Composition‐ and Thickness‐Dependent Electronic and Phonon Properties of Monolayer to Few‐Layer Mo _x W _1−x Se ₂ Alloys

ABSTRACT Mo _x W _1−x Se ₂ 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 X ⁰ and trions T ^s at 25 K, with the T ^s binding energy not depending on the composition. At x = 0.25, anomalous X ⁰ and T ^s blueshift and complex PL structure containing narrow peaks (down to 2.3 meV) are detected, indicating emergence of 0D confinement. 26 MoSe ₂ ‐like and 25 WSe ₂ ‐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, B ¹ _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.