ISSN 0020�1685, Inorganic Materials, 2010, Vol. 46, No. 10, pp. 1072–1076. © Pleiades Publishing, Ltd., 2010.Original Russian Text © Gang Li, 2010, published in Neorganicheskie Materialy, 2010, Vol. 46, No. 10, pp. 1190–1194.

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1 INTRODUCTION

Carbon nanotubes (CNTs) have attracted consider�able interest because of their unique physical proper�ties and may potential applications [1]. Differentdiameter and chirality of nanotubes give rise to diversechemical, physical and mechanical properties. CNTshave been synthesized by numerous techniques, suchas arc discharge [2, 3], laser ablation [4, 5] and chem�ical vapor deposition (CVD) [6–8], etc. Compared toarc discharge and laser ablation, CVD has been used toproduce highly purified CNTs in large quantities andalso to obtain CNTs vertically aligned on substrates foruse as electron emitters. Moreover, this process makesit possible to control the size and growth density ofCNTs by dispersing the catalyst on substrate andadjusting the reaction parameters. Especially, thermalCVD compared to plasma enhanced CVD is better forlarge area synthesis. To research growth mechanismand realize device applications, controlled growth ofwell�aligned CNT is essential. It has been known thatthe morphology of transition metals is of criticalimportance for the CNTs growth. Therefore, controlof the surface morphology of catalytic metal is anessential ingredient prior to the CNTs growth. Fur�thermore, the diameter, density, and length of CNTscan be controlled by the catalyst morphology.

In this study, we used NH3 thermal treatment tocontrol the surface morphologies of the Ni films. NH3

pretreatment of Ni films were adopted to adjust theRMS of surface, the size and density of Ni nanoparti�

1 The article is published in the original.

cles. CNTs were grown on the Ni nanoparticles. Howthe morphology of transition metals after NH3 pre�treatment influences on the vertically aligned carbonnanotube growth was investigated.

EXPERIMENTAL

Preparation of catalysts. The substrates used inexperiments were 6�in. p�type (100) orientated siliconwafers with resistivity of 15–25 Ω cm. In order toremove chemical impurities and particles, the waferswere cleaned by standard RCA cleaning procedures.Ni catalyst films were deposited using K575X PeltierCooled High Resolution Sputtering Coater (Quo�rum/Emitech, English). The film thickness was mon�itored in situ by a quartz�crystal based thickness mon�itor, and calibrated ex situ by atomic fore microscopy(AFM, Explorer EX300500, Veeco, USA). Fifteennanometer Ni films were deposited with a power of120 mA and a sputtering time of 40 s.

Pretreatment of catalysts. The pretreatment of cat�alysts was carried out in a horizontal TCVD system.The as�prepared Si substrates were placed in the reac�tion region of a horizontal quartz tube furnace. Thereaction region was heated to 580°C under nitrogenand then nitrogen substituted by hydrogen at a flowrate of 200 sccm to deoxidize the Fe catalyst for40 min. After that, the reaction region was heated tothe pretreatment temperature (700, 800, and 900°C)in hydrogen. Prior to decomposition reaction, Fe cat�alyst on Si substrates was pretreated in NH3 gas for20 min. The temperature of catalyst pretreatment in

Effect of Morphology of Catalyst Thin Film on Carbon Nanotube Growth

Gang LiLaboratory of Ministry of Education for Conveyance and Equipment, School of Mechanical and Electrical Engineering,

East China Jiaotong University, Nanchang 330013, P.R. Chinae�mail: ligang0794@163.com

Received October 11, 2009

Abstract—The Ni catalyst films were deposited on single crystal Si by magnetron sputtering system. The sur�face morphologies of the films were controlled by varying the pretreatment temperature in NH3. We haveinvestigated how the morphology of Ni after thermal treatment influences on the vertically aligned carbonnanotubes growth during thermal chemical vapor deposition. We found that the pretreatment temperaturedetermined the grain size and density of the Ni catalyst that controlled the diameter, length, and density ofcarbon nanotubes. The degree of crystallization and order of the CNTs as a function of pretreatment temper�ature was further discussed by Raman spectrums. We found that more aligned and ordered samples corre�spond to lower ID/IG ratios.

DOI: 10.1134/S0020168510100080

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NH3 was varied to study its influence on the morphol�ogies of catalyst thin film and aligned carbon nanotubearrays.

Synthesis of carbon nanotubes. Aligned carbon nano�tube arrays were grown using thermal decomposition ofC2H2 at atmospheric pressure. After pretreatment, a mix�ture of H2 and C2H2 at the ratio of 40/200 ml/min wasintroduced into the reaction region. The catalyticdecomposition reaction proceeded for 10 min at850°C. The samples were cooled to room temperatureby the in�flow of N2 and then taken out. During thegrowth process of carbon nanotubes, H2 served asetchant and dilute gas. The purities of the gasemployed in the experiment are all higher than 99.5%.

Characterization. The surface morphologies ofnickel thin films were examined by atomic forcemicroscopy (using a non�contact mode of atomicforce microscopy (AFM). The length, diameter anddensity distributions of CNTs were observed by fieldemission scanning electron microscope (FESEM,JEOL JSM�7001F, Japan). The crystallinity of CNTswas characterized by Raman spectroscope (JY,T64000, France) using 514.5 nm Ar excitation.

RESULTS AND DISCUSSION

Effects of NH3 plasma pretreatment. We observed theeffect of NH3 gas on the formation of Ni nanoparticleduring pretreatment. Figures 1a, 1b, and 1c are theAFM images of surface morphology of Ni catalyticthin films with 15 nm in thickness etched at differentpretreatment temperature in NH3 ambient for 10 min.The corresponding temperature is 800, 900, and1000°C in images Figs. 1a, 1b, and 1c, respectively. Weobserved that the Ni thin was transformed into nano�particles. These nanoparticles could act as the nucle�ation sites for growth of carbon nanotubes. As the pre�treatment temperature increased from 700 to 900°C,an average diameter of Ni particles increased and thedensity of particles deceased. These results suggestingthat Ni film agglomerated to become large particlesdue to the increase of mobility of nanoparticles on thesubstrate surface at higher temperatures during pre�treatment [9, 10]. It was believed that diffusion, nucle�ation and etching would occur during NH3 plasmapretreatment. The NH3 plasma pretreatment also pro�vided cleaning effect which would help growth of car�bon nanotubes. And both small nanoparticles and par�

500 nm 500 nm

500 nm

(a) (b)

(c)

10

0800

RM

S,

nm

Temperature, °C900 1000

20

(d)

Fig. 1. AFM images of surface morphology for Ni films after pretreatment for 15 min: (a) 800°C, (b) 900°C, (c) 1000°C, and(d) height RMS of roughness as a function of the pretreatment temperature.

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tially thin films were etched and then removed by NH3

gas. The RMS surface roughness of the Ni thin filmschanged from 6.14 to 22.68 nm as pretreatment tem�perature increased from 800 to 1000°C, as shown inFig. 1d.

Growth and properties of carbon nanotubes. Figures 2a,2b and 2c show the SEM images of aligned carbonnanotubes grown on the substrate with Ni particlesthat were pretreated with the previously describedconditions. The edges are peeled off using a razor, inorder to be visualized. For the case of NH3 pretreat�ment with 800°C, the nanotubes are oriented perpen�dicular to the surface of the substrate due to high den�sity of Ni nanoparticles as shown in Fig. 2a. WhenNH3 pretreatment temperature of increased up to900°C, because of a low particle density, the nanotubedensity is also low and the nanotubes are not strictlyoriented vertically as shown Fig. 2b. In particular, weobserved that at the pretreatment temperature of1000°C, (I) the CNT lengths were relatively shortcompared to the CNTs grown at other conditions and(II) the CNTs were no longer aligned due to the lowerNi particle density as shown Fig. 2c. So from Fig. 2,

the density of CNTs decreased from 82 to 216 μm–2,and CNTs showed worse alignment as the pretreat�ment temperature increased from 900 to 1000°C.From the inset images, the ranges of the diameter ofCNTs increase from 50–100 to 100–250 nm withincreasing the pretreatment temperature. The CNTlength decrease with the RMS of surface morphology,while the CNTs diameter increase with the RMS, assummarized in Fig. 2d. As we know, catalysts played acritical role in the synthesis of carbon nanotubues.Their densities and sizes controlled the morphologiesof CNT arrays. It strongly suggests that the formationmechanism of the vertically aligned CNTs is the sterichindrance between nanotubes, which is invokes by thehigh number density of Ni grains.

Raman spectroscopy, a powerful technique, iswidely used to characterize the carbon materials.

Crystalline graphite belongs to the space group thezone�center optic phonon modes can be decomposedinto the following irreducible representations

(1)

46 ,hD

opt 2 2 1 2 22 .g u u gE E A BΓ → + + +

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800

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. dia

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f C

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The pretreatrent temperature, °C900 1000

200

(a)

1 µm 1 µm

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100 nm100 nm

100 nm

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(c)

150

250

4

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gth

of

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8

6

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2

(d)

Fig. 2. SEM images of CNTs grown on nanoparticles corresponding to Fig. 1, where insets are High�resolution SEM images ofthe CNTs, respectively.

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Among which only the two E2g modes are Ramanactive [11], the E1u and the A2u are infrared active, andthe B2g modes are optically inactive. Raman spectra ofCNTs grown on grown on nanoparticles correspond�ing to Fig. 1 are shown in Fig. 3. The information col�lected from the Raman spectroscopy investigationincluded the Raman�peak intensities for D and Gpeaks, that is ID and IG peaks, the ID/IG ratios and theposition for the ID and IG as shown in table.

From Fig. 3, all the CNTs apparently consist of twopeaks near 1350 (D band) and 1580 cm–1 (G band) sim�ilar to that of crystalline graphite. The G�band corre�sponds to a phonon high�frequency E2g first ordermode of graphite and is related to the vibration ofsp2�bonded carbon atoms in a 2�dimenstional hexago�nal lattice, such as in graphite layer [12, 13]. On theother hand, the D�band is associated with vibrations ofcarbon atoms with dangling bonds in plane termina�tion of disorder graphite, defects in the curved graph�

eme sheets, as well as the turbostratic structure ofgrapheme in the materials.

Compared to highly oriented pyrolytic graphite(HOPG) with respect to 1580 cm–1 , the G peak posi�tion of sample pretreated at 1000°C didn’t change,while the G peak position of other two samples down�shift from 7–12 cm–1 frequency as shown table. Andfrom Fig. 2, the array order of CNTs and collimationquality of a CNT decreased with the pretreatmenttemperature. We deduce that more aligned andordered CNTs correspond to the higher Einstein shift.This result is in good agreement with SEM observa�tion. Generally, the ID/IG ratio is used to evaluate thequality of as�grown CNTs film. Lower ID/IG ratio valueindicates that the CNTs are pure and have a littledefect. From table, the value of ID/IG increases from0.77 to 1.08 by increasing the pretreatment tempera�ture. The results indicate that the carboneous particlesor the structure defects increased with the pretreat�ment temperature.

1800160014001200

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, a.

u.

D�band

G�band

(a)

(b)

(c)

Raman shift, cm–1

Fig. 3. Raman spectrums of CNTs grown on nanoparticles corresponding to Fig. 1.

Spectral data by Raman measurements for carbon nanotubes arrays grown on nanoparticles corresponding to Fig. 1

Pretreatment temperature,°C D�Peak, cm–1 G�Peak, cm–1 ID, a.u. IG, a.u. ID/IG

800 1341.6 1568.3 133.3 173.7 0.77

900 1344.3 1572.9 110.2 124.8 0.88

1000 1350.6 1580.2 69.6 64.3 1.08

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CONCLUSIONS

In summary, the surface morphologies of the filmswere controlled by varying the pretreatment tempera�ture in NH3. We have demonstrated that the control ofsurface morphology of the Ni film is crucial factor ingrowing vertically aligned CNTs. As the pretreatmenttemperature increased from 800 to 1000°C, the RMSof surface morphology and average diameter of Niparticles increased and the density of particlesdeceased, giving rise to the reduction of the CNTslength, the number density and the alignment. Thedegree of crystallization and order of the CNTs as afunction of pretreatment temperature was further dis�cussed by Raman spectrums. We found that morealigned and ordered samples correspond to lower ID/IG

ratios.

This work is supported by the National NaturalScience of China under grant 30800666.

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