Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, epitaxial graphene/single-crystal 2D gallium, indium, and tin heterostructures. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to graphene overlayer, i.e. they are "half van der Waals" metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals open compelling opportunities for superconducting devices, topological phenomena, and advanced optoelectronic properties. For example, the reported 2D-Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly-freeelectron Fermi surface that closely approaches the Dirac points of the graphene overlayer.Major advances in fundamental science have followed from the exfoliation, stacking, and encapsulation of atomically thin 2D layers 1 . The next step towards technological impact of 2D layers and heterostructures is to transition sophisticated "pick and place" devices to a wafer-scale platform. However, the sensitivity of 2D systems to interfacial reactions and environmental influences -especially for two-dimensional metals or small-gap semiconductors -poses challenges for large-scale integration. Very few metals resist degradation of their top few atomic layers upon environmental exposure, and for a 2D metal, these layers constitute the entire system. A general platform for producing environmentally stable and wafer-scale 2D metals that are not prone to interfacial interactions would represent a significant advance. Inspired by the success of wide-bandgap 2D gallium nitride 2 , we turn focus onto the metal alone and demonstrate a platform dubbed confinement heteroepitaxy (CHet), where the interface between epitaxial graphene (EG) and silicon carbide (SiC) stabilizes crystalline 2D forms of Group-III (Ga, In) and group-IV (Sn) elements. Defect engineering of the graphene overlayer enables uniform, large-area intercalation at the high-energy SiC/EG interface; this interface then templates intercalant crystallization at a thermodynamically defined number of atomic layers. The unreactive nature of as-grown EG on SiC (graphene plus buffer layer) performs multiple services: (1) it only partially passivates the SiC surface underneath, thereby sustaining the high-energy interface that drives intercalation; (2) it lowers the energy of the (otherwise exposed) upper surface of the metal, thus facilitating 2D morphologies; (3) it protects the newly formed 2D metal from environmental degradation after intercalation through in situ healing of the graphene defects. Stability of these 2D metals in air over months gr...

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Engineering atomic-scale defects is crucial for realizing wafer-scale, single-crystalline transition metal dichalcogenide monolayers for electronic devices. However, connecting atomic-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and ReaxFF reactive force field-based molecular dynamics simulations, we provide insights into WS2 crystal growth mechanisms, providing a direct link between synthetic conditions and microstructure. Dark-field TEM imaging of coalesced monolayer WS2 films illuminates defect arrays that atomic-resolution STEM imaging identifies as translational grain boundaries. Electron diffraction and high-resolution imaging reveal that the films have nearly a single orientation with imperfectly stitched domains that tilt out-of-plane when released from the substrate. Imaging and ReaxFF simulations uncover two types of translational mismatch, and we discuss their origin related to relatively fast growth rates. Statistical analysis of >1300 facets demonstrates that microstructural features are constructed from nanometer-scale building blocks, describing the system across sub-Ångstrom to multimicrometer length scales.