Shape and deposition angle control of silver film-over-nanosphere SERS substrates

Abstract

Thin metallic films on dielectric nanospheres are demonstrated to have a high potential for the fabrication of cost-effective SERS substrates. In addition to the morphological advantages that nanospheres offer for attaining a high density of hot spots, possessing shape adjustability by uncomplicated thermal treatment makes them an attractive platform for tuneable SERS substrates. Furthermore, when combined with the oblique angle metal deposition technique, adjustable gaps at a high density and adjustable shape of metal films, such as Ag films, can be achieved on nanospheres. Applying small changes in deposition angle can provide means for fine adjustment of the Raman enhancement factor (EF), resulting in EF up to 10 8 measured using crystal violet dye molecule as a Raman analyte. This practice paves the way for the fabrication of high EF SERS substrates at a reasonable cost using a monolayer of self-organized nanosphere patterns. An ultra-thin Ag film coated at 5°tilt is shown to be an excellent substitute for a film deposited at 0°with double the thickness. There is a strong agreement between the experimental results and finite-elements-method-based Maxwell simulations exhibiting expected field enhancements up to 10 9 at a tilt angle of 5°.

Experimentally evaluating plasmonic sensing performance of silver film over nanosphere surface (AgFON)

“…Some of the widely used nanofabrication techniques for fabrication of Ag nanostructures are; by green synthetic method for antibacterial activity [8], nanoholes by nanoimprint lithography for single-molecule imaging [9], tandom nanodisks by electron beam lithography for colour generation applications [10], armrest nanorods by oblique angle deposition for biomolecular detection [5], nanocubes by the solvothermal method for refractive index sensing applications [11], film-coated monolayer of polystyrene nanospheres for SERS applications [12], tetrahedral nanopyramids by nanosphere lithography for plasmonic applications [13] etc. Even though these methods possess their own merits, nanosphere lithography (NSL) has emerged as a good technique as it is simple, low-cost and possible for large-scale fabrications [14].…”