10 research outputs found
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
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 BiFeO<sub>3</sub> (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
Dopamine-Regulated Plasticity in MoO<sub>3</sub> 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+
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<sub>0.52</sub>Ti<sub>0.48</sub>O<sub>3</sub> (PZT)/La<sub>0.8</sub>Sr<sub>0.2</sub>MnO<sub>3</sub> (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 (10<sup>7</sup>) 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 (VO<sub>2</sub>) is a strong-correlated metal–oxide
with a sharp metal–insulator transition (MIT) for a range of
applications. However, synthesizing epitaxial VO<sub>2</sub> films
with desired properties has been a challenge because of the difficulty
in controlling the oxygen stoichiometry of VO<sub><i>x</i></sub>, 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 VO<sub>2</sub> films. The obvious broadening of the growth window
of stoichiometric VO<sub>2</sub> 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 VO<sub>2</sub> 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
VO<sub>2</sub> 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 WS<sub>2</sub> by Varying the Number of Layers
The optical constants, bandgaps, and band alignments of mono-, bi-, and trilayer WS<sub>2</sub> 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 SiO<sub>2</sub> on a Si substrate. The bandgaps of the few-layer WS<sub>2</sub> 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<sub><i>x</i></sub> and CoO<sub><i>x</i></sub> planar-type devices have exhibited a memristive behavior under
atmospheric pressure, whereas TiO<sub>2‑<i>x</i></sub> planar-type devices did not show a memristive switching even under
the same surroundings. A memristive behavior of TiO<sub>2‑<i>x</i></sub> planar-type devices has emerged when reducing an
ambient pressure and/or employing a SiO<sub>2</sub> 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
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