Index-Tapered Nanostructured Films via Selective Porosification for Enhanced Light Harvesting and Environmental Robustness

Functionally graded materials (FGMs) with physical or chemical continuity are simply reconstructed into graded refractive-index (GRIN) antireflective coatings (ARCs) for optical purposes to suppress reflections at the interfaces. However, achieving seamless continuity in optical properties encounters challenges, primarily due to the limited diversity of materials available. The Seamless-GRIN ARC (SGRIN ARC) addresses this challenge by synergistically combining chemical and physical approaches. AlF₃, Al₂O₃, and intermediate AlO_<i>x</i>F_<i>y</i> thin films were deposited by atomic layer deposition to construct a continuous GRIN profile. To bridge the discontinuity between AlF₃ and air, water-induced porosification of sacrificial Al₂O₃ generated a porous, graded morphology that extended the refractive-index gradient to air, while the AlF₃ layer preserved structural integrity as a pinhole-free barrier. Applied to an optical window, the SGRIN ARC achieved an average transmittance of ∼99.0% from the visible to near-infrared. Coupled with a two-dimensional (2D) SnSe photodetector, it enhanced photoresponse by factors of 4.64 (365 nm), 4.78 (532 nm), and 4.63 (980 nm). The SGRIN structure also provides a moisture-resistant encapsulation layer ensuring durability and an antireflective interface enabling broadband light harvesting. This multifunctional design demonstrates a scalable strategy for next-generation optoelectronic devices requiring both environmental stability and broadband optical performance.