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Metallic phases of a C70 single layer adsorbed on Cu(1 1 1) doped with sodium
Surface Science 532–535 (2003) 892–897 www.elsevier.com/locate/susc
Metallic phases of a C70 single layer adsorbed on Cu(1 1 1) doped with sodium T. Pardini a,b, C. Cepek b, R. Larciprete c,d, L. Sangaletti a, S. Pagliara a, R. Gotter b, L. Floreano b, A. Verdini b, A. Morgante b,e, F. Parmigiani a, A. Goldoni c,* a b
INFM and Universit a Cattolica del Sacro Cuore, Via dei Musei 41, 25121 Brescia, Italy Laboratorio TASC-INFM, s.s. 14 Km 163.5 in Area Science park, 34012 Trieste, Italy c Sincrotrone Trieste, s.s. 14 Km 163.5 in Area Science park, 34012 Trieste, Italy d CNR-IMIP, zona industriale 85050, Tito Scalo (Pz), Italy e Dipartimento di Fisica, Universit a di Trieste, Via Valerio 2, 34127 Trieste, Italy
Abstract The electronic properties of a C70 single layer chemisorbed on Cu(1 1 1) surface, in which the charge state has been modified by Na doping, were studied via synchrotron radiation photoemission and absorption spectroscopy. A twodimensional metallic phase is observed in the whole range of charge states investigated (from 1 up to 3 electrons/C70 ). Moreover, all the results suggest that the orientation of the molecules, with the C5v axis perpendicular to the surface, is not affected by Na doping. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Alkali metals; Fullerenes; Photoemission (total yield); Photon absorption spectroscopy
1. Introduction After C60 , C70 is the next abundant stable fullerene [1]. The D5h group symmetry to which it belongs, gives rise to five inequivalent carbon atoms and eight different carbon–carbon bonds per molecule [1,2]. Many orbital degeneracies, existing in the C60 molecule, are removed by the D5h symmetry in the C70 molecule. For instance, the LUMO which is a three-fold degenerate orbital [3] in the C60 , splits into a lower energy two-fold de-
*
Corresponding author. Fax: +39-040-3758565. E-mail address: [email protected] (A. Goldoni).
generate orbital (e001 ) and into a higher energy onefold degenerate orbital (a001 ) in C70 [4]. Metallic phases (which become superconductive below 40 K) were observed in bulk Ax B3x C60 systems (A ¼ Na, Cs, K, Rb; B ¼ K, Rb, Cs, x ¼ 1,2) [5–10], that is, in compounds where a charge transfer of three electrons from alkali metals to the LUMO of the C60 molecules takes place. While metallic phases are missed in Ax C70 crystals (A ¼ K, Rb, Cs) probably as a consequence of disorder and structural defects in these compounds, very recently metallic phases have been claimed for Na2 CsC70 , where the C70 molecules have the long axis aligned to the cubic {1 1 1} direction [11]. Also, a metallic phase has been
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00179-1
T. Pardini et al. / Surface Science 532–535 (2003) 892–897
obtained for a C70 single layer absorbed on Cu(1 1 1) surface [12]. Motivated by these discoveries, we studied with synchrotron radiation X-ray photoelectron (XPS) and absorption (NEXAFS) spectroscopy the effects of sodium atoms doping on a C70 single layer absorbed on Cu(1 1 1) surface, in order to look for new metallic phases (necessary condition to have superconductive phases) in lowdimensional C70 -derived systems. Sodium atoms were chosen for two different reasons: first, their small dimensions should not appreciably affect the structural order of the system; second, the sodiumcontaining C70 bulk compounds are the only that apparently show metallic properties.
2. Experimental All the measurements were performed at the ALOISA beamline at ELETTRA. C 1s core level and valence band photoemission spectra were measured with photons of 430 eV and 145 eV respectively with an energy resolution of 150 meV. Absorption spectra were obtained by measuring the current on the sample (total yield) due to the absorption of the incident photons. The standard pressure was lower than 5 1010 mbar. All the samples were prepared in situ. The Cu(1 1 1) surface was cleaned by sputtering and annealing (800 K). C70 and sodium atoms were evaporated from a degassed Ta crucible and from commercial SAES getters sources, respectively. The ordered (4 4) single layer (1 ML-C70 /Cu(1 1 1)), having the molecules oriented with the long axis perpendicular to the surface [12], was obtained by sublimating a C70 thick film at 700 K. The binding energies of the photoemission spectra are referred to the position of the Fermi level of the clean Cu substrate.
3. Results and discussions Fig. 1(a) shows the reference valence band photoemission spectra of a thick undoped C70 film and of 1 ML-C70 /Cu(1 1 1) (inset). The spectra were collected with the analyzer normal to the sample surface, whereas the glancing incidence synchrotron beam (7° from the surface) had linear
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polarization almost perpendicular to the surface plane. In the photoemission spectrum of 1 MLC70 /Cu(1 1 1) the contribution from the Cu substrate was subtracted as discussed by Goldoni et al. [12]. Compared to the spectrum of the C70 thick film, the valence band photoemission spectrum of the undoped 1 ML-C70 /Cu(1 1 1) shows a clear Fermi edge (i.e. metallic character) due to the charge transfer of 1 e /C70 from the Cu substrate to the LUMO of the C70 molecules [12]. In Fig. 1(b) we show the evolution of the LUMO emission of 1 ML-C70 /Cu(1 1 1) as a function of the Na evaporation time. The t ¼ 0 min spectrum corresponds to the undoped single layer. By evaporating sodium atoms onto the sample, the system is affected by a further charge transfer from the alkali atoms to the LUMO of the C70 molecules, which is more evident for t > 7 min when a broad feature (indicated by the shaded area) appears in the spectra at about 0.5 eV. We will later discuss the origin of this feature. From Fig. 1(b) it is clear that the system maintains its metallic behavior for all the phases obtained, but the density of states at the Fermi level changes as function of doping. Up to t ¼ 5 min, the density of states at the Fermi level appears quite constant, but it decreases to a minimum at t ¼ 7 min and then increases again for t ! 17 min. It is known that the LUMO of fullerenes has peculiar behaviors as a function of alkali-atoms doping. Photoemission spectra in the LUMO region performed on Ax C70 thick films (A ¼ K [14], Rb [15], Na [16]), show a single feature at 1.5 eV below the Fermi level for ne < 2 e /C70 (ne ¼ number of electrons transferred per molecule). At ne 2 e /C70 a second feature appears at 0.5 eV, which grows by increasing ne until, at ne 6 e /C70 its intensity is about two times larger than the intensity of the previous one. On the basis of the calculations proposed by Saito et al. [17] for the C70 molecule (see inset of Fig. 1(b)) the electrons transferred to the LUMO of the C70 molecule are expected to occupy first the lower energy two-fold degenerate orbital (e001 ), which can accept up to 4 electrons, and after the one-fold degenerate orbital (a001 ). On the other hand, the calculations performed by Tanaka et al. [18] on the electronic structure of the C 70 molecule shows that
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T. Pardini et al. / Surface Science 532–535 (2003) 892–897
a 1'' e 1'' e 1'' a ''2
1 ML-C70 /Cu(111)
LUMO
Intensity (arb. units)
2
0
1
LUMO HOMO
C 70
e 1'' a''1 a 2'' '' C70 e 1
1 ML-C70 /Cu(111)+Na t = 17' 13'
C70 multilayer
10' 7'
HOMO
5' 4' 3' 2' 1' 0'
HOMO-1
6
LUMO
(b)
(a)
4
2
0
Binding Energy (eV)
2
1
0
Binding Energy (eV)
Fig. 1. (a) Valence band photoemission spectra of a C70 multilayer and (inset) of 1 ML-C70 /Cu(1 1 1). (b) Dependence of the LUMO emission of 1 ML-C70 /Cu(1 1 1) as a function of the exposure to Na atoms. The shaded area for t P 7 min roughly indicates the completely filled a001 orbital at 0.5 eV. (Inset) Electronic diagram of the HOMO–LUMO levels of the C70 molecule as proposed by Saito et al. [13] and of the C 70 molecule as proposed by Tanaka et al. [14].
the extra-electron tends to occupy the one-fold degenerate orbital a001 which lowers its energy (see inset of Fig. 1(b)). This observation combined with the relationship between ne and the intensity of the two features observed in the LUMO region of valence band photoemission spectra of Ax C70 thick films [14–16], suggests to identify the two LUMO features observed in photoemission with the a001 and e001 orbitals respectively. By analogy, we suppose that in 1 ML-C70 / Cu(1 1 1) (t ¼ 0 min) the electron transferred from the Cu to the LUMO of the C70 molecules occupies the one-fold degenerate a001 orbital. When sodium atoms are evaporated on the sample (t > 0 min), the first electron transferred from sodium atoms totally fills the a001 orbital, while the others begin to fill the two-fold degenerate e001 orbital. Therefore, the sudden drop in the density of states at the Fermi level, observed in our spectra of 1 ML-C70 /Cu(1 1 1) doped with sodium atoms at t 7 min, could be associated with the complete filling of the a001 orbital. Moreover, for t > 5 min, a
feature appears clearly at 0.5 eV (roughly indicated by the shaded area in Fig. 1(b)), shifting toward higher binding energies with the Na doping. This feature can be associated to the a001 orbital completely filled. This explanation of the LUMO behavior implies that at t 7 min, 2 electrons are transferred to the LUMO of the C70 molecules. More arguments, further supporting this assumption, will be given in the following by discussing the behavior with Na doping of the C 1s core level spectra (XPS and NEXAFS). Fig. 2 show C 1s photoemission spectra (dots) relative to 1 ML-C70 /Cu(1 1 1) at different doping steps [(a) t ¼ 0 min, (b) t ¼ 7 min, (c) t ¼ 17 min]. Particularly evident are the asymmetrical line shape of the peaks, typical of metallic systems, and the higher energy shoulders mainly due to the plasma oscillation of the electrons in the LUMO. The number of electrons (N ) taking part to the plasma oscillation and the plasma energy (Ep ) are related by the well known formula: Ep =h ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p ð4pNe2 Þ=ðm ðe þ 1ÞV Þ, where V is the volume of
Intensity (arb. units)
T. Pardini et al. / Surface Science 532–535 (2003) 892–897
(a)
C1s ML-C 70 /Cu(111) (t = 0 min.)
fit C-C C-Cu
286
285
284
283
Intensity (arb. units)
Binding Energy (eV) (b)
C1s ML-C 70 /Cu(111) + Na (t = 7 min.)
fit C-C C-Cu
286
285
284
283
Intensity (arb. units)
Binding Energy (eV) C1s ML-C 70 /Cu(111) + Na (t = 17 min.)
(c)
fit C-C C-Cu
286
285
284
283
Binding Energy (eV) Fig. 2. C 1s photoemission spectra of 1 ML-C70 /Cu(1 1 1) (dots) at different sodium doping steps and relative simulation (solid lines). The components due to two kinds of carbon atoms are also shown (dashed lines, see the text for detail).
the unit cell, m is the effective mass of the electrons and e is the dielectric constant due to all the other excitations (the term e þ 1 is due to the presence of vacuum at the interface). Therefore, from the plasma energy, is possible to obtain the number of electrons which occupy the LUMO of the molecules. In order to estimate the plasma energy Ep at t ¼ 0 min, t ¼ 7 min, and t ¼ 17 min of sodium
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doping, we made a deconvolution of the C 1s core level, using Doniach–Sunjic functions plus Gaussian functions for the plasma excitation. Using a single Doniach–Sunjic component with the corresponding plasma loss it was not possible to reproduce the experimental data satisfactory. As a consequence we have assumed that for the C70 single layer at least two strongly inequivalent kinds of carbon atoms per molecule should exist: carbon atoms interfacing the substrate, and carbon atoms which cannot interact with the substrate. So each spectrum was simulated by introducing two Doniach–Sunjic peaks with the corresponding plasma losses (see the caption of Table 1). The results (line) are shown in Fig. 2. Table 1 reports the values of the parameters used in the simulations. For the undoped 1 ML-C70 /Cu(1 1 1) (t ¼ 0), we estimated Ep ¼ 0:36 eV. For 1 ML-C70 /Cu(1 1 1) doped with sodium atoms at t ¼ 7 min and t ¼ 17 min we estimated Ep ¼ 0:55 eV and Ep ¼ 0:63 eV respectively. Assuming Nðt¼0 minÞ 1 e /C70 [12] and by supposing m , e and V constant with doping, we estimated Nð7 minÞ 2:3 0:2, and Nðt¼17 minÞ 3 0:2 e /C70 . This result supports the above hypothesis that for t ¼ 7 min about 2 electrons per molecules are transferred from the substrates to the LUMO of the C70 molecules. As already shown in Ref. [12], in 1 ML-C70 / Cu(1 1 1) the C70 molecules are mainly oriented with the C5v axis orthogonal to the surface. From the simulation of the XPS C 1s peak at t ¼ 0 min we note that the intensity of the smaller peak is 7% of the total intensity, which means that 5 carbon atoms per molecules contribute to it. Identifying this feature with the carbon atoms that interact with the substrate we are in agreement with the expected orientation of the C70 molecules [12], since the five carbon atoms belonging to the pentagonal basis of the C70 molecules are the only directly interfacing the substrate. It is also worth noting that the ratio between the intensity of the two structures in each spectrum does not change with the sodium doping. This suggests that the orientation of the molecules does not change during the doping. Fig. 3(a) and (b) show the NEXAFS spectra performed on C 1s absorption edge of 1 ML-C70 /
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T. Pardini et al. / Surface Science 532–535 (2003) 892–897
Table 1 Parameters used in the simulation of the C 1s spectra of 1 ML-C70 /Cu(1 1 1) shown in Fig. 3 0 min 7 min 17 min
E1 (eV)
E2 (eV)
WG (eV)
a
Ep (eV)
284.35 284.55 284.65
285.15 285.25 285.8
0.45 0.39 0.52
0.19 0.22 0.17
0.36 0.55 0.63
Ip (%)
Wp (eV)
0.2 0.1 0.12
0.62 1.39 1.0
E1 , E2 are the energies of the two a Doniach–Sunjic curves, WG is the FWHM of the Gaussian curves convoluted to the Doniach– Sunjic. a is the asymmetry coefficient. Ip , Ep and Wp are the intensity, the energy and the FWHM of the plasmon. The Lorentzian width of the Doniach–Sunijc is kept constant to 0.13 eV. Note the reduction of Wp from t 7 min to t ¼ 17 min This can be explained with an increased conductivity of the sample when there are 3 el/molecule (partial filling of the two-fold degenerate e001 band) with respect to the case of 2 el/molecule (complete filling of the a001 band), in agreement with the reduced DOS at EF (metallicity) measured with photoemission at t 7 min. Accordingly, we expect a decrease of the resistivity up to 4 el/molecule, i.e. up to the half-filling of the e001 band.
Intensity (arb. units)
NEXAFS 1 ML-C 70 /Cu(111)+Na Pol. _|_ surface plane
17' 13' 10' 7' 4' 3' 2' 1' t = 0'
282
LUMO LUMO+2
284
286
(a)
288
290
292
Photon Energy (eV) NEXAFS 1 ML-C 70 /Cu(111)+Na Pol. // surface plane
Intensity (arb. units)
Cu(1 1 1) as a function of sodium doping. The spectra of Fig. 3(a) and (b) were obtained with light polarized perpendicular and parallel to the sample surface respectively, and are aligned to the vacuum level. For all the phases obtained, apart the shift of the features and the gradual reduction in intensity of the first structure with doping, we note that the spectral lineshape modifications with the light polarization direction always mirror those of the undoped monolayer. According to the calculations reported in Ref. [19] this confirms that the orientation of the molecules is not doping dependent, as already argued by analyzing the XPS spectra. Moreover our NEXAFS spectra look very similar to the C 1s absorption edge of Rbx C70 measured with transmission EELS by Sohmen et al. [20]. In particular the NEXAFS spectrum obtained after t ¼ 17 min of sodium doping (ne 3 e /C70 according to the estimation based on the C 1s fit) appears very similar to the EELS spectrum obtained on Rb3 C70 . This further support the correctness of our approximate charge transfer estimation based on the C 1s core level and valence band photoemission. As concerns the lineshape of the NEXAFS spectra, the first peak corresponds to the C 1s ! LUMO transition (first p transition) for the C1, C2, C3 and C4 atoms of the C70 molecule, the second peak (mostly visible with the light polarization parallel to the surface) is the superposition of C 1s ! LUMO for the C5 atoms and of the C 1s ! LUMO + 1 (second p transition) for the remaining atoms, while the third peak is mainly due to C 1s ! LUMO + 2 (third p transition).
17' 13' 10' 7' 4' 3' 2' 1' t = 0'
282
LUMO+1
284
286
(b)
288
290
292
Photon Energy (eV) Fig. 3. NEXAFS spectra of 1 ML-C70 /Cu(1 1 1) as a function of sodium doping with light polarized orthogonal to the sample surface (a) and with light polarized parallel to the sample surface (b). The spectra are aligned to the vacuum level.
According to calculations [12], the strong dependence from the light polarization of the spectral
T. Pardini et al. / Surface Science 532–535 (2003) 892–897
intensity in the region of the second peak shows that the contribution from the C 1s ! LUMO transition of the C5 atoms is dominant. The almost linear shift of the first two p transitions toward higher energies as a function of doping (0.3 eV from the undoped monolayer to the highest doping level reached here)––see Fig. 3(a) and (b)––suggests that the Fermi level of the system is moving rigidly inside the LUMO as these bands were filled with electrons. Nevertheless, the opposite shift of the transition to the LUMO + 2 band indicates that energy gaps between the LUMO, the LUMO + 1 and the LUMO + 2 are reducing. This probably reflects a redistribution of the electronic states due to Jahn-Teller molecular distortions with doping.
4. Conclusions We performed XPS and NEXAFS measurements on 1 ML-C70 /Cu(1 1 1) doped with sodium atoms. We observed fingerprints of a metallic state for all the phases obtained (1 < ne < 3 e /C70 ). The density of states at the Fermi level is always finite, showing a minimum as a function of doping at about 2 e /C70 . We proposed a model to justify this fact on the basis of theoretical calculation performed by Tanaka et al. [18]. We also observed that the molecular orientation with the C5v axis orthogonal to the substrate surface is not affected by the Na doping. For the molecular orientation depends on the competition between the C70 -substrate interaction and the C70 –C70 electrostatic interactions, and the molecular charge state affects both interactions, this conclusion is not trivial. Moreover, we have extended this work by studying the interaction and doping of a C70 monolayer on polycrystalline Al [16], where the molecules form bonds of mainly covalent character, obtaining similar results for the molecular orientation. This surprisingly suggests that the electrostatic interaction between the molecules and the packing efficiency determine the molecular orientation in spite of the C70 -substrate interaction and the substrate surface order. The charge state of the molecules, instead, seems to influence the molecular distortion as suggested
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by the peculiar shifts of the features in the NEXAFS spectra.
References [1] J.R. Heath, S.C. OÕBrien, Q. Zhang, Y. Liu, R.F. Curl, F.K. Tittel, R.E. Smalley, J. Am. Chem. Soc. 107 (1985) 7779. [2] D.R. McKenzie, C.A. Davis, D.J. Cockayne, D.A. Muller, A.M. Vassallo, Nature (London) 355 (1992) 622. [3] S. Saito, A. Oshiyama, Phys. Rev. Lett. 66 (1991) 2637. [4] W. Andreoni, F. Gygi, M. Parrinello, Chem. Phys. Lett. 189 (1992) 241. [5] R.M. Fleming, A.P. Ramirez, M.J. Rosseinsky, D.W. Murphy, R.C. Haddon, S.M. Zahurak, A.V. Makhija, Nature 352 (1991) 787. [6] A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy, S.H. Glarum, T.T.M. Palstra, A.P. Ramirez, A.R. Kortan, Nature 350 (1991) 600. [7] K. Holczer, O. Klein, S.M. Huang, R.B. Kaner, K.J. Fu, R.L. Whetten, F. Diederich, Science 252 (1991) 1154. [8] M.J. Rosseinsky, A.P. Ramirez, S.H. Glarum, D.W. Murphy, R.C. Haddon, A.F. Hebard, T.T.M. Palstra, A.R. Kortan, S.M. Zamurak, A.V. Makija, Phys. Rev. Lett. 66 (1991) 2830. [9] K. Tanigaki, T.W. Ebbesen, S. Saito, J. Mizuki, J.S. Tsai, Y. Kubo, S. Kuroshima, Nature 352 (1991) 222. [10] T.T.M Palstra, O. Zhou, Y. Iwasa, P.E. Sulewski, R.M. Fleming, B.R. Zegarski, Solid State Commun. 93 (1995) 327. [11] M.S. Denning, I.D. Watts, S.M. Moussa, P. Durand, M.J. Rosseinsky, K. Tanigaki, J. Am. Chem. Soc. 124 (2002) 5570. [12] A. Goldoni, C. Cepek, R. Larciprete, L. Sangaletti, S. Pagliara, R. Gotter, L. Floreano, A. Verdini, A. Morgante, Y. Luo, M. Nyberg, J. Chem. Phys. 116 (2002) 7685. [13] X.-D. Wang, V.Yu. Yurov, T. Hashizume, H. Shinohara, T. Sakurai, Phys. Rev. B 49 (1994) 14746. [14] M. Knupfer, D.M. Poirier, J.H. Weaver, Phys. Rev. B 49 (1994) 8464. [15] T. Takahashi, T. Morikawa, S. Hasegawa, K. Kamiya, H. Fujimoto, S. Hino, K. Seki, H. Katayama-Yoshida, H. Inokuchi, S. Suzuki, K. Ikemoto, Y. Achiba, Physica C 190 (1992) 205. [16] T. Pardini, C. Cepek, R. Larciprete, L. Sangaletti, S. Pagliara, R. Gotter, L. Floreano, A. Verdini, A. Morgante, F. Parmigiani, A. Goldoni, unpublished. [17] S. Saito, O. Oshiyama, Phys. Rev. B 44 (1991) 11532. [18] K. Tanaka, M. Okada, K. Okahara, T. Yamabe, Chem. Phys. Lett. 202 (1993) 394. [19] M. Nyberg, Y. Luo, L. Triguero, L.G.M. Pettersson, H. gren, Phys. Rev. B 60 (1999) 7956. A [20] E. Sohmen, J. Fink, Phys. Rev. B 47 (1993) 14532.
Metallic phases of a C70 single layer adsorbed on Cu(1 1 1) doped with sodium T. Pardini a,b, C. Cepek b, R. Larciprete c,d, L. Sangaletti a, S. Pagliara a, R. Gotter b, L. Floreano b, A. Verdini b, A. Morgante b,e, F. Parmigiani a, A. Goldoni c,* a b
INFM and Universit a Cattolica del Sacro Cuore, Via dei Musei 41, 25121 Brescia, Italy Laboratorio TASC-INFM, s.s. 14 Km 163.5 in Area Science park, 34012 Trieste, Italy c Sincrotrone Trieste, s.s. 14 Km 163.5 in Area Science park, 34012 Trieste, Italy d CNR-IMIP, zona industriale 85050, Tito Scalo (Pz), Italy e Dipartimento di Fisica, Universit a di Trieste, Via Valerio 2, 34127 Trieste, Italy
Abstract The electronic properties of a C70 single layer chemisorbed on Cu(1 1 1) surface, in which the charge state has been modified by Na doping, were studied via synchrotron radiation photoemission and absorption spectroscopy. A twodimensional metallic phase is observed in the whole range of charge states investigated (from 1 up to 3 electrons/C70 ). Moreover, all the results suggest that the orientation of the molecules, with the C5v axis perpendicular to the surface, is not affected by Na doping. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Alkali metals; Fullerenes; Photoemission (total yield); Photon absorption spectroscopy
1. Introduction After C60 , C70 is the next abundant stable fullerene [1]. The D5h group symmetry to which it belongs, gives rise to five inequivalent carbon atoms and eight different carbon–carbon bonds per molecule [1,2]. Many orbital degeneracies, existing in the C60 molecule, are removed by the D5h symmetry in the C70 molecule. For instance, the LUMO which is a three-fold degenerate orbital [3] in the C60 , splits into a lower energy two-fold de-
*
Corresponding author. Fax: +39-040-3758565. E-mail address: [email protected] (A. Goldoni).
generate orbital (e001 ) and into a higher energy onefold degenerate orbital (a001 ) in C70 [4]. Metallic phases (which become superconductive below 40 K) were observed in bulk Ax B3x C60 systems (A ¼ Na, Cs, K, Rb; B ¼ K, Rb, Cs, x ¼ 1,2) [5–10], that is, in compounds where a charge transfer of three electrons from alkali metals to the LUMO of the C60 molecules takes place. While metallic phases are missed in Ax C70 crystals (A ¼ K, Rb, Cs) probably as a consequence of disorder and structural defects in these compounds, very recently metallic phases have been claimed for Na2 CsC70 , where the C70 molecules have the long axis aligned to the cubic {1 1 1} direction [11]. Also, a metallic phase has been
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00179-1
T. Pardini et al. / Surface Science 532–535 (2003) 892–897
obtained for a C70 single layer absorbed on Cu(1 1 1) surface [12]. Motivated by these discoveries, we studied with synchrotron radiation X-ray photoelectron (XPS) and absorption (NEXAFS) spectroscopy the effects of sodium atoms doping on a C70 single layer absorbed on Cu(1 1 1) surface, in order to look for new metallic phases (necessary condition to have superconductive phases) in lowdimensional C70 -derived systems. Sodium atoms were chosen for two different reasons: first, their small dimensions should not appreciably affect the structural order of the system; second, the sodiumcontaining C70 bulk compounds are the only that apparently show metallic properties.
2. Experimental All the measurements were performed at the ALOISA beamline at ELETTRA. C 1s core level and valence band photoemission spectra were measured with photons of 430 eV and 145 eV respectively with an energy resolution of 150 meV. Absorption spectra were obtained by measuring the current on the sample (total yield) due to the absorption of the incident photons. The standard pressure was lower than 5 1010 mbar. All the samples were prepared in situ. The Cu(1 1 1) surface was cleaned by sputtering and annealing (800 K). C70 and sodium atoms were evaporated from a degassed Ta crucible and from commercial SAES getters sources, respectively. The ordered (4 4) single layer (1 ML-C70 /Cu(1 1 1)), having the molecules oriented with the long axis perpendicular to the surface [12], was obtained by sublimating a C70 thick film at 700 K. The binding energies of the photoemission spectra are referred to the position of the Fermi level of the clean Cu substrate.
3. Results and discussions Fig. 1(a) shows the reference valence band photoemission spectra of a thick undoped C70 film and of 1 ML-C70 /Cu(1 1 1) (inset). The spectra were collected with the analyzer normal to the sample surface, whereas the glancing incidence synchrotron beam (7° from the surface) had linear
893
polarization almost perpendicular to the surface plane. In the photoemission spectrum of 1 MLC70 /Cu(1 1 1) the contribution from the Cu substrate was subtracted as discussed by Goldoni et al. [12]. Compared to the spectrum of the C70 thick film, the valence band photoemission spectrum of the undoped 1 ML-C70 /Cu(1 1 1) shows a clear Fermi edge (i.e. metallic character) due to the charge transfer of 1 e /C70 from the Cu substrate to the LUMO of the C70 molecules [12]. In Fig. 1(b) we show the evolution of the LUMO emission of 1 ML-C70 /Cu(1 1 1) as a function of the Na evaporation time. The t ¼ 0 min spectrum corresponds to the undoped single layer. By evaporating sodium atoms onto the sample, the system is affected by a further charge transfer from the alkali atoms to the LUMO of the C70 molecules, which is more evident for t > 7 min when a broad feature (indicated by the shaded area) appears in the spectra at about 0.5 eV. We will later discuss the origin of this feature. From Fig. 1(b) it is clear that the system maintains its metallic behavior for all the phases obtained, but the density of states at the Fermi level changes as function of doping. Up to t ¼ 5 min, the density of states at the Fermi level appears quite constant, but it decreases to a minimum at t ¼ 7 min and then increases again for t ! 17 min. It is known that the LUMO of fullerenes has peculiar behaviors as a function of alkali-atoms doping. Photoemission spectra in the LUMO region performed on Ax C70 thick films (A ¼ K [14], Rb [15], Na [16]), show a single feature at 1.5 eV below the Fermi level for ne < 2 e /C70 (ne ¼ number of electrons transferred per molecule). At ne 2 e /C70 a second feature appears at 0.5 eV, which grows by increasing ne until, at ne 6 e /C70 its intensity is about two times larger than the intensity of the previous one. On the basis of the calculations proposed by Saito et al. [17] for the C70 molecule (see inset of Fig. 1(b)) the electrons transferred to the LUMO of the C70 molecule are expected to occupy first the lower energy two-fold degenerate orbital (e001 ), which can accept up to 4 electrons, and after the one-fold degenerate orbital (a001 ). On the other hand, the calculations performed by Tanaka et al. [18] on the electronic structure of the C 70 molecule shows that
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T. Pardini et al. / Surface Science 532–535 (2003) 892–897
a 1'' e 1'' e 1'' a ''2
1 ML-C70 /Cu(111)
LUMO
Intensity (arb. units)
2
0
1
LUMO HOMO
C 70
e 1'' a''1 a 2'' '' C70 e 1
1 ML-C70 /Cu(111)+Na t = 17' 13'
C70 multilayer
10' 7'
HOMO
5' 4' 3' 2' 1' 0'
HOMO-1
6
LUMO
(b)
(a)
4
2
0
Binding Energy (eV)
2
1
0
Binding Energy (eV)
Fig. 1. (a) Valence band photoemission spectra of a C70 multilayer and (inset) of 1 ML-C70 /Cu(1 1 1). (b) Dependence of the LUMO emission of 1 ML-C70 /Cu(1 1 1) as a function of the exposure to Na atoms. The shaded area for t P 7 min roughly indicates the completely filled a001 orbital at 0.5 eV. (Inset) Electronic diagram of the HOMO–LUMO levels of the C70 molecule as proposed by Saito et al. [13] and of the C 70 molecule as proposed by Tanaka et al. [14].
the extra-electron tends to occupy the one-fold degenerate orbital a001 which lowers its energy (see inset of Fig. 1(b)). This observation combined with the relationship between ne and the intensity of the two features observed in the LUMO region of valence band photoemission spectra of Ax C70 thick films [14–16], suggests to identify the two LUMO features observed in photoemission with the a001 and e001 orbitals respectively. By analogy, we suppose that in 1 ML-C70 / Cu(1 1 1) (t ¼ 0 min) the electron transferred from the Cu to the LUMO of the C70 molecules occupies the one-fold degenerate a001 orbital. When sodium atoms are evaporated on the sample (t > 0 min), the first electron transferred from sodium atoms totally fills the a001 orbital, while the others begin to fill the two-fold degenerate e001 orbital. Therefore, the sudden drop in the density of states at the Fermi level, observed in our spectra of 1 ML-C70 /Cu(1 1 1) doped with sodium atoms at t 7 min, could be associated with the complete filling of the a001 orbital. Moreover, for t > 5 min, a
feature appears clearly at 0.5 eV (roughly indicated by the shaded area in Fig. 1(b)), shifting toward higher binding energies with the Na doping. This feature can be associated to the a001 orbital completely filled. This explanation of the LUMO behavior implies that at t 7 min, 2 electrons are transferred to the LUMO of the C70 molecules. More arguments, further supporting this assumption, will be given in the following by discussing the behavior with Na doping of the C 1s core level spectra (XPS and NEXAFS). Fig. 2 show C 1s photoemission spectra (dots) relative to 1 ML-C70 /Cu(1 1 1) at different doping steps [(a) t ¼ 0 min, (b) t ¼ 7 min, (c) t ¼ 17 min]. Particularly evident are the asymmetrical line shape of the peaks, typical of metallic systems, and the higher energy shoulders mainly due to the plasma oscillation of the electrons in the LUMO. The number of electrons (N ) taking part to the plasma oscillation and the plasma energy (Ep ) are related by the well known formula: Ep =h ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p ð4pNe2 Þ=ðm ðe þ 1ÞV Þ, where V is the volume of
Intensity (arb. units)
T. Pardini et al. / Surface Science 532–535 (2003) 892–897
(a)
C1s ML-C 70 /Cu(111) (t = 0 min.)
fit C-C C-Cu
286
285
284
283
Intensity (arb. units)
Binding Energy (eV) (b)
C1s ML-C 70 /Cu(111) + Na (t = 7 min.)
fit C-C C-Cu
286
285
284
283
Intensity (arb. units)
Binding Energy (eV) C1s ML-C 70 /Cu(111) + Na (t = 17 min.)
(c)
fit C-C C-Cu
286
285
284
283
Binding Energy (eV) Fig. 2. C 1s photoemission spectra of 1 ML-C70 /Cu(1 1 1) (dots) at different sodium doping steps and relative simulation (solid lines). The components due to two kinds of carbon atoms are also shown (dashed lines, see the text for detail).
the unit cell, m is the effective mass of the electrons and e is the dielectric constant due to all the other excitations (the term e þ 1 is due to the presence of vacuum at the interface). Therefore, from the plasma energy, is possible to obtain the number of electrons which occupy the LUMO of the molecules. In order to estimate the plasma energy Ep at t ¼ 0 min, t ¼ 7 min, and t ¼ 17 min of sodium
895
doping, we made a deconvolution of the C 1s core level, using Doniach–Sunjic functions plus Gaussian functions for the plasma excitation. Using a single Doniach–Sunjic component with the corresponding plasma loss it was not possible to reproduce the experimental data satisfactory. As a consequence we have assumed that for the C70 single layer at least two strongly inequivalent kinds of carbon atoms per molecule should exist: carbon atoms interfacing the substrate, and carbon atoms which cannot interact with the substrate. So each spectrum was simulated by introducing two Doniach–Sunjic peaks with the corresponding plasma losses (see the caption of Table 1). The results (line) are shown in Fig. 2. Table 1 reports the values of the parameters used in the simulations. For the undoped 1 ML-C70 /Cu(1 1 1) (t ¼ 0), we estimated Ep ¼ 0:36 eV. For 1 ML-C70 /Cu(1 1 1) doped with sodium atoms at t ¼ 7 min and t ¼ 17 min we estimated Ep ¼ 0:55 eV and Ep ¼ 0:63 eV respectively. Assuming Nðt¼0 minÞ 1 e /C70 [12] and by supposing m , e and V constant with doping, we estimated Nð7 minÞ 2:3 0:2, and Nðt¼17 minÞ 3 0:2 e /C70 . This result supports the above hypothesis that for t ¼ 7 min about 2 electrons per molecules are transferred from the substrates to the LUMO of the C70 molecules. As already shown in Ref. [12], in 1 ML-C70 / Cu(1 1 1) the C70 molecules are mainly oriented with the C5v axis orthogonal to the surface. From the simulation of the XPS C 1s peak at t ¼ 0 min we note that the intensity of the smaller peak is 7% of the total intensity, which means that 5 carbon atoms per molecules contribute to it. Identifying this feature with the carbon atoms that interact with the substrate we are in agreement with the expected orientation of the C70 molecules [12], since the five carbon atoms belonging to the pentagonal basis of the C70 molecules are the only directly interfacing the substrate. It is also worth noting that the ratio between the intensity of the two structures in each spectrum does not change with the sodium doping. This suggests that the orientation of the molecules does not change during the doping. Fig. 3(a) and (b) show the NEXAFS spectra performed on C 1s absorption edge of 1 ML-C70 /
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T. Pardini et al. / Surface Science 532–535 (2003) 892–897
Table 1 Parameters used in the simulation of the C 1s spectra of 1 ML-C70 /Cu(1 1 1) shown in Fig. 3 0 min 7 min 17 min
E1 (eV)
E2 (eV)
WG (eV)
a
Ep (eV)
284.35 284.55 284.65
285.15 285.25 285.8
0.45 0.39 0.52
0.19 0.22 0.17
0.36 0.55 0.63
Ip (%)
Wp (eV)
0.2 0.1 0.12
0.62 1.39 1.0
E1 , E2 are the energies of the two a Doniach–Sunjic curves, WG is the FWHM of the Gaussian curves convoluted to the Doniach– Sunjic. a is the asymmetry coefficient. Ip , Ep and Wp are the intensity, the energy and the FWHM of the plasmon. The Lorentzian width of the Doniach–Sunijc is kept constant to 0.13 eV. Note the reduction of Wp from t 7 min to t ¼ 17 min This can be explained with an increased conductivity of the sample when there are 3 el/molecule (partial filling of the two-fold degenerate e001 band) with respect to the case of 2 el/molecule (complete filling of the a001 band), in agreement with the reduced DOS at EF (metallicity) measured with photoemission at t 7 min. Accordingly, we expect a decrease of the resistivity up to 4 el/molecule, i.e. up to the half-filling of the e001 band.
Intensity (arb. units)
NEXAFS 1 ML-C 70 /Cu(111)+Na Pol. _|_ surface plane
17' 13' 10' 7' 4' 3' 2' 1' t = 0'
282
LUMO LUMO+2
284
286
(a)
288
290
292
Photon Energy (eV) NEXAFS 1 ML-C 70 /Cu(111)+Na Pol. // surface plane
Intensity (arb. units)
Cu(1 1 1) as a function of sodium doping. The spectra of Fig. 3(a) and (b) were obtained with light polarized perpendicular and parallel to the sample surface respectively, and are aligned to the vacuum level. For all the phases obtained, apart the shift of the features and the gradual reduction in intensity of the first structure with doping, we note that the spectral lineshape modifications with the light polarization direction always mirror those of the undoped monolayer. According to the calculations reported in Ref. [19] this confirms that the orientation of the molecules is not doping dependent, as already argued by analyzing the XPS spectra. Moreover our NEXAFS spectra look very similar to the C 1s absorption edge of Rbx C70 measured with transmission EELS by Sohmen et al. [20]. In particular the NEXAFS spectrum obtained after t ¼ 17 min of sodium doping (ne 3 e /C70 according to the estimation based on the C 1s fit) appears very similar to the EELS spectrum obtained on Rb3 C70 . This further support the correctness of our approximate charge transfer estimation based on the C 1s core level and valence band photoemission. As concerns the lineshape of the NEXAFS spectra, the first peak corresponds to the C 1s ! LUMO transition (first p transition) for the C1, C2, C3 and C4 atoms of the C70 molecule, the second peak (mostly visible with the light polarization parallel to the surface) is the superposition of C 1s ! LUMO for the C5 atoms and of the C 1s ! LUMO + 1 (second p transition) for the remaining atoms, while the third peak is mainly due to C 1s ! LUMO + 2 (third p transition).
17' 13' 10' 7' 4' 3' 2' 1' t = 0'
282
LUMO+1
284
286
(b)
288
290
292
Photon Energy (eV) Fig. 3. NEXAFS spectra of 1 ML-C70 /Cu(1 1 1) as a function of sodium doping with light polarized orthogonal to the sample surface (a) and with light polarized parallel to the sample surface (b). The spectra are aligned to the vacuum level.
According to calculations [12], the strong dependence from the light polarization of the spectral
T. Pardini et al. / Surface Science 532–535 (2003) 892–897
intensity in the region of the second peak shows that the contribution from the C 1s ! LUMO transition of the C5 atoms is dominant. The almost linear shift of the first two p transitions toward higher energies as a function of doping (0.3 eV from the undoped monolayer to the highest doping level reached here)––see Fig. 3(a) and (b)––suggests that the Fermi level of the system is moving rigidly inside the LUMO as these bands were filled with electrons. Nevertheless, the opposite shift of the transition to the LUMO + 2 band indicates that energy gaps between the LUMO, the LUMO + 1 and the LUMO + 2 are reducing. This probably reflects a redistribution of the electronic states due to Jahn-Teller molecular distortions with doping.
4. Conclusions We performed XPS and NEXAFS measurements on 1 ML-C70 /Cu(1 1 1) doped with sodium atoms. We observed fingerprints of a metallic state for all the phases obtained (1 < ne < 3 e /C70 ). The density of states at the Fermi level is always finite, showing a minimum as a function of doping at about 2 e /C70 . We proposed a model to justify this fact on the basis of theoretical calculation performed by Tanaka et al. [18]. We also observed that the molecular orientation with the C5v axis orthogonal to the substrate surface is not affected by the Na doping. For the molecular orientation depends on the competition between the C70 -substrate interaction and the C70 –C70 electrostatic interactions, and the molecular charge state affects both interactions, this conclusion is not trivial. Moreover, we have extended this work by studying the interaction and doping of a C70 monolayer on polycrystalline Al [16], where the molecules form bonds of mainly covalent character, obtaining similar results for the molecular orientation. This surprisingly suggests that the electrostatic interaction between the molecules and the packing efficiency determine the molecular orientation in spite of the C70 -substrate interaction and the substrate surface order. The charge state of the molecules, instead, seems to influence the molecular distortion as suggested
897
by the peculiar shifts of the features in the NEXAFS spectra.
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