Effect of catalyst on growth behavior of carbon nanotube synthesizing by microwave heating thermal chemical vapor deposition process

heating thermal chemical vapor deposition process

Shi-Cheng Chen, Liang-Yuan Shih, Yu-Chen Chang, George-C Tu, and I-Nan Lin

Citation: Journal of Vacuum Science & Technology B 19, 1026 (2001); doi: 10.1116/1.1375818 View online: http://dx.doi.org/10.1116/1.1375818

View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/19/3?ver=pdfcov

Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

Articles you may be interested in

Effect of catalyst oxidation on the growth of carbon nanotubes by thermal chemical vapor deposition

J. Appl. Phys. 100, 104321 (2006); 10.1063/1.2364381

Carbon nanotubes synthesized by biased thermal chemical vapor deposition as an electron source in an x-ray tube

Appl. Phys. Lett. 86, 123115 (2005); 10.1063/1.1891299

Excellent field emission from carbon nanotubes grown by microwave-heated chemical vapor deposition

J. Vac. Sci. Technol. B 21, 1655 (2003); 10.1116/1.1591750

Effect of catalyst on growth behavior of carbon nanotubes synthesized by microwave heating thermal chemical vapor deposition process

J. Vac. Sci. Technol. B 21, 400 (2003); 10.1116/1.1531150

Effect of Structures and Morphologies of Catalyst on Growth of Carbon Nanotubes Synthesized by Chemical Vapor Deposition

by microwave heating thermal chemical vapor deposition process

Department of Chemistry, National Cheng-Kung University, Tainan, Taiwan 701, Republic of China Yu-Chen Chang and George-C Tu

Department of Materials Science and Engineering, National Chaio-Tung University, Hsin-Chu, Taiwan 300, Republic of China

I-Nan Lin

Materials Science Center, National Tsing-Hua University, Hsin-Chu, Taiwan 300, Republic of China 共Received 19 September 2000; accepted 9 April 2001兲

A modified chemical vapor deposition process, which utilizes a susceptor to absorb the microwave and to self generate the heat, for heating up the Si substrate, was used for growing carbon nanotubes

共CNTs兲. The advantage of such a process is that the deposition chamber can be maintained at around

room temperature with only the substrates localized heated, such that the deposition temperature can be more precisely controlled. The influence of the pretreatment process for catalyst-coated Si substrates on growth behavior of CNTs and the related electron field emission properties were systematically examined. Among the form of catalyst used, the Fe共NO3兲3–ethythersilicate mixture performs much better than the dc sputtered Fe films. The higher the concentration of Fe species in the catalyst mixture, the denser the CNTs formed on the substrates, resulting in better field emission properties. Thus grown CNTs can be turned on at a very low field (E0⬵0.78 V/␮m兲, achieving a very large emission current density (Je⬵13 mA/cm2兲 at 5.5 V/␮m applied field. © 2001

American Vacuum Society. 关DOI: 10.1116/1.1375818兴

I. INTRODUCTION

Since their first successful synthesis by the arc discharg-ing technique,1 and the subsequent method with the metal catalyst in an inert gas atmosphere,2,3 extensive investiga-tions on carbon nanotubes have been pursued due to their unique physical properties4,5 and potential technological applications.6–8 The arc discharging technique, which can synthesize the carbon nanotubes 共CNTs兲 directly from a graphite, is simple but usually produces a mixture of carbon clusters and carbon nanotubes. The chemical vapor deposi-tion 共CVD兲 process can grow CNTs on a catalyst substrate and thus produced materials contain a large proportion of CNTs. The drawback in this process is that the scaling up of the process is difficult due to the high substrate temperature needed. Moreover, gas phase reaction is easily induced in such a high temperature environment.

In this article, we modified the CVD process to improve the quality of CNTs. SiC materials were used as susceptors to absorb the microwave, self generating the heat. The sub-strates can be raised to CNTs deposition temperature, while maintaining the surrounding at low temperature. The nucle-ation and growth of CNTs can be better controlled. The in-fluence of characteristics of substrate, catalyst species, and concentration of catalyst on growth behavior of CNTs were systematically examined.

II. EXPERIMENTS

A modified microwave enhanced plasma CVD process was utilized to synthesize the CNTs. A novel technique, which use SiC as a susceptor for absorbing the microwave to

self generate the heat, was used as a substrate heater, such that the CH4 reacts locally with catalyst coated on Si共100兲 substrates. The catalyst is a mixture of iron nitrate

关Fe共NO3兲3兴 and ethyl–silicate 关Si共OC2H5兲4兴. The Fe共NO3兲3•5H2O was first dissolved in ethanol with a concen-tration of 0.01, 0.1, or 1.0 M and then was added into ethyl– silicate drop by drop. The Fe共NO3兲3–Si共OC2H5兲4 catalyst was then spin coated on silicone substrates, followed by py-rolysis and H₂ plasma reduction processes. Pure methanol

共CH4兲 with 300 sccm flow rate was used as reaction gases. The chamber pressure was maintained at about 700 Torr and substrate temperature was kept at around 1100 °C, during the growth of CNTs.

The morphology of CNTs was examined using scanning electron microscopy共Joel-800, Hitach S-4700兲. The electron field emission properties of CNTs coated on a Si substrate was measured using a diode setup. An indium–tin–oxide-coated glass, which served as an anode, was separated from the cathode, CNT-coated silicon, by a 180 ␮m spacer. The current–voltage (I – V) characteristics of the paralleled plates were measured using Keithley 237 under 10⫺6 mbar and were analyzed using the Fowler–Nordheim共FN兲 model. The turn-on field (E₀) was designated as the voltage at which the ln(I/V2)–(l/V) plot共FN plot兲 deviates from straight line. The

␸/␤ ratio (␸e) is estimated from the slope of the straight

line, where␸is the true work function of the CNTs and␤is the geometric factor of the emission sites. The emission cur-rent density is estimated simply by dividing the emission current by the emitting area, which is defined by the opening of the spacer共i.e., 5 mm⫻5 mm兲.

III. RESULTS AND DISCUSSION

The growth behavior for CNTs in the microwave heating thermal CVD共MHT-CVD兲 process is not very much differ-ent from that of the convdiffer-entional CVD process. As shown in Fig. 1共a兲, most of the CNTs, about 300 nm in diameter, nucleate and grow from the catalysts, which are spherical Fe-containing clusters about the same size as the CNTs. Fig-ure 1共b兲 and the inset illustrate even more clearly how the CNTs grow from a catalyst particle. To successfully synthe-size the carbon nanotubes, the substrate temperature needs to be maintained at around 1000 °C, which is in fact the most critical conditions for the synthesis of carbon nanotubes in the MHT-CVD process. The carbon nanotubes thus obtained, which are curved and about several microns in length, dis-tribute over the substrates uniformly, and the number density of carbon nanotubes increases with the concentration of Fe clusters contained in the catalyst mixture, as illustrated in Figs. 2共a兲, 2共b兲, and 2共c兲. These results clearly imply that, in the Fe共NO3兲3–Si共OC2H5兲4 mixture, the Fe clusters are the acting catalytic sites, promoting the formation of carbon nanotubes. The Si共OC2H5兲4is presumably reduced to Si clus-ters due to preheating under reducing atmosphere in the

CVD process. The Si clusters not only react with Fe clusters, but also interact with the Si substrate, which improves the adhesion of carbon nanotubes. The CNTs to Si interaction is expected to improve the electron transport from substrates to emitting materials, CNTs, since the electron transport barrier is greatly reduced.

The electron field emission properties of the thus obtained carbon nanotubes are shown in Figs. 3共a兲, 3共b兲, and 3共c兲 to reveal that all the CNTs-coated silicons exhibit very good electron field emission properties. The electron field emis-sion current density increases with Fe concentration in the Fe共NO3兲3–Si共OC2H5兲4, Fe–Si–OR mixture. The electron field emission current density is around (Je)0.01 M

⫽75 ␮A/cm2 at 5.5 V/␮m applied field for the CNT layer FIG. 1. Micrographs of CNTs grown on Fe共NO3兲3–Si共OC2H5兲4catalyst

mix-ture, indicating that CNTs stem out from the tip of a catalyst cluster.

FIG. 2. Variation of morphologies of CNTs grown on Fe共NO3兲3–Si共OC2H5兲4

catalyst mixture with the Fe共NO3兲3concentration in the catalyst mixture.

grown on the 0.01 M Fe–Si–OR coated substrate. The Je

value of the CNT layer increases dramatically to (Je)0.1 M

⫽4500 ␮A/cm2 and even to a value as high as (Je)1 M

⫽13 000 ␮A/cm2, as the Fe3⫹content in the catalyst mix-ture increases from 0.1 to 1 M, as shown in Figs. 3共b兲 and 3共c兲, respectively. Moreover, the turn-on field (E0) estimated from the FN plots decreases monotonously with concentra-tion of Fe3⫹ in the catalyst mixture, viz, (E0)0.01 M⫽1.56 V/␮m, (E0)0.1 M⫽1.44 V/␮m, and (E0)1 M⫽0.78 V/␮m.

It should be noted that, although the electron field emis-sion current density (J_e) varies markedly as the Fe content of the catalyst mixture changes, the ␸_e⫽␸/␤ ratio of the three CNT layers changes insignificantly, i.e., ␸e⫽0.016– 0.022

eV关cf. Figs. 3共a兲, 3共b兲, and 3共c兲兴. If we assume that the true work function 共␸兲 of the CNTs, which is a sheet of rolled

s p2 network, is the same as the graphites 关i.e., (␸)CNTs⫽5 eV兴, we can estimate approximately that ␤⫽1000 for the CNTs shown in Fig. 2. These results infer that all of the CNTs, which emit electrons under the application of electric field, are of similar characteristics. The Fe3⫹ content in the catalyst mixture does not alter the diameter, the length, or the electron field emission behavior of individual CNTs. The larger electron field emission current density for the CNT layer grown on 1 M Fe–Si–OR catalyst mixture 关Fig. 2共c兲兴 is mainly due to higher number density of the CNTs. Re-stated, the electron field emission properties of the CNTs insignificantly change with the conditions for synthesizing the CNTs. The detailed structure of the CNTs, viz, chirality, single wall/multiwall, open end/closed end, seems not to markedly influence the electron field emission characteristics of the CNTs. Therefore, the substrate pretreatment process is believed to be the most important factor modifying the elec-FIG. 4.共a兲 Morphology and 共b兲 electron field emission properties of CNTs grown on Si substrates using Fe共OR兲3as catalyst (OR⫽C7H17COO).

FIG. 3. Variation of electron field emission properties of CNTs grown on a Fe共NO3兲3–Si共OC2H5兲4catalyst mixture with the Fe共NO3兲3concentration in

tron field emission characteristics of the CNT layers grown by this technique.

To support the above-described arguments, the character-istics of CNTs synthesized on a different form of Fe catalyst were investigated. Figure 4共a兲 reveals that when spin coated Fe–alkoxides关共Fe共C7H17COO兲3兲, Fe共OR兲3兴, were used as a catalyst, CNTs can also be easily grown on the Si substrate using similar synthesizing parameters. The thus obtained CNTs also possess very good electron field emission proper-ties 关Fig. 4共b兲兴. It takes only E0⫽2.2 V/␮m to induce the emission process and attain a current density about Je

⫽1050 ␮A/cm2under 5.5 V/␮m applied field. The effective work function of these CNTs (␸e⫽0.025 eV兲 is about the

same value as those grown on the Fe共NO3兲3–Si共OC2H5兲4 catalyst mixture. By contrast, the CNTs can hardly form on dc-sputtered Fe films, no matter what are the synthesizing conditions.

IV. CONCLUSION

CNTs exhibiting good electron field emission properties have been successfully synthesized using a novel CVD pro-cess, in which the substrates were heated up by a susceptor via the absorption of microwaves. Morphology of the CNTs coated on the silicon substrate varies with the form of Fe catalyst coated. CNTs own largest number density and

ex-hibit largest emission current density when using spin coated 1 M Fe共NO₃兲₃–Si共OC₂H₅兲₄ as a catalyst. The electron field emission can be turned on at E₀⫽0.78 V/␮m, attaining J_e

⫽13 000␮A/cm2under 5.5 V/␮m applied field, even though the CNTs are randomly oriented. These CNTs possess large enough electron field emission properties and have good po-tential for applications as electron emitters in field emission display.

ACKNOWLEDGMENT

Financial support from National Science Council, R.O.C. through Project No. NSC. 89-2112-M-007-080 was grate-fully acknowledged by the authors.

1

S. Iijima, Nature共London兲 354, 56 共1991兲.

2_{S. Iijima and T. Ichihasi, Nature共London兲 363, 603 共1993兲.}

3_{D. S. Bethune, C. H. Kiang, M. S. deVries, G. Gorman, R. Saroy, J.}

Vazguez, and R. Beyers, Nature共London兲 363, 605 共1993兲.

4

M. S. Dresselhaus, G. Dresselhaus, and R. Saito, Carbon 33, 883

共1995兲.

5_{A. M. Rao et al., Science 275, 187共1997兲.}

6_{W. A. de Heer, J. M. Bonard, K. Fauth, A. Chatelian, L. Forro, and D.}

Ugarte, Adv. Mater. 9, 87共1997兲.

7

G. Nagy, M. Levy, R. Scarmozzino, R. M. Osgood, Jr., H. Dai, R. E. Smalley, and G. F. McLane, Appl. Phys. Lett. 73, 529共1998兲.

8_{R. Martel, T. Schmidt, H. R. Shea, T. Hertel, and Ph. Avouris, Appl.}

Phys. Lett. 73, 2447共1998兲.