Crystal Engineering Pathways Above, Below, and Between 2D Materials

ABSTRACT Two‐dimensional (2D) materials have opened new pathways for 3D thin‐film crystal engineering by overcoming the intrinsic limitations of conventional heteroepitaxy. Their atomically thin van der Waals surfaces enable interfacial interactions fundamentally distinct from those in 3D material systems, allowing the realization of crystal lattices, strain states, defect properties, and reconfigurable architectures unattainable with conventional epitaxy. Despite this promise, a critical gap remains in understanding and harnessing the full potential of 2D‐mediated crystal engineering. Most studies have focused on thin film growth above 2D layers for enhancing the crystallinity and heterogeneous integrability, whereas the equally powerful regimes below and between 2D materials remain largely unexplored. Here, we introduce crystal engineering pathways spanning ‘above (3D on 2D)’, ‘below (3D beneath 2D)’, and ‘between (3D confined within 2D layers)’ 2D layers, highlighting how these regimes collectively enable new crystals and interfaces largely inaccessible through conventional growth techniques. Through a comprehensive analysis of underlying mechanisms, experimental demonstrations, and remaining challenges, we provide a perspective on unlocking the full potential of 2D‐mediated crystal engineering for thin‐film growth and extending it into new regimes of mixed‐dimensional heterostructures.