Epitaxial growth of large-grain-size ferromagnetic monolayer CrI for valley Zeeman splitting enhancement.

Two-dimensional (2D) magnetic CrI has received considerable research attention because of its intrinsic features, including insulation, Ising ferromagnetism, and stacking-order-dependent magnetism, as well as potential in spintronic applications. However, the current strategy for the production of ambient-unstable CrI thin layer is limited to mechanical exfoliation, which normally suffers from uncontrollable layer thickness, small size, and low yet unpredictable yield. Here, via a confined vapor epitaxy (CVE) method, we demonstrate the mass production of flower-like CrI monolayers on mica. Interestingly, we discovered the crucial role of K ions on the mica surface in determining the morphology of monolayer CrI, reacting with precursors to form a KI buffer layer. Meanwhile, the transport agent affects the thickness and size of the as-grown CrI. Moreover, the Curie temperature of CrI is greatly affected by the interaction between CrI and the substrate. The monolayer CrI on mica could act as a magnetic substrate for valley Zeeman splitting enhancement of WSe. We reckon our work represents a major advancement in the mass production of monolayer 2D CrI and anticipate that our growth strategy may be extended to other transition metal halides.