Chemical Physics Letters 634 (2015) 98–100

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Ultraviolet photoelectron spectra of Sc3 C2 @C80
Hajime Yagi a,∗ , Takafumi Miyazaki a,1 , Sosuke Okita a , Takeyuki Zaima a , Haruya Okimoto b ,Noriko Izumi b , Yusuke Nakanishi b , Hisanori Shinohara b , Shojun Hino a,∗ab

Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, JapanDepartment of Chemistry, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japana r t i c l e

i n f oArticle history:
Received 11 May 2015
In ﬁnal form 3 June 2015
Available online 19 June 2015a b s t r a c t
Sc3 C2 @C80 is one of the well studied endohedral fullerenes. Recent band calculation suggests the possibility of its semimetallic behavior due to the small band gap (0.07 eV). We measured the Ultraviolet
Photoelectron Spectra (UPS) of Sc3 C2 @C80 and estimated the lower limit of the band gap to be 0.2 eV. A
simulated spectrum obtained by averaging the results of DFT calculations on two stable Sc3 C2 geometries reproduces the UPS well. In conclusion, Sc3 C2 @C80 thin ﬁlm is not a semimetal but a semiconductor.
The discrepancy between the experimental and previously calculated results might be attributed to thecoexistence of two Sc3 C2 geometries.
© 2015 Elsevier B.V. All rights reserved.

1. Introduction
Since the ﬁrst macroscopic production of La@C82 in 1991 [1],endohedral metallofullerenes have attracted much attention inboth fundamental and applied research. Intramolecular chargetransfer from encapsulated species to the carbon cage gives themunique and different characteristics from empty fullerenes. A
fullerene cage encapsulates not only pure metals but also metalclusters such as metal nitride, metal carbide, metal sulﬁde, andso on [2]. Sc3 C2 @C80 is one of such endohedral cluster fullerenes[3] and well studied because its highly symmetric Ih cage structure is expected to result in unique properties. Recently, Satoet al. reported considerably high carrier mobility (the highest
among the polycrystalline endohedral fullerene ﬁlms) and highelectrical conductivity of polycrystalline Sc3 C2 @C80 ﬁlm comparedto those of La2 @C80 and Sc3 N@C80 [4]. Their DFT calculation suggests that high conductivity of Sc3 C2 @C80 is attributed not only tothe high carrier mobility but also to its small band gap (0.07 eV), andthat Sc3 C2 @C80 can be considered as a semimetal at room temperature. To examine whether Sc3 C2 @C80 really has such a small bandgap, we measured the Ultraviolet Photoelectron Spectra (UPS). Thisletter presents the experimental information on the band gap of∗ Corresponding authors.
E-mail addresses: yagi.hajime@ehime-u.ac.jp (H. Yagi), hino@gakushikai.jp(S. Hino).
1
Present address: Faculty of Science, Okayama University, Okayama 700-8530,Japan.
http://dx.doi.org/10.1016/j.cplett.2015.06.021
0009-2614/© 2015 Elsevier B.V. All rights reserved.

Sc3 C2 @C80 , and discusses the difference between the measured UPS
and the results of DFT calculations.
2. Experimental and calculation method
Sc3 C2 @C80 was synthesized according to the literature procedures [5]. The UPS were measured by the photoelectron
spectrometer at the beamline 8B2 at the Ultraviolet Synchrotron
Orbital Radiation Facility (UVSOR), and by our laboratory equipment which consists of He-I˛ (h = 21.218 eV) discharge lamp
(combination of MB Scientiﬁc L-1 and M-1) and a SCIENTA SES100 electron energy analyzer. All the spectra are referenced to theFermi level of the substrate, which was determined by measuring
the Fermi edge of the gold deposited on a molybdenum disk. TheSc3 C2 @C80 specimen for the UPS measurements were depositedin situ onto this gold-deposited molybdenum disk by sublimationfrom a resistively heated quartz crucible.
The simulated spectra were obtained from the DFT calculation
on a single molecule using Gaussian 03 [6]. The unrestricted B3LYP
functional with the CEP-31G basis set was used for the structuraloptimization. The Kohn–Sham energy levels were calculated on
the optimized structure by using the unrestricted B3LYP functionalwith the TK/NOSeC-V-TZP basis set [7] for scandium atoms and the6-31G(d) basis set for carbon atoms.
3. Results and discussion
Figure 1 shows the UPS of Sc3 C2 @C80 measured at various incident photon energies. The spectra exhibit seven peaks denoted as

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Figure 1. The incident photon energy dependence of the UPS of Sc3 C2 @C80 .

A-G in the ﬁgure. The intensity ratios of those peaks oscillate withthe incident photon energy, which is due to the spherical shapeof the molecule and is the characteristics of the UPS of fullerenes[8–11]. Figure 2 shows the secondary electron cut-off and the onsetof the UPS of Sc3 C2 @C80 . From the secondary electron cut-off, thework function of the Sc3 C2 @C80 thin ﬁlm is estimated to be 4.7 eV.
The onset energy of the UPS (Eonset ), which is the energy separation
between the Fermi level and the valence band maximum, is 0.2 eV
for Sc3 C2 @C80 . This value is much smaller than the Eonset of La2 @C80
(0.77 eV) [12] and Sc3 N@C80 (1.1 eV) [13]. The Fermi level of the ntype semiconducting fullerene thin ﬁlm usually does not situate atthe midgap but closer to the conduction band minimum [13,14],so the band gap of Sc3 C2 @C80 , La2 @C80 and Sc3 N@C80 is probably
between Eonset and twice of the Eonset . The small onset energy of
Sc3 C2 @C80 is qualitatively consistent with the previous DFT calculation by Sato et al. [4] in which the band gap of Sc3 C2 @C80 , La2 @C80
and Sc3 N@C80 is estimated to be 0.07, 0.41 and 1.23 eV, respectively.
Although the underestimate of the band gap is one of the wellknown problems of DFT calculation itself, the difference of theexperimental and calculated band gap of Sc3 C2 @C80 may be partly
attributed to another factor; the motion of the encapsulated cluster. The thermal motion of the Sc3 C2 cluster was implied by theESR experiment [3]. Tan and Lu suggested theoretically that thegeometry of the Sc3 C2 cluster may ﬂuctuate between trifoliate andplanar structure [15], in which the C2 unit is either perpendicular
to or in the Sc3 plane, whereas Sato et al. adopted only a planar-likestructure [4]. In our DFT calculation, we obtained two optimized
structures, isomer 1 (planar) and isomer 2 (trifoliate), and their totalenergy difference is only 0.84 kcal/mol (Figure 3). Simulated spectraof isomer 1 and 2 were obtained by broadening their Kohn–Shamenergy levels with a Gaussian function ( = 0.3 eV), and they are
shown in Figure 4 with the UPS. The simulated spectra for both
structures do not reproduce the UPS well, but a spectrum obtainedby adding the simulated spectra of isomer 1 and 2 reproduces theUPS very well, especially for the width and the position of theFigure 2. (a) The secondary electron cut off and (b) the onset of the UPS of
Sc3 C2 @C80 .

Figure 3. Top and side view of the uB3LYP/CEP-31G optimized structures of
Sc3 C2 @C80 .

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oxidation state of both isomers was (Sc3 C2 )6+ @C80 6− . That is,
entrapped cluster donates 6 electrons to the C80 cage.
4. Summary
Sc3 C2 @C80 thin ﬁlm is not a semimetal as predicted by the previous band calculation, but a semiconductor with a small bandgap (more than 0.2 eV). Considering the coexistence of two different structures of Sc3 C2 @C80 reduces the discrepancy between themeasured UPS and the theoretical spectra calculated on a single
molecule. It seems important for the band calculation of Sc3 C2 @C80
to consider that it could take two stable structures.
Acknowledgements
This study was conducted as part of a Joint Research Program ofUVSOR, Institute for Molecular Science. This work was supportedby JSPS KAKENHI Grant Number 15K05477 and 24550026.
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Figure 4. Comparison between the UPS and the simulated spectra. Vertical bars
represent the Kohn–Sham energy levels of alpha and beta spin orbitals. The abscissaof the simulated spectra is shifted by 3.85 eV so that they can be easily comparedwith the UPS.

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