Geometrically tuned and chemically switched wetting properties of silicon nanotips

The wetting properties of silicon nanotips (SiNTs) are discussed. SiNTs were prepared by single step dry etching of silicon wafers in an electron cyclotron resonance plasma of silane, methane, argon and hydrogen and water contact angles were measured as a function of their aspect ratio (α) and the inter-tip distance. The hydrophilic nature of the SiNTs is tunable with α and the inter-tip distance. Super-hydrophilicity with water contact angles close to 2° was observed with α>12 (length ∼1500 nm). Upon coating a 1500 nm long SiNT with TiO(2), the water contact angle jumped from 2° to ∼140°, demonstrating a switchover from super-hydrophilic to hydrophobic surface properties.