First-principles investigation of the role of Cr in the electronic properties of the two-dimensional MoxCr1-xSe2 and WxCr1-xSe2 alloys

The tuning of the structural and electronic properties of two-dimensional semiconductor monolayers is highly desirable for designing van der Waals heterostructures, which can be employed for several optoelectronic applications. Here, we report a theoretical investigation based on the combination of spin-polarized density functional theory calculations and alloying structures generated by the special quasirandom structure method to investigate the energetic stability and band gap engineering of the compounds MoxCr1-xSe2 and WxCr1-xSe2 as a function of the Cr composition for x = 0 up to 1. We found that even a small concentration of Cr already flattens the low-energy electronic bands and decreases the fundamental electronic band gap. Due the lattice mismatch of the compounds CrSe2 and Mo(W)Se-2, the renormalization of the electronic properties is nonlinear as a function of the Cr composition. We found bowing parameters for the work function and band gap that change in magnitude from 0.066 to 1.178 eV, respectively. From our analyses, Cr alloying decreases the band gap of these monolayers in the direction of the maximum performance band gap predicted by the Shockley-Queisser limit for photovoltaic applications. Band alignment analysis reveals that stacks of Mo(W)(x)Cr1-xSe2 monolayers with particular compositions x can form type II heterojunctions with a high solar harvesting efficiency.