Zigzag-edge Graphene Nanoribbon Electrodes Research Articles

Rotating single molecule-based devices: Single-spin switching, negative differential electrical and thermoelectric resistance

Single molecule-based devices have been one of the most candidates to realize the miniaturization of traditional electronic devices. Here we investigate the spin-polarized transport properties of an iron-complex molecule sandwiched between two ferromagnetic zigzag-edged graphene nanoribbon electrodes based on the non-equilibrium Green’s function formalism combined with the density functional theory. When the iron-complex molecule rotates, a single-spin switching effect is observed at the Fermi level, and meanwhile a perfect spin filtering effect appears in the molecular device. As the electric bias increases, an obvious negative differential electrical resistance is observed. In addition, we also find a negative differential thermoelectric resistance in the absence of the electric bias under a small molecular rotation, and the sign and magnitude of the thermally driven current can be tuned by the temperature. These theoretical findings imply that the iron-complex molecular devices have potential applications in the next-generation spin electric and thermoelectric devices.

Effect of DNA bases on the thermoelectric performance in graphene nanoribbons

The thermoelectric transport properties of four kinds of DNA bases sandwiched between two zigzag-edge graphene nanoribbon electrodes are investigated by first-principles calculation. The modulation of quantum interference, molecular length, and molecular torsion on thermoelectric transport properties are studied. It is found that the thermopower and efficiency can be obviously enhanced in the guanine and electrode coupling geometry that manifests a quantum interference effect. A decrease of thermopower with a number of DNA bases has been found, while the efficiencies have a different trend with the increase of the number of different molecules. In addition, the thermopower and efficiency also can be considerably enhanced by the twisting angle between the molecule and graphene electrodes.

All-carbon multifunctional molecular spintronic device: A first-principles study

Based on the first-principles density functional theory combined with the nonequilibrium Green’s function methodology, we have designed and investigated the spin-polarized transport properties of an all-carbon molecular device, in which a C60 fullerene is sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes. The results show that the spin-polarized properties are strongly dependent on the external magnetic field modulation and spin-filtering, giant magnetoresistance and spin-rectifying effects are realized in this device. The mechanisms are proposed for these interesting phenomena. These results demonstrate that the designed all-carbon device holds great potentials in the development of multifunctional molecular spintronic device.

Ultrahigh Spin Filter Efficiency, Giant Magnetoresistance and Large Spin Seebeck Coefficient in Monolayer and Bilayer Co-/Fe-/Cu-Phthalocyanine Molecular Devices.

The spin related electrical and thermoelectric properties of monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices in a parallel spin configuration (PC) and an anti-parallel spin configuration (APC) between the V-shaped zigzag-edged graphene nanoribbon electrodes and the center bilayer MPc molecules are investigated by combining the density functional theory and non-equilibrium Green’s function approaches. The results show that there is an ultrahigh spin filter efficiency exceeding 99.99995% and an ultra-large total conductance of 0.49996G0 for FePc-CoPc molecular devices in the PC and a nearly pure charge current at high temperature in the APC and a giant MR ratio exceeding 9.87 × 106% at a zero bias. In addition, there are pure spin currents for CuPc and FePc molecular devices in the PC, and an almost pure spin current for FePc molecular devices in the APC at some temperature. Meanwhile, there is a high SFE of about 99.99585% in the PC and a reserved SFE of about −19.533% in the APC and a maximum MR ratio of about 3.69 × 108% for the FePc molecular device. Our results predict that the monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices possess large advantages in designing high-performance electrical and spintronic molecular devices.

Tunable conductance and spin filtering in twisted bilayer copper phthalocyanine molecular devices.

We investigate theoretically the quantum transport properties of a twisted bilayer copper phthalocyanine (CuPc) molecular device, in which the bottom-layer CuPc molecule is connected to V-shaped zigzag-edged graphene nanoribbon electrodes. Based on a non-equilibrium Green's function approach in combination with density-functional theory, we find that the twist angle effectively modulates the electron interaction between the bilayer CuPc molecules. HOMO (highest occupied molecular orbital)–LUMO (lowest unoccupied molecular orbital) gap, spin filtering efficiency (SFE) and spin-dependent conductance of the bilayer CuPc molecular device could be modulated by changing the twist angle. The conductance reaches its maximum when the twist angle θ is 0° while the largest SFE is achieved when θ = 60°. The twist angle-induced exotic transport phenomena can be well explained by analyzing the transmission spectra, molecular energy level spectra and scattering states of the twisted bilayer CuPc molecular device. The tunable conductance, HOMO–LUMO gap and spin filtering versus twist angle are helpful for predicting how a two-molecule system may behave with twist angle.

Spin-dependent transport in all-carbon multifunctional spintronic device

By using density functional theory and non-equilibrium Green’s function method, we investigate the spin-dependent transport properties of an all-carbon spintronic device consisting of a perylene molecule linked to two symmetrical ferromagnetic zigzag-edge graphene nanoribbon (ZGNR) electrodes via carbon atomic chains, where the magnetization of ZGNR electrodes are modulated by applying an external magnetic field. The device exhibits spin filtering and negative differential resistance (NDR) effect in parallel spin configuration. Bipolar spin filtering, spin rectifying and NDR effect are revealed in antiparallel spin configuration. And there is a giant magnetoresistance effect between parallel and antiparallel spin configurations. We discuss these interesting transport mechanisms, and suggest that the proposed all-carbon device holds promise in high-performance multifunctional nanoelectronic devices.

Electrical-thermal transport properties and their applications in graphene nanoribbon with DNA bases

Abstract By applying the density functional theory and the nonequilibrium Green's function formalism, we analyze the thermal, electrical transport properties and their applications of four kinds of bases in DNA sandwiched between two zigzag-edge graphene nanoribbon electrodes. The results show that the currents in different base-sandwiched systems display different current bias voltage characteristics. Conductivity enhancement and negative differential resistance (NDR) behaviors can be observed. The electron transport is relatively smooth, while the phonon transport is depressed obviously. The super-low thermal conductance and the violent fluctuation of electronic transmissions induce the enhanced thermopower.

Spin-filtering and giant magnetoresistance effects in polyacetylene-based molecular devices

Using the non-equilibrium Green's function formalism in combination with density functional theory, we performed ab initio calculations of spin-dependent electron transport in molecular devices consisting of a polyacetylene (CnHn+1) chain vertically attached to a carbon chain sandwiched between two semi-infinite zigzag-edged graphene nanoribbon electrodes. Spin-charge transport in the device could be modulated to different magnetic configurations by an external magnetic field. The results showed that single spin conduction could be obtained. Specifically, the proposed CnHn+1 devices exhibited several interesting effects, including (dual) spin filtering, spin negative differential resistance, odd–even oscillation, and magnetoresistance (MR). Marked spin polarization with a filtering efficiency of up to 100% over a large bias range was found, and the highest MR ratio for the CnHn+1 junctions reached 4.6 × 104. In addition, the physical mechanisms for these phenomena were also revealed.

Modulation of the magnetic properties in zigzag-edge graphene nanoribbons by connection sites

Based on the first-principles method, the magnetic properties for zigzag–edge graphene nanoribbon (ZGNR) junctions are investigated. The results show the system had the ferromagnetic or antiferromagnetic ground state depending on the connection sites between ZGNR electrodes and the central ZGNR. The junction displays a metallic behavior when the central ZGNR is connected at the middle site of electrodes with a ferromagnetic state, but shows obvious spin semiconductor feature when the connection site is shifted to the edge of the ZGNR. For the antiferromagnetic states, all models show a semiconductor behavior, which originates from the spin-degenerate edge states. While for the antiparallel spin configuration, the spin density of the central ribbon is affected by connection sites, and it shows weaker little by little with the central ribbon moving from bottom to the middle site, which is different obviously from ferromagnetic or antiferromagnetic state. When one edge state of the central ZGNR is broken, bipolar spin semiconductor features can be obtained with different band gaps at suitable connection sites.

Negative differential resistance effect in similar right triangle graphene devices

In this work, nanojunctions consisting of two combined similar right triangle graphenes (SRTGs) bonded covalently with zigzag-edged graphene nanoribbon electrodes are designed, and their electron transport properties are investigated using density functional theory and non-equilibrium Green's function method. Results reveal that the SRTG-based devices exhibit an interesting negative differential resistance (NDR) effect and present a rule indicating that the NDR effect increases with the increasing sizes of both SRTGs. The electron transport properties are further tested using two combined SRTGs with different sizes. Overall, this study suggests that the SRTG-based structures are promising candidates in the design of nanoscale NDR devices.

Spin-dependent transport properties in covalent–organic molecular device with graphene nanoribbon electrodes

Based on the first-principles method, the spin-dependent transport properties of covalent-organic molecular device embedded between two zigzag-edge graphene nanoribbon (ZGNR) electrodes have been studied. It is revealed that in the parallel (P) spin configuration of two ZGNR electrodes, the molecular device exhibits a robust spin filtering effect (up to 100%). In another case of antiparallel (AP) spin configuration, dual spin filtering, spin rectifying as well as negative differential resistance (NDR) effects were discovered. These calculated results offer useful ideas for designing high-performance ZGNR-based molecular devices.

Spin-Resolved Transport Properties of a Pyridine-Linked Single Molecule Embedded between Zigzag-Edged Graphene Nanoribbon Electrodes

We study the spin-dependent electron transport through a junction consisting of a single pyridine-linked (PDL) molecule sandwiched between two zigzag-edged graphene nanoribbon (ZGNR) electrodes modulated by an external magnetic field, where 4,4′-bipyridine, 4,4′-vinylenedipyridine, and 4,4′-ethylenedipyridine molecules are considered, respectively. By using ab initio calculations based on the density functional theory combined with nonequilibrium Green’s function formalism, it is shown that the spin-charge transport can be modulated by performing different magnetic configuration in the ZGNR electrodes. Specifically, we demonstrate that the proposed PDL molecular junctions exhibit several interesting effects, including (dual) spin-filtering, rectifying, negative differential resistance (NDR), and magnetoresistance. For the junction consisting of a 4,4′-bipyridine molecule with proper magnetic configuration in the two ZGNRs, it is interesting to note that a perfect spin polarization with filtering efficienc...

The integrated spintronic functionalities of an individual high-spin state spin-crossover molecule between graphene nanoribbon electrodes

The spin-polarized transport properties of a high-spin-state spin-crossover molecular junction with zigzag-edge graphene nanoribbon electrodes have been studied using density functional theory combined with the nonequilibrium Green’s-function formalism. The molecular junction presents integrated spintronic functionalities such as negative differential resistance behavior, spin filter and the spin rectifying effect, associated with the giant magnetoresistance effect by tuning the external magnetic field. Furthermore, the transport properties are almost unaffected by the electrode temperature. The microscopic mechanism of these functionalities is discussed. These results represent a step toward multifunctional molecular spintronic devices on the level of the individual spin-crossover molecule.

Magnetic transport properties of DBTAA-based nanodevices with graphene nanoribbon electrodes

Based on the density functional theory combined with the nonequilibrium Green’s function formalism, the magnetic transport properties of dibenzotetraaza[14]annulene (DBTAA) and transition metal (TM)-DBTAA (TM=Fe and Co) sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes are investigated. The results show that giant magnetoresistance, spin-filtering and spin-polarized current rectifying effects can be realized simultaneously in the DBTAA system by modulating the external magnetic field. Introducing of TM atoms has obvious effects on these spin-related effects. The mechanisms of these intriguing phenomena are proposed and these phenomena would be instructive in the design of high-performance magnetic nanodevices.

Effects of amino-nitro side groups on electron device of oligo p-phenylenevinylene molecular between ZGNR electrodes

We study the electronic transport property for a molecular device of an oligo (p-phenylenevinylene) (OPV) molecule without or with different side groups between two zigzag-edged graphene nanoribbon (ZGNR) electrodes. By using ab initio calculations based on density-functional theory, the effects of negative differential resistance (NDR) and spin-rectifying in I–V characteristics are revealed and explained for the proposed molecular device. Our analysis indicates that the NDR behavior comes from the conduction orbital being suppressed under certain bias voltage, while the rectifying effect is because of the asymmetry distribution of the highest occupied molecular orbital or the lowest unoccupied molecular one as well as the corresponding coupling between the molecule and electrodes. Interestingly, the transport property of the device can be improved by introducing amino-nitro side groups to the OPV molecule. The NDR behavior can be much enhanced for molecule with amino side group, and the rectification can be improved for molecule with amino and nitro side groups, respectively. In particular, the NDR behavior of peak to valley ratio can be much enhanced and a molecular rectifier which offers rectification ratio of more than three orders of magnitude up to 2863 by adding NH2 and NO2 side groups to OPV molecule, respectively.

Spin filtering behaviors for a carbon chain connected with armchair- and zigzag-edged graphene nanoribbon electrodes

Using the non-equilibrium Green’s function method combined with the density functional theory, we investigate the spin transport properties of carbon chains connected with armchair- and zigzag-edged graphene nanoribbon (ZGNR) electrodes at finite bias with the parallel magnetism configuration. When spin polarized electrons are injected into carbon chains from the ZGNR electrode, the β-spin current is nearly zero, while the α-spin current shows large values in the calculated bias region, thus the spin polarization 100% can be achieved, which is a promising pathway for developing a spin filter.

Rectifying, giant magnetoresistance, spin-filtering, newgative differential resistance, and switching effects in single-molecule magnet Mn(dmit)2-based molecular device with graphene nanoribbon electrodes

By applying the density functional theory and the nonequilibrium Green’s function formalism, we investigate the spin transport properties of a single-molecule magnet Mn(dmit)2 sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes. The results show that the system can present large rectifying, giant magnetoresistance, spin-filtering and negative differential resistance effects with the help of magnetic field modulation. Moreover, an improved switching effect can also be realized by changing the orientation between planes of two dmit ligands. Therefore, the system will provide the possibilities for a multifunctional molecular device design.

Perfect Spin-Filtering in 4H-TAHDI-Based Molecular Devices: the Effect of N-Substitution

Based on the non-equilibrium Green's function formalism and spin-polarized density functional theory calculations, we investigate the spin transport properties of HDI and terahydrotetraazahexacene diimide (4H-TAHDI) with two ferromagnetic zigzag-edge graphene nanoribbon electrodes. Compared with HDI, four carbon atoms in the hexacene part of 4H-TAHDI are substituted by nitrogen atoms. The results show that the nitrogen substitution can improve significantly the spin-filtering performance and 4H-TAHDI can be used as a perfect spin filter. Our study indicates that suitable chemical substitution is a possible way to realize high-efficiency spin filters.

Spin polarization effects of zigzag-edge graphene electrodes on the rectifying performance of the D-σ-A molecular diode

Sandwiching the D-σ-A molecule between zigzag-edge graphene nanoribbon (ZGNR) electrodes to construct a particular diode and simultaneously considering the reported experimental results that the demonstration of different spin-polarized states of ZGNRs is possible under different conditions, we perform the first-principles calculations on rectifying behaviors of such a diode. It has been found that its rectification ratio is greatly enhanced by ZGNR electrodes (the maximum rectification ratio >700) as compared with that by metal electrodes where its highest rectification ratio only in the order of 10 was reported. Also predicted is that its rectifying behaviors strongly depends on spin-polarized states of ZGNR electrodes, when electrodes take the AFM state (anti-ferromagnetic state), it has a best rectifying behavior.