Complete Thermally Driven 1T-to-2H Phase Transition in TaSe₂

Phase engineering in two-dimensional materials provides a powerful route to tuning electronic states and enabling functional devices. TaSe₂ is a representative transition metal dichalcogenide with distinct correlated phases associated with its 1T and 2H polytypes. However, experimentally realizing and unambiguously verifying a complete 1T-to-2H transition in TaSe₂ remains a major challenge. Here, we report a complete thermally driven 1T-to-2H transition in TaSe₂. The two phases are distinguished by their electronic and vibrational fingerprints, as revealed by angle-resolved photoemission spectroscopy and Raman spectroscopy. The transition is tracked in situ through the evolution of Raman-active phonon modes, accompanied by pronounced changes in the electronic structure and Ta core-level spectral features. Photoluminescence and electrical transport measurements before and after annealing further confirm the completeness of the phase transition. Our findings deepen the understanding of structural phase transitions in TaSe₂ and support phase engineering in other two-dimensional materials.