Anisotropic Ordering in 1T′ Molybdenum and Tungsten Ditelluride Layers Alloyed with Sulfur and Selenium

Alloyingis an effective way to engineer the band-gap structure of two-dimensional transition-metal dichalcogenide materials. Molybdenum and tungsten ditelluride alloyedwith sulfur or selenium layers (MX2xTe2(1–x), M = Mo, W and X = S, Se) have a large band-gap tunability from metallic to semiconducting due to the 2H-to-1T′ phase transition as controlled by the alloyconcentrations, whereas the alloyatom distribution in these two phases remains elusive. Here, combining atomic resolution Z-contrast scanning transmission electron microscopyimaging and density functional theory (DFT), we discovered that anisotropic ordering occurs in the 1T′ phase, in sharp contrast to the isotropic alloybehavior in the 2H phase under similar alloyconcentration. The anisotropic ordering is presumably due to the anisotropic bonding in the 1T′ phase, as further elaborated by DFT calculations. Our results reveal the atomic anisotropic alloyedbehavior in 1T′ phase layered alloysregardless of their alloyconcentration, shi...