Sequential Deposition of Gold and Polymer Thin Films Onto CaF2 and BaF2 Substrates
Dr. Chad Watson
To protect military personnel and civilians from chemical and biological weapons, the Department of Defense is interested in developing new self-decontaminating materials capable of neutralizing chemical and/or biological weapons upon contact. To further this effort, Boise Technology, Inc. (BTI) is developing an understanding of the molecular level changes that occur at the surface of potential self-decontaminating materials, such as polyethyleneimine (PEI), exposed to different environments. BTI utilizes a surface specific, nonlinear optical spectroscopic technique called vibrational sum frequency generation (VSFG) to investigate polymer properties (e.g., surface molecular orientation, conformation, and functionality) and surface interactions with the surrounding environment in a controlled atmosphere (air, variable humidity, nitrogen, and chemical weapon simulant vapors). VSFG resolves surface mediated effects because the signal generated arises exclusively from the first few molecular layers at an interface. BTI is using CaF2 or BaF2 windows as transmissive substrates on which to coat the polymer samples, due to the high optical transmission of CaF2 and BaF2 in both the red (600-800 nm) and mid-infrared (2.5-10 µm) regions of the electromagnetic spectrum. In conjunction with the resonant VSFG signal obtained from the polymer, the non-resonant signal from gold is used to normalize the VSFG spectrum. In an effort to enhance data reproducibility and accuracy during experimentation, a new design of depositing both PEI and gold on a single optical substrate is under development. The fabrication process involves sputtering a thin layer of gold onto a masked CaF2 or BaF2 substrate with sufficient thickness to reflect incident light (~800 nm), followed by spin coating PEI (~100-400 nm thick) onto the substrate. The thickness, uniformity, and surface roughness of the films will be measured and characterized using scanning electron microscopy (SEM), optical profilometry, ellipsometry, and reflectometry. This design will enable BTI to considerably decrease data acquisition time and cost while greatly improving data accuracy and reproducibility.