- Email: [email protected]
Optical properties of (organic polysilane)–(inorganic matrix) hybrid thin films
Journal of Luminescence 87}89 (2000) 715}717
Optical properties of (organic polysilane)}(inorganic matrix) hybrid thin "lms S. Mimura *, H. Naito , Y. Kanemitsu, K. Matsukawa, H. Inoue Department of Physics and Electronics, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan Plastics Department, Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan
Abstract Polysilane}inorganic hybrid thin "lms } organic polysilane embedded in a silica matrix } were fabricated using a sol}gel method to improve the durability of organic polysilanes as optoelectric devices. Photoluminescence measurements at 10 K show that the durability against ultraviolet light exposure is improved in the polysilane}inorganic hybrid thin "lms. In addition, the luminescence linear polarization memory is found at 10 K, suggesting the decrease in the interchain interaction of polysilanes in the polysilane}inorganic hybrid thin "lms. 2000 Elsevier Science B.V. All rights reserved. Keywords: Polysilane}inorganic hybrid thin "lms; Sol}gel method; Luminescence linear polarization memory
1. Introduction
2. Experiment
Organic polysilanes are chain-like polymers with silicon backbone and organic substituent groups, and exhibit a variety of unique and interesting properties, such as high quantum e$ciency of photoluminescence (PL) and high hole drift mobility. These properties are associated with p conjugation along the Si-backbone [1]. Recently, the applicability of the materials to future optoelectric devices has been examined [2], and unfortunately, the studies for this purpose suggested that the durability of organic polysilanes as optoelectronic devices is not satisfactory because of an irreversible photochemical (photo-oxidation) reaction [1,3,4]. In this paper, we study optical properties of polysilane}inorganic hybrid thin "lms } organic polysilane embedded in silica matrix } to improve the durability of organic polysilanes as optoelectronic devices and discuss new optical properties due to con"nement e!ects of excitons.
2.1. Synthesis
* Corresponding author. Fax: #81-722-54-9908. E-mail address: [email protected] (S. Mimura)
The polysilane}inorganic hybrid materials were prepared from trimethoxysilylpropylmethacrylate}methylphenylsilane block copolymers with a sol}gel method using tetraethoxysilane (TEOS) as an inorganic matrix [5]. The block copolymers having polysilane segments have been synthesized by photopolymerization of trimethoxysilylpropylmethacrylate with polymethylphenylsilane (PMPS) as a macrophotoinitiator. We used here the polysilane}inorganic hybrid thin "lms prepared by spin coating of the block copolymers (M "10 500, M "18 200)/TEOS solution with the di!erent com positions of the block copolymers to TEOS (75, 33 and 7.5 wt% of the copolymers). 2.2. Measurements The optical absorption spectra and the PL spectra of these thin "lms were measured with a Shimadzu UV3100PC spectrometer and a Shimadzu RF-5300PC spectrometer, respectively, at room temperature. For PL measurement at 10 K, a He-Cd laser (Omnichrome
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 7 2 - 5
716
S. Mimura et al. / Journal of Luminescence 87}89 (2000) 715}717
3056-15 ML) was used as an excitation source, and the PL spectra were measured using a 30-cm monochromator (Actron Research Corporation SpectraPro300i) and a CCD camera (Princeton Instrument RTE/CCD-128).
3. Results and discussion Fig. 1 shows the optical absorption and PL spectra under the 3.756 eV excitation of a PMPS thin "lm and polysilane}inorganic hybrid thin "lms at room temperature. It can be seen that the optical absorption and PL spectra of the polysilane}inorganic hybrid thin "lms are essentially the same as those of a PMPS thin "lm. We "nd from these results that organic polysilanes can be embedded in a silica matrix by means of the sol}gel method without any signi"cant spectrum-changes in their optical properties. In what follows, we show the experimental results of the polysilane}inorganic hybrid thin "lm (7.5 wt% of the block copolymer) only in order to highlight the e!ects of the silica matrix on the optical properties of polysilanes. Photoinduced degradation of PL in PMPS and polysilane}inorganic hybrid thin "lms by ultraviolet (UV) light exposure (3.814 eV excitation of 2.5 W/cm at 10 K in vacuum) has been studied (Fig. 2). The PL peak intensity of both PMPS and polysilane}inorganic hybrid thin "lms decays with UV exposure time, but the decay rate of the hybrid thin "lms is much lower than that of the PMPS thin "lms. After the samples were kept at 10 K for 1 h in dark, the PL peak intensity of PMPS thin "lms further decreases, while that of the hybrid thin "lms slightly recovers. These results indicate that the silica matrix in the hybrid thin "lms acts as a passivation cage
Fig. 2. UV light exposure time dependence of PL peak intensity in PMPS and polysilane}inorganic hybrid thin "lms (7.5 wt% of the block copolymer). PL peak intensities after 1 h annealing at 10 K are also shown.
Fig. 3. Angular dependence of the PL peak intensity in polysilane}inorganic hybrid thin "lm (7.5 wt% of the block copolymer) at 10 K under 3.814 eV laser excitation. The angle is made between the polarizers in the excitation and the PL beam.
which prevents from photogenerating dangling bonds from photodegrading irreversibly, thus improving the durability of organic polysilanes. Polarized PL spectra have been examined in order to gain insight into excitation energy migration [6]. The PL peak intensity at di!erent angles between the polarizers in the excitation and the PL beam was measured. Fig. 3 shows the angular dependence of the PL peak intensity, and the degree of anisotropy r is estimated to be 0.33 from r"(I }I )/(I #2I ), where I is PL intensity, and indices v and h denote the vertical and horizontal position of the polarizer in the excitation and the PL beam, respectively. The value of r is close to a theoretical upper limit of 0.4 in the case of an isotropic "lm containing noninteracting molecular emitters, indicating that the interchain energy transfer erasing the polarization memory is considerably suppressed in the polysilane}inorganic hybrid thin "lms (r&0 was found in PMPS thin "lms).
4. Conclusions Fig. 1. Optical absorption spectra and photoluminescence spectra of (a) PMPS and (b) polysilane}inorganic hybrid thin "lms (75, 33 and 7.5 wt% of the block copolymer).
We have fabricated the polysilane}inorganic hybrid thin "lms } organic polysilane embedded in a silica
S. Mimura et al. / Journal of Luminescence 87}89 (2000) 715}717
matrix } by a sol}gel method and studied their optical properties. It is found that the degradation of the polysilane}inorganic hybrid thin "lms induced by UV light is signi"cantly suppressed in comparison with that of PMPS thin "lms. In addition, the linear polarization memory of PL is found in the polysilane}inorganic hybrid thin "lms, and is likely to be due to the increase in the interchain separation, homogeneous dispersion of polysilane chains in the silica matrix by the sol-gel method. It is demonstrated that the organic polysilanes embedded in the silica matrix is a new approach to improve the durability of organic polysilane-based optoelectronic devices.
Acknowledgements Part of this work at Nara is supported by a Grant-inAid for Scienti"c Research from the Ministry of Education, Science, Sport and Culture of Japan.
717
References [1] R.D. Miller, J. Michl, Chem. Rev. 89 (1989) 1359. [2] Y. Kanemitsu, Phys. Rep. 263 (1995) 1. [3] K. Takeda, K. Shiraishi, M. Fujiki, M. Kondo, K. Morigaki, Phys. Rev. B 50 (1994) 5171. [4] H. Naito, S. Zhang, M. Okuda, T. Dohmaru, J. Appl. Phys. 76 (1994) 3612. [5] K. Matsukawa, S. Fukui, N. Higashi, M. Niwa, H. Inoue, Chem. Lett. (1999) 1073. [6] H. Michl, E.W. Thulstrup, Spectroscopy with Polarized Light, VCH, New York, 1989, p.7.
Optical properties of (organic polysilane)}(inorganic matrix) hybrid thin "lms S. Mimura *, H. Naito , Y. Kanemitsu, K. Matsukawa, H. Inoue Department of Physics and Electronics, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan Plastics Department, Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan
Abstract Polysilane}inorganic hybrid thin "lms } organic polysilane embedded in a silica matrix } were fabricated using a sol}gel method to improve the durability of organic polysilanes as optoelectric devices. Photoluminescence measurements at 10 K show that the durability against ultraviolet light exposure is improved in the polysilane}inorganic hybrid thin "lms. In addition, the luminescence linear polarization memory is found at 10 K, suggesting the decrease in the interchain interaction of polysilanes in the polysilane}inorganic hybrid thin "lms. 2000 Elsevier Science B.V. All rights reserved. Keywords: Polysilane}inorganic hybrid thin "lms; Sol}gel method; Luminescence linear polarization memory
1. Introduction
2. Experiment
Organic polysilanes are chain-like polymers with silicon backbone and organic substituent groups, and exhibit a variety of unique and interesting properties, such as high quantum e$ciency of photoluminescence (PL) and high hole drift mobility. These properties are associated with p conjugation along the Si-backbone [1]. Recently, the applicability of the materials to future optoelectric devices has been examined [2], and unfortunately, the studies for this purpose suggested that the durability of organic polysilanes as optoelectronic devices is not satisfactory because of an irreversible photochemical (photo-oxidation) reaction [1,3,4]. In this paper, we study optical properties of polysilane}inorganic hybrid thin "lms } organic polysilane embedded in silica matrix } to improve the durability of organic polysilanes as optoelectronic devices and discuss new optical properties due to con"nement e!ects of excitons.
2.1. Synthesis
* Corresponding author. Fax: #81-722-54-9908. E-mail address: [email protected] (S. Mimura)
The polysilane}inorganic hybrid materials were prepared from trimethoxysilylpropylmethacrylate}methylphenylsilane block copolymers with a sol}gel method using tetraethoxysilane (TEOS) as an inorganic matrix [5]. The block copolymers having polysilane segments have been synthesized by photopolymerization of trimethoxysilylpropylmethacrylate with polymethylphenylsilane (PMPS) as a macrophotoinitiator. We used here the polysilane}inorganic hybrid thin "lms prepared by spin coating of the block copolymers (M "10 500, M "18 200)/TEOS solution with the di!erent com positions of the block copolymers to TEOS (75, 33 and 7.5 wt% of the copolymers). 2.2. Measurements The optical absorption spectra and the PL spectra of these thin "lms were measured with a Shimadzu UV3100PC spectrometer and a Shimadzu RF-5300PC spectrometer, respectively, at room temperature. For PL measurement at 10 K, a He-Cd laser (Omnichrome
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 7 2 - 5
716
S. Mimura et al. / Journal of Luminescence 87}89 (2000) 715}717
3056-15 ML) was used as an excitation source, and the PL spectra were measured using a 30-cm monochromator (Actron Research Corporation SpectraPro300i) and a CCD camera (Princeton Instrument RTE/CCD-128).
3. Results and discussion Fig. 1 shows the optical absorption and PL spectra under the 3.756 eV excitation of a PMPS thin "lm and polysilane}inorganic hybrid thin "lms at room temperature. It can be seen that the optical absorption and PL spectra of the polysilane}inorganic hybrid thin "lms are essentially the same as those of a PMPS thin "lm. We "nd from these results that organic polysilanes can be embedded in a silica matrix by means of the sol}gel method without any signi"cant spectrum-changes in their optical properties. In what follows, we show the experimental results of the polysilane}inorganic hybrid thin "lm (7.5 wt% of the block copolymer) only in order to highlight the e!ects of the silica matrix on the optical properties of polysilanes. Photoinduced degradation of PL in PMPS and polysilane}inorganic hybrid thin "lms by ultraviolet (UV) light exposure (3.814 eV excitation of 2.5 W/cm at 10 K in vacuum) has been studied (Fig. 2). The PL peak intensity of both PMPS and polysilane}inorganic hybrid thin "lms decays with UV exposure time, but the decay rate of the hybrid thin "lms is much lower than that of the PMPS thin "lms. After the samples were kept at 10 K for 1 h in dark, the PL peak intensity of PMPS thin "lms further decreases, while that of the hybrid thin "lms slightly recovers. These results indicate that the silica matrix in the hybrid thin "lms acts as a passivation cage
Fig. 2. UV light exposure time dependence of PL peak intensity in PMPS and polysilane}inorganic hybrid thin "lms (7.5 wt% of the block copolymer). PL peak intensities after 1 h annealing at 10 K are also shown.
Fig. 3. Angular dependence of the PL peak intensity in polysilane}inorganic hybrid thin "lm (7.5 wt% of the block copolymer) at 10 K under 3.814 eV laser excitation. The angle is made between the polarizers in the excitation and the PL beam.
which prevents from photogenerating dangling bonds from photodegrading irreversibly, thus improving the durability of organic polysilanes. Polarized PL spectra have been examined in order to gain insight into excitation energy migration [6]. The PL peak intensity at di!erent angles between the polarizers in the excitation and the PL beam was measured. Fig. 3 shows the angular dependence of the PL peak intensity, and the degree of anisotropy r is estimated to be 0.33 from r"(I }I )/(I #2I ), where I is PL intensity, and indices v and h denote the vertical and horizontal position of the polarizer in the excitation and the PL beam, respectively. The value of r is close to a theoretical upper limit of 0.4 in the case of an isotropic "lm containing noninteracting molecular emitters, indicating that the interchain energy transfer erasing the polarization memory is considerably suppressed in the polysilane}inorganic hybrid thin "lms (r&0 was found in PMPS thin "lms).
4. Conclusions Fig. 1. Optical absorption spectra and photoluminescence spectra of (a) PMPS and (b) polysilane}inorganic hybrid thin "lms (75, 33 and 7.5 wt% of the block copolymer).
We have fabricated the polysilane}inorganic hybrid thin "lms } organic polysilane embedded in a silica
S. Mimura et al. / Journal of Luminescence 87}89 (2000) 715}717
matrix } by a sol}gel method and studied their optical properties. It is found that the degradation of the polysilane}inorganic hybrid thin "lms induced by UV light is signi"cantly suppressed in comparison with that of PMPS thin "lms. In addition, the linear polarization memory of PL is found in the polysilane}inorganic hybrid thin "lms, and is likely to be due to the increase in the interchain separation, homogeneous dispersion of polysilane chains in the silica matrix by the sol-gel method. It is demonstrated that the organic polysilanes embedded in the silica matrix is a new approach to improve the durability of organic polysilane-based optoelectronic devices.
Acknowledgements Part of this work at Nara is supported by a Grant-inAid for Scienti"c Research from the Ministry of Education, Science, Sport and Culture of Japan.
717
References [1] R.D. Miller, J. Michl, Chem. Rev. 89 (1989) 1359. [2] Y. Kanemitsu, Phys. Rep. 263 (1995) 1. [3] K. Takeda, K. Shiraishi, M. Fujiki, M. Kondo, K. Morigaki, Phys. Rev. B 50 (1994) 5171. [4] H. Naito, S. Zhang, M. Okuda, T. Dohmaru, J. Appl. Phys. 76 (1994) 3612. [5] K. Matsukawa, S. Fukui, N. Higashi, M. Niwa, H. Inoue, Chem. Lett. (1999) 1073. [6] H. Michl, E.W. Thulstrup, Spectroscopy with Polarized Light, VCH, New York, 1989, p.7.