Composition-dependent Raman modes of Mo1−xWxS2 monolayer alloys

Abstract

Two-dimensional (2D) transition-metal dichalcogenide alloys with tunable band gaps have promising applications in nanoelectronics and optoelectronics. Characterization of structures of 2D alloys, such as composition and atom mixing, is of fundamental importance to their applications. Here, we have conducted systematic Raman spectroscopic studies on Mo_1−xW_xS₂ monolayers (0 ≤ x ≤ 1). First-order Raman modes and second-order Raman modes have been observed in the range of 100–480 cm⁻¹ in the 2D alloys. The out-of-plane A₁′ modes and in-plane E′ modes showed one-mode and two-mode behaviors, respectively. The broadening of A₁′ and E′ modes in the alloys has been observed. The disorder-related Raman peaks at ∼360 cm⁻¹ were only observed in the 2D alloys but not in the two end materials. Modified random-element-isodisplacement (MREI) model has been adopted to successfully predict mode behaviors of A₁′ and E′ modes in the monolayer alloys. Further, composition-dependent A₁′ and E′ frequencies can be well fitted by the MREI model, giving composition-dependent force constants.