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 ${\mathrm{Mo}}_{x}{\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Se}}_{2}$ and ${\mathrm{W}}_{x}{\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Se}}_{2}$ 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 ${\mathrm{CrSe}}_{2}$ and Mo(W)${\mathrm{Se}}_{2}$, 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 $1.178\phantom{\rule{0.16em}{0ex}}\mathrm{e}\mathrm{V}$, 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)${}_{x}{\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Se}}_{2}$ monolayers with particular compositions $x$ can form type II heterojunctions with a high solar harvesting efficiency.