Ultraschnelles laserinduziertes Heizen und Schmelzen von Gold-Nanopartikeln : Untersuchungen der Struktur und Thermodynamik mit zeitaufgelöster Röntgenstreuung

Abstract

In the present work the structural relaxations of laser excited gold nanoparticles and their surroundings are investigated.The aim is the direct, time-resolved analysis of structural informations of nanostructures, which have been excited using femtosecond laser pulses and thus will transform in a reversible or non-reversible manner.The direct analysis of structural changes on an atomic length scale can be established by using pulsed Xray scattering. The required experimental setup is located at the European synchrotron radiation facility ESRF. At the beamline ID09B the inherent time structure of the synchrotron beam allows us a time resolution from 110 down to 60ps. With this fs-laser pump/ps-Xray probe experiment the system under investigation will be excited to a highly non-stationary state. Then, at a variable delay time and a specific pump energy, various Xray probe techniques will be used to analyse the kinetic evolution of the system after excitation. On the contrary, it is also possible to analyse the excited system at a specific delay time value, while varying the pump power. In this way we can scan a large portion of the kineto-dynamical landscape of the excited nanoparticles. Eventually, by using a variety of different nanoparticle sizes and different surroundings it is possible to acquire an almost complete picture of certain aspects of the different relaxation pathways of laser excited nanoparticles.The Xray scattering techniques used here comprise Bragg and small angle scattering from the nanoparticles, which give us direct access to the crystallinity of the metal lattice and to the overall shape of the nanoparticle, respectively. Furthermore using wide and small angle scattering from the amorphous matrix we obtain informations about the kinetics and dynamics of the immediate surroundings of the nanoparticles. By combining different Xray scattering techniques we are able to acquire clear physical insight into the - often inaccesible - dynamical interactions of the nanoparticles with their surrounding.Specifically, results for the following problems have been achieved:- thermal kinetics of the nanoparticles and emphasis of the meaning of the thermal interface resistance between nanoparticle and matrix- appearance of strong lattice distortions and coherent phonon modes in nanoparticles- dynamics of the melting transition of nanoparticles in water- shape transformation of the nanoparticles well below their melting point due to a non-thermal interaction of the nanoparticles with the laser field- explosive evaporation of a thin water shell surrounding the nanoparticles and its kinetical and dynamical propertiesFor all experiments gold nanoparticles have been used, due to their simple wet-chemical preparation. A wide range of particle sizes are well reproducible and their size distribution is fairly appropriate for most of the experiments. Large amounts can be produced and quite important, the hydrosols are very stable which prevents the samples from too rapid aging. The gold nanoparticles may serve well as prototype systems for other metal nanostructures.