Nanoscale Probing of Elastic–Electronic Response to Vacancy Motion in NiO Nanocrystals

Abstract

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.

Self‐Assembled NiO Nanocrystal Arrays as Memristive Elements

“…In addition, the R-V characteristic between two resistance states (ON-state and OFF-state) is clearly observed at ±5.3 V applied voltage bias (left inset graph of Figure 2d), indicating the formation and deformation of the conducting filament in the nanocrystal via redox reaction occurs at the Pt/NiO interface as described in our previous results. [21] For further test, we observed this on at least twenty NiO nanocrystals taken from the topography image in Figure 2b and we found that the edge resistance is constantly higher than the core resistance (right inset graph of Figure 2d). This suggests that the memristive switching is mainly active at the edge rather than the core of NiO nanocrystals.…”

“…    as shown in our previous work. [21] On the other hand, if the initial state of the nanocrystal is ON-state, then the bias voltage was swept with the order of Figure 1d. The taller the nanocrystal, the higher voltage required to induce the memristive behavior because the field is simply not strong enough to create the modulation due to the migration of the carriers at Pt/NiO interface.…”

“…This is also in agreement with our previous results that the correlation between the I-V and electrochemical strain mapping (ESM) loop with the frequency shifts are substantially stronger at higher switching bias. [21] As the voltage amplitude increases, the different conductivity between the edge and core of the nanocrystal becomes more obvious. At the core of the nanocrystals, the dead zone appears in all voltage range, and the current level remains as low as 0.5 nA with no observable hysteresis window as given in Figure S5.…”

“…[17][18][19][20] We have previously demonstrated oxygen diffusion using electrochemical strain mapping (ESM) in NiO nanocrystals. [21] The cross-correlation between ESM hysteresis loop and I-V measurement provided a vital hint that the elastic-electronic coupling of the oxygen vacancy mobility drives the memristive switching behavior of the NiO nanocrystals. [21] A gradual enhancement of the measured current is obtained as a direct consequence of the growth of the conducting filament which is also a clear indication of an excellent synaptic switching behavior.…”

“…[21] The cross-correlation between ESM hysteresis loop and I-V measurement provided a vital hint that the elastic-electronic coupling of the oxygen vacancy mobility drives the memristive switching behavior of the NiO nanocrystals. [21] A gradual enhancement of the measured current is obtained as a direct consequence of the growth of the conducting filament which is also a clear indication of an excellent synaptic switching behavior. Spatially resolved switching location were accomplished by means of FORC-IV spectroscopy, showing that the memristive switching behavior was mainly observed at the edge region of the nanocrystal rather than at the core.…”