1T-to-2H Phase Evolution During Epitaxial Growth of Transition Metal Dichalcogenides.

Polymorphism in transition-metal dichalcogenides is typically defined based on the post-growth phase (1T or 2H) or stabilized heterophase structures. Transient 1T-to-2H phase evolution is demonstrated during epitaxial growth of MoS2 and WSe2 on sapphire by metalorganic chemical vapor deposition (MOCVD). In both systems, x-ray photoelectron spectroscopy shows that metallic 1T character is prominent during early stages of growth when the film is comprised of small domains and declines as domains enlarge and coalesce and the films approach stoichiometric composition. At low coverage, the XPS-derived ex situ 1T fractions reach ≈40% of detected Mo and ≈48% of detected W and approach zero upon coalescence in both systems. Local structural observations and complementary optoelectronic evidence further corroborate the WSe2 assignment. MoS2-specific ReaxFF molecular-dynamics simulations indicate that small domain size and chalcogen-deficient edge environments reduce the energetic penalty for 1T-like formation within the model, while chalcogen vacancies facilitate local rearrangement. For WSe2, a multivariable growth regime with a smaller early-stage 1T fraction also produces a substantially narrower final ensemble Φ-scan width, motivating a possible path-dependent influence on epitaxial development.