deposition of cnt on moving substrate by laser induced cvd

7/28/2019 Deposition of Cnt on Moving Substrate by Laser Induced Cvd

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Continuous deposition of carbon nanotubes on a moving substrateby open-air laser-induced chemical vapor deposition

Kinghong Kwok, Wilson K.S. Chiu *

Department of Mechanical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269-3139, USA

Received 21 January 2005; accepted 11 May 2005Available online 5 July 2005

Abstract

Continuous deposition of carbon nanotubes under open-air conditions on a moving fused quartz substrate is achieved by pyro-lytic laser-induced chemical vapor deposition. A CO2 laser is used to heat a traversing fused quartz rod covered with bimetallicnanoparticles. Pyrolysis of hydrocarbon precursor gas occurs and subsequently gives rise to rapid growth of a multi-wall carbonnanotube forest on the substrate surface. A mushroom-like nanotube pillar is observed, where a random orientation of carbonnanotubes is located at the top of the pillars while the growth is more aligned near the base. The typical carbon nanotube depositionrate achieved in this study is approximately 50 lm/s. At high power laser irradiation, various carbon microstructures are formed as aresult of excessive formation of amorphous carbon on the substrate. High-resolution transmission and scanning electron micros-copy, and X-ray energy-dispersive spectrometry are used to investigate the deposition rate, microstructure, and chemical composi-tion of the deposited carbon nanotubes. 2005 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes; Chemical vapor deposition; Electron microscopy; X-ray energy-dispersive spectrometry; Reaction kinetics

1. Introduction

A vast amount of research effort has been devoted tothe development of synthesis techniques that would havethe capability of growing high quality, by-product freecarbon nanotubes. Arc-discharge, laser ablation, andhigh-pressure carbon monoxide (HiPco) techniqueshave been widely used for the growth of single- andmulti-wall carbon nanotubes [14]. However, low pro-

duction rate and high deposition temperature or pres-sure (typically between 3000 and 4000 C) required bythese techniques resulted in a scale-up production ofcarbon nanotubes that is prohibitively expensive.Thermal CVD and various forms of plasma CVD tech-niques [57] have been successful in growing a relatively

large quantity (kilogram and even ton level) of carbonnanotubes. However, relatively low deposition tempera-ture in CVD (5501200 C) results in carbon nanotubeswith high defect densities and a low degree of graphiti-zation [4], thereby restricting their usage for advancedapplications.

Recently, Kwok and Chiu [8] have successfully dem-onstrated the feasibility of using pyrolytic LCVD to de-posit carbon nanotubes on stationary fused quartz

substrates in open-air. It was found that different re-gions of carbon growth occur as a result of non-uniformtemperature distribution and the reactant gas jets cross-flow configuration. However, this technique is not suit-able for mass production of nanotubes because asignificant amount of pyrolytic carbon by-products aredeposited as a result of elevated substrate temperatureinduced by direct laser irradiation. Furthermore, ourprevious investigation focused primarily on the funda-mental aspect of open-air LCVD synthesis, so that the

0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2005.05.016

* Corresponding author. Tel.: +1 860 486 3647; fax: +1 860 4865088.

E-mail address: [email protected] (W.K.S. Chiu).

Carbon 43 (2005) 25712578

www.elsevier.com/locate/carbon
mailto:[email protected]:[email protected]


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effect of experimental parameters on carbon nanotubegrowth kinetics, microstructure, and chemical composi-tion has not been examined.

A scanning laser-induced CVD technique is proposedand investigated in order to reduce the amount of unde-sirable by-products and increase the yield of carbon

nanotubes. In this technique, the laser heated spotmoves with respect to the substrate at a prescribedvelocity and direction, thereby reducing the amount oftemperature rise on the substrate surface and the laserresidence time [9]. The formation rate of pyrolytic car-bon film decreases as a result of reduced temperature,which will enhance the carbon nanotube growth byallowing the catalyst particles to remain active for alonger period of time. Furthermore, a moving laserbeam will irradiate a larger substrate surface area in acontinuous manner, which is favorable for mass produc-tion of carbon nanotubes.

The objective of this work is to study the feasibility of

using open-air pyrolytic LCVD to deposit carbon nano-tubes on a moving substrate with direct application toscale-up continuous production of nanotubes. In addi-tion, the influence of laser power intensity and substratemovement on nanotube growth is studied. Extensivesearch in the open literature reveals no documentedexperimental study on the growth of carbon nanotubeson a moving substrate using an open-air CVD tech-nique. This finding is expected because the existingCVD techniques involve (i) large-area deposition thatrequires no moving components and (ii) enclosed depo-sition chamber restricts the movement of the heat source

and the substrate. The main scientific justification forthe study of carbon nanotube growth on a moving sub-strate is the potential in discovering new fundamentalphysics regarding the growth mechanism of carbonnanotubes.

2. Experimental apparatus

The laser-induced CVD system used for the continu-ous growth of carbon nanotubes on a moving substrateis shown in Fig. 1. The overall experimental setup is sim-ilar to that used for laser deposition on a stationary sub-

strate [8], and only a brief description is given here. A30-W CO2 laser with a wavelength of 10.6 lm and aGaussian beam profile is used to heat the substrate. Aplano/convex zinc selenide (ZnSe) lense is used in orderto focus the laser beam to a desired diameter on the sub-strate without altering the beam profile. An integratedcomputer data acquisition and control system is usedto control the laser power output. Fused quartz (SiO2)rods of 3-mm diameter are used as the substrate, andthe procedure for preparing catalyst particles on thesubstrate was outlined in [8].

The catalytic substrate is attached to a supportingarm as shown in Fig. 1. The substrate supporting appa-

ratus sits on a linear traverse mechanism, which is con-nect to a high-torque electric motor that can move thesubstrate at a constant velocity across the laser targetedspot. The mechanism is designed to slide on linear ballbearings to minimize the effect of both static anddynamical friction. During an experimental run, thesubstrate is first mounted onto the traverse mechanism,which is positioned away from the laser heating spot.The CO2 laser, the coaxial nozzle and the exhaust hoodare turned on and set to the appropriate laser power andflow rates. Then the electrical motor is initialized and be-gins driving the linear stage along with the substrate

through the reactor at a constant velocity. Depositionoccurs when the substrate intercepts the laser beamand the reactant gas jet.

In this study, the laser power is varied systematicallyin order to study its effect on the growth kinetics and

Fig. 1. Open-air laser-induced chemical vapor deposition (LCVD) system with a substrate moving mechanism.

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microstructure of the deposited carbon nanotubes whilethe substrate moving velocity and the catalyst particlesize are fixed at 0.14 mm/s and 350 nm, respectively.The hydrocarbon precursor gas used is propane(C3H8) with 99.95% purity and is mixed with ultra-highpurity grade hydrogen gas. The flow rates of propane,

hydrogen and nitrogen are 0.2, 1.0 and 10 SLPM,respectively. A transmission electron microscope (JEOL2010 FasTEM) equipped with an X-ray energy-disper-sion spectrometry (EDS) system, and an environmentalscanning electron microscope (Philips 2020 ESEM) areused to examine the microstructure, reaction kinetics,and chemical composition of the carbon materialsdeposited in this study.

3. Results and discussion

3.1. Continuous synthesis of carbon nanotube forest by

open-air LCVD

In the pyrolytic LCVD technique, the effect of mov-ing the substrate in one dimension perpendicular tothe focused CO2 laser beam is the same as scanningthe laser beam along the substrate. The scanning laserbeam can be described as a moving heat source that in-duces a large temperature gradient on the glass surface,which subsequently leads to thermal decomposition ofgas-phase precursors and results in the deposition of acontinuous stripe of desired material. Fig. 2a shows laserheating at the beginning section of the fused quartz sub-

strate where no catalyst is presented. Rapid temperatureraise becomes obvious by observing the bright spot on

the substrate surface. In this case, the laser power usedfor heating is 13.5 W (5.25 MW/m2) and the substratevelocity is 0.14 mm/s.

As the laser beam enters the section covered with cat-alyst, growth of bulk carbon nanotube material could beimmediately observed by a magnifying camera as shown

in Fig. 2b. The accumulation of carbon nanotubes occurvery rapidly at the trailing edge of the laser heating zoneand also within the laser heating spot. This observationsuggests that both sufficient temperature and residencetime is required for the nucleation and subsequentgrowth of carbon nanotubes since no observable nano-tube growth occurs near the front of the moving laserbeam. The direct observation of the nanotube growingprocess within the laser-heating zone has demonstratedthe rapidness of this laser driven technique. The sideview of the bulk nanotube material, which providesan estimation of its height, is shown in Fig. 2c. Investi-gation of the deposited material by an optical micro-

scope reveals no detailed microstructural informationexcept for the fact that they appear to be grayish andchunky.

Fig. 3a and b shows two ESEM images of the bulkcarbon material shown in Fig. 2c. ESEM investigationreveals pillars of mushroom-like carbon trees thatform a carbon nanotube forest. Higher magnificationESEM studies reveal individual pillars consisting ofcarbon nanotubes in tangled form with the primarygrowth direction perpendicular to the substrate surface.The mushroom-like geometry could be due to ran-dom orientation of the carbon nanotubes locate at the

top of the pillars while the growth is more aligned nearthe base. The height of the carbon nanotube pillars

Fig. 2. (a) Laser heating of an uncoated section on the fused quartz substrate moving at 0.14 mm/s. (b) Laser-induced growth of bulk carbonnanotube material on the moving substrate with the same velocity as in (a), and (c) a side view of the resulting bulk nanotube material.

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deposited in this study ranges from 0.5 to 2.5 mm, but

the length of individual nanotubes within the pillars isvery difficult to measure because of the tangling of nano-tubes and resolution limit of the ESEM. However, byfollowing several relatively straight carbon nanotubesamong the bundles, it is found that their lengths exceed50 lm. The length measurement terminates at a pointwhere further tracing becomes impossible.

High-resolution TEM (HRTEM) study was per-formed in order to examine the detailed microstructuralfeatures of LCVD-grown carbon nanotubes. A stripe ofcarbon nanotube forest that is approximately 3 cm longand 0.3 cm wide is dispersed in an ethanol solution by anultrasonic process. Small fragments of the carbon nano-tube bundle are transferred onto a TEM grid. Fig. 3cshows a TEM image of carbon nanotubes in tangledform with diameter ranging from 5 to 20 nm with themajority of the nanotubes having a diameter of 15 nm.A small amount amorphous carbon is observed amongthe nanotube bundle, which indicates that amorphouscarbon is also deposited during the nanotube growingprocess. Fig. 3d shows a HRTEM image of a pair ofcarbon nanotubes overlapping each other. It clearlyreveals the tubular graphite lattice structure of carbonnanotubes observed by Iijima [10]. In addition, thehollow core and the graphitic walls can also be directly

seen in the image. In this case, the diameter of the

nanotube and its core are approximately 3 and 10 nm,respectively. The distance between graphitic walls is0.34 nm, which matches the separation distance in bulkgraphite.

X-ray energy dispersive spectrometry was performedon a LCVD-grown carbon nanotube sample in order todetermine its chemical composition. Fig. 4 shows theEDS spectrum taken directly from the carbon nanotubebundle shown in Fig. 3c. In the spectrum, the shadedarea represents the characteristic X-ray signal generatedfrom the nanotube sample, while the area under the linerepresents those from the TEM grid which is composedof amorphous carbon and copper. Carbon nanotubebundles within the sample gives rise to the carbon peak,and oxides forming on the nanotube surface after laserdeposition process gives rise to the oxygen peak ob-served in the EDS spectrum. No palladium or gold sig-nal is detected, which implies the absence of nanotubecontamination by catalyst particle intrusion. The ab-sence of catalyst particles observed by TEM and EDSsuggests that base-growth is the primary nanotubegrowth mode in this study. In base-growth mode, cata-lyst particles remain attached to the substrate surfaceas the carbon nanotubes lengthen, as opposed to tip-growth mode where catalyst particles are lifted off from

Fig. 3. (a) ESEM image of the pillars of mushroom-like carbon trees deposited by scanning LCVD. (b) ESEM image of carbon nanotube bundles.(c) TEM image of carbon nanotubes in tangled form with diameter ranging from 5 to 20 nm. (d) HRTEM image of a pair of carbon nanotubesoverlapping each other.



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the substrate and carried along by the nanotube as itgrows in length so that nanoparticle traces will be foundwithin the bundle.

The ESEM and the TEM results have clearly demon-strated the feasibility of using scanning pyrolytic laser-induced chemical vapor deposition to grow a continuousstripe of carbon nanotube forest on a moving glass sur-face. The deposition rate of multi-wall carbon nano-tubes achieved in this study is determined based on theheight of the pillars since each pillar consists of densely

packed carbon nanotubes. The nanotube growth time isdefined as the duration of a given substrate section resid-ing within the laser beam, which is equal to the diameterof the laser beam, divide by the substrate moving veloc-ity. Since this study used 1.8 mm as the laser beam diam-eter and 0.14 mm/s as the substrate velocity, the growthtime is calculated to be 12.5 s. The average height of thepillars deposited with a laser power of 13.5 W is mea-sured by ESEM to be 0.65 mm. The resulting depositionrate is determined to be 50 lm/s, which is relatively highcompared to rates reported by thermal and plasma-enhanced chemical vapor deposition [1114].

3.2. Effect of laser power on carbon nanotube forest

morphology

A second set of experiments were performed in orderto study the effect of intense laser irradiation on themicrostructure and the growth kinetics of carbon nano-tubes by pyrolytic LCVD. The same experimentalparameters are used in this case but with the laser powerincreased to 15.5 W (5.75 MW/m2). Fig. 5a shows aschematic of a continuous stripe of nanotube forest withtwo distinctive regions of growth. The mushroom-likecarbon nanotube pillars are found in Region A as shown

Fig. 5a and b. However, a layer of amorphous carbonfilm forms beneath the pillars as a result of increasedlaser power. No amorphous carbon layer is observedto deposit on the substrate surface when a lower laserpower of 13.5 W was used. The formation of a carbonlayer on the substrate surface covers the catalyst parti-

cles, thereby terminating carbon nanotube growth.Region B forms at the perimeter of Region A, andconsists of loosely packed multi-wall carbon nanotubesgrowing in random orientation as shown in Fig. 5c.Carbon nanotubes with diameter ranging from 30 to50 nm and length ranging from 5 to 50 lm are observedin Region B. This region is located at the edge ofthe Gaussian laser beam, and therefore the surface tem-perature is lower compared to Region A. Since lesshydrocarbon molecules would undergo thermal decom-position at a lower substrate temperature, it is expectedthat less carbon nanotubes will form in Region B. Inaddition, the rapid formation of nanotube pillars in

Region A interferes with the transport of reactants toRegion B, which may cause further reduction in growthrate of carbon nanotubes observed in the region.

A third set of experiments are performed with thelaser power increased to 21.5 W (8.35 MW/m2). In thiscase, deposition of individual bulk carbon chunks wasobserved by visual inspection of the resulting sampleas shown in Fig. 5d. The appearance of this bulk carbonmaterial resembles the nanotube forest observed in theprevious sample except that they are discontinuous is-lands rather than a continuous stripe. ESEM investiga-tion also reveals two distinctive regions of carbon

growth, and the morphology of the carbon deposit with-in each region is shown in Fig. 5e and f. In this case, thebulk carbon islands are labeled as Region C, and theremaining area within the path of the laser beam islabeled as Region D.

Fig. 5e shows that Region C consists of denselypacked carbon fibers grown in random orientation withlength reaching several hundred microns. It is importantto point out that carbon fibers and multi-wall carbonnanotubes have a similar structure, and look similarunder SEM. However, carbon nanotubes have a muchsmaller diameter than the fiber and have well-graphi-tized walls [4,15]. Beside carbon fibers, carbon nanotubebundles are occasionally observed branching out fromnear the tip of the carbon fibers as shown in Fig. 5e.ESEM investigation of Region D shown in Fig. 5f re-veals columns of fiber-like carbon structures sproutingperpendicular to the substrate surface with an averagediameter of 10 lm and height ranging from 30 to100 lm. In addition, a significant amount of amorphouscarbon is deposited on the surface. Carbon nanotubebundles are not found in this region as a result of exces-sive amorphous carbon deposition, which covered uppotential nanotube precipitation sites on the catalystsurface.

Fig. 4. EDS spectrum taken directly from the LCVD-grown multi-wall carbon nanotubes. The shaded area in the EDS spectrumrepresents the characteristic X-ray signal derived from the nanotubesample, while the area under the line represents those from the TEMgrid.



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Fig. 6 shows a series of HRTEM images of carbonnanotubes and carbon fibers found in Region C. Themulti-wall carbon nanotubes observed in Region C,shown in Fig. 6a, have similar diameter and microstruc-ture compared to those deposited in the previous set ofexperiments where a lower laser power is used. Detailedexamination under TEM reveals that a multi-wall car-bon nanotube is embedded into each carbon fiber coreas indicated in Fig. 6b. This observation clearly indicatesthat the formation of carbon fibers begin with thegrowth of carbon nanotubes since it is unlikely for car-

bon nanotubes to grow inside the solid carbon fiber. Theformation of carbon fiber could be explained by the fol-lowing proposed mechanism. Initially, carbon atomsprecipitate from the carbon-saturated metal particle sur-face and form a carbon nanotube. However, rapid pyro-lysis of hydrocarbon molecules due to the intense laserirradiation results in excessive amorphous carbondeposited onto the carbon nanotube. The thickness ofthe amorphous carbon layer increases as the nanotubelengthens and eventually becomes the carbon fiber ob-served in Fig. 5e.

Fig. 5. (a) A schematic of a continuous stripe of nanotube forest showing two distinct regions of growth. (b) Mushroom-like carbon nanotubepillars were found in Region A. (c) Loosely packed multi-wall carbon nanotubes grew in random orientation found in Region B. (d) Bulk carbonchunks deposited at high laser power irradiation with two distinctive regions of growth. (e) Densely packed carbon fibers grown in randomorientation and (f) columns of fiber-like carbon structures sprouting perpendicular to the substrate surface with a significant amount of amorphouscarbon.



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HRTEM investigation of carbon fibers with diameterranging from 75 to 250 nm reveals a polycrystallinestructure as shown in Fig. 6c and d. The selected areaelectron diffraction patterns (inserted in Fig. 6c and d)obtained from the fibers confirm the formation of apolycrystalline carbon layer [15,16] on the fiber surface.The build-up of a polycrystalline carbon layer forms yetanother layer on the fiber. This three-layer carbon fiberstructure is very interesting because it combines threedistinctive forms of solid carbon with sp2 bonding stateinto a single structure. Notice that it is difficult to see thecarbon nanotube within the large diameter carbon fiberbecause the thick layer of carbon prevents the penetra-tion of the electron beam used in TEM imaging, butoccasionally a carbon nanotube core can be seen onthe fiber as shown in Fig. 6c.

Fig. 6bd shows that metal particles are scatteredalong the carbon fiber, which is not observed in the pre-vious case where a lower laser power was used. This re-sult indicates that high power laser irradiation caninduce a sufficiently high surface temperature to meltthe catalyst particles during the initial nanotube growth

stage. The adhesion force between metal particles andthe substrate surface reduces dramatically [17] whenthe metal particles start to melt, which allows the subse-quent growth of carbon nanotubes to lift the unboundedmetal particles from the surface and carry them along onthe nanotube as it lengthens. Notice that only a portionof the original metal particle seed is being lifted since thesize of metal particles found along the nanotube isapproximately 1/7 of the original particle. The liftedmetal particle acts as the catalyst for the growth of car-bon nanotube bundles that branch out from the tip ofthe carbon fibers as shown in Fig. 5e. Similar nanotubebranching phenomenon have been reported in micro-wave plasma-enhanced CVD of multi-wall carbon nano-tubes by Tsai et al. [18].

4. Conclusions

In this study, the feasibility of using open-airlaser-induced chemical vapor deposition to grow car-bon nanotubes on a moving substrate is successfully

Fig. 6. (a) HRTEM image of carbon nanotubes found in Region C. (b) Multi-wall carbon nanotube coated with a thick layer of amorphous carbonwith metal particles scattered along the nanotube. (c, d) are TEM images of large diameter carbon fibers revealing polycrystalline structure. (Inserts

are selected area electron diffraction patterns.)



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demonstrated with direct application to high efficientmass production of carbon nanotubes. Scanning elec-tron microscopy reveals pillars of densely packed carbonnanotubes deposited on the substrate with height reachseveral hundred microns. The typical growth rate ofmulti-wall carbon nanotubes obtained in this study is

50 lm/s, which is relatively high compared to the maxi-mum growth rates achievable in other chemical vapordeposition processes. High-resolution transmission elec-tron microscopy reveals the tubular graphite latticestructure of multi-wall carbon nanotubes with 0.34 nmseparation distance between the graphite walls. Contin-uous growth of carbon nanotube forest with minimumamorphous carbon and catalyst particle contaminationis obtained when a laser power density of 5.25 MW/m2

is used for deposition. At higher laser power irradiation,excessive deposition of amorphous carbon results in theformation of relatively large diameter carbon fibers withmetal particles scattered along the length of the fiber.

Acknowledgments

Financial support by the National Science Founda-tion is gratefully acknowledged. The authors thankRoger Ristau and Mary Anton at the Institute ofMaterials Science at the University of Connecticut fortheir assistance with transmission and scanning electronmicroscopy.

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deposition of cnt on moving substrate by laser induced cvd

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