Photon Factory Activity Report 2004 #22 Part B (2005)
Electronic Structure of Condensed Matter

11D/2004G199

Photoemission Spectra of Carbon Nanotube and C60 Fullerene-Peapod
Hiroyoshi ISHII*1, Hidetsugu SHIOZAWA1, Hideo KIHARA1,Naoya SASAKI1, Satoshi NAKAMURA1, Tetsuo YOSHIDA1,Yasuhiro TAKAYAMA1, Tsuneaki MIYAHARA1, Masashi NAKATAKE2,
1
1
1
3Takeshi KODAMA , Shinzo SUZUKI , Yohji ACHIBA and Hiromichi KATAURA
1
Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
2
HiSOR, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan
3National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8562, Japan
Introduction
A single-wall carbon nanotube (SWNT) encapsulating
C60 fullerenes, which is called a C60 peapod (C60 PPD), has
attracted much attention because of its exotic nano-scale
structure; many theoretical and experimental studies of
the C60 PPDs have been performed. According to theresults of the local density approximation calculation
performed by Okada et al. [1] and Otani et al. [2], theorbital mixing between the nearly-free-electron states of
SWNTs and the π orbitals of C60 fullerenes leads to theintersection of the lowest unoccupied molecular orbital(LUMO) band of the C60 fullerene with the Fermi level(EF); it is expected that the LUMO band of the C60
fullerene appears near EF in the photoemission spectrum.
In this study, we have measured the photoemission
spectra of SWNT and C60 PPD samples.
Experimental
The photoemission experiments were performed usingsynchrotron radiation at the beam line BL-11D of thePhoton Factory, High Energy Accelerator ResearchOrganization (KEK). The instrumental resolution was 50
meV. SWNT samples were prepared by the laser
vaporization method [3]. The mean diameters of SWNTs
in the samples are 1.40 nm.
Results and Discussion
Figure 1 shows the photoemission spectra of theSWNT and C60 PPD samples measured at hν = 65 eV.
The photoemission spectrum of the C60 PPD is, as a whole,similar to that of the SWNT [3].
The spectrum of the C60 fullerenes inside the SWNT
was obtained by subtracting the empty SWNT spectrum
from the C60 PPD spectrum and is shown in the figure.
The overall spectral features are very similar to those of
the C60 solid spectrum [4]. The peaks in the binding
energy region between 5 eV and EF correspond mostly tothe π band structures of an isolated C60 fullerene. Thestructures at the binding energies above 10 eV correspond
mostly to the σ band. The several peaks in the binding
energy region between 5 eV and 10 eV are mixture of theπ and σ bands. The peak at the binding energy of 2.3 eV
is derived from the highest occupied molecular orbital(HOMO) having hu symmetry with 5-fold degeneracy.

The peak at 3.6 eV is derived from the next HOMO
(NHOMO) having gg and hg symmetry with 9-folddegeneracy.
As shown in the figure, the NHOMO and HOMO peak
positions are nearly equal to the respective corresponding
peak positions in the C60 solid spectrum [4]. According to
the theoretical calculation by Okada et al. [1] and Otani etal. [2], the tu1 level of the C60 fullerene intersects with EF
when the SWNT diameter is larger than 1.3 nm. For thepresent C60 PPD sample with the SWNT mean diameter of
1.4 nm, however, there is no structure in binding energies
ranging from the onset of the HOMO peak to EF. This
result is consistent with the experimental results of thepotassium doping effect and the electron-energy-loss
spectroscopy [5]. It is concluded that the tu1 level of theC60 fullerenes inside the SWNT stays above EF even when
the SWNT mean diameter is larger than 1.4 nm.
References[1] S. Okada et al., Phys. Rev. Lett. 86, 3835 (2001).
[2] M. Otani et al., Phys. Rev. B 68, 125424 (2003).
[3] H. Ishii et al., Nature 426, 540 (2003).
[4] J. H. Weaver et al., Phys. Rev. Lett. 66, 1741 (1991).
[5] T. Pichler et al., Phys. Rev. B 67, 125416 (2003).

Figure 1: Photoemission spectra of the C60 peapod andSWNT samples, and C60 fullerenes in SWNTs.
* ishii@comp.metro-u.ac.jp

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