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Fabrication and characterization of field effect transistors using donor and acceptor stacked layers
Applied Surface Science 130–132 Ž1998. 914–918
Fabrication and characterization of field effect transistors using donor and acceptor stacked layers M. Iizuka ) , Y. Shiratori, S. Kuniyoshi, K. Kudo, K. Tanaka Department of Electrical and Electronics Engineering, Faculty of Engineering, Chiba UniÕersity, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan Received 10 September 1997; accepted 12 December 1997
Abstract We fabricated field-effect transistors ŽFETs. using donor ŽTMTSF. and acceptor ŽTCNQ. stacked layers, and we investigated the change of conductivity in the charge transfer ŽCT. complex layer by applying gate voltages. Two types of FETs having TMTSFrTCNQ and TCNQrTMTSF structures are examined. The stacked-layer FET shows a large transconductance compared with a single-layer FET. The experimental results demonstrate that the CT complex layer formed between donor and acceptor films mainly works as a conduction channel. Furthermore, the change in the degree of charge transfer Žcorresponding to conductivity change. is confirmed by infrared absorption spectra. q 1998 Elsevier Science B.V. All rights reserved. PACS: 85.40.V; 73.60; 72.80.L Keywords: TCNQ; TMTSF; CT complex; FET; Organic thin film; Thin-film transistor
1. Introduction Organic thin-film field effect transistors ŽFETs. have recently received increasing interest because of their potential applications in low-cost and large-area devices such as organic electroluminescent devices and liquid crystal displays. Although there are a number of reports w1–7x on using conducting polymers and organic semiconductors, there is little information concerning multi-layer systems and charge transfer ŽCT. complexes. In particular, CT complexes exhibit anisotropic optical and electrical prop)
Corresponding author.
erties, so they are known as very attractive organic compounds. These unique characteristics of CT complexes derive from their crystal structures and a partial charge transfer between donor and acceptor molecules. The degree of charge transfer Ž r . from a donor molecule to an acceptor molecule depends not only on the kind of molecules but also on the crystal structure of the CT complexes, and r is directly related to the electrical conductivity. Most CT crystals have the structure of a segregated system Ždonors and acceptors stack independently. or a mixed system Ždonors and acceptors stack alternately.. The T T F Ž te tra th ia fu lv a le n e . – T C N Q Ž te tra cyanoquinodimethane. complex has a segregated
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 7 6 - 7
M. Iizuka et al.r Applied Surface Science 130–132 (1998) 914–918
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stacked structure organized from columns of TTF and TCNQ, respectively, and its conductivity along the b axis is very high. On the other hand, the TMTSF Žtetramethyltetraserenafulvalene. –TCNQ complex has two types of crystal structure Žred crystal or black crystal., and its r is 0.21 or 0.57 w8x. If r could be modified by the external field, the electrical conductivity of the CT complex could be drastically changed. In this report, we describe the characteristics of organic FETs using donor and acceptor stacked layers, and we explain how to control the conductivity by applying an electric field in the vertical direction.
2. Experimental TMTSF and TCNQ are used as donor and acceptor molecules. Their chemical structures are shown in Fig. 1. The structure of the donor and acceptor stacked-layer FET and the circuit for measuring electric field effect are shown in Fig. 2. The highly doped Si substrate, which works as a gate electrode, was covered with thermally grown SiO 2 with a thickness of approximately 300 nm. The AurCr interdigital source and drain electrodes were formed on the SiO 2 . The channel length and width were 0.2
Fig. 1. Chemical structure of TMTSF Ža. and TCNQ Žb..
Fig. 2. Schematic of fabricated FET structure.
and 56 mm, respectively. TMTSF and TCNQ were deposited on the substrates successively by a standard technique for vacuum evaporation. The substrate temperature was kept at room temperature ŽRT.. We fabricated two types of stacked-layer FETs. One was TMTSFrTCNQrSiO2rgate structure and the other was a TCNQrTMTSFrSiO2rgate structure. The film thickness of the stacked layer in both structures was approximately 500 nm. In-situ electrical characterization was performed at RT in the vacuum chamber without breaking the vacuum under the quasi-static condition. The drain-source voltages Ž V DS . vs. drain-source current Ž I DS . characteristics with applying various gate voltages Ž VGS . were measured immediately after depositing the first layer and the second layer. In-situ field effect measurements are a very promising method for investigating intrinsic electrical properties of organic thin films since the influences of atmosphere gases and impurities are excluded during measurements w7x. The transconductance Ž g m s d I DSrdVGS . of FETs was calculated from field-effect measurements w9x. The change of r was examined by infrared absorption spectra obtained by reflecting FT-IR measurement ŽRAS.. The sample structure for the RAS measurements con-
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M. Iizuka et al.r Applied Surface Science 130–132 (1998) 914–918
sisted of TCNQ and TMTSF evaporated layers sandwiched between a gold electrode and a Si substrate.
3. Results and discussion 3.1. Electrical measurements The I DS vs. V DS characteristics of the FET at various values of VGS are shown in Figs. 3 and 4. Values of g m were calculated by the values of black dots indicated in the figures. Fig. 3a shows the characteristics of a single-layer FET measured after the TCNQ deposition. FET characteristics could be measured at a TCNQ film thickness of more than 50 nm. And I DS of the TCNQ single-layer FET was increased by increasing positive bias of VGS . The TCNQ single-layer FET showed an n-channel FET characteristic and the majority carriers of the accumulation layer were electrons. Fig. 3b shows the characteristics of a TMTSFrTCNQ stacked-layer FET just after the second deposition of TMTSF. The FET characteristics for the stacked-layer FET ŽTMTSFrTCNQrSiO2 . were similar to those of the TCNQ single-layer FET. However, the I DS was one order larger than that of the TCNQ single-layer FET. The values of g m estimated for the TCNQ singlelayer FET and the TMTSFrTCNQ stacked-layer FET are 1.5 = 10y9 S and 5.8 = 10y9 S, respectively. However, no marked change of FET characteristics was observed by increasing the thickness of the TMTSF film by more than 100 nm. These results indicate that the interface layer between TMTSF and TCNQ works mainly as a conductive channel of the TMTSFrTCNQ stacked-layer FET. Fig. 4a shows the characteristics of a single-layer FET after depositing the first layer of TMTSF. This figure shows that I DS for the TMTSF single-layer FET was increased by increasing the negative bias of VGS . The TMTSF single-layer FET showed p-channel FET characteristics and the majority carriers of the accumulation layer were holes. The I DS -VDS characteristics of a TCNQrTMTSF stacked-layer FET are shown in Fig. 4b. In a similar way to the TMTSFrTCNQrSiO2 FET, the VGS characteristics of the stacked-layer FET ŽTCNQrTMTSFrSiO2 . were the same as those of the TMTSF single-layer FET, but I DS was larger than that of the TMTSF
Fig. 3. I DS vs. V DS characteristics of TMTSFrTCNQrgate structure FET.
single-layer FET. Values of g m estimated for the TMTSF single-layer FET and for the TCNQrTMTSF stacked-layer FET are 7.5 = 10y9 S and 1.8 = 10y8 S, respectively. The large values of I DS and g m of the stacked-layer FET compared with those of the single-layer FET demonstrate that these FETs are considered to consist of a trilayer structure, such as TMTSFrCT complexrTCNQrSiO 2 rgate or TCNQrCT complexrTMTSFrSiO2rgate, and the channel-conductance was increased by the formation
M. Iizuka et al.r Applied Surface Science 130–132 (1998) 914–918
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3.2. Infrared absorption spectrum The stacked-layer structure brings about a larger g m than that with both single-layer FETs. These results indicate that the quantities of charge transfer complex and r at the interface layer between two molecular films are varied by the gate voltages. Therefore, the infrared ŽIR. absorption spectra were measured by applying various voltages between the stacked layers. The bias voltages were applied to the TCNQ layer side from the TMTSF layer side. The IR absorption bands of the C[N stretching mode were observed as shown in Fig. 5. The peak of C[N 0 which corresponds to the neutral TCNQ 0 appears at 2222 cmy1 . The peaks of C[Nyx Ž x s 0 to 1. from 2222 to 2181 cmy1 are related to partially charge transferred TCNQ w10x. Compared with peaks at 0 V, these peaks shift toward higher or lower wave numbers by applying voltages of q5 and y5
Fig. 4. I DS vs. V DS characteristics of TCNQrTMTSFrgate structure FET.
of a charge transfer layer. The CT complex layer between donor and acceptor molecular films has high conductivity and works as the main conduction channel in these FETs. As described above, the stacked-layer FETs exhibit both n- and p-channel operation by changing the stacking order, and the values of g m and I DS are almost the same. The symmetrical characteristics on VGS have another advantage for utilizing the stacked-layer FETs as organic elements of complementary circuits.
Fig. 5. Infrared absorption spectra of TCNQrTMTSF stacked layer.
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M. Iizuka et al.r Applied Surface Science 130–132 (1998) 914–918
V. The value of r was estimated by the peak shift of C[Nyx in the IR absorption spectra. The peaks lower than 2190 cmy1 , which correspond to r larger than 1, were sometimes observed in thin films of the CT complex including TCNQ w11–13x. The r value of 0.59 estimated from the peak position of 2202 cmy1 agreed with the r value reported for the black crystal structure w8x. Furthermore, the r values estimated from the peaks of 2198 and 2195 cmy1 at applied biases of 0 and 5 V, were 0.68 and 0.78, respectively. The r values corresponding to the shoulder peaks appearing at 2219 and 2216 cmy1 , under applied bias of 0 and 5 V, were 0.21 and 0.40, respectively. The r value was between those of black and red crystals. The IR absorption spectra demonstrate that a variety of CT states exist in the stacked layers of acceptor and donor molecules and that the r value of the CT complex layer was varied by applying an electric field. These results indicate that the conductivity and g m of the FET are related to the r value and that these electrical parameters can be controlled by an external field.
4. Conclusion We fabricated new-type FETs using donor and acceptor molecules and we investigated the characteristics of single-layer and stacked-layer FETs. The FET characteristics strongly depend on the polarity of VGS and the stacking order of the donor and acceptor layers. Moreover, the values of transconductance are larger than those of each single-layer FET. On the other hand, the IR absorption spectra
measured by applying voltages to the stacked layers indicate that the r and conductivity of the charge transfer complex layer can be varied by applying electric field. These results demonstrate that a newtype organic FET is expected which operates due to the change in the degree of charge transfer or insulator-metal transition in donor-acceptor charge transfer complex layers. References w1x A. Tsumura, H. Koezuka, T. Ando, Appl. Phys. Lett. 49 Ž1986. 1210. w2x H. Akamichi, K. Waragai, S. Hotta, H. Kano, H. Sasaki, Appl. Phys. Lett. 58 Ž1990. 1157. w3x Z. Bao, A.J. Lovinger, A. Dodabalapur, Appl. Phys. Lett. 69 Ž1996. 3066. w4x A. Dodabalapur, H.E. Katz, L. Torsi, R.C. Haddon, Appl. Phys. Lett. 68 Ž1996. 1108. w5x A. Dodabalapur, J. Laquindanum, H.E. Katz, Z. Bao, Appl. Phys. Lett. 69 Ž1996. 4227. w6x K. Tada, H. Harada, K. Yoshinoi, Jpn. J. Appl. Phys. 35 Ž1996. 944. w7x K. Kudo, M. Yamashina, T. Moriizumi, Jpn. J. Appl. Phys. 23 Ž1984. 130. w8x K. Bechgaard, T.J. Kistenmacher, A.N. Bloch, D.O. Cowan, Acta Cryst. B 33 Ž1977. 417. w9x T. Sumimoto, Y. Shiratori, M. Iizuka, S. Kuniyoshi, K. Kudo, K. Tanaka, Synth. Met. 86 Ž1997. 2259. w10x J.G. Robles-Martinez, A. Salmeron-Valverde, C. Soriano, E. Alonso, A. Zehe, Cyst. Res. Technol. 25 Ž1990. 1335. w11x L.R. Melby, R.J. Harder, W.R. Hertler, W. Mahler, R.E. Benson, W.E. Mochel, J. Am. Chem. Soc. 84 Ž1962. 3374. w12x A.S. Dhindsa, G.H. Davies, M.R. Bryce, J. Yarwood, J.P. Lloyd, M.C. Petty, Yu.M. Lvov, J. Mol. Electron. 5 Ž1989. 135. w13x S.-G. Liu, Y.-Q. Liu, D.-B. Zhu, Thin Solid Films 280 Ž1996. 271.
Fabrication and characterization of field effect transistors using donor and acceptor stacked layers M. Iizuka ) , Y. Shiratori, S. Kuniyoshi, K. Kudo, K. Tanaka Department of Electrical and Electronics Engineering, Faculty of Engineering, Chiba UniÕersity, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan Received 10 September 1997; accepted 12 December 1997
Abstract We fabricated field-effect transistors ŽFETs. using donor ŽTMTSF. and acceptor ŽTCNQ. stacked layers, and we investigated the change of conductivity in the charge transfer ŽCT. complex layer by applying gate voltages. Two types of FETs having TMTSFrTCNQ and TCNQrTMTSF structures are examined. The stacked-layer FET shows a large transconductance compared with a single-layer FET. The experimental results demonstrate that the CT complex layer formed between donor and acceptor films mainly works as a conduction channel. Furthermore, the change in the degree of charge transfer Žcorresponding to conductivity change. is confirmed by infrared absorption spectra. q 1998 Elsevier Science B.V. All rights reserved. PACS: 85.40.V; 73.60; 72.80.L Keywords: TCNQ; TMTSF; CT complex; FET; Organic thin film; Thin-film transistor
1. Introduction Organic thin-film field effect transistors ŽFETs. have recently received increasing interest because of their potential applications in low-cost and large-area devices such as organic electroluminescent devices and liquid crystal displays. Although there are a number of reports w1–7x on using conducting polymers and organic semiconductors, there is little information concerning multi-layer systems and charge transfer ŽCT. complexes. In particular, CT complexes exhibit anisotropic optical and electrical prop)
Corresponding author.
erties, so they are known as very attractive organic compounds. These unique characteristics of CT complexes derive from their crystal structures and a partial charge transfer between donor and acceptor molecules. The degree of charge transfer Ž r . from a donor molecule to an acceptor molecule depends not only on the kind of molecules but also on the crystal structure of the CT complexes, and r is directly related to the electrical conductivity. Most CT crystals have the structure of a segregated system Ždonors and acceptors stack independently. or a mixed system Ždonors and acceptors stack alternately.. The T T F Ž te tra th ia fu lv a le n e . – T C N Q Ž te tra cyanoquinodimethane. complex has a segregated
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 7 6 - 7
M. Iizuka et al.r Applied Surface Science 130–132 (1998) 914–918
915
stacked structure organized from columns of TTF and TCNQ, respectively, and its conductivity along the b axis is very high. On the other hand, the TMTSF Žtetramethyltetraserenafulvalene. –TCNQ complex has two types of crystal structure Žred crystal or black crystal., and its r is 0.21 or 0.57 w8x. If r could be modified by the external field, the electrical conductivity of the CT complex could be drastically changed. In this report, we describe the characteristics of organic FETs using donor and acceptor stacked layers, and we explain how to control the conductivity by applying an electric field in the vertical direction.
2. Experimental TMTSF and TCNQ are used as donor and acceptor molecules. Their chemical structures are shown in Fig. 1. The structure of the donor and acceptor stacked-layer FET and the circuit for measuring electric field effect are shown in Fig. 2. The highly doped Si substrate, which works as a gate electrode, was covered with thermally grown SiO 2 with a thickness of approximately 300 nm. The AurCr interdigital source and drain electrodes were formed on the SiO 2 . The channel length and width were 0.2
Fig. 1. Chemical structure of TMTSF Ža. and TCNQ Žb..
Fig. 2. Schematic of fabricated FET structure.
and 56 mm, respectively. TMTSF and TCNQ were deposited on the substrates successively by a standard technique for vacuum evaporation. The substrate temperature was kept at room temperature ŽRT.. We fabricated two types of stacked-layer FETs. One was TMTSFrTCNQrSiO2rgate structure and the other was a TCNQrTMTSFrSiO2rgate structure. The film thickness of the stacked layer in both structures was approximately 500 nm. In-situ electrical characterization was performed at RT in the vacuum chamber without breaking the vacuum under the quasi-static condition. The drain-source voltages Ž V DS . vs. drain-source current Ž I DS . characteristics with applying various gate voltages Ž VGS . were measured immediately after depositing the first layer and the second layer. In-situ field effect measurements are a very promising method for investigating intrinsic electrical properties of organic thin films since the influences of atmosphere gases and impurities are excluded during measurements w7x. The transconductance Ž g m s d I DSrdVGS . of FETs was calculated from field-effect measurements w9x. The change of r was examined by infrared absorption spectra obtained by reflecting FT-IR measurement ŽRAS.. The sample structure for the RAS measurements con-
916
M. Iizuka et al.r Applied Surface Science 130–132 (1998) 914–918
sisted of TCNQ and TMTSF evaporated layers sandwiched between a gold electrode and a Si substrate.
3. Results and discussion 3.1. Electrical measurements The I DS vs. V DS characteristics of the FET at various values of VGS are shown in Figs. 3 and 4. Values of g m were calculated by the values of black dots indicated in the figures. Fig. 3a shows the characteristics of a single-layer FET measured after the TCNQ deposition. FET characteristics could be measured at a TCNQ film thickness of more than 50 nm. And I DS of the TCNQ single-layer FET was increased by increasing positive bias of VGS . The TCNQ single-layer FET showed an n-channel FET characteristic and the majority carriers of the accumulation layer were electrons. Fig. 3b shows the characteristics of a TMTSFrTCNQ stacked-layer FET just after the second deposition of TMTSF. The FET characteristics for the stacked-layer FET ŽTMTSFrTCNQrSiO2 . were similar to those of the TCNQ single-layer FET. However, the I DS was one order larger than that of the TCNQ single-layer FET. The values of g m estimated for the TCNQ singlelayer FET and the TMTSFrTCNQ stacked-layer FET are 1.5 = 10y9 S and 5.8 = 10y9 S, respectively. However, no marked change of FET characteristics was observed by increasing the thickness of the TMTSF film by more than 100 nm. These results indicate that the interface layer between TMTSF and TCNQ works mainly as a conductive channel of the TMTSFrTCNQ stacked-layer FET. Fig. 4a shows the characteristics of a single-layer FET after depositing the first layer of TMTSF. This figure shows that I DS for the TMTSF single-layer FET was increased by increasing the negative bias of VGS . The TMTSF single-layer FET showed p-channel FET characteristics and the majority carriers of the accumulation layer were holes. The I DS -VDS characteristics of a TCNQrTMTSF stacked-layer FET are shown in Fig. 4b. In a similar way to the TMTSFrTCNQrSiO2 FET, the VGS characteristics of the stacked-layer FET ŽTCNQrTMTSFrSiO2 . were the same as those of the TMTSF single-layer FET, but I DS was larger than that of the TMTSF
Fig. 3. I DS vs. V DS characteristics of TMTSFrTCNQrgate structure FET.
single-layer FET. Values of g m estimated for the TMTSF single-layer FET and for the TCNQrTMTSF stacked-layer FET are 7.5 = 10y9 S and 1.8 = 10y8 S, respectively. The large values of I DS and g m of the stacked-layer FET compared with those of the single-layer FET demonstrate that these FETs are considered to consist of a trilayer structure, such as TMTSFrCT complexrTCNQrSiO 2 rgate or TCNQrCT complexrTMTSFrSiO2rgate, and the channel-conductance was increased by the formation
M. Iizuka et al.r Applied Surface Science 130–132 (1998) 914–918
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3.2. Infrared absorption spectrum The stacked-layer structure brings about a larger g m than that with both single-layer FETs. These results indicate that the quantities of charge transfer complex and r at the interface layer between two molecular films are varied by the gate voltages. Therefore, the infrared ŽIR. absorption spectra were measured by applying various voltages between the stacked layers. The bias voltages were applied to the TCNQ layer side from the TMTSF layer side. The IR absorption bands of the C[N stretching mode were observed as shown in Fig. 5. The peak of C[N 0 which corresponds to the neutral TCNQ 0 appears at 2222 cmy1 . The peaks of C[Nyx Ž x s 0 to 1. from 2222 to 2181 cmy1 are related to partially charge transferred TCNQ w10x. Compared with peaks at 0 V, these peaks shift toward higher or lower wave numbers by applying voltages of q5 and y5
Fig. 4. I DS vs. V DS characteristics of TCNQrTMTSFrgate structure FET.
of a charge transfer layer. The CT complex layer between donor and acceptor molecular films has high conductivity and works as the main conduction channel in these FETs. As described above, the stacked-layer FETs exhibit both n- and p-channel operation by changing the stacking order, and the values of g m and I DS are almost the same. The symmetrical characteristics on VGS have another advantage for utilizing the stacked-layer FETs as organic elements of complementary circuits.
Fig. 5. Infrared absorption spectra of TCNQrTMTSF stacked layer.
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M. Iizuka et al.r Applied Surface Science 130–132 (1998) 914–918
V. The value of r was estimated by the peak shift of C[Nyx in the IR absorption spectra. The peaks lower than 2190 cmy1 , which correspond to r larger than 1, were sometimes observed in thin films of the CT complex including TCNQ w11–13x. The r value of 0.59 estimated from the peak position of 2202 cmy1 agreed with the r value reported for the black crystal structure w8x. Furthermore, the r values estimated from the peaks of 2198 and 2195 cmy1 at applied biases of 0 and 5 V, were 0.68 and 0.78, respectively. The r values corresponding to the shoulder peaks appearing at 2219 and 2216 cmy1 , under applied bias of 0 and 5 V, were 0.21 and 0.40, respectively. The r value was between those of black and red crystals. The IR absorption spectra demonstrate that a variety of CT states exist in the stacked layers of acceptor and donor molecules and that the r value of the CT complex layer was varied by applying an electric field. These results indicate that the conductivity and g m of the FET are related to the r value and that these electrical parameters can be controlled by an external field.
4. Conclusion We fabricated new-type FETs using donor and acceptor molecules and we investigated the characteristics of single-layer and stacked-layer FETs. The FET characteristics strongly depend on the polarity of VGS and the stacking order of the donor and acceptor layers. Moreover, the values of transconductance are larger than those of each single-layer FET. On the other hand, the IR absorption spectra
measured by applying voltages to the stacked layers indicate that the r and conductivity of the charge transfer complex layer can be varied by applying electric field. These results demonstrate that a newtype organic FET is expected which operates due to the change in the degree of charge transfer or insulator-metal transition in donor-acceptor charge transfer complex layers. References w1x A. Tsumura, H. Koezuka, T. Ando, Appl. Phys. Lett. 49 Ž1986. 1210. w2x H. Akamichi, K. Waragai, S. Hotta, H. Kano, H. Sasaki, Appl. Phys. Lett. 58 Ž1990. 1157. w3x Z. Bao, A.J. Lovinger, A. Dodabalapur, Appl. Phys. Lett. 69 Ž1996. 3066. w4x A. Dodabalapur, H.E. Katz, L. Torsi, R.C. Haddon, Appl. Phys. Lett. 68 Ž1996. 1108. w5x A. Dodabalapur, J. Laquindanum, H.E. Katz, Z. Bao, Appl. Phys. Lett. 69 Ž1996. 4227. w6x K. Tada, H. Harada, K. Yoshinoi, Jpn. J. Appl. Phys. 35 Ž1996. 944. w7x K. Kudo, M. Yamashina, T. Moriizumi, Jpn. J. Appl. Phys. 23 Ž1984. 130. w8x K. Bechgaard, T.J. Kistenmacher, A.N. Bloch, D.O. Cowan, Acta Cryst. B 33 Ž1977. 417. w9x T. Sumimoto, Y. Shiratori, M. Iizuka, S. Kuniyoshi, K. Kudo, K. Tanaka, Synth. Met. 86 Ž1997. 2259. w10x J.G. Robles-Martinez, A. Salmeron-Valverde, C. Soriano, E. Alonso, A. Zehe, Cyst. Res. Technol. 25 Ž1990. 1335. w11x L.R. Melby, R.J. Harder, W.R. Hertler, W. Mahler, R.E. Benson, W.E. Mochel, J. Am. Chem. Soc. 84 Ž1962. 3374. w12x A.S. Dhindsa, G.H. Davies, M.R. Bryce, J. Yarwood, J.P. Lloyd, M.C. Petty, Yu.M. Lvov, J. Mol. Electron. 5 Ž1989. 135. w13x S.-G. Liu, Y.-Q. Liu, D.-B. Zhu, Thin Solid Films 280 Ž1996. 271.