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Investigation of the surface potential formed in Alq3 films on metal surface by Kelvin probe and nonlinear optical measurement
Current Applied Physics 6 (2006) 877–881 www.elsevier.com/locate/cap www.kps.or.kr
Investigation of the surface potential formed in Alq3 films on metal surface by Kelvin probe and nonlinear optical measurement q Takaaki Manaka *, Kei Yoshizaki, Mitsumasa Iwamoto Department of Physical Electronics, Tokyo Institute of Technology, O-okayama 2-12-1-S3-33, Meguro-ku, Tokyo 152-8552, Japan Received 20 December 2003 Available online 4 January 2006
Abstract Polar orientation of Alq3 (Tris(8-quinolinolato)aluminum) evaporated films on metal electrode was investigated by Kelvin probe method and optical second harmonic generation (SHG) measurement. For as-deposited films under the dark condition, a large potential formed across the films was confirmed and it decreased drastically by light irradiation with a wavelength of 400 nm. From a viewpoint of the interfacial electrostatic phenomena, there are two contributions to the surface potential, i.e. space charge effect and the alignment of dipoles. Before irradiation, alignment dipoles are the main contributor to the surface potential, whereas excess charges contribute mainly to the surface potential after irradiation. According to the SHG measurement, decrease of the signal from Alq3 films was observed after light irradiation. It is also found that using the fundamental light with a wavelength of 900 nm, SH intensity decreases steeply during the SH measurement in the absence of 400 nm irradiation possibly due to the two-photon excitation. 2005 Published by Elsevier B.V. PACS: 42.65.K; 73.40.C; 77.22 Keywords: Surface potential; Alq3; Second harmonic generation
1. Introduction Electrostatic phenomena occurring at the interface between metal/organic and organic/organic materials are fundamentally interesting to the fields of organic electronics and photonics. For the practical application in the nano-scale molecular devices, it is also essential to gain information on electronic structure of the molecules at the interface [1]. For ideal inorganic semiconductor systems, band diagram at the interface can be q
Original version presented at the International Discussion and Conference on Nano Interface Controller Electronic Devices (IDCNICE 2003), Tokyo, Japan, 17–20 December 2003. * Corresponding author. E-mail address: [email protected] (T. Manaka). 1567-1739/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.cap.2005.06.004
understood by the Mott-Schottky model, where the concept of Fermi-level alignment is important [2]. On the other hand, the situation becomes more complex for the metal/organic contact. The interfacial electric double layer is inevitably formed due to many possible origins, e.g., chemical bond formation, image charge effect and permanent dipoles. The band bending due to the charge injection and the dipole alignment also play an important role to the electrostatic phenomena occurring at metal/organic interface [3]. ‘‘Fermi-level alignment’’ is one of the most important and fundamental issues concerning organic materials electronics. Since organic molecule generally forms van der Waals solid in which intermolecular interaction is weak, it is hard to establish thermal equilibrium over the whole region of the organic film [10]. Thus the study
878
T. Manaka et al. / Current Applied Physics 6 (2006) 877–881
of electrostatic interfacial phenomena occurring at organic film/metal interfaces is of specially importance to the fields of organic electronics. From a view point of nano-interfacial electrical phenomena, we have investigated the interfacial states in organic thin films such as polyimide (PI) and phthalocyanine (Pc) on metal surface by the surface potential measurement [4–6]. According to the result, it was revealed that the excess electrons are injected from metal electrode to insulating PI Langmuir–Brogett (LB) films within the region of several nanometers from the metal-film interface. In the same way, for the semiconducting Pc films, the injection of excess electrons are observed within the region of 20 nm. In comparison with inorganic materials, the presence of permanent dipole is one of the most remarkable feature for the organic materials, and the alignment of dipoles also plays an important role to the surface potential. Thus it should be taken into account at least two contributions to the surface potential, i.e. space charge effect and the alignment of dipoles, to discuss the surface potential in polar organic materials. Recently, using Alq3 (Tris(8-quinolinolato)aluminum) molecule, the formation of the vacuum evaporated polar films was confirmed by the surface potential measurement [7,8], though such polar molecules were considered to form a film with centrosymmetric crystal structure by the deposition. Thus Alq3 molecule will be an interesting target for investigating two contributions mentioned above. In our previous paper, we preliminary reported that the space charge effect and alignment dipoles should be considered to understand the surface potential of Alq3 on metal electrodes. In this paper, we focused on the polar orientation of the Alq3 evaporated films on metal electrode, and investigated it by optical second harmonic generation (SHG) method. And besides an ordinary one-photon process, the disappearance of the polar orientation by a two-photon excitation process was examined.
N O N
Al
O
O N
Fig. 1. Molecular structure of the Alq3.
Alq3 was purchased from Wako Pure Chemical Co. Ltd., and was used without further purification. The samples used in the experiments were vacuum evaporated Alq3 films deposited on glass substrate coated with metal and bare glass substrate. During evaporation, process pressure was kept less than 2 · 106 Torr and deposition rate monitored by quartz crystal microbalances (QCM) was controlled to be about 1 nm/min. Since temperature of the substrate was not controlled during deposition, vacuum evaporated Alq3 molecules formed randomly oriented amorphous film. All the evaporation process were performed in the dark condition to avoid the effect of photoillumination. 2.2. Surface potential measurement Kelvin probe method was used to measure the surface potential. Since the details of this method were described in our previous paper [6], the principle of operation is briefly introduced here. For the Kelvin probe methods, capacitance is formed between top probe and metal electrode, half of which is covered with organic film. Assuming that N molecules are placed below the top electrode, charge Qp induced on a top electrode by these molecules is expressed as 1 N l hcos hi; lðtÞ
ð1Þ
2. Experiment
Qp ðtÞ ¼
2.1. Sample preparation
where l and h are the dipole moment and tilt angle of the molecule from the surface normal. h i represents the thermodynamic average of molecules. Here l(t) represents the distance between top and bottom electrode (1 mm), and the top electrode is vibrated with a frequency of 120 Hz during the surface potential measurement. The charge Q induced on the top electrode is sum of the induced charge Qp and CVc, where Vc is the compensation voltage applied to the top electrode and C is the capacitance of air gap. To determine the surface potential of the film, Vc is chosen to satisfy Q = 0 under the up and down vibration of the top electrode with respect to the bare electrode. Thus the surface potential of the organic monolayer can be expressed as [6]
The interest in organic electrical materials for use in organic light-emitting diodes (OLEDs) started with the first report of green electroluminescence from Alq3, by Tang and VanSlyke [9]. In this device, Alq3 is used as the emitting layer and the electron transporting layer of OLEDs. It is known that Alq3 has good electroand photo-luminescence yield. Thus, Alq3 has become the prototype of a whole class of electroluminescent organic compounds in use currently in OLEDs. Fig. 1 shows the molecular structure of the Alq3 in which three hydroxyquinoline ligand groups are attached to the central aluminum atom.
T. Manaka et al. / Current Applied Physics 6 (2006) 877–881
N lhcos hi ; r 0 B
r is the relative dielectric constant of film, 0 is the dielectric constant of a vacuum and B represents the area of top electrode. In this way, molecular alignment of Alq3 molecules can be estimated by the surface potential measurement. 2.3. SHG measurement Fig. 2 shows the experimental configuration for the SHG measurement. For the SHG measurement, s- or p-polarized fundamental light was focused on the sample using a convex lens (f = 100 mm) with an incident angle of 45 after passing through an SH-cut filter to eliminate the SH light from various optical components. The p-polarized SH light generated from the sample was filtered by fundamental cut filter to remove the fundamental light, and was detected by a photomultiplier tube (Hamamatsu photonics: R928) after passing through a monochromator (NIKON: G-250). Obtained signals were averaged by the Boxcar integrator (Stanford Research: SR-250) and processed by the personal computer. The fundamental light was obtained using an optical parametric oscillator (OPO: Continuum Surelite OPO) pumped by the third-harmonic light of a Qswitched Nd-YAG laser (Continuum: Surelite II-10).
3. Results and discussion Fig. 3 shows the thickness dependence of the surface potential observed in Alq3/Al structure. These samples were prepared by the vacuum deposition under dark condition and measurements were also performed in the dark circumstance. As shown in the figure, the surface potential increases steeply and linearly with the film
OPO
π/4 retarder
15
ð2Þ
YAG laser + 3ω generator
sample
monochrometor
optical fiber Xe lump + IF filter
Fig. 2. Experimental setup for the SHG measurement.
PMT
10
before irradiation Al/Alq3
5
after irradiation Al/Alq3 0 0
100
200
thickness [nm] Fig. 3. Thickness dependence of the surface potential observed in Alq3/Al structure.
thickness. Surprisingly, the magnitude of the surface potential is quite large and reaches more than 10 V in the manner same as that reported previously [7,8]. The establishment of such large voltage across Alq3 films cannot be explained by only assuming the electron transfer at the metal–Alq3 interface. It is supposed that the orientational alignment of dipole moment of Alq3 makes a significant contribution. Taking into account both contributions, the appropriate expression for the surface potential Vs built across the film is written as Z d 1 1 Vs ¼ xqðxÞ dx þ P 0 d. ð3Þ r 0 0 r 0 Here, r is the dielectric constant of film, d is film thickness and P0 is the polarization due to the all dipolar molecules. P0 is proportional to the orientational order parameter S 1 ¼ hcos hi, where h is an orientational angle from the normal direction to the substrate. In the first term of Eq. (3), x represents the distance of electrons displaced from the metal–film interface. Assuming the dipole contribution is dominant in the potential (it is reasonable situation before photoirradiation), equation for the surface potential can be rewrite as Vs ¼
fundamental cut filter
SH cut filter
surface potential [V]
V s ¼ ðV c Þ ¼
879
nlS 1 d ; r 0
ð4Þ
where n is the molecular density (number of molecules per volume), l and S1 are the dipole moment and the orientational order parameter, respectively. Taking into account the magnitude of the dipole moment of Alq3 molecules (meridional form, 4.1 D), S1 can be calculated as 0.07. Assuming that tilt angle for all molecule is identical, namely, orientational distribution function of the molecule is assumed to be given by d-function, it is found that molecules are almost lying to the surface. However, this is not a realistic picture. It is reasonable to image that the most of deposited molecules are
880
T. Manaka et al. / Current Applied Physics 6 (2006) 877–881
randomly oriented and the symmetry of deposited film was slightly broken along the z direction. The formation of polar structure of the film can be assigned by the SHG measurement. If the system has an inversion center, the generation of SH signal is not expected under the electric dipole approximation. Therefore, the SHG technique becomes a powerful tool for investigating the orientation of the molecule in the polar structure. Actually, we could get the strong SH signal from the Alq3 evaporated films under the off resonant condition.
400 nm irradiated 6
SH intensity (arb. unit)
5
4
3
2
1
Disappearance of the surface potential by the photoirradiation was also indicated in Fig. 3. After irradiating 400 nm light for a few seconds, potential drastically decreased. At the present stage, detailed mechanism of this phenomena is unclear. Dipole disappearance efficiency shows a clear wavelength dependence. No potential change was observed for the irradiation of 600 nm light where absorption coefficient is negligible. SHG measurement also shows the disappearance of the polar orientation, and interesting result was obtained for the SH behavior with changing measuring wavelength. Fig. 4(a) and (b) show the transient behavior of the SH intensity change with photoirradiation for 1200 nm and 900 nm fundamental light, respectively. As shown in the Fig. 4(a), SH intensity actually decreased to 1/5 value of the initial after photoirradiation. This drastic change cannot be explained only by the refractive index change. Since the absorption spectrum did not change due to the photoirradiation, molecular susceptibility b should not be changed by the photoirradiation. So reasonably, we can assign the SH intensity change to the realignment of the molecule. On the other hand, in Fig. 4(b), SH intensity gradually decreases with the fundamental irradiation of 900 nm. According to the absorption measurement, there is no absorption band at around 900 nm. Thus this change is due to the twophoton excitation. It should be noted that we can see some SH signal even after the photoirradiation, possibly due to the quadrupole or magnetic dipole contribution.
0 0
1000
2000
4. Conclusion
time (s)
a
Polar orientation of Alq3 evaporated films on metal electrode and glass substrate was investigated by Kelvin probe method and SHG, respectively. For as-deposited films under the dark condition, a large potential formed across the films was confirmed and it decreased drastically as light irradiation with a wavelength of 400 nm. Before the irradiation, alignment dipoles are the main contributor to the surface potential, whereas the excess charges contribute mainly to the surface potential after the irradiation. According to the SHG measurement, decrease of the SH signal from the Alq3 films was observed after the light irradiation. It is found that using the fundamental light with a wavelength of 900 nm, SH intensity decreases steeply during the SH measurement in the absence of 400 nm irradiation due to the two-photon excitation.
400 nm irradiated
SH intensity (arb. unit)
80
60
40
20
0 0 b
1000
2000
References
time (s)
Fig. 4. Transient behavior of the SH intensity change with photoirradiation for (a) 1200 nm and (b) 900 nm fundamental.
[1] W.R. Salaneck, K. Seki, A. Kahn, J.J. Pireaux (Eds.), Conjugated Polymer and Molecular Interfaces, Marcell Dekker, New York, 2002.
T. Manaka et al. / Current Applied Physics 6 (2006) 877–881 [2] S.M. Sze, Physics of Semiconductor Devices, Second ed., Wiley, New York, 1981. [3] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 11 (1999) 605. [4] M. Iwamoto, A. Fukuda, E. Itoh, J. Appl. Phys. 75 (1994) 1607. [5] E. Itoh, M. Iwamoto, J. Appl. Phys. 81 (1997) 1790. [6] E. Itoh, H. Kokubo, S. Shouriki, M. Iwamoto, J. Appl. Phys. 83 (1998) 372.
881
[7] E. Ito, Y. Washizu, N. Hayashi, H. Ishii, N. Matsuie, K. Tsuboi, Y. Ouchi, Y. Harima, K. Yamashita, K. Seki, J. Appl. Phys. 92 (2002) 7306. [8] K. Yoshizaki, T. Manaka, M. Iwamoto, Trans. MRS-J. 29 (2004) 739. [9] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 72 (1987) 913. [10] M.A. Lampert, P. Mark, Current Injection in Solids, Academic Press, New York, 1970.
Investigation of the surface potential formed in Alq3 films on metal surface by Kelvin probe and nonlinear optical measurement q Takaaki Manaka *, Kei Yoshizaki, Mitsumasa Iwamoto Department of Physical Electronics, Tokyo Institute of Technology, O-okayama 2-12-1-S3-33, Meguro-ku, Tokyo 152-8552, Japan Received 20 December 2003 Available online 4 January 2006
Abstract Polar orientation of Alq3 (Tris(8-quinolinolato)aluminum) evaporated films on metal electrode was investigated by Kelvin probe method and optical second harmonic generation (SHG) measurement. For as-deposited films under the dark condition, a large potential formed across the films was confirmed and it decreased drastically by light irradiation with a wavelength of 400 nm. From a viewpoint of the interfacial electrostatic phenomena, there are two contributions to the surface potential, i.e. space charge effect and the alignment of dipoles. Before irradiation, alignment dipoles are the main contributor to the surface potential, whereas excess charges contribute mainly to the surface potential after irradiation. According to the SHG measurement, decrease of the signal from Alq3 films was observed after light irradiation. It is also found that using the fundamental light with a wavelength of 900 nm, SH intensity decreases steeply during the SH measurement in the absence of 400 nm irradiation possibly due to the two-photon excitation. 2005 Published by Elsevier B.V. PACS: 42.65.K; 73.40.C; 77.22 Keywords: Surface potential; Alq3; Second harmonic generation
1. Introduction Electrostatic phenomena occurring at the interface between metal/organic and organic/organic materials are fundamentally interesting to the fields of organic electronics and photonics. For the practical application in the nano-scale molecular devices, it is also essential to gain information on electronic structure of the molecules at the interface [1]. For ideal inorganic semiconductor systems, band diagram at the interface can be q
Original version presented at the International Discussion and Conference on Nano Interface Controller Electronic Devices (IDCNICE 2003), Tokyo, Japan, 17–20 December 2003. * Corresponding author. E-mail address: [email protected] (T. Manaka). 1567-1739/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.cap.2005.06.004
understood by the Mott-Schottky model, where the concept of Fermi-level alignment is important [2]. On the other hand, the situation becomes more complex for the metal/organic contact. The interfacial electric double layer is inevitably formed due to many possible origins, e.g., chemical bond formation, image charge effect and permanent dipoles. The band bending due to the charge injection and the dipole alignment also play an important role to the electrostatic phenomena occurring at metal/organic interface [3]. ‘‘Fermi-level alignment’’ is one of the most important and fundamental issues concerning organic materials electronics. Since organic molecule generally forms van der Waals solid in which intermolecular interaction is weak, it is hard to establish thermal equilibrium over the whole region of the organic film [10]. Thus the study
878
T. Manaka et al. / Current Applied Physics 6 (2006) 877–881
of electrostatic interfacial phenomena occurring at organic film/metal interfaces is of specially importance to the fields of organic electronics. From a view point of nano-interfacial electrical phenomena, we have investigated the interfacial states in organic thin films such as polyimide (PI) and phthalocyanine (Pc) on metal surface by the surface potential measurement [4–6]. According to the result, it was revealed that the excess electrons are injected from metal electrode to insulating PI Langmuir–Brogett (LB) films within the region of several nanometers from the metal-film interface. In the same way, for the semiconducting Pc films, the injection of excess electrons are observed within the region of 20 nm. In comparison with inorganic materials, the presence of permanent dipole is one of the most remarkable feature for the organic materials, and the alignment of dipoles also plays an important role to the surface potential. Thus it should be taken into account at least two contributions to the surface potential, i.e. space charge effect and the alignment of dipoles, to discuss the surface potential in polar organic materials. Recently, using Alq3 (Tris(8-quinolinolato)aluminum) molecule, the formation of the vacuum evaporated polar films was confirmed by the surface potential measurement [7,8], though such polar molecules were considered to form a film with centrosymmetric crystal structure by the deposition. Thus Alq3 molecule will be an interesting target for investigating two contributions mentioned above. In our previous paper, we preliminary reported that the space charge effect and alignment dipoles should be considered to understand the surface potential of Alq3 on metal electrodes. In this paper, we focused on the polar orientation of the Alq3 evaporated films on metal electrode, and investigated it by optical second harmonic generation (SHG) method. And besides an ordinary one-photon process, the disappearance of the polar orientation by a two-photon excitation process was examined.
N O N
Al
O
O N
Fig. 1. Molecular structure of the Alq3.
Alq3 was purchased from Wako Pure Chemical Co. Ltd., and was used without further purification. The samples used in the experiments were vacuum evaporated Alq3 films deposited on glass substrate coated with metal and bare glass substrate. During evaporation, process pressure was kept less than 2 · 106 Torr and deposition rate monitored by quartz crystal microbalances (QCM) was controlled to be about 1 nm/min. Since temperature of the substrate was not controlled during deposition, vacuum evaporated Alq3 molecules formed randomly oriented amorphous film. All the evaporation process were performed in the dark condition to avoid the effect of photoillumination. 2.2. Surface potential measurement Kelvin probe method was used to measure the surface potential. Since the details of this method were described in our previous paper [6], the principle of operation is briefly introduced here. For the Kelvin probe methods, capacitance is formed between top probe and metal electrode, half of which is covered with organic film. Assuming that N molecules are placed below the top electrode, charge Qp induced on a top electrode by these molecules is expressed as 1 N l hcos hi; lðtÞ
ð1Þ
2. Experiment
Qp ðtÞ ¼
2.1. Sample preparation
where l and h are the dipole moment and tilt angle of the molecule from the surface normal. h i represents the thermodynamic average of molecules. Here l(t) represents the distance between top and bottom electrode (1 mm), and the top electrode is vibrated with a frequency of 120 Hz during the surface potential measurement. The charge Q induced on the top electrode is sum of the induced charge Qp and CVc, where Vc is the compensation voltage applied to the top electrode and C is the capacitance of air gap. To determine the surface potential of the film, Vc is chosen to satisfy Q = 0 under the up and down vibration of the top electrode with respect to the bare electrode. Thus the surface potential of the organic monolayer can be expressed as [6]
The interest in organic electrical materials for use in organic light-emitting diodes (OLEDs) started with the first report of green electroluminescence from Alq3, by Tang and VanSlyke [9]. In this device, Alq3 is used as the emitting layer and the electron transporting layer of OLEDs. It is known that Alq3 has good electroand photo-luminescence yield. Thus, Alq3 has become the prototype of a whole class of electroluminescent organic compounds in use currently in OLEDs. Fig. 1 shows the molecular structure of the Alq3 in which three hydroxyquinoline ligand groups are attached to the central aluminum atom.
T. Manaka et al. / Current Applied Physics 6 (2006) 877–881
N lhcos hi ; r 0 B
r is the relative dielectric constant of film, 0 is the dielectric constant of a vacuum and B represents the area of top electrode. In this way, molecular alignment of Alq3 molecules can be estimated by the surface potential measurement. 2.3. SHG measurement Fig. 2 shows the experimental configuration for the SHG measurement. For the SHG measurement, s- or p-polarized fundamental light was focused on the sample using a convex lens (f = 100 mm) with an incident angle of 45 after passing through an SH-cut filter to eliminate the SH light from various optical components. The p-polarized SH light generated from the sample was filtered by fundamental cut filter to remove the fundamental light, and was detected by a photomultiplier tube (Hamamatsu photonics: R928) after passing through a monochromator (NIKON: G-250). Obtained signals were averaged by the Boxcar integrator (Stanford Research: SR-250) and processed by the personal computer. The fundamental light was obtained using an optical parametric oscillator (OPO: Continuum Surelite OPO) pumped by the third-harmonic light of a Qswitched Nd-YAG laser (Continuum: Surelite II-10).
3. Results and discussion Fig. 3 shows the thickness dependence of the surface potential observed in Alq3/Al structure. These samples were prepared by the vacuum deposition under dark condition and measurements were also performed in the dark circumstance. As shown in the figure, the surface potential increases steeply and linearly with the film
OPO
π/4 retarder
15
ð2Þ
YAG laser + 3ω generator
sample
monochrometor
optical fiber Xe lump + IF filter
Fig. 2. Experimental setup for the SHG measurement.
PMT
10
before irradiation Al/Alq3
5
after irradiation Al/Alq3 0 0
100
200
thickness [nm] Fig. 3. Thickness dependence of the surface potential observed in Alq3/Al structure.
thickness. Surprisingly, the magnitude of the surface potential is quite large and reaches more than 10 V in the manner same as that reported previously [7,8]. The establishment of such large voltage across Alq3 films cannot be explained by only assuming the electron transfer at the metal–Alq3 interface. It is supposed that the orientational alignment of dipole moment of Alq3 makes a significant contribution. Taking into account both contributions, the appropriate expression for the surface potential Vs built across the film is written as Z d 1 1 Vs ¼ xqðxÞ dx þ P 0 d. ð3Þ r 0 0 r 0 Here, r is the dielectric constant of film, d is film thickness and P0 is the polarization due to the all dipolar molecules. P0 is proportional to the orientational order parameter S 1 ¼ hcos hi, where h is an orientational angle from the normal direction to the substrate. In the first term of Eq. (3), x represents the distance of electrons displaced from the metal–film interface. Assuming the dipole contribution is dominant in the potential (it is reasonable situation before photoirradiation), equation for the surface potential can be rewrite as Vs ¼
fundamental cut filter
SH cut filter
surface potential [V]
V s ¼ ðV c Þ ¼
879
nlS 1 d ; r 0
ð4Þ
where n is the molecular density (number of molecules per volume), l and S1 are the dipole moment and the orientational order parameter, respectively. Taking into account the magnitude of the dipole moment of Alq3 molecules (meridional form, 4.1 D), S1 can be calculated as 0.07. Assuming that tilt angle for all molecule is identical, namely, orientational distribution function of the molecule is assumed to be given by d-function, it is found that molecules are almost lying to the surface. However, this is not a realistic picture. It is reasonable to image that the most of deposited molecules are
880
T. Manaka et al. / Current Applied Physics 6 (2006) 877–881
randomly oriented and the symmetry of deposited film was slightly broken along the z direction. The formation of polar structure of the film can be assigned by the SHG measurement. If the system has an inversion center, the generation of SH signal is not expected under the electric dipole approximation. Therefore, the SHG technique becomes a powerful tool for investigating the orientation of the molecule in the polar structure. Actually, we could get the strong SH signal from the Alq3 evaporated films under the off resonant condition.
400 nm irradiated 6
SH intensity (arb. unit)
5
4
3
2
1
Disappearance of the surface potential by the photoirradiation was also indicated in Fig. 3. After irradiating 400 nm light for a few seconds, potential drastically decreased. At the present stage, detailed mechanism of this phenomena is unclear. Dipole disappearance efficiency shows a clear wavelength dependence. No potential change was observed for the irradiation of 600 nm light where absorption coefficient is negligible. SHG measurement also shows the disappearance of the polar orientation, and interesting result was obtained for the SH behavior with changing measuring wavelength. Fig. 4(a) and (b) show the transient behavior of the SH intensity change with photoirradiation for 1200 nm and 900 nm fundamental light, respectively. As shown in the Fig. 4(a), SH intensity actually decreased to 1/5 value of the initial after photoirradiation. This drastic change cannot be explained only by the refractive index change. Since the absorption spectrum did not change due to the photoirradiation, molecular susceptibility b should not be changed by the photoirradiation. So reasonably, we can assign the SH intensity change to the realignment of the molecule. On the other hand, in Fig. 4(b), SH intensity gradually decreases with the fundamental irradiation of 900 nm. According to the absorption measurement, there is no absorption band at around 900 nm. Thus this change is due to the twophoton excitation. It should be noted that we can see some SH signal even after the photoirradiation, possibly due to the quadrupole or magnetic dipole contribution.
0 0
1000
2000
4. Conclusion
time (s)
a
Polar orientation of Alq3 evaporated films on metal electrode and glass substrate was investigated by Kelvin probe method and SHG, respectively. For as-deposited films under the dark condition, a large potential formed across the films was confirmed and it decreased drastically as light irradiation with a wavelength of 400 nm. Before the irradiation, alignment dipoles are the main contributor to the surface potential, whereas the excess charges contribute mainly to the surface potential after the irradiation. According to the SHG measurement, decrease of the SH signal from the Alq3 films was observed after the light irradiation. It is found that using the fundamental light with a wavelength of 900 nm, SH intensity decreases steeply during the SH measurement in the absence of 400 nm irradiation due to the two-photon excitation.
400 nm irradiated
SH intensity (arb. unit)
80
60
40
20
0 0 b
1000
2000
References
time (s)
Fig. 4. Transient behavior of the SH intensity change with photoirradiation for (a) 1200 nm and (b) 900 nm fundamental.
[1] W.R. Salaneck, K. Seki, A. Kahn, J.J. Pireaux (Eds.), Conjugated Polymer and Molecular Interfaces, Marcell Dekker, New York, 2002.
T. Manaka et al. / Current Applied Physics 6 (2006) 877–881 [2] S.M. Sze, Physics of Semiconductor Devices, Second ed., Wiley, New York, 1981. [3] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 11 (1999) 605. [4] M. Iwamoto, A. Fukuda, E. Itoh, J. Appl. Phys. 75 (1994) 1607. [5] E. Itoh, M. Iwamoto, J. Appl. Phys. 81 (1997) 1790. [6] E. Itoh, H. Kokubo, S. Shouriki, M. Iwamoto, J. Appl. Phys. 83 (1998) 372.
881
[7] E. Ito, Y. Washizu, N. Hayashi, H. Ishii, N. Matsuie, K. Tsuboi, Y. Ouchi, Y. Harima, K. Yamashita, K. Seki, J. Appl. Phys. 92 (2002) 7306. [8] K. Yoshizaki, T. Manaka, M. Iwamoto, Trans. MRS-J. 29 (2004) 739. [9] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 72 (1987) 913. [10] M.A. Lampert, P. Mark, Current Injection in Solids, Academic Press, New York, 1970.