Surface Modifications on Charge-Transfer Complexes Using Scanning Probe Microscopy
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SURFACE MODIFICATIONS ON CHARGE-TRANSFER COMPLEXES USING SCANNING PROBE MICROSCOPY SHOJI YAMAGUCHI*, CARLOS A. VALENZUELA AND RICHARD S. POTEMBER The Johns Hopkins University, Applied Physics Laboratory Johns Hopkins Road, Laurel, Maryland 20723, U.S.A. *Mitsubishi Petrochemical Co., Ltd. Tsukuba Research Center, Advanced Materials Laboratory, 8-3-1 Chuo, Ami, Inashiki, Ibaraki 300-03, Japan ABSTRACT We are exploring high density information storage systems based on organometallic materials. Silver and copper salts of tetracyanoquinodimethane (TCNQ) and its derivatives exhibit an electrical and optical field induced reversible switching phenomenon. We have demonstrated a field-induced, charge-transfer reaction driven by the electric field at the STM tip when the field generated by the STM exceeds the switching threshold of the organic charge-transfer complex. The phase transition induced by the STM tip appears as nanometer-sized domains on the metal-TCNQ and derivatives surface. We also have shown this phase transition occur by means of optical laser irradiation. This paper discusses our plans to combine our research results in optical switching with the scanning near-field optical microscope (NSOM) to develop a very high density optical memory system. In order to assess the feasibility of this, we performed a series of experiments aimed at determining the limitations of information storage using this class of organic charge transfer complexes. INTRODUCTION Today's magnetic storage devices have a storage density of 104-7 bit/cm 2 and optical devices have densities of up to 108 bit/cm 2 . The next generation of high density storage technology will have to be beyond the 1010 bit/cm 2 limit. Different methods have been proposed: multi-optical devices, photochemical hole burning (PHB), and scanning probe microscopy (SPM)-based systems. Most of the presented technologies have significant drawbacks. Some require low temperature environments, others lack the possibility to rewrite the media. Our media exhibits a reversible phase-transition and operates in room temperature. The system that we have proposed combines current optical technology with the advantages of the STM by using the near-field scanning optical2 microscope (NSOM). It is estimated that the highest storage density will be 1015 bit/cm [1] with some scanning probe technique, and will be comparable to the estimated storage density approaching the molecular level. EXPERIMENTAL Materials Polycrystalline charge-transfer complexes films were prepared by a solution growth method. Recrystallized 7,7,8,8- tetracyanoquinodimethane (TCNQ) was dissolved in the dried acetonitrile. Cleaned copper and silver plate (10 x 25 x 0.1 mm) was placed into the above solution for approximately 2 days. Thin films of copper-TCNQ (CuTCNQ) and silver-TCNQ (AgTCNQ) were prepared on several substrates, including highly oriented pyrolytic graphite, single crystal molybdenum disulfide, single crystal potassium chloride, silicon wafer and mica. Samples were prepared by a vacuum co-depositio
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