High Stability Electron Field Emitters Synthesized via the Combination

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High Stability Electron Field Emitters Synthesized via the Combination of Carbon Nanotubes and N2‑Plasma Grown Ultrananocrystalline Diamond Films Ting-Hsun Chang,† Ping-Yen Hsieh,† Srinivasu Kunuku,‡ Shiu-Cheng Lou,§ Divinah Manoharan,∥ Keh-Chyang Leou,‡ I-Nan Lin,*,∥ and Nyan-Hwa Tai*,† †

Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan, R.O.C. Department of Engineering and System Science, National Tsing-Hua University, Hsinchu 300, Taiwan, R.O.C. § Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu 300, Taiwan, R.O.C. ∥ Department of Physics, Tamkang University, New Taipei City 251, Taiwan, R.O.C. ‡

S Supporting Information *

ABSTRACT: An electron field emitter with superior electron field emission (EFE) properties and improved lifetime stability is being demonstrated via the combination of carbon nanotubes and the CH4/N2 plasma grown ultrananocrystalline diamond (NUNCD) films. The resistance of the carbon nanotubes to plasma ion bombardment is improved by the formation of carbon nanocones on the side walls of the carbon nanotubes, thus forming strengthened carbon nanotubes (s-CNTs). The N-UNCD films can thus be grown on s-CNTs, forming N-UNCD/s-CNTs carbon nanocomposite materials. The N-UNCD/s-CNTs films possess good conductivity of σ = 237 S/cm and marvelous EFE properties, such as low turn-on field of (E0) = 3.58 V/μm with large EFE current density of (Je) = 1.86 mA/cm2 at an applied field of 6.0 V/μm. Moreover, the EFE emitters can be operated under 0.19 mA/cm2 for more than 350 min without showing any sign of degradation. Such a superior EFE property along with high robustness characteristic of these combination of materials are not attainable with neither NUNCD films nor s-CNTs films alone. Transmission electron microscopic investigations indicated that the N-UNCD films contain needle-like diamond grains encased in a few layers of nanographitic phase, which enhanced markedly the transport of electrons in the N-UNCD films. Moreover, the needle-like diamond grains were nucleated from the s-CNTs without the necessity of forming the interlayer that facilitate the transport of electrons crossing the diamond-toSi interface. Both these factors contributed to the enhanced EFE behavior of the N-UNCD/s-CNTs films. KEYWORDS: N-UNCD, strengthen carbon nanotubes (s-CNTs), electron field emission properties Diamond has attracted much attention as electron field emitters in vacuum nanoelectronic devices because of their negative electron affinity with low effective work function,10 wide band gap and high electric breakdown strength. Diamondbased EFE devices could possibly be operated with better lifetime stability and reliability due to the strong bonding structure of diamond. It has been demonstrated that coating ultrananocrystalline diamond (UNCD) or hybrid granular structured diamond (HiD) films on CNTs emitters, although slightly degraded the EFE properties of CNTs, markedly enhanced the robustness of the emitters.11 The CH4/N2 plasma grown ultrananocrystalline diamond (N-UNCD) films possess a very unique granular structure, that is, with high aspect ratio needle-like diamond grains as cores encased in nanographitic layers as shell.12 The N-UNCD films

1. INTRODUCTION Nanostructured carbon materials such as carbon nanotubes (CNTs), graphene, carbon nanosheets (CNS) and diamond have been actively investigated in the applications of electronic and photonic nanodevices.1−4 CNTs with excellent properties such as a high aspect ratio, good electrical conductivity, extremely high thermal conductivity and high electron field emission (EFE) properties1,5,6 are regarded as suitable materials for use in cold cathode applications. However, the CNTs materials are subjected to thermal degradation because of the adsorption desorption of gas molecules on their surface or even tip “burn-out” over long-term operation that degrades their electron field emission (EFE) properties and even resulted in the emission ceasing.7 In this aspect, efforts are mainly directed toward increasing the emission stability of CNTs. To overcome this shortcoming, CNTs have been coated with wide band gap materials such as diamond,8 boron nitride,9 SiO27 and ZnO7 to produce promising starting blocks for electronic systems. © 2015 American Chemical Society

Received: October 14, 2015 Accepted: November 24, 2015 Published: November 24, 2015 27526

DOI: 10.1021/acsami.5b09778 ACS Appl. Mater. Interfaces 2015, 7, 27526−27538

Research Article

ACS Applied Materials & Interfaces

diameter) is used as anode. The cathode-to-anode distance was controlled using an adjustable micrometer attached to the anode. The current-to-voltage (I−V) properties of these diamond/s-CNTs films were measured using a Keithley 237 electron source meter operating in a high-vacuum environment below 10−6 Torr. The I−V characteristics were analyzed using the Fowler−Nordheim (F−N) model,13 which describes the field-assisted electrons tunneling from the cathode to the anode in vacuum under a high applied field. This relation was widely used to describe the relationship between current density Je and the local field Eloc at the emitter through the equation Eloc= βE, where β is the field enhancement factor and E is the applied field. Within this frame the F−N equation can be written as

exhibited markedly better EFE properties than the UNCD (or HiD) films. It is proposed that using N-UNCD films to replace for UNCD (or HiD) films in diamond coated CNTs can markedly enhance the EFE performance of the emitters, while maintaining high lifetime stability of the emitters. Although the CNTs materials can survive the bombardment of the species in CH4/Ar plasma for growing UNCD (or HiD) films, they cannot withstand the plasma bombardment in CH4/N2 plasma for growing N-UNCD films. To solve such a difficulty, the CNTs materials were modified using a hot-filament chemical vapor deposition (HF-CVD) process with different deposition parameters to form carbon nanocones along the side walls of CNTs, resulting in strengthened CNTs (s-CNTs). Thus, the N-UNCD films can be grown on s-CNTs, resulting in a nanocomposite material with improved EFE performance. Furthermore, the microstructures of the N-UNCD/s-CNTs films were investigated using transmission electron microscopy (TEM) so as to understand the mechanism which enhanced the robustness of these materials.

Je = (Aβ 2E2 /φ)exp(− Bφ3/2 /βE) where A = 1.54 × 10−6 A eV V−2 and B = 6.83 × 109 eV−3/2 V m−1, and φ is the work function of the emitting materials. To facilitate the comparison on the robustness of the different EFE materials, the plasma illumination (PI) behavior of microplasma device using these materials as cathodes was characterized, as the plasma is the most stringent environment which the EFE materials will encounter in device applications. The microplasma devices were made in a parallel plate configuration, in which the indium tin oxide (ITO) coated glass plate (the anode) was separated from the cathode with a Teflon spacer (1.0 mm in thickness). A cylindrical cavity of the size approximately 8 mm in diameter was formed by cutting a hole in the Teflon spacer. The Ar plasma (2 Torr) was excited between the ITO and cathode materials by applying a positive DC power to the anode. The plasma current versus density-applied field (Jpl−E) for this device was measured using a Keithley 2410 current source electrometer.

2. EXPERIMENTAL SECTION The s-CNTs were fabricated by a HF-CVD process, using Ni-clusters as catalysts. The Ni nanoclusters were synthesized by thermal annealing of a thin Ni coating (2 nm) on Si substrates at 650 °C for 5 min under a low pressure of 10 Torr with a flow rate of 30 sccm of 10% H2/Ar. Two parallel tungsten filaments with the separation of 10 mm were placed at a distance about 10 mm above the substrate holder. The filament temperature was maintained at 2100 °C so as to heat the substrate to a temperature around 650−680 °C. C2H2 with a flow rate of 70 sccm was used as carbon source and 10% H2/Ar with a flow rate of 30 sccm was adopted as buffer gas. The s-CNTs were then decorated with nanosized diamond particulates (NDP) by an electrophoresis process.4 The nanosized diamond particulates serve as nucleation sites and also function as protecting layer against the bombardment etching of s-CNTs induced by species in the growing plasma. The N-UNCD films were grown on the NDP decorated sCNTs by using CH4(6%)/N2 plasma with a microwave power of 1200 W for 60 min. The pressure and the total flow rate were maintained at 50 Torr and 100 sccm, respectively. The substrates were resistantheated up to a temperature of 700 °C, which was monitored by a thermocouple embedded in the stainless steel substrate holder. Thus, obtained diamond films were designated as N-UNCD/s-CNTs. To facilitate the comparison, the UNCD (or HiD) coated s-CNTs films were also prepared. The UCND films were grown on NDP decorated s-CNTs, using a single step microwave plasma enhanced CVD (MPECVD) process (2.45 GHz, IPLAS-CYRANNUS) in CH4(1%)/Ar plasma with a microwave power of 1200 W for 60 min. The pressure and the total flow rate were maintained at 120 Torr and 100 sccm, respectively. The HiD films were grown using a two-step MPE-CVD process, in which the UNCD/s-CNTs films were used as primary films for secondary plasma post-treatment (ppt) process in CH4(1%)/ Ar(49%)/H2(50%) plasma with a microwave power of 1300 W for 60 min. The pressure and the total flow rate in the ppt-process were maintained at 70 Torr and 100 sccm, respectively. Thus, obtained films were designated as UNCD/s-CNTs and HiD/s-CNTs, respectively. The morphology of these diamond/s-CNTs films were examined using field emission scanning electron microscope (FESEM; JEOL6500). The films were further characterized by visible and ultraviolet (UV) Raman spectroscopy (Lab Raman HR800, Jobin Yvon, λ = 632 and 325 nm), X-ray photoemission spectroscopy (XPS; PHI 1600), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. In addition, the microstructure of the carbon materials and diamond films were examined using transmission electron microscope (TEM; JEOL-2100 operated at 200 kV). The EFE properties of these diamond/s-CNTs films were measured using a parallel plate setup, in which a tungsten rod (1.0 mm in

3. RESULTS AND DISCUSSION 3.1. Characterizations of s-CNTs Nanostructures. Figure 1a shows the SEM morphology of s-CNTs nanostructures. Unlike conventional CNTs, which were grown in thermal CVD process and consist of single wall (or multiwalls) with very clean surface (inset Figure 1a), the s-CNTs contain abundant nanostructure extruding from the side walls. Figure 1b shows the TEM micrographs of s-CNTs, revealing that the nanostructure extruded from side wall of s-CNTs are actually carbon nanocones, which is more clearly illustrated by the TEM structure images in Figure 1c. The TEM micrograph of the CNTs shown in inset of Figure 1b reveals that the conventional CNTs are multiwall and the wall of the tubes is free of adhered nanostructure and is very smooth in surface. Inset in Figure 1c revealed that the spacing between the lattice planes is around 0.34 nm, a typical spacing of the (0002) lattice planes of the graphitic materials. These results confirm that the nanostructure extruding from side walls of s-CNTs are carbon nanocones, which are about 25−35 nm in width at the base (i.e., ∼80−100 layers of nanographite sheet). The presence of carbon nanocones is the main factor increasing the resistance of s-CNTs against the plasma species bombarding damage. Raman spectra shown in Figure 2a indicate the bonding characteristics of standard s-CNTs, where the Raman spectrum of conventional CNTs was included to facilitate the comparison. Curve II in Figure 2a shows that the s-CNTs mainly consists of three peaks: the G-band (graphitized carbon) at ∼1580 cm−1 indicates the presence of crystalline graphitic carbon with an sp2 bonding strucrure.14 The D-band (at ∼1345 cm−1) and the shoulder D′-band (at ∼1610 cm−1) of the Gband indicates the disordered state of the sp2 hybridized material, resulting from the symmetry breaking due to the finite crystallite size and different orientations of the domains, defects of the carbon crystallites.15 Such a Raman spectrum is similar to 27527


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Figure 2. (a) Raman spectra of CNTs: (I) conventional CNTs and (II) s-CNTs. (b) Those of diamond/s-CNTS: (I) UNCD/s-CNTs, (II) HiD/s-CNTs, and (III) N-UNCD/s-CNTs.

those of conventional CNTs (cf. curve I, Figure 2a) except that the s-CNTs contains larger proportion of D-band at 1345 cm−1, which is apparently because of the existence of large proportion of nanostructures on the side walls of s-CNTs. The disordered state of the material can be evaluated from the ratio between the intensity of the D-band and G-band (ID/IG), which is considered as the quality factor for these materials.16 A ID/IG ratio greater than unity indicates the presence of the graphene sheets, amorphous carbon (a-C), vacancies or functional groups. The higher value of ID/IG ratio (2.42) for s-CNT, compared with ID/IG ratio (1.61) for CNTs is presumably caused by a greater number of defects from edge planes of nanosheets. The surface bonding structure of the s-CNTs is better analyzed by XPS. Curve II in Figure 3a shows that the C 1s of XPS spectrum of s-CNTs can be deconvoluted into three peaks at around 284.5, 285.5, and 287 eV, which corresponds to the graphite CC species (sp2), C−C species (sp3) and C−O species, respectively.17 The formation of oxygen adsorbates, the C−O species, originated from the physical adsorption of oxygen or H2O on the surface of carbon nanostructures when the sample was exposed to air. These characteristics are similar to the C 1s of XPS spectrum of conventional CNTs (Curve I, Figure 3a). Quantitative peak analysis for these XPS spectra was carried out to determine the surface species concentrations and are summarized in Table 1. The NEXAFS spectroscopy is a better tool to analyze the bonding structure of the materials. Curve I and II in Figure 4a shows the C K-edge of conventional CNTs and s-CNTs, respectively, which were collected in surface-sensitive total-electron-yield mode over the energy of 280−304 eV. There are two major peaks located at 285 and 291.5 eV in both conventional CNTs (curve I) and s-CNTs

Figure 1. (a) SEM micrograph of s-CNTs with the inset showing the SEM micrograph of conventional CNTs, (b) TEM micrograph of sCNTs with the inset showing the TEM micrograph of conventional CNTs and (c) the enlarged TEM micrograph of carbon nanocones extruding from the side wall of s-CNTs. 27528


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Figure 3. (a) XPS spectra of CNTs: (I) conventional CNTs and (II) s-CNTs. (b) Those of diamond/s-CNTS: (I) UNCD/s-CNTs, (II) HiD/s-CNTs, and (III) N-UNCD/s-CNTs.

Figure 4. (a) NEXAFS spectra of CNTs: (I) conventional CNTs and (II) s-CNTs. (b) Those of diamond/s-CNTS: (I) UNCD/s-CNTs, (II) HiD/s-CNTs, and (III) N-UNCD/s-CNTs.

(curve II), corresponding to the C 1s-π* transition and 1s-σ* transition, respectively. These features are typical characteristics of the sp2-bonded carbon graphite. 18,19 The spectrum corresponding to s-CNTs shows an extra weak broad band between 287 and 290 eV, which originates from either 1s-π* (CO) or 1s-π*(C−H) transitions, Presumably, these are surface oxygenated or hydrogenated functionalities,20 indicating that the surface of s-CNT contain more dangling bonds, which tends to bond with the oxygen ions. These observations indicate that the surface chemical bonding characteristics and carbon bonding configurations of s-CNTs are essentially the same as those of the conventional CNTs. The major difference in the two materials is the microstructure. While the conventional CNTs contain very smooth and uniform walls of sp2-bonded carbon hexagons, the s-CNTs consists of abundant carbon nanocones extruding from their side walls, which contain more nanographitic planes. Apparently, the presence of nanocones along the side walls increases markedly the resistance of the s-CNTs against the bombardment damage of the diamond growing plasma, such as CH4/Ar, CH4/N2, etc., and is expected to insignificantly influence the conductivity of the s-CNTs. The s-CNTs thus perform much better in serving as interlayer for growing

diamond films, compared to the conventional CNTs, which are extremely susceptible to plasma bombardment damage. 3.2. Characterizations of Diamond Films Grown on sCNTs. It should be mentioned that although the conventional CNTs have been used as interlayer for growing the UNCD (or HiD) films on Si substrates11 and markedly enhanced the EFE properties of these films, the conventional CNTs cannot survive the bombardment of the species in CH4/N2 plasma, making the growth of N-UNCD films on the interlayer to be rather impossible. In contrast, the s-CNTs contain carbon nanocones extruding from the side walls, which are more resistant to the bombardment of the species in the plasma, no matter whether the plasma is CH4/Ar or CH4/N2. Figure 5 shows that the UNCD, HiD, and N-UNCD films can be grown on s-CNTs with full coverage, provided that they were decorated with the NDP prior to the growth of diamond films. Figure 5a shows the SEM morphology of the UNCD/sCNTs films, indicating that these films contain ultrasmall diamond grains, which cannot be clearly resolved by SEM image. Inset in Figure 5a shows the cross-sectional image of UNCD/s-CNTs. Curve I in Figure 2b shows the UV Raman spectrum of the UNCD/s-CNTs films acquired using a

Table 1. ID/IG Ratio and Relative Concentrations of Surface Species Obtained from Curve Fitting the C 1s Peaks and of s-CNTs and Diamond/s-CNTs Materials materials

ID/IG ratio

CC (sp2) (284.5 eV) (%)

C−C (sp3) (285.5 eV) (%)

C−O (287 eV) (%)

CN (285.8 eV) (%)

C−N (286.1 eV) (%)

CNTs s-CNTs UNCD/s-CNTs HiD/s-CNTs N-UNCD/s-CNTs

1.61 2.41 0.36 0.41 0.45

40.3 55.5 40.0 27.6 42.3

26.2 33.4 48.9 59.8 24.6

33.5 11.1 10.1 12.6 12.0

10.3

10.8

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UNCD/s-CNTs materials is shown as curve I in Figure 3b, which reveals that the C 1s peak contains sp2 CC (284.5 eV), sp3 C−C (285.5 eV), and C−O (287 eV). Table 1 summarizes the concentration of components on the surface of UNCD/sCNTs films. Curve I in Figure 4b shows the NEXAFS C−K edge of UNCD/s-CNTs films. The spectrum shows characteristic feature of diamond: the C 1s-σ* core exciton resonance at 289 eV, and a large dip at 302 eV corresponding to the second absorption band gap of diamond. There exists a small peak at 285 eV, which can be attributed to C 1s-π* transition corresponding to the sp2-bonded carbon contained in these films. This figure demonstrates that the UNCD/s-CNTs materials are predominantly diamond phase with a small amount of sp2-bonded carbon.24−28 Figure 5b shows the SEM morphology of the HiD/s-CNTs films, whereas the inset in Figure 5b shows the cross-sectional image of the films, indicating that these films contain cauliflower-like aggregates, each of which consists of ultrasmall diamond grains, which cannot be clearly resolved by SEM image. Curve II in Figure 2b shows that the UV Raman spectrum of the HiD/s-CNTs films is very similar to those of UNCD/s-CNTs films, except that there exists large D*-band at 1332 cm−1. These results indicate that the HiD/s-CNTs films contain some proportion of large diamond grains, besides the ultrasmall ones. Curve II in Figure 3b shows that the C 1s peak of HiD/s-CNTs films contain the same components as those in UNCD/s-CNTs films. The sp3 C−C bonding is the main part of carbon which possesses a high intensity of 59.8%. Curve II in Figure 4b shows the NEXAFS C−K edge of HiD/s-CNTs films, clearly demonstrating that a major part of carbon is the diamond phase (sp3-bonded carbon) with graphitic phase (sp2bonded carbon) as minor phase in these films. Figure 5c shows the SEM image of the N-UNCD/s-CNTs films with the inset showing the cross-sectional image, revealing a needle-like granular structure for the films. Two prominent Raman resonance peaks are observed in the spectrum for NUNCD/s-CNTs (Curve III in Figure 2b), i.e., a broad D band around 1370−1405 cm−1 and a G-band (1580 cm−1) corresponding to the disorder carbon21 and graphite phase, respectively. The D*-band corresponding to diamond is barely observable. This indicates that the N-UNCD/s-CNTs is dominated by the presence of sp2 bonded carbon. The ID/IG ratio is 0.45 for N-UNCD/s-CNTs. Curve III in Figure 3b shows that the C 1s peak in XPS spectrum of N-UNCD/sCNTs materials can be fitted with five peaks with Lorentzian functions at their corresponding binding energies of sp2 CC (284.5 eV), sp3 C−C (285.5 eV), sp2 CN (285.8 eV), sp3 C−N (286.1 eV), and C−O (287 eV). These films are predominated by sp2 CC bonding and, moreover, there exist N-containing bonding, that is, sp3 C−N bonding and sp2 CN bonding. The sp2 CC bonding is a major part of carbon with a peak intensity of 42.3% and sp3 C−C intensity is 24.6%. The peak occurring at 285.8 eV (CN) and 286.1 eV (C−N) corresponds to the presence of nitrogen on the surface. Based on the XPS spectrum, we found 12.6 at. % N element was included in the N-UNCD/s-CNTs films. Table 1 summarizes the concentration of components on the surface of N-UNCD/ s-CNTs films. The spectrum of NEXAFS C−K edge of NUNCD/s-CNTs (Curve III in Figure 4b) shows specific feature of diamond and graphitic phases resembling with those of UNCD/s-CNTs and HiD/s-CNTs films, except that the NUNCD/s-CNTs films exhibit a much more prominent π*-

Figure 5. SEM micrograph of (a) UNCD/s-CNTs, (b) HiD/s-CNTs, and (c) N-UNCD/s-CNTs films. The inset in each figure shows the corresponding cross-sectional SEM image.

wavelength of 325 nm. There are several Raman resonance peaks observable for UNCD/s-CNTs films, including a broad D band around 1350 cm−1 and a G-band (1580 cm−1) corresponding to the disorder carbon21 and graphite phase, respectively. The signature of diamond, D*-band at 1332 cm−1 was only barely observable that can be ascribed to the phonon confinement effect due to the smaller grain size of diamond film.22 The ratio between the intensity of the D-band and Gband (ID/IG) provides information on the ordering of the sp2bonded carbon.23 The ID/IG ratio is 0.36 for UNCD/s-CNTs films, which is probably due to the presence of significantly larger amount of sp2-bonded carbon, as compared to the diamond phase (sp3-bonded carbon). The XPS spectrum of 27530


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Figure 6. (aI) EFE properties, (bI) F−N plots, and (cI) EFE lifetime spectra of CNTs: (I) conventional CNTs and (II) s-CNTs. (aII, bII, cII) Those of diamond/s-CNTS: (I) UNCD/s-CNTs, (II) HiD/s-CNTs and (III) N-UNCD/s-CNTs. That is, (aII) EFE properties, (bII) F−N plots, and (cII) EFE lifetime spectra of diamond/s-CNTs.

can be turned on at even lower field, (E0)CNTs = 0.79 V/μm, with even higher EFE current density, (Je)CNTs = 4.17 mA/cm2 (at an applied field of 1.37 V/μm). However, the s-CNTs materials show markedly better lifetime stability in EFE measurements than the conventional CNTs materials. Figure 6cI shows that when tested at EFE current density of 0.19 mA/ cm2, the s-CNTs can last 100 min, whereas the conventional CNTs can last only 30 min. These results illustrate again that the carbon nanocones grown on the side walls of s-CNTs materials increase the robustness of the CNTs materials markedly. In contrast, the diamond coating on the s-CNTs slightly degraded the EFE properties but markedly improved the robustness of the diamond/s-CNTs materials. Curve I in Figure 6aII shows that, for UNCD/s-CNTs films, the EFE process can be turned on at (E0)UNCD/s‑CNTs = 5.84 V/μm with EFE current density of (Je)UNCD/s‑CNTs= 0.4 mA/cm2 at an applied field of 7.5 V/μm. Curve II in Figure 6aII shows that the HiD/s-CNTs films possess better EFE properties than the UNCD/s-CNTs films, that is, (E0)HiD/s‑CNTs = 4.82 V/μm with EFE current

bonding, indicating that these films contain more abundant sp2bonded carbon. 3.3. Application: Electron Field Emission and Plasma Illumination Source. The electrical conductivity of these diamond/s-CNTs films were estimated by Hall measurement method in van der Pauw configuration. While the electrical resistance of UNCD/s-CNTs (HiD/s-CNTs) films is too large to be measurable by these technique, the electrical conductivity of N-UNCD/s-CNTs films is observed to be as high as σ = 237 S/cm with a carrier concentration of 2 × 1018 cm−2 and mobility of 692 cm2/(V s) [(σ)s‑CNTs = 758 S/cm]. The EFE measurements of the s-CNTs (CNTs) and diamonds-CNTs materials were shown in Figures 6aI and 6aII, respectively, with the associated Fowler−Nordheim curves plotted in Figures 6bI and 6bII, respectively. Figure 6aI shows that both the conventional CNTs and s-CNTs materials possess very good EFE properties. The s-CNTs require low turn-on field of (E0)s‑CNTs = 1.15 V/μm to turn on the EFE process, achieving high EFE current density of (Je)s‑CNTs = 5.47 mA/cm2 (at an applied field of 2.44 V/μm), whereas the conventional CNTs 27531


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ACS Applied Materials & Interfaces Table 2. EFE and PI Characteristics of s-CNTs and Diamond/s-CNTs Materials materials CNTs s-CNTs UNCD/s-CNTs HiD/s-CNTs NUNCDs-CNTs

σa

(S/cm)

956 758

258

βb

E0c (V/μm)

Jed (mA/cm2)

τefee (min)

(Eth)plf (V/cm)

Jplg (mA/cm2)

neh 1017 cm−3

τpli (min)

7650 3350 1065 1981 2485

0.79 1.15 5.84 4.82 3.58

[email protected] [email protected] [email protected] [email protected] [email protected]

100 100 430 590 350

3600 3600 4000 3800 3600

2.36 2.36 1.67 1.88 2.03

1.47 1.47 1.08 1.20 1.32

70 70 224 314 202

σ: the conductivity evaluated by Hall measurement tequnique in van der Pauw configuration. bβ: the field enhancement factor estimated from the slope of F−N plots. cE0: the turn-on field for inducing EFE process, which was designated as the interception of the straight line segments extrapolated from the of low-field and high field of F−N plots. dJe: the EFE current density at the designated applied field in V/μm. eτefe: the lifetime measured at applied current of 0.19 mA/cm2. f(Eth)pl: the threshold field for igniting the plasma in parallel-plate microplasma devices. gJpl: the plasma current density in parallel-plate microplasma devices measured at an applied field of 5600 V/cm. hne: the plasma density, which is also the electron density in the parallel-plate microplasma devices, measured at an applied field of 5600 V/cm. iτpl: the lifetime measured at 0.8 mA/cm2 applied current in parallel-plate microplasma devices. a

Figure 7. (aI, aII) Plasma illumation images of CNTs based microplasma devices: (I) conventional CNTs and (II) s-CNTs based ones. (bI, bII, bIII) Those of diamond/s-CNTS: (bI) UNCD/s-CNTs, (bII) HiD/s-CNTs, and (bIII) N-UNCD/s-CNTs based one.

density of (Je)HiD/s‑CNTs = 2.94 mA/cm2 at an applied field of 7.5 V/μm. Such a behavior is in accord to the previous report that the HiD-based EFE materials perform better than the UNCD-based ones.29 However, both UNCD and HiD films grown in CH4/Ar plasma contain equi-axed diamond grains with a few nanographitic clusters located along the grain boundary regions of the samples. Such a granular structure does not contain

sufficient amount of nanographitic phase with good enough crystallinity for ensuring good conductivity for the materials. In contrast, N-UNCD films grown in CH4/N2 plasma own an unique granular structure of needle-like grains encased in nanographitic layers. These films are very conducting and possess better EFE properties. Curve III in Figure 6aII shows that, for N-UNCD/s-CNTs films, the EFE process can be turned on at (E0)NUNCD/s‑CNTs = 3.58 V/μm, attaining a large 27532


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Figure 8. (aI) Plasma current density vs applied field properties, (bI) plasma density vs applied field properties, and (cI) plasma illumination lifetime spectra of CNTs: (I) conventional CNTs and (II) s-CNTs. (aII, bII, cII) Those of diamond/s-CNTS: (I) UNCD/s-CNTs, (II) HiD/s-CNTs, and (III) N-UNCD/s-CNTs. That is, (aII) plasma current density vs applied field properties, (bII) plasma density vs applied field properties, and (cII) plasma illumination lifetime spectra of diamond/s-CNTs.

10−6 Torr environment with the current density of 0.19 mA/ cm2. It should be mentioned that in the lifetime test, the applied field required to maintain testing current density of 0.19 mA/cm2 varied with the testing materials. Curve II in Figure 6cII shows that among the diamond/CNTs films, the HiD/sCNTs films exhibit best lifetime stability, that is, τ = 590 min at 0.19 mA/cm2 operation current density. The EFE current density of N-UNCD/s-CNT can last more than 350 min (curve III, Figure 6cII), which is slightly inferior to UNCD/s-CNTs (or HiD/s-CNTs) films (cf. curves I and II, Figure 6cII) but still performed overwhelmingly better than that of the s-CNTs (100 min, curve II, Figure 6cI). Restating, the coating of N-UNCD films on s-CNTs resulted in markedly superior lifetime stability with reasonably good EFE properties that can be considered as beneficial for the EFE device applications. To better illustrate the greater potential of diamond/s-CNTs for device applications, the performance of microplasma devices utilizing these EFE materials as cathode were characterized. Figure 7aI and 7aII shows the series of PI photographs of the

EFE current density of (Je)NUNCD/s‑CNTs= 1.86 mA/cm2 at an applied field of 6.0 V/μm. Moreover, the field enhancement factors were determined by fitting the linear part of the F−N equation. slope = −Bφ3/2 /β

By assuming the work function of diamond to be 5 eV30 and CNTs to be 4.9 eV,31 the β values of s-CNTs and diamond/sCNTs materials are estimated to be approximately (β)CNTs = 7650, (β)s‑CNTs = 3350, (β)UNCD/s‑CNTs = 1065, (β)HiD/s‑CNTs = 1981, and (β)N‑UNCD/s‑CNTs = 2485, respectively. The EFE characteristics of these s-CNTs and diamond/s-CNTs materials are summarized in Table 2. Although the s-CNTs possess very good EFE properties, compared with the diamond/s-CNTs materials, they suffer from low lifetime stability. Stability of the field emission current is of more concern related to practical applications of cold cathode materials. To evaluate the field-emission stability of these EFE materials, the EFE current density was monitored in 27533


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microstructures of the UNCD (HiD) films grown on CNTs/ Si are exactly the same as those grown directly on Si with the presence of a-C interlayer. The same beneficial effect on enhancing the EFE properties was also observed when using sCNTs as interlayer.11 The microstructure of the UNCD (or HiD) films was not altered due to the change of the interlayer from conventional CNTs to s-CNTs, that is, the UNCD films contain uniformly ultrasmall (∼5 nm) diamond grains with relatively wide grain boundaries, whereas the HiD films contain some large diamond aggregates (∼100−200 nm) evenly distributed among the matrix of ultrasmall diamond grains (see Figure S1). Although the conventional CNTs, after decorating with the NDP can withstand the CH4/Ar plasma and enable the growth of the UNCD (or HiD) films very well, they cannot survive the bombardment damage of the species in CH4/N2 plasma and thus making it impossible to grow NUNCD films. Only the s-CNTs films can survive the CH4/N2 plasma enabling the growth of N-UNCD films. Therefore, it is of particular interest to understand the growth behavior of NUNCD films on s-CNTs. The bright field (BF) TEM micrograph shown in Figure 9a reveals that the surface region of N-UNCD/s-CNTs film contain needle-like diamond grains, about 3−5 nm in diameters and 200−1000 nm in length. The selected area electron diffraction (SAED) patterns shown as inset in Figure 9a reveals the presence of the (111), (220), and (311) diffraction rings corresponding to diamond lattices, confirming that the NUNCD films are predominantly diamond, although there are large proportion of regions showing low contrast as if they are noncrystalline materials. Presumably, these low contrast regions contain diamond grains oriented away from the zone-axis and diffracting the electrons weakly. Moreover, there exists a diffused ring in the center, indicating the existence of sp2bonded carbon in the films. The dark field (DF) images were taken from different portion of diffraction rings, including (0002) graphite and (111) diamond diffraction patterns (inset, Figure 9b) and were then superimposed together to form a composed dark field image (c-DF). The c-DF image shown in Figure 9b clearly illustrate that the nanographite layers (green color) is surrounding the needle-like diamond grains (yellow color). The detailed microstructure of the diamond grains is more clearly illustrated by the TEM structure image in Figure 9c, which corresponds to the marked region in Figure 9a. The needle-like diamond grains actually contain clearly a core−shell microstructure, where the diamond grains were encapsulated with a thin graphite layer (∼2−5 nm in thickness). The Fourier-transformed diffractogram (FT0 image) corresponding to the entire structure image illustrates a spotted diffraction pattern arranged in a ring, which corresponds to randomly oriented needle-like diamond grains, and a large diffused ring of donut-shape located at the center of FT0 image, which corresponds to graphitic phase. The FT images, f1 and f2, in Figure 9c highlight the presence of the needle-like diamond grains and nanographite phase, which correspond to designated areas 1 and 2, respectively. Moreover, Figure 10 shows the microstructure of the “interface” region of N-UNCD/s-CNTs films, which is the region near the (N-UNCD)-to-(s-CNTs) interface. The BF(TEM) micrograph in Figure 10a clearly revealed that the needle-like diamond grains were nucleated from the carbon nanowalls of s-CNTs without the formation of a-C interlayers. The SAED shown in inset of Figure 10a indicated the presence of the (111), (220) and (311) diffraction rings corresponding

microplasma devices utilizing the s-CNTs (or conventional CNTs) as cathodes, whereas Figure 7bI, 7bII and 7bIII shows those utilizing the diamond/s-CNTs as cathodes. The brightness of the microplasma devices increases monotonically with the applied voltage. The s-CNTs (conventional CNTs)-based microplasma devices can be triggered by a voltage of 360 V [(Eth)s‑CNTs = 3600 V/cm], while the diamond/s-CNTs microplasma devices need slightly larger voltage of 360−400 V [(Eth)dia. = 3600−4000 V/cm] to trigger the plasma. The PI performance is better illustrated by the Jpl−E curves in Figure 8aI and 8aII for CNTs and diamond/s-CNTs based microplasma devices. The Jpl-value of the microplasma devices increases monotonously with the applied voltage. The s-CNTsbased microplasma devices achieved a large Jpl-value of 2.36 mA/cm2 at an applied field of 5600 V/cm (curve II, Figure 8aI), whereas the Jpl reaches 2.75 mA/cm2 at the same applied field for the device which used conventional CNTs as cathode [curve I of Figure 8aI]. In contrast, the Jpl-values attainable at 5600 V/cm is slightly smaller for microplasma devices which used diamond/s-CNT as cathode, that is, (Jpl)UNCD/s‑CNTs = 1.8 mA/cm2, (Jpl)HiD/s‑CNTs = 2.01 mA/cm2, and (Jpl)N‑UNCD/s‑CNTs = 2.03 mA/cm2 (curves I, II, and III, Figure 8aII, respectively). From each Jpl-value, the corresponding plasma density can be calculated, which is the same as the electron density (ne) of the plasma. The procedures for calculating the plasma density of the microplasma devices from the plasma current density had been described in detail elsewhere.32 The ne−E curves of the sCNTs and N-UNCD/s-CNTs microplasma devices are shown in Figure 8bI and 8bII, respectively, revealing that the ne-values are slightly larger for s-CNTs (CNT)-based microplasma devices and are slightly smaller for diamond/s-CNTs based ones, that is, ne-values at an applied field of 5600 V/cm are around 1.47−1.62 × 1017 cm−3 for CNTs (s-CNTs) based microplasma devices and ∼13.22−14.73 × 1016 cm−3 for diamond/s-CNTs based ones. The most advantageous benefit of coating the diamond films on s-CNTs is the marked improvement in the robustness of the microplasma devices. While the lifetime of s-CNTs microplasma devices is as short as τs‑CNTs = 70 min for a Jpl of 0.8 mA/cm2 (τCNTs = 50 min), that of the diamond/s-CNTs based microplasma devices can be upheld for a period of over 202 min, showing high lifetime stability for N-UNCD/s-CNTs based microplasma devices [curve III, Figure 8cII]. On the contrary, curves I and II in Figure 8cII show that the UNCD/sCNTs and HiD/s-CNTs based devices possess even longer lifetime ((τ)UNCD/s‑CNTs = 224 min and (τ)HiD/s‑CNTs = 314 min). These performances of the microplasma devices are summarized in Table 2. Again, it is to be noted that the lifetime stability is of more concern for the practical applications of the microplasma devices. Among these cathode materials, NUNCD/s-CNTs materials perform better, as they proved good plasma performance (Jpl and ne-values) with reasonably long lifetime stability. Apparently, the better microplasma performance of the NUNCD/s-CNTs-based microplasma devices, as compared with s-CNTs based ones, is primarily owing to the superior robustness of N-UNCD coating on sCNTs. 3.4. TEM Microstructure. It has been shown that conventional CNTs films can be used as interlayer for growing UNCD (or HiD) films on Si substrates.11 The formation of a-C layer which usually occurred when these films were grown directly on Si was effectively circumvented. The EFE properties of diamond/CNTs/Si materials are thus improved. The 27534


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Figure 9. (a) TEM bright field, (b) TEM composed dark field and (c) TEM structure image corresponding to region designated in “a” for surface region of N-UNCD/s-CNTs films. The insets in “a” and “b” are the selected area electron diffraction patterns corresponding to “a” and “b”, respectively. The FT0 is the Fourier-transformed diffractogram corresponding to the entire structure image in “c”, whereas ft1 and ft2 are the FT images corresponding to the designated areas in “c”.

to diamond and a diffused ring in the center corresponding to sp2-bonded carbon. Detailed investigations of the interface regions A and B designated in Figure 10a are shown as TEM structure images in Figure 10b and 10c, respectively. TEM structure image in Figure 10b revealed that this region contains abundant curved fringes. The FT0b image corresponding to the entire structure image in Figure 10b is shown as inset in this figure, which shows a spotted diffraction pattern arranged in ring, implying that these regions contain randomly oriented diamond grains, which were nucleated among the s-CNTs

Figure 10. (a) TEM bright field, (b) TEM structure images corresponding to region A designated in “a” and (c) TEM structure image corresponding to region B designated in “a” for interface region of N-UNCD/s-CNTs films. The inset in “a” is the corresponding selected area electron diffraction patterns. The FT0b and FT0c are the Fourier-transformed diffractogram corresponding to the entire structure image in “b” and “c”, respectively. 27535


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indicating the presence of sp2-bonded carbon along with diamond. However, it is still necessary to differentiate the nature of the sp2-bonded carbons, that is, to differentiate the amorphous carbons from the graphite. It should be noted that the plasmonloss EELS is the most effective technique for distinguishing the crystalline sp2-bonded carbons (the graphite) from the amorphous ones and to differentiate diamond, sp3-bonded carbons, from sp2-bonded carbon. The graphitic phase shows a main peak at 27 eV (ωg-band), whereas the a-C phase shows a peak at 22 eV (ω a -band) in the plasmon-loss EELS spectrum.39,40 Furthermore, the crystalline sp3-bonded carbon in diamond phase shows a peak near 33 eV (ωd2-band), representing the bulk plasma of diamond clusters, with a shoulder near 23 eV (ωd1-band), representing surface plasma of the clusters.41 Figure 11b shows the selected area plasmon-loss EELS spectra of the same regions as those for core-loss EELS. Curve I of Figure 11b exhibits that s-CNTs contains a single peak at 24 eV, which can be deconvoluted into peak at 22 eV (ωa-band) representing the a-C phase and the peak at 27 eV (ωg-band) representing the graphitic phase (dotted curves, Figure 11b), indicating that s-CNTs films is a mixture of amorphous and crystalline sp2-bonded carbons. The amorphous carbons are presumably formed on the surface of sCNTs. In contrast, the plasmon-loss EELS spectrum of NUNCD/s-CNTs (curve II, Figure 11b) is dominated by a wide band near 22−33 eV, in which the ωd1-band (23 eV) and ωd2band (33 eV) are dominating, with ωa-band (22 eV) and ωgband (27 eV) as minor phase, indicating that this region is predominantly diamond and consists of some proportion of graphitic phases. The EELS spectroscopic observations in Figures 11a and 11b are in accord with the TEM microstructural examinations in Figures 1, 9, and 10. The s-CNTs can sustain the CH4/N2 plasma ion bombardment and thus enables the growth of N-UNCD films. The needle-like diamond grains can nucleate directly on s-CNTs without the formation of another interlayer, which facilitates the transport of electrons from Si substrate to N-UNCD films and is readily field emitted. On the other hand, N-UNCD films contain needle-like diamond grains with each of the grains encased in nanographitic phase. It is believed that the graphenelike layers encasing the needle-like diamond grains possess high conductivity and can transport the electrons efficiently along the N-UNCD films to the top of the films and can be field emitted through the tip of diamond grains without any effort due to the negative work function of diamond materials. In other words, the constituents in N-UNCD materials namely the diamond core and nanographitic shell form an ideal nanosized carbon composite, in which the nanographitic phase transports the electrons to the tip of the needle-like diamond grains for field emission. The N-UNCD/s-CNTs thus perform overwhelmingly well compared with other kind of diamond/sCNTs films. A comparison on the EFE property of our work with the published data were made, as shown in Table 3.42−46 It is observed that excellent current densities of the composite films were achieved if the films contained CNTs.

matrix. Whether these diamond clusters are the NPD decorated on s-CNTs or the as MPE-CVD nucleated cannot be clearly resolved. Moreover, the donut-shape diffused diffraction ring located at the center of FT0b image indicates that the curved fringes in these regions are of graphitic phase. They are most probably s-CNTs. The TEM structure image in Figure 10c clearly illustrates the microstructure of a tiny needle-like diamond grain (2−3 nm in diameter and 20−30 nm in length), which seems to be as-nucleated from the matrix of s-CNTs. This “embryo” of a needle-like diamond grain has already been surrounded by nanographitic layers completely. To clarify the localized bonding structure of the s-CNTs and N-UNCD/s-CNTs films, the selective area electron energy loss spectra (EELS) of the films were recorded in the carbon K-edge region that unambiguously distinguished the diamond and nondiamond phases.33 Figures 11a and 11b show the core-loss

Figure 11. (a) Core-loss and (b) plasmon-loss EELS spectra of (I) sCNTs and (II) N-UNCD/s-CNTs.

and plasmon-loss EELS, respectively. The core-loss EELS spectrum of s-CNTs corresponding to BF(TEM) image in Figure 1b is shown as curve I in Figure 11a, revealing the main features of a graphitic materials, that is, they contain a large hump at 284.5 eV, corresponding to transitions from the 1s to the π* states (1s−π*), and a peak at 292 eV, corresponds to transitions from the 1s to the σ* states (1s−σ*).34−36 In contrast, the core-loss EELS spectrum of N-UNCD/s-CNTs (curve II in Figure 11a), which corresponds to BF(TEM) image in Figure 9a, is predominated with the typical EELS signal of sp3-bonded carbon, diamond, that is, a σ*-band near 289.5 eV with a deep valley around 302 eV, which were indicated in Figure 11a.37,38 Moreover, there is a prominent π*band at 284.5 eV in core-loss EELS for N-UNCD/s-CNTs,

4. CONCLUSION Coating diamond films on top of CNTs can increase markedly the robustness of CNTs. For the purpose of growing N-UNCD films, which are highly conducting with superior EFE properties, on CNTs, the resistance of the CNTs materials against the CH4/N2 plasma bombardment needs to be 27536


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Nanosheets Synthesized by C2H2/H2 Plasma Enhanced CVD. Carbon 2011, 49, 2526−2531. (4) Chang, T. H.; Panda, K.; Panigrahi, B. K.; Lou, S. C.; Chen, C.; Chan, H. C.; Lin, I. N.; Tai, N. H. Electrophoresis of Nanodiamond on the Growth of Ultrananocrystalline Diamond Films on Silicon Nanowires and the Enhancement of the Electron Field Emission Properties. J. Phys. Chem. C 2012, 116, 19867−19876. (5) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Hydrogen Storage in Single-Walled Carbon Nanotubes at Room Temperature. Science 1999, 286, 1127−1129. (6) Lee, Y. H.; Jang, Y. T.; Kim, D. H.; Ahn, J. H.; Ju, B. K. Realization of Gated Field Emitters for Electrophotonic Applications Using Carbon Nanotube Line Emitters Directly Grown into Submicrometer Holes. Adv. Mater. 2001, 13, 479−482. (7) Yu, K.; Zhang, Y. S.; Xu, F.; Li, Q.; Zhu, Z. Q.; Wan, Q. Significant Improvement of Field Emission by Depositing Zinc Oxide Nanostructures on Screen-Printed Carbon Nanotube Films. Appl. Phys. Lett. 2006, 88, 153123. (8) Varshney, D.; Sumant, A. V.; Resto, O.; Mendoza, F.; Quintero, K. P.; Ahmadi, M.; Weiner, B. R.; Morell, G. Single-Step Route to Hierarchical Flower-Like Carbon Nanotube Clusters Decorated with Ultrananocrystalline Diamond. Carbon 2013, 63, 253−262. (9) Su, C. Y.; Juang, Z. Y.; Chen, Y. L.; Leou, K. C.; Tsai, C. H. The Field Emission Characteristics of Carbon Nanotubes Coated by Boron Nitride Film. Diamond Relat. Mater. 2007, 16, 1393−1397. (10) May, P. W.; Höhn, S.; Ashfold, M. N. R.; Wang, W. N.; Fox, N. A.; Davis, T. J.; Steeds, J. W. Field Emission from Chemical Vapor Deposited Diamond and Diamond-Like Carbon Films: Investigations of Surface Damage and Conduction Mechanisms. J. Appl. Phys. 1998, 84, 1618−1625. (11) Chang, T. H.; Kunuku, S.; Hong, Y. J.; Leou, K. C.; Yew, T. R.; Tai, N. H.; Lin, I. N. Enhancement of the Stability of Electron Field Emission Behavior and the Related Microplasma Devices of Carbon Nanotubes by Coating Diamond Films. ACS Appl. Mater. Interfaces 2014, 6, 11589−11597. (12) Sankaran, K. J.; Lin, Y. F.; Jian, W. B.; Chen, H. C.; Panda, K.; Sundaravel, B.; Dong, C. L.; Tai, N. H.; Lin, I. N. Structural and Electrical Properties of Conducting Diamond Nanowires. ACS Appl. Mater. Interfaces 2013, 5, 1294−1301. (13) Fowler, R. H.; Nordheim, L. Electron Emission in Intense Electric Fields. Proc. R. Soc. London, Ser. A 1928, 119, 173−181. (14) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett. 2010, 10, 751−758. (15) Ni, Z. H.; Fan, H. M.; Fan, X. F.; Wang, H. M.; Sheng, Z.; Feng, Y. P.; Wu, Y. H.; Shen, Z. X. High temperature Raman spectroscopy studies of carbon nanowalls. J. Raman Spectrosc. 2007, 38, 1449−1453. (16) Kurita, S.; Yoshimura, A.; Kawamoto, H.; Uchida, T.; Kojima, K.; Tachibana, M.; Molina-Morales, P.; Nakai, H. Raman Spectra of Carbon Nanowalls Grown by Plasma-Enhanced Chemical Vapor Deposition. J. Appl. Phys. 2005, 97, 104320. (17) Estrade-Szwarckopf, H. XPS Photoemission in Carbonaceous Materials: A ‘‘Defect’’ Peak Beside the Graphitic Asymmetric Peak. Carbon 2004, 42, 1713−1721. (18) Batson, P. E. Carbon 1s Near-Edge-Absorption Fine-Structure in Graphite. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 2608−2610. (19) Bruhwiler, P. A.; Maxwell, A. J.; Puglia, C.; Nilsson, A.; Anderson, S.; Martensson, N. π* and σ* excitons in C-1s Absorption of Graphite. Phys. Rev. Lett. 1995, 74, 614−617. (20) Abbas, M.; Wu, Z. Y.; Zhong, J.; Ibrahim, K.; Fiori, A.; Orlanducci, S.; Sessa, V.; Terranova, M. L.; Davoli, I. X-ray Absorption and Photoelectron Spectroscopy Studies on Graphite and SingleWalled Carbon Nanotubes: Oxygen Effect. Appl. Phys. Lett. 2005, 87, 051923. (21) Ferrari, A. C.; Roberson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 61, 14095−14107.

Table 3. Comparison on the Electron Field Emission Properties of Diamond Composite Materials electron field emission behavior materials 42

UNCD/SiNWs CNT/diamond composite43 DLC/VACNT44 undoped diamond-coated CNT teepees45 B-doped-diamond-coated CNT teepees45 thin UNCD coating/CNT46 thick UNCD coating/CNT46 N-UNCD/s-CNTsthis study

E0a (V/μm)

Jeb (mA/cm2)

3.70 1.10 2.10 7.46 3.00 1.95 2.30 3.58

5 × 10−9 0.05 0.84 0.08 0.10 0.05 0.11

E0: the turn-on field for inducing EFE process. bJe: the EFE current density at the applied field in 4.0 V/μm.

a

improved. The s-CNTs grown by modified HF-CVD process contains carbon nanocones on the side wall of s-CNTs and can survive the bombardment of CH4/N2 plasma. Therefore, these s-CNTs can be used for growing N-UNCD, as well as UNCD and HiD films. Among the diamond/s-CNTs films, the NUNCD/s-CNTs films show the best EFE properties, that is, lowest E0 of 3.58 V/μm with largest EFE Je of 3.2 mA/cm2 (at 6.5 V/μm), as well as the most excellent PI characteristics, that is, smallest ignition Eth of 3600 V/cm and largest ne of 1.32 × 1017 cm−3 at 5600 V/cm. However, HiD/s-CNTs films exhibit the best lifetime stability in both EFE testing (τefe = 590 min at 0.19 mA/cm2 EFE test current density) and PI testing (τPI = 314 min at 0.8 mA/cm2 PI test current density). All of the diamond/s-CNTs films exhibit tremendously superior robustness, but show slightly inferior EFE and PI performance, compared with the s-CNTs (or CNTs).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09778. TEM images of (a) UNCD/s-CNTs and (b) HiD/sCNTs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support from the National Science Council, Taiwan, Republic of China, for the support of this research through the project No. NSC 104-2221-E-007029-MY3 and NSC 104-2112-M-032-003.

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REFERENCES

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High Stability Electron Field Emitters Synthesized via the Combination

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