Fano Resonance Effect in CO-Adsorbed Zigzag Graphene Nanoribbons

Quantum interference plays an important role in tuning the transport property of nano-devices. Using the non- equilibrium Green＇s Function method in combination with density functional theory, we investigate the influence to the transport property of a CO molecule adsorbed on one edge of a zigzag graphene nanoribbon device. Our results show that the CO molecule-adsorbed zigzag graphene nanoribbon devices can exhibit the Fano resonance phenomenon. Moreover, the distance between CO molecules and zigzag graphene nanoribbons is closely related to the energy sites of the Fano resonance. Our theoretical analyses indicate that the Fano resonance would be attributed to the interaction between CO molecules and the edge of the zigzag graphene nanoribbon device, which results in the localization of electrons and significantly changes the transmission spectrum.

N-Doped Zigzag Graphene Nanoribbons on Si(001):a First-Principles Calculation

The structural and electronic properties of N-doped zigzag graphene nanoribbons （N-ZGNRs） adsorbed on Si（001） substrates are investigated with first-principles density functional cMculations. Compared with the free-standing N-ZGNRs, the energy difference between the substitutional doping at the edge and the inner sites is significantly decreased on the Si substrate. The distribution of the extra charge induced by the N substitutional .dopant keeps the Friedel oscillation feature, and is a main effect that influences the C-Si bonding strength. When N is doped in regions with high C-Si bond densities, the strain induced by the dopant also plays an important role in determining the C-Si bonding interactions. Similar to the undoped case, the strong N-ZGNR/Si interaction destroys the antiferromagnetic coupling of the edge states in N-ZGNR, leading to a non-magnetic ground state for the N-ZGNR/Si heterostructures.

Structural and Electronic Properties of Zigzag Graphene Nanoribbons on Si(001)Substrates

We study the adsorption of zigzag graphene nanoribbons （GNRs） on Si（001） substrates using the first-principles density functional theory, exploring the effects of the interface interaction on the structurM and electronic prop- erties of both GNRs and the substrate. By comparing the adsorption structures predicted by the local density approximation, the generalized gradient approximation, and the DFT-D2 approach, we confirm that both edge and inner C atoms of GNRs can form covalent bonds with the substrate. The GNR/substrate interaction destroys the antiferromagnetic coupling of the edge states in GNB.s. The charge transfer from the substrate to GNRs exhibits a complicated pattern and is mainly localized near the C-Si bonds. We also observe a strong perturbation of the surface states and a surface reconstruction transition induced by the GNR adsorption.

Band structure and edge states of star-like zigzag graphene nanoribbons

Connecting three zigzag graphene nanoribbons（ZGNRs） together through the sp^3 hybrid bonds forms a star-like ZGNR（S-ZGNR）. Its band structure shows that there are four edge states at k = 0.5, in which the three electrons distribute at three outside edge sites, and the last electron is shared equally（50%） by two sites near the central site. The lowest conductance step in the valley is 2, two times higher than that of monolayer ZGNR（M-ZGNR）. Furthermore, in one quasithree-dimensional hexagonal lattice built, both of the Dirac points and the zero-energy states appear in the band structure along the z-axis for the fixed zero k-point in the x-y plane. In addition, it is an insulator in the x-y plane due to band gap 4 eV, however, for any k-point in the x-y plane the zero-energy states always exist at kz = 0.5.

On-surface synthesis and edge states of NBN-doped zigzag graphene nanoribbons

Zigzag graphene nanoribbons(ZGNRs)with spin-polarized edge states have potential applications in carbon-based spintronics.The electronic structure of ZGNRs can be effectively tuned by different widths or dopants,which requires delicately designed monomers.Here,we report the successful synthesis of ZGNR with a width of eight carbon zigzag lines and nitrogen-boronnitrogen(NBN)motifs decorated along the zigzag edges(NBN-8-ZGNR)on Au(111)surface,which starts from a specially designed U-shaped monomer with preinstalled NBN units at the zigzag edge.Chemical-bond-resolved non-contact atomic force microscopy(nc-AFM)imaging confirms the zigzag-terminated edges and the existence of NBN dopants.The electronic states distributed along the zigzag edges have been revealed after a silicon-layer intercalation at the interface of NBN-8-ZGNR and Au(111).Our work enriches the ZGNR family with a new dopant and larger width,which provides more candidates for future carbonbased nanoelectronic and spintronic applications.

Spin-polarized transport in a normal/ferromagnetic/normal zigzag graphene nanoribbon junction

We investigate the spin-dependent electron transport in single and double normal/ferromagnetic/normal zigzag graphene nanoribbon （NG/FG/NG） junctions. The ferromagnetism in the FG region originates from the spontaneous magnetization of the zigzag graphene nanoribbon. It is shown that when the zigzag-chain number of the ribbon is even and only a single transverse mode is actived, the single NG/FG/NG junction can act as a spin polarizer and/or a spin analyzer because of the valley selection rule and the spin-exchange field in the FG, while the double NG/FG/NG/FG/NG junction exhibits a quantum switching effect, in which the on and the off states switch rapidly by varying the cross angle between two FG magnetizations. Our findings may shed light on the application of magnetized graphene nanoribbons to spintronics devices.

Adiabatic quantum pump in a zigzag graphene nanoribbon junction

The adiabatic electron transport is theoretically studied in a zigzag graphene nanoribbon （ZGNR） junction with two time-dependent pumping electric fields. By modeling a ZGNR p-n junction and applying the Keldysh Green＇s function method, we find that a pumped charge current is flowing in the device at a zero external bias, which mainly comes from the photon-assisted tunneling process and the valley selection rule in an even-chain ZGNR junction. The pumped charge current and its ON and OFF states can be efficiently modulated by changing the system parameters such as the pumping frequency, the pumping phase difference, and the Fermi level. A ferromagnetic ZGNR device is also studied to generate a pure spin current and a fully polarized spin current due to the combined spin pump effect and the valley valve effect. Our finding might pave the way to manipulate the degree of freedom of electrons in a graphene-based electronic device.

Valley selection rule in a Y-shaped zigzag graphene nanoribbon junction

The valley valve effect was predicted in a straight zigzag graphene nanoribbon （ZGR） p/n junction. In this work, we address a possible valley selection rule in a Y-shaped ZGR junction. By modeling the system as a three-terminal device and calculating the conductance spectrum, we found that the valley valve effect could be preserved in the system and the Y-shaped connection does not mix the valley index or the pseudoparities of quasiparticles. It is also shown that the Y-shaped ZGR device can be used to separate spins in real space according to the unchanged valley valve effect. Our finding might pave a way to manipulate and detect spins in a multi-terminal graphene-based spin device.

Spin-polarized transport in graphene nanoribbon superlattices

By the Green＇s function method,we investigate spin transport properties of a zigzag graphene nanoribbon superlattice（ZGNS） under a ferromagnetic insulator and edge effect.The exchange splitting induced by the ferromagnetic insulator eliminates the spin degeneracy,which leads to spin-polarized transport in structure.Spin-dependent minibands and minigaps are exhibited in the conductance profile near the Fermi energy.The location and width of the miniband are associated with the geometry of the ZGNS.In the optimal structure,the spin-up and spin-down minibands can be separated completely near the Fermi energy.Therefore,a wide,perfect spin polarization with clear stepwise pattern is observed,i.e.,the perfect spin-polarized transport can be tuned from spin up to spin down by varying the electron energy.