Fig. 3: Top-down fabrication strategies for 3D nanoelectrodes.

a Fabrication of vertical Si nanowires⁶⁵. The process flow shows the key steps, including reactive ion etching (RIE) of an Si-on-insulator (SOI) wafer to form Si cores, followed by thermal oxidation for insulation and metal cap deposition. b Fabrication of nanovolcanoes⁶⁶ via ion beam etching redeposition of a material sputtered from the bottom of the photoresist openings on the sidewalls to build the hollow structure. c Fabrication of nanocrowns^43,101. This multistep process involves the formation of solid SiO₂ pillars, conformal deposition of conductive/insulating layers, directional dry etching of the tip, and final selective wet etching of the core to create the semi-hollow crown structure