Ultrahigh sensitive SERS film of silver nanorods grown on nanohole array template by oblique angle deposition

Abstract

To develop a highly sensitive silver nanorod (AgNR) array SERS film, anodic aluminum oxide (AAO) nanohole array templates were employed as seed substrates for oblique angle deposition (O.A.D.) by thermal evaporation. By pore broadening process, precise hole arrays of various diameters were fabricated from preliminarily prepared nanoholes of 30 nm diameter comprising equilateral triangle unit cells whose side length is 100 nm. Aligned silver nanorods of ∼1 μm length were fabricated on the several kinds of seed templates. The optical properties of the AgNR films were measured using thiophenol self-assembled monolayers on the film surfaces and a micro-Raman spectrometer employing a 785 nm pump laser. Out of the AgNR array samples, the AgNR array deposited on an 80-nm hole array substrate showed the highest SERS activity. The highest surface enhancement factor (E.F.) of Raman signal measured in this experiment was ∼7·107, which is ∼7 times higher than that of the AgNR array fabricated on a flat substrate. The main role of the nanopore array substrate was found to decrease the density of the grown nanorods, which is the key parameter determining the E.F. values of the AgNR arrays. From this and previous works, the optimum nanorod density is anticipated to be 8–9 rods/μm2, where E.F. of > 109 could be obtained. The nanorod density of the optimum AgNR array in this experiment was ∼10/μm2. The origin of the key role conducted by the nanorod density cannot be explained by hot spot effect and waits for understanding in the future studies. The ultrahigh sensitive AgNR array SERS film obtained in this work can be a very powerful medium for various chemical and biological sensors.