Mechanism of Surfactant Removal from Ordered Nanocomposite Silica Thin Films by Deep-UV Light Exposure
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Mechanism of Surfactant Removal from Ordered Nanocomposite Silica Thin Films by Deep-UV Light Exposure Andrew M. Dattelbaum,1 Meri L. Amweg,1,2 Julia D. Ruiz,1 Laurel E. Ecke,1 Andrew P. Shreve1 and Atul N. Parikh2 1 2
Bioscience Division, Los Alamos National Laboratory, Los Alamos, NM 87545 and Department of Applied Science, University of California, Davis, CA 95616
ABSTRACT In recent years, methods have been developed for the generation of complex ordered nanocomposite materials through organic templating of inorganic structures. One approach involves preparation of composite materials by an evaporation induced self-assembly process involving organization of organic surfactants and formation of inorganic silica from soluble precursors. Recently, we have shown that deep-UV light (185-254nm) is efficient at removing the surfactant microphase for a routine production of well-ordered mesoporous silica thin films. Here we probe the evolution of surfactant removal from nanocomposite thin film silica mesophases as a function of deep-UV exposure using a combined application of FTIR and single wavelength ellipsometry. Taken together, these data indicate that surfactant removal occurs in a step-wise fashion with the formation of oxidized intermediates prior to complete removal of the surfactant from the thin film. INTRODUCTION Ordered nanocomposite thin films have been an active area of research due in part to potential application of these materials in catalysis, separations, and photonic applications.1 One approach for the preparation of thin film mesophases uses an evaporation induced self-assembly process involving hierarchical organization of organic surfactant and inorganic silica building blocks.2 Such materials consist of a self-assembled, ordered organic phase that templates the structure of inorganic silica prepared by sol-gel condensation processes. For many applications the utility of these films relies on the ability to remove the organic surfactant phase while maintaining the porous structure of the inorganic framework. High temperature calcination (~450°C) is the most common procedure for template removal. However, high temperature calcination is not applicable for surfactant removal from ordered nanocomposite films deposited on highly temperature sensitive surfaces, like thin gold films, or for the selective removal of surfactant from specific regions of the ordered nanocomposite film. To overcome the limitations of thermal calcination, several surfactant removal techniques have been developed, including solvent extraction3 and UV-treatments.4-6 UV-treatments are particularly attractive and have been extensively used to beak down small amounts of organic impurities on semiconductor surfaces into low vapor pressure molecules, like CO2 and H2O.7 We have recently been employing a photochemical technique that uses a simple Hg-lamp to generate deep-UV light (185-254nm), which efficiently removes the surfactant phase from nanocomposite silica thin films.6 Our process uses inexpensive and readily availab
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