Non-equilibrium growth induced symmetry breaking and valley polarization restoration in radiation-warning-symbol patterned bilayer MoS2

Abstract

Atomically layered transition metal dichalcogenides (TMDCs) exhibit exceptional physical properties dictated by their thickness and stacking-order-dependent symmetry. However, the controllable synthesis of multilayer TMDCs with specialized morphologies remains a significant challenge, hindering fundamental studies of their layer-dependent physics. Herein, we report the non-equilibrium growth of bilayer molybdenum disulfide featuring a unique radiation-warning-symbol (RWS) morphology. By introducing an extremely Mo-rich environment during chemical vapor deposition, we trigger a transition from thermodynamic to kinetic growth regimes, promoting secondary nucleation and out-of-plane fractal expansion. Systematic morphological evolution reveals that the RWS pattern originates from anisotropic growth rates at skeletal vertices, where the precursor supply rate is approximately 1.7 times higher than at the edges, followed by a late-stage thermodynamic-filling process. Furthermore, circularly polarized photoluminescence spectroscopy reveals a spatial restoration of valley polarization (P ≈ 32.2%) at the geometric center of the RWS flakes, contrasting with the quenched polarization in the surrounding 2H-stacked bilayer regions. This local symmetry breaking is attributed to the rapid nucleation-induced lattice strain or rotational twist at the core. This work provides a robust strategy for the morphology-controlled synthesis of hierarchical TMDC architectures and offers insights into the interplay between growth dynamics and valleytronic degrees of freedom.