## 2 Experimental  
A wet coating process was adopted to fabricate a cathode with stable FE at a low driving voltage using metallic SWCNTs (NanoIntegris Technologies Inc., Boisbriand, QC, Canada). The homogeneous dispersion of the SWCNTs in a liquid medium is the key factor for obtaining a planer electron emission homogeneously as a field emitter. Highly crystalline metallic SWCNTs are expected to lower the threshold field and stably emit electrons at a low driving voltage. However, it is difficult to homogeneously disperse highly crystalline SWCNTs in liquids [24]. In general, a dispersant is used to facilitate the homogeneous dispersion of nanocarbon materials in liquids. However, highly crystalline SWCNTs cannot bind to the functional groups in a dispersant because there are very few defective binding sites in their carbon network that can be used for binding to the functional groups in the dispersant. We attempted to physically separate highly crystalline SWCNT aggregates into uniform single nanotubes in a low-viscosity solvent, containing In\({}_{2}\)O\({}_{3}\)-SnO\({}_{2}\) [tin-doped indium oxide (ITO)] precursor solution. A non-ionic dispersant was added to the solvent to facilitate the dispersion of individual SWCNTs, and the mixture was agitated using a homogenizer. To prepare the coating film, butyl acetate (99%) and ethyl cellulose (EC) (\(\sim\)49%, ethoxy 100 cP), which were obtained from Wako Pure Chemical Industries, Ltd., Osaka, Japan, and an ITO precursor solution (Kojundo Chemical Laboratory Co., Ltd., Sakado, Japan) were used. The initial mixture was prepared by mixing metallic SWCNTs, which were purified from solutions from NanoIntegris Technologies Inc., the ITO precursor solution, butyl acetate added as required to control the viscosity of the solution, and EC as a surfactant at a weight ratio of 1:8:600:10. Then, the mixture was agitated using the homogenizer for 30 min at room temperature. The agitated mixture was sprayed on a silicon substrate to form a film. The film was dried at approximately 420 K and sintered at 720 K in vacuum to remove any organic-solvent components. As a result, ITO films were obtained from each type SWCNT, which were used as cathode plates. In addition, to facilitate field emission, the sintered ITO film with dispersed SWCNTs was scratched to activate the film surface and obtain stable FE properties [25]. Figure 1 shows (a) a schematic diagram of SWCNTs that were protruding from a scratched groove, (b) a scanning electron microscopy (SEM, SU-8000, Hitachi High-Technologies Corporation, Tokyo, Japan) overview after activating the FE for the ITO film, including metallic SWCNTs, and (c) an  
enlarged view of the protruding SWCNTs in the circled region of (b). After the film that was coated with the mixture was sintered in vacuum, it was activated by scratching using a thin metal rod to obtain the FE properties at a low turn-on field. We observed that the bundles of SWCNTs protruded from both sides of the edge of the scratched ITO film. The exposed SWCNTs in the ITO film were homogeneously dispersed and were oriented in random directions. Furthermore, the SWCNTs that were used for an FE thin film were checked for high crystallization by transmission electron microscope (TEM) (HR-3000, Hitachi High-Tech. Co. Ltd., Tokyo, Japan) measurement in Figure 1d. Each SWCNT in a bundle of Figure 1d exhibits a clear TEM image of each tube wall identified as straight lines. The TEM image in Figure 1d explains that a SWCNT has highly crystalline carbon network. Namely, we can expect to develop field emitters employing these SWCNTs, and theoretically analyze the effect of the increased crystallinity of SWCNTs on their electrical conductivity by determining their electrical properties (FE properties). Figure 1: Tin-doped indium oxide (ITO) film images after field emission (FE) activation.