Investigation Of The Electronic Structures And Transport Properties For CNTs And SiCNTs

The research progresses on the carbon nanotubes (CNTs) and silicon carbide nanotubes (SiCNTs) are reviewed entirely at first, such as preparation, purification and application, in which the Brillouin zone of the CNTs and the applications related to their electronic structures are described. The density functional theoty and nonequilibrium Green's function are discussed in detail. With this method, achievements have been obtained in the study of the electronic structures and transport properties of the nano-materials and nano-electronics. Using the above method, the electronic structures and transport properties of the CNTs and SiCNTs are calculated in this paper.The structures and electronic properties of the intrinsic and doped (8, 0) CNT are calculated by first-principles calculation based on the density functional theory (DFT). The intrinsic (8, 0) CNT is a direct band-gap semiconductor with a value of 0.46 eV. The electronic properties of the doped CNTs are quite different from the intrinsic CNTs', which broadens the range of their application. From the optimized structure, we can see that the lengths of C-N bonds are longer than that of the C-C bonds. This tendency becomes more obvious with the increase of the doping concentration. It is consistent with the structure of the synthesized nitrogen-doped CNTs, which is bamboo-shaped. As the nitrogen atoms supply excess electrons, these electrons centralize on the doped nitrogen atoms and adjacent carbon atoms. This leads to the increase of the possibility of the charge transfer between different atoms in the CNT. The band-gap of the nitrogen-doped CNT is narrowed. The influence of the boron atoms on the structure of the CNT is similar to the nitrogen and the radius of the boron atom located is increased. Holes are formed by the doped boron atoms, in which the boron atoms localize and the band-gap of the boron-doped CNT is broadened.The transport properties of the isolated (8, 0) CNT and coupled to Au electrodes are investigated with the method combined non-equilibrium Green's function (NEGF) with DFT. The step–shaped equilibrium transmission spectrum of the isolated (8, 0) CNT shows that the CNT is a semiconductor, which is coincident with the result achieved by first-principles calculations. The current voltage curve of the isolated (8, 0) CNT can be divided into two parts, when the bias voltage smaller than 1.2V, the current is near zero, while the voltage greater than 1.2V, the relationship between the current and the voltage is nearly exponential. In practical applications, CNTs are usually connected to metal electrodes. We calculated the transport properties of the (8, 0) CNT coupled to Au electrodes, in which the influence of length on the transport properties of CNT is considered. With the increase of the CNT's length, the two probe system transforms from metallic to semiconductoring. In the formation of the contact between Au electrodes and the CNT, charge transfer between electrodes and the SiCNT will occur due to the difference in their work functions, which results in the metal-induced gap states (MIGS). In short CNTs, the MIGS plays an important role in its transport properties. Its influence weakens with the increase of the CNT's length. This is the reason why no MIGS are observed in experiments.Using the method in the study of CNT's electronic structures, the electronic structures of the intrinsic and doped (8, 0) SiCNT are obtained. The radius of the carbon rings are greater than that of the silicon rings in the optimized intrinsic (8, 0) SiCNT. The intrinsic (8, 0) SiCNT is a direct band-gap semiconductor with a value of 0.94 eV, which is broader than the CNT's. This is owing to the ionicity of the Si-C bonds in SiCNTs. As boron and nitrogen are the common doping material in bulk SiC, they are selected as impurities in the study of the doped SiCNT. In substitution doping of the SiCNT, carbon atoms are replaced by nitrogen atoms, which is the same in bulk SiC doping. When one nitrogen atom is doped into the (8, 0) SiCNT, the lengths of Si-N bonds are shorten. In two nitrogen atoms doped SiCNT, the nitrogen atoms are like to occupy the nearest crystal lattice in the adjacent carbon rings and a salient is formed on the SiCNT's surface. The excess electrons provided by nitrogen atoms locate mainly on the silicon atoms adjacent to the impurity atoms. The band-gap of nitrogen-doped SiCNT is narrowed. The radiuses of the boron atoms located silicon rings are decreased. The appearance probability of the electrons near the doped boron atoms is low and holes are formed, which results in the broadening of the band-gap.The transport properties of isolated (7, 0) SiCNT and coupled to Au electrodes are studied with the same method in the investigation of CNTs'transport properties. The transmission spectrum of the isolated SiCNT shows that is a semiconductor and its turning on voltage is about 2.2 V. In the (7, 0) SiCNT coupled to Au electrodes, the MIGS are found, which leads to the transmission coefficient near the Fermi energy is no zero and under small bias, the relationship between the current and bias voltage is linear. In the bias voltage range from +1.4 V to +1.6 V, the current decreases with the increase of the bias voltage. This means the appearance of the negative differnetial resistance (NDR). The origin of the NDR is the variation of the transmission spectrum caused by the applied voltage.