Endohedral Fullerene Ce@C82 on Cu(111): Orientation, Electronic

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Endohedral Fullerene Ce@C on Cu(111): Orientation, Electronic Structure and Electron-Vibration Coupling Kaliappan Muthukumar, Anna Strozecka, Josef Myslivecek, Aneta Dybek, T. John S. Dennis, Bert Voigtländer, and Andreas Larsson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp305438h • Publication Date (Web): 12 Dec 2012 Downloaded from http://pubs.acs.org on December 19, 2012

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The Journal of Physical Chemistry

Endohedral Fullerene Ce@C82 on Cu(111): Orientation, Electronic Structure and Electron-Vibration Coupling Kaliappan Muthukumar,∗,†,k Anna Stró˙zecka,‡,⊥ Josef Mysliveˇcek,¶ Aneta Dybek,§ T. John S. Dennis,§ Bert Voigtländer,‡ and J. Andreas Larsson† Tyndall National Institute, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland, Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany, and JARA-Fundamentals of Future Information Technology, Charles University, Faculty of Mathematics and Physics, V Holešoviˇckách 2, Praha 8, Czech Republic, and Department of Physics, Queen Mary University of London, Mile End Road, London E1 4NS, UK E-mail: [email protected]

∗ To

whom correspondence should be addressed National Institute, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland ‡ Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany, and JARA-Fundamentals of Future Information Technology ¶ Charles University, Faculty of Mathematics and Physics, V Holešoviˇ ckách 2, Praha 8, Czech Republic § Department of Physics, Queen Mary University of London, Mile End Road, London E1 4NS, UK k Institut für Theoretische Physik, Goethe-Universität Frankfurt am Main, 60438 Frankfurt am Main, Germany ⊥ Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany † Tyndall

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Abstract Structural, electronic and vibrational properties of the endohedral fullerene Ce@C82 on Cu(111) have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). Ce@C82 forms islands on the substrate. Our STM measurements show relatively large differences in morphology and electron spectra of molecules within these islands indicating multiple molecular orientations on the substrate, while the vibrational spectra are more uniform. We have determined molecular orientations by comparing STM and DFT molecular morphology and we have calculated Ce@C82 bound to Cu(111) and found that it is chemisorbed. We show that Ce@C82 adopts orientations on the surface that enables the Ce to remain at its most favorable binding site inside C82 . The effect of chemisorption on the structural and electronic properties of Ce@C82 is thus small, and the orientations are limited to configurations with Ce in the upper hemisphere of the molecular configurational space. We show that the variations in the dI/dV spectra between molecules of different orientations is due to Ce-cage orbitals that are localized in space and their involvement in tunneling depends on the molecular orientation on the substrate. The observed electron-vibration coupling modes in the STM-IETS (in-elastic tunneling spectroscopy) of Ce@C82 arise from cage modes only and therefore electron transport properties are expected to be different compared to Ce2 @C80 which has active Ce-cage vibrations.

Keywords : M@C82 , Density functional theory (DFT), Scanning tunneling techniques (STM,STS,IETS), Single molecule imaging and vibrations, Conductance, Density of states (DOS).


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Introduction Endohedral fullerenes that encapsulate either one or two atoms or a molecule/cluster have attracted great interest because of their electronic properties associated with the charge transfer from the endohedral dopant to the cage. 1–5 Many extraordinary application have been proposed for these molecules, 6 including the recent demonstration of their use as field-effect transistor in nanoelectronic devices. 7,8 Due to the high production yields that are achieved for the endohedral fullerenes of the type M@C82 , research on this class of fullerenes is more common and has focused on understanding their physical, chemical and electronic properties and on learning how to tune these. When C82 encapsulates a Ce atom two cage isomers are found, a Ce@C82 -C2v isomer and a Ce@C82 -Cs isomer, with the C2v isomer being four times more abundant 9,10 although none of these isomers are the most stable for empty C82 . 11–13 In the most stable Ce@C82 -C2v structure, it has been unambiguously proved that the encapsulated Ce atoms are located off-center and bound to the center of a six-membered ring on the C2 -axis (henceforth ’Ce@C82 ’ corresponds to ’Ce@C82 C2v ’ isomer unless otherwise mentioned in this manuscript). 10,14,15 The encapsulated Ce atom has a formal +3 oxidation state 10,14–18 but there is a high covalent contribution to the Ce-cage bonds, which means that the majority of the 3 donated electrons are localized, i.e. not delocalized over the entire cage. 18–20 Though studies examining the structure and the nature of the Ce bonding within the cage are available for gas phase Ce@C82 , a detailed investigation of its electronic structure upon adsorption on different substrates, one of the prerequisite to consider them for practical applications, remain scarce. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are widely used to investigate the electronic structure of adsorbed molecules. Such studies available in the literature have so far investigated empty fullerenes such as C60 and different lanthanoid endohedral systems. 21–31 A few studies are also available for M@C82 systems such as Nd@C82 , Ce@C82 and Dy@C82 on Ag:Si(111), 25–27 and Gd@C82 on Ag(001). 32 In this study, we investigate Ce@C82 molecules bound to Cu(111) using STM and DFT to reveal the nature and strength of the adsorption. In addition we use scanning tunneling spectroscopy (STS), to further probe the 3 ACS Paragon Plus Environment


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electronic structure of adsorbed Ce@C82 . Finally, we analyze the molecular vibrations using single molecule inelastic electron tunneling spectroscopy (IETS) and discuss the origin of the observed modes based on simulated electron-vibration coupling activity.

Experimental Methods The C2v isomer of Ce@C82 was produced by a standard arc discharge method (28 V, 300 A - DC) of suitably doped graphite rods (10 x 10 x 300 mm3 , 1.7 atom-% Ce). The result was a soot-like material that contained a range of both empty and endohedral fullerenes together with amorphous carbon species. The fullerenes were solvent extracted from the soot-like material under anaerobic conditions (ultra-sonication at room temperature in carbon disulphide during ca. 5 hours). 3,33–35 Following filtration of the fullerene solution from the amorphous soot, the CS2 was evaporated and the resulting fullerite powder was dissolved in toluene before purification by high performance liquid chromatography (HPLC), with the purification being monitored by matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectroscopy. A two-stage HPLC treatment was used in the purification. The first-stage involved normal single-pass HPLC during which the fullerite solution was separated into several fractions, one of which contained Ce@C82 as well as traces of the empty fullerenes C84 , C86 and C88 . The fraction that contained Ce@C82 was then subjected to peak-recycling HPLC which resulted in an isomerpure sample (see Fig. 1a), as evidence by the single isolated peak. The quality of Ce@C82 obtained is further demonstrated by the MALDI-TOF mass spectrum (Fig. 1b), which also shows a single peak - indicating no measurable traces of any other fullerenes. Ce@C82 exists in two isomeric forms, C2v and Cs , with the C2v isomer being the more abundant isomer. However, they possess a very different HPLC retention times such that the Cs isomer eluted from the HPLC column in a completely different fraction to that which contained the major C2v isomer. The material studied here was therefore a pure sample of Ce@C82 -C2v . The Cu(111) single-crystal sample was prepared by repeated cycles of Ne+ ion sputtering and


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annealing at 650 ◦ C. The Ce@C82 powder was degassed in ultra-high vacuum (UHV) and the metallofullerenes were evaporated onto a clean Cu(111) surface from a Knudsen-cell operating at 550 ◦ C. During deposition the sample was kept at room temperature. For the STM measurement, chemically etched tungsten tips were prepared by controlled indentation into the copper substrate that results in a coating by Cu.

Figure 1: (a) The Stage-2 HPLC chromatogram of the fraction from Stage-1 that contained the main isomer of Ce@C82 only. Two successive cycles are shown, which demonstrate that remaining traces of empty fullerenes were removed, during the first cycle leaving pure Ce@C82 -C2v . (b) The MALDI-TOF mass spectrum of the HPLC purified Ce@C82 -C2v which shows that there are no detectable traces of other species in our sample.

All measurements were performed on a commercial UHV low temperature-STM operated at T=7 K. The STS was performed by measuring the differential conductance signal dI/dV by a standard lock-in technique under open-feedback conditions. We used the technique of continuously varying the tip-sample distance in order to amplify the tunneling current around zero voltage to a conveniently measurable range. A small AC modulation Vmod ranging from 3-6 mV was added to the bias voltage. The subsequent normalization of the data by the total conductance I/V was performed to remove the distance dependence of the spectra. The d2 I/dV2 signal was recorded simultaneously with the dI/dV spectrum by using a separate lock-in amplifier. The in-elastic tunneling vibrational modes are identified from the d2 I/dV2 signal where a clear peak appears at the threshold voltage for electrons tunneling in both directions: from the tip to the sample and from the sample to the tip.



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Computational Methods We have performed full geometry relaxation simulations of free and Cu(111) surface bound Ce@C82 using unrestricted DFT with the generalized gradient approximation (GGA) exchange-correlation functional PBE 36 employing the RI 37 (resolution of identity) approximation and a dense grid ”m5”, using a relaxation threshold of 10−6 eV as implemented in Turbomole 5.9. 38,39 For free Ce@C82 we have used a polarized valence triple zeta basis set (TZVP) for carbon and cerium, with the accompanying effective core potential (ECP), ECP-28-MWB 40,41 for Ce. 18 For surface bound Ce@C82 the molecule was placed on three different binding sites (hcp, fcc, on-top) on three layered "coin-shaped" Cu55 clusters in a variety of orientations. The top face of the Cu clusters is broader than the diameter of the cage. Usage of similar cluster models have been found to provide a good description of the bonding and electronic structure of several fullerene molecules. 29,42 For these calculations the TZVP basis set was retained for Ce and a double zeta basis sets was used for C and Cu with a 10 electron core ECP (ECP-10-mdf) for the latter. 43 The orientation dependent STM topographs have been simulated by summing up the electron densities of the molecular orbitals of the isolated Ce@C82 that lie in the bias window of the present experiment (0 to −0.2 eV for occupied and 0 to 2 eV for unoccupied orbitals, with the singly occupied HOMO taken to define the Fermi level) in accordance with the Tersoff-Hamann approximation, and electron density isosurface images with a cut-off value of 1.5 × 10−5 e/Å3 were created. 44 The DFT electron density images were oriented in different viewing angles to obtain the best correspondence to the measured constant current STM topographs at both voltages. Mulliken population analysis was used to calculate the partial charges and atomic contribution to the MOs (molecular orbitals), and the PDOS (partial density of states) is simulated by Lorentzian line-width broadening (0.08) of the calculated energy levels, with subsequent Gaussian broadening (0.009) of the spectra. Only the electronic energy levels of the majority spin electrons is shown as it has been found to be sufficient to describe the electronic structure of similar systems. 29 The vibrational spectrum was simulated for free Ce@C82 using numerical derivatives 18 and


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the cross section for exciting an inelastic electron-vibration tunneling channel is evaluated as 2 dE h ¯ EF p ∗ , ∆ρi ≈ dQi 2mi Ωi

(1)

where EEF is the energy of a molecular orbital inelastically coupling at the Fermi level, and Qi , mi , and Ωi the normal coordinate, the reduced mass, and the frequency of the i-th molecular vibration, respectively. Equation (1) has been used as an approximation for discussing the IETS of endofullerenes previously. 28,32 The IETS activity of Ce@C82 was simulated by computing the derivative of the electronic states lying in the energy range (-200 meV to +200 meV) with respect to Qi (

dEEF dQi

) by displacing the atoms by 0.05 Å in directions given by Qi and evaluating the cor-

responding changes to the energy of these orbitals. 45 The vibrational eigenmodes along with the calculated IETS activity are Lorentzian line-width broadened (0.009) with subsequent Gaussian broadening (0.00005). Out of the considered electron states, the electron-vibration coupling activity of the molecular HOMO exhibits the best correspondence with the measured vibration spectra. Vibrational modes are distinguished either as cage or Ce modes based on their effective mass. Modes with mi > 12.5 a.u. (compared to 12.011 a.u. for pure cage modes) are considered to be Ce active. 28

Results and Discussion: Orientation of Ce@C82 on the Cu(111) substrate Ce@C82 has been deposited on Cu(111) at room temperature and the topography of the molecules observed with STM has been compared to DFT computed electron density images (see Fig. 2). The deposited Ce@C82 molecules form closed-packed islands which start to nucleate at step edges. An STM topograph of such an island with contrast on the intramolecular structure of the molecules is shown in Fig. 2a and b for two bias voltages, -0.2 V and +2 V, respectively. A variety of topographic patterns have been observed and were found to be bias dependent. At small sample bias (Fig. 2a) 7 ACS Paragon Plus Environment


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the molecules exhibit complex striped patterns, which change with increasing voltage. Finally, at the bias of 2 V (see Fig. 2b) we observe ring shapes, resembling C60 for which the pentagons are seen in STM topographs at high positive bias. 42,46,47 To further understand the observed variety of patterns for Ce@C82 adsorbed on Cu(111) we investigate the nature of bonding between Ce@C82 and Cu(111). We have performed DFT calculations of Ce@C82 placed at three different Cu(111) surface binding sites ’on-top’, ’fcc-hollow’ and ’hcp-hollow’ that are at the center of different conformers of the Cu55 surface model. The configuration where the double bond on the C2 -axis of the Ce@C82 cage is bonded to the on-top binding site of Cu(111) surface is the lowest energy structure (Fig. 3a, binding energy of Ce@C82 is 2.49 eV) but bonding on the fcc and hcp are not much higher in energy (binding energy of 2.35 and 2.05 eV, respectively). The high adsorption energy confirms that the Ce@C82 molecule is chemisorbed on the Cu substrate. The cage-Cu distance is typically between 2.0 - 2.5 Å which is within a reasonable range compared to 2.2 - 2.4 Å found for C60 bound to Cu surfaces. The chemisorption slightly changes the symmetry of the Ce@C82 molecule resulting in a slight tilt on the Ce atom from the C2 -axis moving it towards one side of the six-membered ring it bonds to. We find that the C-C and Ce-C bond lengths in the chemisorbed Ce@C82 ranges from 1.399−1.470 Å and 2.508−2.525 Å respectively, which is slightly altered compared to the free molecule (DFT calculated values for the free molecule 1.379−1.473 Å and 2.534−2.537 Å). 18 Altogether, we find only a slight perturbation of the geometry of Ce@C82 as a consequence of chemisorbing to copper (see discussion below). Hence, STM topographs for different adsorbed molecules (see Fig. 2a and 2b) were compared to DFT simulated electron density images for free Ce@C82 and an optimal match has been obtained as shown in Fig. 2c. The orientation of the molecule in the DFT model has been chosen in a way that the calculated electron density images match the characteristic features resolved by STM at two bias polarities. This comparison shows that Ce@C82 adsorbs on Cu(111) with many different cage orientations, as found before on Si(111), and also for the Nd@C82 and Dy@C82 molecules on Ag:Si(111). 25,27 Interestingly, however, the comparison between STM and DFT did not yield any orientation of the cage with


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Figure 2: (a) and (b) Ce@C82 as deposited on Cu surfaces for two different bias voltages: (a) -0.2 V and (b) +2 V (Step edges are not visible as the image contrast is enhanced to depict the internal structure of the molecules) (c) Comparison of the experimental STM images of Ce@C82 (first and third column) and calculated isosurface images of Kohn-Sham electron densities (second and fourth column), for different orientations of the cage. STM images of occupied (first column) and empty states (third column) of individual molecules in the island presented in (a) and (b). The orientation of the molecule in the DFT model has been chosen to obtain the best correspondence between the electron density images and the STM images at the two bias polarities. The corresponding geometry of the molecule is presented in the last column.



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the Ce atom pointing towards the Cu substrate. This is in agreement with results of our DFT calculations of Ce@C82 bonded to Cu(111) where we observe that most stable orientations of the Ce@C82 are where the Ce atom is far away from the surface (C2 -axis perpendicular to the substrate surface), followed by a range of orientations with the C2 -axis more parallel to the surface. Least stable are configurations with the Ce close to the substrate surface. In the calculations where the Ce bound six-membered ring was placed pointing towards the surface we find that either the Ce atoms moves within the cage to unfavorable positions (a 0.65 eV energy loss or more, which is the difference in energy between the second lowest and the ground state configuration of free Ce@C82 ) or the cage itself detaches from the Cu surface. The position of the endohedral atom far away from the substrate is in analogy with the results for the most stable position of La in La@C60 bonded to Cu(111), 24 and has its origin in that both bonding to Cu(111) and endohedral bonding of lanthanoids results in a charge transfer to the cage. It is not favorable to have these two charge transfer effects at the same place both from the inside and outside of the cage, since the charge transfer is localized to the carbon atoms directly bonded to the metal atoms and their neighbors, and it would make the charge transfer to these atoms too large. This can also be seen from the picture of MOs (see Fig. 3c) illustrating the interaction of the Ce atom with the cage which does not have any involvement of the lower half of the cage. A good correspondence between the experimental and theoretical data suggests that the part of the carbon cage that is oriented towards the vacuum, which is probed by STM, is affected very little as a result of chemisorption. With this approach, we have successfully mapped many of the observed orientations, with a few exceptions which we attribute to particularly strong moleculesurface interactions arising as a result of defects on the surface and perhaps by the unfavorable orientation of Ce@C82 , in which part of the cage where Ce atom resides faces towards the surface that displaces Ce atom from it’s preferred binding site (i.e., the C2v six-membered ring) on the C82 -C2v cage.


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Electronic structure of Ce@C82 on the Cu(111) substrate STS spectra acquired over Ce@C82 molecules show strong differences depending on the particular orientation of the carbon cage. A series of spectra presented in Fig. 3e has been recorded for differently oriented molecules within the same island (see Fig. 2b). In the occupied states (negative bias voltage) two strong peaks -1.25 (± 0.1) V and -1.7 (± 0.1) V are resolved in most of the spectra, however the relative intensities of the two peaks change. In addition, for certain orientations a characteristic strong feature at -0.65 (± 0.1) V is observed. In the unoccupied states (positive bias voltage), electron spectra vary significantly. We resolve several molecular states, with position and intensity depending on the adsorption configuration. In order to interpret the observed differences in the dI/dV spectra, we have simulated the PDOS of Ce@C82 bonded to three different binding sites (see Fig. 3d). These show a considerable domination of Ce states in the Fermi region especially between 0 and 1 eV, as in the free molecule. 18 The DFT results illustrate that the PDOS is very similar for different adsorption positions, as represented by Ce@C82 bonded to the on-top, hcp and fcc binding sites. Bonding of Ce@C82 to the Cu(111) surface affects Ce bonding to the cage very little, but the electronic structure of the molecule as a whole is altered upon bonding to the Cu(111) surface, rendering the molecule conductive. 18 A simple comparison of the PDOS with the measured spectra shows that not all the peaks in the PDOS are detected in each STS spectrum suggesting that only some states are accessible for tunneling for a given molecular orientation. For the most stable on-top binding site the HOMO of the Ce@C82 −Cu(111) complex has equal contributions from Cu (52 %) and the cage (48 %). In the -1 to +1 eV region of the DFT simulated spectrum, the occupied part has many such mixed cage-Cu orbitals. The conducting HOMO is accompanied by similar states at -0.284 (46 % Cu and 47 % C) and 0.394 eV (60 % Cu and 33 % C) corresponding to three small peaks in the C PDOS in Fig. 3d. We suggest that these states are seen in STS as very small features between -0.5 to 0.5 V (see Fig. 3e) and the three MOs are probably appearing and disappearing as conductive channels seamlessly as the bias voltages change, i.e. without giving rise to individual peaks. With the first shoulder peak in STS at negative 11 ACS Paragon Plus Environment


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Figure 3: (Colour online) (a) The geometry of the most stable configuration of Ce@C82 on Cu(111). Representative delocalized cage orbital (b) and a localized Ce orbital (c). (d) DFT calculated PDOS of Ce@C82 for three different (on-top, hcp, fcc) binding sites of Cu(111) (black and grey represent carbon contribution and the colored lines Ce), and (e) STS spectra obtained for different Ce@C82 molecules on the Cu(111) substrate (different colors represent different orientations). For STS, the STM feedback was switched off at V = −2 V and I = 0.1 nA.


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bias voltage at -0.3 V corresponding to the peak at -0.284 eV in the C PDOS. The STS states at -0.65, -1.25 and -1.7 V are assigned to the peaks at -0.629, -1.239 and -1.627 eV that have carbon contribution of 56, 90 and 47%, respectively, and are delocalized (Fig. 3b). There are no significant difference in STS for different molecules in negative bias (particularly below -1 V), which leads to the logical conclusion that tunneling through these delocalized cage states is possible regardless of the molecular orientation. DFT results illustrate that the unoccupied part of the spectrum is dominated by states that have majority contribution arising from Ce and are localized (see Fig. 3c). In the energy window 0 - 1 eV in the DFT simulated PDOS there are 6 orbitals at 0.294 eV (97% Ce), 0.317 eV (73% Ce), 0.331 eV (97% Ce), 0.647 eV (90% Ce), 0.705 eV (90% Ce), 0.795 eV (95% Ce) with dominant Ce character. Two orbitals with significant contribution from Ce were also found in the occupied part at -0.222 eV (51% Ce), -0.220 eV (25% Ce). These Ce dominated states are strongly localized as a consequence of the stationary position of Ce in the Ce@C82 molecule in contrast to e.g. C60 for which all states are delocalized over the whole carbon cage. 29,42 These strongly localized states can be detected by the STS measurements only if they are overlapping favorably in space and phase with the tip orbitals. Since this condition is fulfilled only for a limited number of molecular orientations, the variation of the bonding geometry yields significant changes in the shape of the electron tunneling spectra of Ce@C82 for the empty states. In the energy range between -1 eV to +1 eV we find a few orbitals with Ce character that also have delocalized cage and Cu character at -0.899 eV (6% Ce), -0.284 eV (7% Ce), 0.124 eV (4% Ce), 0.394 eV (7% Ce). These are expected to be important tunneling channels that become accessible for selected molecular orientations. Recently we have shown for Ce2 @C80 that the presence of Ce localized orbitals hinders the conductance of the molecule, which becomes four times lower than that of free C60 . 29 A similar effect might also be expected for Ce@C82 but our results show that it would not be as enhanced as in the case of the dimetallofullerene as we observe a significant number of delocalized cage orbitals around the Fermi level that were not present for Ce2 @C80 .



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Vibrational Modes of Ce@C82 The vibrational properties of the Ce@C82 molecules adsorbed on Cu(111) were studied by STMIETS. The measured spectra for different Ce@C82 molecules on the surface are shown in Fig. 4. The DFT calculated electron-vibration coupling activity for the vibrational modes of free Ce@C82 and the molecular HOMO is shown as a red line. Vibrations that interact with the electron excitations are detected in STM-IETS and two dominant inelastic tunneling peaks at 60 mV and 88 mV were observed for most Ce@C82 molecules on the Cu(111) surface. The observed peaks also have their counterparts as dips in the opposite bias polarity. The same two peaks are observed for each molecule with differences in intensity irrespective of their orientation on the substrate, which is in contrast to the electronic spectra as discussed in the previous section. The peak at 60 mV resembles the observed inelastic signal for Gd@C82 that was attributed to strong coupling of a specific cage mode to the electronic state of the cage at the Fermi level. 32 The simulated vibrational spectra (IR and Raman) of Ce@C82 shows that Ce dominated modes appear at low energies (4, 4.5 and 18 meV with mi = 50.0, 48.9 and 32.5, respectively) and they are not detected in STM-IETS. 18 No other modes at higher energies (except one at 33 meV with mi =12.9) could be distinguished as Ce modes using the mi > 12.5 a.u. criterion. Of the 243 molecular vibrations, in the energy window of 50-100 meV the vibrational modes at 51, 53, 56, 63, 70, 85, 87 and 91 meV exhibit a significant electron-vibration coupling activity. Outside this energy range, peaks at 33, 178, 196 meV shows IETS activity, with the mode at 33 meV having the highest intensity of all modes, although it is not seen in the measured spectra. This might be due to this mode being a Ce active vibration or the fact that the coupling with the Cu substrate or the fullerene-fullerene interaction hides or quenches this mode. We assign the five calculated modes at 51, 53, 56, 63 and 70 meV to the peak observed at 60 mV and the other three modes at 85, 87 and 91 meV to the observed peak at 88 mV. The calculated modes exhibit similar electron-vibration coupling activity. Some of these vibrations involve a movement of the Ce atom as the part of the cage it is bonded to is buckling, but without any 14 ACS Paragon Plus Environment

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Figure 4: Measured STM-IETS spectra of Ce@C82 recorded over differently oriented molecules. Variations in the intensity of the inelastic signal (green and blue curves) are shown. DFT calculated electron-vibration coupling activity for the vibrational modes of Ce@C82 for the molecular HOMO is shown in red.



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Ce-cage vibration. In the case of Ce2 @C80 and La2 @C80 it has been hypothesized that electronvibration excitation of lower-energy vibrations could induce hopping of Ce and La atoms among the equivalent positions in the C80 cage. This is not observed for Ce@C82 as the C82 -C2v cage only has one preferential position for Ce, all other positions at the inside wall are higher in energy by 0.62 eV or more, 18 which also implies higher barriers for Ce movement in C82 -C2v compared to Ce/La in C80 -Ih . 19 Our comparison between experiment and theory has shown that the two observed STS peaks correspond to the combination of a few molecular vibrational modes that are close in energy, which is a marked difference to the report for Gd@C82 on Ag(001) 32 and highlights the importance of careful simulation as an aid for STM based single molecule spectroscopy.

Conclusions: The electronic and vibrational properties of Ce@C82 have been characterized by STM measurements and DFT calculations. The characteristic intramolecular patterns of Ce@C82 on Cu(111) in STM have been correlated with specific orientations of the cage. The spectroscopic results reveal a strong dependence of the electronic spectra on the molecular orientation of Ce@C82 . Theoretical calculations explain that this behavior is a consequence of tunneling into the hybridized ceriumcage states. Such states are localized on and around the encapsulated atom, and are thus available as tunneling paths only for certain molecular orientations on the substrate. Theory and experiment indicate that only molecular orientations for which the C82 cage’s Ce atom binding site is not facing the Cu substrate are present. Two peaks observed in STM-IETS at 60 mV and 88 mV vary slightly in intensity for different molecular orientations and are assigned to two sets of cage vibrational modes. Modes due to Ce vibrations are not detected in the experiments in contrast to what has been found for Ce2 @C80 .


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Acknowledgement: This work was supported by the FP6 Marie Curie Early Stage Training Network NANOCAGE (MEST-CT-2004-506854). We acknowledge the SFI/HEA Irish Centre for High-End Computing (ICHEC) for generous allotment of computer resources, and also SFI/HEA for the provision of local computing clusters.

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Table of Contents:

Results of DFT calculations performed on Ce@C82 chemisorbed to Cu(111) are compared with STM measurements which illustrates that the molecule gets adsorbed in multiple orientations on the surface but avoids having the Ce binding site close to the substrate.


Endohedral Fullerene Ce@C82 on Cu(111): Orientation, Electronic

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