11414
J. Phys. Chem. B 2001, 105, 11414-11418
Scanning Tunneling Microscopy of Endohedral Metallofullerene Tb@C82 on C60 Film and Si(100) 2 × 1 Surface Bo-Rong Shi,*†,§ Xue-Sen Wang,† Houjin Huang,‡ Shi-He Yang,‡ Wener Heiland,| and Nelson Cue† Department of Physics, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, P. R. China, Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, P. R. China, Department of Physics, Shandong UniVersity, 250100, Jinan, P. R. China, and FB Physik, UniVersita¨ t Osnabru¨ ck, D-49069 Osnabru¨ ck, Germany ReceiVed: May 4, 2001; In Final Form: July 27, 2001
Endohedral metallofullerene Tb@C82 molecules adsorbed on the C60 film and Si(100) 2 × 1 have been investigated by scanning tunneling microscopy (STM). The C60 film was obtained by depositing 2-3 monolayers (ML) of C60 molecules on a highly oriented pyrolytic graphite (HOPG) surface. Tb@C82 molecules have great mobility and aggregate along the edges of terraces on the C60 film, and form a close-packing monolayer with increasing coverage. Very few trimers or dimers were imaged on the surface of C60 film. In contrast, the Tb@C82 molecules were randomly distributed on the surface of Si(100). These nucleation behaviors of Tb@C82 molecules can be explained by the interaction of Tb@C82-Si and Tb@C82-Tb@C82 that were brought on by the permanent dipole moment of Tb@C82 molecules.
I. Introduction Endohedral metallofullerenes (fullerenes with metal atom(s) encapsulated) are novel forms of fullerene-based materials that have attracted wide interest,1 not only in physics and chemistry but also in such interdisciplinary areas as materials and biological sciences. Many experimental results have shown that a group-III metal (M ) Sc, Y, La) or lanthanide atom could be attached to a fullerene C82 molecule.2-4 Synchrotron X-ray diffraction, 13C NMR, and ultrahigh vacuum scanning tunneling microscopy (UHV-STM) studies have revealed that metal atoms are indeed encapsulated by the carbon cage and that the metal atoms are located off the center of the fullerene cage, indicating the presence of a strong metal-cage interaction.5-7 The electron spin resonance (ESR) measurement and theoretical calculation show that substantial electron transfer takes place from the encaged metal atom to the carbon cage: intrafullerene electron transfers. For example, the synchrotron X-ray diffraction experiment on a Y@C82 powder sample revealed the presence of such a charge transfer from the analysis of the total electron density distribution of an Y@C82 microcrystal.8 The X-ray study also reveals that the Y@C82 molecules are aligned along the [001] direction in a head-to-tail order in the crystal, indicating the presence of charge transfer and a strong dipole-dipole interaction among the Y@C82 fullerenes. This kind of dipole moment may be as large as 4 D and may strongly affect the properties of M@C82 molecules. The intermolecular force of M@C82 is commonly considered as mainly caused by their dipole moments. * Corresponding Author. Fax: +852 2358 1652. E-mail: phshibr@ ust.hk. † Department of Physics, The Hong Kong University of Science and Technology. ‡ Department of Chemistry, The Hong Kong University of Science and Technology. § Shandong University. | Universita ¨ t Osnabru¨ck.
STM has been a powerful technique for studying structural and electronic properties of fullerenes. It was first applied to study the morphology of C60 on Au(111),9 highly oriented pyrolytic graphite (HOPG),10 Si(100)11 surfaces. In particular, UHV-STM has been proven to be a crucial technique for the investigation of endohedral metallofullerenes. Sakurai et al. imaged Y@C82 molecules on a Cu(111) 1 × 1 surface at room temperature, showing that the Y@C82 molecules are mobile on the surface and aggregate on the terrace edges.12 Lin et al. studied the initial stage nucleation of endohedral metallofullerene Nd@C82 on a C60 crystalline film with UHV-STM,13 indicating that Nd@C82 molecules form close-packed configurations on the substrate, and they register on the top sites of the C60 lattice. To reveal the similarities and differences of various endohedral metallofullerene molecules, we use in the present work an UHV-STM to directly image endohedral metallofullerene Tb@C82 molecules and investigate their initial stage nucleation behavior on crystalline C60 films and Si(100) surface. The formation of Tb@C82 endohedral metallofullerene was first reported by the groups at UCLA14 and SRI international.15 The absorption spectrum of Tb@C82 in the UV-Vis-NIR region is essentially the same as that of M@C82 (M ) Ce, Pr, Nd, Gd, Dy, Ho, Er, and Lu), and is similar to that of
[email protected] However, it is different from that of Sc@C82. Judging from the similarity of the absorption spectra, Tb@C82 is inferred to have a 3+ charge state similar to that of Nd@C82 and Y@C82. To our knowledge, the nucleation behavior of Tb@C82 has not been studied with UHV-STM. It is important to understand the initial stage nucleation in order to realize thin film and bulk growth process. II. Experiments Soot-containing metallofullerenes were produced by the standard arc vaporization method using composite anodes that contain graphite and terbium oxide.16 First, graphite powder and Tb4O7 in an atomic ratio of Tb:C ) 0.02 were uniformly mixed with a graphite cement. The mixture was then pressed into a
10.1021/jp0117112 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/26/2001
STM of Endohedral Tb@C82 on C60 Film and Si Surface
Figure 1. STM image of C60 film on HOPG substrate at a bias voltage of 2.67 V, tunneling current ) 100 pA, size ) 50 nm × 50 nm.
6-mm-diameter rod under a hydraulic pressure of 3000kg/cm2. After curing at 140 °C for 4 h, the rod was heated to 1100 °C in a vacuum for 3 h. The rod was then subjected to a dc discharge as an anode under a He atmosphere of 125 Torr. The raw soot is collected and extracted in a Soxhlet extractor using N,N-dimethylformamide (DMF) at its boiling temperature (150 °C) for 48 h. After removal of DMF by evaporation, a black powder is obtained. Tb@C82 was collected by HPLC (highperformance liquid chromatography) using a PYE (2-(1-pyrenyl) ethyl) Cosmosil column with toluene as the mobile phase. The injection volume is 5 mL and the elution rate was 4.0 mL/min. The purity of the Tb@C82 sample prepared in this manner is over 99.9% as verified by desorption chemical ionization (DCI) negative-ion mass spectrometry (Finnigan TSQ7000). The measurements were performed with an UHV-STM system made by OMICRON. The system consists of a preparation chamber for sample growing, which includes two evaporation sources (C60 and Tb@C82) and an analysis chamber for STM study. HOPG was used as substrate for growing C60 film since it is a layered material and its (0001) surface is a van der Waals type surface. The atomic scale flat terraces can be easily obtained by cleaving the HOPG surface. The freshly cleaved HOPG was introduced into the preparation chamber, and then C60 film was deposited on HOPG surface by thermal evaporation from C60 powder in a tantalum boat. After the deposition of 2-3 monolayers of C60 on HOPG, the sample was transferred into an STM analysis chamber to make sure that C60 had been grown into a perfect close-packed hexagonal lattice on the HOPG surface. Then, Tb@C82 was sublimated on the substrate. Before its sublimation, Tb@C82 was thoroughly outgassed in the preparation chamber at 400 °C overnight. The Si(100) substrate was prepared under a standard procedure to form 2 × 1 structure on the surface. All of the above depositions were done with the substrate at room temperature. The chamber pressure during scanning was below 4 × 10-11 Torr. III. Results and Discussions The characteristic features of the C60 film on a HOPG substrate are shown in Figure 1. Three monolayers of C60 deposited on the HOPG surface. The first layer grows into large,
J. Phys. Chem. B, Vol. 105, No. 46, 2001 11415 flat, and smooth-edged terraces, more than 95% of the HOPG surface is covered by the first layer. The nearest-neighboring distance between C60 molecules is about 1.0 nm, consistent with the theoretical expectation. The second and third layers form asymmetric orientated islands. The height of the layers is about 0.9 nm. The result is in qualitative agreement with the theoretical simulation, where the Girifalco potential for the C60-C60 interaction and the Ruoff-Hickman potential for the C60-HOPG interaction were used.17 The simulation showed that for a small number of molecules the ground state consisted of hexagonally arranged monolayers supported by the substrate. For layers with very large area, growth of additional layers was energetically favored. Figure 2 shows a large scale (100 nm × 100 nm) STM image of the C60 film at room temperature exposed to a small amount of endohedral metallofullerenes Tb@C82. It is immediately evident that Tb@C82 molecules are mobile on the surface at room temperature and aggregate along the edges of the C60 films. Only a few trimers or dimers are randomly distributed on the surface of C60 film. When the Tb@C82 molecules are sublimated from the tantalum boat onto the C60 film, they impinged on the terrace of the surface with a kinetic energy corresponding to ca. 500 °C (about 0.1 eV). The impinging Tb@C82 molecules migrate to existing step edges following adsorption if their bonding to the C60 substrate surface is relatively weak. The C60 adsorption on the Cu(111) 1 × 1 surface showed a similar tendency.18 The nucleation behavior of Tb@C82 molecules on C60 film is also consistent with that of Y@C82 and Gd@C82 molecules on Cu(111) surface,19 where monomers, dimers, and trimers of Y@C82 and Gd@C82 molecules aggregate on the terrace edges of Cu(111). The preferential dimer formation of Y@C82 was explained as due to the unpaired electron that Y@C82 possesses. Figure 3 is a close up view of a trimer and a dimer of Tb@C82 molecules on C60 film. The bright spheres are Tb@C82 molecules. The nearest neighbor distance is measured to be around 1.15 nm, calibrated by the C60 intermolecules distance of 1.0 nm. The result is nearly the same as Y@C82 and Gd@C82 on Cu(111) (1.18 nm).12 Statistically, the triangle-shaped trimers are more visible than the dimers. It is worthwhile to note that all the Tb@C82 molecules are registering on top sites of the underlying C60 lattice. The same behavior of Nd@C82 molecules on C60 film was reported by Lin et al.13 For a van der Waals interaction expected of C60, the 3-fold hollow sites are energetically favorable to the admolecules since these sites have three nearest-neighboring coordinates, while the top sites only have one. Thus, the top-layer C60 molecules fill the 3-fold hollow sites of the sublayer to form the close-packed fcc structure.17 The particular top-site registration was explained as due to the dipole moment of Nd@C82 molecules. The Nd@C82 polarizes the C60 molecule beneath it, so the dipole-dipole interaction between them makes the top site a preferred site for the Nd@C82 molecule. In this case, Tb@C82 molecules have properties similar to those of Nd@C82 molecules. In contrast to the trimer domination of Tb@C82 and Nd@C82 molecules on C60 film, the preferential dimer formation in the case of Y@C82 adsorbed on Cu(111) was reported by Shirohara et al.6 Such a difference is caused by the nature of the substrates used that results in the different intermolecular interactions. In the case of Tb@C82 and Nd@C82 molecules adsorbed on a C60 film, the closed-shell electronic structure of C60 poses a significant barrier for any charge transfer between the adsorbed endohedral metallofullerene molecules and the C60 substrate. However, the charge transfer is unavoidable in the case of the
11416 J. Phys. Chem. B, Vol. 105, No. 46, 2001
Shi et al.
Figure 2. A typical STM image (100 nm × 100 nm) of endohedral metallofullerene Tb@C82 molecules on the C60 film at the initial stage of adsorption. Trimers are indicated by arrows. The sample bias voltage is 1.69 V and the current is 10 pA.
metal substrate, which altered the intrinsic properties of the endohedral metallofullerene molecules. To investigate the nucleation behavior of endohedral metallofullerene on different substrates, the Tb@C82 molecules were deposited on a Si(100) surface under the same condition as that on C60/HOPG substrate. Figure 4 shows a typical STM image of the initial stage adsorption of Tb@C82 on the Si(100) 2 × 1 surface. The spherical protrusions are individual Tb@C82 molecules. They adsorb randomly and stably, without any aggregation at the step edges. The apparent lateral diameter of Tb@C82 molecules was around 2.2 nm, compared to the nearest neighboring distance is only 1.15 nm for dimers/trimers on C60 film. This can be explained by the convolution effect between the tip and the Tb@C82 molecule. As seen in Figure 4, it is hard to find any dimer or trimer structures of Tb@C82 molecules. The strong interaction between Tb@C82 molecules and Si atoms is evident. Comparing with the behaviors of endohedral metallofullerene molecules on Cu(111) and C60 film, the Si(100) substrate shows the strongest intermolecular interaction. In this context, the dominance of the trimers on the C60 film may simply reflect the dominance of the dipole interaction among Tb@C82 molecules.
Figure 3. A close-up view (25 nm × 25 nm) of trimer and dimer structure of Tb@C82 molecules on the surface of C60 film. Sample bias voltage is 1.46 V and tunneling current is 20 pA.
STM of Endohedral Tb@C82 on C60 Film and Si Surface
Figure 4. A STM image (200 nm × 200 nm) of Tb@C82 molecules adsorbed on the surface of Si(100) 2 × 1. The sample bias voltage is 1.81 V and tunneling current is 40 pA.
J. Phys. Chem. B, Vol. 105, No. 46, 2001 11417 The analysis of X-ray photoelectron spectroscopy and solid fluorescent emission spectroscopy indicates that the encaged Tb atom donates three valence electrons to the C82 cage to form an endohedral metallofullerene of the type
[email protected] The charge transfer between metal atom and carbon cage leads to a Coulomb interaction between them. As a result, Tb@C82 molecules are predicted to have quite large dipole moments. On the basis of a simple model of dipole-dipole interaction, Lin et al. predicted that a triangular trimer is an energetically favorable structure when the dipole moments are off-center of the endohedral metallofullerene molecules.13 According to the study on the chromatographic elution behavior of metallofullerenes (Y, La, Ce, Gd),21 the magnitude of the dipole moments is related to the retention time. This suggests that the metallofullerenes with higher retention time also have a larger effective dipole moment. Huang and Yang systematically investigated the relative yields and relative retention times of endohedral lanthanide metallofullerenes, and found the relative retention time in the order of Tb > Gd > Pr > Ce > Er ∼ Lu > La ∼ Ho > Nd > Dy . Tm ∼ Sm ∼ Eu ∼ Yb.22 The result indicates that Tb@C82 molecules have a larger dipole moment than that of Nd@C82 molecules. With the higher mobility of Tb@C82 on C60, the stronger intermolecular interactions for Tb@C82 molecules force them to form close-packed films along the edges. This is maybe the reason only a few trimers or dimers can be found on the surface of C60 film. IV. Summary The initial nucleation behavior of endohedral metallofullerene Tb@C82 on C60 film and Si(100) 2 × 1 surface was investigated with UHV-STM. The deposited Tb@C82 molecules have great mobility and aggregate at the terrace edges of C60 film, and form close-packed islands on the terraces. Very few trimers or dimers were imaged on the surface of C60 film. However, the monomer of Tb@C82 were randomly adsorbed on the surface of Si(100). The absence of nucleation for Tb@C82 molecules on Si(100) can be explained by the strong intermolecular interaction that freezes Tb@C82 molecules onto the substrate. In contrast, the inert electronic character of the C60 substrate permits the large dipole moments of Tb@C82 molecules to play the crucial role in the nucleation behavior of Tb@C82 on the C60 film.
Figure 5. The close-packed configuration of Tb@C82 molecules on the C60 film imaged by STM. The image size is 50 nm × 50 nm, the sample bias voltage is 1.93 V and tunneling current is 100 pA.
The formation of trimer structure is a particular feature of endohedral metallofullerene molecules on C60 film, but only a few of the trimers or dimers can be imaged. As seen in Figure 2, most of the deposited Tb@C82 molecules on the C60 surface are highly mobile on the terraces and migrated to the step edges, where they stabilized. When the coverage is increased, Tb@C82 molecules continue to aggregate to the step edges until they filled up all adsorption positions on the step edges. Then, Tb@C82 molecules start to form a two-dimensional stripe; initially growing from the step edge and then toward the terrace. Figure 5 is a close up view of the closed-packed Tb@C82 molecules on the top layer of C60 film. The nearest neighboring distance is measured around 1.15 nm, which is consistent with that of the trimer structure of Tb@C82.
Acknowledgment. This work was supported in part by the RGC of Hong Kong SAR (Grant RGC HKUST 6127/97P and HKUST 6192/00P) and the Joint Project of DAAD Germany and RGC Hong Kong. One of the authors (Bo-rong Shi) acknowledges the support of Alexander von Humboldt-Stiftung during his stay in Germany. References and Notes (1) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843. (2) Pan, H.; Pruski, M.; Gerstein, B. C.; Li, F.; Lannin, J. F. Phys. ReV. B 1991, 44, 6741. (3) Wang, C. Z.; Ho, K. M.; Chan, C. T. Phys. ReV. Lett. 1993, 70, 611. (4) Heath, J. R.; O’Brien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Tittel, F. K.; Smalley, R. E. J. Am. Chem. Soc. 1985, 107, 7779. (5) Park, C.; Wells, B. O.; DiCarlo, J.; Shen, Z. X.; Salem, J. R.; Bethune, D. S.; Yannoni, C. S.; Booth, C.; Bridge, F. Chem. Phys. Lett. 1993, 213, 196. (6) Shirohara, H.; Inakuma, M.; Kishida, M.; Yamazaki, S.; Hashizume, T.; Sakurai, T. J. Phys. Chem. 1995, 99, 13690. (7) Lin, N.; Ding, J. Q.; Yang, S. H.; Cue, N. Phys. Lett. 1996, A222, 190.
11418 J. Phys. Chem. B, Vol. 105, No. 46, 2001 (8) Takata, M.; Umeda, B.; Nishibori, E.; Sakata, M.; Saito, Y.; Ohno, M.; Shinohara, H. Nature (London) 1995, 377, 46. (9) Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johson, R. D.; Chambliss, D. D.; de Veres, M. S.; Hunziker, H. E.; Wendt, H. R. Nature 1990, 348, 621. (10) Wragg, J. L.; Chamberlain, J. E.; White, H. W.; Kraetschmer, W.; Huffman, D. R. Nature 1990, 348, 623. (11) Hashizume, T.; Wang, X. D.; Nishina, Y.; Shinohara, H.; Saito, Y.; Kuk, Y.; Sakurai, T. Jpn. J. Appl. Phys. 1992, 31, L880. (12) Sakurai, T.; Wang, X. D.; Xue, Q. K.; Hasegawa, Y.; Hashizume, T.; Shinohara, H. Prog. Surf. Sci. 1996, 51, 263. (13) Lin, N.; Huang, H.; Yang, S. H.; Cue, N. Phys. ReV. B 1998, 58, 2126. (14) Gillan, E. G.; Yeretzian, C.; Min, K. S.; Alvarez, M. M.; Whetten, R. L.; Kaner, R. B. J. Phys. Chem. 1992, 96, 6869.
Shi et al. (15) Moro, L.; Ruoff, R. S.; Becker, C. H.; Lorents, D. C.; Malhotra, R. J. Phys. Chem. 1993, 97, 6801. (16) Huang, H. J.; Yang, S. H.; Zhang, X. X. J. Phys. Chem. B 2000, 104, 1473. (17) Rey, C.; Garcı´a-Rodeja, J.; Gallego, L. J.; Alonso, J. A. Phys. ReV. B 1997, 55, 7190. (18) Hashizume, T.; Sakurai, T. J. Vac. Sci. Technol. B 1994, 12, 1992. (19) Hasegawa, Y.; Ling, Y.; Yamazaki, S.; Hashizume, T.; Shinohara, H.; Sakai, A.; Pickering, H. W.; Sakurai, T. Phys. ReV. B 1997, 56, 6470. (20) Sun, D. Y.; Liu, Z. Y.; Guo, X. H.; Liu, S. Y. Fullerene Sci. Technol. 1998, 6, 707. (21) Fuchs, D.; Roetschel, H.; Michel, R. H.; Fischer, A.; Weis, P.; Kappes, M. M. J. Phys. Chem. 1996, 100, 725. (22) Huang, H. J.; Yang, S. H. J. Phys. Chem. B 1998, 102, 10196.
