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STM-HREELS Investigation of C60 on Cu(111)O. Takaoka, C. Tindall, T. Kobayashi, Y. Hasegawa, T. Sakurai

To cite this version:O. Takaoka, C. Tindall, T. Kobayashi, Y. Hasegawa, T. Sakurai. STM-HREELS Investigation of C60on Cu(111). Journal de Physique IV Proceedings, EDP Sciences, 1996, 06 (C5), pp.C5-179-C5-184.�10.1051/jp4:1996529�. �jpa-00254408�

JOURNAL DE PHYSIQUE IV Colloque C5, supplBment au Journal de Physique 111, Volume 6, septembre 1996

STM-HREELS Investigation of Cso on Cu(ll1)

0. Takaoka, C. Tindall, T. Kobayashi, Y. Hasegawa and T. Sakurai

Institute for Materials Research, Tohoku University, Sendai 980-77, Japan

Abstract . Using an STMJHREELS system, we have investigated the relationship between the surface morphology and vibrational modes of C60 adsorbed on Cu(l11). At 300K, CGO is mobile on the Cu(ll1) terrace, but is more strongly bound at the step edge. Therefore, any C60 which adsorbs on the terrace will migrate to the step edge where it is immobilized. Thus for very low coverages, the ChO preferentially adsorbs at the step edge. Adsorption of the molecule on metal surfacesresults in asignificant amount ofcharge transfer from the substrate to the C60 molecule. Two ofthe four tl, infrared active modes are predicted by theory to shift to lower frequency as the amount of charge transfer increases. However, only a softening of the lowest frequency tl, mode was observed. Furthermore, no further shifting of the frequency of this mode was seen for very low coverages, indicating that any additional charge transfer at the step edge is smaller than one electron per CbO molecule.

1. Introduction

As part of a continuing interest in the structural and electronic properties of C6o molecules adsorbed on clean surfaces[l-31, we have investigated the charge transfer from the Cu(ll1) surface to adsorbed C60 for a variety of coverages using Scanning Tunneling Microscopy (STh4) and High Resolution Electron Energy Loss Spectroscopy (HREELS). The vibrational spectrum of the C60 molecule is very rich, diplaying a total of 46 distinct modes. Ten of these are Raman active and four are IR active. The IR active modes are denoted as the tiu modes. Studying changes in the vibrational spectrum of the C60 molecule as a function of the charge state of the C60 is of interest because its charge state can be varied over a broad range. This is because C60 has a relatively large electron affinity and the three-fold degeneracy of its LUMO level permits it to accommodate up to six additional electrons. For example, the alkali fullerides K3C60 and K6C60 are stable compounds. The charge state of the C60 is -3e in K3C60 and -6e in K6C60[4]. In particular, the frequencies of two of the four tiu (infrared active) modes are significantly red shifted in the alkali compounds when compared with those of the neutral molecules in solid C6o(See Figure 1). However, the vibrational frequencies of the two intermediate energy tlu modes are unaffected by the charge transfer[4]. Previous studies have investigated the amount of charge transfer on clean and K covered Au(ll0) surfaces[5] as well as the Si(111)7x7 surface[6].

In addition, the first monolayer of C60 exhibits interesting structural properties on the Cu(ll1) surface because of the small lattice mismatch[7]. At 300K, the molecule is mobile on the terrace, but is pinned by the step edge. Thus the initial adsorption takes place at the step edge. As the C60 coverage is increased, two dimensional islands form at the step edge. As the coverage is increased, the C60 layer grows out from the step edges. At full monolayer coverage, a 4x4 structure is obtained[l]. Slight annealing to 250°C improves the ordering of the C60 layer. Therefore, the STM can be used to prepare overlayers of C60 with wellldefined structures. For example, at low coverage the C60 molecules are all adsorbed near step edges. So at low coverages the HREELS beam should record the vibrational spectrum of C60 molecules which are adsorbed at the step edge. At one monolayer coverage, those C60 near the step edge will form only a small percentage of the total number of the C60 molecules on the surface. Since the HREELS

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1996529

C5-180 JOURNAL DE PHYSIQUE IV

technique averages over a relatively large surface area, the vibrational spectrum recorded will be characteristic of those C60 molecules adsorbed on the terrace. At multilayer coverages, most of the C60 molecules are situated next to other C6o molecules. Again, since the HREELS beam senses an average value in the probed volume, the vibrational spectrum of the multilayer is dominated by C60 molecules which are adjacent to other C60 molecules, rather than of those adjacent to the metal surface.

With respect to the vibrational spectrum of the molecule there are two points of interest. First, at one monolayer coverage, due to charge transfer from the substrate terraces to the C60 molecules, there should be a shift in the vibrational frequencies of the molecules which can be measured by HREELS. This can be compared to previous studies of charge transfer on other metal surfaces, in particular, Au(110)[5]. Second, since the molecule is mobile on the terrace, but pinned at the step edge, there must be some additional interaction, possibly charge transfer which stabilizes the molecule. The STM can first be used to prepare a substrate on which the C60 is primarily adsorbed on the step edge. Then the HREELS can be used to measure the vibrational frequencies of the molecules adsorbed at the step edge. If there is any additional frequency shift, the amount of additional charge transfer at the step edge can be determined from the mode shift.

0 1 2 1 1 5 h ('harge transfer per molecule

(number of electrons) Figure 1 : Energies of tl, vibrational modes of Ch0 on different metallic substrates (open circle)[4]. The filled circle shows our ChO on Cu(ll1) result.

2. Experimental

The experiments were carried out in a UHV chamber containing an STM equipped with a field-ion microscope (FIM) which is used to monitor and fabricate the tip. The base pressure was in the 10-11 range. In addition, there is an auxiliary chamber which contains an HREELS instrument (LK Technologies, LK- 2000). The sample can be transferred under UHV from the STM chamber to the HREELS chamber. The HREELS incident angle was 30 degrees and a primary beam energy of 3.6eV was used since the scattering cross-section of the C60 modes exhibits a maximum at approximately this energy. All spectra were recorded with the analyzer angle equal to the incident beam angle. The resolution was approximately 10 meV FWHM.

C60 powder (99.95% purity) was placed in a tantalum boat with a pinhole in the side. This boat could be heated causing the C60 to evaporate through the pinhole and impinge on the Cu(ll1) surface. After thorough degassing in UHV, the C60 was dosed onto the surface. The coverage and surface morphology of the C60 on the sample surface could then be checked at a number of different places in order to confirm that the distribution of the C60 conformed with expectations.

3. Results

Figures 2 through 4 show both the HREELS spectrum and the corresponding S W image for the three coverages at which one could expect some change in the amount of charge transfer between the substrate and the C60 molecule. That is to say, in Figure 2, only step edge adsorption is observed, while in Figure 3 a full, well ordered monolayer of C60 is present on the surface. In Figure 4 approximately three layers of C60 have been deposited on the surface. Since as discussed above, the average charge state of the C60 molecules is expected to be different for each of these three coverages, the tlu (infrared active) mode frequencies are expected reflect the amount of charge transfer from the substrate to the C60 molecules.

0 lot) 200 300 4(X) 500

Energy Loss (meV) Figure 2: The HREELS spectrum of C(jO adsorbed on only the step edges of Cu(ll1). The inset show image(~s=-Z.OV, 450Ax450A).

1s the corresponding STM

The most intense vibrational loss peaks visible in the spectrum appear at 66, 93, and 177 meV for multilayer coverage. For monolayer and submonolayer coverages, all of the modes appear at the same frequency, with the exception of the 66 meV mode which is now shifted to 62 meV. The mode which appears at 62 meV for monolayer and submonolayer coverages, and 66 meVfor multilayer coverages is the lowest frequency tiu (IR active) mode. The mode at 93 meV is a Raman active mode. The modes at 154 meV and 177 meV are also tiu modes.

The measured energy shifts of the modes in the bulk compounds of K3C60 and K6C60 are shown in Figure 1[5,8]. Following the method used in reference [4], i. e. assuming a linear relationship between the

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shift in the vibrational frequency and the amount of charge transferred, one can estimate the amount of frequency shift per electron transfered to the molecule from the substrate. In this way, one obtains a value of -1.25 mevlelectron transferred for the 66 meV mode , while the mode at 177 meV shifts by -1.8 mevlelectron. The other two ti, modes at 154 meV and 72 meV do not exhibit any frequency shift. This is in agreement with theory, which indicates that only the 62 meV and 177 meV peaks should shift due to charge transfer [7,9]. However, both on Cu(ll1) and Au(llO), it was experimentally determined that the 177 meV mode does not shift significantly upon adsorption. The authors of the Au(l10) study attributed this discrepancy to non-linearity caused by distortion of the molecule upon adsorption[4]. On the other hand, again consistent with the results of the Au(ll0) study, the mode at 66 meV in the multilayer shifts to 62 meV at monolayer and submonolayer (not shown) coverage.

h

3 .m E 3

e a w

Y I= 3

S

- 100 O I00 200 300 400 500

Energy Loss (meV) Figure 3: The HREELS spectrum of C(jO monolayer adsorbed on Cu(ll1). The inset shows the corresponding STM image ( v s = - ~ . ~ v , ~ooAx~ooA).

Using the numbers given above for calibration, i.e. -1.25 meV1electron shift for the 66 meV mode and -1.8 mevlelectron shift for the 177 meV mode, we can estimate the amount of charge transfer from the Cu(ll1) substrate to the C60 molecules. The lack of shift of the 177 meV peak implies a zero charge transfer while the 4meV shift of the 66 meV peak indicates a 3.2 electron transfer takes place. Again following the method used in reference [4] and taking the average of these numbers one obtains 1.6 electrons/C60 which is slightly higher than the result of 1.2 which was obtained for Au(ll0). The larger value obtained for the charge transfer on Cu(ll1) is plausible since the work function of Cu(ll1) is smaller than that of Au(110)[10].

No additional shifting of the lowest frequency tiu mode was observed for surfaces in which the C60 is adsorbed next to the step edge. A charge transfer of one electron would give rise to an additional shift of between -1.25 and -2.50 meV. This amount of shift should be detectable. Thus we conclude that any additional charge transfer due to the presence of the step edge is small, most likely less than one electron.

4. Conclusion

We have measured the energy shifts of the vibrational modes of C60 adsorbed Cu(ll1). It was found that the energy of the lowest tlu mode, for coverages of one monolayer or less shifted by approximately 4 meV from the frequency observed for a multilayer of C60 on Cu(ll1). However, the highest tlu mode which is also predicted by theory to shift, does not change its position. This may be due to non-linearity of the frequency shift vs. charge transfer caused by distortion of the molecule when it bonds to the substrate. The amount of charge transfer from the substrate to the C60 molecules estimated from this shift is approximately 1.6 electrons per C60. Finally, for the case of very low coverage in which the C60 is predominantly adsorbed at the step edge, no significant additional shifting of the vibrational modes was observed. Thus any additional charge transfer which takes place at the step edge is smaller than one electron/C60.

- 101) 0 100 2(X) 300 400 500 Energy Loss (meV)

Figure 4: The HREELS spectrum of about 2ML C60 ackorbed on Cu(ll1). The inset shows the corresponding STM image (vs=-LOV, ~soAx~soA).

Acknowledgments

C. Tindall would like to thank the Japan Society for the the Promotion of Science (JSPS) and the American National Science Foundation (NSF) for financial suport.

References

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[5] Fu, K.J, Karney, W.L., Chapman, O.L., et. al., Phys. Rev. B 46 (1992) 1937-1940. [6] Suto, S., Kasuya, A., Ikeno, O., et. al., Jpn. J. Appl. Phys. Lett. 33 (1994) L1489-1492. [7] Kohanoff, J., Andreoni, W., and Parrinello, M., Chem. Phys. Lett. 198 (1992) 472-477. [8] Pichler, T., Matos, and Kuzmany, H., Solid State Comm. 86 (1993) 221-225. [9] Rice, M.J. and Choi, H., Phys. Rev. B 45 (1992) 10173-10176. [lo] Lide, D.R., ed., Handbook of Chemistry and Physics, 76th Ed.(CRC Press, Boca Raton, 1995)

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