Investigation Of Electronic Transport Properties For Nanotube Heterojunctions

Nanotube heterojunctions have good application prospects in nano-electronic devices, whose electronic transport properties are foundation of relevant investigations. Studies on electronic transport properties of nanotube heterojunctions are not only of high theoretical value, but also of great practical significance. The electronic transport properties of the carbon/nitrogen-doped carbon nanotube heterojunction, the carbon/silicon carbide nanotube heterojunction and the carbon/boron nitride nanotube heterojunction are studied with the method combined density functional theory (DFT) with nonequilibrium Green's function (NEGF) and the main conclusions are as follows:1. Electronic transport properties of carbon/nitrogen-doped carbon nanotube heterojunctionThe structure and electronic transport properties of the heterojunction may improve related studies. First, the model of the carbon/nitrogen-doped carbon nanotube heterojunction is established and optimized. From the optimized structure, it is can be seen that the diameter of the nitrogen atoms located is enlarged, which is consistent with the structure of the synthesized nitrogen-doped CNT. Then, the electronic transport properties of the heterojunction are studied with the method combined DFT with NEGF. The symmetry of current voltage characteristic for the isolate heterojunction is high. The threshold voltages under positive and negative are±1.0 V respectively. The current voltage curve of the heterojunction coupled to Au electrodes can be divided into two parts: under negative bias and positive bias. Under negative bias, the relationship between the current and the bias voltage is nearly linear. Under positive bias, the current voltage curve consists of three ranges: from 0.0 V to +0.6 V, from +0.6 V to +1.2V and from +1.2 V to +2.0V. In these three ranges, the current voltage curve is nearly linear. While the current in the second bias range increase slower than in the other two bias ranges.2. Electronic transport properties of carbon/silicon carbide nanotube heterojunctionCNT and silicon caride nanotube (SiCNT) are two types of nanotube materials deeply studied. The structure and electronic transport properties of the carbon/SiCNT heterojunction is worth of being studied. Researches on this nanotube heterojunction are focused on two aspects: First, the model of nanotube heterojunction formed by a finite (8, 0) CNT and SiCNT is built and optimized with first principle calculations based on DFT. Results show that the rearrangement of the heterojunction mainly concentrates on its interface and the variation in structure decreses rapidly with the distance increase from the interface. The changes of diameters for the fourth layer in CNT section and SiCNT section can be ignored. Second, the transport properties of the isolate heterojunction and the heterojunction coupled to Au electrodes are studied. From the molecular orbitals of the isolate heterojunction, it can be seen that the conduction band bottom and the valence band top of the heterojunction lie in the CNT. The threshold voltages under negative bias and positive bias are -2.2 V and +1.8 V. In the transport properties of the heterojunction coupled to Au electrodes, negative differential resistance (NDR) effect is discovered. It originates from the variations of the locality for the molecular orbitals caused by the applied bias voltage.3. Electronic transport properties of carbon/boron nitride nanotube heterojunctionThe CNT/boron nitride nanotube (BNNT) heterojunction is the possible fabricated one. The model for an (8, 0) CNT/BNNT heterojunction is established. The geometry optimization of the heterojunction is implemented with the first principle calculations to obtaion the structure of the heterojunction, which is most similar to the fabricated. Results show that the structure variation of the heterojunction is weak in its formation, which is due to the high similarity in structure for two nanotubes. The electronic transport properties of the isolate heterojunction and of the heterojunction coupled to Au electrodes are studed. From the transport properties of the isolate heterojunction, it is can be found that both the conduction band bottom and the valence band top locate on the carbon nanotube section and rectifying property is found in its current voltage characteristic. While the heterojunction is coupled to Au electrodes, its transport properties are influenced by the coupling effect. The most important property is the existence of the NDR under positive bias and negative bias. It derives from the viariation of the locality for molecualr orbitals of the heterojunction caused by the applied bias voltage.The studies on the structure of the heterojunctions are meaningful to their synthesis. Investigations on the transport properties are helpful to modeling and developing novel devices.