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Graphene synthesis
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One-step growth of vertical graphene sheets on carbon nanotubes andtheir field emission propertiesJianlong Liu, Baoqing Zeng, Xiangru Wang, Wenzhong Wang, and Honglong Shi
Citation: Appl. Phys. Lett. 103, 053105 (2013); doi: 10.1063/1.4816751 View online: http://dx.doi.org/10.1063/1.4816751 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i5 Published by the AIP Publishing LLC.
Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors
Downloaded 05 Aug 2013 to 128.118.88.48. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions
One-step growth of vertical graphene sheets on carbon nanotubesand their field emission properties
Jianlong Liu (),1 Baoqing Zeng (),1,2,a) Xiangru Wang (),1
Wenzhong Wang (),3 and Honglong Shi ()31National Key Laboratory of Science and Technology on Vacuum Electronics, School of Physical Electronics,University of Electronic Science and Technology of China, Chengdu 610054, China2Department of Electronic Engineering, Zhongshan Institute, University of Electronic Science andTechnology of China, Zhongshan 528402, China3School of Science, Minzu University of China, Beijing 100081, China
(Received 11 March 2013; accepted 10 July 2013; published online 29 July 2013)
Graphene-carbon nanotube hybrid is prepared by an in situ growth of vertical graphene sheets oncarbon nanotubes (CNTs), using one-step plasma-enhanced chemical vapor deposition, without
catalyst. TEM analysis indicates that the growth of graphene is in accordance with the defects of
carbon nanotubes introduced by high-energy ion bombardment in microwave plasma and expands
by epitaxial growth. The results suggest that the method is ideal for preparing uniform graphene-
carbon nanotube hybrid and demonstrate a categorical explanation for the growth mechanism of
graphene-CNTs hybrid. Because of its uniform networks and multistage structure, the graphene-
CNTs hybrid exhibits good field emission properties.VC 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4816751]
Graphene has gained significant research interest
because of its unique physical properties originating from its
two-dimensional (2D) structure. Furthermore, it is consid-
ered as a potential candidate for applications in energy stor-
age and electrical devices owing to its high surface area and
excellent conductivity.1,2
Despite these exceptional properties, the planar structure
of graphene sheets (GSs) has the inherent limitation of
agglomeration because of van der Waals forces, which tends
to drastically decrease the surface area. However, comparing
with graphene, carbon nanotubes (CNTs) have lower surface
area. Nevertheless, the uniform one-dimensional (1D) struc-
ture of CNTs can hinder agglomeration better than GS. In
order to combine the merits of the 2D GS and 1D CNTs,
many attempts have been made to fabricate GSCNTs hybrid,
with an aim to preserve the high surface area of GS using 1D
CNTs as a matrix.3,4 However, these GSCNTs hybrids are
conventionally prepared by mixing CNTs and GS, which
hardly results in GS uniformly separated by CNTs. In reality,
GSCNTs hybrid prepared by simple mixing CNTs and GS
cannot be considered as an effective approach to overcome
agglomeration. Therefore, it is of critical need to develop new
synthesis methodologies that will enable the uniform growth
of individual GS on CNTs network.
So far, several synthesis methods, including mechanical
exfoliation,5 chemical exfoliation6,7 and chemical vapor dep-
osition (CVD),8 have been proposed for the preparation of
GS. It has been realized that the GS synthesized by mechani-
cal or chemical exfoliation has a planar structure, and
depends on the structure of original graphite. Other method,
such as thermal CVD, results in the growth of GS by planar
catalyst at high temperature. Consequently, the structures of
GS obtained by the methods mentioned above are limited on
in-plane shape, which is difficult to be designed for field
emission research.
On the other hand, plasma-enhanced chemical vapor
deposition (PECVD) is an intensively used technique for the
growth of carbon nanotubes,9,10 GS, and related carbon nao-
nostructure with good conductivity because of their high tem-
perature growth condition. The catalytic PECVD provides
possibility for the well controlled growth of CNTs or carbon
nanofibers (CNFs). However, carbon nanostructures synthe-
sized via PECVD technique suffer from limitations, such as
plasma etching, which results in defects and bamboo struc-
tures. Notably, free-standing GS can also be synthesized by
PECVD on various substrates without catalyst.1114 Recently,
reports indicated that GS can be made from unzipping the
carbon nanotube15 or etching the top side walls of CNTs
imbedded in polymer by argon plasma.16 Because this strat-
egy relies on the conversion of GS from CNTs, the structure
of GS depends on the structure of original CNTs. Moreover,
because CNTs and GS share the same structure and growth
conditions, it is reasonable to assume that it is possible to syn-
thesize GSCNTs compatible structure17 by using PECVD.
Intriguingly, another interesting report indicated that the
shorten cut single-wall carbon nanotube (SWCNT) can act
as a template for elongation growth when catalyst was intro-
duced. The cloned SWCNT grew from short segment with
well defined diameter and chirality.18,19 This indicates the
probable growth of GS on the defects of CNTs. Herein, we
have made an innovative approach to synthesize GSCNTs
hybrid by an in situ growth of GS on the defects of CNTs,without using catalyst during the PECVD. The resulting
GSCNTs hybrid is composed of GS grown on uniform
CNTs network, exhibiting high conductivity and multistage
structure without agglomeration.
Experiment was carried out with 5000W2.45GHz
microwave plasma-enhanced chemical vapor deposition
(MPECVD) system. Prior to the growth, 20 nm nickel layer
a)Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: 86-28-83200158. Fax: 86-28-83203371
0003-6951/2013/103(5)/053105/4/$30.00 VC 2013 AIP Publishing LLC103, 053105-1
APPLIED PHYSICS LETTERS 103, 053105 (2013)
Downloaded 05 Aug 2013 to 128.118.88.48. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions
was deposited on silicon substrates by sputtering as catalyst
for growing the CNTs. First, hydrogen was used as protec-
tion gas and 500W microwave power was applied to heat
the substrate to 950 C. Then mixture gas composed of200 sccm hydrogen and 100 sccm methane was induced to
the reactor and kept the pressures at 1000 Pa. After that, the
microwave power was switched at different condition and
kept for 20min as growth time. Substrates were put on a
platform at the center of reaction cavity, where the electric
field was largest. In contrast with the sample put away from
plasma,20 our samples were put in the center of plasma. To
enhance the bombardment and make the defect, they were
placed on the top of copper pillar. The plasma area was on
the top of the pillar and made the violent bombardment on
the sample.
Scanning electron microscopy (SEM) was employed to
examine the CNTs and the GS-CNTs hybrid. SEM images in
Figs. 1(a) and 1(b) show the CNTs grown with 1000W for
20min. Because of the violent bombardment, the structure of
CNTs is disordered and has a lot of defects. Comparing with
the conventional method, our higher growth temperature
makes the diameter of CNTs bigger. To introduce more
defects on the CNTs, microwave power was increased to
1200W for 20min. Fig. 1(c) shows the CNTs/CNFs struc-
ture after growth. The methane decomposes faster with
higher power plasma, which raises the growth speed and
enlarges the diameter of CNTs/CNFs. The crimple of the
CNTs/CNFs is due to the high bombardment and fast
growing.
Further increasing the microwave power to 1500W, the
grown structure is shown in Figs. 2(a) and 2(b). The structure
of CNTs/CNFs does not obviously change, but the GS is
partly covering on the surface of CNTs. It must be violent
plasma that makes the bombardment and results the defects.
When the defect appears on the CNTs/CNFs, the intensive
electric field and continuous carbonic feedstock makes the
GS grown in situ on the defects. It can be seen from Fig. 2(b)
that the GS is first grown on the corner of the crimple
according to the defects, where the defects and amorphous
carbon would more easily be removed by the plasma etching.
While in other place, the CNTs had a little or no defect, and
it is difficult to be destroyed by plasma etching in this level.
Noteworthy, we did not find the change of the coverage rate
when the growth time is increased.
To make a full coverage of GS on CNTs, the microwave
power was increased to 2000W. SEM images in Figs. 2(c)
and 2(d) show the GS-CNTs hybrid has uniform networks
and is fully covered by vertical GS. Comparing with the
lower growth microwave power, the higher one has higher
local electric field intensity to make more defects during the
growth. This high local electric field intensity also makes the
GS grown in situ on the defects of CNTs to form a uniformGS-CNTs hybrid. Since increasing the growth time could not
change the GS coverage rate, we suggest the defects were
not formed by bombarding the surface of CNTs after grow-
ing, but the intensive bombardment and etching made the
CNTs grown with a lot of defects. The GS was first grown
on the defect and then made the epitaxial growth.
This GS-CNTs hybrid was put in ethanol and ultrasoni-
cated for 1 h, and then deposited on the substrate for trans-
mission electron microscopy (TEM) measurement. TEM
image in Fig. 3(a) shows the structure of CNTs/CNFs that
has ordered walls of multiwalled CNTs (MWCNTs) with
lots of defects. Fig. 3(b) shows the higher magnification
image of section A and defects could be found on the wall of
MWCNTs. These defects appear on the walls during the
growth of CNTs because of the bombardment of local
plasma. With high local electric field and continuous car-
bonic feedstock, multilayered graphene sheets (MLGs) were
grown in situ on the defects. Another high resolution TEMimage from section B also shows, in Fig. 3(c), that bilayer
graphene was first grown on the defect from a double wall of
CNTs. And then the bilayer graphene made epitaxial growth
to turn 3 layer graphene (LG). This 3 LG kept growing and
finally turn to 4 LG with a little disorder on the conjunctionFIG. 1. SEM images of CNTs grown with different microwave power.
(a), (b) 1000W and (c), (d) 1200W.
FIG. 2. SEM images of GS-CNTs hybrid grown with different microwave
power. (a), (b) 1500W and (c), (d) 2000W.
053105-2 Liu et al. Appl. Phys. Lett. 103, 053105 (2013)
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because of the bombardment from local plasma. This point
of view can also be proved in Fig. 3(d) that the GS had 11
LG during the growth, while the epitaxy made the GS turn to
14 LG at the end with a little disorder on the conjunction.
Mechanism of growing this GS-CNTs hybrid is illus-
trated by the scheme in Fig. 4. Fig. 4(a) shows the structure
of MWCNTs, which is composed of multilayer GS. When
the substrate with catalyst was placed on the center of
plasma, the CNTs grow with the help of catalyst. The plasma
consists of high-energy ion and keeps bombarding the CNTs
during the growth, as shown in Fig. 4(b). The high-energy
ion bombardment acts throughout the growth and makes the
CNTs grown with a lot of defects, as shown in Fig. 4(c).
With the high temperature and plentiful carbonic groups, the
GS grow in situ on the defect of CNTs as a start. Without thecatalyst on the defects, the graphene does not grow accord-
ing to the structure of CNTs as before.18 Orientation of origi-
nal graphene may depend on the local electric field.13,14,21
When original graphene was formed on the defects, it not
only makes epitaxial growth to increase the layers but also
transversely stretches and expands, as shown in Fig. 4(d).
After expanding, the thickness of GS is about several nano-
meters while the transversal size of GS has more than two
orders of magnitude with several hundred nanometers, as
shown in Fig. 2(d). So that the GS mainly makes the trans-
versal expansion rather than thickness increase, and makes
the full coverage on CNTs without agglomeration.
Field emission is electron tunneling escape from the
Fermi level when an external electric field is applied. When
specific structure of emitter is subjected to electric field, the
local electric field would be enhanced by the geometry of the
emitter and its surface morphology. Previous reports indicate
that the enhancement factor would be dramatically increased
by multistage geometry.2225 Due to the in situ growth athigh temperature, this GS-CNTs hybrid would also have
FIG. 3. TEM images of exfoliated GS. (a) Low magnification of CNTs/
CNFs with a lot of defects. (b) High magnification image of section A in (a),
the MLG grown on the defect of MWCNTs. (c) High magnification image
of section B in (a), the bilayer GS grows from defect turns to 3 layer GS and
finally turns to 4 layer GS by epitaxial growth. (d) The epitaxial grown GS,
from 11 layers to 14 layers.
FIG. 4. Schematic of GS-CNTs hybrid. (a) Model of MWCNTs consist of
MLG. (b) CNTs are bombarded by high power ion from plasma during the
growing duration. (c) The bombardment results in the defects on the CNTs.
(d) Graphene grows in situ on the defects of CNTs and makes epitaxialgrowth to turn MLG.
FIG. 5. Field emission tests of GS and CNTs. (a) Emission current density
of GS-CNTs hybrid and CNTs (b) corresponding F-N plot.
053105-3 Liu et al. Appl. Phys. Lett. 103, 053105 (2013)
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good conductivity. With the good conductivity and multi-
stage enhancement, this GS structure may have a good field
emission property.
Field emission measurement was carried out with paral-
lel electrodes in a high vacuum chamber. The diameter of
anode was 2mm. Distance between anode and cathode
remained 400 lm. The vacuum was kept at about6.0 107 Torr. Experimental results show that the fieldemission properties of GS-CNTs hybrid are significantly
improved, as shown in Fig. 5(a). Field emission test indicates
that the turn-on field of CNTs grown with the same method
is about 2.1 V/lm, and threshold-field at 1mA/cm2 was3.2V/lm. While for the GS-CNTs hybrid, the turn-on fieldis 0.6V/lm and the threshold-field is 1.6V/lm. Thisimprovement is attributed in great deal to GS, which was
grown in situ on the CNTs and results in multistage geome-try enhancement. Corresponding FowlerNordheim (FN)
plots are shown in Fig. 5(b). By assuming the work function
of GS to be 5 eV, the enhancement factor for CNTs is calcu-
lated at 3670. The field enhancement factor of GS-CNTs
hybrid is 7710. Field emission stability test of GS-CNTs
hybrid is also shown in Fig. 6. The field emission is stable
with only 8% decline after 10 h continuous emission. This
may be due to the more emission sites for GS-CNTs hybrid,
which act as emitters and share the emission current to make
the emission stable.
In summary, we demonstrate a GS-CNTs hybrid grown
by high power microwave PECVD. The GS is grown in situon the elaborately induced defects of CNTs by high energy
ion bombardment of plasma. High resolution TEM indicates
that the GS is grown according to the defect as a start and
then makes epitaxial growth without catalyst. The GS mainly
makes the transversal expand and uniformly covers on CNTs
without agglomeration. This kind of evidence may also hint
the mechanism of various graphene structure grown by
PECVD without catalyst. With multistage structure and
intrinsic low contact resistance in nature, this hybrid struc-
ture has good field emission properties.
This work was partially supported by NSFC (Grant Nos.
60071043 and 11074312), the Doctor Station Foundation of
the Ministry of Education of China (Grant No.
200806140007), National Key Laboratory of Science and
Technology on Vacuum Electronics, and the Fundamental
Research Funds for the Central Universities.
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FIG. 6. Field emission stability tests of GS-CNTs hybrid.
053105-4 Liu et al. Appl. Phys. Lett. 103, 053105 (2013)
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