Although new spintronic devices based on the giant spin-orbit splitting of single-layer ${\mathrm{MoS}}_2$ have been proposed, such splitting has not been studied effectively in experiments. This Letter reports the valence band spin-orbit splitting in single-layer ${\mathrm{MoS}}_2$ for the first time, probed by the triply resonant Raman scattering process. We found that upon 325 nm laser irradiation, the second order overtone and combination Raman modes of single-layer ${\mathrm{MoS}}_2$ are dramatically enhanced. Such resonant Raman enhancement arises from the electron-two-phonon triple resonance via the deformation potential and Fröhlich interaction. As a sensitive and precise probe for the spin-orbit splitting, the triply resonant Raman scattering will provide a new and independent route to study the spin characteristics of ${\mathrm{MoS}}_2$.

Due to the easy intralayer gliding and weak interlayer van der Waals interaction in transition metal dichalcogenides (TMDs), ion (particularly Li⁺) intercalation has been used to modify and tune their atomic structures to obtain the desired optical, electronic and chemical properties for future optoelectronics and energy storage applications. A good understanding of the transformative structures during intercalation is critical. In this paper, we investigate the structural transformation dynamics of 2H-MoS₂ using electrochemical Li⁺ intercalation for 2H-MoS₂. The introduction of Li⁺ changes the local symmetry of the MoS₂ in favor of the dT phase, clearly indicated by the appearance of Raman peaks of the dT phase. Further Li⁺ insertion causes the samples to become single-layer-like, characterized by the disappearance of the 32 cm⁻¹ Raman peak. We also observe for the first time that the photoluminescence (PL) emission gradually redshifts with decreasing intensity, followed by eventual vanishing of the PL peak in the dT-MoS₂ phase. By the nudged elastic band (NEB) calculations, we propose the 2H–1T–dT phase transition mechanism of MoS₂ for Li⁺ intercalated samples. Our claims are supported by high resolution-transmission electron microscopy (HR-TEM). Our study deepens the understanding of the phase transition dynamics upon lithium intercalation, which is of great value to possible optoelectronic devices based on the phase engineering of TMDs. The new Li-stabilized dT-MoS₂ phase does not possess inversion symmetry and may present a feasible way to achieve Weyl state tuning in a single material via phase engineering.