Synthesis of a two-dimensional crystal within a van der Waals gap

Two-dimensional (2D) materials have been a focus in a wide range of fields in material science, engineering and condensed matter physics. Various methods have been developed for creating and optimizing 2D crystals, including mechanical or liquid exfoliations and direct growth on a substrate surface using vapor deposition or epitaxial techniques. While rapid advances have been made in synthesizing and characterizing a large number of single layer crystals, progress so far are largely limited to 2D materials with a known layered bulk parent. Here we demonstrate a distinct approach in which the entire growth process occurs in an atomically confined 2D space inside a van der Waals (vdW) stack and synthesize a new 2D crystal that has no known bulk parent. This approach for crystal synthesis inside a vdW gap paves a new way for creating, engineering, and investigating novel 2D quantum materials.