Formation Mechanism, Structure and Optical properties of Functional Nanoparticle Assemblies

Abstract

Metal nanoparticles (MNPs) concentrate light near their surface because of localized surface plasmon resonances (LSPR). LSPRs depend on MNP morphology, composition, size, and the metal–dielectric interface. LSPRs are further associated with a large optical extinction cross section per unit volume (~10^15 cm^(-1)). Antenna effect of LSPRs has been extensively utilized to direct energy flow around metal particles as well as to locally enhance optical nonlinearities. The utility of LSPRs may be further enhanced by organizing MNPs into assemblies where near-field inter-particle interactions give rise to emergent properties. Assemblies, sometimes referred to as metamaterials, exhibit phenomena ranging from negative index, enhanced dielectric sensitivity, higher harmonic generation as well as proximal field enhancements. Applications include sensing, catalysis, cloaking, super-resolution and medical diagnostics. Emergent properties depend greatly not just on the shape of plasmonic building blocks but also on the separation of building blocks as well as optical parameters associated with the matrix, if any. Development of wet chemical tools that enable the synthesis of practically useful plasmonic assemblies is thus an active area of research. In my work, I first study the mechanistic principles of assembly formation. Structural factors arising from surfactant polymorphism are discussed. Further, I describe mechanistic processes enabling particle nucleation under non-optimal conditions. Assemblies that enable the attainment of strong interparticle coupling are realized. A new analyte sensing mechanism based on these assemblies is proposed and demonstrated. Optical processes occurring within these assemblies are further analysed and generalizations based on optical insights are proposed.