Electrical characterization of large-area donor-acceptor molecular junctions

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The electrical properties of five different donor – bridge – acceptor large-area monolayer molecular junctions show current rectification. Langmuir-Blodgett deposition onto thermally evaporated gold yielded four out of the five monolayers. The fifth system was as a monolayer formed by self-assembly onto template-stripped gold. Of the four Langmuir-Blodgett films studied, three had perylene bisimide as the common acceptor with the donors, pyrene, ferrocene, and tetramethyl-p-phenylenediamine. The fourth Langmuir-Blodgett film is an iron complex. The large extinction coefficient of the three perylene monolayer films make them visible on the water surface. They are also visible on all three substrates used for characterization. The self-assembled monolayer is a dimethoxybenzene donor bridged to a benzoquinone acceptor. X-ray photoelectron spectroscopy, atomic force microscopy, and ultra-violet – visible spectroscopy were the primary tools for surface characterization. The electrical characterization of the monolayer films showed a few things. Primarily the electrical work showed that large-area junctions give better results when a feedback current established contact. The perylene-based monolayers had similar current-voltage behavior, showing a Janus effect of the current enhancement. At low bias (V < ±1.5 V) more current passes at positive bias than at negative bias but switches to more current passing at negative bias than at positive bias at higher voltage (V > ±2.0 V). The largest bias that provided stable molecular junctions was 2.5 V. The rectification ratio increases with increasing donor strength. An additional feature is that the increasing donor strength leads to the formation of a charged state in the molecular junction. This charged state inhibits tunneling and all other mechanisms, resembling a coulomb blockade. The metal complex measured for electrical properties was only stable up to 1.0 V, but all of the molecular junctions have similar current-voltage behavior rectifying at positive bias.

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Physical chemistry