Spectroscopy Studies of Lanthanum

La @ C 82 AND Li @ C 60. V. N. POPOK , A. V. GROMOV , M. JÖNSSON , A. TANINAKA , H. SHINOHARA , E. E. B. CAMPBELL. Nano 2008 03 (03), 155-160 ...
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NANO LETTERS

Scanning Tunneling Microscopy/ Spectroscopy Studies of Lanthanum Endohedral Metallofullerenes

2003 Vol. 3, No. 3 337-341

Atsushi Taninaka,† Kazuhiro Shino,† Toshiki Sugai,† Seiji Heike,§ Yasuhiko Terada,§ Tomihiro Hashizume,§ and Hisanori Shinohara*,†,‡ Department of Chemistry, Nagoya UniVersity, Nagoya 464-8602, Japan, CREST, Japan Science Technology Corporation, c/o Department of Chemistry, Nagoya UniVersity, Nagoya 464-8602, Japan, AdVanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-0395, Japan Received December 30, 2002; Revised Manuscript Received January 23, 2003

ABSTRACT We have investigated monolayer and multilayer islands of La2@C80 and La@C82 molecules grown on a hydrogen-terminated Si(100)-2 × 1 surfaces by ultrahigh vacuum scanning tunneling microscopy. The observed La2@C80 molecule has a spherical shape, consistent with a recent result by synchrotron X-ray measurements. The energy gap for the La2@C80 multilayer islands is measured to be 1.3−1.5 eV, whereas that for La@C82 is 0.5 eV, indicating that the Ih cage of the La2@C80 molecule is highly stabilized by an electron transfer from the encaged La atoms.

1. Introduction. Endohedral metallofullerenes have attracted much attention during the past 10 years1 because of their novel structural and electronic/solid-state properties. Recently, it has been shown experimentally that Gd endohedral metallofullerenes can be inserted into single-wall carbon nanotubes, forming the so-called peapod-like structure.2,3 In this hybrid fullerene/nanotube material, the local band gap of carbon nanotubes can be modulated by encapsulated metallofullerenes.4 To obtain information on electronic properties of the endohedral fullerenes is, therefore, of crucial importance to fabricate and design such peapod hybrid materials. Scanning tunneling microscopy (STM) is a powerful method to directly image the structures and electronic properties of fullerenes and endohedral metallofullerenes with an atomic resolution. A number of STM studies have previously been performed to characterize fullerenes on various surfaces.5-11 A recent result by synchrotron X-ray measurements has shown that La2@C80 has a spherical structure and that the C80 carbon cage possesses Ih symmetry,12 which is similar to C60. The highly symmetrical structure leads to highly degenerated molecular orbitals. Because of the electron transfer from the encaged La atoms to C80 carbon cage, the 4-fold-degenerate HOMOs (highest occupied molecular * Corresponding author. E-mail: [email protected]. Tel +81-52789-2482; fax +81-52-789-1169. † Department of Chemistry, Nagoya University. ‡ CREST, Japan Science Technology Corporation. § Hitachi, Ltd. 10.1021/nl025975r CCC: $25.00 Published on Web 02/25/2003

© 2003 American Chemical Society

orbitals) are occupied by eight electrons to form a closedshell electronic structure with Ih symmetry.13 By incorporating the alkali metal doping into La2@C80 solids as in AxC60 (A: alkali metal atoms), Ax(La2@C80) may exhibit novel properties such as nanoscale superconducting14 and semiconducting devices. Here, we report structures and electronic properties of the La2@C80 and La@C82 metallofullerenes in monolayer and multilayer islands grown on a hydrogen-terminated Si(100)-2 × 1 surface by using STM/STS operating in an ultrahigh vacuum (UHV-STM). We have chosen the hydrogenterminated Si(100)-2 × 1 surface as a substrate on which to grow nanocrystals of these endohedral metallofullerenes. The structure and properties of the hydrogen-terminated Si(100)-2 × 1 surface are well understood.15 The surface is one of the promising substrates for fabricating atomic-scale structures and nanoscale devices.16,17 The dangling bonds of Si atoms on the surface are passivated by H atoms. The mobility of endohedral metallofullerenes on this surface is much higher than that on a clean Si(100) surface, indicating the presence of fewer surface-fullerene interactions. The surface is suitable for investigating the electronic properties of fullerenes using scanning tunneling spectroscopy (STS), because monolayer and multilayer nanoislands of metallofullerenes are well ordered and show less interaction with the substrate. 2. Experimental. Silicon samples of 2 × 14 mm2 rectangular shape were cut from a commercial Si(100) wafer

(n-type, 7 × 10-3 to 13 × 10-3 Ωcm) and were set on a sample holder made of tantalum. After introducing the samples into a UHV preparation chamber, the clean Si(100)-2 × 1 surface was prepared by degassing the surface at 800 °C overnight using resistive heating to 1210 °C. The pressure of the preparation chamber was kept below 5 × 10-10 Torr during the cleaning procedure. Atomic hydrogen was made from gas of hydrogen molecules using a handmade doser.16 To obtain the hydrogenterminated Si(100)-2 × 1-H surface, the clean Si(100)-2 × 1 surface was exposed to atomic-hydrogen flux of typically 3 × 10-2 ML/s for 10 min while the sample was kept at approximately 300 °C (1 ML here is defined as the number of Si atoms on a bulk-terminated ideal Si(100) surface: 6.78 × 1014 atoms/cm2). Sample heating and the hydrogen supply were terminated almost at the same time to quench the growth of the hydrogen-terminated surface. We used scanning tips electrochemically etched from W wires. Each tip apex was cleaned and shaped into a hemisphere using an in situ field ion microscope (FIM) in the preparation chamber.18,19 The La2@C80 and La@C82 metallofullerenes were prepared by the DC arc discharge method followed by purification and isolation with high performance liquid chromatography.20,21 The purity of La2@C80 and La@C82 metallofullerenes was higher than 99.9%. Pure La2@C80 or La@C82 powder was degassed in UHV at approximately 400 °C for 1 h in a doser made of tantalum. Each metallofullerene was sublimated in UHV onto the Si(100)-2 × 1-H surface via a doser kept at 470-530 °C. Typical deposition rate and time were 1 ML/min and for 1-2 min (1 ML here is defined as the number of fullerene molecules on the Si(100) surface forming an ideal (111) face of monolayer fullerene film with the same lattice constant as bulk fcc fullerene crystal). The STM images were taken at a sample bias voltage Vs ranging from -3.0 to +3.0 V and a constant tunneling current It ranging from 10 to 50 pA. All STM/STS measurements were performed at 78 K (liquid N2). 3. Results and Discussion. Figure 1(a) shows an STM image of a La2@C80 fourth layer island grown on the Si(100)-2 × 1-H surface. The La2@C80 islands were formed, whereas the rest of the Si(100)-2 × 1-H surface was not covered with La2@C80, indicating the Volmer-Weber-mode island formation. This clearly demonstrates higher surface mobility of fullerene molecules comparing with the reported Stransky-Krastanov-mode growth on clean Si(111) and Si(100) surfaces.5,21 The inset image in Figure 1(a) is a typical image of the bare Si(100)-2 × 1-H surface, where a missing hydrogen defect on the Si(100)-2 × 1-H surface is indicated by an arrow. A La2@C80 molecule can be stabilized by its adsorption on a single dangling bond;22 we have frequently observed that isolated La2@C80 molecules reside on bare Si(100)-2 × 1-H surfaces. One of the molecular lines of the island is always aligning to one of the two dimer directions on the Si(100)-2 × 1-H surface. However, the molecules in the monolayer island are located in the positions expected for an fcc(111) face. Therefore, the La2@C80 layers were grown 338

Figure 1. (a) STM image of a multilayer La2@C80 island adsorbed on a Si(100)-2 × 1-H surface (23.5 nm × 23.5 nm, the sample bias voltage Vs ) -2.5 V and the tunneling current It ) 10 pA). The inset is a typical image of the bare Si(100)-2 × 1-H surface (7 nm × 7 nm, Vs ) -2.0 V, It ) 20 pA). (b) Zoomed-in STM image of the top layer of a multilayer La2@C80 island (4.3 nm × 4.3 nm, Vs ) -2.5 V, It ) 10 pA).

parallel to the trough between two adjacent dimer rows. The second layer was ordered in a wide range (typically 30 nm × 30 nm). The interactions between the La endohedral metallofullerenes and Si(100)-2 × 1-H are very weak, so that the second layers of La2@C80 were well ordered, because of the van der Waals interactions dominated between La2@C80 molecules. This result is in clear contrast to the growth mode on the Si(111)-7 × 7 or Si(100)-2 × 1 clean surface.5,21 Figure 1(b) shows a zoomed-in image of the La2@C80 multilayer. The La2@C80 molecules formed a hexagonal close-packed array with a nearest neighbor distance of 11.211.4 Å and an interlayer spacing of 9.5 Å. Such interlayer spacing corresponds to the expected value for a close-packed fcc (111) structure of spherical molecules with radius of 11.6 Nano Lett., Vol. 3, No. 3, 2003

Figure 2. (a) STM image of the top layer of a four-layer La@C82 island (10 nm × 10 nm, Vs ) -2.5 V, It ) 20 pA). (b) STM image of the monolayer La@C82 island (10 nm × 10 nm, Vs ) -1.5 V, It ) 10 pA).

Å. A work reported21 that the crystalline islands have the fcc (111) plane parallel to the substrate and that the La2@C80 molecules have quite a spherical structure. These nearest neighbor distance and interlayer spacing are in good agreement with the estimated size of La2@C80 from the synchrotron X-ray structural study of [email protected] Figure 2(a) shows an STM image of the fourth layer of La@C82 grown on the Si(100)-2 × 1-H surface. The La@C82 molecules formed a hexagonal close-packing array with a nearest neighbor distance of 11-12 Å and an interlayer spacing of 9-10 Å. The result is consistent with the previous study that the C82 carbon cage possesses C2v symmetry20 in that the distribution of the nearest neighbor distance and interlayer spacing were much more spread out than in the La2@C80 case. One of the intriguing observations here is that intramolecular structures are clearly observed in the monolayer islands of both La@C82 and La2@C80 adsorbed on the Si(100)-2 × 1-H surface. Figure 2(b) shows such an STM image of La@C82 monolayer grown on the Si(100)-2 × 1-H surface. One can clearly see three or four stripes, two or Nano Lett., Vol. 3, No. 3, 2003

Figure 3. Tip-bias dependent STM images of the monolayer La2@C80 island (6 nm × 6 nm, It ) 30 pA). (a) Vs ) -2.5 V and (b) -1.5 V.

four leaf-like structures, and further complicated patterns, indicating that these metallofullerenes do not rotate freely on the surface. Figure 3(a) and (b) show the STM images of La2@C80 monolayer at -2.5 V and -1.5 V, respectively. Intramolecular structures show bias-voltage dependence. The La2@C80 or La@C82 molecules are adsorbed on the trough between two adjacent dimer rows and are grown into the layer. Since the adsorbed fullerenes cannot rotate in the trough, intramolecular structures are observed in the monolayer islands. Above the third layer, fullerene molecules start to rotate and no intramolecular structures have been observed. Scanning tunneling spectroscopy (STS) is suited to analyze the electronic structure of La2@C80 or La@C82 islands near the Fermi level. Figure 4 shows the STS results for the multilayer islands made of (a) La2@C80 and (b) La@C82 on the Si(100)-2 × 1-H surface, together with that for the bare Si(100)-2 × 1-H surface as a reference (c). Figure 5 shows RHF density-of-state calculations on the C806- and C823- for a reference. The band gap of the Si(100)-2 × 1-H surface has generally been estimated to be 1.1 eV.15,17 The observed gap of Si(100)-2 × 1-H is 1.1-1.2 eV, which confirms that the STM tip does not contribute any significant features to the observed spectra of the La endohedral metallofullerenes. 339

Figure 4. STS spectra measured on the multilayer islands of (a) La2@C80 and (b) La@C82 grown on the H-Si(100)-2 × 1 surface at 78 K. An STS spectrum of bare H-Si(100) surface is also shown (c) for comparison. Normalized tunnel conductance ((dI/dV)/(I/V)) is shown as a constant value in the HOMO-LUMO gap, since in the HOMO-LUMO gap the observed tunneling current is smaller than the inferior limit of measurement (0.03 pA).

The HOMO-LUMO gap of the La@C82 multilayer is evaluated as 0.5 eV, which is much smaller than the observed gap of the Si(100)-2 × 1-H surface. According to the theoretical calculation23,24 and to the result of ultraviolet photoelectron (UPS) studies,24,25 La@C82 has the so-called singly occupied molecular orbital (SOMO). The observed STS peak position of filled state of La@C82 corresponds to the measured SOMO peak by the UPS experiment (0.9 eV). The splitting of the observed peak in the empty state might be caused by interactions with the surrounding La@C82 molecules. In this case, the observed edge of the empty state (i.e., positive bias side) of La@C82 should correspond to the original SOMO level. The fact that the HOMO-LUMO energy gap of La@C82 is small indicates that La@C82 layers are semiconductive. The HOMO-LUMO gap of the La2@C80 multilayer is 1.3-1.5 eV, which is much larger than that of La@C82. The presence of the large gap of La2@C80 suggests that the Ih cage is highly stabilized by an electron transfer from the encaged La atoms. The peak positions of the empty state of La2@C80 molecule were at 1.0 eV and a weak shoulder at 0.8 eV, which can be assigned to the LUMO +1 and LUMO, respectively. These peaks should correspond to the calculated unoccupied state peaks in Figure 5(a). The observed peak positions of the filled state of La2@C80 are at -1.6 eV and a shoulder at -1.0 eV, which are assigned to the HOMO-1 and HOMO, respectively (cf. Figure 5a). 340

Figure 5. Density-of-states calculations of the energy levels of (a) C806- and (b) C823-using HF/3-21G. A Gaussian line broadening (broked lines) was made with a 0.03 Hartree full width at half maxmum. Table 1. Measured HOMO-LUMO Gaps energy gap La2@C80 La@C82 Si(100)-2 × 1-H

1.3-1.5 eV 0.5 eV 1.1-1.2 eV

The normalized tunneling conductance for La2@C80 in Figure 4 shows a salient (filled state) peak at -1.6 eV below the Fermi energy, and the onset of the filled state peak is estimated to be -0.7 eV. In contrast, the filled state peak position of the La@C82 layer is near to the onset of the filled state, suggesting that distribution of HOMOs of the La@C82 layer might be fairly narrow. The HOMOs of the La2@C80 layer might be distributed widely from -0.7 to -1.6 eV, and HOMO and HOMO-1 levels are overlapped with each other to provide the broadened peak at -1.6 eV. Table 1 shows the observed energy gaps of La@C82 and La2@C80. Nano Lett., Vol. 3, No. 3, 2003

In summary, ultrahigh vacuum scanning tunneling microscopy (UHV-STM) has been made on La2@C80 and La@C82 layers on Si(100)-2 × 1-H surfaces. Intramolecular structures were observed in the monolayer of La@C82 and La2@C80 adsorbed on the Si(100)-2 × 1-H surface. The La2@C80 layer has a large energy gap, and that of the La@C82 layer is small. This can be explained by the difference of the number of electrons transferring to the fullerene cages. Acknowledgment. H.S. thanks the CREST Program on Novel Carbon Nanotubes by JST. The present study was also supported by the Special Coordination Funds for Promoting Science and Technology of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References (1) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843. (2) Hirahara, K.; Suenaga, K.; Bandow, S.; Kato, H.; Okazaki, T.; Shinohara, H.; Iijima, S. Phys. ReV. Lett. 2000, 85, 5384. (3) Suenaga, K.; Tence´, M.; Mory, C.; Colliex, C.; Kato, H.; Okazaki, T.; Shinohara, H.; Hirahara, K.; Bandow, S.; Iijima, S. Science 2000, 290, 2280. (4) Lee, J.; Kim, H.; Kahng, S.-J.; Kim, G.; Son, Y.-W.; Ihm, J.; Kato, H.; Wang, Z. W.; Okazaki, T.; Shinohara, H.; Kuk, Y. Nature 2002, 415, 1005. (5) Sakurai, T.; Wang, X. D.; Xue, Q. K.; Hasegawa, Y.; Hashizume, T.; Shinohara, H. Prog. Surf. Sci. 1996, 51, 263. (6) Wang, X. D.; Hashizume, T.; Shinohara, H.; Saito, Y.; Nishina, Y.; Sakurai, T. Jpn. J. Appl. Phys. 1992, 31, L983. (7) Li, Y. Z.; Chander, M.; Patrin, J. C.; Weaver, J. H. Phys. ReV. B 1992, 45, 13837. (8) Hashizume, T.; Motai, K.; Wang, X. D.; Shinohara, H.; Saito, Y.; Maruyama, Y.; Ohno, K.; Kawazoe, Y.; Nishina, Y.; Pickering, H. W.; Kuk, Y.; Sakurai, T. Phys. ReV. Lett. 1993, 71, 2959.

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