Residue-Driven Degradation and Its Stabilization in NaCl-Assisted WS₂ with 3R Stacking Enrichment

Salt-assisted chemical vapor deposition (CVD) enables large-grain synthesis of 2D transition-metal dichalcogenides, yet residual salts at the growth interface remain a critical bottleneck for ambient stability. Here, we investigate NaCl-assisted metal-organic CVD growth of WS₂ and reveal a coupled physical-chemical degradation mechanism driven by Na- and W-containing amorphous residues. Upon air exposure, these residues crystallize and mechanically disrupt the WS₂ layer while fostering a hygroscopic, Na-rich environment that drives oxidative breakdown of W-S bonds. Kinetic analysis establishes a thermally activated degradation process with an apparent activation energy of ∼0.22 eV, consistent with moisture-driven residue crystallization. We demonstrate that post-growth annealing (PGA) eliminates these residues and preserves the morphological integrity and optical emission characteristics over month-scale ambient storage. Beyond residue elimination, PGA also drives a pronounced 3R stacking enrichment in multilayer overgrowth regions. First-principles calculations reveal that interlayer Na⁺ thermodynamically favors the 3R registry, so that 3R is selected during the early, Na-rich stage of annealing and kinetically locked once Na⁺ is removed. These results establish residue management as a central design principle in salt-assisted 2D material synthesis.