3D continuum phonon model for group-IV 2D materials

A general three-dimensional continuum model of phonons in two-dimensional materials is developed. Our first-principles derivation includes full consideration of the lattice anisotropy and flexural modes perpendicular to the layers and can thus be applied to any two-dimensional material. In this paper, we use the model to not only compare the phonon spectra among the group-IV materials but also to study whether these phonons differ from those of a compound material such as molybdenum disulfide. The origin of quadratic modes is clarified. Mode coupling for both graphene and silicene is obtained, contrary to previous works. Our model allows us to predict the existence of confined optical phonon modes for the group-IV materials but not for molybdenum disulfide. A comparison of the long-wavelength modes to density-functional results is included

Martensitic transformation in TiNi based shape memory alloys – a first principles study

The martensitic transformation in TiNi-based shape memory alloys has been investigated using various theoretical methods. The direct method for ab initio lattice dynamics is used for calculating phonon dispersion relations in austenite (B2) and martensites (B19, B19′). Normal mode symmetries in a B2 crystal are determined. Vibrational modes in the B2 crystal are labelled according to symmetry properties of their polarisation vector. Normal modes with imaginary frequencies are observed at the locations of experimentally observed soft modes. While labelling normal modes, the published mode symmetry label of a soft transverse acoustic (TA2) mode with a [Mathematical formula appears here. To view, please open pdf attachment] polarisation at [Mathematical formula appears here. To view, please open pdf attachment] is corrected to Σ2. Irreducible representations of the soft modes are used for determining the order parameters for various martensitic transformation paths experimentally observed using group theoretical methods. Accuracy of the interpolated frequencies in the phonon dispersion relations is assessed by using three different supercell sizes and by calculating the elastic constants from the slopes of the acoustic branches at the Brillouin zone. Born et al. derived a relation between the force constants and elastic constants for a general crystal. Further simplification of this relation is possible for a B2 crystal because of its symmetry. This simplification is derived by applying elementary group theory. Elastic constants calculated with the simplified relation are in agreement with the elastic constants calculated from the slopes of the dispersion relations. The self-consistent ab initio lattice dynamics (SCAILD) formulation is derived for an ordered compound. Precautions that need to be taken while practically implementing this formulation are discussed. The effect of Hf or Zr addition to a binary Ti-Ni system is studied by considering one configuration at each of five compositions from 0 to 25 at.% Hf or Zr. Elastic constants are calculated following a homogeneous deformation method. Selected normal mode frequencies are calculated using the direct method

Thermal conductivity of bulk and monolayer MoS

We show that the lattice contribution to the thermal conductivity of MoS2 strongly dominates the carrier contribution in a broad temperature range from 300 to 800 K. Since theoretical insight into the lattice contribution is largely missing, though it would be essential for materials design, we solve the Boltzmann transport equation for the phonons self-consistently in order to evaluate the phonon lifetimes. In addition, the length scale for transition between diffusive and ballistic transport is determined. The low out-of-plane thermal conductivity of bulk MoS2 ($2.3\ \text{Wm}^{-1}\text{K}^{-1}$ at 300 K) is useful for thermoelectric applications. On the other hand, the thermal conductivity of monolayer MoS2 ($131\ \text{Wm}^{-1}\text{K}^{-1}$ at 300 K) is comparable to that of Si

Universal binding energy relation for cleaved and structurally relaxed surfaces

The universal binding energy relation (UBER), derived earlier to describe the cohesion between two rigid atomic planes, does not accurately capture the cohesive properties when the cleaved surfaces are allowed to relax. We suggest a modified functional form of UBER that is analytical and at the same time accurately models the properties of surfaces relaxed during cleavage. We demonstrate the generality as well as the validity of this modified UBER through first-principles density functional theory calculations of cleavage in a number of crystal systems. Our results show that the total energies of all the relaxed surfaces lie on a single (universal) energy surface, that is given by the proposed functional form which contains an additional length-scale associated with structural relaxation. This functional form could be used in modelling the cohesive zones in crack growth simulation studies. We find that the cohesive law (stress-displacement relation) differs significantly in the case where cracked surfaces are allowed to relax, with lower peak stresses occurring at higher displacements

Is NiCo₂S₄ Really a Semiconductor?

NiCo₂S₄ is a technologically important electrode material that has recently achieved remarkable performance in pseudocapacitor, catalysis, and dye-synthesized solar cell applications.− Essentially, all reports on this material have presumed it to be semiconducting, like many of the chalcogenides, with a reported band gap in the range of 1.2–1.7 eV., In this report, we have conducted detailed experimental and theoretical studies, most of which done for the first time, which overwhelmingly show that NiCo₂S₄ is in fact a metal. We have also calculated the Raman spectrum of this material and experimentally verified it for the first time, hence clarifying inconsistent Raman spectra reports. Some of the key results that support our conclusions include: (1) the measured carrier density in NiCo₂S₄ is 3.18 × 10²² cm^–3, (2) NiCo₂S₄ has a room temperature resistivity of around 10³ μΩ cm which increases with temperature, (3) NiCo₂S₄ exhibits a quadratic dependence of the magnetoresistance on magnetic field, (4) thermopower measurements show an extremely low Seebeck coefficient of 5 μV K^–1, (5) first-principles calculations confirm that NiCo₂S₄ is a metal. These results suggest that it is time to rethink the presumed semiconducting nature of this promising material. They also suggest that the metallic conductivity is another reason (besides the known significant redox activity) behind the excellent performance reported for this material

Thermal conductivity of bulk and monolayer MoS 2

We show that the lattice contribution to the thermal conductivity of MoS2 strongly dominates the carrier contribution in a broad temperature range from 300 to 800 K. Since theoretical insight into the lattice contribution is largely missing, though it would be essential for materials design, we solve the Boltzmann transport equation for the phonons self-consistently in order to evaluate the phonon lifetimes. In addition, the length scale for transition between diffusive and ballistic transport is determined. The low out-of-plane thermal conductivity of bulk MoS2 ($2.3\ \text{Wm}^{-1}\text{K}^{-1}$ at 300 K) is useful for thermoelectric applications. On the other hand, the thermal conductivity of monolayer MoS2 ($131\ \text{Wm}^{-1}\text{K}^{-1}$ at 300 K) is comparable to that of Si