Photon Factory Activity Report 2005 #23 Part B (2006)
Electronic Structure of Condensed Matter

11D/2004G199

Photoemission Spectra of C60 Fullerene-Peapod
Hiroyoshi ISHII*1, Hidetsugu SHIOZAWA1, Hideo KIHARA1, Naoya SASAKI1,Satoshi NAKAMURA1, Tetsuo YOSHIDA1, Yuji NAKAYAMA1, Shinichiro FUJIKI1,Yasuhiro TAKAYAMA1, Tsuneaki MIYAHARA1, Masashi NAKATAKE2,
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3
Takeshi KODAMA , Shinzo SUZUKI , Yohji ACHIBA and Hiromichi KATAURA
1Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
2HiSOR, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan3
National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8562, Japan

Experimental
The photoemission experiments were performed usingsynchrotron radiation at the beam line BL-11D of thePhoton Factory, High Energy Accelerator ResearchOrganization (KEK) at the beam line BL-1 of theHiroshima synchrotron radiation research center (HiSOR),
Hiroshima University. The inverse photoemission spectra
were measured in Tokyo Metropolitan University. SWNT
samples were prepared by the laser vaporization method[4]. The mean diameters of SWNTs in the C60 PPD-A andPPD-B samples were estimated to be 1.40 and 1.28 nm,respectively.

According to the theoretical calculation [1, 2], the t1u
level intersects with EF when the SWNT diameter islarger than 1.28 nm. As shown in the figure, however,there is no structure in binding energies ranging from theonset of the HOMO peak to EF; the t1u peak stays above EF.
These results are consistent with the experimental resultsof the potassium and electrochemical doping effect [6]. It
is concluded that the t1u level of the C60 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. Shiozawa et al.: Phys. Rev. B 73, 075406 (2006).
[4] H. Ishii et al., Nature 426, 540 (2003).
[5] J. H. Weaver et al., Phys. Rev. Lett. 66, 1741 (1991).
[6] T. Pichler et al., Phys. Rev. B 67, 125416 (2003).

NHOMO
HOMO
LUMO
( t1u )

Ei = 76 eV

PPD-A

Intensity

Introduction
Hybrid carbon material consisting of single-walledcarbon nanotubes (SWNTs) and C60 fullerenes, known as
C60 peapod (C60 PPD), has been intensively studied boththeoretically and experimentally because of its exotic
nano-scale structure. Okada et al. [1] and Otani et al. [2]performed the local density approximation calculation;
the orbital mixing between the nearly-free-electron statesof 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). In this study, we have measured the photoemission
spectra of SWNT and C60 PPD samples [3].

hν = 65 eVPPD-B

Results and Discussion
We measured the photoemission spectra of the SWNT
and C60 PPD samples at hν= 65 eV. The photoemission
spectra of the C60 PPD are, as a whole, similar to those ofthe SWNT [4]. The spectrum of the C60 fullerenes inside
the SWNT was obtained by subtracting the empty SWNT
spectrum from the C60 PPD spectrum. The spectra near EF
obtained for the PPD-A and PPD-B samples are shown inFig. 1, together with the spectra of a C60 fcc solid obtainedby Weaver [5]. The overall spectral features are very
similar to those of the C60 solid spectrum [5]. The peak atthe binding energy of 2.3 eV is derived from the highest
occupied molecular orbital (HOMO) with hu symmetry
with fivefold degeneracy. The peak is derived from thenext HOMO (NHOMO) with gg and hg symmetry with
ninefold degeneracy. The peak located at −1.1 eV can be
assigned to a LUMO level with t1u symmetry withthreefold degeneracy.

hν = 65 eVC60420-2Binding Energy (eV)WeaverEi = 19.25 eV-4

Figure1: Photoemission and inverse photoemission
spectra of the C60 fullerene in SWNT together with thespectra of a C60 solid taken from Ref. 5. The inverse
photoemission spectra were measured with tunable
photon energy mode using the incident electron beamwith the constant energy (Ei).
*ishii@comp.metro-u.ac.jp

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