Xinlian Chen

Controllable band structure and topological phase transition in two-dimensional hydrogenated arsenene

Discovery of two-dimensional (2D) topological insulator such as group-V films initiates challenges in exploring exotic quantum states in low dimensions. Here, we perform first-principles calculations to study the geometric and electronic properties in 2D arsenene monolayer with hydrogenation (HAsH). We predict a new σ-type Dirac cone related to the px,y orbitals of As atoms in HAsH, dependent on in-plane tensile strain. Noticeably, the spin-orbit coupling (SOC) opens a quantum spin Hall (QSH) gap of 193 meV at the Dirac cone. A single pair of topologically protected helical edge states is established for the edges, and its QSH phase is confirmed with topological invariant Z2 = 1. We also propose a 2D quantum well (QW) encapsulating HAsH with the h-BN sheet on each side, which harbors a nontrivial QSH state with the Dirac cone lying within the band gap of cladding BN substrate. These findings provide a promising innovative platform for QSH device design and fabrication operating at room temperature.

Tunable Quantum Spin Hall Effect via Strain in two-Dimensional Arsenene Monolayer

The search for a new quantum spin Hall (QSH) phase and effective manipulation of its edge states are very important for both fundamental sciences and practical applications. Here, we use first-principles calculations to study the strain-driven topological phase transition of two-dimensional (2D) arsenene monolayer. We find that the band gap of arsenene decreases with increasing strain and changes from indirect to direct, and then the s-p band inversion takes place at the Г point as the tensile strain is larger than 11.14%, which leads to a nontrivially topological state. A single pair of topologically protected helical edge states is established for the edge of arsenene, and their QSH states are confirmed with the nontrivial topological invariant Z 2 = 1. We also propose high-dielectric BN as an ideal substrate for the experimental synthesis of arsenene, maintaining its nontrivial topology. These findings provide a promising candidate platform for topological phenomena and new quantum devices operating at nanoelectronics.

The magnetic and optical properties of 3d transition metal doped SnO2 nanosheets

Based on first-principles calculations, we study the electronic structure, magnetic properties and optical properties of transition metal (TM) doped SnO2NSs. Computational results indicate that pristine SnO2NSs is a direct gap semiconductor with nonmagnetic states. Cr, Mn, Fe atom doping can induce 2μB, −3μB and 2μB magnetic moment, respectively, while Ni atom doped SnO2NSs keeps the nonmagnetic states. More interestingly, Fe doped SnO2NSs becomes an indirect gap semiconductor, and the Cr, Mn and Ni atom doping maintain the character of direct gap semiconductor. For optical properties, the optical absorption edge shows red shift phenomenon for a TM atom (Cr, Mn, Fe or Ni) doped SnO2NSs. In addition, the tensity of absorption, reflection and refraction coefficient are enhanced significantly in the visible light region, which may be very useful for the design of solar cells, photoelectronic devices and photocatalysts.

Electronic structures and optical properties of TM (Cr, Mn, Fe or Co) atom doped ZnSe nanosheets

The electronic structures and optical properties of pristine and transition-metal (TM) atom doped ZnSe nanosheets (ZnSeNSs) have been studied based on first-principles calculations. The results indicate that the pristine ZnSeNSs are nonmagnetic direct gap semiconductors, while Mn, Fe or Co doped ZnSeNSs are all spin-polarized, and the Cr doped one is half-metallic with 100% spin-polarized currents. Cr or Co doped ZnSeNSs can improve the absorption properties and broaden the absorption range, compared to pristine, Fe or Mn doped ZnSeNSs. Moreover, red-shift phenomena are observed. These results can provide an important reference for designing and fabricating infrared and visible photoelectric nanodevices.

First-principles prediction and Monte Carlo study of half-metallic ferromagnetism of the C-doped honeycomb CdS monolayer

By the full-potential linearized augmented plane wave (FLAPW) method, the electronic structures and magnetic properties of a C-doped honeycomb CdS monolayer (HC-CdS) have been investigated. We find that a C-doped HC-CdS shows a half-metallic character and a 100% spin polarization at the Fermi level with a total magnetic moment of 2.0 μB per unit cell. When two carbon atoms substitute for S atoms in HC-CdS, the interaction between two carbon atoms is antiferromagnetic, but it can turn into ferromagnetic coupling provided that an electron is doped into HC-CdS. The Curie temperature of 354 K is predicted through Monte Carlo simulation. The ferromagnetism of two C-doped HC-CdS monolayer can be explained by the electron-mediated p–p interaction and p–d exchange hybridization. These results provide a new perspective for the potential application of C-doped HC-CdS in spintronics.

Tunable SnO2 Nanoribbon by Electric Fields and Hydrogen Passivation

Under external transverse electronic fields and hydrogen passivation, the electronic structure and band gap of tin dioxide nanoribbons (SnO2NRs) with both zigzag and armchair shaped edges are studied by using the first-principles projector augmented wave (PAW) potential with the density function theory (DFT) framework. The results showed that the electronic structures of zigzag and armchair edge SnO2NRs exhibit an indirect semiconducting nature and the band gaps demonstrate a remarkable reduction with the increase of external transverse electronic field intensity, which demonstrate a giant Stark effect. The value of the critical electric field for bare Z-SnO2NRs is smaller than A-SnO2NRs. In addition, the different hydrogen passivation nanoribbons (Z-SnO2NRs-2H and A-SnO2NRs-OH) show different band gaps and a slightly weaker Stark effect. The band gap of A-SnO2NRs-OH obviously is enhanced while the Z-SnO2NRs-2H reduce. Interestingly, the Z-SnO2NRs-OH presented the convert of metal-semiconductor-metal under external transverse electronic fields. In the end, the electronic transport properties of the different edges SnO2NRs are studied. These findings provide useful ways in nanomaterial design and band engineering for spintronics.

Two-dimensional topological insulators of Pb/Sb honeycombs on a Ge(111) semiconductor surface

Based on first-principles hybrid functional calculations, we demonstrate the formation of two-dimensional (2D) topological insulators (TIs) of Pb/Sb honeycombs on Ge(111) semiconductor surface. We show that 1/3 Cl-covered Ge(111) surface offers an ideal template for metal deposition. When Pb and Sb atoms are deposited on Cl–Ge(111) surface, they spontaneously form a hexagonal lattice (Pb/Sb@Cl–Ge(111)). The Pb/Sb@Cl–Ge(111) exhibits a 2D TI state with large bulk gap of 0.27 eV for Pb@Cl–Ge(111) and 0.81 eV for Sb@Cl–Ge(111). The mechanism of 2D TI state is the substrate orbital-filtering effect that effectively removes the pz bands of Pb(Sb) away from the Fermi level, leaving behind only the px and py orbitals at the Fermi level. Our findings pave another way for future design of 2D topological insulators on conventional semiconductor surface, which promotes the application of 2D TIs in spintronics and quantum computing devices at room-temperature.