Xiaojiao Zhang

Nitrogen doping-induced rectifying behavior with large rectifying ratio in graphene nanoribbons device

By applying nonequilibrium Green’s functions in combination with density-function theory, we investigate the electronic transport properties of armchair graphene nanoribbons devices with one undoped and one nitrogen-doped armchair graphene nanoribbons electrode. For the doped armchair graphene nanoribbons electrode, an N dopant is considered to substitute the center or edge carbon atom. The results show that the electronic transport properties are strongly dependent on the width of the ribbon and the position of the N dopant. The rectifying behavior with large rectifying ratio can be observed and can be modulated by changing the width of the ribbon or the position of the N dopant. A mechanism for the rectifying behavior is suggested.

Theoretical predictions on the electronic structure and charge carrier mobility in 2D Phosphorus sheets

We have investigated the electronic structure and carrier mobility of four types of phosphorous monolayer sheet (α-P, β-P,γ-P and δ-P) using density functional theory combined with Boltzmann transport method and relaxation time approximation. It is shown that α-P, β-P and γ-P are indirect gap semiconductors, while δ-P is a direct one. All four sheets have ultrahigh carrier mobility and show anisotropy in-plane. The highest mobility value is ~3u2009×u2009105u2009cm2V−1s−1, which is comparable to that of graphene. Because of the huge difference between the hole and electron mobilities, α-P, γ-P and δ-P sheets can be considered as n-type semiconductors, and β-P sheet can be considered as a p-type semiconductor. Our results suggest that phosphorous monolayer sheets can be considered as a new type of two dimensional materials for applications in optoelectronics and nanoelectronic devices.

Perfect spin filtering, rectifying and negative differential resistance effects in armchair graphene nanoribbons

Using the non-equilibrium Greens function method combined with the spin-polarized density functional theory, we calculate the electronic and transport properties of the armchair graphene nanoribbons with a special edge hydrogenation (S-AGNRs). The results show S-AGNRs are ferromagnetic bipolar magnetic semiconductors with 2 μ B magnetic moment, and the B or N atom doping can make S-AGNRs convert to up-spin dominated or down-spin dominated half metal. Therefore, a 100% spin-filtering effect has been realized in the corresponding devices. Furthermore, the negative differential resistance phenomenon can also be found. The B and N atoms co-doping can construct a PN junction, and the rectification ratio is as high as 1010.

High performance bipolar spin filtering and switching functions of poly-(terphenylene-butadiynylene) between zigzag graphene nanoribbon electrodes

Using the nonequilibrium Green’s function method combined with spin-polarized density functional theory, we investigate the spin-resolved electronic transport properties of devices made of poly-(terphenylene-butadiynylene) (PTB) between two symmetric ferromagnetic zigzag graphene nanoribbon (ZGNR) electrodes. The bipolar spin filtering effect, rectifying behavior, and negative differential resistance have been found. More interestingly, an on/off ratio in the order of 107 is also predicted by changing the angle between the PTB and ZGNR electrode planes. Further analyses show that the matching of the electronic wave functions among both electrodes and PTB plays a key role in the multi-functional PTB based device. And the coupling between the alkyne triple bond and the phenyl rings of PTB is critical to the value of the spin-resolved current and the on/off ratio. These phenomena suggest that the proposed PTB based devices have potential utilization in molecular spin diodes and molecular switches.

Spin-resolved transport properties in zigzag α-graphyne nanoribbons with symmetric and asymmetric edge fluorinations

Using the non-equilibrium Greens function method and the spin-polarized density functional theory, we investigate the stability and spin-resolved electronic transport properties of zigzag α-graphyne nanoribbons (ZαGYNRs) with symmetric (F-ZαGYNRs-F) and asymmetric (F2-ZαGYNRs-F) edge fluorinations. Our results show edge fluorination can enhance the stability of ZαGYNRs. The spin-resolved transport calculations reveal that the devices of F-ZαGYNRs-F with odd ribbon widths behave as a conductor with a linear current–voltage relationship, while the semiconductor property and perfect bipolar spin-filtering effect can be observed in those devices with even ribbon widths. In contrast, the spin-resolved transport properties of the asymmetric edge fluorinated F2-ZαGYNRs-F systems are independent of the ribbon width. Moreover, the F2-ZαGYNRs-F device is a perfect spin device with nearly 100% bipolar spin-filtering and spin negative differential resistance effects in a wide bias voltage region. And the magnetoresistance effect with the order of 106 and the spin rectification ratio as high as 108 have also been predicted. These phenomena suggest ZαGYNRs with asymmetric edge fluorination can be considered as a promising candidate material for nano-electronics and spintronics.

Tunable negative differential resistance in a single cruciform diamine molecule with zigzag graphene nanoribbon electrodes

We investigate the electronic transport properties of a single cruciform diamine molecule connected to zigzag graphene nanoribbon electrodes by using the non-equilibrium Greens function formalism with density functional theory. Negative differential resistance behavior can be discovered in the current–voltage curve of this molecular device. Then, one hydrogen atom in the molecule is replaced by an electron-donating group (–NH2) or an electron-withdrawing group (–NO2) in order to modulate the devices electronic transport properties. The results show the replacement of –NH2 on two different functionalized sites (R1 and R2) are all ineffective on the devices current–voltage characteristic. However, the replacement of –NO2 on R1 and R2 can enlarge the devices electron transport ability to different extents. More importantly, the original negative differential resistance behavior is also enhanced markedly. The above results offer us a new effective way to control the conductance and to modulate the negative differential resistance behavior in the single molecular device with zigzag graphene nanoribbon electrodes.

Effect of sulphur vacancy and interlayer interaction on the electronic structure and spin splitting of bilayer MoS2

Molybdenum disulfide (MoS2) is one of candidate materials for nanoelectronics and optoelectronics devices in future. The electronic and magnetic properties of MoS2 can be regulated by interlayer interaction and vacancy effect. Nevertheless, the combined effect of these two factors on MoS2 is not clearly understood. In this study, we have investigated the impact of single S vacancy combined with interlayer interaction on the properties of bilayer MoS2. Our calculated results show that S vacancy brings impurity states in the band structure of bilayer MoS2, and the energy level of the impurity states can be affected by interlayer distance, which finally disappears in bulk state when the layer distance is relatively small. Moreover, during the compressing of bilayer MoS2, the bottom layer, where S vacancy stays, gets additional charge due to interlayer charge transfer, which first increases, and then decreasesxa0due to gradually forming the interlayer S-S covalent bond, as interlayer distance decreases. The change of the additional charge is consistent with the change of the total magnetic moment of bottom layers, no magnetic moment has been found in top layer. The distribution of magnetic moment mainly concentrates on the three Mo atoms around S vacancy, each of whose magnetic moment is very much related to the Mo-Mo length. Our conclusion is that the interlayer charge transfer and S vacancy codetermine the magnetic properties of this system, which maybe a useful way to regulate the electronic and magnetic properties of MoS2 for potential applications.

Modulating the spin transport behaviors in ZBNCNRs by edge hydrogenation and position of BN chain

Using the density functional theory and the nonequilibrium Green’s function method, we study the spin transport behaviors in zigzag boron-nitrogen-carbon nanoribbons (ZBNCNRs) by modulating the edge hydrogenation and the position of B-N nanoribbons (BNNRs) chain. The different edge hydrogenations of the ZBNCNRs and the different position relationships of the BNNRs have been considered systematically. Our results show that the metallic, semimetallic and semiconductive properties of the ZBNCNRs can be modulated by the different edge hydrogenations and different position relationships of BN chains. And our proposaled ZBNCNRs devices act as perfect spin-filters with nearly 100% spin polarization. These effects would have potential applications for boron-nitrogen-carbon-based nanomaterials in spintronics nano-devices.

Spin-dependent transport characteristics of nanostructures based on armchair arsenene nanoribbons*

By employing non-equilibrium Greens function combined with the spin-polarized density-functional theory, we investigate the spin-dependent electronic transport properties of armchair arsenene nanoribbons (aAsNRs). Our results show that the spin-metal and spin-semiconductor properties can be observed in aAsNRs with different widths. We also find that there is nearly 100% bipolar spin-filtering behavior in the aAsNR-based device with antiparallel spin configuration. Moreover, rectifying behavior and giant magnetoresistance are found in the device. The corresponding physical analyses have been given.