Vapor-Phase Functionalization of Nanostructured Gradient-Index Titanium Dioxide Thin Films
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0928-GG09-15
Vapor-Phase Functionalization of Nanostructured Gradient-Index Titanium Dioxide Thin Films Andy C. van Popta1, John J. Steele1, Shufen Tsoi1, Enrico Fok2, Jonathan G. C. Veinot2, Michael J. Brett1, and Jeremy C. Sit1 1 Electrical and Computer Engineering, University of Alberta, 9107-116 Street, Edmonton, Alberta, T6G 2V4, Canada 2 Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, T6G 2G2, Canada ABSTRACT Chemical treatments, when applied to nanostructured oxide thin films, can be used to generate added functionality in many devices. In this study, a nanostructured defect-mode optical filter was prepared by glancing angle deposition of titanium dioxide and functionalized with 3,3,3-trifluoropropyl-trichlorosilane to render the thin film insensitive to variable humidity conditions. Electrical characterization and contact angle measurements demonstrate that the hydrophilic thin film becomes hydrophobic when functionalized, and transmission measurements clearly show that the wavelength shift of the defect-mode becomes strongly inhibited for a wide range of humidity levels. INTRODUCTION Thin films are found in a wide range of applications, and are particularly important in the field of optics, where thin film layers are routinely used as antireflection coatings, hot/cold mirrors, bandpass filters, and polarizers. In the majority of cases, the environmental stability of an optical thin film is very important to its performance. Changes in relative humidity are especially detrimental to optical interference filters because the adsorbed water vapor can increase the effective refractive index of the coating, producing an undesirable red-shift in the spectral properties. Water permeation can also lead to altered thin film stress and a lower damage threshold in laser coatings [1]. Conventional methods for reducing humidity effects focus on eliminating thin film pores by using high-energy processes, such as ion-assisted deposition [2]. However, devices incorporating materials such as sol-gel films, photonic crystals, porous silicon, and moth-eye antireflection coatings, are inherently porous and require alternative methods to stabilize their properties under variable humidity conditions. We have fabricated a porous optical interference filter using glancing angle deposition (GLAD), which combines low adatom diffusion, a collimated oblique-incidence vapor stream, and in situ substrate motion to create thin films composed of engineered nanostructures [3]. GLAD is capable of creating optical interference filters using a single material system by changing the film porosity in a controlled manner [4]. The result is an optical coating with nanoscale porosity variations and a high sensitivity to changes in ambient humidity. We will show that it is possible to reduce water permeation by functionalizing the internal surface area of a porous GLAD optical filter with a hydrophobic chemical: 3,3,3-trifluoropropyl-trichlorosilane (TFP-SiCl3). While the goal of applying TFP-SiCl3 is to stabilize the
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