Oxygen and ozone oxidation-enhanced field emission of carbon nanotubes

Article abstract of DOI:10.1063/1.1485315

Vertically aligned carbon nanotube (CNT) arrays were grown on p-type silicon wafer using acetylene and iron phthalocyanine as the sources of hydrocarbons and catalysts, respectively. The CNT arrays were treated by chemical reagents, such as oxygen (O₂), ozone (O₃), bromine, and acids. When treated by O₂ and O₃, the emission current of the CNT array was increased ～800% along with a decrease of the onset field emission voltage from 0.8 to 0.6 V/μm. Other chemical treatments, e.g., bromination and acid oxidation, lead to poorer field emission performance. The effects of these chemical processes on the field emission properties of CNT arrays will be discussed.

Full text of DOI:10.1063/1.1485315

Oxygen and ozone oxidation-enhanced field emission of carbon nanotubes
Sheng-Chin Kung, Kuo Chu Hwang, and I. Nan Lin
Citation: Applied Physics Letters 80, 4819 (2002); doi: 10.1063/1.1485315
View online: http://dx.doi.org/10.1063/1.1485315
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APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 25
24 JUNE 2002
Oxygen and ozone oxidation-enhanced field emission of carbon nanotubes
Sheng-Chin Kung and Kuo Chu Hwanga)
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan
I. Nan Lin
Materials Science Center, National Tsing Hua University, Hsinchu, Taiwan
͑
Received 27 December 2001; accepted for publication 20 April 2002͒
Vertically aligned carbon nanotube ͑CNT͒ arrays were grown on p-type silicon wafer using
acetylene and iron phthalocyanine as the sources of hydrocarbons and catalysts, respectively. The
CNT arrays were treated by chemical reagents, such as oxygen (O ), ozone (O ), bromine, and
2
3
acids. When treated by O and O , the emission current of the CNT array was increased ϳ800%
2
3
along with a decrease of the onset field emission voltage from 0.8 to 0.6 V/␮m. Other chemical
treatments, e.g., bromination and acid oxidation, lead to poorer field emission performance. The
effects of these chemical processes on the field emission properties of CNT arrays will be discussed.
©
2002 American Institute of Physics. ͓DOI: 10.1063/1.1485315͔
Since the first report of using carbon nanotubes ͑CNTs͒
as field emitters, the study of CNT field emission has be-
in the literature.7 Briefly, 0.1 gram iron phthalocyanine
͑FePc͒ and a p-type Si wafer were put into a tube furnace at
850 °C under gas flow ͑5% acetylene in Ar, flow rate: 10
sccm͒ at 1 atm for 30–60 min. Thermal pyrolysis of FePc
provides nanosized Fe particles for catalytic decomposition
of acetylene, which leads to growth of the vertically aligned
CNT array. Scanning electron microscope ͑SEM͒ images of
the CNT array show that the CNTs are quite perpendicular to
1
come one of the major areas of research for CNT
2
–11
nanomaterials.
Field emission properties of CNT films
prepared from both screen printing of CNT slurry ͑i.e., the
postgrowth method͒2 and in situ growth of CNT arrays
from chemical vapor deposition ͑CVD͒ methods have been
–4
5
–13
reported.
Although the screen-printing method has the
2
2
advantages of low cost and ease of preparation, it suffers
from the drawback of few vertically aligned CNTs and thus
low current density as well as a high onset emission
the substrate surface ͑see supplementary Fig. S1 ͒. Trans-
mission electron microscope ͑TEM͒ measurements show that
the CNTs are multiwalled and have 40–50 layers. Raman
2
–4
Ϫ1
voltages.
To have good field emission behavior ͑i.e., low
measurement shows two bands at 1345 and 1576 cm ͑see
2
2
onset of field emission voltages and large emission currents͒,
the emitters should have small, sharp tips with orientation
perpendicular to the cathode surface. In the literature, prepa-
ration of vertically aligned CNT arrays via CVD was
achieved by growing CNTs out of the nanochannels of a
mesoporous silica substrate5 using NH3 plasma treated
supplementary Fig. S2 ͒, which are characteristic bands of
CNTs grown by the CVD method.13
The field emission property of the CNT array was mea-
sured using ITO glass as the anode and a glass plate of 200
␮m thickness as the spacer to separate the CNT cathode from
the ITO anode. The voltage applied was controlled by a Kei-
thley electrometer ͑model 237͒, and the field emission cur-
rent was measured by the same electrometer. The whole
6
nickel thin film as the catalyst, or using iron phthalocyanine
7
as the source of both the hydrocarbon and metal catalysts. In
Ϫ6
the first method, all CNTs were forced to grow along the
direction of the nanochannels. For the latter two processes,
the mechanisms for the growth of well-oriented CNT arrays
are not clear, and are generally attributed to the overcrowd-
ing effect, i.e., the sterical hindrance and van der Waals at-
tractions among growing CNTs. The onset emission voltages
of vertically aligned CNT arrays from the CVD process were
setup was in a vacuum chamber of 10 Torr. As shown in
Fig. 1 ͑open circle͒, at an applied voltage of 4.5 V/␮m, the
2
current density reaches a value of ϳ10 ␮A/cm . According
8
–11
reported to be in the range of 4–0.9 V/␮m.
Recently, Lee
1
2
and co-workers grew CNT arrays on iron/silica substrates.
The CNT film was then peeled off and reversed to allow the
bottom side ͑with open CNT tips͒ to face upward. With open
ended CNTs, a very low turn-on voltage of 0.6–1.0 V/␮m
was observed.
In this study, we grew CNT arrays by the CVD method
and subsequently treated the vertically aligned CNT arrays
with various chemical reagents, such as O and O , bromine
2
3
and acids. Growth of a vertically aligned CNT array on a
p-type silicon wafer was conducted similarly to a procedure
FIG. 1. Current–voltage plots of CNT arrays under different conditions: an
as-grown CNT array ͑open circle͒ and the authentic CNT array treated by
O2 oxidation at 400 °C for 10 ͑closed triangle͒, 20 ͑open triangle͒, and 25
min ͑star͒. The inset is the ln(J/E ) vs 1/E plots of the four CNT arrays
shown, where J is the current density and E is the voltage applied.
2
a͒Electronic mail: kchwang@mx.nthu.edu.tw
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On: Thu, 27 Nov 2014 05:07:02
0

4820
Appl. Phys. Lett., Vol. 80, No. 25, 24 June 2002
Kung, Hwang, and Lin
to Fowler–Nordheim field emission theory, the plot of
2
ln(J/E ) vs 1/E ͑where J is the emission current density and
E is the voltage applied͒ will have a linear relationship. The
2
Fowler–Nordheim ͑FN͒ ͓ln(J/E ) vs 1/E͔ plot of the device
͑see the inset in Fig. 1, closed triangle͒ shows a quite linear
pattern with the onset of electron emission voltage ͑the turn-
ing point, shown by the arrow͒ of 0.8 V/␮m, indicating that
the field emission is intrinsically driven by the electric field.
To modify the electronic properties, CNT arrays grown
on a p-Si wafer under the same conditions were treated with
various oxidants, such as O , O , Br , and acids. In the case
2
3
2
of O oxidation, the CNT array on the p-Si substrate was put
2
in an oven at 400 °C for 10, 20, and 25 min in the presence
of air (O ). As shown in Fig. 1, the field emission of the
2
CNT array with 10 min O treatment ͑closed triangle͒ is
2
about the same as that of the original sample. However, the
samples with 20 ͑open triangle͒ and 25 min ͑star͒ O oxida-
2
tion have much larger emission currents at the same applied
voltages. At an applied voltage of 4.5 V/␮m, the emission
2
current was enhanced from 9 to 72 ␮A/cm ͑or an eightfold
increase͒ after 20 min O treatment at 400 °C. Meanwhile,
2
the onset field emission voltage ͑see the turning point in the
FN plot͒ changes from 0.8 to 0.6 V/␮m. TEM shows that the
end tips of many CNTs were opened or partially opened after
the O oxidation process ͑see Fig. 2͒. Evidently, the increase
2
in the field emission current and the decrease in the onset
field emission voltage observed in Fig. 1 are due to opening
of the CNT end tip by the O oxidation process. As reported
2
1
12
before by Smalley et al., by Lee and co-workers and by
1
4
Wang et al., open-ended CNTs have sharper tips, and thus
lower onset field emission voltage as well as higher emission
current. The emission current is not only determined by the
sharpness of the emitter tips, it is also affected by the con-
ductivity of the emitters. In the array format, all highly
strained CNT tips are exposed to air and are readily oxidized
by O first before oxidation of the less strained CNT walls,
2
which are partially protected from being oxidized by the ar-
ray configuration. However, prolonged O2 oxidation will
lead to damage along the CNT walls, and thus a gradual
decrease in emission current. Selective and preferential oxi-
dation at the CNT tips is the feature of the current process.
Previously, it was reported15 that open-ended multiwalled
nanotubes ͑MWNTs͒ have much poorer field emission prop-
erties than close-ended MWNTs, when the open-ended
FIG. 2. TEM images of CNTs from a CNT array which was treated by O2
oxidation for 20 min at 400 °C.
shown in Fig. 3, the sample with 3 min O treatment at room
3
temperature has an ϳ7.5-fold increase in emission current
together with a decrease in onset emission voltage from 0.8
to 0.6 V/␮m. Prolonged ͑5, 7, or 9 min͒ O treatment leads
3
MWNTs were prepared by O oxidation of carbon soot at
2
high temperatures. Under the carbon soot-O oxidation con-
2
dition, most of the CNT tips were buried within the powder,
and a lot of CNT walls were exposed to O oxidation. Only
2
a small percent of open-ended MWNTs was left after the
oxidation process. Serious oxidative damage along the tube
walls of the surviving MWNTs is most likely responsible for
the reported poor field emission performance.15 The FN plots
of O treated samples ͑see the inset in Fig. 1͒ show a linear
2
slope at the low applied voltage region and a smaller slope at
the high voltage region. This type of FN pattern for MWNT
field emitters was observed before, and the smaller FN slope
at high voltage region is attributed to the saturation effect of
1
5
FIG. 3. Current–voltage plots of CNT arrays treated under different condi-
tions: an as-grown CNT array ͑open circle͒ and CNT arrays treated by O3
oxidation at room temperature for 1 ͑open inverted triangle͒, 3 ͑open
the emitters.
Besides O oxidation, ozone is also known to react with
2
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1
6
olefins and lead to cleavage of CvC double bonds. As
square͒, 5 ͑open triangle͒, 7 ͑star͒, 9 ͑ϩ͒ min, and by Br gas ͑ϫ͒.
2
On: Thu, 27 Nov 2014 05:07:02

Appl. Phys. Lett., Vol. 80, No. 25, 24 June 2002
Kung, Hwang, and Lin
4821
to a decrease in emission current from the maximum ͑at 3
min͒, which, however, is still significantly better than that of
the authentic sample. It was reported that CNT end tips be-
came sharp when treated with ozone.17 Due to the array con-
The authors are grateful to Dr. K. H. Chen ͑Atomic and
Molecular Institute, Academia Sinica͒ for the Raman mea-
surements and to the National Science Council, Taiwan, for
financial support ͑grant number NSC-90-2119-M-002-014͒.
figuration, O preferentially attacks the exposed CNT tips,
3
similar to in the O case. Prolonged ͑5, 7, and 9 min͒ treat-
2
ment of O most probably leads to oxidative damage along
3
the CNT walls and thus a decrease in emission current. It
1
A. G. Rinzler, J. H. Hafner, P. Nikolaev, L. Lou, S. G. Kim, D. Tomanek,
P. Nordlander, D. T. Colbert, and R. E. Smalley, Science 269, 1550 ͑1995͒;
W. A. de Heer, A. Chatelain, and D. Ugarte, ibid. 270, 1179 ͑1995͒.
D. S. Chung, W. B. Choi, J. H. Kang, H. Y. Kim, I. T. Han, Y. S. Park, Y.
H. Lee, N. S. Lee, J. E. Jung, and J. M. Kim, J. Vac. Sci. Technol. B 18,
1054 ͑2000͒.
should be noticed that the reaction condition of O is much
3
milder than the O2 condition ͑room temperature versus
2
4
00 °C; vs 20 min͒, which is due to the higher chemical
1
8
reactivity of O to olefins than O .
3
2
3
In addition to the O and O treatments, the effect of Br
W. B. Choi, Y. H. Lee, N. S. Lee, J. H. Kang, S. H. Park, H. Y. Kim, D. S.
Chung, S. M. Lee, S. Y. Chung, and J. M. Kim, Jpn. J. Appl. Phys., Part 1
2
3
2
treatment was also studied ͑see ϫ in Fig. 3͒. Treatment of
39, 2560 ͑2000͒.
Br gas at room temperature for 20 min leads to only slight
2
4
5
J. L. Kwo, M. Yokoyama, W. C. Wang, F. Y. Chuang, and I. N. Lin,
Diamond Relat. Mater. 9, 1270 ͑2000͒.
W. Z. Li, S. S. Xie, X. L. Qian, B. H. Chang, B. S. Zou, W. Y. Zhou, R. A.
Zhao, and G. Wang, Science 274, 1701 ͑1996͒; Z. W. Pan, S. S. Xie, B. H.
Chang, L. F. Sun, W. Y. Zhou, and G. Wang, Chem. Phys. Lett. 299, 97
changes in both the emission current and turn-on voltage ͑see
ϫ in Fig. 3͒. Smalley and co-workers19 reported that expos-
ing single-walled nanotube ͑SWNT͒ bucky paper to Br2
leads to an increase in the conductivity of the SWNT paper.
͑
1999͒.
It was rationalized that Br serves as an electron acceptor
6
7
2
Z. F. Ren, Z. P. Huang, J. W. Xu, J. H. Wang, P. Bush, M. P. Siegal, and
P. N. Provencio, Science 282, 1105 ͑1998͒.
Y. Yang, S. Huang, H. He, A. W. H. Mau, and L. Dai, J. Am. Chem. Soc.
that oxidizes CNTs and creates ‘‘holes’’ in SWNT ropes
which then leads to an increase in the conductivity. However,
besides the electron transfer oxidation process ͑a nonchemi-
121, 10832 ͑1999͒; S. Huang, L. Dai, and A. W. H. Mau, J. Phys. Chem.
B 103, 4223 ͑1999͒; D. C. Li, L. Dai, S. Huang, A. W. H. Mau, and Z. L.
Wang, Chem. Phys. Lett. 76, 3813 ͑2000͒.
S. Fan, M. G. Chapline, N. R. Franklin, T. W. Tombler, A. M. Cassell, and
H. Dai, Science 283, 512 ͑1999͒.
cal bond formation process͒, Br can also undergo chemical
2
8
9
reactions with the olefin bonds of CNTs at both the tube wall
and tip region and form C–Br bonds.20 The former process
creates free moving holes on the CNT wall and is favorable
to CNT conductivity; the latter process destroys the conju-
gate system and thus decreases the CNT conductivity. In the
current study, the observation of slight changes in the con-
ductivity of the CNT array is probably from combined ͑can-
cellation͒ effects of the above two processes.
C. J. Lee, J. Park, S. Y. Kang, and J. H. Lee, Chem. Phys. Lett. 326, 175
͑
2000͒; 323, 554 ͑2000͒; C. J. Lee, D. W. Kim, T. J. Lee, Y. C. Choi, Y. S.
Park, Y. H. Lee, W. B. Choi, N. S. Lee, G. S. Park, and J. M. Kim, ibid.
312, 461 ͑1999͒; D. S. Chung, W. B. Choi, J. H. Kang, H. Y. Kim, I. T.
Han, Y. S. Park, Y. H. Lee, N. S. Lee, J. E. Jung, and J. M. Kim, J. Vac.
Sci. Technol. B 18, 1054 ͑2000͒.
A. M. Rao, D. Jacques, R. C. Haddon, W. Zhu, C. Bower, and S. Jin, Appl.
Phys. Lett. 76, 3813 ͑2000͒.
10
11
S. H. Tsai, C. W. Chao, C. L. Lee, and H. C. Shih, Appl. Phys. Lett. 74,
Besides bromination, as-grown CNT arrays were also
treated by immersion in an acid mixture ͑concentrated
3462 ͑1999͒; S. L. Sung, S. H. Tsai, C. H. Tseng, F. K. Chiang, X. W. Liu,
H. C. Shih, ibid. 74, 197 ͑1999͒.
Z. W. Pan, F. C. K. Au, H. L. Lai, W. Y. Zhou, L. F. Sun, Z. O. Liu, D. S.
Tang, C. S. Lee, S. T. Lee, and S. S. Xie, J. Phys. Chem. B 105, 1519
H SO and HNO in a 3:2 volume ratio͒ at room temperature
12
2
4
3
for 30 min, followed by gentle washing in de-ionized water
and vacuum drying. The final CNT array was found to have
negligible emission current ͑data not shown͒ and the CNT
film very easily fell off. Most probably, the acid treatment
leads to poorer physical contact between the CNTs and the
supporting substrate via dissolution of metal nanoparticles at
the roots of CNTs. Treatment of CNT arrays by other oxi-
͑
2001͒.
13
W. Li, H. Zhang, C. Wang, Y. Zhang, L. Xu, K. Zhu, and S. Xie, Appl.
Phys. Lett. 70, 2684 ͑1997͒.
W. H. Wang, T. D. Corrigan, J. Y. Dai, R. P. H. Chang, and A. R. Krauss,
Appl. Phys. Lett. 70, 3308 ͑1997͒.
J. M. Bonard, J. P. Salvetat, T. Stockli, L. Forro, and A. Chitelain, Appl.
Phys. A: Mater. Sci. Process. A69, 245 ͑1999͒.
R. W. Murray, Acc. Chem. Res. 1, 313 ͑1968͒.
J. P. Deng, C. Y. Mou, and C. C. Han, Fullerene Sci. Technol. 5, 1033
͑1997͒.
K. Hernadi, A. Siska, L. Thien-Nga, L. Forro, and I. Kiricsi, Solid State
Ionics 141–142, 203 ͑2001͒.
R. S. Lee, H. J. Kim, J. E. Fischer, A. Thess, and R. E. Smalley, Nature
͑London͒ 388, 255 ͑1997͒.
Y. Yoshida, A. Otsuka, O. O. Drozdova, and G. Saito, J. Am. Chem. Soc.
14
15
16
17
dants, such as NO or NO gases, leads to poorer field emis-
2
sion performance ͑data not shown͒. Only oxidants, which are
capable of cleaving CvC double bonds, are able to cut open
CNT tips and improve field emission performance.
18
19
In summary, we have demonstrated a facile gas phase
O /O oxidation process for improvement of the field emis-
2
2
0
1
2
3
122, 7244 ͑2000͒.
sion properties of CNT arrays via selective opening of the
CNT tips in the array format with minimum damage along
the tube walls. With the O /O oxidation treatment, the on-
A reviewer pointed out that the emission current of our CNT array is small
compared to some values in the literature. The magnitude of CNT emis-
sion current is strongly dependent on the number of CNTs in a unit area as
well as on the electric conductivity of CNTs, which is a characteristic of a
given CNT growth process. The main emphasis and the unique feature of
our work are the cap opening of CNT arrays ͑and thus the enhancement in
the field emission current͒, rather than the preparation of vertically aligned
CNT arrays.
2
3
set emission voltage of CNT arrays can be lowered to ϳ0.6
V/␮m along with 700%–800% enhancement in the emission
current. The sharper the CNT end tip, the lower the onset
emission voltage as well as the larger emission current. The
current gas phase O /O oxidation method is a very simple,
22
See EPAPS Document No. E-APPLAB-80-058223 for two figures entitled
2
3
‘‘Oxygen and ozone oxidation-enhanced field emission of carbon nano-
convenient, clean process that can be very easily applied to
any CNT array/film to lower the CNT onset emission voltage
and enhance the emission current.21
tubes.’’ This document may be retrieved via the EPAPS homepage ͑http://
www.aip.org/pubservs/epaps.html͒ or from ftp.aip.org in the directory
/epaps/. See the EPAPS homepage for more information.
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