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Study of polymer blends on polymer light-emitting diodes
Materials Chemistry and Physics 106 (2007) 70–73
Study of polymer blends on polymer light-emitting diodes Sung-Nien Hsieh a , Tzu-Yin Kuo a , Po-Chun Hsu a , Ten-Chin Wen a,∗ , Tzung-Fang Guo b a
b
Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Received 30 September 2006; received in revised form 1 May 2007; accepted 6 May 2007
Abstract The present work reports on the fabrication of polymer light-emitting diodes by blending a small amount of poly(2-methoxy-5-(2 -ethylhexyloxy)1,4-phenylene vinylene) (MEH-PPV) with poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV). The optical spectra of the devices made of various ratios of the DB-PPV/MEH-PPV blends have shown that the inter-chain interactions of MEH-PPV chains were efficiently suppressed in the presence of DB-PPV as the host, resulting in the improvement of thermal stability and electroluminescence (EL) efficiency of the devices. The enhanced performance of the devices is attributed to the efficient F˝orster energy transfer and charge trapping on the polymer blends of the DB-PPV/MEH-PPV host–guest system. © 2007 Elsevier B.V. All rights reserved. Keywords: Polymer light-emitting diode; DB-PPV; MEH-PPV
1. Introduction Poly(2-methoxy-5-(2 -ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) is one of the most widely investigated luminescent polymers for polymer light-emitting diodes (PLEDs). Many studies on controlling the morphology or conformation of MEH-PPV by different methods to affect the optical and electrical properties have been reported [1–6]. These studies revealed that the aggregations of polymer chains play an important role because the aggregation quenching of the excited state is one of the main barriers to high luminescence quantum yield in the luminescent polymer [3,7]. The physical origin of aggregation quenching has been shown to be inter-chain interactions [3,8]. In addition, these aggregations are characterized by delocalization of the electronic wave function among two or three chains in both the ground and excited states [2]. This results in the broad emission spectra to reduce color purity. To reduce the formation of aggregations has been proposed by several techniques, such as using different solvent, film processing conditions and polymer chemical structure modification [2,4,5,9]. Another widely reported approach is to blend MEHPPV with active or inert polymer, which results in the increase of PL efficiency and the reduction of the full width at half maximum (FWHM) to improve the device performance [10–12]. ∗
Corresponding author. Fax: +886 6 2344496. E-mail address: [email protected] (T.-C. Wen).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.05.018
In this study, poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) is used as the host material and blended with a small amount of MEH-PPV. In the blend film, DB-PPV serves as the media isolating the MEH-PPV chains from each other to suppress the inter-chain interactions. In addition, the efficient energy transfer and charge trapping mechanism, which can enhance the EL emission for MEH-PPV, are also observed in this system. Therefore, the thermal stability and EL efficiency of the blend devices are significantly improved compared to the MEH-PPVbased device. 2. Experimental The chemical structures of the EL polymers used are shown in the inset of Fig. 1. DB-PPV and MEH-PPV were dissolved in toluene separately. Both solutions were then mixed in different weight ratios to form blend solutions. A thin layer of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS Bayer Corp. 4083) was spin-cast on pre-cleaned ITO/glass substrate as the hole transport layer. The MEH-PPV or blend solutions were then spun on the top of the PEDOT layer. Finally, a Ca layer capped with Al was thermally evaporated as cathode at a pressure of ∼10−6 Torr. The current density–brightness–voltage (J–L–V) measurements were carried out by the Keithley 2400 source measure units and the Keithley 2000 digital multimeter along with a silicon photodiode, calibrated by Minolta LS-100 luminous meter. UV–vis absorption spectra were recorded by a Shimadzu UV-2101 UV–vis spectrophotometer. PL spectra were measured with a Perkin-Elmer LS55 luminescence spectrometer. The excitation wavelength for DB-PPV and blend films was fixed at 450 nm. For MEH-PPV film, the excitation wavelength was set at 500 nm. The EL spectra were obtained using an Ocean Optics USB 2000 with an optical fiber. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molec-
S.-N. Hsieh et al. / Materials Chemistry and Physics 106 (2007) 70–73
Fig. 1. The UV–vis spectra of MEH-PPV film overlaid with the PL spectra from DB-PPV film. Inset: Chemical structures of MEH-PPV and DB-PPV.
ular orbital (LUMO) of MEH-PPV and DB-PPV were determined by cyclic voltametry and the onset of absorption.
3. Results and discussion The absorption spectrum of MEH-PPV effectively overlaps the PL emission spectrum of DB-PPV with a large portion as shown in Fig. 1. Such guest–host system is eligible for an efficient F˝orster energy transfer from the singlet-excited state of the host, DB-PPV, to the guest, MEH-PPV. Fig. 2 shows the normalized PL spectra of the pure MEHPPV, DB-PPV and their blends at different weight ratios. As can be seen, the PL spectra of MEH-PPV/DB-PPV blends show strong concentration dependence. As the DB-PPV contents are increased, the contribution of DB-PPV emission to the PL spectrum increased. However, the PL emission is still dominated by the emission from MEH-PPV even for the MEH-PPV/DBPPV = 1:200 blend film. This result, as shown in Fig. 1, supports the proposal of the effective F˝orster energy transfer from DBPPV to MEH-PPV. The emission spectrum of pure MEH-PPV is characterized by two main peaks at 590 and 625 nm. With the increase in the ratio of DB-PPV in blend film, the peak at
Fig. 2. Normalized PL spectra of films from pure MEH-PPV, DB-PPV, and their blends at different ratios.
71
Fig. 3. Normalized EL spectra of the pure MEH-PPV and MEH-PPV/DB-PPV blend devices annealed at different temperatures.
590 nm is blue-shifted and the intensity of peak at 625 nm is reduced. The peak at 570 nm was assigned to the unaggregated single chain exciton emission of MEH-PPV [4,10]. Therefore, the blue-shift from peak emission of 590 nm for pure MEH-PPV film to around 570 nm for the blend film indicates that more extended and less coil MEH-PPV chains could be obtained by increasing DB-PPV contents. The reddish peak around 625 nm is related to emission from inter-chain species, such as excimers [4]. The reduced peak intensity at 625 nm indicates that the interchain species of MEH-PPV are suppressed upon blending with DB-PPV. The EL spectra of MEH-PPV/DB-PPV blend films with different ratios presented in Fig. 3 indicate that the emission peak is dominated by MEH-PPV and the inter-chain interactions are reduced with increase in the DB-PPV contents. This is found to be similar to the PL spectra. But the ratio of the emission intensity of the DB-PPV to MEH-PPV is different in the PL and EL spectrum. This is attributed to the difference between the optical and electrical excitation. In the PL process, MEH-PPV is only the acceptors of the F˝orster mechanism. In the EL process, it not only plays a role of the acceptor of the F˝orster mechanism but also the carrier-trapping center. Fig. 4 shows the energy diagram of DB-PPV and MEH-PPV, respectively. The HOMO and LUMO of the MEH-PPV full within the band-gap of DBPPV, indicate that the holes and electrons are readily trapped
Fig. 4. The energy diagram of DB-PPV and MEH-PPV.
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S.-N. Hsieh et al. / Materials Chemistry and Physics 106 (2007) 70–73
Fig. 5. Current density–voltage curves of the pure DB-PPV and MEH-PPV/DBPPV blend devices.
by MEH-PPV. Since the trapping mechanism can effectively promote the recombination of both carriers on MEH-PPV, DBPPV host shows less contribution in the EL spectrum than PL spectrum. The charge trapping in MEH-PPV is also supported by J–V behavior as depicted in Fig. 5. The J–V characteristics of the pure DB-PPV and MEH-PPV/DB-PPV blend devices in Fig. 5 reveal that the driving voltage increased with the increase in MEH-PPV ratio. It can be explained that MEH-PPV could trap both holes and electrons in DB-PPV and slow down their transport. Thus, the apparent resistance increases with increase of MEH-PPV in DB-PPV. The EL spectra of devices annealed at different temperatures are also displayed in Fig. 3. Although, the EL spectra for MEH-PPV-based devices are highly sensitive to the annealing temperature, the dramatic changes are not observed in the blend devices. For the MEH-PPV film annealed at 150 ◦ C, the main peak appears at ∼635 nm and the emitted color is turned into reddish. Because the MEH-PPV chains annealed beyond the glass transition temperature (Tg ) (Tg of MEH-PPV ∼75 ◦ C) [11] start to relax and tend to form aggregations, it would significantly increase the inter-chain interactions [13]. In the blend films, the PL results indicate that the MEH-PPV can be effectively isolated by DB-PPV. Meanwhile, DB-PPV is less sensitive to the annealing conditions than MEH-PPV because the Tg of DBPPV (∼150 ◦ C) [14] is much higher than that of MEH-PPV. Therefore, the relaxation and aggregation of the MEH-PPV chains during the higher annealing temperature can be inhibited in the presence of DB-PPV host. If the device is heated by Joule heating or external heating, the blend film also can prevent the color change of MEH-PPV to improve the thermal stability. Fig. 6 shows the luminescence–current density–EL efficiency curves. Notably, the EL efficiency and luminescence can be improved by blending a small amount of MEH-PPV with DBPPV. At a current density of 100 mA cm−2 , the efficiency is enhanced from about 1.0 cd A−1 for the pure MEH-PPV device to 1.4 and 1.6 cd A−1 for the MEH-PPV/DB-PPV = 1:100 and 1:200 blend devices. The improved performance occurs due to three possible reasons. First, an efficient F˝orster energy transfer from DB-PPV to MEH-PPV in the blend system. Second,
Fig. 6. EL efficiency–current density–luminescence curves of the pure MEHPPV and MEH-PPV/DB-PPV blend devices.
the holes and electrons in the DB-PPV host would be trapped in MEH-PPV to increase the recombination probability of both carriers. Third, the PL efficiency of MEH-PPV in the blend system is enhanced to improve device performance because one of the origins for the low PL efficiency in MEH-PPV is inter-chain interactions [3,8]. 4. Conclusions In summary, the EL efficiency and thermal stability of PLED are improved by blending MEH-PPV with DB-PPV. These improvements are attributed to the isolation of MEH-PPV chains in the presence of DB-PPV, significantly suppressing the interchain interactions. Besides, such guest–host system can satisfy the efficient F˝orster energy transfer and charge trapping in MEHPPV, which benefits the emission of MEH-PPV. Therefore, the device based on blend film with a small amount of MEH-PPV demonstrated the better device performance than MEH-PPVbased device. Acknowledgements This work is partially funded by National Science Council in Taiwan (NSC 94-2214-E-006-010). The authors gratefully thank Dr. Ruei-Tang Chen from Eternal Chemical Co., Ltd. for providing DB-PPV polymer. References [1] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Galvin, T.M. Miller, Phys. Rev. Lett. 73 (1994) 744. [2] T.-Q. Nguyen, V. Doan, B.J. Schwartz, J. Chem. Phys. 110 (1999) 4068. [3] R. Jakubiak, C.J. Collison, W.C. Wan, L.J. Rothberg, B.R. Hsieh, J. Phys. Chem. A 103 (1999) 2394. [4] Y. Shi, J. Liu, Y. Yang, J. Appl. Phys. 87 (2000) 4254. [5] J. Liu, Y. Shi, L. Ma, Y. Yang, J. Appl. Phys. 88 (2000) 605. [6] T.-Q. Nguyen, I.B. Martini, J. Liu, B.J. Schwartz, J. Phys. Chem. B 104 (2000) 237. [7] R. Chang, J.H. Hsu, W.S. Fann, J. Yu, S.H. Lin, Y.Z. Lee, S.A. Chen, Chem. Phys. Lett. 317 (2000) 153. [8] M. Yan, L.J. Rothberg, E.W. Kwock, T.M. Miller, Phys. Rev. Lett. 75 (1995) 1992.
S.-N. Hsieh et al. / Materials Chemistry and Physics 106 (2007) 70–73 [9] B.R. Hsieh, Y. Yu, E.W. Forsythe, G.M. Schaaf, W.A. Feld, J. Am. Chem. Soc. 120 (1998) 231. [10] J. Liu, Y. Shi, Y. Yang, Appl. Phys. Lett. 79 (2001) 578. [11] G. He, J. Liu, Y. Li, Y. Yang, Appl. Phys. Lett. 80 (2002) 1891.
73
[12] Y. Zhang, J. Peng, Y. Mo, Y. Cao, Appl. Phys. Lett. 85 (2004) 5170. [13] J. Liu, T.F. Guo, Y. Yang, J. Appl. Phys. 91 (2001) 1595. [14] M.L. Tu, Y.K. Su, S.J. Chang, T.H. Fang, W.H. Chen, H.L. Yang, Jpn. J. Appl. Phys. 44 (2005) 2787.
Study of polymer blends on polymer light-emitting diodes Sung-Nien Hsieh a , Tzu-Yin Kuo a , Po-Chun Hsu a , Ten-Chin Wen a,∗ , Tzung-Fang Guo b a
b
Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Received 30 September 2006; received in revised form 1 May 2007; accepted 6 May 2007
Abstract The present work reports on the fabrication of polymer light-emitting diodes by blending a small amount of poly(2-methoxy-5-(2 -ethylhexyloxy)1,4-phenylene vinylene) (MEH-PPV) with poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV). The optical spectra of the devices made of various ratios of the DB-PPV/MEH-PPV blends have shown that the inter-chain interactions of MEH-PPV chains were efficiently suppressed in the presence of DB-PPV as the host, resulting in the improvement of thermal stability and electroluminescence (EL) efficiency of the devices. The enhanced performance of the devices is attributed to the efficient F˝orster energy transfer and charge trapping on the polymer blends of the DB-PPV/MEH-PPV host–guest system. © 2007 Elsevier B.V. All rights reserved. Keywords: Polymer light-emitting diode; DB-PPV; MEH-PPV
1. Introduction Poly(2-methoxy-5-(2 -ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) is one of the most widely investigated luminescent polymers for polymer light-emitting diodes (PLEDs). Many studies on controlling the morphology or conformation of MEH-PPV by different methods to affect the optical and electrical properties have been reported [1–6]. These studies revealed that the aggregations of polymer chains play an important role because the aggregation quenching of the excited state is one of the main barriers to high luminescence quantum yield in the luminescent polymer [3,7]. The physical origin of aggregation quenching has been shown to be inter-chain interactions [3,8]. In addition, these aggregations are characterized by delocalization of the electronic wave function among two or three chains in both the ground and excited states [2]. This results in the broad emission spectra to reduce color purity. To reduce the formation of aggregations has been proposed by several techniques, such as using different solvent, film processing conditions and polymer chemical structure modification [2,4,5,9]. Another widely reported approach is to blend MEHPPV with active or inert polymer, which results in the increase of PL efficiency and the reduction of the full width at half maximum (FWHM) to improve the device performance [10–12]. ∗
Corresponding author. Fax: +886 6 2344496. E-mail address: [email protected] (T.-C. Wen).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.05.018
In this study, poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) is used as the host material and blended with a small amount of MEH-PPV. In the blend film, DB-PPV serves as the media isolating the MEH-PPV chains from each other to suppress the inter-chain interactions. In addition, the efficient energy transfer and charge trapping mechanism, which can enhance the EL emission for MEH-PPV, are also observed in this system. Therefore, the thermal stability and EL efficiency of the blend devices are significantly improved compared to the MEH-PPVbased device. 2. Experimental The chemical structures of the EL polymers used are shown in the inset of Fig. 1. DB-PPV and MEH-PPV were dissolved in toluene separately. Both solutions were then mixed in different weight ratios to form blend solutions. A thin layer of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS Bayer Corp. 4083) was spin-cast on pre-cleaned ITO/glass substrate as the hole transport layer. The MEH-PPV or blend solutions were then spun on the top of the PEDOT layer. Finally, a Ca layer capped with Al was thermally evaporated as cathode at a pressure of ∼10−6 Torr. The current density–brightness–voltage (J–L–V) measurements were carried out by the Keithley 2400 source measure units and the Keithley 2000 digital multimeter along with a silicon photodiode, calibrated by Minolta LS-100 luminous meter. UV–vis absorption spectra were recorded by a Shimadzu UV-2101 UV–vis spectrophotometer. PL spectra were measured with a Perkin-Elmer LS55 luminescence spectrometer. The excitation wavelength for DB-PPV and blend films was fixed at 450 nm. For MEH-PPV film, the excitation wavelength was set at 500 nm. The EL spectra were obtained using an Ocean Optics USB 2000 with an optical fiber. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molec-
S.-N. Hsieh et al. / Materials Chemistry and Physics 106 (2007) 70–73
Fig. 1. The UV–vis spectra of MEH-PPV film overlaid with the PL spectra from DB-PPV film. Inset: Chemical structures of MEH-PPV and DB-PPV.
ular orbital (LUMO) of MEH-PPV and DB-PPV were determined by cyclic voltametry and the onset of absorption.
3. Results and discussion The absorption spectrum of MEH-PPV effectively overlaps the PL emission spectrum of DB-PPV with a large portion as shown in Fig. 1. Such guest–host system is eligible for an efficient F˝orster energy transfer from the singlet-excited state of the host, DB-PPV, to the guest, MEH-PPV. Fig. 2 shows the normalized PL spectra of the pure MEHPPV, DB-PPV and their blends at different weight ratios. As can be seen, the PL spectra of MEH-PPV/DB-PPV blends show strong concentration dependence. As the DB-PPV contents are increased, the contribution of DB-PPV emission to the PL spectrum increased. However, the PL emission is still dominated by the emission from MEH-PPV even for the MEH-PPV/DBPPV = 1:200 blend film. This result, as shown in Fig. 1, supports the proposal of the effective F˝orster energy transfer from DBPPV to MEH-PPV. The emission spectrum of pure MEH-PPV is characterized by two main peaks at 590 and 625 nm. With the increase in the ratio of DB-PPV in blend film, the peak at
Fig. 2. Normalized PL spectra of films from pure MEH-PPV, DB-PPV, and their blends at different ratios.
71
Fig. 3. Normalized EL spectra of the pure MEH-PPV and MEH-PPV/DB-PPV blend devices annealed at different temperatures.
590 nm is blue-shifted and the intensity of peak at 625 nm is reduced. The peak at 570 nm was assigned to the unaggregated single chain exciton emission of MEH-PPV [4,10]. Therefore, the blue-shift from peak emission of 590 nm for pure MEH-PPV film to around 570 nm for the blend film indicates that more extended and less coil MEH-PPV chains could be obtained by increasing DB-PPV contents. The reddish peak around 625 nm is related to emission from inter-chain species, such as excimers [4]. The reduced peak intensity at 625 nm indicates that the interchain species of MEH-PPV are suppressed upon blending with DB-PPV. The EL spectra of MEH-PPV/DB-PPV blend films with different ratios presented in Fig. 3 indicate that the emission peak is dominated by MEH-PPV and the inter-chain interactions are reduced with increase in the DB-PPV contents. This is found to be similar to the PL spectra. But the ratio of the emission intensity of the DB-PPV to MEH-PPV is different in the PL and EL spectrum. This is attributed to the difference between the optical and electrical excitation. In the PL process, MEH-PPV is only the acceptors of the F˝orster mechanism. In the EL process, it not only plays a role of the acceptor of the F˝orster mechanism but also the carrier-trapping center. Fig. 4 shows the energy diagram of DB-PPV and MEH-PPV, respectively. The HOMO and LUMO of the MEH-PPV full within the band-gap of DBPPV, indicate that the holes and electrons are readily trapped
Fig. 4. The energy diagram of DB-PPV and MEH-PPV.
72
S.-N. Hsieh et al. / Materials Chemistry and Physics 106 (2007) 70–73
Fig. 5. Current density–voltage curves of the pure DB-PPV and MEH-PPV/DBPPV blend devices.
by MEH-PPV. Since the trapping mechanism can effectively promote the recombination of both carriers on MEH-PPV, DBPPV host shows less contribution in the EL spectrum than PL spectrum. The charge trapping in MEH-PPV is also supported by J–V behavior as depicted in Fig. 5. The J–V characteristics of the pure DB-PPV and MEH-PPV/DB-PPV blend devices in Fig. 5 reveal that the driving voltage increased with the increase in MEH-PPV ratio. It can be explained that MEH-PPV could trap both holes and electrons in DB-PPV and slow down their transport. Thus, the apparent resistance increases with increase of MEH-PPV in DB-PPV. The EL spectra of devices annealed at different temperatures are also displayed in Fig. 3. Although, the EL spectra for MEH-PPV-based devices are highly sensitive to the annealing temperature, the dramatic changes are not observed in the blend devices. For the MEH-PPV film annealed at 150 ◦ C, the main peak appears at ∼635 nm and the emitted color is turned into reddish. Because the MEH-PPV chains annealed beyond the glass transition temperature (Tg ) (Tg of MEH-PPV ∼75 ◦ C) [11] start to relax and tend to form aggregations, it would significantly increase the inter-chain interactions [13]. In the blend films, the PL results indicate that the MEH-PPV can be effectively isolated by DB-PPV. Meanwhile, DB-PPV is less sensitive to the annealing conditions than MEH-PPV because the Tg of DBPPV (∼150 ◦ C) [14] is much higher than that of MEH-PPV. Therefore, the relaxation and aggregation of the MEH-PPV chains during the higher annealing temperature can be inhibited in the presence of DB-PPV host. If the device is heated by Joule heating or external heating, the blend film also can prevent the color change of MEH-PPV to improve the thermal stability. Fig. 6 shows the luminescence–current density–EL efficiency curves. Notably, the EL efficiency and luminescence can be improved by blending a small amount of MEH-PPV with DBPPV. At a current density of 100 mA cm−2 , the efficiency is enhanced from about 1.0 cd A−1 for the pure MEH-PPV device to 1.4 and 1.6 cd A−1 for the MEH-PPV/DB-PPV = 1:100 and 1:200 blend devices. The improved performance occurs due to three possible reasons. First, an efficient F˝orster energy transfer from DB-PPV to MEH-PPV in the blend system. Second,
Fig. 6. EL efficiency–current density–luminescence curves of the pure MEHPPV and MEH-PPV/DB-PPV blend devices.
the holes and electrons in the DB-PPV host would be trapped in MEH-PPV to increase the recombination probability of both carriers. Third, the PL efficiency of MEH-PPV in the blend system is enhanced to improve device performance because one of the origins for the low PL efficiency in MEH-PPV is inter-chain interactions [3,8]. 4. Conclusions In summary, the EL efficiency and thermal stability of PLED are improved by blending MEH-PPV with DB-PPV. These improvements are attributed to the isolation of MEH-PPV chains in the presence of DB-PPV, significantly suppressing the interchain interactions. Besides, such guest–host system can satisfy the efficient F˝orster energy transfer and charge trapping in MEHPPV, which benefits the emission of MEH-PPV. Therefore, the device based on blend film with a small amount of MEH-PPV demonstrated the better device performance than MEH-PPVbased device. Acknowledgements This work is partially funded by National Science Council in Taiwan (NSC 94-2214-E-006-010). The authors gratefully thank Dr. Ruei-Tang Chen from Eternal Chemical Co., Ltd. for providing DB-PPV polymer. References [1] M. Yan, L.J. Rothberg, F. Papadimitrakopoulos, M.E. Galvin, T.M. Miller, Phys. Rev. Lett. 73 (1994) 744. [2] T.-Q. Nguyen, V. Doan, B.J. Schwartz, J. Chem. Phys. 110 (1999) 4068. [3] R. Jakubiak, C.J. Collison, W.C. Wan, L.J. Rothberg, B.R. Hsieh, J. Phys. Chem. A 103 (1999) 2394. [4] Y. Shi, J. Liu, Y. Yang, J. Appl. Phys. 87 (2000) 4254. [5] J. Liu, Y. Shi, L. Ma, Y. Yang, J. Appl. Phys. 88 (2000) 605. [6] T.-Q. Nguyen, I.B. Martini, J. Liu, B.J. Schwartz, J. Phys. Chem. B 104 (2000) 237. [7] R. Chang, J.H. Hsu, W.S. Fann, J. Yu, S.H. Lin, Y.Z. Lee, S.A. Chen, Chem. Phys. Lett. 317 (2000) 153. [8] M. Yan, L.J. Rothberg, E.W. Kwock, T.M. Miller, Phys. Rev. Lett. 75 (1995) 1992.
S.-N. Hsieh et al. / Materials Chemistry and Physics 106 (2007) 70–73 [9] B.R. Hsieh, Y. Yu, E.W. Forsythe, G.M. Schaaf, W.A. Feld, J. Am. Chem. Soc. 120 (1998) 231. [10] J. Liu, Y. Shi, Y. Yang, Appl. Phys. Lett. 79 (2001) 578. [11] G. He, J. Liu, Y. Li, Y. Yang, Appl. Phys. Lett. 80 (2002) 1891.
73
[12] Y. Zhang, J. Peng, Y. Mo, Y. Cao, Appl. Phys. Lett. 85 (2004) 5170. [13] J. Liu, T.F. Guo, Y. Yang, J. Appl. Phys. 91 (2001) 1595. [14] M.L. Tu, Y.K. Su, S.J. Chang, T.H. Fang, W.H. Chen, H.L. Yang, Jpn. J. Appl. Phys. 44 (2005) 2787.