| dc.description.abstract | Part A
Gold nanoparticles are the most stable known metal nanoparticles. These particles exhibit very unique optical properties caused by plasmonic effects. A consequence of these plasmonic effects is the possibility of coupling between a gold particle and an emitter. In a recent paper by Reineck et al., titled ”Distance and Wavelength Dependent Quenching of Molecular Fluorescence by Au@SiO2 Core-Shell Nanoparticles”, this coupling was studied for different dyes as a function of the gold-dye spacing. Gold nanoparticles with a radius of 12.7 nm were used and spacing was induced by a silica shell. A theoretical model was presented in the paper that predicts the obtained results very well which is a great achievement. However, experiments were only performed for small gold nanoparticles and only quenching of the dye emission was observed. For bigger gold nanoparticles (50 nm) enhancement of dye emission is possible which is much more interesting to study since plasmon-enhanced luminescence is promising in a lot of fields. The goal of this research is to extend the work by Reineck et al. by performing similar experiments with bigger particles to study enhancement of the dye emission.
This research started with a two step synthesis of the gold nanoparticles needed for the experiments. In the first step, small gold seeds with diameters close to 15 nm were synthesized via the citrate reduction method. Subsequently, these seeds were grown larger by using hydroquinone as a reductor to selectively reduce additional gold salt onto the surface of the existing nanocrystals. By tuning the number of seeds added in this step, it was possible to synthesize gold nanoparticles with diameters from 50 to 200 nm. Transmission electron microscopy and absorption measurements were used to characterize the samples. It was clearly observed that absorption shifts to longer wavelengths with increasing gold diameters. Furthermore, peak broadening and an increase in scattering was observed with increasing particle size.
To obtain stable dispersions of the particles in ethanol, particles were capped with polyvinylpyrrolidone. These particles were coated with uniform layers of silica via an adjusted St¨ober process. To do so, ammonia was added to the gold nanoparticle solution first. Small volumes of tetraethylorthosolicate were added next for the shells to grow. After some optimization, a procedure was developed to reproducibly coat the gold particles with uniform layers of silica.
After silica coating the outer surface of the particles was functionalised with (3-aminopropyl) triethoxysilane introducing -NH2 groups at the surface. These -NH2 groups were used to bind the activated ATTO700 dye. Photoluminenscence decay measurements showed that there was coupling between the dye molecules attached to the silica coated gold nanoparticles and the gold particles. This was indicated by the presence of a second, much faster decay path that was only observed in the presence of the gold nanoparticles. In order to learn more about the observed gold-dye coupling a comprehensive study is necessary. It would be interesting for example to study the distance dependence of this coupling for bigger nanoparticles and to study this coupling for different sizes of gold nanoparticles. This should be achievable since a reproducible procedure to synthesize the samples is demonstrated in this thesis.
Part B
In this work, CdSe core CdS/CdZnS/ZnS multishell nanocrystals are used to investigate the local-field effects on the spontaneous emission rate in different dielectric media. Multishells are used in the experiments because they shield the electronic transitions in the metal core from chemical influences of the solvent and from high-energy phonons.
CdSe cores were prepared using the hot-injection method. Layers of CdS, CdZnS and Zns were grown around these cores using the SILAR-method. Subsequently, selective precipitation was used to increase the monodispersity of the sample. The quality of the prepared samples was determined by TEM, absorption, emission and photoluminescence decay measurements. The multishell nanocrystals were dispersed in different solvents with refractive indices ranging from 1.375 up to 1.627. Fluorescent lifetimes were quantified assuming either single-exponential decay or a log-normal distribution of decay rates.
The experimental data was compared to four different local-field effect models describing the dependence of the fluorescent lifetime on the dielectric medium. The models used for this are the nanocrystal, full- or virtual cavity, the empty cavity and the fully microscopic model. While all four models qualitatively reproduce the experimental data, it was not possible to conclude which of the four models can be considered as the correct one. | |
