Enhanced Electron Field Emission of One-Dimensional Highly

Feb 20, 2014 - The results depict the best FE performance for Ru-GWCNTs based emitters, ... The Journal of Physical Chemistry C 2015 119 (5), 2917-292...
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Enhanced Electron Field Emission of One-Dimensional Highly Protruded Graphene Wrapped Carbon Nanotube Composites Pranati Nayak,†,‡ P. N. Santhosh,‡ and S. Ramaprabhu*,† †

Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), and ‡Low Temperature Physics Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai−600036, India S Supporting Information *

ABSTRACT: We report the enhanced field emission studies of one-dimensional highly wrinkled graphene wrapped carbon nanotubes measured at a base pressure of 10−6 mbar. The combined advantage of high aspect ratio and protruded graphene layers on CNT surface are envisioned to improve the field emission current density. Furthermore, the emission characteristics are precisely studied by decorating metal/metal oxide (M/ MO like Ru, ZnO, and SnO2) nanoparticles over GWCNTs. It was found that the incorporation of M/MO NPs lowers the work function, which leads to easy electron tunneling and considerably improves the FE performance. The results depict the best FE performance for Ru-GWCNTs based emitters, with a turn-on field of 0.61 V/μm, a current density of 2.5 mA/cm2 at a field of 1 V/μm, and a field enhancement factor of 6958. The enhanced FE behavior of GWCNTs based emitters is attributed to the easy electron tunneling from the protrusions created on CNT surface which increases the emission sites and hence the FE current density. In addition, the surface decorated M/ MO NPs could lower the work function, which contributes to local field enhancement, and hence the low turn-on field. This high performance results for GWCNTs based field emitters are potentially useful for design, fabrication, and optimization of field emission devices. increases the emission current.9 To further increase the performance, several attempts have been made in modifying CNT outer walls by decorating low work function nanostructures which could be fairly achieved at a low turn-on voltage and high field enhancement factor.10,11 Although a number of studies explored CNT as a promising candidate for FE displays, the electron emission predominantly occurs from CNT tips and the defects sites generated by external perturbations like doping or chemical functionalizations.12 Consisting of highly ordered hexagonal carbon lattices, the side walls remain less sensitive to field emission. A report by Nakayama et al. suggests that the intensity of electric field concentrated on the tip of CNT is 2.8 times higher than the side walls.13 Similarly, Chow et al. explores the low turn-on field and high field enhancement factor at bent CNT walls after controlled ion bombardment, which suggests protrusions on CNT sidewalls can effectively improve the field emission current.14 Other than CNTs, several reports came with enhanced FE properties of carbon nanohorn, vertically aligned carbon nanowalls and more recently graphite oxide, single and few layer graphene, free-standing graphene flakes, etc.15−18 Graphene, the parent structure of all these carbon based nanomaterials, is a two-dimensional architecture of sp2 hybridized carbon atoms. The adorable properties of graphene

1. INTRODUCTION Over the last few decades, intensive efforts have been paid to electron field emission, also known as cold cathode emission, due to its promising applications in flat panel display, X-ray generators, microwave tubes, and especially field emission electron guns.1,2 Field emission is the quantum mechanical tunnelling phenomena where at the influence of high external electric field of the order of 109 V/m, the electrons are injected out from the materials into vacuum.3 Under high field, the potential barrier at material−vacuum interface narrows down which in turn enhances the tunnelling probability and hence the emitted electron density. From a technological point of view, a high emission current density at low applied electric field is vastly enviable for commercial fabrication of field emission devices. In order to meet these challenges, tailoring the morphology and shape of the emitter and optimizing the intrinsic properties like work function and electrical conductivity can prominently enhance the field emission properties.4−6 In this regard, materials having sharp tip, high aspect ratio, and low work function are more promising candidates due to the accumulation of localized electrons at the sharp edges which generates a large field enhancement.7 With growing interest in low dimensional carbon based nanomaterials, many efforts have been paid to carbon nanotube (CNT) based FE displays.8 Owing to its one-dimensional geometry with high aspect ratio, sharp nanosize tip, and excellent electrical conductivity, the effective potential barrier lowers at CNT tips and thereby reduces the turn-on field and © 2014 American Chemical Society

Received: December 24, 2013 Revised: February 12, 2014 Published: February 20, 2014 5172

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been done by catalytic chemical vapor deposition technique using 1:1 mixture of GO and rare earth based MmNi3 (Mm = Misch metal) type alloy catalyst.25 Acetylene was used as the carbon precursor. In short, a mechanical fine mixture of MmNi3 and GO was sprinkled on a quartz boat and kept in the center of a CVD reactor under Ar atmosphere. H2 gas was allowed to flow at 240 °C which exfoliates GO to graphene. Then the temperature was allowed to rise up to 700 °C under Ar flow and acetylene (C2H2) for 30 min. Under high temperature, MmNi3 goes to a molton state and MWCNTs growth takes place with simultaneous wrapping of graphene. The assynthesized sample was air oxidized at 400 °C followed by refluxing in conc. HNO3 to remove amorphous carbon and the remaining metal catalyst particles. The M/MO NPs decoration was done by making a mechanical mixture of GWCNTs and metal salt as starting material. Focused sunlight was used as the energy source to reduce metal salt to M/MO NPs. The decoration of M/MO/ Alloy NPs over MWCNTs surface using focused sunlight was discussed in our earlier report.26 Briefly a fine mixture of purified GWCNTs (100 mg) and ruthenium chloride (100 mg) was prepared using a mortar and sprinkled in a glass petri disk. The mixture was irradiated by focused sunlight using a convex lens of 90 mm diameter for 1−2 min. The rapid heating (about 100 °C/s) by lens creates a sudden rise in temperature (150− 450 °C) which decomposes the metal salt to metal NPs and simultaniously deposits on GWCNTs. An apparent release of gaseous byproducts was observed during synthesis which indicates the decomposition of metal salt at high temperature. In the same way SnO2 and ZnO decorated GWCNTs have been prepared by taking a calculated amount of GWCNTs and metal salt. The mechanism behind metal oxide formation relates to the high chemical reactivity of metal precursor with oxygen containing functional groups, atmospheric oxygen, as well as the oxygen present in acetate based salts. Being a noble metal, Ru rarely turns oxide at low temperature. However, metals like Zn and Sn are highly reactive to form oxide easily, which is evident from the characterization techniques. 2.2. Material Characterization Techniques. The synthesized products were characterized by field emission scanning electron microscopy (FESEM, Quanta 3D), high resolution transmission electron microscopy (HRTEM, Tecnai G2 20 STWIN), and PANalytica X’pert pro X-ray diffractometer with Cu−Kα as X-ray source. Raman spectra analysis was performed by WITec alpha 300 confocal Raman spectrometer equipped with Nd:YAG laser (532 nm) as excitation source. Sample heating was avoided by using low laser intensity. The X-ray photoelectron spectra were recorded with SPECS X-ray photoelectron spectrometer using Mg Kα as X-ray source and PHOIBOS 100MCD energy analyzer at ultrahigh vacuum (10−10 mbar). The data were analyzed using CASA XPS software. The electron field emission (FE) properties were studied using an indigeniously fabricated setup comprising a high vacuum chamber with base pressure of 1 × 10−6 mbar. The anode is a cylindrical gold coated copper rod of 1 cm diameter which connects directly to a micrometer screw with a least count of 10 μm in order to maintain the anode at a desired distance from the cathode. The cathode is made up of stainless steel of diameter 5 cm which acts as a substrate holder for field emission studies. The fabricated emitters were fixed to the cathode plate by electrical contact using silver paste. The anode cathode separation was controlled by the micrometer screw gauze vertically fixed on the anode. The field emission current

including large specific surface area, excellent electrical conductivity, high aspect ratio, and tunable work function by incorporation of foreign dopants make it as a potential alternative material for field emission device. In the literature, people explored that the local field enhancement occurs at atomically thin graphene edges.19 Other than this, doping by heteroatoms like nitrogen/boron, metal/metal oxide nanostructures over graphene surface could create more emission sites and contribute significantly to emission current.20−22 More recently, Guo et al. reported that the wrinkles on graphene surface play an important role in improving the field emission current density.23 It is explained that the curvature of the wrinkles in graphene sheet significantly lowers the electron affinity and ionization potential. When electric field is applied, the work function decreases which favors in easy tunnelling of electron from the top part of the wrinkles. In this report, we extensively studied the enhanced field emission properties of highly wrinkled graphene wrapped carbon nanotubes (GWCNTs). Incorporation of highly protruded graphene layers over one-dimensional CNT is done by chemical vapor deposition technique with a vision to create more emission sites and hence large field emission current density. Furthermore, the effect of selective M/MO NPs (Ru, ZnO, and SnO2) decoration in lowering the work function and its effect on electron field emission is investigated systematically. Based on the UV photoelectron spectroscopy (UPS) measurements, it is concluded that the work function of the composite decreases after M/MO nanoparticles decoration over it. This in turn improves the easy electron tunnelling mechanism thereby enhancing the field emission current density which is evident from experimental measurement of field emission studies. Different from widely studied graphene and CNTs based field emitters, the electron emission from GWCNTs is expected to be from the synergistic combined contribution from one-dimensional CNTs as well as the protruded graphene layers over CNTs.

2. EXPERIMENTAL METHODS 2.1. Materials. Flake graphite powder (99.99% SP-1, average particle size 45 μm) was purchased from Bay Carbon, Inc., U.S. Zinc acetate (C4H10O6Zn· 2H2O, 99.5%), tin(II) chloride dihydrate (SnCl2·2H2O), and ruthenium(III) chloride hydrate (RuCl3·xH2O) were procured from Aldrich. Potassium permanganate (KMnO4), sodium nitrates (NaNO3), concentrated sulfuric acid (H2SO4, 99%), and concentrated nitric acid (HNO3, 98%) (Rankem chemicals, India) were used as received. Hydrogen peroxide (H2O2, 30 wt %/V) was purchased from Fisher scientific. Ultrapure water (18.2 MΩ cm) from Millipore system was used throughout all the experiments. 2.2. Synthesis of Materials. Graphite oxide (GO) was synthesized by Hummers method using flake graphite powder as the precursor.24 In brief, about 2 g of flake graphite powder was refluxed in concentrated H2SO4 in an ice bath. One gram of NaNO3 was added to the suspension under continious stirring. Then 6 g of KMnO4 was added to the mixture and allowed to cool down to room temperature by removing from ice bath followed by adding water to it. The above prepared suspension was again diluted by adding warm water. Then 12 mL of H2O2 (3%) was added to it, and the suspension turned bright yellow. It was filtered and washed thoroughly by copious amounts of ultrapure water, and finally the residue was dried in vacuum at 60 °C temperature. Graphene wrapping over MWCNTs has 5173

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was measured by sweeping the voltage using Keithley 237 SMU with nanaoampere current sensitivity. 2.3. Fabrication of Flexible Field Emitter. The field emitter was fabricated on flexible conducting carbon cloth by spin coating. A fine dispersion of about 5 mg of Ru-GWCNT in 0.5 mL of 0.5% nafion solution was made by ultrasonication. Then the prepared dispersion was spin coated on carbon cloth at 500 rpm in first stage for 20 s followed by 2000 rpm in second stage for 30 s. In order to evaporate the solvant, the film was dried at 100 °C for 12 h in vacuum oven. The other MO (ZnO and SnO2) based field emitters were fabricated using equal wt % following the same fabrication technique.

3. RESULTS AND DISCUSSION 3.1. Materials Characterization. In order to investigate the crystallinity of the synthesized material, powder X-ray

Figure 2. (a, b) Field emission scanning electron micrograph (FESEM) of purified GWCNTs at different magnifications, showing the wrinkled surface morphology of GWCNTs. (c, d) HRTEM images of GWCNTs indicating protruded graphene coating over CNT.

on carbon cloth at low as well as high resolutions. As the particle size of M/MO Nps over GWCNTs is very small, we studied the morphology by high resolution transmission electron microscopy (HRTEM). The HRTEM images are shown in Figure 2c, d at different magnification confirming high surface roughness of the synthesized material. Unlike MWCNTs having highly smoothed surfaces, GWCNTs provide protrusions on the surface, which can act as emission site and enhance the field emission characteristics. Figure 3 shows the HRTEM images of Ru-GWCNTs (a and b), SnO2-GWCNTs (c and d), and ZnO-GWCNTs (e and f) at different resolutions, which reveals the structural details of the nanostructures. It exhibits a uniform decoration of corresponding tiny M/MO NPs over GWCNTs. Insets shows the particle size distribution and lattice planes of M/MO NPs. The average particles size calculated are 3.5, 2.5, and 7 nm for RuGWCNTs, SnO2-GWCNTs, and ZnO-GWCNTs, respectively. As shown in the inset of Figure 3b, the lattice fringes measured are ∼2.1 Å, which corresponds to the {002} plane of Ru hexagonal close packed structure.28 Similarly the lattice fringes of ∼3.3 and ∼2.8 Å corresponding to {110} and {100} of SnO2 and ZnO nanostructures are depicted from the insets of Figure 3 d, f.29,30 The good crystallinity of the M/MO nanostructures over GWCNTs agrees well with the previously discussed XRD analysis. Raman spectra analysis was performed in order to analyze the vibrational characteristics of the synthesized materials. Figure 4 represents the micrographs of (a) pure GWCNTs compared with (b) ZnO-GWCNTs, (c) SnO2-GWCNTs, and (d) Ru-GWCNTs, respectively. For pure GWCNTs, two peaks corresponding to D band (∼1338 cm−1) and G band (∼1585 cm−1) appear.31 The G band corresponds to the E2g vibration mode of graphitic carbon, while the D band is mainly sensitive to the defects induced in graphitic structure and surface modification by doping, etc.32 The peak intensity of D band is a

Figure 1. XRD pattern of (a) GWCNTs, (b) ZnO-GWCNTs, (c) SnO2-GWCNTs, and (d) Ru-GWCNTs.

diffraction studies were carried out in the range from 5° to 90° with a step size of 0.016°. Figure 1 shows the X-ray diffraction pattern of (a) pure GWCNTs, (b) ZnO-GWCNTs, (c) SnO2GWCNTs, and (d) Ru-GWCNTs. The characteristic peak corresponding to the C(002) plane of hexagonal lattice for pure GWCNTs is observed at 26.42°. For ZnO-GWCNTs, the diffraction peaks were identified to hexagonal lattice pattern of wurtzite ZnO (JCPDS 36-1451).27 No other characteristic peaks were observed, which indicates complete conversion of zinc salt to ZnO. Similarly, the peaks corresponding to SnO2GWCNTs and Ru-GWCNTs were analyzed which well fits to tetragonal SnO2 (JCPDS 88-0287) and hexagonal Ru lattice structure (JCPDS 65-7646). The sharpness as well as the absence of any other peaks reflects the purity of the sample. The electron microscopy technique has been employed to investigate the surface morphology of the synthesized nanomaterials. Figure 2a, b shows the FESEM image of purified GWCNTs at low and high magnifications. It reveals the coating of graphene layers on MWCNTs which results in a wrinkled morphology covering the entire surface. The average diameter of about 100−120 nm with length of tens of micrometers was observed. Figure S1 in the Supporting Information shows the morphology of GWCNTs based composite emitters fabricated 5174

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measure to scale the defects on the modified structures. Being highly wrinkled surfaces, the D band intensity dominates over G band in GWCNTs with ID/IG ratio of ∼1.40. The ID/IG value for ZnO-GWCNTs, SnO2-GWCNTs, and Ru-GWCNTs are calculated to be 1.44, 1.54, and 1.55, respectively. Compared to pristine GWCNTs, the G band peak blue shifts for M/MO/ GWCNTs based composites shown as the high resolution spectrum in Figure S1 of the Supporting Information. This suggests a coulombic charge transfer between nanoparticles and GWCNTs.33,34 Again the 2D band observed at 2691 cm−1 for pristine GWCNTs is red-shifted after M/MO NPs decorations which are a signature of n-type doping of graphene.35 The 2D band peak intensity appears to be decreased compared to GWCNTs revealing interaction of GWCNTs with attached M/ MO NPs on it. Despite these features, ZnO-GWCNTs exhibit peaks corresponding to A1 symmetry in the transmission optical (TO) mode (∼332 cm−1) and E2 high frequency phonon mode (∼437 cm−1) of ZnO nanostructures. Including this, peaks assigned to first order longitudinal-optical (E1 LO) phonon mode (∼581 cm−1) and second order longitudinaloptical (E2 LO) phonon mode (∼1124 cm−1) appears in the Raman spectra.36 The appearance of these peaks suggests the decoration of ZnO over GWCNTs. For SnO2-GWCNTs, a peak appears at ∼623 cm−1 corresponding to A1g vibrational mode of SnO2 nanostructures along with D and G bands.37 To investigate the chemical state and composition of the composite, XPS analysis was employed in binding energy ranging from 0 to 1050 eV. Figure 5a shows the extensive survey spectra of the composites which depict the presence of carbon, oxygen, and the corresponding metal peaks. No other chemical species was detectable in the spectrum suggesting its high purity and complete reduction of metal salt over GWCNTs without forming any chemical residues. The deconvoluted high resolution XPS spectrum for Ru 3p in RuGWCNTs reveals a doublet at binding energies 462.2 eV (Ru 3p 3/2) and 484.3 eV (Ru 3p 1/2) which is shown in Figure 5b. These peaks are attributed to Ru in metallic Ru(0) state.38 Figure 5d shows the Sn 3d spectrum of SnO2-GWCNTs consisting of deconvoluted peaks at 495.8 and 487.3 eV corresponding to Sn3d3/2 and Sn3d5/2, respectively. These spin−orbit doublets are attributed to Sn(4+) oxidation state and are close to standard data for SnO2.39 The spin−orbit coupled multiplex spectra for Zn 2p in ZnO-GWCNTs appears 1044.7 and 1021.7 eV, which are assigned to Zn 2p1/2 and Zn 2p3/2, respectively. These are approximately the same as reported literature values for Zn(2+) oxidation state.40 The peak shift toward high binding energy confirms the interaction and electron transfer between ZnO and GWCNTs. 3.2. Field Emission Studies. The FE studies of M/MOGWCNTs showed that they are potentially competitive field emitters. Figure 6a shows the comparison plot of emission current density (J) vs applied field (E) measured at anode cathode separation of 500 μm in chamber pressure of ∼10−6mbar for (a) GWCNTs, (b) Ru-GWCNTs, (c) ZnOGWCNTs, and (d) SnO2-GWCNTs. The emission current density increases exponentially with applied field. The turn-on fields (ETO corresponding to J of 10 μA/cm2) calculated for RuGWCNTs, ZnO-GWCNTs, and SnO2-GWCNTs are 0.61, 0.69, and 0.8 V/μm, respectively. Compared to pristine GWCNTs (ETO = 0.81 V/μm), the ETO values change largely with surface decoration of metal (Ru), MO (ZnO and SnO2) nanoparticles over it. Moreover, the current densities (J) at field of 1 V/μm are 2.5, 1.13, 0.3, and 0.16 mA/cm2 for Ru, ZnO,

Figure 3. TEM and HRTEM image of (a, b) Ru-GWCNTs, (c, d) ZnO-GWCNTs, and (e, f) SnO2-GWCNTs. Insets in (a, c, and e): Size distribution plots Ru, SnO2, and ZnO nanoparticles over GWCNTs. In (b, d, f): High resolution lattice image of single Ru, ZnO, and SnO2 nanoparticle. Lattice spacing are of 2.1, 2.8, and 3.3 Å for (002), (110), and (100) directions in Ru, SnO2, and ZnO, respectively.

Figure 4. Raman spectrum of (a) GWCNTs, (b) ZnO-GWCNTs, (c) SnO2-GWCNTs, and (d) Ru-GWCNTs. 5175

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Figure 5. (A) X-ray photoelectron survey spectrum of (a) GWCNTs, (b) ZnO-GWCNTs, (c) SnO2-GWCNTs, and (d) Ru-GWCNTs. High resolution XPS spectra of (B) Ru 3p, (C) Zn 2p, and (D) Sn 3d, respectively.

Figure 6. Field emission current density (J)−electric field (E) plot of (a) GWCNTs, (b) SnO2-GWCNTs, (c) ZnO-GWCNTs, and (d) RuGWCNTs. Insets: The corresponding Fowler−Northeim (FN) ln(J/ E2)−(1/E) plots showing linear dependence in low field regions.

Figure 7. UPS valence band spectra of (a) GWCNTs, (b) RuGWCNTs, (c) ZnO-GWCNTs, and (d) SnO2-GWCNTs.

Furthermore, upon M/MO NPs decoration the FE parameters fairly change. This can be attributed to uniformly decorated nanoparticles over GWCNTs which in turn lowers the work function and hence increases the emission current density accordingly. The field emission J−E characteristics are further analyzed by Fowler−Northeim (F−N) tunnelling mechanism relying on the F−N equation.

SnO2 decorated and pristine GWCNTs, respectively. The highest current density obtained in our study is for RuGWCNTs, which is around 15 times more than that from pristine GWCNTs. Again the threshold field (ETH, field for fixed J = 0.2 mA/cm2) was calculated to be 0.75, 0.77, and 0.93 V/μm for Ru/GWCNTs, ZnO-GWCNTs, and SnO 2 GWCNTs, which are much lower than that for pristine GWCNTs (1.03 V/μm). Compared to similar graphene and CNT based reported literature, GWCNT gives more current density, low ETO and ETh values which can be attributed to wrinkled morphology in one-dimensional GWCNTs. This creates more emission sites and hence more current densities.

⎛ Aβ 2E2 ⎞ ⎛ −Bϕ3/2 ⎞ ⎟ J=⎜ ⎟exp⎜ ⎝ ϕ ⎠ ⎝ βE ⎠

(1)

or 5176

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In the literature, many studies illustrate that the electron tunnelling mechanism is highly subjective to materials having low work function and more emission sites.42 As the field emission current is inversely proportional to the work function of the emitter, accordingly low work function materials are highly preferable. In this work M/MO decoration on GWCNTs surface is employed as an effective way to reduce the work function of the composite. Therefore, here we have measured the work function of the emitters directly using UPS. Figure 7 depicts the UPS spectra of (a) GWCNTs, (b) RuGWCNTs, (c) ZnO-GWCNTs, and (d) SnO2-GWCNTs. The work function was calculated by measuring the edge of the secondary cutoff kinetic energy value. For pristine GWCNTs, the ϕ value was calculated to be 4.97 eV which matches with the reported values for carbon based nanomaterials like graphene (4.89−5.16 eV) and multiwalled carbon nanotubes (4.95 eV).43−45 After decoration of Ru nanoparticles (Figure 7b), the value decreases to about ϕ = 4.09 eV. This illustrates coulombic charge transfer between surface decorated Ru nanoparticles and GWCNTs, which is basically n-type doping effect in graphene based systems. These results agree well with the Raman spectroscopy investigations discussed previously. Again the work function was decreased to 4.15 and 4.18 eV for ZnO-GWCNTs and SnO2-GWCNTs which is shown in Figure 7c, d. Using d = 500 μm and the calculated ϕ values for the composites, the field enhancement factor was calculated to be 6958, 6078, 5821, and 5300 for Ru-GWCNTs, ZnO-GWCNTs, SnO2-GWCNTs, and pristine GWCNTs, respectively. In the literature, Lee et al. have reported on remarkably enhanced field emission of double walled carbon nanotubes improved by Ru nanoparticles decoration.38 It is explained that the Ru nanoparticles lower the work function and increase the field enhancement factor. A recent report by Ran et al. explores a lower work function caused by C−O−C chains in G-ZnO hybrid than only graphene which leads to easy tunnelling of electrons, causing a large field emission current.46 Thus graphene surface decorated with low work function M/MO

Figure 8. Emission current density with respect to time for (a) GWCNTs, (b) SnO2-GWCNTs, (c) ZnO-GWCNTs, and (d) RuGWCNTs depicting stability of the emitters.

⎛ Aβ 2 ⎞ ⎛ Bϕ3/2 ⎞ ⎛ J ⎞ ⎟ ln⎜ 2 ⎟ = ln⎜ ⎟−⎜ ⎝E ⎠ ⎝ ϕ ⎠ ⎝ βE ⎠

(2)

where J is the emission current density, E is the applied field, ϕ is the work function of the field emitter, and β is the field enhancement factor. A and B are constants with values 1.56 × 10−6 A eV V−2 and 6.83 × 103 V/μm eV−3/2, respectively.41 Figure 6b depicts the F−N plots for (a) GWCNTs, (b) RuGWCNTs, (c) SnO2-GWCNTs, and (d) ZnO-GWCNTs. It clearly illustrates linearity of ln(J/E2) vs (1/E), which concludes that the FE from GWCNTs based emitters follows the F−N tunnelling mechanism. The plotted data were linearly fit to calculate the field enhancement factor β based on the following relation β=

Bϕ3/2d slope

where d is anode cathode separation.

Table 1. Comparison Table of ETO, ETh, and β Values for the Present Work Showing Significant Improvements Compared to Carbon Nanotube or Graphene Based Recent Similar Results38,45−49

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NPs gives dramatic performance in electron emission. Further, the emission current stability of each emitter was studied at an applied field of 1 V/μm which is shown in Figure 8. It exhibits no significant degradation of emission current density recorded over a time period of 240 min. This illustrates good stability of GWCNTs based emitters. In this work, upon graphene wrapping, the smooth surface of one-dimensional CNT no longer exists; instead highly wrinkled morphology develops which favors charge accumulation and easy tunnelling of electrons in the presence of external electric field. Based on the combined effect of one-dimensional nature and protruded morphology, the field emission of GWCNTs based emitters has shown significant improvements compared to carbon nanotube or graphene based similar studies shown in Table 1.

4. CONCLUSIONS In summary, highly protruded graphene sheets wrapped CNTs have been synthesized by CVD technique, and surface decoration by Ru, ZnO, and SnO2 nanoparticles has been done by simple solar reduction technique. GWCNTs possess lower turn-on field and larger field-enhancement factor due to combined advantage of high aspect ratio and protruded graphene layers on CNT surface, which could act as emission sites to contribute to high FE current density. Furthermore, the work function of the synthesized composite decreases with M/ MO decoration as confirmed from UPS studies. The field emission measurement shows a significantly high emission current of 2.5 mA/cm2 at low turn-on field of 1 V/μm for RuGWCNTs, which is better than pristine GWCNTs as well as ZnO, SnO2-GWCNTs based emitters. Based on enhanced field emission performance and good stability, GWCNTs based composites may be promising for high performance cold cathode emitters.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images, Raman spectra, and Raman parameters. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-44-22574862. Fax: +91-44-22570509. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thanks the Indian Institute of Technology Madras (IITM), Chennai, India for financial support. REFERENCES

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