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Unoccupied electronic states in phthalocyanine thin films studied by inverse photoemission spectroscopy
Synthetic Metals 133±134 (2003) 673±674
Unoccupied electronic states in phthalocyanine thin ®lms studied by inverse photoemission spectroscopy$ N. Sato*, H. Yoshida, K. Tsutsumi Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Abstract Inverse photoemission (IPE) spectra of thin ®lms of metal-free phthalocyanine (H2Pc), 3d-metal phthalocyanines (MPcs: M Mn, Fe, Co, Ni, Cu and Zn) and lithium phthalocyanine (LiPc) have been measured to directly observe the electronic structures of their unoccupied states. In particular, IPE spectra of MPcs obtained systematically for 3d metals have been studied with the contribution from the 3d-metal orbitals to the electronic structures of the lower unoccupied states focused on. The difference spectra between MPcs (M Mn, Fe, Co, Ni and Cu) and ZnPc as the reference are compared with the reported results from X-ray absorption spectroscopy as well as extended HuÈckel and density functional calculations to show that they re¯ect the density of unoccupied states derived from the central metals with permitting tentative assignments of their features. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Unoccupied states; Electronic structure; Phthalocyanine; Thin ®lm; Inverse photoemission
1. Introduction For the study of organic semiconductors, it is indispensable to grasp their electronic structures both above and below the energy gap as directly as possible. Accordingly, the valence electronic states are often examined by ultraviolet photoemission spectroscopy (UPS), however, the unoccupied states have mostly been observed using indirect methods. Then we have started to directly measure these states in organic thin ®lms using inverse photoemission spectroscopy (IPES) in the vacuum ultraviolet region [1,2]. Metal phthalocyanines (MPcs) attract our attention in various ®elds of basic and applied sciences. Many studies of their valence electronic states using UPS have shown that those electronic structures are quite similar due to the predominant contribution from the p orbitals of the Pc macrocyclic ring [3]. However, there have been only a few research reports on direct observations of unoccupied states in MPcs. In this paper our recent results on IPES measurements of thin ®lms of metal-free phthalocyanine (H2Pc) and MPcs (M Mn, Fe, Co, Ni, Cu, Zn and Li) are $
Yamada Conference LVI, the Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets, ISCOM 2001, abstract number J5Tue. * Corresponding author. Tel.: 81-774-38-3080; fax: 81-774-38-3084. E-mail address: [email protected] (N. Sato).
therefore presented. Especially, the contribution from the 3d-metal orbitals to the electronic structures of the lower unoccupied states in 3d-MPcs was systematically investigated. 2. Experimental All the samples except LiPc, which was provided by Prof. Hitoshi Fujimoto at Kumamoto University, were commercially obtained. They were puri®ed by vacuum sublimation and were evaporated onto polycrystalline gold substrates under a pressure of 10 7 Pa. A sample specimen ®lm of thickness 2±10 nm was transferred under vacuum to the analysis chamber for the IPES measurements. The IPES setup in the Bremsstrahlung isochromat spectroscopy (BIS) mode employed in this work was almost the same as reported previously [1,2]. An electron beam of 3 mm diameter was directed onto the sample ®lm and the electron beam kinetic energy was swept from 4 to 12 eV. The emitted photons were collected by a concave mirror and focussed onto a bandpass photon detector whose sensitivity maximum was at 9.8 eV. Signal pulses from the electron multiplier were counted and normalized to the electron beam current to obtain the IPE spectrum. An overall resolution of 1.0 eV was estimated from the linewidth of a differentiated spectrum of polycrystalline gold at the Fermi edge.
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 2 ) 0 0 4 3 7 - X
674
N. Sato et al. / Synthetic Metals 133±134 (2003) 673±674
3. Results and discussion The IPE spectra observed for H2Pc and MPcs (M Mn, Fe, Co, Ni, Cu and Zn) are marked by two distinct features in the measured energy range and are similar to each other. The spectra of CuPc, NiPc and H2Pc reported by Rocco et al. [4] and that of CuPc by Hill et al. [5] are also very similar to our corresponding spectra in the same energy region. The spectral similarity among the different compounds implies that the spectra are predominantly due to the unoccupied states of the Pc macrocyclic p ring, in the same way as just like their valence electronic states. However, a close look at the observed spectra reveals subtle differences between compounds in the intensity and position of the spectral features. For example, the intensity of the ®rst feature decreases from NiPc to ZnPc, which is consistent with the remark by Rocco et al. that the ®rst feature in their IPE spectra is enhanced for NiPc in comparison with CuPc by increasing occupation of the 3d levels in this order [4]. We regard our spectral variation as different contributions from the unoccupied 3d levels of the central metals. To examine such contributions as precisely as possible, we tried to remove the common contribution from the macrocyclic p system from the spectra. Since ZnPc has no vacant 3d orbitals of the central Zn ion [6], we can employ the ZnPc spectrum as a standard spectrum with no contribution from 3d metal levels. Thus, we will generate spectra that re¯ect the unoccupied density of states (DOS) of the central metals by subtracting the ZnPc one from those of the other MPcs, as shown in Fig. 1. The difference spectra demonstrate signi®cant variation with substitution of the central metal, in contrast to the original IPE spectra. They can be compared with the X-ray absorption (XA) spectra, involving electronic transitions
from a core level to the lower unoccupied state and providing information on the local DOS for the central metal. Thus, the XA spectra, i.e. the 3d, 4s 2p excitation spectra for the central metals, reported for MnPc, FePc, NiPc and CuPc [6] are compared and noted to be remarkably similar with the difference spectra in Fig. 1, especially for FePc, NiPc and CuPc. This indicates that the IPE difference spectrum re¯ects the density of unoccupied states of the central metal in MPcs. With bearing this in mind, the difference spectra are further examined with the help of the reported results of theoretical calculations for 3d-MPcs by means of the extended HuÈckel [7] and density functional (DF) methods [8,9]. The sharp peaks (around 3 eV) in the difference spectra of NiPc and CuPc are assigned to vacant and singly occupied dx2 y2 orbitals (of b1g symmetry in the D4h group, where the x and y axes include the pyrrole nitrogen atoms), respectively, which are derived solely from the central metal atoms. The broad peak (around 3 eV) and the shoulder (around 4 eV) in the CoPc spectrum can be assigned to the b1g orbital (up and down spins) and a singly occupied dz2 orbital (of a1g symmetry with down spin) of cobalt, respectively. In the case of FePc and MnPc, only the b1g orbital (down spin) is tentatively assignable (for the peaks around 1 eV), since more un®lled levels are closely located and the energy resolution of our spectra is insuf®cient to separate them. However, such detailed studies on the contribution from 3d metal orbitals to the unoccupied states in MPcs have been enabled by making use of the IPE difference spectra. In addition, it is notable that LiPc thin ®lms exhibit IPE spectra signi®cantly different from those of H2Pc and M(II)Pc mentioned above due to the difference in electron con®gurations. Acknowledgements This work was partly supported by Grant-in-Aid for Scienti®c Research No. 11304049 and 12CE2005 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References
Fig. 1. IPE difference spectra of thin films of MPcs (M Mn, Fe, Co, Ni and Cu).
[1] N. Sato, H. Yoshida, K. Tsutsumi, J. Elect. Spectrosc. Relat. Phenom. 88±91 (1998) 861. [2] N. Sato, H. Yoshida, K. Tsutsumi, J. Mater. Chem. 10 (2000) 85. [3] W.D. Grobman, E.E. Koch, in: L. Ley, M. Cardona (Eds.), Photoemission in Solids, Vol. II, Springer, Berlin, 1979 (Chapter 5). [4] M.L.M. Rocco, K.-H. Frank, P. Yannoulis, E.E. Koch, J. Chem. Phys. 93 (1990) 6859. [5] I.G. Hill, A. Kahn, Z.G. Soos, R.A. Pascal Jr., Chem. Phys. Lett. 327 (2000) 181. [6] E.E. Koch, Y. Jugnet, F.J. Himpsel, Chem. Phys. Lett. 116 (1985) 7. [7] A.M. Schaffer, M. Gouterman, E.R. Davidson, Theor. Chim. Acta 30 (1973) 9. [8] P.A. Reynolds, B.N. Figgis, Inorg. Chem. 30 (1991) 2294. [9] A. Rosa, E.J. Baerends, Inorg. Chem. 33 (1994) 584.
Unoccupied electronic states in phthalocyanine thin ®lms studied by inverse photoemission spectroscopy$ N. Sato*, H. Yoshida, K. Tsutsumi Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Abstract Inverse photoemission (IPE) spectra of thin ®lms of metal-free phthalocyanine (H2Pc), 3d-metal phthalocyanines (MPcs: M Mn, Fe, Co, Ni, Cu and Zn) and lithium phthalocyanine (LiPc) have been measured to directly observe the electronic structures of their unoccupied states. In particular, IPE spectra of MPcs obtained systematically for 3d metals have been studied with the contribution from the 3d-metal orbitals to the electronic structures of the lower unoccupied states focused on. The difference spectra between MPcs (M Mn, Fe, Co, Ni and Cu) and ZnPc as the reference are compared with the reported results from X-ray absorption spectroscopy as well as extended HuÈckel and density functional calculations to show that they re¯ect the density of unoccupied states derived from the central metals with permitting tentative assignments of their features. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Unoccupied states; Electronic structure; Phthalocyanine; Thin ®lm; Inverse photoemission
1. Introduction For the study of organic semiconductors, it is indispensable to grasp their electronic structures both above and below the energy gap as directly as possible. Accordingly, the valence electronic states are often examined by ultraviolet photoemission spectroscopy (UPS), however, the unoccupied states have mostly been observed using indirect methods. Then we have started to directly measure these states in organic thin ®lms using inverse photoemission spectroscopy (IPES) in the vacuum ultraviolet region [1,2]. Metal phthalocyanines (MPcs) attract our attention in various ®elds of basic and applied sciences. Many studies of their valence electronic states using UPS have shown that those electronic structures are quite similar due to the predominant contribution from the p orbitals of the Pc macrocyclic ring [3]. However, there have been only a few research reports on direct observations of unoccupied states in MPcs. In this paper our recent results on IPES measurements of thin ®lms of metal-free phthalocyanine (H2Pc) and MPcs (M Mn, Fe, Co, Ni, Cu, Zn and Li) are $
Yamada Conference LVI, the Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets, ISCOM 2001, abstract number J5Tue. * Corresponding author. Tel.: 81-774-38-3080; fax: 81-774-38-3084. E-mail address: [email protected] (N. Sato).
therefore presented. Especially, the contribution from the 3d-metal orbitals to the electronic structures of the lower unoccupied states in 3d-MPcs was systematically investigated. 2. Experimental All the samples except LiPc, which was provided by Prof. Hitoshi Fujimoto at Kumamoto University, were commercially obtained. They were puri®ed by vacuum sublimation and were evaporated onto polycrystalline gold substrates under a pressure of 10 7 Pa. A sample specimen ®lm of thickness 2±10 nm was transferred under vacuum to the analysis chamber for the IPES measurements. The IPES setup in the Bremsstrahlung isochromat spectroscopy (BIS) mode employed in this work was almost the same as reported previously [1,2]. An electron beam of 3 mm diameter was directed onto the sample ®lm and the electron beam kinetic energy was swept from 4 to 12 eV. The emitted photons were collected by a concave mirror and focussed onto a bandpass photon detector whose sensitivity maximum was at 9.8 eV. Signal pulses from the electron multiplier were counted and normalized to the electron beam current to obtain the IPE spectrum. An overall resolution of 1.0 eV was estimated from the linewidth of a differentiated spectrum of polycrystalline gold at the Fermi edge.
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 2 ) 0 0 4 3 7 - X
674
N. Sato et al. / Synthetic Metals 133±134 (2003) 673±674
3. Results and discussion The IPE spectra observed for H2Pc and MPcs (M Mn, Fe, Co, Ni, Cu and Zn) are marked by two distinct features in the measured energy range and are similar to each other. The spectra of CuPc, NiPc and H2Pc reported by Rocco et al. [4] and that of CuPc by Hill et al. [5] are also very similar to our corresponding spectra in the same energy region. The spectral similarity among the different compounds implies that the spectra are predominantly due to the unoccupied states of the Pc macrocyclic p ring, in the same way as just like their valence electronic states. However, a close look at the observed spectra reveals subtle differences between compounds in the intensity and position of the spectral features. For example, the intensity of the ®rst feature decreases from NiPc to ZnPc, which is consistent with the remark by Rocco et al. that the ®rst feature in their IPE spectra is enhanced for NiPc in comparison with CuPc by increasing occupation of the 3d levels in this order [4]. We regard our spectral variation as different contributions from the unoccupied 3d levels of the central metals. To examine such contributions as precisely as possible, we tried to remove the common contribution from the macrocyclic p system from the spectra. Since ZnPc has no vacant 3d orbitals of the central Zn ion [6], we can employ the ZnPc spectrum as a standard spectrum with no contribution from 3d metal levels. Thus, we will generate spectra that re¯ect the unoccupied density of states (DOS) of the central metals by subtracting the ZnPc one from those of the other MPcs, as shown in Fig. 1. The difference spectra demonstrate signi®cant variation with substitution of the central metal, in contrast to the original IPE spectra. They can be compared with the X-ray absorption (XA) spectra, involving electronic transitions
from a core level to the lower unoccupied state and providing information on the local DOS for the central metal. Thus, the XA spectra, i.e. the 3d, 4s 2p excitation spectra for the central metals, reported for MnPc, FePc, NiPc and CuPc [6] are compared and noted to be remarkably similar with the difference spectra in Fig. 1, especially for FePc, NiPc and CuPc. This indicates that the IPE difference spectrum re¯ects the density of unoccupied states of the central metal in MPcs. With bearing this in mind, the difference spectra are further examined with the help of the reported results of theoretical calculations for 3d-MPcs by means of the extended HuÈckel [7] and density functional (DF) methods [8,9]. The sharp peaks (around 3 eV) in the difference spectra of NiPc and CuPc are assigned to vacant and singly occupied dx2 y2 orbitals (of b1g symmetry in the D4h group, where the x and y axes include the pyrrole nitrogen atoms), respectively, which are derived solely from the central metal atoms. The broad peak (around 3 eV) and the shoulder (around 4 eV) in the CoPc spectrum can be assigned to the b1g orbital (up and down spins) and a singly occupied dz2 orbital (of a1g symmetry with down spin) of cobalt, respectively. In the case of FePc and MnPc, only the b1g orbital (down spin) is tentatively assignable (for the peaks around 1 eV), since more un®lled levels are closely located and the energy resolution of our spectra is insuf®cient to separate them. However, such detailed studies on the contribution from 3d metal orbitals to the unoccupied states in MPcs have been enabled by making use of the IPE difference spectra. In addition, it is notable that LiPc thin ®lms exhibit IPE spectra signi®cantly different from those of H2Pc and M(II)Pc mentioned above due to the difference in electron con®gurations. Acknowledgements This work was partly supported by Grant-in-Aid for Scienti®c Research No. 11304049 and 12CE2005 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References
Fig. 1. IPE difference spectra of thin films of MPcs (M Mn, Fe, Co, Ni and Cu).
[1] N. Sato, H. Yoshida, K. Tsutsumi, J. Elect. Spectrosc. Relat. Phenom. 88±91 (1998) 861. [2] N. Sato, H. Yoshida, K. Tsutsumi, J. Mater. Chem. 10 (2000) 85. [3] W.D. Grobman, E.E. Koch, in: L. Ley, M. Cardona (Eds.), Photoemission in Solids, Vol. II, Springer, Berlin, 1979 (Chapter 5). [4] M.L.M. Rocco, K.-H. Frank, P. Yannoulis, E.E. Koch, J. Chem. Phys. 93 (1990) 6859. [5] I.G. Hill, A. Kahn, Z.G. Soos, R.A. Pascal Jr., Chem. Phys. Lett. 327 (2000) 181. [6] E.E. Koch, Y. Jugnet, F.J. Himpsel, Chem. Phys. Lett. 116 (1985) 7. [7] A.M. Schaffer, M. Gouterman, E.R. Davidson, Theor. Chim. Acta 30 (1973) 9. [8] P.A. Reynolds, B.N. Figgis, Inorg. Chem. 30 (1991) 2294. [9] A. Rosa, E.J. Baerends, Inorg. Chem. 33 (1994) 584.