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

Abstract 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 (Cs 3 Bi 2 X 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 Cs 3 Bi 2 I 9 photodetector reaches up to 3.7 × 10 12 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.