Solid-State Electronics 47 (2003) 2075–2080

www.elsevier.com/locate/sse

DC electric field assisted alignment of carbon nanotubeson metal electrodes

M. Senthil Kumar a, S.H. Lee b, T.Y. Kim b, T.H. Kim a, 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, South Koreab Surface Reaction Engineering Laboratory, School of Chemical Engineering and Technology, Chonbuk National University,

Chonju 561-756, South Korea

Received 5 May 2003; received in revised form 4 June 2003; accepted 7 June 2003

Abstract

Single walled carbon nanotubes (SWCNTs) have been aligned across the metal electrodes using a dc electric field.

Effects of electric field strength, nanotube concentration in the suspension, and the solvents used for CNT dispersion

were examined on the aligning nature of nanotubes. An improved dispersion of CNTs has been found in dimethyl-

formamide solution compared to ethanol. CNTs mostly moved towards anode for the applied electric field indicating

the negative charge of the nanotubes. Experimental results exhibit the possibilities of precise positioning of nanotubes

on pre-patterned electrodes by controlling the magnitude of electric field as well as the concentration of CNT sus-

pension.

� 2003 Elsevier Ltd. All rights reserved.

PACS: 81.07.De; 73.63.Fg; 85.35.Kt; 68.37.Hk

Keywords: SWCNT; DC electric field; Alignment; FE-SEM

1. Introduction

The one-dimensional structure of carbon nanotube

(CNT), a cylinder of rolled graphite sheet, has shown

intriguing mechanical, electronic, and structural prop-

erties that have stimulated a great amount of interest and

significant research progress in nanotechnology over a

decade. CNTs show both semiconducting and metallic

properties depending upon their diameter and folding

angle and therefore could be employed for building

electronic devices and circuit interconnects. CNT growth

process has now matured to fulfill the practical demands.

The capability of producing both n- and p-type CNTs

has been shown possible, which is necessary to construct

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

E-mail address: eksuh@moak.chonbuk.ac.kr (E.-K. Suh).

0038-1101/$ - see front matter � 2003 Elsevier Ltd. All rights reserv

doi:10.1016/S0038-1101(03)00258-2

electronic logic circuits. Accordingly, several devices

based on CNT have been demonstrated in recent years

such as scanning probes [1], field emitters [2], field ef-

fect transistors [3], chemical [4] and biological sensors

[5], computer logic circuits [6], memory devices [7], etc.

Nevertheless, difficulties in handling an individual

nanotube and positioning it at a desired location hamper

the absolute exploration of CNT properties and also

devices for practical applications. Most of CNT devices

have been realized by dropping nanotube-dispersed sol-

vent on pre-patterned metal electrodes on the substrates.

In this case, the nanotubes are expected to bridge a gap

between two of the electrodes only by �some chance�.Researchers have employed a number of techniques

in order to orient the nanotubes at specific sites on the

pre-patterned substrates. Individual single walled CNTs

have been found to deposit on chemically functionalized

nanolithographic templates [8]. High-density selective

placement methods for CNTs have been developed using

ed.

2076 M. Senthil Kumar et al. / Solid-State Electronics 47 (2003) 2075–2080

aminopropyltriethoxysilane (APTS) patterns on silicon

substrates [9]. Zhang et al. utilized the electric field dur-

ing nanotube growth process to orient CNTs along a

particular direction by avoiding randomization of na-

notubes due to thermal fluctuations and gas flows [10].

Likewise, there are few reports deal with CNT alignment

onmetal electrodes accomplished using applied ac and dc

electric fields [11–13]. Electric field alignment technique is

very powerful and of great importance since nanotubes

and wires can be placed at specific locations in a more

simple way to realize functional devices and circuits [14].

In this report, we demonstrate the alignment of CNTs

between metal electrodes using dc electric fields and

discuss the influence of applied electric fields and CNT

concentration of the suspension on the alignment

behavior of nanotubes.

2. Experimental

The samples used for our experiments were single

walled carbon nanotubes (SWCNTs) fabricated by

arc discharge method. The length and diameter of the

SWCNTs in the form of ropes were measured about 2-4

lm and 10-15 nm, respectively. The CNT suspension

was prepared with two different solvents such as ethanol

and dimethylfloramine (DMF) and the dispersion of

CNTs in the solution was obtained by proper sonication

for several hours. The metal electrode consisted of finger

patterns was defined on oxidized silicon substrate using

UV lithography and lift-off technique followed by suc-

Fig. 1. FE-SEM pictures of aligned SWCNTs with various dc elect

concentration of 12.5 lg/cm3 in ethanol.

cessive evaporations of Ti (30 nm) and Pt (70 nm)

metals. The width of electrode finger and the spacing

between two-neighbor electrodes were about 3 and 2 lm,

respectively. Alignment experiments were conducted by

dispensing a drop of CNT suspension onto metal pat-

terns biased with a dc electric field. The applied dc

voltage to the electrodes was varied from 0 to 20 V. The

concentration of CNTs in the suspension was altered as

6.25 and 12.5 lg/cm3 in order to study the relative

density of aligned CNTs on electrodes. The alignment

was allowed to occur until the placed CNT-suspended

ethanol was completely dried on the electrode patterns

whereas the alignment duration for DMF based sus-

pension was maintained as 30 min. The results of vari-

ous �pipetting time� after sonication of CNT suspension

have also been investigated on the alignment nature.

The nature of CNT alignment on electrode patterns

was observed through field emission scanning electron

microscopy (FE-SEM) with an accelerating voltage of

15 kV. The current flow across the metal electrodes

through aligned nanotubes was analyzed using current–

voltage (I–V ) characteristics.

3. Results and discussion

The observed FE-SEM pictures of aligned CNTs on

metal electrodes are shown in Fig. 1 for various applied

electric fields of (a) 0, (b) 25, (c) 50, and (d) 100 kV/cm

with a CNT concentration of 12.5 lg/cm3 in ethanol.

Preferred alignment of nanotubes on the electrodes has

ric fields of (a) 0, (b) 25, (c) 50, and (d) 100 kV/cm for CNT

M. Senthil Kumar et al. / Solid-State Electronics 47 (2003) 2075–2080 2077

not been detected without an applied dc electric field as

shown in Fig. 1a. Only some nanotubes and particles

appeared randomly placed on the electrodes and in the

gap as well. It could be seen from the picture that the

nanotubes positioned parallel to the metal electrodes

under zero applied electric field without an alignment.

When a dc electric field of 25 kV/cm is applied between

the electrodes, few CNTs and nanoparticles aligned

vertically to the electrodes as observed in Fig. 1b. To

understand the alignment mechanism, we consider that

the dipole moments are induced in the nanotubes by the

applied electric field, and subsequently, the nanotubes

move towards the electrodes for the alignment due to

Coulomb force. Owing to strong dipole moment in the

axis parallel to the length of the nanotubes, they attempt

to align perpendicular to the parallel electrodes and

along the electric field direction. Meanwhile, the dipole-

induced nanoparticles are drawn by the strong electro-

static force at electrode edges and largely deposited on

the electrodes. The length of the aligned CNTs is mea-

sured about 2–3 lm; it reveals that the as-prepared

nanotubes have not been cut into small pieces due to

sonication. As the nanotubes have enough length larger

than the electrode spacing, they orient well between the

electrodes by constructing a bridge for possible current

flow. At some places, several nanotubes aligned at one

electrode edge and linked up to other electrode by

forming a rope. The density of aligned nanotubes and

nanoparticles is found to increase with increase of ap-

plied dc electric fields to 50 and 100 kV/cm as given in

Fig. 1c and d, respectively. A stronger electric field at-

Fig. 2. FE-SEM pictures of aligned SWCNTs with CNT concentratio

kV/cm. Lower magnification pictures of the same with dc electric fiel

tracts a larger number of nanotubes and nanoparticles

towards electrodes for the alignment. This result in the

formation of network structures consisting nanotubes

and nanoclusters between the electrodes, especially for

the higher electric fields. These observations are evident

that the applied electric field only aligns the nanotubes

and has a strong influence on the density of collected

CNTs and nanoparticles. The density of CNTs and

nanoparticles were found relatively higher at the elec-

trode peripheries than electrode spacing since the electric

field is concentrated at electrode edges and is nonuni-

form in the electrode gaps. Yamamoto et al. have ex-

plained the alignment of nanotubes along the electric

field based on the anisotropy of their electrophoresis

velocity [11].

Fig. 2a shows the CNT orientation on electrodes

aligned with diluted nanotube concentration of 6.25 lg/cm3 in ethanol for the applied electric field of 50 kV/cm.

Here, the number of collected nanotubes and nanopar-

ticles between the electrodes has significantly reduced.

Comparing Figs. 1c and 2a, we may strongly declare that

the density of CNT alignment between the metal elec-

trodes could be controlled by the alteration of nanotube

concentration in the suspension. The FE-SEM pictures

of CNT-aligned electrode patterns, taken at lower mag-

nification, for various dc electric fields of 15, 30, and 50

kV/cm are given in Fig. 2b, c, and d, respectively. As we

discussed previously, these pictures provide clear evi-

dence that the number of collected nanotubes and par-

ticles on the metal electrodes increases considerably as a

function of magnitude of dc electric fields.

n of 6.25 lg/cm3 in ethanol for applied dc electric field of (a) 50

ds of (b) 15, (c) 30, and (d) 50 kV/cm.

Fig. 3. FE-SEM pictures of aligned SWCNTs with CNT con-

centration of 12.5 lg/cm3 in DMF solution for applied dc

electric field of 25 kV/cm: (a) higher and (b) lower magnifica-

tions.

2078 M. Senthil Kumar et al. / Solid-State Electronics 47 (2003) 2075–2080

The FE-SEM picture of CNTs aligned from the

suspension containing dispersed nanotubes in the DMF

solution is shown in Fig. 3a. The nanotube concentra-

tion and applied electric field for this experiment were

kept as 12.5 lg/cm3 and 25 kV/cm. The picture exposes

the alignment of nanotubes with less attachment of

nanoparticles whereas many nanoparticles were found

attached to CNTs aligned using CNT dispersed ethanol

solution (Fig. 1). The nanotubes are exceptionally sep-

arated in the suspension made up of DMF solution, and

hence, an improved dispersion of nanotubes is achieved

for better alignment. This result is in good agreement

with the earlier report [15].

In all of our experiments, it is found that the na-

notubes and nanoparticles deposited more densely on

the metal electrodes in addition to alignment of few

nanotubes between the electrodes and they massively

gathered near the electrode ends rather than the full

length of the electrodes. Because, the applied electric

field is quite stronger at the end of electrode fingers and

hence more number of nanotubes and particles are at-

tracted towards the ends. As a result, CNTs and nano-

particles were found massively crowded at the electrode

ends and the regions surrounding them as seen in Fig.

3b. Besides, a heavy deposition of nanotubes and

nanoparticles was found mainly on the positive elec-

trode, i.e. anode. This behavior of CNT deposition on

anode reveals that the nanotubes are negatively charged.

Chen et al. have reported the similar movement of

SWCNTs towards anode [13] while Wakaya et al. ob-

served that the MWCNTs in isopropyl alcohol moved

towards the cathode under the influence of an external

electric field [16]. The real mechanism involved in the

movement of nanotubes towards a particular type of

electrode is not yet clear. However, it is reasonable to

consider that CNTs with a perfect structure will move

towards both the electrodes (anode and cathode) due to

uniform strength of electric field at both electrodes. The

selective deposition of the CNTs is probably attributed

to the structure defects or the charge of the carbon na-

notubes, which could be made in the sample preparing

process [13].

CNT alignments were made with different �pipettingtime� after sonication of the CNT suspension and the

respective results on the nanotube alignment were also

examined. The CNT dispersed solution was pipetted for

the experiment from the surface of the suspension res-

ervoir. The time delay between sonication and pipetting

of the suspension for the experiment was intentionally

delayed upto 3 min and the corresponding FE-SEM

pictures of CNT alignment (in lower magnification) are

given in Fig. 4. The density of aligned nanotubes and

nanoparticles gradually decreased when the pipetting

time was increased from 30 s to 3 min as seen in Fig.

4a–d. The large particles and clusters are slowly brought

down to the suspension reservoir due to gravitational

effect with respect to increased time delay in pipetting

after the sonication. Therefore, the density of the sus-

pension itself degreases near the surface with time and

this effect accordingly reflects in the alignment outcome.

The selective deposition of nanotubes and nanoparticles

on the anode has been perceived very clearly in this case,

which we have already discussed.

The room temperature current–voltage (I–V ) char-

acteristic of CNTs aligned on metal electrodes with

the nanotube concentration of 12.5 lg/cm3 and the dc

electric field of 10 kV/cm is presented in Fig. 5. The

current transport between the electrodes shows a non-

linear dependence of applied voltage with the presence

of Schottky barrier at nanotube–metal contact and a

measured current of few microamperes. This room tem-

perature non-linearity of I–V curve indicates the semi-

conducting nature of aligned CNTs and their ability to

be used for the fabrication of electronic nanodevices. In

some cases, the measured current was rather high de-

scribing an accumulation of nanotubes and nanoparti-

cles between the electrode gaps, which conduct more

current. Generally, the amount of current flow across

the electrodes depends on the possible alignment of

nanotubes and nanoparticles connecting two electrodes,

that is unpredictable. The efforts for the quantita-

tive study of current flow across the aligned CNTs are

underway.

Fig. 4. FE-SEM pictures of SWCNTs aligned on electrode patterns for various pipetting time of (a) 30, (b) 90, (c) 120, and (d) 180 s

with CNT concentration of 12.5 lg/cm3 in DMF and applied dc electric field of 10 kV/cm.

-1.0 -0.5 0.0 0.5 1.0-25

-20

-15

-10

-5

0

5

10

15

20

25

Cur

rent

(µA

)

Voltage (V)

Fig. 5. The room temperature current–voltage (I–V ) charac-

teristic of aligned SWCNTs with dc electric field of 10 kV/cm

and CNT concentration of 12.5 lg/cm3 in DMF.

M. Senthil Kumar et al. / Solid-State Electronics 47 (2003) 2075–2080 2079

4. Conclusions

A dc electric field assisted alignment of SWCNTs

between metal electrodes has been attained. The density

of aligned nanotubes is greatly influenced by the mag-

nitude of applied electric field and increases with in-

crease of electric field strength. The reduction of CNT

concentration in the suspension decreases the density of

aligned nanotubes on the electrodes. Electric field

alignment technique shows the promise for handling

individual nanotubes by altering the concentration in

CNT suspension and electrode patterns, which is im-

portant not only to construct nanodevices but also to

explore the fundamental properties of nanotubes.

Acknowledgement

This work was supported by the Korea Research

Foundation grant no. 2001-005-D0036.

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