Article
pubs.acs.org/JPCC

STM and STS Studies on the Density of States Modulation of Pr@C82
and Sc3C2@C80 Binary-Metallofullerene Peapods
Yuki Iijima,† Kazunori Ohashi,† Naoki Imazu,† Ryo Kitaura,† Ken Kanazawa,‡ Atsushi Taninaka,‡
Osamu Takeuchi,‡ Hidemi Shigekawa,‡ and Hisanori Shinohara*,††Department of Chemistry and Institute for Advanced Research, Nagoya University, Nagoya, Aichi 464-8602, Japan
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571, Japan‡S Supporting Information*

ABSTRACT: The density of states modulation of single-wall carbon nanotubes(SWCNTs) is induced by the encapsulation of two diﬀerent kinds ofmetallofullerenes (Pr@C82 and Sc3C2@C80). We report the observation of themodulated density of states using scanning tunneling microscopy/spectroscopy,
where the modulation is successfully reproduced by density functional theory
calculation. The metallofullerene molecules in SWCNTs strongly aﬀect the local
band structures of SWCNTs. The present ﬁndings show that the local electronic
structures of SWCNTs can be modiﬁed by not only varying the geometricstructures of SWCNTs but also the varieties of metallofullerenes encapsulated.

■

INTRODUCTION

We have so far reported that the two main factors
determining the electronic structure of peapods are theencapsulated metallofullerene and the chirality of carbon
nanotubes (CNTs). In this respect, atomic resolution scanningtunneling microscopy/spectroscopy (STM/STS) observationof the electronic local density of states (DOS) near the Fermi
level is a powerful method to investigate the electronic
structures of CNTs14,15 and peapods.7,11 The defects on the
CNT surface show localized states in the band gap of the
CNT.14,16 We found moiré patterns showing local electronic
modulations of double-wall CNTs.15 Substantial spatial band
gap modiﬁcation of SWCNTs was observed in a lowtemperature STM/STS investigation of Gd@C82 peapods.
We found that the band gap of 0.5 eV was reduced to 0.1 eV
when Gd@C82 fullerenes were tightly encapsulated in smalldiameter SWCNTs.7 This band gap reduction occurs locally at
the locations of Gd@C82 metallofullerenes encapsulated. In theprevious study, we observe a band gap reduction only for a
Gd@C82@(11, 9) SWCNT peapod. To further shed light on
the origin of the band gap modulation of SWCNTs
encapsulating various fullerenes or metallofullerenes, it isnecessary to study the electronic local band structures ofmetallofullerene peapods having similar chiralities.
In the present study, we encapsulate two diﬀerent kinds ofmetallofullerenes in the same SWCNT, the binary-metallofullerene peapods. The two diﬀerent metallofullerenes
encapsulated might induce two diﬀerent modiﬁcations in theelectronic structure of the SWCNT. STM/STS analyses of suchbinary-metallofullerene peapods thus enable us to investigate

Single-wall carbon nanotubes (SWCNTs) have been attracting
considerable interest due to their promising potential tominiaturize electronics beyond the current scale of microelectromechanical systems as a result of their inherent
nanoscale structures and unique electronic properties.1,2 In
particular, their band gap can be varied between 0 and 1.5 eV
by varying their geometrical structure, while their electricalconductivity can exhibit metallic or semiconducting behavior
depending on their chirality. To apply SWCNTs to nanoelectronic devices such as ﬁeld-eﬀect transistors3 and sensors,4
it is critical to control their electronic structures.
One of the most eﬀective ways to realize this is toencapsulate atoms or molecules such as fullerenes andendohedral metallofullerenes5 in their central hollow spaces.6−9Electron microscopy studies have revealed the one-dimensional alignment of metallofullerenes inside CNTs.6,10
Encapsulation of C60 induces a diameter-dependent variationin the band gap of SWCNTs11 so that their photoluminescence
properties vary with their diameters.12 We found that theencapsulating Gd@C82 metallofullerenes had signiﬁcantly
reduced the band gap of SWCNTs.7 In addition, the ﬁeldeﬀect transistor characteristics of SWCNTs encapsulating Gd@
C82 molecules (known as metallofullerene peapods) exhibit
ambipolar characteristic that diﬀer completely from those of theconventional empty SWCNTs.8 We also found that the band
gap modulation of SWCNTs in fullerene peapods strongly
depends on the fullerenes used.13 These studies clearly
demonstrate that the peapod formation is an eﬀective way tocontrol the electronic band structure of SWCNTs. However, todate the details of the mechanism for the change in theelectronic structure have not suﬃciently been understood.
© 2013 American Chemical Society

Received: December 29, 2012
Revised: March 14, 2013
Published: March 20, 2013
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Articlefour (2s22p2), three (3d24s), and 11 (5s25p65d6s2) valence
electrons of carbon (C), scandium (Sc), and praseodymium(Pr), respectively. The atomic positions are relaxed withresidual Hellman−Feynman forces smaller than 0.01 eV/Å byusing the conjugate gradient method incorporating one
irreducible k-point with the coordinate (0, 0, 1/4)(2π/T),
where T is the translational period.
Band structure calculations were fully performed selfconsistently using 101 irreducible k-points for SWCNTs [(0,0, n/200)(2π/T), n = 0, 1,..., 100] and 21 irreducible k-points
for metallofullerene peapods (Pr@C 82 and Sc 3 C 2 @C 80
peapods) [(0, 0, n/40)(2π/T), n = 0, 1,..., 20]. Gaussian
smearing with a parameter of σ = 0.02 eV was applied tobroaden the one-electron eigenenergies. The metallofullerenes
were found to be repeatedly placed along the CNT axis with anintermolecular distance of approximately 1 nm, which is equal
to the periodicity of the SWCNT. For STS calculations, weused partial (band decomposed) charge densities that
correspond to the local DOS.

metallofullerene-dependent DOS modulation of SWCNTs in asingle peapod without considering the chirality dependence.
Here, we report a STM/STS study on binary-metallofullerene
peapods, in which Pr@C82 and Sc3C2@C80 are encapsulated in
the exactly same SWCNT. STM/STS measurements are
performed by low-temperature STM (LT-STM) under ultrahigh vacuum (UHV) at 5 K. In addition, we performed density
functional theory (DFT) band structure calculations to analyze
the observed STM/STS results.

■

EXPERIMENTAL SECTION
Experimental Procedures. SWCNTs were synthesized by
the enhanced direct-injection pyrolytic synthesis (e-DIPS)
method.17 The SWCNT sample was puriﬁed by thermaloxidation at 773 K for 30 min in an electric furnace followed byhigh-temperature vacuum heat treatment at 1473 K for 12 h in
an infrared furnace. The average diameter of the SWCNT isestimated to be 1.6 nm with a standard deviation of 0.2 nm
from high-resolution transmission electron microscopy(HRTEM) observations. Prior to the encapsulation of
metallofullerenes in SWCNTs, the SWCNTs were heated indry air at 823 K for 30 min in an electric furnace to open the
tube ends.
Pr@C82 and Sc3C2@C80 metallofullerenes were synthesized
by DC arc discharge and were puriﬁed by multistage highperformance liquid chromatography (HPLC)5 using toluene as
the eluent. The purities of Pr@C82 and Sc3C2@C80 metallofullerenes were checked by positive and negative laser
desorption time-of-ﬂight mass spectrometry and by HPLC
analyses.
The Pr@C 82 and Sc 3 C2 @C 80 metallofullerenes wereencapsulated in SWCNTs by a gas-phase reaction.18 Isolated
Pr@C82 and Sc3C2@C80 metallofullerenes and the open-ended
SWCNTs were placed in a quartz tube and vacuum sealed to
2.0 × 10−7 Torr at 573 K for 1 h. The quartz tube was thenheated at 823 K for 3 days in the electric furnace to encapsulate
Pr@C82 and Sc3C2@C80 metallofullerenes in SWCNTs.
Au(111) substrates were purchased from Unisoku Co., Ltd..
The Au(111) substrates were ﬂame annealed in air prior todeposition. The binary-metallofullerene peapods were dispersed in 1,2-dichloroethane by sonication for more than 10 h.
After sonication, 10 drops of the dispersion solution were
deposited by drop casting to the Au(111) substrates at 573 K.
The samples were annealed at 573 K for 1 h before introducing
into an UHV-STM chamber.
The STM/STS measurements were performed at 5 K under
UHV conditions (base pressure <7.5 × 10−11 Torr) using an
Omicron UHV LT-STM system in constant-current mode
operated using an electrochemically sharpened tungsten tip.
This STM system has a thermal drift of less than 0.002 nm/s.
We used the WSxM software to process the obtained STM
images.19
Calculation Details. Theoretical calculations were performed on the structures and electronic properties of the
SWCNTs, Pr@C82 peapods, and Sc3C2@C80 peapods. All
calculations were performed based on DFT using the Vienna ab
initio simulation package (VASP).20,21 In these calculations, weused the generalized gradient approximation (GGA) and aplane-wave basis set with a cutoﬀ energy of 250 eV since this
cutoﬀ energy has been reported to provide reliable results forthe present purpose.22 The interaction between the ionic coresand the valence electrons was implemented through the
projector augmented wave (PAW) method.23 We treat all■

RESULTS AND DISCUSSIONHRTEM and EDX Characterization of Binary Metallofullerene Peapods. Figure 1a shows a HRTEM image of a

Figure 1. HRTEM images and EDX spectra of binary-metallofullerene
peapods: (Pr@C82 and Sc3C2@C80)n@SWCNTs. (a) HRTEM imageof binary-metallofullerene peapod bundles, (b) EDX spectra of binarymetallofullerene peapods in panel a, and (c) HRTEM image ofisolated binary-metallofullerene peapod. The scale bars in both TEM
images are 5 nm.

bundle of (Pr@C82 and Sc3C2@C80)n@SWCNTs peapods. It
clearly shows that the metallofullerenes are densely packed in
the SWCNTs, giving rise to the formation of a one-dimensional
crystalline array. The metallofullerenes are estimated to have aﬁlling ratio of typically over 90% from the observed HRTEM
images.
Figure 1b shows an energy-dispersive X-ray (EDX) spectrum
measured in the exactly same area shown in Figure 1a. The Lα1(5.03 keV), Lβ2 (5.85 keV), and Lγ1 (6.32 keV) emission linesof Pr are all spectrally resolved. The Lβ1 (5.49 keV) emission
line of Pr is obscured by the Kα (5.42 keV) line of chromium(Cr), which is attributed to the contamination from theultrasonication tip used for dispersion when preparing the TEM
sample. The Ll (4.45 keV) line of Pr is also indicated but it
overlaps with Kβ1 Sc emission. For Sc, lines corresponding toKα (α1 and α2 at 4.09 keV) and Kβ1 (4.46 keV) emissions arepresent as indicated in the spectrum but only the Kα doublet isspectrally resolved. A strong correlation is found between thepositions of the Pr and Sc lines of (Pr@C82 and Sc3C2@C80)n@SWCNT in Figure 1b and those of PrSc@C80.24 Therefore, we
can identify the peapods in Figure 1a to be the binary6967

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Articleand diameter, the chiral indices of the SWCNT are estimated tobe (15, 5), (16, 5), and (17, 5).
In the STM topographic images of the binary-metallofullerene peapods obtained at the bias voltages of +1.0,+1.2, and −1.2 V, no modulations due to modiﬁcation of thelocal electronic structure by the encapsulated metallofullerenes
were observed besides that for the atomic lattice of SWCNT.
Therefore, the exact locations of the encapsulated metallofullerenes cannot be determined solely from these STM
images. To observe the local variation of the electronic
structure, we performed site-dependent STS measurementson the binary-metallofullerene peapods. Since minimizing thethermal drift during measurements is crucial to obtain reliable
results, STM/STS measurements were performed with athermal drift of less than 0.002 nm/s. Figure 2c shows anintensity plot obtained from the STS spectra taken along thepeapod axis. The STM topographic imaging was performed byusing 512 × 512 pixels to image a 30 × 30 nm2 region, whereas
the STS spectra were obtained at every four pixels with eightlines along the x and y direction, respectively. The STS spectral
resolutions in the x and y directions are 0.234 and 0.469 nm,
respectively. There are 400 data points between −1.522 and+1.522 V; the bias interval is thus 0.0076 V. The ordinate in
Figure 2c indicates the sample voltage bias. The normalized
diﬀerential conductance (dI/dV)/(I/V) shown in Figure 2ccorresponds to the local DOS at each position.28
As is clearly seen in Figure 2c, two DOS features located at
6.6 and 9.4 nm along the peapod axis diﬀer distinctly fromthose at other positions. When we performed the STS
measurements at a large defect-like hole on the CNT surface,
we normally observed this kind of DOS shift. However, since
this STM topographic image does not show any evidence of
defects, we infer that these DOS shifts exhibiting similar shapes
do not originate from CNT surface defects. Moreover, theseshifted DOS are separated by 2.8 nm, which is twice as large as
the intermolecular distance (see Supporting Information).
These results thus suggest the presence of an array of the
Pr@C82 and Sc3C2@C80 metallofullerenes with an interval of
1.4 nm and the intensity diﬀerence is due to the local DOS
modulation resulting from the two diﬀerent metallofullerenes.
However, this DOS shift could not be directly observed in thecurrent study.
Figure 2d shows the representative (dI/dV)/(I/V) spectra(black and red lines) as a function of bias voltage obtained fromtwo locations (5.2 and 9.4 nm, as indicated by the two triangles
in Figure 2a) along the peapod axis. From Figures 2c and d)
and Table 1, the positions of the ﬁrst van Hove singularity(vHs) peak in the conduction band (peak 2) and the valence

metallofullerene peapods (Pr@C82 and Sc 3 C 2 @C 80 ) n @SWCNTs. Moreover, the ratio of Pr@C82 to Sc3C2@C80
metallofullerenes in SWCNTs is estimated to be 1.0:5.2
based on the intensities of Sc Kα, Pr Lα1, Pr Lβ2, and Pr Lγ1emissions in Figure 1b.
Figure 1c shows an HRTEM image of an isolated binarymetallofullerene peapod. The intermolecular distance is about
1.1 nm, which is reasonable for the van der Waals separation
generally observed in C80 and C82 fullerenes,6,25 indicating that
this intermolecular distance corresponds to the monomeric
form of C80 or C82.
STM Image and Position-Dependent STS Mapping of
Peapods. Figure 2a shows a schematic diagram of an

Figure 2. (a) Schematic diagram of a binary-metallofullerene peapod.
(b) STM topographic image (Vs = +1.0 V; It = +1.0 nA). (c) Intensity
plot obtained from the STS spectra as a function of voltage bias
(ordinate) and position along the peapod (abscissa). (d) Two
representative (dI/dV)/(I/V) spectra (black and red lines) as afunction of bias obtained from the two locations indicated by thetriangles in (a). The color scale indicates the normalized diﬀerential
conductance (dI/dV)/(I/V).

individual binary-metallofullerene peapod. The abscissas in
Figures 2a−c indicate the position along the peapod axis, where
the two kinds of metallofullerene molecules are aligned witheach other. The open and ﬁlled circles in Figure 2a denote thePr@C82 and Sc3C2@C80 metallofullerenes, respectively, as
inferred from an intensity plot obtained from the STS spectra
taken along the peapod axis (Figure 2c). STM/STS (dI/dV,
where I is the current and V is the voltage) provides biasdependent topographic images and the local DOS. Figure 2b
shows a typical STM topographic image of a binarymetallofullerene peapod, (Pr@C 82 and Sc 3 C 2 @C 80 ) n @SWCNT, obtained with a bias voltage of +1.0 V and a set
point current of +1.0 nA. This STM topographic image wasobtained by using 512 × 512 pixels to image a 25 × 25 nm2
region; thus, the STM topographic imaging has a resolution of
0.049 nm in the horizontal (x) and vertical (y) directions. As
Figure 2b clearly shows, a typical lattice image originating fromthe atomic lattice of SWCNT was observed. The chiralangle26,27 (i.e., the angle between the hexagon rows and the
CNT axis) is estimated to be 11−14° from the atomicallyresolved STM image. The SWCNT diameter is estimated to be
1.4−1.6 nm based on the apparent height and width of the
SWCNT. On the basis of these estimates for the chiral angle

Table 1. vHs Peak Positions and Shiftsa
peak 1 in C
Δ1peak 2 (1st−vHs) in C
Δ2peak 3 (1st−vHs) in V
Δ3fullerene 1 (V)fullerene 2 (V)

+0.79
+0.18
+0.17
−0.11
−0.20
−0.13

+0.61
+0.28
−0.07

a

Peaks 1 and 2 in C and peak 3 in V indicate the positions of the vHs
peak at 9.4 nm (fullerene 1) and 5.2 nm (fullerene 2) along thepeapod axis. Δ1 and Δ2 indicate the diﬀerence in the positions of thevHs peak (fullerene 1 − fullerene 2).
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Articleobtained from an X-ray diﬀraction study of M@C82 andSc3C2@C80.29 The C2 axis of the C82−C2v cage and the S10 axisof the C80−Ih cage are placed perpendicular and parallel to the
CNT axis, respectively. The top and middle rows in Figure 3
show the atomic positions of the structural models after
structural optimization.
As mentioned in the Experimental Section, we used thepartial charge densities calculated by VASP for the STS
calculations. We have to emphasize here that the STS measures
the tail of the wave function rather than the usual DOS at theatomic position.30 The charge density corresponds to the normof the wave function. The partial charge densities are calculated
at a distance of 3.3 Å above the CNT surface along the CNT
length. To simulate the actual resolution of the STM tip, they
are averaged across a 7.3 Å long horizontal line perpendicular
to the CNT axis.
The bottom row in Figure 3 shows the calculated STS
intensity plots, which provide more extensive data as a functionof energy (ordinate) and position of the tip along the CNT axis
(abscissa). In addition, we set the vacuum level to zero tocompare the vHs peak positions of SWCNT and peapod DOS.
These normalized STS intensity plots indicate the calculated
local DOS near the Fermi level of the CNT surface modiﬁed by
the encapsulating metallofullerenes. These local DOS modulations strongly depend on the metallofullerene used.
Regardless of the tip position (i.e., irrespective of whether the
tip was above or between the metallofullerenes), we observed
negative shifts in the ﬁrst-vHs peak positions of Pr@C82@(15,5) peapod in both the conduction and valence bands ascompared with those of (15, 5) SWCNT and Sc3C2@C80@(15,5) peapod in Figures 3a and b, respectively. In addition to thesecalculated DOS shifts, DOS gaps are also observed. The DOS
intensities in both the conduction and valence bands above the
Pr atom in the Pr@C82@(15, 5) peapod are weaker than thoseof the (15, 5) SWCNT and the Sc3C2@C80@(15, 5) peapod.
These results exhibit good agreement with the experimental
results shown in Figures 2c and d. We therefore conclude thatthe experimental results can be attributed to the local DOS
modulation of SWCNTs by the encapsulating two diﬀerent
metallofullerenes. On the basis of the results above, one of thetwo metallofullerenes at the two DOS features observed at 6.6
and 9.4 nm along the peapod axis diﬀer distinctly from those atother positions in Figure 2c and are thus considered to be Pr@
C82 metallofullerenes.
First-Principles Calculations (One-Dimensional DOS
Plot). Figure 4 shows the site-projected DOS of (15, 5)
SWCNT, Sc3C2@C80@(15, 5) and Pr@C82@(15, 5) peapods.
Comparison of Figure 4c with Figures 4a and b reveals that the
DOS conﬁguration of (15, 5) in Pr@C82@(15, 5) peapod is
integrally shifted by about 0.09 eV in the negative direction. For
example, the ﬁrst vHs peak positions in the conduction band
(peak 2) and valence band (peak 3) of (15, 5) in Pr@C82@(15,5) peapod shift by 0.08 V (from −4.17 to 4.25 V) and 0.11 V(from −4.63 to −4.74 V) relative to that of (15, 5) in Sc3C2@
C80@(15, 5) peapod, respectively (see Table 2). The diﬀerencein the relative peak positions of the (15, 5) SWCNT in the Pr@
C82@(15, 5) peapod and the (15, 5) SWCNT in the Sc3C2@
C80@(15, 5) peapod relative to the vacuum level has important
implications for the experimental results shown in Figures 2c
and d. Namely, these negative shifts can be interpreted as therelative reduction in the energies of the electrons in the (15, 5)
SWCNT due to the eﬀect of the local electrostatic potentialfrom the Pr@C82 metallofullerene originating from theband (peak 3) at 9.4 nm, which is estimated to be above one of
the two metallofullerenes (fullerene 1, denoted by ﬁlled
circles), are shifted by −0.11 V (from +0.28 to +0.17 V) andby −0.13 V (from −0.07 to −0.20 V) relative to that at 5.2 nm,
which is estimated to be above the other metallofullerene(fullerene 2, denoted by open circles), respectively. The
position of the vHs peak in the conduction band (peak 1) at
9.4 nm above fullerene 1, shifts by +0.18 V from 0.61 to 0.79 V
relative to that at 5.2 nm above fullerene 2. We infer that thepeak energy positions depend on the metallofullerene. In
addition to these shifts in the vHs peak positions, DOS gaps areobserved. The DOS intensity between −0.34 and −0.74 V in
the valence band at 6.6 and 9.4 nm is weaker than that at otherpositions along the peapod axis. These DOS modulations are,therefore, considered to originate from the two diﬀerent
metallofullerenes.
First-Principles Calculations (STS Two-Dimensional
Plot). To investigate the origin of the DOS energy shift
shown in Figure 2, we performed DFT calculations on the STS
intensities of SWCNT, Pr@C82 peapod, and Sc3C2@C80
peapod. As discussed above, there are three possible chiralities
for the SWCNTs in (Pr@C82 and Sc3C2@C80)n@SWCNT:(15, 5), (16, 5), and (17, 5). In the present DFT calculations,
we chose (15, 5) SWCNT due to the computational limitationthat the translational vector along the CNT axis should have asimilar size to that of metallofullerenes under periodic
boundary conditions. A (15, 5) SWCNT of 1.43 nm indiameter has the chiral angle of 13.9° and a translation vector
length of 1.56 nm. Figures 3a−c shows the calculated STS
intensity plots and the corresponding structure models of (15,5) SWCNT, Pr@C82@(15, 5) peapod, and Sc3C2@C80@(15,5) peapod, respectively. The initial atomic positions in themetallofullerenes used for structural optimization are those

Figure 3. Calculated STS intensity plots for (a) (15, 5) SWCNT, (b)Sc3C2@C80@(15, 5), and (c) Pr@C82@(15, 5) peapods. The top andmiddle rows show the structural models. (15, 5) SWCNTs are shownin gray. Fullerene C80, Sc atoms, and C atoms are shown in blue, red,and yellow, respectively. Fullerene C82 and Pr atoms are shown ingreen and red, respectively. The bottom row shows two-dimensional
map of calculated dI/dV spectra as a function of energy (ordinate) andthe position of the STM tip along the CNT (abscissa). The arrows in
the top row indicate the theoretical STM tip direction.
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Articleon the metallofullerene encapsulated. This variation is thought
to be due to the local electrostatic potential from diﬀerent
electrical charge distributions of Pr@C82 and Sc3C2@C80 in
SWCNTs.

■ASSOCIATED CONTENT

S Supporting Information*

Intensity plot obtained from the STS spectra taken along
peapod axis. Atomic positions of metallofullerenes prior to
optimization. This material is available free of charge via theInternet at http://pubs.acs.org.

■

Corresponding Author

Figure 4. Site-projected density of states (pDOS) of (a) (15, 5)
SWCNT, (b) Sc3C2@C80@(15, 5), and (c) Pr@C82@(15, 5) peapods
decomposed in their constituents. Gray regions contained in blacklines denote the pDOS of (15, 5) SWCNT. Blue, yellow, and red linesin (b) denote the pDOS of fullerene C80, C atoms, and Sc atoms,
respectively. Green and red lines in (c) denote the pDOS of fullerene
C82 and Pr atom, respectively.

Table 2. Positions and shift of vHs peaks
peak 1 (2nd−vHs) in C
Δ1peak 2 (1st−vHs) in C
Δ2peak 3 (1st−vHs) in V
Δ3peak 4 (2nd−vHs) in V
Δ4

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

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work has been supported by Grant-in-Aids for Speciﬁc
Area Research (No. 19084008) on Carbon Nanotube Nanoelectronics and for Scientiﬁc Research A (No. 19205003) from
MEXT, Japan, and the Global COE Program in Chemistry,
Nagoya University.

aempty (eV)

Sc3C2@C80 (eV)Pr@C82 (eV)

−3.89
0
−4.17
0
−4.66
−0.03
−4.91
0

−3.89

−3.97
−0.08
−4.25
−0.08
−4.74
−0.11
−5.01
−0.10

−4.17
−4.63
−4.91

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a
Peaks 1−4 indicate the positions of the second and the ﬁrst vHs
peaks in conductance band and ﬁrst and second vHs peaks in valenceband, respectively. Δ1−Δ4 indicate the diﬀerences in the positions of
the vHs peaks of (15, 5) SWCNT and Pr@C82@(15, 5) peapod −Sc3C2@C80@(15, 5) peapod.

diﬀerence in the electrical charge distributions of Pr@C82 andSc3C2@C80 metallofullerenes. As a result, the position of the
DOS is shifted, whereas the shape of the DOS does not change.
The lowest unoccupied molecular orbital (LUMO) of
Sc3C2@C80 is located in the middle of the CNT gap according
to Figure 4b. In this small bias range, we assume that this
LUMO state evades the STS tip in the present experiments,
partly because it has a low tunneling probability. The C80 stateat −4.47 eV is hardly visible because this state has a short decay
length in the theoretical STM tip direction.

■

CONCLUSION
We used LT-STM/STS to observe the electronic states
modiﬁcation of SWCNTs, which is induced by encapsulating
two diﬀerent kinds of metallofullerenes. To observe themetallofullerene-dependent DOS modulation of SWCNTs
without considering the chirality dependence, we used binarymetallofullerene peapods that encapsulate two diﬀerent metallofullerenes, the Pr@C82 and Sc3C2@C80 molecules, in an
identical SWCNT. From the experimental results and DFT
calculations, we successfully determined the positions of Pr@
C82 and Sc3C2@C80 in the SWCNTs. The STS results revealthat the DOS shifts originate from the encapsulation of the two
diﬀerent metallofullerenes. We found that the relative positionsof the conduction band and valence band peaks vary depending
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