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Molecular rectifying behaviors of a planar binuclear phthalocyanine studied by scanning tunneling microscopy
Synthetic Metals 128 (2002) 43–46
Molecular rectifying behaviors of a planar binuclear phthalocyanine studied by scanning tunneling microscopy Yan Jie Zhanga, Yingshun Lib, Qingsheng Liua, Jian Jina, Baoquan Dinga, Yanlin Songb, Lei Jiangb, Xiguang Duc, Yingying Zhaoa,*, Tie Jin Lia a
b
Department of Chemistry, Jilin University, Changchun 130023, PR China Research Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China c Department of Chemistry, Northeast Normal University, Changchun 130025, PR China Received 18 January 2001; received in revised form 13 August 2001; accepted 2 October 2001
Abstract A casting film of a kind of binuclear cobalt phthalocyanine-sulphonate (CoPc-CoPc) deposited on highly oriented pyrolytic graphite (HOPG) substrate was studied by scanning tunneling microscopy (STM) using a Pt/Ir nanotip in air at 22 8C. Molecules CoPc-CoPc crystallized on HOPG surface along one direction in a large scale. The asymmetrical I–V curves for the monolayer and the double layer reveal a rectifying behavior for a negative bias. The rectification mechanism can be explained by a modification of the Aviram–Ratner model. The electron transfer in the molecule can only along one direction and along another direction is disfavored. The electric resistance for the monolayer is smaller than that for the double layer, and thus the rectification of the monolayer is better than that of the double layer. The STM experiments have indicated that the asymmetry for I–V curves arises from the material asymmetry. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Planar binuclear phthalocyanine; Asymmetric I–V; Molecular rectification; STM
1. Introduction The term molecular electronics evokes varied image, everything from circuits consisting of single molecular to biosensors. One of the goals of molecular electronics is to mimic macroscopic electronics functions and devices with molecules. For this reason, the observation of molecular rectification is an important early step [1–4]. In 1974, Aviram and Ratner [5] described an analogy in the current versus voltage response of conventional rectifiers and bridged organic donor (D)–acceptor (A) molecules. The current flowing through the molecule should have the essential characteristics of the electronic device known as a rectifier, a device that allows current to flow when a voltage is applied with one polarity but not with the opposite one. This accomplishment demonstrates the potential of single molecules as electronic components. Recently, nanoscopic rectification attempts were conducted by scanning tunneling microscopy (STM) [6–9]. Phthalocyanines are promising candidates to investigate molecular rectification [10–12]. *
Corresponding author. Tel.: þ86-431-892-2331; fax: þ86-431-894-9334. E-mail address: [email protected] (Y. Zhao).
Many binuclear phthalocyanines have been synthesized and various properties have been reported in the last decade [13–16]. However, few studies about the rectification behaviors of planar binuclear phthalocyanines have been reported. In previous work, we have described the synthesis, Langmuir–Blodgett film preparation and photovoltaic characteristics [17,18] of a kind of planar binuclear cobalt phthalocyanine (CoPc-CoPc). Here, we are interested in the current–voltage characteristics of CoPc-CoPc studied by STM. The asymmetric current–voltage characteristics of CoPc-CoPc suggest that it has a potential application as a unimolecular rectifier.
2. Experimental details The synthesis of CoPc-CoPc has been described elsewhere [17]. The chemical structure and molecular dimension are shown in Fig. 1. CoPc-CoPc was dissolved in water to form a solution with the concentration of about 1 106 M. The STM investigations were conducted using a commercial system (Seiko Instruments, SPA 300HV, Japan) with a 20 mm scanner. The probe was Pt/Ir alloy wire and the substrate was a piece of freshly cleaved plane of
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 6 5 5 - 5
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Y.J. Zhang et al. / Synthetic Metals 128 (2002) 43–46
studied by STM after water had evaporated. Current–voltage (I–V) profiles were collected in the constant height mode. Each image presented here is representative of several images taken at different times to ensure reproducibility. X-ray photoelectron spectroscopy (XPS) was performed on a VGESCALAB NKII (UK) spectrometer excited by aluminum Ka X-radiation.
3. Results and discussion
Fig. 1. (a) The chemical structure of CoPc-CoPc. (b) The molecular dimension of H2Pc-H2Pc estimated by Chem3d model.
highly oriented pyrolytic graphite (HOPG, 10 mm 10 mm). All the images were recorded under ambient condition at 22 8C. Typical operating conditions were 0.1 nA current and 0.1 V bias voltage. After the atoms of HOPG were resolved, a drop of CoPc-CoPc solution was applied to the HOPG and
In the XPS experiment, two characteristic peaks were found at 781.0 eV (binding energy) and 796 eV for Co2p corresponding to Co(III) and Co(I), respectively (figure is not shown here). This certified that CoPc-CoPc exist in the configuration of Co(I)Pc-Co(III)Pc. Moreover, the total energies (E (a.u.)) of two different configuration of CoPc-CoPc, Co(II)Pc-Co(II)Pc and Co(I)Pc-Co(III)Pc, were calculated by INDO/S method [19]. The E (a.u.) for Co(II)Pc-Co(II)Pc and Co(I)Pc-Co(III)Pc are 280.8229 and 281.4510, respectively. These results indicate that Co(I)Pc-Co(III)Pc is more stable than Co(II)Pc-Co(II)Pc and phthalocyanine rings can stabilize the different valent of the same metal ions. There is a tendency to disproportionate from Co(II)Pc-Co(II)Pc to Co(I)Pc-Co(III)Pc with the aid of large conjugate system. A large-scale STM topographic image obtained on the dried Co(I)Pc-Co(III)Pc on HOPG is shown in Fig. 2a. In this picture one can see clearly that Co(I)Pc-Co(III)Pc crystallized along one direction on HOPG. The inset showed the crystal lattice of HOPG. Fig. 2b is a zoomed-in image obtained from the area indicated in Fig. 2a. A thinner layer (Fig. 3a) was selected to investigate the I–V characteristics at molecular level. Cross-section analysis (Fig. 3c and d) along the direction marked A and B in Fig. 3a shows steps with the height of 3.97 and 1.95 nm, respectively. If we accept the molecular dimension of H2Pc-H2Pc estimated by Chem3d model shown in Fig. 1b, 1.95 nm will be attributed to the
Fig. 2. (a) A large-scale STM topographic image of Co(I)Pc-Co(III)Pc on highly oriented pyrolytic graphite (HOPG), the inset showed an atomic scale image of HOPG surface, the bias voltage ¼ 0:1 V, the current ¼ 0:1 nA. (b) A zoom-in image obtained from the area indicated in (a).
Y.J. Zhang et al. / Synthetic Metals 128 (2002) 43–46
45
Fig. 3. (a) STM topographic image of Co(I)Pc-Co(III)Pc on HOPG obtained at thinner area. (b) Asymmetric current–voltage (I–V) curves taken at the points marked as þ1 and þ2. (c) and (d) Height cross-section taken in the direction shown as A and B.
thickness of monolayer in an edge-on stack. In the monolayer, Co(I)Pc-Co(III)Pc molecules stand up with the long axial inclined to the substrate surface by about 428 ðsin y ¼ 1:95=2:93Þ and the short axial parallel to the HOPG surface. The step of 3.97 nm is corresponding to the thickness of double layers. The molecules Co(I)Pc-Co(III)Pc packed closely in each layer as illustrated in Fig. 4. The tunneling current versus bias voltage (I–V) relations of Co(I)Pc-Co(III)Pc on HOPG were studied by STM using a Pt/Ir nanotip at a set point of 0.1 nA and 0.1 V. The dc current–voltage characteristics of the casting film taken at the points marked as þ1 and þ2 are shown in Fig. 3b. Curves 1 and 2 show an asymmetric property to zero, indicating rectification behavior. Co(I)Pc-Co(III)Pc is an asymmetrical molecule because the valent of two coupled ions in molecule are different to each other. The asymmetry creates a dipole moment and lead to intramolecular one-direction charge transfer. In this molecule, Co(I) and Co(III) can be regarded as donor and acceptor, respectively, and the benzene-ring linked the two phthalocyanine rings can be regarded as the
bridge mediated electronic coupling between the two Pc rings. The rectification of Co(I)Pc-Co(III)Pc can be viewed in a modification of Aviram–Ratner model [4,9]. At a negative bias, an electron transfers from the nanotip to the lowest unoccupied electronic orbital on the molecule localized on the acceptor species to create DBA (Co(I)PcCo(II)). A molecular orbital localized on the donor site transfers an electron to the HOPG, creating DþBA (Co(II)Pc-Co(II)). Finally, the electron transfers from the reduced acceptor to the oxidized donor, regenerating the DBA species. The intramolecular electron transfer reaction Dþ BA ) DBA is facilitated by weak bridge-mediated electronic coupling between the donor and acceptor units. If the coupling between the acceptor and donor is too strong, the asymmetry of the current–voltage characteristic will be broken, and bidirectional charge transfer should become possible. When the voltage polarity is reversed, the current flow is almost shut. The electron transfer is only within the molecule and a schematic diagram of the current flow at a negative voltage is shown in Fig. 4. The charge transfer is
46
Y.J. Zhang et al. / Synthetic Metals 128 (2002) 43–46
Fig. 4. Schematic diagram of the packing of molecules Co(I)Pc-Co(III)Pc in each layer and the current flow at a negative voltage for Co(I)Pc-Co(III)Pc molecule on HOPG substrate.
easier in one direction of polarity. Moreover, the electric resistance for the monolayer is smaller than that for the double layer, and thus the rectification of the monolayer is better than that of the double layer. The STM experiments have indicated that the asymmetry for I–V curves arises from the material asymmetry.
[3] [4] [5] [6]
[7]
4. Summary The STM studies have indicated that the asymmetry for the I–V curves arises from the asymmetry of this binuclear phthalocyanine (Co(I)Pc-Co(III)Pc). It has a potential application as a unimolecular rectifier that will be of great significance in molecular electronics. Acknowledgements
[8] [9] [10] [11] [12] [13]
The authors acknowledge the National Natural Science Foundation of China (NNSFC), State Key Project Fundamental Research and Chinese Academy of Sciences for financial supports.
[14]
References
[17]
[1] D.H. Waldeck, D.N. Beratan, Science 261 (1993) 576. [2] A.C. Brady, B. Hodder, A.S. Martin, J.R. Sambles, C.P. Eweles, R. Jones, P.R. Briddon, A.M. Musa, C.A. Panetta, D.L. Mattern, J. Mater. Chem. 9 (1999) 2271.
[15] [16]
[18] [19]
R.M. Metzger, J. Mater. Chem. 9 (1999) 2027. R.M. Metzger, Acc. Chem. Res. 32 (1999) 950. A. Aviram, M.A. Ratner, Chem. Phys. Lett. 29 (1974) 277. R.M. Metzger, B. Chen, U. Ho¨ pfner, M.V. Lakshmikantham, D. Vuillaume, T. Kawai, X. Wu, H. Tachibana, T.V. Hughes, H. Sakurai, J.W. Baldwin, C. Hosch, M.P. Cava, L. Brehmer, G.J. Ashwell, J. Am. Chem. Soc. 119 (1997) 10455. M. Pomerantz, A. Aviram, R.A. McCorkle, L. Li, A.G. Schrott, Science 255 (1992) 1115. A. Stabel, P. Herwig, K. Mu¨ llen, J.P. Rabe, Angew. Chem. Int. Ed. Engl. 34 (1994) 1609. A. Dhirani, P.-H. Lin, P. Guyot-Sionnest, R.W. Zehner, L.R. Sita, J. Chem. Phys. 106 (1997) 5249. S. Zhou, Y. Liu, Y. Xu, W. Hu, D.B. Zhu, X. Qiu, C. Wang, C. Bai, Chem. Phys. Lett. 297 (1998) 77. S. Hiller, D. Schlettwein, N.R. Armstrong, D. Wo¨ hrle, J. Mater. Chem. 8 (1998) 945. C.M. Fischer, M. Burghard, S. Roth, K.V. Klitzing, Europhys. Lett. 28 (1994) 129. N. Kobayashi, H. Lam, W.A. Nevin, P. Janda, C.C. Leznoff, Y. Koyama, A. Monden, H. Shirai, J. Am. Chem. Soc. 116 (1994) 879. N. Kobayashi, H. Lam, W.A. Nevin, P. Janda, C.C. Leznoff, A.B.P. Lever, Inorg. Chem. 29 (1990) 3415. J.H. Kim, T.M. Cotton, A.R. Uphaus, C.C. Leznoff, Thin Solid films 159 (1988) 141. Y. Liu, K. Shigehara, M. Hara, A. Yamada, J. Am. Chem. Soc. 113 (1991) 440. Y.J. Zhang, L.S. Li, J. Jin, S. Jiang, Y. Zhao, T.J. Li, X. Du, S. Yang, Langmuir 15 (1999) 2183. Y.J. Zhang, L.S. Li, J. Jin, X. Du, Y. Zhao, T.J. Li, Mol. Cryst. Liq. Cryst. 337 (1999) 257. B. Chen, The Doctoral Thesis of Northeast Normal University, 1997.
Molecular rectifying behaviors of a planar binuclear phthalocyanine studied by scanning tunneling microscopy Yan Jie Zhanga, Yingshun Lib, Qingsheng Liua, Jian Jina, Baoquan Dinga, Yanlin Songb, Lei Jiangb, Xiguang Duc, Yingying Zhaoa,*, Tie Jin Lia a
b
Department of Chemistry, Jilin University, Changchun 130023, PR China Research Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China c Department of Chemistry, Northeast Normal University, Changchun 130025, PR China Received 18 January 2001; received in revised form 13 August 2001; accepted 2 October 2001
Abstract A casting film of a kind of binuclear cobalt phthalocyanine-sulphonate (CoPc-CoPc) deposited on highly oriented pyrolytic graphite (HOPG) substrate was studied by scanning tunneling microscopy (STM) using a Pt/Ir nanotip in air at 22 8C. Molecules CoPc-CoPc crystallized on HOPG surface along one direction in a large scale. The asymmetrical I–V curves for the monolayer and the double layer reveal a rectifying behavior for a negative bias. The rectification mechanism can be explained by a modification of the Aviram–Ratner model. The electron transfer in the molecule can only along one direction and along another direction is disfavored. The electric resistance for the monolayer is smaller than that for the double layer, and thus the rectification of the monolayer is better than that of the double layer. The STM experiments have indicated that the asymmetry for I–V curves arises from the material asymmetry. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Planar binuclear phthalocyanine; Asymmetric I–V; Molecular rectification; STM
1. Introduction The term molecular electronics evokes varied image, everything from circuits consisting of single molecular to biosensors. One of the goals of molecular electronics is to mimic macroscopic electronics functions and devices with molecules. For this reason, the observation of molecular rectification is an important early step [1–4]. In 1974, Aviram and Ratner [5] described an analogy in the current versus voltage response of conventional rectifiers and bridged organic donor (D)–acceptor (A) molecules. The current flowing through the molecule should have the essential characteristics of the electronic device known as a rectifier, a device that allows current to flow when a voltage is applied with one polarity but not with the opposite one. This accomplishment demonstrates the potential of single molecules as electronic components. Recently, nanoscopic rectification attempts were conducted by scanning tunneling microscopy (STM) [6–9]. Phthalocyanines are promising candidates to investigate molecular rectification [10–12]. *
Corresponding author. Tel.: þ86-431-892-2331; fax: þ86-431-894-9334. E-mail address: [email protected] (Y. Zhao).
Many binuclear phthalocyanines have been synthesized and various properties have been reported in the last decade [13–16]. However, few studies about the rectification behaviors of planar binuclear phthalocyanines have been reported. In previous work, we have described the synthesis, Langmuir–Blodgett film preparation and photovoltaic characteristics [17,18] of a kind of planar binuclear cobalt phthalocyanine (CoPc-CoPc). Here, we are interested in the current–voltage characteristics of CoPc-CoPc studied by STM. The asymmetric current–voltage characteristics of CoPc-CoPc suggest that it has a potential application as a unimolecular rectifier.
2. Experimental details The synthesis of CoPc-CoPc has been described elsewhere [17]. The chemical structure and molecular dimension are shown in Fig. 1. CoPc-CoPc was dissolved in water to form a solution with the concentration of about 1 106 M. The STM investigations were conducted using a commercial system (Seiko Instruments, SPA 300HV, Japan) with a 20 mm scanner. The probe was Pt/Ir alloy wire and the substrate was a piece of freshly cleaved plane of
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 6 5 5 - 5
44
Y.J. Zhang et al. / Synthetic Metals 128 (2002) 43–46
studied by STM after water had evaporated. Current–voltage (I–V) profiles were collected in the constant height mode. Each image presented here is representative of several images taken at different times to ensure reproducibility. X-ray photoelectron spectroscopy (XPS) was performed on a VGESCALAB NKII (UK) spectrometer excited by aluminum Ka X-radiation.
3. Results and discussion
Fig. 1. (a) The chemical structure of CoPc-CoPc. (b) The molecular dimension of H2Pc-H2Pc estimated by Chem3d model.
highly oriented pyrolytic graphite (HOPG, 10 mm 10 mm). All the images were recorded under ambient condition at 22 8C. Typical operating conditions were 0.1 nA current and 0.1 V bias voltage. After the atoms of HOPG were resolved, a drop of CoPc-CoPc solution was applied to the HOPG and
In the XPS experiment, two characteristic peaks were found at 781.0 eV (binding energy) and 796 eV for Co2p corresponding to Co(III) and Co(I), respectively (figure is not shown here). This certified that CoPc-CoPc exist in the configuration of Co(I)Pc-Co(III)Pc. Moreover, the total energies (E (a.u.)) of two different configuration of CoPc-CoPc, Co(II)Pc-Co(II)Pc and Co(I)Pc-Co(III)Pc, were calculated by INDO/S method [19]. The E (a.u.) for Co(II)Pc-Co(II)Pc and Co(I)Pc-Co(III)Pc are 280.8229 and 281.4510, respectively. These results indicate that Co(I)Pc-Co(III)Pc is more stable than Co(II)Pc-Co(II)Pc and phthalocyanine rings can stabilize the different valent of the same metal ions. There is a tendency to disproportionate from Co(II)Pc-Co(II)Pc to Co(I)Pc-Co(III)Pc with the aid of large conjugate system. A large-scale STM topographic image obtained on the dried Co(I)Pc-Co(III)Pc on HOPG is shown in Fig. 2a. In this picture one can see clearly that Co(I)Pc-Co(III)Pc crystallized along one direction on HOPG. The inset showed the crystal lattice of HOPG. Fig. 2b is a zoomed-in image obtained from the area indicated in Fig. 2a. A thinner layer (Fig. 3a) was selected to investigate the I–V characteristics at molecular level. Cross-section analysis (Fig. 3c and d) along the direction marked A and B in Fig. 3a shows steps with the height of 3.97 and 1.95 nm, respectively. If we accept the molecular dimension of H2Pc-H2Pc estimated by Chem3d model shown in Fig. 1b, 1.95 nm will be attributed to the
Fig. 2. (a) A large-scale STM topographic image of Co(I)Pc-Co(III)Pc on highly oriented pyrolytic graphite (HOPG), the inset showed an atomic scale image of HOPG surface, the bias voltage ¼ 0:1 V, the current ¼ 0:1 nA. (b) A zoom-in image obtained from the area indicated in (a).
Y.J. Zhang et al. / Synthetic Metals 128 (2002) 43–46
45
Fig. 3. (a) STM topographic image of Co(I)Pc-Co(III)Pc on HOPG obtained at thinner area. (b) Asymmetric current–voltage (I–V) curves taken at the points marked as þ1 and þ2. (c) and (d) Height cross-section taken in the direction shown as A and B.
thickness of monolayer in an edge-on stack. In the monolayer, Co(I)Pc-Co(III)Pc molecules stand up with the long axial inclined to the substrate surface by about 428 ðsin y ¼ 1:95=2:93Þ and the short axial parallel to the HOPG surface. The step of 3.97 nm is corresponding to the thickness of double layers. The molecules Co(I)Pc-Co(III)Pc packed closely in each layer as illustrated in Fig. 4. The tunneling current versus bias voltage (I–V) relations of Co(I)Pc-Co(III)Pc on HOPG were studied by STM using a Pt/Ir nanotip at a set point of 0.1 nA and 0.1 V. The dc current–voltage characteristics of the casting film taken at the points marked as þ1 and þ2 are shown in Fig. 3b. Curves 1 and 2 show an asymmetric property to zero, indicating rectification behavior. Co(I)Pc-Co(III)Pc is an asymmetrical molecule because the valent of two coupled ions in molecule are different to each other. The asymmetry creates a dipole moment and lead to intramolecular one-direction charge transfer. In this molecule, Co(I) and Co(III) can be regarded as donor and acceptor, respectively, and the benzene-ring linked the two phthalocyanine rings can be regarded as the
bridge mediated electronic coupling between the two Pc rings. The rectification of Co(I)Pc-Co(III)Pc can be viewed in a modification of Aviram–Ratner model [4,9]. At a negative bias, an electron transfers from the nanotip to the lowest unoccupied electronic orbital on the molecule localized on the acceptor species to create DBA (Co(I)PcCo(II)). A molecular orbital localized on the donor site transfers an electron to the HOPG, creating DþBA (Co(II)Pc-Co(II)). Finally, the electron transfers from the reduced acceptor to the oxidized donor, regenerating the DBA species. The intramolecular electron transfer reaction Dþ BA ) DBA is facilitated by weak bridge-mediated electronic coupling between the donor and acceptor units. If the coupling between the acceptor and donor is too strong, the asymmetry of the current–voltage characteristic will be broken, and bidirectional charge transfer should become possible. When the voltage polarity is reversed, the current flow is almost shut. The electron transfer is only within the molecule and a schematic diagram of the current flow at a negative voltage is shown in Fig. 4. The charge transfer is
46
Y.J. Zhang et al. / Synthetic Metals 128 (2002) 43–46
Fig. 4. Schematic diagram of the packing of molecules Co(I)Pc-Co(III)Pc in each layer and the current flow at a negative voltage for Co(I)Pc-Co(III)Pc molecule on HOPG substrate.
easier in one direction of polarity. Moreover, the electric resistance for the monolayer is smaller than that for the double layer, and thus the rectification of the monolayer is better than that of the double layer. The STM experiments have indicated that the asymmetry for I–V curves arises from the material asymmetry.
[3] [4] [5] [6]
[7]
4. Summary The STM studies have indicated that the asymmetry for the I–V curves arises from the asymmetry of this binuclear phthalocyanine (Co(I)Pc-Co(III)Pc). It has a potential application as a unimolecular rectifier that will be of great significance in molecular electronics. Acknowledgements
[8] [9] [10] [11] [12] [13]
The authors acknowledge the National Natural Science Foundation of China (NNSFC), State Key Project Fundamental Research and Chinese Academy of Sciences for financial supports.
[14]
References
[17]
[1] D.H. Waldeck, D.N. Beratan, Science 261 (1993) 576. [2] A.C. Brady, B. Hodder, A.S. Martin, J.R. Sambles, C.P. Eweles, R. Jones, P.R. Briddon, A.M. Musa, C.A. Panetta, D.L. Mattern, J. Mater. Chem. 9 (1999) 2271.
[15] [16]
[18] [19]
R.M. Metzger, J. Mater. Chem. 9 (1999) 2027. R.M. Metzger, Acc. Chem. Res. 32 (1999) 950. A. Aviram, M.A. Ratner, Chem. Phys. Lett. 29 (1974) 277. R.M. Metzger, B. Chen, U. Ho¨ pfner, M.V. Lakshmikantham, D. Vuillaume, T. Kawai, X. Wu, H. Tachibana, T.V. Hughes, H. Sakurai, J.W. Baldwin, C. Hosch, M.P. Cava, L. Brehmer, G.J. Ashwell, J. Am. Chem. Soc. 119 (1997) 10455. M. Pomerantz, A. Aviram, R.A. McCorkle, L. Li, A.G. Schrott, Science 255 (1992) 1115. A. Stabel, P. Herwig, K. Mu¨ llen, J.P. Rabe, Angew. Chem. Int. Ed. Engl. 34 (1994) 1609. A. Dhirani, P.-H. Lin, P. Guyot-Sionnest, R.W. Zehner, L.R. Sita, J. Chem. Phys. 106 (1997) 5249. S. Zhou, Y. Liu, Y. Xu, W. Hu, D.B. Zhu, X. Qiu, C. Wang, C. Bai, Chem. Phys. Lett. 297 (1998) 77. S. Hiller, D. Schlettwein, N.R. Armstrong, D. Wo¨ hrle, J. Mater. Chem. 8 (1998) 945. C.M. Fischer, M. Burghard, S. Roth, K.V. Klitzing, Europhys. Lett. 28 (1994) 129. N. Kobayashi, H. Lam, W.A. Nevin, P. Janda, C.C. Leznoff, Y. Koyama, A. Monden, H. Shirai, J. Am. Chem. Soc. 116 (1994) 879. N. Kobayashi, H. Lam, W.A. Nevin, P. Janda, C.C. Leznoff, A.B.P. Lever, Inorg. Chem. 29 (1990) 3415. J.H. Kim, T.M. Cotton, A.R. Uphaus, C.C. Leznoff, Thin Solid films 159 (1988) 141. Y. Liu, K. Shigehara, M. Hara, A. Yamada, J. Am. Chem. Soc. 113 (1991) 440. Y.J. Zhang, L.S. Li, J. Jin, S. Jiang, Y. Zhao, T.J. Li, X. Du, S. Yang, Langmuir 15 (1999) 2183. Y.J. Zhang, L.S. Li, J. Jin, X. Du, Y. Zhao, T.J. Li, Mol. Cryst. Liq. Cryst. 337 (1999) 257. B. Chen, The Doctoral Thesis of Northeast Normal University, 1997.