Motivated by the recent study of inspiring thermoelectric properties in bulk SnSe [Zhao et al., Nature, 2014, 508, 373] and the experimental synthesis of SnSe sheets [Chen et al., J. Am. Chem. Soc., 2013, 135, 1213], we have carried out systematic calculations for a single-layered SnSe sheet focusing on its stability, electronic structure and thermoelectric properties by using density functional theory combined with Boltzmann transport theory. We have found that the sheet is dynamically and thermally stable with a band gap of 1.28 eV, and the figure of merit (ZT) reaches 3.27 (2.76) along the armchair (zigzag) direction with optimal n-type carrier concentration, which is enhanced nearly 7 times compared to its bulk counterpart at 700 K due to quantum confinement effect. Furthermore, we designed four types of thermoelectric couples by assembling single-layered SnSe sheets with different transport directions and doping types, and found that their efficiencies are all above 13%, which are higher than those of thermoelectric couples made of commercial bulk Bi2Te3 (7%-8%), suggesting the great potential of single-layered SnSe sheets for heat-electricity conversion.

Using semiclassical Boltzmann transport theory and density functional formalism, we have systematically studied the thermoelectric performance of layered GeAs 2 . The figure of merit, ZT value, of this layered structure is found to be 2.78 along the out-of-plane direction, with optimal carrier concentration at 800 K. Analysis of the charge density difference and phonon transport properties allows us to attribute such exceptional thermoelectric properties to strong interlayer interaction between the adjacent layers where quasicovalent bonding is responsible for the enhanced electrical conductivity, while the layered structure accounts for the suppressed lattice thermal conductivity. This study highlights the potential of layered crystals for highly efficient thermoelectric materials.

Motivated by the unique geometry and novel properties of penta-graphene proposed recently as a new carbon allotrope consisting of pure pentagons [Zhang et al. Proc. Natl. Acad. Sci. 2015, 112, 2372, we systematically investigated its phonon transport properties by solving exactly the linearized phonon Boltzmann transport equation combined with first principles calculations. The intrinsic lattice thermal conductivity K lat of penta-graphene is found to be about 645 W/mK at room temperature, which is significantly reduced as compared to that of graphene. The underlying reason is the strong anharmonic effect introduced by the buckled pentagonal structure with hybridized sp 2 and sp 3 bonding. A detailed analysis of the phonons of penta-graphene reveals that the ZA mode is the primary heat carrier (nearly 60%). The K lat is dominated by three-phonon scattering where the scattering rate of the Normal scattering process is comparable to that of the Umklapp scattering process. The phonon mean free path of the collective phonon excitations is in the order of micrometers. Complementing the high thermal conductivity of graphene, the low thermal conductivity of penta-graphene adds additional features to the family of carbon materials for thermal applications.

Penta-graphene has recently been proposed as a new allotrope of carbon composed of pure pentagons, and displays many novel properties going beyond graphene [Zhang et al., Proc. Natl. Acad. Sci. U. S. A., 2015, 112, 2372]. To further explore the property modulations, we have carried out a theoretical investigation of the hydrogenated and fluorinated penta-graphene sheets. Our first-principles calculations reveal that hydrogenation and fluorination can effectively tune the electronic and mechanical properties of penta-graphene: turning the sheet from semiconducting to insulating; changing the Poisson's ratio from negative to positive, and reducing the Young's modulus. Moreover, the band gaps of the hydrogenated and fluorinated penta-graphene sheets are larger than those of fully hydrogenated and fluorinated graphene by 0.37 and 0.04 eV, respectively. The phonon dispersions and ab initio molecular dynamics simulations confirm that the surface modified penta-graphene sheets are dynamically and thermally stable, and show that the hydrogenated penta-graphene has more Raman-active modes with higher frequencies as compared to the fluorinated penta-graphene.