First-principles Studies Of Field Emission Properties Of Some New Type Nano-materials

Field emission properties of nano-materials have always attracted larger numbers of researching interests. Studying the emission properties of nano-materials is very helpful for people to understand the mechanism of current emissions in theory and to design new electronic devices in practice. And the spin polarized field emission is also very useful in condensed matter physics and nuclear physics. On the other hand, the density functional theory (the DFT), as a theory that was used widely and successfully in condensed matter physics and quantum chemistry, would be the best tool to calculate the electronic structures and field emission properties of nano-systems. In this dissertation, we mainly apply the density functional theory to calculate the field emission and spin polarized field emission from some new type of nano-materials, including some nano-ribbons and nano-tubes.In the first chapter, we introduce the basic idea of field emission and a kind of self-consistent method for calculating the emission currents used in this article. Also we give a brief introduction of graphene and nanotubes as well as the basic idea of the density functional theory. The concept of field emission is the process that electrons in an atom overcome a potential barrier and tunnel into the vacuum. We introduce the basic idea of field emission, including the definition, the usefulness and the basic theory of field emission including some methods for calculating the emission currents. Then we introduce graphene and nanotubes in brief. At last, we sketch the framework of the density functional theory. The DFT tells that we would know any properties of a many body system so long as we know the electron density of a system at ground state.In chapter 2, we mainly discuss the spin polarized field emission properties of edge decorated graphene nano-ribbon in lateral and longitudinal electric field, respectively. Zigzag edge graphene nano-ribbon is semiconductor with two degenerate electronic edge states near the Fermi level occupied by different spin channel each other. When decorated by different chemical group on each edge, one spin channel would be metallic while the other keeps the same and the whole system becomes spin selective. Spin polarization near the Fermi level may be excellent spin polarized electron emission sources. So we study the spin polarization of field emission of these systems. We got highly spin polarized emission currents in some electric field and we find that we can easily control the emission spin polarization just by changing the electric field in some situation. The spin abruption of the electronic structure can prove the results we discussed above.In chapter 3, we study the field emission properties of pure boron-nitride nanotube and boron-nitride nanotube encapsulated with natrium atoms. There are nearly free electron states existing in the conduction band of boron-nitride nanotube. And these states would fall into the Fermi level to form a hybridized state and be occupied by electrons by doping or aikali atom ornament. It is well known that the nearly free electron states existing in the boron-nitride nanotube have a good response with a lateral electric field and would be ideal electron transport channels for emission. We calculate the emission currents and confirm that the NFE states are more active than the bound states and have the distribution feature of dispersion, as a result of which, the NFE states have a pretty performance of field emissions.