Synthesis And Field Emission Properties Of AlN-based Hybrid Nanostructures

One-dimensional (1D) AIN nanomaterials are important candidates of nanoscale cold cathodes due to the excellent physical and chemical properties of AIN including high thermal conductivity, high melting point, and low electron affinity, etc. Up to now, the research on AlN-based cold cathodes is still on the experimental stage. It is still difficult for the AlN-based field emitters to achieve low turn-on field and threshold field as well as large emission current density. The related studies could provide fundamental information for the practical application of AlN-based nanoscale cold cathodes.In our previous reports, AIN nanocones were prepared by chemical vapor deposition (CVD) method. Compared with other nanoscale cold cathode materials such as carbon nanotubes, AIN nanocones do not show excellent field emission (FE) properties due to the following reasons:(1) AIN has poor conductivity because of its large band gap and also has a strong tendency toward oxidization and hydrolysis; (2) AIN nanostructures own relatively high work function (-3.7eV) in comparison with some substances, e.g., Cs-based materials; (3) the obtained AIN nanocones usually have large density, which would result in large screening effect. Based on the above problems, we developed several strategies such as patterned growth, direct growth on conductive substrate and doping to optimize the FE properties of AIN nanomaterials. In this thesis, we attempt to construct hybrid nanostructures of AIN nanocones and some materials with complementary functions to optimize the FE performances, namely by decorating with low work function materials, and constructing core-shell nanostructures and heterojunction nanostructures. The results are briefly summarized as follows:1. According to the Fowler-Nordheim (FN) theory, field emission properties of a material strongly depend on its work function and field enhancement factor. Hence the materials with low work function or large aspect ratio are usually used as field emitters. However, many materials with low work function are hard to be considered as FE cathodes due to their unsuitable other properties, e.g., Cs with the lowest work function is not robust enough to maintain the 1D geometry. If combining the advantages of 1D scaffold and low work function material, the obtained hybrid nanostructures may exhibit low work function.In this thesis, we report the construction of CsI-AlN hybrid nanostructures by evaporating CsI in vacuum onto the preformed AlN nanocone arrays. The FE performances of the hybrid samples are much improved depending on the size and density of the CsI nanoparticles. The results indicate that decorating low-work-function nanoparticles is an effective way to enhance the FE performance of the AlN nanocones.2. The application of AlN-based field emitters in FE community was limited due to its poor conductivity and a strong tendency toward oxidization and hydrolysis. If 1D AlN nanomaterials were wrapped by certain substances, they may show increased stability and conductivity. In this thesis, the AlN-C, AlN-CN and AlN-BCN core-shell nanocones were prepared by coating three different sheaths of C, CN, and BCN on the surface of the preformed AIN nanocones. These AlN-based core-shell nanostructures present much improved FE performances, i.e., with low turn-on field and threshold field, as well as large current density, in comparison with the uncoated counterparts in the order of AlN-BCN>AlN-CN>AlN-C> AlN, because of the enhanced conductivity, the lower work function arising from the coating layer and the synergetic effect between the inner core and the outer shell. In addition, the coating thickness could influence the FE performance of the core-shell nanocones and thin coating layers are preferred for optimization.3. Longer AlN nanocones are expected to have greater resistance, in other words, good conductivity and large aspect ratio were difficult to be achieved simultaneously for the AlN nanomaterials. Growing AlN nanostructures on the top of 1D nanomaterials with good conductivity can retain the low electron affinity of AlN tips and the large field enhancement factor of the emitters, while the conductivity of the heterojunction structures was enhanced because of the short length of AlN nanocones. With this consideration, we first prepared ZnO nanorod arrays by a seed-mediated wet chemistry method, and then coated CNX shell on the surface of ZnO nanorod array by a CVD process, and finally deposited AlN nanocones on the top of ZnO-CNx core-shell nanostructures. The FE performances of the obtained ZnO-CNx-AlN nanostructures have been obviously improved in the order of ZnO-CNx-AlN> ZnO-CNx>ZnO>AlN. It is revealed the deposition time of AlN nanostructures could influence the FE properties of the heterojunction nanostructures, and the deposition time of ca.40 min is preferred for optimization. The decay of FE performances for the products with longer deposition time could be attributed to the screening effect of the dense AlN nanocones.The preceding results suggest that the constructing of hybrid nanostructures with some complementary materials does improve the FE performance of the AIN-based nanomaterials, which could be extended to other nanoscale cold cathode systems.