Heterogranular-Structured Diamond–Gold Nanohybrids: A New Long

Nov 24, 2015 - In the age of hand-held portable electronics, the need for robust, stable and long-life cathode materials has become increasingly impor...
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Heterogranular-Structured Diamond−Gold Nanohybrids: A New Long-Life Electronic Display Cathode Kamatchi Jothiramalingam Sankaran,*,†,‡ Bohr-Ran Huang,§ Adhimoorthy Saravanan,§ Divinah Manoharan,∥ Nyan-Hwa Tai,† and I-Nan Lin∥ †

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, R.O.C. Institute for Materials Research (IMO), Hasselt University, 3590 Diepenbeek, Belgium § Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, R.O.C. ∥ Department of Physics, Tamkang University, Tamsui 251, Taiwan, R.O.C. ‡

S Supporting Information *

ABSTRACT: In the age of hand-held portable electronics, the need for robust, stable and long-life cathode materials has become increasingly important. Herein, a novel heterogranular-structured diamond−gold nanohybrids (HDG) as a long-term stable cathode material for field-emission (FE) display and plasma display devices is experimentally demonstrated. These hybrid materials are electrically conductive that perform as an excellent field emitters, viz. low turn-on field of 2.62 V/μm with high FE current density of 4.57 mA/cm2 (corresponding to a applied field of 6.43 V/μm) and prominently high lifetime stability lasting for 1092 min revealing their superiority on comparison with the other commonly used field emitters such as carbon nanotubes, graphene, and zinc oxide nanorods. The process of fabrication of these HDG materials is direct and easy thereby paving way for the advancement in next generation cathode materials for high-brightness FE and plasma-based display devices. KEYWORDS: heterogranular-structured diamond, gold nanoclusters, nanographitic filaments, lifetime stability, field-emission display, plasma display

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INTRODUCTION Because the display is such a decisive human interface of electronic entertainment systems such as television, hand-held computers, and mobile phones, there has been enormous development in this foresaid sector so as to implement practically user-friendly display technology that is being considered benign. The fabrication of conducting cold cathode field-emission (FE) materials that are robust and reproducible and yield adequate and uniform electronic properties are of particular concern for display applications.1 Right since their discovery, carbon nanotubes (CNTs)2 are viewed as one of the most likely cold cathode materials for FE from both fundamental aspects and practical point of views.3−5 However, CNTs-based FE displays (FEDs) show poor uniformity and low current stability.6 Researchers are actively looking for alternative materials. What would the ideal field emitter look like? The cold cathode field emitter is expected to possess certain characteristics, namely, resistance against chemical attack, resistance upon ion bombardment by residual gases, sustaining plasma discharges, withstanding the difference in pressure, and stability in various gas environments.7 Other than the low onset fields, the long-lasting stability and stable emission current with very little/no fluctuations are necessary for using cold cathode materials in FEDs. © XXXX American Chemical Society

Extensive research effort has been devoted to fabricate various nanohybrid heterostructures with new functionalities by merging two or more types of building blocks. Compared with single-component ones, nanohybrid heterostructures reveal superior properties. For instance, they possess better emission efficiency8 and high electron mobility,9 which are considered very crucial factors for many devices’ performance.10,11 By the effort of a large number of research groups, a myriad of nanohybrid heterostructured materials have been developed. In the past decade, heterostructures such as single-walled CNTs− CdSe quantum dots hybrid ultrathin films,12 CNT and graphene hybrid nanostructures,13 graphenated MWCNTs decorated with gold,14 gold−ultrananocrystalline diamond (UNCD) hybrid structured materials,15 CNT−UNCD-coated Si tips,16 hybrid diamond-like carbon and CNT,17 lanthanum hexaboride coated MWCNTs,18 and gold nanoparticle decorated ZnO nanopillars19 have been widely studied. The research on synthesis of heterostructures not only is essential for fundamental studies but also accounts for various advanced functional devices, such as interconnects and emitters, etc. Received: June 26, 2015 Accepted: November 24, 2015

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250 μm. The current−voltage (I−V) characteristics were measured using an electrometer (Keithley 2410) under pressure below 10−6 Torr. The FE parameters were extracted from the obtained I−V curves by using the Fowler−Nordheim (F−N) model.31 The turn-on field (E0) was designated as the point of interception of the straight lines extrapolated from the low- and the high-field segments of the F−N plots, namely, ln(Je/E2) versus 1/E plots, where Je is the FE current density and E is the applied field. The fabrication of PD device is given elsewhere.16

Utilizing the unique possibility to merge two materials with extremely dissimilar electronic properties, an electrically conductive heterogranular-structured diamond−gold nanohybrids (HDG) for long lifetime, stable electron emitters for a FED, and a plasma display (PD) is being introduced. It is known that diamond materials own excellent physical as well as chemical properties such as good thermal conductivity, tunable negative electron affinity, high secondary electron emission coefficient, high hardness, high chemical inertness, and wide band gap.20 All these characteristics render diamond materials as a promising candidate for application as cathodes in FEDs and PDs. In contrast, the noble metal gold possesses special electronic properties such as conduction via activated electron hopping, distinct optical properties and unusually high catalytic activity.21 Contradictory to the extremely unreactive and catalytically inactive bulk gold, the properties of gold drastically change when scaled down to nanosized particles; Au nanoparticles thus can catalyze a numerous reactions.22−27 The reason behind the catalytic activity might be due to the soluble nature of carbon in gold nanoparticles,28 which also suggested size dependency.29 However, the mechanism of growth of diamond based on the catalytic activity of Au is unknown.

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RESULTS AND DISCUSSION The plane view SEM image shown in Figure 1a depicts the presence of equi-axed ultrasmall granular structure exhibiting a

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EXPERIMENTAL METHODS Following a simple and feasible method, the HDG materials being fabricated. First, Au is deposited on Si substrates. The process of Au deposition on Si substrate and the preseeding process for the growth of UNCD on these Au−Si substrates is described in detail elsewhere.30 Second, UNCD materials were grown on Au−Si substrates using microwave plasma-enhanced chemical vapor deposition (MPECVD; 2.45 GHz, 6″ IPLASCYRANNUS-I, Troisdorf, Germany) system in a CH4(2%)/ Ar(98%) plasma with a microwave power of 1000 W for 30 min, with a pressure and total flow rate of 150 Torr and 200 sccm (standard cubic centimeter). Third, the UNCD/Au−Si materials were post-treated in a CH4(2%)/Ar(48%)/H2(50%) plasma, which was excited by 1200 W microwave power in 65 Torr chamber pressure, with negative bias voltage (−300 V) applied on UNCD/Au−Si for 30 min; thus, the HDG materials were obtained. During the growth process of diamond, the substrate was not heated using an external heater. However, because of plasma bombardment heating, the substrate temperature increased to around 450 °C, which was monitored using a thermocouple embedded in a stainless-steel substrate holder. The morphology, the crystalline quality, and the depth profile of the films were characterized using scanning electron microscopy (SEM; Jeol JSM-6500F), visible−Raman spectroscopy (λ = 632.8 nm, Lab Raman HR800, Jobin Yvon), nearedge X-ray absorption spectroscopy (NEXAFS), and secondary ion mass spectroscopy (SIMS; Cameca IMS-4f), respectively. The local microstructure and bonding structure of the samples were examined using TEM (Jeol 2100F) and electron energy loss spectroscopy (EELS) (Gatan Enfina), respectively. Hall measurements were carried out in van der Pauw configuration (ECOPIA HMS-3000) to observe the conducting behavior of these films. The electrical resistivity of these films was measured by a four-probe technique. FE light-emitting performance was tested using a green phosphor-coated indium tin oxide (ITO) glass as anode and the cold cathode materials used in this study as cathode. The cathode-to-anode separation was fixed by a Teflon spacer (250 μm in thickness) and hence the cathode-anode distance, say d =

Figure 1. Typical SEM micrographs of (a) UNCD materials grown on Au-coated Si substrates and (b) HDG materials grown on Au−Si substrates with the inset showing the evolution of bias current in the post-treated process. (c) SIMS depth profiles of C, Au, and Si species in HDG materials with the inset showing the cross-sectional SEM micrograph of the HDG materials. (d) NEXAFS spectrum with the inset showing the Raman spectrum of the HDG materials.

very smooth surface of UNCD materials grown on Au−Si substrates. The UNCD materials grown on Au−Si substrates (UNCD/Au−Si) reach a thickness of 300 nm in a time period of 30 min (not shown). Notably, UNCD materials contain ultrasmall diamond grains (5−10 nm) interspersed with amorphous carbon (a-C) grain boundaries ensuing better transport of electrons through the grain boundaries that guarantees the superior FE properties of UNCD materials, in comparison with that in microcrystalline diamond (MCD).32 Figure 1b shows the SEM morphology of HDG materials, which contain cauliflowerlike geometry with spherical diamond aggregates of size around 50−80 nm. The addition of H2 in CH4/Ar plasma leads to the enlargement of the grain size from ultranano- to nanosized diamond grains.32 Moreover, the inset in Figure 1b shows that the bias current increased rapidly reaching a saturation value of 160 mA at about 3.8 min after −300 V bias voltages was applied onto the UNCD/Au−Si in the post-treatment process. Usually, the increase in bias current during the bias enhanced nucleation and growth of diamond on Si infers the onset of the formation of diamond nuclei on Si substrates, and the saturation of bias current implies the full B

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ACS Applied Materials & Interfaces coverage of diamond clusters on the Si.33,34 Unlike the conventional bias-enhanced growth process for growing MCD (or UNCD),33,34 which requires tens of minutes to nucleate diamond clusters, the bias current shown in the inset of Figure 1b implies that no incubation time is needed for the diamond nucleation process because the Si substrates were already covered with a layer of UNCD materials prior to the bias process. Notably, the top layer of the UNCD films were converted into HDG materials with a thickness of ∼350 nm, whereas the thickness of UNCD films was decreased to around 150 nm (inset, Figure 1c) because of the plasma post-treatment process. The depth profiles of HDG materials were examined using SIMS to understand the distribution of Au in the HDG materials. The SIMS depth profiles of HDG materials in Figure 1c reveals that in the interface layer, there are pronounced Auand Si-ion counts being observed, indicating clearly the diffusion of Au (Si) species into the interface layer. The various types of carbon bonding configuration in the materials was explicitly differentiated using the NEXAFS spectrum. The diamond nature of the materials is confirmed from the observed sharp rise at 289.5 eV (σ* band) and the large dip at 302.2 eV (second diamond gap) exhibited by the HDG materials as shown in Figure 1d.35 The presence of sp2-bonded carbon in these materials is inferred from the presence of a hump near 285.5 eV (π* band).35 The NEXAFS confirms unambiguously that the diamond phase is predominant in the HDG materials. The crystalline quality of the HDG materials was characterized by visible−Raman spectroscopy. The inset of Figure 1d shows the deconvoluted Raman spectrum (using the multipeak Lorentzian fitting method) of HDG materials. The Raman spectrum shows that the HDG materials exhibit broad Raman resonance peaks, which reflects the presence of nanodiamond grains. In contrast to the commonly observed sharp Raman peak at 1332 cm−1 in the diamond materials with grains in micrometer size, the nanosized diamond grains do not exhibit such a sharp visible-Raman peak. This is due to the fact that Raman resonance is more sensitive to sp2 sites, as compared with that to sp3 bonds.36 However, the HDG materials also show a small kink (indicated by arrow) at 1332 cm−1 (D band), indicating the presence of large diamond grains. Moreover, there are broad Raman resonance peaks at 1140 cm−1 (ν1 band) and at 1480 cm−1 (ν3 band), corresponding to the transpolyacetylene phase locating at grain boundaries,32 and resonance peaks at 1350 cm−1 (D* band) and 1580 cm−1 (G band), corresponding to the disordered carbon and graphite.37,38 The Hall measurements were carried out with the measuring probes directly in contact with the HDG materials. The Hall measurements of the HDG materials in van der Pauw configuration shows a high electrical conductivity of 207.2 (Ω cm)−1 with a sheet carrier concentration of 3.64 × 1020 cm−2 and a mobility of 2.3 × 102 cm2 V−1 S1−. It is to be noted that the resistivity of the UNCD materials directly grown on Si substrates (UNCD/Si) is too large and hence could not be measured in the van der Pauw configuration.30 Compared to the conducting diamond materials that are being reported until now, the HDG material’s electronic parameters are observed to be better.14,35,39−42 To enhance the transport efficiency of electrons crossing the diamond-to-Si interface, there is a need to eliminate the formation of a-C interlayer, which can be effectively accomplished by the utilization of Au-coating on Si substrates. Generally, there is no interdiffusion taking place

between C and Si species in heterogranular-structured diamond grown directly on the Si substrates (Figure S1). Usually the a-C transition layer is necessary for the nucleation of diamond, which is shown clearly in the TEM micrograph in Figure S2. This a-C interlayer hinders the transport of electrons crossing the diamond-to-Si interface because of its resistive character. The heterogranular-structured diamond directly grown on Si substrates (without Au interlayer) possesses the electrical conductivity value of only 9.5 (Ω cm)−1. The SIMS depth profile in Figure 1c confirms the interdiffusion of Au and Si in HDG materials, which has been efficiently induced by the Au-coating, thereby facilitating the interface conductivity. How the utilization of Au-coating effectively enhances the transport of electron crossing the diamond-to-Si interface, is confirmed using the four-probe technique. The four-probe technique is used to analyze the electron transport properties on the surface and the interface of the HDG materials. The surface resistance of the HDG materials is 0.026 Ω (Figure S3a), whereas those of the UNCD/Si materials is 3.94 × 103 Ω, which concurs well with the Hall measurements.43 Moreover, the interface resistance is around 1.17 Ω for HDG materials (Figure S3b), which is markedly lower than the interface resistance of UNCD/Si materials (7.24 × 104 Ω).43 The FE behaviors of these highly conducting HDG materials are shown in Figure 2, which demonstrates the interesting FE properties. The schematic of the measurement is shown in the inset of Figure 2a. Generally, FE can be defined as a quantum tunneling phenomenon in which by a strong electric field the electrons are emitted from a solid surface into the vacuum. The Fowler−Nordheim (F−N) equation could very well explain this phenomenon.31 The FE behaviors of CNTs, graphene, and zinc oxide nanorods (ZNR) were also tested for the purpose of comparison. The surface morphologies of these materials are shown in the SEM images (Figures S4a−c). The plots of ln(Je/ E2) versus 1/E (inset, Figure 2b) show the E0 value of 2.68 V/ μm for HDG materials (solid spheres, Figure 2b), which is comparatively similar to the E0 values of CNTs (2.33 V/μm) and graphene (2.42 V/μm) and lower than that of ZNR (8.23 V/μm). The Je reached 5 mA/cm2 at an applied field of Ea = 6.45 V/μm for HDG materials (Figure 2a), which is slightly larger than those of CNT and graphene ((Ea)CNTs = 4.01 V/μm and (Ea)graphene = 5.30 V/μm to attain the same FE current density of 5 mA/cm2) and is much lower than that of ZNR ((Ea)ZNR = 14.8 V/μm for reaching 5 mA/cm2). The uniformity of the emission current can be confirmed from the luminescence of the phosphor-coated anode plate at a Je value of 0.2 mA/cm2, which are shown in the insets I−IV of Figure 2c, respectively. A luminescence image taken at 1092 min during field emission of HDG materials at Je of 0.2 mA/ cm2 is shown in the inset I of Figure 2c, revealing uniform emission pattern from the whole cathode area. This states that the HDG material proves itself as a stable FED cathode. Restated, low E0 with high Je was achieved, which emphasizes the excellent FE characteristics of HDG materials. The FE properties of the HDG materials compared with the other nanohybrid heterostructured materials are listed in Table S1.13−19 Furthermore, the F−N plots enable the estimation of field enhancement factor (β) from the equation given as follows: β = [−6.8 × 103φ3/2]/m C

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because of oxidation of the surface, and resistive heating resulting in the melting of the emitting centers. The Je versus time curve of these HDG materials is presented as curve I in Figure 2c, which shows the variations of emission current recorded over a period of 1092 min before the onset of degradation at a working field of 4.91 V/μm corresponding to Je of 0.2 mA/cm2. Neither notable fluctuation nor current degradation is being detected in this period of observation, as evident from Figure 2c. However, as expected, the CNTs, graphene, and ZNR cannot last very long. Curves II−IV in Figure 2c show that these materials can only last up to a maximum of 106 min corresponding to Je of 0.2 mA/cm2, which is 1 order of magnitude shorter than the lifetime of HDG materials. Recently, Kato et al. measured stable emission for 1308 h at a current density of 102 mA/cm2 using a coniferous carbon nanostructure emitter as a FE electron source, which could be a typical lifetime for good FE cathode material.48 Although the lifetime of HDG materials are not as good as those reported by Kato et al., it is better than the lifetime of most of the materials reported in the literature13−19 (Table S1). Moreover, the luminescence images shown in the insets of Figure 2c are the best luminescence obtained before the degradation of these cathode materials corresponding to Je of 0.2 mA/cm2. Once these materials start degrade, they show an inhomogeneity in luminescence behavior and eventual burn out (figure not shown). Consequently, the lifetime stability measurement illustrates the salient feature of the HDG materials, i.e., these emitters possess overwhelmingly superior emission reliability when compared to other nanohybrid heterostructured field emitters (Table S1).13−19 The fascinating phenomena in the robustness of the HDG materials can be very well demonstrated by the fabrication of plasma display (PD) devices utilizing these FE emitters as cathodes because the emitters experience the bombardment of energetic Ar ion in such a device environment, proving their robustness. The schematic of the measurement is shown in the inset of Figure 3b. A series of plasma illumination images of the device utilizing the HDG materials as cathodes is shown as Figure 3a, which was ignited at 2 Torr. The threshold breakdown voltage (Vth) is taken as the onset of ignition of the plasma. The threshold breakdown voltage of 340 V (panels I−IV, Figure 3a) is required to trigger the plasma devices using these materials as cathodes. With increasing applied voltage, the intensity of the plasma is found to increase monotonically. The PD device plasma illumination behavior that uses the HDG materials (panel I, Figure 3a) as cathode is comparable with those using CNTs (panel II, Figure 3a) and graphene (panel III, Figure 3a) as cathodes, and is superior to those using ZNR (panel IV, Figure 3a) as cathodes. The PD device plasma illumination behavior is better illustrated by variation of the plasma current with applied voltage measured using an electrometer (Keithley 237) and is shown as Figure S5. The variation of the plasma density (npl) with the applied field (Figure 3b) could very well explain the PD characteristics. The detailed method of calculating the plasma density (npl) from the plasma current density (Jpl) (Figure S5) is reported elsewhere.49 With increasing applied voltage, the plasma density (npl) was observed to increase monotonously and attained almost the same plasma density values at an applied voltage of 0.55 V/μm for all these field emitters used as cathodes in plasma devices (curves I−IV, Figure 3b). For practical applications, the stability of the device is considered to be an important parameter. The plasma devices

Figure 2. (a) Field-emission current density (Je) as a function of applied field (E) for cathode materials with the inset showing schematic of measuring setup, (b) comparison on the turn-on field (E0, solid squares) and the field enhancement factor (β, open circles) among the cathode materials with the inset showing the corresponding Fowler−Nordheim plots, i.e., log(Je/E2) vs 1/E plots, and (c) the FE lifetime stability measurements, i.e., Je vs time at Je = 0.2 mA/cm2 of cathode materials, with the insets showing the photographic images of the luminous FED device. Cathode materials are I, HDG; II, CNTs; III, graphene; and IV, ZNR.

where m is the slope of the F−N plot. From the slopes of the F−N plots (inset, Figure 2b) and using the φ value of diamond, CNT, graphene, and ZNR as 5.0,44 4.8,45 4.62,46 and 5.3 eV,47 respectively, the β values of the materials were calculated. The HDG materials exhibit the β value of 3080, which is comparable with the β values of CNTs (4532) and graphene (3986) and is larger than the β value of ZNR (2654) (open circles, Figure 2b). In the FE process, the tip of emitting materials will be heated up to very high temperature. The thermionic emission current also contributes to the total current density which is the reason behind the overestimation of the FE current density in F−N plots and thereafter the β factor. Nevertheless, these β factors can still serve as good parameters for comparing the characteristics of the emitting materials. For potential application of cold cathode materials, the stability of the FE current is a very crucial parameter. Prolonged usage may deteriorate the emission efficiency of the material D

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To elucidate how the heterogranular-structured diamond and the Au interlayer enhance the lifetime stability of the HDG materials in FED and PD devices, the materials were examined using TEM to clearly know the microstructure of these materials. The typical cross-sectional bright-field TEM micrograph of HDG materials is shown in Figure 4a with clearly marked regions of diamond and Au-interlayer. The selective area electron diffraction (SAED) pattern shown as inset in Figure 4a reveals the presence of the ring shaped pattern, corresponding to the lattice planes (111), (220), and (311) of diamond, indicating the randomly oriented nanosized diamond grains. The presence of Au is inferred from the weak diffraction ring inside the ring pattern of diamond. Besides these, weak diffraction spots are being observed between Au ring pattern and the central diffuse ring corresponding to the SiC phase. Moreover, in the center of this SAED there exists a prominent diffused ring, which indicates that these hybrid materials contain some sp2-bonded carbon phases. The distribution of each phase in the HDG materials is clearly illustrated by the composed dark-field (c-DF) image obtained by superimposing dark field images acquired for different phases. The c-DF image of HDG materials shown in Figure 4b reveals that nanosized diamond (green) and Au (red) were distributed randomly in the materials which confirms the diffusion of Au in the diamond materials (cf. Figure 1c). Moreover, the Si or SiC clusters (blue) are observed between the UNCD and the Au coating. Hence, the diamond clusters nucleated in the vicinity of the Au coating, presumably via the SiC clusters, without the necessity to form the a-C layer. Probably because of the evident interdiffusion between Cr layer, Si substrates, and Au coating, the Cr layer could not be explicitly observed. High-resolution TEM (HRTEM) studies were done in order to obtain still more detailed information about the different phase constituents present in HDG materials. The structure image of the region near to the UNCD-Au interface of HDG materials (designated as region A in Figure 4a) is shown in Figure 4c. Evenly dispersed Au nanoclusters of size around 5 nm in the matrix of ultrasmall diamond grains is clearly observed in this region. (High-magnification HRTEM image of region 3 in Figure 4c reflects the Au phase with a lattice spacing of 0.204 nm.) The crystalline nature of diamond with a lattice spacing of 0.21 nm is evident from the high-magnification HRTEM image of region 2 in Figure 4c. In addition to the spotted diamond diffraction pattern arranged in a ring (designated as D) in the Fourier-transformed (FT) diffractogram image of the whole structure image (FT0c), there exists a diffraction ring outside the diamond ring and spots inside the diamond ring corresponding to the Au phase and SiC phase, respectively. Moreover, the presence of a diffused ring at the center of FT0c image indicates that these materials contain sp2bonded carbon. In addition, the ft1-image corresponding to the region 1, the ft2-image corresponding to the region 2, and the ft3-image corresponding to the region 3 evidently confirm the existence of Si, diamond, and Au nanoclusters, respectively. In between the diamond and the Au nanoclusters, there is a mixed phase of SiC and nanographitic phase, which is highlighted by the ft4-image corresponding to the region 4. The cross-sectional HRTEM image of the designated region B in Figure 4a is shown in Figure 4d. The FT image of the whole structure image (FT0d) shows the presence of discrete (111) diffraction spots arranged in a circular pattern, which indicates the existence of numerous diamond grains oriented randomly. Moreover, in the FTod image, the central diffuse

Figure 3. (a) Photographs of the plasma illumination, (b) plasma density vs applied field of the plasma display devices, and (c) stability of the plasma display devices measured at 450 V. The plasma display devices were fabricated utilizing the ITO coated glass as anode and I, HDG, II, CNTs, III, graphene, and IV, ZNR as cathodes. The inset in b shows the schematic of a plasma display device.

utilizing these cathode materials were examined over an extensive period with a constant applied voltage of 450 V as shown in Figure 3c to determine their lifetime stability. The HDG-based plasma devices PI behavior shows the highest stability (curve I, Figure 3c) as it is observed to be stable for over 772 min with the plasma current density of 1.58 mA/cm2. Dissimilar to the above results, the CNTs-, graphene- and ZNR-based plasma devices sustain even less than 100 min (curves II−IV, Figure 3c), and these materials were damaged completely after plasma discharge (Figures S6b−c). However, the HDG materials can withstand even 772 min of plasma discharge (Figure S6a), which discloses the robustness of the HDG materials in plasma devices. The highest lifetime plasma stability of the HDG-based plasma devices as compared with those of other cathode materials are directly associated with the FE lifetime stability of the HDG cathode materials (cf. Figure 2c). The stability of the plasma devices at different voltages was also measured. Figure S7 shows that the lifetime of HDG-based plasma devices tested under an applied voltage of 350 V is around 860 min (curve I, Figure S7), which is longer than the lifetime observed at 450 V (τ450 = 772 min, curve I, Figure 3c). In contrast, the lifetime tested under 550 V (curve II, Figure S7) is inferior to τ450 = 772 min (τ550 = 700 min). Restated, the lifetime of the HDG-based plasma devices decreased as the testing voltage is larger because the kinetic energy of Ar species bombarding the HDG cathode materials is larger when testing under larger applied voltage. E

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Figure 4. Typical cross-sectional (a) bright-field and (b) composed dark-field TEM micrographs of the HDG materials with the inset showing the corresponding SAED pattern; (c−f) HRTEM images of HDG materials, corresponding to the regions A−D of a, respectively. The insets (FT0c, FT0d, FT0e, and FT0f) show the Fourier-transformed images corresponding to the whole structure images in c−e, respectively, whereas the ft1 to ft6 images show the FT images corresponding to regions 1−6, respectively. (g−i) Selective area EELS spectra corresponding to the cross-sectional TEM micrograph of HDG materials shown in a. (g) Core-loss EELS spectrum, (h) the core-loss EELS spectrum corresponding to Au, and (i) plasmonloss EELS spectrum corresponding to carbon. Inset in h shows the core-loss EELS spectrum corresponding to SiC.

the applied negative bias voltage. Notably, the encapsulation of CH species on the as-nucleated diamond clusters to passivate them is the accepted model for explaining the formation mechanism of the UNCD grains grown in CH4/Ar plasma.51 The CH species will accumulate on the growing diamond films, irrespective of the presence of the bias voltage. Apparently, the rate of accumulation of CH species on the diamond clusters is proportional to the abundance of CH species contained in the CH4/Ar or CH4/Ar/H2 plasma. Bonding in CH species is easy to break via the bombardment of C2+ and Ar+ species, especially when these species were accelerated by the negative bias voltage and possess a large kinetic energy. Both the higher accumulation rate of CH species and large kinetic energy of bombarding species facilitate the conversion of CH species into crystalline sp2-bond phase, the nanographite filaments.52 Such a phenomenon is similar to the observation of Teng et al.33 and Saravanan et al.34 who reported the enhanced FE properties

diffraction ring represents to the presence of graphitic phase. The presence of large proportion of sp2-bonded carbon in these regions is highlighted by the ft6 image corresponding to the marked region 6. These results support the findings from Almeida et al. that the graphitic phases were present in the interfaces of nano/microcrystalline multigrade diamond coatings.50 Moreover, closer inspection of the TEM image in regions C and D of Figure 4a reveals that there are abundant interconnected nanographitic clusters forming a filament. A typical nanographitic filament is illustrated in the HRTEM images in Figures 4e,f, which is designated by a pair of yellow dotted lines. These observations indicate that the formation of nanographitic filaments due to the application of negative bias voltage (−300 V) is one of the prime factors, which resulted in enhanced FE properties of HDG materials. The origin of nanographitic filament in HDG materials seems to correlate more closely with the increase in the number density of CH+ species contained in the CH4/Ar or CH4/Ar/H2 plasma with F

DOI: 10.1021/acsami.5b10569 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces due to the induction of nanographite filaments in the diamond materials grown using CH4/H2 and CH4/Ar plasma. Furthermore, we used TEM-EELS in TEM to investigate the characteristics of the HDG materials. The core-loss EELS spectrum for HDG materials recorded in the carbon K-edge region is shown in Figure 4g, indicating that, besides a sharp peak at 289.5 eV (σ* band) and a dip in the vicinity of 302 eV,53 there exist a large hump at 285 eV (π* band). The σ* band and the dip represent the typical EELS signal of sp3bonded carbon (the diamond), whereas the π* band represents the sp2-bonded carbon. Because the plasmon-loss EELS spectrum shows a prominent peak at 27 eV (s3) and 22 eV (s1) corresponding to the graphitic phase and a-C phase, respectively, the plasmon-loss EELS spectrum is a better way to distinguish the graphitic phase (crystalline sp2-bonded carbon) and the a-C phase.54,55 The plasmon-loss EELS spectrum shown as Figure 4i reveals that the sp2-bonded carbon present in these materials is graphitic in nature because it shows a large diffuse peak near 27 eV.54,55 These observations confirm that the HDG materials are predominated with diamond phase, coexisting with some proportion of nanographitic phase. The presence of a peak at 18 eV in plasmon-loss EELS is due to the presence of Si species in these materials.56 Moreover, the coreloss EELS spectrum shown in Figure 4h shows a peak at 62 eV corresponding to Au57 and a peak at 109 eV (inset, Figure 4h) corresponding to SiC,56 which evidently proves the SiC phase present in the HDG materials induced because of the application of Au interlayer. The layers made up of the heterogranular-structured diamond and hybrid Au−Si in the HDG materials that coexist with nanographitic clusters that are confirmed from the TEM-EELS observations. To have adequate electron supply to the diamond emitting sites from the Si substrates, the diamond conductivity as well as the diamond-to-Si interface resistance must be improved. The application of negative bias voltage and the utilization of Au coating simultaneously fulfills the above-mentioned criterion for a good electron field emitter. This is evident from the TEM investigations, which illustrates that in the heterogranularstructured diamond region of HDG materials, negative bias voltage induces nanographitic filaments thereby improving the conductivity of the HDG materials.52 In contrast, in the interface region of these materials, the Au−Si hybrid materials suppress the formation of a-C phase while inducing the graphitic phase formation and hence enhancing the electron transport across the interface. The role of Au coating on Si in improving the diamond nucleation in the vicinity of diamondto-Si interface region and the nanographite phase formation in the interface region of HDG materials can be explained on the basis of the previous report on Au nanoparticle−UNCD hybrid structured materials.15 Earlier reports have brought into the spotlight that the graphitic grain boundary phases coexisting with the diamond grains of the UNCD materials can result in large improvement in electrical conductivity and FE properties of UNCD materials.15,34,40,41 Here the application of negative bias voltage benefits the formation of nanographitic filaments in the interior of the materials and could be one of the factors leading to high conductivity for HDG materials. However, an equally important factor, which enhances the FE properties of HDG materials, is believed to be the elimination of the formation of resistive a-C phase due to the utilization of Au−Si eutectic layer. This can therefore enable the transfer of the electrons easily from Si substrates to the diamond region through the

interface layer. Further, the electrons move through the conduction channels of the diamond grains to the emitting surface easily. Once they reach the emitting surface, the electrons are effortlessly emitted into vacuum because of the negative electron affinity property of the diamond surfaces.58,59

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CONCLUSIONS A simple and easily reproducible fabrication of HDG materials for utilization as FED and PD device with excellent properties is being established. The application of bias voltage and the Au nanoclusters−UNCD hybrid materials distinctly enhanced the electrical conductivity (207.2 (Ω cm)−1) and the FE properties (viz. low turn-on field of 2.62 V/μm, high FE current density of 4.57 mA/cm2 at 6.43 V/μm, and the lifetime stability for 1092 min) of the HDG materials. The combination of the material is the basis for the enhancement in the conductivity/FE behavior. The SiC cluster formation in the interface region is induced via the Au−Si eutectic layers formed because of the presence of Au coating, thereby eliminating the a-C phase formation. The SiC layer efficiently nucleated the diamond and thus improved the transport of electrons across the diamond to Si interface. Moreover, the Au nanoclusters formed in the interface catalytically induce the formation of nanographitic phases, which leads to the improved conductivity of the materials. Alternatively, the application of negative bias voltage in the growth of heterogranular-structured diamond encourages the formation of nanographitic filaments around the diamond grains. Both the factors resulted in the enhanced FE properties of HDG materials. Such HDG materials not only exhibit excellent FE properties with low turn-on field and high FE current density but also show better robustness viz. longer lifetime as an FED and a PD device. Therefore, this method of fabrication of HDG materials is an easy and direct process that paves way for the progress of next generation cold cathode electron sources for FED and PD devices owing to their stable emission current with low fluctuations, high brightness, and longer lifetimes.

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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.5b10569. SIMS depth profile and HRTEM image of heterogranular-structured diamond directly grown on Si substrates, schematics of four probes technique, SEM images, plasma current density against applied field curves, and lifetime of HDG-based plasma display (PD) devices tested under an applied voltages. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank for financial support the Ministry of Science and Technology, Republic of China, through the project no. MOST 103-2112-M-032-002. K.J.S. is a FWO Postdoctoral Research Fellow of the Research Foundations-Flanders (FWO). G

DOI: 10.1021/acsami.5b10569 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.5b10569 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX