Full potential linearized augmented plane wave (FP-LAPW) method is applied to study the electronic and optical properties of MoSe_2 monolayer under uniaxial tensile strain. The bandgap of MoSe_2 monolayer increases for the application of 1% uniaxial tensile strain and then reduces with increasing the uniaxial tensile strain. The direct bandgap nature of MoSe_2 monolayer remain unchanged for 1% and 3% tensile strain. An applied tensile strain of ε=5%, causes a direct to indirect bandgap transition. Analyzing partial density of states (PDOS), the transition of bandgap from direct to indirect is due to the changes of orbital contribution from Mo-d_(x~2-y~2) & d_(z~2) to Mo-d_(z~2) states in the conduction band minima (CBM) and Mo-d_(x~2-y~2) and Se-p_X & p_Y to Mo-d_(z~2) and Se-p_Z states in the valence band maxima (VBM). The enhancement of ε_1(ω), ε_2(ω) and a(ω) values are observed with uniaxial tensile strain in the visible region. The ε_1(0) value increases with increasing tensile strain. The possibility of achieving tunable electronic and optical properties of MoSe_2 monolayer with the implementation of uniaxial tensile strain, makes them a potential candidate for optoelectronics.
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