Effect of oxygen plasma on field emission characteristics of single-wall carbonnanotubes grown by plasma enhanced chemical vapour deposition systemAvshish Kumar, Shama Parveen, Samina Husain, Javid Ali, Mohammad Zulfequar, Harsh, and MushahidHusain Citation: Journal of Applied Physics 115, 084308 (2014); doi: 10.1063/1.4866995 View online: http://dx.doi.org/10.1063/1.4866995 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhanced field emission from cerium hexaboride coated multiwalled carbon nanotube composite films: Apotential material for next generation electron sources J. Appl. Phys. 115, 094302 (2014); 10.1063/1.4866990 Carbon nanowalls grown by microwave plasma enhanced chemical vapor deposition during the carbonization ofpolyacrylonitrile fibers J. Appl. Phys. 113, 024313 (2013); 10.1063/1.4774218 Effect of parameters on carbon nanotubes grown by floating catalyst chemical vapor deposition AIP Conf. Proc. 1502, 242 (2012); 10.1063/1.4769148 Ultrathin ultrananocrystalline diamond film synthesis by direct current plasma-assisted chemical vapor deposition J. Appl. Phys. 110, 084305 (2011); 10.1063/1.3652752 Transition from single to multi-walled carbon nanotubes grown by inductively coupled plasma enhanced chemicalvapor deposition J. Appl. Phys. 110, 034301 (2011); 10.1063/1.3615945

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Effect of oxygen plasma on field emission characteristics of single-wallcarbon nanotubes grown by plasma enhanced chemical vapourdeposition system

Avshish Kumar,1 Shama Parveen,1 Samina Husain,1 Javid Ali,1 Mohammad Zulfequar,1

Harsh,2 and Mushahid Husain1,2,a)

1Department of Physics, Jamia Millia Islamia (A Central University), New Delhi 110025, India2Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia, New Delhi 110025, India

(Received 27 September 2013; accepted 14 February 2014; published online 28 February 2014)

Field emission properties of single wall carbon nanotubes (SWCNTs) grown on iron catalyst film

by plasma enhanced chemical vapour deposition system were studied in diode configuration. The

results were analysed in the framework of Fowler-Nordheim theory. The grown SWCNTs were

found to be excellent field emitters, having emission current density higher than 20 mA/cm2 at a

turn-on field of 1.3 V/lm. The as grown SWCNTs were further treated with Oxygen (O2) plasma

for 5 min and again field emission characteristics were measured. The O2 plasma treated SWCNTs

have shown dramatic improvement in their field emission properties with emission current density

of 111 mA/cm2 at a much lower turn on field of 0.8 V/lm. The as grown as well as plasma treated

SWCNTs were also characterized by various techniques, such as scanning electron microscopy,

high resolution transmission electron microscopy, Raman spectroscopy, and Fourier transform

infrared spectroscopy before and after O2 plasma treatment and the findings are being reported in

this paper. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4866995]

I. INTRODUCTION

Carbon nanotube (CNT) based field emission display

(FED) devices have attracted considerable attention for

future display devices due to its high current density, low

turn-on field, low power consumption, and rapid response

time.1–4 The high aspect ratio (�1000) of CNT and its

atomically sharp apex enhances the local field and lowers

the threshold field for electron emission.5–7 In addition to

the geometrical features, the high electrical and thermal

conductivity along with high chemical and temperature

stability8–10 make them further attractive for stable field

emitter. Intensive work on field emission properties of

CNT emitters prepared by screen-printing method, dielec-

trophoresis deposition, dip coating, and spraying has been

reported.11–14 However, for large useful life and high cur-

rent density FED device applications, these techniques are

of limited use especially for large size devices with high

emission current density and uniform emission. Therefore,

in order to optimize the field emission properties for large

area applications, it is highly desirable to produce verti-

cally aligned CNTs with optimal combinations of density,

diameter, and length on the substrates.15 Synthesis of

CNTs on catalyst film by chemical vapour deposition

(CVD) has been extensively used for obtaining better con-

trol of these parameters,16 but the growth of CNTs by CVD

requires a high synthesis temperature.17,18 To overcome

this limitation of higher temperature growth, single wall

carbon nanotubes (SWCNTs) can be synthesized by low

temperature process like Plasma Enhanced Chemical

Vapor Deposition (PECVD) technique.19–21 The PECVD

growth at low temperatures is also attractive for heat-

sensitive substrates. At low temperature, the geometry of

the catalyst particle remains unchanged throughout the

process and a strong correlation between metal catalyst

particle size and CNT growth has been reported.22–25 This

further facilitates the enhancement in the field emission

behaviour of CNTs.

The covalently bonded structures of SWCNTs make

them more stable than traditional metallic structures and

immune to electro migration. The field emission behaviour

of SWCNTs can be further improved by exploiting the sur-

face states arising from the CNTs structures itself and exter-

nal molecular interactions. Plasma treatment has been

widely used for surface activation of various materials, rang-

ing from organic polymers to inorganic ceramics and met-

als.26 Few studies have been reported recently on the effect

of plasma treatment on to the surface of SWCNTs.27–32

Plasma treatment is an environment friendly, solvent free,

and time efficient process with room temperature processing

ability to enhance molecular interaction with CNTs on a

large scale. During plasma treatment, excited electrons, ions,

and free radicals are generated through inelastic collisions

between energetic electrons and molecules. These plasma

species thus formed are very reactive toward surfaces, lead-

ing to surface modification.

In order to improve the emission characteristics of the

SWCNT field emitters, we report in the present research work

the growth of uniform and vertically aligned SWCNTs which

were further treated under oxygen (O2) plasma to enhance the

field emission characteristics of the SWCNTs field emitters.

The Iron (Fe) catalyst film deposited on Silicon (Si) wafer

a)Author to whom correspondence should be addressed. Electronic mail:

mush_reslab@rediffmail.com. Tel.: þ91-11-26988332. Fax: þ91-11-

26981753.

0021-8979/2014/115(8)/084308/6/$30.00 VC 2014 AIP Publishing LLC115, 084308-1

JOURNAL OF APPLIED PHYSICS 115, 084308 (2014)

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170.140.105.10 On: Mon, 24 Nov 2014 16:04:33

was used to grow vertically aligned SWCNTs by PECVD at

an optimised growth temperature of 550 �C and pressure of

15 mbar. The as grown SWCNTs were treated under O2

plasma for 5 min using Radio Frequency (RF) sputtering sys-

tem at a RF power of 100 W and at a gas pressure of 120 mbar

under the vacuum of the order of 5� 10�6 Torr.

II. EXPERIMENTAL

Ultrasonically cleaned Si substrate was coated with Fe

catalyst using RF sputtering system. The Fe catalyst film

substrate was then placed upon a graphite heater inside the

quartz bell jar chamber. The desired pressure of the order

of 15 mbar was achieved inside the chamber. The catalyst

film was then pre-treated under hydrogen (H2) atmosphere

for 10 min at 500 �C. The hydrogen flow was kept at

of 750 sccm. The temperature was measured by a thermo-

couple directly connected to the graphite heater. After

pre-treatment, the source gas acetylene (C2H2) at the rate of

20 sccm was inserted in the chamber and flow rate of H2

was increased to 1380 sccm and the heater temperature was

quickly raised to 550 �C to start the SWCNT growth. The

growth time was kept 15 min. During growth process, dc

plasma was struck with dc power of 40 W, to facilitate ver-

tically aligned growth of the CNTs on the substrate. The

growth was terminated by turning off the power supply of

the heater and C2H2/H2 gas flow. The sample was then

cooled down to room temperature.

The grown samples were characterized by using number

of techniques like Scanning Electron Microscope (SEM),

Raman Spectroscopy, Fourier Transform Infra Red (FTIR)

spectroscopy, and also the measurement of field emission

characteristics of the as grown samples. Field emission scan-

ning electron microscope (FESEM) of FEI (Nova Nano) was

used to study the surface morphology of the as grown

SWCNTs. High resolution transmission electron microscope

(HRTEM) was recorded using a Tecnai G2 F30 S-Twin

(FEI; Super Twin lens with Cs¼ 1.2 mm) instrument operat-

ing at an accelerating voltage at 300 kV. The structure of as

grown SWCNTs was also studied by Raman Spectrometer of

HORIBA Jobin Yvon (LABRAM HR 800 JY) at wavelength

of 633 nm. The field emission measurement of the as grown

SWCNT sample was carried out at room temperature in a

high vacuum chamber using diode type arrangement. Data

obtained from the current density versus electric field (JE)

and Fowler Nordheim (FN) plots were used to calculate the

field enhancement factor.

The sample was then treated with O2 plasma for 5 min

using RF sputtering system with a RF power of 100 W and

gas pressure of 120 mbar under vacuum of the order of

5� 10�6 Torr. The treated sample was subjected to all the

above characterization techniques earlier reported. We

observed the change in morphology and structure of

SWCNTs. A comparative study of as grown SWCNTs and

O2 plasma treated SWCNTs were carried out by FTIR spec-

troscopy to see the attachment of O2 molecule on the surface

of SWCNTs. The field emission measurements had shown

dramatic enhancement in the emission characteristics of the

O2 plasma treated SWCNTs.

III. RESULTS AND DISCUSSION

A. SEM study

FESEM micrograph shown in Fig. 1(a) indicates the sur-

face morphology of as grown SWCNTs with the diameter of

the SWCNTs in the range of 1–2 nm. The high resolution

image (Fig. 1(a)) also reveals that the film was essentially

composed of high density, long, and vertically aligned

SWCNTs. It was worth noting that up to the acceleration

voltage (30 kV) used for the morphological observations, no

charging effect was observed, which was an indication of a

high electrical conductivity of the SWCNT film. The SEM

micrograph of the O2 treated SWCNTs shown in Fig. 1(b)

clearly indicates the change in morphology. Fig. 1(b) also

shows the clear attachment of some molecule which was fur-

ther verified by FTIR study.

B. HRTEM study

TEM micrograph of the as-grown SWCNTs sample is

shown in Fig. 2(a). The sample was dispersed in ethanol and

ultrasonicated for 5 min. The carbon coated grid was used to

prepare the sample of dispersed solution for HRTEM study.

FIG. 1. FESEM micrograph of (a) as-grown SWCNTs (b) O2 plasma treated

SWCNTs.

084308-2 Kumar et al. J. Appl. Phys. 115, 084308 (2014)

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From Fig. 2(a), the HRTEM micrograph reveals the SWCNT

diameter distribution in the range of 0.87–1.3 nm.

The sample after O2 Plasma treatment was again prepared

on carbon coated grid. Figure 2(b) shows the HRTEM image

of O2 plasma treated SWCNTs. From Fig. 2(b), we can clearly

see the change in the structure of the SWCNT sample.

C. Raman spectroscopic study

Raman spectroscopy has been known as a convenient

and very powerful tool to probe the individual tube structure

and determine diameter. To evaluate the quality of as-grown

SWCNTs, a laser beam with an excitation wavelength of

633 nm was used. Raman spectra of SWCNTs grown on Si

substrate are shown in Fig. 3(a). The diameter of SWCNTs

shown in the inset of Fig. 3(a) was estimated using the corre-

lation d¼ 248/�, where d is the diameter of SWCNT in nm

and � is the Raman shift in cm�1. According to this relation,

the peaks at 189, 257, and 285 cm�1 correspond to the diam-

eters 1.3, 0.96, and 0.87 nm, respectively, indicating the exis-

tence of SWCNTs. The peaks at 1339, 1575, and 2667 cm�1

correspond to the D-band, G-band, and G0-band, respec-

tively. The G-band or Tangential Mode (TM) sometimes

also called the high energy band corresponds to the stretch-

ing modes in the graphite plane. The D-band which is a

result of a photon-defect interaction originates from defect

induced double-resonant Raman scattering and involves pho-

nons from the graphite K-point. It lies in the range of

1200–1400 cm�1. This D-mode shifts with energy of the

exciting laser. The G0-band arises due to photon-second pho-

non interaction. These above mentioned peaks are character-

istic of both CNTs and graphite. Fig. 3(b) shows the Raman

spectra of the same sample after O2 plasma treatment. Peak

intensity in all the modes was almost same except for the

presence of an extra peak at 975 cm�1. This new peak indi-

cates the existence of O2 as mentioned by Lee et al.33,34 Our

results are also in good agreement with the results obtained

by Zhao et al.35

D. FTIR spectroscopic study

FTIR spectroscopy was performed using Biorad FTS 40

spectrometer for the structural analysis of O2 molecules

FIG. 2. HRTEM micrograph of (a) as-grown SWCNTs (b) after O2 plasma

treated SWCNTs.

FIG. 3. Raman spectra of (a) as grown SWCNTs and inset indicates the

enlarge view of RBM mode for as-grown SWCNTs. (b) SWCNTs after O2

treatment and inset indicates the enlarge view of RBM mode for O2 plasma

treated SWCNTs.

084308-3 Kumar et al. J. Appl. Phys. 115, 084308 (2014)

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attached with SWCNTs. The SWCNTs samples (as grown

and after plasma treatment) were scratched and mixed with

KBr to prepare pellets under hydraulic press force of

10 ton. A comparative study of FTIR spectroscopy in the

range 400–4000 cm�1 was used to identify the O2 func-

tional groups attached on the surface of the SWCNTs.

Figures 4(a) and 4(b) show the infrared spectra recorded for

as grown SWCNTs samples and the O2 plasma treated

SWCNTs. In the FTIR spectra of as grown sample (Fig.

4(a)), there was almost no signal except for indication of

small C-C stretch, however, after plasma treatment, quite a

number of new peaks were observed. The analysis of the

spectra Fig. 4(b) showed the presence of infrared bands at

1390–1580 cm�1, which can be associated with carbon dou-

ble bond (C¼C) stretching, typical of CNTs. The band

around 1650–1740 cm�1 indicates the presence of C¼O ox-

ygen functionalities in the nanotube surface. Other bands

seen at 2925 cm�1 and 3452 cm�1 were the characteristic of

C-H and O-H stretches, respectively. The C-C vibrations

occur due to the internal defects, and the O-H vibration was

associated with the amorphous carbon because amorphous

carbon easily forms a bond with atmospheric air.

E. Field emission study

FN theory of field emission dates back to the beginnings

of quantum tunnelling mechanics and still widely used today

to confirm field emission from materials. The FN theory was

therefore used to describe field emission from SWCNT based

electron emitters. According to FN theory, emission current

density (J) from the surface of emitting material is expressed

as a function of the electric field (E) and work function (/)

of the emitting material, i.e.,

J ¼ AE2 exp �B/3=2

E

� �; (1)

where A¼ 1.56� 10�6 AeVV�2, B¼ 6.83� 107eV�3/2V cm�1

are constants and applied electric field (E) is defined as bV/d,

where V is the voltage between anode and the CNT emitters as

cathode, d is the distance between cathode and anode, and b is

field enhancement factor.

In the present case, emission occurs from the multiple

SWCNT emitters and hence the measured current is an aver-

age of currents due to all field emitters. The exact analysis of

field emission behaviour of the SWCNTs is quite difficult. It is

useful to find the value of geometrical field enhancement factor

b from the slope of FN plot instead of the simple ratio of geo-

metric factor of the SWCNT. The value of b was determined

from the slope of FN plot by using the following relation:

b ¼ B/3=2d

m: (2)

Field emission measurements of the as grown SWCNTs

were performed in a diode mode by applying negative volt-

age on the copper plate (cathode) with respect to stainless

steel anode plate. SWCNTs film used as electron emitter

source was pasted on the copper plate with silver epoxy. The

effective area of anode was �78.5 mm2 for field emission

measurements. The emission measurements were carried out

at chamber vacuum of 10�6 Torr to minimize the electron

scattering and degradation of the emitters. Cathode and an-

ode distances were kept 250 lm (constant) during entire field

emission measurements. After O2 plasma treatment, the field

emission characteristics of SWCNTs were again measured

under the similar conditions.

JE plots of plasma untreated/treated SWCNTs emitters

were recorded to determine the effect of plasma on field

emission behaviour of SWCNTs. JE plot in Fig. 5 shows a

comparison between the emission properties of as grown and

plasma treated SWCNT emitter. As seen from this plot, the

sample after plasma treatment shows about �6 times higher

current density in comparison with untreated SWCNTs sam-

ple. A low current density of 20.0 mA/cm2 at 1.4 V/lm field

and a high turn-on field (Eto) of 1.3 V/lm were recorded for

the untreated SWCNTs emitters. In contrast after plasma

treatment, SWCNT emitters gave high current density of

111.25 mA/cm2 at 1.4 V/lm and with low Eto of 0.8 V/lm.

Thus, the comparison of these two typical results indicates

drastic improvement in the field emission properties after

plasma treatments, a favourable requirement for field emis-

sion based devices. Actually, during plasma treatmentFIG. 4. FTIR spectra of SWCNT (a) as grown SWCNT (b) O2 plasma

treated SWCNT.

084308-4 Kumar et al. J. Appl. Phys. 115, 084308 (2014)

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process, the as grown SWCNTs emitters with a smaller

diameter, longer length, and having large b seem to be

deformed by the positive ions, resulting in the emitters hav-

ing smaller values. Due to this, a large number of emitters

are activated which in turn improve the emission stability

and hence a reward for a higher electrical field application.

To better understand the effect of plasma on field emis-

sion performance of SWCNTs, the measurement results

were plotted on semi-log scale as depicted in Figure 6. The bestimated from slope of FN plots came out as 1202 and

17 452 for as grown sample and plasma treated SWCNTs

sample, respectively, assuming work function (/) to be 5 eV

as for carbon. The typical summary result obtained is shown

in Table I.

IV. CONCLUSIONS

Uniform and vertically aligned SWCNTs with diameter

0.87 nm–2 nm were successfully grown by PECVD system.

The as grown SWCNTs had shown good field emission prop-

erties. The field emission characteristics of SWCNTs were

enhanced dramatically after O2 plasma treatment. The

plasma treated SWCNTs emitter showed high current density

of 111.25 mA/cm2 at a field of 1.4 V/lm and a low Eto of

0.8 V/lm. Observed improvement in current density after

plasma treatment was about 6 times of the original value at

the same field.

ACKNOWLEDGMENTS

The authors are thankful to DeitY for the financial sup-

port in the form of major research project. One of the

authors, Samina Husain, is also thankful to CSIR for the fi-

nancial support in the form of Research Associateship.

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FIG. 5. J-E plot of as-grown SWCNTs cathode.

FIG. 6. F-N plot of as-grown SWCNTs cathode.

TABLE I. Field emission parameters of SWCNTs before and after O2

plasma treatment.

SWCNTs Turn-on field Current density Beta (b)

Before O2 treatment 1.3 V/lm 20.0 mA/cm2 1.2� 103

After O2 treatment 0.8 V/lm 111 mA/cm2 1.7� 104

084308-5 Kumar et al. J. Appl. Phys. 115, 084308 (2014)

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