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Electronic structure and growth mode of the early stages of C60 adsorption at the Ag(001) surface
Surface Science 454–456 (2000) 766–770 www.elsevier.nl/locate/susc
Electronic structure and growth mode of the early stages of C adsorption at the Ag(001) surface 60 C. Cepek a, *, L. Giovanelli a, M. Sancrotti a, G. Costantini b, C. Boragno b, U. Valbusa b a Laboratorio Nazionale TASC-INFM, Padriciano 99, I-34012 Trieste, Italy b INFM and Dipartimento di Fisica, Universita´ di Genova, Via Dodecaneso 33, I-16144 Genova, Italy
Abstract The electronic structure and growth mode of C molecules deposited on Ag(001) have been studied as a function 60 of deposition parameters and annealing temperature. The measurements show that C molecules are chemically 60 bound to the Ag(001) surface even when deposited at 150 K, and the bond properties do not change significantly after annealing up to 670 K. In the resulting ordered C overlayer, a mixed contrast of the buckyballs, as seen by 60 scanning tunneling microscopy, is discussed in terms of non-equivalent orientations of the adsorbed molecules. © 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Fullerenes; Photoelectron spectroscopy; Scanning tunneling microscopy
1. Introduction The electronic structure and the growth mode of C molecules on different metal and semicon60 ductor surfaces is of great interest for the control of the epitaxial growth of C based materials, and 60 for the understanding of the interaction character between these molecules and different elements (see, for instance, [1]). The C /Ag(001) ordered 60 monolayer (ML) has been found to be an interesting system where C molecules strongly interact 60 with the substrate, being in a charge state close to −2 [2,3]. The morphology of C films deposited 60 on Ag(001) at room temperature (RT ) or above has been studied by scanning tunneling microscopy (STM ) [4,5] which shows the existence of two * Corresponding author. Fax: +39-040-226767. E-mail address: [email protected] (C. Cepek)
different types of C molecules on the surface and 60 the authors suggest that this is due to two different bonding states. This behavior is not observed in films deposited at temperatures lower than RT, where all C cages appear equivalent in the STM 60 images, suggesting a different kind of interaction between C and Ag(001). A similar behavior has 60 been observed when C is adsorbed on other 60 surfaces [6,7,12] which was explained in terms of substrate reconstruction, and differences in the electronic structure of the molecules in different adsorption sites as seen by scanning tunneling spectroscopy. An additional key point is that, in all systems reported on so far, the bright–dim contrast seen in STM persisted over the whole temperature range investigated; while in the case of the C /Ag(001) the bright–dim contrast van60 ishes when T< RT. This, in turn, might help in discriminating between the differing contributions to the valence band photoemission spectra.
0039-6028/00/$ - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 1 4 -4
C. Cepek et al. / Surface Science 454–456 (2000) 766–770
In the present work, the electronic structure and growth mode of C on Ag(001), deposited 60 both at RT and 150 K, are studied and compared as a function of the annealing temperature.
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surface, leaving a complete single layer of molecules on the Ag(001) surface.
3. Results
2. Experimental The electronic properties were investigated in an ultra-high vacuum ( UHV ) apparatus. The valence band ( VB) photoemission spectra were obtained using a conventional helium discharge lamp (Bn=21.2 eV ) in normal emission geometry by means of a 50 mm radius hemispherical analyser (acceptance angle ~1°) with an overall energy resolution of ~0.12 eV. Core level (CL) photoemission data were measured using a non-monochromatized MgKa X-ray source (Bn=1253.6 eV ) using a 100 mm hemispherical analyzer with an overall energy resolution of ~1.0 eV. All binding energy (BE ) values of VB and CL spectra have been referenced, respectively, to the Fermi level and the Ag 3d CL of a silver target in direct contact with the sample. STM measurements were performed in a variable temperature UHV-STM apparatus, able to maintain a continuous temperature control during both sample preparation and measurement. A complete description of the system can be found in Ref. [9]. Typical measurement conditions were a tunneling current of ~0.5 nA and a tip–sample voltage of ~1 V. Much care was taken in order to reproduce the same deposition conditions in both pieces of apparatus. Surface cleanliness and order were checked by CL, VB photoemission and low energy electron diffraction measurements, as well as direct STM images. C sublimation (99.9% purity) was per60 formed at a rate of ~0.2 ML/min (we define 1 ML as a complete single layer of C molecules on the 60 Ag(001) surface), which corresponded to a chamber pressure always below 1.0×10−9 mbar. C 60 coverage was calibrated by means of CL spectroscopy taking as a calibration point the Ag 3d and C 1s intensity ratio of a single C monolayer. In 60 fact, as shown in Refs. [4,5], for temperatures above 500 K all the C molecules which do not 60 belong to the first monolayer desorb from the
In order to determine the nature of the bond between the Ag(001) surface and the C molecules 60 adsorbed at RT and 150 K, we deposited a C 60 film at a coverage corresponding to ~0.7 ML. Since at RT, and below, the film grows in a 3-D fashion [5], this coverage value is a good compromise between a low fraction of second layer molecules, which are mutually bound as in solid C 60 (see further discussion), and a number of first layer molecules large enough to obtain a high photoemission signal. Fig. 1a shows an STM image of 0.7 ML of C deposited at RT. It clearly appears 60 that the growth does not proceed layer by layer, but that second layer islands begin to form before the completion of the first layer. From the direct analysis of the STM measurements, we derived that at this coverage (0.7 ML) and with our evaporation rate (~0.2 ML/min), about 17% of all the C sublimated molecules are arranged in the 60 second layer. Fig. 2 shows the VB photoemission spectra of 0.7 ML of C adsorbed on Ag(001) at 150 K and 60 subsequently annealed to different temperatures (RT and 600 K for ~5 min), compared to a thick C multilayer. The 150 K spectrum was measured 60 at 150 K and all the others at RT. The deposition of the same quantity of C molecules at RT 60 produces a photoemission spectrum (not shown) identical to that of the 150 K deposited film annealed at RT. The same is true for the spectrum of a 600 K deposited or annealed film. The spectrum of the clean Ag substrate (not shown) is characterized by a featureless plateau in the 0– 4 eV range, due to the Ag 5s delocalised states, so all the structures observed in this energy range can be unambiguously attributed to the C adsorbed 60 molecules [2,10–12].
4. Discussion We note that, for all the reported spectra, adsorption of C induces an increase of the photo60
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C. Cepek et al. / Surface Science 454–456 (2000) 766–770
Fig. 1. STM topographies of 0.7 ML C deposited at 60 0.2 ML/min at RT: (a) growth proceeds in a 3-D fashion with hexagonal compact first layer islands and ramified dendritic second layer islands (scan area 170×170 nm2); (b) higher magnification image (scan area 30×30 nm2) — the upper part shows second layer island with identical molecules; the lower part shows first layer molecules characterized by bright/dim contrast.
emission intensity at the Fermi level with respect to the clean surface thereby indicating that in all samples the bond is characterized by charge transfer from silver atoms to C molecules. The 60 150 K and RT VB photoemission spectra show that the highest occupied molecular orbitals (HOMO) and HOMO-1 derived bands consist of two components: one centered at the same BE of the multilayer spectrum, the other at the BE of
Fig. 2. Ultraviolet VB photoemission spectra of 0.7 ML of C molecules adsorbed at 150 K on Ag(001) and annealed to 60 RT and 670 K, compared to a C multilayer (>5 ML) 60 spectrum (bottom). In the 150 K and RT spectra the superimposed solid lines come from a model explaining the temperature evolution (see text (Section 4) for details). The inset shows a magnified view of the 150 K and RT spectra close to the Fermi level energy region.
one ordered C monolayer as reported in [2]. 60 This suggests that at both 150 K and RT there are at least two different kinds of molecule: one with an electronic structure similar to the face centred cubic solid, the second with an electronic structure very close to the C monolayer. Based on STM 60 images, as in Fig. 1a, we attribute the former to C cages in the second layer, and the latter to 60 C molecules in direct contact with the substrate. 60 In order to quantitatively check this hypothesis, we have tried to reproduce the 150 K and RT photoemission spectra in a numerical simulation.
C. Cepek et al. / Surface Science 454–456 (2000) 766–770
Following our assumption, we used the photoemission spectrum of the annealed film to reproduce the photoemission intensity coming from the first layer molecules, while the photoemission signal from second layer molecules was taken to be the same as that from the multilayer spectrum. The percentage of second layer molecules was treated as an adjustable parameter and used to normalize the corresponding photoemission spectrum. We neglected contributions of third or higher molecular layers and we added a polynomial background. ˚ We assumed a photoelectron escape depth of 4 A in C [11], and we supposed that every C layer 60 60 ˚. has a reasonable thickness of ~10 A The results are shown by the solid lines in Fig. 2 which correspond to parameter values for the percentage of the molecules of the second layer, x$24% for the 150 K spectrum and x$20% for the RT spectrum. The very good agreement of these results with the value of 17% obtained by STM confirms that the molecules in direct contact with the substrate present the same bond at 150 K, RT and 600 K, while the second layer molecules do not interact with the substrate, as already observed for different systems [11]. Taking into account that CL spectra indicate that no C 60 molecules desorb passing from 150 K to RT, this indicates that the increased thermal energy enables a fraction of the second layer molecules to descend the step edge and to incorporate in the first layer islands. From a careful inspection of Fig. 2, it can be seen that the 150 K deposited film spectrum is slightly sharper than the simulation result. This is due to the specific phonon broadening of the experimental data (measured at 150 K ) which is lower than in the simulation, performed using spectra measured at RT. However, it also presents a lower photoemission intensity at the Fermi level with respect to the simulation, and this can not be due to the different temperatures and may indicate a lower C charge state, or a higher disorder. 60 In spite of these minor differences, on passing from 150 K to RT there is no evidence for the appearance of two different electronic states corresponding to the two different kinds of molecule (bright and dim) observed in STM images. One way to reconcile the STM and photoemission data is to suppose that there are two different orienta-
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tions of the fullerene cages with respect to the substrate. This situation could indeed generate differences in the STM images, even though the electronic structure of the molecules always remains the same. In fact the height of a C 60 ˚ as a function molecule can vary by up to ~0.8 A of its orientation, and this value may also increase taking into account the fact that the charged molecules on the Ag(100) may be distorted by the Jahn–Teller effect [13,14]. In addition, the fact that the charge is not uniformly distributed on the C cage [15–17] can further contribute to the 60 observed height differences in STM for two different molecular orientations. These effects are likely to be additive, resulting in the measured ˚ [4,5] between the bright height difference of ~2A and dim molecules of Fig. 1b. The hypothesis of two different orientations is also confirmed in recent X-ray photoelectron diffraction measurements [3], and similar conclusions have already been drawn for C adsorption on other substrates 60 [6,8]. We remark that the case of the C /Ag(001) interface is significantly different 60 from the C /Al(111) system [6 ] where scanning 60 tunneling spectroscopy revealed different electronic structures for, bright and dim molecules. If such a difference were present for the Ag(100) substrate one would expect a change in the ultraviolet photemission spectra moving from the co-existence of the bright and dim molecules and the homogeneous pattern seen at 150 K. We also remark that the temperature dependent evolution of the bright–dim contrast suggests the occurrence of an energy potential barrier between two distinct molecular configurations, corresponding to two different orientational arrangements of the cages.
5. Conclusion It has been found that C molecules are already 60 bound to Ag(001) at 150 K and that this bond is characterized by charge transfer from Ag atoms to C molecules in all the cases investigated while 60 changing the annealing temperature. A possible explanation for the two different types of C 60 molecules seen at RT and in 600 K annealed films
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C. Cepek et al. / Surface Science 454–456 (2000) 766–770
has been given in terms of different orientations with respect to the substrate.
Acknowledgements The authors are indebted to Kevin Prince for providing the experimental apparatus for the photoemission experiment. This project was partially financed within the PRA-CLASS of the INFM.
References [1] P. Rudolf, in: H. Kuzmany, J. Fink, H. Hehring, S. Roth (Eds.), Proc. 10th International Winterschool on Electronic Properties of Novel Materials, World Scientific, Singapore, 1996. [2] A. Goldoni, C. Cepek, E. Magnano, A.D. Laine, S. Vandre´, M. Sancrotti, Phys. Rev. B 58 (1998) 2228. [3] C. Cepek, M. Sancrotti, T. Greber, J. Osterwalder, unpublished results.
[4] E. Giudice, E. Magnano, S. Rusponi, C. Boragno, U. Valbusa, Surf. Sci. 405 (1998) L561. [5] G. Costantini, S. Rusponi, E. Giudice, C. Boragno, U. Valbusa, Carbon 37 (5) (1999) 727. [6 ] M.K.J. Johansson, A.J. Maxwell, S.M. Gray, P.A. Bru¨hwiler, D.C. Mancini, N. Ma˚rtensson, Phys. Rev. B 54 (1996) 13 472. [7] J.K. Gimzewski, S. Modesti, R.R. Schlittler, Phys. Rev. Lett. 72 (1994) 1036. [8] E.I. Altman, R.J. Colton, Phys. Rev. B 48 (1993) 18 244. [9] R. Conti, S. Rusponi, D. Pagnotta, C. Boragno, U. Valbusa, Vacuum 48 (1997) 639. [10] J.J. Yeh, I. Lindau, in: Atomic Data and Nuclear Data Tables 32 (1985) 1. [11] A. Goldoni, C. Cepek, S. Modesti, Synth. Met. 77 (1996) 189. [12] B.W. Hoogenboom, R. Esper, L.H. Tjeng, G.A. Sawatzky, Phys Rev. B 57 (1998) 11 939. [13] A. Auerbach, N. Manini, E. Tosatti, Phys. Rev. B 49 (1994) 12 998. [14] N. Manini, E. Tosatti, A. Auerbach, Phys. Rev. B 49 (1994) 13 008. [15] S. Saito, A. Oshiyama, Phys. Rev. Lett. 66 (1991) 2637. [16 ] D.L. Novikov, V.A. Gubanov, A.J. Freeman, Physica C 191 (1992) 399. [17] W. Andreoni, P. Giannozzi, M. Parinello, Phys. Rev. B 51 (1995) 2087.
Electronic structure and growth mode of the early stages of C adsorption at the Ag(001) surface 60 C. Cepek a, *, L. Giovanelli a, M. Sancrotti a, G. Costantini b, C. Boragno b, U. Valbusa b a Laboratorio Nazionale TASC-INFM, Padriciano 99, I-34012 Trieste, Italy b INFM and Dipartimento di Fisica, Universita´ di Genova, Via Dodecaneso 33, I-16144 Genova, Italy
Abstract The electronic structure and growth mode of C molecules deposited on Ag(001) have been studied as a function 60 of deposition parameters and annealing temperature. The measurements show that C molecules are chemically 60 bound to the Ag(001) surface even when deposited at 150 K, and the bond properties do not change significantly after annealing up to 670 K. In the resulting ordered C overlayer, a mixed contrast of the buckyballs, as seen by 60 scanning tunneling microscopy, is discussed in terms of non-equivalent orientations of the adsorbed molecules. © 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Fullerenes; Photoelectron spectroscopy; Scanning tunneling microscopy
1. Introduction The electronic structure and the growth mode of C molecules on different metal and semicon60 ductor surfaces is of great interest for the control of the epitaxial growth of C based materials, and 60 for the understanding of the interaction character between these molecules and different elements (see, for instance, [1]). The C /Ag(001) ordered 60 monolayer (ML) has been found to be an interesting system where C molecules strongly interact 60 with the substrate, being in a charge state close to −2 [2,3]. The morphology of C films deposited 60 on Ag(001) at room temperature (RT ) or above has been studied by scanning tunneling microscopy (STM ) [4,5] which shows the existence of two * Corresponding author. Fax: +39-040-226767. E-mail address: [email protected] (C. Cepek)
different types of C molecules on the surface and 60 the authors suggest that this is due to two different bonding states. This behavior is not observed in films deposited at temperatures lower than RT, where all C cages appear equivalent in the STM 60 images, suggesting a different kind of interaction between C and Ag(001). A similar behavior has 60 been observed when C is adsorbed on other 60 surfaces [6,7,12] which was explained in terms of substrate reconstruction, and differences in the electronic structure of the molecules in different adsorption sites as seen by scanning tunneling spectroscopy. An additional key point is that, in all systems reported on so far, the bright–dim contrast seen in STM persisted over the whole temperature range investigated; while in the case of the C /Ag(001) the bright–dim contrast van60 ishes when T< RT. This, in turn, might help in discriminating between the differing contributions to the valence band photoemission spectra.
0039-6028/00/$ - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 1 4 -4
C. Cepek et al. / Surface Science 454–456 (2000) 766–770
In the present work, the electronic structure and growth mode of C on Ag(001), deposited 60 both at RT and 150 K, are studied and compared as a function of the annealing temperature.
767
surface, leaving a complete single layer of molecules on the Ag(001) surface.
3. Results
2. Experimental The electronic properties were investigated in an ultra-high vacuum ( UHV ) apparatus. The valence band ( VB) photoemission spectra were obtained using a conventional helium discharge lamp (Bn=21.2 eV ) in normal emission geometry by means of a 50 mm radius hemispherical analyser (acceptance angle ~1°) with an overall energy resolution of ~0.12 eV. Core level (CL) photoemission data were measured using a non-monochromatized MgKa X-ray source (Bn=1253.6 eV ) using a 100 mm hemispherical analyzer with an overall energy resolution of ~1.0 eV. All binding energy (BE ) values of VB and CL spectra have been referenced, respectively, to the Fermi level and the Ag 3d CL of a silver target in direct contact with the sample. STM measurements were performed in a variable temperature UHV-STM apparatus, able to maintain a continuous temperature control during both sample preparation and measurement. A complete description of the system can be found in Ref. [9]. Typical measurement conditions were a tunneling current of ~0.5 nA and a tip–sample voltage of ~1 V. Much care was taken in order to reproduce the same deposition conditions in both pieces of apparatus. Surface cleanliness and order were checked by CL, VB photoemission and low energy electron diffraction measurements, as well as direct STM images. C sublimation (99.9% purity) was per60 formed at a rate of ~0.2 ML/min (we define 1 ML as a complete single layer of C molecules on the 60 Ag(001) surface), which corresponded to a chamber pressure always below 1.0×10−9 mbar. C 60 coverage was calibrated by means of CL spectroscopy taking as a calibration point the Ag 3d and C 1s intensity ratio of a single C monolayer. In 60 fact, as shown in Refs. [4,5], for temperatures above 500 K all the C molecules which do not 60 belong to the first monolayer desorb from the
In order to determine the nature of the bond between the Ag(001) surface and the C molecules 60 adsorbed at RT and 150 K, we deposited a C 60 film at a coverage corresponding to ~0.7 ML. Since at RT, and below, the film grows in a 3-D fashion [5], this coverage value is a good compromise between a low fraction of second layer molecules, which are mutually bound as in solid C 60 (see further discussion), and a number of first layer molecules large enough to obtain a high photoemission signal. Fig. 1a shows an STM image of 0.7 ML of C deposited at RT. It clearly appears 60 that the growth does not proceed layer by layer, but that second layer islands begin to form before the completion of the first layer. From the direct analysis of the STM measurements, we derived that at this coverage (0.7 ML) and with our evaporation rate (~0.2 ML/min), about 17% of all the C sublimated molecules are arranged in the 60 second layer. Fig. 2 shows the VB photoemission spectra of 0.7 ML of C adsorbed on Ag(001) at 150 K and 60 subsequently annealed to different temperatures (RT and 600 K for ~5 min), compared to a thick C multilayer. The 150 K spectrum was measured 60 at 150 K and all the others at RT. The deposition of the same quantity of C molecules at RT 60 produces a photoemission spectrum (not shown) identical to that of the 150 K deposited film annealed at RT. The same is true for the spectrum of a 600 K deposited or annealed film. The spectrum of the clean Ag substrate (not shown) is characterized by a featureless plateau in the 0– 4 eV range, due to the Ag 5s delocalised states, so all the structures observed in this energy range can be unambiguously attributed to the C adsorbed 60 molecules [2,10–12].
4. Discussion We note that, for all the reported spectra, adsorption of C induces an increase of the photo60
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C. Cepek et al. / Surface Science 454–456 (2000) 766–770
Fig. 1. STM topographies of 0.7 ML C deposited at 60 0.2 ML/min at RT: (a) growth proceeds in a 3-D fashion with hexagonal compact first layer islands and ramified dendritic second layer islands (scan area 170×170 nm2); (b) higher magnification image (scan area 30×30 nm2) — the upper part shows second layer island with identical molecules; the lower part shows first layer molecules characterized by bright/dim contrast.
emission intensity at the Fermi level with respect to the clean surface thereby indicating that in all samples the bond is characterized by charge transfer from silver atoms to C molecules. The 60 150 K and RT VB photoemission spectra show that the highest occupied molecular orbitals (HOMO) and HOMO-1 derived bands consist of two components: one centered at the same BE of the multilayer spectrum, the other at the BE of
Fig. 2. Ultraviolet VB photoemission spectra of 0.7 ML of C molecules adsorbed at 150 K on Ag(001) and annealed to 60 RT and 670 K, compared to a C multilayer (>5 ML) 60 spectrum (bottom). In the 150 K and RT spectra the superimposed solid lines come from a model explaining the temperature evolution (see text (Section 4) for details). The inset shows a magnified view of the 150 K and RT spectra close to the Fermi level energy region.
one ordered C monolayer as reported in [2]. 60 This suggests that at both 150 K and RT there are at least two different kinds of molecule: one with an electronic structure similar to the face centred cubic solid, the second with an electronic structure very close to the C monolayer. Based on STM 60 images, as in Fig. 1a, we attribute the former to C cages in the second layer, and the latter to 60 C molecules in direct contact with the substrate. 60 In order to quantitatively check this hypothesis, we have tried to reproduce the 150 K and RT photoemission spectra in a numerical simulation.
C. Cepek et al. / Surface Science 454–456 (2000) 766–770
Following our assumption, we used the photoemission spectrum of the annealed film to reproduce the photoemission intensity coming from the first layer molecules, while the photoemission signal from second layer molecules was taken to be the same as that from the multilayer spectrum. The percentage of second layer molecules was treated as an adjustable parameter and used to normalize the corresponding photoemission spectrum. We neglected contributions of third or higher molecular layers and we added a polynomial background. ˚ We assumed a photoelectron escape depth of 4 A in C [11], and we supposed that every C layer 60 60 ˚. has a reasonable thickness of ~10 A The results are shown by the solid lines in Fig. 2 which correspond to parameter values for the percentage of the molecules of the second layer, x$24% for the 150 K spectrum and x$20% for the RT spectrum. The very good agreement of these results with the value of 17% obtained by STM confirms that the molecules in direct contact with the substrate present the same bond at 150 K, RT and 600 K, while the second layer molecules do not interact with the substrate, as already observed for different systems [11]. Taking into account that CL spectra indicate that no C 60 molecules desorb passing from 150 K to RT, this indicates that the increased thermal energy enables a fraction of the second layer molecules to descend the step edge and to incorporate in the first layer islands. From a careful inspection of Fig. 2, it can be seen that the 150 K deposited film spectrum is slightly sharper than the simulation result. This is due to the specific phonon broadening of the experimental data (measured at 150 K ) which is lower than in the simulation, performed using spectra measured at RT. However, it also presents a lower photoemission intensity at the Fermi level with respect to the simulation, and this can not be due to the different temperatures and may indicate a lower C charge state, or a higher disorder. 60 In spite of these minor differences, on passing from 150 K to RT there is no evidence for the appearance of two different electronic states corresponding to the two different kinds of molecule (bright and dim) observed in STM images. One way to reconcile the STM and photoemission data is to suppose that there are two different orienta-
769
tions of the fullerene cages with respect to the substrate. This situation could indeed generate differences in the STM images, even though the electronic structure of the molecules always remains the same. In fact the height of a C 60 ˚ as a function molecule can vary by up to ~0.8 A of its orientation, and this value may also increase taking into account the fact that the charged molecules on the Ag(100) may be distorted by the Jahn–Teller effect [13,14]. In addition, the fact that the charge is not uniformly distributed on the C cage [15–17] can further contribute to the 60 observed height differences in STM for two different molecular orientations. These effects are likely to be additive, resulting in the measured ˚ [4,5] between the bright height difference of ~2A and dim molecules of Fig. 1b. The hypothesis of two different orientations is also confirmed in recent X-ray photoelectron diffraction measurements [3], and similar conclusions have already been drawn for C adsorption on other substrates 60 [6,8]. We remark that the case of the C /Ag(001) interface is significantly different 60 from the C /Al(111) system [6 ] where scanning 60 tunneling spectroscopy revealed different electronic structures for, bright and dim molecules. If such a difference were present for the Ag(100) substrate one would expect a change in the ultraviolet photemission spectra moving from the co-existence of the bright and dim molecules and the homogeneous pattern seen at 150 K. We also remark that the temperature dependent evolution of the bright–dim contrast suggests the occurrence of an energy potential barrier between two distinct molecular configurations, corresponding to two different orientational arrangements of the cages.
5. Conclusion It has been found that C molecules are already 60 bound to Ag(001) at 150 K and that this bond is characterized by charge transfer from Ag atoms to C molecules in all the cases investigated while 60 changing the annealing temperature. A possible explanation for the two different types of C 60 molecules seen at RT and in 600 K annealed films
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C. Cepek et al. / Surface Science 454–456 (2000) 766–770
has been given in terms of different orientations with respect to the substrate.
Acknowledgements The authors are indebted to Kevin Prince for providing the experimental apparatus for the photoemission experiment. This project was partially financed within the PRA-CLASS of the INFM.
References [1] P. Rudolf, in: H. Kuzmany, J. Fink, H. Hehring, S. Roth (Eds.), Proc. 10th International Winterschool on Electronic Properties of Novel Materials, World Scientific, Singapore, 1996. [2] A. Goldoni, C. Cepek, E. Magnano, A.D. Laine, S. Vandre´, M. Sancrotti, Phys. Rev. B 58 (1998) 2228. [3] C. Cepek, M. Sancrotti, T. Greber, J. Osterwalder, unpublished results.
[4] E. Giudice, E. Magnano, S. Rusponi, C. Boragno, U. Valbusa, Surf. Sci. 405 (1998) L561. [5] G. Costantini, S. Rusponi, E. Giudice, C. Boragno, U. Valbusa, Carbon 37 (5) (1999) 727. [6 ] M.K.J. Johansson, A.J. Maxwell, S.M. Gray, P.A. Bru¨hwiler, D.C. Mancini, N. Ma˚rtensson, Phys. Rev. B 54 (1996) 13 472. [7] J.K. Gimzewski, S. Modesti, R.R. Schlittler, Phys. Rev. Lett. 72 (1994) 1036. [8] E.I. Altman, R.J. Colton, Phys. Rev. B 48 (1993) 18 244. [9] R. Conti, S. Rusponi, D. Pagnotta, C. Boragno, U. Valbusa, Vacuum 48 (1997) 639. [10] J.J. Yeh, I. Lindau, in: Atomic Data and Nuclear Data Tables 32 (1985) 1. [11] A. Goldoni, C. Cepek, S. Modesti, Synth. Met. 77 (1996) 189. [12] B.W. Hoogenboom, R. Esper, L.H. Tjeng, G.A. Sawatzky, Phys Rev. B 57 (1998) 11 939. [13] A. Auerbach, N. Manini, E. Tosatti, Phys. Rev. B 49 (1994) 12 998. [14] N. Manini, E. Tosatti, A. Auerbach, Phys. Rev. B 49 (1994) 13 008. [15] S. Saito, A. Oshiyama, Phys. Rev. Lett. 66 (1991) 2637. [16 ] D.L. Novikov, V.A. Gubanov, A.J. Freeman, Physica C 191 (1992) 399. [17] W. Andreoni, P. Giannozzi, M. Parinello, Phys. Rev. B 51 (1995) 2087.