An experimental and computational approach to the development of SERS substrates for water quality monitoring sensors

Moazzez, Behrang (2014) An experimental and computational approach to the development of SERS substrates for water quality monitoring sensors. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This PhD thesis presents three essential aspects for an optical sensor suitable for use in harsh environments. The first of these is a robust gold film layer. The second is the metallic rough film with tendency to engage the hydrocarbon on the surface for detection with surface enhanced Raman spectroscopy (SERS). The third is predicting the Raman responses of the confined hydrocarbon molecules for altered laser field directions. The first project delineates a promoter-free protocol to improve the adhesion of thermally evaporated gold thin films by depositing the gold layer on SU-8 photoresist prior to UV exposure, and post-exposure baking steps of SU-8 processing. The higher adhesion of the top gold film to the post-deposition cured SU-8 sublayer is a product of shrinkage and distribution of residual stresses, due to cross-linking of the SU-8 polymer layer in the post-exposure baking step. The SU-8 underlayer can also impact the resulting gold film morphology. This approach is easily integrated with existing processes and can be used in a wide range of applications including, but not limited to, several types of optical sensors, where good adhesion of gold to a substrate is important and where controlled topography/roughness is key. In this project, a stable gold thin film substrate was developed specifically to address the mechanical properties required to avoid delamination and functionality in harsh environments, because gold thin films are naturally vulnerable to thermal shocks introduced during different fabrication steps of the sensor package and when introduced to cold ocean water where the sensor package will be used. In addition, the findings are also beneficial for the development of gold coated cantilevers for atomic force microscopy (AFM) and for MEMS-based sensors. For the first time, a mixed gold/chromium substrate is proposed and developed to tune surface features to generate a better SERS signal of the analyte in use. This alloy (SERS) substrate allows for detection of phenanthrene, a prominent member of the polycyclic aromatic hydrocarbons (PAHs), at low levels. Gold/chromium bilayer films with three different thickness ratios were deposited on glass by thermally evaporating layers of chromium and gold. Subsequent thermal annealing procedures either at 325ﾟC or 400ﾟC for either one or two hours were performed on bilayer films. Annealing led to the creation of gold and chromium islands. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) reveal that islands resulting from annealing-based segregation could reach the morphology and composition giving SERS activity for detection of phenanthrene. UV-Vis reflectance illustrates that films annealed at 400ﾟC show more intense scattering behavior than bilayer films. SERS activity tests with 532 nm excitation using confocal Raman spectroscopy show better response for annealed substrates with thicker top gold layers, with the best enhancement for those annealed at 400ﾟC for two hours. A reproducible SER spectrum has been the target of many studies for the last five decades. One of the difficulties with reproducibility and quantification of analytes in trace concentrations comes from the changing interaction angle of the analyte with the laser electric field due to random movements. Here, quantum chemical timedependent Hartree-Fock (TDHF) calculations are conducted to indicate the effect of the laser field direction and energy on the Raman spectra (peak shape and intensity) of oriented and free phenanthrene and naphthalene molecules. These findings can inform and improve SERS experiment and sensor design by determining the appropriate molecular orientation(s) for optimum signal acquisition.

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