Copper halide diselenium: predicted two-dimensional materials with ultrahigh anisotropic carrier mobilities

On the basis of first-principles calculations, we discuss a new class of two-dimensional materials-CuXSe₂ (X = Cl, Br) nanocomposite monolayers and bilayers-whose bulk parent was experimentally reported in 1969. We show the monolayers are dynamically, mechanically and thermodynamically stable and have very small cleavage energies of ∼0.26 J m^-2, suggesting their exfoliation is experimentally feasible. The monolayers are indirect-gap semiconductors with practically the same moderate band gaps of 1.74 eV and possess extremely anisotropic and very high carrier mobilities (e.g., their electron mobilities are 21 263.45 and 10 274.83 cm² V^-1 s^-1 along the Y direction for CuClSe₂ and CuBrSe₂, respectively, while hole mobilities reach 2054.21 and 892.61 cm² V^-1 s^-1 along the X direction). CuXSe₂ bilayers are also indirect band gap semiconductors with slightly smaller band gaps of 1.54 and 1.59 eV, suggesting weak interlayer quantum confinement effects. Moreover, the monolayers exhibit high absorption coefficients (>10⁵ cm^-1) over a wide range of the visible light spectra. Their moderate band gaps, very high unidirectional electron and hole mobilities, and pronounced absorption coefficients indicate the proposed CuXSe₂ (X = Cl, Br) nanocomposite monolayers hold significant promise for application in optoelectronic devices.