Photostructural modifications in poly(methylphenylsilylene) thin films: Excitation wavelength and atmosphere dependence
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K. Simmons-Potter Electrical and Computer Engineering Department, University of Arizona, Tucson, Arizona 85718
G.M. Jamison and W.J. Thomes, Jr. Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received 28 April 2006; accepted 7 June 2006)
Electronic (UV-vis) and vibrational absorption spectroscopies were used to examine the impact of incident photon energy and local atmospheric composition on the development of photo-induced structural changes in poly[(methyl)(phenyl)silylene] thin films. The relative impact of atmosphere on the magnitude and nature of photostructural modifications in this material is found to be enhanced under 3.68 eV photon exposure where the incident photon is resonant with the lowest energy absorption of the Si-Si-conjugated backbone structure. This is in contrast to a greater overall magnitude of photo-induced structural change, with limited atmospheric dependence, observed under 5.1 eV exposure, resonant with absorption transitions associated with the -conjugated phenyl ring side group. These results provide insight into the underlying structural mechanisms contributing to the large refractive index changes typically observed in these materials.
I. INTRODUCTION
In the context of rapid, in situ formation of photonic device structures under nonlaboratory environments, the production of a photo-induced refractive index change in polysilane thin films has been examined. These efforts have confirmed the primary mechanism for photodegradation in linear chain polysilylene systems, i.e., the photolysis of the Si-Si-conjugated bond structure responsible for the lowest energy intrinsic optical absorption.1–5 Our previous work has demonstrated that the magnitude of the observed change in material refractive index resulting from this photo event is dependent upon the local atmospheric composition during the optical exposure. A difference in photo-induced index modification of nearly 50% was exhibited, dependent upon the oxygen content of the surrounding atmosphere.6,7 Thus, it is apparent that although the bond scission process may initiate the photostructural modification of these polysilylene materials, the final photoreaction products, and the associated photo-induced changes in optical constants, can be significantly influenced by the local environment. In addition, we have shown that the magnitudes of the observed
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0288 J. Mater. Res., Vol. 21, No. 9, Sep 2006
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photo-induced optical property changes are also dramatically influenced by the excitation photon energy used during exposure.8 In the application of present interest, the optical processing of refractive index structures is intended only after deployment of the material in the field and, thus, under potentially uncontrolled local environmental conditions and with available solid-state optical sources. Therefore, the sensitivity of the observed refractive index change to
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