www.elsevier.com/locate/tsf

Thin Solid Films 476

Electrodeposited carbon nanotube thin films

A.K. Pal*, R.K. Roy, S.K. Mandal, S. Gupta, B. Deb

Department of Materials Science, Indian Association for the Cultivation of Science, Calcutta-700 032, India

Received 23 July 2003; received in revised form 23 September 2004; accepted 23 September 2004

Available online 2 December 2004

Abstract

A successful attempt to grow carbon nanotubes (CNTs) by electrodeposition technique for the first time is reported here. Carbon

nanotubes were grown on Si (001) substrate using acetonitrile (1% v/v) and water as electrolyte at an applied d.c. potential ~20 V. The films

were characterized by X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), Raman, optical absorbance, Fourier Transform Infra

Red spectroscopy (FTIR) and Electron Spin Resonance (ESR) measurements. The effect of magnetic field on the growth of nanotubes was

studied critically. It was found that the presence of magnetic field during electro-deposition played a crucial role on the growth of carbon

nanotubes and hence the electronic properties. Photoluminescence (PL) studies indicated band edge luminescence ~0.72–0.83 eV. Field

emission studies indicated lower turn-on voltage and higher current density for films deposited with magnetic field.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Carbon; Nanostructures; Luminescence

1. Introduction

Since the discovery in 1991 by Iijima et al. [1], a flurry of

research activities on carbon nanotubes (CNTs) has been

witnessed over the past few years. Due to their fascinating

physical and chemical properties, carbon nanotubes have

emerged as ideal candidates for nanoscale devices [2–7]. To

date, a number of techniques are available to synthesis

carbon nanotube, most common of which are arc discharge

[8], laser ablation [9] and Chemical Vapour Deposition

processes [10]. However, all these synthesis techniques

inherently produce carbon nanotubes along with various

impurities in the form of amorphous carbon, metal catalysts

and many carbonaceous particles, etc. It needed further

purification to produce high quality CNTs for device

applications.

Also, they are to be suspended in solvent before

depositing them on suitable substrates. All these purification

and suspension procedures further lead to the CNTs

containing various defects on their surfaces which pro-

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.09.064

* Corresponding author. Tel.: +91 33 473 4971; fax: +91 33 473 2805.

E-mail address: msakp@iacs.res.in (A.K. Pal).

foundly affect the electronic properties, and in turn the

device performance. So the challenge still lies on the large-

scale synthesis of CNTs directly onto the wafer/substrate in

their purest form and in a very cost-effective way.

In this report, we described here for the first time to the

best our knowledge an attempt to synthesis CNTs directly

onto the Si (001) by a simple electrodeposition technique.

This technique besides being scalable and cost-competitive

would allow coating on irregular surfaces.

2. Experimental details

CNTs were synthesized by electrolysis using acetonitrile

(1% v/v) and deionized water as electrolyte. Electrolysis

was carried out at atmospheric pressure and the bath

temperature was kept at ~300 K. Carbon nanotubes were

deposited onto Si (001) wafers (resistivity ~15 V cm; size

~10�8�0.3 mm) attached to a copper cathode. Graphite

was used as the counter electrode (anode). Before mounting

the substrates on the cathode, they were thoroughly cleaned

and rinsed with deionized water and ethanol solution,

respectively. The electrodes were separated by a distance

of ~8 mm. The applied d.c. voltage between the electrodes

(2005) 288–294

A.K. Pal et al. / Thin Solid Films 476 (2005) 288–294 289

was kept ~20 V by using a d.c. power supply capable of

generating stabilized voltage (30 V, 2 A). The deposition

was carried for ~4–6 h. The typical thickness of the films as

measured by an interferometer was ~300 nm.

The films were characterized by X-ray Diffraction

(XRD) (Rich Seifert S-3000P) using Cu Ka line (0.154

nm), Raman (Horiba U-1000), Ultra Violet–Visible–Near

Infra Red (UV–VIS–NIR) spectrophotometer (Hitachi U-

3410), Fourier Transform Infra Red spectroscopy (FTIR)

Spectroscopy (Nicolet, MAGNA-IR-750), X-ray Photoelec-

tron Spectroscopy (XPS) (Perkin Elmer, PHI-1257) system

with a 280-mm HAS and a dual anode Mg K-alpha X-ray

source (1253.6 eV) as primary beam and Scanning Electron

Microscope (SEM) (Hitachi S-2300) respectively. Electron

Spin Resonance (ESR) measurements were carried out with

a Varian X-band E-109 Century Series spectrometer

equipped with a variable temperature cryostat.

Fig. 1. SEM micrographs of three representative CNT films: (a) deposited

without magnetic filed, (b) with 0.1 T magnetic field and (c) micrograph

showing circular pattern for growth in combined electric and magnetic

field.

3. Results and discussion

The possibility of depositing carbon structures in various

forms like diamond-like carbon (DLC), diamond, a-C:H and

CNx films by electrodeposition technique were demonstra-

ted by several authors in recent years [11–16]. The choice of

electrolytes was different for different groups. Also, the

electric field applied between the electrodes was quite high

~1–5 kV for most of the workers. It was thought that a high

potential was a prerequisite to the formation of sp3-

structured carbon and the methyl radicals and hydrogen

ions present in the solvents would play a critical role in the

formation of films. In contrast to these, recently Gupta et al.

[17] demonstrated the possibility of depositing DLC and a-

C:H films by electrolysis carried out at significantly lower

applied potential 1 kV. Carbon nanotubes could be

considered as a result of folding graphite layers into carbon

cylinders of single shell (single-walled nanotubes) or of

multi shells (multi-walled nanotubes). Thus, by suitably

choosing the electrolyte and the deposition parameter, it

should be possible to deposit carbon nanotube by electro-

deposition technique. In this communication, the possibility

of the formation of carbon nanotube structures by electrol-

ysis at an applied voltage of ~20 V using acetonitrile as the

organic precursor is demonstrated. Application of magnetic

field applied perpendicular to the electric field between the

electrodes during deposition was also examined.

Fig. 1(a–c) shows the scanning electron micrographs of

three representative films deposited on Si (100). Both the

micrographs for a film deposited without a magnetic field

(Fig. 1a) and with magnetic field (Fig. 1b) clearly indicated

the formation of carbon nanotube structures, presumably

interconnected multi-walled nanotube bundles in a web-like

network. Y-type interconnected nanotubes are also visible in

the films deposited without magnetic field. The typical

diameters of CNT bundles were 20–30 nm for the films on

Si. It may be seen that the growth of CNTs on Si surface was

more or less uniform and regular. The XPS spectra (not

shown here) indicated the presence of carbon and oxygen

only. No trace of nitrogen incorporation from the bath fluid

was observed in the deposits from the XPS studies. It may

be noted here that Fu et al. [14] observed the formation of

carbon nitride on Si substrate kept at the anode using the

same precursor. The presence of oxygen may arise due to

surface contamination after the removal of the films from

the electrolytic bath.

Application of magnetic field during electrodeposition

was found to have a remarkable effect on the alignment of

Fig. 3. Reflection spectra for a representative CNT film deposited with

magnetic field.

A.K. Pal et al. / Thin Solid Films 476 (2005) 288–294290

the carbon nanotubes. Fig. 1(b) shows the micrograph of the

films deposited onto Si substrate when the deposition was

carried out at a magnetic field of ~0.1 T applied

perpendicular to electric field. It may be noticed that the

carbon nanotubes obtained without using magnetic field

(Fig. 1a) are randomly arranged while application of

magnetic field culminated in aligning the nanotubes in the

plane of the substrate surface (Fig. 1b). One would expect

the radicals (CH3+) and ions (H+; OH�) to experience

Lorentz force and move in the direction of the cathode in a

helical path whose diameter would depend on the magnitude

of both the electric and magnetic fields. It may be seen (Fig.

1c) that the signature of preferential deposition along the

periphery of circles whose diameter increased with the

increase in magnetic field while the electric field was kept

constant. One can also observe that carbon nanotubes have

grown in bunches and aligned parallel to the substrate

surface with longer length than those obtained without the

application of magnetic field. It may be noted here that

increase in magnetic field culminated in lower number

density of carbon nanotube bunches as the diameter of the

helical path became very large so that the whole film was

covered by fewer number of circles with larger diameters.

The XRD spectra (Fig. 2) of the films showed (002) and

(004) reflections which could be assigned to the hexagonal

ring structure of graphite sheets forming the carbon nano-

tube. All the peaks are slightly shifted to lower angle from

that of graphite indicating the wider interlayer spacing. The

(002) peak position for the CNT deposits on Si was located

at 2h~26.48. Additional peaks for graphitic carbon for the

reflections from (102) and (105) planes could also be

observed. The presence of peaks around the reflection for

(002) planes indicated the presence of carbon impurities

other than the graphitic one (Fig. 2).

The optical reflectance spectra of CNTs were recorded in

the range 200–2200 nm and are shown in Fig. 3. The spectra

indicated an inevitable mixture of metallic and semiconduct-

Fig. 2. XRD trace for a representative films deposited with magnetic field.

ing phase of CNTs. The CNT film on Si showed two broad

reflection bands at 1880 and 935 nm along with a weaker and

broader one at 700 nm. The band located at 1880 nm has a

distinct shoulder indicating a reflection band at 1730 nm.

The first two peaks located at higher wavelength for the films

could be assigned to the first and second interband optical

transitions of the CNTs and the one at ~935 nm to the optical

transitions from valance band to the conduction bands in

semiconducting CNTs while that at ~700 nm may be due to

metallic CNTs respectively [18,19]. The relative intensities

of the reflections corresponding to the semiconducting

density of states are higher than that of metallic tubes. This

indicates the presence of higher content of the semiconduct-

ing CNT bundles in the films than the metallic CNT bundles.

The peaks are broad and asymmetric which are believed to

be arising due to size distribution of the nanotubes.

The Raman spectra (Fig. 4a and b) are found to bear

more confirmative signature of the presence of carbon

nanotubes in both the films deposited without and with

magnetic field. The films deposited without magnetic field

(Fig. 4a) showed a sharp feature ~1354 cm�1 (D-band)

and ~1604 cm�1 (G-band). In addition to this, the film

showed a shoulder at 1370 cm�1 in the high frequency

region. This band centered around ~1354 cm�1 (D-band) is

generally attributed to defects in the curved graphite sheet

and tube ends. This band also corresponds to either a

disordered or small crystallites of sp2 networks and could

be attributed to finite size effects [20]. Basca et al. [21]

attributed some of the D-band scattering to curvature in the

tube wall. This could be explained in terms of the

relaxation of the wave vector selection rules arising out

of the finite size effects allowing the M point phonon to

contribute to the Raman scattering. The Raman peak (G

band) for films deposited without magnetic field (Fig. 4a)

is centered around 1604 cm�1 with a number of additional

features at 1526, 1550, 1590 cm�1, respectively. The peak

at 1526 cm�1 could be attributed to the longitudinal A

Fig. 5. FTIR traces for representative films deposited (a) without magnetic

field and (b) with magnetic field.

Fig. 4. Raman spectra for representative films deposited: (a) without

magnetic field and (b) with magnetic field.

A.K. Pal et al. / Thin Solid Films 476 (2005) 288–294 291

vibrations corresponding to the metallic single-walled

carbon nanotubes [22]. Peaks at 1550, 1590 and 1604

cm�1 could be assigned to the C–C stretching vibrations

E2, A, E1 of CNTs, respectively [23,24]. The 1590-cm�1

peak, although debated, is generally assigned to the Raman

resonance of semiconducting CNTs [23]. It is also

worthwhile to mention that D-band could also arise from

the surface oxidized graphite due to a CO stretching mode

or a hexagonal ring stretching mode in micro-crystallites

with surfaces covered with CO complexes [25].

It is interesting to examine the G-band for the films

deposited with magnetic field (Fig. 4b). It may be seen that

the G-band shifted to lower wave number at ~1598 cm�1

which corresponded to C–C stretching modes for CNT

while the main peak centered around 1602 cm�1 is absent.

The downshift of the frequency of the G-line with respect to

highly oriented pyrolitic graphite signifies the presence of

sharply curved and closed graphitic structures of CNTs [26]

which is consistent with the SEM observation shown in Fig.

1(b). The D-line located at ~1348 cm�1 may be attributed to

the presence of amorphous carbonaceous products or

defects in curve graphene sheets [26]. The intensity of the

D-band has decreased substantially compared to that for

film deposited without magnetic field.

The FTIR spectra recorded in absorbance mode for the

CNTs deposited without and with magnetic filed are shown

in Fig. 5a and b, respectively. The FTIR spectrum for films

deposited without magnetic field (Fig. 5a) indicates three

main features centered around 1060, 1411 and 1583 cm�1

could be observed in the lower frequency range. The broad

IR absorbance at ~1060 cm�1 is a characteristic to Si–O–Si

stretching vibrations [27]. The other two peaks centered at

1411 cm�1 and 1583 cm�1 along with other satellite peaks

at 1475, 1478, 1606 and 1630 cm�1, respectively, may be

attributed to the vibrational modes for CNTs [28]. Features

at 2854 and 2918 cm�1 in the higher frequency region of

the spectra correspond to the C–H stretching vibrations of

chemisorbed hydrogen and these peaks are quite strong

compared to those related to CNTs. The broad band

centered at ~3400 cm�1 could be attributed to the presence

of –OH groups (possibly Si–(OH)) and molecular water in

the films. In contrast, the spectrum for the films grown

with the magnetic field (Fig. 5b) is dominated by strong

peaks at ~1579 and 1414 cm�1 for CNTs while those at

~2973 cm�1 for sp3-CH symmetric and ~3274 cm�1 for

sp2 C–H symmetric modes are significantly depressed.

Thus, the FTIR spectra indicate that the films deposited in

presence of magnetic field favours the growth of CNTs.

This observation is in conformity with that obtained from

Raman measurements.

ESR studies were carried out by using the sample

scrapped out from the indium tin oxide coated glass

substrate. ESR could be used to detect the conduction

electrons of metallic or very narrow gap semiconducting

carbon nanotubes [9,29]. The ESR spectra of the as-grown

sample measured at 300 and 80 K are shown in Fig. 6. The

thermal dependence of the ESR spectra showed changes in

linewidth and g-values without any appreciable change in

the intensity. The ESR intensity of the carbon nanotubes is

Fig. 7. PL spectra recorded at 80 K and 300 K for representative films

deposited: (a) without magnetic field and (b) with magnetic field.

Fig. 6. ESR spectra for a representative film recorded at 80 and 300 K.

A.K. Pal et al. / Thin Solid Films 476 (2005) 288–294292

found to be independent of temperature similar to that of

graphite except that it shows a Curie-like behavior at very

low temperature due to the exchange interaction between

the conduction electrons and localized spins arising out of

the presence of defects and unsaturated bonds, etc. [30].

With the reduction in temperature, the ESR signal is

largely broadened with a corresponding increase in g-

values. The linewidth increased from 320 G at 300 K to

1050 G at 80 K with a corresponding increase in g-value

from 2.0 to 2.03, respectively. The average g-value of

graphite powder is ~2.018 at room temperature. Compared

to graphite, the g-values in tube geometry get attenuated due

to the curvature-induced reduction in anisotropy. The shift

in g-values (Dg) with decreasing temperature (T) reflects the

change in the spin-orbit coupling energy (k) and the energy

separation of the spin-orbit coupled states (D) and is given

by Dg~k/D [31]. With lowering in temperature, the 1-D

wires would exhibit lowering in symmetry due lattice

distortion and give rise to atomic/molecular disorder. The

anomalous change in linewidth with decreasing temperature

could be caused by the increase in disorder of the graphitic

network, and enhancement of the spin-lattice relaxation

[32]. The ESR results clearly support the formation of tubes

in our samples, and the thermal dependence of linewidth,

intensity and g-values are found to correspond closely with

those for graphite.

Photoluminescence (PL) measurements of the films were

carried out in the temperature range of 80–300 K by using a

xenon arc lamp as the emission source. A Hamamatsu

photomultiplier was used as the detector along with a 1/4-m

monochromator. The PL spectra of two representative films

deposited without and with magnetic field on Si substrates

are shown in Fig. 7a and b, respectively. The spectra were

recorded at temperatures at 80 and 300 K with excitation

radiation at 800 nm. One could observe that the PL spectra

are dominated by a sharp peak centered ~0.72–0.83 eV

followed by a small peak at ~1.03 eV (Fig. 7b). The peak at

~0.72–0.83 eV could be related to the band-edge lumines-

cence and the position of the peak shifted to higher energy

with lowering of temperature of measurement. It may be

noted here that the presence of a shoulder at 1730 nm in the

broad band in reflection spectra also indicated possible band

edge transitions in these films. Temperature coefficient of

band gap evaluated from this shift was found to be

~2.4�10�4 eV/K. It may be noted here that the peak

positions for the band edge luminescence shifted to lower

energy for films deposited with magnetic field. This observed

Fig. 8. I–V characteristics of representative CNT films deposited: (a)

without magnetic field and (b) with magnetic field. Insets show the plot of

ln[I/Eo2] vs. 1/Eo.

A.K. Pal et al. / Thin Solid Films 476 (2005) 288–294 293

variation in the position of band edge luminescence for films

deposited with and without magnetic field may be associated

with the change in diameter of the CNTs. This PL peak

position for the band edge luminescence compares favour-

ably with the predicted value of band gap (~0.9 eV) for the

[9,2] nanotubes [33] obtained from self-consistent calcula-

tions of the band structure performed with modest number of

evenly spaced points in the Brillouin zone. The higher energy

PL peaks (Fig. 7b) with low intensity at ~1.03 eV may be

associated with the possible radiative recombination via

localized states arising out of the defect states caused by

breaking of the network of a-C:H films [34,35]. It may be

stressed here that there may not be one to one correspondence

between the peaks observed in reflectance and PL spectra.

The optical absorbance of the CNTs may generally be

insensitive to small changes in the radius of nanotube.

However, they are strongly affected by the polarization

direction of an external field and also the k-band structures

(e.g., the number of sub-bands and the first Brilloun zone) of

CNTs would change with chiral angles.

Field emission properties were studied utilizing a simple

diode structure [36]. The anode, a SnO2-coated glass, was

separated from the CNT coating on Si substrate acting as a

cathode by a mica sheet (~25 m thick) having appropriate

holes. A regulated d.c. power supply (0–5 kV; 100 mA) and

a Keithley electrometer were used to record the I–V

characteristics of the above diode structure. The diode

structure was kept inside a stainless steel vacuum chamber

which could be evacuated to a pressure ~10�6 Torr. The I–V

characteristic of a representative CNT film deposited with-

out and with magnetic field is shown in Fig. 8a and b,

respectively. The turn-on voltage was found to vary between

4 and 16 V/Am. It may be observed that the films deposited

with magnetic field (Fig. 8b) have much lower turn-on

voltage (~5 V/Am) than that for films deposited without

magnetic field (Fig. 8a). The current density could be seen

to improve in films deposited with magnetic field, i.e. for

films with aligned nanotubes.

Electron field emission data were analyzed using the

Fowler–Nordheim [36] model:

I ¼ 154ab2 E2o=/

� �exp � 6830/3=2

� �=Eo

h ið1Þ

where Eo is the electric field corresponding to a flat plate

capacitor geometry, / is the work function of the material in

eV, a is the emitting area, b is the field factor (E=bEo)

giving the enhancement of local field (E) relative to Eo. Eq.

(1) may be rewritten as:

I=E2o ¼ 154ab2

� �=/

� �exp � 6830/3=2

� �=Eo

h ið2Þ

ln I=E2o

� �¼ ln 154ab2

� �=/

� �� 6830/3=2� �

=Eo ð3Þ

Eq. (3) indicates that a plot of ln[I/Eo2] vs. 1/Eo would be a

straight line, the slope of which would give the value of /.

One can then evaluate b from the intercept if the area (a) is

known. The above Fowler–Nordheim (FN) plots (ln[I/Eo2]

vs. 1/Eo) for the film grown without and with magnetic field

are shown in the inset of Fig. 8a and b, respectively. The

value of / was obtained from the slope of the above plot

and the intercept could be utilized to evaluate b if the area

(a) is known. The values of effective work function (/e=//

b) and b derived from FN plots of the films varied between

130–150 meV and 2000–2200, respectively.

4. Conclusion

In summary, a novel electrodeposition technique for the

deposition of carbon nanotubes directly onto Si substrates

by applying a very low voltage ~20 V has been demon-

strated. We observed that the initial stages of nucleation (b2

h) were due to amorphous carbon and graphitic micro-

crystallites. The subsequent increase in deposition time ~4–

6 h resulted in the growth of carbon nanotubes in a web-like

pattern onto Si substrate. Effect of magnetic field applied

perpendicular to the direction of the electric field during

deposition had significant effect on the nanotube growth.

Nanotubes grew more or less aligned to the substrate surface

when deposited in the presence of magnetic field. PL studies

indicated band edge luminescence ~0.72–0.83 eV. Films

deposited with magnetic field indicated lower turn-on

A.K. Pal et al. / Thin Solid Films 476 (2005) 288–294294

voltage and higher current density than those obtained for

films deposited without magnetic field.

Acknowledgement

The authors wish to thank the Defence Research and

Development Organization (DRDO), Ministry of Defence,

Government of India, for sanctioning financial assistance for

executing this programme. Two of us R.K.R. and S.G. wish

to thank the Council of Scientific and Industrial Research,

Government of India, for granting them fellowships for

executing this programme. Thanks are also due to Professor

S. N. Sahu, Institute of Physics, Bhubaneswar, India for his

help in recording Raman spectra of the films.

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