Hybrid phthalocyanine/lead sulphide nanocomposite for bistable memory switches

Abstract

A simple, one-step method is employed to produce, at room temperature, a single layer of an organic-inorganic nanocomposite containing non-aggregated lead sulphide (PbS) quantum dots (QDs) embedded in a 130 nm thick solution processed film of the organic semiconductor 6PcH2 (metal-free, non-peripherally substituted octahexyl phthalocyanine) on indium tin oxide. The mean size of PbS QDs is found from x-ray diffraction and transmission electron microscopy techniques to be much smaller than the Bohr radius. Further evidence of the quantum confinement effect is provided by a blue shift in the absorption spectrum and the increased band gap of 1.95 eV with respect to bulk PbS. The current–voltage characteristics of the hybrid and pristine 6PcH2 films, both in a sandwich configuration with the aluminium top electrode, exhibit bistable switching type hysteresis. The on-off ratio of the nanocomposite device is at least three orders of magnitude higher than that for 6PcH2 organic films, while both devices operate at a very low bias voltage of 0.5 V. The inclusion of the PbS QDs into the 6PcH2 film enhances the conductivity by nearly two orders of magnitude over that of a comparable pristine 6PcH2 film due to the formation of a charge transfer complex with PbS QDs and 6PcH2 acting as acceptors and donors of electrons, respectively.