Synthesis And Field Electron Emission Properties Of Carbon Nanotubes, Carbon Nanosheets, Graphene And Their Composites

In recent years, carbon is a widely used material that has been intensively investigated theoretically and experimentally because of its varied structures, exceptional physical and chemical properties, and potential applications. Both carbon nanotubes (CNTs) and other forms of carbon nanomaterials (nanodiamond, carbon nanofibres, carbon nanosheets (CNSs) et al.) can be good candidates for field emission displays and other nanoelectronic devices due to their excellent performance of electron emission and electron transport. Especially, the high surface-to-volume ratio and sharp edge of two-dimensional carbon nanomaterials (CNSs and graphene) are attractive for field electron emission, fuel cells, high performance capacitors, and micro- and nanoelectronic technologies.Among a variety of methods for fabricating carbon nanomaterials, Plasma enhanced chemical vapor deposition (PECVD) technique has been widely utilized for the synthesis of carbon nanomaterials(CNTs, CNSs, few layer graphene sheets (FLGSs) and so on) because its feasibility and potentiality for large-area production with reasonable growth rates at relatively low temperatures. Although the effect of oxygen on the growth of CNTs has attracted much attention, up till now, few reports have focused on the role of oxygen that acts as an etching gas for the pretreatment of catalysts and consequently the growth of CNTs. In addition, although many types of carbon nanomaterials have been synthesized by PECVD, the systematical investigations on the growth conditions for different carbon nanomaterials are rare. For example, the effects of reactive gases including their species and total gas pressure during PECVD on the growth of carbon nanomaterials have not been well understood.On the other hand, the systematical studies on the field electron emission properties of a hybrid CNSs/CNTs material and FLGSs is also limited.In this dissertation, we study the influence of oxygen on the Co catalyst film and the growth of CNTs by PECVD. Secondly, we synthesize the carbon nanomaterials using PECVD via only adjusting one experimental parameter. Finally, the field emission properties of a hybrid CNSs/CNTs have been explored, and plasma treatment in Ar gas on FLGSs has been performed for enhancing their field emission properties.In chapter 1, we first give a brief introduction to the progress in investigating the structures, properties, applications and preparation of some kinds of carbon nanomaterials. Furthermore, some unresolved issues for carbon nanomaterials and the objective of this dissertation are given.In chapter 2, we give a description on the synthesis and characterization of carbon nanomaterials in this work, and the principle of of magnetron sputtering and PECVD system is introduced.In chapter 3, the influence of oxygen on the growth of carbon nanotubes (CNTs) using PECVD has been studied, in which we synthesize pure and vertical aligned CNTs with highly-yielding growth rate on cobalt catalyst. The O2-etched Co catalyst film has small cobalt oxide grain particles, and the small size for the particles is benefit to the growth of CNTs when the cobalt oxide particle is deoxidization during the CNT growth, resulting in highly yielding CNTs. In addition to the key role of oxygen in decreasing catalyst particle size, the addition of small amount of O2 into CH4/H2 atmosphere can increase the purity of CNTs and decrease the negative effect of H species on the growth of CNT.In chapter 4, using CoO particles as catalyst, we have synthesized various types of carbon nanomaterials including CNTs/cobalt particles shelled with graphitic layers, pure CNTs, a hybrid CNTs/CNSs material, and pure CNSs using PECVD in discharging different mixed gases (CH4/H2, CH4/H2/O2, CH4/Ar, and CH4/Ar/O2 ). We find that the deoxidization of CoO particles is very important for the growth of various carbon nanomaterials, which is governed by the discharging gases in PECVD process. Only when the CoO substrate is deoxidized completely, CNTs can be obtained. In contrast, as the deoxidizing rate is low enough, the CNSs with a high purity can be obtained. However, when the deoxidizing rate is not two high or low, a hybrid material, including CNTs/cobalt particle shelled with graphitic layers and CNTs/carbon nansheets, will be realized.In addition, using one-step method, we have synthesize a composite consisting of aligned CNTs and graphite shell-encapsulated cobalt nanoparticles on Si (100) substrate using PECVD technique after pre-treating Co film by O2-etching. The graphite shells encapsulated cobalt nanoparticles aggregate on the surfaces of aligned CNTs. Also, we find that the size of graphite coated Co particles and the density of CNTs could be controlled by varying H2/CH4.In chapter 5, it is found that the total gas pressure during PECVD significantly influences the structures of deposited carbon nanomaterials, and through adjusting the total gas pressure, pure CNTs, a composite of CNTs/GSs, and pure GSs can be obtained on Si (100) substrate covered by CoO nanoparticles. The reasons why the total gas pressure influences the structures of the obtained carbon nanomaterials can be explained that the total CH4+Ar gas pressure determines the concentration of atomic hydrogen which in turn controls the deoxidizing rates of CoO nanoparticles and Ar ion bombardment energy.For the composite of CNTs/GSs, the well dispersed GSs with a small size and sharp edges can act as independent emitters on the surface of CNTs, which significantly enhances the electron field emission, compared to either pure CNTs or pure GSs. This investigation is important for the shape-controlled synthesis of carbon nanomaterials towards their technological applications.In chapter 6, we find that the field emission properties of FLGSs, grown by PECVD, can be improved by Ar plasma treatment for a proper period of time, which can be attributed to an enhanced geometrical factor due to the removal of the folded edges and the formation of an extremely sharp edge for the FLGSs, the reduced screening effect between neighbour FLGSs, and the increased number of defects acting as additional emission sites on the FLGSs'surface due to Ar ion bombardment. It is expected that plasma treatment will provide an efficient way to improve the field emission properties of FLGSs.In conclusion, oxygen plays a key role in highly yielding CNTs, and various types of carbon nanomaterials including CNTs/cobalt particles shelled with graphitic layers, pure CNTs, a hybrid CNTs/CNSs material, and pure CNSs on O2-etched Co catalyst film can be obtained using PECVD. In addition, a hybrid CNTs/CNSs material exhibits better field emission performance than either CNTs or CNSs. Ar plasma treatment on FLGSs can enhance the field emission properties for CNSs effectively.