Stress-Induced Microstructures and Nanogaps Embedded in Plasmonic Gratings

Nanomaterials and nanostructures exhibit unique properties that are far different from bulk properties. Our group has utilized these unique properties to enhance signal performance in the detection of biomarkers for biosensing applications. Currently, we are developing metallic nanostructured sensor platforms to obtain ultrasensitive levels of detection. One useful property of metallic nanostructures, such as metallic nanogratings, is the ability to couple incident photons to a metallic/dielectric interface to form concentrated electromagnetic (EM) fields by surface plasmon resonance (SPR). When surrounding fluorescent dye molecules are exposed to these high intensity EM fields, the fluorescent emission intensity can be greatly enhanced to provide much better signal-to-noise ratios, reducing false positives, and improved detection limits, even down to single molecule detection. Our group has been developing a modified microcontact printing process to inexpensively replicate the periodic structure of commercially available DVD discs and Blu-ray discs using polydimethylsiloxane (PDMS). The PDMS gratings are then coated with thin metals (e.g., gold films) to produce plasmonic gratings that can couple light at specific wavelengths and incidence angles, which appear as sharp dips in spectroscopic reflectivity measurements. In addition to the plasmonic gratings, we demonstrate a fabrication method to create additional structure and nanogaps in the plasmonic gratings. Controlled stress is applied to the plasmonic grating resulting in stress-induced structures in the plasmonic gratings. Different levels of stresses cause changes in physical properties of the structure including the sizes depths, and lengths of nanogaps. Nanogaps embedded in plasmonic gratings show much stronger SPR compared to plasmonic gratings without additional structure and nanogaps. Fabrication methods of creating the microstructure and nanogaps parallel to the gratings and perpendicular to the gratings are demonstrated as well as their characterization and properties.