Mengqiu Long

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 ~3 × 105 cm2V−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.

Designing of spin-filtering devices in zigzag graphene nanoribbons heterojunctions by asymmetric hydrogenation and B-N doping

Using nonequilibrium Greens function in combination with the spin-polarized density functional theory, the spin-dependent transport properties of boron and nitrogen doped zigzag graphene nanoribbons (ZGNRs) heterojunctions with single or double edge-saturated hydrogen have been investigated. Our results show that the perfect spin-filtering effect (100%), rectifying behavior and negative differential resistance can be realized in the ZGNRs-based systems. And the corresponding physical analysis has been given.

Theoretical Prediction of Electronic Structure and Carrier Mobility in Single-walled MoS2 Nanotubes

We have investigated the electronic structure and carrier mobility of armchair and zigzag single-walled MoS2 nanotubes using density functional theory combined with Boltzmann transport method with relaxation time approximation. It is shown that armchair nanotubes are indirect bandgap semiconductors, while zigzag nanotubes are direct ones. The band gaps of single-walled MoS2 nanotubes are along with the augment of their diameters. For armchair nanotubes (5 ≤ Na ≤ 14), the hole mobility raise from 98.62 ~ 740.93 cm2V−1s−1 at room temperature, which is about six times of the electron mobility. For zigzag nanotubes (9 ≤ Na ≤ 15), the hole mobility is 56.61 ~ 91.32 cm2V−1s−1 at room temperature, which is about half of the electron mobility.

First-Principles Prediction of the Charge Mobility in Black Phosphorus Semiconductor Nanoribbons

We have investigated the electronic structure and carrier mobility of monolayer black phosphorus nanoribbons (BPNRs) using density functional theory combined with Boltzmann transport method with relaxation time approximation. It is shown that the calculated ultrahigh electron mobility can even reach the order of 10(3) to 10(7) cm(2) V(-1) s(-1) at room temperature. Owing to the electron mobility being higher than the hole mobility, armchair and diagonal BPNRs behave like n-type semiconductors. Comparing with the bare BPNRs, the difference between the hole and electronic mobilities can be enhanced in ribbons with the edges terminated by H atoms. Moreover, because the hole mobility is about two orders of magnitude larger than the electron mobility, zigzag BPNRs with H termination behave like p-type semiconductors. Our results indicate that BPNRs can be considered as a new kind of nanomaterial for applications in optoelectronics, nanoelectronic devices owing to the intrinsic band gap and ultrahigh charge mobility.

Electronic transport properties on transition-metal terminated zigzag graphene nanoribbons

By using non-equilibrium Green’s functions in combination with the density-functional theory, we investigate the spin transport properties of molecular junctions based on 3d transition terminated zigzag graphene nanoribbons. The results show that the electronic transport properties are strongly depending on the type of terminated atom at the edge of ribbon. The currents of spin-up and spin-down display different behaviors, and the spin-filter effects can be observed. These unconventional doping effects could be used to design novel nanospintronics devices.

Effects of van der Waals interaction and electric field on the electronic structure of bilayer MoS2

The modification of the electronic structure of bilayer MoS2 by an external electric field can have potential applications in optoelectronics and valleytronics. Nevertheless, the underlying physical mechanism is not clearly understood, especially the effects of the van der Waals interaction. In this study, the spin orbit-coupled electronic structure of bilayer MoS2 has been investigated using the first-principle density functional theory. We find that the van der Waals interaction as well as the interlayer distance has significant effects on the band structure. When the interlayer distance of bilayer MoS2 increases from 0.614 nm to 0.71 nm, the indirect gap between the Γ and Λ points increases from 1.25 eV to 1.70 eV. Meanwhile, the energy gap of bilayer MoS2 transforms from an indirect one to a direct one. An external electric field can shift down (up) the energy bands of the bottom (top) MoS2 layer and also breaks the inversion symmetry of bilayer MoS2. As a result, the electric field can affect the band gaps, the spin-orbit interaction and splits the valance bands into two groups. The present study can help us understand more about the electronic structures of MoS2 materials for potential applications in electronics and optoelectronics.

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.

Hydrogenations and electric field induced magnetic behaviors in armchair silicene nanoribbons

Using the first-principles calculations, we investigate the geometric, electronic and magnetic properties of armchair silicene nanoribbons with different edge hydrogenations. Our results show that the interesting magnetic behaviors such as the bipolar magnetic semiconductor can be found. Moreover, the addition of the transverse electric field can modulate the bipolar magnetic semiconductor to half-metal or spin-splitting metal. And the spin-up electrons are localized at one edge, the spin-down holes localized at the opposite edge under the external electric field. These results may present a new avenue for band engineering of silicene nanoribbons and benefit the design of silicon-based nano-spin-devices in nanoelectronics.

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.