- Email: [email protected]
The effect of C60 doping on the electroluminescent performance of organic light-emitting devices
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 642–645 www.elsevier.com/locate/jlumin
The effect of C60 doping on the electroluminescent performance of organic light-emitting devices Denghui Xu, Zhenbo Deng, Jing Xiao, Dong Guo, Jingang Hao, Yuanyuan Zhang, Yinhao Gao, Chunjun Liang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, P R China Available online 22 March 2006
Abstract Organic light-emitting devices (OLEDs) with the PVK hole transport layer were fabricated. The effect of C60 doping in the hole transport PVK layer on the performance of the devices was investigated by changing the C60 content from 0 to 3.0 wt%. The OLEDs had a structure of ITO\PEDOT:PSS\PVK:C60 (0, 0.5, 1.0, 2.0, 3.0 wt%)\AlQ\LiF\Al. The doping led to a higher conductivity in C60-doped PVK layer and the hole mobility of PVK was improved from 4.5 107 to 2.6 106 cm2/Vs with the doping concentration of C60 changing from 0 to 3.0 wt%. Moreover, the doping led to a high density of equilibrium charges carriers, which facilitated hole injection and transport. Doping of C60 in PVK resulted in efficient hole injection and low drive voltage at high luminance. r 2006 Elsevier B.V. All rights reserved. Keywords: Electroluminescence (EL); Mobility; Organic light-emitting devices (OLEDs)
1. Introduction The light-emitting properties of organic light emitting devices (OLEDs) are greatly dependent on the charge injection and transport layers as well as a light-emitting layer [1–3]. In particular, the driving voltage, luminance efficiency and stability of OLEDs are closely related to charge injection and transport in the devices. In optoelectronic devices based on inorganic semiconductors, controlled doping of the transport layers is a key technique for the realization of efficient devices. Basically, doping is important in these devices in two ways: first, doping is needed to achieve high ohmic conductivity to attain a low voltage drop over even thick transport layers, and second, thin space charge layers created by high doping levels close to the ohmic contacts lead to efficient carrier injection by tunneling, even for large contact barriers. There have been many attempts to improve the device performance of OLEDs by modifying the charge injection and transport layers [1]. At the interface of ITO anode with Corresponding author. Tel.: +86 10 51688675; fax: +86 10 51683933.
E-mail address: [email protected] (Z. Deng). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.248
organic materials, enhanced hole injection was desired to increase the internal quantum efficiency by balancing charge carriers in the active layer. C60 has been used in solar cells because photo induced charge transfer between C60 and conducting polymers leads to a photovoltaic effect [2]. It has been found that C60 can be used as an electron acceptor dopant in conducting polymers, and the enhancement of photoconductivity was observed in C60/conducting polymer system. However, the dissociation of the excitons in C60/polymers systems leads to weak electroluminescence in C60 containing devices. Therefore, C60 cannot be used as the light emitting materials in OLEDs. In this paper, we present an alternative way to help in improving the hole-inject ability of the device. The effect of C60 doping in the hole-transport layer on the device performance of the OLEDs was investigated by changing its content from 0 to 3.0 wt% and the bulk properties of C60-doped PVK were also analyzed. 2. Experimental PEDOT:PSS as a hole-injection layer was spin-coated onto the ITO-coated glass substrate and then baked in the
ARTICLE IN PRESS D. Xu et al. / Journal of Luminescence 122–123 (2007) 642–645
oven at 120 1C for 1 h. The pristine PVK and the composites blended with C60 were prepared by dissolving them in toluene. They then were directly spin-coated on the substrate. For the hole-only devices (ITO/PEDOT: PSS (65 nm)/PVK: C60 (x wt%) (80 nm)/Al (150 nm)), the aluminum cathode was deposited by thermal evaporation. For the heterojunction devices with the configuration of ITO/ PEDOT: PSS (50 nm)/PVK: C60 (x wt%) (40 nm)/AlQ (50 nm)/LiF (0.3 nm)/Al (150 nm), AlQ was selected as the electron transport/emitting layer. A very thin layer of LiF (0.3 nm) was used as the electron-injecting layer (EIL). Details of the device fabrication are described elsewhere. The AlQ, LiF and the Al cathode were sequentially deposited by conventional vacuum vapor deposition. A quartz-crystal oscillator thickness monitored the thickness and deposition rates of the organic layers and LiF layer. The thickness of the layers made by spin coating was measured by XP-2 Stylus Profiler. The current–voltage–luminescence characteristics of these devices were measured by a Keithley 234 source meter and a PR-650 spectrometer. All the measurements were carried out in the air at room temperature. 3. Results and discussion 3.1. Properties of hole-only devices Fig. 1 shows the forward bias J–V characteristics of the hole-only devices fabricated to understand the effects of C60 doping on the carrier transport and the energy level diagram of the device. The overall characteristics show three different regimes and each can be fitted with the power law IpVm, with m varying from 1 at low voltage through 2.570.5 at intermediate voltage to 8.570.5 at high voltage, respectively. Such behavior is characteristic of the bulk-limited carrier transport for the thick film devices with traps. The first evolution corresponds to the transition from ohmic conduction to space–charge-limited conduction (SCLC). With the increase in voltage, the behavior 2.2 eV
Current Density (mA/cm2)
1000
PEDOT :PSS
100
10
ITO 4.8 eV
Al 4.3 eV
0.0 wt% 0.5 wt% 1.0 wt% 2.0 wt% 3.0 wt%
4.5 eV 5.2 eV
PVK 5.8 eV
1
0.1
0.01 0.1
could be taken as an indication for trap charge limited conduction. The canonical technique for measuring charge carrier mobilities in disordered systems such as molecularly doped polymers, PMMA and conjugated polymers is the time-of-flight (TOF) experiment. However, due to the highly dispersive transient photocurrent traces, the determination of the charge-carrier mobilities is often very complicated and at low field it is particularly impossible. The effective hole carrier mobility (m) can be determined from the region with m ¼ 2 through the child equation m¼
8 d3 J SCL 2 , 9er e0 V
where er, e0, JSCL, d, V are the dielectric constant of the organic materials (er ¼ 3 for PVK), the permittivity in vacuum, the current density, the thickness of the organic materials and the applied voltage, respectively [3]. The mobility of the device without C60 was estimated to be about 4.5 107 cm2/Vs, and the mobility increased as the C60 concentration in PVK increased. The hole mobility of PVK film with the C60 concentration of 3.0 wt% reached up to 2.6 106 cm2/Vs, which was higher than that of pure PVK film by 6 times. The hole mobilities of PVK deduced from the TOF by several groups were between 107 and 106 cm2/Vs [4], which were in agreement with our results. In Fig. 1, it was clearly demonstrated that as the fraction of C60 increases, higher current density was observed compared to that without C60. It is well known that C60 has extended surface p orbitals and shows a band gap of only 1.9 eV [5]. As shown in the inset of Fig. 1, the lowest unoccupied molecular orbital (LUMO) of C60 is only 4.5 eV [6], which is low enough to accept electrons from common charge transport materials. The electron trapping character may contribute to the formation of a positively charged molecule. Further more, this character of C60 might increase the interchain hopping of positive polarons, which led to high-hole mobility in the C60-doped PVK film [7]. 3.2. OLEDs with C60-doped PVK as HTL
2.3 eV
C60
643
1
10
Voltage (V) Fig. 1. The energy diagram (the insert) and J–V characteristics of the hole only transport devices as different content of C60.
The devices showed the green emission of AlQ, which showed that the carriers’ recombination only occured in the emitting layer. The J–V and B–V characteristics of double-layer devices are depicted in Figs. 2 (a) and (b). Here the doping concentration of C60 was from 0.5 to 3.0 wt%. The performance of doping devices was shown comparable to that of undoped reference device. It was obvious that the doping strikingly decreased the driving voltage and the J–V and B–V characteristics were dependent on the doping ratio of C60. The devices with doped PVK generated a substantially larger current density than the undoped devices at the same forward driving voltage. It is shown in Fig. 2 (a) that the current density of the PVK doped with 3.0 wt% C60 was 94.1 mA/cm2 compared with 66.1 mA/cm2 in pure PVK at 10 V. There was about
ARTICLE IN PRESS D. Xu et al. / Journal of Luminescence 122–123 (2007) 642–645
644
11000
500
400
0.0 0.5 1.0 2.0 3.0
300
9000
wt% wt% wt% wt% wt%
EL Brightness (cd/m2)
Current Density (mA/cm2)
10000
200
0.0 wt% 0.5 wt% 1.0 wt% 2.0 wt% 3.0 wt%
8000 7000 6000
2.2 eV
5000
3.0 eV
4000
LiF/Al
3000 ITO
100
2000
5.2 eV
0
0
2
4
(a)
6 8 Voltage (V)
10
12
14
PEDOT: C60 PSS 4.5 eV
4.1 eV AlQ
4.8 eV
1000
0
2.3 eV
PVK 5.8 eV
0
50
5.7 eV
250 100 150 200 Current Density (mA/cm2)
300
Fig. 3. The energy diagram (the insert) and B–J characteristics of the double layer devices at different concentration of C60.
4000
EL Brightness (cd/m2)
3500 0.0 wt% 0.5 wt% 1.0 wt% 2.0 wt% 3.0 wt%
3000 2500 2000 1500 1000 500 0 3
(b)
4
5
6
7
8
9
10
11
Voltage (V)
Fig. 2. J–V (a) and B–V (b) characteristics of double-layer OLEDs at different molecular doping ratios.
40% improvement of current density of 3.0 wt%-doped devices. In general, the driving voltage of the device was dependent on the hole and electron mobility of the materials in the device, and high mobility led to low driving voltages. Fig. 3 shows the energy diagram and the B–J characteristics of double-layer devices at different concentration of C60. In terms of luminance, the luminance at a given current density was improved in both doped devices to a similar extent. This improvement corresponds to an increased current efficiency. Moreover, it can be seen in this figure, the efficiency of doped devices was higher than the undoped device. The improvement in current efficiency of the doped devices may be qualitatively explained by more balanced hole and electron injections. The balance of the hole and electron injection is one of the main points that affect the efficiency of the devices [1]. With an increase in current density, the efficiency of the device of 3.0 wt% was lower than the other doped devices. This might be caused by the imbalance of the carriers at a high voltage. The enhanced hole injecting ability of the PVK layer may cause the increase of the leakage of current and part of the
carriers quenched near the electrode. Thus, the efficiency will decrease. When discussing the influence of doping on the turn-on voltage, one has to consider two effects of doping: improved injection and reduction of bulk resistance losses. Our calculation shows that even low doping levels are enough to raise the conductivity. The current density of the device was determined mainly by the interfacial energy barrier and charge mobility of the organic material. A low interfacial energy barrier and high charge mobility usually give low driving voltage. With an increase in concentration, the distance between the C60 molecules decreases, leading to direct hopping which results in a further increase in current density. As a second effect of the doping on turnon voltage, one has to consider the reduction of the space–charge layers at the contact, leading to efficient injection due to tunneling. Further increase of current densities with doping indicated that higher doping levels further improved the carrier injection into the holetransporting layers. 4. Conclusion In summary, the effect of C60 doping on the device performance of OLEDs was investigated. The doping led to high conductivity of doped PVK layers and the hole mobility of PVK was improved. Moreover, the doping led to a high density of equilibrium charge carriers, which facilitated hole injection and transport. Doping of C60 in PVK resulted in efficient hole injection and low drive voltage at high luminance. Acknowledgments We gratefully acknowledge the financial support of National Natural Science Foundation of China under Contract no. 90201004 and Beijing Science Foundation under Contract no. H0304300204.
ARTICLE IN PRESS D. Xu et al. / Journal of Luminescence 122–123 (2007) 642–645
References [1] Z.B. Deng, X.M. Ding, S.T. Lee, W.A. Gambling, Appl. Phys. Lett. 74 (1999) 2227. [2] G. Priebe, B. Pietzak, R. Konenkamp, Appl. Phys. Lett. 71 (1997) 7160. [3] J.-Y. Kim, M. Kim, J.-H. Choi, Synth. Met. 139 (2003) 565.
645
[4] M.B. Khalifa, D. Vaufrey, A. Bouazizi, et al., Mater. Sci. Eng. C 21 (2002) 277. [5] J.Y. Lee, J.H. Kwon, Appl. Phys. Lett. 86 (2005) 063514. [6] K. Iizumi, K. Saiki, A. koma, Surf. Sci. 518 (2002) 126. [7] S.M. Silence, J.C. Scott, F. Hache, J. Opt. Soc. Am. B 10 (1993) 2307.
Journal of Luminescence 122–123 (2007) 642–645 www.elsevier.com/locate/jlumin
The effect of C60 doping on the electroluminescent performance of organic light-emitting devices Denghui Xu, Zhenbo Deng, Jing Xiao, Dong Guo, Jingang Hao, Yuanyuan Zhang, Yinhao Gao, Chunjun Liang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, P R China Available online 22 March 2006
Abstract Organic light-emitting devices (OLEDs) with the PVK hole transport layer were fabricated. The effect of C60 doping in the hole transport PVK layer on the performance of the devices was investigated by changing the C60 content from 0 to 3.0 wt%. The OLEDs had a structure of ITO\PEDOT:PSS\PVK:C60 (0, 0.5, 1.0, 2.0, 3.0 wt%)\AlQ\LiF\Al. The doping led to a higher conductivity in C60-doped PVK layer and the hole mobility of PVK was improved from 4.5 107 to 2.6 106 cm2/Vs with the doping concentration of C60 changing from 0 to 3.0 wt%. Moreover, the doping led to a high density of equilibrium charges carriers, which facilitated hole injection and transport. Doping of C60 in PVK resulted in efficient hole injection and low drive voltage at high luminance. r 2006 Elsevier B.V. All rights reserved. Keywords: Electroluminescence (EL); Mobility; Organic light-emitting devices (OLEDs)
1. Introduction The light-emitting properties of organic light emitting devices (OLEDs) are greatly dependent on the charge injection and transport layers as well as a light-emitting layer [1–3]. In particular, the driving voltage, luminance efficiency and stability of OLEDs are closely related to charge injection and transport in the devices. In optoelectronic devices based on inorganic semiconductors, controlled doping of the transport layers is a key technique for the realization of efficient devices. Basically, doping is important in these devices in two ways: first, doping is needed to achieve high ohmic conductivity to attain a low voltage drop over even thick transport layers, and second, thin space charge layers created by high doping levels close to the ohmic contacts lead to efficient carrier injection by tunneling, even for large contact barriers. There have been many attempts to improve the device performance of OLEDs by modifying the charge injection and transport layers [1]. At the interface of ITO anode with Corresponding author. Tel.: +86 10 51688675; fax: +86 10 51683933.
E-mail address: [email protected] (Z. Deng). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.248
organic materials, enhanced hole injection was desired to increase the internal quantum efficiency by balancing charge carriers in the active layer. C60 has been used in solar cells because photo induced charge transfer between C60 and conducting polymers leads to a photovoltaic effect [2]. It has been found that C60 can be used as an electron acceptor dopant in conducting polymers, and the enhancement of photoconductivity was observed in C60/conducting polymer system. However, the dissociation of the excitons in C60/polymers systems leads to weak electroluminescence in C60 containing devices. Therefore, C60 cannot be used as the light emitting materials in OLEDs. In this paper, we present an alternative way to help in improving the hole-inject ability of the device. The effect of C60 doping in the hole-transport layer on the device performance of the OLEDs was investigated by changing its content from 0 to 3.0 wt% and the bulk properties of C60-doped PVK were also analyzed. 2. Experimental PEDOT:PSS as a hole-injection layer was spin-coated onto the ITO-coated glass substrate and then baked in the
ARTICLE IN PRESS D. Xu et al. / Journal of Luminescence 122–123 (2007) 642–645
oven at 120 1C for 1 h. The pristine PVK and the composites blended with C60 were prepared by dissolving them in toluene. They then were directly spin-coated on the substrate. For the hole-only devices (ITO/PEDOT: PSS (65 nm)/PVK: C60 (x wt%) (80 nm)/Al (150 nm)), the aluminum cathode was deposited by thermal evaporation. For the heterojunction devices with the configuration of ITO/ PEDOT: PSS (50 nm)/PVK: C60 (x wt%) (40 nm)/AlQ (50 nm)/LiF (0.3 nm)/Al (150 nm), AlQ was selected as the electron transport/emitting layer. A very thin layer of LiF (0.3 nm) was used as the electron-injecting layer (EIL). Details of the device fabrication are described elsewhere. The AlQ, LiF and the Al cathode were sequentially deposited by conventional vacuum vapor deposition. A quartz-crystal oscillator thickness monitored the thickness and deposition rates of the organic layers and LiF layer. The thickness of the layers made by spin coating was measured by XP-2 Stylus Profiler. The current–voltage–luminescence characteristics of these devices were measured by a Keithley 234 source meter and a PR-650 spectrometer. All the measurements were carried out in the air at room temperature. 3. Results and discussion 3.1. Properties of hole-only devices Fig. 1 shows the forward bias J–V characteristics of the hole-only devices fabricated to understand the effects of C60 doping on the carrier transport and the energy level diagram of the device. The overall characteristics show three different regimes and each can be fitted with the power law IpVm, with m varying from 1 at low voltage through 2.570.5 at intermediate voltage to 8.570.5 at high voltage, respectively. Such behavior is characteristic of the bulk-limited carrier transport for the thick film devices with traps. The first evolution corresponds to the transition from ohmic conduction to space–charge-limited conduction (SCLC). With the increase in voltage, the behavior 2.2 eV
Current Density (mA/cm2)
1000
PEDOT :PSS
100
10
ITO 4.8 eV
Al 4.3 eV
0.0 wt% 0.5 wt% 1.0 wt% 2.0 wt% 3.0 wt%
4.5 eV 5.2 eV
PVK 5.8 eV
1
0.1
0.01 0.1
could be taken as an indication for trap charge limited conduction. The canonical technique for measuring charge carrier mobilities in disordered systems such as molecularly doped polymers, PMMA and conjugated polymers is the time-of-flight (TOF) experiment. However, due to the highly dispersive transient photocurrent traces, the determination of the charge-carrier mobilities is often very complicated and at low field it is particularly impossible. The effective hole carrier mobility (m) can be determined from the region with m ¼ 2 through the child equation m¼
8 d3 J SCL 2 , 9er e0 V
where er, e0, JSCL, d, V are the dielectric constant of the organic materials (er ¼ 3 for PVK), the permittivity in vacuum, the current density, the thickness of the organic materials and the applied voltage, respectively [3]. The mobility of the device without C60 was estimated to be about 4.5 107 cm2/Vs, and the mobility increased as the C60 concentration in PVK increased. The hole mobility of PVK film with the C60 concentration of 3.0 wt% reached up to 2.6 106 cm2/Vs, which was higher than that of pure PVK film by 6 times. The hole mobilities of PVK deduced from the TOF by several groups were between 107 and 106 cm2/Vs [4], which were in agreement with our results. In Fig. 1, it was clearly demonstrated that as the fraction of C60 increases, higher current density was observed compared to that without C60. It is well known that C60 has extended surface p orbitals and shows a band gap of only 1.9 eV [5]. As shown in the inset of Fig. 1, the lowest unoccupied molecular orbital (LUMO) of C60 is only 4.5 eV [6], which is low enough to accept electrons from common charge transport materials. The electron trapping character may contribute to the formation of a positively charged molecule. Further more, this character of C60 might increase the interchain hopping of positive polarons, which led to high-hole mobility in the C60-doped PVK film [7]. 3.2. OLEDs with C60-doped PVK as HTL
2.3 eV
C60
643
1
10
Voltage (V) Fig. 1. The energy diagram (the insert) and J–V characteristics of the hole only transport devices as different content of C60.
The devices showed the green emission of AlQ, which showed that the carriers’ recombination only occured in the emitting layer. The J–V and B–V characteristics of double-layer devices are depicted in Figs. 2 (a) and (b). Here the doping concentration of C60 was from 0.5 to 3.0 wt%. The performance of doping devices was shown comparable to that of undoped reference device. It was obvious that the doping strikingly decreased the driving voltage and the J–V and B–V characteristics were dependent on the doping ratio of C60. The devices with doped PVK generated a substantially larger current density than the undoped devices at the same forward driving voltage. It is shown in Fig. 2 (a) that the current density of the PVK doped with 3.0 wt% C60 was 94.1 mA/cm2 compared with 66.1 mA/cm2 in pure PVK at 10 V. There was about
ARTICLE IN PRESS D. Xu et al. / Journal of Luminescence 122–123 (2007) 642–645
644
11000
500
400
0.0 0.5 1.0 2.0 3.0
300
9000
wt% wt% wt% wt% wt%
EL Brightness (cd/m2)
Current Density (mA/cm2)
10000
200
0.0 wt% 0.5 wt% 1.0 wt% 2.0 wt% 3.0 wt%
8000 7000 6000
2.2 eV
5000
3.0 eV
4000
LiF/Al
3000 ITO
100
2000
5.2 eV
0
0
2
4
(a)
6 8 Voltage (V)
10
12
14
PEDOT: C60 PSS 4.5 eV
4.1 eV AlQ
4.8 eV
1000
0
2.3 eV
PVK 5.8 eV
0
50
5.7 eV
250 100 150 200 Current Density (mA/cm2)
300
Fig. 3. The energy diagram (the insert) and B–J characteristics of the double layer devices at different concentration of C60.
4000
EL Brightness (cd/m2)
3500 0.0 wt% 0.5 wt% 1.0 wt% 2.0 wt% 3.0 wt%
3000 2500 2000 1500 1000 500 0 3
(b)
4
5
6
7
8
9
10
11
Voltage (V)
Fig. 2. J–V (a) and B–V (b) characteristics of double-layer OLEDs at different molecular doping ratios.
40% improvement of current density of 3.0 wt%-doped devices. In general, the driving voltage of the device was dependent on the hole and electron mobility of the materials in the device, and high mobility led to low driving voltages. Fig. 3 shows the energy diagram and the B–J characteristics of double-layer devices at different concentration of C60. In terms of luminance, the luminance at a given current density was improved in both doped devices to a similar extent. This improvement corresponds to an increased current efficiency. Moreover, it can be seen in this figure, the efficiency of doped devices was higher than the undoped device. The improvement in current efficiency of the doped devices may be qualitatively explained by more balanced hole and electron injections. The balance of the hole and electron injection is one of the main points that affect the efficiency of the devices [1]. With an increase in current density, the efficiency of the device of 3.0 wt% was lower than the other doped devices. This might be caused by the imbalance of the carriers at a high voltage. The enhanced hole injecting ability of the PVK layer may cause the increase of the leakage of current and part of the
carriers quenched near the electrode. Thus, the efficiency will decrease. When discussing the influence of doping on the turn-on voltage, one has to consider two effects of doping: improved injection and reduction of bulk resistance losses. Our calculation shows that even low doping levels are enough to raise the conductivity. The current density of the device was determined mainly by the interfacial energy barrier and charge mobility of the organic material. A low interfacial energy barrier and high charge mobility usually give low driving voltage. With an increase in concentration, the distance between the C60 molecules decreases, leading to direct hopping which results in a further increase in current density. As a second effect of the doping on turnon voltage, one has to consider the reduction of the space–charge layers at the contact, leading to efficient injection due to tunneling. Further increase of current densities with doping indicated that higher doping levels further improved the carrier injection into the holetransporting layers. 4. Conclusion In summary, the effect of C60 doping on the device performance of OLEDs was investigated. The doping led to high conductivity of doped PVK layers and the hole mobility of PVK was improved. Moreover, the doping led to a high density of equilibrium charge carriers, which facilitated hole injection and transport. Doping of C60 in PVK resulted in efficient hole injection and low drive voltage at high luminance. Acknowledgments We gratefully acknowledge the financial support of National Natural Science Foundation of China under Contract no. 90201004 and Beijing Science Foundation under Contract no. H0304300204.
ARTICLE IN PRESS D. Xu et al. / Journal of Luminescence 122–123 (2007) 642–645
References [1] Z.B. Deng, X.M. Ding, S.T. Lee, W.A. Gambling, Appl. Phys. Lett. 74 (1999) 2227. [2] G. Priebe, B. Pietzak, R. Konenkamp, Appl. Phys. Lett. 71 (1997) 7160. [3] J.-Y. Kim, M. Kim, J.-H. Choi, Synth. Met. 139 (2003) 565.
645
[4] M.B. Khalifa, D. Vaufrey, A. Bouazizi, et al., Mater. Sci. Eng. C 21 (2002) 277. [5] J.Y. Lee, J.H. Kwon, Appl. Phys. Lett. 86 (2005) 063514. [6] K. Iizumi, K. Saiki, A. koma, Surf. Sci. 518 (2002) 126. [7] S.M. Silence, J.C. Scott, F. Hache, J. Opt. Soc. Am. B 10 (1993) 2307.