A study of photophysical processes using raman and fluorescence spectroscopy at the ensemble and single molecule levels

This work investigates the phenomenon of charge transfer probed using Raman and fluorescence spectroscopy techniques. Under Raman spectroscopy study, we chose a system of nitrothiophenol (NTP) isomers chemisorbed onto TiO2 and system of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in a diode-like device. Under fluorescence spectroscopy study, we chose a system of poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) nanoparticle under the influence of electric field and energy transfer in a capacitor-like device. Chapter 1 provides general introduction of Raman spectroscopy and the mechanism of Surface Enhanced Raman Spectroscopy (SERS). Particular attention will be given to chemical enhancement mechanism of SERS which involves charge transfer. Since charge movement is influenced by electric field, the fabrication of proper electrical device is necessary. Application of charge transfer in the fluorescence of conjugated polymer in an electrical device will be described, along with the combined effect of charge transfer and energy transfer. In Chapter 2, surface enhanced Raman signal of meta- and para-nitrothiophenol (m-NTP and p-NTP, respectively) were observed when they were chemisorbed onto TiO2. The enhancement mechanism was deduced from absorption spectrum and Density Functional Theory (DFT) and was found to be due to charge transfer from NTP isomers to TiO2. In Chapter 3, fluorescence intensity of single dye molecule (ATTO 647N) and backscattered light intensity were studied in an open and a closed circuit device. In an open circuit, electric field did not influence fluorescence and backscattered light intensity. In a closed circuit, the resulting alternating electric current modulated both fluorescence and backscattered light intensity. This artificial result was found to be due displacement (i.e. mechanical jerk) of the substrate. In Chapter 4, Raman spectrum of PEDOT:PSS in OLED (Organic Light Emitting Diode) device was investigated. During OLED operation, holes are injected into PEDOT:PSS, oxidizing it to form a benzoid structures. The vibrational frequency of PEDOT:PSS was found to gradually shift to higher wavenumber. After the operation, it gradually shifted back to its initial wavenumber. In Chapter 5, fluorescence intensity of a single MEH-PPV nanoparticle was studied in an open circuit. The alternating electric fieldinduces modulation of fluorescence intensity. This result was interpreted in the light dissociation of singlet exciton to become electron and hole polaron. Electric field then induced the movement of hole polaron within the nanoparticle, modulating its fluorescence intensity. An energy acceptor, meso-Tetraphenylporphyrin (TPP), was added to the nanoparticle. The resulting composite nanoparticle showed lesser modulation of fluorescence intensity under the influence of electric field. This is interpreted in the light of quenching of singlet exciton via Förster Resonance Energy Transfer (FRET), thereby reducing the probability of dissociation of singlet exciton to become electron and hole polaron.DOCTOR OF PHILOSOPHY (SPMS