Theoretical Studies On The Electronic Transport Properties And Mechanism Of Carbon Nanotube/Graphene Heterojunctions

Because of the miniaturization of electronic devices, traditional silicon-based semiconductor devices will gradually access their physical and geometrical limits.Carbon-based molecular devices are considered as the most ideal substitution of traditional silicon-based semiconductor devices. New carbon-based materials, such as grapheme and carbon nanotubes, have been expected to be applied to electronic nanodevices because of their excellent mechanical and electrical properties. Recently,graphene/carbon nanotube composite materials have attracted more and more attention, because they not only combine the excellent properties of both graphene and carbon nanotubes, but also show more excellent electromagnetical and mechanical properties. Recently, they have been successfully applied in the capacitors,optoelectronic devices, storage battery, electrochemical sensors, and other fields. In this thesis, we calculate the electronical transport properties of various graphene/carbon nanotube composite heterojunctions using first principles calculation method, which is based on density functional theory and nonequilibrium green’s function. Our attention was mainly concentrated on the effects of contact structure details on transport properties of graphene/carbon nanotubes composite heterojunctions. It is observed obvious rectification, resonance effect and spin filtering effect from these graphene/carbon nanotube composite heterojunctions and the mechanisms of these effects are analyzed, respectively.we propose a spin device made of GNR/CNT/GNR heterojunction constructed by attaching two semi-infinite zigzag- GNRs at the side-wall of a segment of CNT. This GNR/CNT/GNR heterojunction contains two sp3-hybridized contacts. A good thermal stability of the heterojunction was proved from ab initio molecular dynamics stimulations at room temperature, and resonant transport and spin-selective transport characteristics were revealed from nonequilbrium Green’s functions(NEGFs)calculations. It is revealed that the two sp3 contacts formed at the interface can be regarded as two isolators, which let the CNT act as a resonator and only allow the electron of one spin-orientation and resonant energy to transport through the system,resulting in a remarkable spin-selective transport behavior. Our findings will be useful for building all-carbon spin injection and spin filter devices.we evaluate the electronic transport properties of CNT/GNR devices from firstprinciples calculations. It was found that the transmission function of the CNT/GNR is sensitive to the contact structure in junction region. The device with an ideal contact shows a metallic characteristic and the device with a realistic contact is semiconducting. Electronic rectifying is an inherent characteristic of the system. The realistic contact can enhance the rectifying and the obtained rectifying is insensitive to the details of the edge atomic structure of the contact. Our findings would be useful in designing and manufacturing all-carbon rectifications using CNT/GNR junctions in nanoelectronics.we put forward an all-carbon molecules spin filter with another GNR-CNT-GNR configuration. It is constructed by directly connecting two semi-infinite zigzag graphene nanoribbons to a segment of armchair carbon nanotube, which is different from the GNR-CNT-GNR configuration mentioned in the third chapter. In this chapter,all of GNR-CNT-GNR configurations considered have two junction regions with the same contact structrue. We calculate the electronic transport of the GNR-CNT-GNR heterojunction using the non-equilibrium green function combined with spin-polarized density functional theory method. It is found that the electron transport properties of GNR-CNT-GNR heterojunctions are sensitive to the contact structures.The GNR-CNT-GNR heterojunction with two ideal contacts is metal and has not obvious spin filtering properties. But GNR-CNT-GNR heterojunction with two same realistic contacts is semiconductor and has obvious spin filtering properties. The remarkable spin-selective transport behavior is attributed to two realistic contacts formed at interfaces. The two realistic contacts can be regarded as two isolators,which let the CNT act as a resonator and only allow the electron of one spin-orientation and resonant energy to transport through the system. Our findings will be useful for building all-carbon spin injection and spin filter devices.we further research the effect of contact microstructures on the electronic transport properties of GNR-CNT-GNR heterojunction, which is contructed by directly connecting two semi-infinite zigzag graphene nanoribbons to a segment of armchair carbon nanotube. In this chapter, we calculate the electronic transport of GNR-CNT-GNR heterojunctions with two same contacts and two different contacts. It is found that when the two contacts are two ideal contacts, GNR-GNR-CNT composite heterojunction is metal. When two contacts are the same realistic contact,GNR-CNT-GNR composite heterojunction can be characterized by metal or semiconductor depending on the realistic contact strctures. It is because different realistic contact structures have different barrier height. When two contacts are thedifferent realistic contacts, GNR-CNT-GNR composite heterojunctions are semiconductor. Our findings will be useful for designing and manufacturing highperformance CNT/graphene molecular nanodeviceswe studied the effect of doping N atoms on the electronic transport properties and electronic rectifying behavior of GNR-CNT composite heterojunction. We considered these three situations: doping N atom only in the part of carbon nanotube,doping N atom only in the part of grapheme nanoribbon, doping N atom in both carbon nanotube and grapheme nanoribbon of GNR-CNT composite heterojunction.We periodically doped N atoms along the longitudinal direction of one-dimensional CNT-GNR heterojunction. When the distance of N atom from each other is the distance of three cells, doping N atoms can’t change the semiconductor character of CNT-GNR heterojunction, but can significantly reduce the gap width. When doping N atom only in the part of CNT, doping N atoms can strengthen the transport of electrons. When doping N atom only in the part of GNR, doping N atoms can strengthen the transport of holes. When doping N atom in both carbon nanotube and grapheme nanoribbon, doping N atoms can strengthen the transport of both electrons and holes. Doping N atoms in CNT and GNR respectively can not change the rectification behavior of CNT-GNR heterojunction. But the rectification behavior disappeared when doping N atoms in both of CNT and GNR.These results suggested the feasiblity for designing realistic all-carbon molecule spin filter and rectifiers nanoelectronics based GNR-CNT composite heterojunction.This work is of benefit to develop functional molecular devices.