Phase‐Locked Growth of Superconducting Ultrathin Monoclinic WS 2 Single Crystals via Chemical Vapor Deposition

ABSTRACT Two‐dimensional (2D) WS 2 in the unconventional monoclinic (2 M ) phase displays a remarkable array of quantum phenomena discovered in exfoliated samples. However, the high formation energy of this metastable phase hinders the direct synthesis. Here, for the first time, we realized the phase‐locked growth of 2M WS 2 single crystals via a one‐step chemical vapor deposition (CVD). By establishing the growth phase diagram using K 2 WS 4 as precursor, we selectively synthesized 2D 2M WS 2 and, through theoretical calculations, elucidated the mechanism driving the stabilization of the 2M phase over the competing T d phase. Comprehensive characterization using Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) revealed high‐phase‐purity, distinct in‐plane anisotropy, and superior dynamics stability of the synthesized 2D crystals. Notably, the resulting ultrathin 2M WS 2 exhibits thickness‐dependent 2D superconductivity with onset critical temperatures up to ∼8.6 K. Remarkably, magnetotransport measurements reveal that the in‐plane upper critical field of 2M WS 2 far exceeds the Pauli limit. This indicates the presence of strong spin–orbit‐parity coupled superconducting mechanisms and demonstrates crystalline quality comparable to that of mechanically exfoliated samples. This work establishes a scalable bottom‐up synthesis for 2D metastable WS 2 , paving the way for exploring atomic‐scale quantum phenomena in this unconventional phase.