Ultrafast carrier dynamics of CVD- grown WS2: role of sodium induced defects

Two-dimensional tungsten disulfide (WS2) is one the most studied materials in transition metal dichalcogenides owing to its excellent electronic and optical properties. Understanding of ultrafast carrier dynamics of 2D WS2 is essential for its applications in various optoelectronic devices. In this work, we report a study of influence of sodium induced defects on the ultrafast carrier dynamics of layered CVD-grown WS2 by using time-resolved Kerr rotation measurements at room temperature. This ultrafast pump-probe spectroscopic technique enables the study of a wide range of decay dynamics from sub-picosecond to several nanoseconds and therefore allows one to access both the ultrafast valley dynamics and influence of induced defects. Different bi-exponential decay dynamics were observed for various levels of induced sodium. The fast decay component was related to the valley exchange interaction while the slow decay components were associated to several origins including the defect-related localized states and hole valley relaxation. Our study provides valuable feedback on the doping mechanism of layered CVD-grown WS2 in particular, and 2D transition metal dichalcogenides in general.