Design and fabrication of nanostructured thin films and applications in optics and interfacial phenomena

Abstract

In the second part, we designed a lossless, zero-index photonic crystal structure to enable the emission-pattern control near 920 nm, and we proved the feasibility of fabricating the designed structure with OAD method by successfully making its prototype. In the third part, a composite film made of a stable, gold nanoparticle array with well-controlled separation and size on the top a nanocolumnar TiO2 OAD film was fabricated. The fabrication of the nanoparticle array is based on governing the solid-state dewetting of as-deposited gold aggregates on the TiO2 nanorods with Rayleigh-instability. We found that the topology of TiO2 nanorod tips can induce contact line pinning and increase the contact angle along the vapor flux direction to the supported gold aggregates, and dewetting dynamics will therefore act differently on TiO2 nanorod tips than on a featureless, planar TiO2. Contact line pinning and contact angle increase can inhibit the coalescence between dewetting geometries and facilitate the instability growth on the supported materials, which makes nanocolumnar OAD films as desired templated substrates to govern Rayleigh-instability and produce monodisperse NPs.

The main body of this dissertation is composed by three parts. The first and second part focus on using OAD method to fabricate sculptured thin films with engineered refractive indices, and applying the product films to address some key challenges in the LED industry. The third part focuses on studying how the surface topology of OAD films could possibly mediate solid-state dewetting. In the first part, a zig-zag nanospiral-structured graded-index anti-reflection coating was fabricated on the top surface of a YAG:Ce ceramic phosphor plate (CPP) in the effort of enhancing its transmission in the normal direction. The anti-reflection coating enhances the normal-direction transmission of YAG:Ce CPP over the visible-light region of the spectrum. At 764 nm, the transmission is enhanced by 7.82%, compared to the potential maximum enhancement of 8.53% . For 633-nm light incident at 5°, the reported coating is measured to induce a scattering loss of no greater than 1.27% .

The interaction between light and matter is not only dependent on the chemical composition of the matter, but also on the size and topology. In this sense, nanotechnology is the driving force of the development of modern optics as people are enabled to structure matter at atomic scale with engineering methods. Oblique angle deposition (OAD), a fabrication technique widely used in physical vapor deposition systems, has emerged as a powerful tool for the fabrication of nanostructured thin films. It allows the realization of arbitrary film porosity, and thereby a continuous spectrum of refractive index. The ability to control refractive index is especially important in LED industry where enhancing light-extraction efficiency and enabling emission pattern control are the primary interest. On the other hand, nanocolumnar thin films fabricated by OAD method also serve as a valuable tool for the studies of interfacial phenomena. Surface topology of these nanocolumnar thin films affects wetting or dewetting mechanisms of the supported materials. By supporting noble-metal nanoislands onto the nanocolumns, noble-metal nanoparticle arrays could be produced on the nanocolumns by letting the nanocolumn tips mediate the dewetting process.