Electrically tunable band gap in strained h-BN/silicene van der Waals heterostructures.

Single layers of hexagonal boron nitride (h-BN) and silicene are brought together to form h-BN/silicene van der Waals (vdW) heterostructures. The effects of external electric fields and compressive strain on their structural and electronic properties are systematically studied through first principles calculations. Two silicene phases are considered: the low-buckled Si(LB) and the dumbbell-like Si(DB). They show exciting new properties as compared to the isolated layers, such as a tunable band gap that depends on the interlayer distance and is dictated by the charge transfer and orbital hybridization between h-BN and silicene, especially in the case of Si(LB). The electric field also increases the band gap in h-BN/Si(DB) and causes an asymmetric charge rearrangement in h-BN/Si(LB). Remarkably, we found a great potential of h-BN layers to function as substrates for silicene, enhancing both the strain and electric field effects on its electronic properties. These results contribute to a more detailed understanding of h-BN/Si 2D-based materials, highlighting promising possibilities in low-dimensional electronics.

Stability, and optical and electronic properties of ultrathin h-BNC.

Spin polarized density functional theory has been used to study the stability, and electronic and optical properties when BN nanodomains are embedded in graphene and carbon patches are embedded in a single layer of h-BN forming h-BNC nanosystems. Our results show that graphene doped with BN nanodomains exhibits a non-zero gap, which depends on the nanodomain's shape and width. For h-BN with C domains we observe that we can tune the h-BN gap into the visible region, making the h-BNC a promising material for catalysis using solar energy. Furthermore, n-type and p-type semiconductors can be obtained by controlling the bond (C-N or C-B) in the border of the domain. These findings open the possibility to use h-BNC nanosheets for future applications in photocatalysis and optoelectronic devices.

The effect of impurities in ultra-thin hydrogenated silicene and germanene: a first principles study.

Spin polarized density functional theory within the GGA-PBE and HSE06 approach for the exchange correlation term has been used to investigate the stability and electronic properties of nitrogen and boron impurities in single layers of silicane and germanane. We have observed that these impurities have lower formation energies in silicane and germanane when compared to their counterparts in graphane. We have also noticed that the adsorption of H atoms in the vicinity of defects stabilizes the system. In addition, we have shown that the electronic properties of silicane and germanane can be tuned when N and B are incorporated in the Si and Ge network. N-doping and B-doping give rise to n-type and p-type semiconductor properties. However, the adsorption of H atoms quenches the doping effects.

Size- effect induced high thermoelectric figure of merit in PbSe and PbTe nanowires.

The fundamental properties that compose the thermoelectric figure of merit are investigated in the confined systems of PbSe and PbTe nanowires, with the goal to improve the thermoelectric efficiency. Using the Landauer electronic transport theory, we verify that the figure of merit can be several times larger than the bulk value for nanowires with diameters down to the one nanometer scale. This enhancement in the thermoelectric efficiency is primarily due to the reduction of the thermal conductivity and an increase in the power factor. The origin of these desireable properties, that enable the transformation of heat into electricity, comes from the confinement effect which increases the density of states around the Fermi level, either for an n- or p-type system.

Noncentrosymmetric two-dimensional Weyl semimetals in porous Si/Ge structures.

In this work we predict a family of noncentrosymmetric two-dimensional (2D) Weyl semimetals (WSMs) composed by porous Ge and SiGe structures. These systems are energetically stable graphenylene-like structures with a buckling, spontaneously breaking the inversion symmetry. The nontrivial topological phase for these 2D systems occurs just below the Fermi level, resulting in nonvanishing Berry curvature around the Weyl nodes. The emerged WSMs are protected byC3symmetry, presenting one-dimensional edge Fermi-arcs connecting Weyl points with opposite chiralities. Our findings complete the family of Weyl in condensed-matter physics, by predicting the first noncentrosymmetric class of 2D WSMs.

2D Dumbbell Silicene as a High Storage Capacity and Fast Ion Diffusion Anode for Li-Ion Batteries.

First-principles calculations within DFT have been performed to investigate the use of a recently synthesized form of silicene, the dumbbell (DB) silicene as an anode material for Li-ion batteries (LiBs). The energetically most stable geometries for Li adsorption on DB silicene were investigated, and the energy barriers for Li-ion diffusion among the possible stable adsorption sites were calculated. We found that DB silicene can be lithiated up to a ratio of 1.05 Li per Si atom, resulting in a high storage capacity of 1002 mA h g-1 and an average open-circuit potential of 0.38 V, which makes DB silicene suitable for applications as an anode in LiBs. The energy barrier for Li-ion diffusion was calculated to be as low as 0.19 eV, suggesting that the Li ions can easily diffuse on the entire DB silicene surface, decreasing the time for the charge/discharge process of the LiBs. Our detailed investigations show that the most stable form of two-dimensional silicon has characteristic features suitable for application in high-performance LiBs.

Hydrogen adsorption on carbon-doped boron nitride nanotube.

The adsorption of atomic and molecular hydrogen on carbon-doped boron nitride nanotubes is investigated within the ab initio density functional theory. The binding energy of adsorbed hydrogen on carbon-doped boron nitride nanotube is substantially increased when compared with hydrogen on nondoped nanotube. These results are in agreement with experimental results for boron nitride nanotubes (BNNT) where dangling bonds are present. The atomic hydrogen makes a chemical covalent bond with carbon substitution, while a physisorption occurs for the molecular hydrogen. For the H(2) molecule adsorbed on the top of a carbon atom in a boron site (BNNT + C(B)-H(2)), a donor defect level is present, while for the H(2) molecule adsorbed on the top of a carbon atom in a nitrogen site (BNNT + C(N)-H(2)), an acceptor defect level is present. The binding energies of H(2) molecules absorbed on carbon-doped boron nitride nanotubes are in the optimal range to work as a hydrogen storage medium.

Rationalizing the Hydrogen and Oxygen Evolution Reaction Activity of Two-Dimensional Hydrogenated Silicene and Germanene.

We have undertaken first-principles electronic structure calculations to show that the chemical functionalization of two-dimensional hydrogenated silicene (silicane) and germanene (germanane) can become a powerful tool to increase the photocatalytic water-splitting activity. Spin-polarized density functional theory within the GGA-PBE and HSE06 types of exchange correlation functionals has been used to obtain the structural, electronic, and optical properties of silicane and germanane functionalized with a series of nonmetals (N, P, and S), alkali metals (Li, Na, and K) and alkaline-earth metals (Mg and Ca). The surface-adsorbate interaction between the functionalized systems with H2 and O2 molecules that leads to envisaged hydrogen and oxygen evolution reaction activity has been determined.