Surface and Coatings Technology 169–170(2003) 348–352

0257-8972/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž03.00108-7

Comparison of source gases and catalyst metals for growth of carbonnanotube

Tae Young Lee , Jae-Hee Han , Sun Hong Choi , Ji-Beom Yoo * , Chong-Yun Park , Taewon Jung ,a a a a, ,1 a b

SeGi Yu , Junghee Lee , Whikun Yi , Jong Min Kimb b b b

Center for Nanotubes and Nanostructured Composite Sungkyunkwan University, 300, Chunchun-Dong, Jangan-Gu, Suwon 440-746,a

South KoreaNCRI, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Koreab

Abstract

Effects of different carbon source gases and catalyst metals on the growth characteristics and emission properties of carbonnanotubes(CNTs) were investigated. As the flow rate ratio of NHyC H increased, the growth rate of CNTs was enhanced and3 3 4

the average diameter of CNTs became smaller. Ni was more efficient for the field emission of CNTs than any other catalystmetals when using the C H gas, and in case of C H as the carbon source, Co played an important role as a catalyst. When2 2 3 4

using the CO gas, Fe was the most activated catalyst for the CNTs growth under the flow rate ratio of COyNH of 18. CNTs3

grown using different catalyst metals had their corresponding catalyst particles at the top of the tips.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Carbon nanotube; Source gas; Catalyst metal; Field emission

1. Introduction

Among various synthesis methods for the carbonnanotube (CNT) growth, chemical vapor deposition(CVD) method has been used for the growth of CNTsw1–3x. Especially, plasma enhanced chemical vapordeposition (PECVD) method has been reported as apromising candidate for the synthesis of CNTsw4–6x.In PECVD method for the CNTs growth, many gaseousprecursors such as CH , C H , C H and CO have been4 2 2 6 6

employed. These precursors play important roles in thegrowth characteristics and properties of CNTs becauseof their own binding energy, type, and role of reactivespecies under plasma, and thermodynamic properties.However, there have been few reports on the effectthese precursors on the growth characteristics and prop-erties of CNTs.

In this study, we used C H , CO and C H for the2 2 3 4

growth of CNTs as a carbon source and investigateeffects of different precursors on the growth character-

*Corresponding author.E-mail addresses: jbyoo@sait.samsung.co.kr(J.-B. Yoo),

jbyoo@yurim.skku.ac.kr(J.-B. Yoo).Currently on sabbatical leave at SAIT.1

istics and emission properties of CNTs. The effect ofthe catalyst such as Ni, Co, and Fe on the growthcharacteristics of CNTs with different precursors wasalso studied and compared.

2. Experiments

In this work, the growth of vertically aligned CNTswas carried out using various source gases with NH as3

a catalytic gas on different catalyst metals(Ni, Co andFe) coated glass substrate with Cr buffer layers. The Crbuffer layer of 1700 A and the catalyst layer of 300 A˚ ˚were deposited on the glass substrate by electron beamevaporation. The CNT growth was performed at thetemperature at approximately 5508C by the PECVDmethod. A C H , C H and CO gas was used as the2 2 3 4

carbon source and the NH gas was used as the catalyst3

and a dilution gas, respectively. The experimental detailsare listed in Table 1. A d.c. plasma was used to growvertically aligned CNTs. Detailed processes for thegrowth of CNTs were described in our previous reportsw7–9x. Field-emission scanning electron microscopy andhigh-resolution transmission electron microscopy wereused for the analysis of the surface morphology, crosssection and the microstructure of CNT films. The emis-

349T.Y. Lee et al. / Surface and Coatings Technology 169 –170 (2003) 348–352

Table 1The experimental conditions used in this study for the growth of CNTs

Source gas Flow rate(sccm) Plasma intensity(VyA) Pressure(Torr) Time (min)

(catalyst NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3metal) Pre- q Pre- q Pre- q Pre- q

etching Source gas etching Source gas etching Source gas etching Source gas

C H (Ni) or C H (Co)2 2 3 4 240 60 540y0.04 620y0.12 2.8 3.3 5 20CO (Fe) 10 180 540y0.04 620y0.12 2.8 3.3 5 20

Catalyst metals shown are the most effective ones for the growth of CNTs when using the corresponding source gases.

Fig. 1. SEM micrographs showing the morphology of CNTs using various source gases and catalyst metals:(a) C H –Ni (source gas–catalyst);2 2

(b) C H –Co;(c) C H –Co;(d) CO–Fe;(e) C H –Ni; (f) C H –Co, where the flow rate of NH was 240 sccm for(a–c) and 300 sccm for(e2 2 3 4 3 4 3 4 3

and f) (C H and C H were 60 sccm equally). For (d), the flow rates of NH and CO were 10 and 180 sccm, respectively. Scale bars indicate2 2 3 4 3

500 nm equally.

sion characteristics of CNTs were measured in a vacuumchamber with a parallel diode-type configuration at5=10 mbar.y7

3. Results and discussion

Fig. 1 shows morphologies of CNTs grown on theglass substrate with various catalyst metals(Ni, Co andFe) and precursors. When C H gas was used as the2 2

carbon source, vertically aligned CNTs were grown onNi (denoted by C H –Ni, in this paper) and Co catalysts2 2

(denoted by C H –Co, in this paper) under the condition2 2

that the flow rate of a NH and the C H gas was 2403 2 2

and 60 sccm, respectively. The average diameters ofCNTs with C H –Ni and C H –Co were 100 and 1202 2 2 2

nm, respectively(Fig. 1a and b). However, CNTs wererarely grown on Fe catalyst under this condition andonly the initial stage of growth morphology wasobserved(not shown here). Fig. 1c shows CNTs withthe average diameter of CNTs of 140 nm grown usingC H gas, which indicates that the CNTs growth in3 4

C H is activated especially with Co catalyst(denoted3 4

350 T.Y. Lee et al. / Surface and Coatings Technology 169 –170 (2003) 348–352

Fig. 2. Current vs. applied electric field(I–V) curves with different source gases and catalyst metals. Current saturations are indicated by arrows.Inset shows F–N plots achieved from CNTs.

by C H –Co, in this paper) when the flow rates of the3 4

NH and the C H gas were 240 and 60 sccm,3 3 4

respectively.Using CO gas, CNTs with the average diameter of

CNTs 40 nm were well grown on Fe catalyst(denotedby CO–Fe, in this paper) when the flow rates of NH3and CO were 10 and 180 sccm, respectively(Fig. 1d).Under such flow rates as mentioned above, CNTs werealso well grown on Invar catalysts(consisting of Fe52%, Ni 42% and Co 6%, respectively) too. To furtherstudy growth characteristics of CNTs using the C H3 4

gas, the flow rate of the NH gas was only varied to3

300 sccm at the constant flow rate of the C H gas of3 4

60 sccm(Fig. 1e and f). As shown in Fig. 1e, CNTswith Ni catalyst (denoted by C H –Ni, in this paper)3 4

were grown with the relatively large diameter(;240nm). The average diameter of CNTs with C H –Co was3 4

reduced to 70 nm(Fig. 1f) compared to those shown inFig. 1c.

Fig. 2 shows the emission current plots as a functionof applied voltage(I–V) of CNTs grown with differentcatalysts. The turn-on electric field,E , was defined asto

the electric field at 1mAycm of the current density.2

E of CNTs with C H –Ni, C H –Co, C H –Co, andto 2 2 2 2 3 4

CO–Fe(corresponding to Fig. 1a–d, respectively) were4.5, 7.7, 10.3 and 6.4 Vymm, respectively. It is notedthat the current saturation(indicated by arrows) fromCNTs with CO–Fe appeared at the relatively low current

density of 0.01mAycm at the electric field of 2.8 Vy2

mm and the rate of increase in the emission currentrapidly decreased. However, in the samples using theC H gas(marked bys andn), the current saturation2 2

occurred at higher current density(f40–50mAycm ).2

Thus,E of CNTs with CO–Fe was higher than that ofto

CNTs with C H –Ni although the increment in the2 2

emission from CNTs with CO–Fe in the low field region(-2.8 Vymm) was the promptest among the otherCNTs.

From the Fowler–Nordheim(F–N) model, the fieldenhancement factor(b) can be calculated from the slopeof the F–N plotwln(IyV ) vs. 1yVx w10x. As shown in2

the inset of Fig. 2, F–N plots for CNTs using differentthe carbon source gas and the catalyst metal showeddistinct features in the slope of the plot. The fieldenhancement factors,b, of samples with C H –Ni,2 2

C H –Co and C H –Co, are determined to 820, 460,2 2 3 4

and 380ycm, respectively.Based on results following, two points should be

noticed. First, the growth rate of CNTs was much higherand the average diameter became smaller as the flowrate ratio of NHyC H increased. Second, Ni was more3 3 4

efficient for field emission of CNTs as the catalyst metalwhen using the C H gas; on the other hand, in case of2 2

C H as the carbon source, Co played an effective role3 4

as the catalyst.

351T.Y. Lee et al. / Surface and Coatings Technology 169 –170 (2003) 348–352

Fig. 3. (a) TEM images of CNT grown with C H –Ni. The upper-left image shows Ni cap at the CNT tip and the upper-right image shows the2 2

Ni (420) facet of Ni particle. The lower image shows the interior and the wall structures of CNTs.(b) TEM images of CNT grown with CO–Fe. The left image shows the interior and wall structures of the CNTs and the right image indicate an Fe cap at the end of the CNT.

Fig. 3a is the TEM (300 kV, H-9000NA) cross-sectional views showing the Ni tip and wall structuresof CNTs with C H –Ni. It shows a multiwalled structure2 2

with the inner and outer diameter of the tube is approx-imately 7.5 and 70 nm, respectively. The graphitic wallsof the CNT looks like having some cut-off defects,

which may be due to the low temperature growth. A Nicap of 17 nm was observed at the end of each CNTs asshown in the upper-left image of Fig. 3a. The upper-right image of Fig. 3a shows the forefront facet of Niparticle was identified as Ni(4 2 0), and the ring fringesmay stem from disordered{ 0 0 0 2} planes, which

352 T.Y. Lee et al. / Surface and Coatings Technology 169 –170 (2003) 348–352

indicates spacing of 0.34 nm between graphite planes.Microstructure of CNT with CO–Fe was also examinedby TEM as shown in Fig. 3b. The interior and wallstructures of the CNT were shown in the left image ofFig. 3b. It was shown that the inner and outer diameterof CNT is approximately 6 and 50 nm, respectively. AnFe cap with the diameter of approximately 30 nm wasobserved at the end of some of CNTs as shown in theright image of Fig. 3b.

4. Conclusion

As the flow rate ratio of NHyC H increased, the3 3 4

growth rate of CNTs was enhanced and the averagediameter became smaller. Ni was more efficient for thefield emission of CNTs than any other catalyst metalwhen using the C H gas, and in case of C H as the2 2 3 4

carbon source, Co played an important role as thecatalyst. When using the CO gas, Fe was the mostactivated catalyst for the CNTs growth under the flowrate ratio of COyNH of 18. CNTs grown using different3

catalyst metals had their corresponding catalyst particlesat the tips.

Acknowledgments

This work was supported by the Ministry of Com-merce, Industry and Energy of Korea.

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