Promotion of Probabilistic Bit Generation in Mott Devices by Embedded Metal Nanoparticles.

Considerable attention has been drawn to the use of volatile two-terminal devices relying on the Mott transition for the stochastic generation of probabilistic bits (p-bits) in emerging probabilistic computing. To improve randomness and endurance of bit streams provided by these devices, delicate control of the transient evolution of switchable domains is required to enhance stochastic p-bit generation. Herein, it is demonstrated that the randomness of p-bit streams generated via the consecutive pulse inputs of pump-probe protocols can be increased by the deliberate incorporation of metal nanoparticles (NPs), which influence the transient dynamics of the nanoscale metallic phase in VO2 Mott switches. Among the vertically stacked Pt-NP-containing VO2 threshold switches, those with higher Pt NP density show a considerably wider range of p-bit operation (e.g., up to ≈300% increase in ΔVprobe upon going from (Pt NP/VO2)0 to (Pt NP/VO2)11) and can therefore be operated under the conditions of high speed (400 kbit s-1), low power consumption (14 nJ per bit), and high stability (>105 200 bits) for p-bit generation. Thus, the study presents a novel strategy that exploits nanoscale phase control to maximize the generation of nondeterministic information sources for energy-efficient probabilistic computing hardware.

Anionic Flow Valve Across Oxide Heterointerfaces by Remote Electron Doping.

As an analogue of charged electron flows, the ionic flow could be controlled by the electronic band alignment due to the ambipolar nature of diffusion in the ionic crystal. Here, we demonstrate the active control of the anionic diffusion across heterointerfaces through remote electron doping in the capping layers. In contrast to the spontaneous ionic flux from the underlying VO2 layers to the undoped TiO2 capping layers, the activated Nb dopants in the TiO2 capping layers substantially restrict the ionic flux, despite identical growth conditions. The increase of Fermi level by Nb donors in TiO2 prevents electron flux from being generated across the interfaces by the heightening of a Schottky barrier; this electron shortage generates a kinetic close valve for the flow of negatively charged oxygen ions. Thus, these results demonstrate the importance of electron supply on charged ionic flow, thereby suggesting an unprecedented strategy for ionic-defect-induced emergent properties at interfaces.

Embedded metallic nanoparticles facilitate metastability of switchable metallic domains in Mott threshold switches.

Mott threshold switching, which is observed in quantum materials featuring an electrically fired insulator-to-metal transition, calls for delicate control of the percolative dynamics of electrically switchable domains on a nanoscale. Here, we demonstrate that embedded metallic nanoparticles (NP) dramatically promote metastability of switchable metallic domains in single-crystal-like VO2 Mott switches. Using a model system of Pt-NP-VO2 single-crystal-like films, interestingly, the embedded Pt NPs provide 33.3 times longer 'memory' of previous threshold metallic conduction by serving as pre-formed 'stepping-stones' in the switchable VO2 matrix by consecutive electical pulse measurement; persistent memory of previous firing during the application of sub-threshold pulses was achieved on a six orders of magnitude longer timescale than the single-pulse recovery time of the insulating resistance in Pt-NP-VO2 Mott switches. This discovery offers a fundamental strategy to exploit the geometric evolution of switchable domains in electrically fired transition and potential applications for non-Boolean computing using quantum materials.

Directional ionic transport across the oxide interface enables low-temperature epitaxy of rutile TiO2.

Heterogeneous interfaces exhibit the unique phenomena by the redistribution of charged species to equilibrate the chemical potentials. Despite recent studies on the electronic charge accumulation across chemically inert interfaces, the systematic research to investigate massive reconfiguration of charged ions has been limited in heterostructures with chemically reacting interfaces so far. Here, we demonstrate that a chemical potential mismatch controls oxygen ionic transport across TiO2/VO2 interfaces, and that this directional transport unprecedentedly stabilizes high-quality rutile TiO2 epitaxial films at the lowest temperature (≤ 150 °C) ever reported, at which rutile phase is difficult to be crystallized. Comprehensive characterizations reveal that this unconventional low-temperature epitaxy of rutile TiO2 phase is achieved by lowering the activation barrier by increasing the "effective" oxygen pressure through a facile ionic pathway from VO2-δ sacrificial templates. This discovery shows a robust control of defect-induced properties at oxide interfaces by the mismatch of thermodynamic driving force, and also suggests a strategy to overcome a kinetic barrier to phase stabilization at exceptionally low temperature.

All-Solid-State Synaptic Transistors with High-Temperature Stability Using Proton Pump Gating of Strongly Correlated Materials.

Designing energy-efficient artificial synapses with adaptive and programmable electronic signals is essential to effectively mimic synaptic functions for brain-inspired computing systems. Here, we report all-solid-state three-terminal artificial synapses that exploit proton-doped metal-insulator transition in a correlated oxide NdNiO3 (NNO) channel by proton (H+) injection/extraction in response to gate voltage. Gate voltage reversibly controls the H+ concentration in the NNO channel with facile H+ transport from a H+-containing porous silica electrolyte. Gate-induced H+ intercalation in the NNO gives rise to nonvolatile multilevel analogue states due to H+-induced conductance modulation, accompanied by significant modulation of the out-of-plane lattice parameters. This correlated transistor operated by a proton pump shows synaptic characteristics such as long-term potentiation and depression, with nonvolatile and distinct multilevel conductance switching by a low voltage pulse (≥ 50 mV), with high energy efficiency (∼1 pJ) and tolerance to heat (≤150 °C). These results will guide the development of scalable, thermally-stable solid-state electronic synapses that operate at low voltage.

Super-resolution visible photoactivated atomic force microscopy.

Imaging the intrinsic optical absorption properties of nanomaterials with optical microscopy (OM) is hindered by the optical diffraction limit and intrinsically poor sensitivity. Thus, expensive and destructive electron microscopy (EM) has been commonly used to examine the morphologies of nanostructures. Further, while nanoscale fluorescence OM has become crucial for investigating the morphologies and functions of intracellular specimens, this modality is not suitable for imaging optical absorption and requires the use of possibly undesirable exogenous fluorescent molecules for biological samples. Here we demonstrate super-resolution visible photoactivated atomic force microscopy (pAFM), which can sense intrinsic optical absorption with ~8 nm resolution. Thus, the resolution can be improved down to ~8 nm. This system can detect not only the first harmonic response, but also the higher harmonic response using the nonlinear effect. The thermoelastic effects induced by pulsed laser irradiation allow us to obtain visible pAFM images of single gold nanospheres, various nanowires, and biological cells, all with nanoscale resolution. Unlike expensive EM, the visible pAFM system can be simply implemented by adding an optical excitation sub-system to a commercial atomic force microscope.