Very recently, two-dimensional nanosheets of MoSe₂, MoTe₂ and WS₂ were successfully synthesized experimentally [Science, 2011, 331, 568]. In the present work, the electronic and magnetic properties of perfect, vacancy-doped, and nonmetal element (H, B, C, N, O, and F) adsorbed MoSe₂, MoTe₂ and WS₂ monolayers are systematically investigated by means of first-principles calculations to give a detailed understanding of these materials. It is found that: (1) MoSe₂, MoTe₂ and WS₂ exhibit surprising confinement-induced indirect-direct-gap crossover; (2) among all the neutral native vacancies of MoSe₂, MoTe₂ and WS₂ monolayers, only the Mo vacancy in MoSe₂ can induce spin-polarization and long-range antiferromagnetic coupling; (3) adsorption of nonmetal elements on the surface of MoSe₂, MoTe₂ and WS₂ nanosheets can induce a local magnetic moment; H-absorbed WS₂, MoSe₂, and MoTe₂ monolayers and F-adsorbed WS₂ and MoSe₂ monolayers show long-range antiferromagnetic coupling between local moments even when their distance is as long as ∼12 Å. These findings are a useful addition to the experimental studies of these new synthesized two-dimensional nanosheets, and suggest a new route to facilitate the design of spintronic devices for complementing graphene. Further experimental studies are expected to confirm the attractive predictions.

The geometric and electronic structures of graphene adsorption on MoS₂ monolayer have been studied by using density functional theory. It is found that graphene is bound to MoS₂ with an interlayer spacing of 3.32 Å and with a binding energy of −23 meV per C atom irrespective of adsorption arrangement, indicating a weak interaction between graphene and MoS₂. A detailed analysis of the electronic structure indicates that the nearly linear band dispersion relation of graphene can be preserved in MoS₂/graphene hybrid accompanied by a small band-gap (2 meV) opening due to the variation of on-site energy induced by MoS₂. These findings are useful complement to experimental studies of this new synthesize system and suggest a new route to facilitate the design of devices where both finite band-gap and high carrier mobility are needed.

Here we present first-principles calculations to investigate systematically the electronic behavior and the electron energy low-loss spectra (EELS) of monolayer, bilayer, four-layer, and bulk configurations of periodic GaX (X = S, Se), as well as the effect of mechanical strain on the electronic properties of the GaX monolayer. We predicate that the GaX monolayer is a semiconductor with an indirect band gap, however, the difference between the direct and indirect gaps is so small that electrons can transfer easily between this minimum with a small amount of thermal energy. Owning to strong surface effects, the electronic and dielectric properties of GaX vary drastically with number of layers in a sheet. In detail, the band gap increases from multilayer-to-single layer and EELS shifts towards larger wavelengths with a decrease in the layer thickness. Moreover, we demonstrate that the band gaps of GaX monolayers can be widely tuned by mechanical deformation, making them potential candidates for tunable nanodevices. The present study provides theoretical insight leading to a better understanding of these novel 2D structures.

${\text{MoS}}_2$, a member of transition metal dichalcogenides (TMDs), has recently emerged as an interesting two-dimensional material due to its unique mechanical, thermal, electronic and optical properties. Unlike graphene which possesses massless Dirac fermions with ultrahigh electron mobility, monolayer ${\text{MoS}}_2$ is a direct band gap semiconductor. An interesting question arises: Can monolayer ${\text{MoS}}_2$ also possess massless Dirac fermions with ultrahigh electron mobility? Here, using first-principles calculations, we show that a monolayer ${\text{MoS}}_2$ allotrope, which consists of repeated square-octagon rings (abbreviated as so-${\text{MoS}}_2$ to distinguish it from the normal hexagonal lattice, h-${\text{MoS}}_2$) possesses both massless Dirac fermions and heavy fermions. Distinct from the $p$-orbital Dirac fermions of graphene, the Dirac fermions of so-${\text{MoS}}_2$ are $d$ electrons and possess a Fermi velocity comparable to that of graphene. The Dirac cone structure in so-${\text{MoS}}_2$ demonstrated here greatly enriches our understanding on the physical properties of TMDs and opens up possibilities for developing high-performance electronic or spintronic devices.