Visualizing Heteroepitaxy of Angle-Oriented GeSe/WSe2 Heterostructures for Near-Infrared Ultrasensitive Photodetectors.

Precise heteroepitaxial control in van der Waals (vdW) heterostructures remains a challenge. Here, we demonstrate a visualized and directed heteroepitaxy strategy for GeSe on WSe2 by integrating in situ imaging with symmetry-guided physical vapor deposition. Real-time observation captures the nucleation kinetics and orientation evolution, accelerating process optimization. Combined density functional theory calculations and transmission electron microscope characterizations identify a preferential epitaxial angle of 20° (occurrence: 59.37%) for most GeSe nanosheets, corresponding to the global energy minimum governed by lattice symmetry. Coverage-tunable photodetectors are constructed by modulating GeSe nanosheets' number, delivering a three-orders-of-magnitude enhancement in responsivity at 808 nm (823.8 A/W at maximum coverage). This is attributed to optimized light harvesting in GeSe and a photogating effect via back-to-back type-II band alignment. High-contrast single-pixel imaging is also demonstrated. This work establishes a paradigm for visualized synthesis and performance optimization of monochalcogenide-based vdW heterostructures.