Text

theses

Plasmonics is a rapidly growing field of optics that utilizes plasmon resonances, which arise from collective oscillations of the free electrons in metallic nanostructures. Plasmonic nanomaterials have been incorporated into optoelectronic devices, spectroscopic methods, and nanophotonic applications to enhance the emission or absorption of light, with the aim of improving efficiency. By utilizing plasmons instead of light, traditional optical devices, such as lasers, can become nanoscale in physical size; however, demonstrations of plasmon-enhanced light emission at blue wavelengths are not common. The main reasons are the increase in interband electronic absorption (i.e., loss) that occurs in plasmonic materials at short wavelengths and the challenge of finding materials or materials combinations that can overcome these losses.

Traditional optics rely on light, which is diffraction limited, meaning there is a limit to how small of a volume the light can be confined due to the bending of light in or around small objects. By utilizing plasmonics, light can be manipulated below this diffraction limit, allowing for thinner films and devices. There are practical reasons for desiring thinner films (e.g. lower costs for production); but there are also interesting phenomena that occur in thin films that do not necessarily translate to bulk films. For example, due to differences in the refractive index of the layers (e.g. the film and air) light tends to be reflected and transmitted differently in thin-films (due to pronounced interference) compared to how light interacts with bulk films. Working with dimensions below the diffraction limit of light allows for the observation of new phenomena which have previously been unexplored such as local interactions between nanoparticles and light-emitting materials.

In this thesis, two topics of plasmonics are investigated, metal-molecule interactions, and metal-nanoparticle synthesis. To investigate metal-molecule interactions, the combination of plasmonic silver nanoparticles with light-emitting conjugated polymer materials is studied in two different systems. The use of solution-processable conjugated polymers allows for easy fabrication of thin-films; providing controllability of the thickness of the light-emitting material used below the diffraction limit of light. This enables local interactions between silver nanoparticles and the light-emitting thin-films to be studied in ultra-thin layers in ensemble measurements as well as at the single particle level.

In the first part of the thesis, thin-film composites of silver nanoparticles and an organic conjugated polymer (poly(9.9-di-n-octylfluorenyl-2,7-diyl); PFO) are investigated for their ability to behave as spasers (surface plasmon amplification by stimulated emission of radiation). This combination of materials is chosen so that there is good spectral overlap of the localized surface plasmon resonance of the silver nanoparticles and the high-gain emission peak of PFO. The composite films demonstrate stimulated emission of surface plasmons when ultra-thin-films of PFO are employed, ranging from 30 nm - 70 nm in thickness. Additionally, the quality factor of the spaser is enhanced in some instances, which is attributed to occurrences of interparticle coupling. These spasers emit at a wavelengths between 444.8 nm and 449.4 nm, which is the first demonstration of blue-emitting spasers.

Next, ultra-thin-film composites of silver nanoparticles and a different organic conjugated polymer, (poly(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3] thiadiazol-4,8-dily); (F8BT)), are investigated. The F8BT film thickness is between ~ 3.75 nm – 5 nm. In this particular composite, there is good spectral overlap of the localized surface plasmon resonance of the silver nanoparticle and the absorption band of F8BT. The occurrence of plasmon-exciton coupling and the extent of enhancement of the polymer’s emission is investigated in this system. It is demonstrated that enhanced emission occurs at the single nanoparticle level. Additionally, absorption induced scattering, which is a form of plasmon-exciton coupling, is clearly identified in this system. The study of optical and photonic interactions between silver nanoparticles and conjugated polymer thin-films enable advancements in demonstrations and potential applications of plasmonic materials at shorter wavelengths and at smaller size scales.

In the metal-molecule investigations spherical silver nanoparticles are used, due to their blue localized surface plasmon resonance wavelength, which overlaps well with the conjugated polymers studied. However, utilizing nanorod-shaped nanoparticles, that have two localized surface plasmon resonance wavelengths (one corresponding to the width and one corresponding to the length) would allow for tunability of the molecules utilized in metal-molecule interactions and, potentially, stronger plasmon-exciton coupling. However, nanorod shaped silver nanoparticles are not commercially available; therefore, they need to be synthesized in the laboratory. In the last part of this thesis, the polyol synthesis method for anisotropic silver nanoparticles is investigated, in order to better understand the challenges associated with their synthesis, which is one reason that silver is not utilized as often for plasmonic applications. Parameters such as polyol solvent molecular weight, reaction time, reaction temperature, and ratio of reagents are investigated extensively. The synthesis of silver nanoparticles is found to be very sensitive to the molecular weight of the polyol solvent, and the size and shape of the silver nanoparticles are restricted to low aspect ratios for higher molecular weight polyols.

ETD_10136

Ph.D.

Includes bibliographical references

doi:10.7282/t3-3b77-ws85

ETD doctoral

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