Open Journal of Microphysics, 2011, 1, 23-27
doi:10.4236/ojm.2011.12004 Published Online August 2011 (
Copyright © 2011 SciRes. OJM
Eelectrical Transport Properties of C59N Azafullerene
Encapsulated Double-Walled Carbon Nanotube
Y. F. Li*, T. Kaneko, R. Hatakeyama
Department of Electronic Engineering, Tohoku University, Sendai, Japan
May 18, 2011; revised June 22, 2011; accepted July 18, 2011
Electrical transport properties of double-walled carbon nanotubes (DWNTs) are modulated by encapsulating
the azafullerene C59N which is synthesized via a plasma ion-irradiation method. The encapsulation of C59N
molecules inside DWNTs has been confirmed by both transmission electron microscopy and Raman spec-
troscopy. The pristine DWNTs with outer diameter 4 - 5 nm are found to exhibit an ambipolar semiconduct-
ing behavior due to their small band gap. It is found that C60 fullerene encapsulated DWNTs exhibit a unipo-
lar p-type semiconducting behavior. By comparison, C59N encapsulated DWNTs display an n-type semi-
conducting behavior. Our findings demonstrate that C59N operates as an electron donor compared with the
acceptor behavior of C60, which is further clarified by photoelectron emission spectroscopy.
Keywords: Carbon Nanotubes, FET, Encapsulation, Azafullerene
1. Introduction
Recently, double-walled carbon nanotubes (DWNTs)
serving as nanoelectrical materials has received extensive
attentions owing to their great potential applications [1,2].
DWNTs represent a good candidate since they possess
more stable mechanical properties and thermal stability
than single-walled carbon nanotubes (SWNTs) because
of their intrinsic coaxial structure. In particular, the large
inner diameter of DWNTs makes them especially ad-
vantageous as an effective atom/molecule container.
Therefore, DWNTs are interesting as material in engi-
neering various kinds of nanoelectronic devices. How-
ever, most of previous experiments to date focus on the
empty DWNTs which initially show an ambipolar or a
p-type behavior when fabricated as the channels of field-
effect transistor (FET) devices [3-5]. The extensive re-
search using different kinds of nanotubes with controlla-
ble electronic properties to construct nanoelectronic de-
vices is extremely important for the progress in this field.
Unfortunately, the number of reports on the transport
properties of DWNTs is still limited. Moreover, there are
few systematic studies concerning the electronic proper-
ties of DWNTs modified with electron dopants [6,7].
Here, we have investigated the transport properties of
FET devices fabricated based on DWNTs which are
modulated with the C59N azafullerene for the first time.
The encapsulation of C59N in DWNTs is proven by
transmission electron microscopy and Raman spectros-
copy. Pristine DWNTs are found to show either metallic
or ambipolar semiconducting behavior owing to their
narrow bandgap. However, after the C59N encapsulation,
DWNTs can exhibit a unipolar n-type semiconducting
behavior in contrast to the p-type behavior of C60 encap-
sulated DWNTs, indicating that the electronic structure
of DWNTs is strongly modified upon the insertion of
C59N azafullerene in contrast to the case of pristine
DWNTs and C60 encapsulated DWNTs.
2. Experimental
The azafullerene C59N was synthesized by a nitrogen
plasma irradiation method [8]. A plasma was produced
by applying an RF power with a frequency of 13.56 MHz,
and nitrogen ions in the plasma were generated and ac-
celerated toward a substrate by a sheath electric field in
front of the deposited C60 fullerene. Detailed experimen-
tal conditions are given as follows: plasma density np ~
109 cm–3, electron temperature Te ~ 0.5 eV, and nitro-
gen-ion irradiation energy Ei = 10 - 40 eV. The fullerene
C60 after the plasma irradiation was dissolved in toluene
and its mixture was separated into a residue and a solu-
tion. The mass spectroscopy analysis of the formed C59N
azafullerene was performed using a laser-desorption time-
24 Y. F. LI ET AL.
of-flight mass spectrometer (LD-TOF-MS, Shimadzu
The DWNTs used in this work were fabricated by an
arc discharge method with Fe as catalyst. C59N aza-
fullerene or C60 fullerene molecules encapsulated
DWNTs were synthesized by a vapor diffusion method.
The purified DWNTs together with azafullerene or
fullerene powders were first sealed in a glass tube under
the vacuum condition ~10–5 Torr. After that, the sealed
glass tube was heated at 500˚C for 48 h to encapsulate
the C59N azafullerene or C60 fullerene in DWNTs. The
encapsulated samples were obtained after the above
process, and examined in detail by field emission trans-
mission electron microscopy (FE-TEM, Hitachi HF-2000)
operated at 200 kV and Raman Spectroscopy with a laser
wavelength of 633 nm.
The electronic transport properties of various DWNTs
are investigated by fabricating them as the channel of
FET devices. During the fabrication process, DWNT
samples are firstly dispersed by sonication in N, N-di-
methylformamide (DMF) solvent and then spincoated on
a substrate, which consists of Au electrodes on a SiO2
insulator layer. A heavily doped Si substrate is used as a
backgate, and the back-gate electrode is prepared by Al
evaporation. The fabrication process for nanotube FET
devices has been described in detail in our previous studies
[9,10]. The transport property measurements are carried
out at room temperature in a vacuum using a semicon-
ductor parameter analyzer (Agilent 4155C).
3. Results and Discussions
3.1. TEM and Raman Spectroscopy
Figure 1(a) shows the mass spectrum of synthesized
C59N, in which the peak at 722 is the most distinct, cor-
responding to the C59N azafullerene. While the peak at
720 is well known for the C60 fullerene, its peak density
is much lower than that of C59N, suggesting that C59N is
the dominant material in the sample. Such C59N mole-
cules are encapsulated into DWNTs by a vapour diffu-
sion method. Figures 1(b) and (c) give TEM images of
individual pristine DWNT and DWNT filled with the
C59N molecules. In Figure 1(b), a pristine DWNT with
inner diameter 4 nm and outer diameter 4.8 nm is clearly
observed. In contrast, Figure 1(c) shows the TEM image
of an individual DWNT filled with the C59N molecules.
Our results indicate that they have been filled in DWNTs
in the amorphous-phase state (indicated by arrows),
which is similar to the case of C60 encapsulated DWNTs
[11], but is different from the chain-like C59N observed
in SWNTs [12] because of large diameter of DWNTs.
Interestingly, the dimer form of C59N, that is (C59N)2, is
observed in the DWNT (as indicated by a circle), which
is in agreement with the original property of C59N aza-
fullerene which primarily exists in the stable form of
dimer, as illustrated in the inset of Figure 1(c). Further-
more, Raman spectra reveal a definate difference be-
tween the C59N and C60 encapsulated DWNTs, as given
in Figure 2. After the C59N encapsulation, only the in-
tensity-ratio decrease in the G/D band is found compared
with that of pristine DWNTs. In contrast, big changes are
recognized on the sample of C60-filled DWNTs. Apart
from the decrease in the G/D ratio, two distinct peaks
between D-band (1378 cm–1) and G-band (1584 cm–1)
715 720725 73
Intensity (a.u.)
Mass number (m/z)
(b) (c)
Figure 1. (a) Mass spectrum of synthesized C59N, TEM im-
ages for a pristine DWNT (b) and a C59N encapsulated
DWNT (c).
Figure 2. Raman spectra for pristine DWNTs, C59N encap-
sulated DWNTs (C59N@DWNTs) and C60 encapsulated
DWNTs (C60@DWNTs).
Copyright © 2011 SciRes. OJM
are observed. One strong peak at 1476 cm–1 corresponds
to the intermolecular Raman active frequency (tangential
mode) Ag (2) of C60 molecules, and the other weak peak
at 1437 cm–1 near the D-band can be attributed to the Hg
(7) mode of C60 molecules. A very weak peak for the Ag
(2) mode observed in C59N encapsulated DWNTs com-
pared with that observed for C60 encapsulated DWNTs
may possibly be explained in terms of their different
electronic structure.
3.2. Transport Properties of C59N Encapsulated
The electrical transports properties of DWNTs are meas-
ured based on an FET configuration, as schematically
illustrated in the inset of Figure 3. Our measurements
demonstrate that the transport properties of pristine
semiconducting DWNTs show an ambipolar behavior, as
shown in Figure 3. The characteristics of source-drain
current versus gate voltage (IDS-VG) curves indicate that
the device conducts either electrons or holes depending
on the gate bias when different source-drain voltages
(VDS) from 0 to 1 V are applied. The region on the
left-hand for VG –20 V corresponds to the p-type con-
duction and the n-type conductance is observed in the
right-hand region for VG –20 V. The current-voltage
characteristics of the device indicate that the source-drain
current increases strongly with increasing the negative
gate voltage in the p-channel and increasing the positive
gate voltage in the n-channel, respectively. Particularly,
the observed saturated conductance in the p-channel
typically appears to be two or three times larger than that
observed in the n-channel for pristine DWNT-FETs. In
contrast, unipolar n-type DWNT-FETs can be obtained
by the C59N-encapsulation, as shown in Figure 4(a),
where the characteristics of IDS-VG measured at different
VDS ranging from 0 to 0.1 V in steps of 0.02 V indicate
clearly that the FET device exhibits an excellent n-type
semiconducting behavior, and no amibipolar behavior is
found due to the strong electron-donating property of
C59N. The threshold voltage (Vth) necessary to com-
pletely deplete the nanotubes is about –20 V at VDS = 0.1
V, which is similar to the value of Vth for the n-type re-
gion in the pristine ambipolar DWNTs. To further esti-
mate the performance of the n-type FET device, the
IDS-VDS curves are measured with VDS ranging from –0.1
to 0.1 V by applying different gate voltages from –30 V
to 20 V, as shown in Figure 4(b). The conductance of
device is significantly suppressed by decreasing the gate
voltages from 20 V until the gate voltage reaches about
–40 V, which also exhibits a reproducible characteristic
for the n-type nanotube FETs, being consistent with the
result in Figure 4(a). The above result demonstrates
-40 -2002040
IDS (nA)
VDS =0 V
VDS =0.2 V
VDS =0.4 V
VDS =0.6 V
VDS =0.8 V
VDS =1.0 V
Figure 3. (a) Drain-source current versus gate voltage
(IDS-VG) characteristics for an amibipolar semiconducting
DWNT-FET measured with bias voltage (VDS) ranging from
0 to 1 V in steps of 0.2 V. The inset shows the FET configu-
evidently that there is the strong electron transfer from
C59N to the encapsulated DWNT; as a result, the electron
density of conduction band of DWNT is strongly modi-
fied. In contrast, for the C60-encapsulation, the transport
characteristic is completely opposite to that observed for
the C59N encapsulated DWNTs, and the unipolar p-type
semiconducting DWNTs are obtained, as given in Fig-
ures 4(c) and (d). The IDS-VG characteristics demonstrate
that no n-type conductance is found during the meas-
urements performed with VDS in the range of –1 ~ 1 V.
The observed Vth near –10 V at VDS = 1 V shows a clear
upshift compared with that (–20 V) observed for the
p-type region of pristine DWNTs. In addition, the IDS-VDS
curve characteristics in Figure 4(d) indicate that the
conductance of the FET device is reduced by increasing
the gate voltage from –40 to 40 V, which is opposite to
that observed in Figure 4(b) for the device based on the
C59N encapsulated DWNT. The present experiments
suggest that the C59N molecules exert a strong elec-
tron-donating effect on DWNTs compared with the elec-
tron-accepting behavior of C60.
In order to analyze electronic structures of C59N and
C60, their work functions are investigated by ultraviolet
photoemission spectroscopy (UPS), which can provide a
mechanical insight into the charge transfer process be-
tween the encapsulated azafullerene and DWNTs. Fig-
ure 5 presents the photoelectron emission spectra of C60
and C59N, in which the work function of C59N is deter-
mined to be about 5.5 eV, much smaller than that (6.1 eV)
observed for C60, suggesting evidently that the electronic
property of C59N is significantly different from that of
C60. Namely, the nitrogen-atom bonding with C atom
makes the release of electrons easier in the case of C59N
azafullerene. Therefore, by combining the electrical
transport properties of FETs and photoelectron emission
spectra, the n-type semiconducting behavior of DWNTs
Copyright © 2011 SciRes. OJM
Copyright © 2011 SciRes. OJM
-40 -2002040
IDS (nA)
VDS= 0.1 V
-0.10 -
VG = 20 V
IDS (nA)
-40 -2002040
VDS= -1 V
VG (V)
VDS= 1 V
-1.0 -
VG= -40 V
Figure 4. (a) IDS-VG characteristics for an n-type C59N encapsulated DWNT with VDS ranging from 0 to 0.1 V in steps of 0.02
V. (b) IDS-VDS curves for an n-type C59N encapsulated DWNT with VG ranging from 20 to –30 V. (c) IDS-VG characteristics for
a p-type C60 encapsulated DWNT with VDS ranging from –1 to 1 V in steps of 0.2 V. (c) IDS-VDS output characteristics for a
p-type C60 encapsulated DWNT measured with VG ranging from –40 to 40 V.
Energy (ev)
Counting Rate (cps)
: C59N
: C60
Figure 5. Photoelectron emission spectra of C59N and C60.
can be understood by the charge transfer from C59N to
DWNTs, which shifts the Fermi level towards the con-
duction band. In other words, the Fermi level of DWNTs
is strongly modified by the interaction between DWNTs
and C59N azafullerene.
4. Summary
TEM observations and Raman spectra have confirmed
that the C59N azafullerene has successfully been filled
inside DWNTs. Electrical transport measurements indi-
cate that pristine DWNTs can exhibit the amibipolar
semiconducting behavior. On the other hand, unipolar
n-type semiconducting DWNTs are significantly ob-
served after the C59N encapsulation, proving the elec-
tronic structure of DWNTs is strongly modified. Com-
pared with C60 with the electron accepting behavior, the
C59N azafullerene shows the interesting electron-donat-
ing behavior, which is confirmed by photoelectron emis-
sion spectra. The DWNTs are a promising candidate for
creating FET devices showing various properties includ-
ing p-type, n-type and ambipolar behaviors.
5. Acknowledgments
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan, and JSPS-CAS Core-
University Program on Plasma and Nuclear Fusion. We
are grateful to Professor K. Tohji and Mr. K. Motomiya
for their help in TEM observation.
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