Fabrication of Anisotropic Nanostructures on Solid Substrates for Applications as Optically Active Surfaces

Abstract

Analysis of the vibrational energy levels in molecules using Raman Spectroscopy is a popular analytical method amongst today's optical technologies. Unlike fluorescence microscopy, the Raman emission doesn't undergo the process of photobleaching, which leads to short lived signal collection. The only downside with this approach is the small absorption cross-section resulting in a low intensity of the emission signal. One of the approaches used to boost this weak signal is Surface Enhanced Raman Spectroscopy (SERS). My research efforts have been focused on how to create a novel SERS substrate with aligned patterns of Au/Ag nanoparticle deposited metals of a particular shape and size. Although Raman is our targeted emission process, this substrate will also be of great benefit to fluorescence emission signals as well. Metal nanoparticles with various shapes were synthesized using different chemical approaches. These synthesized metal nanoparticles were placed on surface modified silicon substrates. The surface modification was created by using a modified version of Electron-Beam Lithography (EBL), one of the popular micro- and nano-lithographic techniques in the semiconductor field. In a typical EBL process, a positive photoresist, polymethylmethacrylate (PMMA) is exposed to the electron beam in specific patterns. This exposed polymer gets selectively dissolved in a combination of solvents called a developer solution. If the electron exposure is increased beyond what is needed, the patterns cross-link to the substrate, and become permanent features. Such patterns created in this experimental approach are insoluble in both the developer solution and in the lift-off solution. Metal coatings and nanoparticle depositions on top of these cross-linked patterns make the substrate suitable for creating isolated nanostructures on solid substrates. Advancing a step further, these nanostructures were confined in "nano-walls", thereby creating "nanowells" to hold the sample under investigation within the local environment of the nanostructure. Effects of the metal plane near this nanostructure were studied using computational calculations.