Micro-spacing In-air Sublimation Growth of Organic Crystals

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Article Cite This: Chem. Mater. 2018, 30, 412−420

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Microspacing In-Air Sublimation Growth of Organic Crystals Xin Ye, Yang Liu,* Quanxiang Han, Chao Ge, Shuangyue Cui, Leilei Zhang, Xiaoxin Zheng, Guangfeng Liu, Jie Liu, Duo Liu, and Xutang Tao* State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P.R. China S Supporting Information *

ABSTRACT: Organic single crystals manifest the intrinsic physical properties of materials. However, traditional growth of organic single crystals is limited by low solubility from solutions or complexity from physical vapor deposition. Here we report a new method to grow organic single crystals by microspacing in-air sublimation, which avoids costly vacuum system and time-consuming procedures and is practical for a wide range of organic crystals. In situ crystal growth observation revealed an unprecedented vapor-to-melt-tocrystal mechanism, resulting from the micrometer scale spacing distance between the source and the growth position. FET devices based on the rubrene crystals directly grown on Si/SiO2 substrate exhibited higher mobility than the best record using SiO2 as the gate dielectric. This effective organic crystal growth technique can be affordable and handled for almost every lab, which may be beneficial for future research and application of organic crystals.

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INTRODUCTION Organic functional materials have attracted significant research and industrial interest because of their key roles in a diverse range of electronic, optical, and optoelectronic applications.1−3 The traditional techniques for the incorporation of organic materials in most of the applications prefer a morphology of thin films, because of their large area, light weight, flexibility, and reproducibility of characteristics of individual elements.4,5 However, extrinsic structural defects in amorphous or polycrystalline films impede the photon, electron, and/or exciton manipulation and migration inside the semiconducting layers.6−8 Intrinsic properties of organic semiconductors can be measured on single crystals with the highest degree of order and purity among the variety of different material forms.6,9−12 Considering the ease of integration into devices, organic crystals with dimensions of nano- to micrometers that were grown in situ on the substrates or can be manually laminated onto the prepared surfaces were preferred in most of the applications.13−17 The most well developed growth methods of organic crystals are based on vapor and solution processing techniques.18−20 The physical vapor transport (PVT) usually produced single crystals with high quality and dense contact with the substrate; however, it was relatively complex and high cost, because it must include a vacuum and/or an inert carrier gas environment.15,18,20 And what is more, the material utilization (the ratio from starting powders to single crystals grown on substrate) was low. On the other hand, the solution based methods are favored because of their simplicity and tailorability in fabrication procedures; however, they were limited by the poor solubility of the rigid organic semiconductors in common solvents and the possible inclusion of solvent molecules into the crystals.10,15,21 © 2017 American Chemical Society

Although a large number of modified growth methods have been developed during the last decades,7−9,16,22−27 their use is still impeded by the low production efficiency and nonuniversality especially for the materials with poor solubility and stability. It is important to develop an efficient and simple method to evaluate crystalline materials for their performances in devices. In this paper, we reported a facile in-air sublimation method to grow organic single crystals on the substrate which can be affordable and easily handled for almost every lab at low cost and yield crystals with just as good of a performance as those approaches that require very specialized, expensive equipment run by researchers with considerable experience.

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RESULTS AND DISCUSSION Apparatus and Crystal Characterization. The apparatus, as schematized in Figure 1, consists merely of a hot stage and two silicon wafers as the substrates. The starting organic powders (typically much less than 1 mg) were put dispersedly on the bottom substrate. Then the substrates were placed on the hot stage, with a tiny space between the bottom and top ones separated by two small glass sleepers. The growth procedure was implemented by heating the bottom substrate to sublime the materials and formation of microcrystals on the down surface of the top substrate. The crucial part of this method to obtain high quality single crystals is the spacing distance between the two wafers, which was found to be a key factor in determining the morphology of the grown crystals. Received: October 3, 2017 Revised: December 17, 2017 Published: December 18, 2017 412

DOI: 10.1021/acs.chemmater.7b04170 Chem. Mater. 2018, 30, 412−420

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Chemistry of Materials

Figure 1. Microspacing in-air sublimation setup for growth of organic crystals on substrate and the grown rubrene. (a) Schematic drawing of the setup for crystal growth, which consists of a hot stage and two silicon wafer substrates separated by small glass sleepers, with a spacing distance set at 150 μm. (b) Optical microscopy image of rubrene single crystals grown on the top substrate (inset: enlarged image of a single rubrene crystal). Scale bar, 50 μm.

Figure 2. Morphology characterization of rubrene single crystals grown by microspacing the in-air sublimation method. (a) SEM image of rubrene crystals grown directly on the Si/SiO2 substrate. Inset shows the chemical structure of rubrene. (b) TEM image of a single rubrene crystal. (c) AFM image and cross-section thickness profile of a typical rubrene single crystal. (d) Corresponding PXRD pattern of the crystals in a, showing a set of (00l) reflections of their orthorhombic structure. (e) Corresponding SAED pattern of the crystal in b, confirming the orientation of the exposed ab crystalline plane. (f) Fluorescence microscopy images of rubrene crystals grown on the Si/SiO2 substrate. Excitation wavelength: 375 nm.

diffraction (XRD) spectra of the crystals on the substrate (Figure 2d), the pattern is indexed to an orthorhombic rubrene single-crystalline phase and suggests an oriented growth of the rubrene crystals on the substrate, with c-axis perpendicular to the surface. It is further verified by the selected area electron diffraction (SAED) patterns (Figure 2e) confirming the orientation and the high crystallinity of the formed single crystals. Figure 2f shows the fluorescence microscopy images of rubrene crystals, where the contour outlines exhibit bright red fluorescence while the bodies are invisible, due to the optical waveguide effect of the crystals. Growth process and mechanism. This microspacing inair growth of rubrene crystals is quite efficient and economic. The growth process by heating the powders on the bottom substrate to 280 °C could be accomplished in 10 min. One may be concerned about the stability and sublimation rate of the compound during the process. According to both the literature29 and our differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) measurements on rubrene (Figure S1), the melting point, which appeared as an endothermic peak in Figure S1a,c, was observed at 331 °C. Upon cooling, the occurrence of recrystallization depended on

Figure 1a shows a typical growth setup configuration with the spacing distance set at 150 μm, where micrometer-scale single crystals of rubrene grew uniformly on the down surface of the top substrate (Figure 1b). This new method is totally carried out in an ambient environment, devoid of any vacuum equipment or inert gases, and even the quartz tube or furnace in the conventional PVT-based techniques. To clarify the practicability of this method, we first probe the growth process and mechanism by employing a superstar organic semiconductor, rubrene.12 Quite a few techniques have been developed to fabricate rubrene crystals based on both solution and vapor methods.12,28 Here by using our method, high quality rubrene single crystals can be facilely grown on the Si/SiO2 or OTS-modified Si/SiO2 substrate via in-air sublimation. The characterization of the formed crystals is shown in Figure 2. From the scanning electron microscopy (SEM) (Figure 2a) and transmission electron microscopy (TEM) (Figure 2b), we can see the rod-like rubrene single crystals are grown well-dispersed on the substrate. The thickness of the single crystals was about 60 nm, with quite smooth surfaces as revealed by the atomic force microscopy (AFM) (Figure 2c and its inset). According to the X-ray 413


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Figure 3. Crystal formation process from melt droplets on the top quartz substrate observed in real time via hot-stage microscope. (a) Before and (b−e) after heating to 280 °C for (b) 1 min, (c) 3 min, (d) 5 min, (e) 8 min, and (f) 10 min. The six pictures were taken at the same location, and the red circles in d−f indicate the same positions. Scale bar, 100 μm. (g) Microscopy and (h) fluorescence microscopy images of a cooled top substrate removed from the hot stage, exhibiting formed crystals in the solidified melt. Excitation wavelength: 375 nm.

the test conditions, normally ranging from 250 to 280 °C. The TG curve reveals that rubrene started to lose weight abruptly after 365 °C. However, when rubrene was kept at a constant temperature of 280 °C, it continued to lose its weight, with 40% of mass loss after 5 h (Figure S1b). We believe this mass loss resulted from the volatilization but not decomposition. Actually the sublimation of rubrene in our microspacing crystal growth apparatus was even more obvious and effective under atmospheric environment. To ensure the stability of rubrene in the in-air sublimation process, we redid its TG and DSC in air. As shown in Figure S2, the profiles of the TG and DSC curves are similar to those measured in N2. This may indicate that the oxidation or decomposition of rubrene is not obvious during the heating process in air in our test conditions. Figure S3 shows the real time observation of the sublimation process on the bottom substrate during a temperature raising procedure. The volatilization of rubrene powders occurred after 220 °C, and rubrene thin film (deposited on the bottom substrate as starting material) started to detach from the substrate at 200 °C. These results manifest that the sublimation of rubrene at a temperature much lower than the melting point is a considerable thermal effect, although the mass loss in the meantime measured by the changing temperature thermogravimetry is negligible. The sublimed molecules, almost all of which can be deposited on the top substrate, were enough to grow a large amount of microscale single crystals. Because of the microspacing distance between the bottom and the top substrates, the material utilization ratio of this mothed is much higher compared with other PVT and vacuum deposition methods. This case can refer to the close space sublimation (CSS), a modified chemical vapor deposition technique proposed in 1963 for heteroepitaxial growth of inorganic film. The close space between the precursor source and the substrate was believed to greatly decrease the materials consumption.30,31 The method has some similarity with the contact printing, but there is no direct contact between starting materials and the substrate for crystal growth.32 Having been aware of the way the molecules sublimed out from the bottom substrate, we investigated the second step of

the growth process, deposition, and crystal formation on the top substrate. To our surprise, the deposition of rubrene on the top substrate presented entirely differently to the conventional vapor deposition. It is well-known that in the PVT and vacuum evaporation process, that the molecular vapor transforms directly to the solid state crystals or films. However, in the process of our microspacing in-air sublimation growth, after the vapor molecules reach the top substrate, they liquefied into a liquid-like melt first. The process of the melt formation and crystal growth from the melt could be also monitored unequivocally under a hot-stage microscope in situ and in real time. As shown in Figure 3, the top quartz substrate displayed a bare and lime-green surface before growth (Figure 3a). After heating the bottom substrate to 280 °C for 1 min, we could see a mass of tiny melt droplets densely distributed on the down surface of the top substrate, appearing to be a frosted glass (Figure 3b). As the heating went on, individual droplets become larger, owing to the Ostwald ripening effect (Figure 3c). To make sure the droplets are a melt, but not vitreous solid, we intentionally pressed the droplets parts, where liquid-like back and forth flow occurred correspondingly (Movie 1). The formation of the melt droplets, which resulted from the condensation of the sublimed rubrene vapor, is another unique characteristic of this method, and decides the direction of crystal growth subsequently. After about 5 min, some of the droplet sizes become even larger, and notably, we found small crystals nucleated from some melt droplets (Figure 3d). Then the formed crystals grow larger; meanwhile, the melt droplets surrounding the crystal tended to shrink and some were even absorbed by the crystals or the adjacent ones, because of mass transportation (Figure 3e). During this period the rotation movement of the crystals embedded within the melt was also noticed, verifying the fluent nature of the liquid melt droplets (Figure S7). If the top substrate from the apparatus was removed at this period, the droplet melt would solidify into the glassy state, as shown in Figure 3g,h, where we can see clearly the formed crystals surrounded by the solidified melt. The fluorescence microscopy image witnessed the red luminescent rubrene crystals buried in the orange amorphous 414


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Figure 4. Schematic outline of crystallization mechanism in the microspacing in-air sublimation method: vapor-to-melt-to-crystal. The process followed these steps: materials vaporization → vapor condensation into melt droplets → melt droplets ripening → nucleation from melt → crystal growth. The up and down arrows indicate buoyancy driven convection.

the seed crystals, finally growing into dendritic structures. To produce well distributed and high quality single crystals, we must minimize the distance to below 400 μm, where all of the vapor molecules condensed into melt on the top substrate; the crystals crystallized from individual melt droplets. The spacing distance variation then must have a subtle effect to change the growth conditions. The temperature of the top substrate was believed to be one of the key variables, because of the severe through-space heat conduction in such a close distance. We measured the temperature dependence of the top substrate on the different spacing distance in a temperature rising and stabilizing procedure. As shown in Figure S10, the temperature of the top substrate raises synchronously along heating of the bottom substrate when the distance is below 400 μm. At a spacing distance of 150 μm, the top substrate could be stabilized at 270 °C when the bottom substrate was heat to 280 °C, with a temperature difference of only 10 °C. While when the spacing distance was set at 4 mm, the temperature of the top substrate lagged obviously behind that of the bottom substrate, with a temperature difference as much as 140 °C. According to the Maxwell−Boltzmann distribution equation33,34

solid. The identity and purity of the rubrene crystals and the droplet melt were further proved by measuring their MALDITOF-MS (matrix assisted laser desorption ionization time-offlight mass spectrometry), 1H NMR spectra, 13C NMR spectra, elemental analysis, and IR spectra (Figures S4−S6 and Table S1). Comparison with starting materials shows no notable difference. These results, plus the PXRD patterns, demonstrate that rubrene remained steady during the sublimation growth in our method, without notable oxidation and degradation. We regard that a short source−substrate distance and fast evaporation are factors that limited the amount of oxygen in the confined space to prevent oxidation. After about 10 min of heating, almost all of the melt had grown into crystals (Figure 3f). The Movie 2 represents the whole dynamic process of crystal formation from the melt droplets on the top substrate. Actually here in order to facilitate the observation of crystal growth from the melt, we used a superabundant amount of rubrene sample, resulting overcrowded and low quality crystals. If we used a moderate amount of the starting material, the distribution and the quality of the grown crystals improved a lot, as shown in Figure 1b and Figure 2a. This indicated that the size of the grown crystals and their distribution on the substrate can be adjusted by the dosage of the starting materials. Figure S8 and Movie 3 demonstrate a real time process of using a much smaller amount of the starting material (about one hundredth to that of Figure 3), where we can see the small dosage formed small size melt droplets, and subsequently inducing small size crystals and uniform position distribution. As has been mentioned above, the spacing distance between the two substrates is a most important factor of the method, which determines the morphology of the grown crystals. Thus, to investigate this distance-dependent growth process, we tuned the spacing distance by changing the layers of the glass sleepers between the two substrates and probed the growth process and the resulted morphology variation of the formed crystals. As shown in Figure S9, when the distance is bigger than 4 mm, the materials deposited on the top substrate as polycrystalline films. Because at this distance the vapor transformed to solids directly, like in vacuum deposition, no liquefied melt was involved. When the distance is down to about 2 mm, dendrite crystals formed on the substrate. As revealed by the real time observation shown in Movie 4, there were actually two different deposition mechanisms being involved contemporarily. In the initial stage of the formation of a large amount of melt droplets, a few tiny crystals directly nucleated. Then along with the condensation of more and more melt droplets, new crystals continually developed from

λ=

kBT 2 πd 2p

(1)

where T is the temperature, d is the molecular diameter, p is the pressure, and kB is the Boltzmann constant, although the mean free path (λ) of the vapor molecules was not significantly reduced because the process was implemented at ambient pressure, the elapsed time for the gas molecules to get to the top substrate during the sublimation is in a microsecond scale due to the microspacing. More importantly, the microspacing distance makes the vapor transport regime to be molecular flow limited but not convection-limited which is normally dominant at atmospheric pressure conditions. The Rayleigh number (Ra),18 a dimensionless number associated with buoyancy-driven flow,35 is used to demarcate the vapor and heat transport regimes: Ra =

gβ ΔTh3 νκ

(2)

where g is acceleration due to gravity, β is the thermal expansion coefficient, ν is the kinematic viscosity, κ is the thermal diffusivity, ΔT is the temperature difference, and h is the spacing length between the source and deposition positions. From eq 2 we know Ra is in direct proportion to the third power of the spacing distance; thus, a microspacing leads to an extremely small Ra number, which is estimated to 415


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Figure 5. Using microspacing in-air sublimation to grow various kinds of organic crystals on a substrate. (a) Chemical structures of the tested compounds anthrance, perylene, pyrene, TCNQ, pentacene, 1DOAFO, TPE, Alq3, and telmisartan. (b) Microscope images, (c) fluorescence microscopy images, and (d) PXRD patterns of the respective as-grown crystals.

be 1.7 × 10−7. This value indicates a molecular flow vapor transport mode.18 Although here ΔT is only 10 K, the temperature gradient is quite steep due to the microspacing distance. Thus, the buoyancy driven motion is mainly responsible for the molecular flow transport.18,35 The molecular flow vapor transport will facilitate the volatilization rate of the starting materials, which is even more effective than the inert gas flowing in conventional PVT.18 That is why the volatilization in our apparatus is much faster than that in the TGA measurements with nitrogen gas flowing. The buoyancy driven convection, with a circulation pattern upward in the substrate area and downward in the marginal area (as revealed in Figure 4), will also produce a transient close environment with relative negative pressure within the two substrates, protecting the vapor molecules from oxidation or decomposition. In a word, the microspacing would generate effects similar to those of vacuum or reduced pressure. The efficient volatilization and vapor transport, which is too much faster than the possible speed for nucleation on the top substrate, were believed to be an important contributor for the condensation of the vapor into liquid, but not crystallization directly. Furthermore, because of the small temperature difference between the two substrates, the temperature of the top substrate (270 °C) is even higher than the crystallization point of rubrene (250 °C in our DSC test condition). If we hypothesize the crystal nucleated from the melt is homogeneous (maybe not exactly true in practice), from the aspect of nucleation energy barrier, the free energy to form a nuclei with the critical radius, r*, is given by Volmer analysis:34 ΔGr* =

a critical nucleus would be lowered if a liquid phase is involved and the crystals nucleate from the liquid, compared to the case where vapor crystallizes directly. Thus, for both kinetic and thermodynamic view, the growth from an intermediate supercooling melt is more favored in theory, which can be attributed to a synergistic result from the microspacing distance and a small temperature difference. To verify the inference, we did control experiments, by reducing the temperature difference between the two substrates in a larger spacing distance or increasing the temperature difference in a microspacing distance. As shown in Figure S11, when we heated the top substrate in a big spacing distance of 4 mm to 270 °C, no molecules deposit on the substrate, differing from the formation of thin film as in a normal temperature (145 °C, Figure S11a). A saddle-shaped temperature distribution, being depressed in the spacing gap due to the large distance, was considered to produce a less efficient vapor transport, breaking the equilibrium with the evaporation from the top substrate. When we cool the top substrate in a microspacing distance to 80 °C, glassy state solids formed on the substrate, either being different with the case in its normal temperature (260 °C, Figure S11c). These results confirm the synergy of the microspacing and its induced temperature distribution, witnessing a mechanism substantially different from traditional PVT and other sublimation methods. The mechanism model, based on the in situ hot-stage optical microscopies and the above analysis, was schematized in Figure 4. Our microspacing in-air sublimation followed a vapor-to-melt-to-crystal crystallization process. The process, being characterized by the meltintermediate, formed crystals with intimate contact with the substrate. And because of the surface tension of the liquid, the well distributed melt droplet made for the distribution of the microcrystals on the substrate. It should be noted that compared with the previous large-distance sublimation growth of Cl2−NDI crystals reported by collaboration of Frank Würthner’s and our groups,9 the microspacing sublimation indeed represents a brand-new transport and growth mechanism. Universality to More Materials. To check the universality of the method, we test nine other kinds of starting materials for crystal growth. A broad range of materials, including five typical organic semiconductorsanthrance, perylene, pyrene,

16πTm 2σ 3 3L2ΔT 2

(3)

where σ is the surface tension and L is the latent heat. The equation indicates the free energy is depending heavily on the degree of supercooling and the surface tension for a specific material. It is well-known that for organic substances which can be supercooled a lot below the melting point, a relashenship34 σv − l + σl − s < σv − s (4) is valid, where σv−s is the surface tension of the vapor−solid interface and so on. According to eqs 3 and 4, in a circumstance of a small ΔT, the free energy barrier to form 416


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Figure 6. Single-crystal transistors and the corresponding device performance based on the grown rubrene crystals on Si/SiO2 substrate. (a) Microscopy image of a representative single-crystal transistor, with a channel direction along the b-axis of the crystal. (b) Relative rubrene molecular stacking in the basal (a−b) plane, with a stronger overlap along the b-axis. (c) Schematic of device configuration. (d) Representative transfer curve and (e) output characteristics curve. A drain voltage (VD) of −40 V was applied when the transfer characteristics were measured. (f) Mobility distribution of 15 single-crystal transistors on two substrates.

TCNQ (7,7,8,8-tetracyanoquinodimethane), and pentacene,20 an intermolecular hydrogen bond-induced solid emitter 2-(4(diphenylamino)phenyl)fluorenone (1DPAFO),36 a propellershaped compound with aggregation-induced-emission tetraphenylethene (TPE),37 a representative luminescent metal coordination compound tris(8-hydroxyquinoline) aluminum (Alq3),38 and even a pharmaceutical compound against hypertension telmisartan39were employed in the microspacing in-air sublimation growth. Figure 5 shows the respective chemical structures and the as-grown crystals on the substrate. Our method worked well on all of the mentioned materials, obtaining single crystals with size of about 10 to 100 micrometers. The morphologies of the grown crystals were related to their intrinsic crystal habits, for which anthrance, perylene, and pentacene prefer two-dimensional plate-like and rubrene, 1DPAFO, Alq3, and telmisartan prefer one-dimensional rod-like, while pyrene, TCNQ, and TPE prefer threedimensional diamond-like (Figure S12). The morphology preference and size distribution was believed to be affected by the limited mass transfer during crystallization process from the small-size melt droplets. The XRD patterns shown in Figure 5c proved the identity of the corresponding crystals and, furthermore, indicate the oriented growth for most of the crystals. The fluorescence of the crystals (except TCNQ and pentence) are clearly visualized under fluorescence microscopy (Figure 5d). Another noteworthy point is the sublimation temperature and duration for different materials. For the five organic semiconductors, the sublimation temperature was about 100 or more lower than its melting point (e.g., anthrance Tm = 215 °C, Tsublimation = 130 °C; pentacence Tm = 373 °C, Tsublimation = 255 °C), and the growth could be accomplished in about 5 min. While for telmisartan, the sublimation temperature is 265 °C, close to its melting point (271 °C);, and the growth needed 2 h. We ascribe the ease or difficulty to the difference of the sublimation enthalpy of various materials. The results suggest a broad generality of our microspacing in-air sublimation method, being applicable to materials with various thermal properties. As shown in the TG and DSC measurements of the materials (Figures S13 and S14), whether the

difference between Tm and Td (decomposition temperature) and that between Tm and Tc are large or small for a certain material, in-air sublimation and growth could proceed without thermal degradation, even for a pharmaceutical crystal containing a carboxylic acid group and multiple hydrogen bonds. Crystallization from melting intermediate was also observed for the materials (Figure S15), validating the proposed the vapor-to-melt-to-crystal growth mechanism. In order to verify the stability of the materials, especially pentacene in the process of in-air sublimation, we measured the elemental analysis (Table S1) and the IR spectra (Figure S16) of the starting materials and the grown crystals. The results indicated that no oxidation or decomposition occurred during the in-air sublimation process. Additionally, we performed TG and DSC analyses of pentacene in air atmosphere (Figure S17), and its degradation started after 335 °C. Because the growth temperature of pentacene in our method is at 255 °C, much lower than the decomposition temperature, it is relatively safe at the growth temperature. Application in Single-Crystal Devices. Based on the grown rubrene crystals on Si/SiO2 substrate, field effect transistors (FET) were fabricated to verify the practical applications in single-crystal FET devices. A thin layer of Au as source and drain electrodes was deposited onto the individual rubrene single crystals by using organic-ribbon shadow masks, with a top-contact, bottom-gate configuration. Figure 6 shows the microscopy image and a schematic representation of a representative rubrene single-crystal transistor and the corresponding transfer and output characteristics curves. The transistor channel is along the b-axis of the crystal, which is the fastest crystal-growth direction, and has a stronger overlap of the electronic π orbitals along it (Figure 6a−c). All the devices were measured at room temperature and under ambient conditions. The devices showed typical p-type field-effect modulation, with on/off current ratios as high as 107 and threshold voltages between −1.5 and −7.4 V. The saturation regime electron mobility was extracted, where an average hole mobility of above 4 cm2 V−1 s−1 was achieved from 15 devices on two substrates. The statistical distribution 417


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Hot-Stage Microscope. The hot-stage microscope system consists of a Linkam THMS600 heating stage and Leica DM2700P optical microscope. Contrast of the inset of Figure 1b was enhanced with Photoshop to make the crystal edge clearer. Device Fabrication. SiO2/Si wafer (SiO2 300 nm thick, 10 nF cm−2) was first washed by deionized water, hot H2SO4:H2O2(2:1) solution, deionized water, and isopropyl alcohol and dried by a N2 gun. Then the wafer was cleaned by O2 plasma to remove organic compounds on its surface, followed by the octadecyltrichlorosilane (OTS) modification. OTS was thermally evaporated onto wafer at 200 °C for 2 h in vacuum. Then the SiO2/Si wafer was washed by nhexane, trichloromethane, and isopropyl alcohol and finally dried by N2 gun. Crystals were grown directly onto the OTS modified SiO2/Si wafer by microspacing in-air sublimation. Then gold (Au) electrodes (20 nm thick) were thermally evaporated onto crystals, using the “organic ribbon mask method”. Device Characterization. For characterization, a Keithley 4200A semiconductor parameter analyzer was used. Note: no precautions were taken to exclude light and air, either for the fabrication of the active layer of the discrete as well as integrated devices or during electrical characterization. All measurements were performed at room temperature in air. Powder X-ray Diffraction. The powder X-ray diffraction was performed on a Bruker-AXS D8 ADVANCE X-ray diffractometer equipped with a diffracted beam monochromator set for Cu Kα radiation (λ = 1.54056 Å) in the range 9−40° (2θ), with a step size of 0.08° and a step time of 0.2 s at room temperature. Fluorescence Microscope Measurement. Fluorescence images were obtained using a Nikon Ti−U Inverted Microscope System equipped with a Nikon C-SHG 1 mercury lamp. The exposure time to acquire a bright photo on a fluorescence microscope for rubrene, TPE, 1DPAFO, and Alq3 is as short as 100 ms and 4 s for telmisartan. Electron Microscopy Measurement. Scanning electron microscopy (SEM) was obtained by a Hitachi S-4800 ultrahigh resolution (UHR) field emission (FE) scanning electron microscope, and the pictures were taken at an accelerating voltage of 5.0 kV. Transmission electron microscopy (TEM) images were performed on a JEOL JEM2100F transmission electron microscope operating at an acceleration voltage of 200 kV. Rubrene microcrystals were prepared on a carboncovered copper grid by microdistance in-air sublimation at ambient pressure. The thickness and surface morphology of crystals were measured by AFM at ambient conditions with a using a Digital Veeco Instruments atomic force microscope operating in the tapping mode. Thermal Analysis. The differential scanning calorimetry (DSC) measurements were implemented on a NETZSCH DSC 200F3 analyzer. Differential scanning calorimetry and thermogravimetric analysis (DSC-TGA) were carried out using a TGA/DSC1/1600HT analyzer (METTLER TOLEDO Instruments) with a heating rate of 10 °C/min.

of the mobility values is shown in Figure 6f. The highest hole mobility was up to 7.8 cm2 V−1 s−1. According to plenty of previous studies on rubrene crystal-based FETs, the best results when using SiO2 as the gate dielectric is 4−7 cm2 V−1 s−1 (the mobilities can be up to 15−20 cm2 V−1 s−1 when the vacuum gate dielectric was used).40−44 Thus, the devices based on our grown crystals show even higher mobilities than the best record. The remarkably high mobility should be attributed to the high single crystallinity of the rubrene crystals and their conformal contact with the substrate, which proved the great usability of our method on device applications.

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CONCLUSION In conclusion, we developed a new method for growth of organic crystals on substrate. The method, which we called microspacing in-air sublimation, most characterized by the microspacing distance between the source and the growth position, represents an advance on both growth mechanism and implementation techniques. The synergy effect between the microspacing and the temperature distribution induced an unprecedented vapor transport regime and crystallization mechanism. A vapor-to-melt-to-crystal process was revealed via in situ observation. The method, by employing 10 kinds of organic crystals, including different semiconductors and a pharmaceutical crystal, that were successfully grown with high crystallinity proved a broad generality to various materials. The method, totally being implemented at ambient conditions, requiring neither vacuum systems, nor inert gases, nor even a growth chamber, possesses advantages in many aspects such as growth time, materials utilization ratio, applicability for real time observation, etc. (Table S2 in Supporting Information presents a detailed list of comparison of our method to traditional PVT.) FET devices based on the rubrene crystals grown on the Si/SiO2 substrate exhibited higher mobilities than the best record ever reported. Thus, the new method on one side serves as an effective organic crystal growth technique that can be affordable and handled for almost every lab and, on the other hand, provides a platform to probe the realistic mechanism of organic crystal formation. In future work we will tackle other technologically relevant challenges on location and orientation of the grown single crystals, via substrate patterning or other strategies to control the distribution and solidification of the melt droplets and on issues when scaling to larger areas. We hope the new method will open a new era for the future research and application of organic crystals.

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ASSOCIATED CONTENT

S Supporting Information *

EXPERIMENTAL SECTION

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04170.

Materials. Materials used in this work were purchased as follows: 5,6,11,12-tetraphenylnaphthacene (rubrene; sublimed grade) and 2,3:6,7-dibenzanthracene (pentacene; sublimed grade) from TCI; 4′[(1,4′-dimethyl-2′-propyl[2,6′-bi-1H-benzimidazol]-1′-yl)methyl][1,1′-biphenyl]-2-carboxylic acid (telmisartan; 99%) and tris(8hydroxyquinoline) aluminum(III) (Alq3; 99%) from Alfa Aesar; and anthrance, perylene, and pyrene (99%) from Aladdin. Tetraphenylethene (TPE) and 2-(4-(diphenylamino) phenyl) fluorenone (1DPAFO) were synthesized according to refs 37 and 36. Growth of Single Crystals. The detailed growth temperature and time are as follows: rubrene (280 °C for 10 min), anthracene (130 °C for 10 min), perylene (200 °C for 5 min), pyrene (90 °C for 5 min), TCNQ (180 °C for 5 min), pentacene (255 °C for 5 min), Alq3 (300 °C for 10 min), TPE (185 °C for 15 min), 1DPAFO (185 °C for 2 h), and telmisartan (265 °C for 2 h). The surface density and thickness of the crystals were controlled by adjusting the distance between the bottom Si/SiO2 and the top substrate for crystal growth.

Video showing the back and forth flow of the melt (AVI) Video showing the whole dynamic process of crystal formation from the melt droplets on the top substrate (AVI) Video showing a real time process of using a much smaller amount of the starting material (AVI) Video showing the real time observation of two different deposition mechanisms (AVI) Experimental details and additional characterization data (PDF) 418


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AUTHOR INFORMATION

Corresponding Authors

*(Y.L.) E-mail: [email protected]. *(X.T.) E-mail: [email protected]. ORCID

Duo Liu: 0000-0002-5503-2566 Xutang Tao: 0000-0001-5957-2271 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (Grant Nos. 21772115, 51321091, 51227002, 51272129), National key Research and Development Program of China (Grant No. 2016YFB1102201), and the Program of Introducing Talents of Disciplines to Universities in China (111program no. b06015). Y.L. is grateful for the support from Qilu Young Scholars and Tang Scholars. We thank Weigang Zhu, Huanli Dong, and Wenping Hu for help with OFET fabrication and measurements.

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