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Preparation and characterization of Rhodamine B and Alq3 thin films deposited by solution jet beam method
Applied Surface Science 244 (2005) 217–220 www.elsevier.com/locate/apsusc
Preparation and characterization of Rhodamine B and Alq3 thin films deposited by solution jet beam method Y. Okabayashia,*, T. Mitaraia, S. Yamazakia, R. Matsuyamaa, K. Kanaia, Y. Ouchia, K. Sekib a
Department of Chemistry, Graduate School of Science, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-8602, Japan b Research Center for Materials Science, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-8602, Japan Received 31 May 2004; accepted 18 October 2004 Available online 15 January 2005
Abstract We report the solution jet beam method as a novel technique to fabricate organic thin films in vacuum. The method can be applied even to ionic organic materials, which cannot be vacuum evaporated due to thermal decomposition. Thin films of RhB (Rhodamine B) and Alq3 (tris(8-hydroxyquinoline)aluminum) were fabricated and characterized. Organic light emitting diodes (OLEDs) were also fabricated using Alq3 films prepared by this method. # 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Ef; 81.15.Rs Keywords: Organic thin film; Jet; Spray; Alq3; Rhodamine B
1. Introduction There are various methods to fabricate organic thin films, but each method has some disadvantage. Vacuum evaporation is the most common method, but it cannot be applied to ionic materials, which decompose at evaporation. Spin coating can be applied to ionic materials, but there are problems * Corresponding author. Tel.: +81 52 789 2944; fax: +81 52 789 2944. E-mail address: [email protected] (Y. Okabayashi).
with the preparation of organic multilayers, as the previously deposited layer may dissolve. In this article, we describe the solution jet beam method as a new method to fabricate organic thin films. The film is fabricated by spraying the solution of the sample material, dissolved in a volatile solvent, onto the substrate in vacuum. This method is generally applicable to all samples, which can be dissolved in a volatile solvent. Recently new techniques based on similar ideas have also been reported with applications to polymers, liquid crystals and ionic materials [1–3]. In this study, tris(8-hydroxyquinoline)aluminum (Alq3) and Rhodamine B (RhB) were used to examine
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.082
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Y. Okabayashi et al. / Applied Surface Science 244 (2005) 217–220
the applicability of this method for fabricating organic thin films. Alq3 is the most common material for organic light emitting diodes (OLEDs), and RhB is a representative ionic dye used as a biological tracer and a laser dye.
2. Experimental
was injected through the pulse valve (General Valve Co.). Typical working conditions of the pulse valve were an opening width of 1–10 ms at repetition rate of 1 Hz. The deposition rate was measured by a crystal microbalance (INFICON). The morphology of fabricated thin films was examined by an optical microscope (Nikon, OPTIPHOT2-POL). Organic light emitting diodes were fabricated as described later, and were driven by a regulated DC power supply. The current– voltage characteristics were measured by a Keithley 487 Picoammeter.
Fig. 1 shows the experimental apparatus. It consisted of three chambers (A–C) separated by two skimmers with diameters of 2 and 1 mm, respectively. Chamber A was evacuated by a rotary pump (10 L/s) through a liquid-nitrogen-cooled cold trap. Chamber B was evacuated by a diffusion pump (570 L/s), also through a cold trap. Chamber C was evacuated by a turbomolecular pump (250 L/s) to a base pressure of 3 10 2 Pa. The sample solution was ejected into vacuum from a pulse valve. Droplets of the solution broke into small fragments, which passed through two skimmers, and reached the substrate. The droplets may have dried before reaching the substrate. The valve was heated up to 473 K to help the evaporation of the solvent and to prevent freezing due to evaporation heat. RhB (Tokyo Kasei Kogyo Co.) and Alq3 (Aldrich) were used without further purification. They were dissolved in methanol and acetone, respectively (1 mmol dm 3). The sample solution, pressurized by a pump designed for liquid chromatography (LC-10AT; Shimadzu Co.),
The morphology of the films prepared by this method depended on parameters such as solvent, temperature of the pulse valve, rate of solution pumping, frequency of ejection, and pulse width of ejection. All these parameters were investigated, but the effects of only a few parameters are reported here. Fig. 2 depicts optical microscopic images of films prepared with pulse widths of: (a) 0.16 ms, (b) 1 ms, and (c) 10 ms from RhB methanol solution at valve temperature of 383 K, and (d) 0.5 ms, (e) 1 ms, and (f) 10 ms from Alq3 acetone solution at valve temperature of 413 K. The pumping rate of the solution was 0.01 mL/min for (a)–(c), 0.1 mL/min for (d) and (f), and 1 mL/min for (g); the average thickness was (a) 140 nm, (b) 110 nm, (c) 80 nm, (d) 18 nm, (e) 20 nm, and (f) 120 nm. All the films had island-like structures,
Fig. 1. Apparatus for solution jet beam method. Sample solution was directly ejected into vacuum, passed through the two skimmers, and reached the substrate.
Fig. 2. Optical microscopic images of the films of RhB prepared from methanol solution (a–c) and Alq3 from acetone solution (d–f) fabricated by the solution jet beam method. They show dependence of morphology on pulse width of the pulse valve: (a) 0.16 ms, (b) 1 ms, (c) 10 ms, (d) 0.5 ms, (e) 1 ms, and (f) 10 ms.
3. Results and discussion
Y. Okabayashi et al. / Applied Surface Science 244 (2005) 217–220
with typical sizes of: (a) 37 mm, (b) 20 mm, (c) 16 mm, (d) 12 mm, (e) 10 mm, and (f) 5 mm. The island-like structure indicated that the droplets of the sample solution reached the substrate before they had dried completely. The films (c) and (f), with pulse width of 10 ms, had the finest film structure from each solution. The reason why longer pulse width produced a finer structure is unclear at present. Comparing the films (a)–(c) with (d)–(f), fabricated from methanol and acetone solutions, respectively, films (d)–(f) had finer structure. Other experiments using methanol and acetone solutions with the same solute showed that morphology did not depend much on the solute. Thus the large difference between (a)–(c) and (d)–(f) is mainly ascribed to the difference of the solvents. Such solvent dependence might be due to higher vapor pressure of acetone than that of methanol. Fig. 3 shows the dependence of morphology on the valve temperature for acetone solution of Alq3 at the pulse width of 10 ms, and pumping rate of 1.0 mL/ min. For (a) 393 K, (b) 413 K, and (c) 433 K, the typical sizes of the islands were 8, 7, and 1 mm, respectively. Film thickness was (a) 60 nm, (b) 120 nm, and (c) 60 nm. The film prepared at the highest temperature (433 K) shown in Fig. 3(c) had the finest structure, possibly because higher temperature enhanced the evaporation of the solvent, with droplets broken into small fragments. These valve temperatures were higher than the boiling point of acetone (329 K), but the pressure inside the valve reached 1.2 MPa, raising the boiling point. Once the acetone solution was injected into vacuum, it immediately evaporated. The preparation of finer films with less solvent suggests that the quality of the films with well volatile solution may become close to the good uniformity of vacuum evaporated film, which can be regarded as the case without solvent.
Fig. 3. Optical microscopic images of Alq3 thin film fabricated by the solution jet beam method from acetone solution at the pulse width of 10 ms. They show dependence of morphology on temperature of the pulse valve: (a) 393 K, (b) 413 K, and (c) 433 K.
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Fig. 4. Current–voltage characteristics of OLEDs with Al/Alq3/ITO structure fabricated by the solution jet beam method (open circles) and by vacuum evaporation method (crosses). The inset is a photograph of the OLED fabricated by the solution jet beam method, and operated at 15 V, 30 mA.
As mentioned above, OLEDs were fabricated by using Alq3 films (600 nm) deposited by this method onto indium tin oxide (ITO) glasses (inset of Fig. 4). Al films (80 nm) were vacuum evaporated as cathodes onto these films. The current–voltage characteristics were measured for the OLEDs fabricated in this manner and also by vacuum evaporation. Although the former drained larger current than the latter (Fig. 4), their luminescence was much less intense. This reduced efficiency may be caused by the roughness of the films (75 nm rms by AFM), since the vacuum evaporated film had roughness of only 17 nm rms.
4. Conclusion RhB and Alq3 thin films were fabricated by the solution jet beam method. The dependence of morphology on deposition conditions was studied, and the most finely structured films were obtained by using acetone solution, with the valve temperature of 433 K, with the valve pulse width of 10 ms, and at the pumping rate of 0.5 mL/min. OLEDs were fabricated by both the present method and by vacuum evaporation. Current–voltage curves were measured, and compared with each other.
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Y. Okabayashi et al. / Applied Surface Science 244 (2005) 217–220
Although the quality of the film is still not satisfactory, this method will become a powerful tool to fabricate thin films of materials which have been difficult to prepare, e.g. ionic materials, macromolecules, and polymers.
Molecular Functions’’ from the Ministry of Education, Culture, Science, Sports, and Technology (MEXT), Japan. We also acknowledge the cooperation of Prof. Nishi of Institute for Molecular Science at the initial stage of this study.
Acknowledgements References We acknowledge the financial support of this work by Grant-in-Aid for Creative Scientific Research ‘‘Elucidation and control of interfaces related to organic electronic devices’’ (No. 14GS0213) and the 21st Century COE Program ‘‘Establishment of COE of Materials Science: Elucidation and Creation of
[1] T. Yamada, M. Ge, H. Shinohara, K. Kimura, S. Mashiko, Chem. Phys. Lett. 379 (2003) 458. [2] T. Ishikawa, M. Nakamura, K. Fujita, T. Tsutsui, Appl. Phys. Lett. 84 (2004) 2424. [3] X. Mo, T. Mizokuro, H. Mochizuki, N. Tanigaki, T. Hiraga, Jpn. J. Appl. Phys., Part 1 43 (2004) 307.
Preparation and characterization of Rhodamine B and Alq3 thin films deposited by solution jet beam method Y. Okabayashia,*, T. Mitaraia, S. Yamazakia, R. Matsuyamaa, K. Kanaia, Y. Ouchia, K. Sekib a
Department of Chemistry, Graduate School of Science, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-8602, Japan b Research Center for Materials Science, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-8602, Japan Received 31 May 2004; accepted 18 October 2004 Available online 15 January 2005
Abstract We report the solution jet beam method as a novel technique to fabricate organic thin films in vacuum. The method can be applied even to ionic organic materials, which cannot be vacuum evaporated due to thermal decomposition. Thin films of RhB (Rhodamine B) and Alq3 (tris(8-hydroxyquinoline)aluminum) were fabricated and characterized. Organic light emitting diodes (OLEDs) were also fabricated using Alq3 films prepared by this method. # 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Ef; 81.15.Rs Keywords: Organic thin film; Jet; Spray; Alq3; Rhodamine B
1. Introduction There are various methods to fabricate organic thin films, but each method has some disadvantage. Vacuum evaporation is the most common method, but it cannot be applied to ionic materials, which decompose at evaporation. Spin coating can be applied to ionic materials, but there are problems * Corresponding author. Tel.: +81 52 789 2944; fax: +81 52 789 2944. E-mail address: [email protected] (Y. Okabayashi).
with the preparation of organic multilayers, as the previously deposited layer may dissolve. In this article, we describe the solution jet beam method as a new method to fabricate organic thin films. The film is fabricated by spraying the solution of the sample material, dissolved in a volatile solvent, onto the substrate in vacuum. This method is generally applicable to all samples, which can be dissolved in a volatile solvent. Recently new techniques based on similar ideas have also been reported with applications to polymers, liquid crystals and ionic materials [1–3]. In this study, tris(8-hydroxyquinoline)aluminum (Alq3) and Rhodamine B (RhB) were used to examine
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.10.082
218
Y. Okabayashi et al. / Applied Surface Science 244 (2005) 217–220
the applicability of this method for fabricating organic thin films. Alq3 is the most common material for organic light emitting diodes (OLEDs), and RhB is a representative ionic dye used as a biological tracer and a laser dye.
2. Experimental
was injected through the pulse valve (General Valve Co.). Typical working conditions of the pulse valve were an opening width of 1–10 ms at repetition rate of 1 Hz. The deposition rate was measured by a crystal microbalance (INFICON). The morphology of fabricated thin films was examined by an optical microscope (Nikon, OPTIPHOT2-POL). Organic light emitting diodes were fabricated as described later, and were driven by a regulated DC power supply. The current– voltage characteristics were measured by a Keithley 487 Picoammeter.
Fig. 1 shows the experimental apparatus. It consisted of three chambers (A–C) separated by two skimmers with diameters of 2 and 1 mm, respectively. Chamber A was evacuated by a rotary pump (10 L/s) through a liquid-nitrogen-cooled cold trap. Chamber B was evacuated by a diffusion pump (570 L/s), also through a cold trap. Chamber C was evacuated by a turbomolecular pump (250 L/s) to a base pressure of 3 10 2 Pa. The sample solution was ejected into vacuum from a pulse valve. Droplets of the solution broke into small fragments, which passed through two skimmers, and reached the substrate. The droplets may have dried before reaching the substrate. The valve was heated up to 473 K to help the evaporation of the solvent and to prevent freezing due to evaporation heat. RhB (Tokyo Kasei Kogyo Co.) and Alq3 (Aldrich) were used without further purification. They were dissolved in methanol and acetone, respectively (1 mmol dm 3). The sample solution, pressurized by a pump designed for liquid chromatography (LC-10AT; Shimadzu Co.),
The morphology of the films prepared by this method depended on parameters such as solvent, temperature of the pulse valve, rate of solution pumping, frequency of ejection, and pulse width of ejection. All these parameters were investigated, but the effects of only a few parameters are reported here. Fig. 2 depicts optical microscopic images of films prepared with pulse widths of: (a) 0.16 ms, (b) 1 ms, and (c) 10 ms from RhB methanol solution at valve temperature of 383 K, and (d) 0.5 ms, (e) 1 ms, and (f) 10 ms from Alq3 acetone solution at valve temperature of 413 K. The pumping rate of the solution was 0.01 mL/min for (a)–(c), 0.1 mL/min for (d) and (f), and 1 mL/min for (g); the average thickness was (a) 140 nm, (b) 110 nm, (c) 80 nm, (d) 18 nm, (e) 20 nm, and (f) 120 nm. All the films had island-like structures,
Fig. 1. Apparatus for solution jet beam method. Sample solution was directly ejected into vacuum, passed through the two skimmers, and reached the substrate.
Fig. 2. Optical microscopic images of the films of RhB prepared from methanol solution (a–c) and Alq3 from acetone solution (d–f) fabricated by the solution jet beam method. They show dependence of morphology on pulse width of the pulse valve: (a) 0.16 ms, (b) 1 ms, (c) 10 ms, (d) 0.5 ms, (e) 1 ms, and (f) 10 ms.
3. Results and discussion
Y. Okabayashi et al. / Applied Surface Science 244 (2005) 217–220
with typical sizes of: (a) 37 mm, (b) 20 mm, (c) 16 mm, (d) 12 mm, (e) 10 mm, and (f) 5 mm. The island-like structure indicated that the droplets of the sample solution reached the substrate before they had dried completely. The films (c) and (f), with pulse width of 10 ms, had the finest film structure from each solution. The reason why longer pulse width produced a finer structure is unclear at present. Comparing the films (a)–(c) with (d)–(f), fabricated from methanol and acetone solutions, respectively, films (d)–(f) had finer structure. Other experiments using methanol and acetone solutions with the same solute showed that morphology did not depend much on the solute. Thus the large difference between (a)–(c) and (d)–(f) is mainly ascribed to the difference of the solvents. Such solvent dependence might be due to higher vapor pressure of acetone than that of methanol. Fig. 3 shows the dependence of morphology on the valve temperature for acetone solution of Alq3 at the pulse width of 10 ms, and pumping rate of 1.0 mL/ min. For (a) 393 K, (b) 413 K, and (c) 433 K, the typical sizes of the islands were 8, 7, and 1 mm, respectively. Film thickness was (a) 60 nm, (b) 120 nm, and (c) 60 nm. The film prepared at the highest temperature (433 K) shown in Fig. 3(c) had the finest structure, possibly because higher temperature enhanced the evaporation of the solvent, with droplets broken into small fragments. These valve temperatures were higher than the boiling point of acetone (329 K), but the pressure inside the valve reached 1.2 MPa, raising the boiling point. Once the acetone solution was injected into vacuum, it immediately evaporated. The preparation of finer films with less solvent suggests that the quality of the films with well volatile solution may become close to the good uniformity of vacuum evaporated film, which can be regarded as the case without solvent.
Fig. 3. Optical microscopic images of Alq3 thin film fabricated by the solution jet beam method from acetone solution at the pulse width of 10 ms. They show dependence of morphology on temperature of the pulse valve: (a) 393 K, (b) 413 K, and (c) 433 K.
219
Fig. 4. Current–voltage characteristics of OLEDs with Al/Alq3/ITO structure fabricated by the solution jet beam method (open circles) and by vacuum evaporation method (crosses). The inset is a photograph of the OLED fabricated by the solution jet beam method, and operated at 15 V, 30 mA.
As mentioned above, OLEDs were fabricated by using Alq3 films (600 nm) deposited by this method onto indium tin oxide (ITO) glasses (inset of Fig. 4). Al films (80 nm) were vacuum evaporated as cathodes onto these films. The current–voltage characteristics were measured for the OLEDs fabricated in this manner and also by vacuum evaporation. Although the former drained larger current than the latter (Fig. 4), their luminescence was much less intense. This reduced efficiency may be caused by the roughness of the films (75 nm rms by AFM), since the vacuum evaporated film had roughness of only 17 nm rms.
4. Conclusion RhB and Alq3 thin films were fabricated by the solution jet beam method. The dependence of morphology on deposition conditions was studied, and the most finely structured films were obtained by using acetone solution, with the valve temperature of 433 K, with the valve pulse width of 10 ms, and at the pumping rate of 0.5 mL/min. OLEDs were fabricated by both the present method and by vacuum evaporation. Current–voltage curves were measured, and compared with each other.
220
Y. Okabayashi et al. / Applied Surface Science 244 (2005) 217–220
Although the quality of the film is still not satisfactory, this method will become a powerful tool to fabricate thin films of materials which have been difficult to prepare, e.g. ionic materials, macromolecules, and polymers.
Molecular Functions’’ from the Ministry of Education, Culture, Science, Sports, and Technology (MEXT), Japan. We also acknowledge the cooperation of Prof. Nishi of Institute for Molecular Science at the initial stage of this study.
Acknowledgements References We acknowledge the financial support of this work by Grant-in-Aid for Creative Scientific Research ‘‘Elucidation and control of interfaces related to organic electronic devices’’ (No. 14GS0213) and the 21st Century COE Program ‘‘Establishment of COE of Materials Science: Elucidation and Creation of
[1] T. Yamada, M. Ge, H. Shinohara, K. Kimura, S. Mashiko, Chem. Phys. Lett. 379 (2003) 458. [2] T. Ishikawa, M. Nakamura, K. Fujita, T. Tsutsui, Appl. Phys. Lett. 84 (2004) 2424. [3] X. Mo, T. Mizokuro, H. Mochizuki, N. Tanigaki, T. Hiraga, Jpn. J. Appl. Phys., Part 1 43 (2004) 307.