RESUMO
Two-dimensional (2D) membranes consisting of a single layer of Mo atoms were recently manufactured [ Adv. Mater. 2018 , 30 , 1707281 ] from MoSe2 sheets by sputtering Se atoms using an electron beam in a transmission electron microscope. This is an unexpected result as formation of Mo clusters should energetically be more favorable. To get microscopic insights into the energetics of realistic Mo membranes and nonstoichiometric phases of transition-metal dichalcogenides (TMDs) MaXb, where M = Mo and W and X = S, Se, and Te, we carry out first-principles calculations and demonstrate that the membranes, which can be referred to as metallic quantum dots embedded into a semiconducting matrix, can be stabilized by charge transfer. We also show that an ideal neutral 2D Mo or W sheet is not flat but a corrugated structure, with a square lattice being the lowest-energy configuration. We further demonstrate that several intermediate nonstoichiometric phases of TMDs are possible as they have lower formation energies than pure metal membranes. Among them, the orthorhombic metallic 2D M4X4 phase is particularly stable. Finally, we study the properties of this phase in detail and discuss how it can be manufactured by the top-down approaches.
RESUMO
The possibility of a FeS2 phase formation in the 2D MoS2 structure was investigated by an ab initio DFT approach. Various concentrations of FeS2 in MoS2 have been analyzed, and it is shown that the energy favorable structures of the Mo1-xFexS2 composition are in-plane hybrid phases, FeS2 and MoS2 domains. After increasing the Fe/Mo concentration ratio up to 0.68, a complete transformation of the whole structure is predicted. We have found that the introduction of only a small amount of Fe atoms leads to a change in the electronic and magnetic properties of the film. An increase of the FeS2 nucleus size leads to the nearly monotonous increase of the magnetic moment governed by the exponential law.
RESUMO
On the basis of ab initio density functional calculations, we performed a comprehensive investigation of the general graphitization tendency in rocksalt-type structures. In this paper, we determine the critical slab thickness for a range of ionic cubic crystal systems, below which a spontaneous conversion from a cubic to a layered graphitic-like structure occurs. This conversion is driven by surface energy reduction. Using only fundamental parameters of the compounds such as the Allen electronegativity and ionic radius of the metal atom, we also develop an analytical relation to estimate the critical number of layers.
RESUMO
We present a theoretical study of current-voltage characteristics of different junctions of graphene nanoribbons. We considered isolated Y- and T-junctions of graphene nanoribbons (GNRs) with various geometry parameters and a graphene Y-junction in the graphane sheet. Our ab initio calculations based on the nonequilibrium Green's functions formalism displayed the influence of the geometry parameters of different ribbons on the I-V curves e.g. the shifting of zero voltage regions. We showed that not only the shape of the structure, but also the arrangement of electrodes attached to the structure will lead to changes in the transport properties.
RESUMO
The common opinion that diamond is the stiffest material is disproved by a number of experimental studies where the fabrication of carbon materials based on polymerized fullerenes with outstanding mechanical stiffness was reported. Here we investigated the nature of this unusual effect. We present a model constituted of compressed polymerized fullerite clusters implemented in a diamond matrix with bulk modulus B0 much higher than that of diamond. The calculated B0 value depends on the sizes of both fullerite grain and diamond environment and shows close correspondence with measured data. Additionally, we provide results of experimental study of atomic structure and mechanical properties of ultrahard carbon material supported the presented model.