Charge injection, transport and trapping in nanoparticle based memory devices

Blends of metallic nanoparticles in a semiconducting organic host show bistable electrical resistance at low ($\sim $1 V) reading voltage. When a layer of the blend, of order 100 nm thick, is sandwiched between electrodes, a low resistance (on) state is set by applying a voltage pulse of order 2 V to 3 V, and is switched to the high resistance (off) state by a pulse of about 6 V to 8 V. This behavior is observed for several different metals (e.g. Au, Ag, Al, Mg) and for both polymeric and small-molecule semiconductors. We interpret the switching and bistability in terms of charge trapping and storage on the nanoparticles. When trapped charge density is high, the resulting space-charge field inhibits charge injection, yielding the off-state, and vice versa. A simple model based on Fowler-Nordheim tunneling shows that particles of order 2 - 5 nm in diameter exhibit sharp discharge thresholds in the range of a few volts, as observed in the experiments.