Novel bimetallic plasmonic nanomaterials

Plasmonic nanomaterials have attracted a lot of attention recently due to their application in various fields such as chemical and biological sensing, catalysis, energy harvesting and optical devices. However, there is a need to address several outstanding issues with these materials, including cost-effective synthesis, tunability in plasmonic characteristics, and long term stability. In this thesis, we have focused on bimetallic nanoparticles (NPs) of Ag and Co due to their immiscibility as well as their individual properties. First, a pulsed laser induced dewetting route was used to synthesize Ag-Co bimetallic plasmonic NPs. An synthesis parameter space was derived to show the range of achievable particle diameter and composition. Subsequently, the localized surface plasmon resonance (LSPR) characteristics were studied by broadband spectroscopy which showed that LSPR in Ag-Co NPs can be readily tuned over a wavelength range one order magnitude greater than that for pure Ag NPs. In another important result, significantly better optical stability with 10 times higher life-time in Ag-Co bimetallic NPs was observed as compared to pure Ag NPs under ambient conditions due to cathodic protection of Ag by galvanic coupling of Ag and Co within the NPs. During this study, various new methods, that can be broadly applied to other material systems, were developed to aid in sample preparation and analysis. One was a novel and simple technique to make NPs on electron transparent substrates (i.e. Carbon) for improved the quality of TEM and EELS analysis. A method to quantitatively analyze low-loss EELS spectra was also developed in which necessary number of Lorentzians were fitted in spectrum to obtain detailed information of surface plasmons. By EELS analysis of bimetallic Ag-Co NPs, the first known discovery of a ferroplasmon, i.e. strong visible and ultraviolet wavelength plasmons in ferromagnetic materials, was made. It was observed that Co, when in contact with Ag in a NP, shows strong surface plasmons, which was highly unusual based on the known highly damped plasmonic nature of pure Co. These investigations helped advancing the science of bimetallic plasmonic nanomaterials and unequivocally demonstrate Ag-Co NP system as a promising material for plasmonics based applications.