The XRD patterns were plotted as a function of the scattering vector q = (4 π/λ) sin θ, where λ is the X-ray wavelength and θ is the angle of diffraction, which enabled comparison of the XRD patterns obtained using different X-


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ray wavelengths. Single crystals for crossed-Nicol imaging were collected from a mechanically dispersed solution of the crystals using a pipette and were transferred onto glass substrates. (a)

(b)

QQT(CN)4 (c)

C8-BTBT (d)

solution observation

capillary

water

capillary

observation

dish

Figure 1. Chemical structures of (a) QQT(CN)4 and (b) C8-BTBT. Schematic illustrations of (c) preparation of glass capillary sample and (d) observation directions for closed glass capillary and open dish.


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Results and discussion

Optical microscopy observation of crystallization of QQT(CN)4 The apparent critical temperatures were roughly estimated by the direct observation of the appearance and disappearance of the QQT(CN)4 crystals in the closed glass capillaries. In the slow cooling processes of the QQT(CN)4 in TCE from an isotropic solution state, the temperature at which the first crystal was visually detected was defined as Tn. The temperature at which the formed crystal disappeared was defined as Td. The advantage of the use of the capillaries is the easy examination of bulk oil phases. To examine the bulk phases, the QQT(CN)4 solutions were covered with pure water to suppress the crystallization at the air/TCE interface and solvent evaporation. It is possible to perform repetition experiments with good reproducibility using sealed glass capillaries. Using capillaries, temperature change experiments for various concentrations were conducted using a similar temperature management process at a rate of 1 °C per 5 min to determine Tn and Td.


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(a)

(b)

crystal

TCE

TCE

200 µm (d)

(c) air

200 µm

crystal

TCE

air

200 µm (e)

TCE

200 µm (f)

200 µm

200 µm

Figure 2. Optical microscopy images of QQT(CN)4 crystal in TCE at concentration of 13 mM in (a–d) closed glass capillaries and (e, f) open glass dishes. (a–d) Images taken in the TCE bulk phase upon (a) cooling from 8 °C to 7 °C and (b) upon heating from 26 °C to 27 °C and at the air/TCE interface upon (c) cooling from 19 °C to 18 °C and (d) heating from 26 °C to 27 °C. (e, f) Images captured upon cooling (e) from 21 °C to 20 °C and (f) from 17 °C to 16 °C. The black arrows in (e) and (f) indicate the regions magnified in the inset images with each side = 50 µm. The sample was heated or cooled at a rate of 0.2 °C min−1.

Figure 2a–d present typical optical microscope images of the closed capillaries filled with QQT(CN)4 in TCE at 13 mM. Above 9 °C, the solution was entirely homogeneous, and no crystal was observed. In the TCE bulk phase shown in Figure 2a, the QQT(CN)4 crystals were formed at 8 °C cooled from 9 °C (9 °C → 8 °C), indicating that Tn is 8 °C in the TCE bulk phase.


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The crystals did not drift in the solution because of the small space constraints of the capillaries. In addition, the color of the solution near the crystals became lighter, indicating a decrease of the concentration of QQT(CN)4 caused by the crystal growth. In addition, no crystals were detected at the water/TCE interface. Therefore, crystallization at the water/TCE interface could not be achieved because crystallization in the bulk TCE occurred first. During the next heating process (26 °C → 27 °C), the crystals were dissolved, as observed in Figure 2b.

At the air/TCE interface during cooling from the isotropic state, the solution above 19 °C was entirely homogeneous, and no crystal was detected. Upon cooling from 19 °C to 18 °C, crystals were observed at the air/TCE interface, as shown in Figure 2c. No crystal appeared to form in the TCE bulk phase or on the glass surface. The air/TCE interface at 18 °C is thought to exceed the critical point of crystal nucleation, whereas the glass surface and TCE bulk phase are considered to be below the critical point of crystal nucleation. In addition, the presence of QQT(CN)4 prevented adsorption on the water/TCE interface (negative Gibbs absorption), leading to suppression of the crystal formation. The expected value of Tn at the water/TCE interface was lower than those at the air/TCE interface and in the bulk TCE phase. The crystals formed during the cooling process were dissociated upon heating from 26 °C to 27 °C, as observed in Figure 2d. Td at the air/TCE interface was almost the same as that in the bulk TCE phase. There appears to be essentially no difference between Td in the TCE bulk phase and that at the air/TCE interface. Crystallization experiments at the air/TCE interface in an open dish were also conducted to confirm the morphology of a relatively large crystal grown at an air/oil interface. In the slow cooling of the solution of 13 mM QQT(CN)4, tiny crystals suddenly appeared at the air/TCE


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interface upon cooling from 21 °C to 20 °C, as observed in Figure 2e. The critical temperature for apparent nucleation was 1–2 °C higher than the corresponding temperature in the capillaries. In the capillary experiment, the decrease of the concentration near the crystals due to crystallization might have led to a decrease of the critical temperature. The magnified optical microscope image in Figure 2e reveals the achieved ideal rhombus shape, providing evidence of the single crystal growth of QQT(CN)4. When the temperature reached below 16 °C during the continuous cooling process, harsh and bumpy precipitates expected for polycrystalline materials were formed at the air/TCE interface instead of the ideal rhombus-shaped crystals, as shown in Figure 2f. As the crystal size was obviously increased, the observed precipitates were polycrystalline with the original singlecrystal cores generated by aggregation of single crystals and secondary nuclear growth from the crystal surface. The complex shape without regularity indicates that the crystals were the result of complex competitive processes, including nucleation, nuclear growth, secondary nucleation on the crystal surface, and crystal aggregation.

Quasi-phase diagrams of QQT(CN)4 in TCE According to the basics of crystal theory, a lower temperature or higher concentration is required as a driving force for initial nuclear formation rather than crystal growth. To create a single crystal, fine kinetic management of the two essential processes of nuclear formation and crystal growth is required. Elucidation of the critical conditions as a function of concentration and temperature is a first step to management of crystal structures. Even though the resolution achieved by optical observation is too low to capture the true moment of nucleation, Td and Tn


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can be roughly estimated as the critical temperatures for nuclear formation and nuclear growth, respectively. The Tn and Td values of QQT(CN)4 in TCE at each concentration are summarized in Figure 3 in two-quasi phase diagrams for the (a) air/TCE interface and (b) TCE bulk phase. Td is higher than Tn, and the temperature region between Td and Tn is attributed to the supercooling condition. “Nucleation” (green or blue), “supercooling” (white), and “dissolution” (pink) phases are indicated in the quasi-phase diagrams. The lines between the “supercooling” and “dissolution” phases and between the “supercooling” and “nucleation” phases are the apparent critical “dissolution” and “nucleation” curves, respectively. The dissolution curves for the air/TCE interfaces and TCE bulk phases essentially matched. However, the nucleation curve for the air/TCE interface changed drastically. As previously mentioned, the crystallization of QQT(CN)4 was promoted at the air/TCE interface compared with that in the bulk TCE phase. For instance, for 13 mM QQT(CN)4 in TCE, Tn at the air/TCE interface was approximately 12 °C higher than that in the TCE bulk phase. As Td at the air/TCE interface and in the bulk were coincident, the bulk TCE phase exhibited a wider “supercooling” region between Tn and Td compared with that for the air/TCE interface. The difference in Tn for the air/oil interface and bulk oil phase (∆Tn) increased with increasing QQT(CN)4 concentration, with values of 13 °C, 20 °C, and 25 °C for concentrations of 6, 13 and 19 mM, respectively.


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(b)

(a)

(closed capillary) Tn 20

Td

20

nucleation

super cooling

-1

Concentration [mmol L ]

-1


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15 nucleation

Tn

Td

10

dissolution 5

15

10

dissolution

5

Tn Td (open dish) 0

0 0

10 20 Temperature [℃]

30

40

0

10 20 Temperature [℃]

30

40

Figure 3. Quasi-phase diagrams of QQT(CN)4 in TCE (a) in bulk TCE phase and (b) at air/TCE interface showing Td and Tn. The blue and red lines are guides for the nucleation and dissolution regions. The black circles and triangles and white circles and triangles were obtained from experiments with open dishes and closed capillaries, respectively.

Surface pressure measurements of QQT(CN)4 in TCE Gibbs adsorption of QQT(CN)4 at the air/TCE interface was investigated using surface pressure measurements. The increase of the surface pressure indicates adsorption of QQT(CN)4 at the air/TCE interface. Figure 4a plots the surface pressure as a function of temperature in the QQT(CN)4 in TCE at different QQT(CN)4 concentrations. A surface pressure of zero was sustained in the Td region at each concentration. Even in a dissolution phase, Gibbs adsorption might be occurred, however an increase in absolute surface tension was not observed due to a limited resolution of 0.05 mN m-1. The critical rising points of the surface pressure at 11 °C, 22 °C, and 32 °C were in good agreement with the values of 12 °C, 22 °C, and 32 °C obtained by visual inspection in the closed glass capillaries for solutions at concentrations of 6, 13, and 19 mM, respectively. This finding clearly indicates that the critical rising points of the surface


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pressure can be attributed to Tn. The drastic increases of the surface pressures at 9 °C, 17 °C, and 28 °C at each concentration were attributed to the end of curves. At these points for all the experiments with different concentrations, large islands of QQT(CN)4 crystals appeared at the interface, as observed by naked eye in Figure 2f. These islands indicate uncontrolled rapid crystallization, including secondary aggregation due to overcooling. The Gibbs adsorption of QQT(CN)4 at the air/TCE interface can be explained by the solvophobic effect but not the hydrophobic effect. The alkyl chain moieties of QQT(CN)4 would face the air side (Figure 4b). The surface free energy of the alkyl chain moieties of QQT(CN)4 expected from the value of hexane (20 mN m−1) measured at 20 °C, as a typical alkane, should be lower than that of TCE (37 mN m−1). Figure 4c shows the negative Gibbs absorption of QQT(CN)4 at the water/TCE interface and on hydrophilic glass surfaces. (a) 3 Surface pressure [mN m -1]

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2 19 mM 13 mM

1 6 mM 0 0

10

20

30

40

Temperature [℃]

(b)

(c) air

TCE

water or glass

TCE


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Figure 4. (a) Surface pressure curves as a function of temperature at cooling rates of 0.10–0.20 °C min−1 of QQT(CN)4 in TCE at concentrations of 6, 13, and 19 mM. The inset shows a photograph of an open dish of QQT(CN)4 at 22 °C and 13 mM. The black arrows indicate the critical rising point and drastic increase point of the surface pressure. Schematic illustrations of QQT(CN)4 in TCE at (b) air/TCE interface and (c) water (glass)/TCE interface.

Crystallization properties of C8-BTBT modified with two alkyl chains Crystallization of C8-BTBT modified with two alkyl chains in a DCB solution as well as QQT(CN)4 modified with four alkyl chains was also studied for comparison of the bulk phase and air/oil interface. Figure 5 presents optical microscopy images of closed capillaries filled with 100 mM of C8-BTBT in DCB. Although crystals were not observed above 2 °C in the bulk DCB phase (Figure 5a and c), the formation of the crystals was observed upon cooling to 1 °C, which is considered Tn. The formed crystals exhibited polycrystalline structures, as confirmed by crossed-Nicol imaging (Figure 5e), suggesting the aggregation of crystals and/or secondary nucleation on the crystal surface. The crystal size of C8-BTBT was larger than that of QQT(CN)4. The crystallization speed of C8-BTBT was faster than that of QQT(CN)4 because of the higher concentration. In addition, upon cooling from 11 °C to 10 °C, crystal formation was only observed at the air/DCB interface (Figure 5f). Similar to the case of QQT(CN)4 in TCE, an increase of concentration at the air/DCB interface also led to the promotion of nuclear formation for C8BTBT. These crystals formed in the bulk and at the air/DCB interface were dissolved at 21 °C and 18 °C, as observed in Figure 5b and d, respectively. The difference in Td was due to the


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large size compared with that of QQT(CN)4 in TCE. In addition, no crystal growth of C8-BTBT was observed at the water/DCB interface or on a hydrophilic glass surface.

(a)

(b)

air

DCB

DCB

200 µm

200 µm (d)

(c)

air

DCB

DCB

(f)

(e)

DCB crystal

air

DCB crystal

Figure 5. (a, b) Optical microscopy and (c–f) crossed-Nicol microscope images of C8-BTBT in bulk DCB at 100 mM in a closed capillary at (a, c) 21 °C and (e) upon cooling from 1 °C to 0 °C and at an air/DCB interface at (b, d) 18 °C and (f) upon cooling from 11 °C to 10 °C. The dotted lines represent the inner diameters of the glass capillaries. The sample was heated or cooled at a rate of 0.2 °C min-1.

Figure 6 presents quasi-phase diagrams (a) in the bulk DCB phase and (b) at the air/DCB interface in C8-BTBT solutions in closed capillaries at various concentrations. The Tn values at the air/DCB interface were higher than those at the bulk DCB interface at relatively lower concentrations, indicating that the air/DCB interface also promotes the crystallization of C8BTBT. The differences in Tn at the air/oil interface and in the bulk oil phase (∆Tn) were 18 °C, 11


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°C, and 1 °C for concentrations of 30, 100, and 200 mM, respectively. At the higher concentration of 200 mM, Tn at the air/DCB interface produced a similar value as that at the bulk DCB interface. Although no difference in Td was expected between the air/DCB interface and the bulk as for QQT(CN)4, the Td values of C8-BTBT at the air/DCB interface were roughly 8 °C lower than those at the bulk DCB interface in 30 mM. This finding is due to the 10 times higher concentration in the C8-BTBT systems. As observed in Figure 5e and f, the size of the bulk crystal was larger than those at the interface. Because Td was defined as the temperature at which the crystal disappeared entirely, a larger crystal may have resulted in a higher Td because of kinetic dissolution. A slight difference in Td between the air/DCB interface and the bulk was observed at high concentrations because of the higher diffusion at higher temperature. The faster crystallization of the C8-BTBT system might be disadvantageous for the determination of thermodynamic parameters in terms of accuracy. Comparison of the two quasi-phase diagrams reveals that both systems of QQT(CN)4 and C8BTBT exhibited Gibbs adsorption for the promotion of nucleation. Additionally, the temperature differences for supercooling (∆Tsc) at the air/oil interfaces were smaller than those of the bulk oil phases for both systems. Notably, ∆Tsc only increased with increasing concentration in the bulk TCE phase of QQT(CN)4. In other words, the increase of Tn with increasing concentration was smaller than that of Td for the system. For the air/DCB interface and bulk DCB phase of C8BTBT and the air/TCE interface of QQT(CN)4, ∆Tsc were almost similar at each concentration. This phenomenon can be explained by the fluctuation of alkyl chain moieties, which increases with temperature and hinders nucleation. The four alkyl chains on QQT(CN)4 may have induced


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a larger effect compared with the two alkyl chains on C8-BTBT, although the π-conjugated frameworks are essentially different. The ∆Tn values of QQT(CN)4 were larger than those of C8-BTBT, and the supercooling regions of QQT(CN)4 at the air/oil interface were smaller than those in C8-BTBT. This finding can be explained by considering that the larger number of alkyl chains on QQT(CN)4 assists the Gibbs adsorption at the air/oil interface. (a)

(b) 250

250

nucleation

-1

200


-1


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150

100

Tn

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0 -20

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0 10 20 Temperature [℃]

30

40

200

nucleation

150

Tn

100

dissolution

50

0 -20

Td

-10

0 10 20 Temperature [℃]

30

40

Figure 6. Quasi-phase diagrams of C8-BTBT (a) in bulk DCB phase and (b) at air/DCB interface in glass capillaries.

Crossed-Nicol imaging and XRD analysis of QQT(CN)4 and C8-BTBT crystals The QQT(CN)4 and C8-BTBT crystals were prepared by crystallization only at the air/oil interfaces by considering the critical conditions for each quasi-phase diagram. As a typical condition to produce crystals, the solution was first set at a dissolution phase temperature to start from a homogeneous phase. The temperature was gradually decreased to the nucleation lines. Nucleation at the air/oil interfaces was induced by allowing the solution to stand at a temperature


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slightly below the nucleation line (typically 1–2 °C below Tn) for several minutes. Then, the temperature was increased to the supercooling region (typically 1–2 °C above Tn) to suppress further nucleation and was maintained overnight for crystal growth. Micron-scale crystalline precipitates were observed at the air/oil interface. The results for the crystals formed at the air/oil interface are consistent with powder XRD measurements previously reported for QQT(CN)4 crystals27 and C8-BTBT crystals20,28,29. Figure 7a and b present XRD patterns of powders of the QQT(CN)4 and C8-BTBT crystals. In the XRD pattern of QQT(CN)4, the signals at 3.7, 4.3, 8.7, 13.1, and 18.1 nm−1 are attributed to the (001), (001)’, (002), (021), and (211) diffraction peaks, respectively, and are consistent with those previously reported27. Two types of (001) signals have been reported25, which indicates two types of lamellar structures with different interlayer distances in the crystals. The larger signals for (001), (001), and (002) attributed to the lamellar structures indicate that the lamellar structures in the crystals lied nearly parallel to the substrates. These crystals at the major axis would grow perpendicularly to the (211) face. In the XRD pattern of C8-BTBT, the signals at 2.1, 4.2, 6.5, 13.4, 16.0, and 19.4 nm−1 are assigned to the (001), (002), (003), (110), (020), and (120) peaks obtained from a single crystal.20,28,29 The (00l) planes are also attributed to the lamellar structures of C8-BTBT, and the other planes are based on the herringbone packing of BTBT core structures. Figure 7c–f presents crossed-Nicol microscopy images of the crystals of QQT(CN)4 and C8BTBT. The crystals formed on the air/water interface were transferred on glass substrates by casting. A rhombus shape with a major axis of 20 µm and a minor axis of 10 µm for the QQT(CN)4 crystal and a hexagonal shape with a diagonal of 400 µm for C8-BTBT were observed as typical shapes of the single crystals. The appearance and disappearance of the


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crystals based on the rotational angles against the polarized directions marked by arrows clearly indicates the homogeneous crystallinity. In addition to single crystals, polycrystals and/or aggregated crystals were also formed at the air/oil interfaces. The formation of polycrystals and aggregates indicate secondary nucleation on the crystal and aggregation of nuclei at the air/oil interface after the primary nucleation, suggesting that more delicate temperature control is required for the mass production of single crystals. In addition, no crystals or precipitates were observed in the bulk phase.

(a)

(b)

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30

001 15

3

Count [10 ]

001

3

Count [10 ]

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021

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002111 121 001 200 211

5

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10

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002 003

005 110

120

0 0

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10 q [nm-1]

15

20

(c)

0

5

10 q [nm-1]

15

20

(d)

20 µm (e)

200 µm (f)

20 µm

200 µm

Figure 7. XRD patterns of powders of (a) QQT(CN)4 and (b) C8-BTBT crystals. Crossed-Nicol

microscopy images of (c, e) QQT(CN)4 and (d, f) C8-BTBT crystals cast on glass plates. The


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crystals were at angles of (a, b) 45° and (c, d) 0° against the polarization direction. The white dotted lines indicate the positions of the crystals.

Conclusion We constructed quasi-phase diagrams consisting of nucleation, supercooling, and dissolution states for QQT(CN)4 with four alkyl chains and C8-BTBT with two alkyl chains and compared the air/oil interfaces and bulk oil phases. In both systems, the Tn values at the air/oil interfaces were higher than those in the bulk oil phase; however, the air/oil interfaces had no effect on Td, indicating Gibbs adsorption. In the other words, this is the heterogeneous nuclear generation at an air/oil interface induced by Gibbs adsorption. No nucleation or crystal growth were observed on the hydrophilic interfaces, unlike those observed at the water/oil and oil/glass interfaces. The critical temperatures of “nucleation” and “dissolution” obtained by the proposed method must only be considered apparent temperatures for the following reasons: (1) the size of the real critical nucleus expected must be at least hundred times smaller than the above-micron scale of the crystal observed by optical microscopy, (2) the observation time was too short to reach equilibrium, (3) the concentration reduction around the crystal formed due to crystallization cannot be ignored, because the volume of the solution in the capillary is relatively small with respect to the size of the crystal, and (4) diffusion in the closed capillary would be limited compared with that in an open space solution. Nonetheless, the quasi-phase diagrams of soluble small-molecular semiconductors combined with the use of nucleation at air/oil interfaces would enable mass production of organic semiconductor single crystals at desired regions on a substrate as an advanced crystal engineering and interfacial science approach.


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AUTHOR INFORMATION Corresponding Authors *Satoshi Watanabe, *Masashi Kunitake Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources This work was partly supported by Grant-in-Aid for challenging Exploratory Research (15K13818) and Scientific Research on Innovative Areas “Coordination Asymmetry” (17H05378) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

ACKNOWLEDGMENT We appreciate Dr. Xingmei Ouyang and Dr. Atsuya Muranaka for the synthesis of QQT(CN)4.

ABBREVIATIONS


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QQT(CN)4, quinoidal oligothiophene derivative; 2,7-dioctyl [1] benzothieno [3, 2-b] [1] benzothiophene, (c8-BTBT); Tn, crystal nucleation temperature; Td, crystal dissolution temperature; Ta, aggregate formation temperature; TCE, 1,1,2,2-tetrachloroethane; DCB, odichlorobenzene. REFERENCES (1)

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TABLE OF CONTENTS (TOC) GRAPHIC. air

Crystallization

Gibbs adsorption

oil water or glass

For Table of Contents Use Only


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Oil Interfaces and Bulk Oil Phases for

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