Thermal Phase‐Modulation of Thickness‐Dependent CVD‐Grown 2D In ₂ Se ₃

Abstract 2D indium selenide (In 2 Se 3 ) exhibits a rich landscape of polymorphism and metastable phases that are highly responsive to external stimuli, making it an attractive platform for next‐generation phase‐engineered applications. Here, a comprehensive study of the thickness‐dependent and thermally modulated phase behavior of chemical‐vapor‐deposition (CVD)‐grown 2D In 2 Se 3 is presented. The findings reveal a coexistence of α and β ″ phases governed by layer thickness: the α ‐phase stabilizes in ultrathin flakes, while thicker flakes (≥four layers) preferentially adopt the β ″‐phase at room temperature. This work reveals previously unexplored thermally driven phase transformation behavior of the β ″ variant, characterized by a unique three superlattice‐spot electron diffraction pattern. Temperature‐dependent electron diffraction and in situ Raman spectroscopy demonstrate that, between 210–220 K, the β ″‐phase undergoes a reversible transition to a low‐temperature β* ‐phase. The β ″↔ β* phase transition, exclusive to the β ″‐phase, and the thermal robustness of the α ‐phase in ultrathin geometries (evidenced by its structural invariance across the investigated temperature range) underscore the potential for controlled phase engineering in 2D systems. This study illuminates the polymorphic landscape of In 2 Se 3 , delineating phase stability regimes and advancing understanding of the intricate interplay between 2D material thickness, temperature, phase selection, and transition dynamics in low‐dimensional In 2 Se 3 .