Ultrafast energy transfer dynamics of molecules near plasmonic metal nanosurface

Abstract

Many researches have been made on the principle of metal enhancement fluorescence phenomenon, which has been used as an effective method for increasing the fluorescence intensity of a fluorescent substance for the past several decades. Recently, it has been interpreted that the increase of the absorption by the enhancement of the electric field around the nanoparticles and the enhancement of the radiative decay rate through the energy transfer are concurrently operated but it is not clarified experimentally yet. In particular, it is known that when the extinction spectrum of nanoparticles and the fluorescence spectrum of the dye overlap well, the fluorescence enhancement is superior, but there are various theories about the cause. In addition, organic solar cells (polymer solar cells) are attracting attention as next generation solar cells because of their advantages such as abundant raw materials, inexpensive production process, and flexible shape compared to inorganic solar cells. Among them, carotenoid, which is a natural dye, has a low polarity due to symmetrical structure, and has fast energy transfer characteristic, which is suggested as an electron donor of organic solar cell. However, due to the lack of research on the interaction between carotenoid and metal nanoparticles and the excited state dynamics of carotenoids in the polymer, the application was very limited. In this paper, we use TCSPC and femtosecond transient absorption spectroscopy to measure the effects of excitation pulses of different wavelengths on the excitation state dynes of dyes themselves and dyes around nanoparticles in solvent and polymer matrix environment. In addition, by controlling the size of the nanoparticles, the positions of the SPR bands of the nanoparticles were varied, and the metal enhancement fluorescence characteristics according to the fluorescence spectrum of the dye and the position of the SPR band of the nanoparticles were investigated. In addition, fluorescence intensities and excited state dynamics of carotenoids, a natural dye, were measured in polymer films. In Chapter 3, the changes in excited state dynamics of 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) dyes with silver island films (SIF) in methanol were measured using femtosecond transient absorption spectroscopy with excitation pulses of 403 and 480 nm. The intensity of the stimulated emission (SE) of the DCM with SIF was increased by 10% regardless of the time when the 480 nm excitation pulse was used. However, when 403 nm excitation pulse was used, the intensity did not appear before ~ 40 ps, After that, SE enhancement gradually increased. The gradual increase in SE intensity enhancement after 40 ps with an increase in lifetime appears to be an energy transfer from the nanoparticles to the dye. In Chapter 4, a dye-doped polymer was coated on the SIF surface by spin coating to form a polymer film of ~ 100 nm. Through this, it is possible to prevent direct increase of contact between the dye and the nanoparticles, and to keep the distance between the nanoparticles and the dye within the film thickness, thereby effectively observing the metal enhancement fluorescence of the dye. Time-resolved fluorescence measurements of DCM and Rhodamine 6G around the nanoparticles using TCSPC showed that a new component with a fast lifetime of ~ 400 ps as seen by the energy transfer between nanoparticles and dye was observed. In chapter 5, all-trans--carotene (carotene) and 8-apo--carotene-8-al (carotenal) were applied to polymer films coated on SIF instead of laser dyes. These carotenoids showed a fluorescence enhancement of 4-47 times depending on the type of polymer in the vicinity of SIF, and a rapid decrease in lifetime was observed. The energy transfer between the dye and the nanoparticle can be calculated through the reduction of the lifetime. The energy transfer efficiency is related to the relative position of the SRP band of the nanoparticles and the excited energy level of the carotenoid in the polymer. Chapter 6 used a seed-growth method to fabricate nanoparticles with an average size between 70 and 200 nanometers within a standard deviation of about 10%. Then, silver colloidal films (SCF), which can adjust the position of SPR band, were fabricated by adsorbing the prepared nanoparticles on a substrate. Through the fluorescence enhancement measurement of DCM coated polymer on the surface of SCFs via TCSPC and the measurement of the excited state dynamics of DCM with SCFs using femtosecond transient absorption spectroscopy, the overlap of the SRP band of the nanoparticles with the fluorescence spectrum of the dye is large It was confirmed that the energy transfer was large. Chapter 7 moves the sample two-dimensionally to minimize photo-damage of the sample during the measurement of the transient absorption spectra. The transient absorption spectra of carotene and carotenal in thin polymer films were measured. As a result, carotene and carotenal in the polymer showed the components which appeared as S* state with long lifetime in addition to the three kinetics components appearing in hexane.