Home

SearchCollections

Journals

About

Contact us

My IOPscience

Probing of electronic structures of La@C82 superatoms upon clustering realized using glycine
nanocavities

This content has been downloaded from IOPscience. Please scroll down to see the full text.
2015 Appl. Phys. Express 8 125503(http://iopscience.iop.org/1882-0786/8/12/125503)
View the table of contents for this issue, or go to the journal homepage for more

Download details:
IP Address: 130.158.129.94
This content was downloaded on 01/12/2015 at 06:10

Please note that terms and conditions apply.

Applied Physics Express 8, 125503 (2015)
http://dx.doi.org/10.7567/APEX.8.125503

Probing of electronic structures of La@C82 superatoms upon clustering realizedusing glycine nanocavities
Atsushi Taninaka1, Takahiro Ochiai1,2, Ken Kanazawa1, Osamu Takeuchi1, and Hidemi Shigekawa1*
1Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
TAKANO Co., Ltd., Miyada, Nagano 399-4301, Japan
E-mail: hidemi@ims.tsukuba.ac.jp
2

Received September 29, 2015; accepted November 4, 2015; published online November 25, 2015
We have succeeded in the ﬁrst direct probe of the change in the electronic structures of La@C82 superatoms upon clustering by scanning tunnelingmicroscopy/spectroscopy (STM/STS). An array of >1.3-nm-diameter glycine nanocavities self-assembled on a Cu(111) surface was used as atemplate. Isolated La@C82 superatoms were stably observed on terraces without diffusion to step edges, which enabled us to observe the change
in the electronic structures associated with single, dimer, and clustered La@C82. A cluster with four La@C82 superatoms showed electronic
structures similar to those obtained for thin ﬁlms in previous works. © 2015 The Japan Society of Applied Physics

wing to the electron transfer from the endohedral
metal to the fullerene cage to form an atomic structure, endohedral metallofullerenes are called superatoms and have been attracting considerable attention.1–3) In
contrast to the σ and π orbitals of fullerenes, which are highly
degenerate and tightly bound to individual C atoms, superatom molecular orbitals (SAMOs) are rather loosely bound to
the screening potential of the hollow molecular cage, producing nearly free electron-like characteristics.1–4) Furthermore, endohedral metallofullerenes are promising candidates
for nanotechnological applications because of their electronic
and magnetic ﬂexibilities.1,5–9) However, the formation ofisolated structures of endohedral metallofullerenes has beendiﬃcult owing to their high reactivity, reducing the highpotential for some functions such as catalysis. In addition,
despite the importance of understanding the electronic
structures of individual endohedral metallofullerenes, studies
have generally been carried out on ﬁlms, for example, thoseformed on Si surfaces or adsorbed at the steps of metal
surfaces.7,10–13) On the other hand, glycine is known to form
an array of nanocavities of ∼1.3 nm diameter on a Cu(111)
surface by self-assembly, as shown in Fig. 1.14) Since thediameter of a fullerene is ∼1 nm, the array of nanocavities is
expected to be a desirable template for studying the electronic
structures of individual endohedral metallofullerenes fromthe fundamental and practical viewpoints.
Here, we demonstrate the ﬁrst direct probe of the change inthe electronic structures of La@C82 superatoms upon clustering by scanning tunneling microscopy=spectroscopy (STM=
STS) using glycine nanocavities self-assembled on a Cu(111)
surface.
La@C82 metallofullerenes were prepared by the DC arc
discharge method.1,7) The produced soot was extracted byultrasonication with carbon disulﬁde for 2 h, and the residueof the extraction was further reﬂuxed with pyridine for 3 h.
The La@C82 metallofullerenes were separated and isolated by
two-stage high-performance liquid chromatography (HPLC)
using a Buckyprep (ϕ20 × 250 mm2; Nacalai Tesque) and a
Buckyclutcher (ϕ21 × 500 mm2; Regis Chemical). The purityof the La@C82 sample was higher than 99.9%.1,15)
After glycine molecular deposition on a clean Cu(111)
surface, the obtained surface was annealed at 350 K for 1 h
to form an array of glycine nanocavities,14) and then La@C82
molecules were evaporated from a crucible heated at 700 K
for 5 s to the Cu surface with the array of glycine nano-

(a)

O

(b)

(c)

(d)

Fig. 1. (a) STM image of Cu(111) surface covered with glycine nanocavities formed by self-assembly (Vs = −0.15 V, It = 1.0 nA). (b) Magniﬁed
STM image of the nanocavity array. (c) Cross section along the line in (a).
(d) Schematic model of a glycine nanocavity.

cavities, which was kept at room temperature (RT). All
STM=STS measurements were carried out at ∼5 K.
Figure 2(a) shows a wide-scan STM image of the La@C82=
glycine nanocavities=Cu(111) surface. With the glycine nanocavity template, as expected, La@C82 superatoms were stably
observed on terraces without diﬀusion to the step edges, eventhough the evaporation of La@C82 was carried out at RT
without cooling. Figure 2(b) shows a magniﬁed image of thearea indicated by the square in Fig. 2(a), where the Cu surface
in each nanocavity was imaged and formed an array of circles,
similar to the image shown in Fig. 1(a). The bare Cu surface
is imaged bright because the tunneling current from the area
is higher than that above the glycine molecules for this bias

125503-1© 2015 The Japan Society of Applied Physics

Appl. Phys. Express 8, 125503 (2015)

A. Taninaka et al.

(a)

(c)

(d)

(b)

(e)

Fig. 2. (a) STM image of the La@C82=glycine nanocavities=Cu surface (Vs = −0.11 V, It = 1.0 nA). (b) Magniﬁed image of the area indicated by the blacksquare in (a) (Vs = +0.5 V, It = 1.0 nA). (c)–(e) Cross sections obtained along the lines shown in (b) labeled (c), (d), (e)-1, and (e)-2, respectively. The lines
were drawn from left (low) to right (up).

voltage,14) and the nanocavity has a structure shown in
Fig. 1(d). The circle Cu area is surrounded by six glycine
trimers. Since the diameter of each nanocavity is ∼1.3 nm(estimated from the circle image in the cavity), La@C82
with a diameter of ∼1 nm1,16) (≲ 1:3 nm) can be located in
the nanocavities. Figures 2(c)–2(e) show the cross sections
along the lines (c), (d), (e)-1, and (e)-2 in Fig. 2(b), i.e., aglycine nanocavity, two isolated La@C82 superatoms locatedin neighboring nanocavities, and two La@C82 superatoms
located close to each other. The distance of 1.59 nm between
the two La@C82 superatoms shown in Fig. 2(d) is close to theperiodicity of 1.68 nm for the nanocavities shown in Fig. 2(c).
In contrast, the distance of 0.91 nm between the two La@C82
superatoms shown in Fig. 2(e) is close to the diameter ofC82 (∼1 nm), indicating that the structure is a dimer.
Although the heights of the two La@C82 superatoms ina dimer are almost the same, the STM images of the two
dimers (e)-1 and (e)-2 show a slightly asymmetric shape, depending on the bias voltage, which is more clearly shownfor (e)-1 in Fig. 3(a) in the next section. This asymmetry
was generally observed in this experiment independent of theordering of the glycine molecules around La@C82. The eﬀectof the Cu substrate is negligible in the case of La@C82
because the additional charge transfer is small.7,11,17) La@C82
has two isomers with diﬀerent cage structures;1) however,since the sample was puriﬁed by two-stage HPLC in thiscase, this asymmetry is not due to the mixture of the two
isomers. Taking into account the cross sections shown in
Figs. 2(c) and 2(e), two bottoms of a dimer may be barely
located on a Cu surface in a nanocavity or one of them may
be placed above glycine molecules forming corrals. Although
the charge transfer is not large, its eﬀect may appear. Anotherpossible mechanism for this is the relative rotation of thetwo La@C82 superatoms in a dimer, as was observed in thecase of Ce@C82; that is, Ce@C82 with a diﬀerent adsorption geometry in a ﬁlm formed on a Cu(111) surface showed
diﬀerent characteristics, especially in terms of the relative
intensity in their spectra.18) For further understanding of thisissue, theoretical analysis including the glycine template isnecessary.
To observe the electronic structures of the La@C82 superatoms, we carried out STS. Figure 3(a) shows a magniﬁcation of the area indicated by the white square in Fig. 2(b).
The current–voltage (I–V ) curves acquired and averaged overthe squares on La@C82 labeled 1 to 6 are shown in Fig. 3(c).
The spectra obtained from the I–V curves in Fig. 3(c) are
shown in Fig. 3(e). The four spectra 1 to 4 are similar to eachother, showing that the La@C82 superatoms with the crosssection shown in Fig. 2(d) are isolated and that the interaction between them is small even at a distance of ∼0.6 nm(1.59–1 nm). The singly occupied molecular orbital (SOMO)of the La@C82 superatom, which is formed by the charge

125503-2© 2015 The Japan Society of Applied Physics

Appl. Phys. Express 8, 125503 (2015)(a)A. Taninaka et al.

(e)

(f)

(g)

(h)

(i)

(b)

(c)

(d)

Fig. 3. (a) Magniﬁed image of the area indicated by the white square in Fig. 2(b) (Vs = −0.8 V, It = 0.5 nA). (b) STM image of a La@C82 cluster(Vs = −1.5 V, It = 1.0 nA). (c) and (d) I–V curves obtained over the area indicated by the squares labeled 1–6 in (a) and 7–10 in (b). The intensity of signal 5 islower than that of 6, which may be due to the fact that the fullerene 5 in the dimer was not in contact with the Cu surface. For the measurements on 7–10, thewaiting time to start the I–V measurement after moving the STM tip to the next grid was shortened to reduce the eﬀects of thermal and mechanical drifts.
Therefore, the feedback loop gain was set at a higher value (5%), which resulted in the shift of the set point and a decrease in current intensity. This change does
not aﬀect the normalized (dI=dV)=(I=V) spectra. (e) Spectra obtained from the I–V curves labeled 1–6 in (c) and 7–10 in (d). (f) STM image of the clustershown in (b) with high contrast. (g)–(i) Cross sections along the three blue lines in (f). Labels A–F correspond to those in (f).

transfer of one electron from La to the lowest unoccupied
molecular orbital (LUMO) of fullerene,1) is not clear,probably owing to the density of states of the Cu surface
near the Fermi level. The signals around 1 V are considered
to indicate the density of states related to the hybrid orbitalsof unoccupied states of the La@C82 superatom, as previously
reported.1,10,15,17,18) The electronic structures of endohedral
metallofullerenes are very complicated. For example, as
shown in Fig. 2 in Ref. 11, Fig. 5(b) in Ref. 15, and Fig. 3
in Ref. 18, one STS peak is considered to include somedegenerate and quasi-degenerate orbitals that have spatial
distributions. Therefore, it is diﬃcult to assign the STS peaks
at higher energy regions here.
In contrast, spectra 5 and 6, measured and averaged over
the two squares on the dimer labeled 5 and 6, are diﬀerent
from those obtained for the isolated La@C82 superatoms. The
hybrid orbitals observed at ∼1 eV in spectra 1 to 4 clearlyshifted toward a higher energy in spectra 5 and 6. The
diﬀerence in the hybrid orbitals between spectra 5 and 6 is
not caused by the shift but by the changes in the intensitiesof the three peaks indicated by the three red dashed lines.
Since hybrid orbitals are complex, the origin of these peaks is

not clearly determined at present. The two peaks appeared asindicated by BS and ABS in Fig. 3(e). Taking into accountthe characteristic of the electronic structures of the superatoms, i.e., the existence of SOMOs, the two peaks BS and
ABS are considered to be assigned to the bonding andantibonding states, respectively, which were produced by thebonding of the SOMOs. The SOMO is not degenerated
and its position is similar to those observed in previous
works.1,10,15,17) The diﬀerence in intensity between the peaks
BS and ABS and that in the hybrid orbitals between spectra
5 and 6 are considered to be caused by the asymmetry of thetwo La@C82 superatoms in a dimer, which was discussed
above. To understand the spectra in more detail, theoretical
calculations are necessary. However, since La@C82 has a
SOMO, theoretical analysis with the optimization of the La
state is rather diﬃcult at present, which is left for future work.
To examine the observed diﬀerence in electronic structure
between an isolated La@C82 superatom and a dimer structure, we carried out STS measurement on the cluster shownin Fig. 3(f), in which four La@C82 superatoms are connectedto each other. As shown in Figs. 3(g)–3(i), the distance
between the two La@C82 superatoms in the cluster was

125503-3© 2015 The Japan Society of Applied Physics

Appl. Phys. Express 8, 125503 (2015)

A. Taninaka et al.

∼1 nm, which is close to that in the dimer shown in Fig. 2(e),indicating that the superatoms are bound to each other. The
I–V curves and the spectra obtained over the blue squares
in Fig. 3(b) are shown in Figs. 3(c) and 3(e), respectively.
The characteristic structures of the spectra coincide withthat previously obtained for a La@C82 ﬁlm formed on ahydrogen-terminated Si(001) surface,15) as the peaks of BS
and ABS were broadened with the formation of the cluster offour La@C82 superatoms, suggesting that the electronic
structure becomes bulk-like in a cluster of this size.
To understand the observed characteristics in more detail,it will be interesting to analyze the electronic structures oftrimers consisting of an odd number of La@C82 superatoms
in which a single spin remains. Unfortunately, all the trimerlike structures observed in this experiment had a structure inwhich three La@C82 superatoms were located in threeneighboring nanocavities. This experiment is under consideration using STM with the equipment for applying a magnetic ﬁeld to characterize the spin eﬀects. In any case, theseexperiments can be realized using a glycine nanocavity template. As shown in previous works,11–13,15,18) the electronic
structures of M@C82 (M: lanthanoid) have spatial distributions. Here, spatially averaged spectra were considered in thepresent work because we were interested in the change innonlocalized SOMO upon clustering. Taking into account therotation of molecules under STS measurement, a lower setpoint current condition used in Ref. 12 should be adopted forthe discussions including local electronic structures. A moredetailed study with theoretical analysis is expected to open
new insights into the physics of superatoms.
In conclusion, we have succeeded in the ﬁrst directprobe of the electronic structures of La@C82 superatoms
upon clustering using an array of glycine nanocavities
self-assembled on a Cu(111) surface as a template. Isolated
La@C82 superatoms were stably observed on terraces without
diﬀusion to step edges. In addition to the change in hybrid
orbitals of around 1 V, two peaks appeared near the Fermi
level after the formation of a dimer structure; such peaks
are attributed to the bonding and antibonding states of the

SOMO of La@C82 superatoms. A cluster with four La@C82
superatoms showed electronic structures similar to thoseobtained for thin ﬁlms in previous works. Further experiments with theoretical analysis on these results are expected
to open new insights into the physics of superatoms.
Acknowledgment H.S. acknowledges the support from the Japan Society
for the Promotion of Science (Grant-in-Aid for Scientiﬁc Research, 15H05734).

1) H. Shinohara, Rep. Prog. Phys. 63, 843 (2000).
2) M. Feng, Y. Shi, C. Lin, J. Zhao, F. Liu, S. Yang, and H. Petek, Phys. Rev.
B 88, 075417 (2013).
3) T. Huang, J. Zhao, M. Feng, H. Petek, S. Yang, and L. Dunsch, Phys. Rev.
B 81, 085434 (2010).
4) M. Feng, J. Zhao, and H. Petek, Science 320, 359 (2008).
5) H. Kato, Y. Kanazawa, M. Okumura, A. Taninaka, T. Yokawa, and H.
Shinohara, J. Am. Chem. Soc. 125, 4391 (2003).
6) J. Zhao, X. Huang, P. Jin, and Z. Chen, Coord. Chem. Rev. 289–290, 315(2015).
7) T. Sakurai, X. D. Wang, Q. K. Xue, Y. Hasegawa, T. Hashizume, and H.
Shinohara, Prog. Surf. Sci. 51, 263 (1996).
8) R. M. Brown, Y. Ito, J. H. Warner, A. Ardavan, H. Shinohara, G. A. D.
Briggs, and J. J. L. Morton, Phys. Rev. B 82, 033410 (2010).
9) Y. Ito, J. H. Warner, R. Brown, M. Zaka, R. Pfeiﬀer, T. Aono, N. Izumi, H.
Okimoto, J. J. L. Morton, A. Ardavan, H. Shinohara, H. Kuzmany, H.
Peterlik, and G. A. D. Briggs, Phys. Chem. Chem. Phys. 12, 1618 (2010).
10) C. Ton-That, A. G. Shard, S. Egger, A. Taninaka, H. Shinohara, and M. E.
Welland, Surf. Sci. 522, L15 (2003).
11) M. Grobis, K. H. Khoo, R. Yamachika, X. Lu, K. Nagaoka, S. G. Louie,
M. F. Crommie, H. Kato, and H. Shinohara, Phys. Rev. Lett. 94, 136802(2005).
12) M. Iwamoto, D. Ogawa, Y. Yasutake, Y. Azuma, H. Umemoto, K. Ohashi,
N. Izumi, H. Shinohara, and Y. Majima, J. Phys. Chem. C 114, 14704(2010).
13) K. D. Wang, J. Zhao, S. F. Yang, L. Chen, Q. X. Li, B. Wang, S. H. Yang,
J. L. Yang, J. G. Hou, and Q. S. Zhu, Phys. Rev. Lett. 91, 185504 (2003).
14) K. Kanazawa, A. Taninaka, H. Huang, M. Nishimura, S. Yoshida, O.
Takeuchi, and H. Shigekawa, Chem. Commun. 47, 11312 (2011).
15) A. Taninaka, K. Shino, T. Sugai, S. Heike, Y. Terada, T. Hashizume, and H.
Shinohara, Nano Lett. 3, 337 (2003).
16) E. Nishibori, M. Takata, M. Sakata, H. Tanaka, M. Hasegawa, and H.
Shinohara, Chem. Phys. Lett. 330, 497 (2000).
17) C. Ton-That, M. E. Welland, J. A. Larsson, J. C. Greer, A. G. Shard, V. R.
Dhanak, A. Taninaka, and H. Shinohara, Phys. Rev. B 71, 045419 (2005).
18) K. Muthukumar, A. Stróżecka, J. Mysliveček, A. Dybek, T. J. S. Dennis, B.
Voigtländer, and J. A. Larsson, J. Phys. Chem. C 117, 1656 (2013).

125503-4© 2015 The Japan Society of Applied Physics