Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig. 6.9 Schematic illustration of effective potential area (shadowed parts) of electron tunneling varying with the change of the thickness of the coating ultrathin film. Evac represents the vacuum level, EF donates the Fermi energy, Ø is the work function of CNT, CBM means the conduction band minimum, and VBM is the valence band maximum. In addition, it is noted that the F-N F plots of some samples comprise two linear regions with a knee point in between, one in the lower field region and the other in the higher field region. The deviation of the F-N F N plot in the high electric field region is commonly observed for semiconductor field emitters and it is probably due to the overheating of the emitter tips and the space charge effect [20-23]. More specifically, 134 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures with the increase of ambient temperature during emission process, the work function of the emitter would change such that the emitter exhibited enhanced FE performance. On the other hand, space charge would be generated around the emitter tips during emission, which sharply reduced the local electric field on the emission sites. As the ta-C mostly consists of sp3 carbon bonds while CNTs are rich in sp2 bonds, deposition of ta-C onto the CNT surface would result in a decreased conductivity, which explains the more severe space charge effect of the ta-C coated samples during emission process. 6.4 Hydrogenation Effect on FE Properties of the Composite Emitters Hydrogenation treatment is a commonly used method to improve the surface conductivity of diamond [24-28]. Recently, surface hydrogenation has been found to be capable of significantly reducing the work functions of DLC thin films [29]. Theoretically, higher conductivity and lower surface work function for diamond or ta-C films would enhance the FE properties of the ta-C coated CNT emitters. Therefore, in this project, hydrogen plasma treatments with varied durations (10, 20 and 30 s) were conducted on the 50 nm ta-C coated CNT composite emitters in order to investigate the hydrogenation effect on their FE performances. The same experiment was also carried out on the 100 nm ta-C coated CNT samples for confirmation purpose. Approximately µm long high density vertically-aligned CNTs 135 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures were used as substrate in this project. 6.4.1 Characterization by SEM and TEM Fig. 6.10 shows the top and cross-sectional view SEM images of the 50 nm ta-C coated composite emitters with and without hydrogen plasma post-treatment. From the top view it can be clearly observed that the nanotubes become increasing thinner as the duration of hydrogen plasma treatments was increased. From the cross-sectional view it is obvious that without hydrogenation treatment, the CNT tips are wrapped with thin films, which look like a layer of whiskers formed with the average diameter larger than that of the pristine CNTs and the whiskers extended from the tips of the CNTs. With a 10 s surface hydrogenation treatment, the sample appears similar with the one without hydrogenation treatment in cross-sectional image. However, the tips of these whiskers seem to protrude out and stand freely rather than exhibiting curly shape at the tips like the pristine ta-C coated CNTs do. With 20 s hydrogenation, the surface of the sample becomes very flat, without many random protrusions of individual nanotubes. Moreover, little or no thick whiskers can be observed from this image. With an even longer hydrogen plasma treatment, i.e., 30 s hydrogenation duration, strictly no thick whiskers can be found on the CNT tips. The surface is rather flat and the average diameter of these nanotubes is comparable to that 136 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig. 6.10 Top and cross-sectional sectional view SEM images of composite emitters. (a) and (b) 50 nm ta-C C coated CNTs; (c) and (d) 50 nm ta-C ta coated CNTs with a 10 s hydrogenation treatment; (e) and (f) 50 nm ta-C C coated CNTs with a 20 s hydrogenation treatment; (g) and (h) 50 nm ta-C C coated CNTs with a 30 s hydrogenation treatment. 137 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures of the pristine CNTs. Hence, it is reasonable to assume that the coated tips of these nanotubes were fully etched away by the hydrogen plasma. This assumption was further confirmed by the emitters’ average length reduce, which was around 0.5 µm comparing the 30 s hydrogenation sample to the 20 s sample. The same features can be extracted as well from the SEM images of the 100 nm ta-C coated composite emitters with and without hydrogen plasma post-treatment as shown in Fig. 6.11. High resolution TEM images of the composite emitters with and without hydrogenation treatment are shown in Fig. 6.12. Fig. 6.12(a) confirms the core-shell structure of the composite emitter. After the 10 s hydrogen plasma treatment, the edge of the composite nanotube was slightly etched as shown in Fig. 6.12(b). With the 30 s hydrogenation treatment, the nanotube was severely etched at the surface such that the CNT can be hardly observed, which was believed to be entirely etched away by plasma as shown in Fig. 6.12(c). 138 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig. 6.11 Top and cross-sectional sectional view SEM images of composite emitters. emitters. (a) and (b) 100 nm ta-C C coated CNTs; (c) and (d) 100 nm ta-C ta coated CNTs with a 10 s hydrogenation treatment; (e) and (f) 100 nm ta-C C coated CNTs with a 20 s hydrogenation treatment; (g) and (h) 100 nm ta-C C coated CNTs with a 30 s hydrogenation treatment. 139 . Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core- Shell Nanostructures 138 of the pristine CNTs. Hence, it is reasonable to assume that the coated. ta- C coated CNTs with ta- C coated CNTs with Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core- Shell Nanostructures sectional view SEM images of composite. density vertically-aligned CNTs Chapter 6 Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core- Shell Nanostructures 136 were used as substrate in