The electronic properties of two-dimensional honeycomb structures of molybdenum disulfide (MoS 2 ) subjected to biaxial strain have been investigated using first-principles calculations based on density functional theory. On applying compressive or tensile bi-axial strain on bi-layer and mono-layer MoS 2 , the electronic properties are predicted to change from semiconducting to metallic. These changes present very interesting possibilities for engineering the electronic properties of two-dimensional structures of MoS 2 .

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Using ab initio calculations we demonstrate that extra electrons in pure amorphous SiO2 can be trapped in deep band gap states. Classical potentials were used to generate amorphous silica models and density functional theory to characterize the geometrical and electronic structures of trapped electrons. Extra electrons can trap spontaneously on pre-existing structural precursors in amorphous SiO2 and produce ≈ 3.2 eV deep states in the band gap. These precursors comprise wide (≥132 • ) O-Si-O angles and elongated Si-O bonds at the tails of corresponding distributions. The electron trapping in amorphous silica structure results in an opening of the O-Si-O angle (up to almost 180 • ). We estimate the concentration of these electron trapping sites to be ≈ 4 × 10 19 cm −3 . The structure of these centers is similar to that of Ge and Li electron centers in α-quartz.

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The electronic properties of two-dimensional hexagonal germanium, so called germanene, are investigated using first-principles simulations. Consistent with previous reports, the surface is predicted to have a “poor” metallic behavior, i.e., being metallic with a low density of states at the Fermi level. It is found that biaxial compressively strained germanene is a gapless semiconductor with linear energy dispersions near the K points—like graphene. The calculated Fermi velocity of germanene is almost independent of the strain and is about 1.7×106 m/s, quite comparable to the value in graphene.

Amorphous (a)-HfO2 is a prototype high dielectric constant insulator with wide technological applications. Using ab initio calculations we show that excess electrons and holes can trap in aHfO2 in energetically much deeper polaron states than in the crystalline monoclinic phase. The electrons and holes localize at precursor sites, such as elongated Hf-O bonds or under-coordinated Hf and O atoms and the polaronic relaxation is amplified by the local disorder of amorphous network. Single electron polarons produce states in the gap at ∼2 eV below the bottom of the conduction band with average trapping energies of 1.0 eV. Two electrons can form even deeper bipolaron states on the same site. Holes are typically localized on under-coordinated O ions with average trapping energies of 1.4 eV. These results advance our general understanding of charge trapping in amorphous oxides by demonstrating that deep polaron states are inherent and do not require any bond rupture to form precursor sites.Electron and hole states with energy levels lying deep in the bandgap impair the dielectric quality of insulating layers. In particular, electron transitions facilitated by these states account for multitude of degradation phenomena including enhanced leakage current and charge trapping eventually leading to the dielectric barrier failure and breakdown. Routinely, however, these deep electron states are seen as not inherent to the perfect material but rather associated with the presence of defects and/or impurity centers in the atomic network of an insulator. This belief offers some hope that imperfections can be eliminated by using more clean and optimized synthesis and proper processing of the insulators. On the other hand, self-trapping of excess charges in the form of small electron and hole polarons is well known to occur even in perfect crystalline oxide insulators. However, it is usually shallow, with trapping energies of the order of 0.2 eV (see e.g. [1-3]). As a result, the electron and hole polarons are mobile at room temperature in crystalline reduced TiO 2 and NiO [4], CeO 2 [5, 6], doped ZrO 2 [7,8], and in plethora of other oxides (see e.g. [1,[9][10][11][12]). The intrinsic localization of excess electrons and holes in noncrystalline materials and liquids has also been a subject of extensive experimental and theoretical studies pioneered in [13]. Structural disorder typically induces shallow electron states near the bottom of the conduction band, below the so-called mobility edge (see, e.g. [14] . In these systems, the polaronic relaxation is amplified by the local disorder of amorphous network.Here we turn to amorphous (a)-HfO 2 , which represents a wide class of high dielectric constant oxides recently emerged as the major contenders to replace SiO 2 in a broad spectrum of nano-electronic devices ranging from deep-scaled transistors to DRAM and non-volatile memory cells (see, e.g. [21,22]). Amorphous oxides make the backbone of most electronic devices and charge trapping appears to be the key factor determining devic...

We study the structural, mechanical and electronic properties of the two-dimensional (2D) allotrope of tin: tinene/stanene using first-principles calculation within density functional theory, implemented in a set of computer codes. Continuing the trend of the group-IV 2D materials graphene, silicene and germanene; tinene is predicted to have a honeycomb lattice with lattice parameter of a0 = 4.62 Å and a buckling of d0 = 0.92 Å. The electronic dispersion shows a Dirac cone with zero gap at the Fermi energy and a Fermi velocity of m s−1; including spin–orbit coupling yields a bandgap of 0.10 eV. The monolayer is thermally stable up to 700 K, as indicated by first-principles molecular dynamics, and has a phonon dispersion without imaginary frequencies. We explore applied electric field and applied strain as functionalization mechanisms. Combining these two mechanisms allows for an induced bandgap up to 0.21 eV, whilst retaining the linear dispersion, albeit with degraded electronic transport parameters.