006) 1851–1856www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Enhanced field emission characteristics of nitrogen-dopedcarbon nanotube films grown by microwave plasma

enhanced chemical vapor deposition process

Sanjay K. Srivastava a,1, V.D. Vankar a,⁎, D.V. Sridhar Rao b, Vikram Kumar c

a Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, Indiab Electron Microscopy Division, Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad-500056, India

c National Physical Laboratory, Dr. Krishnan Marg, Pusa, New Delhi-110012, India

Received 27 November 2005; received in revised form 23 June 2006; accepted 7 July 2006Available online 22 August 2006

Abstract

Nitrogen-doped carbon nanotube (CNT) films have been synthesized by simple microwave plasma enhanced chemical vapor depositiontechnique. The morphology and structures were investigated by scanning electron microscopy and high resolution transmission electronmicroscopy. Morphology of the films was found to be greatly affected by the nature of the substrates. Vertically aligned CNTs were observed onmirror polished Si substrates. On the other hand, randomly oriented flower like morphology of CNTs was found on mechanically polished ones.All the CNTs were found to have bamboo structure with very sharp tips. These films showed very good field emission characteristics withthreshold field in the range of 2.65–3.55 V/μm. CNT film with flower like morphology showed lower threshold field as compared to verticallyaligned structures. Open graphite edges on the side surface of the bamboo-shaped CNT are suggested to enhance the field emission characteristicswhich may act as additional emission sites.© 2006 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotubes; Field emission; Scanning electron microscopy (SEM); Transmission electron microscopy (TEM)

1. Introduction

The discovery of carbon nanotubes (CNTs) by Iijima in 1991[1] has created immense interest worldwide due to their uniquestructure and properties. In particular, field electron emissionfrom CNTs has shown to be one of the most promisingproperties as far as its practical application is concerned. Theseproperties are observed because of high aspect ratio, chemicalstability, highest mechanical strength and the possibility of largescale production of these materials at relatively low costscompared to conventional spindt (Mo, Si etc.) field emitters.Both vertically aligned and randomly oriented CNTs have beenreported to be very good emitters at low operating voltages [2–12]. There are limitations for vertically aligned CNTs due toscreening effect, presence of metal at the tip, etc. It has been

⁎ Corresponding author. Tel.: +91 11 2659 1329; fax: +91 11 26581114.E-mail address: vdvankar@physics.iitd.ernet.in (V.D. Vankar).

1 Present address: Department of Physics, Agra College, Agra-282002, India.

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.07.016

reported that density, length, diameter, alignment and degree ofdefects affect the performance of CNT films as field emitters.Efforts are being made to control these parameters to havestable, high density of emission current at low operatingvoltages required for a long time reliable field emission electronsource. In this regard plasma enhanced chemical vapor depo-sition technique is well suited because of its low temperaturesynthesis and controllability of the process parameters. Theo-retical studies have suggested that electronic properties of CNTscan be modified by introducing topological defects. Theoreticaland experimental studies revealed that doping CNTs by N or Batoms can alter the structural and electronic propertiessignificantly. These studied showed that nitrogen doping inCNTs enhances the local density of states near the Fermi levelwhich in turn enhance the field emission characteristics atrelatively lower applied voltage [13,14]. Capped nanotubes andone with open graphite edges are the two most suitablegeometries for field emission [15]. These proposed theoreticalmodels are able to explain the enhanced field emission of open

Fig. 1. SEM micrographs showing (a) vertically aligned CNTs (sample A) and(b) flower like morphology of CNTs around pits (sample B). Cross sectionalview of vertical alignment is shown in inset of (a). Magnified views of flowersare shown in the insets of (b).

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and N/B-doped CNTs to some extent only. There are still veryfew experimental reports in this field. Recently Chan et al. [16]reported a comparative field emission study of normal CNTswith N/B-doped CNTs. But they followed an additional postsynthesis process for N doping. Similarly Che et al. [17]proposed superior field emission properties of horizontallyaligned N-doped CNTs prepared by a tedious slow CVDprocess using nitrogen containing compound as precursor. Theyobserved that side emission geometry of CNTs is superior to topemission geometry. The motivation of this study was to find therole of different configurations of CNTs which give enhancedfield emission properties.

In this report, a high density of both randomly oriented andhighly aligned N-doped CNT films were synthesized by simple,fast and low temperature microwave plasma enhanced chemicalvapor deposition (MPECVD) technique. Systematic study offield emission measurements of CNT films was carried out. Itwas observed that nature of substrate affected the growthmorphology of CNTs in plasma CVD process and hence thefield emission characteristics.

2. Experimental details

Carbon nanotube films were prepared using a tubularMPECVD technique on two kinds of p-Si (100) substrates:(a) mirror polished, (b) mechanically polished containing pits.CNT films deposited on mirror polished Si-substrates will bereferred to as sample A and those on mechanically polished Sias sample B hereafter. The results of only the representativefilms are described here. Details of the experimental set-up aredescribed in our previous article [18]. Prior to CNT growth, iron(Fe) layer of 10 nm thickness was deposited on Si substrates bya thermal evaporation technique. No buffer layer was usedbetween metal film and Si substrate. Fe coated Si substrate wasthen pretreated in ammonia (NH3) plasma for 10 min atmicrowave power of 500 W, pressure of 6.67×102 Pa and NH3

flow rate of 40 sccm. After pretreatment, acetylene (C2H2) wasintroduced in the plasma for CNT growth at flow rate of20 sccm keeping other parameters constant. Growth time was10 min. All growth parameters for both samples A and B wereidentical except the nature of the substrate. Scanning electronmicroscopy (SEM, LEO 435 VP) operated at 15 kV wasemployed to examine the morphology of the films. Transmis-sion electron microscopy (TEM) (Philips, CM12) operating at100 kV and high resolution transmission electron microscopy(HRTEM) (TECNAI 20UT, 200 kV) were used for structuralanalysis of CNTs. For TEM specimens, a piece of film waspeeled off from the substrate and ultrasonicated in acetone for10 min. Few drops of the suspension were then placed on a holycarbon coated copper grid. Incorporation of nitrogen wasconfirmed by X-ray photoelectron spectroscopy (XPS) (PerkinElmer-1257 spectrometer with a dual anode Mg/Al Kα 25 kVX-ray source and high resolution hemispherical sectionanalyzer) and Fourier transform infra-red (FTIR) spectroscopy(Perkin Elmer-Spectrum BX with an instrumental resolution of2 cm−1) studies. The XPS measurements were carried out usingAl Kα (1486.6 eV) as X-ray source operating at 150 W.

Sampling step was 0.05 eV. The X-ray source was kept at anincident angle of 45° with respect to the substrate normal andaxis of the hemispherical analyzer was kept parallel to the latter.For analysis, appropriate smoothing for the raw data andbackground removal using the Shirley method [19] was applied.Prior to XPS measurement, sample was given surface cleaningtreatment by ion sputtering method using 4 keV Ar+ ions for2 min. The nitrogen concentration was calculated by ratio ofareas of atomic C and N factors with application of appropriateatomic sensitivity factor (ASF). The ASF values as availablefrom the manufacturers for C 1s and N 1s are 0.205 and 0.38respectively. For field emission, simple planar diode geometrywas used with CNT film as cathode and plane circular Cuelectrode as anode [18]. Field emission measurements wereperformed at a base pressure of 2.67×10−4 Pa. Field emissioncurrent was measured as a function of applied positive voltageat the anode using an electrometer (Keithley, model 617). Thegap between electrodes (d) was kept ∼300 μm using a spacer.Current density was determined by dividing current with thearea of the sample exposed to the anode.

3. Results and discussion

SEMmicrographs of samples A and B are shown in Fig. 1(a)and (b), respectively. Sample A has high density of vertically

Fig. 3. XPS spectra of representative CNT film; (a) C 1s and (b) N 1s core levelspectrum.

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aligned CNTs. Sample B has flower like morphology of CNTsaround each pit on the substrate. Side view of vertically alignedCNTs is shown in inset of Fig. 1(a).Magnified view of a flower ofCNTs is presented in the inset of Fig. 1(b), which clearly showsthat CNTs are aligned around the pit present on the mechanicallypolished Si substrate. The shape and size of the CNTs' flower wasfound to depend on the shape and size of the pits. Alignment ofCNTs was found to be controlled by the plasma density or inother words electrostatic potential developed on the substratesurface due to plasma which is always oriented normal to thesubstrate. Due to presence of pits and hence curved surface, localorientation of the electric field lines may have changed aroundthe pits, resulting in flower like growth of aligned CNTs. Metalnanoparticles on the substrate in the peeled off region (inset Fig.1(a)) and metal-particle free closed tips suggest that growth ofCNTs was governed by ‘base’ growth mechanism [20]. Fig. 2(a)is a typical TEM micrograph of CNT samples. Irrespective ofmorphology of the two samples, all the CNTs were found tohave bamboo-shaped microstructure with a very sharp tip (5–10 nm) at the top (Fig. 2(a) and (b) insets) and to be open at thebase (Fig. 2(a)). A typical diffraction pattern taken at the joint oftwo compartments is shown in the inset of Fig. 2(a). Few CNTswere found to have elongated metal particles at their base asshown in Fig. 2(b). This gives clear indication for the ‘base’growth of CNTs as observed by SEM. Although no elementalanalysis of the particle was carried out, this metal particle issupposed to be of iron. The outer diameter of the bamboo-shaped CNTs is in the range of 30–50 nm but few of them havediameter of ∼150 nm.

Elemental analysis of CNT films was carried out by XPS andFTIR. Fig. 3 shows the C 1s and N 1s core level spectra of thetypical CNT film. The C 1s and N 1s spectra were fitted in fourand three Gaussian sub-peaks respectively. In the C 1sspectrum, peak I at 284.4 eV represents the C–C bond of thegraphite. Peak II at 285.6 eV and peak III at 286.5 eV areattributed to the different types of carbon–nitrogen bonds. The

Fig. 2. Typical TEM micrograph of samples A and B showing (a) bamboo-shaped sshowing elongated metal catalyst particle at base and generation of compartments o

peaks II and III correspond to the sp2 trigonal C–N bonding andsp3 tetrahedral C–N bonding respectively [21,22]. Peak IV at289.1 eV is attributed to C–O type bonds. In the N 1s spectrum,peak I at 397.2 eV and peak II at 398.9 eV are assigned to thetetrahedral nitrogen bonded to an sp3C and trigonal nitrogen

tructure with sharp tip and open base, SAED pattern is shown in the inset, (b)ver the top of catalyst particle. Magnified view of tip is shown in the inset.

Fig. 5. HRTEM micrograph of typical bamboo-shaped CNTs showing manyopen graphite edges (shown by arrows) on the outer surface of the tube.

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bonded to an sp2C respectively. Peak III at 401 eV is attributedto N–O type of bonds. The nitrogen doping concentration wasfound to be ∼1 at.% from XPS measurement. The nitrogendoping was further confirmed by the FTIR measurements. Fig.4 shows the typical FTIR spectrum of CNT film. The band at∼1230 cm−1 is assigned to C–N stretching mode [21,23]. BandII at ∼1450 cm−1 is attributed to both the C–N bond and the IRactive G and D bands of graphite [21]. Band III at ∼1600 cm−1

is due to the CfN stretching mode.Typical high resolution TEMmicrograph of a compartment of

N-doped CNTs is shown in Fig. 5. This shows that CNTs havehigh degree of crystallinity. The wall thickness at the joint sectionof two compartments is maximum and it decreases continuouslytowards the base of the compartment leaving many open graphiteedges on the outer surface of nanotube as shown by arrowheads inFig. 5. Growth models of the bamboo-shaped structure have beenproposed in prior works [24,25] suggesting the role of nitrogen intube closure and re-nucleation of new compartment which is alsosupported by our experimental observations. The periodicappearance of cone structure in one tube is supposed to be dueto periodic precipitation of the graphite sheets on the top surface ofcatalyst particle under steady state.

Fig. 6(a) shows the typical field emission current density vs.applied macroscopic field of samples A and B. In this plot, ‘R’and ‘F’ correspond to the emission measurements withincreasing and decreasing fields respectively. Measurementswere carried out for two cycles in each sample (extended over aperiod of 60 min each). First sweep of the voltage in both thesamples showed slight fluctuation in emission current in the lowfield region. However the emission current was fairly stable inthe high field region. Also in the first cycle of voltage sweep, theemission current showed some hysteresis. However, emissioncurrent in the second cycle was repeatable. Both the samplesshowed similar behavior of cyclic emission performance.However, in the first cycle, sample A showed stronger hysteresisas compared to that for sample B. The fluctuation in the emissioncurrent in the first cycle could be due to adsorbates and/or someuneven CNTs emitting preferably. Once the sample is scannedover high field these adsorbates get desorbed and the cleansurface of all the CNTs take part in emission process. Thisresulted in stable and repeatable emission performance.

Fig. 4. FTIR spectrum of representative CNT film.

Field emission is usually analyzed using Fowler–Nordheim(F–N) theory, according to which emission current density (J)of a metal tip is dependent on the local electric field (Eloc) and

Fig. 6. (a) Current density vs. macroscopic field plots of samples A and B fortwo cycles of electric field scan. (b) Corresponding Fowler–Nordheim plots forthe second cycle.

Table 1Comparative field emission characteristics of samples A and B

Sample βL βH Turn-on (10 μA/cm2)field (V/μm)

Threshold field at1 mA/cm2 (V/μm)

A 1500 6252 1.95 3.55B 1820 12,140 1.50 2.65

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chemical state (i.e. work function ϕ) of the emitter tip asJ∝ (Eloc

2 /ϕ)exp(−Bϕ3/2/Eloc) with B=6.83×109 VeV−3/2 m−1.

In case of CNT films, emission occurs from the multipleemitters and an average current is measured. There is lot ofvariation of local field on emitter tip in CNT films due tovarious geometries. Also work function of each emitter is notthe same because the exact analysis of field emissioncharacteristics of CNT films is a difficult task. There are twoparameters, ϕ and Eloc, where Eloc is related to the macroscopicelectric field (E) by geometrical enhancement factor (β ) asEloc=βE and β can be determined experimentally from the slopeof ln(I/V2) vs. 1/V as β=−Bϕ3/2d/slope, provided ϕ is known.F–N plots of samples A and B are shown in Fig. 6(b). Twoslopes in different regions were observed in both the samples.Slope of the F–N plot in high field region is significantly less ascompared to low field region. The origin of such dual slopebehavior has been discussed in the prior works. It is suggestedthat such deviation from normal F–N model in high field regionis due to space charge effect, presence of defects (or changes inthe local density of states) at the emitter's tip, interactionbetween neighboring tubes, absorptions and desorptions ofgaseous species under the emission assisted surface reactionprocess [3,6,26,27]. The value of βwas estimated in both the low(βL) and high (βH) filed regions assuming ϕ∼5 eV. The fieldemission properties of two samples are compared in Table 1.Such a high field enhancement factor of N-doped CNTs issupposed to be due to very sharp tip (5–10 nm) of tube assuggested by Bonard et al. [6] and open edges on the surface ofCNTs. The CNT film with flower like morphology has superior(low threshold field and high current density) emissioncharacteristics compared to that with vertically aligned one.This could be due to the existence of many open graphite edgesnear the joint of two consecutive cones in a nanotube as shownby arrowheads in Fig. 5. These kinds of defects are more likely toappear in N-doped CNTs. The higher the number of cones (morejoints), the higher the density of defects on the sidewalls of N-doped CNTs [15,28–30]. These open edges on the side circularwalls of N-doped CNTs act as additional emission sites [17,31].On the other hand, in case of vertically aligned N-doped CNTs,emission is supposed to occur mainly from the tip section whichcan be further limited by the screening of the electric field due toneighboring tubes.

Field emission performance of both samples was tested formore than 20 h continuously at an emission current density of1 mA/cm2. The emission current remains fairly constant forboth the samples. The emission current was fairly stable with afluctuation of ±2%. The fall in average emission currents overthis period was ∼2% in sample A and ∼3 % in sample B. Nochange in the morphology of the samples was observed by SEMeven after 20 h of continuous emission performance test. It is

possible that some of the graphite edges on the surface of CNTsmay get damaged or modified under the high local field or thecrystallinity of CNTs may get affected. However, no significantchanges in the emission current were observed. It is possiblethat the average number of emission sites being active all thetime remains unchanged during the measurement. Thedynamics of the process needs critical investigation.

4. Conclusions

In conclusion, we have synthesized and investigated the fieldemission characteristics of vertically aligned and flower likealigned N-doped bamboo-shaped CNT films. Nature of thesubstrate was found to strongly affect the growth morphology ofCNTs in MPECVD process. Flower like morphology of CNTsaround pits was observed onmechanically polished Si substratesand vertically aligned CNTs on the mirror polished one. CNTfilm with flower like morphology was found to have superioremission properties as compared to vertically aligned one. Thisis suggested to be due to the large number of open graphite edgeson the surface of nanotubes which may act as additionalemission sites and enhance the emission characteristics.

Acknowledgements

One of the authors (S. K. Srivastava) is highly thankful toMr. Sandeep Chhoker for his assistance in field emissionmeasurements. S.K.S. is also thankful to Dr. S. M. Shivprasadfor carrying out XPS study and related discussions.

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