We report ab initio calculations showing that a single one-dimensional extended defect can originate topologically-protected metallic states in the bulk of two-dimensional topological insulators. We find that a narrow extended defect composed of periodic units consisting of one octogonal and two pentagonal rings embedded in the hexagonal bulk of a bismuth bilayer introduces two pairs of one-dimensional Dirac-fermion states with opposite spin-momentum locking. Although both Dirac pairs are localized along the extended-defect core, their interactions are screened due to the trivial topological nature of the extended defect. studies due to the coexistence of an insulating bulk band structure with a non-trivial topology that, when interfaced with a topologically trivial insulator such as the vacuum, gives rise to time-reversal-protected metallic surface states, with Dirac-fermion dispersions spanning the bulk band gap in the one-dimensional (1D) edges in twodimensional (2D) TIs. Existence of the edge states is a requirement imposed by the different topologies of the band structures across the interface. In these edge states, the spin quantization axis and the momentum direction are locked-in, implying that the metallic edge states are protected from backscattering, rendering their electronic conductance robust against the presence of disorder. Robust conduction and spin polarization may allow the manipulation of edge modes of TIs in many applications such as spintronics [7] and quantum computation [4,5].While topological-insulating band structures are also found in three-dimensional (3D) systems, manipulation of the metallic surface modes in 3D TIs is commonly hampered by the difficulty in tuning the Fermi level to achieve sufficiently low (ideally null) levels of bulk carriers, and the metallic surface carriers are significantly outnumbered by bulk carriers in most 3D TI samples [8,9]. Hence, 2D TIs can be advantageous in transport applications, because the 2D bulk is fully exposed to chemical manipulation and, besides, the bulk Fermi level can also be tuned by proper gating. When an insulating bulk is achieved, electrons can conduct only along the edge in these structures. From this, many efforts have been made to find candidate 2D TI systems.

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Recent experimental observations suggested that the presence of oxygen vacancies on TiO 2 surfaces affects the adsorption mode of formic acid (Xu, M.; et al. Catal. Today 2012, 182, 12). Here we use density functional theory and the hybrid density functional HSE06 form for the exchange− correlation functional to determine the atomic geometry and band structure of single molecules on TiO 2 (101) surfaces. We show that formic acid adsorbs dissociatively on both perfect and defective surfaces with no overlap between oxygen defect states and molecular states, leading to no change in the adsorption mode. We propose that both relaxation experienced by the surface atoms due to the presence of vacancies and molecule adsorption affect the electronic structure of the surface, leading to stabilization of the monodentate mode.

We have performed first-principles calculations of electronic and dielectric properties of singlelayer bismuth (bismuthene) adsorbed with -COOH. We show that the Bi-COOH hybrid structure is a two-dimensional topological insulator with protected edge Dirac states. The adsorption process of -COOH induces a planar configuration to the initially pristine buckled bismuthene. We claim that the stability of these planar structure mainly stem from strain induced by the adsorption of the -COOH organic group, but it is also related to ligand-ligand interactions. Using charge density analysis we show that the role of this organic group is not only to stabilize the layer but also to functionalize it, which is very important for future applications such as sensing, biomolecules immobilization, as well in electronic spintronic and even optical devices, due to its large band gap.Finally we demonstrate that many body corrections are crucial to obtain a better description of the electronic and dielectric properties of these systems.

The efficiency of nanoscale electronic devices usually is limited by the decrease in the carrier mobilities when the dimensionality is reduced. Using first principles calculations our results reveal that the hole effective masses of InAs nanowires decrease significantly below a threshold diameter. The mobilities have been estimated, and it is shown that for an optimal range of diameters, the hole mobilities exceeds the bulk value by up to five times, whereas the electron mobilities remain comparable to the bulk one. These results indicate that there exists a diameter window where p-type InAs based high-speed nanodevices can be fabricated.

We employ first-principles density-functional calculations to study structural and topological electronic transitions in two-dimensional bismuth layers. Our calculations reveal that a free-standing hexagonal bismuthene phase (the most stable one in the absence of strain) should become thermodinamically unstable against transformation to a putative ‘pentaoctite’ phase (composed entirely of pentagonal and octagonal rings), under biaxial tensile strain. Moreover, our results indicate that 2D bismuth layers in the pentaoctite phase should undergo a topological electronic phase transition under either a biaxial or uniaxial tensile strain. More specifically, at its equilibrium lattice parameters the pentaoctite lattice is a topologically trivial system with a direct band gap. Strain-induced parity inversion of valence and conduction bands is obtained, and the pentaoctite structure undergoes a transition to a topological-insulator phase at a biaxial tensile strain of 5%. In the case of uniaxial tensile strains, the topological transition happens at a tensile strain of 6% along the armchair direction of the pentaoctite lattice, and at a 5% tensile strain in the zigzag direction. Our study indicates that 2D bismuth layers may prove themselves a rich platform to realize topologically non-trivial 2D materials upon strain engineering.

In the present work we predict, through first-principles calculations, that bilayers of the recently synthesized Ni₃C₁₂S₁₂ and Pt₃C₁₂S₁₂ layered materials are topological insulators upon electron doping, and that that their topological insulator properties can be modulated by the application of electric fields with magnitudes achievable in devices. The electronic structures of both bilayers are characterized by spin-orbit split graphene-like bands, with gap magnitudes that are three orders of magnitude larger than graphene's. In ribbon geometries, chiral edge modes develop at each side with band dispersions similar to that of Kane-Mele graphene model. Surprisingly, the edge states' spin-propagation locking occurs even for very thin ribbons. We also find that the response of the electronic structure of both materials to applied electric fields are similar to both graphene and the Kane-Mele model with a Rashba term. All these findings indicate that these bilayer systems can be considered as large-spin-orbit graphene analogues with a strong sensitivity to applied electric fields.

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