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We investigate the tuning of the electronic properties of pentagonal PdSe2 monolayer by applying external strain. We find that the electronic structure of PdSe2 monolayer can be flexibly modulated by applying the uniaxial as well as biaxial strains. Under the tensile strain (uniaxial or biaxial), the bandgap remains indirect with CBM at M point and VBM at a k-point along the Γ -X path. The bandgap decreases monotonically from 1.34 to 0.83 eV (0.34 eV) under uniaxial (biaxial) tensile strains of 8%. Under compressive strain, the bandgap transforms from indirect to quasi-direct with both CBM and VBM at k-points along the Γ -X path. The change in bandgap under compressive strain is not significant as compared to corresponding value of tensile strains. [ABSTRACT FROM AUTHOR]

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.