Dongchao Wang

Pressure-induced polymerization of nitrogen in potassium azides

The phase transition and structural evolution of KN3 are systematically studied using first-principles density functional (DFT) methods and the particle swarm optimization (PSO) structure search algorithm under pressures up to 400 GPa. For the first time, we identify three stable phases with , and structure at pressures of 15.7, 41.4 and 298.6 GPa. The analysis of the crystal structures of three new predicted phases reveals that the transition of N3− ions goes from linear molecules to benzene-like rings and then to polymer chains induced by pressure. The study of atomic and electronic structures of three predicted phases reveals that the structural changes are accompanied and driven by the change of atomic orbital hybridization, first from sp to sp2, and then from sp2 to sp3. Our result provides a new view of the pressure-induced polymerization process of metal azides.

Quantum spin Hall insulator in halogenated arsenene films with sizable energy gaps

Based on first-principles calculations, the electronic and topological properties of halogenated (F-, Cl-, Br- and I-) arsenene are investigated in detail. It is found that the halogenated arsenene sheets show Dirac type characteristic in the absence of spin-orbital coupling (SOC), whereas energy gap will be induced by SOC with the values ranging from 0.194u2009eV for F-arsenene to 0.255u2009eV for I-arsenene. Noticeably, these four newly proposed two-dimensional (2D) systems are verified to be quantum spin Hall (QSH) insulators by calculating the edge states with obvious linear cross inside bulk energy gap. It should be pointed out that the large energy gap in these 2D materials consisted of commonly used element is quite promising for practical applications of QSH insulators at room temperature.

Highly sensitive H 2 S sensors based on Cu 2 O/Co 3 O 4 nano/microstructure heteroarrays at and below room temperature

Gas sensors with high sensitivity at and below room temperature, especially below freezing temperature, have been expected for practical application. The lower working temperature of gas sensor is better for the manufacturability, security and environmental protection. Herein, we propose a H2S gas sensor with high sensitivity at and below room temperature, even as low as −30u2009°C, based on Cu2O/Co3O4 nano/microstructure heteroarrays prepared by 2D electrodeposition technique. This heteroarray was designed to be a multi-barrier system, and which was confirmed by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and scanning probe microscopy. The sensor demonstrates excellent sensitivity, sub-ppm lever detection, fast response, and high activity at low temperature. The enhanced sensing property of sensor was also discussed with the Cu2O/Co3O4 p-p heterojunction barrier modulation and Cu2S conductance channel. We realize the detection of the noxious H2S gas at ultra-low temperature in a more security and environmental protection way.

Strain Effect on Electronic Structure and Work Function in α-Fe2O3 Films

We investigate the electronic structure and work function modulation of α-Fe2O3 films by strain based on the density functional method. We find that the band gap of clean α-Fe2O3 films is a function of the strain and is influenced significantly by the element termination on the surface. The px and py orbitals keep close to Fermi level and account for a pronounced narrowing band gap under compressive strain, while unoccupied dz2 orbitals from conduction band minimum draw nearer to Fermi level and are responsible for the pronounced narrowing band gap under tensile strain. The spin polarized surface state, arising from localized dangling-bond states, is insensitive to strain, while the bulk band, especially for pz orbital, arising from extended Bloch states, is very sensitive to strain, which plays an important role for work function decreasing (increasing) under compressive (tensile) strain in Fe termination films. In particular, the work function in O terminated films is insensitive to strain because pz orbitals are less sensitive to strain than that of Fe termination films. Our findings confirm that the strain is an effective means to manipulate electronic structures and corrosion potential.

Effects of Intrinsic Defects and Substrate Transfer Doping on the Electronic Structure of Sb2Te3 Thin Films on Graphene

We study how the electronic structure of the Sb2Te3 thin films on graphene changes with intrinsic defects and substrate transfer doping using first-principles calculations. We find that for freestanding Sb2Te3 thin films without defects band gap decreases with increasing film thickness, and that the behavior of three-dimensional topological insulators does not appear up to seven quintuple layers. The orbital degeneracy is broken by defects and substrate transfer doping, leading to the Rashba effect and a reduced band gap. By decreasing the defect ratio of Sb vacancies (VSb) as the primary source of p-type conductivity and increasing that of Te-on-Sb antisites (TeSb) as the primary source of n-type conductivity, coupled with a transfer doping using a graphene substrate, the energy band of Sb2Te3 thin films can be tuned from the p-type to the n-type. Differences in the local defects of Sb2Te3 can result in variations in the work function and intrinsic defects have a stronger effect on the work function than...

Strain induced band inversion and topological phase transition in methyl-decorated stanene film

The researches for new quantum spin Hall (QSH) insulators with large bulk energy gap are of much significance for their practical applications at room temperature in electronic devices with low-energy consumption. By means of first-principles calculations, we proposed that methyl-decorated stanene (SnCH3) film can be tuned into QSH insulator under critical tensile strain of 6%. The nonzero topological invariant and helical edge states further confirm the nontrivial nature in stretched SnCH3 film. The topological phase transition originates from the s-pxy type band inversion at the Γ point with the strain increased. The spin-orbital coupling (SOC) induces a large band gap of ~0.24u2009eV, indicating that SnCH3 film under strain is a quite promising material to achieve QSH effect. The proper substrate, h-BN, finally is presented to support the SnCH3 film with nontrivial topology preserved.