Dopamine-Regulated Plasticity in MoO₃ Synaptic Transistors

Field-effect transistor-based biosensors have gained increasing interest due to their reactive surface to external stimuli and the adaptive feedback required for advanced sensing platforms in biohybrid neural interfaces. However, complex probing methods for surface functionalization remain a challenge that limits the industrial implementation of such devices. Herein, a simple, label-free biosensor based on molybdenum oxide (MoO3) with dopamine-regulated plasticity is demonstrated. Dopamine oxidation facilitated locally at the channel surface initiates a charge transfer mechanism between the molecule and the oxide, altering the channel conductance and successfully emulating the tunable synaptic weight by neurotransmitter activity. The oxygen level of the channel is shown to heavily affect the device’s electrochemical properties, shifting from a nonreactive metallic characteristic to highly responsive semiconducting behavior. Controllable responsivity is achieved by optimizing the channel’s dimension, which allows the devices to operate in wide ranges of dopamine concentration, from 100 nM to sub-mM levels, with excellent selectivity compared with K+, Na+, and Ca2+

Direct Observation of Domain Motion Synchronized with Resistive Switching in Multiferroic Thin Films

The room-temperature resistive switching characteristics of ferroelectric, ferroelastic, and multiferroic materials are promising for application in nonvolatile memory devices. These resistive switching characteristics can be accompanied by a change in the ferroic order parameters via applied external electric and magnetic excitations. However, the dynamic evolution of the order parameters between two electrodes, which is synchronized with resistive switching, has rarely been investigated. In this study, for the first time, we directly monitor the ferroelectric/ferroelastic domain switching dynamics between two electrodes in multiferroic BiFeO₃ (BFO) planar devices, which cause resistive switching, using piezoresponse force microscopy. It is demonstrated that the geometrical relationship between the ferroelectric domain and electrode in BFO planar capacitors with only 71° domain walls significantly affects both the ferroelectric domain dynamics and the resistive switching. The direct observation of domain dynamics relevant to resistive switching in planar devices may pave the way to a controllable combination of ferroelectric characteristics and resistive switching in multiferroic materials

Ion Current Oscillation in Glass Nanopipettes

Ion currents detected by glass nanopipettes in solutions depended on the diameters of pipettes and ion species in the solutions. The ion current oscillation with frequency of 2.7 mHz was observed using the pipet with inner diameter of 50 nm in 0.1 M KCl solution. However, nonoscillatory currents were observed using the same pipet in 0.1 M KOH and HCl solutions or using pipettes with inner diameters of 15 nm, 500 nm, and 0.7 mm in 0.1 M KCl solution. Oscillation of the double layer thickness due to the change of ion concentration in the nanopipette perturbs the path of the ion current through the bulk layer, which results in the nonlinear current oscillation

Dual Defects of Cation and Anion in Memristive Nonvolatile Memory of Metal Oxides

The electrically driven resistance change of metal oxides, called bipolar memristive switching, is a fascinating phenomenon in the development of next-generation nonvolatile memory alternatives to flash technology. However, our understanding of the nature of bipolar memristive switching is unfortunately far from comprehensive, especially the relationship between the electrical transport and the local nonstoichiometry. Here we demonstrate that the coexistence of anion and cation defects is critical to the transport properties of NiO, one of the most promising memristive oxides, by utilizing first-principles calculations. We find that, in the presence of both nickel and oxygen defects, which must exist in any real experimental systems, carrier concentrations of holes generated by nickel defects can be modulated by the presence or absence of oxygen defects around the nickel defect. Such alternation of local nonstoichiometry can be understood in terms of an oxygen ion drift induced by an external electric field. This implication provides a foundation for understanding universally the nature of bipolar memristive switching in various p-type metal oxides

Synaptic Plasticity Selectively Activated by Polarization-Dependent Energy-Efficient Ion Migration in an Ultrathin Ferroelectric Tunnel Junction

Selectively activated inorganic synaptic devices, showing a high on/off ratio, ultrasmall dimensions, low power consumption, and short programming time, are required to emulate the functions of high-capacity and energy-efficient reconfigurable human neural systems combining information storage and processing (Li et al. Sci. Rep. 2014, 4, 4096). Here, we demonstrate that such a synaptic device is realized using a Ag/PbZr_0.52Ti_0.48O₃ (PZT)/La_0.8Sr_0.2MnO₃ (LSMO) ferroelectric tunnel junction (FTJ) with ultrathin PZT (thickness of ∼4 nm). Ag ion migration through the very thin FTJ enables a large on/off ratio (10⁷) and low energy consumption (potentiation energy consumption = ∼22 aJ and depression energy consumption = ∼2.5 pJ). In addition, the simple alignment of the downward polarization in PZT selectively activates the synaptic plasticity of the FTJ and the transition from short-term plasticity to long-term potentiation

Enhanced Metal–Insulator Transition Performance in Scalable Vanadium Dioxide Thin Films Prepared Using a Moisture-Assisted Chemical Solution Approach

Vanadium dioxide (VO₂) is a strong-correlated metal–oxide with a sharp metal–insulator transition (MIT) for a range of applications. However, synthesizing epitaxial VO₂ films with desired properties has been a challenge because of the difficulty in controlling the oxygen stoichiometry of VO_<i>x</i>, where <i>x</i> can be in the range of 1 < <i>x</i> < 2.5 and V has multiple valence states. Herein, a unique moisture-assisted chemical solution approach has been developed to successfully manipulate the oxygen stoichiometry, to significantly broaden the growth window, and to significantly enhance the MIT performance of VO₂ films. The obvious broadening of the growth window of stoichiometric VO₂ thin films, from 4 to 36 °C, is ascribed to a self-adjusted process for oxygen partial pressure at different temperatures by introducing moisture. A resistance change as large as 4 orders of magnitude has been achieved in VO₂ thin films with a sharp transition width of less than 1 °C. The much enhanced MIT properties can be attributed to the higher and more uniform oxygen stoichiometry. This technique is not only scientifically interesting but also technologically important for fabricating wafer-scaled VO₂ films with uniform properties for practical device applications

Graphene/Pentacene Barristor with Ion-Gel Gate Dielectric: Flexible Ambipolar Transistor with High Mobility and On/Off Ratio

High-quality channel layer is required for next-generation flexible electronic devices. Graphene is a good candidate due to its high carrier mobility and unique ambipolar transport characteristics but typically shows a low on/off ratio caused by gapless band structure. Popularly investigated organic semiconductors, such as pentacene, suffer from poor carrier mobility. Here, we propose a graphene/pentacene channel layer with high-k ion-gel gate dielectric. The graphene/pentacene device shows both high on/off ratio and carrier mobility as well as excellent mechanical flexibility. Most importantly, it reveals ambipolar behaviors and related negative differential resistance, which are controlled by external bias. Therefore, our graphene/pentacene barristor with ion-gel gate dielectric can offer various flexible device applications with high performances

Engineering Optical and Electronic Properties of WS₂ by Varying the Number of Layers

The optical constants, bandgaps, and band alignments of mono-, bi-, and trilayer WS₂ were experimentally measured, and an extraordinarily high dependency on the number of layers was revealed. The refractive indices and extinction coefficients were extracted from the optical-contrast oscillation for various thicknesses of SiO₂ on a Si substrate. The bandgaps of the few-layer WS₂ were both optically and electrically measured, indicating high exciton-binding energies. The Schottky-barrier heights (SBHs) with Au/Cr contact were also extracted, depending on the number of layers (1–28). From an engineering viewpoint, the bandgap can be modulated from 3.49 to 2.71 eV with additional layers. The SBH can also be reduced from 0.37 eV for a monolayer to 0.17 eV for 28 layers. The technique of engineering materials’ properties by modulating the number of layers opens pathways uniquely adaptable to transition-metal dichalcogenides

Prominent Thermodynamical Interaction with Surroundings on Nanoscale Memristive Switching of Metal Oxides

This study demonstrates the effect of surroundings on a memristive switching at nanoscale by utilizing an open top planar-type device. NiO_<i>x</i> and CoO_<i>x</i> planar-type devices have exhibited a memristive behavior under atmospheric pressure, whereas TiO_2‑<i>x</i> planar-type devices did not show a memristive switching even under the same surroundings. A memristive behavior of TiO_2‑<i>x</i> planar-type devices has emerged when reducing an ambient pressure and/or employing a SiO₂ passivation layer. These results reveal that a thermodynamical interaction with surroundings critically determines the occurrence of memristive switching via varying a stability of nonstoichiometry. Since this effect tends to be more significant for smaller devices with larger specific surface area, tailoring the surrounding effect by an appropriate passivation will be essential for high density devices

Mechanical Control of Electroresistive Switching

Hysteretic metal–insulator transitions (MIT) mediated by ionic dynamics or ferroic phase transitions underpin emergent applications for nonvolatile memories and logic devices. The vast majority of applications and studies have explored the MIT coupled to the electric field or temperarture. Here, we argue that MIT coupled to ionic dynamics should be controlled by mechanical stimuli, the behavior we refer to as the piezochemical effect. We verify this effect experimentally and demonstrate that it allows both studying materials physics and enabling novel data storage technologies with mechanical writing and current-based readout