[en] The PCB molecules have been self-assembled on Au(111) surface and the self-assembly behavior has been studied based on the first principle calculation. The results show that the PCB molecules are antiparallel phase between line and line observed by scanning tunneling microscopy (STM). Moreover, the lattice parameter are matched between the PCB molecules and the Au(111) substrate. Based on the first-principle calculation, it is found that the self-assembly behavior are affected by the molecule–substrate (MS) and molecule–molecule (MM) interactions (cyano coupling force), in which the molecule–substrate (MS) interactions is stronger than that of between the molecules. And the effect of MS interaction plays a dominate role during the PCB molecules self-assembly. This work is helpful to achieve rational design, accurate prediction, and controllable construction of assembled molecular nanostructures. (paper)

[en] 2D covalent organic framework-1 (COF-1) membrane is a potential hydrogen storage material. The hydrogen storage capacity of Li-decorated COF-1 has been studied by first-principles calculation. The results show its hydrogen storage capacity has been improved significantly by Li decoration, which is 7.69 wt%. Then ab initio molecular dynamics simulations at 300 K have been carried out and the results show that 12 H₂ molecules are stably absorbed on the double sides of COF-1 unit cell decorated by 6 Li atoms and the hydrogen storage capacity is 5.26 wt%. (paper)

[en] Despite many excellent properties, the synthesis of high quality graphene with low-cost way is still a challenge, thus many different factors have been researched. In this work, the effect of surface roughness to the graphene quality was studied. Graphene was synthesized by plasma enhanced chemical vapor deposition (PECVD) method on copper substrates with different roughness from 0.074 μm to 0.339 μm, which were prepared via annealing, corrosion or polishing, respectively. Ar⁺ plasma cleaning was applied before graphene growth in order to accommodate similar surface chemical reactivity to each other. Scanning electron microscope and Raman spectroscope were employed to investigate the effect of surface roughness, which reveals that the graphene quality decrease first and then increase again according to the ratio of I_D/I_G in Raman spectroscopy. When the ratio of I_D/I_G reaches the largest number, the substrate roughness is 0.127 μm, where is the graphene quality changing point. First principle calculation was applied to explain the phenomenon and revealed that it is strongly affected by the graphene grain size and quantity which can induce defects. This strategy is expected to guide the industrial production of graphene. (paper)

[en] The fascinating Dirac cone in honeycomb graphene, which underlies many unique electronic properties, has inspired the vast endeavors on pursuing new two-dimensional (2D) Dirac materials. Based on the density functional theory method, a 2D material Zn₃Si₂ of honeycomb transition-metal silicide with intrinsic Dirac cones has been predicted. The Zn₃Si₂ monolayer is dynamically and thermodynamically stable under ambient conditions. Importantly, the Zn₃Si₂ monolayer is a room-temperature 2D Dirac material with a spin–orbit coupling energy gap of 1.2 meV, which has an intrinsic Dirac cone arising from the special hexagonal lattice structure. Hole doping leads to the spin polarization of the electron, which results in a Dirac half-metal feature with single-spin Dirac fermion. This novel stable 2D transition-metal-silicon-framework material holds promises for electronic device applications in spintronics. (paper)

[en] Using first-principle calculations, we predict a new family of stable two-dimensional (2D) topological insulators (TI), monolayer Be₃ X ₂ (X = C, Si, Ge, Sn) with honeycomb Kagome lattice. Based on the configuration of Be₃C₂, which has been reported to be a 2D Dirac material, we construct the other three 2D materials and confirm their stability according to their chemical bonding properties and phonon-dispersion relationships. Because of their tiny spin–orbit coupling (SOC) gaps, Be₃C₂ and Be₃Si₂ are 2D Dirac materials with high Fermi velocity at the same order of magnitude as that of graphene. For Be₃Ge₂ and Be₃Sn₂, the SOC gaps are 1.5 meV and 11.7 meV, and their topological nontrivial properties are also confirmed by their semi-infinite Dirac edge states. Our findings not only extend the family of 2D Dirac materials, but also open an avenue to track new 2DTI. (paper)

[en] Cu₂Se monolayer (ML) synthesized experimentally is a member of transition metal chalcogenides materials, which has attracted significant attention due to its diversity and unique properties. However, the feature of an indirect band gap of Cu₂Se ML in the low-temperature phase limits its’ application in electronics devices. Our study results based on the first principle calculations show that indirect-direct band gap transitions can occur in Cu₂Se ML under appropriate uniaxial or biaxial strains. The band gap of Cu₂Se ML is controllable due to the different responses of the edge-states near the Fermi level to the strain. The phonon dispersion suggests that the semiconducting Cu₂Se ML can maintain dynamic stability in a wide range of strains. With the tunable electronic structure, semiconducting Cu₂Se ML would become a promising candidate for electronic devices. (paper)

[en] To obtain high-performance spintronic devices with high integration density, two-dimensional (2D) half-metallic materials are highly desirable. Based on first-principles calculations, we predicted a potential candidate of 2D half-metals, named monolayer Mg₃Si₂, which has a honeycomb-kagome lattice. The ground state of monolayer Mg₃Si₂ is an antiferromagnetic (AFM) semiconductor. However, it will transfer to be ferromagnetic (FM) half-metals within electrons or holes doping. Our result shows that its half-metallicity comes from p _y orbitals of Si and Mg atoms in the carriers doping case. Our findings highlight a new promising material with controllable magnetic and electronic properties, which will have potential applications in 2D spintronic devices. (paper)

[en] We have implanted boron (B) ions (dosage: 5x10¹⁴ cm^-2) into diamond and then hydrogenated the sample by implantating hydrogen ions at room temperature. A p-type diamond material with a low resistivity of 7.37 m Ω cm has been obtained in our experiment, which suggests that the hydrogenation of B-doped diamond results in a low-resistivity p-type material. Interestingly, inverse annealing, in which carrier concentration decreased with increasing annealing temperature, was observed at annealing temperatures above 600 deg. C. In addition, the formation mechanism of a low-resistivity material has been studied by density functional theory calculation using a plane wave method.