Phase-Locked Growth of Superconducting Ultrathin Monoclinic WS2 Single Crystals via Chemical Vapor Deposition.

Two-dimensional (2D) WS2 in the unconventional monoclinic (2M) 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 WS2 single crystals via a one-step chemical vapor deposition (CVD). By establishing the growth phase diagram using K2WS4 as precursor, we selectively synthesized 2D 2M WS2 and, through theoretical calculations, elucidated the mechanism driving the stabilization of the 2M phase over the competing Td 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 WS2 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 WS2 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 WS2, paving the way for exploring atomic-scale quantum phenomena in this unconventional phase.