Researches On Influences Of Defects On The Electronic Transport Properties Of Single Walled Carbon Nanotubes And Graphene

We have made great progress in silicon-based electronic devices that have been widely used in computing, communications, automation and other applications for more than four decades, and the miniaturization of electronic devices is the remarkable characteristic to a large extent. As a matter of fact, denser, faster and more power-efficient circuitry is obtained through the approach of continuously silicon-based transistors miniaturizing. At present, the challenge in face of us is that the miniaturization approach will soon encounter both scientific and technical limits. It is urgent for us to make an effort to develop alternative device technologies after we realize the approaching limits. Owing to the excellent electrical properties, carbon-based nanomaterials, such as one-dimensional (1D) carbon nanotubes (CNT) and two-dimensional (2D) graphene layers, are considered as candidates to make next generation electronic devices. With the development of high performance cluster computers and the improvement of algorithm for calculating, numerical simulations based on the first-principles are able to be performed to understand physical background of atomistic structures and to investigate device engineering issues. More importantly, simulation becomes the most important method to investigate the nanoelectronic.In this dissertation we make use of the first-principles density functional theory combined with nonequilibrium Green's function technology to systematically explore the influence of curvature, intramolecular junction, topologic defects and vacancy defects on the electronic structure and transport properties of single-walled carbon nanotubes. In addition, we discuss the effect of substitute nitrogen-doping on the electronic structure of zigzag-edged metallic graphene nanoribbons (GNRs) and come to some meaningful conclusions. It is significant for the practical preparation and development of the nanotubes-based electronic devices.The simulation results of carbon nanotube with small diameter show that the electronic structure and transport properties remarkablely change due to curvature effects. For example, the calculation taken into account of curvature effect suggests that the (2,2) carbon tube has a certain transmission gap; it is confirmed by the results of its current-voltage curves with semiconducting characteristics. It can be seen that the curvature effect shifts down theπ* band crossing the Fermi level, which gives rise to the (5,0) nanotube metallic character. The emergence of negative differential conductance (NDR) in small diameter nanotube indicates that electron transmission channels are suppressed under certain bias. We point out that curvature effect on zigzag tube with diameter larger than (9,0) tube or armchair tube with diameter larger than (4,4) tube is very little.The dissertation focuses on the issue of various topologic defects in carbon nanotube. We systematically investigate the influence of three typical kinds of heterojunctions, including semiconducting-semiconducting (S-S), metallic-semiconducting (M-S) and metallic-metallic (M-M), on the transport properties of carbon nanostructure. It is found that the introducing of three kinds of heterojunctions all result in localized states. Under bias the transmission gap shifts in all heterojunction cases and the emergence of NDR in M-S heterojunction. In addition, our simulation shows that the gate voltage can effectively mediate the current through S-S heterojunction nanostructure.Atomic vacancy is an important type of point defects that widely exists in carbon nanotubes. We perform the calculation in the cases of the metallic (12,0) and (7,7) carbon nanotube, exploring the effects of different numbers of atomic vacancies as well as various vacancy configurations on the transport properties by comparison the results for the defective nanotube and for the pristine tube. From the point of transformation energy, carbon nanotube with divacancy defect is the most stable nanostructure among defective nanotubes. Defects in carbon nanotubes give rise to defective states. Monovacancy configurations usually contain one pentagon and one dangling bond forming a so-called 5-1DB defect, which does not have much impact on the electronic transport. We find current in (12,0) carbon nanotube does not monotonously decrease with increasing number of vacancies since the conductance for the six vacancies case is larger than that for the divacancy configurations results in the charge density near the defects for the former is larger than that for the latter. However, the current monotonously decrease with increasing number of vacancies in the (7,7) carbon nanotube. In addition, Stone-Wales defect in carbon nanotube is extremely unfavorable for the electron transport.We also pay attention to the newly found graphene material and simulate electronic structure and transport properties of graphene nanoribbon with ziazag edge. The band structure of zigzag-edged nanoribbon has metallic character, and its electronic transport properties can be considerablely improved by nitrogen doping as a result of density of states at the Fermi level increases, which makes the transition of electrons from the valence band to the conduction band becomes easier. Moreover, the electronic transports in these nanoribbons depend on the nitrogen doping site and the nitrogen atom energetically prefers to lie on the nanoribbon edge.