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Applied Surface Science 322 (2014) 236–241

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

comparative study of nitrogen plasma effect on field emissionharacteristics of single wall carbon nanotubes synthesized by plasmanhanced chemical vapor deposition

vshish Kumara, Shama Parveena, Samina Husaina, Javid Alia, Mohammad Zulfequara,b,arshb, Mushahid Husaina,b,∗

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

r t i c l e i n f o

rticle history:eceived 17 July 2014eceived in revised form 10 October 2014ccepted 20 October 2014vailable online 27 October 2014

eywords:ingle wall carbon nanotubelasma enhanced chemical vaporepositionield emissioncanning electron microscopeaman spectrometerourier transform infrared spectrometer

a b s t r a c t

Vertically aligned single wall carbon nanotubes (SWCNTs) with large scale control of diameter, lengthand alignment have successfully been grown by plasma enhanced chemical vapor deposition (PECVD)system. The nickel (Ni) as catalyst deposited on silicon (Si) substrate was used to grow the SWCNTs.Field emission (FE) characteristics of the as grown SWCNTs were measured using indigenously designedsetup in which a diode is configured in such a way that by applying negative voltage on the copperplate (cathode) with respect to stainless steel anode plate, current density can be recorded. To measurethe FE characteristics, SWCNTs film pasted on the copper plate with silver epoxy was used as electronemitter source. The effective area of anode was ∼78.5 mm2 for field emission measurements. The emissionmeasurements were carried out under high vacuum pressure of the order of 10−6 Torr to minimize theelectron scattering and degradation of the emitters. The distance between anode and cathode was kept500 �m (constant) during entire field emission studies. The grown SWCNTs are excellent field emitters,having emission current density higher than 25 mA/cm2 at turn-on field 1.3 V/�m. In order to enhancethe field emission characteristics, the as grown SWCNTs have been treated under nitrogen (N2) plasmafor 5 min and again field emission characteristics have been measured. The N2 plasma treated SWCNTs

2

show a good enhancement in the field emission properties with emission current density 81.5 mA/cmat turn on field 1.2 V/�m. The as-grown and N2 plasma treated SWCNTs were also characterized byfield emission scanning electron microscope (FESEM), high resolution transmission electron microscope(HRTEM), Raman spectrometer, Fourier transform infrared spectrometer (FTIR) and X-ray photoelectronspectroscopy (XPS).

. Introduction

Carbon nanotube (CNT) reveals an extraordinary field emissionroperty due to its high electrical conductivity and the high aspectatio leads to optimum geometrical field enhancement and remark-ble thermal stability [1–6]. CNTs as a field emitting materialsave a wide range of potential applications in vacuum electron-

cs, microwave amplification [7], electron microscopy [8], X-ray

eneration [9,10], plasma processing, gas ionization, nano klystron11], electron beam evaporation and electron beam lithography12]. Any system that uses an electron source could potentially

∗ Corresponding author at: Department of Physics, Jamia Millia Islamia (A Centralniversity), New Delhi 110025, India. Tel.: +91 11 26988332; fax: +91 11 26981753.

E-mail address: mush reslab@rediffmail.com (M. Husain).

ttp://dx.doi.org/10.1016/j.apsusc.2014.10.116169-4332/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

host a CNT based field emission device. A typical example is fieldemission flat panel display (FED) [13], which is having the largestpotential market and could replace most displays used today indesktop and laptop computers as well as televisions. A field emit-ter should have high current density, low turn-on field, low powerconsumption and fast response time [14–16]. Due to high aspectratio, the single wall carbon nanotubes (SWCNTs) show an excel-lent field emission properties such as low turn on field and highelectric field enhancement factor (ˇ) [17–19]. However, field emis-sion properties of SWCNTs are generally considered to be brittlebecause the single tube CNTs are less flexible to emission degra-dation mechanisms. Recently, various researchers have shown the

field emission properties of MWCNTs [20–25] but not much workhas been focused upon improving the field emission properties ofSWCNTs. The major complications to fabricate SWCNTs based FEDsare the rigorous requirement like high synthesis temperatures of

face Science 322 (2014) 236–241 237

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A. Kumar et al. / Applied Sur

WCNTs [26,27]. Further, SWCNTs as field emitters require an opti-al combination of high density array of vertically aligned CNTsith large scale control of diameter, length, alignment, location

nd orientation [28,29]. In this connection, SWCNTs can be grownn catalyst film by low temperature process like plasma enhancedhemical vapor deposition (PECVD) technique [30–32]. This lowemperature process is also suitable for heat sensitive substratesnd further, it also provides a strong adhesion between catalystnd CNTs which further facilitates the enhancement in the fieldmission behavior of CNTs [33–36].

There are certain points important in the development ofWCNT based FEDs such as; (i) a huge tip enhancement of anpplied field and (ii) a homogeneous current distribution i.e. a largeurrent density resulting in a giant current per emitter. These issuesan be prevailed over by surface modification of SWCNTs. Varioustudies have been reported on the chemical functionalization ofarbon nanotubes in order to improve the characteristic proper-ies of CNTs in the fabrication of field emission devices. But, fewesearchers have worked on the post treatment of SWCNTs filmsuch as plasma treatment on to the surface of SWCNTs [37–43]hich can also be called as physical functionalization. Recently, weave reported the enhanced field emission behavior of SWCNTs byhysical functionalization in which SWCNTs were treated under O2lasma which leads to the surface modification of SWCNTs [44].lasma treatment is a time efficient process with low tempera-ure processing ability having environment friendly and solventree. During plasma treatment process, excited electrons, ions, andree radicals produced through inelastic collisions between ener-etic electrons and molecules, are very reactive toward surfaces,eading to surface modification and enhance molecular interaction

ith CNTs on a large scale [45]. The as grown SWCNTs emittersith a smaller diameter, longer length and having large ̌ seems to

e deformed by the positive ions, resulting in the emitters havingmaller values. Therefore, a large number of emitters are activatedhich in turn improve the emission stability and hence excellent

or a higher electrical field application.In the present work, we report the synthesis of vertically aligned

WCNTs on nickel (Ni) coated silicon (Si) substrate which are fur-her treated under nitrogen (N2) plasma in order to enhance theeld emission characteristics of the SWCNTs based FEDs.

. Experimental

The Ni catalyst coated Si substrate was placed upon the graphiteeater inside the quartz chamber/bell jar. The chamber was thenvacuated to 10−3 Torr and the catalyst film is pre-treated underydrogen (H2) atmosphere at a temperature of 600 ◦C which is mea-ured by a thermocouple directly connected to the graphite heater.cetylene (C2H2) as a source gas at the rate of 15 sccm was thendded for 10 min in continuation with hydrogen (H2) to start theWCNTs growth while the heater temperature was quickly raisedo 675 ◦C (growth temperature). During growth process, the pres-ure inside the chamber was kept at 15 mbar. In this process, dclasma at a power of 40 W was struck, to assist vertically alignedrowth of the SWCNTs. Finally, the growth process was turned offnd the sample was then cooled down to room temperature.

Field emission scanning electron microscope (FESEM) of FEINova Nano) was used to study the surface morphology of the asrown SWCNTs. High resolution transmission electron microscopeHRTEM) was recorded using a Tecnai G2 F30 S-Twin (FEI; Superwin lens with Cs¼1.2 mm) instrument operating at an accelerat-

ng voltage at 300 kV. The structural analysis of as grown SWCNTs

as also studied by Raman spectrometer of HORIBA Jobin YvonLABRAM HR 800 JY) at a wavelength of 633 nm. The existencef nitrogen and its bonding with carbon was confirmed by XPS

Fig. 1. FESEM micrographs of (a) as-grown SWCNTs and (b) N2 plasma treatedSWCNTs.

of Omicron Nano Technology GmbH, Germany. The field emis-sion characteristics of the as grown SWCNT sample were measuredusing indigenously designed system. All the data were recorded atroom temperature in a high vacuum chamber using cathode–anodearrangement. The current density and field enhancement factorwere calculated using J–E plot and Fowler–Nordheim (F–N) plotsrespectively.

Further, sample was treated under N2 plasma for 5 min usingRF sputtering system at an RF power of 100 W and gas pressure of100 mbar. During treatment process, the vacuum of the order of10−6 Torr was achieved. The treated sample was subjected to allthe above characterization techniques earlier reported.

3. Results and discussion

3.1. SEM study

FESEM micrograph {Fig. 1(a)} shows the morphological imageof as-grown vertically aligned single wall carbon nanotubes (VA-

SWCNTs) grown on Ni catalyst film. The micrograph gives a roughidea of the diameter distribution in the range of 1–2 nm. More-over, the growth is uniform and vertically aligned which is good forspecific applications particularly for field emission display devices.

238 A. Kumar et al. / Applied Surface Science 322 (2014) 236–241

F

TeSipwtlNRX

3

ascp

0 500 1000 1500 2000 2500 3000

(a)

2593

1590

1303

Si

251

183

124

Inte

nsit

y (

a.u

.)

Raman Shift (cm-1 )

0 500 1000 1500 2000 2500 3000

(b)

G

D

Si

2661

1581

1347

251233In

ten

sit

y (

a.u

.)

Raman Shift (cm-1 )

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

ig. 2. HRTEM image of (a) as grown SWCNTs and (b) N2 plasma treated SWCNTs.

he high resolution image {Fig. 1(a)} also reveals that the film isssentially composed of high density with vertically alignment ofWCNTs. The morphology of N2 plasma treated sample is shownn Fig. 1(b). From the micrograph, we observe the change in mor-hology which is essentially due to the attachment of N2 moleculesith the surface of SWCNTs. However, diameter distribution seems

o be little bit higher than before which may be due to accumu-ation of nanotubes during plasma treatment. The attachment of

2 molecules onto the surface of SWCNTs were further verified byaman spectroscopy, Fourier transform infrared spectroscopy andPS.

.2. HRTEM study

The as grown as well as plasma treated SWCNTs were char-cterized by HRTEM {Fig. 2(a) and (b)}. For HRTEM study, the

amples were ultrasonically dispersed in ethanol for 5 min. Thearbon coated grid was then used to prepare the sample of dis-ersed solution. From HRTEM observations, the as grown SWCNTs

Fig. 3. Raman spectra of (a) as grown SWCNTs sample and (b) SWCNTs sampleafter N2 plasma treatment and inset shows the enlarge view of peak intensity at1467 cm−1 indicating attachment of nitrogen with SWCNTs.

sample (Fig. 2(a)) essentially display long bundles of SWCNTs withnegligible deposits of amorphous carbon on their walls.

However, the SWCNTs treated with N2 plasma (Fig. 2(b)) showsome defects and seem to be deformed at some places. In fact, thesamples are dominated by agglomerates of defective single layernanotubes with some external attachment. Moreover, the defectsare still free of amorphous carbon.

3.3. Raman spectroscopic study

Raman spectroscopy is very powerful technique to evaluate thequality of nanotubes with individual tube structure and also usedto determine tube diameter [46]. A laser beam with an excitationwavelength of 633 nm was used to characterize the as-grown sam-ple as well as plasma treated sample. Fig. 3(a) shows Raman spectraof SWCNTs grown on Ni deposited Si substrate. The radial breathingmode (RBM) peak of SWCNTs at 124, 183 and 251 cm−1 clearly con-firms the existence of SWCNTs. The diameter of as-grown SWCNTswas estimated using the correlation, d = 248/� where d is the diam-eter of SWCNT in nm and � is the Raman shift in cm−1. In this case,it was measured to be 2, 1.35 and 0.98 nm which is also in accor-dance with the TEM results. The peaks at 1303, 1590 and 2593 cm−1

corresponding to the D-band, G-band and G′-band respectively.The G-band also known as the high energy band corresponds tothe stretching modes in the graphite plane. The D-band originatesfrom defect induced double-resonant Raman scattering and lies in

the exciting laser. The G′-band arises due to photon-second phononinteraction. These above mentioned peaks are characteristic of bothCNTs and graphite. The Raman spectra of the N2 plasma treated

A. Kumar et al. / Applied Surface Science 322 (2014) 236–241 239

4000 3000 2000 1000 0

(a)T

ran

sm

itta

nce

Wavenu mbe r (cm-1)

4000 3000 2000 1000 0

(b)

C=C

C=N

C-H

O-H

Tra

nsm

itta

nce

-1

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3

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0 20 0 40 0 60 0 80 0 100 0

O 1s

N 1s

C 1s

Inte

ns

ity

(a

.u.)

Binding Energy (eV)

Wavenu mbe r (cm )

ig. 4. FTIR spectra of (a) as grown SWCNTs and (b) N2 plasma treated SWCNTs.

WCNTs {Fig. 3(b)} were also recorded and observed the presencef an extra peak at 1467 cm−1. It appears that the peak is relatedo C N bonds. This new peak at 1467 cm−1 is the indication ofttachment of N2 molecule as already reported by Matsui et al. andang et al. [47,48]. On the other hand, the peak intensity (ID) of-band has been increased in height. If we compare ID/IG of the asrown sample and plasma treated sample, we observe an increasen this ratio which indicates that some defects and disorder are pro-uced due to attachment of N2 plasma treatment. Before plasmareatment, the calculated ratio ID/IG was 0.22 while after plasmareatment it was 0.70 indicating the increase in number of defectsue to plasma treatment.

.4. FTIR spectroscopic study

A comparative study of FTIR spectroscopy in the range00–4000 cm−1 was used to identify the attachment of N2olecules on the surface of the SWCNTs. Biorad FTS 40 spectrome-

er was used for the structural analysis of N2 molecules. The powderorm the surface of SWCNTs grown samples (as grown and N2lasma treated) were scratched and mixed with KBr to prepareellets under hydraulic press using force of 10 ton. Fig. 4(a) and (b)hows the FTIR spectra of as grown SWCNTs and N2 plasma treatedWCNTs respectively. From Fig. 4(a), no signal except for indicationf small C C stretch was observed. However, after N2 plasma treat-

ent quite a number of new peaks were observed. From the spectra

f Fig. 4(b), the presence of infrared bands at 1350–1490 cm−1

an be associated with carbon double bond (C C) stretching, typ-cal of CNTs. The band observed at 2100–2440 cm−1 point toward

Fig. 5. XPS spectrum of N2 plasma treated SWCNTs.

the presence of C N nitrogen functionalities in the nanotube sur-face. Other bands observed around 2925 cm−1 and 3452 cm−1, werethe characteristic of C H and O H stretches respectively. The C Cvibrations occur due to the internal defects and the O H vibrationis linked with the amorphous carbon because amorphous carboneasily forms a bond with atmospheric air.

3.5. XPS study

To further confirm the bonding of nitrogen with SWCNTs, wehave used XPS which is a very powerful technique to study nitro-gen incorporation in the carbon nanotube tube structure. The XPSmeasurement of the nitrogen plasma treated SWCNTs was doneusing multi-technique surface analysis system from Omicron NanoTechnology GmbH, Germany with EA 125 energy analyser. Fig. 5exhibits a typical wide XPS spectrum of N2 plasma treated SWNTs.The peaks at 289, 399 and 531 eV were identified to C 1s, N 1s andO 1s, peaks which were attributed to the electronic 1s core levelsof carbon, nitrogen and oxygen atoms. From Fig. 5, we observe avery sharp peak at 399 eV, which clearly confirms the existence ofnitrogen and its bonding with carbon nanotubes.

3.6. Field emission study

Field emission is the extraction of electrons from surface of solidmaterial by tunneling through the potential barrier under the influ-ence of a strong external electric field and is completely explainedby quantum-tunneling effects. The potential barrier is rectangu-lar when no electric field is present and becomes triangular whena negative potential is applied to the solid. Fowler and Nordheimpresented the first quantum mechanical model for describing fieldinduced electron emission from a metallic surface; a model still inuse today [49]. The Fowler–Nordheim (F–N) theory was thereforeused to describe field emission from SWCNT based electron emit-ters. According to this theory, emission current density (J) from thesurface of emitting material can be expressed as a function of theelectric field (E) and work function (�) of the emitting material i.e.

J = AE2 exp

(B�3/2

E

)(1)

where A = 1.56 × 10−6 A eV V−2, B = 6.83 × 107 eV−3/2 V cm−1 are

linear constant and exponential factor respectively at room tem-perature. Electric field (E) is defined as ˇV/d, where V is the voltagebetween anode and the CNT emitters as cathode, d is the distancebetween cathode and anode and ̌ is field enhancement factor.

240 A. Kumar et al. / Applied Surface Science 322 (2014) 236–241

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

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sity (

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/cm

2)

Electric Field (V/ m) µ

Before plasma treatment

After plasma treatment

FSl

JsficpTf

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pcpSc∼sod5ni

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-11.5

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Before Plasma treatment

After Plasma treatment

ig. 6. J–E plot of as-grown SWCNTs cathode (in black color) and N2 plasma treatedWCNTs (in red color). (For interpretation of the references to color in these figureegends, the reader is referred to the web version of this article.)

Graph between current density versus electric field known as–E curve (Fig. 6) and shows exponential increments in current den-ity with applying electric field. In the present case, emission occursrom the multiple SWCNT emitters and hence the measured currents an average of currents due to all field emitters. The geometri-al field enhancement factor ̌ is calculated from the slope of F–Nlot instead of the simple ratio of geometric factor of the SWCNT.he value of ̌ was calculated from the slope of F–N plot using theollowing relation

= B�3/2d

m(2)

Field emission measurements of the as-grown SWCNTs wereerformed using indigenously designed system in which a diodeonfiguration is setup by applying negative voltage on the cop-er plate (cathode) with respect to stainless steel plate (anode).WCNTs film used as electron emitter source was pasted on theopper plate with silver epoxy. The effective area of anode was78.5 mm2 for field emission measurements. The emission mea-

urements were carried out in a vacuum chamber under pressuref the order of 10−6 Torr to minimize the electron scattering andegradation of the emitters. Cathode and anode distance was kept00 �m. In order to enhance the electron emission from the carbonanotubes, after N2 plasma treatment, the field emission character-

stics of SWCNTs were again measured under the similar conditions.J–E plot in Fig. 6 shows a comparison between the emission

roperties of as grown and plasma treated SWCNT emitter. A highurrent density of 25.0 mA/cm2 at 1.9 V/�m field and a high turn-n field (Eto) of 1.3 V/�m was recorded for the as grown SWCNTsmitters. In contrast, after plasma treatment, SWCNT emitters havehown huge current density of 81.5 mA/cm2 at 2.0 V/�m and withow Eto of 1.2 V/�m. Thus, after N2 plasma treatment more thanhree times higher current density in comparison to untreatedWCNTs sample was observed. The comparison of these two typicalesults indicates a drastic enhancement in the field emission prop-rties after plasma treatments. Actually, during plasma treatmentrocess, the as grown SWCNTs emitters with a smaller diameter,

onger length and having large ̌ seems to be deformed by the pos-tive ions, resulting in the emitters having smaller values. Due to

his, a large number of emitters are activated which in turn enhancehe current density with low turn on field and improve the emis-ion stability which is a favorable requirement for field emissionisplay devices.

Fig. 7. F–N plot of as-grown SWCNTs cathode (in black color) and after plasmatreatment (in red color). (For interpretation of the references to color in these figurelegends, the reader is referred to the web version of this article.)

The results were plotted on semi-log scale as depicted in Fig. 7for better understanding of effect of plasma treatment in order tocalculate the field enhancement factor ˇ. The work function of theCNTs before plasma treatment was assumed to be 5 eV. In order tocalculate the work function after plasma treatment, we have usedthe formula [50]

�2 = �1 ×(

slope2

slope1

)2/3

where �1 is the work function of the as grown SWCNTs sam-ple (5 eV), �2 is the work function of the plasma treated SWCNTssample, slope1 is the slope of the F–N plot (Fig. 7) before plasmatreatment, slope2 is the slope of the F–N plot (Fig. 7) after plasmatreatment. By using this formula, the work function of the plasmatreated SWCNTs was calculated to be 3.3 eV.

From slope of F–N plots (Fig. 7), the ̌ was calculated at 6.2 × 103

and 6.4 × 103 for as grown sample and plasma treated SWCNTssample respectively, keeping work function (�) of the SWCNTs tobe 5 and 3.3 eV before and after plasma treatment respectively.

4. Conclusions

Vertically aligned SWCNTs with diameter 1–2 nm and length ofseveral micrometer were grown, on Ni deposited Si substrate, byPECVD system. We have successfully grown the SWCNTs in a wayso that high current density at low turn on field can be achieved.Therefore, the as grown SWCNTs have shown good field emissionproperties. In order to further enhance the electron emission fromthe carbon nanotubes, the grown SWCNTs were effectively treatedunder N2 plasma and found dramatically enhancement in the fieldemission characteristics of SWCNTs. The plasma treated SWCNTsemitter showed high current density of 81.5 mA/cm2 at a field of2.0 V/�m and a low Eto of 1.2 V/�m. The enhancement in currentdensity after plasma treatment is more than three times of theoriginal value at almost same field.

Acknowledgements

We are thankful to Department of Electronics and Informa-

tion Technology (DeitY), India for the financial support in theform of major research project. We are also thankful to Dr. AshokKapoor, Characterization Division, Solid State Physics Laboratory,New Delhi, for XPS study of the sample.

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