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Articles Periodic Submicrocylinder Diamond Surfaces Using Two-Dimensional Fine Particle Arrays Suguru Okuyama,† Sachiko I. Matsushita,‡ and Akira Fujishima*,† Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Dissipative-Hierarchy Structures Laboratory, Spatio-Temporal Function Materials Research Group, Frontier Research System, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Received July 16, 2001. In Final Form: January 3, 2002 Periodic submicroscale diamond-cylinder arrays were fabricated on diamond surfaces using twodimensionally ordered arrays of SiO2 particles. For the preparation, the diamond surface was etched by means of reactive ion etching with oxygen plasma through two-dimensionally ordered arrays as masks. The etching time has an important influence on the diameter and depth of the diamond cylinders. The Raman spectra of these films indicate that they consist mostly of diamond, with small amounts of sp2 carbon.
Introduction Submicrostructured materials have recently attracted much attention because of their various applications in electronic,1 electrochemical,2,3 magnetic,4,5 and photonic6-9 systems. Many efforts have been made to prepare submicrostructured materials with controlled periodicity.10,11 Diamond is a material that is difficult to prepare in such a form. However, due to its unique properties, for example, high hardness, extreme chemical stability, high refractive index, wide electrochemical potential window, wide band gap, and negative electron affinity (NEA),12 diamond films with periodic submicrostructured surfaces are expected to be applicable in a number of fields. For example, the periodicity can be controlled at a high degree of regularity with a scale on the same order of magnitude as the optical wavelength, and the resulting structured diamond could be used as a photonic crystal,9,13 particularly due to the * To whom correspondence should be addressed. † Department of Applied Chemistry, School of Engineering, The University of Tokyo. ‡ Dissipative-Hierarchy Structures Laboratory, Spatio-Temporal Function Materials Research Group, Frontier Research System, RIKEN. (1) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (2) Hao, Y.; Yang, M.; Yu, C.; Cai, S.; Liu, M.; Fan, L.; Li, Y. Sol. Energy Mater. Sol. Cells 1998, 56, 75. (3) Schwarzburg, K.; Willing, F. J. Phys. Chem. B 1999, 103, 574. (4) Bergman, D. J.; Strelniker, Y. M. Physica B 2000, 279, 1. (5) Vavassori, P.; Donzelli, O.; Metlushko, V.; Grimsditch, M. B.; Ilic, P. N.; Kumar, R. J. Appl. Phys. 2000, 88, 999. (6) Yin, J. S.; Wang, Z. L. Appl. Phys. Lett. 1999, 74, 2629. (7) Yagi, Y.; Matsushita, S. I.; Tryk, D. A.; Koda, T.; Fujishima, A. Langmuir 2000, 16, 1180. (8) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56. (9) Masuda, H.; Ohya, M.; Nishio, K.; Asoh, H.; Nakao, M.; Nohtomi, M.; Yokoo, A.; Tamamura, T. Jpn. J. Appl. Phys. 2000, 39, 1039. (10) Okano, K.; Yamada, T.; Ishihara, H.; Koizumi, S.; Itoh, J. Appl. Phys. Lett. 1997, 70, 2201. (11) Temst, K.; Bael, M. J. V.; Moshchalkov, V. V.; Bruynseraede, Y. J. Appl. Phys. 2000, 87, 4216. (12) Himpsel, F. J.; Knapp, J. A.; VanVechen, J. A.; Eastman, D. E. Phys. Rev. B 1979, 20, 624. (13) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals; Princeton University Press: Princeton, NJ, 1995.
large difference in its refractive index compared to that of air.14,15 Since conductive boron-doped diamond also has outstanding properties as an advanced electronic material, including a wide electrochemical potential window, low electrochemical background current, high carrier mobility, high electrical breakdown voltage, and high thermal conductivity, it can also be used in high-sensitivity electrochemical and electromechanical sensors.16,17 From the viewpoint of NEA, electron emitters10,18-20 for field emission displays are expected to be one application of submicroscale diamond structures. We have recently reported a new technique for the preparation of periodic submicroscale diamond structures using two-dimensional (2D) arrays of monodisperse solid particles as masks.15 In the 2D arrays, fine particles21 or protein molecules22 are packed in high-density, highly oriented layers over a wide surface area by water evaporation and lateral capillary forces (also referred to as capillary immersion forces23). Since 2D arrays have several unique features, including a large range of periodicity (from 19 nm24 to 20 µm25) and a simple and (14) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; Khayrullin, I.; Dantas, S. O.; Marti, J.; Ralchenko, V. G. Science 1998, 282, 897. (15) Okuyama, S.; Matsushita, S. I.; Fujishima, A. Chem. Lett. 2000, 534. (16) Rao, T. N.; Yagi, I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Anal. Chem. 1999, 771, 2506. (17) Davidson, J. L.; Kang, W. P.; Gurbuz, Y.; Holmes, K. C.; Davis, L. G.; At, A. W.; Kerns, D. V.; Eidson, R. L.; Henderson, T. Diamond Relat. Mater. 1999, 8, 1741. (18) Kleps, I.; Nicolaeascu, D.; Stamatin, I.; Correia, A.; Gil, A.; Zlatkin, A. Appl. Surf. Sci. 1999, 146, 152. (19) Saito, Y.; Hamaguchi, K.; Mizushima, R.; Uemura, S.; Nagasako, T.; Yotani, J.; Shimojo, T. Appl. Surf. Sci. 1999, 146, 305. (20) Shiomi, H. Jpn. J. Appl. Phys. 1997, 36, 7745. (21) Yamaki, M.; Higo, J.; Nagayama, K. Langmuir 1995, 11, 2975. (22) Nagayama, K.; Takeda, S.; Endo, S.; Yoshimura, H. Jpn. J. Appl. Phys. 1995, 34, 3947. (23) Dushkin, C. D.; Kralchevsky, P. A.; Paunov, V. N.; Yoshimura, H.; Nagayama, K. Langmuir 1996, 12, 641. (24) Dushkin, C. D.; Lazarov, G. S.; Kotsev, S. N.; Yoshimura, H.; Nagayama, K. Colloid Polym. Sci. 1999, 277, 7. (25) Tatsuma, T.; Ikezawa, A.; Ohko, Y.; Miwa, T.; Fujishima, A. Electrochem. Solid-State Lett. 2000, 3, 467.
10.1021/la011107i CCC: $22.00 © 2002 American Chemical Society Published on Web 09/27/2002
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Figure 2. SEM images of the SiO2 particle array on the diamond thin film.
Figure 1. Schematic diagrams showing the preparation procedures for cylinder-like microstructured diamond films.
inexpensive preparation apparatus, this approach utilizing 2D arrays promises to provide a versatile means of producing highly textured surfaces.26-28 In this paper, we report details of the procedure for the preparation of periodic submicroscale diamond structures using 2D arrays, particularly the effects of particle shapes on the texture, which are important in controlling the dimensions of the diamond cylinders. The properties of these diamond cylinders as electron emitters were also observed briefly. Experimental Section The preparation processes utilize the intrinsic submicrostructures of two-dimensionally ordered arrays.26 The schematic processes are shown in Figure 1. In the initial step of preparation, polycrystalline diamond was grown (1.2 × 104 Pa for 9 h at 860 °C) on a Si(100) wafer by microwave-assisted plasma chemical vapor deposition (CVD) with a commercial microwave plasma reactor (ASTeX Corp., Woburn, MA).16,29 Acetone-methanol (volume ratio, 9:1) solution was used as the carbon source. The thickness of the diamond film was approximately 20 µm. While the as-grown surface of the diamond has high roughness, its interface with the Si wafer has a flat face. We removed the Si wafer from the diamond with HFHNO3 (HF 24 wt %, HNO3 30 wt %) solution in order to utilize the flat face of the diamond, instead of polishing the as-grown side, because the former method can be performed quickly and inexpensively. On the flat side (interface), single layers of 2D arrays were prepared, using capillary force and water evaporation.30 Details of the preparation of the 2D arrays have been reported elsewhere.31 We selected a 1.0 µm SiO2 particle (26) Matsushita, S.; Miwa, T.; Tryk, D. A.; Fujishima, A. Langmuir 1998, 14, 6441. (27) Matsushita, S.; Miwa, T.; Fujishima, A. Chem. Lett. 1997, 925. (28) Miguez, H.; Meseguer, F.; Lopez, C.; Holgado, M.; Andreasen, G.; Mifsud, A.; Fornes, V. Langmuir 2000, 16, 4405. (29) Swain, G. M. J. Electrochem. Soc. 1994, 141, 3382.
suspension (Seahostar KE-P, 10.0 w/w%, Nihon Syokubai Japan), because the particles remain monodisperse and stable throughout the subsequent processes. After preparation of the 2D array, reactive ion etching (RIE) was carried out with oxygen plasma through the SiO2 arrays for 5-120 min in a plasma etching apparatus (SAMCO, BP-1)1,32,33 with a radio frequency (rf) generator (13.56 MHz). Operating oxygen pressure, flow speed, and plasma power were 20 Pa, 20 cm3 s-1, and 150 W, respectively. Oxygen ions, which were accelerated by the rf generator, attacked the carbon atoms on the diamond surface, which might oxygenate the carbon to gas. Although the mass spectrum of the emitted gas has not been measured, we believe that a large proportion of the gas is carbon dioxide. Finally, the SiO2 particles were removed from the diamond with the HF-HNO3 solution (HF 24 wt %, HNO3 30 wt %). The morphologies of the samples were observed with a scanning electron microscope (SEM, S-4200, Hitachi, Ltd., Japan). MicroRaman spectroscopy was carried out with a Raman spectrograph (System 2000, Renishaw Transducer Systems, Ltd.) and a 514.5 nm Ar+ ion laser beam (Ion Laser Technology). X-ray photoelectron spectroscope (XPS) measurement was carried out with a monochromated PHI model 5600 (Perkin-Elmer) spectrometer, with Mg KR radiation (400 W) at the pass energy of 187.85 eV.
Results and Discussion 2D Arrays on Diamond. When a particle array is arranged on a substrate, it is desirable for the contact angle of the solvent used in the particle suspension to be less than approximately 10°. If the substrate has a high contact angle, the suspension cannot spread, and an array does not form. Because the as-deposited diamond surface has a high contact angle (40-60°), it is not suitable for array preparation. To decrease the contact angle, we carried out oxygen plasma treatment (70 W, 15 s) of the diamond surface and obtained a much lower contact angle (nearly 0°), due to the conversion of H-termination to O-termination. An SEM image of a 2D array on the O-terminated diamond surface is shown in Figure 2. We can observe that the SiO2 particles could be arranged sufficiently well to function as a mask for periodic submicroscale structures. Morphology of the Cylinder. The completed diamond films were opalescent because the periodicity of the structures is on the same size scale as the wavelength of visible light. Figure 3 shows SEM images of a film etched for 60 min with oxygen plasma through a monolayer of 1.0 µm SiO2 particles. The diamond was vertically etched by the oxygen plasma through the spaces between the SiO2 particles, and the resulting cylinder-like structures are a projection of the original SiO2 particle array. It can (30) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26. (31) Matsushita, S.; Miwa, T.; Fujishima, A. Langmuir 1997, 13, 2582. (32) Honda, K.; Rao, T. N.; Tryk, D. A.; Fujishima, A.; Yasui, K.; Masuda, H. J. Electrochem. Soc. 2000, 147, 659. (33) Masuda, H.; Yasui, K.; Watanabe, M.; Nishio, K.; Rao, T. N.; Fujishima, A. Chem. Lett. 2000, 1112.
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Figure 3. SEM images of the diamond cylinder surfaces observed at (a) 0° and (b) 30° from the surface normal. Oxygen reactive ion etching was carried out for 60 min through the 1.0 µm SiO2 particle arrays.
Figure 4. Raman spectra of (a) the diamond films with the cylinder structures, (b) the interface with the Si wafer before RIE, and (c) the as-grown surface.
be said generally that the film has a hexagonal closepacked array of cylinder-like structures on the surface. The chemical reactions during RIE can be summarized simply as follows: oxygen ions accelerated by the rf generator attacked the carbon atoms at the surface, and the carbon was oxygenated into carbon dioxide and adsorbed into vapor. In examining this processing technique, we have not regarded the composition of the emitted gas in the RIE process to be very important. Though the mass spectrum of the emission gas has not been measured, we believe that a large proportion of the emitted gas is carbon dioxide. Raman Spectra. To determine the state of combination of the carbon, we measured the Raman spectra (Figure 4). Raman spectra enable one to distinguish carbon allotropes.34 In the spectrum of the structured film (Figure 4a), the characteristic peak for crystalline diamond at 1332 cm-1 is clearly observed, and broad, low-intensity peaks at 1350 and 1600 cm-1 (sp2 carbon) are also visible. On (34) Bou, P.; Vandenbulcke, L. J. Electrochem. Soc. 1991, 138, 2991.
the basis of the known high sensitivity of Raman spectroscopy for sp2 carbon versus sp3 carbon, these data indicate that both diamond and smaller amounts of sp2 carbon exist on the submicrocylinder surface. The spectrum of the interface side between bulk diamond and Si wafer before the periodic structure preparation (Figure 4b) resembled that after the preparation (Figure 4a). In brief, RIE had almost no effect on the amount of sp2 carbon. On the other hand, in the spectrum of the as-deposited side (Figure 4c), only the peak for crystalline diamond at 1332 cm-1 is observed. Thus, the existence of sp2 carbon appears to be intrinsic to the interface with the Si substrate, irrespective of the presence of the periodic structure. When the film is used as an emitter, the presence of sp2 carbon is advantageous. It is known that the existence of sp2 carbon facilitates electron emission from the diamond surface.35 If a higher diamond purity is required for other applications, the polished as-deposited side should be selected. X-ray Photoelectron Spectroscopy. The surface chemistry of the diamond cylinder after the RIE process was observed with XPS. It is well-known that diamond films deposited from hydrogen plasma have a hydrogenterminated surface, but oxygen plasma etching can change it to an oxygenated surface. Parts a and b of Figure 5 show the XPS spectra of diamond films before and after RIE, respectively. C 1s peaks at around 285 eV were clearly confirmed in the cases of both before and after RIE. On the other hand, the existence of oxygen on the surface was indicated by the O 1s peak at 532 eV only in the spectrum obtained after RIE (Figure 5b). The oxygen content of the surface after RIE was estimated to be approximately 10% from the quantitative analysis. Change of the Diamond Cylinder Shape. RIE was carried out for periods of 10-120 min to investigate the effect on the diamond cylinder shape. It was revealed that as the etching time was increased, both the height and the diameter changed.36 Let us illustrate the effect on the diameter using top-view SEM images (Figure 6). When plasma etching was carried out for 10 min, the cylinders were connected to each other (Figure 4a). The cylinder diameter was nearly the same as that of the SiO2 particles (1 µm). At the tops of the cylinders, small scratches were observed (Figure 6a). It is well-known that artificial (35) Robertson, J. Diamond Films Technol. 1999, 8, 225. (36) Haginoya, C.; Ishibashi, M.; Koike, K. Appl. Phys. Lett. 1997, 71, 2934.
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Figure 5. XPS spectra of the diamond film (a) before and (b) after RIE.
Figure 7. SEM images of the diamond films (a) after and (b,c) before the final HF treatment to remove SiO2 particles. (c) Part of the surface where the SiO2 particles were detached from the diamond cylinders.
Figure 6. SEM images of the diamond cylinders. Oxygen reactive ion etching was carried out for (a) 10, (b) 60, (c) 90, and (d) 120 min.
diamond films prepared by CVD have polycrystalline structures that have many twinning planes.37 These scratches might be grain boundaries or faults of crystals.
When etching was carried out for 60 min (Figure 6b), the cylinder shape became narrower than that for the 10min-etched cylinders, and they became separated. The cylinder diameter became less than the original particle diameter (about 730 nm). After 90 min of etching, the cylinder diameter was not significantly changed from that of the cylinder etched for 60 min, but vertical roughness began to be observed (Figure 6c). This type of roughness increased after 120 min of etching, and the diamond structures had the appearance of cones rather than cylinders (Figure 6d). From this series of observations, the change of cylinder shape could be divided roughly into two stages: (1) diameter decrease and (2) cylinder shape collapse. It thus appears that changes in the SiO2 particles themselves might play an important role in the transition from the first to the second stage. It is considered that not only the diamond film but also the mask composed of SiO2 particles was etched by the oxygen plasma. Changes in the SiO2 particle shape by RIE were confirmed from the SEM images before and after the last HF treatment (Figure 7). From these images, we observe that the SiO2 particles, which correspond to the gray circles in Figure 7b,c, remained on the tops of the cylinders after RIE. It is clear that the SiO2 particle diameters (originally about 1.0 µm, see Experimental Section) decreased and became separated from each other. Moreover, from observing the fallen SiO2 particles (Figure 7c), it is clear that the particle shape became saucerlike (diameter, 800 nm). From these results, we can conclude that the oxygen plasma etched both the diamond and the SiO2 particles. Next, we discuss the visual change of the height using Figure 6. All of the cylinders etched for 30 min exhibit (37) Matsumoto, S.; Matsui, Y. J. Mater. Sci. 1983, 18, 1785.
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Figure 8. Relationship between the etching time and the diameter and height of the diamond cylinders.
Figure 9. Schematic model of the change of the SiO2 particles and diamond cylinders during oxygen reactive etching.
nearly uniform height values (Figure 6a′), as does the sample etched for 60 min (Figure 6b′). In contrast, for the samples etched for 90 and 120 min, the tops of the cylinders became rough. The relationship between the diameter and height of the cylinders and the etching time is shown in Figure 8. Diameters for cylinders etched for 120 min were not plotted, because the surface of the sample became so rough that we could not measure the height (Figure 6d,d′). Initially, both the diameter and height were negligibly changed. In the second stage, as the etching time increased, the diameter decreased and the height increased up to 60 min. In the third stage, the diameter did not change rapidly but the height increased up to 90 min. The height reached a maximum after etching for approximately 90 min, and the maximum aspect ratio was 6. After this point, the cylinder height decreased. Figure 9 shows a schematic model of the changes in the SiO2 particles and diamond cylinders. As the etching time increased, the particles became smaller and the cylinders
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Figure 10. Emission currents of the sample etched for 60 min: for consistency (a) 12th measurement (first emission), (b) 13th measurement, (c) 14th measurement, and (d) 100th measurement data.
became narrower without any change of pitch. Eventually (between 60 and 90 min), the SiO2 particles were completely etched and finally disappeared. As a result, the height has a limiting value. After this height is reached, the tops of the cylinders are also etched and the cylinder shape gradually collapses, because the tops, which are nearer to the oxygen plasma, are etched more rapidly than the narrow bottom areas between the cylinders. On the basis of these results, we can consider that the RIE time can be used to control the heights and diameters, and consequently the aspect ratios, of the cylinders within a certain range. The pitch can be controlled by the particle diameter, that is, from 19 nm to 20 µm. If the particles are replaced with those of another material, it can be considered that the diamond cylinders may take on a different aspect ratio. For example, if harder and more difficult to etch particles were used as a mask, the aspect ratio might become high. Conversely, if softer and easier to etch particles were used, the aspect ratio might become low. Field Emission Measurement. One of the applications of these diamond cylinders is an electron emitter array,10,18-20 and therefore we attempt to demonstrate the electron emission of our diamond cylinders. As discussed above, the diamond cylinder has the largest aspect ratio and a flat top surface after etching at 150 W for 60 min. The ability of electron emission was thus evaluated on this sample (Figure 10). The distance between the sample and the anode probe was 70 µm. In the first several experiments, electron emission from the sample could not be observed. However, in the 12th experiment of applying voltage from 0 to 20 V µm-1, the first electron emission was observed, and at that time, the emission starting voltage was 12 V µm-1 (Figure 10a). After the first emission, the value of the threshold field dropped to 6 V µm-1 (Figure 10b,c). This electron emission could be constantly observed in this threshold field, until at least the 100th experiment (Figure 10d). There is a strong possibility that this change of threshold field between the first electron emission and the latter electron emission was caused by admolecules on the diamond surface. The admolecules were desorbed from the surface at the first emission, and therefore the threshold field became damped from the second emission. These results confirmed the potential of the diamond cylinders as an emitter. Conclusions Two-dimensionally periodic submicroscale cylinders were fabricated on smooth diamond surfaces, with the use of two-dimensionally ordered particle arrays as masks.
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Even though nanocylinder diamond was prepared previously by using anodic porous alumina1,38 as a substrate for deposition, it is difficult to make the heights of these cylinders uniform. On the other hand, the cylinders prepared by the process described in this work have uniform height and flat top surfaces. Raman spectrum data indicate that diamond exists, along with smaller amounts of sp2 carbon, on the submicroscale cylindrical surface. It seems that the power of the etching plasma also might have some influence on the aspect ratio. Here, we have succeeded in demonstrating the potential of 2D arrays as useful masks based on the RIE technique and the ability of electron emission of these diamond cylinders. The pitch and aspect ratio can be controlled by the etching (38) Masuda, H.; Yanagishita, T.; Yasui, K.; Nishio, K.; Yagi, I.; Rao, T. N.; Fujishima, A. Adv. Mater. 2001, 13, 247.
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conditions, in particular the oxygen plasma etching time, the particle diameter, and the etching rate of the particle. Acknowledgment. Thanks are due to Professor D. A. Tryk (Tokyo Metropolitan University, Japan) for useful discussions, to Dr. A. S. Dimitrov (L’OREAL Tsukuba Center, Japan) for the development of the glass cell used in the preparation of the particle coatings, and to Dr. K. Honda and Mr. H. Notsu (The University of Tokyo) for their valuable assistance. The present work has been partially supported by the Japan Scholarship Foundation and a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport and Culture of Japan. LA011107I
