What does dark field spectroscopy study

Abstract

In the present dissertation, hybrid systems based on alternative nanoparticle morphologies were examined with regard to their optical properties in order to largely eliminate the known limitations and inadequacies of the established nanoparticle systems and shapes, such as rod-shaped or spherical nanoparticles. Star-shaped gold nanoparticles and their special plasmonic properties are presented. Methods of dark field spectroscopy, photoemission electron microscopy and scanning electron microscopy (SEM) are used to correlate the near and far field properties of individual nanostars with their morphology. The first experimental proof of the localization of optically excited hot spots at the tips of the individual nanostars could be provided. Through a suitable choice of polarization and wavelength, hot spots are selectively excited at several or exclusively at individual tips of a nanostar and the detected plasmon resonances are assigned to individual nanostar tips only on the basis of experimental data. Furthermore, field enhancement factors in the range of 58-79 at the nanostar tips are determined directly and quantitatively for the first time. The high and easily accessible hot spots at the tips of the nanostars are used to demonstrate the excellent amplification of Raman scattering at the level of individual nanostars. The Raman amplification factors of 10 ^ 7 for the investigated nanostars without exploiting plasmonic coupling effects make the nanostars an important candidate for use in complex dynamic environments. Another possibility of using the hot spots located at the tips are nanobipyramids. By manipulating with an atomic force microscope, the hot spots can be positioned up to 1 nm above the surface and anywhere on a sample substrate. Nanoresonators made of two nanobipyramids can be produced mechanically in three different adsorption configurations with regard to the tips of the two nanoparticles pointing towards one another. As a result of the mechanical change in the configuration of the nanoresonator, the position of the hot spot created in the particle gap can be moved in height without any significant change in the spectral position of the coupled plasmon resonance.