## 5 Experimental Results
### Microwave Plasma Jet Chemical Vapor Deposition of Diamond Films and Characterization  
The treated graphite substrate was placed on the quartz stage in the MPJCVD chamber (see Figure 9) with the following operation settings: microwave power of 600 W at a frequency of 2.45 GHz, a gas pressure of 70 Torr, and a combination of hydrogen and methane gases (a methane concentration percentage of 0.25%) for two hours. To improve the uniformity of the deposited diamond film and increase the deposition rate, the geometry configuration was optimized to enhance the electron density, ion density, electric field, and electron temperature. Finally, according to the optimal simulation parameters, the MPJCVD system is operating successfully. The scanning electron microscope is used to examine the surface morphology and microstructure of the diamond film. Figure 10 shows the SEM image of the diamond film grown on the graphite substrate with working gas containing 0.25% methane, exhibiting distributed particles with a size of \(\sim\)1 \(\upmu\)m. The film thickness is also measured with the SEM, as shown in Figure 10(a), giving a uniform thickness of about 435 nm. Figure 10(b) shows the Raman spectroscopy of this fabricated diamond film, and as one can see clearly, the diamond (D) peak appears in the spectrum [44]. The SEM images reveal the presence of a diamond film uniformly distributed with particles of a size of \(\sim\)1 \(\upmu\)m. The deposited diamond film has relatively high uniformity and continuity. Figure 8: Distribution of (**a**) electron density, (**b**) ion density, (**c**) electric field, and (**d**) electron temperature at the sample holder height of 118 mm, microwave power of 600 W, and hydrogen gas pressure of 70 Torr in the MPJCVD system. Figure 11: Measurement of (**a**) the film thickness and (**b**) Raman spectroscopy of the diamond film grown on the graphite substrate with working gas containing 0.25% methane. Figure 10: SEM image of the diamond film, exhibiting distributed particles with a size of \(\sim\)1 \(\upmu\)m.  
Figure 9: Photo image of the sample holder stage and antenna. Using the same fabrication method and settings (microwave power of 600 W at a frequency of 2.45 GHz, a gas pressure of 70 Torr with a combination of CH\({}_{4}\) and H\({}_{2}\) gases), and the methane concentration changing from 8% to 40% at a step of 8%, diamond films are deposited on silicon and graphite substrates for 2 h. In addition to SEM analysis and examining surface morphology, electrical properties are also tested, and field emission from the diamond film grown from this MPJCVD system shows extraordinary properties, i.e., extremely low turn-on voltage and high current density. The field emission current densities from the diamond films fabricated under different conditions and measured at different electric fields are shown in Figure 12. The diamond film grown on the graphite substrate with a 32% methane concentration has the most significant electrical property. The turn-on electric field determined for this case is as low as 4 V/\(\upmu\)m. Figure 13 shows the SEM images with (a) 300 X and (b) 50,000 X of the diamond film grown on the graphite substrate with working gas containing 32% methane. It is obvious that large particles with sizes ranging from 10s to 100 \(\upmu\)m can be seen in Figure 13a, while a zoom in SEM in Figure 13b shows a graphite structure. As the sample exhibits extraordinary field emission properties, we further study the diamond film thickness and its composition. Figure 14a is the SEM image of the diamond film for thickness measurement, and the film thickness is around 115.5 \(\upmu\)m. Figure 14b shows the Raman spectroscopy of the diamond film, and there are two main peaks found in the spectrum: in addition to the D (left) peak, which shows the presence of the diamond, one can clearly see a graphite (G) peak on the right side.