Unlocking Room‐Temperature Thermoelectric and Flexibility in Freestanding Single‐Crystalline SnSe Membrane through Metastable‐Phase Engineering

Abstract

Despite SnSe's high thermoelectric performance, its application in flexible devices is limited by a negligible room‐temperature power factor (PF) and poor flexibility from inherent rigidity. Here, first‐principles calculations of Gibbs free energy and phonon spectra for SnSe phases revealed that metastable Fm3m‐SnSe exhibits relative stability. Freestanding single‐crystalline metastable Fm3m‐SnSe membranes are then fabricated. These membranes exhibit a remarkable room‐temperature PF of 2.39 µW cm⁻¹ K⁻², nearly two orders of magnitude higher than that of stable Pnma‐phase SnSe films (0.03 µW cm⁻¹ K⁻²). Notably, the freestanding Fm3m‐SnSe membranes demonstrate exceptional mechanical robustness: they spontaneously curl into tubular architectures and retain over 80% of their electrical conductivity after 200 bending cycles (bending radius: 12 mm). Microstructural analyses confirm polycrystalline Pnma‐SnSe films suffer from grain boundary cracking under bending due to stress concentration, but single‐crystal Fm3m‐SnSe membranes—without grain boundaries to concentrate stress—exhibit better flexibility and resistance to cracking. This work establishes freestanding single‐crystalline metastable Fm3m‐SnSe as a highly efficient and flexible thermoelectric material and provides a fundamental strategy for designing room‐temperature SnSe‐based self‐powered flexible electronics.