Sheath-Engineered Nanoscale Silicon Pillar Arrays with Sunflower-Like Radial Tilt Gradients for Angle-Selective Optical Sensing

Silicon nanopillar arrays enable versatile optical functionalities through geometric design, yet most fabrication methods produce vertical or uniformly tilted structures, limiting the spatial programmability of their nanoscale morphology. Here, we present a simple strategy for fabricating silicon nanopillar arrays with a sunflower-like radial tilt gradient using capacitively coupled plasma reactive ion etching (CCP-RIE). The fabricated silicon pillars have an average diameter of approximately 500 nm, a height of approximately 2 μm, and an array periodicity of 5 μm, forming an ordered nanostructured array with submicrometer lateral dimensions and micrometer-scale height. By introducing a metal base on the powered electrode, sheath distortion is induced, generating a radial electric-field component that continuously modulates ion incidence across the substrate. As a result, the nanopillars exhibit a centrosymmetric tilt distribution, evolving from near-vertical at the center to increasingly oblique toward the edges. Systematic experiments show that at the same outermost radial position of the silicon sample, the nanopillar tilt angle increases from approximately 8° to approximately 19° as the metal-base height increases from 2 to 8 mm, while the inner radius has a negligible influence on the spatial tilt-angle distribution. Increasing the sample size preserves the tilt angles at the center and edges but reduces the spatial gradient. Numerical simulations support the conclusion that the tilt originates from the coupling between sheath deformation and ion-trajectory deflection. The radially graded pillar orientations break the uniform geometrical symmetry of conventional vertical arrays and provide a spatially varying structural anisotropy that may be relevant to angle-dependent light−structure interactions. Such arrays could therefore be considered candidate nanostructures for angle-selective optical sensing, in which the incident angle or direction of light may be distinguished through spatial variations in optical response. Although the optical functionality remains to be experimentally validated, this work establishes a direct relationship between electrode geometry, plasma dynamics, and nanoscale morphology and provides a route to spatially programmable silicon pillar arrays for future angle-dependent optical studies and sensing-device development.