Evolution of Granular Structure and the Enhancement of Electron Field

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Functional Inorganic Materials and Devices

Evolution of granular structure and the enhancement of electron field emission properties of nanocrystalline and ultrananocrystalline diamond films due to plasma treatment process Wei-En Chen, Chengke Chen, Chien-Jui Yeh, Xiaojun Hu, Keh-Chyang Leou, I-Nan Lin, and Chii-Ruey Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02799 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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ACS Applied Materials & Interfaces

Evolution of Granular Structure and the Enhancement of Electron Field Emission Properties of Nanocrystalline and Ultrananocrystalline Diamond Films due to Plasma Treatment Process

Wei-En Chen1, Chengke Chen2, Chien-Jui Yeh3, Xiao-June Hu2, Keh-Chyang Leou3, I-Nan Lin4*, Chii-Ruey Lin1* 1

Department of Mechanical Engineering and Institute of Manufacturing Technology, National Taipei University of Technology, Taipei, Taiwan 2

College of Materials Science and Engineering, Zhejiang Industrial University of Science and Technology, Zhejiang, China

3

Department of Engineering System and Science, National Tsing-Hua University, Hsin-Chu, Taiwan 4

Department of Physics, Tamkang University, New-Taipei, Taiwan

*Chii-Ruey Lin: [email protected]; *I-Nan Lin: [email protected]

ABSTRACT: The present work reports the plasma post treatment (PPT) process that instigates the evolution of granular structure of nanocrystalline diamond (NCD), consequently conducing the enhancement of the electron field emission (EFE) properties. The NCD films contain uniform and nano-sized diamond grains (~20 nm) with negligible thickness for grain boundaries that is distinctly different from the microstructure of ultra nano crystalline (UNCD) films with uniformly sized ultra-nano-diamond grains (~5 nm) having relatively thick grain boundaries (~0.1 nm). The turn-on of the electron field emission (EFE) process occurs at (Eo)NCD = 24.1 V/µm and (Eo)UNCD = 18.6 V/µm for the pristine NCD and UNCD materials, respectively. The starting diamond films granular structure largely influenced the microstructure evolution behavior and EFE properties of the materials subject to plasma annealing. The CH4/(Ar-H2) ppt-process leads to formation of hybrid granular structured diamond (HiDNCD) via isotropic conjoining of nano-sized diamond grains, whereas the CH4/N2 ppt-process leads to the formation of acicular granular structured diamond films (NNCD) via inducing aeolotropic growth of nano-diamond grains. While both of the HiDNCD films contain hybrid granular structure, which contain larger proportion of nanographite phase and result in improved EFE 1

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properties, viz. (Eo)HiD-UNCD = 7.7 V/µm. In contrast, when the films were CH4/N2 ppt-processed, the acicular diamond grains were formed for NUNCD films, but these pretends no carbon nanoclusters attached to the diamond grains and the nano-graphitic layers encasing diamond cores are not crystallized very well; as compared with NNCD films. Therefore, the NNCD films exhibit slightly inferior EFE properties than the NUNCD films, viz. (Eo)N-UNCD = 5.3 V/µm and (Eo)N-NCD = 11.8 V/µm. The difference in EFE properties for ppt-processed NCD and UNCD films corresponds to the dissimilar granular structure evolution behavior due to these films that is, in turn, resulted from the distinct different microstructure of the pristine NCD and UNCD films.

KEYWORDS: Nanocrystalline diamond films, Ultrananocrystalline diamond films, Transmission Electron Microscopy, Electron Field Emission properties, Post-annealing plasma treatment process

I.

Introduction Diamond film possesses remarkable field emission properties with added advantage

of exemplary characteristics namely good hardness, chemical stability and prospect of doping to improve p-/n- type conductivities. Owing to these distinctive and beneficial properties , the diamond films have gained interest among researchers in past decades.1-3 Unlike conventional microcrystalline diamond (MCD) films with large and faceted diamond grains (~1 µm) that are grown in the presence of CH4/H2 plasma, , the ultrananocrystalline diamond (UNCD) films with ultra-small diamond grains (~ 5 nm) are grown in the presence of CH4/Ar plasma.4-7 Accounting to the higher proportion of the grain boundary phase, the conductivity and electron field emission (EFE) properties of UNCD films surpasses that of the MCD films.. A noticeable expansion of width of the grain boundary region was resulted, owing to the inclusion of N2 in CH4/Ar plasma in the preparation of UNCD films that primarily improved the n-type conductivity to an elevated range of σ=143 S/cm.8-10 Gruen et. al.11 reported that the films grown in the presence of CH4/[80%Ar-20%N2] plasma as well as at a substrate temperature >700℃ results in the creation of acicular diamond grains, which possess a 2


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high conductivity of σ=200 S/cm. Moreover, Sankaran et. al.12 studied in detail the effect of substrate temperature (in CH4/N2 plasma) on the diamond film’s microstructure and observed that only at necessarily higher substrate temperature (>700℃) can the acicular diamond grains be obtained. Moreover, the intrinsic reason for the observed high conductivity in the diamond films grown in CH4/(100%N2) plasma (at 700℃) was stated as the explicit structure encompassing the nano-graphitic phase surrounding the acicular diamond grains. Further, it was found that the aeolotropic growth of diamond grains forming acicular nanostructures is primarily due to the presence of C2 and CN species in the plasma. Notably, one of the drawback of these processes is the requirement of higher substrate temperature for the growth of diamond with acicular grains. In this regard, it is obviously necessary to design a new stratagem for the preparation of acicular granular structured diamond without the need of high substrate temperature. Nevertheless, the limitation imposed on the EFE properties of these films due to the incompetent conductivity of the sp2-bonded carbon present in the grain boundaries of the UNCD films is yet a challenge. Wang13 and Chen14 observed that the post-annealing treatment of UNCD films in CH4/[Ar(50%)-H2(50%)] (or CH4/N2) plasma can induce the conjoining of ultra-small diamond grains thereby creating nanographitic clusters, which consequently improved the EFE properties of UNCD films. Additionally, it is disclosed that the post-annealing process in the presence of CH4/Ar/H2 plasma contained C2 and CH species and only caused the creation of equi-axed diamond masses via the conjoining process. Contrariwise, CH4/N2 plasma containing primarily the C2 and CN species tends to induce aleotropic growth of diamond. The latter post-treatment process can thus prompt the creation of acicular diamond grains which can even more significantly enhance the EFE properties of the films.15, 16 While it is observed that post-annealing plasma treatment (ppt) of diamond films can effectively furnish the UNCD films with improved conductivity and greatly enhance the EFE performance of the materials. It is of great technological importance to understand whether or not such kind of ppt-process can also be applied for enhancing conductivity/EFE properties of nanocrystalline diamond films, which possess different granular structure.

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In this paper, we performed CH4/(Ar-H2) and CH4/N2 ppt-processes on nanocrystalline diamond (NCD) films prepared by a microwave plasma jet chemical vapor deposition (MPJCVD) process with distinctly different granular structure from conventional UNCD films prepared by microwave plasma enhanced CVD process. The EFE properties of NCD films were markedly improved attributable to these ppt-processes but in a different manner from these of UNCD films. Such a different behavior is observed to be closely related to the unique microstructure of the pristine diamond films. How the microstructure of pristine diamond films influenced the granular structure evolution behavior of diamond due to the ppt-process was methodically investigated using transmission electron microscopy (TEM). Furthermore, the obvious mechanism which resulted in the different EFE properties of the ppt-processed diamond films has been elucidated.

II.

Experimental method The nanocrystalline diamond (NCD) films were synthesized on Si substrates with

bulk conductivity (σ) of 10 S/cm using a microwave plasma jet chemical vapor deposition (MPJCVD) process, which was described in detail elsewhere.17, 18 In brief, the microwave power of 700 W was fed via a specially designed antenna to a large chamber to excite a plasma jet consisting of CH4(3%)/H2 (balanced) gas mixture (at 700 torr and 200 sccm). The Si substrates were pre-seeded in methanol solution containing nano-sized diamond powders (∼5 nm) and Ti powders (∼32.5 nm) mixtures via ultrasonication before the growth of NCD films.19 The process of post-annealing plasma treatment (ppt-process) was proceeded in CH4(3%)/(Ar50%-H250%) plasma or CH4(6%)/N2 plasma for 30 min (50 torr, 1200 W). A was applied onto The Si substrates were subjected to the application of negative bias voltage (- 250 V) throughout the ppt-process. The plasma used for the ppt-process was monitored using an optical emission spectroscopy (OES, BWTEK). The structural bonding of the materials in the film was analyzed using visible-Raman spectroscopy (λ: 632.8 nm, Lab Raman HR800, Jobin Yvon). The morphology of the films was observed using a scanning electron 4


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microscopy (SEM, Jeol JSM-6500F). Detailed microstructural analysis of the films was investigated using transmission electron microscopy (TEM, Jeol 2100F). A set-up comprising of a tunable parallel plate and a Cu-rod (3 mm in diameter) acting as the anode, where the cathode-to-anode distance could be controlled by a micrometer was utilized to measure the EFE properties of the films. Electrometer (Keithley 2410) was used to observe the current-voltage (I-V) characteristics and the experiment was carried out under pressure below 10-6 Torr. Subsequently, using the Fowler-Nordheim (F-N) model, the EFE parameters were derived from the acquired I-V curves .20 The as The point of intersection of the straight lines extrapolated from the low- and the high-field segments of the F-N (ln (Je/E2) vs. 1/E) plots corresponds to the turn-on field (E0) , where Je and E are the EFE current density and applied field, respectively.

III.

Results and Discussion The SEM morphology of pristine NCD films as shown in Figure 1 reveals the

presence of very small diamond grains and their surface appears to be very smooth. Curve 1 in Figs. 2a indicates the Raman spectra of pristine NCD films. The NCD films consist of very broad Raman peaks, which implies that the diamond grains in the prepared films are quite small. There is a D-band detected at 1,350 cm-1 and a G-band detected at 1,580 cm-1, corresponding to disordered carbon and nanographite phases, respectively.21, 22 The presence of ν1 at 1,140 cm-1 and ν3 at 1,480 cm-1 correspond to transpoly-acetylene phases,23,

24

which are probably positioned along the grain

boundaries in the diamond films. The Raman peaks corresponding to Γ2g of diamond, D*-band at 1,332 cm-1, is only scarcely recognizable for NCD films (curve 1, Fig. 2a). The invisibility of D*-band is due to the much stronger florescence for sp2-bonded carbon in comparison to the sp3-bonded carbon. To more clearly investigate the occurrence of sp3-bonded carbon in the present materials under consideration, NEXAFS spectra of pristine NCD films was examined and were shown as curve I in Fig. 2b. The pristine NCD films shows an rapid escalation near 289 eV (σ*-band) with a profound dip near 302 eV, which are an authentic sign of sp3-bonded carbon, confirming that the films are predominately diamond.25,26 A small lump is present near 5



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284.5 eV (π*-band) for pristine NCD films27,28 (cf. curve I, Fig. 2b), implying that the pristine NCD films encloses a rare part of sp2-bonded carbon, most probably in the grain boundary region. Figure 3a shows the bright field (BF) TEM microstructure of pristine NCD films, revealing that these films contain large diamond masses about 100 nm in size evenly spread amongst the sample. Inset in Fig. 3a shows selected area electron diffraction pattern (SAED), corresponding to the BF-TEM micrograph. The SAED contains (111), (220) and (311) diffraction ring pattern, signifying the small and randomly oriented diamond grains in NCD films. It should be noted that although most of the areas in micrograph show grey contrast, they are also diamond grains but are oriented away from a zone-axis, diffracting very few electrons and thus show low contrast. The phase composition of the films is well illustrated by the composed dark field (C-DF) TEM micrograph in Fig. 3b. The C-DF TEM micrograph is the superposition of several dark field TEM micrographs taken using several different portions of (111) ring (indicated in inset of Fig. 3b). The detailed microstructure of diamond clusters in NCD films was further examined using high resolution TEM. Figure 3c shows the structure image of the region designated as a dashed square in Fig. 3a. It is intriguing to observe that the large diamond cluster in NCD films is actually a cluster of nano-sized diamond grains (~20 nm in size). Fourier transformed diffractrogram corresponding to the whole structure image (FT0 image) indicates that these diamond grains are oriented to nearly the same orientation, i.e., [110]. The FT0 image contains very few diffuse rings in the center, implying that there is a very small amount of sp2-bonded carbon contained in the grain boundaries of NCD films. The Raman, NEXAFS, and TEM analyses reveal the fact that the NCD films are predominantly diamond, which form clusters and each cluster consists of nano-sized diamond grain about 20 nm in size with very few grain boundary phase. Fig. S1a in supplementary information indicates that the UNCD films also contain very smooth surface, whereas the Fig. S2 in supplementary information shows the TEM microstructure of UNCD films. The UNCD films contain ultrasmall diamond grains with relatively wide grain boundarie (~0.1 nm). Figure S3 in supplementary information shows that the NCD films contain essentially the same bonding structure as the UNCD films and only the relative intensity of the peaks is slightly different. 6


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The distinctly dissimilar granular structure of these two materials lead to a large difference in EFE behavior, which is shown in Fig. 4a with the corresponding Fowler-Nordheim plots shown in Fig. 4b. In these figures, the EFE properties of pristine UNCD films were also presented for ease of comparison. The EFE process of NCD films with the bulk conductivity of σ=0.5 S/cm can be turned on at (E0)NCD=24.1 V/µm, reaching EFE current density of (J0)NCD = 0.4 mA/cm2 at an applied field of Ea=35 V/µm (curve I), whereas the EFE properties of UNCD films (σ=1.0 S/cm) can be initiated at a smaller field ((E0)UNCD=18.6 V/µm), achieving (J0)UNCD = 0.4 mA/cm2 at an applied field of Ea=32 V/µm (curve II). The EFE parameters of the pristine NCD and/or UNCD films along with bulk conductivity of the materials are summarized in Table I. The EFE behavior of the NCD and/or UNCD films were also plotted in semilogarithmic scale in Fig. S4 of the supporting information. The EFE parameters (mainly the turn-on field) of the NCD and/or UNCD films evaluated from the curves in semi-logarithmic scale are similar with those deduced from Fowler-Nordheim plots (cf. Table I). The better EFE properties for pristine UNCD films compared to the pristine NCD films can apparently be ascribed to the presence of a large proportion of trans-polyacetylene (t-PA) grain boundary phase, surrounding each of the nano-sized (~5 nm) diamond grains in UNCD films. The t-PA phase in these grain boundaries is more conductive than the diamond grains and can transport electrons more easily, hence establishing better EFE properties for UNCD films in comparison to the NCD films, as the NCD films contain larger diamond grains (~20 nm) with negligible grain boundary phase. The dissimilar granular structure of the pristine NCD and UNCD diamond films not only resulted in distinctly different EFE behaviour, but also led to dramatically different granular structure evolution behavior (along with different EFE properties) when they were post-treated under CH4/(Ar-H2) (or CH4/N2) plasma. So as to cognize the granular structure evolution behavior, we first characterized the ppt-process plasma, using optical emission spectroscopy. Curve I in Fig. 5a indicates that the OES of CH4/(Ar-H2) plasma is predominated with H species (Hα and Hβ) with C2 and CH species as minor ingredients. In contrast, curve II in this figure shows that the CH4/N2 plasma contains mainly C2 and CN species with a small proportion of N2 species and no H species. It should be mentioned that the CH peaks at 384 nm, which are located at a 7



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very close position to that of CN peaks, cannot be clearly differentiated from CN peaks. They were so designated because of the main ingredients in the starting gas containing CH4, H2, and Ar in the former (no N2), and CH4 and N2 in the latter (containing abundant N2). Figure 5(b) shows the bias current-time characteristics during post-annealing plasma treatment (ppt) process of NCD films. The bias current represents the electrons emitted from diamond due to the negative bias voltage applied to the Si substrates and is a good parameter for monitoring the ppt-process. It should be mentioned that in conventional bias enhanced growth (beg) of diamond film,29 the rise of bias current was delayed about 3-5 min after switching on the bias voltage that is ascribed to the incubation of diamond nuclei on the Si substrates. The saturation value of bias current is reached after a while that is attributed to the full coverage of diamond nuclei on the substrates. In the bias enhanced ppt-process for the NCD films (Fig. 5b), the bias current jumped to a saturation value instantaneously after starting the ppt-process that is due to the fact that the starting materials were by this time fully covered by the pristine NCD films. The bias current vs. time characteristics do not seem markedly different for the plasma used for the ppt-process. The Ibias-t characteristics during the ppt of UNCD film, as shown in Fig. S5 in supporting information, indicated that they are essentially of the same characteristics as those for NCD films (cf. Fig. 5b). That is, the constituent produced in the plasma depends mainly on the ingredient in the ppt gas mixture and is insensitive to the features of diamond films (no matter whether they are NCD or UNCD films). However, the ppt-processes resulted in totally different behavior for the evolution of microstructure for the two films. Figure 1(b) shows that the surface of NCD films was roughened, indicating the marked transformation in granular structure attributable to the ppt-process in CH4/(Ar-H2) plasma (cf. Fig. 1a for SEM of pristine NCD films). These films were designated as NCD-derived hybrid granular structured diamond (HiDNCD) for the occurrence of duplex granular structure, which will be shown shortly. In contrast, Fig. 1c indicates even more rigorous transformation in granular structure occurred when the pristine NCD films were ppt-processed in CH4/N2 plasma. These films consist of acicular diamond grains about 10-20 nm in diameter (hundreds nm in length) and were designated as NNCD films. Curve II in Figs. 2a and 2b reveal, respectively, that the HiDNCD material possesses slightly different Raman and 8


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NEXAFS spectra from those of the pristine NCD films. Curve II of Fig. 2a shows that the D*-band (at 1332 cm-1) is more clearly detected in Raman spectrum of HiDNCD, which infers the increase in diamond grains in these samples due to CH4/(Ar-H2) plasma ppt-process. This Raman spectrum still contains prominent D-, G-, ν1- and ν2-bands, indicating the co-existence of diamond grains with the t-PA grain boundary phase in HiDNCD films. Moreover, in the NEXAFS of HiDNCD films (curve II of Fig. 2b), the π*-band decreased markedly to a level almost not observable, which is presumably due to the marked decline in the proportion of grain boundaries that resulted from the growth of diamond grains. These Raman and NEXAFS spectra indicate that the bonding structure and phase contents of NCD films were altered markedly due to the CH4/(Ar-H2) plasma ppt-process. In contrast, curve III in Fig. 2a points out that the t-PA peaks (ν1 and ν3) were no longer observable in Raman spectrum of NNCD films due to the CH4/N2 plasma ppt-process, suggesting that the grain boundary phase (t-PA) in NCD films has been effectively eliminated, whereas the corresponding NEXAFS spectrum (curve III in Fig. 2b) implies that the CH4/N2 plasma ppt-processed NCD materials are still predominantly diamond. That is, the ppt-process in either CH4/Ar-H2 or CH4/N2 plasma modified markedly the surface morphology and the granular structure of the NCD films. The bulk conductivity of ppt-processed NCD films was increased to σHiD-NCD=1.2 S/cm and σN-NCD=21.0 S/cm. Figure 6(a) shows that the turn-on field for initiating EFE process was lowered from (E0)NCD=24.1 V/µm for pristine NCD films to around (E0)HiD-NCD=12.3 V/µm, (E0)N-NCD=11.8 V/µm for CH4/(Ar-H2), and CH4/N2 ppt-processed diamond films, respectively. That is, the bulk conductivity and the EFE properties of NCD films were evidently enhanced owing to the transformation in granular structure. It is interesting to observe that, for UNCD films, the CH4/(Ar-H2) plasma ppt-process also roughened the surface, and the CH4/N2 plasma ppt-process also led to the formation of acicular diamond grains (Figs. S1b and S1c supplementary information) that are similar to the cases for the ppt-process of NCD films. The Raman and NEXAFS of UNCD films also change in similar manner due to the ppt-processes as they do for NCD films, as shown in curves II and III in Fig. S3a (supplementary information) for the modification on Raman and curves II and III in Fig. S3b 9



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(supplementary information) for that on NEXAFS of UNCD films. Again, curves II in Figs. S3a and S3b imply that the ppt-process in the presence of CH4/(Ar-H2) plasma induced the growth of diamond grains to a larger size and decreased the proportion of grain boundaries, whereas curves III in Figs. S3a and S3b infer that the CH4/N2 plasma ppt-process changed the geometry of diamond grains but with the size of diamond grains remained at a small size. There presents enormous amount of sp2-bonded carbon. However, these ppt-processes improved the EFE properties of UNCD films in a different way dissimilar to that of NCD films. The CH4/(Ar-H2) plasma ppt-process increased the bulk conductivity of UNCD films to σHiD-UNCD=68 S/cm and reduced the turn-on field from (E0)UNCD=18.6 V/µm for pristine UNCD films to (E0)HiD-UNCD=7.7 V/µm (curve II, Fig. 6b) for HiDUNCD films, whereas the CH4/N2 plasma ppt-process increased the bulk conductivity of the films to even a higher range of σN-UNCD=186 S/cm and lowered the E0-value even more, to (E0)N-UNCD=5.3 V/µm (curve III, Fig. 6b). The EFE parameters of these ppt-processed NCD and/or UNCD films are summarized in Table I together with the corresponding bulk conductivity values. The above results indicate that plasma ppt-processes enhanced the EFE properties of NCD films less efficiently than that on the EFE behavior of UNCD films. Apparently, it occurs due to the less effective transformation on the granular structure of NCD films than that on UNCD films. However, the SEM microstructure analysis in Figs. 1b and 1c cannot elucidate the genuine mechanism, which enhanced the EFE properties of the NCD and films in a different manner from the improvement of these on UNCD films owing to the ppt-process. To understand why the ppt-process enhanced the EFE properties of NCD and UNCD films in a different way, the detailed microstructures of these ppt-processed films were investigated by the TEM. Figures 7(a) and 7(b) show, respectively, the BF and C-DF TEM micrographs of HiDNCD films, which were NCD films ppt-processed in CH4/(Ar-H2) plasma. These micrographs indicate that the HiDNCD films have large diamond grains about 100 nm in size (dark contrast in BF image). Note that the material in the other regions are also diamond grains but are oriented in non-diffraction direction and thus show low contrast that can be verified by tilting the samples during TEM investigation (not shown). Although the BF-TEM microstructure of HiDNCD films looks like those for pristine NCD films (cf. Figs. 3a and 3b), the high resolution TEM micrograph shown in Fig. 7c indicates that they are 10


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actually quite different. The aggregate in pristine NCD films consist of an agglomeration of nano-diamond grains (~20 nm in size, cf. Fig. 3c), whereas those in HiDNCD films (Fig. 7c) are of single large diamond grains, i.e., the nano-size diamond grains in the clusters have conjoined together due to CH4/(Ar-H2) ppt-process. Such a conjoining process eliminated the grain boundaries between nano-sized diamond grains, which contain amorphous carbon (a-c) and t-PA phase. But it induced the formation of nano-graphitic phase around the resultant large diamond clusters, as indicated by arrows in high resolution TEM micrograph (Fig. 7c). The nano-graphitic phases surrounding the large diamond clusters in HiDNCD films are much more conductive than the a-C phase (or t-PA phase) distributed among nano-sized diamond grains in pristine NCD films, and thus resulted in improved conductivity and EFE properties for HiDNCD films. It should be reminded that, when ppt-processed in the same plasma (CH4/(Ar-H2)), the HiDUNCD films derived from UNCD films unveiled much superior EFE properties than the HiDNCD films. Apparently, the key factor is that the two materials, HiDNCD and HiDUNCD, possess a markedly different granular structure from each other. Figures S6(a) and S6(b) of the supplementary information show, respectively, the BF and C-DF TEM micrographs of the HiDUNCD films, revealing that these films contain large diamond grains above 100 nm in size evenly spread all over the samples that is similar to BF(C-DF) TEM micrographs for HiDNCD films (cf. Figs. 7a and 7b). However, the comprehensive examination using high resolution TEM micrographs indicate the obvious difference in granular structure of the two materials. The HR-TEM micrograph for HiDUNCD films, as shown in Fig. S6c of the supplementary information, reveals that when the nano-sized diamond grains in UNCD films conjoined due to the CH4/(Ar-H2) ppt-process in some region (such as region A in Fig. S6c), the region nearby the large diamond clusters (such as region B in Fig. S6c) remained as ultra-small grain granular structure and there presented thick nanographitic phase between them (indicated by arrow in Fig. S6c). In contrast, for HiDNCD films, the nearby region B in Fig. 7c, neighboring to large diamond clusters in region A in Fig. 7c, is another diamond aggregation (region B) with very thin boundaries between them (cf. Fig. 7c). Such a phenomenon can be confirmed by tilting the samples during TEM investigation (not shown). 11



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The ppt-process using CH4/N2 plasma has been shown to be an even more effective way for enhancing the EFE properties of the diamond films via the conversion of spherical diamond grains in the films into acicular ones (cf. Figs. 1b and 1c). However, the extent of improvement on EFE properties is less marked on NNCD films than that on NUNCD films. The evolution of microstructure is again investigated for understanding such a phenomenon. Figures 8(a) and 8(b) show the BF and C-DF TEM micrographs for NNCD films, which were CH4/N2 ppt-processed NCD films. NNCD films possess evidently superior EFE properties than the pristine NCD (and HiDNCD) diamond films due to the formation of acicular diamond grains. Although the NNCD films contain a core-shell structure with acicular diamond grains as the core and a nano-graphitic layer as the shell (an idea granular structure for transporting electrons), the NNCD films exhibit much inferior EFE properties to the NUNCD films, i.e., (E0)N-UNCD=5.3 V/µm and (E0)N-NCD=11.8 V/µm (cf. Figs. 6a and 6b). TEM micrographs shown in Figs. 8a and 8b indicate that such a phenomenon stems from the imperfection in core-shell granular structure for NNCD films compared with those for NUNCD films. While most of the NUNCD diamond grains are thin and lengthy (cf. Figs. S7a and S7b), the NNCD acicular diamond grains are thicker and shorter (cf. Figs. 8a and 8b). Moreover, some acicular diamond grains in NNCD films were attached with diamond nanoclusters, which are indicated by arrows in Fig. 8a. Such kind of “dirt” hindered the transport of electronics. TEM structure images shown in Fig. 8c points out that even for the clean acicular diamond grains in NNCD films, which are free of attached “dirt”, the perfection in core-shell structure for NNCD grains is not as good as that in NUNCD film (Fig. S7c). The FT0- image corresponding to structure image for NNCD films (inset, Fig. 8c) contains a full-moon shaped central diffuse ring, implying that the matrix of the NNCD films is predominantly a-C phase. In contrast, the FT0b-image corresponding to those for NUNCD films (inset, Fig. S7c) contains a central diffuse ring of donut geometry, implying that the matrix of the NUNCD films is mostly nano-graphitic phases. Such characteristics were further illustrated by the more ordered and clean nano-graphitic fringes surrounding acicular diamond core for NUNCD films (Fig. S7c), compared with blurred and broken nano-graphitic fringes cores for NNCD films (Fig. 8c). That is, although CH4/N2 can induce aeolotropic growth of nano-sized diamond grains for NCD films, the acicular diamond grains and the associated 12


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nanographitic phase in the two films have slightly different degrees of perfection. The subtle differences in granular structure of the NNCD and NUNCD films lead to dramatically dissimilar EFE behavior. Since the UNCD films is composed of ultra-small diamond grains (~5 nm, Fig. S2) with proportionately broad grain boundaries (containing a-C phase) that allows the aeolotropic growth of diamond grains to proceed more freely. The sufficient supply of sp2-bonded carbons in grain boundaries results in better crystallization for nanographite phase. On contrary, the NCD films is composed of larger diamond grains (~20 nm, Fig. 3c) with much thinner grain boundaries. Therefore, there is insufficient supply of sp2-bonded carbons for the formation of nano-graphitic layers, and the aeolotropic growth process is hindered. Therefore, the NUNCD films possessed better granular structure and thus exhibited superior EFE properties compared with NNCD films. It should be mentioned that the quantitative analysis of the phases in NCD films from their Raman spectra is extremely difficult owing to the broadness of the Raman peaks that even rendered large ambiguous in assigning the phases exist in these materials. The large broadness in Raman peaks arise from the small feature size of the phase constituents in these films.30-37 Higher fluorescence of Raman signal corresponding sp2-bonded carbon when compared to sp3-bonded carbon makes the analysis of diamond materials even more difficult. Such kind of difficulty can partly solved by using multi-wavelength Raman spectroscopy as the laser of short wavelength is less sensitive in inducing the fluorescence due to the presence of sp2-bonded carbon.33,34 However, the quantitate analyses on the phase constituents in UNCD films is still quite difficult. In this study, we utilized NEXAFS and TEM analyses as complementary analyzing technique to understand the bonding structure for the pristine and ppt-processed NCD (and/or UNCD films). The NEXAFS supply the bulk information on sp3-banded carbon (the σ*-band) and sp2-banded carbon (the π*-band), whereas the TEM analyses provides the geometry and structure of each phase constituents. By combining these analyses together, we can get more detailed information on the microstructure of the materials and gain better understanding on how the granular structure of NCD films evolved into HiD and N series diamond films.

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Moreover, it should be mentioned that many different groups38-44 have worked on the local EFE behavior of UNCD films and observed that electrons were primarily emitted from the grain boundaries amidst the asperities of diamond surfaces. It is because that the electrons, which were supplied by the bottom external electrodes (through the substrates) were transported along the path with the smallest resistance in the diamond materials upward to the top surface of the UNCD films and were then emitted there. The grain boundaries will serve as the transport path as they are of better conductivity than the diamond grains and better conductivity of grain boundaries will apparently reduce the applied field needed to induce EFE process, leading to smaller turn-on field and improve the overall EFE properties. However, the locally measured emission properties cannot be directly correlated with the macroscopic field emission behavior due to several different aspects in the measuring setup. In SPM-based local EFE measuring setup, the measuring tip and the emitting surface is very close (smaller than 100 nm) such that the anode and cathode (hundreds of nanometers) were arranged almost in parallel with each other and there is no field enhancement effect exerted onto the diamond films. Moreover, the precise control of the gap concerning anode-and-cathode is rather difficult, the emission current is comparably smaller and the precise estimation on measuring area is difficult that inhibits the precise quantitative analyses of the local J-E behavior of the materials. In contrast, in macroscopic measuring setup, there always presence large field enhancement factor due to small size of emitting sites. The measured current and field can be more precisely evaluated. Therefore, the direct correlation between the local EFE behavior evaluated using SPM-based techniques are extremely difficult to compare with those measured macroscopically. However, it provides very valuable information on the role of grain boundary phase on the conductivity/EFE behavior of the materials. IV.

Conclusion The evolution of granular structure of NCD films due to post-annealing plasma treatment

(ppt) process was studied in comparison with the UNCD films to better understand the underlying dissimilar mechanism that lead to the superior EFE properties of diamond materials. Starting with NCD films of nano-sized diamond grains (~20 nm) with negligible thickness of 14


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grain boundaries, the diamond grains grew to a larger size (~100 nm), forming large diamond clusters and resulting in a hybrid granular structured diamond (HiDNCD) films when ppt-processed under CH4/(Ar-H2) plasma. The EFE properties of NCD films were enhanced markedly, i.e., the turn-on field was reduced from (E0)NCD=24.1 V/µm for pristine NCD films to (E0)HiD-NCD=12.31 V/µm for HiDNCD films (the bulk conductivity, σ, was increased from 0.5 S/cm to 1.2 S/cm). When the NCD films were ppt-processed under CH4/N2 plasma, a acicular diamond grain granular structure around tens of nanometers in diameter and hundreds of nanometers in length were resulted, and the turn-on field for initiating EFE properties was lowered to (E0)N-NCD=11.0 V/µm (σ was increased from 21.0 S/cm). In contrast, for UNCD films of ultra-nano grain (~5 nm) granular structure (with ~0.1 nm grain boundaries), a hybrid diamond granular structure (HiDUNCD) was obtained and the EFE properties of the diamond films were evidently upgraded due to the CH4/(Ar-H2) ppt-process. The turn-on field was reduced from (E0)UNCD=18.6 V/µm to (E0)HiD-UNCD=7.7 V/µm (the σ was increased from 1.0 S/cm to 68.0 S/cm). Compared with HiDNCD films, which contain conjoined uniformly large diamond grains about 100 nm in size in adjacent to one anothers, the HiDUNCD films consist of large clusters (~100 nm) evenly spread among the matrix of ultra-nano diamond grains (~5 nm). Nanographitic layers were induced due to conjoined of nano-grains which occurred in the CH4/(Ar-H2) ppt-process for both HiDNCD and HiDUNCD films. But the crystallinity of nanographitic phase was better when the granular structure evolved from ultra-nanocrystalline diamond granular structure (i.e. HiDUNCD), resulting in better EFE properties for HiDUNCD films compared with those evolved from nano-crystalline diamond granule structure (i.e. HiDNCD). On the other hand, both NNCD and NUNCD films, which were NCD and UNCD films ppt-processed in CH4/N2, respectively, consisted of acicular diamond grain granular structure. However, the acicular diamond grains contained in NNCD films were thicker, shorter, and attached with some nano-carbon clusters, compared with thinner, larger, and cleaner acicular diamond grains for NUNCD films that led to superior EFE properties for NUNCD films. The formation of superior granular structure for NUNCD films compared with that for NNCD films is attributed to unique granular structure of starting materials. The turn-on field for initiating EFE properties was as low as (E0)N-UNCD=5.3 V/µm and (E0)N-NCD=11.8 V/µm with bulk conductivity of σN-UNCD=186 S/cm and σN-NCD=21.0 S/cm for NUNCD and NNCD films, respectively.

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Acknowledgement: The authors would like to thank the financial support of Ministry of Science and Technology, Republic of China, through the project No. MOST 105-2221-E-027-050 and MOST 106-2221-E-159-00.

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Table I The EFE properties and bulk conductivity of the pristine and ppt-processed NCD and/or UNCD films materials

ppt-process

σ∗ (S/cm)

E0*

JEFE (Ea)*

(V/µ µm)

(V/µ µm)

NCD

-

0.5

24.1

35 (V/µm)

HiDNCD

CH4/(Ar-H2)

1.2

12.3

25 (V/µm)

NNCD

CH4/N2

21.0

11.8

20 (V/µm)

UNCD

-

1.0

18.6

32 (V/µm)

HiDUNCD

CH4/(Ar-H2)

68.0

7.7

17(V/µm)

NUNCD

CH4/N2

186.0

5.3

8 (V/µm)

∗

σ: the bulk conductivity measured by 4 probe technique; E0: the turn-on field for EFE process; JEFE(Ea): The applied field needed to induce 0.4 mA/cm2 EFE current density.

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Figure captions

Fig. 1 SEM micrograph for (a) pristine, (b) CH4/(Ar-H2) and (c) CH4/N2 ppt-processed NCD films, showing smooth surface due to small diamond grains contained in the pristine films and the surface become rough in ppt-processed films. Fig. 2 The (a) Raman and (b) NEXAFS films spectra of NCD, where spectra (I)’s are for pristine diamond films and spectra (II)’s and (III)’s are for CH4/(Ar-H2) plasma and CH4/N2 plasma post-treated diamond films, respectively. Fig. 3 (a) Bright field (BF) and (b) composed dark field (C-DF) TEM micrographs of pristine NCD films, with the insets showing the corresponding selected area electron diffraction (SAED) patterns. The inset of “b” indicates the portion of diffraction rings used for acquiring the dark field images. (c) The structure images corresponding to designated region in “b”. The FT image is the Fourier-transformed diffractograms corresponding to the whole structure images in “c”. Fig. 4 The (a) electron field emission properties, the JEFE-E curves, and (b) the corresponding Fowler-Northeim plots of (I) pristine NCD and (II) pristine UNCD films. Fig. 5 (a) The optical emission spectra of the plasma used for post-annealing plasma treatment (ppt) process and (b) the bias current during the ppt-process of NCD films, where (I)’s are ppt in CH4/(Ar-H2) and (II)’s are ppt in CH4/N2 plasma. Fig. 6 The EFE properties of ppt-processed (a) NCD and (b) UNCD films, where (I)’s are for pristine films, (II)’s are CH4/(Ar-H2) ppt-processed and (III)’s are CH4/N2 ppt-processed films. Fig. 7 (a) Bright field and (b) composed dark field TEM micrograph of CH4/(Ar-H2) ppt-processed NCD films. (c) Structure image of CH4/(Ar-H2) ppt-processed NCD films, where FT0 image show Fourier-transformed diffractograms corresponding to entire structure image in (c), respectively, and ft1-ft2 are FT-images corresponding to areas 1 and 2, respectively.

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Fig. 8 (a) Bright field and (b) composed dark field TEM micrograph of CH4/N2 ppt-processed NCD films. (c) Structure image of CH4/N2 ppt-processed NCD films, where FT0 image show Fourier-transformed diffractograms corresponding to entire structure image in (c), respectively, and ft1-ft2 are FT-images corresponding to areas 1 and 2, respectively.

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Fig. 1 SEM micrograph for (a) pristine, (b) CH4/(Ar-H2) and (c) CH4/N2 ppt-processed NCD films, showing smooth surface due to small diamond grains contained in the pristine films, and the surface becomes rough in ppt-processed films.

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Fig. 2 The (a) Raman and (b) NEXAFS films spectra of NCD, where spectra (I)’s are for pristine diamond films and spectra (II)’s and (III)’s are for CH4/(Ar-H2) plasma and CH4/N2 plasma post-treated diamond films, respectively.

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Fig. 3 (a) Bright field (BF) and (b) composed dark field (C-DF) TEM micrographs of pristine NCD films, with the insets showing the corresponding selected area electron diffraction (SAED) patterns. The inset of “b” indicates the portion of diffraction rings used for acquiring the dark field images. (c) The structure images corresponding to designated region in “b”. The FT image is the Fourier-transformed diffractograms corresponding to the whole structure images in “c”.

26


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Fig. 4 The (a) electron field emission properties, the JEFE-E curves, and (b) the corresponding Fowler-Northeim plots of (I) pristine NCD and (II) pristine UNCD films.

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Fig. 5 (a) The optical emission spectra of the plasma used for post-annealing plasma treatment (ppt) process and (b) the bias current during the ppt-process of NCD films, where (I)’s are ppt in CH4/(Ar-H2) and (II)’s are ppt in CH4/N2 plasma.

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Fig. 6 The EFE properties of ppt-processed (a) NCD and (b) UNCD films, where (I)’s are for pristine films, (II)’s are CH4/(Ar-H2) ppt-processed and (III)’s are CH4/N2 ppt-processed films.

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Fig. 7 (a) Bright field and (b) composed dark field TEM micrograph of CH4/(Ar-H2) ppt-processed NCD films. (c) Structure image of CH4/(Ar-H2) ppt-processed NCD films, where FT0 image show Fourier-transformed diffractograms corresponding to entire structure image in (c), respectively, and ft1-ft2 are FT-images corresponding to areas 1 and 2, respectively. 31



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Fig. 8 (a) Bright field and (b) composed dark field TEM micrograph of CH4/N2 ppt-processed NCD films. (c) Structure image of CH4/N2 ppt-processed NCD films, where FT0 image shows Fourier-transformed diffractograms corresponding to entire structure image in (c), respectively, and ft1-ft2 are FT-images corresponding to areas 1 and 2, respectively. 32


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TOC image

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Evolution of Granular Structure and the Enhancement of Electron Field

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