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Electronic switches based on the migration of high-density point defects, or memristors, are poised to revolutionize post-digital electronics. Despite significant research, key mechanisms for filament formation and oxygen transport remain unresolved, thus hindering our ability to predict and design crucial device properties. For example, predicted retention times based on current models can be 10 orders of magnitude lower than ones experimentally realized. Here, using electrical measurements, chemical spectroscopy, and first-principles calculations on tantalum oxide memristors, we reveal that the formation and stability of conductive filaments crucially depend on the stability of the amorphous oxygen-rich and oxygen-poor compounds, which undergo composition phase separation. Including the previously neglected effects of this amorphous phase separation reconciles unexplained discrepancies and enables predictive design of key performance indicators such as retention stability. This result emphasizes non-ideal thermodynamic interactions as key design criteria in post-digital devices with defect densities substantially exceeding those of today’s covalent semiconductors.

The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, plays a key role in many symmetry-lowering crystal deformations1. Lattices of Jahn-Teller ions can induce a cooperative distortion, as exemplified by LaMnO3 (refs.2–4). While many examples occur in octahedrally5 or tetrahedrally6 coordinated transition metal oxides due to their high orbital degeneracy, this effect has yet to be manifested for square-planar coordination, as found in infinite-layer Cu (refs.7-9), Ni (refs.10–12), Fe (refs.13–15), and Mn oxides16. Here we synthesize single-crystal CaCoO2 thin films by topotactic reduction of the Brownmillerite CaCoO2.5 phase. We observe a dramatically distorted infinite-layer structure, with Angstrom-scale displacements of the cations from their high-symmetry positions. This can be understood to originate from the Jahn-Teller degeneracy of the dxz and dyz-orbitals in the d7 electronic configuration along with substantial ligand-transition metal mixing. A complex pattern of distortions arises in a 2√2× 2√2×1 tetragonal supercell, reflecting the competition between an ordered Jahn-Teller effect of the CoO2 sublattice, and geometric frustration of the associated displacements of the Ca sublattice. As a result of this competition, the CaCoO2 structure forms an extended two-in/two-out type of Co distortion following “ice rules”17.

Monitoring the charged defect concentration at the nanoscale is of critical importance for both the fundamental science and applications of ferroelectrics. However, up-to-date, high-resolution study methods for the investigation of structural defects, such as transmission electron microscopy, X-ray tomography, etc., are expensive and demand complicated sample preparation. With an example of the lanthanum-doped bismuth ferrite ceramics, a novel method is proposed based on the switching spectroscopy piezoresponse force microscopy (SSPFM) that allows probing the electric potential from buried subsurface charged defects in the ferroelectric materials with a nanometer-scale spatial resolution. When compared with the composition-sensitive methods, such as neutron diffraction, X-ray photoelectron spectroscopy, and local time-of-flight secondary ion mass spectrometry, the SSPFM sensitivity to the variation of the electric potential from the charged defects is shown to be equivalent to less than 0.3 at% of the defect concentration. Additionally, the possibility to locally evaluate dynamics of the polarization screening caused by the charged defects is demonstrated, which is of significant interest for further understanding defect-mediated processes in ferroelectrics. published

The coupling between ionic degrees of freedom and ferroelectricity has received renewed attention in recent years, given that surface electrochemical processes have been shown to be intrinsically linked to ferroelectric phase stability in ultrathin ferroelectric films. However, the coupling between bulk ionic transport and local polarization switching has received less attention, as typically the bulk ionic mobilities are low for common ferroelectrics at room temperature. Here, we use the coupled band-excitation method in conjunction with site-correlated time-of-flight secondary ion mass spectrometry, to determine the coupling between ferroelectric switching and ionic motion in single crystal KTiOPO

Ever since the invention of the scanning probe and electron microscope, a goal has been to image liquid specimens as easily as in a light microscope, and to measure liquid processes with a good resolution in both spatial and temporal. Direct observation of electrochemical processes is essential to develop electrochemical models, optimize nanostructures, and understand reaction mechanisms. We have developed and improved some in-situtechniques and tested them on electrochemical processes. In an attempt to have improved topography and surface reactivity information, we have designed and fabricated nanometer sized scanning probe tips for combined atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM). We report here the characterization of these tips using an in-house fabricated calibration sample and a model anode. In another approach we have used scanning near-field microwave microscopy (sMIM) for imaging in liquids using ultrathin molecular impermeable membranes separating scanning probes from samples enclosed in environmental cells. We imaged a process of dendritic growth during model electroplating reaction. Due to very small energy of few gigahertz microwaves we demonstrate the advantages of this microscopy for non-destructive artifact-free imaging compared to electron microscopy [1]. Different to TEM in liquids which has been used successfully to study the electrochemical processes very little research has been performed using liquid SEM [2]. We explore this imaging modality and describe here a novel chip [3,4] and cell designs enabling us to study Ag electrodeposition in real time using SEM in liquids. All these techniques are complementary and well suited for in situelectrochemistry. These examples demonstrate the versatility of these techniques and can be extended to other applications such as corrosion, battery and bio-medical research. References: [1] A. Tselev, J. Velmurugan, A. Ievlev, S. Kalinin,and A. Kolmakov ACS Nano, 10 (3),3562, 2016 [2] E. Jensen, C. Kobler, P. S. Jensen and K. Molhave, Ultramicroscopy, 129, 63, 2013 [3] F. Yi, W. Osborn, J. Betz, J and D.A. LaVan Microelectromechanical Systems, 24, 1185, 2015 [4] M. D. Grapes, T. LaGrange, L.H. Friedman, B. W. Reed, G. H. Campbell, T. P. Weihs and D. A. LaVan Review of Scientific Instruments, 85, 084902, 2014