Tunable interlayer coupling in MXY (M = Mo, W; (X/Y) = S, Se, Te) tri-layered van der Waals heterostructures

Using first principles calculations, we investigated the electronic band structure and optical spectra of Janus MXY (M = Mo, W; X/Y = S, Se, Te) tri-layered van der Waals heterostructures (vdWHs). The binding energies, phonon spectra and mechanical properties were calculated and AIMD simulations performed, which confirm the stabilities of these tri-layered vdWHs. The electronic band structures, calculated using the PBE and HSE06 functionals, indicate that MoSSe-WSSe-MoSSe, WSSe-MoSSe-WSSe, MoSeTe-WSeTe-MoSeTe and WSeTe-MoSeTe-WSeTe vdWHs are Γ-K indirect bandgap semiconductors, arising not only from interlayer hybridization but also from intrinsic Janus dipole-induced electric fields. The carrier effective masses were determined using parabolic fitting of the conduction band minima (CBM), confirming a small effective mass relative to higher carrier mobility, useful for high-speed nanoelectronic device applications. The qualitative and quantitative behaviors for these tri-layered vdWHs are investigated by using charge density difference and Bader charge analysis. A heavier chalcogen atom (Mo-W) in the middle of these layers blue shifts the position of the exciton, while a red shift is observed with compressive strain. A strong absorption from the infrared to visible to UV light range is observed for these tri-layered vdWHs.