- Email: [email protected]
Characterizing reactions to fabricate thin films of charge transfer complexes by synchrotron photoelectron spectroscopy: A case study of DCNQI-Cu
a -_
__ lii!iB
ELSVIER
applied
surface science Applied Surface Science 113/I 14 (1997) 322-325
Characterizing reactions to fabricate thin films of charge transfer complexes by synchrotron photoelectron spectroscopy: A case study of DCNQI-Cu Toshihiro Shimada *, Michihiro Mochida, Atsushi Koma Department
of Chemistn,
The lJniwrsi@
of
Tokyo, Bunkyo-ku,
Tokw
113. Japtrn
Abstract Ultraviolet photoelectron spectroscopy with various photon energies using xynchrotron radiation was used to characterize chemical reactions associated with thin film growth of organic charge transfer complex (DMe-DCNQI)Ju. Other molecular systems H2Pc, CuPc and C,, were also studied to clarify the origin of the systematic relation between the spectra and the incident photon energy. Characteristic photon energy dependence of the photo-ionization cross section of molecular orbitals is useful to analyze the intermolecular reactions. PACS:
79.60.Fr.; 82.65.Yh; 68.55.G -
1. Introduction Charge
transfer
complexes
of organic
molecules,
such as Cu salts of DMe-DCNQI (dimethyl-N,N’-dicyanoquinonediimine) are gathering much attention recently because of their chemical variation and interesting physical properties. The preparation of these materials, however, has been performed only by conventional crystal growth techniques using molecular diffusion in solution, or electrochemical reaction at electrodes. Therefore the size and shape of the specimen are determined by subtle changes in growth conditions, which forms a large obstacle for the practical application of these materials. On the other hand, sophisticated crystal growth techniques such as
* Corresponding author. Tel.: +81-3-381221 1I ext. 4603; fax: + 81-3-56890654: e-mail: [email protected].
molecular beam epitaxy has been successfully applied to fabricate atomically tailored inorganic materials. Once such methods can be applied to the organic charge transfer complexes, the hetero structures will bring novel physical properties and single crystals with large surface area will facilitate the application. Chemical reactions of these materials accompanied with charge transfer at surfaces must be characterized in order to find the optimum conditions for the ultrathin film growth. Although core level characterization in X-ray photoelectron spectroscopy (XPS) is very useful to study the surface reactions of inorganic materials, it is not easily applicable to the organic materials. They consist of almost only C. H and N with various chemical states. which broadens the peak shapes. Valence band spectra in ultraviolet photoelectron spectroscopy (UPS), which represent the energy levels of molecular orbitals in organic
0169.4332/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SOl69-4332(96)00810-O
T. Shimada et o/./Applied
Surface Science 113/114
molecules, will provide much better measures for the characterization. In this paper we investigate the possibility of identifying the molecular orbitals for the characterization of the chemical reactions of the organic molecular complexes. The attempted method utilizes the different relationship of photo-ionization cross section versus photon energy, obtained from synchrotron radiation. The materials studied are (DMe-DCNQI)2Cu polycrystalline bulk grown by a wet process ((DCNQI),Cu (WET)) and a thin film prepared by a deposition-anneal process in ultrahigh vacuum (DCNQI-CU (DRY)). AS a reference, molecular films of copper phthalocyanine (CuPc), hydrogen phthalocyanine (H? PC) and C,,, were also studied.
2. Experimental Ultraviolet photoelectron spectroscopy was performed using BL-2 of SOR-RING, ISSP, University of Tokyo, with photon energy between 40 and 130 eV. The electron energy was analyzed using a double pass cylindrical mirror analyzer (CMA; PHI 15 2550). The ultraviolet light was irradiated to the specimen with an incident angle of 45” to the sample surface and the axis of the CMA was perpendicular to the UV light and was 45” to the surface normal of the samples. The thin film specimens DCNQI-Cu (DRY), H2Pc. CuPc and C,, were grown in a vacuum chamber, which was turbo-molecular-pumped to 2 X lo-’ Pa. Chemically oxidized Si(1 1 11 wafers and cleaved (0001) surfaces of MoS2 were used as substrates. Copper, DMe-DCNQI, H,Pc, CuPc and C,, were evaporated from separate Knudsen cells. For the DCNQI-Cu (DRY) specimen, 20 nm thick equivalent of Cu was deposited on the substrate at room temperature. Then much excess amount of DCNQI was deposited and after that the specimen was annealed at 150°C for 3 h. Reflection high energy electron diffraction of a sample fabricated by the same process gave a halo pattern and did not show any indication of incomplete coverage of the substrate. The thickness of other samples were more than 100 nm. The samples were transferred to the UPS chamber quickly using a gate-valved magneticcoupled transfer rod without breaking the vacuum.
(IYY71322-325
323
After the UPS measurement, the surface morphology of the samples was examined by an atomic force microscope (AFM; SII SPI-3700). The (DCNQI),Cu (WET) sample was prepared according to the method described by Schmeisser et al. [I], which is known to make stoichiometric (DMe-DCNQI),Cu bulk polycrystals. A Cu plate was immersed in CH,CN solution of DMe-DCNQI in a glove box filled with purified He gas. After waiting until the sample surface was completely covered with black microcrystals of (DMeDCNQ&Cu, the sample was attached to a sample stub for the UPS and then transferred to a portable vacuum vessel filled with pure He gas. The vessel was evacuated by a turbo molecular pump to the vacuum better than 1 X lo-’ Pa and an ion pump was working during the transportation to the synchrotron facility. No oxygen was detected by Auger electron spectroscopy.
3. Results and discussion The surface roughness of the samples was of the order of 100 nm as measured by AFM, suggesting that the molecular orientation was random and does not affect the UPS spectra. Fig. 1 shows the UPS spectra from CuPc and (DCNQH2Cu (WET) samples taken with various photon energies (Izv). They are normalized by the maximum at each photon energy. The peaks marked with ‘Cu’ increase their relative intensities as the photon energy is increased. It is due to the difference in the photo-ionization cross section of Cu 3d and other constituent elements as suggested by other groups studying the DCNQI-Cu system [l-3]. The dominant contribution for the UPS at h Y = 40- 130 eV is from Cu 3d, C 2p and N 2p as calculated by Yeh and Lindau [4]. Although the photo-ionization cross sections of these atomic levels are nearly the same at 20-40 eV. those of C 2p and N 2p steeply decrease with hv. The difference with Cu 3d is almost two orders of magnitude 141. We therefore conclude the peaks marked with ‘Cu’ in Fig. 1 are due to the Cu 3d levels. Fig. 2 shows the UPS spectra of C, and H2Pc. While the spectra of C,, does not show distinctive difference in the relative peak height except for a
324
T. Shimada et al/Applied
Surface Science 113/114
DCNQI-Cu(WET)
I
Binding Energy (eV)
Binding
Energy (eV)
Fig. I. UPS spectra of CuPc and (DMe-DCNQI),Cu (prepared by wet process) polycrystalline films taken with various incident photon energies.
small ups and downs in the relative intensity [5], those of H,Pc change systematically with the increase of the photon energy. The peak marked ‘N’ at 5-6 eV binding energy increases with the photon energy. As the photo-ionization cross section of C 2p decreases more steeply than N 2p as hv increases
(19971322-325
and the ratio is more than 15 at hv = 100 eV [4], we consider the peak ‘N’ is due to the molecular orbital of HzPc with large contribution from N 2p. Although interference of photoelectrons must be incorporated in the precise analysis of the photoelectron intensity [6], this result shows the hv dependence is helpful for the peak assignment of UPS of the samples which are not single crystalline. Fig. 3 shows the UPS of DCNQI-CU (DRY). While the overall feature of the spectra and hv dependence are similar to those of (DCNQI),Cu (WET) shown in Fig. 1, the peak ‘A’, which is strong at lower hv, is not seen in (DCNQI),CU (WET). Since the contributions of C 2p and N 2p are strong at lower hv, it can be concluded that the additional peak ‘A’ is due to the non-stoichiometric DMe-DCNQI molecules. Physisorption of the excess molecule at the surface is not likely because the sample was annealed to 150°C which is higher than the sublimation temperature of DMe-DCNQI in the Knudsen cell. It suggests that the electronic state of the molecule corresponding to the peak ‘A’ (ordinarily anion with - 1 charge) is different, or some of DMe-DCNQI molecules were decomposed by the reaction with Cu. The information shown above cannot be obtained by other methods for the amor-
/DCNQI-CLI(DRY)
HzPc
I
C&N1
I
20 20 3
15 IO 5 Binding Energy (eV)
15
10
5
0
Bmdmg Energy (eV)
Fig. 2. UPS spectra of C,, and H, PC polycrystalline with various incident photon energies.
films taken
15 Binding
0
io Energy
5
0
(eV)
Fig. 3. UPS spectra of a DCNQI-Cu amorphous film prepared (DRY)) taken with various incident photon
in-vacua (DCNQI-CU energies.
T. Shimada et al. /Applied Surjace Science I13 / 114 f 19971322-325
phous sample. It indicates that the approach attempted here will be useful for the study of organic intermolecular reactions.
4. Conclusion Synchrotron radiation UPS with the photon energy from 40 to 130 eV was used to characterize a chemical reaction intended to make thin films of a charge transfer complex, (DMe-DCNQI),Cu. Bulk polycrystalline (DMe-DCNQI),Cu and thin films of H,Pc, CuPc and C,, were used as references. It has been found that materials containing Cu and N shows characteristic change in the spectra as the photon energy was changed. It is due to the different relationship of the photo-ionization cross section of each atomic level to the incident photon energy. The DCNQI-Cu thin film prepared by a dry process showed additional peak in the UPS taken with the lower photon energies. This peak was assigned to the excess DMe-DCNQI molecules or to decomposed species of the molecule. The technique demonstrated here will be helpful for the peak assignment of organic intermolecular compounds, which are increasing in importance in the materials science.
325
Acknowledgements The authors are indebted to Professor R. Kato (ISSP, The University of Tokyo) and to Professor H. Suematsu (Department of Physics, The University of Tokyo) for the discussion on the Cu-DCNQI systems and for allowing the use of the He glove box, respectively. They also thank Dr. Y. Tezuka (ISSP, The University of Tokyo), Dr. K. Ueno and Messieurs T. Sakurada, N. Akama and Ms. K. Hamaguchi for the collaboration in the experiment.
References 111D. Schmeisser,
K. Graf, W. Gopel, J.U. von Schitz, P. Erk and S. Hlnig, Chem. Phys. Lett. 148 (1988) 423. 121A. Tanaka, A. Chainani, T. Miura, T. Takahashi, T. Miyazaki, S. Hasegawa and T. Mori, Solid State Commun. 93 (1995) 1. A. Fujimori, A. [31 I.H. moue. A. Kakizaki, H. Namatame, Kobayashi, R. Kato and H. Kobayashi, Phys. Rev. B 45 (19921 5828. 141J.J. Yeh and I. Lindau. At. Data Nucl. Data Tables 32 (1985) 1. 151J.H. Weaver, J.L. Martins, T. Komeda, Y. Chen, T.R. Ohno. G.H. Kroll, N. Troullier, R.E. Haufler and R.E. Smalley, Phys. Rev. Lett. 66 (1991) 1741. [61S.M. Goldberg, C.S. Fadley and S. Kono. J. Electron Spectrosc. Relat. Phenom. 21 (1981) 285.
__ lii!iB
ELSVIER
applied
surface science Applied Surface Science 113/I 14 (1997) 322-325
Characterizing reactions to fabricate thin films of charge transfer complexes by synchrotron photoelectron spectroscopy: A case study of DCNQI-Cu Toshihiro Shimada *, Michihiro Mochida, Atsushi Koma Department
of Chemistn,
The lJniwrsi@
of
Tokyo, Bunkyo-ku,
Tokw
113. Japtrn
Abstract Ultraviolet photoelectron spectroscopy with various photon energies using xynchrotron radiation was used to characterize chemical reactions associated with thin film growth of organic charge transfer complex (DMe-DCNQI)Ju. Other molecular systems H2Pc, CuPc and C,, were also studied to clarify the origin of the systematic relation between the spectra and the incident photon energy. Characteristic photon energy dependence of the photo-ionization cross section of molecular orbitals is useful to analyze the intermolecular reactions. PACS:
79.60.Fr.; 82.65.Yh; 68.55.G -
1. Introduction Charge
transfer
complexes
of organic
molecules,
such as Cu salts of DMe-DCNQI (dimethyl-N,N’-dicyanoquinonediimine) are gathering much attention recently because of their chemical variation and interesting physical properties. The preparation of these materials, however, has been performed only by conventional crystal growth techniques using molecular diffusion in solution, or electrochemical reaction at electrodes. Therefore the size and shape of the specimen are determined by subtle changes in growth conditions, which forms a large obstacle for the practical application of these materials. On the other hand, sophisticated crystal growth techniques such as
* Corresponding author. Tel.: +81-3-381221 1I ext. 4603; fax: + 81-3-56890654: e-mail: [email protected].
molecular beam epitaxy has been successfully applied to fabricate atomically tailored inorganic materials. Once such methods can be applied to the organic charge transfer complexes, the hetero structures will bring novel physical properties and single crystals with large surface area will facilitate the application. Chemical reactions of these materials accompanied with charge transfer at surfaces must be characterized in order to find the optimum conditions for the ultrathin film growth. Although core level characterization in X-ray photoelectron spectroscopy (XPS) is very useful to study the surface reactions of inorganic materials, it is not easily applicable to the organic materials. They consist of almost only C. H and N with various chemical states. which broadens the peak shapes. Valence band spectra in ultraviolet photoelectron spectroscopy (UPS), which represent the energy levels of molecular orbitals in organic
0169.4332/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SOl69-4332(96)00810-O
T. Shimada et o/./Applied
Surface Science 113/114
molecules, will provide much better measures for the characterization. In this paper we investigate the possibility of identifying the molecular orbitals for the characterization of the chemical reactions of the organic molecular complexes. The attempted method utilizes the different relationship of photo-ionization cross section versus photon energy, obtained from synchrotron radiation. The materials studied are (DMe-DCNQI)2Cu polycrystalline bulk grown by a wet process ((DCNQI),Cu (WET)) and a thin film prepared by a deposition-anneal process in ultrahigh vacuum (DCNQI-CU (DRY)). AS a reference, molecular films of copper phthalocyanine (CuPc), hydrogen phthalocyanine (H? PC) and C,,, were also studied.
2. Experimental Ultraviolet photoelectron spectroscopy was performed using BL-2 of SOR-RING, ISSP, University of Tokyo, with photon energy between 40 and 130 eV. The electron energy was analyzed using a double pass cylindrical mirror analyzer (CMA; PHI 15 2550). The ultraviolet light was irradiated to the specimen with an incident angle of 45” to the sample surface and the axis of the CMA was perpendicular to the UV light and was 45” to the surface normal of the samples. The thin film specimens DCNQI-Cu (DRY), H2Pc. CuPc and C,, were grown in a vacuum chamber, which was turbo-molecular-pumped to 2 X lo-’ Pa. Chemically oxidized Si(1 1 11 wafers and cleaved (0001) surfaces of MoS2 were used as substrates. Copper, DMe-DCNQI, H,Pc, CuPc and C,, were evaporated from separate Knudsen cells. For the DCNQI-Cu (DRY) specimen, 20 nm thick equivalent of Cu was deposited on the substrate at room temperature. Then much excess amount of DCNQI was deposited and after that the specimen was annealed at 150°C for 3 h. Reflection high energy electron diffraction of a sample fabricated by the same process gave a halo pattern and did not show any indication of incomplete coverage of the substrate. The thickness of other samples were more than 100 nm. The samples were transferred to the UPS chamber quickly using a gate-valved magneticcoupled transfer rod without breaking the vacuum.
(IYY71322-325
323
After the UPS measurement, the surface morphology of the samples was examined by an atomic force microscope (AFM; SII SPI-3700). The (DCNQI),Cu (WET) sample was prepared according to the method described by Schmeisser et al. [I], which is known to make stoichiometric (DMe-DCNQI),Cu bulk polycrystals. A Cu plate was immersed in CH,CN solution of DMe-DCNQI in a glove box filled with purified He gas. After waiting until the sample surface was completely covered with black microcrystals of (DMeDCNQ&Cu, the sample was attached to a sample stub for the UPS and then transferred to a portable vacuum vessel filled with pure He gas. The vessel was evacuated by a turbo molecular pump to the vacuum better than 1 X lo-’ Pa and an ion pump was working during the transportation to the synchrotron facility. No oxygen was detected by Auger electron spectroscopy.
3. Results and discussion The surface roughness of the samples was of the order of 100 nm as measured by AFM, suggesting that the molecular orientation was random and does not affect the UPS spectra. Fig. 1 shows the UPS spectra from CuPc and (DCNQH2Cu (WET) samples taken with various photon energies (Izv). They are normalized by the maximum at each photon energy. The peaks marked with ‘Cu’ increase their relative intensities as the photon energy is increased. It is due to the difference in the photo-ionization cross section of Cu 3d and other constituent elements as suggested by other groups studying the DCNQI-Cu system [l-3]. The dominant contribution for the UPS at h Y = 40- 130 eV is from Cu 3d, C 2p and N 2p as calculated by Yeh and Lindau [4]. Although the photo-ionization cross sections of these atomic levels are nearly the same at 20-40 eV. those of C 2p and N 2p steeply decrease with hv. The difference with Cu 3d is almost two orders of magnitude 141. We therefore conclude the peaks marked with ‘Cu’ in Fig. 1 are due to the Cu 3d levels. Fig. 2 shows the UPS spectra of C, and H2Pc. While the spectra of C,, does not show distinctive difference in the relative peak height except for a
324
T. Shimada et al/Applied
Surface Science 113/114
DCNQI-Cu(WET)
I
Binding Energy (eV)
Binding
Energy (eV)
Fig. I. UPS spectra of CuPc and (DMe-DCNQI),Cu (prepared by wet process) polycrystalline films taken with various incident photon energies.
small ups and downs in the relative intensity [5], those of H,Pc change systematically with the increase of the photon energy. The peak marked ‘N’ at 5-6 eV binding energy increases with the photon energy. As the photo-ionization cross section of C 2p decreases more steeply than N 2p as hv increases
(19971322-325
and the ratio is more than 15 at hv = 100 eV [4], we consider the peak ‘N’ is due to the molecular orbital of HzPc with large contribution from N 2p. Although interference of photoelectrons must be incorporated in the precise analysis of the photoelectron intensity [6], this result shows the hv dependence is helpful for the peak assignment of UPS of the samples which are not single crystalline. Fig. 3 shows the UPS of DCNQI-CU (DRY). While the overall feature of the spectra and hv dependence are similar to those of (DCNQI),Cu (WET) shown in Fig. 1, the peak ‘A’, which is strong at lower hv, is not seen in (DCNQI),CU (WET). Since the contributions of C 2p and N 2p are strong at lower hv, it can be concluded that the additional peak ‘A’ is due to the non-stoichiometric DMe-DCNQI molecules. Physisorption of the excess molecule at the surface is not likely because the sample was annealed to 150°C which is higher than the sublimation temperature of DMe-DCNQI in the Knudsen cell. It suggests that the electronic state of the molecule corresponding to the peak ‘A’ (ordinarily anion with - 1 charge) is different, or some of DMe-DCNQI molecules were decomposed by the reaction with Cu. The information shown above cannot be obtained by other methods for the amor-
/DCNQI-CLI(DRY)
HzPc
I
C&N1
I
20 20 3
15 IO 5 Binding Energy (eV)
15
10
5
0
Bmdmg Energy (eV)
Fig. 2. UPS spectra of C,, and H, PC polycrystalline with various incident photon energies.
films taken
15 Binding
0
io Energy
5
0
(eV)
Fig. 3. UPS spectra of a DCNQI-Cu amorphous film prepared (DRY)) taken with various incident photon
in-vacua (DCNQI-CU energies.
T. Shimada et al. /Applied Surjace Science I13 / 114 f 19971322-325
phous sample. It indicates that the approach attempted here will be useful for the study of organic intermolecular reactions.
4. Conclusion Synchrotron radiation UPS with the photon energy from 40 to 130 eV was used to characterize a chemical reaction intended to make thin films of a charge transfer complex, (DMe-DCNQI),Cu. Bulk polycrystalline (DMe-DCNQI),Cu and thin films of H,Pc, CuPc and C,, were used as references. It has been found that materials containing Cu and N shows characteristic change in the spectra as the photon energy was changed. It is due to the different relationship of the photo-ionization cross section of each atomic level to the incident photon energy. The DCNQI-Cu thin film prepared by a dry process showed additional peak in the UPS taken with the lower photon energies. This peak was assigned to the excess DMe-DCNQI molecules or to decomposed species of the molecule. The technique demonstrated here will be helpful for the peak assignment of organic intermolecular compounds, which are increasing in importance in the materials science.
325
Acknowledgements The authors are indebted to Professor R. Kato (ISSP, The University of Tokyo) and to Professor H. Suematsu (Department of Physics, The University of Tokyo) for the discussion on the Cu-DCNQI systems and for allowing the use of the He glove box, respectively. They also thank Dr. Y. Tezuka (ISSP, The University of Tokyo), Dr. K. Ueno and Messieurs T. Sakurada, N. Akama and Ms. K. Hamaguchi for the collaboration in the experiment.
References 111D. Schmeisser,
K. Graf, W. Gopel, J.U. von Schitz, P. Erk and S. Hlnig, Chem. Phys. Lett. 148 (1988) 423. 121A. Tanaka, A. Chainani, T. Miura, T. Takahashi, T. Miyazaki, S. Hasegawa and T. Mori, Solid State Commun. 93 (1995) 1. A. Fujimori, A. [31 I.H. moue. A. Kakizaki, H. Namatame, Kobayashi, R. Kato and H. Kobayashi, Phys. Rev. B 45 (19921 5828. 141J.J. Yeh and I. Lindau. At. Data Nucl. Data Tables 32 (1985) 1. 151J.H. Weaver, J.L. Martins, T. Komeda, Y. Chen, T.R. Ohno. G.H. Kroll, N. Troullier, R.E. Haufler and R.E. Smalley, Phys. Rev. Lett. 66 (1991) 1741. [61S.M. Goldberg, C.S. Fadley and S. Kono. J. Electron Spectrosc. Relat. Phenom. 21 (1981) 285.