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Enhanced photo-stability of conjugated polymer nanocomposites doped with functionalized nanoparticles
Optical Materials 21 (2002) 585–589 www.elsevier.com/locate/optmat
Enhanced photo-stability of conjugated polymer nanocomposites doped with functionalized nanoparticles Yong Taik Lim, Tae-Woo Lee, Ho-Chul Lee, O Ok Park
*
Department of Chemical Engineering, Center for Advanced Functional Polymers, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong -gu, Taejon 305-701, South Korea
Abstract A new nanoparticle approach to enhance the lifetime of conjugated polymer devices was suggested. We prepared poly(p-phenylenevinylene) (PPV) nanocomposites films doped with gold-coated silica (SiO2 @ Au) nanoparticles and characterized the photo-luminescent properties of them. The optical resonance of SiO2 @ Au nanoparticles was specifically designed to interact with the triplet excitons which play a primary role in the photo-oxidation of PPV. The rate of photo-oxidation in PPV was drastically reduced by doping with SiO2 @ Au nanoparticles, while the absorption and photo-luminescent spectra vary slightly between pristine PPV and PPV nanocomposite films. This effect has important implications on the enhancement of lifetimes in conjugated polymers-based optoelectronic devices. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.65.Vh; 61.46:þw Keywords: Photo-oxidation; Triplet excitons; Poly(p-phenylenevinylene); Coated nanoparticles; Plasmon resonance
1. Introduction Poly(p-phenylenevinylene) (PPV) and its derivatives are versatile conjugated polymers which have been employed for polymer-based light emitting devices (LEDs) [1], lasers [2] and photovoltaic cells [3]. The widespread adoption of conjugated polymer-based devices is blocked by the relatively their short lifetime due to the rapid photo-oxidation of the films under ambient conditions. Therefore a precise understanding of the effects of photo-oxidation in conjugated poly-
*
Corresponding author. Tel.: +82-42-869-3923; fax: +82-42869-3910. E-mail address: [email protected] (O Ok Park).
mer films is a necessary step in the optimization of polymer-based materials and devices. Fig. 1 illustrates the mechanism of photo-oxidation in luminescent conjugated polymer devices and impediment of the photo-oxidation by doping with triplet state quencher. Because photo-oxidation of many PPV derivatives begins with formation of singlet oxygen via energy transfer from long-lived triplet excitons (3 ! 4), triplet exciton energetics and dynamics play a primary role in the photooxidation process [4,5]. The oxygen is excited to a highly reactive singlet site, which reacts with the conjugated polymer backbone, resulting in the formation of luminescence-quenching defects (4 ! 5). Since the triplet exciton drives this process, controlling the triplet exciton dynamics will slow the photo-oxidation process.
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 5 - 7
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Y. Taik Lim et al. / Optical Materials 21 (2002) 585–589
2. Experimental
Fig. 1. Mechanism of photo-oxidation in luminescent conjugated polymers and impediment of the photo-oxidation with triplet quencher.
The utilization of nanoparticles in optoelectronic devices leads to enhancement of optical, electrical properties and stability [6–11]. In these studies, it has been suggested that nanoparticles such as SiO2 and TiO2 can act as charge carrier promoters, or electro-optically active centers to influence the optical and electronic properties of PPV. Metal-coated nanoparticles are composite nanoparticles consisting of a dielectric core coated with a thin metal shell [8,12]. The plasmon resonance of the metal-coated nanoparticles may be shifted throughout the visible and near-IR by varying the ratio of the core radius to the shell thickness. By incorporating metal-coated nanoparticles specifically designed to interact with the triplet excitons in conjugated polymers, the rate of photo-oxidation can be slowed and the density of luminescence-quenching traps reduced (3 ! 6). In this article, we have prepared PPV nanocomposite films doped with SiO2 @ Au nanoparticles, whose optical resonance was specifically designed to interact with the triplet excitons of PPV. It has been observed that the rate of photooxidation in PPV was drastically reduced by doping with SiO2 @ Au nanoparticles.
SiO2 @ Au nanoparticles were synthesized as followings. To briefly summarize, silica nanoparticles were synthesized by the Stober method and then coated with an Sn layer, which acts as a linker site for gold deposition. Gold layers were coated on the Sn-functionalized silica nanoparticles by reduction of HAuCl4 . Finally, the gold-coated silica nanoparticles were covered by another SiO2 layer, which acts as a stabilizer for the composite particles. A PPV precursor (poly(xylylidene tetrahydrothiophenenium chloride)) was also prepared by well-known methods [13]. The SiO2 @ Au nanoparticles were then dispersed in the PPV precursor solutions which were subsequently spin coated onto glass substrates. The substrates were held under vacuum at 180 °C for five hours for thermal conversion of the film. PL spectra were measured by exciting 410 nm monochromatic light. The PL decay of pristine PPV and PPV nanocomposite films was measured by exciting 310 nm light. While filtering the light through a UV cutting filter, the PL intensity was detected using an optical power meter connected to a photodiode with the function of time. There was no visible difference between the prinstine PPV thin film and the PPV nanocomposite film.
3. Results and discussion Recently, optical properties of triplet excitons which play a primary role in the photo-oxidation process in PPV, have been studied in detail [14]. From the threshold energy for singlet fission, it was deduced that the lowest lying, odd parity triplet excitonic state is located at 1.55 eV (k ¼ 799:9 nm) from the ground state, which is about 0.9 eV lower than the lowest lying, oddparity singlet state. And, the plasmon resonance of SiO2 @ Au nanoparticles was specially designed to interact with the triplet energy state of PPV. The TEM images and absorption spectrum of SiO2 @ Au nanoparticles are shown in Fig. 2. The diameter of composite nanoparticles is about 380 nm. The radius of silica core particles is about 170 nm, while the thickness of the gold shell is about
Y. Taik Lim et al. / Optical Materials 21 (2002) 585–589
587
Fig. 2. TEM images (a) and optical absorption spectra (b) of gold-coated silica nanoparticles (Davg ¼ 340 nm).
20 nm. The outer silica layer has a thickness of 20 nm. The optical resonance of SiO2 @ Au nanoparticles is located at a broad region around 720– 800 nm, which nearly overlaps with the triplet excitons in PPV. Fig. 3 shows the optical absorption and emission properties of PPV and PPV nanocomposite films doped with SiO2 @ Au nanoparticle. The optical and luminescent properties of our PPV agreed well with previously reported results [15]. The absorbance of the nanoshells at the excitation wavelength was negligible. The relative height of the two features observed in each PL spectrum vary slightly between samples, but no dependence on SiO2 @ Au nanoparticle concentration, suggesting that the small height difference arises from minor variations in the local structure or film thickness of the PPV and PPV–SiO2 @ Au nanoparticles composites.
The PL decay patterns for polymer nanocomposites films with two different SiO2 @ Au nanoparticles are shown in Fig. 4. The rate of PL decay in PPV nanocomposites doped with SiO2 @ Au nanoparticles was drastically reduced compared with that of the prinstine PPV. This suggests that the rate of luminescence quenching by exciton trap formation is reduced considerably. The protection against photo-oxidation obtained with SiO2 @ Au nanoparticles in PPV could be accomplished with an extremely low concentration, corresponding to about 0.05% volume fraction. It is noticeable that such a low concentration of nanoshells yields a high level of protection against photo-oxidation. We can explain that this enhanced energy transfer could be related to the nature of polymer excitons and the fact that the SiO2 @ Au nanoparticles can exhibit greatly enhanced local field intensities and large effective cross-sections [8,16].
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Y. Taik Lim et al. / Optical Materials 21 (2002) 585–589
Since solid metal nanoparticle resonance have excitation lifetimes of only a few picoseconds, the donor–acceptor interaction between the comparatively long-lived excitons of the PPV and nanoparticles will result in strong quenching of the triplet state of PPV to which the composite nanoparticles resonance have been tuned. The excitons in the conjugated polymer are highly mobile along the polymer backbone and can hop between chains readily, finding the lowest-energy region of the film. It is possible that the SiO2 @ Au nanoparticles dispersed in the polymer film will cause variations in the local energy environment of the triplet excitons, attracting them to the SiO2 @ Au nanoparticles, where the triplet exciton–SiO2 @ Au nanoparticles interaction can take place easily. The large effective cross-section of the nanoshells, which may be a larger than electromagnetic waves, should in effect reduce the apparent distance between the SiO2 @ Au nanoparticles in the film.
4. Conclusion
Fig. 3. Optical absorption (a) and PL (b) spectra of pristine PPV and PPV nanocomposites (doped 0.05 vol% gold-coated silica nanoparticles).
We have studied the use of SiO2 @ Au nanoparticles to impede the photo-oxidation of PPV films. By doping with nanoparticles which were specially designed to interact with triplet excitons of PPV, the rate of photo-oxidation in PPV nanocomposites was drastically reduced. This effect may have significant implications on the commercialization of conjugated polymer-based devices. The addition of SiO2 @ Au nanoparticles to conjugated polymers-based optoelectronic devices, along with careful processing techniques and device encapsulation, may result in improved lifetimes.
Acknowledgements
Fig. 4. Retarded photo-oxidation in PPV nanocomposite films doped with gold-coated silica nanoparticles. (nanocomposite-1 (0.05 vol%), nanocomposite-2 (0.1 vol%)).
The authors are thankful for the financial supports of the Center for Advanced Functional Polymers (CAFPoly) appointed by the Korea Science and Engineering Foundation. This work is also partially supported by the Brain Korea 21 Project of the Ministry of Education (MOE) of Korea.
Y. Taik Lim et al. / Optical Materials 21 (2002) 585–589
References [1] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 402. [2] M.A. Diaz-Garcia, H. Fumitomo, B.J. Schwartz, Synth. Met. 84 (1997) 455. [3] Y. Cao, G. Yu, A.J. Heeger, Adv. Mater. 10 (1998) 692. [4] B.H. Cumpston, K.F. Jensen, Synth. Met. 73 (1995) 195. [5] R.D. Scurlock, B.J. Wang, P.R. Ogilby, J.R. Sheats, R.L. Clough, J. Am. Chem. Soc. 117 (1995) 10194. [6] S.A. Carter, J.C. Scott, P.J. Brock, Appl. Phys. Lett. 71 (1997) 1145. [7] W.P. Chang, W.T. Whang, Polymer 37 (1996) 4229. [8] G.D. Hale, J.B. Jackson, O.E. Shmakova, T.R. Lee, N.J. Halas, Appl. Phys. Lett. 78 (2001) 1502.
589
[9] T.-W. Lee, J. Yoon, J.-J. Kim, O.O. Park, Adv. Mater. 13 (2001) 211. [10] T.-W. Lee, O.O. Park, J.-J. Kim, J.-M. Hong, Y.C. Kim, Chem. Mater. 13 (2001) 2217. [11] H.-C. Lee, T.-W. Lee, Y.T. Lim, O.O. Park, Appl. Clay Sci., submitted for publication. [12] Y.T. Lim, O.O. Park, J. Colloid Interf. Sci., submitted for publication. [13] J. Gmeiner, S. Karg, M. Meier, W. Riess, P. Strohriegel, M. Schwoerer, Acta Polym. 44 (1993) 201. [14] R. Osterbacka, M. Wohlgenannt, D. Chinn, Z.V. Vardeny, Phys. Rev. B 60 (1999) R11253. [15] M. Herold, J. Gmeiner, W. Riess, P. Strohriegel, Synth. Met. 76 (1996) 109. [16] G.D. Hale, Ph.D. Thesis, 2000.
Enhanced photo-stability of conjugated polymer nanocomposites doped with functionalized nanoparticles Yong Taik Lim, Tae-Woo Lee, Ho-Chul Lee, O Ok Park
*
Department of Chemical Engineering, Center for Advanced Functional Polymers, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong -gu, Taejon 305-701, South Korea
Abstract A new nanoparticle approach to enhance the lifetime of conjugated polymer devices was suggested. We prepared poly(p-phenylenevinylene) (PPV) nanocomposites films doped with gold-coated silica (SiO2 @ Au) nanoparticles and characterized the photo-luminescent properties of them. The optical resonance of SiO2 @ Au nanoparticles was specifically designed to interact with the triplet excitons which play a primary role in the photo-oxidation of PPV. The rate of photo-oxidation in PPV was drastically reduced by doping with SiO2 @ Au nanoparticles, while the absorption and photo-luminescent spectra vary slightly between pristine PPV and PPV nanocomposite films. This effect has important implications on the enhancement of lifetimes in conjugated polymers-based optoelectronic devices. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.65.Vh; 61.46:þw Keywords: Photo-oxidation; Triplet excitons; Poly(p-phenylenevinylene); Coated nanoparticles; Plasmon resonance
1. Introduction Poly(p-phenylenevinylene) (PPV) and its derivatives are versatile conjugated polymers which have been employed for polymer-based light emitting devices (LEDs) [1], lasers [2] and photovoltaic cells [3]. The widespread adoption of conjugated polymer-based devices is blocked by the relatively their short lifetime due to the rapid photo-oxidation of the films under ambient conditions. Therefore a precise understanding of the effects of photo-oxidation in conjugated poly-
*
Corresponding author. Tel.: +82-42-869-3923; fax: +82-42869-3910. E-mail address: [email protected] (O Ok Park).
mer films is a necessary step in the optimization of polymer-based materials and devices. Fig. 1 illustrates the mechanism of photo-oxidation in luminescent conjugated polymer devices and impediment of the photo-oxidation by doping with triplet state quencher. Because photo-oxidation of many PPV derivatives begins with formation of singlet oxygen via energy transfer from long-lived triplet excitons (3 ! 4), triplet exciton energetics and dynamics play a primary role in the photooxidation process [4,5]. The oxygen is excited to a highly reactive singlet site, which reacts with the conjugated polymer backbone, resulting in the formation of luminescence-quenching defects (4 ! 5). Since the triplet exciton drives this process, controlling the triplet exciton dynamics will slow the photo-oxidation process.
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 5 - 7
586
Y. Taik Lim et al. / Optical Materials 21 (2002) 585–589
2. Experimental
Fig. 1. Mechanism of photo-oxidation in luminescent conjugated polymers and impediment of the photo-oxidation with triplet quencher.
The utilization of nanoparticles in optoelectronic devices leads to enhancement of optical, electrical properties and stability [6–11]. In these studies, it has been suggested that nanoparticles such as SiO2 and TiO2 can act as charge carrier promoters, or electro-optically active centers to influence the optical and electronic properties of PPV. Metal-coated nanoparticles are composite nanoparticles consisting of a dielectric core coated with a thin metal shell [8,12]. The plasmon resonance of the metal-coated nanoparticles may be shifted throughout the visible and near-IR by varying the ratio of the core radius to the shell thickness. By incorporating metal-coated nanoparticles specifically designed to interact with the triplet excitons in conjugated polymers, the rate of photo-oxidation can be slowed and the density of luminescence-quenching traps reduced (3 ! 6). In this article, we have prepared PPV nanocomposite films doped with SiO2 @ Au nanoparticles, whose optical resonance was specifically designed to interact with the triplet excitons of PPV. It has been observed that the rate of photooxidation in PPV was drastically reduced by doping with SiO2 @ Au nanoparticles.
SiO2 @ Au nanoparticles were synthesized as followings. To briefly summarize, silica nanoparticles were synthesized by the Stober method and then coated with an Sn layer, which acts as a linker site for gold deposition. Gold layers were coated on the Sn-functionalized silica nanoparticles by reduction of HAuCl4 . Finally, the gold-coated silica nanoparticles were covered by another SiO2 layer, which acts as a stabilizer for the composite particles. A PPV precursor (poly(xylylidene tetrahydrothiophenenium chloride)) was also prepared by well-known methods [13]. The SiO2 @ Au nanoparticles were then dispersed in the PPV precursor solutions which were subsequently spin coated onto glass substrates. The substrates were held under vacuum at 180 °C for five hours for thermal conversion of the film. PL spectra were measured by exciting 410 nm monochromatic light. The PL decay of pristine PPV and PPV nanocomposite films was measured by exciting 310 nm light. While filtering the light through a UV cutting filter, the PL intensity was detected using an optical power meter connected to a photodiode with the function of time. There was no visible difference between the prinstine PPV thin film and the PPV nanocomposite film.
3. Results and discussion Recently, optical properties of triplet excitons which play a primary role in the photo-oxidation process in PPV, have been studied in detail [14]. From the threshold energy for singlet fission, it was deduced that the lowest lying, odd parity triplet excitonic state is located at 1.55 eV (k ¼ 799:9 nm) from the ground state, which is about 0.9 eV lower than the lowest lying, oddparity singlet state. And, the plasmon resonance of SiO2 @ Au nanoparticles was specially designed to interact with the triplet energy state of PPV. The TEM images and absorption spectrum of SiO2 @ Au nanoparticles are shown in Fig. 2. The diameter of composite nanoparticles is about 380 nm. The radius of silica core particles is about 170 nm, while the thickness of the gold shell is about
Y. Taik Lim et al. / Optical Materials 21 (2002) 585–589
587
Fig. 2. TEM images (a) and optical absorption spectra (b) of gold-coated silica nanoparticles (Davg ¼ 340 nm).
20 nm. The outer silica layer has a thickness of 20 nm. The optical resonance of SiO2 @ Au nanoparticles is located at a broad region around 720– 800 nm, which nearly overlaps with the triplet excitons in PPV. Fig. 3 shows the optical absorption and emission properties of PPV and PPV nanocomposite films doped with SiO2 @ Au nanoparticle. The optical and luminescent properties of our PPV agreed well with previously reported results [15]. The absorbance of the nanoshells at the excitation wavelength was negligible. The relative height of the two features observed in each PL spectrum vary slightly between samples, but no dependence on SiO2 @ Au nanoparticle concentration, suggesting that the small height difference arises from minor variations in the local structure or film thickness of the PPV and PPV–SiO2 @ Au nanoparticles composites.
The PL decay patterns for polymer nanocomposites films with two different SiO2 @ Au nanoparticles are shown in Fig. 4. The rate of PL decay in PPV nanocomposites doped with SiO2 @ Au nanoparticles was drastically reduced compared with that of the prinstine PPV. This suggests that the rate of luminescence quenching by exciton trap formation is reduced considerably. The protection against photo-oxidation obtained with SiO2 @ Au nanoparticles in PPV could be accomplished with an extremely low concentration, corresponding to about 0.05% volume fraction. It is noticeable that such a low concentration of nanoshells yields a high level of protection against photo-oxidation. We can explain that this enhanced energy transfer could be related to the nature of polymer excitons and the fact that the SiO2 @ Au nanoparticles can exhibit greatly enhanced local field intensities and large effective cross-sections [8,16].
588
Y. Taik Lim et al. / Optical Materials 21 (2002) 585–589
Since solid metal nanoparticle resonance have excitation lifetimes of only a few picoseconds, the donor–acceptor interaction between the comparatively long-lived excitons of the PPV and nanoparticles will result in strong quenching of the triplet state of PPV to which the composite nanoparticles resonance have been tuned. The excitons in the conjugated polymer are highly mobile along the polymer backbone and can hop between chains readily, finding the lowest-energy region of the film. It is possible that the SiO2 @ Au nanoparticles dispersed in the polymer film will cause variations in the local energy environment of the triplet excitons, attracting them to the SiO2 @ Au nanoparticles, where the triplet exciton–SiO2 @ Au nanoparticles interaction can take place easily. The large effective cross-section of the nanoshells, which may be a larger than electromagnetic waves, should in effect reduce the apparent distance between the SiO2 @ Au nanoparticles in the film.
4. Conclusion
Fig. 3. Optical absorption (a) and PL (b) spectra of pristine PPV and PPV nanocomposites (doped 0.05 vol% gold-coated silica nanoparticles).
We have studied the use of SiO2 @ Au nanoparticles to impede the photo-oxidation of PPV films. By doping with nanoparticles which were specially designed to interact with triplet excitons of PPV, the rate of photo-oxidation in PPV nanocomposites was drastically reduced. This effect may have significant implications on the commercialization of conjugated polymer-based devices. The addition of SiO2 @ Au nanoparticles to conjugated polymers-based optoelectronic devices, along with careful processing techniques and device encapsulation, may result in improved lifetimes.
Acknowledgements
Fig. 4. Retarded photo-oxidation in PPV nanocomposite films doped with gold-coated silica nanoparticles. (nanocomposite-1 (0.05 vol%), nanocomposite-2 (0.1 vol%)).
The authors are thankful for the financial supports of the Center for Advanced Functional Polymers (CAFPoly) appointed by the Korea Science and Engineering Foundation. This work is also partially supported by the Brain Korea 21 Project of the Ministry of Education (MOE) of Korea.
Y. Taik Lim et al. / Optical Materials 21 (2002) 585–589
References [1] A. Kraft, A.C. Grimsdale, A.B. Holmes, Angew. Chem. Int. Ed. 37 (1998) 402. [2] M.A. Diaz-Garcia, H. Fumitomo, B.J. Schwartz, Synth. Met. 84 (1997) 455. [3] Y. Cao, G. Yu, A.J. Heeger, Adv. Mater. 10 (1998) 692. [4] B.H. Cumpston, K.F. Jensen, Synth. Met. 73 (1995) 195. [5] R.D. Scurlock, B.J. Wang, P.R. Ogilby, J.R. Sheats, R.L. Clough, J. Am. Chem. Soc. 117 (1995) 10194. [6] S.A. Carter, J.C. Scott, P.J. Brock, Appl. Phys. Lett. 71 (1997) 1145. [7] W.P. Chang, W.T. Whang, Polymer 37 (1996) 4229. [8] G.D. Hale, J.B. Jackson, O.E. Shmakova, T.R. Lee, N.J. Halas, Appl. Phys. Lett. 78 (2001) 1502.
589
[9] T.-W. Lee, J. Yoon, J.-J. Kim, O.O. Park, Adv. Mater. 13 (2001) 211. [10] T.-W. Lee, O.O. Park, J.-J. Kim, J.-M. Hong, Y.C. Kim, Chem. Mater. 13 (2001) 2217. [11] H.-C. Lee, T.-W. Lee, Y.T. Lim, O.O. Park, Appl. Clay Sci., submitted for publication. [12] Y.T. Lim, O.O. Park, J. Colloid Interf. Sci., submitted for publication. [13] J. Gmeiner, S. Karg, M. Meier, W. Riess, P. Strohriegel, M. Schwoerer, Acta Polym. 44 (1993) 201. [14] R. Osterbacka, M. Wohlgenannt, D. Chinn, Z.V. Vardeny, Phys. Rev. B 60 (1999) R11253. [15] M. Herold, J. Gmeiner, W. Riess, P. Strohriegel, Synth. Met. 76 (1996) 109. [16] G.D. Hale, Ph.D. Thesis, 2000.