Electronic junction devices featuring redox electrodes

Abstract

The electronic properties of molecular junctions of the general type carbon/molecule/TiO/Au as examples of 'molecular heterojunctions' consisting of a molecular monolayer and a semiconducting oxide. Junctions containing fluorene bonded to pyrolyzed photoresist film (PPF) were compared to those containing AlOinstead of fluorene, and those with only the TiOlayer. The responses to voltage sweep and pulse stimulation were strongly dependent on junction composition and temperature. A transient current response lasting a few milliseconds results from injection and trapping of electrons in the TiOlayer, and occurred in all three junction types studied. Conduction in PPF/TiO/Au junctions is consistent with space charge limited conduction at low voltage, then a sharp increase in current once the space charge fills all the traps. With fluorene present, there is a slower, persistent change in junction conductance which may be removed by a reverse polarity pulse. This 'memory' effect is attributed to a redox process in the TiOwhich generates Tiand/or Ti, which have much higher conductance than TiOdue to the presence of conduction band electrons. The redox process amounts to “dynamic doping” of the TiOlayer by imposed electric field. The memory effect arises from a combination of the properties of the molecular and oxide layers, and is a special property of the molecular heterojunction configuration.