Article
pubs.acs.org/JPCC

Modulation of the Local Density of States of Carbon Nanotubes by
Encapsulation of Europium Nanowires As Observed by Scanning
Tunneling Microscopy and Spectroscopy
Terunobu Nakanishi,† Ryo Kitaura,† Takazumi Kawai,‡,§ Susumu Okada,§ Shoji Yoshida,∥
Osamu Takeuchi,∥ Hidemi Shigekawa,∥ and Hisanori Shinohara*,††Department of Chemistry & Institute for Advanced Research, Nagoya University, Nagoya 464-8602, Japan
Security Research Laboratory, NEC Corporation, 1753 Shimonumabe, Nakahara, Kawasaki, Kanagawa 211-8666, Japan§
Graduate School of Pure and Applied Sciences and ∥Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki
305-8571, Japan‡S Supporting Information*

ABSTRACT: Modulation of the local density of states of single-wall carbon nanotubes(SWCNTs) is induced by the encapsulation of europium nanowires (EuNWs). The
observation of these modulated density of states using scanning tunneling microscopy/
spectroscopy combined with density functional theory calculations is reported. The
electronic modulation of SWCNTs by encapsulation of EuNWs is revealed as a Fermi
level shift, band gap reduction, and the emergence of localized states in the gap. The
present results show that the electronic interaction between EuNWs and SWCNTs ismuch stronger than that previously reported for nanomaterials encapsulated in
SWCNTs.

1. INTRODUCTION
One-dimensional (1D) materials have been studied because of
their fundamental importance and interests in nanometer-scalematerials science together with potential applications in
nanoelectronic devices for charge, spin, and heat transport.
Single-wall carbon nanotubes (SWCNTs) are 1D materials thathave excellent mechanical and electronic properties1,2 forpotential applications in various ﬁelds. In particular, the band
gap of SWCNTs can be varied from 0 to 1.5 eV, depending on
the geometrical structure of the SWCNTs, and their electricalconductivity can be metallic or semiconducting according totheir chirality.3,4
SWCNTs are 1D hollow structures that enable the synthesisof materials restricted to wire structures encapsulated in the
SWCNTs. One-dimensional materials encapsulated in
SWCNTs are novel hybrid materials that should exhibit
speciﬁc low dimensional properties and structures. Various
1D structures inside carbon nanotubes (CNTs), such as
molecular arrays,5,6 long linear carbon chains,7−9 metal complex
nanowires,10−13 and metal atomic nanowires (NWs),14−16 havebeen synthesized during the past couple of decades. Metal NWs
that are normally synthesized by a direct nanoﬁlling method
originally developed in our laboratory form 1D ultrathin NWs
of 1 nm to several nanometers in diameter.16 The properties of
1D ultrathin NWs, such as their structure, optical, electrical,
and magnetic properties,15,16 are diﬀerent from those of thebulk crystals. NWs are typically mechanically fragile and© 2017 American Chemical Society

chemically reactive/unstable under ambient conditions. NWs
encapsulated in CNTs (NW@CNTs) are, by contrast, totallystable under ambient conditions and their novel properties canbe well-characterized.
Density functional theory (DFT) band structure calculations
can explain the properties of NWs from the calculated
electronic structure. DFT band structure calculations17,18 formetal NWs encapsulated in CNTs have suggested that chargetransfer from the metal NWs to the CNTs occurs and that theelectronic structure of NW@CNTs cannot be explained as asimple superposition of neutral NWs and CNTs. To understand the basic properties and the structure of NW@CNTs andto elucidate their electronic structure, it is thus important toinvestigate the interactions exerted between NWs and CNTs.
Scanning tunneling microscopy and spectroscopy (STM/
STS) is a powerful method used to obtain information on theelectronic local density of states (DOS) near the Fermi level,
which dominates the electronic properties of these materials.
This technique reveals the electronic structure of CNTs andhybrid CNTs including NW@CNTs. STM studies on C60
peapods,19 Gd@C82 peapods,20,21 and other metallofullerene
peapods22 have revealed periodic band gap modulation of
CNTs. The band gap modulation in these systems accounts for
Received: April 29, 2017
Revised: July 11, 2017
Published: July 31, 2017
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Figure 1. (a) High magniﬁcation TEM images of EuNW@CNTs. (b) EDX spectra of EuNW@CNTs. (c) STM image of EuNW@CNTs dispersedin Au(111)/mica. A bundle of EuNW@CNTs is in the place sandwiched between the blue arrows. The bias voltage was 1.01 V, and the current set
point was 30.3 pA. (d) STM image of the EuNW@CNT bundle. The black hexagonal lines of the CNT are superimposed to indicate the lattice
structure. The bias voltage was −98 mV, and the current set point was 72 pA.

atoms, the SWCNTs were heated in dry air at 823 K for 30 min
in an electric furnace to open the ends of the SWCNTs. Openended CNTs and ground europium metal powder were placedin a glass tube under an Ar atmosphere. After vacuum heat
treatment at 550 K for 1 h, the glass tube was vacuum sealed at
10−4 Pa. The glass tube was then heated at 823 K for 2 days inan electric furnace for encapsulation of Eu atoms. The metal
encapsulating CNTs were washed with diluted HCl to remove
residual metal atoms attached to the outer surface of the CNTs.
The CNTs were then dispersed in tetrahydrofuran (THF) withammonium carbonate27 by sonication for 4 h. After sonication,
the dispersion solution was deposited by drop-casting onto
Au(111) substrates, which were ﬂame annealed in air andannealed at 400 K in air prior to the deposition.
STM Measurements. The samples were introduced to an
ultrahigh vacuum (UHV) environment and degassed at 500 K
prior to STM measurements. The STM/STS measurements
were conducted using a microscope (Omicron VT-STM andUnisoku LT-STM) in constant current mode operated at 90 K
with an electrochemically etched Ni tip (UNISOKU Co., Ltd.).
A lock-in detection technique was used to acquire dI/dV
curves. WSxM software was used to process the STM images.28
Computational Details. Theoretical calculations for the
structural and electronic properties of EuNW@CNTs were also
performed. All calculations were performed based on DFT
using the Vienna ab initio simulation package (VASP),29,30
where the generalized gradient approximation (GGA) of
Perdew−Burke−Ernzerhof (PBE) for the exchange−correlation energy was used. To account for the appropriate orbital
dependence of the Coulomb and exchange interactions of thespin-polarized Eu 4f states, the GGA+U method was employed.
The Hubbard parameter for the 4f states was set to U = 6.0 eV
to reproduce the appropriate 4f energy. A plane wave basis set
was employed with a cutoﬀ energy of 500 eV. The interactions
between the ionic cores and valence electrons were included
through the projector augmented wave (PAW) method.31 A
special pseudopotential for Eu supplied with VASP wasthe orbital hybridization and local charge transfer between
fullerenes and CNTs.23,24 On the other hand, metal NW@
CNTs may induce much stronger interactions than fullerene
peapods due to the large amount of electron doping17,18,25,26
derived from the diﬀerence in the electronegativity between
metals and carbon. Therefore, the strong interaction between
NWs and CNTs is expected to cause the change in theelectronic structures of NW@CNTs such as Fermi level shift,band gap reduction, and the emergence of localized states in
the gap.
Here, the electronic nature of europium nanowires (EuNWs)
encapsulated in CNTs (EuNW@CNTs) is investigated byusing STM/STS and DFT calculations. The band gaps of
EuNW@CNTs modulate between the EuNW encapsulatedarea and the empty area, in contrast to the semiconducting
CNTs, where those are uniform throughout the entire
nanotube axes. The electronic structure is strongly modiﬁed
due to the interactions between EuNWs and CNTs. The
electronic modulation of CNTs by encapsulation of EuNWs is
identiﬁed as a Fermi level shift and band gap shrinking, as wellas the appearance of localized states in the gap. Theoretical
DFT results for EuNW@CNTs well explain the electronic
modulation of the Fermi level shift and the localized statesobserved experimentally. Strong interaction between EuNWs
and CNTs can actually occur due to the large amount ofelectron doping, and this leads to the electronic band structure
modulation.

2. EXPERIMENTAL SECTION
Sample Preparation. SWCNTs (EC1.0, Meijo Nano
Carbon Co., Ltd.) were synthesized by the enhanced directinjection pyrolytic synthesis (e-DIPS) method. The average
diameter of the SWCNTs was estimated from high-resolution
transmission electron microscopy (HRTEM) observations tobe 1.0 nm. A direct nanoﬁlling method was used to synthesize
EuNW@CNTs.15,16 Prior to the encapsulation of europium
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Figure 2. (a) dI/dV mapping of a EuNW@CNT. The horizontal and vertical axes indicate the position along the CNT and the energy, respectively,
and the color scale shows the intensity of dI/dV. Modulation amplitude of tunneling voltage is 20 mV, and lock-in frequency is 971 Hz. (b) dI/dV
intensity plots for selected vertical line proﬁles marked by the triangles in (a). The dashed lines indicate the peak position transitions in the STS
spectra. Arrows indicate the emerged peaks in the gap area. (c) Structural model of EuNW@CNT that corresponds to (a).

bundle, which does not show any change in the features due totopographic modulation. A typical dI/dV spectrum of a CNT atthe left side position of the dI/dV mapping area marked by ablack triangle in Figure 2a is plotted as the black line in Figure
2b. The dI/dV spectra of the CNT is dominated by van Hove
singularities (vHs) derived from the 1D nature of the CNT.
The black line in Figure 2b has ﬁve vHs peaks. The ﬁrst vHs
peak of the valence band that originates from the CNT isdenoted as V1, whereas those of the conduction bands aredenoted as C1, C2, and C3. In contrast, the other areas of the dI/dV mapping marked by colored triangles in Figure 2a show
various features that are not evident along the black triangle
line. The dI/dV lines indicated by the blue, green, and red
triangles in Figure 2a are shown in Figure 2b with thecorresponding colors. The modulated area of the CNT has adiﬀerent dI/dV line from that of a typical CNT, and its length isgreater than 10 nm. A model of the EuNW@CNTs illustrated
based on the acquired STS lines is shown in Figure 2c.
The dI/dV spectra for the EuNW@CNTs are signiﬁcantly
modiﬁed due to the encapsulation of EuNWs. The peak
positions of C1−C3, V1, and V2, and the localized states L1 andL2 in the dI/dV spectra in Figure 2b are summarized in Table 1.
The various Δ values designate the diﬀerences between line 1

employed. The atomic positions are relaxed with residual forces
smaller than 0.03 eV/Å using a conjugate-gradient algorithm in
20 k-points. One hundred k-points were used for the energy
calculation. Europium atoms were placed along the direction of
the CNT axis about every 0.42 nm to be commensurate withthe periodicity of the CNT and also for simplicity of thecalculation.

3. RESULTS AND DISCUSSION
Figure 1a shows a TEM image of EuNW@CNTs. It shows darkareas overlapping with the CNTs in contrast to that of empty
CNTs. Figure 1b is the energy dispersive X-ray (EDX)
spectrum of EuNW@CNTs, which shows strong peaks ofarising from Lα and Lβ edges of the Eu atom. Therefore, thedark dots in the CNTs can be assigned to Eu atoms
encapsulated in CNTs. The structure of EuNW in Figure 1a
exhibits a ladder structure due to their restricted internal spaceof CNTs.
Figure 1c shows an STM image of EuNW@CNTs dispersedin Au(111)/mica. There is a bundle of EuNW@CNTs in theplace sandwiched between the blue arrows. The terrace andstep structures of Au(111) are observed. EuNW@CNTs wasdispersed in tetrahydrofuran (THF) with ammonium carbonate.27 A small bundle of EuNW@CNTs was deposited on thesubstrate. Figure 1d shows a corresponding STM image of thebundle of EuNW@CNTs.
A schematic of the CNT lattice structure is superimposed on
the image for clarity. This image is taken from the top of a
CNT bundle. The chirality index of the bottom CNT was
estimated from the image. The chiral angle was measured to bealmost 0°, which suggests that these CNTs are of the zigzag
type.3,4 The width of the CNTs is not clear because they are ina bundle state. The STM image of the bundled EuNW@CNTs
is not signiﬁcantly diﬀerent from that of an empty CNT at the
selected bias.3,4 However, the dI/dV curves in Figure 2a show asigniﬁcant diﬀerence between the encapsulated and empty sitesof the EuNW@CNTs.
Figure 2a shows dI/dV spectra of EuNW@CNTs used toinvestigate the eﬀect of EuNW encapsulation. This is the ﬁrsttime observation of the electronic structure of EuNW@CNTs.
The position of the acquired dI/dV spectra is the top of theTable 1. Peak Positions and Peak Shifts in the STS Spectraof EuNW@CNTs

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line 1 (eV)line 2 (eV)line 3 (eV)line 4 (eV)peak C3
ΔC3

+0.89
0

+0.82
−0.07

+0.70
−0.19

+0.66
−0.23

peak C2
ΔC2

+0.48
0

+0.43
−0.05

+0.38
−0.10

+0.35
−0.13

peak C1
ΔC1

+0.12
0

+0.10
−0.02

+0.09
−0.03

+0.09
−0.03

L1
L2
peak V1
ΔV1

−0.60
0

−0.05
−0.18
−0.59
+0.01

−0.05
−0.18
−0.46
+0.14

−0.05
−0.19
−0.46
+0.14

peak V2
ΔV2

−0.91
0

−0.87
+0.04

−0.74
+0.07

−0.69
+0.22

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Figure 3. Structural model of geometrically optimized EuNW@CNT: (a) front and (b) side views. The chirality of the CNT is (13,0), the Eu−Eu
distance is 0.553 nm, and the periodicity of the unit cell is 0.427 nm. Each unit cell has two Eu atoms.

of positively charged Eu atoms and ﬁxing of the CNT axis
length, which is shorter than the observed distance.
Table 2 provides a summary of the results from the DFT
calculations, such as the binding energies per number of Eu(empty area) and lines 2−4. For example, C3 is shifted by−0.23 V (from +0.89 to +0.66 V) from line 1 to line 4, and V2
is shifted by +0.22 V (from −0.91 to −0.69 V). The STS
spectra reveal that the modiﬁcation of the band structure has
three distinctive features. First, the conduction and valencebands shift to the negative direction with respect to the STS
spectrum for the empty CNT. C1 and V1 are typicallysymmetric with respect to the Fermi energy, or C1 is slightly
closer to the Fermi level due to electron transfer to thesubstrate. C1 is positioned around +0.1 eV and V1 is positioned
around −0.6 eV with respect to the Fermi energy in line 2.
Second, the diﬀerence between C1 and V1 decreases at the
EuNW encapsulated area. C1 and V1 and higher bands are
shifted toward the Fermi level. The transitions of peaks areindicated by the dashed lines in Figure 2b. The size of the peakshifts increases in the order of line 4 to line 1. Third, thelocalized states in the gap, gap states L1 and L2 marked byarrows, appear just under the Fermi level. These states are notobserved in the empty CNT area, indicating that the localizedstates are derived from the encapsulation of EuNWs whichcause the band modulations.
To obtain further information on the origin of these band
modulations, DFT calculations were performed for the bands,
DOS, and STS spectra of EuNW@CNTs. For thesecalculations, the chirality of the CNTs was properly assumed.
The observed diameter of the CNT is 1.02 nm, and the dI/dV
map shows semiconducting features. Therefore, a (13,0)
SWCNT was selected due to its semiconducting properties,
in which the computational limitations for the translational
vector along the CNT should be small with a ﬁtted diameter tothat of EuNWs under periodic conditions. The diameter of the(13,0) CNT is 1.01 nm and the translational vector is 0.427
nm, which is almost comparable to the distance of Eu−Eu
obtained from transmission electron microscopy (TEM)
observation (0.437 nm).15 Figure 3 shows the optimized
structure of the EuNW@CNTs from DFT simulations. Two
atomic rows of Eu were employed because the observed CNT
diameter was 1.02 nm. The most probable structure of the
EuNW for this diameter CNT is a two-channel staggered
structure. The Eu−Eu distance in the unit cell of Figure 3 is0.553 nm, which is larger than that from TEM observations.
The main reason for this diﬀerence is the electrostatic repulsion

Table 2. Number of Eu Atoms per Unit Cell, Binding
Energy, Distance of Nearest Neighbor Eu and C, and Charge
Transfer from Eu Atoms to CNTs in Current and Previous
Work18
Eu2@CNT(13,0)
Eu1@CNT(10,0)18

na

Ebb (eV/n)d(Eu−C)c (nm)Qd (e)

2
1−1.024
−1.62

0.283
0.273

1.57e
1.6f

a

n is the number of Eu atoms per unit cell. bEb is the binding energy of
Eu atoms. cd(Eu−C) is the distance between nearest-neighbor Eu
atoms and C atoms. dQ is the amount of charge transfer from Eu
atoms to CNTs. eBader charge analysis was used to calculate theamount of charge transfer from EuNW to CNTs. fMulliken population
analysis was used for calculating the amount of charge transfer from
EuNW to CNTs in previous work.18

atoms, the nearest Eu−C distance, and charge transfer from Eu
atoms. Eu2@CNT contains two Eu atoms per unit cell
calculated in this work, whereas Eu1@CNT contains one Eu
atom per unit cell calculated in a past work.18 The binding
energy of Eu atoms encapsulated in a CNT can be deﬁned as
E b = (Etot − E Eu − ECNT)/n(1)where Etot represents the total energy of the EuNW@CNT, EEu
and ECNT are the total energies of an isolated EuNW andpristine CNT, respectively, and n is the number of Eu atoms
per unit cell. The binding energy per Eu atom is −1.024 eV,
which is exothermic and implies strong interaction between the
EuNW and CNT. In previous calculations, Eb was −1.62 eV for
the Eu1@(10,0) CNT system.18 The diﬀerence between these
two results is the number of Eu atoms. In the present
calculations, the unit cell contains two Eu atoms, and theelectrostatic repulsion from two Eu atoms decreases thebinding energy.
The binding energy of Eu2@CNT with an encapsulated NW
is −2.048 eV per unit cell, which is slightly larger than that for a
fullerene peapod. In previous calculations for C60 fullerene
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Figure 4. Band structures of (a) pristine (13,0) CNT, (b) EuNW, and (c) EuNW@CNT. The position of EuNW in (b) is same as that in (c). Red,
majority spin; black, minority spin.

peapods, the binding energies of C60 were −1.61 and −0.51 eV
for a (17,0) CNT24 and (10,10) CNT,23 respectively. Bothcharge transfer from the fullerene to CNT and orbital
hybridization of the lowest unoccupied molecular orbital(LUMO) state of the CNT conduction band are important
for the observed band modulation in the STS spectra. The largebinding energy in Eu2@CNT originates not only from van der
Waals interactions but also from charge transfer and orbital
hybridization.
The large charge transfer causes strong interaction between
EuNWs and CNTs. Bader charge analysis32 was used to
calculate the charge transfer from the EuNW to CNT in
EuNW@CNTs. The total charge from the EuNW that contains
two Eu atoms in the CNT is determined to be +1.57 e. In aprevious study,18 the charge transfer from Eu1@CNT(10,0) is+1.6 e per Eu atom using Mulliken population analysis. The
total amount of charge transfer is almost the same in these two
results, suggesting that the magnitude of interaction between
Eu atoms and the CNT in these two systems is of the same
degree.
Figure 4 shows a band diagram for a pristine (13,0) CNT, an
intact EuNW, and an EuNW@CNT. The Eu atom position in
the EuNW is the same as that for the structure encapsulated in
the CNT. The EuNW is metallic and the band crosses the
Fermi energy, whereas the (13,0) CNT is semiconducting witha band gap of 0.61 eV. Figure 4c shows that the bands of the
CNT are negatively shifted due to the electron transfer from
EuNW in consistency with the charge analysis results. By
comparing the band structures of the pristine CNT, intact
EuNW, and EuNW@CNT, the rigid ﬁlling approximation
remains invalid due to the interaction between the EuNW and(13,0) CNT. The entire band shift toward the negative
direction in the STS spectra should be represented in thecalculation.
To compare the observed STS spectra and band structure,and to determine what bands inﬂuence the experimental STS
spectra, the partial DOS (PDOS) plot of the EuNW@CNT
was examined. Figure 5a shows atom- and spin-resolved PDOS
plots for EuNW@CNT. The Eu 6s bands appear below theFermi level and this band is indeed hybridized, especially with
the Eu 5d and C 2p bands. The DOS peak of the Eu state has aﬁnite DOS of the C state at the same energy level. The C states
can easily be detected by the STM tip above the CNT surface.
Figure 5b shows an electron density plot of the gap states,
where the charge density corresponds to the norm of the wave
function. The red lines in Figure 5b show an electron

Figure 5. (a) Atom- and spin-resolved partial DOS (PDOS) of
EuNW@CNT from −2 to 0 eV. The positive and negative values
represent majority and minority spins, respectively. (b) Charge density
plots of the gap state at the Γ point. The gap state is around −1 eV.
The isosurface values are deﬁned in the color map. The gray circle
represents the frame of the CNT.

accumulation contour, and the blue lines show an electron
depletion area. Electron density accumulated on the carbon andeuropium atoms. The electronic states of C atoms can beprobed by the STM tip, and the results suggest that Eu
encapsulation and orbital hybridization can cause the additional
peak in the STS spectrum. The gap states in the STS spectra for
the EuNW@CNT emerge upon orbital hybridization of the Eu
6s and 5d states with the C 2p states.
The band gap reduction is qualitatively reproduced in thecalculation. The C 2p band, which is the original conduction
band before encapsulation, undergoes spin-splitting due to theorbital hybridization with the Eu 6s state. This causes the
reduction of the vHs gap. Some higher vHs states do notundergo hybridization with the Eu state, even though all of thepeaks in the experimental STS spectra, except for L1 and L2, areshifted. This suggests that the proposed model is still somewhat
diﬀerent from the actual structure and that some electroninteractions such as the screening eﬀect33 are not represented
in the calculation.

4. CONCLUSION
Low-temperature STM/STS is used to observe the electronic
modulation of CNTs induced by encapsulation of EuNWs,
such as a negative band shift, band gap reduction, and theappearance of localized states due to strong EuNW and CNT
interactions. DFT calculations well describe the electronic
modulation in the Fermi level shift, the change in the localizedstates by charge transfer from EuNWs to CNTs, and the orbital
hybridization of the Eu and C states. The orbital hybridization
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of the Eu and C states provides accessible localized states,
which can be probed by the STM tip. These observed
electronic modulation is attributed to the strong charge transfer
interactions exerted between the EuNWs and CNTs.

■

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.7b04047.
STM image of point of STS acquired area; TEM images
and EDX of EuNW@CNTs; total energy of EuNW@
CNTs as a function of number of k-points; PDOS andsimulated STS spectra of EuNW@CNT based on DFT
simulations; contour plots of total charge density of
EuNW@CNT (PDF)■

AUTHOR INFORMATION

Corresponding Author*Tel.: +81-52-789-2482. Fax: +81-52-747-6442. E-mail: noris@
nagoya-u.jp.
ORCID

Hidemi Shigekawa: 0000-0001-9550-5148
Hisanori Shinohara: 0000-0001-9400-7622
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
The present study was supported by MEXT/JSPS KAKENHI
Grant JP16H06350 and JP25107002.

■

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