Composition- and Thickness-Dependent Electronic and Phonon Properties of Monolayer to Few-Layer Mo_xW_1-xSe₂ Alloys

Mo_xW_1-xSe₂ alloys exhibit composition-tunable properties critical for optoelectronics. This study investigates their composition-dependent excitonic behavior using photoluminescence (PL) spectroscopy at 300 and 25 K and their phonon properties across stoichiometries and layer numbers from 1 to 10 using Raman spectroscopy. PL spectroscopy reveals nonlinear bowing of 0.106 and 0.149 eV for A and B excitons at 300 K, and of 0.097 and 0.100 eV for neutral excitons X⁰ and trions T^s at 25 K, with the T^s binding energy not depending on the composition. At x = 0.25, anomalous X⁰ and T^s blueshift and complex PL structure containing narrow peaks (down to 2.3 meV) are detected, indicating emergence of 0D confinement. 26 MoSe₂-like and 25 WSe₂-like identified Raman modes show composition- and layer-dependent frequency shifts. A modified random element isodisplacement model is used to quantify force constant variations for 1-7 layers, highlighting roles of Mo-W mass disparity and interlayer interactions. A critical transition from 2D to 3D behavior occurs at 4-5 layers, marked by nonlinear force constant variations, B¹ _2g mode inflections, and saturation of phonon frequencies for N > 5. These findings provide fundamental insights for designing TMD-based devices with tailored optoelectronic and thermal properties.