Study Of Charge Transport Mechanism And Calculational Design For Low-dimensional Carbon-based Molecular Devices

Using first-principles calculation based on density functional theory in combined with non-equilibrium green function method, the electronic structures and charge transport properties for several molecular scale devices have been studied in this thesis. There are many important problems, such as, the charge transfer between electrodes and molecules, the conjugated molecular orbitals (MOs), the doping atoms and the side groups on molecules, have been discussed deeply. Moreover, we also put forward some suggestions on functional molecular devices, such as DNA sequencing sensors and PN junctions.The electronic transport properties of C60,C59N and C59B have been studied. The results show that all the three molecular junctions display metallic transport behavior. At low bias, the introduction of the donor atom nitrogen and acceptor atom boron can weaken the electron transport, especially the acceptor doping significantly suppresses the electron transport. When the bias is in the range from0.6V to1.0V, the currents of C60and C59N molecule junctions are remarkably suppressed, especially, in the former, negative differential resistance (NDR) behavior appears in this bias range. However, the current of C59B molecule junction increases rapidly with the increase in the bias. These results show that the electronic transport properties of molecular junction can be modulated by doped atoms, which results from the fact that the introduction of the doping atoms leads to an increase in HLG and shift in the molecular orbitals obviously, as well as the change in the coupling degree between the molecular orbitals and electrodes.We calculated the effects of construction torsion and side groups for electronic transport properties of a molecular wire with tetrathiafulvalene (TTF) as center, which sandwiched with two gold electrodes. The theoretical results show that conjugated molecular orbitals can be changed by the construction torsion and side groups, and the transport characteristics have been modulated. When all of atoms in TTF are lying at the same plane, the frontier molecular orbitals are expanded and contribute to the electronic conductions of molecular devices. Otherwise, the molecular orbitals are localized under construction torsion, and the electronic conductions are suppressed. In addition, the TTF molecule connected with donor and acceptor side groups have been investigated. The calculated results show that the donor (-NH2) side group is contributed to electronic conduction, however, the current is slightly decreased for acceptor (-NO2) one.We have investigated spin-dependent electronic transport properties of molecular junctions constructed by Zigzag graphene nanoribbons (ZGNRs) with V-shaped notches. The effects of armchair and zigzag V-shaped notches on symmetry and asymmetry ZGNRs have been considered. The results show that when the spin polarization is considered, all of the calculated systems present semiconductor behavior. When introducing a V-shaped notch on the edge of ZGNR, the current is reduced remarkably, moreover, the spin-up and spin-down display different behaviors. Especially, the effects of reduction play a more important role in asymmetric5-Z than in symmetric4-Z, and also, for the zigzag V-shaped notched ZGNR, the transmission spectrum will be suppressed with the bias increasing and NDR behaviors can be observed. It is suggested that the breakdown of the edge states should result in the effects of current increasing and the spin-dependent current characteristics, and the NDR behavior originates from the changes of the frontier molecular orbitals of zigzag V-shaped notched ZGNRs with the bias increasing.Based on V-notched graphene nanoribbons, we designed a fast DNA sequencing molecular sensor. The results show that the electronic transport properties of GNRs junctions can be effect by the interaction of the base molecules (A, G, C and T) and the V-notched graphene nanoribbons. In special bias region, the electronic conductions are very different for base molecules A, G, C and T. We found that the molecular orbitals will be changed by the interactions of base molecules and V-notched graphene nanoribbons. The changed MOs will be contributed to the electronic conductions, so the different base molecules can be identified.The charge transport properties of GNRs heterojunctions have been investigated. Firstly, we studied the transport of the different width AGNRs heterojunctions, the results show that the electronic transport properties of heterojunctions are depended on the electronic structures of perfect AGNRs. A good electronic conduction can be observed for the AGNRs heterojunction is constructed by an AGNRs with width of3N and another with width of3N+2, otherwise, the electronic conduction is very smaller for3N+1. Secondly, the transport properties of ZGNRs heterojunctions with donor and acceptor side groups also have been considered. We find that the heterojunction constructed by symmetric ZGNR can perform single direction conductivity and exhibits an effect of P-N junction. However, a NDR effect can be observed in the heterojunction of asymmetric ZGNR. These interesting results will be useful for development of practical functional molecular devices.