AFRL-AFOSR-JP-TR-2016-0104 Synthetic Approach to Controlled Assembly of Metal Nanoparticles

The aim of this one-year project is to develop synthetic methods to form well-defined colloidal assemblies of metal nanoparticles and to understand their unique optical properties focusing on magnetic resonance scattering and surface enhanced Raman scattering (SERS). Our synthetic approach is based on the templated surfactant-assisted seed growth method, where polymer particles decorated with small metal nanoparticles are used as seed-decorated templates to grow metal nanoparticles of varying sizes and shapes. Of particular interest in this study is closely packed gold nanobeads assembled on a polymer core, which is termed raspberry-like metamolecules (raspberry-MM) due to their strong magnetic resonances. Here, we first report surprisingly weak distance dependence in Raman enhancement from the raspberry-like gold nanoparticles. Due to the abundant “built-in” hot spots between adjacent gold nanobeads, bright and uniform Raman signals were observed from isolated single raspberry-MMs. Interestingly, dimers of raspberry-MMs also showed highly reproducible Raman signals, indicating that the dimer SERS signal is not strongly dependent on the nanoparticle separation. Finite-difference time-domain (FDTD) modeling shows that a strong hot spot is created at the dimer gap, as expected. However, since there are many more built-in hot spots in each raspberry-MM, the contribution of the dimer gap hot spot to the total Raman enhancement remains low even for 2 nm separation, which explains the observed weak distance dependence. This result is in stark contrast with many previous SERS studies on nanoparticle dimers and clusters, and provides an important guideline on how to design bright and highly reproducible Raman substrates. Secondly, we have developed a synthetic method to prepare “silver” raspberry-type nanoparticles. Silver particles possess higher extinction cross section than gold particles, and show surface plasmon resonance (SPR) peaks blue-shifted from those of gold particles. Therefore, this capability allows for the fabrication of raspberry-MMs with enhanced magnetic resonances in the visible region. DISTRIBUTION A. Approved for public release: distribution unlimited.