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Abstract The monoclinic 2M phase of tungsten disulfide (2M-WS₂) has emerged as a promising platform for topological superconductivity, combining topological surface states, anisotropic thermoelectric response, and superconductivity with a critical temperature of ∼8.8K. A detailed understanding of its electron–phonon coupling and phonon anharmonicity is, however, still lacking. Here, we investigate the phonon properties of few-layer 2M-WS₂as functions of laser power, temperature, polarization, and magnetic field using polarized Raman spectroscopy down to 12.5 K. We uncover an unusual, non-linear temperature dependence of the Raman shifts in selected modes, consistent with strong, mode-dependent electron–phonon interactions that dominate over phonon–phonon scattering at low temperatures. Angle-resolved polarized Raman measurements further reveal a pronounced change in the polarization behavior of the $A_g$ (4) mode at ∼316cm⁻¹upon cooling, indicating a modification of the underlying lattice and electronic anisotropy. In contrast, the applied magnetic field has a negligible influence on the Raman response over the measured temperature range. Together, these results advance the microscopic understanding of vibrational dynamics and electron–phonon coupling in few-layer 2M-WS₂and provide important insight into the lattice-electronic interplay relevant for its unconventional superconductivity and potential quantum and thermoelectric device applications. 

Recent research has demonstrated the potential for topological superconductivity, anisotropic Majorana bound states, optical nonlinearity, and enhanced electrochemical activity for transition metal dichalcogenides (TMDs) with a 2M structure. These unique TMD compounds exhibit metastability and, upon heating, undergo a transition to the thermodynamically stable 2H phase. The 2M phase is commonly made at high temperatures using traditional solid-state methods, and this metastability further complicates the growth of large 2M WS2 crystals. Herein, a novel synthetic method was developed, focusing on a molten salt reaction to synthesize large 2H crystals and then inducing transformation to the 2M phase through intercalation and thermal treatment. The 2H crystals were intercalated via a room-temperature sodium naphthalenide solution, producing a previously unreported Na-intercalated 2H WS2 phase. Thermal heating was required to facilitate the phase transition to the intercalated 2M crystal structure. This phase transition was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), electron dispersive X-ray spectroscopy (EDS), and Raman spectroscopy, which confirmed the synthesis of the intercalated 2M phase. Upon deintercalation, crystal and powder samples showed superconductivity with a Tc of 8.6–8.7 K, similar to previously reported values. The generality of this process was further demonstrated using alkali metal triethyl borohydride to intercalate 2H WS2 and produced the desired 2M phase. This novel synthetic method has broad implications for discovering metastable phases in other TMD families and layered materials. Separation of the intercalation and phase transition also has the potential to allow for large-scale synthesis of this technologically important phase with greater control over each step of the reaction.