First-principles Investigation On The Properties Of Nanocone And Graphene

With the rapid development of nanotechnology and the experimental progress in scanning tunneling microscope (STM), especially upon discovering fullerence, nanotube, nanocone and graphene, there exist many possibilities to design and realize molecular devices by using the properties of the nanomaterials. Simultaneity, it becomes more and more important to explain the phenomenon via the.computational method and theoretically design active electronic devices. In this dissertation, we calculate the geometrical structure, electronic and field emission properties of carbon nanocones, N/B doped carbon nanocones and BN nanocones using first-principles. Meanwhile, we perform density functional theory (DFT) combined with nonequilibrium Green's function on the electronic and transport properties of symmetric (two edges are terminated with the same atoms or groups) and asymmetric (one edge is terminated with H atom, and the other edge is terminated by other atoms or groups) zigzag graphene nanoribbons (ZGNRs). This work will be very helpful in designing the devices with two electrodes and synthesis of nanometer-sized devices.In Chapter 1, we give a brief introduction to the background, structures, properties, applications and synthesis of carbon nanocone, BN nanocone and graphene. Furthermore, critical unresolved issues in the research of these materials and the theory about the field emission and spin-dependent electronic transport properties have also been introduced. Finally, we simply describe the purpose of this thesis.In Chapter 2, we introduce the progress and basic concept of the theoretical method used in the thesis, including first-principles, density functional theory and nonequilibrium Green's function. We also give a brief description of the simulation package DMol3 and ATK, which are used in this work. Then we introduce the method for calculating the field emission current.In Chapter 3, we investigate the stability, electronic and field emission properties of five different carbon nanocones. Our calculations reveal that with increasing the disclination angle, the stability of carbon nanocone decreases. Under the applied field, the pentagonal rings have more opportunity to be the emission site. The calculation on work function and electronic density reveals that the electronic properties do not depend critically on the cone angle, but the position and the number of pentagons. We also find that the orbitals having the largest emission current locate at the pentagon. The calculated field emission current for the CNC with three pentagons is the largest among CNCs.In Chapter 4, we present a theoretical investigation on N/B-doped CNCs with 60°disclination. Under the consideration that the concentration of the foreign atoms in carbon materials typically does not exceed 2-4% and the effect of the doping position, we focus on how the electronic and field emission properties are modified via N or B atom doping at different positions and with different concentrations. In this work, we perform first-principles DFT calculations to optimize all the geometrical structures of doped CNCs, and calculate the electronic structure and the emission current. The calculation results reveal that the enhanced electron emission due to N doping is related to a decrease in the work function and increase in the emission current, which attributes to the upshift of the Fermi level induced by the local state. N-doped CNCs exhibits a low turn-on voltage for field electron emission. Especially, with two N doping at the pentagon, the systems has the largest emission current. However, B-doping has no significant influence on the emission properties of CNCs.In Chapter 5, we perform first-principles DFT calculations to investigate the geometrical structures and field emission properties of different boron nitride nanocones with 240°disclination. It is found that the nanocones can be stable under applied electric field and the emission current is sensitively dependent on the tips of nanocones. The nanocones with homonuclear bonds at the tip can introduce additional energy states near Fermi level, which can reduce the ionization potential and increase the emission current of boron nitride nanocones.In Chapter 6, we perform DFT combined with nonequilibrium Green's function calculations on the symmetric and asymmetric terminated ZGNRs. The calculation results reveal that these different species of atoms and groups have a significant impact on the edge states near Fermi level as well as the spin-depandent electronic transport properties. Our results indicate that graphene nanoribbons have great potential to serve as future molecular sensors and spin filters.In conclusion, we have performed first-principles DFT calculations on the properties of carbon nanocones, N/B doped carbon nanocones, BN nanocones and graphene nanoribbons. This investigation on the field emission properties of nanocones and the spin-dependent transport properties of graphene nanoribbons will give a helpful guidance in synthesis and design of new electronic devices.