We report on the non-volatile resistive switching properties of epitaxial nickel oxide (NiO) nanostructures, 10-100 nm wide and up to 30 nm high grown on (001)-Nb:SrTiO3 substrates. Conducting-atomic force microscopy on individual nano-islands confirms prominent bipolar switching with a maximum ON/OFF ratio of ∼103 at a read voltage of ∼+0.4 V. This ratio is found to decrease with increasing height of the nanostructure. Linear fittings of I-V loops reveal that low and high resistance states follow Ohmic-conduction and Schottky-emission mechanism, respectively. The switching behavior (dependence on height) is attributed to the modulation of the carrier density at the nanostructure-substrate interface due to the applied electric field.

Measuring the diffusion of ions and vacancies at nanometer length scales is crucial to understanding fundamental mechanisms driving technologies as diverse as batteries, fuel cells, and memristors; yet such measurements remain extremely challenging. Here, we employ a multimodal scanning probe microscopy (SPM) technique to explore the interplay between electronic, elastic, and ionic processes via first-order reversal curve I-V measurements in conjunction with electrochemical strain microscopy (ESM). The technique is employed to investigate the diffusion of oxygen vacancies in model epitaxial nickel oxide (NiO) nanocrystals with resistive switching characteristics. Results indicate that opening of the ESM hysteresis loop is strongly correlated with changes to the resonant frequency, hinting that elastic changes stem from the motion of oxygen (or cation) vacancies in the probed volume of the SPM tip. These changes are further correlated to the current measured on each nanostructure, which shows a hysteresis loop opening at larger (∼2.5 V) voltage windows, suggesting the threshold field for vacancy migration. This study highlights the utility of local multimodal SPM in determining functional and chemical changes in nanoscale volumes in nanostructured NiO, with potential use to explore a wide variety of materials including phase-change memories and memristive devices in combination with site-correlated chemical imaging tools.

Nickel oxide (NiO) nanocrystals epitaxially grown on (001) strontium titanate (SrTiO3) single crystal substrates were characterized to investigate interface morphology and chemistry. Aberration corrected high angle annular dark field scanning transmission electron microscopy reveals the interface between the NiO nanocrystals and the underlying SrTiO3 substrate to be rough, irregular, and have a lower average atomic number than the substrate or the nanocrystal. Energy dispersive x-ray spectroscopy and electron energy loss spectroscopy confirm both chemical disorder and a shift of the energy of the Ti L2,3 peaks. Analysis of the O K edge profiles in conjunction with this shift, implies the presence of oxygen vacancies at the interface. This sheds light into the origin of the previously postulated minority carriers’ model to explain resistive switching in NiO [J. Sullaphen, K. Bogle, X. Cheng, J. M. Gregg, and N. Valanoor, Appl. Phys. Lett. 100, 203115 (2012)].

This report investigates the phase evolution pathway of magnetite nanocrystal synthesis on oxidesupported substrates. A template-free phase separation approach, which exploits the thermodynamic instability of ternary perovskite BiFeO 3 and inherent volatility of bismuth oxide in low oxygen pressure and high temperature is presented. The formation of an intermediate hematite nanocrystal phase is found as a key step that controls the eventual size and morphology of the magnetite nanocrystals. X-ray absorption spectra measurements and X-ray magnetic circular dichroism confirm that the spectral fingerprints of the magnetite nanocrystals match with that of bulk crystals. Magnetic measurements show that magnetic anisotropy is directly attributed to the nanocrystal morphology. V

Abstract

Nickel oxide (NiO) is a binary metal oxide that is found to have resistive switching (RS) properties, which makes it a viable candidate for next generation resistive random access memories (RRAMs). The demand for high-density memories has concentrated on RS materials with scalable dimensions. In these nanoscaled devices, the presence of point and line defects, composition inhomogeneity, and atomic interdiffusion interfaces have been shown to have a significant impact on properties.The objective of this study is to investigate the role of interface morphology and chemistry in resistive switching behavior of NiO epitaxial nanocrystals fabricated on (001) Aberration corrected high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) reveals the interface between the NiO nanocrystals and the underlying STO substrate to be rough, irregular, and have a lower average atomic number than the substrate or the nanocrystal [1]. Energy dispersive x-ray spectroscopy and electron energy loss spectroscopy (shown in Figure 1 HRTEM and HAADF-STEM reveal that the substrate is deformed (see Figure 2) so as to back-fill around the base of the nanostructure in order to promote wetting. Remarkably, the substrate under the nanocrystal is physically pulled towards the nanostructure, thus forming a rim around the nanocrystal [4].