2, University of Nebraska-Lincoln, Lincoln, Nebraska, Lincoln, Nebraska, United States
3, Nebraska Center for Materials and Nanoscience, Lincoln, Nebraska, Lincoln, Nebraska, United States
Nanoclusters are an interesting research area due to their different catalytic, electronic, magnetic, and optical properties from their corresponding bulk counterparts. Among the various fabrication methods, inert gas condensation (IGC) effectively produces monodispersed nanoclusters with sizes typically in the 5-20 nm range. Further, IGC is a proven route to produce core/shell structures directly from the gas phase. Here, we produce Co core/ZnO shell nanoclusters to study electric field-induced resistive switching as these structures are candidates for resistive random-access- memory, which has shown great potential to lead the next generation nonvolatile memory technology.
IGC was utilized to produce initially Co core/Zn shell nanoclusters. The Zn was subsequently oxidized to form ZnO both in situ by bleeding oxygen into the gas condensation chamber and ex situ by exposing the nanoclusters to air. The core-shell Co/ZnO nanoclusters were characterized by transmission electron microscopy (TEM) to study the crystal structure of nanoparticles and scanning transmission electron microscopy (STEM) for imaging and elemental mapping. High resolution TEM images displayed Moiré fringes in core and shell regions indicated both core and shell are crystalline, with fcc cobalt in the core and epitaxially grown zinc oxide in the shell. STEM using high angle annular dark field (HAADF) imaging and elemental energy dispersive X-ray spectroscopy (EDS) mapping provided more evidence of the core-shell structure of the nanoparticles. The resistive switching behavior in Co/ZnO nanoclusters were studied by using two methods to obtain I-V characteristics of these nanoparticles. In the first method, which is done in electrical modes of an atomic force microscope (AFM), the conductive tip of the AFM acts as a mobile top electrode, and the voltage can be varied on a single nanocluster deposited on a silicon substrate. The second method deposited the nanoclusters between two metal electrodes and the I-V characteristics were obtained using a two probe configuration. I-V graphs obtained in both methods indicated the bipolar resistive switching behavior which can be explained by the formation and disruption of conducting nano-filaments induced by oxygen vacancy migration and the contribution of Co nanoclusters. With applying a positive voltage, a “SET” process from high resistance OFF state to low resistance ON state appears. However, upon applying negative voltage a transition from a low resistance state to a high resistance state as a “RESET” process appears.