Digital versus analog resistive switching depending on the thickness of nickel oxide nanoparticle assembly

The thickness-dependent digital versus analog resistive switching of nickel oxide (NiOx) nanoparticle assemblies was investigated in a Ti/NiOx/Pt structure. The NiOx nanoparticles were chemically synthesized with ∼5 nm diameter. The Ti/NiOx/Pt structure with assembly thickness of ∼60 nm exhibited the digital-type bipolar resistive switching. However, the assembly with a thickness of ∼90 nm presented analog resistive switching with gradually decreasing resistance when sweeping −V while increasing resistance after applying +V. Repeating −V pulses decreased the resistance sequentially, but the high resistance was restored sequentially by repeating +V pulses, which is analogous to the potentiation and depression of adaptive synaptic motion.