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HREELS study of C70 molecules adsorbed on a Si(1 1 1)-(7×7) surface
Applied Surface Science 169±170 (2001) 147±152
HREELS study of C70 molecules adsorbed on a Si(1 1 1)-(7 7) surface Takanori Wakita, Kazuyuki Sakamoto, Shozo Suto* Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan Received 2 August 1999; accepted 13 December 1999
Abstract We have measured the coverage dependence of vibrational excitation spectra of C70 molecules adsorbed on a Si(1 1 1)(77) surface using high-resolution electron-energy-loss spectroscopy. At the monolayer coverage, the intensity of the 57 meV peak increases, and those of the 83 and 178 meV peaks decrease. Taking into account the dipole selection rule, the change in intensity of the 57 meV peak indicates that the average angle between the long axes of C70 molecules and surface normal is about 40 . The decreases in intensities of the 83 and 178 meV peaks suggest that the rotational motion of molecules is quenched upon adsorption. We will discuss the Coriolis interaction between the accidentally degenerate A002 and E10 modes. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Fullerenes; Vibrations of adsorbed molecules; Electron energy loss spectroscopy; Silicon; Coriolis interaction
1. Introduction C70 is the second stable and abundant fullerene, but little is known about the interaction of C70 molecules with metal and semiconductor surfaces. It is important to understand the interaction in order to develop a new functionality. The rugby ball-shaped C70 molecule has a D5h symmetry. Due to the symmetry, a C70 molecule has 31 infrared (IR) active modes (10A002 21E10 ) and all vibrational energies of these modes lie in the range from 30 to 200 meV. The A002 modes have oscillating dipole moments parallel to the long axis (®ve-fold axis) and the E10 modes have those perpendicular to the axis [1]. Yu et al. studied the oscillator strength of IR active modes for the C70 thick ®lm using high-resolution electron-energy-loss spectroscopy (HREELS)
[2]. Fujikawa et al. reported the vibrational modes for the 1 ML ®lm at a MoS2 surface and that the interaction is very weak [1]. We studied the thermal reaction and the SiC ®lm formation from C70 molecules at Si surfaces [3]. In this paper, we report the measurements of vibrational excitation spectra of C70 molecules adsorbed on a Si(1 1 1)-(77) surface at the coverage lower than 6 ML (monolayers) to investigate the interaction. We found that the average angle between the long axes of C70 molecules in the 1 ML ®lm is about 40 judging from the dipole selection rule. The rotational motion of C70 molecules are discussed in terms of the Coriolis interaction. 2. Experiment
*
Corresponding author. Tel.: 81-22-217-7752; fax: 81-22-217-7746. E-mail address: [email protected] (S. Suto).
We performed the HREELS measurements in a UHV system which consists of an analysis chamber
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 1 3 - 3
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T. Wakita et al. / Applied Surface Science 169±170 (2001) 147±152
and a sample preparation chamber. The analysis chamber is equipped with HREELS (VSW IB2000) and a low energy electron diffraction (LEED). The base pressure was 1:3 10ÿ8 Pa in the analysis chamber and 1:3 10ÿ7 Pa in the sample preparation chamber. The samples were transferred between chambers under UHV condition. In the HREELS measurements, the incident energy of the electron beam was 5.0 eV. The scattered angle (ys ) was ®xed at 60 from the surface normal. We changed the incident angle (yi ) to measure the angular dependence of scattering intensity. Commercial n-type Si(1 1 1) wafers (phosphorus doped, 1000 O cm) were used as sample substrates. The sample preparation and estimation procedures are reported in [4]. The C70 powder was loaded into a quartz crucible and was heated by a tungsten wire in the sample preparation chamber. The C70 powder was carefully outgassed below 573 K for over 24 h prior to evaporation. The thickness of the C70 ®lm was monitored by a quartz Ê /min. We crystal balance. The deposition rate was 2 A Ê and assumed the hcp structure (a b 10:56 A c 17:18 A) of C70 ®lm, though C70 ®lms grown on room temperature substrates tend to adopt a mixture of fcc and hcp phases with numbers of stacking faults [5]. The thickness of 17:18 A is estimated to be 2 ML. All measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows electron-energy-loss spectra of C70 molecules adsorbed on the Si(1 1 1)-(77) surface measured with the specular condition (yi ys 60 ). The coverages are 6 ML (a), 1 ML (b), 0.5 ML (c) and 0.25 ML (d). At 6 ML, two intense peaks appear at 68 and 178 meV. Small peaks and shoulders are observed at 43, 57, 83, 96, 111, 144, 164 and 194 meV. The spectral pro®les are the same as those previously reported for C70 thick ®lms [2,6]. At 1 ML, peaks and shoulders appear at 43, 57, 68, 96, 111, 164 and 178 meV. The intensities at 83 and 178 meV peaks dramatically decrease. At 0.5 and 0.25 ML, two peaks appear at 65 and 94 meV, and a broad peak is observed at 170 meV. Fig. 2 shows electron-energy-loss spectra of C70 molecules adsorbed on the Si(111)-(77) at 6 ML in
Fig. 1. Electron-energy-loss spectra of C70 molecules adsorbed on a Si(1 1 1)-(77) surface measured with the specular condition (yi ys 60 ). The coverages are 6 ML (a); 1 ML (b); 0.5 ML (c) and 0.25 ML (d).
(a) and 1 ML in (b). The thick solid lines are measured with the specular condition, and the thin solid lines with the off-specular condition (yi 51 , ys 60 ). The ordinate is plotted by absolute intensity to compare the cross sections between off-specular and specular conditions. In off-specular spectra, peaks and shoulders are observed at 67, 94, 163 and 184 meV for the 6 ML ®lm, and at 66, 94, 166, 179 and 192 meV for the 1 ML ®lm. It is noticed that the spectral pro®les in Fig. 1(c) and (d) are the same as those of the off-specular spectra in Fig. 2. In the following sections, we discuss the differences in the relative peak intensities between the 6 ML and the 1 ML ®lms in terms of the surface loss function. In general, an HREEL spectrum consists of two components. One is due to the dipole-scattering process and the other is due to the impact-scattering process [7]. The dipole-scattered electrons sharply peak about the specular direction, while the impact-scattered electrons have rather isotropic angular dependence on the whole [2]. We observed drastic decreases in the
T. Wakita et al. / Applied Surface Science 169±170 (2001) 147±152
149
Fig. 2. Electron-energy-loss spectra of C70 molecules adsorbed on a Si(1 1 1)-(77) at the coverages of 6 ML (a) and 1 ML (b). The thick solid lines are measured with the specular condition, and the thin solid lines with the off-specular condition (yi 51 , ys 60 ). The dashed lines are the backgrounds we assumed.
Fig. 3. The thick lines are obtained by subtracting the off-specular spectra and the backgrounds from the specular spectra in Fig. 2. The thick dashed lines are the simulated spectra by convoluting the surface loss function with the instrumental function. The thin lines are the Gaussian components of the simulated spectra. The hatched Gaussian components are A002 modes and the other Gaussians are E10 modes.
intensities of the 68 and 178 meV peaks as yi decreases, but little change between the off-specular spectra measured with yi 51 and 48 . This result indicates that we are able to ignore the angular dependence of the impact-scattered electrons for C70 ®lms. Due to the difference in the angular distributions, it is possible to extract the dipole-scattering components from the experimental data by subtracting the off-specular spectra [2]. Thick solid lines in Fig. 3 are the residual spectra which display the dipole-scattering components at 6 ML in (a) and 1 ML in (b). The dipole-scattering components are reproduced by using the surface loss function, I / Imfÿ1= e o 1g [7]. We assume that the dipole polarizability of a C70 molecule is expressed by a simple model of harmonically bound charges with viscous damping. Then, the dielectric function e o of a C70
®lm is given by e o e 1
X j
o2j
rj o2j ÿ o2 ÿ ioj ogj
;
(1)
where oj , rj and gj are the oscillator frequency, the oscillator strength and the damping ratio of the jth dipole active mode of a C70 molecule. e 1 is the optical dielectric constant and the value is 4.96 for C70 [8]. Eq. (1) is successfully applied to the HREELS data of C60 and C70 thick ®lms with parameters obtained from optical absorption measurements [2,9]. In the case of a 1 ML ®lm, the interaction between molecules through their dipole moments and the in¯uence of the substrate, should result in revisions to the parameters determined for thick ®lms. It is reported that the extrapolated frequencies at 0 K for the C70 molecules in gas phase are quite close to
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T. Wakita et al. / Applied Surface Science 169±170 (2001) 147±152
the solid-state line positions at room temperature in IR spectra [10]. This result indicates that the dipole± dipole interaction between C70 molecules is negligible. The in¯uence of the Si surface will be discussed below. When one inserts Eq. (1) into the surface loss function, the function has peaks at the energies which are approximately equal to oj 's and each peak has a Lorentzian lineshape. In order to reproduce our experimental data, the convolution of the surface loss function with the instrumental function is required. In the ®rst approximation, the instrumental function has a Gaussian lineshape with the full widths at half maximum of elastic peaks (10 meV) and the convoluted spectrum consists of the Gaussian components. Thick dashed lines in Fig. 3 are simulated spectra by using Eq. (1). The thin solid lines are each component. We use a set of parameters determined by optical absorption spectra for 13 modes which have stronger intensities than the others [2]. The hatched Gaussian components are A002 modes and the other Gaussians are E10 modes [11]. Each simulated spectrum is normalized to the 68 meV peak intensity. At 6 ML, the thick solid and dashed lines are in good agreement with each other. This result con®rms that the thick solid lines are produced by electrons which lose their energies almost only by the dipole-scattering process. Nevertheless, the spectrum at 1 ML is not well reproduced. Particularly, the intensity at 57 meV increases and those at 83 and 178 meV decrease. Fujikawa et al. reported that the intensities at 83 and 178 meV exhibit no change on a MoS2 substrate at 1 ML [1]. These results clearly indicate the strong in¯uence of the Si(1 1 1)-(77) surface on the dielectric response of the 1 ML ®lm. The parameters in Eq. (1) should be subject to review for the 1 ML ®lm on the Si(1 1 1)-(77) surface. A probable modi®cation to the oscillator strengths in Eq. (1) is demanded by the dipole selection rule. The dielectric constant of silicon with E 11:7 is large enough to provide for effective screening of dipole moments parallel to the surface, especially for a 1 ML ®lm. Fig. 3(b) shows that both the E10 and A002 modes contribute to the spectra. This result indicates that the orientations of the long axes are neither perpendicular nor parallel to the surface uniformly. We note that the decreases in the peak intensities at 83 and 178 meV are attributed to both
the E10 and the A002 modes, whereas the increase in the peak intensity at 57 meV is related only to the increase of the A002 modes intensity. We consider that the observed intensity ratio IA002 =IE10 is given by following equation IA002 I
E10
rA002 cos2 yl 2
rE10 sin yl
rA002 rE10
cot2 yl ;
(2)
where yl is the angle between the long axis of C70 molecule and surface normal, rA002 the oscillator strength of A002 mode at 57 meV and rE10 the sum of the oscillator strength of E10 modes at 66, 70 and 72 meV. We obtained yl 40 by adjusting the relative intensities of the Gaussian components to reproduce the spectrum at 1 ML in the shoulder at 57 meV. There are two ideal con®gurations of C70 molecules. One is an ordered and the other is a random con®guration. In fact, C70 molecules form a ¯at monolayer ®lm with a short-range order on a Si(1 0 0)-(21) surface [12]. In the case of C60 molecules, it is reported that only 23% of the C60 molecules in a 1 ML ®lm strongly interact with Si dangling bonds on a Si(1 1 1)-(77) surface resulting in a short-range order [4]. The chemical bond can prevent a long range order of C70 molecules. We consider that also on a Si(1 1 1)-(77) surface, the 1 ML ®lm has just a shortrange order due to the chemical bond formation between a small fraction of C70 molecules and the Si surface. The ordered molecules can interact with Si surface by van der Waals force. It is important as well that C70 molecules have an anisotropic chemical reactivity. Balch et al. reported that the ®ve double bonds between two of the ®ve hexagons in a polar cap are relatively chemically reactive for iridium complex adsorption than those in equator belt hexagons [13]. This fact suggests that the polar caps are rather `C60 like' [14] and mainly react with the Si dangling bonds in the process of the chemical bond formation. A possible geometry of strongly interacting C70 molecules is that one of the ®ve hexagons in a polar cap is parallel to the surface (yl 32 ). Such molecules can in¯uence the orientations of short-range ordered molecules. Our estimation of yl 40 suggests that the long axes of short-range ordered molecules incline by 40 as an average. Finally, we discuss the decreases in the intensities at 83 and 178 meV peaks. As discussed above, these
T. Wakita et al. / Applied Surface Science 169±170 (2001) 147±152
peaks consist of both the E10 and the A002 modes and the decreases are not explained by the dipole selection rule. Moreover, it seems that the decreases are free from both splitting and broadening of the peaks, and these intensities simply decrease as shown in Fig. 3. We interpret these decreases in terms of the quenching of the rotational motion of C70 molecules upon adsorption. It is known that the intramolecular vibrational energies of fullerenes reside in the range from 30 to 200 meV, and it is separated into two regions, one (30± 110 meV) in which the normal modes of fullerenes consist of primarily radial motions, and the other (130±200 meV) in which the tangential motions are dominant [15]. We refer the modes lying in each region the radial modes and the tangential modes, respectively. For the four T1u modes of the C60 molecule, the two modes at 66 and 72 meV are the radial modes and those at 147 and 179 meV are the tangential modes. The tangential modes have much smaller oscillator strengths than those of the radial modes as manifested by infrared absorption spectroscopy and HREELS for C60 molecules [4,9,16]. In the case of C70 molecules, the tangential mode at 178 meV has a rather comparable magnitude of intensity with that of the radial modes at 68 meV as shown for the 6 ML ®lm. Nemes [17] suggested that this discrepancy is attributed to the vibration±rotation interaction in the C70 molecule. The C70 molecules in solid-state rotate anisotropically at room temperature. For such a rotating molecule, there is a large mixing of two accidentally degenerate vibrational states due to Coriolis interaction between them [18,19]. The accidental degeneracy is not directly related to the symmetry. It means that A002 and E10 modes can degenerate accidentally. For C70 molecule, Jahn's rule reveals that a rotational motion around the short axis mixes the A002 and E10 modes [17]. The oscillator strength in a dielectric function is proportional to the square of the transition dipole moment rj / jhf jmz jiij2 / j@mz =@Qj j2Qj 0 ;
(3)
where jii and jf i denote the initial state and the ®nal state of vibrational modes, respectively, mz is the component of the dipole moment relative to the axis normal to the surface and Qj the jth normal coordinate. Due to the mixing of the parallel and perpendicular
151
modes, the derivative of induced dipole moment in the new excited state may have a larger value than that in the non-perturbed states. A lot of theoretical and experimental works have been performed on the vibrational modes of C70 molecules, but not all of the fundamentals have been determined and the real structure of C70 molecule may not completely corresponds with the ones assumed in the reported calculations. The presence of the 31 dipole active modes, rather a lot of modes, may result in an accidental degeneracy between the A002 and E10 modes. If the A002 and E10 modes which are components of the 83 and 178 meV peaks are accidentally degenerate, they will strongly interact through the rotational motion for the 6 ML ®lm and the enhancement of the peak intensity may be observed. We consider that the decreases in intensities at 83 and 178 meV for the 1 ML ®lm indicate that the rotational motion of C70 molecule is quenched upon adsorption and the Coriolis interaction disappears at the surface. 4. Conclusion We have investigated the coverage dependence of vibrational excitation spectra of C70 molecules adsorbed on a Si(1 1 1)-(77) surface using HREELS at room temperature. Our analysis shows that the dielectric response of the 1 ML ®lm is strongly in¯uenced by the Si(1 1 1)-(77) surface. Judging from the probable modi®cation to the oscillator strengths demanded by the dipole selection rule, we found that for the 1 ML ®lm the average angle between long axes of C70 molecules and surface normal is about 40 . We consider that the decreases in intensities at the 83 and 178 meV for 1 ML ®lm indicate the quenching of the rotational motion of the molecules upon adsorption. Acknowledgements The authors would like to thank Professor Wakio Uchida and Professor Atsuo Kasuya for their valuable discussions. This work was supported in part by Grant-in-Aid for Scienti®c Research from the Ministry of Education, Science, Sports and Culture and the Izumi Science and Technology Foundation.
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References [1] Y. Fujikawa, K. Saiki, A. Koma, Surf. Sci. 357-358 (1996) 176. [2] L.-M. Yu, K. Hevesi, B.-Y. Han, J.-J. Pireaux, P.A. Thiry, R. Caudano, Ph. Lambin, Surf. Rev. Lett. 2 (1995) 705. [3] T. Wakita, K. Sakamoto, A. Kasuya, Y. Nishina, S. Suto, Appl. Surf. Sci. 144/145 (1999) 653. [4] S. Suto, K. Sakamoto, T. Wakita, C.-W. Hu, A. Kasuya, Phys. Rev. B 56 (1997) 7439. [5] V.P. Dravid, S. Liu, M.M. Kappes, Chem. Phys. Lett. 185 (1991) 75. [6] Y. Fujikawa, M. Sakurai, A. Koma, Jpn. J. Appl. Phys. 34 (1995) L188. [7] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982. [8] K.M. Chen, K. Wu, Y. Chen, Y.Q. Jia, S.X. Jin, C.Y. Li, Z.N. Gu, X.H. Zhou, Appl. Phys. Lett. 67 (1995) 1683. [9] G. Gensterblum, L.-M. Yu, J.J. Pireaux, P.A. Thiry, R. Caudano, Ph. Lambin, A.A. Lucas, W. KraÈtschmer, J.E. Fischer, J. Phys. Chem. Solids 53 (1992) 1427.
[10] L. Nemes, R.S. Ram, P.F. Bernath, F.A. Tinker, M.C. Zumwalt, L.D. Lamb, D.R. Huffman, Chem. Phys. Lett. 218 (1994) 295. [11] W. Brockner, F. Menzel, J. Mol. Struct. 378 (1996) 147. [12] X.-D. Wang, Q. Xue, T. Hashizume, Phys. Rev. B 49 (1994) 7754. [13] A.L. Balch, V.J. Catalano, J.W. Lee, M.M. Olmstead, S.R. Parkin, J. Am. Chem. Soc. 113 (1991) 8953. [14] A.M. Rao, M. Menon, K.-A. Wang, P.C. Eklund, K.R. Subaswamy, D.S. Cornett, M.A. Ducan, I.J. Amster, Chem. Phys. Lett. 224 (1994) 106. [15] D.E. Weeks, W.G. Harter, Chem. Phys. Lett. 176 (1991) 209. [16] D.S. Bethune, G. Meijer, W.C. Tang, H.J. Rosen, W.G. Golden, H. Seki, C.A. Brown, M.S. de Vries, Chem. Phys. Lett. 179 (1991) 181. [17] L. Nemes, J. Mol. Struct. 436/437 (1997) 25. [18] G. Herzberg, Infrared and Raman Spectra, Van Nostrand, Princeton, NJ, 1945. [19] C. Di Lauro, I.M. Mills, J. Mol. Spectrosc. 21 (1966) 386.
HREELS study of C70 molecules adsorbed on a Si(1 1 1)-(7 7) surface Takanori Wakita, Kazuyuki Sakamoto, Shozo Suto* Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan Received 2 August 1999; accepted 13 December 1999
Abstract We have measured the coverage dependence of vibrational excitation spectra of C70 molecules adsorbed on a Si(1 1 1)(77) surface using high-resolution electron-energy-loss spectroscopy. At the monolayer coverage, the intensity of the 57 meV peak increases, and those of the 83 and 178 meV peaks decrease. Taking into account the dipole selection rule, the change in intensity of the 57 meV peak indicates that the average angle between the long axes of C70 molecules and surface normal is about 40 . The decreases in intensities of the 83 and 178 meV peaks suggest that the rotational motion of molecules is quenched upon adsorption. We will discuss the Coriolis interaction between the accidentally degenerate A002 and E10 modes. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Fullerenes; Vibrations of adsorbed molecules; Electron energy loss spectroscopy; Silicon; Coriolis interaction
1. Introduction C70 is the second stable and abundant fullerene, but little is known about the interaction of C70 molecules with metal and semiconductor surfaces. It is important to understand the interaction in order to develop a new functionality. The rugby ball-shaped C70 molecule has a D5h symmetry. Due to the symmetry, a C70 molecule has 31 infrared (IR) active modes (10A002 21E10 ) and all vibrational energies of these modes lie in the range from 30 to 200 meV. The A002 modes have oscillating dipole moments parallel to the long axis (®ve-fold axis) and the E10 modes have those perpendicular to the axis [1]. Yu et al. studied the oscillator strength of IR active modes for the C70 thick ®lm using high-resolution electron-energy-loss spectroscopy (HREELS)
[2]. Fujikawa et al. reported the vibrational modes for the 1 ML ®lm at a MoS2 surface and that the interaction is very weak [1]. We studied the thermal reaction and the SiC ®lm formation from C70 molecules at Si surfaces [3]. In this paper, we report the measurements of vibrational excitation spectra of C70 molecules adsorbed on a Si(1 1 1)-(77) surface at the coverage lower than 6 ML (monolayers) to investigate the interaction. We found that the average angle between the long axes of C70 molecules in the 1 ML ®lm is about 40 judging from the dipole selection rule. The rotational motion of C70 molecules are discussed in terms of the Coriolis interaction. 2. Experiment
*
Corresponding author. Tel.: 81-22-217-7752; fax: 81-22-217-7746. E-mail address: [email protected] (S. Suto).
We performed the HREELS measurements in a UHV system which consists of an analysis chamber
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 1 3 - 3
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T. Wakita et al. / Applied Surface Science 169±170 (2001) 147±152
and a sample preparation chamber. The analysis chamber is equipped with HREELS (VSW IB2000) and a low energy electron diffraction (LEED). The base pressure was 1:3 10ÿ8 Pa in the analysis chamber and 1:3 10ÿ7 Pa in the sample preparation chamber. The samples were transferred between chambers under UHV condition. In the HREELS measurements, the incident energy of the electron beam was 5.0 eV. The scattered angle (ys ) was ®xed at 60 from the surface normal. We changed the incident angle (yi ) to measure the angular dependence of scattering intensity. Commercial n-type Si(1 1 1) wafers (phosphorus doped, 1000 O cm) were used as sample substrates. The sample preparation and estimation procedures are reported in [4]. The C70 powder was loaded into a quartz crucible and was heated by a tungsten wire in the sample preparation chamber. The C70 powder was carefully outgassed below 573 K for over 24 h prior to evaporation. The thickness of the C70 ®lm was monitored by a quartz Ê /min. We crystal balance. The deposition rate was 2 A Ê and assumed the hcp structure (a b 10:56 A c 17:18 A) of C70 ®lm, though C70 ®lms grown on room temperature substrates tend to adopt a mixture of fcc and hcp phases with numbers of stacking faults [5]. The thickness of 17:18 A is estimated to be 2 ML. All measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows electron-energy-loss spectra of C70 molecules adsorbed on the Si(1 1 1)-(77) surface measured with the specular condition (yi ys 60 ). The coverages are 6 ML (a), 1 ML (b), 0.5 ML (c) and 0.25 ML (d). At 6 ML, two intense peaks appear at 68 and 178 meV. Small peaks and shoulders are observed at 43, 57, 83, 96, 111, 144, 164 and 194 meV. The spectral pro®les are the same as those previously reported for C70 thick ®lms [2,6]. At 1 ML, peaks and shoulders appear at 43, 57, 68, 96, 111, 164 and 178 meV. The intensities at 83 and 178 meV peaks dramatically decrease. At 0.5 and 0.25 ML, two peaks appear at 65 and 94 meV, and a broad peak is observed at 170 meV. Fig. 2 shows electron-energy-loss spectra of C70 molecules adsorbed on the Si(111)-(77) at 6 ML in
Fig. 1. Electron-energy-loss spectra of C70 molecules adsorbed on a Si(1 1 1)-(77) surface measured with the specular condition (yi ys 60 ). The coverages are 6 ML (a); 1 ML (b); 0.5 ML (c) and 0.25 ML (d).
(a) and 1 ML in (b). The thick solid lines are measured with the specular condition, and the thin solid lines with the off-specular condition (yi 51 , ys 60 ). The ordinate is plotted by absolute intensity to compare the cross sections between off-specular and specular conditions. In off-specular spectra, peaks and shoulders are observed at 67, 94, 163 and 184 meV for the 6 ML ®lm, and at 66, 94, 166, 179 and 192 meV for the 1 ML ®lm. It is noticed that the spectral pro®les in Fig. 1(c) and (d) are the same as those of the off-specular spectra in Fig. 2. In the following sections, we discuss the differences in the relative peak intensities between the 6 ML and the 1 ML ®lms in terms of the surface loss function. In general, an HREEL spectrum consists of two components. One is due to the dipole-scattering process and the other is due to the impact-scattering process [7]. The dipole-scattered electrons sharply peak about the specular direction, while the impact-scattered electrons have rather isotropic angular dependence on the whole [2]. We observed drastic decreases in the
T. Wakita et al. / Applied Surface Science 169±170 (2001) 147±152
149
Fig. 2. Electron-energy-loss spectra of C70 molecules adsorbed on a Si(1 1 1)-(77) at the coverages of 6 ML (a) and 1 ML (b). The thick solid lines are measured with the specular condition, and the thin solid lines with the off-specular condition (yi 51 , ys 60 ). The dashed lines are the backgrounds we assumed.
Fig. 3. The thick lines are obtained by subtracting the off-specular spectra and the backgrounds from the specular spectra in Fig. 2. The thick dashed lines are the simulated spectra by convoluting the surface loss function with the instrumental function. The thin lines are the Gaussian components of the simulated spectra. The hatched Gaussian components are A002 modes and the other Gaussians are E10 modes.
intensities of the 68 and 178 meV peaks as yi decreases, but little change between the off-specular spectra measured with yi 51 and 48 . This result indicates that we are able to ignore the angular dependence of the impact-scattered electrons for C70 ®lms. Due to the difference in the angular distributions, it is possible to extract the dipole-scattering components from the experimental data by subtracting the off-specular spectra [2]. Thick solid lines in Fig. 3 are the residual spectra which display the dipole-scattering components at 6 ML in (a) and 1 ML in (b). The dipole-scattering components are reproduced by using the surface loss function, I / Imfÿ1= e o 1g [7]. We assume that the dipole polarizability of a C70 molecule is expressed by a simple model of harmonically bound charges with viscous damping. Then, the dielectric function e o of a C70
®lm is given by e o e 1
X j
o2j
rj o2j ÿ o2 ÿ ioj ogj
;
(1)
where oj , rj and gj are the oscillator frequency, the oscillator strength and the damping ratio of the jth dipole active mode of a C70 molecule. e 1 is the optical dielectric constant and the value is 4.96 for C70 [8]. Eq. (1) is successfully applied to the HREELS data of C60 and C70 thick ®lms with parameters obtained from optical absorption measurements [2,9]. In the case of a 1 ML ®lm, the interaction between molecules through their dipole moments and the in¯uence of the substrate, should result in revisions to the parameters determined for thick ®lms. It is reported that the extrapolated frequencies at 0 K for the C70 molecules in gas phase are quite close to
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T. Wakita et al. / Applied Surface Science 169±170 (2001) 147±152
the solid-state line positions at room temperature in IR spectra [10]. This result indicates that the dipole± dipole interaction between C70 molecules is negligible. The in¯uence of the Si surface will be discussed below. When one inserts Eq. (1) into the surface loss function, the function has peaks at the energies which are approximately equal to oj 's and each peak has a Lorentzian lineshape. In order to reproduce our experimental data, the convolution of the surface loss function with the instrumental function is required. In the ®rst approximation, the instrumental function has a Gaussian lineshape with the full widths at half maximum of elastic peaks (10 meV) and the convoluted spectrum consists of the Gaussian components. Thick dashed lines in Fig. 3 are simulated spectra by using Eq. (1). The thin solid lines are each component. We use a set of parameters determined by optical absorption spectra for 13 modes which have stronger intensities than the others [2]. The hatched Gaussian components are A002 modes and the other Gaussians are E10 modes [11]. Each simulated spectrum is normalized to the 68 meV peak intensity. At 6 ML, the thick solid and dashed lines are in good agreement with each other. This result con®rms that the thick solid lines are produced by electrons which lose their energies almost only by the dipole-scattering process. Nevertheless, the spectrum at 1 ML is not well reproduced. Particularly, the intensity at 57 meV increases and those at 83 and 178 meV decrease. Fujikawa et al. reported that the intensities at 83 and 178 meV exhibit no change on a MoS2 substrate at 1 ML [1]. These results clearly indicate the strong in¯uence of the Si(1 1 1)-(77) surface on the dielectric response of the 1 ML ®lm. The parameters in Eq. (1) should be subject to review for the 1 ML ®lm on the Si(1 1 1)-(77) surface. A probable modi®cation to the oscillator strengths in Eq. (1) is demanded by the dipole selection rule. The dielectric constant of silicon with E 11:7 is large enough to provide for effective screening of dipole moments parallel to the surface, especially for a 1 ML ®lm. Fig. 3(b) shows that both the E10 and A002 modes contribute to the spectra. This result indicates that the orientations of the long axes are neither perpendicular nor parallel to the surface uniformly. We note that the decreases in the peak intensities at 83 and 178 meV are attributed to both
the E10 and the A002 modes, whereas the increase in the peak intensity at 57 meV is related only to the increase of the A002 modes intensity. We consider that the observed intensity ratio IA002 =IE10 is given by following equation IA002 I
E10
rA002 cos2 yl 2
rE10 sin yl
rA002 rE10
cot2 yl ;
(2)
where yl is the angle between the long axis of C70 molecule and surface normal, rA002 the oscillator strength of A002 mode at 57 meV and rE10 the sum of the oscillator strength of E10 modes at 66, 70 and 72 meV. We obtained yl 40 by adjusting the relative intensities of the Gaussian components to reproduce the spectrum at 1 ML in the shoulder at 57 meV. There are two ideal con®gurations of C70 molecules. One is an ordered and the other is a random con®guration. In fact, C70 molecules form a ¯at monolayer ®lm with a short-range order on a Si(1 0 0)-(21) surface [12]. In the case of C60 molecules, it is reported that only 23% of the C60 molecules in a 1 ML ®lm strongly interact with Si dangling bonds on a Si(1 1 1)-(77) surface resulting in a short-range order [4]. The chemical bond can prevent a long range order of C70 molecules. We consider that also on a Si(1 1 1)-(77) surface, the 1 ML ®lm has just a shortrange order due to the chemical bond formation between a small fraction of C70 molecules and the Si surface. The ordered molecules can interact with Si surface by van der Waals force. It is important as well that C70 molecules have an anisotropic chemical reactivity. Balch et al. reported that the ®ve double bonds between two of the ®ve hexagons in a polar cap are relatively chemically reactive for iridium complex adsorption than those in equator belt hexagons [13]. This fact suggests that the polar caps are rather `C60 like' [14] and mainly react with the Si dangling bonds in the process of the chemical bond formation. A possible geometry of strongly interacting C70 molecules is that one of the ®ve hexagons in a polar cap is parallel to the surface (yl 32 ). Such molecules can in¯uence the orientations of short-range ordered molecules. Our estimation of yl 40 suggests that the long axes of short-range ordered molecules incline by 40 as an average. Finally, we discuss the decreases in the intensities at 83 and 178 meV peaks. As discussed above, these
T. Wakita et al. / Applied Surface Science 169±170 (2001) 147±152
peaks consist of both the E10 and the A002 modes and the decreases are not explained by the dipole selection rule. Moreover, it seems that the decreases are free from both splitting and broadening of the peaks, and these intensities simply decrease as shown in Fig. 3. We interpret these decreases in terms of the quenching of the rotational motion of C70 molecules upon adsorption. It is known that the intramolecular vibrational energies of fullerenes reside in the range from 30 to 200 meV, and it is separated into two regions, one (30± 110 meV) in which the normal modes of fullerenes consist of primarily radial motions, and the other (130±200 meV) in which the tangential motions are dominant [15]. We refer the modes lying in each region the radial modes and the tangential modes, respectively. For the four T1u modes of the C60 molecule, the two modes at 66 and 72 meV are the radial modes and those at 147 and 179 meV are the tangential modes. The tangential modes have much smaller oscillator strengths than those of the radial modes as manifested by infrared absorption spectroscopy and HREELS for C60 molecules [4,9,16]. In the case of C70 molecules, the tangential mode at 178 meV has a rather comparable magnitude of intensity with that of the radial modes at 68 meV as shown for the 6 ML ®lm. Nemes [17] suggested that this discrepancy is attributed to the vibration±rotation interaction in the C70 molecule. The C70 molecules in solid-state rotate anisotropically at room temperature. For such a rotating molecule, there is a large mixing of two accidentally degenerate vibrational states due to Coriolis interaction between them [18,19]. The accidental degeneracy is not directly related to the symmetry. It means that A002 and E10 modes can degenerate accidentally. For C70 molecule, Jahn's rule reveals that a rotational motion around the short axis mixes the A002 and E10 modes [17]. The oscillator strength in a dielectric function is proportional to the square of the transition dipole moment rj / jhf jmz jiij2 / j@mz =@Qj j2Qj 0 ;
(3)
where jii and jf i denote the initial state and the ®nal state of vibrational modes, respectively, mz is the component of the dipole moment relative to the axis normal to the surface and Qj the jth normal coordinate. Due to the mixing of the parallel and perpendicular
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modes, the derivative of induced dipole moment in the new excited state may have a larger value than that in the non-perturbed states. A lot of theoretical and experimental works have been performed on the vibrational modes of C70 molecules, but not all of the fundamentals have been determined and the real structure of C70 molecule may not completely corresponds with the ones assumed in the reported calculations. The presence of the 31 dipole active modes, rather a lot of modes, may result in an accidental degeneracy between the A002 and E10 modes. If the A002 and E10 modes which are components of the 83 and 178 meV peaks are accidentally degenerate, they will strongly interact through the rotational motion for the 6 ML ®lm and the enhancement of the peak intensity may be observed. We consider that the decreases in intensities at 83 and 178 meV for the 1 ML ®lm indicate that the rotational motion of C70 molecule is quenched upon adsorption and the Coriolis interaction disappears at the surface. 4. Conclusion We have investigated the coverage dependence of vibrational excitation spectra of C70 molecules adsorbed on a Si(1 1 1)-(77) surface using HREELS at room temperature. Our analysis shows that the dielectric response of the 1 ML ®lm is strongly in¯uenced by the Si(1 1 1)-(77) surface. Judging from the probable modi®cation to the oscillator strengths demanded by the dipole selection rule, we found that for the 1 ML ®lm the average angle between long axes of C70 molecules and surface normal is about 40 . We consider that the decreases in intensities at the 83 and 178 meV for 1 ML ®lm indicate the quenching of the rotational motion of the molecules upon adsorption. Acknowledgements The authors would like to thank Professor Wakio Uchida and Professor Atsuo Kasuya for their valuable discussions. This work was supported in part by Grant-in-Aid for Scienti®c Research from the Ministry of Education, Science, Sports and Culture and the Izumi Science and Technology Foundation.
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References [1] Y. Fujikawa, K. Saiki, A. Koma, Surf. Sci. 357-358 (1996) 176. [2] L.-M. Yu, K. Hevesi, B.-Y. Han, J.-J. Pireaux, P.A. Thiry, R. Caudano, Ph. Lambin, Surf. Rev. Lett. 2 (1995) 705. [3] T. Wakita, K. Sakamoto, A. Kasuya, Y. Nishina, S. Suto, Appl. Surf. Sci. 144/145 (1999) 653. [4] S. Suto, K. Sakamoto, T. Wakita, C.-W. Hu, A. Kasuya, Phys. Rev. B 56 (1997) 7439. [5] V.P. Dravid, S. Liu, M.M. Kappes, Chem. Phys. Lett. 185 (1991) 75. [6] Y. Fujikawa, M. Sakurai, A. Koma, Jpn. J. Appl. Phys. 34 (1995) L188. [7] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982. [8] K.M. Chen, K. Wu, Y. Chen, Y.Q. Jia, S.X. Jin, C.Y. Li, Z.N. Gu, X.H. Zhou, Appl. Phys. Lett. 67 (1995) 1683. [9] G. Gensterblum, L.-M. Yu, J.J. Pireaux, P.A. Thiry, R. Caudano, Ph. Lambin, A.A. Lucas, W. KraÈtschmer, J.E. Fischer, J. Phys. Chem. Solids 53 (1992) 1427.
[10] L. Nemes, R.S. Ram, P.F. Bernath, F.A. Tinker, M.C. Zumwalt, L.D. Lamb, D.R. Huffman, Chem. Phys. Lett. 218 (1994) 295. [11] W. Brockner, F. Menzel, J. Mol. Struct. 378 (1996) 147. [12] X.-D. Wang, Q. Xue, T. Hashizume, Phys. Rev. B 49 (1994) 7754. [13] A.L. Balch, V.J. Catalano, J.W. Lee, M.M. Olmstead, S.R. Parkin, J. Am. Chem. Soc. 113 (1991) 8953. [14] A.M. Rao, M. Menon, K.-A. Wang, P.C. Eklund, K.R. Subaswamy, D.S. Cornett, M.A. Ducan, I.J. Amster, Chem. Phys. Lett. 224 (1994) 106. [15] D.E. Weeks, W.G. Harter, Chem. Phys. Lett. 176 (1991) 209. [16] D.S. Bethune, G. Meijer, W.C. Tang, H.J. Rosen, W.G. Golden, H. Seki, C.A. Brown, M.S. de Vries, Chem. Phys. Lett. 179 (1991) 181. [17] L. Nemes, J. Mol. Struct. 436/437 (1997) 25. [18] G. Herzberg, Infrared and Raman Spectra, Van Nostrand, Princeton, NJ, 1945. [19] C. Di Lauro, I.M. Mills, J. Mol. Spectrosc. 21 (1966) 386.