Yun Ni

Spin Seebeck Effect and Thermal Colossal Magnetoresistance in Graphene Nanoribbon Heterojunction

Spin caloritronics devices are very important for future development of low-power-consumption technology. We propose a new spin caloritronics device based on zigzag graphene nanoribbon (ZGNR), which is a heterojunction consisting of single-hydrogen-terminated ZGNR (ZGNR-H) and double-hydrogen-terminated ZGNR (ZGNR-H2). We predict that spin-up and spin-down currents flowing in opposite directions can be induced by temperature difference instead of external electrical bias. The thermal spin-up current is considerably large and greatly improved compared with previous work in graphene. Moreover, the thermal colossal magnetoresistance is obtained in our research, which could be used to fabricate highly-efficient spin caloritronics MR devices.

The transport properties and new device design: the case of 6,6,12-graphyne nanoribbons

By performing first-principle quantum transport calculations, we studied the transport properties of three kinds of 6,6,12-graphyne nanoribbons with different edges and different cutting directions. The nanoribbon with zigzag edges shows metallic properties and the spin-polarized currents show an obvious negative differential resistance effect, the other two nanoribbons terminated by a phenyl ring are semiconductors and spin-unpolarized. We also designed several nanowire devices based on these 6,6,12-graphyne nanoribbons, such as rectifier, spin filter diode, spin FET and spin caloritronics devices. These results indicate that 6,6,12-graphyne is a potential candidate for spintronics and spin caloritronics.

Nearly Perfect Spin Filter, Spin Valve and Negative Differential Resistance Effects in a Fe 4 -based Single-molecule Junction

The spin-polarized transport in a single-molecule magnet Fe4 sandwiched between two gold electrodes is studied, using nonequilibrium Greens functions in combination with the density-functional theory. We predict that the device possesses spin filter effect (SFE), spin valve effect (SVE), and negative differential resistance (NDR) behavior. Moreover, we also find that the appropriate chemical ligand, coupling the single molecule to leads, is a key factor for manipulating spin-dependent transport. The device containing the methyl ligand behaves as a nearly perfect spin filter with efficiency approaching 100%, and the transport is dominated by transmission through the Fe4 metal center. However, in the case of phenyl ligand, the spin filter effect seems to be reduced, but the spin valve effect is significantly enhanced with a large magnetoresistance ratio, reaching 1800%. This may be attributed to the blocking effect of the phenyl ligands in mediating transport. Our findings suggest that such a multifunctional molecular device, possessing SVE, NDR and high SFE simultaneously, would be an excellent candidate for spintronics of molecular devices.

The spin-dependent transport properties of zigzag α-graphyne nanoribbons and new device design

By performing first-principle quantum transport calculations, we studied the electronic and transport properties of zigzag α-graphyne nanoribbons in different magnetic configurations. We designed the device based on zigzag α-graphyne nanoribbon and studied the spin-dependent transport properties, whose current-voltage curves show obvious spin-polarization and conductance plateaus. The interesting transport behaviours can be explained by the transport spectra under different magnetic configurations, which basically depends on the symmetry matching of the electrodes’ bandstructures. Simultaneously, spin Seebeck effect is also found in the device. Thus, according to the transport behaviours, zigzag α-graphyne nanoribbons can be used as a dual spin filter diode, a molecule signal converter and a spin caloritronics device, which indicates that α-graphyne is a promising candidate for the future application in spintronics.

Perfect spin-filter, spin-valve, switching and negative differential resistance in an organic molecular device with graphene leads

By performing first-principle quantum transport calculations, we proposed a multiple-effect organic molecular device for spintronics. The device is constructed of a perylene tetracarboxylic diimide molecule sandwiched between graphene electrodes. Our calculations show that the device has several perfect spintronics effects such as a spin-filter effect, a magnetoresistance effect, a negative differential resistance effect and a spin switching effect. These results indicate that our one-dimensional molecular device is a promising candidate for the future application of graphene-based organic spintronics devices.

Spin transport properties of partially edge-hydrogenated MoS2 nanoribbon heterostructure

We report ab initio calculations of electronic transport properties of heterostructure based on MoS2 nanoribbons. The heterostructure consists of edge hydrogen-passivated and non-passivated zigzag MoS2 nanoribbons (ZMoS2NR-H/ZMoS2NR). Our calculations show that the heterostructure has half-metallic behavior which is independent of the nanoribbon width. The opening of spin channels of the heterostructure depends on the matching of particular electronic orbitals in the Mo-dominated edges of ZMoS2NR-H and ZMoS2NR. Perfect spin filter effect appears at small bias voltages, and large negative differential resistance and rectifying effects are also observed in the heterostructure.

Carbon doping induced peculiar transport properties of boron nitride nanoribbons p-n junctions

By applying nonequilibrium Greens function combined with density functional theory, we investigate the electronic transport properties of carbon-doped p-n nanojunction based on hexagonal boron nitride armchair nanoribbons. The calculated I-V curves show that both the center and edge doping systems present obvious negative differential resistance (NDR) behavior and excellent rectifying effect. At low positive bias, the edge doping systems possess better NDR performance with larger peak-to-valley ratio (∼105), while at negative bias, the obtained peak-to-valley ratio for both of the edge and center doping systems can reach the order of 107. Meanwhile, center doping systems present better rectifying performance than the edge doping ones, and giant rectification ratio up to 106 can be obtained in a wide bias range. These outstanding transport properties are explained by the evolution of the transmission spectra and band structures with applied bias, together with molecular projected self-consistent Hamiltonian eigenvalues and eigenstates.

A first principles study of novel one-dimensional organic half-metal vanadium-cyclooctatetraene wire

The structural, electronic, and magnetic properties of one-dimensional vanadium-cyclooctatetraene[(V-COT)]∞ wire and sandwich clusters are investigated by means of density functional theory. It is found that the (V-COT)∞ SMW is half-metallic. Through the spin transportation calculations, the system for V-COT clusters coupled to gold electrodes performs nearly perfect spin filters. In addition, the I-V curve shows obviously negative differential resistance effects. These results suggest the potential applications of (V-COT)∞ in spintronics.

The study of interaction and charge transfer at black phosphorus-metal interfaces

Using density-functional theory, we analyze the potential barrier, charge transfer and atomic orbital overlap at the metal-black phosphorus interface in an optimized structure to understand how efficiently carriers could be injected from a metal contact to the black phosphorus. We investigate a monolayer black phosphorus directly in contact with five representative metal substrates Ag(1 1 1), Au(1 1 1), Al(1 1 1), Cu(1 1 1) or Zn(0 0 0 1), having varying work functions but each with minimal lattice mismatch with the black phosphorus overlayer. We find that the contact nature is ohmic versus Schottky. For different kinds of contact, Cu and Al show better conductivity than the other metals. The dependence of the barrier height exhibits partial Fermi-level pinning character. These findings may prove to be instrumental in the future design of BP-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.

Efficient spin-filter and negative differential resistance behaviors in FeN4 embedded graphene nanoribbon device

In this paper, we propose a new device of spintronics by embedding two FeN4 molecules into armchair graphene nanoribbon and sandwiching them between N-doped graphene nanoribbon electrodes. Our first-principle quantum transport calculations show that the device is a perfect spin filter with high spin-polarizations both in parallel configuration (PC) and antiparallel configuration (APC). Moreover, negative differential resistance phenomena are obtained for the spin-down current in PC, and the spin-up and spin-down currents in APC. These transport properties are explained by the bias-dependent evolution of molecular orbitals and the transmission spectra.