Field Emission Properties Studies Of Chemically Modified Graphene-materials

Since the nanotechnology was put forward and has been developed, traditional manufacturing technical process has been innovated. Nanotechnology intends to improve the performance of devices in the micro-scale world at the nano-scale. It is thought to be the most important science technology at the twenty-first century. As long as the rapidly growing of nano-technology, it promotes the emerging of a series of new subjects. Numerous scientific researchers in different fields of nanotechnology make great forward to solve the challenging problems on the way, and their contributions promote the practical application of nano-technology. Nano-device is a hot area of research as the foundation of the nanotechnology. The field electron emitters built of nano-materials belongs to it. As new type of the field emitter, nano-materials have raised high expectations with its unique advantage in structure and property. Only by keeping gaining insight into the microcosmic mechanism, people can get directional information when they try to improve the properties of nano-devices, avoiding much waste on resource and time. The strong and stable current is not the only goal. Magnetic nano-structures have raised high expectations since they may bring high spin-polarization of the emission current. On the other hand, density functional theory (DFT) owns a wide range of applications, and has been widely used in various subjects such as solid-state physics and quantum chemistry. The DFT has great advantage in calculating the electronic structures of nano-systems. In this paper, by employing DFT-based theoretical simulation, we mainly studied the field-emission properties of graphene nanoribbons and graphene surface with chemistry modification by different functional group, involving spin-unpolarization and spin-polarization condition. In the process, we used the first-principle theory to decrease the computational complexity on the basis of accuracy.In chapter1, we first introduce the basic knowledge of electron field emission. Then we give detailed information of the method applied in this paper for studying the field emission properties, which combines the typical wavefunction-matching method and the first-principles theory. Besides, we give a brief review of the researches and application progresses of some graphene-based nano-materials, including graphene, graphene nanoribbons and carbon nanotubes. The basic knowledge of the DFT is described at the last. The field emission is the short name of the electron field emission. It is also named as cold-cathodes style of field emission. It refers to electrons are emitted from the classic forbidden zone into vacuum under a strong electrostatic field, leading to an observable current at the macro level. In the article, we give a concrete introduction of the mechanism and current developments of the field emission. Considering the specificity of the simulation method of the field emission current for the nano-materials, we give specific discuss on the related issues. We introduce some basic concepts of the modern DFT because we combine it with the traditional wavefuntion matching method when we calculate the field emission current. Besides, we also give its detailed step on theory. Graphene-based nano-materials are our main research object in this paper, so we introduce some basic knowledges of them, including graphene, graphene nanoribbons and carbon nanotubes, in order to help people to get a clear understanding on their applications in the field emission area.In chapter2, the content consists of two parts. In the first part, field emission properties of zigzag graphene nanoribbons terminated with C-O-C ether groups (including cyclic and alternative ether groups at edge, denoted as ZGNR-CE and ZGNR-AE), which have been found experimentally, are studied by the method which we give a detailed description at the previous section. The results show that the field emissions of these two nanoribbons are dominated by states around Brillouin zone center and close to Fermi level. Because of lower work function, the ZGNR-CE can produce much stronger emission current than reconstructed zigzag graphene nanoribbon. The ZGNR-AE has nearly completely spin-polarized emission current, although its emission current is not strong enough. It is also found that under the lower E-field, the uniaxial strain can effectively modulate their emission currents but the spin polarization of ZGNR-AE keeps unchanged with the varied strain. The underlying mechanisms are revealed by combining the analyses of their work functions and band structures with edge dipole model. At the second part, we introduce the field emission properties of zigzag graphene nanoribbons terminated with N-H secondary amine-group (denoted as ZGNR-NH). The atoms of ZGNR-NH are not on the same plane. It has extraordinarily low work function. The band structure has a flat band attached to the Γ point at the Fermi level. The field emission current of ZGNR-NH is very strong, benefiting from its strong edge dipole moment. The field emission properties of ZGNR-NH are even better than the graphene nanoribbon chemically modified by C-O-C ether groups.In chapter3, we introduce the field emission properties of bilayer graphene with surface chemically modified by H and F atoms. The Γ state occupied by electron of the bilayer graphene is far away from the Fermi level, and as a result, the bilayer graphene owns a bad field emission property. When both layers of the bilayer graphene are modified by H atoms, we get bilayer graphane, which has a slightly lower work function than the bilayer graphene. But the most important is that the H modification leads to that the occupied states near the Fermi level mainly exist near the Γ point, so the field emission properties is improved obviously. According to relevant researches of other group, the electronic structure of the bilayer graphane is effectively modified by replacing H atoms on a single side with F atoms. The nearly free electron states would fall into the Fermi level and be occupied by electrons, along with an extraordinarily low work function. A deep analysis is given about the field emission properties of this new material, and we can find that its field emission properties are greatly improved compared with the bilayer graphane and the bilayer graphene.