High Quality Aluminum Doped Zinc Oxide Plasmonic and Hyperbolic Metamaterials via Atomic Layer Deposition
The ability to transform energy from electromagnetic radiation into oscillations of free electrons at the surface of certain materials forms the basis of surface plasmonics. The majority of research in this area has utilized noble metals such as gold and silver owing to their appropriate optical response and low loss at visible frequencies. However, these materials are not suited for many applications operating out of the visible spectrum and at high temperatures. Therefore many researchers are investigating new plasmonic materials. At the forefront of this search is aluminum-doped zinc oxide. By adjusting the free electron concentration, this high melting point material can be tuned to operate throughout the infrared spectrum up to the important telecommunication wavelength (ca. 1550 nm) with lower loss than any other material studied. However, to date, high quality aluminum-doped zinc oxide capable of operating at telecommunications wavelengths has only been demonstrated with the non-scalable and non-conformal method of pulsed laser deposition. Herein, a simple process is devised that enables the use of atomic layer deposition, an ultraconformal, highly scalable technique, to synthesize tunable aluminum doped zinc oxide thin films with plasmonic properties that rival the best films achieved by pulsed laser deposition. This method is proven to be a powerful tool for plasmonic applications by exploiting the ultraconformal properties of atomic layer deposition to make materials that cannot be made by any other process. This is accomplished by creating tunable localized surface plasmonic resonant cavities made of vertically aligned aluminum doped zinc oxide coated silicon nanopillars and solid aluminum doped zinc oxide nanotubes. In addition, hyperbolic metamaterials comprised of aluminum doped zinc oxide and zinc oxide are created in the multilayer and the embedded nanowire geometry. Finally, this method is used to create the first demonstration of a new class of transferable hyperbolic metamaterials particles in the form of vertically aligned hyperbolic nanotubes. When closely packed, these particles show broadband absorbance with a single monolayer. More importantly, they can be transferred to virtually any substrate paving the way for flexible and visibly transparent materials.