Electrochemically Assembled CuO Nanoparticles Using Crystallographically Anisotropic Functional Metal Ions and Highly Expeditious Resistive Switching via Nanoparticle Coarsening.

We have developed an artificially controllable strategy of an electrodeposition process adequate for resistive random-access memory (ReRAM) applications of binary CuO. Typically, the precise control of OH ion concentration (the intermediate supplier of oxygen ions) at the electrode's surface decides the overall reaction rate of the CuO. Here, the suggested Pb and Sb metal additives preferentially contribute to the consumption of OH ions and the supply of OH ions, respectively, during the CuO electrochemical reaction so that the final products are the (200) preferential quadrangular pyramids and the (111) preferential triangular pyramids. Interestingly, the coexistence of Sb/Pb precursors in the Cu electrolytes results in extraordinarily decreased reaction rate from the opposite action of OH ion utilization as well as intense progressive growth behavior, and the resultant CuO films consist of crystallized small-size nanoparticles (NPs) in an amorphous-like matrix. In the case of ReRAM applications, while the polycrystalline film induces irregular device performance and the amorphous layer shows an easily irreparable electrical breakdown, our NP-assembled CuO films from Pb/Sb metal ions reveal the formation of a conduction bridge via phase change to a crystalline filament with no need for forming voltage and with superior electrical stability. It is attributed to the coalescence of crystal NPs into large grains during the set/reset cycle process for the heat dissipation of Joule heating. The CuO sample prepared with a 3 mM Sb + 3 mM Pb mixture solution exhibits forming-free ReRAM devices with high on/off resistance ratios of 1.2 × 10 and long-term electrical/thermal stability.