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Electronic structures of organic salt DMTSA-BF4 using photoelectron spectromicroscopy
Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 1013–1018 www.elsevier.nl / locate / elspec
Electronic structures of organic salt DMTSA-BF 4 using photoelectron spectromicroscopy a, b c c Yuichi Haruyama *, Toyohiko Kinoshita , Kazuo Takimiya , Tetsuo Otsubo , Chikako Nakano d , Kyuya Yakushi d a
Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology, 3 -1 -2 Kouto, Kamigori, Ako 678 -1205, Japan b Synchrotron Radiation Laboratory, Institute for Solid State Physics, University of Tokyo, Kashiwa 277 -8581, Japan c Faculty of Engineering, Hiroshima University, 1 -4 -1 Kagamiyama, Higashi-Hiroshima 739 -8527, Japan d Institute for Molecular Science, Okazaki 444 -8585, Japan Received 8 August 2000; received in revised form 23 September 2000; accepted 3 October 2000
Abstract The electronic structures of organic salt DMTSA-BF 4 , where DMTSA is 2,3-dimethyltetraseleno-anthracene, have been studied with photoelectron spectromicroscopy at various photon energies. The atomic orbital characters of the spectral features observed in the valence band region were determined from the photon energy dependence of the photoionization cross-section. The features between |1 and |4 eV are predominantly derived from both Se 4p and C 2p states. The features at |6 eV, between |7 and |10 eV, and between |10 and |20 eV are predominantly derived from Se 4p, C 2p, and Se 4s and / or C 2s states, respectively. The F 2p states originating from the counter anion BF 4 are located at |8 eV. By comparing the Se 3d core-level photoemission spectra between DMTSA-BF 4 and DMTSA, it is suggested that DMTSA-BF 4 has a mixed valence state. 2001 Elsevier Science B.V. All rights reserved. Keywords: DMTSA-BF 4 ; Organic crystals; Electronic structure; Photoelectron spectroscopies; Photoelectron spectromicroscopies; Synchrotron radiation
1. Introduction Until recently, it has been considered that the charge-transfer salt with 1:1 stoichiometry becomes a Mott insulator due to the correlation effect since the 1:1 charge-transfer salt has a half-filled band. However, the 1:1 charge-transfer salt DMTSA-BF 4 , where DMTSA is 2,3-dimethyltetraseleno-anth*Corresponding author. Tel.: 181-791-58-0474; fax: 181-791-580242. E-mail address: [email protected] (Y. Haruyama).
racene, has attracted the interest of many researchers because of the high electrical conductivity and metallic physical properties [1,2]. In the temperature region between the room temperature and |150 K, the electrical resistivity of DMTSA-BF 4 decreases with a positive slope with decreasing the temperature and DMTSA-BF 4 behaves as a metal. At |150 K DMTSA-BF 4 shows the metal–insulator transition and the electrical resistivity below |150 K increases with a negative slope as an insulator with decreasing the temperature. DMTSA-BF 4 has an orthorhombic crystal structure with a space group Cmcm and
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00328-5
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DMTSA molecules are stacked along the c-axis alternatively turning over the direction of the molecular axis. The column structure along the c-axis suggests that DMTSA-BF 4 is one-dimensional conductor with p-electronic interaction and the metal– insulator transition around 150 K is regarded as Peierls transition, not as Mott transition [2]. In addition, as a reason for the metallic physical properties, Dong et al. suggested that the correlation effect is weak. However, whether there is another reason for the metallic physical properties is not clear at present. So far, the electronic structures such as the band structure, the density of states, the orbital characters and the charge state for many organic salts have been studied by the photoemission spectroscopy [3]. In this experiment, in order to investigate the reason for the metallic physical properties, we have performed the photoemission experiments for the organic salt DMTSA-BF 4 . Since the size of synthesized DMTSA-BF 4 crystal is not large enough to carry out the ordinary photoemission experiments, we used the photoelectron spectromicroscope equipment that the photoemission measurements of specific small area are possible [4,5]. In this paper, we report the photon energy (hn ) dependence of the valence band photoemission spectra for DMTSA-BF 4 and DMTSA. The atomic orbital characters of the observed spectral features are investigated from the hn dependence of the photoionization cross-section. The Se 3d core-level photoemission measurements were also performed and the difference in the electronic structure between DMTSA-BF 4 and DMTSA is discussed.
In the case of the Mg Ka X-ray source with hn 5 1253.6 eV, the total energy resolution was |1.0 eV FWHM. The base pressure was |2310 28 Pa. All photoemission spectra shown in this paper were recorded at room temperature. No spectral change caused by the light radiation damage and the contaminations was observed during the photoemission measurements. Needle-like single crystals DMTSA-BF 4 and DMTSA were synthesized by electrochemical oxidation and were characterized by X-ray diffraction, electrical resistivity, magnetic susceptibility and ESR measurements [2]. The typical sample size used here was less than 230.230.1 mm 3 . The rectangular plate with 1 mm height and 1.530.05 mm 2 cross-section that is smaller than the sample cross-section was attached on the sample holder. Each single crystal was glued with carbon paste on a rectangular plate not to detect the signals from the sample holder. By using this sample holder and choosing the detection area of f 50 mm in the photoelectron spectromicroscope equipment, we succeeded in obtaining the signals from the only sample region. The clean surfaces were obtained by scraping the sample surface using an edge of a razor. In addition to the scraping method, the in situ evaporation was performed for DMTSA. As the clean surfaces were prepared in these methods, the angle-integrated photoemission spectra were obtained in this experiment. The cleanliness was checked by X-ray photoemission spectroscopy for the absence of extra features arising from the contaminations and the photoemission spectra obtained from the clean surfaces were always reproducible. The evaporated Au films on the sample holder were used for the Fermi level (EF ) reference.
2. Experimental Photoemission measurements were carried out at the UVSOR facility (BL5B), Institute for Molecular Science using the photoelectron spectromicroscope equipment (Fisons, ESCALAB-220i-XL), which is mounted with the X-ray source and the hemispherical electrostatic analyzer combined with the electrostatic and magnetic lens systems for magnification [4,5]. The total instrumental energy resolution was 0.3–0.6 eV full width at half maximum (FWHM), depending on hn in the energy range of 30–100 eV.
3. Results and discussion Fig. 1 shows the valence band photoemission spectra of the evaporated DMTSA taken with hn between 30 and 1253.6 eV. The observed photoemission spectra were normalized to the maximum intensity of the valence band. Twelve features in the valence band region were observed and marked by a bar as shown in the figure, respectively. The relative intensity of these features varies with hn. At hn 530–
Y. Haruyama et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1013 – 1018
Fig. 1. The valence band photoemission spectra of the evaporated DMTSA taken with hn between 30 and 1253.6 eV.
40 eV the feature at |6 eV is the most prominent and the features at |7, |8 and |9 eV have comparable intensities with the feature at |6 eV, taking the background signal caused by the secondary electron into consideration. The relative intensities of features at |7, |8 and |9 eV to the feature at |6 eV increase with increasing hn to |60 eV. With further increasing hn, they decrease gradually and the feature at |6 eV becomes most prominent at hn 51253.6 eV. The relative intensities of features at |1, |2 and |3 eV to the feature at |6 eV behave similarly to features at |7, |8 and |9 eV. That is, at hn 5|60 eV, the intensities of features at |1, |2 and |3 eV increase relatively. When hn becomes higher, the relative intensities of features at |1, |2 and |3 eV to the feature at |6 eV become small. At the binding energy region between 10 and 20 eV, five features were observed with smaller intensity as compared with the feature at |6 eV in the hn region between 30 and 100 eV. These features are enhanced a little around hn 560–80 eV. At hn 51253.6 eV, these
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features, especially at |13.5, |16 and |17.5 eV, are enhanced with comparable intensity to the feature at |6 eV and become very broad. The observed broad features resemble to those of the previous photoemission study for Se that the contribution of Se 4s electrons ranges widely between 8–17 eV binding energy [6,7]. The hn dependence of the photoemission spectra is predominantly ascribed to that of the photoionization cross-section. Here, we discuss the hn dependence of the photoemission spectra, comparing with the Yeh and Lindau’s calculation of the photoionization cross-section of atomic state [8]. Although there may be a solid-state effect, the overall tendency of the hn dependence agrees with that in the solid state [9–12]. Therefore, we compared Yeh and Lindau’s calculation with the hn dependence of our obtained photoemission spectra. According to Yeh and Lindau’s calculation, the photoionization cross-section of Se 4p electrons is comparable with that of C 2p electrons at hn 5|30 eV. The relative photoionization cross-section of C 2p electrons to Se 4p electrons increases with hn and has a maximum at hn 550–60 eV. With further increasing hn, the relative photoionization cross-section of C 2p electrons to Se 4p electrons decreases monotonically. From the hn dependence of the relative photoionization crosssection, the contribution of the C 2p electrons would enhance at hn 550–60 eV and decrease considerably at higher hn 51253.6 eV. Therefore, the features at |7, |8 and |9 eV are predominantly derived from C 2p electrons. On the other hand, the feature at |6 eV, which is dominant at hn 51253.6 eV, is predominantly from Se 4p electrons. Since the features at |1, |2 and |3 eV are enhanced at hn 560 eV and the intensity of these features is preserved at hn 51253.6 eV, the features at |1, |2 and |3 eV are predominantly derived from both Se 4p and C 2p electrons. As for the features at |7, |8 and |9 eV, there is a small contribution of Se 4p electrons in the same reason for features at |1, |2 and |3 eV. The relative photoionization cross-sections of Se 4s and C 2s electrons to Se 4p electrons have a maximum around hn 560–80 eV. With increasing hn, the relative photoionization cross-section of C 2s electrons decreases gradually while that of Se 4s electrons decreases once and increases at higher hn. At hn 5 1253.6 eV the photoionization cross-section of Se 4s
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electrons is comparable with that of Se 4p electrons. The above hn dependence of the relative photoionization cross-section indicates that the features between 10 and 20 eV are predominantly derived from Se 4s and / or C 2s electrons. For the scraped DMTSA, the hn dependence of the valence band photoemission spectra was shown in Fig. 2a. Seven features at |1, |3, |6, |8.5, |11, |13.5 and |17 eV are observed in this valence band region. The hn dependence of these features is similar to that for the evaporated DMTSA but the spectral features for the scraped DMTSA are unresolved and broad as compared with those for the evaporated DMTSA. These features for the scraped DMTSA are ascribed to the increased pass energy because the small detection area of f 50 mm was selected. Although the core-level photoemission spectra for the scraped DMTSA were compared with those for the evaporated DMTSA, the clear difference was not observed (not shown). This indicates that the electronic structure for DMTSA does not change by the scraping the sample. Therefore, the orbital characters of the observed features for the scraped DMTSA are identical to those for the evaporated DMTSA. Fig. 2b shows the valence band photoemission spectra of the scraped DMTSA-BF 4 taken with hn between 30 and 1253.6 eV. Seven features at |1, |3, |6, |8, |11, |13.5 and |17 eV are observed in this valence band region. The features at |1, |3, |6, |13.5 and |17 eV have a similar hn dependence to those for DMTSA but features at |8 and |11 eV are enhanced as compared with features at |8.5 and |11 eV for DMTSA. The enhancement is ascribed to the contribution of the counter anion BF 4 . According to Yeh and Lindau’s calculation of the photoionization cross-section [8], the photoionization cross-section of F 2p electrons is considerably larger than that of B 2p electrons at hn ,100 eV and the contribution of the B 2p electrons is negligible. Therefore, the features at |8 and |11 eV are predominantly derived from the F 2p and C 2p electrons since there is a contribution of C 2p electrons for DMTSA in this binding energy region. As for the other features, the orbital characters of observed features are identical to those for DMTSA. Features at |1 and |3 eV for DMTSA-BF 4 are ascribed to both C 2p and Se 4p electrons. Features at |6 eV and 11–17 eV are
Fig. 2. The valence band photoemission spectra of the scraped (a) DMTSA and (b) DMTSA-BF 4 taken with hn between 30 and 1253.6 eV. The inset of Fig. 2(b) shows the photoemission spectrum near EF of DMTSA-BF 4 . The detection area was f 50 mm.
ascribed to Se 4p and Se 4s and / or C 2s electrons, respectively. The photoemission spectrum near EF of DMTSA-
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BF 4 is shown in the inset of Fig. 2(b). The used hn was 21.2 eV. From the above mentioned assignment, the feature near EF is ascribed to both C 2p and Se 4p electrons. The obtained orbital characters near EF shows the importance of the p-electronic interaction. This partially supports the previous result that the one-dimensional conductivity consists of uniform donor stacking columns with effective p-electronic and strong non-bonded Se–Se interaction [1]. Although it is reported from the band calculation and the electrical conductivity [2] that DMTSA-BF 4 shows a metallic character at the room temperature, the clear Fermi edge was not observed. The density of states at EF is considerably suppressed as compared with that of the band calculation [2]. The spectral suppression at EF is probably due to the one-dimensional nature and / or the correlation effect as reported in the pseudo-one dimensional conductor DCNQI complexes [13]. In Fig. 3, the Se 3d core-level photoemission spectrum for DMTSA-BF 4 is compared with that for DMTSA. The Se 3d core-level photoemission spectrum for DMTSA-BF 4 is broader than that for DMTSA and shows the tail at the higher binding energy side. Now, we discuss some possibilities for the reason of the spectral difference between DMTSA-BF 4 and DMTSA. At the room tempera-
Fig. 3. The Se 3d core-level photoemission spectra of the scraped DMTSA-BF 4 (closed dots) and DMTSA (open dots) taken with hn 51253.6 eV. The detection area was f 50 mm. Using the photoemission spectrum of DMTSA, the photoemission spectrum of DMTSA-BF 4 is deconvoluted into two components. The another component and resulting curve are shown in broken and solid lines, respectively.
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ture, DMTSA-BF 4 is in a metallic state while DMTSA is not in a metallic state. In general, it is known that the core-level photoemission spectrum in a metallic state shows the asymmetric line shape (called Doniach-Sunjic line shape) with the tail at the higher binding energy side [14]. The Doniach-Sunjic line shape arises from the many body effect that conduction electrons are scattered by the potential due to a suddenly created core hole. If this is the case, the Se 3d core-level photoemission spectrum below the metal–insulator transition temperature would be identical to that for DMTSA. However, the temperature dependence of the Se 3d core-level spectra for DMTSA-BF 4 did not show any spectral changes (not shown). For the reason, the possibility of Doniach-Sunjic line shape is excluded. In addition, charging is also excluded since DMTSA-BF 4 is in a metallic state at the room temperature. Next, we consider the possibility that the asymmetric shape is caused by the other chemical components for some reasons. In the previous photoemission study for various TMTSF complexes [15], Ikemoto et al. found that the Se 3d core-level photoemission spectrum in a mixed valence state is broader and shows the tail for the higher binding energy side as compared with that in a neutral state. With regard to the spectral shape and width, our observed Se 3d core-level photoemission spectra for DMTSA and DMTSA-BF 4 are similar to those in a neutral state and in a mixed valence state, respectively. Therefore, it is suggested that DMTSA-BF 4 is in a mixed valence state. In order to confirm that DMTSA-BF 4 is in the mixed valence state, the deconvolution analysis of the photoemission spectrum of DMTSA-BF 4 was performed using the photoemission spectrum of DMTSA. As shown in Fig. 3, it is found that the photoemission spectrum of DMTSA-BF 4 is deconvoluted into two components centered at 56.0 and 57.5 eV, respectively. The ratio of another component centered at 57.5 eV to the dominant component centered at 56.0 eV is estimated to be about 10%. From the reflectance spectrum [2], Dong et al. pointed out the fluctuation of the lattice dimerization even above the phase transition temperature. If the lattice dimerization occurred, the charge distribution of neighboring DMTSA molecules would change with the symmetry break. Then, two components with identical intensity should
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appear. Therefore, our observed another component would not be due to the charge fluctuation caused by the lattice dimerization. Although we presumably infer that the metallic physical properties observed in the electric resistivity are caused by another component, the reason why another component appears is not clear at present. As a possible reason of another component, it may be considered a little aberration from the stoichiometry and the influence of the impurity. The more precise element specific measurements such as X-ray fluorescence analysis are desired in order to clarify the stoichiometry and the influence of the impurity.
4. Conclusion We have succeeded in measuring the electronic structures of small organic salts DMTSA-BF 4 and DMTSA using the photoelectron spectromicroscopy. The atomic orbital characters of observed spectral features in the valence band region are determined from the hn dependence of the photoionization cross-section. In addition, by comparing the Se 3d core-level photoemission spectra between DMTSABF 4 and DMTSA, it is suggested that DMTSA-BF 4 has a mixed valence state.
Acknowledgements This work is partially supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Science, Sports and Culture, and supported by the Joint Studies Program (1998–1999) of the Institute for Molecular Science. We are pleased to thank the staff of the UVSOR facility for excellent support.
References [1] K. Takimiya, A. Ohnishi, Y. Aso, T. Otsubo, F. Ogura, K. Kawabata, K. Tanaka, M. Mizutani, Bull. Chem. Soc. Jpn. 67 (1994) 766. [2] J. Dong, K. Yakushi, K. Takimiya, T. Otsubo, J. Phys. Soc. Jpn. 67 (1998) 971. ¨ [3] S. Hufner, in: Photoelectron Spectroscopy Springer Series in Solid State Science, Vol. 82, Springer, Berlin, 1995, and Refs. therein. [4] T. Kinoshita, K.G. Nath, Y. Haruyama, M. Watanabe, S. Yagi, S. Kimura, A. Fanelsa, J. Electron Spectrosc. Relat. Phenom. 92 (1998) 165. [5] T. Kinoshita, K.G. Nath, M. Watanabe, S. Yagi, S. Kimura, A. Fanelsa, UVSOR Activity Report 1996, p. 154. [6] N.J. Shevchik, J. Tejeda, M. Cardona, D.W. Langer, Solid State Commun. 12 (1973) 1285. [7] T. Takahashi, K. Murano, K. Nagata, Y. Miyamoto, Phys. Rev. B 28 (1983) 4893. [8] J.J. Yeh, I. Lindau, Atomic Data Nucl. Data Tables 32 (1985) 1. [9] I.H. Inoue, M. Watanabe, T. Kinoshita, A. Kakizaki, R. Kato, A. Kobayashi, H. Kobayashi, A. Fujimori, Phys. Rev. B 47 (1993) 12917. [10] A. Tanaka, A. Chainani, T. Yokoya, T. Takahashi, T. Miyazaki, S. Hasegawa, T. Mori, Phys. Rev. B 52 (1995) 7951. [11] T. Takahashi, T. Yokoya, A. Chainani, H. Kumigashira, O. Akaki, R. Kato, Phys. Rev. B 53 (1996) 1790. [12] A. Sekiyama, A. Fujimori, S. Aonuma, R. Kato, T. Iwasaki, S. Suga, H. Ishii, T. Matsushita, T. Miyahara, J. Electron Spectrosc. Relat. Phenom. 88–91 (1998) 939. [13] Y. Haruyama, K.G. Nath, S. Kimura, Y. Ufuktepe, T. Kinoshita, K. Hiraki, K. Kanoda, Solid State Commun. 110 (1999) 17. ¨ [14] S. Hufner, in: Photoelectron Spectroscopy, Springer Series in Solid State Science, Vol. 82, Springer, Berlin, 1995, p. 112. [15] I. Ikemoto, K. Kikuchi, K. Yakushi, H. Kuroda, K. Kobayashi, Solid State Commun. 42 (1982) 257.
Electronic structures of organic salt DMTSA-BF 4 using photoelectron spectromicroscopy a, b c c Yuichi Haruyama *, Toyohiko Kinoshita , Kazuo Takimiya , Tetsuo Otsubo , Chikako Nakano d , Kyuya Yakushi d a
Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology, 3 -1 -2 Kouto, Kamigori, Ako 678 -1205, Japan b Synchrotron Radiation Laboratory, Institute for Solid State Physics, University of Tokyo, Kashiwa 277 -8581, Japan c Faculty of Engineering, Hiroshima University, 1 -4 -1 Kagamiyama, Higashi-Hiroshima 739 -8527, Japan d Institute for Molecular Science, Okazaki 444 -8585, Japan Received 8 August 2000; received in revised form 23 September 2000; accepted 3 October 2000
Abstract The electronic structures of organic salt DMTSA-BF 4 , where DMTSA is 2,3-dimethyltetraseleno-anthracene, have been studied with photoelectron spectromicroscopy at various photon energies. The atomic orbital characters of the spectral features observed in the valence band region were determined from the photon energy dependence of the photoionization cross-section. The features between |1 and |4 eV are predominantly derived from both Se 4p and C 2p states. The features at |6 eV, between |7 and |10 eV, and between |10 and |20 eV are predominantly derived from Se 4p, C 2p, and Se 4s and / or C 2s states, respectively. The F 2p states originating from the counter anion BF 4 are located at |8 eV. By comparing the Se 3d core-level photoemission spectra between DMTSA-BF 4 and DMTSA, it is suggested that DMTSA-BF 4 has a mixed valence state. 2001 Elsevier Science B.V. All rights reserved. Keywords: DMTSA-BF 4 ; Organic crystals; Electronic structure; Photoelectron spectroscopies; Photoelectron spectromicroscopies; Synchrotron radiation
1. Introduction Until recently, it has been considered that the charge-transfer salt with 1:1 stoichiometry becomes a Mott insulator due to the correlation effect since the 1:1 charge-transfer salt has a half-filled band. However, the 1:1 charge-transfer salt DMTSA-BF 4 , where DMTSA is 2,3-dimethyltetraseleno-anth*Corresponding author. Tel.: 181-791-58-0474; fax: 181-791-580242. E-mail address: [email protected] (Y. Haruyama).
racene, has attracted the interest of many researchers because of the high electrical conductivity and metallic physical properties [1,2]. In the temperature region between the room temperature and |150 K, the electrical resistivity of DMTSA-BF 4 decreases with a positive slope with decreasing the temperature and DMTSA-BF 4 behaves as a metal. At |150 K DMTSA-BF 4 shows the metal–insulator transition and the electrical resistivity below |150 K increases with a negative slope as an insulator with decreasing the temperature. DMTSA-BF 4 has an orthorhombic crystal structure with a space group Cmcm and
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00328-5
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DMTSA molecules are stacked along the c-axis alternatively turning over the direction of the molecular axis. The column structure along the c-axis suggests that DMTSA-BF 4 is one-dimensional conductor with p-electronic interaction and the metal– insulator transition around 150 K is regarded as Peierls transition, not as Mott transition [2]. In addition, as a reason for the metallic physical properties, Dong et al. suggested that the correlation effect is weak. However, whether there is another reason for the metallic physical properties is not clear at present. So far, the electronic structures such as the band structure, the density of states, the orbital characters and the charge state for many organic salts have been studied by the photoemission spectroscopy [3]. In this experiment, in order to investigate the reason for the metallic physical properties, we have performed the photoemission experiments for the organic salt DMTSA-BF 4 . Since the size of synthesized DMTSA-BF 4 crystal is not large enough to carry out the ordinary photoemission experiments, we used the photoelectron spectromicroscope equipment that the photoemission measurements of specific small area are possible [4,5]. In this paper, we report the photon energy (hn ) dependence of the valence band photoemission spectra for DMTSA-BF 4 and DMTSA. The atomic orbital characters of the observed spectral features are investigated from the hn dependence of the photoionization cross-section. The Se 3d core-level photoemission measurements were also performed and the difference in the electronic structure between DMTSA-BF 4 and DMTSA is discussed.
In the case of the Mg Ka X-ray source with hn 5 1253.6 eV, the total energy resolution was |1.0 eV FWHM. The base pressure was |2310 28 Pa. All photoemission spectra shown in this paper were recorded at room temperature. No spectral change caused by the light radiation damage and the contaminations was observed during the photoemission measurements. Needle-like single crystals DMTSA-BF 4 and DMTSA were synthesized by electrochemical oxidation and were characterized by X-ray diffraction, electrical resistivity, magnetic susceptibility and ESR measurements [2]. The typical sample size used here was less than 230.230.1 mm 3 . The rectangular plate with 1 mm height and 1.530.05 mm 2 cross-section that is smaller than the sample cross-section was attached on the sample holder. Each single crystal was glued with carbon paste on a rectangular plate not to detect the signals from the sample holder. By using this sample holder and choosing the detection area of f 50 mm in the photoelectron spectromicroscope equipment, we succeeded in obtaining the signals from the only sample region. The clean surfaces were obtained by scraping the sample surface using an edge of a razor. In addition to the scraping method, the in situ evaporation was performed for DMTSA. As the clean surfaces were prepared in these methods, the angle-integrated photoemission spectra were obtained in this experiment. The cleanliness was checked by X-ray photoemission spectroscopy for the absence of extra features arising from the contaminations and the photoemission spectra obtained from the clean surfaces were always reproducible. The evaporated Au films on the sample holder were used for the Fermi level (EF ) reference.
2. Experimental Photoemission measurements were carried out at the UVSOR facility (BL5B), Institute for Molecular Science using the photoelectron spectromicroscope equipment (Fisons, ESCALAB-220i-XL), which is mounted with the X-ray source and the hemispherical electrostatic analyzer combined with the electrostatic and magnetic lens systems for magnification [4,5]. The total instrumental energy resolution was 0.3–0.6 eV full width at half maximum (FWHM), depending on hn in the energy range of 30–100 eV.
3. Results and discussion Fig. 1 shows the valence band photoemission spectra of the evaporated DMTSA taken with hn between 30 and 1253.6 eV. The observed photoemission spectra were normalized to the maximum intensity of the valence band. Twelve features in the valence band region were observed and marked by a bar as shown in the figure, respectively. The relative intensity of these features varies with hn. At hn 530–
Y. Haruyama et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 1013 – 1018
Fig. 1. The valence band photoemission spectra of the evaporated DMTSA taken with hn between 30 and 1253.6 eV.
40 eV the feature at |6 eV is the most prominent and the features at |7, |8 and |9 eV have comparable intensities with the feature at |6 eV, taking the background signal caused by the secondary electron into consideration. The relative intensities of features at |7, |8 and |9 eV to the feature at |6 eV increase with increasing hn to |60 eV. With further increasing hn, they decrease gradually and the feature at |6 eV becomes most prominent at hn 51253.6 eV. The relative intensities of features at |1, |2 and |3 eV to the feature at |6 eV behave similarly to features at |7, |8 and |9 eV. That is, at hn 5|60 eV, the intensities of features at |1, |2 and |3 eV increase relatively. When hn becomes higher, the relative intensities of features at |1, |2 and |3 eV to the feature at |6 eV become small. At the binding energy region between 10 and 20 eV, five features were observed with smaller intensity as compared with the feature at |6 eV in the hn region between 30 and 100 eV. These features are enhanced a little around hn 560–80 eV. At hn 51253.6 eV, these
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features, especially at |13.5, |16 and |17.5 eV, are enhanced with comparable intensity to the feature at |6 eV and become very broad. The observed broad features resemble to those of the previous photoemission study for Se that the contribution of Se 4s electrons ranges widely between 8–17 eV binding energy [6,7]. The hn dependence of the photoemission spectra is predominantly ascribed to that of the photoionization cross-section. Here, we discuss the hn dependence of the photoemission spectra, comparing with the Yeh and Lindau’s calculation of the photoionization cross-section of atomic state [8]. Although there may be a solid-state effect, the overall tendency of the hn dependence agrees with that in the solid state [9–12]. Therefore, we compared Yeh and Lindau’s calculation with the hn dependence of our obtained photoemission spectra. According to Yeh and Lindau’s calculation, the photoionization cross-section of Se 4p electrons is comparable with that of C 2p electrons at hn 5|30 eV. The relative photoionization cross-section of C 2p electrons to Se 4p electrons increases with hn and has a maximum at hn 550–60 eV. With further increasing hn, the relative photoionization cross-section of C 2p electrons to Se 4p electrons decreases monotonically. From the hn dependence of the relative photoionization crosssection, the contribution of the C 2p electrons would enhance at hn 550–60 eV and decrease considerably at higher hn 51253.6 eV. Therefore, the features at |7, |8 and |9 eV are predominantly derived from C 2p electrons. On the other hand, the feature at |6 eV, which is dominant at hn 51253.6 eV, is predominantly from Se 4p electrons. Since the features at |1, |2 and |3 eV are enhanced at hn 560 eV and the intensity of these features is preserved at hn 51253.6 eV, the features at |1, |2 and |3 eV are predominantly derived from both Se 4p and C 2p electrons. As for the features at |7, |8 and |9 eV, there is a small contribution of Se 4p electrons in the same reason for features at |1, |2 and |3 eV. The relative photoionization cross-sections of Se 4s and C 2s electrons to Se 4p electrons have a maximum around hn 560–80 eV. With increasing hn, the relative photoionization cross-section of C 2s electrons decreases gradually while that of Se 4s electrons decreases once and increases at higher hn. At hn 5 1253.6 eV the photoionization cross-section of Se 4s
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electrons is comparable with that of Se 4p electrons. The above hn dependence of the relative photoionization cross-section indicates that the features between 10 and 20 eV are predominantly derived from Se 4s and / or C 2s electrons. For the scraped DMTSA, the hn dependence of the valence band photoemission spectra was shown in Fig. 2a. Seven features at |1, |3, |6, |8.5, |11, |13.5 and |17 eV are observed in this valence band region. The hn dependence of these features is similar to that for the evaporated DMTSA but the spectral features for the scraped DMTSA are unresolved and broad as compared with those for the evaporated DMTSA. These features for the scraped DMTSA are ascribed to the increased pass energy because the small detection area of f 50 mm was selected. Although the core-level photoemission spectra for the scraped DMTSA were compared with those for the evaporated DMTSA, the clear difference was not observed (not shown). This indicates that the electronic structure for DMTSA does not change by the scraping the sample. Therefore, the orbital characters of the observed features for the scraped DMTSA are identical to those for the evaporated DMTSA. Fig. 2b shows the valence band photoemission spectra of the scraped DMTSA-BF 4 taken with hn between 30 and 1253.6 eV. Seven features at |1, |3, |6, |8, |11, |13.5 and |17 eV are observed in this valence band region. The features at |1, |3, |6, |13.5 and |17 eV have a similar hn dependence to those for DMTSA but features at |8 and |11 eV are enhanced as compared with features at |8.5 and |11 eV for DMTSA. The enhancement is ascribed to the contribution of the counter anion BF 4 . According to Yeh and Lindau’s calculation of the photoionization cross-section [8], the photoionization cross-section of F 2p electrons is considerably larger than that of B 2p electrons at hn ,100 eV and the contribution of the B 2p electrons is negligible. Therefore, the features at |8 and |11 eV are predominantly derived from the F 2p and C 2p electrons since there is a contribution of C 2p electrons for DMTSA in this binding energy region. As for the other features, the orbital characters of observed features are identical to those for DMTSA. Features at |1 and |3 eV for DMTSA-BF 4 are ascribed to both C 2p and Se 4p electrons. Features at |6 eV and 11–17 eV are
Fig. 2. The valence band photoemission spectra of the scraped (a) DMTSA and (b) DMTSA-BF 4 taken with hn between 30 and 1253.6 eV. The inset of Fig. 2(b) shows the photoemission spectrum near EF of DMTSA-BF 4 . The detection area was f 50 mm.
ascribed to Se 4p and Se 4s and / or C 2s electrons, respectively. The photoemission spectrum near EF of DMTSA-
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BF 4 is shown in the inset of Fig. 2(b). The used hn was 21.2 eV. From the above mentioned assignment, the feature near EF is ascribed to both C 2p and Se 4p electrons. The obtained orbital characters near EF shows the importance of the p-electronic interaction. This partially supports the previous result that the one-dimensional conductivity consists of uniform donor stacking columns with effective p-electronic and strong non-bonded Se–Se interaction [1]. Although it is reported from the band calculation and the electrical conductivity [2] that DMTSA-BF 4 shows a metallic character at the room temperature, the clear Fermi edge was not observed. The density of states at EF is considerably suppressed as compared with that of the band calculation [2]. The spectral suppression at EF is probably due to the one-dimensional nature and / or the correlation effect as reported in the pseudo-one dimensional conductor DCNQI complexes [13]. In Fig. 3, the Se 3d core-level photoemission spectrum for DMTSA-BF 4 is compared with that for DMTSA. The Se 3d core-level photoemission spectrum for DMTSA-BF 4 is broader than that for DMTSA and shows the tail at the higher binding energy side. Now, we discuss some possibilities for the reason of the spectral difference between DMTSA-BF 4 and DMTSA. At the room tempera-
Fig. 3. The Se 3d core-level photoemission spectra of the scraped DMTSA-BF 4 (closed dots) and DMTSA (open dots) taken with hn 51253.6 eV. The detection area was f 50 mm. Using the photoemission spectrum of DMTSA, the photoemission spectrum of DMTSA-BF 4 is deconvoluted into two components. The another component and resulting curve are shown in broken and solid lines, respectively.
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ture, DMTSA-BF 4 is in a metallic state while DMTSA is not in a metallic state. In general, it is known that the core-level photoemission spectrum in a metallic state shows the asymmetric line shape (called Doniach-Sunjic line shape) with the tail at the higher binding energy side [14]. The Doniach-Sunjic line shape arises from the many body effect that conduction electrons are scattered by the potential due to a suddenly created core hole. If this is the case, the Se 3d core-level photoemission spectrum below the metal–insulator transition temperature would be identical to that for DMTSA. However, the temperature dependence of the Se 3d core-level spectra for DMTSA-BF 4 did not show any spectral changes (not shown). For the reason, the possibility of Doniach-Sunjic line shape is excluded. In addition, charging is also excluded since DMTSA-BF 4 is in a metallic state at the room temperature. Next, we consider the possibility that the asymmetric shape is caused by the other chemical components for some reasons. In the previous photoemission study for various TMTSF complexes [15], Ikemoto et al. found that the Se 3d core-level photoemission spectrum in a mixed valence state is broader and shows the tail for the higher binding energy side as compared with that in a neutral state. With regard to the spectral shape and width, our observed Se 3d core-level photoemission spectra for DMTSA and DMTSA-BF 4 are similar to those in a neutral state and in a mixed valence state, respectively. Therefore, it is suggested that DMTSA-BF 4 is in a mixed valence state. In order to confirm that DMTSA-BF 4 is in the mixed valence state, the deconvolution analysis of the photoemission spectrum of DMTSA-BF 4 was performed using the photoemission spectrum of DMTSA. As shown in Fig. 3, it is found that the photoemission spectrum of DMTSA-BF 4 is deconvoluted into two components centered at 56.0 and 57.5 eV, respectively. The ratio of another component centered at 57.5 eV to the dominant component centered at 56.0 eV is estimated to be about 10%. From the reflectance spectrum [2], Dong et al. pointed out the fluctuation of the lattice dimerization even above the phase transition temperature. If the lattice dimerization occurred, the charge distribution of neighboring DMTSA molecules would change with the symmetry break. Then, two components with identical intensity should
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appear. Therefore, our observed another component would not be due to the charge fluctuation caused by the lattice dimerization. Although we presumably infer that the metallic physical properties observed in the electric resistivity are caused by another component, the reason why another component appears is not clear at present. As a possible reason of another component, it may be considered a little aberration from the stoichiometry and the influence of the impurity. The more precise element specific measurements such as X-ray fluorescence analysis are desired in order to clarify the stoichiometry and the influence of the impurity.
4. Conclusion We have succeeded in measuring the electronic structures of small organic salts DMTSA-BF 4 and DMTSA using the photoelectron spectromicroscopy. The atomic orbital characters of observed spectral features in the valence band region are determined from the hn dependence of the photoionization cross-section. In addition, by comparing the Se 3d core-level photoemission spectra between DMTSABF 4 and DMTSA, it is suggested that DMTSA-BF 4 has a mixed valence state.
Acknowledgements This work is partially supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Science, Sports and Culture, and supported by the Joint Studies Program (1998–1999) of the Institute for Molecular Science. We are pleased to thank the staff of the UVSOR facility for excellent support.
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