Photo-induced Deposition of Nanostructured Thin Films by UV-exposure of Heteroleptic Ti-alkoxide Solutions
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Photo-induced Deposition of Nanostructured Thin Films by UV-exposure of Heteroleptic Ti-alkoxide Solutions J. David Musgraves1, B.G. Potter, Jr. 1, and Timothy J. Boyle2 1 Department of Materials Science and Engineering, University of Arizona, Tucson, AZ, 85712 2 Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, NM, 87185 ABSTRACT Photodeposited physical relief structures have been formed via the UV-irradiation of solutions of the heteroleptic titanium alkoxide, (OPy)2Ti(TAP)2. Raman studies of the films produced confirm the photoinitiation of hydrolysis and condensation reactions in the molecular structure, leading to the development of an insoluble solid phase. Using a shadow mask technique, these films were deposited directly from solution with microscale patterning and their structure and properties were studied as a function of both irradiation conditions (UV-fluence and intensity) as well as precursor solution chemistry (water content). Variations in nanostructure are visible in scanning electron micrographs, with a higher solution water content producing a more condensed, i.e. less porous, film. INTRODUCTION The optical activation of reactive species is being investigated as a means to catalyze intermolecular bonding processes and the development of longer range network structure in solution-synthesized inorganic and molecular hybrid materials. The molecule currently under study is the mononuclear heteroleptic titanium alkoxide (OPy)2Ti(TAP)2 (see Figure 1, where OPy = pyridine carbinoxide and TAP = 2,4,6 tris(dimethylamino)phenoxide).[1,2] Patterned exposure of such photosensitive systems offers the opportunity to control material structural development over multiple length scales (i.e., nanostructural evolution to micron-scale patterning), providing new options for the manipulation of associated physical properties.
Figure 1. Molecular structure of (OPy)2Ti(TAP)2 (hydrogens removed for clarity)
This concept is being pursued in the context of sol-gel systems based on complex, heteroleptic titanium alkoxide precursors. Engineered to inhibit conventional hydrolysis and condensation kinetics via steric effects, UV-excitation of the precursor is used to disrupt the alkoxide ligand structure and to initiate the molecular assembly process in solution. Photodeposition of thin film oxide materials directly from alkoxide solutions has been demonstrated in the present work. Control of irradiation conditions and solution chemistry has been shown to influence the resulting film nanostructure. EXPERIMENT Solutions of (OPy)2Ti(TAP)2 were synthesized under inert atmosphere glove box conditions. To 1 mL of anhydrous pyridine, 84 mg of (OPy)2Ti(TAP)2 was added to generate a 106 mM stock solution. From this stock solution, water-containing solutions were produced by adding either 1.90, 7.59, or 15.18 µL of water yielding a 1:1, 4:1, or 8:1 molar ratio of water to (OPy)2Ti(TAP)2 in solution. These water-containing solutions were then stirred at 500 rpm for 30 min before use to ensure tho
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