The influence of roughness on surface enhanced effects

Surface enhanced effects, like surface enhanced absorption, fluorescence, or Raman scattering, allow for the ultra- sensitive detection and investigation of molecule-light interactions, eventually reaching the single molecule limit. This project aims at a detailed characterization and modelling of surface enhanced effects on lithographically fabricated substrates (regular arrays of polycrystalline metal nanoparticles). As result, we will obtain a complete picture of the contributions of the nanoparticles` surface roughness, crystallinity, and non-local dielectric response to surface enhancement. Additionally, routes towards controlled crystallinity of lithographically fabricated metal nanoparticles and geometry optimization of the particles and particle arrangements will be explored. The aim of the project is to improve surface enhanced effects on lithographically fabricated substrates, which is of importance for novel sensor applications. The electromagnetic coupling of molecular optical transitions to the surface plasmons allows to influence the molecule-light interaction, and to enhance scattering rates by several orders of magnitude. Chemically prepared and deposited metal colloidal particles are usually very efficient substrates for SEE, but their disadvantage lies in the random nature with regard to particle size and shape, as well as to particle arrangements. An alternative approach to get well controlled, reproducible substrates for SEE is by nano-lithographic means, such as electron beam lithography. However, due to the fabrication process there is usually considerable surface roughness on lithographic samples, in contrast to colloidal particles, which so far has inhibited to exploit the full potential of SEE on well- controlled nanostructures. The influence of this surface roughness on the optical near fields and SEE is not known. It is the purpose of this project to address the effects caused by surface roughness by a systematic comparison between lithographically fabricated nanostructures and chemically synthesized ones, and to seek for improved lithography schemes that would allow fabricating well-controlled metallic nanostructures with optimized optical properties. Our analysis will include the investigation of both single nanoparticles and ensembles of particles, partly in collaboration with other groups, and will be supported by detailed theoretical model calculations. For the investigation of single nanoparticles we will be using atomic force microscopy and scanning electron microscopy, optical near field characterization, and electron-energy loss spectroscopy and cathodoluminescence. For the investigation of SEE, we will be using conventional far-field spectroscopy on nanoparticle ensembles covered with probe-molecules. Our theoretical analysis will be based on calculations performed within the boundary-element- method approach, and will address SEE, surface roughness in single nanoparticles and ensembles of particles, as well as non-local dielectric functions and possible particle shape optimizations.

Metallic nano-particles with dimensions in the range of a few 100,000th of a millimetre have the nature to concentrate electromagnetic light fields at their surface, i.e. in spatial regions which are orders of magnitude smaller than the smallest possible focal region of a traditional optical lens. Responsible for this is the resonant excitation of so-called surface plasmons, which are coherent oscillation states of the metals electron-system. If a a molecule is placed close to the nano-particle surface, this light concentration leads to a by orders of magnitude enhanced light-molecule interaction (surface enhancement) with the result, that fluorphores shine brighter or the otherwise undetectably weak Ramans scattering of single molecules can be traced. For the controlled fabrication of suitable metallic nano-particles, often the metal is deposited at previously lithographically defined regions by means of thermal vacuum- deposition. The thus produced particles are polycrystalline and have a very small surface roughness of less than one millionth of a millimetre. These properties where hitherto not considered in the description of surface enhanced fluorescence or -Raman scattering of these particles. In the project Influence of surface roughness on surface enhanced effects, this was investigated systematically for the first time. By combining experimental investigations with numerical electromagnetic simulations, it could be demonstrated that the surface roughness as well as the polycrystallinity have a clear influence on the surface enhanced effects, and thus have to be controlled for quantitative sensor-applications. Within the frame of this project it could be further demonstrated, that a defined thermal annealing of lithographically fabricated metal nano-particles allows for such a control. These results are seminal for future development of improved and reproducibly fabricated metal nano-particles for the application in quantitative, highly sensitive biochemical sensors.