Fabrication and characterization of nano-porous surfaces on silicon and mica

Nano-porous surfaces are becoming increasingly more relevant due to their unique structural and functional properties. Depending on the scientific question to be addressed, both single nanopores and large-area nano-porous surfaces (i.e., above 100 µm × 100 µm) are particularly valuable model systems for studying fundamental physical effects, especially in contact with fluids. This study explores two fabrication approaches to generate defined nanopores on initially flat surfaces of silicon and mica substrates: one using focused ion beam (FIB) milling, and the other employing anodized aluminum oxide (AAO) as an etching mask for ion etching (RIE). Both methods yield nano-porous surfaces with inter-pore maximum roughness below 2 nm and diameters ranging from 40 nm up to hundreds of nanometers. Despite progress in understanding fluid behavior in nanopores, direct surface force measurements on such surfaces remain scarce. To address this, we performed atomic force microscopy (AFM) measurements in air and water, revealing a locally enhanced van der Waals attraction around the pores, compared to the flat surfaces. This enhanced attraction is discussed in terms of contact mechanics and the distinct configuration of the electric double layer (EDL) around the orifice of such nanopores.