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Temperature- and size-effects on optical properties of perylene microcrystals
Optical Materials 21 (2002) 595–598 www.elsevier.com/locate/optmat
Temperature- and size-effects on optical properties of perylene microcrystals Tsunenobu Onodera a, Hitoshi Kasai a,b, Shuji Okada a,b, Hidetoshi Oikawa a,b, Ken-ichi Mizuno c, Mamoru Fujitsuka a, Osamu Ito a, Hachiro Nakanishi a,b,* a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan b CREST (Core Reseach for Evolution Science and Technology) of Japan Science and Technology Corporation (JST), Japan c Department of Applied Chemistry, Faculty of Science, Konan University, Okamoto 8-9-1, Higashinada-ku, Kobe 658-8501, Japan
Abstract Perylene microcrystals with different sizes were prepared by the reprecipitation and subsequent microwave irradiation method. Their excitonic absorption peaks were blue-shifted with decreasing crystal size in the size range of submicrometer. Perylene microcrystal films were also fabricated by means of the electrostatic adsorption method to investigate the dependence of the excitonic absorption spectra on temperature. As a result, the temperature-dependence of half-width of the excitonic absorption spectra was well explained by ToyozawaÕs theoretical prediction, which was based upon an interaction between exciton and lattice vibration. In addition, it was revealed that the peak position of emission spectra from the self-trapped exciton (STE) state was relatively blue-shifted in comparison with the bulk crystal, and that the lifetime of STE emission became shorter in perylene microcrystal in any temperature. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 78.66.V; 81.05.L; 78.20; 63.20.L Keywords: Perylene; Microcrystals; Optical properties; Exciton; Lattice vibration
1. Introduction Microcrystals are of much interest because of their unique optical and electronic properties. For example, the semiconductor and metal microcrystals have been extensively investigated for understanding their basic physicochemical phenomena such as quantum confinement effect [1], and for
* Corresponding author. Tel.: +81-22-217-5585; fax: +81-22217-5645. E-mail address: [email protected] (H. Nakanishi).
some applications to non-linear optics [2]. On the other hand, little organic microcrystals have been undertaken, due to thermal unstability during the preparation procedure. However, we have recently proposed that some kind of organic microcrystals with the crystals size ranges from several tens nanometer to sub-micrometer could be fabricated under mild conditions by the reprecipitation method and subsequent microwave irradiation method [3,4], and clarified experimentally the sizeeffect of their linear optical properties. In particular, p-conjugated polydiacetylene microcrystals, such as poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (poly-DCHD), have been discussed in detail the
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 2 0 7 - 0
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T. Onodera et al. / Optical Materials 21 (2002) 595–598
relationship between their visible absorption spectra, crystal size and temperature [5,6]. In the present research, perylene microcrystals and their thin films with high optical quality could be fabricated, and the temperature- and size-dependence will be discussed in the visible absorption and time-resolved fluorescence spectra.
2. Experimental Perylene used was commercially available, and purified by vacuum sublimation and zone-refining. First, perylene microcrystals were prepared by the reprecipitation method: 1 ml of perylene–acetone solution (2 mM) was injected into vigorously stirred ultra-pure water (10 ml) using a microsyringe, and then microwave (2.45 GHz, 50 W) was irradiated for 5 min, so that microcrystallization may proceed smoothly by removing acetone. The resulting perylene microcrystals were obtained as pale-yellow ones dispersed in water. The crystal size was controlled by changing water temperature. By the measurement with powder X-ray diffraction, the crystal form was identified to be the a-type of dimeric form [7]. The f-potential of perylene microcrystals in an aqueous dispersion was 30 mV, which was measured with ELS-8000 (Otsuka Electronics Co.). By utilizing electrostatic adsorption between perylene microcrystals and cationic polyelectrolyte [(poly(dimethyldiallylammonium chloride): PDAC] (Mw ¼ 2:0 105 –3:5 105 , ALDRICH) coated on the glass substrate, perylene microcrystal films were fabricated as follows [8]. The glass substrate washed previously with alkali aqueous solution was first immersed into 1 wt.% PDAC aqueous solution, and then subsequently into perylene microcrystal dispersion in the similar manner. The immersing times were 20 min for PDAC and several hours for perylene microcrystals, respectively. After each immersing process, the substrate was slightly rinsed with pure water. The crystal size and morphology of perylene microcrystals were evaluated by scanning electron microscope (SEM; Hitachi, S-900). The UV–VIS spectrometer (JASCO, V-570DS) was used to evaluate linear optical properties. The time-
resolved emission spectra were measured with the system equipped with Ti:sapphire laser (SpectraPhysics, Tsunami 3950-L2S) pumped by an argon ion laser (Spectra-Physics, BeamLok 2060-10-SA), a pulse selector (Spectra-Physics, Model 3980), a harmonic generator (GWU-23PS), and a streak camera (HAMAMATSU, C4334-01). The excitation wavelength used was 400 nm. The perylene microcrystal films were mounted in a cryostat which was connected with the above-mentioned spectrometers. Liquid He was flowed into the cryostat for stably keeping a given temperature.
3. Results and discussion The crystal size of perylene microcrystals adsorbed on PDAC was evaluated directly from SEM photograph as shown in Fig. 1. The crystal size in Fig. 1 was approximately the same as that when dispersed in an aqueous medium. Fig. 2 shows the absorption spectra of the perylene microcrystal films with different sizes. The excitonic absorption peak positions were observed to be shifted to high energy region with decreasing crystal size. The similar size-dependent shift has been confirmed not only in perylene microcrystal dispersion [7] but also in poly(DCHD) microcrystals system [5,6]. Fig. 3 indicates the temperature-dependence of the excitonic absorption spectra for perylene mi-
Fig. 1. SEM photograph of perylene microcrystals electrostatically adsorbed on PDAC film.
T. Onodera et al. / Optical Materials 21 (2002) 595–598
Fig. 2. Absorption spectra of perylene microcrystal films: (a) crystal size 300 nm and (b) 130 nm.
597
Fig. 4. Temperature- and size-dependence of STE peak positions for perylene microcrystal with 166 53 nm in size (N), and for perylene bulk crystal with above 1 lm ( ). STE is the self-trapped exciton.
Fig. 3. Temperature dependence of the excitonic absorption spectra for perylene microcrystals (average crystal size: 198 48 nm): half-width ( ) and maximum absorbance peak positions (N).
crocrystal films, when the averaged crystal size was 198 48 nm. At low temperature below 140 K, the half-width (m1=2 ) was almost constant, approximately proportional to temperature from 140 to 200 K, and was scaled as temperature to the 0.5th power at more than 200 K. These three-step behaviors were semi-quantitatively explained by ToyozawaÕs theoretical prediction [9], which was discussed on the basis of a certain coupled interaction between exciton and lattice vibration. On the contrary, the absorption peak positions (kmax ) themselves were almost independent of temperature within the experimental error. Fig. 4 exhibits the temperature- and size-dependence of emission peak positions (mmax and/or kmax )
from the self-trapped exciton (STE) state. The STE peak positions for perylene microcrystals were high energy-shifted at any temperature above 70 K in comparison with the bulk crystal, and the difference was spread out with increasing temperature. On the other hand, the both STE peak positions were almost the same around 70 K. In addition, the fluorescent lifetime (sSTE ) of microcrystal was always shorter than that of bulk crystal above 120 K as shown in Fig. 5, whereas both the sSTE were converged below 120 K. As discussed previously in Fig. 3, these emission phenomena were closely related to a certain surface effect and/or lattice vibration in softened perylene microcrystal lattice, depending on temperature and size. Thus, a certain interaction
Fig. 5. Temperature dependence of luminescence lifetime at STE state in perylene microcrystal (N) and bulk crystal ( ).
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T. Onodera et al. / Optical Materials 21 (2002) 595–598
between exciton and phonon in microcrystals may lead to the changes of the electronic band structure and of the optical transition. 4. Conclusions We have investigated the absorption and timeresolved emission spectra of perylene microcrystal film, and revealed crystal size- and temperaturedependence of excitonic absorption peaks, its halfwidth, STE peak positions, and luminescence lifetime. As a result, the optical properties of perylene microcrystals were strongly influenced by temperature and size, which might be due to the changes of the electronic band structure through a certain surface effect and/or thermally softened lattice in a microcrystal caused by microcrystallization.
References [1] R. Kubo, J. Phys. Soc. Jpn. 17 (1962) 975. [2] E. Hanamura, Phys. Rev. B 37 (1988) 1273. [3] H. Kasai, H.S. Nalwa, H. Oikawa, S. Okada, H. Matsuda, N. Minami, A. Kakuta, K. Ono, A. Mukoh, H. Nakanishi, Jpn. J. Appl. Phys. 31 (1992) L1132. [4] K. Baba, H. Kasai, S. Okada, H. Oikawa, H. Nakanishi, Jpn. J. Appl. Phys. 39 (2000) L1256. [5] H. Katagi, H. Kasai, S. Okada, H. Oikawa, H. Matsuda, H. Nakanishi, J. Macromol. Sci. -Pure Appl. Chem. A 34 (1997) 2013. [6] H. Katagi, H. Oikawa, S. Okada, H. Kasai, A. Watanabe, O. Ito, Y. Nozue, H. Nakanishi, Mol. Cryst. Liq. Cryst. 314 (1998) 285. [7] H. Kasai, Y. Yoshikawa, T. Seko, S. Okada, H. Oikawa, H. Matsuda, A. Watanabe, O. Ito, H. Toyotama, H. Nakanishi, Mol. Cryst. Liq. Cryst. 294 (1997) 173. [8] S.K. Tripathy, H. Katagi, H. Kasai, S. Balasubramanian, H. Oshikiri, J. Kumar, H. Oikawa, S. Okada, H. Nakanishi, Jpn. J. Appl. Phys. 37 (1998) L343. [9] Y. Toyozawa, Prog. Theor. Phys. 20 (1958) 53.
Temperature- and size-effects on optical properties of perylene microcrystals Tsunenobu Onodera a, Hitoshi Kasai a,b, Shuji Okada a,b, Hidetoshi Oikawa a,b, Ken-ichi Mizuno c, Mamoru Fujitsuka a, Osamu Ito a, Hachiro Nakanishi a,b,* a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan b CREST (Core Reseach for Evolution Science and Technology) of Japan Science and Technology Corporation (JST), Japan c Department of Applied Chemistry, Faculty of Science, Konan University, Okamoto 8-9-1, Higashinada-ku, Kobe 658-8501, Japan
Abstract Perylene microcrystals with different sizes were prepared by the reprecipitation and subsequent microwave irradiation method. Their excitonic absorption peaks were blue-shifted with decreasing crystal size in the size range of submicrometer. Perylene microcrystal films were also fabricated by means of the electrostatic adsorption method to investigate the dependence of the excitonic absorption spectra on temperature. As a result, the temperature-dependence of half-width of the excitonic absorption spectra was well explained by ToyozawaÕs theoretical prediction, which was based upon an interaction between exciton and lattice vibration. In addition, it was revealed that the peak position of emission spectra from the self-trapped exciton (STE) state was relatively blue-shifted in comparison with the bulk crystal, and that the lifetime of STE emission became shorter in perylene microcrystal in any temperature. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 78.66.V; 81.05.L; 78.20; 63.20.L Keywords: Perylene; Microcrystals; Optical properties; Exciton; Lattice vibration
1. Introduction Microcrystals are of much interest because of their unique optical and electronic properties. For example, the semiconductor and metal microcrystals have been extensively investigated for understanding their basic physicochemical phenomena such as quantum confinement effect [1], and for
* Corresponding author. Tel.: +81-22-217-5585; fax: +81-22217-5645. E-mail address: [email protected] (H. Nakanishi).
some applications to non-linear optics [2]. On the other hand, little organic microcrystals have been undertaken, due to thermal unstability during the preparation procedure. However, we have recently proposed that some kind of organic microcrystals with the crystals size ranges from several tens nanometer to sub-micrometer could be fabricated under mild conditions by the reprecipitation method and subsequent microwave irradiation method [3,4], and clarified experimentally the sizeeffect of their linear optical properties. In particular, p-conjugated polydiacetylene microcrystals, such as poly[1,6-di(N-carbazolyl)-2,4-hexadiyne] (poly-DCHD), have been discussed in detail the
0925-3467/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 2 ) 0 0 2 0 7 - 0
596
T. Onodera et al. / Optical Materials 21 (2002) 595–598
relationship between their visible absorption spectra, crystal size and temperature [5,6]. In the present research, perylene microcrystals and their thin films with high optical quality could be fabricated, and the temperature- and size-dependence will be discussed in the visible absorption and time-resolved fluorescence spectra.
2. Experimental Perylene used was commercially available, and purified by vacuum sublimation and zone-refining. First, perylene microcrystals were prepared by the reprecipitation method: 1 ml of perylene–acetone solution (2 mM) was injected into vigorously stirred ultra-pure water (10 ml) using a microsyringe, and then microwave (2.45 GHz, 50 W) was irradiated for 5 min, so that microcrystallization may proceed smoothly by removing acetone. The resulting perylene microcrystals were obtained as pale-yellow ones dispersed in water. The crystal size was controlled by changing water temperature. By the measurement with powder X-ray diffraction, the crystal form was identified to be the a-type of dimeric form [7]. The f-potential of perylene microcrystals in an aqueous dispersion was 30 mV, which was measured with ELS-8000 (Otsuka Electronics Co.). By utilizing electrostatic adsorption between perylene microcrystals and cationic polyelectrolyte [(poly(dimethyldiallylammonium chloride): PDAC] (Mw ¼ 2:0 105 –3:5 105 , ALDRICH) coated on the glass substrate, perylene microcrystal films were fabricated as follows [8]. The glass substrate washed previously with alkali aqueous solution was first immersed into 1 wt.% PDAC aqueous solution, and then subsequently into perylene microcrystal dispersion in the similar manner. The immersing times were 20 min for PDAC and several hours for perylene microcrystals, respectively. After each immersing process, the substrate was slightly rinsed with pure water. The crystal size and morphology of perylene microcrystals were evaluated by scanning electron microscope (SEM; Hitachi, S-900). The UV–VIS spectrometer (JASCO, V-570DS) was used to evaluate linear optical properties. The time-
resolved emission spectra were measured with the system equipped with Ti:sapphire laser (SpectraPhysics, Tsunami 3950-L2S) pumped by an argon ion laser (Spectra-Physics, BeamLok 2060-10-SA), a pulse selector (Spectra-Physics, Model 3980), a harmonic generator (GWU-23PS), and a streak camera (HAMAMATSU, C4334-01). The excitation wavelength used was 400 nm. The perylene microcrystal films were mounted in a cryostat which was connected with the above-mentioned spectrometers. Liquid He was flowed into the cryostat for stably keeping a given temperature.
3. Results and discussion The crystal size of perylene microcrystals adsorbed on PDAC was evaluated directly from SEM photograph as shown in Fig. 1. The crystal size in Fig. 1 was approximately the same as that when dispersed in an aqueous medium. Fig. 2 shows the absorption spectra of the perylene microcrystal films with different sizes. The excitonic absorption peak positions were observed to be shifted to high energy region with decreasing crystal size. The similar size-dependent shift has been confirmed not only in perylene microcrystal dispersion [7] but also in poly(DCHD) microcrystals system [5,6]. Fig. 3 indicates the temperature-dependence of the excitonic absorption spectra for perylene mi-
Fig. 1. SEM photograph of perylene microcrystals electrostatically adsorbed on PDAC film.
T. Onodera et al. / Optical Materials 21 (2002) 595–598
Fig. 2. Absorption spectra of perylene microcrystal films: (a) crystal size 300 nm and (b) 130 nm.
597
Fig. 4. Temperature- and size-dependence of STE peak positions for perylene microcrystal with 166 53 nm in size (N), and for perylene bulk crystal with above 1 lm ( ). STE is the self-trapped exciton.
Fig. 3. Temperature dependence of the excitonic absorption spectra for perylene microcrystals (average crystal size: 198 48 nm): half-width ( ) and maximum absorbance peak positions (N).
crocrystal films, when the averaged crystal size was 198 48 nm. At low temperature below 140 K, the half-width (m1=2 ) was almost constant, approximately proportional to temperature from 140 to 200 K, and was scaled as temperature to the 0.5th power at more than 200 K. These three-step behaviors were semi-quantitatively explained by ToyozawaÕs theoretical prediction [9], which was discussed on the basis of a certain coupled interaction between exciton and lattice vibration. On the contrary, the absorption peak positions (kmax ) themselves were almost independent of temperature within the experimental error. Fig. 4 exhibits the temperature- and size-dependence of emission peak positions (mmax and/or kmax )
from the self-trapped exciton (STE) state. The STE peak positions for perylene microcrystals were high energy-shifted at any temperature above 70 K in comparison with the bulk crystal, and the difference was spread out with increasing temperature. On the other hand, the both STE peak positions were almost the same around 70 K. In addition, the fluorescent lifetime (sSTE ) of microcrystal was always shorter than that of bulk crystal above 120 K as shown in Fig. 5, whereas both the sSTE were converged below 120 K. As discussed previously in Fig. 3, these emission phenomena were closely related to a certain surface effect and/or lattice vibration in softened perylene microcrystal lattice, depending on temperature and size. Thus, a certain interaction
Fig. 5. Temperature dependence of luminescence lifetime at STE state in perylene microcrystal (N) and bulk crystal ( ).
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T. Onodera et al. / Optical Materials 21 (2002) 595–598
between exciton and phonon in microcrystals may lead to the changes of the electronic band structure and of the optical transition. 4. Conclusions We have investigated the absorption and timeresolved emission spectra of perylene microcrystal film, and revealed crystal size- and temperaturedependence of excitonic absorption peaks, its halfwidth, STE peak positions, and luminescence lifetime. As a result, the optical properties of perylene microcrystals were strongly influenced by temperature and size, which might be due to the changes of the electronic band structure through a certain surface effect and/or thermally softened lattice in a microcrystal caused by microcrystallization.
References [1] R. Kubo, J. Phys. Soc. Jpn. 17 (1962) 975. [2] E. Hanamura, Phys. Rev. B 37 (1988) 1273. [3] H. Kasai, H.S. Nalwa, H. Oikawa, S. Okada, H. Matsuda, N. Minami, A. Kakuta, K. Ono, A. Mukoh, H. Nakanishi, Jpn. J. Appl. Phys. 31 (1992) L1132. [4] K. Baba, H. Kasai, S. Okada, H. Oikawa, H. Nakanishi, Jpn. J. Appl. Phys. 39 (2000) L1256. [5] H. Katagi, H. Kasai, S. Okada, H. Oikawa, H. Matsuda, H. Nakanishi, J. Macromol. Sci. -Pure Appl. Chem. A 34 (1997) 2013. [6] H. Katagi, H. Oikawa, S. Okada, H. Kasai, A. Watanabe, O. Ito, Y. Nozue, H. Nakanishi, Mol. Cryst. Liq. Cryst. 314 (1998) 285. [7] H. Kasai, Y. Yoshikawa, T. Seko, S. Okada, H. Oikawa, H. Matsuda, A. Watanabe, O. Ito, H. Toyotama, H. Nakanishi, Mol. Cryst. Liq. Cryst. 294 (1997) 173. [8] S.K. Tripathy, H. Katagi, H. Kasai, S. Balasubramanian, H. Oshikiri, J. Kumar, H. Oikawa, S. Okada, H. Nakanishi, Jpn. J. Appl. Phys. 37 (1998) L343. [9] Y. Toyozawa, Prog. Theor. Phys. 20 (1958) 53.