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-CuXSe 2 (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 cm 2 V -1 s -1 along the Y direction for CuClSe 2 and CuBrSe 2 , respectively, while hole mobilities reach 2054.21 and 892.61 cm 2 V -1 s -1 along the X direction). CuXSe 2 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 (>10 5 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 CuXSe 2 (X = Cl, Br) nanocomposite monolayers hold significant promise for application in optoelectronic devices.