Nanoscale scanning near-field ellipsometric microscopy (SNEM) imaging of heterogeneous polymers
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Nanoscale scanning near-field ellipsometric microscopy (SNEM) imaging of heterogeneous polymers Aysegul Cumurcu1, Joost Duvigneau1, Ian D. Lindsay2, Peter Schön1 and G. Julius Vancso*,1 1
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Materials Science and Technology of Polymers, MESA Institute for Nanotechnology, University of Twente, Enschede, NL-7500, The Netherlands. 2 Nanophysics and Soft Matter Group, H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom. *Corresponding author: Prof. G. Julius Vancso, Tel: +31(0)53-4892967 Fax: +31 (0)53 489 3823 E-mail: [email protected]
ABSTRACT In this study a scanning near-field ellipsometric microscope (SNEM), a hybrid device of an atomic force microscope (AFM) and an ellipsometer, is used to obtain optical images of heterogeneous polymer thin films with a resolution below the diffraction limit of light. SNEM optical images of a microphase separated PS-b-P2VP block copolymer film collected with gold coated and bare silicon AFM probe tips were compared to obtain a deeper insight into the nature of the SNEM contrast mechanism. Furthermore, intensity vs. distance curves were recorded on a PS-b-PMMA block copolymer film simultaneously during the acquisition of force-displacement curves to study the far-field contribution of the optical signal to the optical image. INTRODUCTION In the past 20 years, the atomic force microscope (AFM) has evolved into a powerful tool for the nanoscale investigation of polymeric materials down to the single macromolecule level[1]. Although AFM can provide images with nanometer spatial resolution of the topographic and mechanical properties of heterogeneous materials, there is a fundamental need to obtain further material specific information, for instance optical properties with nanoscale resolution[2]. In this regard the integration of AFM with optical instrumentation into hybrid devices has opened novel avenues for nanoscale material characterization. Karageorgiev et al. introduced a hybrid apertureless scanning near-field optical microscopy (a-SNOM) system consisting of a combination of an AFM with an ellipsometer to study the optical properties of transparent thin films[3]. In their instrumental arrangement an evanescent field created at the sample surface was scattered by an AFM tip and collected by the ellipsometer detector. An alternative SNEM configuration where the dielectric constants and thicknesses of thin films were measured by using an aperture fiber probe was proposed by Liu et al[4]. More recently, the same authors have reported a SNEM set-up that is operated in a reflection configuration allowing the characterization of bulky or opaque samples[5]. We proposed an alternative SNEM configuration consisting of an AFM combined with an ellipsometer in which the AFM probe tip is illuminated at an incident angle θ smaller than the critical angle[6, 7]. This approach is based on localized electric field enhancement at the apex of
the gold coated AFM tip and, therefore, provides an increased spatial resolution. Local field enha
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