Tailoring of optical response for sensing: Simulations, roughness effect, and Raman

Abstract

The manipulation and control of light is on the forefront of sensing technology. Humans have taken inspiration from nature (vibrant colours of butterfly’s wings, iridescence in peacock feathers, camouflage of squid skin, etc.) to create periodic structures that are able to confine light in micro and even nano sized structures. The incorporation of plasmonic material into periodic structures produces sharp and strong resonances due to a combination of near field and far field effects. These nanostructures can be used as surface enhanced Raman spectroscopy (SERS) substrates to significantly enhance the intensity of the Raman signal. In this thesis, nanostructures optimized for Raman signal enhancement were used for sensing agrochemicals, which are causing food and water contamination due to their excessive use. The plasmonic response of nanostructures from single nanoparticles to periodic nanoarrays was studied using finite element method (FEM) simulations. The geometric parameters of the arrays were optimized for fabrication. An experimental study was performed to establish the effect of roughness on the optical response of thin gold films. Gold nanodisk arrays of various periods were fabricated using laser interference lithography (LIL). Thermal annealing was performed to tune the geometric parameters and decrease the roughness of the nanodisks to enhance their plasmonic response. Annealing the arrays improved the coupling effect of the near filed and far field resonance. The simulations strongly supported the experimental results. The period of the nanodisk arrays fabricated using the LIL technique was optimized for Raman sensing. The overlap of the resonance wavelength of the nanodisk array and the Raman excitation created the highest SERS enhancement. The most commonly used agrochemicals, herbicide (Atrazine), fungicide (Mancozeb), acaricide (Clofentezine), weedicide (Metribuzin) and insecticides (Chlorantraniliprole, Thiamethoxam, Tau-fluvalinate, and Flubendiamide) were collected for testing their limit of detection (LOD) in water using the optimized LIL sample as a SERS substrate. Density functional theory (DFT) was used to theoretically confirm Raman spectra of the agrochemicals. DFT is quantum mechanical method which can be used to calculate and visualize the vibrational modes of molecules. The optimal basis set was selected at the B3LYP level theory which provided a good compromise between accuracy and computational cost. The DGDZVP basis set was used to perform the vibrational mode analysis and validate the Raman spectra of the agrochemicals.