This thesis is mainly focused on the development of nano-structured noble metal substrates for applications in surface-enhanced Raman (SERS) and hyper Raman scattering (SEHRS) spectroscopies.
Chapter 1 is a brief introduction and an overview of SERS and SEHRS.
Chapter 2 demonstrates a new methodology for the preparation of Ag fractal nanostructures by electrodeposition. The morphology of as-prepared fractals is characterized by SEM and TEM. The valence of the Ag fractal is shown to be zero by X-ray photoelectron spectroscopy (XPS). The UV-vis absorption spectra of the as-prepared Ag fractals span from the blue to the red. These Ag fractal substrates shown good and stable SERS active by using pyridine and rhadomine-6G (R6G) as the probing molecules. The enhancement factor for pyridine is estimated to be ~10
6 at 514.5 nm excitation, and ~10
5 at 1064 nm excitation, respectively.
Chapter 3 extends the preparation methodology of Ag fractal to the synthesis of Au, Cu, and bimetallic fractals, including Au/Ag, Au/Pt and Au/Pd pairs. The as-prepared metal fractals have been characterized by SEM, UV-vis absorption spectrum and XPS. Important parameters, such as the electrodeposition methods, pH value of the electrolytes, and the multilayer layer matrix of polyelectrolytes have been systematically optimized and studied to generate the metal fractals. It is shown that metal fractals with different morphologies can be prepared by varying these parameters. For bimetallic fractals, comparisons are made between the reagent ratio and the composition ratio in fractals by XPS.
Chapter 4 reports high quality SEHRS, SEHRayS and SERS spectra of 1,4-bis(4’-pyridylethenyl)benzene (1,4-BPENB), 1,4-bis(4’-pyridylethynyl)benzene (1,4-BPYNB) , 1,3-bis(4’-pyridylethynyl)benzene (1,3-BPYNB), and 1,3,5-tri(4’-pyridylethynyl)benzene (1,3,5-TPYNB) adsorbed on Ag fractal substrates supported on electrode at different potentials. The spectra suggest that the molecules adsorb on the substrate via its pyridyl groups. The comparison between these spectra demonstrates that the adsorbing orientation of these molecules on the substrate change with the applied potentials. Comparisons are also made between the isotropic normal spectra and surface-enhanced spectra (SEHRS and SERS) for each molecule in order to check the effect of surface adsorption on the selectivity and enhancement. Ab Initio/DFT method was employed to calculate normal modes of the molecules as well as their intensities in IR and Raman spectra. The finite difference method was used to calculate hyper-Raman, SERS, and SEHRS to explore the adsorption effect and molecular orientation.
Chapter 5 reports SEHRS, SEHRayS and SERS spectra of 4,4’-bis(4-pyridy)biphenyl (4,4’-BPBP), 9,10-di(4-pyridyl)anthracene (9,10-DPANE), 1,4-di(4-pyridyl)benzene (1,4-DPB), 1-(Br-phenyl)-4(4’-pyridyl)benzene (1-BP-4-PB), and 1-Br-4-(4’-pyridyl)benzene (1-Br-4-PB), adsorbed on Ag fractal substrates supported on Ag electrode measured at different potentials. For each molecule, its orientation on the Ag fractals surface in SERS, SEHRS and SEHRayS is discussed in detail. Comparisons are also made between infrared, Raman and hyper-Raman spectra, with the aim of checking the complementarities between SEHRS and SERS as well as between SEHRS and infrared absorption. Finally, we compared isotropic normal spectra with surface-enhanced spectra (SEHRS and SERS), with the aim to examine the effect of surface adsorption on the selectivity and enhancement.
In Chapter 6 reports the spectroscopic study of an octupolar TPD-1. Resonance Raman spectra (RRS) were measured with excitations into the absorption band attributed to the intramolecular charge-transfer (CT) transition of TPD-1. Fluorescence excitation profile of the TPD-1 solution in pyridine is blue-shifted with respect to the absorption maximum. It indicates that at least two excited electronic states with strongly different internal conversion efficiencies are localized within the CT absorption profile. Thus, this observation is in conformity with an exciton model for the electronic structure of the octupolar molecules which predicts one blue-shifted one-photon forbidden state and one allowed double degenerated excited electronic state within the CT band. RRS contains the most intense bands at 1175 and 1589 cm
-1 assigned to the vibrations of single (C-C) and double (C=C) bonds of the ethylenic chain groups, thus indicating the importance of the π-conjugated bridges in the electron transfer from donor to acceptor in TPD-1. Raman excitation profiles for fundamental and overtone bands show no contribution of the forbidden transition on the high-energy side. This finding shows that the molecule keeps three-fold symmetry in solution. Surface-enhanced resonance hyper-Raman (SERHRS) and surface-enhanced resonance Raman (SERRS) spectra are also measured for TPD-1. Both SERHRS and SERRS spectra contain the most intense bands at 1175 and 1589 cm
-1 assigned to the vibrations of single (C-C) and double (C=C) bonds of the ethylenic chain groups. It was found that SERRS excitation profiles for the fundamental and overtone bands have an intense maximum on the high-energy side of the CT band. It means that at least two excited electronic states are localized within the CT band. This observation is consistent with the exciton model for the electronic structure of the octupolar molecules which predicts one blue-shifted one-photon forbidden state and one allowed double degenerated excited electronic state within the CT band. However, the ordinary Raman excitation profile shows no contribution of the forbidden transition on the high-energy side. These findings show that the molecule keeps the three-fold symmetry in solution but the symmetry is lifted when adsorbed on the Ag sol. SERHRS spectrum is found to be similar to the SERRS spectra in respect of number of bands observed and their relative intensities.
In chapter 7, we demonstrate the application of Ag fractal substrate in studying molecules with biological relevance. SERHRS and SERRS study of free-base tetra(4-pyridyl)-porphyrin (H
2TPyP) and iron (III) tetra(4-pyridyl)-porphyrin chloride (Fe(III)TPyP) are carried out on Ag fractal substrate. We made comparison between IR (one-photon), Raman (two-photon), and hyper-Raman (three-photon) spectra of H
2TPyP and Fe(III)TPyP in order to check the complementarity between SERHRS and SERRS as well as between SERHRS and IR.
In chapter 8, we report the design of a simple approach to carry out the SERS study on a single Au and Ag nanoflakes, respectively. The Au and Ag nanoflakes were deposited on the polystyrene sphere with a diameter of 2.6 μm. The nanoflake has a diameter of ~300 nm which is characterized by AFM and SEM. The Raman enhancement factor for Au at 632.8 nm excitation is on the order of ~10
6, and the Ag has a Raman enhancement factor of ~10
7 at 514.5 nm excitation. Resonance energy transfer (RET) mechanism was also employed in the study of Raman and SERS both on Ag mirror substrate and on the single Ag nanoparticle for the first time. Two dyes, R6G and PSS-TRITC, are chosen as the donor and acceptor in the experiments, respectively.
At the end, a brief summary and perspective is given on the basis of the studies carried out in this thesis.
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