Optical properties of hybrid titanium chevron sculptured thin films coated with a semiconducting polymer
Introduction
Engineering three-dimensional geometries on the nanometer scale is possible with advanced deposition techniques such as glancing angle deposition [1], [2]. Sculptured thin films (STFs) can be tailored from many materials by dynamically varying the orientation of the substrate during growth. Such nanostructured thin films possess physical properties, which are closely related to their morphology, and which vary significantly from bulk properties due to size and confinement effects. Therefore, physical properties of nanostructured materials can be controlled by engineering of, for instance, the morphology, porosity, and composition [3]. Photonic characteristics, for example, can be modified and tuned by infiltrating a dielectric material in void [4], [5]. Active control of optical properties of hybrid materials can be achieved by combining nanoparticles with polymers, which change their properties upon exposure to gases [6], and inorganic porous layers with temperature sensitive liquid crystals [7]. The optical constants of hybrid nanoporous thin films are effective optical constants, which depend not only on the geometry but also on the dielectric properties of the material infiltrated into void. Determination of changes of the anisotropic optical response upon infiltration of semiconducting poly(3-dodecylthiophene) (P3DDT), a commonly used hole-conducting material [8], in STFs is the subject of this paper. This might be particularly interesting for the emerging field of hybrid photovoltaic applications [9].
Section snippets
Theory
Generalized ellipsometry (GE), a non-destructive and non-invasive optical technique, has proven to be highly suitable for determining optical properties of highly anisotropic structured films from metals such as slanted columnar thin films (SCTFs) or helical (chiral) STFs [10], [11], [12]. Measurement of the complex ratio ρ of the s- and p-polarized reflection coefficients is presented here in terms of the Stokes descriptive system, where real-valued Mueller matrix elements Mij connect the
Experimental
Titanium chevron nanostructures were deposited by electron-beam glancing angle deposition in a customized ultra-high vacuum chamber. The (100) p-type silicon substrate with 2.5 nm native oxide was mounted on a water-cooled sample manipulator, which allows for adjustment of the deposition angle as well as the sample in-plane orientation. The deposition angle, measured between the incident particle flux direction and the substrate normal, was set to 85°. The particle flux during growth was kept
Results and discussion
The optical constants of the 20.3 nm thick P3DDT layer have been determined prior to this study by analyzing experimental ellipsometry data taken on our reference sample and are in agreement with P3DDT optical constants recently published by Müller et al. [8].
Cross-sectional SEM images of the Ti chevrons after hybridization by spin coating semiconducting P3DDT reveal that the nanostructures are homogeneously coated with a cladding. The polymer penetrated all the way through to the film/substrate
Conclusion
We have studied structural and optical properties of as-deposited and hybridized titanium chevron STFs. The inorganic nanoscaffold remains unaffected by the hybridization process (infiltrating the semiconducting polymer P3DDT by spin-coating) and structural information derived from best-model calculations is in excellent agreement with SEM investigations. A complete set of optical constants has been obtained for the anisotropic STFs before and after hybridization. Refractive indices ni and
Acknowledgments
D.S., T.H., E.S., and M.S. gratefully acknowledge financial support from the National Science Foundation in CAREER (ECCS-0846329), and MRSEC (DMR-0820521), the University of Nebraska-Lincoln, and the J. A. Woollam Foundation. C.M. and O.I. would like to acknowledge funding from the Swedish Science Foundation (SSF) through the programme Organic Hybrid Printed Electronics and Nanoelectronics (OPEN). The Knut and Alice Wallenberg Foundation is acknowledged for support to instrumentation.
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