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Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 137–140 Study of excited states of fluorinated copper phthalocyanine by inner shell excitation K.K. Okudaira a,b,∗ , H. Setoyama a , H. Yagi a , K. Mase c , S. Kera a,b , A. Kahn d , N. Ueno b a b Graduate school of Science and Technology, Chiba University, Chiba 263-8522, Japan Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan c Institute of Materials Structure Science, Tsukuba 305-0801, Japan d Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA Available online 20 March 2004 Abstract Near edge X-ray absorption fine structure (NEXAFS) spectra of hexadecafluoro copper phthalocyanine (FCuPc) films (thickness of 50 Å) on MoS2 substrates were observed near the carbon (C) and fluorine (F) K-edges. From the analysis of the dependence of C and F K-edge NEXAFS spectra on the photon incidence angle (α), the average molecular tilt angle was determined to be 30◦ . The lowest and second lowest peaks in the F K-edge NEXAFS were assigned to the transition to ∗ . In the ion time-of-flight mass spectra of FCuPc excited by photons near the F K-edge, F+ , CF+ , and CF3 + ions were mainly observed. These results indicate that C–C bonds as well as C–F bonds are broken by the photon irradiation. From the analysis of the partial ion yield spectra of F+ and CF+ near the F K-edge, the lowest and second lowest peaks in the F K-edge NEXAFS spectra could be assigned to transitions to (C–F)∗ and (C–C)∗ , respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Fluorinated copper phthalocyanine; Inner shell excitation; Unoccupied state; Site-specific chemical bond scission 1. Introduction In general, photoabsorption spectroscopy provides information on occupied states as well as unoccupied states. As the inner shell electron is excited in near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, the character of unoccupied states can be easily studied. For the assignment of the spectral structure of NEXAFS for large and complex molecules, the building block approach is very useful and has been widely used [1]. For ordered films of planar -conjugated organic molecules, the polarization dependence of NEXAFS spectra provides the symmetry of the ∗ and ∗ unoccupied states. The analysis of the dependence of photon-stimulated ion desorption (PSID) on photon energy (hν) is expected to help in the assignment of NEXAFS spectra, since the chemical bond scission by inner shell excitation depends on the electronic configuration of the excited state. In ∗ Corresponding author. Tel.: +81-43-290-3446; fax: +81-43-290-3449. E-mail address: [email protected] (K.K. Okudaira). 0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2004.02.078 fact, for poly(methyl methacrylate) and fluorocarbons such as poly(tetrafluoroethylene) (PTFE) and perfluorinated oligo(p-phenylene) (PF8P), it was reported that partial ion yields (PIYs) depend on the photon energy near absorption edges [2–5]. These studies demonstrated that PSID in molecular systems is highly dependent on the character of the excited state. Fluorinated copper phthalocyanine is a very interesting material, since it is a promising electron-transport material for organic devices [6]. To develop a highly efficient organic light-emitting diodes (OLED), it is important to clarify the characteristics of unoccupied states of the electron-transport layer in the OLED consisting of organic molecules. In this paper, we show the polarization dependence of NEXAFS of a hexadecafluoro copper phthalocyanine (FCuPc) film on a MoS2 substrate near the carbon and fluorine K absorption edges. It provides both molecular orientation and characteristics of the unoccupied states. PIY spectra of FCuPc near the fluorine K-edge are observed. The comparison of FCuPc PIY spectra with those of fluorocarbons such as PTFE and PF8P provides the needed peak assignment of NEXAFS spectra. 138 K.K. Okudaira et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 137–140 2. Experimental (a) α Total Electron Yield (arb. units) (*) α=70˚ α=70˚ α=55˚ α=55˚ α=35˚ α=0˚ α=35˚ α=0˚ 280 290 300 310 320 330 280 290 Photon energy (eV) (c) hν 2 Iπ*(α,β) Experiments were performed at the beamline 13 C at the Photon Factory, Institute of Materials Structure Science. This beam line was designed based on a cylindrical element monochromator (CEM) concept using undulator radiation from a 27-pole multiple wiggler/undulator. NEXAFS spectra were measured by the total electron yield (TEY) method. TEY spectra were obtained by measuring the sample current. The incidence angle of the photons (α) was defined by the angle between the direction of incidence of the photons and the surface normal. PIY spectra were measured using a time-of-flight (TOF) mass spectrometer system at α = 55◦ [2]. Soft X-ray pulses with a period of 624 ns, which is obtained during single-bunch operation of the Photon Factory storage ring, were incident on the sample through a photon-flux monitor [7]. Ion yields were normalized to the incident photon flux. The incident photon flux spectrum was recorded as the photocurrent at the photon-flux monitor consisting of a gold-evaporated mesh. All measurements were performed at room temperature. Commercially obtained FCuPc was purified by one-time sublimation in an Ar gas stream of about 0.2 Torr. The thin films were prepared by vacuum evaporation onto a MoS2 single-crystal surface. The thickness of the film was 50 Å. (b) E hν 1 E β=90˚ α β Molecular Plane 300 β=0˚ β=20˚ β=30˚ β=45˚ β=60˚ 00 20 40 60 Incidence Angle (α) (˚) β=75˚ 80 Fig. 1. (a, b) Carbon K-edge NEXAFS spectra of FCuPc film (50 Å thick) on MoS2 at incidence angle (α) = 0◦ (normal incidence), 35, 55, and 70◦ (grazing incidence). (c) Comparison between observed (solid symbols) and calculated (solid curve) incidence angle dependence (α) of the transition intensity from 1s to ∗ (the second lowest peak marked by an asterisk (∗) in (b)). 3. Results and discussion Figure 1a and b show carbon (C) K-edge NEXAFS spectra of FCuPc at α = 0◦ (normal incidence), 35, 55, and 70◦ (grazing incidence). Four sharp peaks at hν = 284.6, 285.5, 287.8, and 289.5 eV and one broad peak at about 295 eV appear at grazing incidence. The first three peaks show strong polarization dependence. The intensity of these peaks becomes larger as the incidence angle decreases. On the contrary, the broad peak at about 295 eV becomes larger at normal incidence. From the assignment of the lowest peak in the carbon K-edge NEXAFS spectrum of CuPc [8], which is expected to be similar to that of FCuPc, the lowest peak of the FCuPc NEXAFS spectrum is attributed to the transition from C 1s to ∗ . Since the second and third peaks at 285.5 and 287.8 eV show a polarization dependence similar to that of the first peak, these two peaks can also be assigned to transitions from C 1s to ∗ . This polarization dependence of the ∗ transition indicates that the angle between the FCuPc molecular plane and the MoS2 substrate surface is small. The opposite polarization dependence of the broad peak at about 295 eV indicates that its origin is in the transition to the ∗ state. Indeed, the ∗ orbitals are oriented along the chemical bonds within the molecular plane, and the direction of the dipole moment of the ∗ state is perpendicular to that of the ∗ state for a planar -conjugated molecule. In NEXAF spectroscopy, molecular orientation can be determined by analyzing the polarization dependence of either the ∗ or ∗ transition intensities [1]. The intensity of the 1s → ∗ transition for a planar -conjugated molecule under perfect polarization conditions has been given as I(α, β) ∝ 2 cos2 β cos2 (90 − α) + sin2 β sin2 (90 − α) (1) where β is the tilt angle of the molecular plane with respect to the substrate surface [9]. We assumed here that the azimuthal orientation is disordered. The calculated results of Eq. (1) for several tilt angles are presented as solid curves in Fig. 1c. We compare the intensity of the second lowest peak at hv = 285.5 eV marked by an asterisk in Fig. 1b with calculated results. The observed dependence of the intensity of the second peak on the angle α is in good agreement with the calculated result for β = 30◦ . It indicates that the average molecular tilt angle of FCuPc is 30◦ for the 50 Å-thick film on MoS2 substrate. Figure 2a shows fluorine (F) K-edge NEXAFS spectra of FCuPc at α = 0, 35, 55, and 70◦ . Two intense peaks appear at hν = 689.8 and 695 eV. The intensity of these peaks is larger at normal incidence than at grazing incidence. The two peaks in the fluorine K-edge NEXAFS spectrum show a polarization dependence opposite to that of the first three peaks (the transitions to ∗ states) in the carbon K-edge spectrum, indicating that these two peaks are attributed to the transition from F 1s to the ∗ states. Since the direction of the ∗ state is perpendicular to that of the ∗ state for a planar -conjugated molecule, the intensity of the transition from 1s to ∗ could be obtained by substituting β in Eq. (1) with α α=70˚ α=55˚ α=35˚ α=0˚ Iσ*(α,β) 690 2 1 0 0 (b) 700 710 720 730 Photon Energy (eV) E hν α Molecular Plane β β=0˚ 740 β=90˚ β=75˚ β=60˚ β=45˚ 20 40 60 Incidence Angle (α) (˚) β=30˚ β=20˚ 80 Fig. 2. (a) Fluorine K-edge NEXAFS spectra of FCuPc film (50 Å thick) on MoS2 at incidence angle α = 0◦ (normal incidence), 35, 55, and 70◦ (grazing incidence). (b) Comparison between observed (solid symbols) and calculated (solid curve) incidence angle (α) dependence of the transition intensity from 1s to ∗ (the lowest peak is marked by an asterisk (∗) in (a)). 90−β. The calculated results of the 1s to ∗ transition for several tilt angles are presented as solid curves in Fig. 2b. We compare the intensity of the lowest peak at hν = 689.8 eV, marked by an asterisk in Fig. 2a, with calculated results. The observed dependence of the intensity of the first peak on angle α is in good agreement with the calculated result for β = 30◦ . This result is consistent with that obtained for the carbon K-edge. From the analysis of the dependences of the carbon and fluorine K-edge NEXAFS spectra on α, it is found that the average molecular tilt angle of FCuPc is 30◦ for the 50 Å-thick film on the MoS2 . This tilt angle of FCuPc is larger than that of the CuPc film on MoS2 (β = 10◦ ) [10]. A typical ion time-of-flight mass spectrum of FCuPc near the fluorine K-edge is shown in Fig. 3. The main desorption ions are F+ , CF+ , CF3 + , and H+ . The H+ peak can be ascribed to (i) surface contamination (the film was exposed to air after the deposition) and (ii) high sensitivity of mass spectrometer for H+ ion. The appearance of F+ and CF+ ions indicates that C–C bonds as well as C–F bonds are broken by the irradiation with photons near the fluorine K-edge. The mass spectra of typical fluorocarbons such as hexafluoroethylene and hexafluorobenzene show relatively high intensity for the CF3 + ion [11]. Since the simple chemical bond breaking can not account for the process of CF3 + ion generation, we focus here on the ion yields of F+ and CF+ . Figure 4a and b show partial ion yield (PIY) spectra of F+ and CF+ ions, respectively, for FCuPc near the flu- 139 hν=689.8eV F+ 2 1.5 H+ + CF hν 1 0 CF3+ hν 200 400 600 Time of Flight (nsec) Fig. 3. Typical ion time-of-flight mass spectrum of FCuPc at hν = 689.8 eV. orine K-edge. The NEXAFS spectrum of FCuPc is also shown for comparison. The PIY of F+ ions becomes larger near the energy position of the lowest peak in the NEXAFS spectrum, and reaches a maximum at hν = 691.2 eV. This indicates that C–F bond scission is effective due to absorption of hν = 689.8 eV photons (corresponding to the energy position of the lowest peak in fluorine K-edge NEXAFS) and hν = 691.2 eV photons. In general, chemical bond scission by the excitation to ∗ is very effective. We reported that for fluorocarbons such as PTFE and PF8P thin films, F+ ion desorption shows an intense peak at the photon energy corresponding to the lowest peak in the fluorine K-edge NEXAFS spectrum and that these lowest peaks in the fluorine K-edge NEXAFS of PTFE and PF8P are assigned to transitions to (C–F)∗ [4,5]. The fluorine K-edge NEXAFS spectrum of FCuPc shown in Fig. 4a is 20 (a) + PIY(F ) TEY PIY 10 TEY 0 (b) PIY + PIY(CF ) TEY 0.8 TEY Intensity (arb. units) hν PIY Intensity (CPS/pA) E (*) Total Electron Yield (arb. units) (a) Ion yield (normlized to photon flux) (CPS/pA/channel) K.K. Okudaira et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 137–140 0.6 TEY 0.4 680 690 700 710 720 730 Photon energy (eV) Fig. 4. PIY spectra of (a) F+ and (b) CF+ for FCuPc near the fluorine K absorption edge. NEXAFS spectra (broken curve) are also shown for comparison. NEXAFS spectra are renormalized at hν = 682 and 732 eV to fit the PIY intensities. 140 K.K. Okudaira et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 137–140 similar to those of PTFE and PF8P. From these results, the lowest peak in the fluorine K-edge NEXAFS of FCuPc can be assigned to the transition to (C–F)∗ . This assignment is consistent with the result of polarization dependence of fluorine K-edge NEXAFS as discussed above. We also observe a maximum in the F+ ion yield at hν = 691.2 eV, which is higher by 1.4 eV than the photon energy of the lowest NEXAFS peak position. At hν = 691.2 eV, the fluorine K-edge NEXAFS spectra do not show a peak at any incidence angle (see Fig. 2a), indicating that (i) a transition to an unoccupied state which is distributed at the C–F bond in FCuPc molecule exists at hν = 691.2 eV and (ii) the transition dipole moment from F 1s to this unoccupied state is small. It is expected that a possible assignment of this unoccupied state is (C–F(s))∗ , since the transition from 1s to unoccupied states with s-symmetry is forbidden. Next we focus on the PIY spectrum of CF+ ion near the fluorine K-edge shown in Fig. 4b. The hν-dependence of CF+ yield is different from that of F+ . This difference suggests that the site-specific chemical bond scission by the inner shell excitation occurs in FCuPc thin film near the fluorine K-edge. The PIY spectrum of CF+ ion gives two maximum at hν = 692.5 and 695 eV. It indicates that C–C bond scission occurs effectively by the irradiation at these two photon energies. The energy position of the second peak in the PIY spectrum of CF+ is in agreement with that of the second lowest peak of the fluorine K-edge NEXAFS spectrum. The second lowest peak in the fluorine K-edge NEXAFS at hν = 695 eV could be assigned to the transition to (C–C)∗ . On the other hand, in the fluorine K-edge NEXAFS spectrum no peaks appear at hν = 692.5 eV, corresponding to the energy position of the first peak in PIY spectrum of CF+ ion. It indicates that (i) a transition to the unoccupied state which is distributed at the C–C bond exists at hν = 692.5 eV and (ii) the transition dipole moment from F 1s to this unoccupied state is small. For a more detail assignment of the transition, we need extensive studies such as theoretical analyses, since there are many candidates for the unoccupied states which are distributed on the C–C bonds. 4. Conclusions The polarization dependence of carbon and fluorine K-edge NEXAFS spectra of FCuPc provides the molecular orientation. The average molecular tilt angle of FCuPc molecules in a 50 Å thick film on MoS2 is 30◦ . The lowest peak in fluorine K-edge NEXAFS is assigned to the transition to the ∗ unoccupied state, not to ∗ . PIY spectra of FCuPc for F+ and CF+ ions near the fluorine K-edge are observed. The PIY spectra are different from the total electron yield (NEXAFS) spectrum near the fluorine K-edge. In particular, the PIY of F+ ions becomes larger at hν = 689.8 eV, corresponding to the lowest peak in the NEXAFS spectrum and reaches a maximum at hν = 691.2 eV. From the comparison of the PIY spectrum of F+ ions for FCuPc with those of PTFE and PF8P, the lowest peak in the fluorine K-edge NEXAFS spectrum of FCuPc can be assigned to the transition from F 1s to (C–F)∗ . Furthermore, it is expected that the transition from F 1s to (C–F(s))∗ exists at hν = 691.2 eV. From PIY of the CF+ ion, the second lowest peak in the fluorine K-edge NEXAFS spectrum can be assigned to the transition to (C–C)∗ . The analysis of the PIY spectra is useful for the assignment of NEXAFS spectra. Acknowledgements This research was partially supported by the Ministry of Education, Science, Sports and Culture; Grant-in-Aid for Creative Scientific Research, 14GS021; and a grant from the New Energy and Industrial Technology Development Organization (NEDO). References [1] J. Stöhr, NEXAFS Spectroscopy, Springer-Verlag, 1996. [2] C.K. Tinone, K. Tanaka, J. Maruyama, N. Ueno, M. Imamura, N. Matsubayashi, J. Chem. Phys. 100 (1994) 5988. [3] N. Ueno, K. Kamiya, Y. Harada, M.K.C. Tinone, T. Sekitani, K. Tanaka, Optoelectronics 11 (1996) 91. [4] K.K. Okudaira, H. Yamane, K. Ito, M. Imamura, S. Hasegawa, N. Ueno, Surf. Rev. Lett. 9 (2002) 335. [5] K.K. Okudaira, K. Ohara, H. Setoyama, T. Suzuki, Y. Sakamoto, M. Imamura, S. Hasegawa, K. Mase, N. Ueno, Nucl. Inst. Meth. Phys. Res. B 199 (2002) 265. [6] Z. Bao, A.J. Lovinger, J. Brown, J. Am. Chem. Soc. 120 (1998) 207–208. [7] N. Matwsubayashik, H. Shimada, K. Tanaka, T. Sato, Y. Yoshimura, A. Nishijima, Rev. Sci. Instrum. 63 (1992) 1362. 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