Optical Characterization of Localized Surface Plasmon Resonances in Doped Metal Oxide Nanocrystals

AbstractOptical Characterization of Localized Surface Plasmon Resonances in Doped Metal Oxide NanocrystalsbyRobert Walker JohnsDoctor of Philosophy in ChemistryUniversity of California, BerkeleyProfessor Tanja Cuk, Co-Chair Professor Delia Milliron, Co-ChairElectronically doped metal oxide nanocrystals exhibit tunable infrared localized surface plasmon resonances (LSPRs). Semiconductors provide an alternative dielectric environment than metallically bonded solids, such as noble metals, for metallic behavior. The ways in which the electronic structure of the semiconductor and of the dopants used to make them metallic hybridize substantially changes the plasmonic behavior. Choice of dopant element, dopant placement within the nanocrystal, and dopant interaction with other defects in the lattice all lead to changes in the observed optical properties of these nanocrystals. Here, the methods for optical characterization of LSPRs in doped metal oxides are discussed with particular attention directed at how undetermined heterogeneous contributions to ensemble measurements lead to misattributing inhomogenous broadening to poor plasmonic performance. Electronic damping in these materials is incredibly low compared to coinage metals, and they tout the added benefit of spectral tuning through chemical composition rather than morphology. The result is a class of materials that can both have their optical response tuned separately from other application relevant factors like nanocrystal size, and yield high performance LSPR for directing far-field radiation to the near-field. Learning that doped metal oxides have high quality factor LSPR was found through the first single nanocrystal measurements of LSPR made in the mid-IR through the use of near-field optics to interrogate these nanocrystals separately. The result was uncovering substantial nanocrystal-to-nanocrystal variation within batches of nanocrystals making ensemble measurements appear to have broad LSPR, while in fact these materials have high quality factors individually. These measurements were enabled by broadband synchrotron based scattering type- scanning near field optical microscopy (s-SNOM). Broadband s-SNOM in the IR can yield the single nanocrystal optical spectrum and dielectric function of an isolated signal nanocrystal when the proper considerations are made to backgrounding signal over such a wide spectral range. The methodology as well as new understanding of the materials learned through this instrumentation advance are outlined. Finally, the lessons learned about the properties of LSPR in doped metal oxides from single nanocrystal measurements are extended to an adaptation of applying Mie theory to the nanocrystal dielectric function in order to assign reasonable dielectric constants to nanocrystals even from ensemble optical measurements over any energy range, not just those obtained from mid-IR s-SNOM. Further, these advances in assigning optical density to an ensembles of doped metal oxide nanocrystals are applied to understanding how energy relaxation out of the LSPRs occurs in these materials through the use of the two-temperature model, using constants obtained from NIR ultrafast transient absorption measurements. The low free carrier concentrations of metal oxide nanocrystals lead to less efficient heat generation as compared to metallic nanocrystals such as Ag. This suggests that metal oxide nanocrystals may be ideal for applications wherein untoward heat generation may disrupt the application’s overall performance, such as solar energy conversion and photonic gating.