Thin Solid Films 518 (2010) 6915–6920

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Enhanced electron emission from titanium coated multiwalled carbon nanotubes

Himani Sharma, V. Kaushik, P. Girdhar, V.N. Singh, A.K. Shukla, V.D. Vankar ⁎Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India

⁎ Corresponding author. Tel.: +91 11 26591329; faxE-mail address: vdvankar@physics.iitd.ernet.in (V.D.

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.07.043

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 January 2010Received in revised form 30 June 2010Accepted 8 July 2010Available online 14 July 2010

Keywords:Carbon nanotubesMicro Raman spectroscopyHigh resolution transmission electronmicroscopyField emission

Enhanced field emission properties and improved crystallinity of titanium (Ti) coated multiwalled carbonnanotubes (MWCNTs), prepared by microwave plasma enhanced chemical vapour deposition have beenobserved. Ti films of extremely low thicknesses (0.5 nm, 1.0 nm and 1.5 nm) were coated over carbonnanotubes (CNTs) and their field emission behaviour was investigated. The turn on field of Ti coated CNTswas found to be low (~0.8 V/μm) as compared to pristine CNTs (~1.8 V/μm). The field enhancement factorfor Ti coated CNTs was quite large (~1.14×104) as compared to pristine CNTs (~6×103). This enhancementin electron emission is attributed to the passivation of defects and improved crystallinity of CNTs. Surfacemorphological and microstructural studies were carried out to investigate the growth of pristine and Ticoated CNTs. It was observed that Ti nanoclusters adsorb on the edges of MWCNTs and increase theircrystallinity. This increase is directly correlated with the thickness of Ti film deposited. Micro Ramanspectroscopy confirmed the improved crystallanity of Ti coated CNTs.

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1. Introduction

Since their discovery in 1991 [1], carbon nanotubes (CNTs) are ofspecial interest due to their promising applications in field emissiondisplays [2,3], nanoelectronics [4,5], hydrogen sensing [6] and fuelcells [7]. CNTs exhibit excellent field emission properties on account oftheir unique geometry [8], high aspect ratio and good chemicalproperties [9]. Field emission characteristics depend upon variousparameters such as field enhancement factor and work function [10].Lower work function and higher field enhancement factor lead toenhanced field emission. Many efforts have beenmade to improve theemission properties of CNTs, such as coating, doping, and various posttreatments [11–13]. Not only emission properties, but also consider-able efforts have been made to improve the structural properties ofcarbon nanotubes.

Composite materials made of metals or metal oxides and CNT havegained much importance, as it results in the modification in theelectrical, structural and mechanical properties of carbon nanotubesthrough orbital hybridization and thus results in the passivation ofdefect sites. Recently, various studies have been carried out to modifythe electrical and structural properties of CNTs, by coating of variousmetals, oxides andfluorides such as Ru, Pd, ZnO, RbF, CrO3 [14–18]. Choet al. investigated the effect of various metal coatings (Co, Ti, Pd, W,and Ru) on the electronic structure of CNTs by ab initio calculationsand field emission experiments [14]. According to their theoreticalcalculations, adsorption of metal atoms leads to substantial change in

the band structures andwork function of nanotubes [14]. It is reportedthat Titanium (Ti) affects the work function of CNTsmost significantlyand thus enhances the emission current andmodifies the properties ofCNTs [14,19]. Cho et al. in their studies on Ti coated CNTs using achemical reduction method have shown that the field emissionproperties improved almost two times as compared to pristine CNTs[14]. Similarly, Uh et al. reported that CNTs sputter coated with a thinlayer of Ti results in good emission characteristics [19]. Both Cho et al.and Uh et al. in their studies reported the enhanced emissionproperties of Ti coated CNTs because of low work function of Ti aswell as strong adhesion between the CNTs and the substrate [14,19].Theoretical simulation studies suggest that Ti can be firmly adsorbedon CNT surface due to its negative interfacial energy and high diffusionbarrier to form continuous or quasicontinuous layers as compared toother metals (Al, Au) [20]. However these studies have been carriedout for thicknesses of more than 5 nm of Ti films. At low thicknessesthe effects are expected to be different as the field emission is verysensitive to surface electronic structure.

In this paper, we report the effect of Ti coating of thickness ~0.5 nmto 1.5 nm on the field emission properties of CNT films and suggest apossiblemechanism. This is also correlatedwith themicrostructure andimproved crystallanity of MWCNTs post Ti coating as investigated byhigh resolution transmission electronmicroscope (HRTEM) studies andmicro Raman spectroscopy.

2. Experimental

The multiwalled carbon nanotubes (MWCNT) samples were grownbymicrowave plasma enhanced chemical vapour deposition [21] using

Table 1Details of pristine and Ti coated CNTs.

S. no. Samples Catalyst Precursor gas (C2H2: H2) Flow ratio

A Pristine Fe C2H2/H2 1:5B 0.5 nm Ti coating Fe C2H2/H2 1:5C 1 nm Ti coating Fe C2H2/H2 1:5D 1.5 nm Ti coating Fe C2H2/H2 1:5

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iron coated p-Si (100) as a substrate. The microwave power of 550Wwas applied to generate the plasma at 1.3×102 Pa. The samples werepretreated in the presence of hydrogen and argon plasma at 720 °C for15 min at 3.3×102 Pa. The reactant gases were a mixture of hydrogenandacetylenewitha ratioof1:5 at apressureof 6.6×102 Pa. Thepristinecarbon nanotubes were named as A. In the second set, CNTs weremodified by Ti coating of 0.5 nm, 1 nm and 1.5 nm thicknesses. The Ticoated CNTs were named as B, C and D, as seen from Table 1. Timodification on the CNT samples was carried out using thermalevaporation technique at base pressure of 2.6×10−4 Pa.

The surface morphology of pristine and Ti coated MWCNTs werecharacterized by scanning electron microscopy (SEM: ZEISS EVO 50)operating at 20 kV accelerating voltage by secondary electron imaging.Carbon films were scratched from the substrate and ultrasonicated inacetone at least for 15 min so that CNTs could disperse properly. Fewdrops of the suspension were then transferred on to carbon coatedcopper grid. Their microstructures were analyzed by transmissionelectronmicroscope (TEM:Philips CM12) andHRTEM: TechnaiG2 20 S-Twin model, operating at 200 kV. Energy dispersive analysis of X-rays(EDAX)was carried out for the elemental analysis.Micro Raman studies(Micro-RamanT64000 Jobin Yvon triplemonochromator system) of thesamples were carried out at an excitation wavelength of 514.5 nm toanalyze the structure of the samples. Field Emission measurementswere carried using a diode set up in the vacuum chamber at2.6×10−4 Pa. CNTs were used as cathode and stainless steel plate was

Fig. 1. SEM images showing (a) pristine CNTs and Ti coated CNT

used as anode. Emission current density was determined by dividingcurrent with the area of the sample.

3. Results and discussion

3.1. Surface morphology of pristine and Ti coated MWCNTs

The surface morphologies of pristine and Ti coated MWCNTs areshown by SEM images in Fig. 1. Fig. 1(a) shows the pristine CNTs thatare quite dense along with some nanoflakes. On depositing thin layerof Ti (b2 nm) over CNTs, their surface morphology gets modified, asseen in Fig. 1(b), (c) and (d). Although, little bit of densification can beseen in the micrograph of Ti coated CNTs, it is quite less as comparedto that in Fig. 1(a). There are no flakes formation in the Ti coated CNTs.In Fig. 1(b) and (c), entangled morphology of CNTs can be seen,having Ti coating of 0.5 nm and 1 nm respectively. When Ti layer of1.5 nm is deposited on CNTs, it remains no more entangled but isaligned in one direction, as can be seen in Fig. 1(d).

3.2. Microstructure of pristine and Ti coated MWCNTs

Fig. 2 shows the TEM images of pristine and Ti coated CNTs. InFig. 2(a), pristine CNTs were seen having diameters in the range of10–25 nm. After Ti coating, as seen in Fig. 2(b–f), Ti nanoclustersconfine on the edges of MWCNTs. This is expected to lead topassivation of defects. HRTEM studies of Ti coated MWCNTs werecarried out to observe the effect of defect passivation. Fig. 3(a) and (b)are the HRTEM images of as grown and Ti coated CNTs. As evidentfrom the images, pristine CNTs have several defect planes which maybe attributed to the commonly occurring Stone–Wales defects [22]. Inthe present study, it was observed that due to very thin coating of Ti(b2 nm), Ti nanoparticles get adsorbed on the CNT walls, because ofthe presence of these defects. The defects reduce the cohesive energyof Ti to a negative value which improves its adhesion of Ti to the CNT

s with a thin layer of (b) 0.5 nm (c) 1.0 nm and (d) 1.5 nm.

Fig. 2. TEM micrographs showing (a) pristine MWCNTs (b), (c), (d), (e) and (f) Ti coated MWCNTs. The encircled areas in figures (b), (c), (d) and (f) are showing Ti.

Fig. 3. HRTEM images showing (a) defect planes in bare MWCNTs by dotted line and (b) Ti coated MWCNTs, where Ti nanoparticles have been encircled, the arrow shows the Tiplanes. Inset images showing: (a) (002) planes of pristine MWCNT (b) (i) (002) planes of Ti coated MWCNT, with d=0.34 nm; (ii) (102) planes of Ti, with d=0.23 nm (c) aschematic presentation of adhering of Ti nanoparticles to the CNT walls and passivation of defects. Here, d is the interplanar distance.

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Fig. 4. EDAX image presenting the elemental analysis (a) for pristine CNTs (b) Ti coatedCNTs, showing the presence of Ti (drawn on the same scale).

Fig. 5. (a) First-order Raman spectra showing the increasing order of crystallinity of samplesA, B, C and D, where (A) pristine CNTs, Ti coated CNTs of thickness (B) 0.5 nm (C) 1.0 nm(D) 1.5 nm. (b) Second-order Raman spectra showing decreasingG*mode of samples A, B, Cand D (drawn on the same scale and shifted one above the other for comparison).

Table 2Raman parameters of pristine and Ti modified CNTs.

S. no. Peak position (I order)(cm−1)

ID/IG Peak position (II order)(cm−1)

D G D* G*

A 1355 1584 1625 1.24 2689B 1355 1584 1623 0.98 2691C 1355 1582 1625 0.82 2691D 1355 1582 1625 0.57 2691

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walls [22]. This improved adhesion and bonding with CNTs may bedue to the unfilled d-shell, resulting in the modification of theirstructure. Incorporation of Ti overcomes the defects and thusenhances the emission and crystallinity of CNTs. Fig. 4(a) shows theEDAX spectra of pristine CNTs. The Ti peak in Fig. 4(b) confirms thepresence of Ti. The presence of oxygen peak in EDAX spectra may bedue to the partial oxidation of Ti nanoclusters at the surface. Tinanoclusters are strongly adhered to CNTs at the interface due tonegative interfacial energy of Ti [14,20]. Thus, there will be nooxidation at the interface between Ti and CNTs, and Ti will be stronglyadhered to CNTs [14].

3.3. Micro Raman spectroscopy of pristine and Ti coated MWCNTs

Fig. 5(a) shows the first-order Raman spectra of pristine and Ticoated CNT samples (A, B, C and D). Two major peaks observed at1355 cm−1 and 1584 cm−1 are referred to as D and G-band [23,24]. Inaddition, a shoulder peak, referred to as D*, is evident towards thehigher wave number side of the G-band peak, located at 1625 cm−1.The D-band corresponds to the defects and structural disorders inpolycrystalline graphite. The G-band corresponds to the tangentialvibrations of carbon atoms. The intensity ratio of the D band to G band(ID/IG) is used as one of the parameter to ascertain the degree ofcrystallanity of CNT samples. It is commonly defined as the relativeresponse of sp3 bonded carbon originating from intrinsic defects inthe CNTs or amorphous carbon located on the CNTs to sp2 bondedgraphitic carbon.

The ID/IG ratios, as seen in Table 1, are 1.24, 0.98, 0.82, 0.57 for thesamples of A, B, C and D. It is clear that Raman spectra exhibit adecreasing trend in the ID/IG ratio of Ti coated CNTs. This trend withincreasing Ti thickness corresponds to the increased crystallanity ofCNTs and decreased sp3 bonded carbon. It was found that Ti coatedCNTs were more crystalline than the pristine CNT samples. This waspossibly due to passivationof defect sites and resulting changes in localorder. The various parameters as obtained from the Raman spectra aresummarized in Table 2. The peaks at 2689 and 2691 cm−1 are referredto as G* mode of the pristine and Ti coated CNTs, respectively, thatcorresponds to the second-order overtone ofDmode, seen in Fig. 5(b).The intensity of G* mode progressively decreases with the increase in

the thickness of Ti coating for samplesA, B, C andD. This corresponds tothe increasing crystallinity of Ti coated CNTs. It may be mentionedthat the sensitivity of second-order Raman scattering to shortrange compositional disorder is the reason for its dependence on Tithickness.

Fig. 6. (a) J vs. E plot for samples A, B, C and D at the distance of 200 μm(b) correspondingF–Nplot. d is the distance between the cathode and anode. The turn on and thresholdfieldvalues are taken at 10 μA/cm2 and 1 mA/cm2.

Fig. 7. (a) J vs. E plot for samples A, B, C and D at the distance of 400 μm(b) correspondingF–Nplot. d is the distance between the cathode and anode. The turn on and thresholdfieldvalues are taken at 10 μA/cm2 and 1 mA/cm2.

Table 3Field emission characteristics at d=200 μm.

S. no. Sample name Turn on field(V/μm)

Threshold field(V/μm)

Enhancementfactor

A FE-18 1.8 4.0 6066B FE-5 1.7 3.5 8542C FE-11 1.71 3.3 8542D FE-7 1.4 2.1 7676

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4. Field emission studies

The field emission behaviour of pristine and Ti coated CNTs wasinvestigated using diode setup. The emission characteristics werestudied at two distances between cathode and anode, viz. 200 and400 μm.

Field emission behaviour was analyzed using Fowler–Nordheim(F–N) equation [25]:

J = ðAβ2E2 =φÞ exp ð−Bφ3=2= βEÞ

where, J is the emission current density; φ the work function, E is theelectric field and β the field enhancement factor.

In general, it is explained on the basis of parameters achieved fromF–N plot. The F–N plot is obtained by plotting between ln (J/E2) and 1/E.The enhancement factor was calculated from the slope of F–N plot byconsidering the work function of CNTs as 5 eV [26,27]. Figs. 6 and 7shows the emission current density versus electric field (J vs. E) andFowler–Nordheimplots (F–N) plots for the four samples A, B, C andD atthe distance of 200 and 400 μm, respectively. The turn on and thresholdfield values at 10 μA/cm2 and 1 mA/cm2 are summarized in Tables 3and 4. A significant change in field emission behaviour of pristine and Ticoated CNTs were observed, as seen from Figs. 6, 7 and Tables 3 and 4.For the distance of 200 μm, as seen in Fig. 6(a) the turn on field is foundto be 1.8, 1.7, 1.71 and 1.4 V/μm for samples A, B, C and D, respectively.

The thresholdfield for the samesampleswas found to be 4.0, 3.5, 3.3 and2.1 V/μm (Fig. 6(b)). For the distance of 400 μm, the turn on field wasfound to be 0.95, 1, 0.9 and 0.8 V/μm for samples A, B, C and D,respectively, as seen in Fig. 7(a). The threshold field for the samples Cand D at the same distance was found to be 2.95 and 2.65 V/μm. Therewas no threshold field for the samples A and B (Fig. 7(b)). The turn onand threshold field (given in Tables 3 and 4) found in our studies areslightly lower as reported in earlier studies (3.7 V/μm and 6.19 V/μm)[14]. β factor is an important parameter in the field emissioninvestigations. The value of β calculated in our Ti coated CNTs are

Table 4Field mission characteristics at d=400 μm.

S. no. Sample name Turn on field(V/μm)

Threshold field(V/μm)

Enhancementfactor

A FE-18 0.95 – 8740B FE-5 1.0 – 8858C FE-11 0.9 2.95 11,432D FE-7 0.8 2.65 9386

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quite large (~1.14×104) which is not reported in earlier studies [14].The emissionpropertieswere enhanceddue to very thin coatingof Ti. Asthe thickness of Ti is increased, field emission properties getdeteriorated [19]. The enhancement in the field emission properties ofTi coated CNTs as compared to pristine CNTs are due to the lower workfunction of the Ti (4.29 eV). As very thin layer (b2 nm) of Ti is coated onCNTs, Ti nanolusters adhere on the tip and edges of MWCNTs, thusshifting the Fermi level towards the higher energy side. The coating of Tileads to the adsorption of Ti nanoclusters on the edges and sidewalls ofCNTs as confirmed by TEM studies. These nanoclusters should slightlyaffect the aspect ratio (length to diameter) of CNTS. The presence of Tinanoparticles is expected to reduce the local surface work function ofCNT film leading to the enhancement in the electron emission [14]. Alsothe charge transfer between the Ti nanoclusters and the CNTs lower thework function of the CNTs. [19] .These factors leads to the increase in thefield enhancement factor. Consequently, the density of states increasesnear the Fermi level resulting in the enhancement of tunnelling current.

5. Conclusion

In conclusion, we have reported the enhanced field emissionproperties and crystallanity of Ti coatedmultiwalled CNTs as comparedto pristine CNTs. It was observed that Ti coated CNTs have low workfunction and higher field enhancement factor as compared to pristineCNTs. The enhancement in the field emission properties of Ti coatedCNTs is considered to be due to the shifting of the Fermi level towardsthe higher energy side. From HRTEM studies, it was found that Tinanoparticles adhere to the CNT walls and thus passivated the defectswhich reaffirm the Raman observations.

Acknowledgments

Oneof the authors (H.S.) is thankful todirector IITDelhi forprovidinga research scholarship. Authors are thankful toMr. Rajkumar, Mr. Vivekand Mr. Kapil for their assistance in field emission measurements andmicro Raman spectroscopy. Authors are also grateful to Dr. C. Singh andMr. D.C. Sharma for carrying out scanning electron microscopy.

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