Imaging and Elemental Analysis of Polymer/Fullerene Nanocomposite Memory Devices

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Imaging and Elemental Analysis of Polymer/Fullerene Nanocomposite Memory Devices Ari Laiho1, Jayanta K. Baral2,3, Himadri S. Majumdar2, Daniel Tobjörk2,3, Janne Ruokolainen1, Ronald Österbacka2, and Olli Ikkala1 1 Department of Engineering Physics and Center for New Materials, Helsinki University of Technology, P.O. Box 5100, FIN-02015 TKK, Espoo, Finland 2 Department of Physics and Center for Functional Materials, Åbo Akademi University, Porthansgatan 3, FIN-20500, Turku, Finland 3 Graduate School of Materials Research, Universities of Turku, Turku, Finland ABSTRACT In this report we study the morphology and chemical composition of a nanocomposite memory device where the active device layer is sandwiched between two aluminum electrodes and consists of a nanocomposite of polystyrene (PS) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The morphology of the active layer is imaged both in plan-view and crosssectional view by using transmission electron microscopy (TEM). We introduce two techniques to prepare the cross-sections for TEM, namely, a conventional technique based on microtoming and secondly nanostructural processing with focused ion beam (FIB). Based on the morphology studies we deduce that within the used concentrations the PCBM forms spherical nanoscale clusters within the continuous PS matrix. The chemical composition of the device is determined by using X-ray photoelectron spectroscopy (XPS) and it shows that the thermal evaporation of the aluminum electrodes does not lead to observable inclusion of the aluminum into the active material layer. INTRODUCTION The research on organic memory devices is an emergent branch in the field of organic electronics. When such a memory device is subjected to an electrical bias, the device can exhibit two distinct levels of conductivity that can then be used to define the “0” and “1” state of the memory bit. Another mechanism, besides the electrical bistability, that could possibly be used for memory applications [1] is the so called negative differential resistance (NDR) where within a certain voltage range, the current density decreases as the applied voltage is increased. So far, several different device compositions have been introduced [2-6] but their working mechanism still remains a topic of ongoing discussion [7]. It has been even suggested that the electrical characteristics of such sandwich structure devices can be explained by unintentional inclusion of aluminum into the intermediate polymer thin film while thermally evaporating the top aluminum electrode onto the polymer layer [8] or doping of the metals into the organics by application of electric field i.e. “electroforming” [9]. Moreover, Cölle et al. emphasized the role of the electrodes for obtaining reversible switching and showed that the organic material in between the sandwiched device had only minor influence [10]. They concluded that the resistive switching could be due to the breakdown of a native oxide layer at the aluminum electrode interface and transport through filaments.