Study On The Electronic Structures And Transport Mechanisms Of Low-dimension Graphene Nanoribbon Systems

Based on the review of the progress of molecular device, we have performed the density-functional theory combined with the nonequilibrium Green’s functions approaches to systematically study electron-transport properties of molecular junctions based on the first-principles calculations. The effects of substituted atoms,the length of molecules, electrode-molecule contacts, intermolecular interaction, chemical functionalization and gate voltage on the electronic transport properties of molecular devices are studied. The interesting results such as the obvious negative differential resistance (NDR) behaviors, the molecular rectifying behavior, specially the switching behaviors, the spin-filtering effect and the spin polarization, et al have been observed obviously, and the related mechanism on modulating the electronic transport properties is presented. The dissertation is organized as following:(1)The electronic band structures and transport properties of N2AA -doped armchair graphene nanoribbons (aGNRs) with two quasi-adjacent substitu-tional nitrogen atoms incorporated in pairs of neighboring carbon atoms in the same sublattice A, are investigated by using non-equilibrium Green function formalism in combination with density functional theory. The results show that the N2AA -doped aGNRs can be semiconductor or metal by different locations of N2AA -doped aGNRs and the coupling effect between the Pz orbitals of carbon and nitrogen atoms plays an important role in the transition. And the striking NDR behaviors can be found in such devices. Therefore, the transport properties of aGNR systems are mainly dominated by the positions of N2AA.(2)We investigated the electronic transport properties of aGNR devices in which one lead is undoped and the other is N2AA-doped with two quasi-adjacent substitutional nitrogen atoms incorpora-ting pairs of neighboring carbon atoms in the same sublattice A. Two kinds of N2AA-doped style are considered, for N dopants substitute the center or the edge carbon atoms. Our results show that the rectification behavior with a large rectifying ratio can be found in these devices and the rectifying characteristics can be modulated by changing the width of graphene nanoribbons or the position of the N2AA dopant. The results show that the width of N=10 have large rectifying ratio in N2AA-doped aGNRs. The large rectifying ratio in A10-E device may result fromthe fact that as applied the positive bias, the well-matched energy bands encountered between both electrodes and the good coupling appeared in its corresponding molecularly projected selfconsistent Hamiltonian(MPSH).(3)Using the non-equilibrium Greens function method in combination with the density functional theory, we investigate the electronic transport properties of zigzag graphene nanoribbons (zGNRs) passivated with hydrogen atoms. The coexistence of sp2-edges and sp3-edges in zGNRs can induce quite stable conductance gaps in a large range from 0 to 3.5 eV. We found that the orbital symmetry mismatch between the sections with sp2-and sp3-edges is responsible for the totally suppressed conductivity of the edge states, and the gap size is determined by the minimal energy difference between the second highest valence band and the second lowest conduction band of the corresponding sp2-edged ribbons. Beside, the gaps of sp2-and sp3-edges zGNRs are also depended on the width of ribbons. Whereas, gaps in even ribbons were keep large and they are decreased relatively slow to compare with the odd ribbons. Moreover, the size of gap decreases rapidly with the width increasing.(4)The spin-dependent electron transport properties through a single-carbon atomic chain (SCAC) sandwiched between two zGNR electrodes are investigated. Our calculations show that the spin transport behavior of 53 configuration is more obvious than the 56/36 configuration. Moreover, the SCAC connecting two zGNRs with asymmetry-contacting points is a perfect spin filter in the transmission function within a large energy range. The spin-dependent electron transmission spectra exhibit robust transport polarization characteristics and a strong current polarization behavior (almost 100%) can be found. It is suggested that the coupling strength between molecular orbitals and electrodes is mainly responsible for these spin transport phenomena.(5)We studied the spin-dependent electron transport of a molecular device constructed from a chromium porphyrin molecule linking with two carbon chains sandwiched between two semiinfinite zigzag-edged graphene nanoribbons(GNR) electrodes. The results show that the spin-up electrons can easily pass through the device, while the spin-down electrons were wholly suppressed in the P configuration, while the peak moves right into the higher energy region with transmission spectra at the EF being suppressed in the AP configuration. The single spin-conducting can be obtained by performing different magnetic configuration of the leads. The coexistence of spin-filtering with 100% spin polarization, rectifying and negative differential resistance (NDR) behaviors in our model device is demonstrated. Moreover, these special spin-related effects are attributed to the intrinsic transport selection rules of π and π* subbands and the coupling strength between the molecular orbital and electrodes in zigzag-edge GNR.