In‐Plane Adaptive Heteroepitaxy of 2D Cesium Bismuth Halides with Engineered Bandgaps on <i>c</i> ‐Sapphire

The heteroepitaxy of 2D materials with engineered bandgaps are crucial to broaden the spectral response for their integrated optoelectronic devices. However, it is a challenge to achieve the high-oriented epitaxy and integration of multicomponent 2D materials with varying lattice constants on the same substrate due to the limitation of lattice matching. Here, in-plane adaptive heteroepitaxy of a series of high-oriented 2D cesium bismuth halide (Cs₃Bi₂X_9, X = I, Br, Cl) single crystals with varying lattice constants from 8.41 to 7.71 Å is achieved on c-plane sapphire with distinct lattice constant of 4.76 Å at a low temperature of 160 °C in an air ambient, benefiting from tolerable interfacial strain by switching compressive stress to tensile stress during a 30° rotation of crystal orientation. First-principles calculation demonstrates that those are all thermodynamically stable phases, deriving from multiple minima of interfacial energy between single crystals and sapphire substrate. The detectivity of Cs₃Bi₂I₉ photodetector reaches up to 3.7 × 10¹² Jones, deriving from high single-crystal quality. This work provides a promising experimental strategy and basic theory to boost the heteroepitaxy and integration of 2D single crystals with varying lattice constants on low-cost dielectric substrate, paving the way for their applications in integrated optoelectronics.