Xianqing Lin

Adsorption capacity of H2O, NH3, CO, and NO2 on the pristine graphene

First-principles together with statistical mechanics calculations have been performed to study the adsorption behavior of H2O, NH3, CO, and NO2 on the pristine graphene. In the first-principles calculations, we find that the most recent van der Waals (vdW) density functional vdW-DF2 gives even larger binding energies (Eb) that those obtained with the local density approximation, indicating vdW-DF2 may be inappropriate for describing the interaction between these molecules and graphene. With the potential energy curves of the molecules on graphene calculated by the density functional theory, the adsorption capacity (n) of the molecules on the pristine graphene is calculated with the statistical mechanics method. NO2 has the largest n of the order of 108 cm−2 among the four molecules on graphene at room temperature and concentration of 1.0 ppm, but still smaller by almost two order than that on graphene devices estimated from the experimental results. This is probably due to the strong binding of NO2 to the...

Charge and magnetic states of Mn-, Fe-, and Co-doped monolayer MoS2

First-principles calculations have been performed to investigate the electronic and magnetic properties of monolayer MoS2 substitutionally doped with Mn, Fe, and Co in possible charge states (q). We find that the Mn, Fe, and Co dopants substituting for a Mo atom in monolayer MoS2 (Mn@Mo, Fe@Mo, and Co@Mo) are all magnetic in their neutral and charge states except in the highest positive charge states. Mn@Mo, Fe@Mo, and Co@Mo have the same highest negative charge states of q=−2 for chemical potential of electron just below the conduction band minimum, which corresponds to the electron doping. In the q=−2 state, Mn@Mo has a much larger magnetic moment than its neutral state with the antiferromagnetic coupling between the Mn dopant and its neighboring S atoms maintained, while Fe@Mo and Co@Mo have equal or smaller magnetic moments than their neutral states. The possible charge states of Mn@Mo, Fe@Mo, and Co@Mo and the variation of the magnetic moments for different dopants and charge states are due to the ch...

Flat bands near Fermi level of topological line defects on graphite

Here, we report direct experimental evidence for the presence of flat bands, close to the Fermi level, in one-dimensional topological defects of graphite. The flat bands are manifested by a pronounced peak in the tunnelling density of states. Our ab initio calculations indicate that the flat bands with vanishing Fermi velocity originate from sp2 dangling bonds (with antibonding nature) of undercoordinated carbon atoms at the edges of the defects. We further demonstrate that the presence of flat bands could be an inevitable behavior of 1D defects of graphene/graphite with undercoordinated carbon atoms at the edges of the defects.

Dirac points and van Hove singularities of silicene under uniaxial strain

First-principles calculations have been performed to investigate the low energy electronic properties and van Hove singularities (VHSs) of silicene under uniaxial strain. The Dirac points (DPs) persist when silicene is stretched uniaxially, while they are shifted away from the corners (K points) of the first Brillouin zone (FBZ). The relative positions of DPs with respect to the K points for silicene strained along the armchair (AC) or zigzag (ZZ) direction show opposite tendency compared with strained graphene, which is due to the larger deformation of the unit cell of strained silicene than that of strained graphene. Moreover, for silicene under AC or ZZ strain, the Fermi velocities around DPs along the positive and negative directions of the FBZ show rather significant difference. The nature of the VHS just above the Fermi energy undergoes a transition from the π* band to the σ* band for silicene under increasing AC or ZZ strain. These observations suggest uniaxial strain as an effective route to tune the electronic properties of silicene for potential applications in future electronic devices.

The extraordinary magnetoelectric response in silicene doped with Fe and Cr atoms

We have investigated the magnetic properties of the silicene doped with Fe and Cr metal atoms under external electric field by the first-principles calculations. We find that the doped systems show a variety of interesting magnetoelectric (ME) behaviors: (1) The magnetic moment of Fe doped silicene show a sharp jump at a threshold electric field, which indicates a good switching effect; (2) For the low concentrations of Fe or Cr doped silicene, there are two structures in which the changes of magnetism are significantly different under external electric field; (3) The magnetic moment of the doped systems has a nearly linear region with the electric field. We find that the changes of magnetic moment strongly depend on the direction of the electric field. In particular, one structure of Fe doped silicene shows an interesting ME response which can be considered as a magnetoelectric diode. With the electric field, the good controllability and sharp switching of the magnetism may offer a potential applications in the ME devices.

Tailoring of the structural, energetic and electronic properties of silicene-based nanostructures

Silicene-based nanostructures are highly promising for future applications in electronics as silicon is an important element for conventional semiconductor industry. In our recent works, we have studied the effect of external electric filed, strain and metal adatoms on tailoring the structural, energetic and electronic properties of silicene-based nanostructures using first-principles and tight-binding methods. We find that half-metallicity can be realized in zigzag silicene nanoribbons when applying electric field across the nanoribbon width. Under small tensile strain, some armchair silicene nanoribbons are predicted to have linear energy dispersion around the Fermi level. And strain-induced structural phase transitions are observed in silicene bilayers in which strong covalent interlayer bonds form. When metal atoms are adsorbed on silicene, several metal adatoms obtain a larger binding energy than the cohesive energy of the bulk metal and the bonding between the metal atoms and silicene can be ionic or covalent.

Magnetism and electronic phase transitions in monoclinic transition metal dichalcogenides with transition metal atoms embedded

First-principles calculations have been performed to study the energetic, electronic, and magnetic properties of substitutional 3d transition metal dopants in monoclinic transition metal dichalcogenides (TMDs) as topological insulators ( 1T′-MX2 with M = (Mo, W) and X = (S, Se)). We find various favorite features in these doped systems to introduce magnetism and other desirable electronic properties: (i) The Mn embedded monoclinic TMDs are magnetic, and the doped 1T′-MoS2 still maintains the semiconducting character with high concentration of Mn, while an electronic phase transition occurs in other Mn doped monoclinic TMDs with an increasing concentration of Mn. Two Mn dopants prefer the ferromagnetic coupling except for substitution of the nearest Mo atoms in 1T′-MoS2, and the strength of exchange interaction shows anisotropic behavior with dopants along one Mo zigzag chain having much stronger coupling. (ii) The substitutional V is a promising hole dopant, which causes little change to the energy disper...

Topologically insulating states in ternary transition metal dichalcogenides

The topological and electronic properties of monolayered monoclinic transition metal dichalcogenide (TMD) alloys (1T ′-M1−xNxX2 with M, N = Cr, Mo, W and X = S, Se) have been studied through calculations based on the projected Wannier functions obtained from first-principles calculations. We predict that the ternary compounds 1T ′-Mo1−xCrxS2 with x up to 7/12 and all 1T ′-Mo1−xWxSe2 host topologically insulating states with band gaps comparable to the pure systems. For Cr contained alloys, the mechanism of sign changing of Berry curvature is proposed to explain the trivial band topology of some configurations. The predicted topologically insulating ternary TMDs may be promising candidates for future realization of topological devices.