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Achieving high valley degeneracy (i.e. "band convergence") in a material usually results in considerably enhanced thermoelectric properties. However, it is still unclear why this strategy of designing efficient thermoelectric materials is so successful, since the benefit of increased density of states may be severely degraded by intervalley scattering. Using first-principles calculations, we investigate these effects in $p$-type PbTe, where temperature induces alignment of the $L$ and $\Sigma$ valleys at $\sim$~620~K. We explicitly show that the thermoelectric power factor and figure of merit peak near the band convergence temperature. The figure of merit maximum is larger than those of the individual $L$ and $\Sigma$ valleys. Surprisingly, intervalley scattering does not considerably affect the figure of merit near the band convergence temperature and optimal doping conditions, although it reduces the power factor by almost a factor of 2. Our results suggest that band convergence will significantly increase the figure of merit if intervalley scattering is roughly proportional to the density of states and the lattice thermal conductivity is considerably lower than the electronic thermal conductivity, even if intervalley scattering is strong., Comment: 15 pages, 6 figures, Journal publication

We present a first principles based model of electron-phonon scattering mechanisms and thermoelectric transport at the L and $\Sigma$ valleys in <200b>$p$-type PbTe, accounting for their thermally induced shifts. Our calculated values of all thermoelectric transport parameters at room temperature are in very good agreement with experiments for a wide range of doping concentrations. Scattering due to longitudinal optical phonons is the main scattering mechanism in $p$-type PbTe, while scattering due to transverse optical modes is the weakest. The L valleys contribute most to thermoelectric transport at 300 K due to the sizeable energy difference between the L and $\Sigma$ valleys. We show that both scattering between the L and $\Sigma$ valleys and additional transport channels of the $\Sigma$ valleys are beneficial for the overall thermoelectric performance of $p$-type PbTe at 300 K. Our findings thus support the idea that materials with high valley degeneracy may be good thermoelectrics., Comment: 16 pages, 6 figures

Two-dimensional group IV transition-metal dichalcogenides have encouraging thermoelectric applications since their electronic and lattice properties can be manipulated with strain. In this paper, we report the thermoelectric parameters such as electrical conductivity, Seebeck coefficients, electrical relaxation times, and the mode dependent contributions to the lattice thermal conductivity of ZrX$_2$ (X = S, Se, Te) from first principles methods. Our calculations indicate that due to tensile strain, the powerfactor increases while simultaneously decreasing the lattice thermal conductivity thus enhancing the thermoelectric figure of merit. Tensile strain widens the bandgap which corresponds to higher powerfactor. The lattice thermal conductivity decreases due to the stiffening of the out-of-plane phonon modes thus reducing the anharmonic scattering lifetimes and increasing the thermoelectric figure-of-merit., Comment: 10 pages, 8 figures

Using first-principles density functional perturbation theory based calculations of length-dependent lattice thermal conductivity (\k{appa} L ) and using our previously calculated results (Phys Rev B 95 085435 (2017)) of electrical transport, we report results of thermoelectric figure-of-merit (ZT ) of monolayer and bilayer Graphene. We find nearly ten-fold increase in ZT for the graphene sample doped with boron nitride and reduced sample length. We also compare \k{appa} L calculated using the iterative real space method with conventional analytical Callaway-Klemens method and obtain the flexural (ZA) phonon modes to be dominant in thermal transport unlike in the latter method. Our calculations are in good agreement with available experimental data., Comment: 11 pages, 9 figures. Recently accepted in Journal of Applied Physics

The phonon dispersion, density of states, Gr\"{u}neisen parameters, and the lattice thermal conductivity of single- and multi-layered boron nitride were calculated using first-principles methods. For the bulk {\it h}-BN we also report the two-phonon density of states. We also present simple analytical solutions to the acoustic vibrational mode-dependent lattice thermal conductivity. Moreover, computations based on the elaborate Callaway-Klemens and the real space super cell methods are presented to calculate the sample length and temperature dependent lattice thermal conductivity of single- and multi-layered hexagonal boron nitride which shows good agreement with experimental data., Comment: 13 pages, 7 figures

Using combination of Density Functional Theory and Monte Carlo simulation, we study the phase stability and electronic properties of two dimensional hexagonal composites of boron nitride and graphene, with a goal to uncover the role of the interface geometry formed between the two. Our study highlights that preferential creation of extended armchair interfaces may facilitate formation of solid solution of boron nitride and graphene within a certain temperature range. We further find that for band-gap engineering, armchair interfaces or patchy interfaces with mixed geometry are most suitable. Extending the study to nanoribbon geometry shows that reduction of dimensionality makes the tendency to phase segregation of the two phases even stronger. Our thorough study should form an useful database in designing boron nitride-graphene composites with desired properties., Comment: 8 pages, 6 figures

We report the transport properties of monolayer and bilayer graphene from first principles calculations and Boltzmann transport theory (BTE). Our resistivity studies on monolayer graphene show Bloch-Gr${\rm \ddot{u}}$neisen behavior in a certain range of chemical potentials. By substituting boron nitride in place of a carbon dimer of graphene, we predict a twofold increase in the Seebeck coefficient. A similar increase in the Seebeck coefficient for bilayer graphene under the influence of a small electric field $\sim 0.3$ eV has been observed in our calculations. Graphene with impurities shows a systematic decrease of electrical conductivity and mobility. We have also calculated the lattice thermal conductivities of monolayer graphene and bilayer graphene using phonon BTE which show excellent agreement with experimental data available in the temperature range 300-700\ K., Comment: 10 pages, 9 figures

We present first principles study of thermoelectric transport properties of sandwiched heterostructure of Graphene (G)/hexagonal Boron Nitride (BN)/G, based on Boltzmann transport theory for band electrons using the bandstructure calculated from the Density Functional Theory (DFT) based plane-wave method. Calculations were carried out for three, four and five BN layers sandwiched between Graphene layers with three different arrangements to obtain the Seebeck coefficient and Power factor in $T\sim 25-400$K range. Moreover, using Molecular Dynamics (MD) simulations with very large simulation cell we obtained the thermal conductance ($K$) of these heterostructures and obtained finally the Figure-of-Merit ($ZT$). These results are in agreement with recently reported experimental measurements., Comment: 11 pages, 6 figures

We use density functional theory based first-principles method to investigate the bandstructure and phase stability in the laterally grown hexagonal C$_x$(BN)$_{1-x}$, two-dimensional Graphene and $h$-BN hybrid nanomaterials, which were synthesized by experimental groups recently (Liu $et al$, Nature Nanotech, 8, 119 (2013)). Our detail electronic structure calculations on such materials, with both armchair and zigzag interfaces between the Graphene and $ h$-BN domains, indicate that the band-gap decreases non-monotonically with the concentration of Carbon. The calculated bandstructure shows the onset of Dirac cone like features near the band-gap at high Carbon concentration ($x \sim 0.8$). From the calculated energy of formation, the phase stability of C$_x$(BN)$_{1-x}$ was studied using a regular solution model and the system was found to be in the ordered phase below a few thousand Kelvin. Furthermore, using the Boltzmann transport theory we calculate the electrical resistivity from the bandstrcture of C$_x$(BN)$_{1-x}$ at different temperature ($T$), which shows a linear behaviour when plotted in the logarithmic scale against $T^{-1}$, as observed experimentally, Comment: 10 pages, 5 figures