Evolution of photoluminescence and Raman spectra in laterally gradient composition Mo1− <i>x</i> W <i>x</i> S2 alloys

Alloying two-dimensional semiconductors holds great promise to tailor electronic and optical properties for device applications. In this study, we report the synthesis of monolayer Mo1-xWxS2 alloys with a laterally gradient composition using an improved one-step chemical vapor deposition method. With increasing W composition, the intensity of exciton peak A exhibits a slow increase followed by a near-exponential enhancement, resulting in a significantly enhanced photoluminescence efficiency and a nonlinear blueshift of the peak. First-principles calculations reveal that this nonlinear blueshift is due to the predominant regulation of the conduction band by the Mo atom over a broad composition range. Furthermore, the asymmetry evolution within the conduction band can be attributed to the Mo-dz2 and W-dz2 orbital hybridizations in Mo1-xWxS2 alloys. At high W composition, the Raman spectra exhibit four peaks. The newly emerged A1g-mixed peak originates from the splitting of A1g peak in monolayer MoS2, corresponding to other out-of-plane vibrational modes of S atoms induced by W atoms. Furthermore, the evolution of Raman spectra is driven by lattice distortions and alterations in atomic vibrational modes. This work unravels the mechanism of composition-gradient-tuned electronic and optical properties of 2D Mo1-xWxS2 alloys, which could facilitate the development of next-generation electronic and optoelectronic devices.