ZnO-based oxide-metal vertically-aligned nanocomposites for tunable hyperbolic metamaterials

Abstract

Vertically-aligned nanocomposite thin films have recently emerged as a platform to combine multifunctionalities. These materials are made up of two co-grown immiscible phases where one phase grows as anisotropic vertical pillars in the matrix of the second phase. Many of the classic systems of VAN consist of oxide-oxide nanocomposites but more recently the oxide-metal combinations have been realized with great promise of combining metal and oxide functionalities. In particular for streamlined processing of hybrid plasmonic metamaterials with nanoscale light matter manipulation. The work presented in this dissertation is towards realizing oxide-metal VAN with ZnO as the matrix. ZnO is particularly interesting for composite design due to its breadth of properties including piezoelectricity, semiconductivity, relative non-toxicity, and wide-spread availability making it applicable for sustainable and compact device designs. In the first part of this dissertation, ZnO is combined with plasmonic Au to form ZnO-Au VAN. The ZnO-Au design demonstrates highly ordered and tunable in-plane periodicity which results in strong hyperbolic dispersion and plasmonic response, making it desirable for hybrid plasmonic hyperbolic metamaterials. The resultant morphology has spontaneous and controllable quasi-hexagonal in-plane order and a tailorable microstructure through deposition parameter control. In the second part of this work, a new oxide-nanoalloy VAN system of ZnO-AuxAu1-x is designed to reduce the optical losses of Au in hyperbolic metamaterials application. Ag is combined with Au to reduce losses and overcome particle-in matrix morphology of ZnO-Ag, with the result being reduced losses as compared to ZnO-Au. The third part investigated the tunability of the new oxide-nanoalloy VAN ZnO-AuxAg1-x through the oxygen background pressure. Both optical properties and morphology were shown to be strongly correlated with the background pressure. In the final part, ZnO is combined with ferromagnetic metals of Co, Ni, and their alloy CoxNi1-x for ferromagnetic/piezoelectric coupling. Oxidation was found to occur due to the high reactivity of Ni and Co to oxygen ambient, with ZnO-Co growing as an oxide-oxide VAN of ZnO-CoO. ZnO-Ni and ZnO-CoxNi1-x demonstrated anisotropic ferromagnetic and optical response. The work presented in the body of this thesis serve to demonstrate ZnO-based nanocomposite towards future device and heterostructure integrations in both optical and acoustic applications.