Preparation And Field Emission Of Carbon Nanotubes Film

Carbon nanotubes (CNTs) have been studied extensively as cold cathodes for microelectronics devices, such as field emission display (FED), field emission lighting, microwave amplifier, X-ray tube, due to their high aspect ratio, small tip radio of curvature, and high conductivity. During the CNTs application to the products, there are many problems such as the low cost, low temperature, large-scaled and controllable fabrication, the uniformity and stability of the field emission and the processing compatibility. The solution to these problems will speed up the industrialization of the field emission devices based on CNTs and raise the market competition ability.In this study, the CNTs films are grown using low pressure chemical vapor deposition (LPCVD) method, which enables low cast, low temperature and large area growth. The controllable growth of CNTs film is realized simply by using binary catalyst system. The aging, annealing and patterning effect on the field emission of the CNTs films are investigated for practical applications. The prototype of the CNT field emission pressure sensor is fabricated. Main concludes and innovative results are listed as follow:1) Optimization of the CNT films growth using LPCVD techniqueThe growth temperature, flow ratio of C₂H₂ and H₂, pressure and the buffer layer effect on the growth of CNT films are systematically studied. The result shows that the CNTs can be synthesized at relatively low temperature in the range of 500℃ to 700℃, which is enough for most applications. The growth rate of CNTs increased first with increasing temperature from 500℃, and reached a maximal value at 650℃, then deceased when the temperature increased further. The diameter distribution of CNTs became narrow with the temperature increasing, but their average length increased. Moreover, with the increasing temperature, the defects and amorphous phase decreased, the crystalline phase increased, and the size of graphite nanoparticles increased in the films. In addition, the flow ratio of C₂H₂ and H₂, pressure and the buffer layer also have impact on the growth behavior of the CNTs. By adjusting these processing parameters, the growth and field emission properties of the CNTs can be controlled to some extent.2) The controllable growth of the CNTs film by binary catalyst system for the field emission usage Improved field emission property of CNTs is achieved by using NiTi/NiCr alloyfilm as catalyst under optimized condition. This method is simple, compatible with IC techniques, large processing latitude and applicable to industrial production. In particular, the field emission property is significantly enhanced when the Ni/Ti ratio is about 76 at.%, and it is supposed attributing to the combined effect of the hills-like surface enhancement and the intrinsic emission properties of CNTs.3) The aging and annealing effect on the field emission of the CNTs filmsThe field emission of CNTs films become stable and uniform after aging. When the longest CNTs are cut or vapor by joule heating during emission, the .second long CNTs in mumber become emitters. Therefore, the emission becomes uniform for more emitters,and stale for less current density and joule heating in each emitter.The CNTs film annealed at 400℃ in air contains less amorphous carbon phase, and shows better field emission properties comparing to the as-grown CNTs film. Annealing at 450℃ causes serious oxidative damage along the tube walls, resulting in the poor field emission performance. The CNTs annealed at 500℃ are all burned out. The experiment shows that the sealing temperature at 400℃ or below can be conducted in air, while the sealing temperature above 400℃ should be done in N₂ or Ar ambient.4) Fabrication of the CNT field emission pressure sensor (FEPS) A novel field emission pressure sensor has been developed utilizing CNTs as the emitter. The prototype pressure sensor has a measured sensitivity of 0.94-5.30 nA/Pa (2-550 kPa). The fabrication process is easier than the reported field emission pressure sensor using silicon micro-tips. The experimental results show the method is promising and will develop a new path in the fabrication of the field emission pressure sensor.