Interface-Directed Two-Dimensional Growth of Non-Layered NiO on MoS₂ Under Coupled External Fields

The controlled growth of two-dimensional (2D) non-layered transition metal oxides remains a key challenge in low-dimensional material synthesis. Achieving atomically precise construction of 2D structures requires a fundamental understanding of how external fields and interfacial interactions govern nucleation and growth processes. In this work, amorphous NiO_x (a-NiO_x) was deposited onto monolayer MoS₂, and its transformation into crystalline 2D NiO nanosheets was directly visualized in situ using environmental transmission electron microscopy. The electron beam induces the decomposition of a-NiO_x into mobile NiO_x species, which migrate and nucleate on MoS₂, demonstrating the synergistic roles of the electron beam and thermal field in NiO cluster formation and interface-dominated epitaxial growth. Combined notch-filtered imaging and theoretical calculations reveal that Ni-O units preferentially attach at the NiO/MoS₂ interface to form one-dimensional growth fronts, while oriented attachment proceeds via nanosheet rotation, interfacial fusion, and defect-mediated reconstruction, culminating in lattice relaxation into a single-crystalline structure. High-throughput selected-area electron diffraction further uncovers the modulation of external fields on the multi-path structural evolution of NiO_x. This work establishes an in situ strategy to resolve nucleation and growth pathways of 2D heterostructures, enabling active control over the 2D growth of non-layered metal oxides and advancing deterministic atomic-level construction.