Composition- and Thickness-Dependent Electronic and Phonon Properties of Monolayer to Few-Layer MoxW1-xSe2 Alloys.

MoxW1-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 Ts at 25 K, with the Ts binding energy not depending on the composition. At x = 0.25, anomalous X0 and Ts 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.