Electronic properties of WS2 and WSe2 monolayers with biaxial strain: A first-principles study

In the present work, we consider electronic properties of WX2 (X = S, Se) monolayers under a biaxial strain εb using the first principles study. Our calculations indicate that, at equilibrium, the WS2 and WSe2 monolayers are semiconductors with a direct band gap of respectively 1.800 eV and 1.566 eV while their bulk structures are indirect semiconductors. The electronic properties of the WX2 monolayers are very sensitive with the biaxial strain, especially compression strain. The biaxial strain εb is the cause of the band gap of the WX2 monolayers and especially the semiconductor-metal phase transition has occurred in the WS2 monolayer at εb=-10%. In addition, the direct-indirect band gap transition was observed in both WS2 and WSe2 monolayers at a certain elongation of biaxial strain εb. The phase transitions in these monolayers can be very useful for their applications in nanoelectromechanical devices.