Band offsets engineering in asymmetric Janus bilayer transition-metal dichalcogenides.

Using the first-principles calculation, we systematically studied the electronic properties of the bilayer transition metal dichalcogenides (TMDs) MX2 (M  =  Mo, W; X  =  S, Se, Te) with replacing one, two, three or four layers of X atoms as Y atoms (X  ≠  Y  =  S, Se, Te). By comparison, it is found that when the inner two layers of chalcogenide atoms are different, the system has both valence band offset (VBO) and conduction band offset (CBO). Among them, values of the band offsets reach maxima when the inner one layer of X atoms is replaced by Y atoms, namely forming the asymmetric Janus bilayer XMX/YMX. We take SMoS/SeMoS as an example to analyze the formation of the band offsets and the improvement of optoelectronic properties. Importantly, it is also found that both external electric field and biaxial strain can regulate electronic structures of asymmetric Janus bilayer TMDs with noticeable modulation of the values of band offsets. When the external electric field changes from negative to positive continually, CBO decreases and VBO increases. While when the biaxial strain changes from compression to stretch continually, CBO increases and VBO decreases. These findings enrich the study of bilayer TMDs that can be used as optoelectronic, nanoelectronic and valleytronic devices.