Non‐Vacuum Phase‐Selective Growth of Na(Lu ₁₋ <i> _x </i> Er <i> _x </i> )S ₂ Phosphor for High‐Contrast Infrared Tracing and Ultrasensitive Thermal Sensing

ABSTRACT Lanthanide‐doped sulfides materials are synthesized with persistent limitations deriving from costly inert atmospheres or intricate vacuum operations. Herein, a non‐vacuum oversulfurization strategy is proposed to achieve phase‐selective growth and effective defect suppression in NaLuS 2 through thermodynamic regulation of the sulfur chemical potential. The sulfur‐rich environment stabilizes S–Lu bond, suppresses deep‐level defects associated with sulfur vacancies, and produces chemically and thermally stable Na(Lu 1− x Er x )S 2 crystals featuring ultra‐low phonon energy and highly asymmetric crystal fields. Consequently, parity‐forbidden 4f–4f transitions of Er 3+ ions are significantly enhanced with upconversion luminescence increasing up to 2.8‐fold under 1530 nm excitation, exhibiting a pronounced non‐linear growth trend with elevating pump powers. Furthermore, the functionalization and integration of NaLuS 2 microcrystals into polyacrylonitrile‐thermoplastic polyurethane (PAN‐TPU) nanofibers through electrospinning achieves high thermal sensitivity of 0.93% K −1 at 313 K whilst maintaining high luminescence intensity, thereby providing an ideal material platform for flexible sensor devices. This work reports a novel, cost‐effective and atmosphere‐tolerant synthetic paradigm for ternary rare‐earth sulfides, paving the way for high‐contrast infrared tracing and ultrasensitive thermal sensing.