Plasmonics and Thermal Stability of Anisotropic Nanoparticles: Heating and Fs-Laser Excitation

Abstract

Nowadays, we are constantly surrounded by the word ’nano’. We find nanoparticles in food, cosmetics and electronic devices. Nanoparticles (NPs) are in a size regime between the atomic and the macroscopic world. The size confinement leads to intriguing new properties which cannot be observed for macroscopic systems. For instance, confining metals to the nanoscale gives rise to localized surface plasmons, a collective motion of the conduction band electrons. The phenomenon of localized surface plasmons results in strong absorption and scattering resonances of metal nanoparticles compared to bulk metals and in strong electric fields in the vicinity of the particle. Anisotropic shapes like nanorods are especially interesting from a plasmonic point of view due to increased absorption and scattering cross sections and intense field enhancements. The strong absorption at the plasmon resonance leads to an increase in particle temperature due to plasmon damping with electron-phonon scattering. Hundreds of degrees Celcius can be reached locally on a short time scale when e.g. fs-laser pulses are used for the excitation. Thus, metal NPs can act as nano heat sources. When non-toxic materials like gold are used, these properties can be even exploited in biology and medicine. For example, gold NPs are already used in hyperthermic cancer treatment where tumour cells are locally destroyed by this heat. A downside of the heating of the particle is that anisotropic shapes are not stable and deform towards the thermodynamically stable more sphere-like shape. We address several of the above mentioned points in the different chapters in the thesis. We start by exploring how different theoretical approaches can be used to model the optical properties of bimetallic NRs, e.g. NRs with a gold core and a silver shell and compare that to experimental results. We furthermore look at radiation damping of metal nanoparticles and develop an empirical formula to calculate the strength of radiation damping for different metals. We thenstudy the shape instability of silica-coated gold nanorods induced by femtosecond-laser pulses on a single particle level. We continue to explore the shape instability of silica-coated and ‘bare’ ligand-coated gold nanorods by heating them with different means in and specifically look at the dependence on aspect ratio and volume. Furthermore, we make bimetallic alloyed nanorods via heating and study the properties of mesoporous-silica-coated gold-silver, gold-palladium and gold-platinum alloyed nanorods. We extend our methodology to non-metallic nanorods, specifically semiconductor silica-coated CdSe/CdS nanorods, where we study their transformations upon femtosecond-laser pulse excitation. Finally, we look at the impact of femtosecond-laser pulses on small spherical gold nanoparticles on a glass substrate.