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High Performance Electron Field Emitters and Microplasma Cathodes Based on Conductive Hybrid Granular Structured Diamond Materials Adhimoorthy Saravanan, Bohr-Ran Huang, Divinah Manoharan, and I-Nan Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12375 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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ACS Applied Materials & Interfaces

High Performance Electron Field Emitters and Microplasma Cathodes Based on Conductive Hybrid Granular Structured Diamond Materials Adhimoorthy Saravanan,† Bohr-Ran Huang,† Divinah Manoharan,¶ and I-Nan Lin¶,* †

Graduate Institute of Electro-Optical Engineering and Department of Electronic and computer 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.

ABSTRACT: High performance diamond electron field emitters (EFE) with extremely low turn-on field (E0=1.72 V/μm) and high current density (1.70 mA/cm2 at an applied field of 3.86 V/μm) was successfully synthesized by using a modified two-step microwave plasma chemical deposition process. Such kind of emitters possess EFE properties comparable with most of carbon based or semiconductor based EFE materials, but with markedly better lifetime stability. The superb EFE behavior of these materials was achieved owing to the reduction on the diamond-to-Si interfacial resistance and the increase in the conductivity of the bulk diamond films (HBD-400V), via the applications of high bias voltage during the preparation of the UNCD primary layer and the subsequent plasma post-treatment (ppt) process, respectively. The superior EFE properties along with enhanced robustness of HBD-400V films compared with the existing diamond based EFE materials rendered these materials of greater potential for applications as high brightness display and multifunctional microplasma.

KEYWORDS: Hybrid granular structured diamond films, ultra-low electron field emission properties, nanographite, bias-enhanced growth process, long-life UNCD films

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 INTRODUCTION Electron field emitters (EFE) play significant roles in micro/nano vacuum devices and optoelectronic applications such as field emission display (FPD), vacuum amplifiers, field emission X-ray sources and electron microscopies.15 Besides the great potential of EFE emitters in these applications, the utilization of high EFE emitters as cathode in microplasma (MP) devices has been demonstrated to be critical in enhancing the performance of the devices.6,7 Generally, the MP devices are useful in many remarkable applications such as ozone production, display panels, and several bioelectronics and biomedical applications.8-10 Several cold cathode materials, which perform very well in emission of electrons, were developed for these applications such as Mo, ZnO, Si, and CNTs.7,11-14 However, these EFE materials mostly suffer from stability issues,1517 as cathode materials in a MP device are subjecting continuous plasma ion bombardment. Researchers are keen to develop new generation of materials for MP devices that can overcome the deficiencies of existing the cathode materials.1820 Diamond based materials have been attractive in the pursuit of good cathode materials for MP device applications due to their unique combination of superior EFE performance and MP related characteristics,21 since diamond possesses negative electron affinity (NEA), superb electron field emission properties, large secondary electron emission behavior (large γ-coefficient) and high bonding strength (robustness).2224 Among the diamond films conventionally synthesized by microwave plasma enhanced CVD (MPE-CVD) process, the ultrananocrystalline diamond (UNCD) films comprising of ultra-nano sized grains with abundant grain boundaries exhibit enhanced conductivity and superior EFE properties than the diamond films with micron sized diamond grians.2527 However, the amorphous carbon (a-C), or trans-polyacetylene (t-PA) phase contained, in the grain boundaries of UNCD films own limited conductivity that still need relatively large field (15~20 V/μm) to turn the EFE 2 ACS Paragon Plus Environment

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process.19,27 Such an EFE behavior is not satisfactory as compare to other carbon based or semiconductor based materials such as CNTs (1.7 V/μm),28 MWCNTs (0.5 V/μm),29 graphene (2.3 V/μm),30 and La/MWCNT (2.4),31 ZnO/SWCNT (1.8 V/μm),32 ZnO/graphene (2.3/ V/μm).33 However, UNCD films can sustain longer term operation, especially when used as cathode materials in a MP devices, compare with those high EFE materials. Further improvement in EFE properties of UNCD films is thus a more efficient approach in enhancing the performance of a MP device. Fortunately, the EFE properties of UNCD films can be enhanced via the modification on the granular structure of UNCD films by various kind of methods, such as in-situ incorporation of doping species,34 post ion implantation process, 35 heavy ion irradiation process,36 bias enhanced growth process37 and plasma post treatment process.38 Among all the processes, which can effectively modify the granular structure of the UNCD films toward the improve EFE characteristics, the in-situ bias enhanced growth of diamond films and the bias enhanced plasma post-treatment (PPT) process have been observed to be more efficient. The enhancement on the EFE properties of UNCD films were presumably owing the increase in conductivity of the films. The other phenomenon, which limited the EFE properties of UNCD films, is the formation of a-C phase between UNCD and Si substrates when the films were grown directly on Si substrates. The presence of a-C interface layer hindered the transport of electrons from Si substrates crossing the interface to diamond films.39,40 There have been great effort in improving the EFE of UNCD films via the suppression on the appearance of a-C phase present in diamond-and-Si interface.41,42 It has been reported that the turn-on field of diamond films can be lowered markedly due to the elimination of a-C phase via the utilization of Au coating as interlayer.41,42 However, it is a complicated process, as the thickness of Au-coating needs to be critically controlled. Insufficient 3 ACS Paragon Plus Environment


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thickness in Au-coating tends to result in the formation of Au islands rather than a continuous Au layer due to the inevitable Au-Si eutectic reaction, whereas too thick Au coating hindered the nucleation of diamond. The search for a simpler yet more effective process for suppressing the formation of interface a-C layer prior to the growth of diamond films is apparently an important issue in the development of diamond films with better EFE emitters. In this paper, we modified the two-step MPE-CVD process to synthesize a diamond film with extremely superb EFE properties. Firstly, in the preparation of UNCD primary layers, we used bias enhancement technique to circumvent the formation of a-C layer at the UNCD-to-Si interface. Subsequently, in the plasma post-treatment process for modifying the granular structure of the primary UNCD layer, we used high negative bias voltage (-400 V) to enhance the kinetic energy of species bombarding the UNCD primary layer so as to result in more conductive grain boundary phase for the hybrid granular structure (HBD) films. The EFE properties of the HBD films were thus improved significantly, achieving a low-turn on field of 1.7 V/μm with large current density of 1.7 mA/cm-2 (at 3.86 V/μm) that are better than the other kind of high EFE diamond films ever reported. The potential applications of these superb EFE materials as FDP and MP devices were demonstrated.  EXPERIMENTAL METHODS The HBD materials were grown on silicon (Si) substrates (1cm x 1cm) in a 2.45 GHz microwave plasma enhanced CVD system (MPECVD, IPLAS-CYRANNUS)). Prior to the deposition of diamond films, the Si was preseeded by ultrasonication in methanol solution containing mixture of nano-sized diamond particles (~5 nm) and Ti powders for 0.75 hr. The Si was ultrasonicated again for 60 s in methanol to remove the excess nanoparticles adhered on the Si substrate. UNCD films were first grown on Si substrate as primary layer using a CH4 (2%)/Ar


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(98%) plasma with a microwave power of 1000 W for 30 min (with 150 Torr pressure). A negative bias of -200 V was applied in the growth of UNCD primary layer to facilitate the nucleation of diamond so as to suppress to formation of a-C phase at diamond-to-Si interlayer. These films were designated as UNCD-200V films. To facilitate the comparison on the effect of this negative bias voltage on improving the UNCD-to-Si interface characteristics, UNCD films were also grown without the application of bias voltage that is designated as UNCD0V films. Subsequently, the granular structure of the UNCD films were modified using a CH4 (2%)/Ar (48%)/H2 (50%) plasma post treatment (ppt) process. During the ppt process, a bias voltage of -400 V was applied to the substrate and the ppt process was proceeded for 30 min. It should be noted that in order to apply a large bias voltage (i.e., -400 V) in ppt process, a special care has to be taken to avoid the occurrence of arching in the plasma. Any sharp edges or debris on the substrates (or holders) were removed by carefully polishing before the ppt-process. A square shallow dish was cut from the stainless steel (SS) substrate holder, which fits the Si substrate, such that the surface of the Si-substrate is in flush with the SS holder. The bias voltage was increased steadily during ppt process. Even the vibration in the environment was minimized. Thus obtained diamond films were designated as hybrid-granular structure diamond (HBD-400V) films. The morphology and bonding structure of diamond films were investigated by using field emission scanning electron microscopy (FESEM; Jeol JSM-6500F) and Raman spectroscopy (λ: 632.8 nm, Lab Raman HR800, Jobin Yvon), respectively. The detailed microstructure and local bonding structure of the HBD-400V films were examined by using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) (Gatan Enfina) in TEM, respectively. The EFE properties of the HBD-400V films were measured using a tunable parallel plate setup in which the cathode-to-anode distance was controlled by a micrometer. The current-voltage (I-V)



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characteristics were measured under a pressure below 10-6 Torr using an electrometer (Keithley 2410). The EFE behavior of the HBD-400V films was extracted from the I-V curves using FowlerNordheim (F-N) model: 43 3

𝐽𝐸𝐹𝐸

𝐴𝛽 2 𝐸 2 𝐵∅2 =( ) exp (− ) 𝜑 𝛽𝐸

(1)

where A = 1.54 × 10-6 A eV/V2 and B = 6.83 × 109 eV-3/2 V/m, JEFE is the EFE current density,  is the field-enhancement factor, E is the applied field and φ is the work function of the electron field emitting materials. The turn-on field (E0) for the EFE process was designated as the point of intersection of the straight lines extrapolated from the low-and high-field segments of the F-N plots, viz., ln (JEFE/E2) versus 1/E curves. The  factor can be estimated from the slope (m) of the F-N plots, viz.  =[6.8 × 103 φ3/2]/m, with the proper work function value (φ) assigned to the emitting materials. To illustrate the potential for the microplasma application of HBD-400V films, a microplasma device with parallel-plate configuration was fabricated, which used indium tin oxide (ITO) glass as anode and HBD-400V films as cathode. The ITO anode was separated from the HBD400V cathode by means of a polytetrafluoroethylene (Teflon) spacer (227 μm in thickness), on which

a circular hole about 3 mm in diameter was cut through to form a cylindrical cavity. The microplasma was ignited by using a DC pulsed voltage under 2 torr of Ar. The current density versus applied voltage (J-V) characteristics were acquired using Keithley 2410 electrometer.  RESULTS (a) Interfacial characteristics of UNCD primary layers grown on Si-substrates Figures 1a and 1b show the morphology of UNCD0V and UNCD-200V films, which were grown on Si-substrates without bias and with bias of -200V, respectively. Both films contain equi-axed 6 ACS Paragon Plus Environment

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diamond grains with very smooth surface, but the UNCD-200V films possesses slightly smaller grains than UNCD0V films. The insets of figures 1a and 1b show the corresponding cross-sectional SEM micrographs, revealing that the thickness of the films is around 400-500nm for both UNCD0V and UNCD-200V films. Figure 2 shows the bonding structure of the UNCD0V and UNCD-200V films (curve I and II, respectively). Both of the films contain broad resonance peak due to the smallness of diamond grains in these films.26 The Raman spectra contain D-band resonance peak near 1350 cm-1, which signifies disordered carbon, and G-band resonance peak near 1580 cm-1, which corresponds to the graphite materials.44,45 A G-band resonance peak at 1600 cm-1 was observed for UNCD-200 V films (curve II, figure 2), indicating that application of -200 V bias voltage for growing UNCD primary layer has induced the formation of nano-graphite phase in these films.44,45 The D*-band resonance peak (1320 cm-1), which typically represents large sized diamond grains, is almost absent, implying, again that these films mainly contain ultra-small diamond grains.46 Additionally, there exist other resonance bands of ν1 at 1140 cm-1 and ν3 at 1480 cm-1, which correspond to the transpolyacetylene (t-PA) phase located at diamond grain boundaries.47

The application of negative bias voltage for growing UNCD primary layer seems not markedly influence the morphology and bonding structure of UNCD films. However, such a bias voltage significantly altered the EFE properties of the UNCD films. The EFE properties of UNCD0V and UNCD-200V films are plotted as emission current density vs applied electrical field (J-E) curves in figure 3a (curves I and II, respectively) and the corresponding F-N plots, log (J/E2)-1/E curves, are shown in figure 3b, from which the turn-on field for initiating the electron field emission process was evaluated (designated as arrows in figure 3b). These curves indicate that the UNCD0V films need large electric field to turn-on the EFE process, i.e., (E0)UNCD(0V) = 15.32 V/μm with emission current density (Je)UNCD(0V) value of 1.20 mA/cm2 at an applied field of 34 V/μm. In contrast, the 7 ACS Paragon Plus Environment


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UNCD-200V films can be turned on at much smaller field, i.e.,(E0)UNCD(-200V) = 6.56 V/μm with (Je) UNCD (-200V)

value of 1.35 mA/cm2 at an applied field of 10.46 V/μm. Moreover, by assigning φ

value of 5.2eV for diamond materials,10 large field enhancement factor ()UNCD(0V) = 672 and ()UNCD (-200V) = 1873 were obtained for UNCD0V and UNCD-200V films, respectively. Previous studies reported that the nucleation of diamond was facilitated by pre-coating a thin layer of nano-sized diamond particulates on Si substrates and the formation of a-C phase at UNCDto-Si interface was circumvented.39 The EFE properties of UNCD films grown directly on Sisubstrates were thus markedly enhanced. Presumably, the same mechanism can be applied for explaining the beneficial effect of negative bias voltage on enhancing the EFE properties of UNCD-200V films compared with those for UNCD0V films, viz. the application of -200 V bias voltage improved the EFE properties of UNCD films via the enhancement on the UNCD-to-Si conductance. To support such an argument, we characterized the diamond-to-Si resistance by using an electrochemical impedance spectroscopic (EIS) technique to measure complex impedance, 𝑧 ∗ =z′ − 𝑖𝑧′′, of the diamond/Si films. For the first, we used a lump-circuit, i.e., parallel combination of a resistance (R) and a capacitance (C), to model each of the bulk diamond, diamond-to-Si interface and Si materials of the diamond/Si films. It should be noted that the UNCD diamond films consist of ultra-small diamond grains separated by grain boundaries, which are too small to be resolved by EIS measurements. The diamond films with ultra-small grains can thus be modelled by a simple RC lump circuit, which is the parallel combination of Rdia and Cdia (designated in the equivalent circuit in inset of figure 4). The contribution of Rdia-Cdia lump circuit to the overall impedance is relatively small as the diamond is relatively conductive (i.e., small Rdia). The Si substrate also very conductive and the contribution of the Si reactance capacitance ( 𝑋𝑆𝑖 ≅


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1 𝑗𝜔𝐶𝑠𝑖

) to the overall impedance of Si materials is also negligible. The Si substrate can thus be

modelled as Rsi in the equivalent circuit (cf. inset, figure 4). The interface located in between the diamond and the Si substrate can also be modelled by a lump circuit, which is the parallel combination of interfacial resistance (𝑅𝑖 ) and capacitance (𝐶𝑖 ) (inset, figure 4.). The interfacial resistance 𝑅𝑖 can be large if there exists a-C phase formed prior to the nucleation of diamond clusters. The interfacial capacitance 𝐶𝑖 is also large due to the small thickness of this interfacial layer. The overall impedance of the diamond/Si materials is thus the summation of the 3 lump circuits, i.e., ∗ ∗ (𝜔) + 𝑧𝑅𝑑𝑖𝑎−𝐶𝑑𝑖𝑎 (𝜔) =z′(𝜔) − 𝑖𝑧′′(𝜔)------------- (2) 𝑧 ∗ (𝜔)=𝑧 ∗ (𝜔)𝑆𝑖 +𝑧𝑅𝑖−𝐶𝑖 ∗ (𝜔) are the impedance of the lump circuit where, 𝑧 ∗ (𝜔)𝑆𝑖 , 𝑧 ∗ 𝑅𝑑𝑖𝑎−𝐶𝑑𝑖𝑎 (𝜔) and 𝑧𝑅𝑖−𝐶𝑖

corresponding to Si-substrates, diamond films and interface layer, respectively. The plots of the imaginary part of impedance 𝑧 ′′ (𝜔) against the real part of impedance 𝑧 ′ (𝜔) at various measuring frequencies form Cole-Cole plots. The special feature of the Cole-Cole plot is that at extremely low measuring frequency, the capacitance of the lump circuits are open circuit and the impedance is approximately the summation of the 3 resistance, i.e., 𝑧 ′ = 𝑅𝑆𝑖 +𝑅𝑖 + 𝑅𝑑𝑖𝑎 ------------------------------------ (3) As the measuring frequency increased, the resistance of the interfacial lump circuits are short 1

circuited by the interfacial capacitance first, since 𝑧 ′′ 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒 ≅ 𝑗𝜔𝐶 ≅ 0, and the overall impedance equals approximately to the sum of Si and diamond resistances i.e., 𝑧 ′ ≅ 𝑅𝑆𝑖 + 𝑅𝑑𝑖𝑎 (we have assumed that the contact resistance is negligible in this EIS measurements). In principle, the 3 lump circuits form three semicircle in Cole-Cole plots. However, since the interfacial resistance/capacitance are relatively large compared with those of the substrate and 9 ACS Paragon Plus Environment


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diamond, the behavior of interfacial lump circuit 𝑅𝑖 − 𝐶𝑖 dominates the Cole-Cole plots, viz. only one semicircle is observed, which is corresponds to the frequency response of the lump circuit of the interfacial layer (figure 4). The right-hand-side interception of semi-circle with abcisa (low frequency end) represents the sum of diamond, Si substrate and interfacial resistance (𝑅𝑑𝑖𝑎 + 𝑅𝑆𝑖 +𝑅𝑖 ) whereas the left-hand-side (high frequency end) interception represents the sum of diamond and Si substrate resistance, 𝑅𝑑𝑖𝑎 + 𝑅𝑆𝑖 (i.e., the interfacial resistance, 𝑅𝑖 was short circuited). Curves I and II in figure 4 shows the Cole-Cole plots of UNCD0V and UNCD-200V films, respectively, revealing that sum of Si and diamond resistance measured at high frequency end of Cole-Cole plots is around hundreds ohms, which is more than one order of magnitude smaller than the interface resistance, which corresponds to the low-frequency end of Cole-Cole plots. These curves indicate that the interface resistance of UNCD-200V films is around 7,900 Ω, whereas the interface resistance of UNCD0V films is around 20,090 Ω. Apparently, the large interfacial resistance (𝑅𝑖 ) in UNCD0 V films is the factor, which hindered the transport of electrons from substrates, crossing the diamond-to-Si interface to diamond, and resulted in inferior EFE behavior for UNCD0V films. (b) Bulk EFE properties of ppt-processed HBD-400V films The UNCD-200V films were then ppt-processes to further enhance the EFE properties of these films. Figure 1c shows the SEM micrograph of HBD-400V films, indicating that these films comprise of large diamond aggregates. Inset in figure. 1c displays the cross-sectional view of HBD-400V films, revealing that the diamond grains are densely packed and the thickness of the HBD-400V films is very uniform (~1 μm). The TEM examination to be shown shortly, will indicate that the HBD-400V films actually consist of large diamond aggregates evenly distributed among the matrix of ultra-nano diamond grains. Curve III in figure 2 indicates that the bonding structure of 10 ACS Paragon Plus Environment

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HBD-400V films is very alike those of the UNCD films, i.e., it contains disordered carbon (D-band), graphite (G-band) and t-PA phase (ν3- bands). The signature of Fd3m, the diamond, (D*-band) is also not observable. However, the TEM micrograph to be show later will reveal that the granular structure of the UNCD films has been altered markedly due to the ppt-process. While the primary UNCD-200V films are too resistive to be measurable by this method, the electrical conductivity of HBD-400V films is very high, viz. σ =22.2 Ω•cm-1 with electron concentration of n=8.6 x1018 cm-3 and mobility of =236 cm-2/Vs, which were measured using standard Hall measurement in van-der Pauw configuration (ECOPIA, HMS-3000). The pptprocess under high bias voltage (-400 V) is very effective in increasing the electrical conductivity and the EFE properties of the diamond films. The EFE properties of HBD-400V films are shown as curve III in figure 3a, whereas the corresponding F-N plot (ln J/E2 vs. 1/E) is shown as curve 3 in figure 3b, from which the turn-on field for the EFE process was estimated (indicated by arrow). These curves indicate that the HBD-400V films possess very low E0-value of 1.72 V/μm and the high Je-value of 1.70 mA/cm2 at the applied field of 3.86 V/μm, which is significantly superior to those of primary UNCD-200V films. It should be mentioned that the EFE properties for the different locations on HBD-400V films were tested (supporting information, figures S1a-c), and the measurements reveal that EFE properties of the HBD-400V films are quite consistent over the whole samples. This implies that the precaution taken in ppt-process is effective in preventing the occurrence of arcing during the ppt-process. Interestingly, by assuming φ = 5.2eV for diamond material, the FN plots (cf. figure 3b) reveal a high field enhancement factor for HBD-400V films of

=2916, compared those of UNCD0V films (672) and UNCD-200V films (1873). Moreover, the lifetime stability of HBD-400V films were tested at a constant applied field of 3.12 V/μm, which corresponds to an applied voltage of 420V and a current density of 0.71 mA/cm2.



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The lifetime was described as the variations in emission current density versus measuring time (Jet). Figure 3c shows that the HBD-400V films can sustain more than 44 hrs (2640 min) without showing sign of current degradation (curve II). In contrast, the UNCD-200V films, which were primary layer used for ppt process, can last only for ~12 hrs (720 min) under the same test voltage (curve I, figure 3c). It is interesting to observe that the ppt process not only enhanced the EFE behavior of UNCD-200V films, but also significantly improved the robustness of the materials. This is contrary to the commonly observed phenomena that the improvement in EFE of a material usually resulted in the degradation in stability (e.g. CNTs).17,18 Moreover, we have performed a test on field emission display behavior of the HBD-400V field emission materials using phosphor coated ITO glass as anode. The display images are shown as inset in figure 3c, revealing that the increase in EFE current density with applied field is mostly due to the increase in emission site density. These results indicate that the HBD-400V films can serve as high bright FPD devices due to the low field required to ignite the EFE process. The performance of a MP device using HBD-400V material as cathode was tested to illustrate the potential of these high EFE materials for practical application. As, in a MP device, the cathode materials were subjected to continuous bombardment of active Ar ions that is considered as harshest environment in device applications. Figure 5a (curve II) shows that the MP devices using the HBD-400V films as cathode can be ignited by a voltage as low as 280V, which corresponds to an applied field of 1.23 V/μm. The MP current density (JMP) increased monotonically with applied voltage, reaching a high value of 2.6 mA/cm2 at an applied voltage 540 V. In contrast, the UNCD200V

films based MP devices needs higher voltage of 390 V to activate the microplasma, and JMP-

value can reach only 1.6 mA/cm2 at 540 V (curve I, figure 5a).


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The plasma density (np) of the MP devices under each applied voltage can be derived from corresponding JMP-value by using modified Child law.48 The np-value of HBD-400V based MP devices is large than those of UNCD-200V based MP devices. Curve II in figure 5b shows that for HBD-400V based MP devices, the np-value also increased monotonically with applied voltage and the np value reaches up to 1.41 × 1017 m-3 at an applied voltage of 540 V, whereas curve I in this figure shows that the np-value for UNCD-200V based MP devices can attain only 8 × 1016 m-3. These results indicated clearly that EFE emitters from the cathode materials contributed large proportion of electrons for ionizing the Ar species in the MP devices. The better EFE properties of the cathode materials are, the higher the plasma density can be attained. However, in practical application, the lifetime stability of the MP devices is actually of more concern than the plasma illumination behavior. The lifetime stability of the MP device was thus characterized. Curve II in figure 5c shows that, when tested under applied voltage of 400 V, which corresponding to an operation current density of 1.4 mA/cm2, the plasma current density for HBD-400V based devices can last for 1152 min without showing sign of current degradation. In contrast, curve I in figure 5c indicates that, when tested under the same applied voltage, the UNCD-200V based devices can last only for 540 min. Some of the microplasma illumination images of the HBD-400V based MP devices are shown as insets in figure 5c. The plasma illumination performance and robustness of HBD-400V based MP devices is overwhelmingly better than those of the devices using other kind of diamond as cathode reported in the literatures (Table I).15, 21, 38 Apparently, the noteworthy enhancement on EFE properties and MP device performance of HBD-400V films are due the modification on granular structure of prime UNCD layer. To understand the mechanism, which enhanced these properties of HBD-400V films, the microstructure of these films were examined by TEM and EELS spectroscopic analyses. Figure 6a shows the



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bright field TEM microstructure of HBD-400V films, indicating that these films consist of large diamond aggregates surrounded by ultranano grains with the size around 5nm. The inset of figure 6a discloses the selective area electron diffraction (SAED) patterns corresponding to HBD-400V films, which contains typical ring pattern corresponding to (111), (220), and (311) lattice planes of diamond. Moreover, the SAED contains very bright central diffuse ring, implying the existence of large proportion of sp2-bonded carbon (graphitic or a-C) in the HBD-400V films. The phenomenon that the HBD-400V films comprise mixture of large and ultra-small diamond grains along with large amount of nano graphitic (ng) layers is highlighted as yellow (diamond) and green (ng phases) in composed dark field (c-DF) TEM micrograph (figure 6b). The c-DF TEM micrograph is the superposition of dark field TEM micrographs, which were acquired using different segment of SAED (designated in inset of Figure 6b). The region with other colored (red, pink, blue and light blue) are also diamond, as they also DF image corresponding to some segment on (111) diffraction ring. The high resolution TEM (HRTEM) microstructure (the structure image) corresponds to the region nearby large diamond aggregates of HBD-400V films (designated as region I in figure 6a.) is shown in figure 6c. Fourier-transformed diffractogram analysis corresponding to the entire structure image is shown as inset in figure 6c (FT0). The FT0 image consists of diffraction spots arranged in a ring geometry, which correspond to the diamond, and donut appearance diffuse ring in the middle, which is indicative of the presence of sp2-bonded carbon, mostly the ng phase. The existence of large diamond grains, small diamond grains and ng phase are highlighted by areas 1, 2 and 3 in figure 6c with corresponding ft images shown as ft1, ft2 and ft3, respectively.

The bonding structure of the HBD-400V films were further investigated by EELS spectroscopy. Figure 6(d) shows that there is an abrupt rise near 289.5 eV (σ* band) and a large deep in the 14 ACS Paragon Plus Environment

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vicinity of 302 eV in core-loss EELS clearly demonstrates the diamond nature of HBD-400V films.49 The nature of the sp2-bonded carbon materials contained in HBD-400V films were further analyzed using plasmon-loss EELS spectroscopy. Figure 6(e) shows that the plasmon-loss EELS spectrum of HBD-400V films contains a broad peak at 33 eV (S4 band), the bulk plasma of diamond, with a shoulder at 23 eV (S2 band), the surface plasma of diamond confirming again that the materials shown in figure 6c are predominantly diamond. The deviation of S2/S4 ratio from 1/√2 implies the existence of a small intensity peak present around 22 eV (S1 band), showing the presence of a-C phase, and the peak around 27 eV (S3 band) indicating the existence of ng phase in the HBD-400V materials.50,51 Presumably, the ng materials are interconnected, creating conducting paths for transporting electrons, that is recognized to be the key factor resulting in superior electrical conductivity and EFE properties for HBD-400V films. The HRTEM micrograph shown in figure 7 reveals the structure image of the regions away from the large diamond aggregates (designated as region II in figure 6(a)), revealing the existence of the ultra-small diamond grains with the size around 5-10 nm. Fourier-transformed diffractogram corresponding to entire HRTEM images (FT0 in figure 7) shows the presence of diamond diffraction spots arranged in a ring geometry and donut shaped central diffuse ring, indicating again, the coexistence of diamond and ng (or a-C) phase. The existence of nano-sized diamond grains and ng phase in this region is highlighted by ft1 and ft2 images corresponding to areas 1 and 2 in figure 7, respectively. It is fascinating to observe that the HBD-400V films contains large sized diamond grains, coexisting with ultranano sized diamond grains, a hybrid granular structure, which is totally different from uniformly small grain microstructure of UNCD-200V films. The ppt-process has effectively altered the granular structure of UNCD films.



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 DISCUSSION The HBD-400V films synthesized by the modified two-step MPE-CVD process possess superior properties, which are even better than other kind of diamond based EFE materials (cf. Table I). Notably, this table reveals that, in general, the diamond films with hybrid granular structure (HiD, table Ib) possess markedly better EFE properties than those with uniform granular structure (UNCD or NCD, Table, Ia). Such a phenomenon can be accounted for by the induction of ng phase by the ppt-processing for UNCD films. The ng phase was induced when ultra-small diamond grains in UNCD primary layer coalesced, resulting in the formation of large diamond aggregates during ppt-process. The coalescence process was triggered due to the interaction of energetic species in the post-treatment plasma, C2, H and CH species, with the grain boundary phase in the UNCD primary layer, breaking the bonding in t-PA (or a-C phase). This generates the active carbon species, which attached to the existing diamond grains, inducing the growth of diamond grains, and at the same time triggered, the formation of crystalline sp2-bonded carbon, the ng phase.27 Such a microstructure evolution process is schematically illustrated in figure 8. Restated, as-grown UNCD films contain the ultra-nano grains about 5-10 nanometer in size. The grain boundaries mainly comprises of t-PA (or a-C) phases, which is highlighted as red background in figure 8. During the ppt process, the grain boundary t-PA (or a-C) phase was dissociated and was converted into ng phase (highlighted as blue background). At the same time, this process was induced the coalescence process, resulting in the formation of hybrid granular structure for diamond films along with the presence of large proportion of ng phase. Moreover, HBD-400V films show superior EFE properties compared with other kind of hybrid granular structured diamond films, regardless whether Si substrates were precoated with the Au interlayer or not (cf. table Ib and Ic). This is due to the factor that marked larger bias voltage (-400 16 ACS Paragon Plus Environment

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V) was applied in ppt-process for the preparation of HBD-400V films, whereas, in the preparation of other kind of HiD films, either 0 V or lower bias voltage (

High-Performance Electron Field Emitters and ... - ACS Publications

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