highly - ordered carbon nanotube arrays for electronics applications

Moskovits Department of Chemistry, University of Toronto, Toronto M5S 3H6, Canada 共Received 1 March 1999; accepted for publication 23 May 1999兲 Highly-ordered arrays of parallel carbon n

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Highly-ordered carbon nanotube arrays for electronics applications

J Li, C Papadopoulos, and J M Xua)

Department of Electrical and Computer Engineering, University of Toronto, Toronto M5S 3G4, Canada

M Moskovits

Department of Chemistry, University of Toronto, Toronto M5S 3H6, Canada

共Received 1 March 1999; accepted for publication 23 May 1999兲

Highly-ordered arrays of parallel carbon nanotubes were grown by pyrolysis of acetylene on cobalt

within a hexagonal close-packed nanochannel alumina template at 650 °C The nanotubes are

characterized by a narrow size distribution, large scale periodicity, and high densities Using this

method ordered nanotubes with diameters from 10 nm to several hundred nm and lengths up to 100

␮m can be produced The high level of ordering and uniformity in these arrays is useful for

applications in data storage, field emission displays and sensors, and offers the prospect of deriving

computational functions from the collective behavior of symmetrically coupled nanotubes The

fabrication method used is compatible with standard lithographic processes and thus enables future

American Institute of Physics. 关S0003-6951共99兲01929-4兴

Carbon nanotubes1are among the most promising

mate-rials anticipated to impact future nanotechnology Their

unique structural and electronic properties2,3have generated

great interest for use in a broad range of potential

nanodevices.4–7Most of these applications will require a

fab-rication method capable of producing uniform carbon

nano-tubes with well-defined and controllable properties

reproduc-ibly In addition, electronic and photonic devices such as

field emission displays and data storage8,9would need high

density, well-ordered nanotube arrays While efforts to

fab-ricate high-quality crystalline ropes or bundles of carbon

nanotubes10 and aligned arrays of isolated carbon

nanotubes11–14have been successful, to date it is still a

chal-lenge to produce arrays of isolated carbon nanotubes with

uniform diameters and periodic arrangement to meet device

requirements In this letter, we describe a method for

fabri-cating large arrays of parallel carbon nanotubes with an

un-precedented level of periodicity and uniformity by pyrolysis

of acetylene on cobalt within a hexagonal close-packed

nanochannel alumina共NCA兲 template

The method we used is based on template growth,

re-cently used by us and a rapidly increasing number of

work-ers, as a possible alternative route to the future

nanofabrica-tion of electronic devices.8When compared with mainstream

semiconductor fabrication techniques this template method

has the important advantages of being nonlithographic and

does not involve a cleanroom process In addition, the

method is not material specific; we have been successful in

fabricating semiconductor, metallic and magnetic nanowire

and nanodot arrays using related template-based methods.8

Here, the template approach was extended to produce

peri-odic carbon nanotube arrays by first electrochemically

de-positing a small amount of cobalt into the pores of a

hexago-nally ordered nanochannel alumina template and then

growing carbon nanotubes by pyrolysis of acetylene under

cobalt catalysis in the nanochannels

An illustration of a typical fabrication process flow is

shown in Fig 1共a兲 The process begins with the anodization

of high purity共99.999%兲 aluminum on a desired substrate It has been observed15 that under appropriate anodizing condi-tions the pores of the anodic alumina film can self-organize into a highly ordered hexagonal array of parallel vertically-oriented pores After subsequent investigations,16 it is now well established that by varying anodizing conditions hex-agonal close-packed arrays with selectable diameters, densi-ties and lengths can be formed defect-free over large areas.17–19The scanning electron microscope共SEM兲共Hitachi S-4500兲 images in Figs 2共a兲 and 2共b兲 show results of a two-step anodization method that was used to create a NCA tem-plate consisting of a hexagonal array of 32 nm diameter channels, 6␮m in length, by anodizing an aluminum sheet in

a 0.3 M oxalic acid solution at 15 °C under a constant volt-age of 40 V The next step is to electrochemically deposit a

a 兲Electronic mail: xujm@eecg.utoronto.ca

FIG 1 共a兲 Schematic of fabrication process 共b兲 SEM image of the resulting

hexagonally ordered array of carbon nanotubes fabricated using method in

共a兲.

367

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small amount of cobalt catalyst into the bottom of the

tem-plate channels8,10 关Fig 1共a兲, center兴 The ordered array of

nanotubes are grown by first reducing the catalyst by heating

the cobalt-loaded templates in a tube furnace at 600 °C for

4–5 h under a CO flow 共100 cm3min⫺1兲 The CO flow is

then replaced by a mixture of 10% acetylene in nitrogen at a

flow rate of 100 cm3min⫺1 关Fig 1共a兲, right兴 In a typical

experiment, the acetylene flow is maintained for 2 h at

650 °C The samples are then annealed in nitrogen for 15 h at

the same temperature An SEM image of a highly ordered

carbon nanotube array formed in this way is shown in Fig

1共b兲

The resultant carbon nanotube arrays were characterized

using SEM Figures 3共a兲 and 3共b兲 show SEM micrographs of

carbon nanotube arrays which have been ion milled to

re-move residual amorphous carbon from the template surface

The tubes in Fig 3共a兲 were partially exposed by etching the

alumina matrix using a mixture of phosphoric and chromic

acid The SEM micrographs show several important features

of the carbon nanotube arrays produced by this technique

First, all of the nanotubes are parallel to each other and

per-pendicular to the template forming a periodic hexagonal

close-packed array Second, the nanotubes are of uniform

length and are open ended Third, each pore of the template

is filled with one nanotube, which defines the tube diameter

In addition, the tube diameter distribution throughout the

ar-ray is narrow, typically 5% of the mean diameter关Fig 3共a兲,

lower right inset兴—much narrower than heretofore reported using other methods of nanotube array synthesis The mean diameter is approximately 47 nm; slightly larger than the original template diameter due to uniform widening of the template channels during processing.10Finally, the array has

a very high density of tubes—approximately 1010cm⫺2.

Further sample characterization was carried out using transmission electron microscopy 共TEM兲共H7000 or JEOL 2021F兲 and electron diffraction Figures 4共a兲 shows a TEM image of a carbon nanotube bundle which resulted from completely dissolving the NCA matrix which supported the nanotube array using a chemical etch The nanotubes are straight and have uniform lengths of 6␮m equal to the thick-ness of the NCA film in which they were grown The elec-tron diffraction patterns of the nanotube bundle 关Fig 4共a兲, inset兲 imply that the tubes are graphitic with an interwall

distance (d002) of approximately 3.6 Å, slightly larger than

the interplanar separation in graphite (d002⫽3.35 Å) The tube wall thickness was found to lie in the range 4–5 nm, suggesting the tubes are composed of approximately 12 gra-phitic shells

Several significant features of the nanotubes produced

by this fabrication technique are noted: Aside from the ex-cellent uniformity in size and disposition, the nanotubes

FIG 2 Nanochannel alumina templates A two-step anodization method

共see Ref 19兲 was used to obtain the hexagonal close-packed nanochannel

alumina templates: 共a兲 SEM image of etched alumina template after first

anodization showing top view of the resultant surface 共b兲 Second

anodiza-tion; The patterned surface from the previous step is anodized again under

identical conditions as in 共a兲 The SEM image shows an oblique

cross-section view of the resultant highly-ordered nanochannel alumina having 6

␮ m long channels, 32 nm in diameter with a density of approximately

10 10 cm 2

FIG 3 Highly-ordered carbon nanotube arrays 共a兲 SEM image showing

oblique view of periodic carbon nanotube array The inset at the lower left is

an enlarged view of the tubes The inset at the lower right is a histogram of the nanotube diameter showing a narrow size distribution around 47 nm 共b兲

Top-view SEM image of the carbon nanotubes showing hexagonal close-packed geometry The hexagonal cells have sides approximately 57 nm long and the intercell spacing is 98 nm The slight splitting of the tube ends and the apparent increase in tube wall thickness is an artifact of the nonspecial-ized ion-milling apparatus that was used in our experiments The inset shows a close-up view of a typical open-ended carbon nanotube in its hex-agonal cell.

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grow naturally perpendicular to a rigid substrate without

ex-tra processing steps In addition, this method of nanotube

array synthesis is not inherently area limited and can be

scaled up with the template size.18The approach of

nanofab-rication presented need not involve lithography and is

there-fore inexpensive The gentle electrochemical methods used

and moderate growth temperature 共650 °C兲 also make the

method compatible with standard lithographic processes

since the aluminum film used to form the NCA template can

be deposited and processed on a variety of surfaces including

standard silicon wafers The controlled variation of the

nano-tube size, density, and array spacing depends on easily

ad-justable parameters such as the anodizing voltage, electrolyte

composition, and temperature resulting in ordered arrays

with selectable diameters ranging from approximately 10 nm

to several hundred nm and densities of up to 1011cm⫺2.

Tube lengths of up to 100␮m can be obtained by varying the

length of the pores in the NCA template in which the

nano-tubes are grown, which can be achieved by varying the time

of anodization

The above properties are important for fundamental and

applied purposes; Precise and reproducible control of

nano-tube dimensions should allow the reliable study of their

physical properties In addition, our technique arranges

indi-vidual carbon nanotubes in a periodic superstructure creating

the unique possibility of studying novel mesoscopic

collec-tive excitations and cooperacollec-tive phenomena due to

electro-magnetic coupling of tubes in the array.20 Finally, our

method allows inexpensive production of large arrays of

or-dered carbon nanotubes with controllable dimensions needed

for practical applications

The physical mechanism of carbon nanotube growth by

the catalytic decomposition of organic vapors has been

pos-tulated as either base or tip growth.21At this stage an

expla-nation of the nanotube growth mechanism within NCA

tem-plates must remain speculative, but some observations can

help point the way to an eventually understanding of the

growth process; using SEM we have observed residual co-balt catalyst in the base of the tubes 关Fig 4共b兲兴 indicating that a tip growth mechanism cannot be entirely responsible for the tube growth In addition, the appearance of catalyst-free tube ends in the SEM images further argues for base growth However, the situation is complicated by the pres-ence of the alumina template which may also act as a catalyst

in the nanotube growth.10 Determining the exact nature of the growth process will require further detailed study

In summary, we have synthesized highly-ordered carbon nanotube arrays over large areas by pyrolysis of acetylene on cobalt within a hexagonally-disposed nanochannel alumina template The method presented in this letter offers precise control of nanotube length共up to 100 ␮m兲, diameter 共⬃10–

350 nm兲 and array density 共up to 1011cm⫺2兲 These ex-tremely uniform arrays could be used in a variety of appli-cations including high-density data storage, inert membranes for biomedical use, field emission displays, and infrared im-aging detectors Looking further, our method allows indi-vidual carbon nanotube devices to be periodically assembled into ultradense nanoelectronic networks whose collective be-havior could then be used to perform computational func-tions

The authors would like to thank A Rakitin, A J Ben-nett, and D Levner for valuable discussions Support from NSERC, OCMR, and Nortel is greatly appreciated

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FIG 4 共a兲 TEM image of carbon nanotube bundle completely removed

from the NCA template The nanotubes are very straight and have uniform

lengths of 6 ␮ m corresponding to the dimensions of the NCA template they

were grown in The insets are electron diffraction patterns of the nanotubes.

共b兲 Cross-section SEM image of the nanotube array partially exposed from

NCA template; the cobalt catalyst is at the base of the tubes separated from

the aluminum substrate The inset is an enlarged view TEM image of a

single nanotube showing the cobalt particle at the base.

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