Changmin Shi

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 0.194 eV for F-arsenene to 0.255 eV 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.

Robust large-gap quantum spin Hall insulators in chemically decorated arsenene films

Based on first-principles calculations, we propose one new category of two-dimensional topological insulators (2D TIs) in chemically functionalized (-CH3 and -OH) arsenene films. The results show that the surface decorated arsenene (AsCH3 and AsOH) films are intrinsic 2D TIs with sizeable bulk gap. The bulk energy gaps are 0.184 eV, and 0.304 eV in AsCH3 and AsOH films, respectively. Such large bulk gaps make them suitable to realize quantum spin Hall effect in an experimentally accessible temperature regime. Topologically helical edge states in these systems are desirable for dissipationless transport. Moreover, we find that the topological properties in these systems are robust against mechanical deformation by exerting biaxial strain. These novel 2D TIs with large bulk gaps are potential candidate in future electronic devices with ultralow dissipation.

Adsorption and gas-sensing characteristics of a stoichiometric α-Fe2O3 (0 0 1) nano thin film for carbon dioxide and carbon monoxide with and without pre-adsorbed O2

Herein, for the first time, the adsorption and gas-sensing characteristics of the CO2 and CO molecules on a stoichiometric α-Fe2O3 (0 0 1) nano thin film with and without pre-adsorbed O2 molecules have been studied using the density functional theory (DFT) method. Without pre-adsorbed O2 molecules, the CO2 molecule plays the role of an acceptor and obtains electrons from the stoichiometric α-Fe2O3 (0 0 1) nano thin film. For the O2 pre-adsorption α-Fe2O3 (0 0 1) nano thin film system, the CO2 molecule also plays the role of an acceptor. However, less number of electrons are transferred to the CO2 molecule as compared to the pre-adsorbed O2 molecules. Different from the CO2 molecule, the CO molecule always plays the role of a donor for the α-Fe2O3 (0 0 1) nano thin film system with and without pre-adsorbed O2. The theoretical results verify that the CO molecule can react with the lattice oxygen or adsorbed oxygen of the α-Fe2O3 (0 0 1) nano thin film. The electrons transferred to the stoichiometric α-Fe2O3 (0 0 1) nano thin film from the CO molecule or newly formed CO2 molecule are more than that transferred to the O2 pre-adsorption α-Fe2O3 (0 0 1) nano thin film. For the stoichiometric or O2 pre-adsorption α-Fe2O3 (0 0 1) nano thin film, the CO2 and CO molecules exhibited opposite behaviors of charge transformation. In addition, pre-adsorbed O2 molecules displayed competitive adsorption with the CO2 or CO molecule. The pre-adsorbed O2 molecules hinder electron transfer to the CO2 molecules from the α-Fe2O3 (0 0 1) nano thin film or hinder electron transfer to the α-Fe2O3 (0 0 1) nano thin film from the CO molecule. Theoretical results demonstrate that the surface of α-Fe2O3 materials (0 0 1) could be prepared for use as adsorbents or gas sensors for CO2 and CO molecules. Their structures are stable after CO2 molecules are adsorbed or after the reaction of CO molecules with the lattice oxygen or adsorbed oxygen of the α-Fe2O3 (0 0 1) nano thin film.

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 −30 °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.

Synthesis and characterization of Ag@Cu nano/microstructure ordered arrays as SERS-active substrates

We fabricated an Ag decorated Cu (Ag@Cu) nano/microstructure ordered array by facile template-free 2D electrodeposition combined with a galvanic reduction method for SERS applications. The Cu nano/microstructure ordered arrays were first synthesized by a 2D electrodeposition method, then Ag nanocubes were decorated on the arrays by galvanic reduction without any capping agent. The pollution-free surface and edge-to-face heterostructure of Ag nanocubes and Cu nano/microstructure arrays provide the powerful field-enhancements for SERS performance. The results verified that the Ag@Cu nano/microstructure ordered arrays have excellent activity for 4-Mercaptopyridine, and the sensitivity limit is as low as 10−8 M. Therefore, this facile route provides a useful platform for the fabrication of a SERS substrate based on nano/microstructure ordered arrays.

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 ~0.24 eV, 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.