Selected Papers

Processing and Electric Property Control of Carbon
Nanotubes by Low-energy Electron Irradiation
Satoru Suzuki† and Yoshihiro Kobayashi
Abstract
We have recently discovered that low-energy electron irradiation damages single-walled carbon nanotubes. This phenomenon can be exploited to achieve spatially selective removal of single-walled carbon nanotubes and control their electric properties over a wide range. The selective removal will makeit possible to fabricate various nanotube architectures. The electric properties of metallic nanotubes canbe converted to semiconducting, and further electron irradiation finally makes them insulating. It appears
that these irradiation-induced changes in the electric properties are similar to those that would be expected if chirality were controlled.

1. Introduction
Single-walled carbon nanotubes (SWNTs) arepromising for future nanoelectronics because of their
quasi-one-dimensional structure and excellent electric properties. Carbon nanotubes can be semiconducting or metallic, depending on their chirality (thechirality determines the nanotube structure, such as
the diameter and wrapping angle). A semiconducting
SWNT can be used as a field effect transistor (FET)with high mobility [1]. On the other hand, a metallic
SWNT can work as an interconnecting wire with highchemical stability and large current capability [2],[3].
However, some serious problems have prevented
the fabrication of nanotube-based integrated circuits.
First, there is no established way to place individual
SWNTs precisely at desired positions. Although it ispossible to control where SWNTs start to grow by
patterning a catalytic metal layer by conventionallithography, it is still impossible to control the growth
direction of individual SWNTs independently. Thus,
high-density nanotube growth almost always resultsin the growth of unnecessary SWNTs at undesired
positions, which could cause short circuits. Anotherproblem is that it is still impossible to control the chi† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan
Email: ssuzuki@will.brl.ntt.co.jp

Vol. 4 No. 11 Nov. 2006

rality of SWNTs as they grow. Thus, synthesized
SWNT samples contain both metallic and semiconducting types. A metallic SWNT does not work as a
FET. The bandgap of semiconducting SWNTs varies
widely, which causes irregularities in the FET performance.
Due to the high chemical stability of graphenesheets, it had been generally assumed that electron
irradiation does not cause any structural damage to
SWNTs when the electron energy is below thethreshold energy (86 keV [4]) for knock-on damage.
However, our recent study clearly showed that lowenergy (500 eV to 25 keV) electron irradiation causes severe damage to SWNTs [5]. In this paper, weshow that we can exploit this low-energy electron
irradiation damage to selectively remove unnecessary
SWNTs at undesired positions [6], [7] and convert
the electric properties of metallic SWNTs to semiconducting [8] or insulating [9].
2. Experimental
SWNTs were grown on SiO2/Si substrates by thethermal chemical vapor deposition (CVD) method,using methane or ethanol as a carbon source and Co
as a catalyst. SWNT-FETs were fabricated bydepositing Ti/Au electrodes on the SWNTs using
conventional lithography and lift-off techniques. The
substrate (heavily doped p-type Si with 100-nm-thickthermally oxidized SiO2) worked as a back gate.
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Selected Papers

Electron irradiation was performed at room temperature in a Hitachi S-4300 cold cathode field-emission
scanning electron microscope (SEM). Four microprobes were installed in the SEM for in-situ electricmeasurements during electron irradiation. Eachmicroprobe can be actuated by piezo-devices independently. Micro Raman spectra were obtained in air
at the excitation laser wavelength of 785 nm.
3. Spatially selective removal of SWNTs
The procedure for the spatially selective removal of
SWNTs is schematically shown in Fig. 1 [6], [7]. In
this study, suspended SWNTs were grown on a patterned substrate because that makes them easy toobserve by SEM. The acceleration voltage of theelectrons was 1 kV, which causes severe damage [5].
As shown in the figure, part of the SWNT sample wasirradiated by the electron beam for five minutes using
the line-scan mode. The scanned line length was

e–

(a)

(b)

Fig. 1. Selective removal of SWNTs. (a) Some SWNTs
are irradiated by the electron beam and damaged.
(b) Damaged SWNTs are preferentially removed
by annealing in air.

about 125 µm and the beam current was about 50 pA.
Assuming that the spot size of the electron beam was
5 nm, we estimated that the irradiation dose was 1.5
× 1019 cm–2. The irradiated positions of the SWNTs
were severely damaged by low-energy electron irradiation damage. Raman spectra of the irradiated
SWNTs were measured to identify the irradiation
damage [5]. Figure 2 shows (a) tangential mode (G)and disorder-derived (D) bands and (b) the radial
breathing mode (RBM) region of the Raman spectraof SWNTs before and after electron irradiation. The
SWNTs were irradiated by 1-keV electrons up to adose of 8 × 1017 cm–2 in the normal observation
mode. The irradiation significantly decreased the Gband intensity and this resulted in a drastic decreasein the G/D ratio, which is often used as an indicatorof the crystallinity of SWNTs. The RBM peaks
almost vanished after the irradiation, as shown in Fig.
2(b), indicating that the characteristic optical properties of the SWNTs disappeared because of the irradiation damage. Note that the irradiation dose given by
the line scan was much larger than the dose in thiscase, so the local damage caused by the line scanwould be much severer than that shown in Fig. 2. We
think that the damage is due to the formation of dangling bonds caused by C-C bond breaking, which isstabilized by the strain energy release in the sidewallof a SWNT. A detailed discussion is given elsewhere[10]-[12].
After the electron irradiation, the sample wasannealed in air at 420°C for 30 min. The annealing
preferentially eliminated the irradiated SWNTs, as
shown in Fig. 1(b), because the damage reduced theirrobustness to annealing in air [5].
A SEM image of the sample after the procedure is(b)– before– after

G

D

1200

1300

1400

1600

1500
–1

RBM

Intensity (arb. unite)

Intensity (arb. unite)(a)

Raman shift (cm )1700– before– after

100

200

300

400

–1

Raman shift (cm )Fig. 2. Raman spectra of SWNTs before and after electron irradiation: (a) G- and D-bands and(b) RBM regions. In (b), the small humps at 300 cm–1 are from the substrate.

26

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verticalhorizontal

(b)

2
1
0
–1
–2

Relative position

(a)

5
4
3
2
1
0
–1
–2
–3
–4
–5

0

2

4

6

8

10

12

Number of suspended SWNTs

Fig. 3. (a) SEM image of the SWNT sample after the selective removal procedure. The irradiation line is denoted by the dashedline. Scale bar: 2.5 µm. (b) Position dependence of the number of SWNTs suspended between neighboring pillars.

4. Electric property changes induced by electron
irradiation
4.1 Metal-semiconductor transition
The electric properties are much more sensitively
changed by low-energy electron irradiation than the
Raman spectra are. The initial gate voltage characteristics of a SWNT-FET device measured at 28 K areshown in Fig. 4(a) [8]. The gate-dependent oscillation of the conductance is due to single-electron transistor characteristics caused by a Coulomb blockade.
Vol. 4 No. 11 Nov. 2006(a)

4 × 10–4

Pristine metallic

2 × 10–4
0
G (e2/h units)

shown in Fig. 3(a) [6], [7]. The dashed line indicatesthe line along which the electron beam was scanned.
No SWNTs cross the line, although there are still
many suspended SWNTs far from the line. Thismeans that the irradiated i.e., damaged, SWNTs wereselectively removed by the annealing in air. Using
electron beam lithography, it should therefore be possible to fabricate various kinds of SWNT architectures by removing unnecessary SWNTs from a highdensity SWNT network.
We estimated the spatial resolution of the selective
removal procedure from Fig. 3(a). The position
dependence of the number of remaining SWNTs suspended vertically and horizontally between the pillars
is shown in Fig. 3(b) (diagonally suspended SWNTs
were neglected). The relative positions are indicatedin increments of the pillar pitch of 400 nm. The
results show that the effects of the irradiation almost
completely disappeared about 800 nm from the irradiation line. Such a high spatial resolution can be easily obtained because of the small spot size of the electron gun.

–0.8

–0.4

0.0

0.4

0.8

0

2

(b) After first exposure

8 × 10–5
4 × 10–5
0

–6

–4

–2

(c) After second exposure

6 × 10–6
3 × 10–6
0

–6

–4

–2

0

2

Vg (V)

Fig. 4. Gate voltage characteristics of an initially metallic
SWNT device: (a) before irradiation, (b) after the
first irradiation, and (c) after the second irradiation.

Other than the Coulomb oscillation, the device
showed no off-region in the whole gate voltage range,
indicating that the SWNT was metallic. The device
was exposed to an electron beam of 1 keV up to adose of about 1015 cm–2. Despite the very small dose,
the gate voltage characteristics after the irradiation
changed considerably, as shown in Fig. 4(b): an offregion distinctly appeared around the gate voltage of
0 V, although the conductance peaks still existed on
both the negative and positive gate voltage sides. The
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appearance of the off-region is a characteristic ofsemiconducting SWNTs. These results indicate thatthe electric properties of the SWNT device were converted from metallic to ambipolar semiconducting
simply by the low-energy electron irradiation. Further irradiation resulted in an increase of the offregion, as shown in Fig. 4(c). In general, the width ofthe off-region reflects the bandgap of SWNTs. The
device characteristics changed as if second irradiation had caused an increase in the bandgap. In appearance, the irradiation-induced bandgap would be tunable. What is important here is that the metal-semiconductor transition and the extension of the offregion caused by the low-energy electron irradiation
are phenomena that would be expected if the chirality of the SWNT were changed.
Here, we discuss a possible mechanism for the
metal-semiconductor transition [13]. The tempera-

2 × 10–3
1 × 10–3

8 × 10

–4

(a)
295 K

0
–4

–2

0

2

0

2

0

2

0

2

140 K

4 × 10–4
0

G (e2/h units)

2 × 10

–5

–4

–2
60 K

1 × 10–5
0
8 × 10–6

–4

–2
50 K

4 × 10–6
0
1 × 10–5–4–2

ture dependence of the device characteristics after thesecond irradiation is shown in Fig. 5 [8]. The offregion in the gate voltage characteristics was distinctly observed below 60 K, but not at room temperature(295 K). This means that electrons encountered anenergy barrier that they could overcome at room temperature, but not at low temperatures. From an Arrhenius plot of the conductivity at around a gate voltageof 0 V (off-region), the typical barrier height wasdetermined to be 50-60 meV. It has been shown that adefect acts as a transport barrier in SWNT-FETs
regardless of the defect formation method, such asion irradiation [14], chemical processing [15], or
atomic force microscopy (AFM) manipulation [16].
The origin of the energy barrier in our case seems to
be defects caused by low-energy electron irradiation.
The device characteristics in the off-region in Fig. 5
can be explained by the barrier formation. The nominal bandgap increases because the barrier heightincreases with increasing irradiation dose. The nextquestion [13] is: why does the device turn on at larger gate voltages? We think this is due to gate-voltageinduced band bending (dE) in metallic SWNTs. The
gate-induced charge (Q) is calculated by Q = C·VG,
where C is the gate capacitance and VG is the gate
voltage. C is given by C = 2πεεo/ln(2h/r), where ε isthe relative dielectric constant of gate insulator material, εo is the dielectric constant of vacuum, h is the
thickness of the gate insulator, and r is the radius of
the SWNT channel. The gate-induced electrons(holes) occupy the unoccupied (occupied) electronic
states of the SWNT. The density of states near the
Fermi level of a metallic SWNT is very low (0.02 to
0.04 states/eV/atom), and this results in a considerable amount of band bending. The estimated value ofdE per VG approaches 40 to 50 meV. Due to the band
bending, the effective barrier height is reduced, as
schematically shown in Fig. 6. The typical height of

30 K
VG = 0

5 × 10–6
0
8 × 10–6
4 × 10

–4

–2

0

D

2

Barrier

EF

S

20 K
VG < 0–6

VG > 0
e–

0

D
–4

–2

0

2

Vg (V)

Fig. 5. Gate voltage characteristics of the SWNT device
obtained at various temperatures after the second
irradiation.

28

S
h+

D

S

Band bending

Fig. 6. Schematic model of the mechanism for the metalto-semiconductor transition induced by low-energy
electron irradiation.

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10–6
D
e–

SEM observation
10–7

V
_

I D (A)G
SLine scan
10–8

A
V
_

5µm

10–9
Stop

10–10
10–11

Fig. 7. SEM image of a device and a schematic of in-situ
electrical measurements during electron
irradiation. Neighboring electrodes were used asdrain (D) and source (S) electrodes.

10–12
0

50

100

150

Time (s)the barrier formed by the irradiation-induced defects
is 50 to 60 meV, as mentioned above. Thus, at a gate
voltage of several volts, the carriers could go throughthe channel without being blocked by the barrier.
4.2 Making SWNTs insulating
The effects of intensive electron irradiation were
measured by in-situ electric measurements during
electron irradiation in the SEM [9]. A typical SWNTFET structure and a schematic of the measurement
are shown in Fig. 7. Two microprobes were attachedto pads connected to the drain and source electrodes.
Then, the device was exposed to an electron beam
while the drain-source current (ID) was monitored at
a constant drain-source voltage (VD) and VG. In thiscase, electron kinetic energy was set to 20 keV.
The dependence of the drain-source current (ID) of
a metallic SWNT device on irradiation time is shownin Fig. 8. The drain and back-gate voltages were 0.1
and 0 V, respectively. The entire device was first irradiated while the SEM was in the normal observation
mode. The irradiated area was about 26 × 16 µm2(magnification of 5000), and the beam current was
290 pA. The resulting dose was estimated to be about
3.7 × 1014 cm–2·s–1. The current drastically decreased
just after the irradiation started. Note that the electronenergy of 20 keV used in this measurement is muchless destructive than lower energies, such as 1 keV[5]. At 10 s, part of the SWNT was intensively irradiated using the line scan mode. The dose rate was estimated to be about 1.4 × 1018 cm–2·s–1. The current
rapidly decreased and stayed almost constant at 10–11
A. After the line scan was stopped at 140 s, current of
only 10–12 A was observed. That is, the two-probe
conductivity (ID/VD) decreased by about six orders ofmagnitude from that of the initial state. A similar
conductivity decrease was also observed for a semiVol. 4 No. 11 Nov. 2006

Fig. 8. Time evolution of I D of a metallic SWNT device
during electron irradiation.

conducting SWNT in the on state [9]. These results
indicate that intensive electron irradiation makes
metallic and semiconducting SWNTs almost insulating.
5. Conclusions
Low-energy electron irradiation damage can beused to remove unnecessary SWNTs with high spatial resolution to control the electric properties of
SWNTs. The spatially selective removal of SWNTs
from a high density random SWNT network will
make it possible to fabricate various SWNT architectures. The electric properties of SWNTs can be controlled over a wide range, from metallic throughsemiconducting to almost insulating, simply by electron irradiation. The irradiation effects on the electric
properties are similar to those expected if the chirality of a SWNT were controlled.
Acknowledgments
Part of this work was done by Dr. Aravind Vijayaraghavan, a former trainee at NTT from Rensselaer
Polytechnic Institute, New York, USA, and by Dr.
Kenichi Kanzaki, a former group member. We would
like to thank Dr. Hiroshi Inokawa, the former Leader ofNanodevices Research Group at NTT, for valuable discussions. This work was partly supported by CREST
of the Japan Science and Technology Agency (JST).
References[1]

A. Javey, J. Guo, D. B. Farmer, Q. Wang, E. Yenilmez, R. G. Gordon,29

Selected Papers

M. Lundstrom, and H. Dai, “Self-Aligned Ballistic Molecular Transistors and Electrically Parallel Nanotube Arrays,” Nano Lett., Vol. 4,
No. 7, pp. 1319-1322, 2004.
[2] Y. Homma, Y. Kobayashi, T. Ogino, and T. Yamashita, “Growth ofsuspended carbon nanotube network on 100-nm scale Si pillars,”
Appl. Phys. Lett., Vol. 81, No. 12, pp. 2261-2263, 2002.
[3] Y. Homma, Y. Kobayashi, and T. Ogino, “Growth and Architectureof Single-walled Carbon Nanotubes on Patterned Silicon Substrates,”
NTT Technical Review, Vol. 2, No. 2, pp. 28-35, 2004.
[4] B. W. Smith, and D. L. Luzzi, “Electron irradiation effects in single
wall carbon nanotubes,” J. Appl. Phys., Vol. 90, No. 7, pp. 35093515, 2001.
[5] S. Suzuki, K. Kanzaki, Y. Homma, and S. Fukuba, “Low-acceleration-voltage electron irradiation damage in single-walled carbon nanotubes,” Jpn. J. Appl. Phys., Vol. 43, No. 8B, pp. L1118-L1120, 2004.
[6] S. Suzuki, D. Takagi, Y. Homma, and Y. Kobayashi, “Selective
removal of carbon nanotubes utilizing low-acceleration-voltage electron irradiation damage,” Jpn. J. Appl. Phys., Vol. 44, No. 4, pp.
L133-L135, 2005.
[7] S. Suzuki, S. Fukuba, K. Kanzaki, Y. Homma, and Y. Kobayashi,“Spatially selective removal of carbon nanotubes for fabricating nanotube circuits,” Proc. 5th IEEE Conf. Nanotechnol., WE-A4-2,
Nagoya, Japan, July 2005.
[8] A. Vijayaraghavan, K. Kanzaki, S. Suzuki, Y. Kobayashi, H.
Inokawa, Y. Ono, S. Kar, and P. M. Ajayan, “Metal-semiconductortransition in single-walled carbon nanotubes induced by low-energy
electron irradiation,” Nano Lett., Vol. 5, No. 8, pp. 1575-1579, 2005.
[9] S. Suzuki and Y. Kobayashi, “Conductivity decrease in carbon nanotubes caused by low-acceleration-voltage electron irradiation,” Jpn.
J. Appl. Phys., Vol. 44, No. 49, pp. L1498-L1501, 2005.
[10] S. Suzuki and Y. Kobayashi, “Diameter dependence of formation andhealing of defects in single-walled carbon nanotubes induced by lowenergy electrons and photons,” Seventh Int. Conf. Sci. Appl. Nanotubes, E052, Nagano, Japan, June 2006.
[11] S. Suzuki and Y. Kobayashi, “Diameter dependence of low-energy
electron and photon irradiation damage in single-walled carbon nanotubes,” Chem. Phys., Lett., Vol. 430, No. 4-6, pp. 370-374, 2006.
[12] S. Suzuki and Y. Kobayashi, “Low-energy irradiation damage in carbon nanotubes,” The Papers of Technical Meeting on Electronic
Materials, IEE Japan, EFM-06-13 (in Japanese).
[13] S. Suzuki, K. Kanzaki, Y. Ono, H. Inokawa, A. Vijayaraghavan, andY. Kobayashi, “Mechanism of metal-semiconductor transition in theelectric properties of single-walled carbon nanotubes induced by lowenergy electron irradiation,” Seventh Int. Conf. Sci. Appl. Nanotubes,
G007, Nagano, Japan, June 2006.
[14] T. Kamimura, K. Yamamoto, and K. Matsumoto, “Effects of ultra low
energy nitrogen ion irradiation on carbon nanotube channel singleelectron transistor,” Jpn. J. Appl. Phys., Vol. 43, No. 5A, pp. 27712773, 2004.
[15] T. Kamimura, M. Maeda, K. Sasamoto, and K. Matsumoto, “Roomtemperature single-hole transistors made using semiconductor carbon
nanotube with artificial defects near carrier depletion region,” Jpn. J.
Appl. Phys., Vol. 44, No. 1A, pp. 461-464, 2005.
[16] Y. Gotoh, K. Matsumoto, and T. Maeda, “Room temperature
Coulomb diamond characteristic of single electron transistor made by
AFM nano-oxidation process,” Jpn. J. Appl. Phys., Vol. 41, No. 4B,
pp. 2578-2582, 2002.

30

Satoru Suzuki
Senior Research Scientist, NTT Basic
Research Laboratories
He received the B.S. and M.S. degrees inphysics and the Ph.D. degree in chemistry fromTohoku University, Miyagi, in 1990, 1992, and1999, respectively. He joined NTT Interdisciplinary Research Laboratories in 1992. In 1994, hestarted researching structural properties of dopedcarbon nanotubes. He moved to NTT Basic
Research Laboratories in 1998. Since then, he
has been researching the electronic structure andphysical properties of carbon nanotubes. His current interest is the control of the physical properties of carbon nanotubes. He is a member of the
Physical Society of Japan (JPS), the Japan Society of Applied Physics (JSAP), the Fullerenes
and Nanotubes Research Society, and the Japanese Society for Synchrotron Radiation Research.

Yoshihiro Kobayashi
Group Leader, Materials Science Laboratory,
NTT Basic Research Laboratories.
He received the B.S. degree in chemistry andthe M.S. and Ph.D. degrees in applied chemistryfrom Waseda University, Tokyo, in 1983, 1985,
and 1994, respectively. He joined NTT
Musashino Electrical Communication Laboratories in 1985. Since then he has been researching
reaction control on semiconductor surfaces andthe fabrication of ordered nanomaterials. His current interest is the functionalization of carbon
nanotubes and other semiconductor nanostructures. He is a member of JPS, JSAP, and the Surface Science Society of Japan.

NTT Technical Review