Electrochemically Assembled Cu₂O 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 Cu₂O. 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 Cu₂O. 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 Cu₂O 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 Cu₂O 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 Cu₂O 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 Cu₂O 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 × 10⁴ and long-term electrical/thermal stability.