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

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 108measured 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 109at a tilt angle of 5°.

Facile preparation of nanoparticle based SERS substrates for trace molecule detection.

In this work, we demonstrate that a polished Si wafer surface can be converted to possess strong surface-enhanced Raman scattering (SERS) activity by spray coating of polyol synthesized colloidal silver nanoparticles (AgNPs) at as low as 1% surface coverage. The SERS activity assays of substrate surfaces prepared with different production procedures (spray and spin coating) at different surface coverages are realized using population statistics. The resulting Raman enhancement factors (EFs) are discussed with the help of distance-dependent electromagnetic simulations for single particles and dimers. Statistics on the SERS effect and the corresponding EF calculations show that polyol synthesized AgNPs exhibit extremely strong SERS activity with EFs up to 108 at as low as 1% surface coverage. We discuss in this work that this is possible due to the distinct properties of polyol synthesized AgNPs such as atomically flat surfaces, sharp edges and corners naturally occurring in this synthesis method, which favor strong plasmonic activity. The method can be generalized to convert virtually any surface into a SERS substrate.