On the kinetics of carbon nanotube growth by thermal CVD method

www.elsevier.com/locate/diamond

Diamond & Related Materials 13 (2004) 2140–2146

On the kinetics of carbon nanotube growth by thermal CVD method

Z.Y. Juanga, J.F. Laib, C.H. Wenga, J.H. Leea, H.J. Laic, T.S. Laib, C.H. Tsaia,*

aDepartment of Engineering and System Science, National Tsing Hua University, 101, Section 2 Kuang Fu Road, Hsinchu 300, Taiwan, ROCbDepartment of Physics, Chung-Yuan Christian University, Taiwan, ROC

cMaterials Research Laboratories, Industrial Technology Research Institute, Taiwan, ROC

Available online 18 May 2004

Abstract

The role of ammonia (NH3) on obtaining good quality vertically aligned multi-walled carbon nanotubes (CNTs) in thermal chemical

vapor deposition (CVD) method has been widely studied. It was generally agreed that NH3 helps to maintain catalyst metal surface active by

reacting with amorphous carbon. In this article, a systematic study in varying the temperature and mixing ratio of gases was conducted in

order to clarify the role of NH3 and revealed a criterion for optimized condition window in the growth processes. In addition, this study has

also carried out a statistical analysis through intensive TEM observations on the tube diameters, bamboo spacing, and the formation rate of

each diaphragm under various temperatures and carbon source/NH3 ratios. While the formation of the separation diaphragms were indeed a

result of bulk diffusion of carbon atoms from bottom of the Ni nanoparticle following thermal dehydrogenization to the top of the Ni

nanoparticle, there were other carbon atoms diffusing presumably via surface diffusion to the CNT-metal interface and contributed to the

growth of tube wall; in other words, the CNTs growth is simultaneous renucleation and growth processes, instead of a continuous

renucleation and growth process. This kinetics-based mechanism in combination with the proposed role of NH3 could not only successfully

explain the effects of the process parameters including temperature and the mixing gas ratio, but also could be used for pursuing the goal of

lower growth temperature for thermal CVD method which is very important for many applications of CNTs.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotube; Mechanism; Thermal CVD; Ammonia

1. Introduction

Carbon nanotubes (CNTs) have great potential for vari-

ous nanotechnology applications. Since their first successful

synthesis by arc-discharge technique and observation by the

transmission electron microscope (TEM) analysis [1], and

the subsequent methods with the metal catalyst in an

hydrocarbon/inert mixing gas atmosphere [2,3], extensive

investigations on CNTs have been carried out due to their

unique physical properties [4,5] and potential applications

[6–8]. Compared to the earlier gas phase synthesis techni-

ques, the catalytic chemical vapor deposition (CVD) meth-

ods [9–15], either thermal pyrolysis or plasma-enhanced,

have the advantageous ability in in-situ selective growth on

patterned area for large area array fabrication in the appli-

cations such as field emission display (FED), chemical

0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.diamond.2004.03.007

* Corresponding author. Tel.: +886-3571-5131x5804; fax: +886-3572-

0724.

E-mail address: [email protected] (C.H. Tsai).

sensors, etc. Thus far, catalytic metals deposited on sub-

strates, such as nickel (Ni) film on silicon or glass sub-

strates, have been widely used for this purpose. Methane

(CH4), acetylene (C2H2) or ethylene (C2H4) was often used

as carbon source and ammonia (NH3) was sometimes used

as reactive gas for the growth of aligned CNTs in thermal

CVD method [12,13,16,17]. Even thought it was general-

ly agreed that NH3 helps to maintain catalyst metal sur-

face active [18], whether NH3 is needed in every stage of

CNTs synthesis is controversial and the detailed mecha-

nism of how CNTs grow vertically aligned is still unclear

[12,18,19]. In our previous work [20], we found that the

effects of NH3/CH4 ratios on the CNTs growth in atmo-

spheric thermal CVD process at a fixed temperature of 900

jC showed distinct trends with and without diluting gas

argon (Ar). Our experimental results suggested that there

exists a critical supply of CH4 or other carbon source when

CNTs growth is limited by carbon atom diffusion that might

be determined by the growth temperature or/and effective

active area of catalyst particles. In the case of low carbon

supply and high carbon diffusion, so called carbon supply

Z.Y. Juang et al. / Diamond & Related Materials 13 (2004) 2140–2146 2141

on catalyst surface limited (CSL), either graphite structure

formation/CNTs growth is hampered, or CNTs grow in

spaghetti-like morphology; whereas in the case of high

carbon supply and low carbon diffusion, so called carbon

diffusion rate limited (CDL), carbon atoms will saturate on

nanoparticle surface and excess carbon atoms form amor-

phous carbon and CNTs growth is hampered due to catalyst

passivation. It is therefore believed that the optimal CNTs

growth situated on the condition that carbon atom supply

and diffusion reach an identical rate; that is, CNTs grow

with ‘‘best quality’’ at this optimal growth condition. In this

article, systematic experiments were performed furthermore

to examine the role of NH3 on the growth of CNTs in

atmospheric thermal CVD under various temperatures and

carbon source/NH3 ratios. In addition, since there have been

numerous papers attempted to investigate the growth mech-

anisms by examining the bamboo-like structure [21–25],

this study has also carried out a statistical analysis through

an intensive TEM observations on the tube diameters,

bamboo spacing (or called inter-bamboo-layer distance),

and the formation rate of each diaphragm under various

temperatures and carbon source/NH3 ratios. It is hoped that,

in combining the kinetics-based model and the examined

role of NH3 on CNTs growth as well as bamboo-like

morphology, a better understanding and better control of

CNTs catalytic growth can be achieved.

2. Experimental

Patterned (10� 10 Am2) Ni catalyst films of thickness 10

nm were deposited on SiO2/p-Si(100) wafer by optical

lithography and e-gun evaporation followed by lift-off pro-

cess. The pieces of catalyst deposited substrates were then

placed on a quartz plate and leaded into a 46-mm inner-

diameter resistance heated quartz tube furnace at atmospheric

pressure. The quartz tube was pumped down to f 10� 3

mbar using amechanical pump, before purgingAr gas back to

atmospheric pressure. The substrates were then heated to 900

jC at a heating rate of 60 jC/min with Ar gas supplied

continuously.When the temperature was stable at 900 jC,Ar/NH3 mixing gas (1:1) was introduced into the quartz tube for

10 min pretreatment. At the end of pretreatment process, the

C2H4 gas was introduced into the quartz tube for CNTs

growth. In order to precisely control the synthesis tempera-

ture, the furnace was equipped with three individual thermal

couples to in-situ monitor the temperature independently to

avoid overheating. All specimens of different growth con-

ditions were treated with the same pretreatment process. Note

that most of the reports in the literature used the temperature

of pretreatment the same as that for growth process. That

means the experimental results in different growth temper-

atures were not only influenced by the difference of the

growth temperature, but also affected by different pretreat-

ment temperatures. For this reason, a better experimental

design to study the effects of temperatures should be that with

fixed pretreatment temperature. The time of CNTs growth

was 10 min and the pressure was 1 atm. At the end of the

growth, the quartz tube was pumped down to f 10� 3 mbar

again to avoid the interference with remaining gaseous

species. After that, the specimens were furnace-cooled to

room temperature in an Ar flowing environment at 1 atm.

Note that the total flow rate, even in varying the ratio of Ar/

NH3/C2H4 mixed gases, was always kept at 400 sccm via

mass flow controller (MFC, LINTEC MC-2100ENC). The

field emission scanning electron microscopy (SEM, JEOL

JSM-6330F) and transmission electron microscopes (TEM,

JEOL JEM 2000FXII/2010/2010F) were used to examine the

morphologies, dimensions, bamboo structure and the atomic-

resolution images of CNTs, respectively.

3. Results and discussion

There were two sets of experiments, one with fixed C2H4

flow rate of 30 sccm (7.5% of total flow rate) and the other

with fixed NH3 flow rate of 30 sccm. The SEM micrographs

of the grown CNTs were shown in Fig. 1a–f and d–i for

each set respectively. Between the two sets, Fig. 1d–f was a

reference group in common with the gas mixing ratio of

C2H4/NH3 (Rm) equal to one at various temperatures.

Although not evidently shown in Fig. 1, examinations under

higher magnification showed that all CNTs were grown in

base-growth mode.

For the first set of experiments with fixed 7.5% C2H4, at

all temperatures, lower C2H4/NH3 ratio (i.e. more NH3) was

beneficial for growing vertically aligned CNTs. It was

observed that amorphous carbon (a-C) was heavily depos-

ited on CNTs for those grown at 900 jC so that the apparent

CNTs diameters were much larger than those grown at lower

temperatures. Comparing Fig. 1a–c, the optimal growth

temperature seems to appear between 850 and 900 jC,whereas the optimal growth temperature by comparing

Fig. 1d–f the optimal growth temperature is between 800

and 850 jC. In other words, optimal temperature is higher

when more NH3 is provided. On the other hand, for the

second set of experiments with fixed 7.5% NH3, at lower

temperatures (800 jC for example), lower C2H4/NH3 ratio

was preferred for CNTs growth, whereas at higher temper-

ature (900 jC), higher C2H4/NH3 ratio was beneficial. By

comparing Fig. 1g–i, it was found that the optimal growth

temperature for less C2H4 is most likely below 800 jC.The TEM micrographs as shown in Fig. 2 clearly illus-

trated that the CNTs were of bamboo-like structure in all

conditions. Table 1 lists the statistics of a numerous number

of CNTs examined under TEM, in which the inner and outer

diameters are the average inner and outer diameters of CNTs;

bamboo spacing is the average diaphragm spacing between 2

adjacent separation layers averaged over a large portion of

straight CNTs of at least more than 10 tubes for each growth

condition, length is the average CNT length estimated from

SEM, and Rb is obtained by dividing the estimated total

Fig. 1. Two sets of experiments, (a)– (f) and (d)– (i). One with fixed C2H4 flow rate of 30 sccm (7.5% of total flow rate) and the other with fixed NH3 flow rate

of 30 sccm (7.5% of total flow rate) for each set respectively. (d)– (f) were a reference group in common with Rm= 1 at various temperatures. (a) Rm= 1/3 and

temperature is 800 jC, (b) Rm= 1/3 and temperature is 850 jC, (c) Rm= 1/3 and temperature is 900 jC, (d) Rm= 1 and temperature is 800 jC, (e) Rm= 1 and

temperature is 850 jC, (f) Rm= 1 and temperature is 900 jC, (g) Rm= 1/3 and temperature is 800 jC, (h) Rm= 1/3 and temperature is 850 jC and (i) Rm= 1/3

and temperature is 900 jC.

Z.Y. Juang et al. / Diamond & Related Materials 13 (2004) 2140–21462142

number of bamboo diaphragms by the growth time, meaning

the number of diaphragms formed per unit time. Table 1

clearly shows that the inner diameters are independent of the

growth conditions and the outer diameters increase with

growth temperature. Especially at 900 jC, an abrupt increaseof diameter was observed due to deposition of a-C, which is

consistent with SEM observations. It is also found that the

bamboo spacing decreases and Rb increases with increasing

temperature, which are also illustrated in Figs. 3 and 4. Fig. 5

shows the dependence of CNT length on the growth tem-

perature at different C2H4/NH3 ratios.

In discussing the observations described in the previous

paragraphs, the role of NH3 is assumed to maintain the

catalytic activity of the nickel particle surface so that higher

NH3 ratio will provide more active surface sites and higher

carbon atom is supplied from the catalyst surface. In the first

set of experiments with fixed C2H4 flow rate, higher carbon

supply at higher NH3 ratio needs higher carbon diffusion in

the catalyst particle to attain steady and straight CNTs

growth, which explains why higher optimal growth temper-

ature at higher NH3 ratio was observed. On the other hand,

with fixed NH3 flow rate that catalyst activity is fixed, lower

optimal growth temperature was observed at lower C2H4

supply.

Among the proposed bamboo structure formation mech-

anisms [21–25], Lee et al. [21,22] suggested that the

separation diaphragms be grown mainly by bulk diffusion

of carbon atoms, and the motive force for the cap graphitic

sheets departing from the catalyst particle was due to the

stress accumulated under the graphite cap and then the wall

grew toward the vertical direction with a certain small angle.

When the carbon atoms were supplied continuously under

steady-state condition, the diaphragm layers appeared peri-

odically. And they found that the separation layers appeared

at longer distance as the catalyst particle size decreased. By

the way, they grew CNTs using thermal CVD using C2H2

gas at 950 jC on various catalyst metals, and no reactive gas

such as NH3 was added. Cui et al. [23] investigated the

effects of methane/ammonia ratio and temperature on the

bamboo-like structure. They proposed that periodic bam-

boo-like structure suggested a continuous growth and renu-

cleation mechanism, and the inter-diaphragm distance

represented a renucleation of subsequent multi-walled nano-

tube segments and thus variations in this dimension might

lead to better understanding of the process. Their results of

the inter-bamboo-diaphragm distance for different CH4/NH3

concentrations at temperature of 810 jC showed an increase

in spacing with partial pressure of CH4 (Fig. 4 of Ref. [23]),

Fig. 2. The CNTs were of bamboo-like structure in all conditions. Each CNT of (a)– (g) corresponding to Fig. 1(a), (d), (b), (e), (h), (c) and (f). Details of each

CNTs in different conditions are shown within Table 1 in order. Note that each TEM image was just one CNT picked from numerous numbers of CNTs and

hard to avoid some error comparing with the average value listed in Table 1. (f) and (g) only show the hollow part of CNT and their outer surfaces are not

included in each image.


which suggested that the formation of the bamboo-dia-

phragm sections was a diffusion limited process if it was

due to precipitation and renucleation at the catalyst surface

and the nanotube renucleation event was the rate-limited

step. They also found that the inter-bamboo-diaphragm

distance increased exponentially with temperature for a

constant CH4/NH3 ratio (Fig. 5 of Ref. [23]). However,

they claimed that, taking into account the simultaneous

variations in both tube length and diameter, more experi-

Table 1

List of the statistics of a numerous number of CNTs examined under TEM, in whic

CNTs; bamboo spacing is the average diaphragm spacing between 2 adjacent sepa

than 10 tubes for each growth condition, length is the average CNT length estimat

bamboo diaphragms by the growth time, meaning the number of diaphragms for

C2H4

(%)

NH3

(%)

Rm Inner

diameter

(nm)

O

d

(n

Fig. 1(a) 7.5 22.5 1/3 7.25

Fig. 1(d) 7.5 7.5 1 6.09

Fig. 1(b) 7.5 22.5 1/3 5.29

Fig. 1(e) 7.5 7.5 1 4.86

Fig. 1(h) 2.5 7.5 1/3 5.13

Fig. 1(c) 7.5 22.5 1/3 9.44 1

Fig. 1(f) 7.5 7.5 1 6.17 1

mental evidence was required to confirm the speculation.

Their observations were based on the deposition using

microwave plasma enhanced CVD, in which CNTs were

grown in tip-growth mode. On contrast, our results as

shown in Fig. 3 revealed that (i) the inter-bamboo spacing

is higher for lower C2H4/NH3 ratio (relatively higher NH3 at

fixed C2H4 flow rate); (ii) the inter-bamboo spacing de-

creased with increasing temperature at all gas mixing ratios,

which are completely opposite to those observed by Cui et

h the inner and outer diameters are the average inner and outer diameters of

ration layers averaged over a large portion of straight CNTs of at least more

ed from SEM, and Rb is obtained by dividing the estimated total number of

med per unit time

uter

iameter

m)

Bamboo

spacing

(nm)

Length

(Am)

Rb

(1/min)

Temp.

(jC)

22.13 44.49 10 22.48 800

23.36 47.13 20 42.43 800

30.00 44.91 20 44.54 850

39.36 22.2 9 40.54 850

24.25 49.34 19 38.51 850

41.11 24.37 17 69.74 900

45.83 17.23 14 81.26 900

Fig. 3. The bamboo spacing decreases with increasing temperature from 800 to 900 jC at different C2H4/NH3 ratios and revealed that (i) the inter-bamboo

spacing is higher for lower C2H4/NH3 ratio (relatively higher NH3 at fixed C2H4 flow rate), (ii) the inter-bamboo spacing decreased with increasing temperature

at all gas mixing ratios.


al. [23]. To explain this distinction, we propose a hypothesis

as follows. While the formation of the separation dia-

phragms were indeed a result of bulk diffusion of carbon

atoms from one side (bottom) of the catalyst particle

following thermal dehydrogenization to the other side

(top) of the particle, there were other carbon atoms diffusing

presumably via surface diffusion to the CNT-metal interface

and contributed to the growth of tube wall [26]; in other

words, the CNTs growth is simultaneous renucleation and

growth processes, instead of a continuous renucleation and

growth process. The high-resolution TEM micrograph as

shown in Fig. 6 (high-resolution TEM micrograph of Fig.

1b) is typically observed in our experiments that the

separation graphite layers on top of the catalyst are not

connected to the tube wall graphite layers. Consequently, at

Fig. 4. The formation rate of each diaphragm (Rb) increases with increasing temper

when bulk diffusion is hampered, the surface diffusion is relatively fast that h

temperatures the spacing is relatively short due to faster bulk diffusion rate. On the

higher temperatures was attributed to higher carbon diffusion rates, both through

low temperatures when bulk diffusion is hampered, the

surface diffusion is relatively fast that higher spacing

between two adjacent bamboo structures; whereas at high

temperatures the spacing is relatively short due to faster bulk

diffusion rate. On the other hand, higher Rb (the number of

compartments formed per unit time) at higher temperatures

was attributed to higher carbon diffusion rates (as shown in

Fig. 4), both through bulk and surface.

In Fig. 6, for the CNT grown under C2H4/NH3 = 30:90

sccm (high NH3 flow) and 850 jC, we also found that there

are different number of graphene layers (wall thicknesses) at

locations a and b of the same CNT, which means that the

wall thickness increases even at the wall far away from the

Ni particle (location b). Comparing this to the cases at low

NH3 flow and high temperature of 900 jC (e.g. Fig. 1f), it

ature from 800 to 900 jC at different C2H4/NH3 ratios. At low temperatures

igher spacing between two adjacent bamboo structures, whereas at high

other hand, higher Rb (the number of compartments formed per unit time) at

bulk and surface.

Fig. 5. The dependence of CNT length on the growth temperature at different C2H4/NH3 ratios. Even though the trends are not apparent in each ratio, but the

trends appear after consider with the growth time, length and bamboo spacing of CNTs (as shown in Fig. 4).


reveals that the thickening of CNTs was due to deposition

by a-C at low NH3 flow and high temperature, whereas the

deposition of ordered graphene layers were responsible for

the wall thickness increase at high NH3 flow and relatively

low temperature. This not only further confirmed the ability

of a-C removal by NH3, but also revealed that NH3 assisted

in ordered graphene layer deposition on the outer surface of

CNTs if excess carbon atoms were supplied either through

surface diffusion or from decomposition of C2H4 in the gas

phase.

4. Conclusion

It is believed that the optimal CNTs growth situated on

the condition that carbon atom supply and diffusion reach an

identical rate; that is, CNTs grow with ‘‘best quality’’ at this

optimal growth condition. In this article, systematic experi-

ments were performed furthermore to examine the role of

NH3 on the growth of CNTs in atmospheric thermal CVD

under various temperatures and carbon source/NH3 ratios.

Besides, a temperature fixed pretreatment process and rapid

Fig. 6. The high-resolution TEM micrograph of Fig. 1b with CNT grown under C

separation graphite layers on top of the catalyst are not connected to the tube w

grapheme layers (wall thicknesses) at the locations a and b (which is far away fr

pumping method at the end of process could avoid the

inaccuracy of results indeed. In addition, this study has also

carried out a statistical analysis through intensive TEM

observations on the tube diameters, bamboo spacing and a

new parameter Rb (the formation rate of each diaphragm)

under various temperatures and carbon source/NH3 ratios.

The role of NH3 is assumed to maintain the catalytic activity

of the nickel particle surface so that higher NH3 ratio will

provide more active surface sites and higher carbon atom is

supplied from the catalyst surface. While the formation of

the separation diaphragms were indeed a result of bulk

diffusion of carbon atoms from the bottom of the Ni nano-

particle following thermal dehydrogenization to the top of

the Ni nanoparticle, there were other carbon atoms diffusing

presumably via surface diffusion to the CNT-metal interface

and contributed to the growth of tube wall; in other words,

the CNTs growth is simultaneous renucleation and growth

processes, instead of a continuous renucleation and growth

process. Furthermore, we found that the thickening of CNTs

was due to deposition by a-C at low NH3 flow and high

temperature, whereas the deposition of ordered graphene

layers were responsible for the wall thickness increase at

2H4/NH3 = 20:90 sccm and temperature of 850 jC. It was observed that the

all graphite layers. It was also observed that there are different number of

om Ni particle).


high NH3 flow and relatively low temperature. This not only

further confirmed the ability of a-C removal by NH3, but

also revealed that NH3 assisted in ordered graphene layer

deposition on the outer surface of CNTs if excess carbon

atoms were supplied either through surface diffusion or

from decomposition of C2H4 in the gas phase.

Acknowledgements

Financial support for this work from the National Science

Council under Contract No. NSC-92-2622-E-007-016 and

some TEM analysis supported by Chung-Shan Institute of

Science and Technology are gratefully acknowledged.

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On the kinetics of carbon nanotube growth by thermal CVD method

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