Mechanistic study of surface enhanced Raman scattering for high sensitivity PAH detection in water

Chatterjee, Abhijit (2018) Mechanistic study of surface enhanced Raman scattering for high sensitivity PAH detection in water. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The main objective of this thesis is the study of surface enhanced Raman scattering (SERS) mechanisms in order to fabricate efficient SERS substrates for the trace detection of PAHs. SERS is gaining tremendous attention in environmental pollutant sensing. SERS mainly relies on the plasmonic enhancement of the Raman signal from metal nanoparticles on the SERS substrate. In addition, metal-molecule charge transfer upon analyte adsorption is also important. Therefore, a study of the SERS enhancement mechanism is crucial in order to design an efficient SERS substrate. Polycyclic aromatic hydrocarbons (PAHs; phenanthrene, pyrene etc.) gained our attention due to their impact on the environment. PAHs dissolve in water at trace concentrations. Therefore, PAH sensing is challenging using typical environmental monitoring techniques. In this thesis, the SERS technique was employed to detect PAHs (phenanthrene and pyrene) with enhancement factors (EFs) of 10⁵ to 10⁶. The SERS activities of different types of substrates (metal-semiconductor, bimetallic, metal-insulator) were tested, and the Raman enhancement mechanism was investigated using plasmon absorption studies, scanning probe microscopy, and other spectroscopic techniques. A synergistic effect of electromagnetic and chemical enhancement on the SERS performance was illustrated with scanning probe microscopy. This thesis presents new applications or extensions of materials characterization methods, including Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM), to the SERS mechanism question. A range of materials was used to create the SERS substrates through multilayer deposition. In all cases, the top layer consisted of Au, Ag, or both. These plasmonic materials were supported by ZnO or silica spheres, whose morphology contributed to hotspot formation, plasmon tuning, and surface hydrophobicity. Substrate's surface morphology was tuned by varying the film preparation methods. Some methods allowed for independent tuning of surface roughness and surface area, two distinct contributors to enhancement. The semiconducting and insulating supports also electronically impacted the top metal layer. By varying the method of preparation of the ZnO, or by adding an insulating poly(methyl methacrylate) (PMMA) interlayer, both morphology and the electronic interactions were tuned. Different surface electronic interations between the metal and the analyte were linked to chemical enhancement. Among Au/ZnO substrates, Au possessed a partial positive surface charge as a result of Fermi level equilibrium with certain types of ZnO, while defect-rich ZnO led to a negative surface charge on Au. Bimetallic films with Au and Ag similarly generated a positive or negative surface charge on the substrate depending on which metal was on top. The direct measurement of surface charge, enabled by EFM explained why these surface charges impacted the EFs in an analyte-specific way. Tuning the SERS activity of Au/ZnO by the elimination of impurities in ZnO was also studied. The types of ZnO impurities and defects were identified by X-ray diffraction (XRD), Raman, thermogravimetric analysis (TGA), cathodoluminescence (CL), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), which were also used to identify changes in defects and impurities during the optimization of substrate preparation.

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