Stacked-Disk Nanotower Arrays for Use as Omniphobic Surface-Enhanced Raman Scattering Substrates

Abstract

Herein, a simple interference lithographic technique to design omniphobic surface-enhanced Raman scattering active surfaces composed of stacked-disk nanotower arrays is reported. To create the nanotower arrays in a highly reproducible fashion, the authors use a single laser exposure onto the photoresist-on-dielectric mirror using a phase mask. The interference by the phase shift produces a nanotower array with a submicrometer periodicity, whereas the standing wave effect by reflection from the mirror defines the stacked-disk structure at each nanotower at sub-100 nm periodicity. The stacked-disk nanotowers with silver outer-coating provide a strong electric field localization at the vertically integrated nanogaps on the side wall. At the same time, the series of reentrant geometry on the side walls can efficiently pin the liquid-air interface for a variety of liquids, including water and toluene, thereby providing omniphobic surface property. The molecules dissolved in either water or toluene at low concentration can be enriched in the drop on the surface in a course of evaporation, which are finally deposited onto a tiny spot. Raman spectra of the molecules are further amplified by the strong electric field at the nanogaps on the side walls of the nanotowers. Therefore, the surfaces provide high sensitivity of Raman analysis for both polar and nonpolar molecules.