8.1 STM studies of metallofullerenes on clean surfaces
STM has been a powerful technique for studying structural and electronic properties of endohedral metallofullerenes. In fullerenes, the STM tech- nique was first applied to study the morphology of C60 on Au(111) (Wilson et al., 1990), highly oriented pyrolytic graphite (Wragg et al., 1990), GaAs(110) (Li et al., 1991), Si(100) (Hashizume et al., 1992), Si(111) (Wang et al., 1992), Cu(111) (Cuberes et al., 1996; Hashizume et al., 1993a) and Au(110) (Joachim et al., 1995) surfaces. In particular, UHV-STM has been proven to be a crucial technique for the study of endohedral metallofullerenes, and was reviewed extensively by Sakurai et al. (1996).
Figure 25 shows a UHV-STM image of a small (141 A˚141 A˚ ) area of the Si(100) 21 surface covered with Sc2C2@C82molecules with a cover- age of approximately three mono-layers at room temperature (Shinohara et al., 1993b, X Wang et al., 1993d). Pure scandium fullerenes were eva- porated from a tantalum boat heated to approximately 700C onto the clean Si(100) 21 surface in an UVH condition (51011 Torr). The Sc2C2@C82molecules reside in the trough separated by the Si dimer rows and they are distributed randomly on the surface with a minimum sepa- ration of 11.7 A˚ . The STM image of the Sc2C2@C82molecules shows small deviation (within ca 10%) from the perfectly circular shape. The first layer of Sc2C2@C82 has provided only a short-range local ordering. When the first layer was completed, the second Sc2C2@C82layer began to form and island formation was observed. The second layer was still somewhat irregular. However, the Sc2C2@C82 molecule overlayers (the third layer and up) grown on the second layer were well ordered and perfectly close- packed, indicating that the overlayer film was basically formed by van der Waals interaction without interference from the Si substrate, similar to the case of the C60 (Hashizume et al., 1992) and C84(Wang et al., 1992) depositions on the Si(100) 21 surface.
The STM images present strong evidence that the two scandium atoms are indeed encapsulated by the C84fullerene cage: the STM images of the Sc2C2@C82molecules show no characteristic bright (or dark) spots (which may correspond to the position of scandium atoms) on and around the carbon cage, and all images are essentially the same as those of hollow C84
FIGURE 25 STM image of the third layer of Sc2C2@C82fullerenes on the Si(100) 21 clean surface at a bias voltage of3.0 V (tunneling current¼20 pA). The white contrasts correspond to the Sc2C2@C82fullerenes which are slightly get ahead of the other close-packed fullerenes.
molecules (Hashizume et al., 1993b, X Wang et al., 1993e). The two scandium atoms are trapped securely inside the C82cage, which is consis- tent with HRTEM (Beyers et al., 1994), high-resolution 13C NMR (Yamamoto et al., 1996) and synchrotron X-ray diffraction (Iiduka et al., 2006; Nishibori et al., 2006a,b; Takata et al., 1997) results.
In contrast to the STM observation on the Si(100) 21 surface, Gimzewski (1996) found that STM images of Sc2C2@C82 on a Au(110) (Joachim et al., 1995) surface show a characteristic internal structure.
The appearance of the internal structure indicates that the interaction between Sc2C2@C82 and the Si and Au surfaces differ from each other and that the electronic structures near the Fermi levels might be also different.
8.2 Metallofullerenes as superatoms
A typical large-scale (400 A˚400 A˚ ) STM image of a small amount [23 molecules (1,000 A˚ )2] of Y@C82 on Cu(111) 11 surface at room temperature shows a preferential adsorption at the terrace edges (Figure 26) (Hasegawa et al., 1997; Shinohara et al., 1995a,b). The Y@C82
molecules are sublimated from the tantalum boat onto the copper surface and impinge on the terrace of the surface with a kinetic energy corresponding to about 400C. The Y@C82molecules are mobile on the surface and segregate to the terrace edges. The impinging Y@C82
molecules migrate to the edges following adsorption since the bonding to the substrate surface is relatively weak. The C60 adsorption on the
FIGURE 26 Typical large-scale (400 A˚ 400 A˚ ) STM image of (Y@C82)2dimers adsorbed on a terrace edge of Cu(111) 11 clean surface.
Cu(111) 11 surface showed a similar mobile tendency (Hashizume et al., 1993a). This is in sharp contrast to the adsorption of fullerenes on Si(100) and Si(111) surfaces in which the fullerenes such as C60
(Hashizume et al., 1992), C70(Wang et al., 1994) and C84(X Wang et al., 1993e) do not freely migrate.
One of the most intriguing observation here is that the Y@C82 full- erenes predominantly form clusters, (Y@C82)n(n¼2–6), and in particular dimers, (Y@C82)2, on the Cu(111) surface even at the very initial stage of adsorption (Shinohara et al., 1995a,b). The distribution of the (Y@C82)n
clusters has the maximum at dimers and shows that more than 60% of the Y@C82molecules on the Cu(111) surface exist as dimers or larger clusters (Hasegawa et al., 1997). Previous STM results on higher fullerenes indi- cate that, like C60, the higher fullerenes, such as C70 and C84, also exist mostly as monomers in the initial stage of deposition on the copper and silicon surfaces (X Wang et al., 1993e; Wang et al., 1994). These results indicate that the Y@C82metallofullerenes have a very special tendency to form dimers and larger clusters on the copper surface. The dimerization energy of Y@C82 on the Cu(111) surface was estimated to be about 180 meV (Hasegawa et al., 1997). In addition, large dipole moments of metallofullerenes may also play a crucial role in the dimerization, since the calculated and experimental dipole moments of La@C82 are 3–4 D (debye) (Laasonen et al., 1992; Poirier et al., 1994) and 4.40.4 D (Fuchs and Rietschel, 1996), respectively.
As discussed in Section 6.1 hfs analysis of the ESR measurements of Y@C82indicates that the encaged Y atom donates three valence electrons to the C82 cage to form an endohedral metallofullerene of the type Y3þ@C823 (Shinohara et al., 1992a; Weaver et al., 1992). An ab initio theoretical calculation (Nagase and Kobayashi, 1994a,c) reveals that the charge on the encaged yttrium, i.e., 3þ, is little changed even when Y@C82
ejects or accepts an additional electron. Namely, the Y@C82metallofuller- ene can be regarded as a positively charged core metal and a negatively charged carbon cage. Such a molecule has a great similarity to the supera- tom concept proposed first by Watanabe and Inoshita (1986) and Inoshita et al. (1986) in a semiconductor heterostructure composed of spherically symmetric positively charged core. Superatoms have also been discussed theoretically in relation to endohedral metallofullerenes by Rosen and Waestberg (1988, 1989), Saito (1990) and Nagase and Kobayashi (1994a,c).
The STM observation of the Y@C82dimers and clusters is direct experi- mental evidence that Y@C82molecules exhibit the superatom feature. The observed interfullerene distance is 11.2 A˚ , which is shorter than that of the simple Y@C82–Y@C82van der Waals distance (11.4 A˚ ), suggesting that the interfullerene interaction is not a simple dispersion type of weak interaction but a relatively strong interaction. A large dipole moment of Y@C82also plays an important role in the tight binding between Y@C82
molecules, particularly in the solid state. In fact, a synchrotron X-ray diffraction study on a powder Y@C82sample (Takata et al., 1995) reveals the presence of such a charge-transfer-type interaction from the analysis of the total electron density distribution map of the Y@C82microcrystal.
In an Y@C82–Y@C82fullerene interaction, the positively charged (Z¼ þ3) yttrium core on one side of Y@C82 attracts the negatively charged (Z¼ 3) C82cage of the other Y@C82molecule. The Y@C82–Y@C82mole- cule can be viewed just like a Li–Li molecule but with much weaker interaction. The superatom character of such a metallofullerene might in future lead to novel solid state properties.