Structural transformation and field emission enhancement of carbon nanofibers by energetic argon plasma post-treatment

Article abstract of DOI:10.1063/1.1815062

Vertically aligned carbon nanofibers (CNFs) grown by plasma enhanced chemical vapor deposition (PECVD) were transformed into cone-shaped nanostructures after treatment by argon (Ar) plasma. Significant enhancement of field emission characteristics of the post-treated CNFs has been achieved. Analysis by electron microscopy and energy dispersive spectroscopy (EDS) suggests that the structural transformation is a result of a cosputtering/deposition process by energetic plasma ions. The enhancements can be attributed to the combining effects of an additional Si/C layer coverage, catalytic nanoparticles removal and the sharpening of CNFs tips. The argon plasma post-treatment processes developed here can be easily extended to in situ PECVD processes for fabricating CNFs based emitters.

Full text of DOI:10.1063/1.1815062

Structural transformation and field emission enhancement of carbon nanofibers by
energetic argon plasma post-treatment
C. H. Weng, K. C. Leou, H. W. Wei, Z. Y. Juang, M. T. Wei, C. H. Tung, and C. H. Tsai
Citation: Applied Physics Letters 85, 4732 (2004); doi: 10.1063/1.1815062
View online: http://dx.doi.org/10.1063/1.1815062
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 20
15 NOVEMBER 2004
Structural transformation and field emission enhancement of carbon
nanofibers by energetic argon plasma post-treatment
a)
C. H. Weng, K. C. Leou, H. W. Wei, Z. Y. Juang, M. T. Wei, C. H. Tung, and C. H. Tsai
Department of Engineering and System Science, National Tsing Hua University, Taiwan,
Republic of China
(
Received 24 March 2004; accepted 9 September 2004)
Vertically aligned carbon nanofibers (CNFs) grown by plasma enhanced chemical vapor deposition
PECVD) were transformed into cone-shaped nanostructures after treatment by argon (Ar) plasma.
(
Significant enhancement of field emission characteristics of the post-treated CNFs has been
achieved. Analysis by electron microscopy and energy dispersive spectroscopy (EDS) suggests that
the structural transformation is a result of a cosputtering/deposition process by energetic plasma
ions. The enhancements can be attributed to the combining effects of an additional Si/C layer
coverage, catalytic nanoparticles removal and the sharpening of CNFs tips. The argon plasma
post-treatment processes developed here can be easily extended to in situ PECVD processes for
fabricating CNFs based emitters. © 2004 American Institute of Physics.
[
DOI: 10.1063/1.1815062]
Carbon nanotubes (CNTs) or carbon nanofibers (CNFs)
have been considered one of the most promising materials
for field emission (FE) applications due to their high aspect
ratio geometry, small radius of curvature at tips, and high
coupled plasma CVD (ICP-CVD). In contrast to previous
1
7
studies, regimes of more energetic ions were chosen for
post-treatments. Mechanisms for enhancement of field emis-
sion characteristics and the structural transformation of as-
grown CNFs resulting from Ar plasma treatments were in-
vestigated.
1
–3
physical/chemical stability.
CNTs/CNFs based electron
emitters thus potentially allow low turn-on voltage operation,
large emission current density, and long-time stability at
The CNFs growth and post-treatment processes were
4
,5
12
large current loading. Their FE properties at device level,
however, are often affected by additional factors, such as the
alignments and density/length/diameter of the tubes/fibers,
absorbents on the tube/fiber surface, conductivity between
both carried out in a home-built ICP-CVD system. Two
separate 13.56 MHz radio-frequency (rf) power generators
were used for high density plasma generation and substrate
stage bias, respectively. The graphite substrate stage was
temperature controlled in an operating temperature in the
range of 400–900 °C. For CNFs growth, the chamber was
fed with a gas mixture of acetylene ͑C H ͒, H and Ar at a
3
,6–8
CNTs/CNFs and substrates, etc.
In order to optimize the
FE properties, it is highly desirable to produce vertically
aligned CNTs/CNFs with optimal combinations of density,
length, and diameter on substrates. Therefore, direct synthe-
sis of CNTs/CNFs on catalytic transition metal patterned
substrates by chemical vapor deposition (CVD) have been
widely employed for better control of those properties of
2
2
2
total pressure typically below 50 mTorr. For the results
shown in this letter, p-type Si (100) wafers with square-
patterned ͑10 ␮mϫ10 ␮m͒ Ni film of thickness 10 nm de-
posited by electron beam evaporation were used as sub-
strates. Figure 1(a) shows the scanning electron microscopy
(SEM) image of the vertically aligned CNFs grown in this
ICP-CVD reactor with the following process conditions:
flow rates C H /H /Ar=8/24/0.5 sccm, total pressure
2
,9
CNTs/CNFs.
In particular, synthesizing by plasma-
enhanced CVD (PECVD) provides further benefits of pro-
ducing isolated and better vertically aligned CNTs/CNFs at
1
0
much lower substrate temperatures. This feature is essential
for fabricating FE devices on large-area glass substrates in
2
2
2
20 mTorr, substrate temperature 580 °C, ICP power
1000 W, bias power 300 W, and growth time 10 min. The
resulting CNFs have an average diameter of 60–80 nm, a
1
0–12
the field emission displays (FEDs) application.
However,
impurities produced during CNTs/CNFs growth by CVD/
PECVD methods, for example, the catalytic nanoparticles at
9
10
−2 12
length of 1–2 ␮m, and a density of ϳ10 –10 cm . Cata-
1
3,14
5,14
tips
or amorphous phase carbons, are believed to ham-
lytic Ni nanoparticles are found encapsulated at CNFs tips,
11
per the FE characteristics. Post-treatment techniques, such as
indicating the tip-growth mode for CNFs growth. Good
uniformity of CNFs was obtained in terms of their morphol-
ogy, density and Raman spectroscopy analysis (intensity ra-
tio of D–G-band modes). The as-grown CNFs samples were
then further treated separately in the same reactor by Ar
plasma under various process conditions.
1
5
16
ultraviolate laser irradiation, hydrogen ͑H2͒, or argon
1
7
(
Ar) plasma treatments, and oxidation by oxygen or ozone
1
8
gases, etc., have been developed to process the as-grown
CNTs. Significant enhancement of FE has been obtained as a
result of one or combination of the following mechanisms:
catalytic nanoparticles removal or caps opening of
Figures 1(b)–1(e) show the SEM images of CNFs post-
treated by Ar plasma for different treatment time. The pro-
cess conditions are ICP power 1000 W, rf bias power 300 W
(self-bias voltage ϳ410 V), and total pressure 20 mTorr.
Post-treated CNFs became more like cone shaped as the
treatment time was increased. In addition, catalytic nanopar-
ticles encapsulated at CNF tips were gradually removed by
Ar plasma and almost disappeared after 5 min treatment.
1
3–18
13–15,17
CNTs,
formation on CNTs surface, etc.
purifications of CNTs,
or C–H dipole layer
1
6
In this study, an Ar plasma post-treatment process was
developed for CNFs grown by high density inductively
a)
Author to whom correspondence should be addressed; electronic mail:
kcleou@mail.ess.nthu.edu.tw
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003-6951/2004/85(20)/4732/3/$22.00 4732 © 2004 American Institute of Physics
49.106.210.114 On: Sun, 23 Nov 2014 20:14:27
0
1

Appl. Phys. Lett., Vol. 85, No. 20, 15 November 2004
Weng et al.
4733
FIG. 3. Raman spectra of the CNFs. Peaks at ϳ520 and ϳ950 cm−1 were
from the Si substrates: (a) as-grown CNFs, and after treatment time of (b)
2
min, (c) 5 min, (d) 10 min, and (e) 20 min, respectively.
2
(e). Furthermore, additional EDS analysis (not shown)
showed that Ni nanoparticles of size ϳ20 nm were buried
inside cone-shaped CNFs. The residual nanoparticles may be
attributed to the incomplete removal of large nanoparticles
by Ar ion bombardments. Due to the smaller size reshaped
by Ar ion sputtering, nanoparticles might be melted as a
result of heating by ion bombardments, and eventually be
buried inside the deposited layer of Si/C atoms which were
not able to be observed by SEM.
FIG. 1. SEM images of the CNFs: (a) as-grown CNFs, and after treatment
time of (b) 2 min, (c) 5 min, (d) 10 min, and (e) 20 min, respectively. The
insets show the images of CNT tips at higher magnification.
Figure 2 shows the transmission electron microscopy (TEM)
images of the as-grown and the post-treated CNFs, revealing
the evolution of catalytic Ni nanoparticles and the morpho-
logical transformation. The TEM images, Figs. 2(c) and 2(d),
also show that CNFs surfaces were covered by a thin amor-
phous layer of thickness 10–50 nm, dependent on the treat-
ment time. The layer coverage was identified to be composed
of Si and/or C atoms by qualitative energy-dispersive spec-
troscopy (EDS) analysis auxiliary to TEM, as shown in Fig.
From the electron microscopic and EDS analysis, we
propose that the transformation procedure into cone-shaped
nanostructures is to follow a simultaneously “sputtering” and
“
deposition” process. The Si and/or C atoms (from the sub-
strate and CNFs, respectively) were first sputtered off by
energetic Ar ions and then deposited back on CNF surfaces
to form the amorphous layer coverage. This process occurred
repeatedly during the entire post-treatment period. The mor-
phological change into cone shaped is suggested to be origi-
nated from the different sputtering yields of Ni nanoparticles
and graphitic layers, as indicated in Fig. 2(f), where the
black-dash arrows represent the incidence of Ar ions. The
lengths of the arrows correspond to the sputtering yields with
typical dependence of the incident angles of ions. It can be
observed that the graphitic layers over CNF tip were more
easily removed than the encapsulated Ni particle. In addition,
the white-curved arrows stand for the amorphous layer depo-
sition over the entire surface, which might also be sputtered
off simultaneously.
The CNFs were also characterized by micro-Raman
spectroscopy where a He–Ne laser of wavelength 632.8 nm
was employed to induce the spectra, as depicted in Figure 3.
The spectrum of as-grown CNFs, as shown in Fig. 3(a), ex-
1
9
hibited the normal spectra of carbonaceous materials. The
intensity ratio of D to G-band modes, which represents the
amorphous phase content or the degree of crystallinity of
2
0
carbonaceous materials, of the as-grown and the 2 min
treated CNFs is ϳ1.3 and ϳ1.2, respectively. The decrease
of this value indicates that Ar plasma sputtered off the amor-
phous phase carbons and random-oriented CNFs at the be-
ginning stage of post-treatment [see Fig. 1(a) and 1(b)].
However, for CNFs with longer treatment durations, an extra
photoluminescence-like (PL-like) background signal was
found in addition to the normal Raman spectrum. Qualita-
tively, it is believed that the PL-like background signals pro-
vided an indirect evidence for certain structural transforma-
FIG. 2. TEM images of the CNFs, and the EDS analysis: (a) as-grown
CNFs, and after treatment time of (b) 2 min, (c) 10 min, (d) 20 min, where
the black arrow indicated the buried particle, and (e) EDS analysis obtained
by directly focusing the electron beam on the post-treated CNFs body of (c)
as depicted. The copper (Cu) signal was from the TEM sample grids; (f) tip
part of (b) at higher magnification. The insets of (c) and (d) show the thin
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amorphous layers by white double-headed arrows.
tion from the spectroscopic point of view.
1
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4734
Appl. Phys. Lett., Vol. 85, No. 20, 15 November 2004
Weng et al.
teristics than the untreated one, regardless the final CNFs
morphology, length, density, or the appearance of catalytic
nanoparticles [see Fig. 1(b)]. Therefore, it is believed that the
first factor, an additional layer coverage, plays a dominate
role in the FE enhancement of post-treated CNFs.
In conclusion, we have demonstrated an effective
method for FE characteristics enhancement of as-grown
CNFs by energetic Ar plasma post-treatment. The Ar plasma
treatment process and mechanisms described in this work
could also be applied to other types of field emitters having a
vertically aligned and rod/tube like geometry to improve FE
properties, but special care must be taken to avoid extensive
structure damage by the energetic plasma ions. Further stud-
ies, such as in situ post-treatment process integration, as well
as other FE characterizations, e.g., long-time stability of
emission current and large-area uniformity of luminance, are
required to demonstrate the viability of this approach.
This work was supported by the National Science Coun-
cil under Contract No. NSC-92-2622-E-007-016. Partial
TEM supports from Chung-Shan Institute of Science and
Technology, and field emission measurements supported by
S. C. Kung, Material Research Laboratories, Industrial Tech-
nology Research Institute, Taiwan, are also acknowledged.
FIG. 4. Field emission measurements of the CNFs: (a) Typical J–E curves,
while the insets are the corresponding F–N plots and (b) the dependence of
turn-on field (open-circle) and threshold field (solid-square) on the treatment
time.
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