Optical properties and alloying of metal nanorods

Abstract

In this thesis we study the plasmonic properties and alloying process of metallic nanoparticles (NPs) by means of experiments and calculations. Specifically, we mainly focus on Au nanorods (NRs) and bimetallic Au-Ag core-shell or alloyed NRs. Firstly, Mie-Gans theory, modified to fit our needs, was employed to analytically calculate extinction cross sections for these systems. We found that alloyed NRs had a relatively bad plasmonic performance compared to pure Au NRs. Subsequently, we studied the radiative decay of the localized surface plasmon resonance (LSPR) of metal NPs using finite-difference time-domain (FDTD) calculations and discovered a dependence on resonance energy and dielectric constant of the medium for various metals, including a reduced radiative decay rate at resonance energies for which interband transitions occur. Furthermore, we analyzed experimental ensemble extinction spectra by using our knowledge of Mie-Gans modelling and damping mechanisms, and determined important parameters from the extinction spectrum like the aspect ratio distribution and sample concentration. By comparing the extracted results with proven characterization methods, the accuracy and limits of this approach were investigated. In the second part of the thesis we employed the from calculations obtained knowledge to analyze measurements on silica-coated Au-Ag core-shell and alloyed NRs. Core-shell NRs were first synthesized and then characterized via transmission electron microscopy (TEM) and visible-near infra-red (vis-NIR) Fourier Transform Infra-red (FTIR) spectroscopy. With the synthesized silica-coated Au-Ag core-shell NRs we studied the alloying process of Au-Ag NRs with thermal heating experiments, that we followed in-situ with vis-NIR FTIR spectroscopy. From these measurements we determined activation energies (EA) for diffusion in the NRs that were two to three times lower than the activation energies for Au and Ag atoms in bulk Au-Ag alloys. To determine whether the alloying process depends on the shape of the NPs, we compared nanospheres and NRs of the same volume and Au-Ag composition in in-situ heating experiments in an electron microscope. No difference in alloying temperature was observed, indicating that the NR shape did not facilitate the alloying process.