Analysis of biomimetic polymer brushes by XPS and optical spectroscopy

Abstract

A grand challenge for the past two decades has been to discover how to achieve efficient long- range transport of excitation in molecular photonic materials. Excitons (electron-hole pairs) are transported via incoherent hopping processes, with the effect that recombination rates are high and exciton diffusion lengths are short. The goal of this PhD has been to explore a new approach to the design of molecular photonic materials that combines biologically-inspired design with strong light-matter coupling. In strong light-matter coupling, a confined optical mode such as a localised surface plasmon resonance (LSPR) is hybridised with molecular excitons to yield new states that mix the properties of light and matter. Recent work demonstrated that light-harvesting complexes from plants and photosynthetic bacteria are coupled strongly to LSPRs associated with gold nanostructure arrays, yielding large coupling energies and coherent, ultra-fast energy transfer. However, proteins are not suitable for applications in electronic and optical devices. Thus, the goal of this thesis is to explore the construction of biomimetic plexcitonic antenna complexes, in which a synthetic polymer scaffold is used to organise pigment molecules within the near field associated with a metal nanoparticle. Chlorophyll a was isolated from spinach and the central magnesium ion replaced by Zn2+ to stabilise the molecule (ZnChla). The binding of ZnChla to poly(dimethyl aminoethyl methacrylate) (PDMA) was studied by X-ray photoelectron spectroscopy and optical spectroscopy. Concentrations of ZnChla of ~1 M were obtained, somewhat in excess of the concentrations found in plant light-harvesting complexes (~0.6 M). ZnChla is thought to coordinate to the PDMA scaffolds via the formation of coordination bonds from the tertiary nitrogens to the central metal ion. When these structures were formed on gold nanostructure arrays, splitting of the plasmon band was observed, consistent with the expected plasmon- exciton coupling. The coupling strength was found to be correlated with the concentration of ZnChla in the film, consistent with the expected behaviour if strong coupling occurred. However, 2 the largest coupling energy achieved was 0.16 eV, short of the strong coupling limit and suggesting that further optimisation of the system is required. Nile Red (NR) was derivatised to introduce an iodo-alkane linker for quaternisation to the PDMA scaffold. Binding of the dye to the scaffolds was characterised by XPS and UV-Vis measurements. Extinction spectra of gold nanostructure arrays displayed dramatic changes after binding of iodinated Nile Red (INR), consistent with strong plasmon-exciton coupling. However, time did not permit a detailed investigation of the correlation between the coupling strength and the concentration of dye in the film. As an alternative approach, scaffolds were grown from glass surfaces and metal nanoparticles were subsequently embedded in the polymer layer, after which ZnChla was attached to the scaffold. For gold nanoparticles (AuNPs), beside red shift of the LSPR of AuNPs, a new feature is observed at 2.48 eV, which is probably due to week interaction resulted from possible interference between the LSPR of AuNPs and the vibrational transitions of ZnChla. For aluminium nanoparticles (AlNPs), obtaining useful optical properties of AlNPs that can be used to create plexcitonic complexes using this route is supressed by the fast oxidation of AlNPs.