Wärmeeintrag und Temperaturprofile an der Oberfläche und in der Umgebung von plasmonischen durch Nanosekunden-Laserpulse geheizten Nanopartikeln und Nanoheteropartikeln

Abstract

Diese Doktorarbeit behandelt Untersuchungen der Auswirkungen von Nanosekunden-Laserpulsen auf plasmonische Nanopartikel. Der Fokus liegt auf den Temperaturen, welche bei der direkten Anregung der beweglichen Ladungsträger der Partikel und durch die Wechselwirkungen mit den Gitteratomen innerhalb der Partikel erreicht werden können und darauf, wie stark und lange die Umgebung der Partikel aufgeheizt werden kann.

This thesis deals with investigations of the effects of nanosecond laser pulses on plasmonic nanoparticles. The focus is set on the temperatures that can be reached during the direct excitation of the mobile charge carriers of the particles and through their interactions with the lattice atoms within the particles, and how strongly and how long the environment of the particles can be heated. In order to investigate these questions, two different approaches were followed, which provide new insights into nanoscopic heat generation. The results and discussion section is divided into three parts. In the first part, an approach to temperature determination on laser-heated gold particles in solution is pursued. This is based on the thermal decomposition of analyte molecules dissolved in the colloidal suspension of the nanoparticles. Dye molecules with different charges were used for this purpose. Their decomposition can be quantified with the help of extinction spectroscopy. The dyes are not directly excited and decomposed by the laser radiation, but by the plasmonic heating of the gold particles. From the change in the dye concentration, a reaction rate of the decomposition can be determined, which changes with the variation of the laser energy density. Using an Arrhenius-like plot of the energy density and the reaction verlocity, a possible temperature range that is reached at the nanoparticle surface is estimated. This includes temperatures between a few hundred Kelvin up to 10000 Kelvin and more. Investigations of the particles after laser irradiation using TEM, DLS and XPS measurements show that temperatures at least in the range of the melting and boiling points of gold can be reached. The loss of the surface ligands of the particles is already given at low laser energy densities and leads to agglomeration of the particles. The second part follows an approach using gold particles encapsulated within a matrix of the metal-organic framework compound ZIF-8. The particles can be heated within the material. In the process, ZIF-8 is decomposed around the gold particles. The size dependence of the radius of the resulting cavities was investigated as a function of the laser energy density. TEM images show that the particles fragment into smaller particles at similar energy densities as in the experiments with the dye molecules. The temperature in the vicinity of the particles can be estimated. The distance from the particle surface at which ZIF-8 decomposes ranges from a few nanometres at energy densities less than 10 mJ·cm−2 to 60 nanometres at 700 mJ·cm−2. Based on the volume of the cavities in the ZIF-8 matrix, the amount of decomposed material can be estimated and the amount of heat required to decompose this amount of material can be calculated. The Fourier-equation can be used to determine the temperatures at the particle surface for different energy densities. These temperatures range from a few hundred to several thousand Kelvin. At higher energy densities, a sudden increase up to several 10,000 Kelvin can be observed. Experiments with encapsulated hetero-particles show that heat is transferred from a plasmonically heated component of the hetero-particle to the non-heated component. At sufficiently high energy densities, this is sufficient to thermally decompose the component not directly heated by the laser pulse and parts of the ZIF-8 matrix. In the third part, the temperature ranges obtained using the two methods for temperature estimation based on the Arrhenius equation and the Fourier equation are compared. The morphology changes observable on the particles are observed in both systems at similar laser energy densities. The temperature data are used to determine temporal temperature profiles for the cooling processes using Newtons law of cooling. Very small particles cool from several thousand Kelvin to room temperature within several hundred femtoseconds, while large (about 100 nm) particles take up to 600 picoseconds or more. The data from both methods are combined to make a theoretical estimate of the dye turnover of the golddye system. The comparison of the theoretically and experimentally determined turnovers leads to the conclusion that a large part of the dye molecules do not decompose during the influence of the laser pulse. The molecules are probably removed from the heated area very quickly by temperature-induced forces or currents and cannot heat up enough to be decomposed. Keywords: