## 2 Results and Discussion  
### Detailed iPC-APPJ Device and Optical, Electronical, and Discharge Characteristics  
Figure 1 shows a schematic diagram of the proposed iPC-APPJ with the optical and electrical measurement system employed in this study. The three jet tubes, arranged in a triangle, were combined and wrapped through the powered electrode with copper tape, 10 mm from the end of the jet, which resulted in a more compact design, with each jet in physical contact with the adjacent jets. Each jet tube was 13 cm in length, with an inner diameter (i.d.) of 1.5 mm and an outer diameter (o.d.) of 3 mm, with the center-to-center distance between the two adjacent tubes at 3 mm. It is noted that  
the proposed plastic tube and PTFE bottom cap were installed at the jet end to confine the jet flow in the recombination region and to produce an intense plasma cloud. Moreover, it is well-advised to leave a gap between the end of the plastic tube and the substrate with the PTFE bottom cap for a smoother jet flow. A sinusoidal power supply was connected to the powered electrode with a peak value of 8 kV and a frequency of 26 kHz. As shown in the plasma jet images, with and without the PTFE bottom cap (Figure 1), in the case of a jet without the PTFE bottom cap, the short plasma plumes were only produced in the recombination regions, however, in the case of a jet with the PTFE bottom cap, strong plasma plumes were produced with an additionally broad glow plasma region in a farther downstream region. It can be inferred that the proposed PTFE bottom cap plays a role in minimizing the quenching from ambient air and in confining the jet flow in the recombination region by reducing exhaust gas flow. Figure 2 shows the applied voltages, discharge currents, and optical intensities measured in recombination region for proposed iPC-APPJ with and without PTFE bottom cap. As shown in Figure 2, the optical intensity was observed to have increased significantly due to the presence of the PTFE bottom cap without any changes of discharge voltage and current. It should be noted that there was a definite shift in the optical emission peak to the left, compared to that of the case without the PTFE bottom cap. It indicates that the proposed iPC-APPJ system can produce an efficient and intensive plasma jet due to the effect of the PTFE bottom cap, by analyzing optical intensities using a photo-sensor amplifier. Accordingly, the iPC-APPJ can produce a broad and intense glow plasma in the recombination regions during plasma polymerization. Figure 3 shows the emission spectra from 270 to 880 nm, measured in the recombination region for the proposed iPC-APPJ with and without the PTFE bottom cap, further indicating that the excited N\({}_{2}\), Ar, OH, and carbonaceous species exist in the plasma plumes. In the full spectrum measurements, significant increases in the various nitrogen (N\({}_{2}\); 337, 357, and 380 nm) peaks and the oxygen (OH; 309 nm) peak were observed in the case with the PTFE bottom cap, while the Ar peak was slightly changed. It also indicates that the proposed iPC-APPJ system can produce an efficient and intensive  
Figure 1: Schematic diagram of experimental setup in this study and images of plasmas produced in the recombination region of intense plasma cloud type atmospheric pressure plasma jets (iPC-APPJ), with and without the polytetrafluoroethylene (PTFE) bottom cap. plasma jet due to the effect of the PTFE bottom cap, by analyzing spectrums of various reactive species peaks using the OES technique [44; 45; 46; 47; 48]. The various N\({}_{2}\) and OH peaks indicate a higher concentration of reactive nitrogen species (RNS) and reactive oxygen species (ROS), respectively, present in the iPC-APPJ with the PTFE bottom cap; both of these free radicals have been shown to play an important role in many biological/medical and industrial applications, and imply the presence of a more useful, efficient, and energy-dense plasma.