Influence of Substrate on In-Plane Electrical Conduction of CuPc Nano-Crystals
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G8.11.1
Influence of Substrate on In-Plane Electrical Conduction of CuPc Nano-Crystals Masakazu Nakamura, Masatoshi Watanabe, Tsuyoshi Maruyama, Masaaki Iizuka and Kazuhiro Kudo Department of Electronics and Mechanical Engineering, Faculty of Engineering, Chiba University, Chiba, 263-8522, JAPAN. ABSTRACT We have investigated the conductivity variation of copper phthalocyanine nano-crystals deposited on SiO2 and mica. The density of surface hydroxyl group on SiO2 was estimated to be more than 10 times larger than that on mica by X-ray photoelectron spectroscopy. The grown crystals on both substrates were categorized to be a-axis orientation of α–form. Conductance-distance (G-d) measurements for various crystalline grains having different heights have been carried out with current imaging using atomic force microscope. The obtained conductivity vs. crystal height plot indicates that lower height crystals have higher conductivities on both surfaces. Besides, the lower height crystals on SiO2 have higher conductivity than those on mica. These results suggest that the electrons in the bottommost molecular layer are transferred to the electrophilic functional groups on the substrate surface. Such a phenomenon at organic semiconductor/insulator interface is reported for the first time.
INTRODUCTION Since the ratio of interfacial molecules to ‘bulk’ ones is not negligible for nano-scale molecular clusters or ultra-thin films, the interaction between the film and the underlying substrate strongly influences the electrical properties of the film. Copper phthalocyanine (CuPc) is a typical organic semiconductor of which conductivity is known to be sensitive to adsorption of electronegative molecules such as NO2 [1]. We, therefore, assume that the same carrier doping effect would take place even at the film/substrate interface due to the charge transfer between the molecules and electrophilic functional groups on the substrate surface. Such a doping effect will emerge from that induced by bulk impurity when we measure electrical conductance of ultra-thin films of organic semiconductors grown on strongly electronegative or electropositive surfaces of insulating substrates. The interfacial doping affects the characteristics of an organic thin-film transistor (OTFT) because the carrier channel of an OTFT is formed near the interface with gate insulator. However, to the best of our knowledge, experimental works that reveal such an effect have not been done so far. In this work, we have therefore investigated the conductivity variation of CuPc nano-crystals deposited on SiO2, of which surface has dense hydroxyl groups (–OH), and on mica, few –OH, as a function of crystal height. Conductance-distance (G-d) measurements between Pt electrodes and Au-coated atomic force microscope (AFM) cantilevers were carried out under high vacuum, and conductivities of crystals were estimated by fitting a theoretical curve to the G-d curves. The results were strongly suggestive of the existence of the interfacial doping effect.
G8.11.2
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