Optically Switchable Metal Hydride Films: Properties and Structures

University Of Texas At Austin, Austin TX

Investigators

Abstract

Thin films synthesized by electron-beam evaporation will be used to explore and exploit the remarkable optical switching properties of certain recently-discovered metal-hydride thin films, whereby some thin films (e.g., yttrium) can quickly and reversibly transform from a metallic mirror to a transparent insulator, by inducing a relatively small change in the hydrogen content of the thin film. The objectives of this project include: 1) to gain an understanding of the mechanism responsible for the optical switching (electron correlation, interstitial site occupancy, and other mechanisms have been proposed); 2) to achieve an improvement in switching times; 3) to understand the stability and strain effects in the films; and 4) to provide a demonstration of pixel switching in an array of micro-mirrors. Control of hydrogen content using gas pressure, electrochemical charge-transfer, and a novel electrotransport technique will be explored. Switching speed and the diffusivity of hydrogen will be explored by investigating specific thin film alloys, substrate-film strain effects, and overlayer thickness and composition. The first arrays of micro-mirrors with driving electrodes will be patterned by photo-lithographic techniques. Various characterizational and analytical techniques, including the new capabilities offered by nuclear magnetic resonance force microscopy, as well as conventional Hall effect, x-ray diffraction, and atomic force microscopy, will be employed to delineate systematics in diffusion coefficients, transport properties, optical transmission, and surface and film morphology. Students and post doctoral research associates will participate in this research. %%% This work will explore and exploit the remarkable properties of certain thin films. These films (typically, these would be metallic coatings on windows and mirrors, or would make up the small dots ("pixels") on video displays) can be made to quickly switch from being a mirror to being transparent. This "on-off mirror" behavior is controlled by changing the amount of hydrogen that the film is allowed to contain. Such materials have immediate potential for applications, from "smart windows" and other energy-saving large-scale products, to switchable micro-mirrors for flat-panel displays. This project represents a multifaceted program to optimize and develop such materials. Objectives include: 1) understanding why the optical switching occurs; 2) making the switching times faster, in order to make the materials useful for video devices; 3) understanding the structure of the films; and 4) demonstrating pixel switching in an array of micro-mirrors. The work will study the ways that the hydrogen content can be changed; in particular, a new technique whereby the hydrogen is swept along with an electrical current will be examined. The structure of the films and variations in the speed of hydrogen motion in the material will be investigated. The first arrays of micro-mirrors with driving electrodes will be patterned to demonstrate the utility of these switchable optical materials for video displays. This research will be conducted with the assistance of graduate and undergraduate students as well as postdoctoral research associates. They will thereby receive training in one of the current forefront areas of condensed matter physics and materials science. This will facilitate their entry into the scientific/technological workforce during the coming decades of this century. ***