Thermal stability and anisotropic thermal expansion of WS₂ annealed in different atmospheres

Two-dimensional transition metal dichalcogenides (TMDCs) are promising semiconductors for next-generation nanoelectronic and optoelectronic devices due to their thickness-dependent band structure and direct band gap in the monolayer limit. However, their integration into CMOS-compatible platforms requires exposure to high-temperature processing under various ambient conditions, raising concerns about their thermal stability and structural robustness. Here, we systematically investigate the effects of high-temperature annealing on the structural and optical properties of monolayer WS₂. Temperature-dependent photoluminescence and micro-Raman spectroscopy reveal a strong dependence of lattice dynamics, thermal expansion, and degradation pathways on the annealing atmosphere. Samples annealed in air exhibit triangular etch pit formation at approximately 650 K, accompanied by pronounced anisotropic thermal expansion. In contrast, WS₂ annealed under argon shows nearly isotropic thermal expansion and remains structurally stable up to ∼950 K. We demonstrate that degradation initiates at sulphur-deficient regions, such as flake edges and intrinsic sulphur vacancies, which act as nucleation centres for oxidative decomposition. The triangular pits align along metal-terminated zigzag directions, indicating anisotropic bond dissociation governed by defect chemistry and edge energetics. These results provide critical insight into atmosphere-dependent thermal degradation mechanisms and establish guidelines for the reliable integration of TMDCs into high-temperature semiconductor processing.