Yipeng An

Abnormal electronic transport and negative differential resistance of graphene nanoribbons with defects

Electronic transport properties of zigzag graphene nanoribbons (GNRs) with two kinds of triangular defects are explored by using an ab-initio method. At a certain bias, the current of the GNR with an upward-triangle defect can be surprisingly larger than that of the perfect GNR due to the defect-induced symmetry breaking and more conductive channels. Dissimilarly, if the orientation of the triangle is changed rightward, the current is depressed much and shows negative differential resistance behavior. Our findings indicate that defect designs can be an efficient way to tune the electronic transport of GNR nanodevices.

Improving electronic transport of zigzag graphene nanoribbons by ordered doping of B or N atoms.

Using an ab initio method, we explored electronic structures and transport properties of zigzag graphene nanoribbons (ZGNRs) with ordered doping of B or N atoms. We find B or N atoms doping can increase significantly the conductance of the ZGNRs with an even number of zigzag chains due to additional conducting channels being induced and the breakdown of parity limitation. The higher the doping concentration, the larger the current amplification factor obtained. For the nanojunctions with one row B (or N) atoms, the current amplification factor can be larger when the doping position is near to the center, while for the junction with two rows, the trend is subtle due to the interactions between the two rows of B (or N) atoms. Negative differential resistive phenomena are found for the case of B doping at low concentrations and the case for N doping. The conductance of the ZGNR with odd numbers of zigzag chains can also be increased by doping of B or N atoms. More interestingly, the B or N doping can almost completely remove the even-odd effect on electronic transport of the ZGNRs. Our studies provide avenues to drastically improve the electronic transport of ZGNRs, helpful for graphene applications.

Tunable electronic structures and magnetism in arsenene nanosheets via transition metal doping

Based on density-functional theory, the electronic structures and magnetism of 3d transition metal (TM)-doped arsenene nanosheets are investigated by means of first-principles methods. The results show that Sc- and Co-doped arsenene nanosheets possess the nonmagnetic semiconducting properties, while Ti, Cr, and Cu substituting As atom can induce dilute magnetic semiconductor phase. Moreover, half-metal properties are induced in the V-, Mn-, Fe-, and Ni-doped arsenene nanosheets. In addition, results also show that Ti-, V-, Mn-, and Fe-doped arsenene nanosheets present ferromagnetic coupling, whereas Cr substitutional doping results in an antiferromagnetic coupling under their most stable configuration. These results indicate that TM doping can tune effectively the electronic and magnetic properties in the arsenene nanosheets.

Intrinsic negative differential resistance characteristics in zigzag boron nitride nanoribbons

We investigate the charge transport properties of zigzag boron nitride nanoribbons (ZBNNRs) with various hydrogen passivations by employing density functional theory (DFT) combined with the non-equilibrium Greens function (NEGF) formalism. The calculated results reveal that the ZBNNR-based devices exhibit negative differential resistance (NDR) characteristics except those models whose both edges are passivated, due to the mechanism in which the overlap of bands near the Fermi level between the left and right electrodes gets smaller or disappears under a high bias. The NDR characteristics of the perfect ZBNNRs with one or two bare edges are weakly dependent on their widths. This is one intrinsic NDR characteristic of the ZBNNR-based devices, including some defective structures. The intuitive electronic current channels are plotted and analyzed to better understand the charge transport mechanisms. Our results suggest that the ZBNNR-based structures could be favorable candidates for preparing nanoscale NDR devices.

The electronic transport properties of transition-metal dichalcogenide lateral heterojunctions

Two-dimensional (2D) heteromaterials have aroused a new interest in many applications, and have emerged as a unique family of nanomaterials in physics and materials science. Recently, vertical and in-plane heterostructures of MoS2–WS2 monolayers, were successively prepared and observed in experiments [Gong et al., Nat. Mater., 2014, 13, 1135]. Herein, using a first-principles technique, we study the electronic transport properties of several types of zigzag MoS2–WS2 lateral heterojunctions. The results demonstrate that the MoS2–WS2 lateral heterojunctions show an interesting negative differential resistive (NDR) effect, due to owning very similar band structures to that of the pristine MoS2 and WS2 nanoribbons. The electrons always propagate through the heterojunctions along the metal-terminations, while never along the S-termination. The results demonstrate that our proposed transition-metal dichalcogenide (TMD) heterojunctions could become candidates of NDR devices, and have potential applications in nanoelectronics.

Magnetic vanadium sulfide monolayers: transition from a semiconductor to a half metal by doping

Two dimensional crystals, befitting nanoscale electronics and spintronics, can have versatile applications due to their ultrathin and flexible nature. We show by first-principles calculations that suitable doping can influence the exchange splitting of the spin and electronic states in VS2 monolayers using the meta-GGA (MGGA) method by self-consistently introducing the exact-exchange interaction to the sub d shell of the V atoms. Among the large number of non-metallic atoms, H, B, C, N, P, As, O, Se, Te, F, Cl, and Br, As appears to be a candidate for p-type doping of the VS2 monolayer with an appropriate formation energy. The enhancement of the conductivity is limited and the spin polarization of the As doped system is also slightly lowered from the mixed V-s state with an inverse spin orientation. H and halogen atoms (F, Cl, Br) are ideal candidates for n-type doping with very small and even negative formation energy. Particularly, a switch from a half semiconductor to a half metal is realized with enhanced conductivity and spin polarization in VS2 monolayers from the increased magnetic moment of the nearest neighboring V atoms of the dopants under a rather wide range of doping density, which is very promising for spintronic applications.

Spin-dependent thermoelectronic transport of a single molecule magnet Mn(dmit)2

We investigate spin-dependent thermoelectronic transport properties of a single molecule magnet Mn(dmit)2 sandwiched between two Au electrodes using first-principles density functional theory combined with nonequilibrium Greens function method. By applying a temperature difference between the two Au electrodes, spin-up and spin-down currents flowing in opposite directions can be induced due to asymmetric distribution of the spin-up and spin-down transmission spectra around the Fermi level. A pure spin current and 100% spin polarization are achieved by tuning back-gate voltage to the system. The spin caloritronics of the molecule with a perpendicular conformation is also explored, where the spin-down current is blocked strongly. These results suggest that Mn(dmit)2 is a promising material for spin caloritronic applications.

The rectifying and negative differential resistance effects in graphene/h-BN nanoribbon heterojunctions

We investigate the electronic transport properties of four types of lateral graphene/h-BN nanoribbon heterojunctions using the non-equilibrium Greens function method in combination with the density functional theory. The results show that the heterojunction displays an interesting rectifying effect when the interface has a left-right type structure, while a pronounced negative differential resistance (NDR) effect when the interface has an up-down type structure. Moreover, when the interface of the heterojunction has a left-bank or right-bank type structure, it presents the rectifying (with a larger rectification ratio) and NDR effects. This work is helpful to further construct and prepare a nanodevice based on the graphene/h-BN heterojunction materials according to the proposed structures.

Width and defect effects on the electronic transport of zigzag MoS2 nanoribbons

Using first-principles methods, we investigate the electronic transport properties of zigzag MoS2 nanoribbons (Z-MoS2NRs). The current?voltage (I?V) curves of Z-MoS2NRs show a negative differential resistive (NDR) effect, and are independent of nanoribbon width. The current flowing through the nanoribbon is mainly along the Mo-edge, with two different local current channels (Mo?????Mo hop current and S?????Mo?????S bond current). The current will be suppressed when introducing a Mo vacancy-defect at the Mo-edge under low biases?while, under high biases, the current through the defected Z-MoS2NRs will increase a little, due to the other S-edge channel being opened.

Spin-dependent electronic transport properties of zigzag silicon carbon nanoribbon

Spin-dependent electronic transport properties of the zigzag silicon carbon nanoribbon (Z-SiCNR) are studied by employing the non-equilibrium Greens function method in the framework of density functional theory. It is found that the Z-SiCNR exhibits a variety of exotic physical properties. While the Z-SiCNR in the metallic FM state presents spin filtering and current-limited effects, it is shown that the abnormal oscillation of spin-polarized currents with spin polarization as high as 100% under a certain bias voltage emerges in the half-metallic AFM state. The results demonstrate that tuning the spin state of the zigzag SiC nanoribbon provides a possible avenue to design next generation spin nanodevices with novel functionalities.