Physica E 44 (2011) 29–36

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

Jordans

E-m

journal homepage: www.elsevier.com/locate/physe

A comparative study of the growth, microstructural and electrical propertiesof multiwall CNTs grown by thermal and microwave plasma enhancedCVD methods

A. Mathur a, S. Wadhwa a, M. Tweedie a, K.S. Hazra b, C. Dickinson c, S.S. Roy a,d,n, S.K. Mitra d,D.S. Misra b, J.A. McLaughlin a

a NIBEC, School of Engineering, University of Ulster, Jordanstown, BT37 0QB, UKb Department of Physics, IIT Bombay, Powai, Mumbai 400076, Indiac Material and Surface Science Institute, University of Limerick, Irelandd Micro and Nano-Scale Transport Laboratory, Department of Mechanical Engineering, University of Alberta, Edmonton, Canada T6G 2G8

a r t i c l e i n f o

Article history:

Received 11 February 2011

Received in revised form

14 June 2011

Accepted 28 June 2011Available online 21 July 2011

77/$ - see front matter & 2011 Elsevier B.V. A

016/j.physe.2011.06.035

esponding author at: NIBEC, School of Engin

town BT37 0QB UK.

ail address: sinharoy@uslberta.ca (S.S. Roy).

a b s t r a c t

Multiwalled carbon nanotubes (CNTs) were grown on silicon substrates by thermal chemical vapour

deposition (CVD) and microwave plasma CVD (MPCVD). In this manuscript, an attempt has been made

to compare the growth mechanism and electrical and micro-structural properties of CNTs grown by

two different methods. CNTs in MPCVD is grown by tip-growth mechanism whereas in TCVD it is

grown by base-growth mechanism. To compare these techniques, the CNT samples were examined

using scanning and transmission electron microscopy, Raman spectroscopy, X-ray diffraction (XRD),

and X-ray photoelectron spectroscopy. Growth rates were 10 and 0.6 mm/min, for thermal CVD and

MPCVD, respectively. Diameters ranged from �40–60 nm for thermal CVD grown CNTs, and 20–50 nm

for MPCVD-grown CNTs. Deposition over 200 diameter silicon wafers was achieved for the MPCVD

growth technique. XRD and Raman studies revealed good crystallinity for both growth methods.

However, thermal CVD grown CNTs showed more defects, with a crystallite size of �20 nm, compared

to �37 nm for MPCVD. Static water contact angles of thermal CVD and MPCVD samples were 1551 and

1361, respectively, indicating greater hydrophobicity for thermal CVD-grown CNTs. The electrical

resistance was 75.40 O for MPCVD-grown CNTs, but 782.10 O for thermal CVD samples. Field emission

studies revealed b values for thermal CVD and MPCVD samples, of 8010 and 13,570, with turn-on fields

of 0.96 and 0.60 V/mm, respectively.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The physical, chemical, and mechanical properties of CNTshave been extensively measured for numerous applications[1–13]. Furthermore, CNT-based electronic devices, such asbio-sensors and chemical sensors, have been fabricated andcharacterised [14–17]. CNTs have been synthesised using a rangeof methods, including arc-discharge, laser deposition, thermalchemical vapour deposition (CVD), and microwave plasma che-mical vapour deposition (MPCVD) [18]. Basic structural andphysical properties, such as dimensions, density, alignment, andcrystallite size, along with electrical and mechanical properties,have been shown to be strongly affected by the method of CNT

ll rights reserved.

eering, University of Ulster,

deposition [18]. Detailed comparisons between CNTs depositedby various techniques are, however, rare. Thermal CVD andMPCVD are the main techniques commonly employed for grow-ing CNTs. Recently, a direct comparison between thermal CVD-and MPCVD-grown non-aligned single wall carbon nanotubes,using a supported bimetallic catalyst, was published [19]. How-ever, no focused research compares micro-structural studies onaligned multiwall carbon nanotube (MWCNT) grown by the lattertwo methods. To determine the more appropriate growth methodfor different applications, it is essential to characterize themicrostructural properties of CNTs grown by these. This presentstudy aims to measure the key differences between the micro-structural properties of CNTs grown by the above two methods.

In the present study, we grew multiwalled CNTs by boththermal CVD and MPCVD. We compared the structural differencesbetween CNTs grown by each method, characterizing the samplesusing scanning electron microscopy (SEM), transmission electronmicroscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD),

A. Mathur et al. / Physica E 44 (2011) 29–3630

and X-ray photoelectron spectroscopy (XPS). The electrical prop-erties of the nanotubes were, also, characterised by field emissionand I–V measurements.

2. Experimental techniques

2.1. CNT growth by thermal CVD

CNTs can be grown over a temperature range of 600–900 1Cusing thermal CVD [20]. The thermal CVD system was found togive optimum vertically aligned CNTs grown at a nominal tubetemperature of 750 1C, with an argon carrier gas atmosphere.A mixture of ferrocene and toluene (2%) was used as a precursor.Ferrocene acts as a catalyst, whereas toluene supplies therequired carbon feedstock for CNT growth by thermal dissociationof the toluene molecule. The furnace used in this work is a singlezone furnace, in which both the precursor and substrates wereplaced for CNT growth. Two separate molybdenum boats wereused for the precursor liquid and the substrates. The substratewas centred in the main heating zone of the furnace, while theprecursor was positioned near the cool input tube end, so that itwas at a much lower temperature than the substrate. Thetemperature gradient across the tube was �10 1C/cm, from thecentre to each end, at a nominal furnace centre temperature of750 1C. The temperature difference from the substrate to theprecursor, at the CNT growth temperature, was around 500 1C.Substrates used were unoxidised p-type, /1 0 0S, silicon.

2.2. CNT growth by MPCVD

Carbon nanotubes were produced using an MPCVD system,described in detail elsewhere [21]. The substrates used werep-type Si, /1 0 0S, on which a cobalt (Co) catalyst, with a layerof thickness 3 nm, was deposited by DC sputtering. A mixture ofN2 and CH4 (methane) was used for the synthesis of CNTs. Prior toCNT growth, the samples were pre-treated for 2 min in a nitrogenplasma of 300 W power at 750 1C. The nitrogen plasmapre-treatment on the substrates breaks the cobalt ultrathin filminto nanoparticles, which act as catalytic sites for CNT growth.The microwave power was increased immediately after thepre-treatment phase, to 600 W, for CNT growth. During thegrowth phase, the CH4 provides carbon for nanotube growth,whereas the N2 etches the amorphous carbon (a-C) by-productsfrom the deposition process, thereby reducing the amount of a-Cdeposited with the CNTs. The growth phase terminated when themethane gas supply was turned off, the system then beingallowed to cool to room temperature. The three stage growthprocess is shown schematically in Fig. 1.

Fig. 1. Schematic showing the three stages of CNT growth process by MPCVD

system.

2.3. Growth mechanism of CNTs in thermal CVD and MPCVD

The three principal stages of CNT synthesis in MPCVD growth[19,22,23] are (a) catalyst nanoparticle formation at thepre-treatment temperature, (b) initial diffusion of carbon on thecatalyst nanoparticle surface creating CNT nucleation, and(c) growth of CNTs to form a CNT film on the substrate. Usually,plasma pre-treatment breaks the catalyst ultrathin film intonanoparticles [24]. According to the tip-growth model, thegrowth rate of CNTs can be increased by enhancing the carbondiffusion rate in the catalyst [25]. In contrast, for thermal CVD, thefirst stage was different as catalyst precursor solutions were used.In this case, ferrocene thermally decomposes to form Fe particles,which enhances toluene decomposition and leads to the initiationof the nanotube growth process. Although the mechanistic stepsare wellknown, some important details about nanotube formationare still not well understood. Nanotube growth has, also, beenproposed to occur by either root growth [26], in which thenanotube base interfaces directly with the nanoparticle, or a‘‘folded growth mode’’ [27] in which the carbon shell that formsthe nanotube wraps around the nanoparticle, leading to curvedgraphitic layers that extrude from the particle surface. Theliterature shows that both of these latter mechanisms can occur,depending on the synthesis conditions [26,27].

Nanoparticle formation occurs during pre-treatment of thecatalyst film on substrates, by a thermal or a combined thermal/plasma process, where the size of the catalyst nano-islandsdirectly controls the CNT diameters [21,28]. Comparing the AFMimages (as shown in Fig. 2), an increase in particle size occursbetween plasma and thermally treated samples. The difference innano-island size in Fig. 2 could be related to higher effectivesurface temperatures during plasma annealing, than indicated bythe substrate thermocouple reading, because of sample bombard-ment by highly energetic plasma ions. As a result, where thetemperature reading is identical for both plasma and thermaltreatments, in the former, the substrate could be additionallyheated by the plasma, allowing particles to redistribute on thesurface in order to find a more stable energetic state. Further-more, in MPCVD, the nanofilm surface is strongly eroded bybombardment from plasma ions, thinning the film and creatingsmaller catalyst nano-islands than for thermal CVD at the sametemperature.

3. Results and discussion

3.1. SEM and TEM analysis

The SEM images in Fig. 3 show the general morphology of theCNTs grown by both CVD methods, on unpatterned ultrathin filmcatalyst and on lithographically patterned micro-islands of ultra-thin film catalyst. Patterned CNT growth may have applications inbio-sensors, as capture and sensing elements when grown onelectrodes, and in microfluidic filters as particle traps. Fig. 3(a)–(b) shows vertically aligned CNT forests obtained from MPCVDand thermal CVD, respectively. The MPCVD-grown CNTs areobserved to have less dense growth compared to the thermalCVD-grown CNTs, when Fig. 3(d) is compared to Fig. 3(c).Furthermore, the MPCVD-grown CNTs are nearly 20 mm in length,whereas the thermal CVD-grown CNTs are nearly 300 mm inlength. The thermal CVD-grown CNTs are highly dense, and theSEMs show a significant amount of bright material on the topsurface, associated with amorphous carbon and residual catalyst.The CNT growth rate was much higher for thermal CVD than forMPCVD growth.

Fig. 2. Comparative AFM images of the catalyst particles before CNT growth using the (a) plasma and (b) non-plasma (thermal-only) methods.

Fig. 3. SEM micrographs of CNTs produced by MPCVD (a, c) and thermal CVD (b, d). The images were taken with a 451 tilt.

A. Mathur et al. / Physica E 44 (2011) 29–36 31

Fig. 4. shows wafer scale growth of MWCNTs on a 2 inchsilicon wafer, using 3 nm of cobalt catalyst for synthesis byMPCVD. Detailed characterisation showed results comparable tothe literature for MWCNTs. In our thermal CVD system, due tolimited diameter of the thermal CVD tube furnace, a complete2 in. wafer could not be inserted, but we successfully grewuniform carpets of MWCNTs on rectangular/square sections ofquartz, molybdenum and silicon substrates. Large scale synthesisof CNTs has, also, been reported in the literature [29,30].

A representative high resolution TEM image, of CNTs preparedby MPCVD, is shown in Fig. 5(a), where a typical bamboo-typegrowth structure is evident. It is reported in the literature thatnitrogen incorporation in the tube walls is the cause of theformation of bamboo-shaped CNTs [31–34], where the growthmechanism has, also, been reported [35,36]. From multiple TEMobservations, it has been confirmed that the nanotubes are ofgood quality, with very little amorphous carbon or residualcatalyst particles present.

Fig. 4. Wafer scale synthesis of vertically aligned CNTs on a 2 in. silicon wafer using MPCVD. Inset showing the corresponding SEM image of the CNTs.

Fig. 5. TEM images of CNTs produced by (a) MPCVD and (b) thermal CVD.

Table 1Comparison of experimental parameters and growth measurements for thermal

CVD and MPCVD growth techniques.

Parameters Thermal CVD MPCVD

Substrate n-Si n-Si

Carbon source Toluene (C6H5CH3) Methane (CH4)

Catalyst Ferrocene VLS phase Cobalt thin film

Growth temperature 750 1C 750 1C

Growth rate 10 mm/min 0.6 mm/min

CNT length 300 mm 30 mm

CNT diameter 8–60 nm 10–150 nm

Other gases Argon Nitrogen

A. Mathur et al. / Physica E 44 (2011) 29–3632

For thermal CVD-grown CNTs, good nanotube growth qualitywas, also, observed, in terms of wall structural integrity, and thevery low quantity of catalyst particles. A representative TEMimage of CNTs prepared by the thermal CVD method is shownin Fig. 5(b). The HRTEM images confirm that the tubes aremultiwalled in nature, with no catalyst visible in the nanotubecores. It was found from TEM studies that the diameter distribu-tion of the CNT samples has a wide range, from 10–150 nm overvarious samples. However, these MWCNTs have always beenshown to have a central tube with a diameter range from6–10 nm. Generally, the diameter of inner tubes varies locallywithin the same MWCNT sample. The purity of the sample wasfound to be high, based on TEM observations covering themajority of the copper TEM sample grid. For a simple comparison,all major experimental parameters and growth measurements areshown in Table 1.

3.2. Raman spectroscopy and X-ray diffraction

Raman spectra were measured for the CNT samples, using anAr ion laser at 514.5 nm, with power density reduced using built-in filters so as not to burn the CNTs. The Raman spectra of allcrystalline graphitic materials exhibit at least three to four majorbands, denoted as D, G, D0, and G0 (2D). The G peak corresponds tothe tangential stretching (E2g) mode of highly oriented pyrolyticgraphite (HOPG). The origin of the disorder-induced D and D0

peaks, and the G0 overtone of the D peak (or the 2D peak, which isalways observed in crystalline graphite materials), in CNTs, hasbeen explained by double resonance theory [37,38]. The presenceof the D and D0 peaks indicate defects in the crystallite structureand edges of the material. The Raman spectra in Fig. 6 showtypical MWCNT peaks. Prominent D, G, G0 (2D) peaks, and faint D0

shoulders on the G peak were observed in all samples, similar totypical MWCNT peaks reported in the literature [39]. Ramanparameters obtained after fitting Gaussian functions to thespectra are listed in Table 2.

Using the ID/IG values, and the excitation energy (EL), for theArþ ion laser used (2.41 eV, for a laser wavelength of 514.5 nm),the in-plane size of graphite crystallites La,was determined fromthe following general formula [40,41]:

LaðnmÞ ¼ 560=E4L ðID=IGÞ

�1ð1Þ

The in-plane size of graphite crystallites for MPCVD-grownCNTs is higher than that of thermal CVD-grown CNTs. The lowercluster size in the thermal CVD-grown samples indicates a higherdegree of disorder. The lower ID/IG value of 0.44, for MPCVD-grown CNTs, compared to an ID/IG value of 0.81 for thermalCVD-grown CNTs, indicates a decrease in the amount of

1000 1000

D'

D'

G

Raman Shift (cm-1)

MPCVD

D

Ram

an In

tens

ity (a

.u.)

TCVD

G'

GD

1500 2000 2500 3000 1200 1400 1600 1800

Fig. 6. Comparative Raman spectra of CNTs produced by (upper) MPCVD and

(lower) thermal CVD.

Table 2Comparison of the Raman parameters for thermal CVD and MPCVD-grown

samples for 2.41 eV excitation energy (Ar ion laser at 514.5 nm).

Raman parameters MPCVD Thermal CVD

D peak position 1350.88 1322.69

G peak position 1576.93 1569.77

D0 peak position 1613.80 1605.51

G0 peak position 2691.15 2646.93

ID/IG value 0.44 0.81

La value 37.68 20.42

Table 3

Field enhancement factors, b, and turn-on fields, from Field Emission analysis of

thermal CVD and MPCVD-grown CNT samples.

Sample b high field b low field Turn-on field (V/lm)(for 10 lA/cm2)

Thermal CVD 8010 4011 0.96

MPCVD 13,570 5700 0.60

0

1

2

3

20

CNT-MPCVDC

(100)

C (002)

C (100)

SiC

(101)

C (002)

CNT-thermal CVD

Inte

nsity

(a.u

.)

2 theta

(Fe2 O

3 )

30 40 50

Fig. 7. Comparative X-ray diffraction patterns from CNTs produced by thermal

CVD (upper trace) and MPCVD (lower trace) growth.

Fig. 8. Comparative XPS spectra of CNTs produced by (a) MPCVD and (b)

thermal CVD.

A. Mathur et al. / Physica E 44 (2011) 29–36 33

amorphous carbon or in the degree of disorder in the nanotubes, andan increase in the graphitic content (Table 2). All the Raman peaksfor thermal CVD-grown CNTs occurred at lower wavenumbers thanfor MPCVD samples, possibly, due to less stress and higher amor-phous carbon content in the former. The Raman results, suggestinghigher amounts of a-C and higher defect levels in the thermal CVD-grown CNTs, are consistent with the observations made from SEMimaging. These show the top surface of thermal CVD samples isvisibly more defective, with numerous bright spots suggesting thepresence of a-C and metal catalyst Table 3.

The XRD diffraction patterns from CNTs grown by bothmethods are shown in Fig. 7. It is well known that a typicalcarbon structure obtained at low temperatures (500–1000 1C)exhibits an XRD pattern consisting of broad bands located nearthe (0 0 2), (1 0 0), (1 1 0), and (1 1 2) reflections of graphite. Thec-axis lattice parameter values for the CNT films were obtainedfrom the strongest reflection of the (0 0 2) peak. The full width athalf maximum of the (0 0 2) peak is seen to be narrow, whichindicates good crystallinity of the CNTs, for both MPCVD andthermal CVD samples. However, the (0 0 2) peak of the thermal

CVD sample is broader at the base (circled in Fig. 7), which mayindicate a two phase crystalline system, or a slight increase in thed-spacing between the walls in a relatively small number oftubes. The prominent peak around 2y¼260 can be attributed tothe (0 0 2) reflection of carbon. The d002 value derived from thisXRD pattern is 3.366 A, which is slightly higher than that ofperfect graphite (d002¼3.354 A), but is in good agreement withthe TEM measurements. XRD measurements for thermal CVDsamples indicate that, besides CNTs, the main phases are Fe andFe3C, the latter being known to be an active phase for CNTformation [42,43]. The diffraction peaks associated with Fe3Cand Fe phases are comparable to other reports [44,45].

3.3. Surface chemistry and hydrophobicity

XPS data gives information about the chemical composition ofthe nanotube samples. Fig. 8(a) and (b) shows the deconvolutedC1s peaks of both MPCVD and thermal CVD samples. A mixture of

A. Mathur et al. / Physica E 44 (2011) 29–3634

Gaussian and Lorentzian function was used for deconvolution ofthe C1s peaks, centred at 284.470.2 (C1), 284.870.2 (C2),285.370.2 (MPCVD), 285.570.1 (TCVD) (C3), and 286.070.1(C4) eV. The main peak at 284.4 eV (C1) originates from thegraphite signal (CQC). The peak at 284.8 eV (C2) is assigned tosp3 carbon bonding (C–C/C–H). The peaks in the range 285.3–285.5 (C3) and at 286.6 (C4) eV correspond to C–O and CQOsignals, respectively [46,47]. For both sets of samples, oxygencontent was significantly lower at the surface, demonstrating thepresence of less dangling carbon bonds and pure nanotubes.Qualitative discussion can be made as follows. C2 and C4 peakscontribution was relatively larger in TCVD grown samples, indi-cating larger sp3 defects. A relatively broader C3 peak shiftedtowards lower binding energy (285.3 eV) is observed in MPCVDsample. The reason is not clear however indicating abundance ofC–O functional groups on the CNT surface of MPCVD-grownsample.

Contact angle measurements were performed using the sessiledrop method, on MPCVD- and thermal CVD-grown CNT surfaces.Both samples showed a very high contact angle, greater than 901,indicating that the surface is super-hydrophobic, i.e. the waterdroplet does not adhere to the surface of the CNTs. The highcontact angle for both samples is clearly visible in Fig. 9. Contactangles were averaged from three different spots per sample,giving average water droplet contact angles of 1361 and 1551 forthe MPCVD and thermal CVD samples, respectively. The measuredvalues are comparable to previous reports [48,49]. The as-grownnanotubes are covered by amorphous carbon and/or the metalcatalyst, which may leave no free bonds to attach easily to water.Our XPS study, also, confirms very few oxygen-related bonds

Fig. 9. Comparative water contact angle images of CN

-0.2

-6

-4

-2

0

2

4

6

Cur

rent

(mA

)

TCVD

-0.2

0

20

40

60

80

100

120

0

200

400

600

800

1000

1200

R (i

n O

hms)

V (in Volts)

MPCVD

TCVD

-0.1 0.0

-0.1 0.0 0.1 0.2

Fig. 10. Typical I–V characteristics of the CNT samples (both from thermal CV

available on the surface of the CNTs. Adsorption of water on theCNT surface varies with time. Thermal CVD samples had highercontact angles for two reasons: firstly, surface roughness washigher than that of MPCVD-grown samples, and secondly, CNTswere much more closely packed, hence, there were less air voids,resulting in less water adsorption.

3.4. Electrical resistivity and field emission measurements

Electrical measurements were performed on vertically alignedas-grown carbon nanotube samples using a 2-probe method [21].All nanotube samples exhibited I–V characteristics that wereohmic, being linear at low positive and negative applied voltages.A typical I–V characteristic at room temperature for MWCNTs isshown in Fig. 10. The inset of Fig. 10 shows a V vs. R plot, showingconstant R, irrespective of the polarity of V. The linearity of thisI–V curve indicates that there is no contact barrier between themetal electrodes used and the carbon nanotube. It is known thatin a two probe measurement, the resistance measured is the sumof the contact resistance and the nanotube resistance, and hence,the resistances measured here are slightly higher than the actualresistance of the nanotubes. The measured value of the resistancefor MPCVD-grown CNTs was 75.47 O, whereas that for thermalCVD samples was 782.19 O. As the length of thermal CVDnanotubes is approximately 10 times higher than that ofMPCVD-grown CNTs, the observed resistance also differs by afactor of approx. 10, as these MWCNTs are, effectively, purelymetallic. The values of electrical resistivity are 1.8�10�4 O cmand 19.32�10�4 O cm for MPCVD and thermal CVD samples,

Ts produced by (a) MPCVD and (b) thermal CVD.

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Cur

rent

(mA

)

MPCVD

R (i

n O

hms)

0.1 0.2

D and MPCVD) showing linear behaviour. Inset showing the R Vs V plot.

Fig. 11. (a) Field-emission current density vs. applied field, for thermal CVD- and

MPCVD-grown CNTs, (b) F–N plots for thermal CVD and MPCVD-grown CNTs.

A. Mathur et al. / Physica E 44 (2011) 29–36 35

respectively. In general, our measured values of resistivity com-pare well with the available literature [21,50,51].

Fig. 11(a) shows the results of field emission measurement forCNTs grown by thermal CVD and MPCVD methods. Turn-on fields(defined at an emission current density of 10 mA/cm2), for thermaland MP CVD samples, were 0.96 and 0.60 V/mm, respectively. In orderto obtain stable and reproducible field emission characteristics,initially high currents were generated between the anode and CNTsto remove any loosely attached nanotubes present on the top surface.The measured current was observed to follow the Fowler–Nordheim(F–N) law, where the current density, J, is related to applied electricfield, E, as J¼ Aðb2E2=fÞexpð�Bf3=2=bEÞ, where J is current densityin A/cm2, A¼ 1.56�10�6 AV�2 eV, B¼6.83�107 eV�3/2 V cm�1,b is a field enhancement factor, f is the work function (eV), and E

is applied electric field in V/mm. We used a value of work function,�5 eV, to calculate the CNT field enhancement factor, b [52,53].Fig. 11(b) shows F–N behaviour, indicative of field emission from theCNT samples. The F–N plots in Fig. 11(b) show two linear regions ofdifferent gradients, with a knee point in between. The appearance of aknee point in F–N plots indicates saturation of emission currents athigh electric field [54]. The estimated field enhancement factors forthermal CVD and MPCVD samples, respectively, are 4011 and 5700 inthe low field region, and 8010 and 13570 in the high field region.

4. Conclusions

Vertically aligned multiwall carbon nanotubes were grownusing MPCVD and thermal CVD techniques. Microstructure wasanalysed using SEM, TEM, Raman spectroscopy, and XRD analysis.Although both growth techniques are useful for synthesisingvertically aligned CNT forests, there is a substantial difference

between the CNT microstructures. The samples grown by thermalCVD show higher growth density, and higher topological defectdensity, compared to MPCVD-grown samples. Temperature playsa vital role in the growth of CNTs. The diameter and length ofCNTs were directly related to the cluster size of the catalyst nano-islands. Micro-structural comparisons revealed that MPCVD-grown CNTs are �10 times shorter in length as compared totheir thermal CVD counterparts. The diameter distribution ofthermal CVD-grown CNTs was in the range of �40–60 nm, whileit was �20–50 nm for MPCVD CNTs. A sharp G0 (2D) peak in theRaman spectra, and a narrow C (0 0 2) peak in XRD analysis, for allCNT samples, indicated good crystalline quality, irrespective ofgrowth method. Raman studies show a higher ID/IG value and alower G peak wavenumber for CNTs grown by thermal CVD. Thisindicates a higher degree of disorder in thermal CVD samples.Surface hydrophobicity was higher for thermal CVD-grownsamples, with the contact angle being �201 higher than forMPCVD samples. A rougher top surface in thermal CVD samplesmay be responsible for the increased super-hydrophobicity. Thedifferences in the microstructures for both growth methods arenot reflected in the room temperature electrical resistivity mea-surements. Turn-on field and field enhancement factors weresmaller for the MPCVD-grown samples. The above studies and theoptimised deposition conditions could be useful for growingCNTs with specific dimensions. Furthermore, a clear understand-ing of micro-structural properties can be utilised for specificapplications.

Acknowledgement

A.M. is grateful to UKIERI and VCRS for funding this project.S.S.R. is grateful to DEL, NI, for the cross-border R&D fundingprogramme. C.D’s work was conducted under the framework ofthe INSPIRE programme, funded by the Irish Government’sProgramme for Research in Third Level Institutions, Cycle 4,National Development Plan 2007–2013.

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