Strain-induced enhancement in the electronic and thermal transport properties of the tin sulphide bilayer.

The enhancement in the thermoelectric figure of merit (ZT) of a material is limited by the interplay between the electronic transport coefficients. Here we report the greatly enhanced thermoelectric performance of the SnS bilayer with the application of isotropic strain, due to the simultaneous increase in the Seebeck coefficient and low lattice thermal conductivities. Based on first-principles calculations combined with Boltzmann transport theory, we predict that the band structure of the SnS bilayer can be effectively tuned using the strain, and the Seebeck coefficient is significantly improved for the tensile strain. The lattice thermal conductivities for the bilayer under the tensile strain are quite low (0.21-1.89 W m-1 K-1 at 300 K) due to the smaller frequencies of the acoustic phonon modes. Along the zigzag (armchair) direction, the room temperature peak ZT value of 4.96 (2.40) is obtained at a strain of 2% (4%), which is 5.3 (2.03) times higher than the peak ZT of the unstrained bilayer along the zigzag (armchair) direction. Thus the strain-tuned SnS bilayer is a good thermoelectric material with low lattice thermal conductivities and promising ZT values at room temperature.

Exceptionally high open circuit thermoelectric figure of merit in two-dimensional tin sulphide.

Thermoelectric materials with high values of power factor and thermoelectric figure of merit (ZT) are in great demand to make efficient thermoelectric devices. In this work, we explore the thermoelectric transport properties of layered tin sulphide (SnS) using first-principles method combined with Boltzmann transport theory. Our calculations show that the two-dimensional (2D) SnS materials have exceptionally high charge carrier mobilities and low lattice thermal conductivities as compared to other 2D materials such as graphene, phosphorene, MoS2, etc. Consequently, these 2D SnS materials have high power factor andZTvalues.

Nuclear quantum effects induce metallization of dense solid molecular hydrogen.

We present an accurate computational study of the electronic structure and lattice dynamics of solid molecular hydrogen at high pressure. The band-gap energies of the C2/c, Pc, and P63/m structures at pressures of 250, 300, and 350 GPa are calculated using the diffusion quantum Monte Carlo (DMC) method. The atomic configurations are obtained from ab initio path-integral molecular dynamics (PIMD) simulations at 300 K and 300 GPa to investigate the impact of zero-point energy and temperature-induced motion of the protons including anharmonic effects. We find that finite temperature and nuclear quantum effects reduce the band-gaps substantially, leading to metallization of the C2/c and Pc phases via band overlap; the effect on the band-gap of the P63/m structure is less pronounced. Our combined DMC-PIMD simulations predict that there are no excitonic or quasiparticle energy gaps for the C2/c and Pc phases at 300 GPa and 300 K. Our results also indicate a strong correlation between the band-gap energy and vibron modes. This strong coupling induces a band-gap reduction of more than 2.46 eV in high-pressure solid molecular hydrogen. Comparing our DMC-PIMD with experimental results available, we conclude that none of the structures proposed is a good candidate for phases III and IV of solid hydrogen. © 2017 Wiley Periodicals, Inc.

Spin-polarized density functional investigation into ferromagnetism in C-doped (ZnO)n clusters; n = 1-12, 16.

We investigate the ferromagnetism in ZnO clusters due to vacancy defects and C impurities doped at substitutional O or Zn sites, and interstitial sites. The total energy calculations suggest C at the O site is more stable than that at the Zn site in ZnO clusters. The total magnetic moments of Zn(n)O(n-m)C(m) clusters are 2.0 µ(B)/C. However, when two C atoms are bonded to the same Zn atom they interact antiferromagnetically and the total magnetic moment becomes less than 2.0 µ(B)/C. The interstitial C defects in ZnO clusters induce small magnetic moments. The combination of substitutional and interstitial C defects in ZnO clusters leads to magnetic moments of 0.0-2.0 µ(B)/C. The presence of vacancy defects in addition to substitutional C defects gives magnetic moments of greater than 2.0 µ(B)/C. These results suggest that the experimentally observed sample dependence of magnetic moments in ZnO systems is largely due to the different concentrations of substitutional and interstitial C impurities and the presence of vacancy defects in ZnO samples prepared under different growth conditions.

Lower bound for the excitonic fine structure splitting in self-assembled quantum dots.

The excitonic fine structure splitting describes the splitting of the bright excitons as a consequence of the atomistic symmetry of the lattice and the electron-hole exchange interaction. Efforts are underway to eliminate this natural splitting by external constraints in order to use quantum dots in quantum optics. We show by million atom empirical pseudopotential calculations that for realistic structures a lower bound for this splitting exists. We underpin our numerical calculations by an insightful symmetry analysis.

Nanowire quantum dots as an ideal source of entangled photon pairs.

We predict that heterostructure quantum wires and [111] grown quantum dots have a vanishing fine-structure splitting on the grounds of their symmetry, and are therefore ideal candidates to generate entangled photon pairs. We underpin this proposal by atomistic million-atom many-body pseudopotential calculations of realistic structures and find that the vanishing fine-structure splitting is robust against possible variations in morphology.

Magnetism in graphene due to single-atom defects: dependence on the concentration and packing geometry of defects.

The magnetism in graphene due to single-atom defects is examined by using spin-polarized density functional theory. The magnetic moment per defect due to substitutional atoms and vacancy defects is dependent on the density of defects, while that due to adatom defects is independent of the density of defects. It reduces to zero with decrease in the density of substitutional atoms. However, it increases with decrease in density of vacancies. The graphene sheet with B adatoms is nonmagnetic, but with C and N adatoms it is magnetic. The adatom defects distort the graphene sheet near the defect perpendicular to the sheet. The distortion in graphene due to C and N adatoms is significant, while the distortion due to B adatoms is very small. The vacancy and substitutional atom (B, N) defects in graphene are planar in the sense that there is in-plane displacement of C atoms near the vacancy and substitutional defects. Upon relaxation the displacement of C atoms and the formation of pentagons near the vacancy site due to Jahn-Teller distortion depends upon the density and packing geometry of vacancies.