Controlled Engineering of 2D Alkali‐Iron‐Chloride Compounds and Lateral Quantum Heterostructures Within Confined Graphene Nanoreactors

Using bilayer graphene (BLG) as Å‐scale reaction cavity and focused electron beam as both a stimulus and imaging tool, we report the controlled synthesis of a new class of two‐dimensional (2D) alkali‐iron‐chloride compounds (M@FeCl 3 , M = Li, Na, K) and lateral heterostructures formed by monolayer alkali halides (LiCl, NaCl, KCl) and iron chlorides. The electron beam locally decomposes alkali carbonate precursors deposited on the BLG surface, stimulating ion migration into pre‐intercalated FeCl 3 layers, inducing redox‐mediated phase transitions, and enabling confined crystallization. All these processes are recorded with atomic resolution using scanning transmission electron microscopy (STEM). The resulting monolayer heterostructures feature narrow, insulating halide nanostripes embedded within a magnetic FeCl 2 matrix. First‐principles calculations reveal spin‐polarized ground states in M@FeCl 3 compounds and tunable quantum‐confined behaviors modulated by the insulating halide domains. This work introduces a versatile platform for selective solid‐state synthesis and programmable interface engineering at the atomic scale, with implications for quantum information, ultrathin spintronic devices, and solid‐state ionic circuitry.