Multiple resistive switching in core-shell ZnO nanowires exhibiting tunable surface states

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Samuele Porro , francesca risplendi , Giancarlo Cicero , Katarzyna Bejtka , Gianluca Milano , Paola Rivolo , Alladin Jasmin , Alessandro Chiolerio , Candido Fabrizio Pirri and Carlo Ricciardi

Received 31st May 2017 , Accepted 4th August 2017

First published on 4th August 2017

Surface and quantum confinement effects in one-dimensional systems such as ZnO nanowires are responsible for novel electrical properties, and can be exploited to tune the electrical transport at the nanoscale. The investigation of new physical mechanisms for resistive switching can be fulfilled by studying metal/insulator/metal memristive devices that take advantage of the unique properties of one-dimensional nanoscale metal oxides. In particular, the mechanisms of resistive switching between multiple resistance states in such nanostructures can be associated to the variation of internal physical states. Here we demonstrate both experimentally and theoretically that core-shell structures based on polyacrylic acid coated ZnO nanowires exhibit a resistive switching behavior characterized by internal multiple resistance states, owing to changes in surface states induced by redox reactions occurring at their surfaces. The introduction of a thin layer of polymer coating resulted in a resistive switching between more than two states. Specifically, the existence of two intermediate states in addition to the high and low resistance states was revealed during DC measurements in voltage sweep mode. All resistive states showed low variability over cycling. The mechanism of switching between multiple steps, as probed by Density Functional Theory calculations, was associated to redox reactions involving species at the interface (e.g. methanal or hydroxyl groups), each characterized by a given redox potential. Therefore, the multiple resistance states were induced by specific and stable threshold voltages, as shown experimentally.