| Developments in thin films are allowing better control over the light, energy, and chemical interactions that take place at the sub-visible wavelength and nanoscopic levels. A better understanding of how to fabricate and alter new forms of thin film metamaterials will enhance their efficiency and open up new ways to filter and bend light to achieve the desired conversion or redirection of energy at the most fundamental level. A special breed of porous thin films are studied here that demonstrate improved light transmission through transparent media or can initiate chemical reactions by ultraviolet light absorption. This is accomplished using a fabrication technique known as oblique angle deposition which provides a way to control the amount of void space as a function of thickness. Through demonstration of the efficacy of two applications, terahertz anti-reflection and titanium dioxide photocatalysis, we are able to show the limits of the current understanding of the fabrication process, and improve on the models used to predict the porosity. Time domain transmission spectroscopy, surface area, porosity, crystalline structure, and photocatalytic activity are measured and connected back to the parameters used during the deposition process. Models for transmission and porosity demonstrate deviation from experimental results, and a better understanding of the morphology of the films leads to an improved model and can be used to engineer more efficient films. A 98% improvement in the transmission of terahertz radiation through a graded index coating demonstrates the scalability of the anti-reflection coating theory, and reveals a difference between the design criteria used to construct the multiple layers of the film. Titanium dioxide nanorod films are tested in both the liquid and gas phases for their use as photocatalyists. We show that the surface area of the film is directly related to the deposition tilt angle, and that maximum photocatalysis is achieved for 1000 nm films deposited at 70° tilt. This condition is exceeds the performance of a gram-per-gram comparison with that of commercially available anatase. |
