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An STM and SXPS study of the interaction of C60 with the ten-fold surface of the Al72Ni11Co17 quasicrystal
Surface Science 566–568 (2004) 1200–1205 www.elsevier.com/locate/susc
An STM and SXPS study of the interaction of C60 with the ten-fold surface of the Al72Ni11Co17 quasicrystal E.J. Cox a, J. Ledieu a, V.R. Dhanak a, S.D. Barrett a, C.J. Jenks b, I. Fisher b, R. McGrath a,* a
Department of Physics, Surface Science Research Centre, The University of Liverpool, Liverpool L69 3BX, UK b Ames Laboratory, Iowa State University, Ames, IA 50011, USA Available online 17 June 2004
Abstract The adsorption of C60 on the ten-fold surface of the decagonal Al–Ni–Co quasicrystal at room temperature has been investigated using scanning tunnelling microscopy (STM) and soft X-ray photoemission spectroscopy (SXPS). STM indicates disordered adsorption of intact C60 molecules on the surface up to a coverage of 1 ML, followed by the formation of a C60 multilayer. No step decoration is observed indicating that the molecules are immobile at room temperature. SXPS data of the Al 2p core level indicate a strong interaction of the C60 molecules with the Al atoms of the substrate in the sub-monolayer regime. Valence band data indicate that the C60 multilayer desorbs after annealing to 700 K leaving only the strongly bonded layer on the surface. 2004 Elsevier B.V. All rights reserved. Keywords: Soft X-ray photoelectron spectroscopy; Scanning tunneling microscopy; Surface structure, morphology, roughness, and topography; Alloys; Fullerenes
1. Introduction Quasicrystals are a class of bi- and tri-metallic alloys which have long-range order but without translational symmetry [1]. They therefore exhibit rotational symmetries, which are disallowed in conventional periodic crystals. The study of quasicrystal surfaces is at a relatively early stage compared with single crystal metal surfaces where most clean surface structures are known and a large variety of ordered overlayer structures have *
Corresponding author. Tel.: +44-151-794-3873; fax: +44151-708-0662. E-mail address: [email protected] (R. McGrath).
been investigated. This is because large single grains of quasicrystals became available only in the mid 1990s [2]. Nevertheless previous STM studies indicate that the surface of the d-Al–Ni–Co quasicrystal can be prepared in a highly ordered form with extended flat terraces and that it is consistent with bulk structural models [3,4]. A natural progression is to see how the surface geometry is modified in the presence of adsorbates. From a technological viewpoint this is potentially important: for example it may be possible to improve frictional properties by forming a thin coating of a suitable molecule. There is additionally the possibility of forming a two-dimensional single-species quasicrystalline molecular overlayer.
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.089
E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205
A molecule adsorbing at a unique site on a quasicrystal surface could form such an overlayer by transference of the quasicrystallinity from the substrate ‘template’ to the adsorbate structure. A further point of interest is the comparison of the adsorption characteristics of C60 adsorbed on a quasicrystal surface with its behaviour on a single crystal metal surface. Maxwell et al. [5] have collated the available room temperature data for C60 monolayers adsorbed at room temperature on a variety of metal and semiconductor substrates. These materials are periodic, in contrast to Al–Ni– Co which is periodic along the axis perpendicular to the planes having ten-fold rotational symmetry, but quasicrystalline within these planes. The structure can therefore be thought of as a periodic stacking of quasicrystalline planes AB where A and B are rotated by 36 to each other and exhibit tenfold rotational symmetry. In this paper we describe the results of a study of the interaction of C60 with the ten-fold surface of the d-Al–Ni–Co quasicrystal. The experimental details are in Section 2; results of SXPS investigations of the clean surface are in Section 3.1. STM images of the C60 dosed surface are presented in Section 3.2 and SXPS results from this surface are shown in Section 3.3. The results are compared with other work on metal surfaces and on the fivefold surface of the icosahedral Al–Pd–Mn surface in Section 4.
2. Experimental details The surface was prepared by ex situ polishing with 6 , 1 and 1/4 lm diamond pastes on Kemet (PSU-M type) cloth. For each grade the polishing time was approximately 30 min. This was followed by cycles of in situ sputtering (1 kV, 15 lA at 4 · 105 mbar of Argon) and annealing (via e beam heating to 1085 K for 3–4 h.) All experiments were performed in the ultra-high-vacuum (UHV) regime with the sample at room temperature. The adsorption of C60 was monitored by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), STM and by SXPS using synchrotron radiation. The LEED, AES and STM measurements were performed at the University of
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Liverpool. The C60 doser was made from a glass tube wound with tungsten wire to act as a heating filament, the C60 being ejected at around 490 K. Photoemission experiments were carried out at beamline 4.1 at Daresbury Synchrotron Radiation Facility. Here the dosing was achieved by means of a tungsten ‘envelope’ with a small hole, again emission of the C60 occurs around 750 K. A polycrystalline aluminium foil was also used as a reference sample for comparison. The Al 2p, valence band and Fermi edge of both the clean quasicrystalline surface and the dosed surface were studied. Due to their low cross sections the Ni and Co core level spectra were not observed. The experimental resolution for the Al 2p data was estimated at 250 ± 50 meV. In the STM experiments, the C60 coverage was estimated based on a calibration of the dosing flux performed by measurements of the surface area coverage from images of the dosed surface. In the photoemission experiments, the C60 coverage was estimated from a pre-calibration of the doser. The estimated error on coverages quoted below is ±0.1 ML.
3. Results and analysis 3.1. Clean surface studies: SXPS After preparation as described above, the Al– Ni–Co sample exhibits a ten-fold LEED pattern with sharp spots and a low background. Fig. 1 shows the Al 2p spin–orbit split doublet for the clean quasicrystalline surface. After subtraction of an appropriate background the spectra were fitted with a doublet peak having a Voigt lineshape. A spectrum from a sputtered polycrystalline Al foil (not shown) was taken for comparison. The peakwidth for the foil was considerably narrower than for the Al–Ni–Co surface, which we attribute to the fact that there are many different possible local environments for the Al atoms in the quasicrystal, leading to multiple chemically shifted components which cause the observed broadening of the doublet. We also observed, in Fermi edge spectra (not shown), the well known ‘‘pseudogap’’, which is the low density of states at the Fermi edge characteristic of the quasicrystalline state [6].
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E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205
3.2. C60 adsorption: STM
Fig. 1. The Al 2p spectrum for the clean Al–Ni–Co surface. The spectrum is fitted with a doublet after background subtraction. Residuals are shown ·10. Beam energy is 130 eV.
· 1500 Fig. 2(a)–(c) show a sequence of 1500 A STM images taken after dosing with C60 . In Fig. A 2(c) an almost complete monolayer is imaged. There is no evidence for localised clustering, or migration to the step edges; rather the C60 adsorbs randomly on the surface. Successive STM images of the same area of the surface show that the C60 molecules are not mobile. No internal structure of the C60 molecules was observed. Fig. 2(d) shows a · 150 A STM image of the dosed surface at 150 A 0.9 ML coverage. The lack of adsorbate ordering is confirmed by the fast Fourier transform (FFT) of this image which shows no sharp peaks. The C60 deposition causes the LEED pattern to disappear
· 1500 A STM images of the surface of Al–Ni–Co with increasing C60 coverage––0.2, 0.8 and 0.9 ML. (d) 150 Fig. 2. (a)–(c) 1500 A · 150 A STM image of C60 on the Al–Ni–Co surface at a coverage of 0.9 ML. (Vbias ¼ 1 V, Itunnelling ¼ 0:34 nA). A
E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205
at low coverages. These results indicate that there is no preferential adsorption site for C60 on the surface. 3.3. C60 adsorption: SXPS The evolution of the Al 2p peak with increasing coverage of C60 was monitored. A spectrum corresponding to a coverage of 0.85 ML is shown in Fig. 3. Upon C60 dosing a change in the peakshape necessitates the addition of a second doublet in the fitting procedure to obtain a good fit. This doublet is shifted by 0.15 ± 0.05 eV towards lower binding energy. The relative size of this second doublet increases with C60 coverage. A peak shifted to lower binding energy has been observed previously in the C60 /Al(1 1 1) system. Al 2p SXPS data for 1 ML C60 /Al(1 1 1) show a broadening of the main peak in the spectrum and the formation of a new, shifted peak situated at 0.55 eV lower binding energy [7]. This chemical shift in the energy of the Al 2p electrons was interpreted in terms of a change in the chemical environment of a significant fraction of Al surface atoms, consistent with a picture of a covalent bonding interaction between C60 and aluminium [7]. Although the shift of the peak observed in the
Fig. 3. Quasicrystal Al 2p peak after dosing 0.85 ML C60 . Residuals are shown ·10. Beam energy is 130 eV.
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present study is lower in magnitude, such a picture of a strong interaction with the substrate is consistent with the STM data which show that the C60 molecules are essentially immobile on the surface. In Fig. 4, spectra for the valence band are shown for the clean surface and for increasing coverages of C60 . Fig. 4(b) corresponds to a coverage of 0.6 ML. Fig. 4(c) corresponds to a multilayer of C60 . The set of peaks from 0 to 13 eV binding energy are characteristic of a C60 multilayer [8], the one closest to the Fermi edge being the highest occupied molecular orbital (HOMO). There is a clear correspondence between the C60 peaks from 5 to 15 eV binding energy, indicating intact C60 adsorption at the lower coverage. The HOMO and HOMO-1 peaks are masked by the strong Ni and Co d emission between 0 and 5 eV binding energy. Spectra after a multilayer dose followed by an anneal of the surface to 700 K for 10 min (not shown) are comparable to that of Fig. 4(b), indicating desorption of all but the most tightly bound C60 molecules. This gives a picture of a two-stage adsorption process: initial strong bonding of the C60 to the substrate, followed by formation of a C60 multilayer.
Fig. 4. (a) SXPS spectrum of the clean valence band. The large increase of the total density of states close to the Fermi edge is due to the Ni and Co 3d bands. (b) Valence band after 0.6 ML dosing C60 . (c) Valence band for a C60 multilayer. (Incident beam energy 130 eV).
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E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205
4. Discussion There has been one previous study using STM of C60 adsorbed on a quasicrystal surface, which was the five-fold surface of the icosahedral i-AlPdMn quasicrystal [9]. In that study preferential adsorption at sites of high symmetry was observed for very low coverages leading to s-scaling relationships in the intermolecular distances [9]. However, these scaling relationships are not carried over to long-range correlations, with the result that long-range ordered 2-D structures are not found in this system. A disordered structure was observed, similar to the current findings. One of the main conclusions of the present study is the strong interaction of the first C60 layer with the substrate and the weak bonding of the subsequent C60 layers evidenced by their removal during thermal annealing. This behaviour is common for fullerene deposition on a widerange of metallic [5] and semiconducting [10] substrates. The observation of disorder in the C60 overlayer, even after annealing of the overlayer, is perhaps the most surprising conclusion of this study. For inert substrates, such as graphite, overlayer formation is dominated by the adsorbate–adsorbate interaction and hexagonal overlayers are formed [11]. On noble metals C60 is mobile at room temperature; step edge decoration is observed and hexagonal overlayers may be formed by annealing to around 700 K [12,13]. On Al surfaces, a strong covalent interaction is found, but again the C60 molecules are mobile at room temperature and form ordered overlayers after annealing [5]. On silicon surfaces covalent bonding is found, the adsorbate–substrate interaction dominates and the molecules are immobile at room temperature. For sub-monolayer coverages the adsorption is disordered [14], but at monolayer coverages local ordering is found [10]. The behaviour of C60 on the Al–Ni–Co surface has the closest match to this latter case, although no ordering was found even for the monolayer case. This close comparison to the case of silicon may be related to the relatively low density of states observed at the Fermi edge (the pseudogap) which will have a considerable effect on the nature of the
chemical bond between the substrate and the adsorbate.
5. Conclusions A consistent picture for this adsorption system emerges from the combination of the STM and SXPS data. Initial dosing leads to C60 molecules strongly bonded to the surface Al atoms up to monolayer coverage. The molecules are randomly distributed and immobile on the surface. This initial adsorption phase is followed by the formation of a less tightly bound C60 multilayer. Although C60 was chosen as a candidate for possible ordered adsorption, this was not observed.
Acknowledgements The EPSRC (Grant numbers GR/N18680, GR/ N25718 and GR/S19080/1) are acknowledged for funding. George Miller is thanked for technical assistance and Michael Hunt for the use of the C60 doser in the photoemission experiments.
References [1] D. Shechtman, I. Blech, D. Gratias, J. Cahn, Phys. Rev. Lett. 53 (1984) 1951. [2] R. McGrath, J. Ledieu, E. Cox, R. Diehl, J. Phys.: Condens. Matter 14 (2002) R119. [3] E. Cox, J. Ledieu, R. McGrath, R. Diehl, C. Jenks, I. Fisher, Mater. Res. Soc. Symp. Proc. (2001) K11.1. [4] M. Kishida, Y. Kamimura, R. Tamura, K. Edagawa, S. Takeuchi, T. Sato, Y. Yokoyama, J. Guo, A. Tsai, Phys. Rev. B 65 (9) (2002), art. no.–094208. [5] A. Maxwell, P. Br€ uhwiler, D. Arvanitis, J. Hasselstr€ om, M.-J. Johansson, N. Martensson, Phys. Rev. B 57 (1998) R16330. [6] D. Naumovic, P. Aebi, L. Schlapbach, C. Beeli, T. Lograsso, D. Delaney, Phys. Rev. B 60 (1999) R16330. [7] A.J. Maxwell, P.A. Br€ uhwiler, S. Andersson, Phys. Rev. B 52 (1995) R5546. [8] S. Hasegawa, T. Miyamae, K. Yakushi, H. Inokuchi, K. Seki, N. Umo, Phys. Rev. B 58 (1998) 4927. [9] J. Ledieu, C. Muryn, G. Thornton, R. Diehl, T. Lograsso, D. Delaney, R. McGrath, Surf. Sci. 472 (1–2) (2001) 89. [10] D. Chen, D. Sarid, Surf. Sci. 318 (1994) 74. [11] M. Hunt, R. Palmer, Surf. Rev. Lett. 3 (1996) 937.
E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205 [12] E. Altman, R. Coltan, Surf. Sci. 295 (1993) 13. [13] S. Modesti, R. Schlittler, J. Gimzewski, Surf. Sci. 331–333 (1995) 1129.
1205
[14] T. Hashizume, X. Wang, Y. Nishina, H. Shinohara, Y. Saito, Y. Kuk, T. Sakurai, Appl. Phys. Lett. 31 (1992) L880.
An STM and SXPS study of the interaction of C60 with the ten-fold surface of the Al72Ni11Co17 quasicrystal E.J. Cox a, J. Ledieu a, V.R. Dhanak a, S.D. Barrett a, C.J. Jenks b, I. Fisher b, R. McGrath a,* a
Department of Physics, Surface Science Research Centre, The University of Liverpool, Liverpool L69 3BX, UK b Ames Laboratory, Iowa State University, Ames, IA 50011, USA Available online 17 June 2004
Abstract The adsorption of C60 on the ten-fold surface of the decagonal Al–Ni–Co quasicrystal at room temperature has been investigated using scanning tunnelling microscopy (STM) and soft X-ray photoemission spectroscopy (SXPS). STM indicates disordered adsorption of intact C60 molecules on the surface up to a coverage of 1 ML, followed by the formation of a C60 multilayer. No step decoration is observed indicating that the molecules are immobile at room temperature. SXPS data of the Al 2p core level indicate a strong interaction of the C60 molecules with the Al atoms of the substrate in the sub-monolayer regime. Valence band data indicate that the C60 multilayer desorbs after annealing to 700 K leaving only the strongly bonded layer on the surface. 2004 Elsevier B.V. All rights reserved. Keywords: Soft X-ray photoelectron spectroscopy; Scanning tunneling microscopy; Surface structure, morphology, roughness, and topography; Alloys; Fullerenes
1. Introduction Quasicrystals are a class of bi- and tri-metallic alloys which have long-range order but without translational symmetry [1]. They therefore exhibit rotational symmetries, which are disallowed in conventional periodic crystals. The study of quasicrystal surfaces is at a relatively early stage compared with single crystal metal surfaces where most clean surface structures are known and a large variety of ordered overlayer structures have *
Corresponding author. Tel.: +44-151-794-3873; fax: +44151-708-0662. E-mail address: [email protected] (R. McGrath).
been investigated. This is because large single grains of quasicrystals became available only in the mid 1990s [2]. Nevertheless previous STM studies indicate that the surface of the d-Al–Ni–Co quasicrystal can be prepared in a highly ordered form with extended flat terraces and that it is consistent with bulk structural models [3,4]. A natural progression is to see how the surface geometry is modified in the presence of adsorbates. From a technological viewpoint this is potentially important: for example it may be possible to improve frictional properties by forming a thin coating of a suitable molecule. There is additionally the possibility of forming a two-dimensional single-species quasicrystalline molecular overlayer.
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.089
E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205
A molecule adsorbing at a unique site on a quasicrystal surface could form such an overlayer by transference of the quasicrystallinity from the substrate ‘template’ to the adsorbate structure. A further point of interest is the comparison of the adsorption characteristics of C60 adsorbed on a quasicrystal surface with its behaviour on a single crystal metal surface. Maxwell et al. [5] have collated the available room temperature data for C60 monolayers adsorbed at room temperature on a variety of metal and semiconductor substrates. These materials are periodic, in contrast to Al–Ni– Co which is periodic along the axis perpendicular to the planes having ten-fold rotational symmetry, but quasicrystalline within these planes. The structure can therefore be thought of as a periodic stacking of quasicrystalline planes AB where A and B are rotated by 36 to each other and exhibit tenfold rotational symmetry. In this paper we describe the results of a study of the interaction of C60 with the ten-fold surface of the d-Al–Ni–Co quasicrystal. The experimental details are in Section 2; results of SXPS investigations of the clean surface are in Section 3.1. STM images of the C60 dosed surface are presented in Section 3.2 and SXPS results from this surface are shown in Section 3.3. The results are compared with other work on metal surfaces and on the fivefold surface of the icosahedral Al–Pd–Mn surface in Section 4.
2. Experimental details The surface was prepared by ex situ polishing with 6 , 1 and 1/4 lm diamond pastes on Kemet (PSU-M type) cloth. For each grade the polishing time was approximately 30 min. This was followed by cycles of in situ sputtering (1 kV, 15 lA at 4 · 105 mbar of Argon) and annealing (via e beam heating to 1085 K for 3–4 h.) All experiments were performed in the ultra-high-vacuum (UHV) regime with the sample at room temperature. The adsorption of C60 was monitored by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), STM and by SXPS using synchrotron radiation. The LEED, AES and STM measurements were performed at the University of
1201
Liverpool. The C60 doser was made from a glass tube wound with tungsten wire to act as a heating filament, the C60 being ejected at around 490 K. Photoemission experiments were carried out at beamline 4.1 at Daresbury Synchrotron Radiation Facility. Here the dosing was achieved by means of a tungsten ‘envelope’ with a small hole, again emission of the C60 occurs around 750 K. A polycrystalline aluminium foil was also used as a reference sample for comparison. The Al 2p, valence band and Fermi edge of both the clean quasicrystalline surface and the dosed surface were studied. Due to their low cross sections the Ni and Co core level spectra were not observed. The experimental resolution for the Al 2p data was estimated at 250 ± 50 meV. In the STM experiments, the C60 coverage was estimated based on a calibration of the dosing flux performed by measurements of the surface area coverage from images of the dosed surface. In the photoemission experiments, the C60 coverage was estimated from a pre-calibration of the doser. The estimated error on coverages quoted below is ±0.1 ML.
3. Results and analysis 3.1. Clean surface studies: SXPS After preparation as described above, the Al– Ni–Co sample exhibits a ten-fold LEED pattern with sharp spots and a low background. Fig. 1 shows the Al 2p spin–orbit split doublet for the clean quasicrystalline surface. After subtraction of an appropriate background the spectra were fitted with a doublet peak having a Voigt lineshape. A spectrum from a sputtered polycrystalline Al foil (not shown) was taken for comparison. The peakwidth for the foil was considerably narrower than for the Al–Ni–Co surface, which we attribute to the fact that there are many different possible local environments for the Al atoms in the quasicrystal, leading to multiple chemically shifted components which cause the observed broadening of the doublet. We also observed, in Fermi edge spectra (not shown), the well known ‘‘pseudogap’’, which is the low density of states at the Fermi edge characteristic of the quasicrystalline state [6].
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E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205
3.2. C60 adsorption: STM
Fig. 1. The Al 2p spectrum for the clean Al–Ni–Co surface. The spectrum is fitted with a doublet after background subtraction. Residuals are shown ·10. Beam energy is 130 eV.
· 1500 Fig. 2(a)–(c) show a sequence of 1500 A STM images taken after dosing with C60 . In Fig. A 2(c) an almost complete monolayer is imaged. There is no evidence for localised clustering, or migration to the step edges; rather the C60 adsorbs randomly on the surface. Successive STM images of the same area of the surface show that the C60 molecules are not mobile. No internal structure of the C60 molecules was observed. Fig. 2(d) shows a · 150 A STM image of the dosed surface at 150 A 0.9 ML coverage. The lack of adsorbate ordering is confirmed by the fast Fourier transform (FFT) of this image which shows no sharp peaks. The C60 deposition causes the LEED pattern to disappear
· 1500 A STM images of the surface of Al–Ni–Co with increasing C60 coverage––0.2, 0.8 and 0.9 ML. (d) 150 Fig. 2. (a)–(c) 1500 A · 150 A STM image of C60 on the Al–Ni–Co surface at a coverage of 0.9 ML. (Vbias ¼ 1 V, Itunnelling ¼ 0:34 nA). A
E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205
at low coverages. These results indicate that there is no preferential adsorption site for C60 on the surface. 3.3. C60 adsorption: SXPS The evolution of the Al 2p peak with increasing coverage of C60 was monitored. A spectrum corresponding to a coverage of 0.85 ML is shown in Fig. 3. Upon C60 dosing a change in the peakshape necessitates the addition of a second doublet in the fitting procedure to obtain a good fit. This doublet is shifted by 0.15 ± 0.05 eV towards lower binding energy. The relative size of this second doublet increases with C60 coverage. A peak shifted to lower binding energy has been observed previously in the C60 /Al(1 1 1) system. Al 2p SXPS data for 1 ML C60 /Al(1 1 1) show a broadening of the main peak in the spectrum and the formation of a new, shifted peak situated at 0.55 eV lower binding energy [7]. This chemical shift in the energy of the Al 2p electrons was interpreted in terms of a change in the chemical environment of a significant fraction of Al surface atoms, consistent with a picture of a covalent bonding interaction between C60 and aluminium [7]. Although the shift of the peak observed in the
Fig. 3. Quasicrystal Al 2p peak after dosing 0.85 ML C60 . Residuals are shown ·10. Beam energy is 130 eV.
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present study is lower in magnitude, such a picture of a strong interaction with the substrate is consistent with the STM data which show that the C60 molecules are essentially immobile on the surface. In Fig. 4, spectra for the valence band are shown for the clean surface and for increasing coverages of C60 . Fig. 4(b) corresponds to a coverage of 0.6 ML. Fig. 4(c) corresponds to a multilayer of C60 . The set of peaks from 0 to 13 eV binding energy are characteristic of a C60 multilayer [8], the one closest to the Fermi edge being the highest occupied molecular orbital (HOMO). There is a clear correspondence between the C60 peaks from 5 to 15 eV binding energy, indicating intact C60 adsorption at the lower coverage. The HOMO and HOMO-1 peaks are masked by the strong Ni and Co d emission between 0 and 5 eV binding energy. Spectra after a multilayer dose followed by an anneal of the surface to 700 K for 10 min (not shown) are comparable to that of Fig. 4(b), indicating desorption of all but the most tightly bound C60 molecules. This gives a picture of a two-stage adsorption process: initial strong bonding of the C60 to the substrate, followed by formation of a C60 multilayer.
Fig. 4. (a) SXPS spectrum of the clean valence band. The large increase of the total density of states close to the Fermi edge is due to the Ni and Co 3d bands. (b) Valence band after 0.6 ML dosing C60 . (c) Valence band for a C60 multilayer. (Incident beam energy 130 eV).
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E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205
4. Discussion There has been one previous study using STM of C60 adsorbed on a quasicrystal surface, which was the five-fold surface of the icosahedral i-AlPdMn quasicrystal [9]. In that study preferential adsorption at sites of high symmetry was observed for very low coverages leading to s-scaling relationships in the intermolecular distances [9]. However, these scaling relationships are not carried over to long-range correlations, with the result that long-range ordered 2-D structures are not found in this system. A disordered structure was observed, similar to the current findings. One of the main conclusions of the present study is the strong interaction of the first C60 layer with the substrate and the weak bonding of the subsequent C60 layers evidenced by their removal during thermal annealing. This behaviour is common for fullerene deposition on a widerange of metallic [5] and semiconducting [10] substrates. The observation of disorder in the C60 overlayer, even after annealing of the overlayer, is perhaps the most surprising conclusion of this study. For inert substrates, such as graphite, overlayer formation is dominated by the adsorbate–adsorbate interaction and hexagonal overlayers are formed [11]. On noble metals C60 is mobile at room temperature; step edge decoration is observed and hexagonal overlayers may be formed by annealing to around 700 K [12,13]. On Al surfaces, a strong covalent interaction is found, but again the C60 molecules are mobile at room temperature and form ordered overlayers after annealing [5]. On silicon surfaces covalent bonding is found, the adsorbate–substrate interaction dominates and the molecules are immobile at room temperature. For sub-monolayer coverages the adsorption is disordered [14], but at monolayer coverages local ordering is found [10]. The behaviour of C60 on the Al–Ni–Co surface has the closest match to this latter case, although no ordering was found even for the monolayer case. This close comparison to the case of silicon may be related to the relatively low density of states observed at the Fermi edge (the pseudogap) which will have a considerable effect on the nature of the
chemical bond between the substrate and the adsorbate.
5. Conclusions A consistent picture for this adsorption system emerges from the combination of the STM and SXPS data. Initial dosing leads to C60 molecules strongly bonded to the surface Al atoms up to monolayer coverage. The molecules are randomly distributed and immobile on the surface. This initial adsorption phase is followed by the formation of a less tightly bound C60 multilayer. Although C60 was chosen as a candidate for possible ordered adsorption, this was not observed.
Acknowledgements The EPSRC (Grant numbers GR/N18680, GR/ N25718 and GR/S19080/1) are acknowledged for funding. George Miller is thanked for technical assistance and Michael Hunt for the use of the C60 doser in the photoemission experiments.
References [1] D. Shechtman, I. Blech, D. Gratias, J. Cahn, Phys. Rev. Lett. 53 (1984) 1951. [2] R. McGrath, J. Ledieu, E. Cox, R. Diehl, J. Phys.: Condens. Matter 14 (2002) R119. [3] E. Cox, J. Ledieu, R. McGrath, R. Diehl, C. Jenks, I. Fisher, Mater. Res. Soc. Symp. Proc. (2001) K11.1. [4] M. Kishida, Y. Kamimura, R. Tamura, K. Edagawa, S. Takeuchi, T. Sato, Y. Yokoyama, J. Guo, A. Tsai, Phys. Rev. B 65 (9) (2002), art. no.–094208. [5] A. Maxwell, P. Br€ uhwiler, D. Arvanitis, J. Hasselstr€ om, M.-J. Johansson, N. Martensson, Phys. Rev. B 57 (1998) R16330. [6] D. Naumovic, P. Aebi, L. Schlapbach, C. Beeli, T. Lograsso, D. Delaney, Phys. Rev. B 60 (1999) R16330. [7] A.J. Maxwell, P.A. Br€ uhwiler, S. Andersson, Phys. Rev. B 52 (1995) R5546. [8] S. Hasegawa, T. Miyamae, K. Yakushi, H. Inokuchi, K. Seki, N. Umo, Phys. Rev. B 58 (1998) 4927. [9] J. Ledieu, C. Muryn, G. Thornton, R. Diehl, T. Lograsso, D. Delaney, R. McGrath, Surf. Sci. 472 (1–2) (2001) 89. [10] D. Chen, D. Sarid, Surf. Sci. 318 (1994) 74. [11] M. Hunt, R. Palmer, Surf. Rev. Lett. 3 (1996) 937.
E.J. Cox et al. / Surface Science 566–568 (2004) 1200–1205 [12] E. Altman, R. Coltan, Surf. Sci. 295 (1993) 13. [13] S. Modesti, R. Schlittler, J. Gimzewski, Surf. Sci. 331–333 (1995) 1129.
1205
[14] T. Hashizume, X. Wang, Y. Nishina, H. Shinohara, Y. Saito, Y. Kuk, T. Sakurai, Appl. Phys. Lett. 31 (1992) L880.