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Mechanism Studies Of Electronic Transport In Molecular Scale Devices

Posted on:2011-12-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y RenFull Text:PDF
GTID:1101360308468535Subject:Condensed matter physics
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In this thesis, we have investigated the electronic structure and transport properties of molecular scale systems such as organic molecules, carbon nanotubes (CNTs), graphene nanoribbons (GNRs) etc. by using the density functional theory in combination with nonequilibrium Green's function technique. The length effect, deformation effect, spin-polarized effect, symmetric effect, side group effect and interaction between molecules effect are studied systematically. Simultaneously, some significant fruits such as multiple negative differential resistance (NDR) and molecular switch, etc. have been found.We investigate the electronic transport properties of PPE molecular junction(<8 nm) (MJ) constructed by PPE molecule with different lengths sandwiched between two gold electrodes. The results show that the electronic transport properties depend on the length of molecule. A NDR behavior with large peak-to-valley ratio can be observed and can be modulated with the length of molecule:When molecular length is below 6 nm NDR appears, while the length of molecule is further increased, the NDR behavior disappears. A mechanism is proposed for the length effect and NDR behavior. Thus, it is concluded that the NDR behavior can be modulated with length.we investigate the electronic transport properties of molecular junctions constructed by (8,0) and (9,0) CNTs with radial and elongation deformation. Our results show that the transitions between semiconductor and metal can be induced by the mixed deforma-tion. The results also show that the electronic transport properties of molecular junction can be modulated by deformation degree. It is suggested the change of electronic struc-ture due to symmetry breaking is responsible for these transitions. It can be applied in mechanical sensors and molecular switches in the future.The spin-dependent electronic transport properties in zigzag GNRs (ZGNRs) are studied. The effects of symmetry and defect have been considered. The results show that when the spin polarization is considered, both symmetric and asymmetric ZGNRs present semiconductor behavior, which is different from spin-unpolarized result. The symmetry of ZGNRs plays an important role in electron transport behavior. Asymmetric ZGNR displays monotonic transport behavior. However, in symmetric ZGNRs systems, a NDR is observed. The influence of defect is more obvious in symmetric ZGNRs than in asymmetric systems. A physical analysis of these results is given.A compound molecular device constructed by the CNT, OPE molecule and Au(111) electrodes has been proposed and its electronic transport properties have been studied. The results show that an excellent mechanical molecular switch can be realized by ad-justing the distance between gold and CNT. For larger distance between gold and CNT, the influence of the inserted OPE and side group on electronic transport of the device is more obvious. Furthermore, a NDR behavior with a big PVR can be observed. The be-havior is closely related to the delocalized degree of some frontier molecular orbitals in bias window.We simulate mechanical deformation of armchair GNRs (AGNRs) by applying the ab initio calculation in combination with molecular mechanism method. Then, we re-search the electronic transport properties of deformed AGNRs and double deformed AG-NRs. The results show the mechanical deformation has different influences on various kinds of AGNRs. A molecular switch has also been efficiently preformed through me-chanical relative motion between two deformed AGNRs. The results show that the inter-action between molecules has an remarkable influence on electronic transport properties.
Keywords/Search Tags:Molecular device, First-principles, Nonequilibrium Green function, Density functional theory, Electronic transport, Negative differential resistance, Carbon nanotube, Graphene nanoribbons
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