Engineering Dielectric Properties of Nanomaterials

Abstract

Metallic nanostructures can exhibit plasmonic modes due to coherent oscillations of the free electrons under electromagnetic excitations. Therefore, they find diverse applications in optical sensing, photonics, and photocatalysis etc. Further metallic interfaces have unusual dielectric properties that are partly understood by conventional means. For example, Au and Ag interfaces on the nanoscale exhibit dielectric functions distinct from the bulk metals. However, these effects remain somewhat small pronounced even for a core-shell structure. As a novel approach, our group prepared an engineered Au-Ag nanostructure with > 10 times the interfacial area of ordinary nanoparticles. In chapter 2, I have presented a simplified route to fabricate nanostructured devices, following a colloidal destabilisation route for optical and electrical applications. The material, discussed here, is comprised of ~ 1 nm sized Ag nanoclusters embedded into Au matrix. The optical properties of these nanostructures and their oxygen sensitivity are discussed. Further a stabilization procedure via encapsulation was employed to prevent the oxidation. These nanostructures exhibited unconventional electrical and magnetic properties that indicated the emergence of a possible superconducting state at ambient temperature and pressure. In chapter 3, I have investigated the dielectric properties of these nanostructure at a single particle level via Electron Energy Loss Spectroscopy. Here I have observed a significant damping and a consequential broadening of Mie resonance due to increased electron scattering at the interfaces. To explain the findings, I have employed the Coronado-Schatz dielectric model, a linear perturbation to the traditional Drude model. We further find that Mie resonances can be restored gradually by growing a thin shell of Au on the nanostructures. In chapter 4, I have structurally analysed the formation of Janus-like Au-Ag nanoparticles after degradation. Through DFT calculations it was shown that charge transfer can take place at Au-Ag interface. This can accelerate the surface oxidation of these nanostructure and the formation of large sheet like structure of Ag2O. However, incorporation of Au restricts this sheet formation up to nanoscale level. In a different study these nanostructures were proven to improve the efficiency of Zn-Ag2O batteries. By a detailed structural analysis, I have tried to rationalize this finding and the advantages of Au incorporation. In chapter 5, I have investigated the applicability of the Coronado-Schatz model at radio frequencies. It was calculated that the Coronado-Schatz dielectric values can deviate from Drude dielectric values by ~ 100 % at kHz-MHz frequency range. Here I have performed 2-probe impedance analysis measurements to visualise such deviation. Unlike bulk Au, these nanostructures exhibit negative capacitance response, accompanied by a 10-fold reduction in conductivity values. I have carried out the measurements for these nanostructures with 3 different molar ratios. In all the cases, our observations suggest a quantum mechanical approach to the dielectric model, as proposed by Kreibig, instead of the Coronado-Schatz dielectric picture.