Research Of Field Emission Electron Source Based On Carbon Nano-materials

Field emission electron sources have wide application in various vacuum devices,such as flat panel display, micro-wave amplifier, X-ray source, ion thruster, etc. Due to their high mechanical and chemical stability, good electrical and thermal conductivity,carbon nano-materials, such as carbon nanotube and graphene, have been attracted much attention for their field emission properties. However, until so far, the large current emitting ability, emitting uniformity, and the emission current tunability of the nano-carbon field emission cathode still can not satisfy the requirement of the practical vacuum devices. The key issue is how to improve the emission performance of the nano-carbon cathode. It is also important to design and realize triode field emission structure for the sensitive tune of the emission current. Consequently, this research aims to improve the emission current, uniformity, current tune sensitivity of the nano-carbon cathodes,focuses on the synthesis of carbon nanotube and graphene,characterization of field emission performance and fabrication of triode structure, and hence promote their application in high current density field emission devices. The research work can be concluded as follows.1 .Screen effect in nano-carbon field emission cathode will lead to some problems,such as low emission current density, non-uniformity, high driving electric field. To reduce the screen effect, and then improve the emission current density and uniformity,and reduce driving electric field, we have fabricated two kinds of patterned nano-carbon film cathode structure employingthermal chemical vapor depostion method and nano-fabrication technology. (1)Carbon nanotube field emitter based on metal-mesh electrode. Compared with the carbon nanotube emitter on flat-metal electrode, the turn-on field reduced 71%, while the threshold field reduced 74%. The total emission current reached 5mA (corresponding to a current density of 5A/cm2) at an electric field of 5 V/?m. The driving field of this cathode is much smaller than references reported cathodes at the similar emission current level. (2) graphene-nanomesh field emitter and its triode structure. The experimental results show that the emission current can be sensitively modulated with a low gate voltage (0-20V). At the meanwhile, the total emission current density reached 135 mA/cm2, at an anode field of 5V/?m and a gate voltage of 20V. The current density and current modulation sensitivity of the cathode have been greatly improved compared with the references reported graphene field emitter. These two results are important for the practical application of nano-carbon cold cathodes, and have been already published in Applied Surface Science and IEEE Electron Device Letters.2. Although we have obtained excellent field emission properties from patterned nano-carbon field emitters, their stable emission will require an extremely high vaccum condition. To improve the emission stability, untilizing the advantages of ZnO nanowires, we proposed two kinds of three-dimensional nano-carbon materials-ZnO nanowires hybrid structure cathodes. (1) 3D carbon nanotube mesh-ZnO nanowires hybrid structure. Compared with bare carbon nanotube mesh, the emission stability of hybrid structure has been greatly improved (no obvious degradation in 80 hours for hybrid structure, while the emission current of the bare carbon nanotube mesh degraded more than 60%). At the meanwhile, this method can also improve the emission current of the ZnO nanowires film. (2) 3D graphene foam-ZnO nanowires hybrid structure.Compared with bare graphene foam emitter, the emission fluctuation decreased from 20% to 5%, turn-on field reduced 50%, and threshold field reduced 60%. These two results will have a good application prospect in low power vacuum devices with low vacuum requirement, and have been published in Thin Solid Film and Applied Surface Science.3. The sensitive modulation of the large emission current and electron beam focusing of large current are two key issues,which have not been solved very well in current field emission electron sources. For instance,metal-mesh is widely used in high power field emission devices, the aperture ratio determined the electron transmission rate. However,larger aperture ratio will lead to a low modulation sensitivity of the emission current. To solve this problem,this research prosposed a novel graphene-metal mesh hybrid gate structure, utilizing the ultra-thin nature of graphene, high mechanical strengthen, good electrical conductivity. Both experiments and theory calculation were carried out to investigate the feasibility. The testing results show that, compared with bare metal-mesh gate, the cathode-gate electric field is much more uniform, the driving field reduced 50%, the emission angular is smaller, and the emission transmission ratio is much higher, using the hybrid gate. The hybrid gate field emission structure proposed here will improve the practical application of nano-carbon based vacuum electronic devices. This result has been published in Advanced Functional Materials.