Optical properties of silver nanostructures prepared by electron beam lithography

Abstract: In this thesis, electron beam lithography (EBL) has been used to fabricate substrates for studies of nanoparticle optics, in particular for the investigation of noble metal nanoparticles and their tunable optical properties. The strong interaction of metal nanoparticles with light has its origin in the resonant excitation of collective oscillations of conduction electrons, so called localized surface plasmons (LSP). The resonance frequencies of the LSPs depend strongly on the nanoparticles material (e.g. Au, Ag, Cu), size, shape, neighboring particles and dielectric environment of the nanoparticle. Associated with the excitation of a LSP is a strong field enhancement in the vicinity of the metal particle. This field enhancement results in a phenomenon known as surface-enhanced Raman scattering (SERS), which occurs for molecules adsorbed on metallic nanostructures. In this work, I have specifically focused on interparticle coupling effects by systematically varying the interparticle distances and arrangements. Microextinction measurements of two-dimensional arrays of silver and gold nanoparticles revealed the importance of radiative coupling and retardation effects, seen as a clear blue shift of the LSP with decreasing interparticle distance. The SERS efficiencies of similar arrays of silver particles showed a strong increase as the interparticle distance was decreased. Additionally, dark-field scattering spectra of single particle pairs with varying interparticle distances were recorded. With light polarized parallel to the pair axis, we observed a strong redshift of the scattering peak position as the interparticle distance decreases, in good agreement with theoretical estimations. In addition to the work mentioned above, one project dealt with the field of nanoelectronics, namely the fabrication and current-voltage characterization of a self-assembled single-electron transistor. The fabrication scheme was based on a combination of electron-beam lithography and self-assembly of chemically synthesized gold nanoparticles covered with organic molecules. These devices show single-electron characteristics, such as Coulomb blockade at room temperature and Coulomb staircase and gate-voltage modulation of the current at 4.2 K.