MoSe₂ nanosheets and MoSe₂/graphene hybrids have been prepared by a facile hydrothermal method. The number of layers of the MoSe₂ nanosheets is typically <10 as confirmed directly by transmission electron microscopy and indirectly by a red shift of the characteristic A_1g Raman peak. The hydrogen evolution reaction (HER) studies show that the onset potentials of MoSe₂ and MoSe₂/RGO hybrids are only ∼0.15 V vs. RHE and ∼0.05 V vs. RHE, respectively, about 20–30 mV lower than those of MoS₂ and its graphene hybrids reported previously. Density functional theory calculations reveal that the Gibbs free energy for atomic hydrogen adsorption (ΔG⁰ _H) on MoSe₂ edges is closer to thermoneutral than that on MoS₂, with an H coverage of about 75% on the edge under operating conditions, which is also higher than that of MoS₂ reported in the literature. The consistency between the experimental and computational results indicates that MoSe₂ nanosheets have potential to be a better HER catalyst than their MoS₂ counterpart.

Interlayer coupling is of vital importance for manipulating physical properties, e.g., electronic band gap, in two-dimensional materials. However, tuning magnetic properties in these materials is yet to be addressed. Here, we found the in-plane magnetic orders of $\mathrm{Cr}{\mathrm{S}}_2$ mono and few layers are tunable between striped antiferromagnetic (sAFM) and ferromagnetic (FM) orders by manipulating charge transfer between Cr $t_{2g}$ and $e_g$ orbitals. Such charge transfer is realizable through interlayer coupling, direct charge doping, or substituting S with Cl atoms. In particular, the transferred charge effectively reduces a portion of ${\mathrm{Cr}}^{4+}$ to ${\mathrm{Cr}}^{3+}$, which, together with delocalized S $p$ orbitals and their resulting direct S-S interlayer hopping, enhances the double-exchange mechanism favoring the FM rather than sAFM order. An exceptional interlayer spin-exchange parameter was revealed over $-10\mathrm{meV}$, an order of magnitude stronger than available results of interlayer magnetic coupling. It addition, the charge doping could tune $\mathrm{Cr}{\mathrm{S}}_2$ between $p$- and $n$-doped magnetic semiconductors. Given these results, several prototype devices were proposed for manipulating magnetic orders using external electric fields or mechanical motion. These results manifest the role of interlayer coupling in modifying magnetic properties of layered materials and shed considerable light on manipulating magnetism in these materials.

We report ab initio analysis of current-voltage $(I-V)$ characteristics of carbon nanotubes with nitrogen substitution doping. For zigzag semiconducting tubes, doping with a single N impurity increases current flow and, for small radii tubes, narrows the current gap. Doping a N impurity per nanotube unit cell generates a metallic transport behavior. Nonlinear $I-V$ characteristics set in at high bias and a negative differential resistance region is observed for the doped tubes. These behaviors can be well understood from the alignment/mis-alignment of the current carrying bands in the nanotube leads due to the applied bias voltage. For a armchair metallic nanotube, a reduction of current is observed with substitutional doping due to elastic backscattering by the impurity.

We have calculated charge transport properties of molecular wires from first principles. The wires are made of oligophenylene molecules of three different lengths, in contact with atomic scale Au electrodes. The current-voltage characteristics of the wires are found to be linear, but the small bias resistances increase exponentially with the wire length. These behaviors are in qualitative agreement with recent experimental data. Quantitatively, our calculated slope of the resistance-wire-length curve is the same as the experimental data, while values of current and resistance are at the same order of magnitude as the data.