Homoepitaxy Growth of Well-Ordered Rubrene Thin Films - American

Apr 1, 2008 - as Au, SiO2, and Al2O3 by organic molecular beam deposition or ordinary vacuum evaporation. In this work, rubrene organic single...
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CRYSTAL GROWTH & DESIGN

Homoepitaxy Growth of Well-Ordered Rubrene Thin Films Xionghui Zeng,†,‡ Liduo Wang,† Lian Duan,† and Yong Qiu*,† Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China, and Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou, 215123, China

2008 VOL. 8, NO. 5 1617–1622

ReceiVed October 23, 2007; ReVised Manuscript ReceiVed January 13, 2008

ABSTRACT: Rubrene is a nonplanar molecule, it is recognized that only amorphous rubrene films can be obtained on such substrates as Au, SiO2, and Al2O3 by organic molecular beam deposition or ordinary vacuum evaporation. In this work, rubrene organic single crystals with very smooth surfaces were used as substrates. Well-ordered rubrene thin films were obtained by ordinary vacuum evaporation, and the substate temperature is room temperature. Two-dimensional nucleation-monolayer by monolayer growth and homoepitaxy was demonstrated when the deposition rate was kept at 0.2 nm/min. Furthermore, when the deposition rate was increased to 0.6 nm/min, two-dimensional hexagons were observed in the growth process of rubrene thin films and are similar to the twodimensional islands with regular shapes observed in homoepitaxy growth of inorganic thin films. Introduction In recent years, organic electronic materials and devices have attracted much attention because of their potential applications in large-area and flexible electronics displays.1–3 At present, a well-developed microscopic description of the charge transport in organic materials is still lacking. The intrinsic electronic properties of organic semiconductor can be studied by using single crystals, because organic single crystals are free from grain boundaries and molecular disorder.4 Rubrene (Figure S1, Supporting Information) is a nonplanar molecule, and under ordinary conditions, only amorphous rubrene films can be obtained even by organic molecular beam deposition (OMBD).5,6 Surprisingly, rubrene single crystals with high quality can be grown by physical vapor transport7,8 and exhibit extraordinary large charge carrier mobility up to 20 cm2/ V · s and strong anisotropic charge transport.9–12 Some progress has also been made in achieving rubrene crystalline thin films. By the incorporation of a glass-inducing diluent, rubrene crystalline thin films were obtained from vitreous solutionprocessed rubrene hypereutectics, and the mobility of transistors based on the rubrene crystalline thin film is up to 0.7 cm2/V · s.13 The growth of crystalline films of rubrene on SiO2 and Au(111) substrates has been investigated by employing “hot wall” deposition. But according to the scanning electron microscopy (SEM) graphs in this research,6 the flatness of these crystalline films is not good. Orderly rubrene thin films were also obtained on the surface of pentacene thin films by OMBD14 and vacuum evaporation.15 The mobilities of the related thin film transistors were 0.07 cm2/V · s14 and 0.6 cm2/V · s,15 respectively. It can be noticed that the mobility of all above-mentioned rubrene thin film field-effect transistors13–15 is much lower than that of rubrene single-crystal field-effect transistors.9–12 However, for pentacene, the best mobility of its thin film transistors is up to 3 cm2/V · s16 and is on the same order as that of pentacene singlecrystal field-effect transistors.17 We think that the poor crystalline quality of rubrene thin films13–15 owing to the nonplanar molecular structure of rubrene is responsible for the poor performance of rubrene thin film field-effect transistors. Recently, Bao and her colleagues fabricated large arrays of high* Corresponding author. E-mail: [email protected]. † Tsinghua University. ‡ Suzhou Institute of Nano-tech and Nano-bionics.

performance rubrene single crystals with mobilities as high as 2.4 cm2/V · s and on/off ratios greater than 107.10 They also pointed out it is very important to study the crystal growth mechanism for controlling the orientation and alignment of crystals and achieving high device uniformity. Therefore, the growth mechanism study of rubrene crystalline thin films and rubrene single crystals will be very helpful for the research of charge transport theory in organic semiconductors4 and large-area flexible electronics.1–3 Generally, the growth of inorganic crystalline thin films relies on the rather strong covalent or ionic bonds of the adsorbates to the substrates; therefore, lattice matching is usually a requisite, because it avoids stress buildup.18 For organic crystalline thin film growth, although lattice matching is not a requisite, lattice strain should also be considered.19 To make the mechanism of organic thin film growth more clear, it is necessary to adopt a single crystal substrate.19 At present, substrates commonly used for growing organic thin films are mainly divided into three types as follows:19 (a) metal substrates, (b) metal oxides, and (c) alkali halides and mica. Therefore, if an organic single crystal can be used as substrate for growing organic thin films,20–22 it will be more powerful for elucidating the growth mechanism of organic crystalline thin films. In this work, rubrene single crystals with very good flatness were used as substrates for growing rubrene thin films. Experimental Section Rubrene materials with 99% purity were purchased from Acros and used as received in all experiments. Rubrene single crystals were grown by physical vapor transport.7,23 The source temperature and the deposition temperature are 290 and 260 °C, respectively. The flowing rate of argon is 30 mL/min, and the growth time is 30 h. The surfaces of single crystals were scanned by atomic force microscopy (AFM) (SPA 400 made by Seiko Instruments, Inc.) in tapping mode. Figure 1A shows a photo of a rubrene single crystal. Figure 1B shows an AFM image of an 80 × 80 µm2 area of the rubrene crystal surface, and a steplike structure is observed. It can be seen from Figure 1C that the height of the typical step is about 1.5 nm and agrees very well with the (100)-layer spacing of d(100) ) 1.34 nm in rubrene crystals.23,24 The width of the steps is larger than 15 µm, and the density of steps on the surface of rubrene crystal is much less than that described by B. D. Chapman, et al.24 Therefore, our crystals have a smoother surface. The rms (root mean square) of roughness in 80 × 80 µm2 area as shown in Figure 1B is 1.607 nm. We also scanned other rubrene crystals by AFM and typically the of roughness in 5 × 5 µm2 area as shown in

10.1021/cg701046h CCC: $40.75  2008 American Chemical Society Published on Web 04/01/2008

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Figure 1. (A) Photograph of rubrene single crystals grown by physical vapor transport. (B) AFM image of 80 × 80 µm2 area of rubrene crystal surface, rms ) 1.607 nm. (C) Cross-section curve indicated by the line in B with a step height of about 1.5 nm. (D) Typical AFM image of 5 × 5 µm2 area on the surface of rubrene single crystal, rms ) 0.132 nm. Figure 1D is less than 0.2 nm. It is well-known that the flatness of substrate is very important for thin films growth.25 In this work, the rubrene single crystals provide a very flat substrate for thin film growth. To demonstrate the growth process, a rubrene single crystal was used as the substrate for preparing rubrene thin films with nominal thicknesses of 4, 8, 12, 16, 21, and 28 Å, successively. For example, after we prepared a rubrene thin film with 4 Å thickness, we scanned the surface of rubrene thin film by AFM, and then the sample was used as a substrate for growing a rubrene thin film again and we obtained a rubrene thin film with 8 Å thickness. In this successive process, the deposition rate was kept at 0.2 nm/min. In addition, we prepared rubrene thin films with 10 and 20 nm thicknesses on the surface of a rubrene single crystal with the deposition rate of 0.2 nm/min. For comparison, we also prepared rubrene thin films with a 20 nm thickness on the surface of a rubrene single crystal and a quartz crystal with the

deposition rate of 0.6 nm/min. In all evaporating experiments, the substrate temperature was room temperature and the background pressure was about 1 × 10-3 Pa. The thickness of the thin films and the deposition rate were monitored by a quartz crystal thickness monitor.

Results and Discussion Figures 2-7 shows the AFM images of rubrene thin films with different thickness on the same rubrene single-crystal substrate grown by successive evaporation. As shown in Figure 2A, the nominal thickness of rubrene thin film is 4 Å, and some two-dimensional islands nucleate on the surface of the rubrene single crystal. The island density is about 108/5 × 5 µm2, and the average island area is about 0.09 µm2. The height of these

Homoepitaxy Growth of Rubrene Thin Films

Figure 2. (A) AFM image of rubrene thin film with 4 Å thickness, rms ) 0.7903 nm. (B) Cross-section curve indicated by the line in A with a step height of about 1.65 nm.

Figure 3. (A) AFM image of rubrene thin film with 8 Å thickness, rms ) 0.7234 nm. (B) Cross-section curve indicated by the line in A with a step height of about 1.69 nm.

islands is about 1.65 nm, as shown in Figure 2B, corresponding to a monolayer height of rubrene molecules.21 As shown in Figure 3A, the two-dimensional islands grow in size, and some islands even begin to connect with each other when the thickness of the rubrene film reaches 8 Å. The average island area is about 0.23 µm2. When the thickness of the rubrene thin film is up to 12 Å, as shown in Figure 4A, nearly all two-dimensional islands have already been connected with each other and small interspaces between the islands can be observed. Furthermore, some new and very small two-dimensional islands with about 1.65 nm height as shown in Figure 4B nucleate on the top of the first monolayer, indicating that the second monolayer begins to grow. As shown in Figure 5A, when the thickness is 16 Å, the first monolayer is nearly completely formed and only some small holes with about 1.59 nm depth are observed in Figure 5B. At this stage, the two-dimensional islands in the second layer have grown up. It is noted that Figure 5A is very similar

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Figure 4. (A) AFM image of rubrene thin film with 12 Å thickness, rms ) 0.4847. (B) Cross-section curve indicated by the line in A with a step height of about 1.65 nm.

Figure 5. (A) AFM image of rubrene thin film with 16 Å thickness. rms ) 0.7644 nm. (B) Cross-section curve indicated by the line in A with a step height (depth of monolayer hole) of about 1.59 nm.

to Figure 2A. As shown in Figure 6A, when the thickness is 21 Å, most two-dimensional islands begin connecting with each other and some sparse and new two-dimensional islands nucleate on the top of the second monolayer. It can be seen that Figure 6A is similar to Figure 4A. As shown in Figure 7A, when the thickness is 28 Å, the second monolayer is nearly completely formed and only some small holes of 1.61 nm depth are observed in Figure 7B. At the same time, the two-dimensional islands of the third layer also grow up. It can be observed that Figure 7A is also very similar to Figures 5A and 2A. According to Figures 2-7 and our analysis, two-dimensional nucleation-monolayer by the monolayer growth of rubrene thin films on the surface of a rubrene single crystal has been demonstrated. Furthermore, we also demonstrated that the formation process of the second layer is nearly the same as that of the first one, indicating that the first layer and subsequent

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Figure 6. (A) AFM image of rubrene thin film with 21 Å thickness, rms ) 0.6388 nm. (B) Cross-section curve indicated by the line in A with a step height of about 1.64 nm.

layers have the same nature as the surface of a rubrene single crystal. Therefore, homoepitaxy growth was observed. In the other hand, the island density of the second layer and the third layers is greater than that of the first layer. For example, as shown in Figures 2 and 5, the island density of the second layer is about one and a half that of the first layer. It should be noticed that the rubrene single crystals were grown at thermodynamic equilibrium,23 and the growth stress and strain in the single crystals can be eliminated in the growth process. In the growth process of rubrene thin films, the substrate temperature was room temperature and the background pressure was about 1 × 10-3 Pa. Therefore, the rubrene thin films were grown at thermodynamic nonequilibrium, and growth stress and strain in the interior of the rubrene thin films can not be completely avoided, although they were grown on the surface of the rubrene single crystals. We think that the growth stress and strain in the interior of the rubrene thin films lead to the island density of the second layer and the third layer being bigger than the first layer. From this point of view, the growth process of rubrene thin films as shown in Figures 2-7 is very close in resemblance to the layerby-layer growth mode (Frank-van der Merwe growth) but only strictly belongs to the layer-plus-island growth mode. Under the same conditions as those in Figures 2-7, we prepared rubrene thin films with 10 and 20 nm thickness (Figures S2 and S3, Supporting Information). Two-dimensional islands with irregular shapes were observed on the surfaces of these two rubrene thin films and are very similar to the twodimensional islands in Figures 2-7. When the deposition rate was increased to 0.6 nm/min, two-dimensional hexagon islands were observed, as shown in Figure 8A. To our knowledge, up to now there have been no reports of two-dimensional islands with regular shapes observed in the growth process of organic thin films. However, two-dimensional islands with regular shapes including triangle, square, and hexagon were observed in the homoepitaxy growth process of inorganic thin films.26,27 By comparison of Figures 2-7, S2A, and S3A (the latter two in Supporting Information) with Figure 8A, it can be seen that the shape of two-dimensional islands changes from irregular shape to regular hexagon shape by changing the deposition rate. Therefore, it seems that the formation of two-dimensional hexagon islands depends on the dynamic process. Furthermore,

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Figure 7. (A) AFM image of rubrene thin film with 28 Å thickness, rms ) 0.7892 nm. (B) Cross-section curve indicated by the line in A with a step height (depth of monolayer hole) of about 1.61 nm.

Figure 8. (A) AFM image of rubrene thin film with 20 nm thickness, rms ) 0.8379 nm. (B) Cross-section curve indicated by the line in A with a step height of about 1.0 nm.

an obvious layer-plus-island mode is observed in Figure 8A. It is known that surface diffusion and interlayer mass transport are the two most important factors for inorganic film growth.28 In our experiments, when the deposition rate is as low as 0.2 nm/min, the deposited molecules may have enough time to diffuse to the lower layer before they are “pinned” in the position by the arrival of one or more additional molecules. Therefore, monolayer-by-monolayer growth has been observed, as shown in Figures 2-7, S2, and S3 (the latter two in Supporting Information). When the deposition rate is as high as 0.6 nm/ min, the deposition molecules may have small chance to diffuse to the lower layer before they are pinned in the position by the arrival of one or more additional molecules. Therefore, new two-dimensional islands more easily nucleate on the top of previously formed two-dimensional islands and layer-plus-island mode is obviously observed, as shown in Figure 8. In our previous work,23 hexagon rubrene crystals and hexagon etching

Homoepitaxy Growth of Rubrene Thin Films

pits were observed. Other researchers also obtained hexagon rubrene crystals.6,10 We think the two-dimensional hexagon islands as shown in Figure 8 are mainly determined by the nature of rubrene crystals and the dynamics process. On the other hand, the formation of the two-dimensional hexagon islands is not just a dynamics process, but is also favored by thermodynamics. J. E. Northrup et al. adopted first-principles pseudopotential density-funtional calculations to determine the cohesive and surface energies for polyacenes such as pentacene and anthracene and constructed an equilibrium crystal shape of pentacene. For pentacene, calculations predicts that the (001) surface has a much lower surface energy than the other surfaces and thus the pentacene films exibit (001) orientation. They think that the calculation of cohesive and surface energies is useful for understanding the temperature dependence of the growth morphology of pentacene. Therefore, we think the formation of hexagonal-shaped two-dimensional islands may also be related to the surface energies of facets of rubrene crystal. However, only amorphous rubrene thin film was obtained on the surface of quartz (Figure S4, Supporting Information). In fact, it is recognized that only amorphous films can be obtained on such substrates as Au,6 SiO2,6,15 and Al2O314 by OMBD or ordinary vacuum evaporation. With the use of density functional theory, the geometry of a free rubrene molecule in gas phase has been calculated, yielding D2 symmetry with a twisting of the tetracene backbone of about 42°.6 In contrast, in the crystal phase rubrene adopts a conformation with planar tetracene backbone (C2h symmetry), and the calculated energy difference between the two molecular conformations amounts to 210 meV.6 Käfer and Witte’s experimental results indicate that the thermal energy at room temperature is too small to activate the required conformational change and only results in rather amorphous films.6 In addition, they obtained crystalline films on SiO2 and Au substrates by hot wall deposition.6 Unfortunately, they did not provide AFM graphs but SEM graphs. A high degree of roughness can be observed on the SEM graphs of the rubrene thin films grown by hot wall deposition. However, from our experiments, well-ordered and very flat rubrene thin films were grown on the surface of rubrene single crystals by ordinary vacuum evaporation, and the substrate temperature is room temperature. As shown in Figures 2-8 and S2-S3 (Supporting Information), the rms of roughness of rubrene thin films on the surface of rubrene single crystals is typically 0.8 nm in a 5 × 5 µm2 area. Therefore, it can be concluded that although the calculated energy difference between the two rubrene molecular conformations in gas phase and in crystal phase amounts to 210 meV, because we adopted rubrene single crystals with very smooth surface as substrates for growing rubrene thin films and the deposition rate was controlled in the range of 0.2-0.6 nm/min, rubrene molecules in the gas phase can be adsorbed on the surfaces of rubrene single crystals according to a certain way and formed into wellordered thin films by homoepitaxy. Prospects and Conclusion This work exhibits the following prospects. First, growing well-ordered rubrene thin films on the surface of rubrene single crystals is very similar to the homoepitaxy growth of inorganic thin films on single crystal substrates. Therefore, the possibility that the growth theories of inorganic thin films can be applied to the research of the growth mechanism of organic crystalline thin films is very promising. Second, since rubrene single crystals show very smooth surfaces, new and interesting results

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should be expected by growing other organic materials on the surface of rubrene crystals. Third, since rubrene single crystals can be used as substrates for thin films growth, then how about other organic crystals? We also grew pentacene and tetracene single crystals by physical vapor transport. And AFM measurements proved that their surfaces were even smoother than the surface of carefully polished silicon wafer (Figures S5–S7, Supporting Information). Then, similar work can be done by using pentacene and tetracene single crystals as the substrates (Figures S8–S16, Supporting Information). Fourth, organic homoepitaxy, which was demonstrated in our work, provides an excellent method to probe the growth mechanism of organic crystals. In our experiments, we can freely control the thickness of rubrene thin films, and then we can study the morphology and growth process of every growth stage of rubrene thin films by controlling the growth parameters including substrate temperature, deposition rate, background pressure, etc. Therefore, it is reasonable to elucidate the growth mechanism of rubrene single crystals through the study of the growth process of rubrene thin films grown by homoepitaxy. Certainly, this method is also very promising for the study of the growth mechanisms of other organic single crystals. Therefore, there are many research avenues left to be explored. Acknowledgment. This work was supported by the National Key Basic Research and Development Programme of China under Grant No. 2006CB806203 and the Postdoctoral Science Foundation of China (No. 2005038299). Supporting Information Available: AFM images and their crosssection curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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