heterostructures by van der Waals epitaxy*

Recently, two-dimensional (2D) van der Waals (vdW) heterostructures enable the exploration of new physics and applications. Many reported 2D heterostructures are formed by exfoliation and restacking of 2D flakes of small sizes and are restricted to vdW materials. In this work, we report epitaxial growth of covalent 2D magnet Cr₅Te₈ on vdW semiconducting monolayer WSe₂ template, with a tunable thickness down to a single unit cell, via two-step chemical vapor deposition. Moiré superlattices were clearly identified by high-resolution scanning transmission electron microscopy images. Strong coupling between Cr₅Te₈ and WSe₂ was demonstrated by the reduction of the Cr valence state near the interface and strong quenching of the photoluminescence of WSe₂, which is expected to be absent in fully vdW heterostructures. Ferromagnetism in 2D Cr₅Te₈ crystals down to two unit cells thicknesses was measured by both reflective magnetic circular dichroism and X-ray magnetic circular dichroism. Rich magnetic domain patterns were identified by photoemission electron microscopy. Our work expands 2D heterostructures into covalent material systems.