Composition-dependent Raman modes of Mo(1-x)W(x)S2 monolayer alloys. 
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 Mo1-xWxS2 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.