One-Step Growth of Vertical Graphene Sheets on Carbon Nanotubes and Their Field Emission Properties

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.

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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)


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)


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.



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.



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One-Step Growth of Vertical Graphene Sheets on Carbon Nanotubes and Their Field Emission Properties