Chemical Physics Letters 383 (2004) 235–239

www.elsevier.com/locate/cplett

Influence of electric field type on the assemblyof single walled carbon nanotubes

M. Senthil Kumar a, T.H. Kim a, S.H. Lee b, S.M. Song a,J.W. Yang a,*, K.S. Nahm b, E.-K. Suh a

a Semiconductor Physics Research Center and Department of Semiconductor Science and Technology, Chonbuk National University,

Chonju 561-756, Republic of Koreab Surface Reaction Engineering Laboratory, School of Chemical Engineering and Technology, Chonbuk National University,

Chonju 561-756, Republic of Korea

Received 16 July 2003; in final form 11 November 2003

Published online: 2 December 2003

Abstract

Electric field assisted assembly of single walled carbon nanotubes (SWCNTs) on lithographically patterned electrodes has been

studied using both dc and ac electric fields. The nanotube alignment is strongly dependent on the magnitude and the frequency of the

applied electric field. An improved carbon nanotube (CNT) orientation is achieved with ac electric field of high frequency due to

alternating force exerted rapidly on field-induced dipoles of the nanotubes and the nanotubes orient nearly at right angle to the

metal electrodes. The purification of as-prepared CNTs from background nanoparticles appears possible by aligning nanotubes with

an ac electric field.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

The ever-reducing device dimension is currently lead-ing us to the new field of nanotechnology where the de-

vices are made with materials in nano-scale. The amazing

progress and demonstration of several electronic, opto-

electronic and sensor nanodevices has sparked a huge

amount of interest among researchers to deal with the

science and technology of this new field. The present,

well-developed growth technologies to produce high

quality nanowires and nanotubes offer us an excellentopportunity to realize efficient and reliable nanodevices.

Among the reported one-dimensional nanostructures,

carbon nanotube (CNT) is unique because of their

promising structural, mechanical and electronic proper-

ties which have been evident behind the achievements of

CNT based field emitters [1], field effect transistors [2],

chemical and biological sensors [3], computer logic cir-

* Corresponding author. Fax: +82-63-270-3585.

E-mail address: jwyang@moak.chonbuk.ac.kr (J.W. Yang).

0009-2614/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2003.11.032

cuits [4], memory devices [5], and so on. To fabricate

nano-scale devices, especially, in electronics, the na-

notubes should be given metal contacts to electricallycommunicate with the devices. Providing electrical con-

tacts to such nanostructures is, in fact, a challenging task.

Also, the nanotubes have to be assembled in a particular

direction to build functional electronic and photonic

nanodevices. Several techniques have been adopted in

order to perform this including the followings: alignment

through introduction of argon gas in the laser ablation

reactor [6], electric and magnetic field assisted alignmentof post-grown CNTs [7,8], deposition of individual na-

notubes using chemically functionalized nanolitho-

graphic templates [9], controlling the shape and position

of CNT with AFM tip [10], and electric field directed

growth of aligned SWCNTs [11]. Assembling the na-

notubes and nanowires on electrodes using an electric

field is practically attractive and efficient technique for

various fundamental and potential applications. Therehave already been a few efforts to align the CNTs using dc

and ac electric fields [7,12,13]. In this report, we describe

236 M. Senthil Kumar et al. / Chemical Physics Letters 383 (2004) 235–239

the alignment of SWCNTs using both the dc and ac

electric fields and compare the results obtained.

Fig. 1. FE-SEM images of the aligned SWCNTs under different biasing

conditions. (a) aligned with zero bias, (b) aligned by applying a dc

voltage of 4 V, (c) aligned by applying an ac voltage of 3V (peak value)

with a frequency of 6.5 kHz.

2. Experimental

Single walled carbon nanotubes (SWCNTs) produced

using DC arc discharge technique have been employed

for our alignment experiments. The length and diameter

of the nanotubes were measured, respectively, as about

3–5 lm and 10–15 nm in the form of ropes. The solution

taken for the preparation of CNT suspension was dim-

ethylformamide (DMF) since DMF had already beenreported as a good solvent [14]. The suspension was

prepared by dispersing CNTs for various concentra-

tions. A well-dispersed CNT suspension was achieved by

sonicating the solution for several hours. The Ti/Au

(40 nm/160 nm) electrode patterns were made using

conventional UV lithography and lift-off technique on

semi-insulating GaAs and oxidized silicon substrates.

Two kinds of electrode patterns, such as two co-planarand finger type electrodes, were used for the alignment.

The distance between neighboring electrodes was about

2–4 lm. Alignment experiments were conducted by

dispensing the CNT suspension on metal patterns for

3 min after properly biasing the electrodes with applied

electric fields. Both dc and ac electric fields were utilized

to align the nanotubes. Besides, the influence of applied

electric field strength, concentration of CNT in thesuspension, and ac electric field frequency on the CNT

alignment behavior have also been investigated. The

applied dc voltage to the electrodes ranged from 0 to 5 V.

The applied ac voltage was varied from 0 to 9 V (peak

value) while the ac frequency was tuned from 10 Hz to

10 MHz. The concentration of CNTs in the DMF so-

lution was changed from 0.02 to 12.5 lg/cm3. After

alignment, the samples were cleaned with acetone andthen dried in air. The alignment nature of CNTs ori-

ented with various experimental conditions has been

observed through field emission scanning electron mi-

croscope (FE-SEM) operating with an accelerating

voltage of 15 kV. Electrical properties of the oriented

CNTs have been analyzed using current–voltage (I–V )characteristics.

3. Results and discussion

Preferred orientation of the nanotubes between metal

electrodes has not been obtained when no voltage was

applied to the electrodes. The CNTs and nanoparticles

were distributed randomly on the electrodes and also in

the electrode gaps (Fig. 1a). But, an alignment of CNTsbetween the metal electrodes was observed for both

applied dc and ac voltages. The FE-SEM image of

CNTs oriented with an applied dc voltage of 4 V and

CNT concentration of 12.5 lg/cm3 in the suspension is

given in Fig. 1b. Fig. 1c presents the FE-SEM image of

CNT orientation with an applied ac voltage of 3 V

(frequency 6.5 kHz) and CNT concentration of 4.2 lg/cm3 in the suspension. Both cases demonstrate a good

alignment of nanotubes preferably connecting two metal

electrodes for a possible current flow. The nanotubeshave enough length larger than the electrode spacing to

link the electrodes. A few nanoparticles were found at-

tached to the aligned nanotubes along with many sep-

arate particles. The density of nanotubes aligned with

Fig. 2. FE-SEM images of the aligned SWCNTs with applied dc

voltages (lower magnification). Deposition of CNTs and nanoparticles

on anodes of: (a) co-planar and (b) finger type electrodes. (c) Crowding

of nanotubes around the ends of finger electrodes for the applied dc

voltage of 4 V.

M. Senthil Kumar et al. / Chemical Physics Letters 383 (2004) 235–239 237

the dc electric field has been observed as significantly

lower compared to that aligned with ac electric field,

even if the CNT concentration in the suspension and the

applied field strength are quite higher. From FE-SEM

observations, the length of the aligned CNTs is mea-sured about 2–4 lm indicating that as-prepared na-

notubes have not been cut into pieces during sonication

of the suspension.

The alignment of nanotubes between electrodes oc-

curs due to forces that direct the nanowires toward re-

gions of high electric field. The basic mechanism of

electric field assisted nanotube alignment is understood

as follows. The applied electric field induces electric di-poles in the nanotubes and nanoparticles in the sus-

pension. In other words, the nanotubes and particles are

electronically polarized as a result of applied electric

field. The movement of nanotubes is not restricted in the

suspension, and hence, they are attracted for the align-

ment by strong electrostatic force present between the

metal electrodes. Because of the structural anisotropy of

CNTs, the induced dipole moment in the directionparallel to the length of the tube axis is much stronger

than that in perpendicular direction while the nano-

particles have uniform dipole moment irrespective of

axes. Owing to the strong polarization across their

length, CNTs move faster towards metal electrodes and

align between them by Coulomb force. In contrast,

compared to nanotubes, nanoparticles respond with

slow movement to the applied electric field due to theirisotropic dipole moment. The nanotubes align in the

direction of applied electric field by adjusting themselves

along tube axis (strongly polarized axis) parallel to

the electric field direction, as it is clear from FE-SEM

observations.

It has been observed that most of the nanotubes and

nanoparticles moved towards positive electrodes (an-

odes) under the influence of dc electric field as seen fromFig. 2a and b. Hence, only fewer CNTs were found at-

tached between electrodes when compared with ac field

assisted alignments. Yamamoto et al. [12] have reported

that MWCNTs moved towards the cathodes for an

applied electric field while Chen et al. [7] reports the

deposition of SWCNTs on anodes. Though the reason

for the selective deposition of CNTs and nanoparticles

on anodes is not completely clear, it is possible to con-sider that nanotubes and nanoparticles used for the

alignment possess some structural defects or native

charges, which could be made in the sample preparation

process. As the CNTs are mostly driven to the anode by

the electrostatic force, the density of aligned CNTs be-

tween electrodes is very poor in case of dc electric field

as observed in Fig. 1b.

For ac electric field assisted alignment, the tunedfrequency shows a great influence on the nanotube

alignment. The tangled nanotubes are straightened with

increasing frequency as the ac electric field exerts an

alternating force on both ends of the tubes at a higher

speed. Also, the nanotubes align more perpendicular to

the metal electrodes in this case (Fig. 1c) whereas the

CNTs aligned with dc field showed some inclinations

with respect to metal electrodes as seen in Fig. 1b. The

perfection of the CNT orientation at right angle to the

electrode increased with increase of applied ac fre-quency. Also, the nanotubes behave very rapidly to the

applied ac frequency and orient well between the elec-

trodes compared to nanoparticles. Therefore, the num-

ber of deposited nanoparticles decreases considerably as

the applied ac frequency is increased. Hence, as an

Fig. 4. FE-SEM image of CNT aligned by applying an ac voltage

of 3 V peak value at 1 MHz with the suspension concentration of

0.2 lg/cm3.

238 M. Senthil Kumar et al. / Chemical Physics Letters 383 (2004) 235–239

another advantage, ac electric field assisted alignment at

higher frequencies could be used to purify the as-pre-

pared CNTs from background nanoparticles consisted

of carbon and catalysts. Our observations on electric

field assisted purification of CNTs are in good agree-ment with earlier report by Yamamoto et al. [13]. This

method of purification is superior to oxidation treat-

ment that causes damage to surviving nanotubes on

their walls and at the end caps during the process [15].

At some places, few nanotubes of short length have

been found combined together and linked the electrodes

by forming a rope. After a CNT joins with an electrode

edge, the electric field is concentrated at its end thatattracts another nanotube to connect the electrodes.

Fig. 3 presents a representative FESEM image of

alignment of combined nanotubes that bridge the elec-

trodes due to concentrated electric field at their ends.

The density of aligned nanotubes and nanoparticles in-

creases with increase of applied voltage, which clearly

suggests that the electrostatic force determines the

aligning speed.The alignment with high-applied voltages results in

the formation of network structures consisting nanotu-

bes and nanoclusters between the electrodes. The density

of CNTs and nanoparticles is found relatively higher at

the electrode peripheries than at electrode gaps (Fig. 2a),

because the applied electric field is concentrated at

electrode peripheries and is non-uniform in the electrode

gaps. In case of finger type electrodes, the CNTs andnanoparticles are drawn by the strong dc field at elec-

trode ends and largely crowded around these high field

regions as shown in Fig. 2c. The density of nanotubes

aligned on electrodes has been shown possible to control

by diluting the CNT suspension. The FE-SEM picture

of CNT alignment with the suspension concentration of

Fig. 3. FE-SEM image of alignment of CNTs between metal electrodes

with an applied ac voltage of 3 V peak value at 1 MHz. Short na-

notubes are found combined at their ends (indicated by circle) due to

concentrated electric field and bridge the electrodes.

0.2 lg/cm3 and an applied ac voltage of 3 V at 1 MHz is

shown in Fig. 4. Only few nanotubes are found aligned

over the electrode length of about 100 lm. Controlling

the alignment of an individual CNT is crucial to fabri-

cate electronic devices and circuit interconnects and alsoto study their fundamental properties.

The charge transport characteristics of aligned CNTs

with both dc and ac electric fields have been studied

using I–V analysis. In case of dc electric field, the

amount of current flow across the electrodes depends on

the possible alignment of nanotubes connecting two

electrodes that is unpredictable. The I–V curves of

CNTs aligned using applied ac voltage of 3 V withvarious CNT suspension concentrations are shown in

Fig. 5. The amount of current flow increases with sus-

pension concentration as the density of aligned CNTs

increases. The current flow across CNTs aligned from

-6 -4 -2 0 2 4 6-20

-15

-10

-5

0

5

10

15

20

Cur

rent

(µA

)

Voltage (V)

0.2 µg/cm3

0.5 µg/cm3

1.0 µg/cm3

Fig. 5. Current–voltage characteristics of CNTs aligned with various

CNT suspension concentrations. The applied ac voltage and frequency

are 3 V (peak value) and 6.5 kHz, respectively.

M. Senthil Kumar et al. / Chemical Physics Letters 383 (2004) 235–239 239

high concentrated suspension is measured as few mic-

roamperes whereas it is in nano–ampere scale for the

alignment with low suspension concentration. The cur-

rent flow for aligned CNTs also increases with increas-

ing applied voltage due to high-density nanotubealignment.

In conclusion, we have demonstrated the alignment

of SWCNTs using both dc and ac electric fields. The dc

electric field assisted alignment shows a deposition of

nanotubes and nanoparticles on positive electrodes with

less number of aligned nanotubes across the electrodes.

However, the ac electric field proves to be effective for

the alignment of CNTs nearly perpendicular to theelectrodes with less attracted nanoparticles and also

straightens the tangled nanotubes at high frequencies.

The nanotubes could also be purified from carbon par-

ticles using ac electric field of high frequency. By suit-

ably optimizing the suspension concentration and the

strength of ac electric field, nanotubes can be manipu-

lated individually, which is essential to investigate their

basic properties.

Acknowledgements

This work was supported by the grant of Post-Doc.

Program, Chonbuk National University (2002).

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