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The paradigm shift of information carriers from charge to spin has long been awaited in modern electronics. The invention of the spin-information transistor is expected to be an essential building block for the future development of spintronics. Here, we introduce a proof-of-concept experiment of a magnetic skyrmion transistor working at room temperature, which has never been demonstrated experimentally. With the spatially uniform control of magnetic anisotropy, we can maintain the shape and topology of a skyrmion when passing the controlled area. Our findings will open a new route towards the design and realization of skyrmion-based spintronic devices in the near future. This article is protected by copyright. All rights reserved.

Atomically thin two-dimensional (2D) materials have emerged as promising candidates for flexible and transparent electronic applications. Here, we introduce non-volatile charge trapping memory devices, based on the 2D heterostructure field-effect transistor consisting of a few-layer MoS2 channel and CrPS4 charge-trapping gate stack. Clockwise hysteresis behaviors in transfer curves measured at room temperature show strong dependence on the thickness of CrPS4, which are attributed to charge trapping at trap sites in the CrPS4 layers. Our heterostructure memory device with 75 nm-thick CrPS4 layer exhibits both large memory windows up to 100 V and a high on/off current ratio (3 $$\times$$ 105) with good endurance during 625 cycles because of excellent trapping ability of trap sites in the CrPS4. Especially, the memory window size can be effectively tuned from 7.6 to 100 V by changing the sweep range of gate voltage. Such high performances of the charge-trapping memory device with a simple heterostructure provide a promising route towards next-generation memory devices utilizing 2D materials.

Semiconductors have long been perceived as a prerequisite for solid-state transistors. Although switching principles for nanometer-scale devices have emerged based on the deployment of two-dimensional (2D) van der Waals heterostructures, tunneling and ballistic currents through short channels are difficult to control, and semiconducting channel materials remain indispensable for practical switching. In this study, we report a semiconductor-less solid-state electronic device that exhibits an industry-applicable switching of the ballistic current. This device modulates the field emission barrier height across the graphene-hexagonal boron nitride interface with ION/IOFF of 106 obtained from the transfer curves and adjustable intrinsic gain up to 4, and exhibits unprecedented current stability in temperature range of 15–400 K. The vertical device operation can be optimized with the capacitive coupling in the device geometry. The semiconductor-less switching resolves the long-standing issue of temperature-dependent device performance, thereby extending the potential of 2D van der Waals devices to applications in extreme environments. In field-effect transistors, a semiconducting channel is indispensable for device switching. Here, the authors demonstrate semiconductor-less switching via modulation of the field emission barrier height across a graphene-hBN interface with ON/OFF ratio of 106.

Temperature-independent magnetoresistance (TIMR) has been studied for applications in magnetic field sensors operating in wide temperature ranges. Graphene is considered as one of the best candidates for achieving nonsaturating and large TIMR through engineering disorders. Nevertheless, large TIMR has not been achieved in disordered graphene with intrinsic defects, such as chemical doping and atomic dislocations. In this work, by introducing extrinsic defects, we realize nonsaturating and large TIMR in monolayer graphene transferred on a BiFeO3 nanoisland array (G/BFO-NIA). Furthermore, the G/BFO-NIA device exhibits a significantly larger MR (∼250% under 9 T) than other materials without gating operation, demonstrating its application feasibility. It is shown that the large MR is a result of the coexistence of electrons and holes with almost the same density, and the observed TIMR originates from the temperature dependence of carrier transport in graphene and of the dielectric property of BFO-NIA.

Summary Mammalian embryos exhibit a transition from head morphogenesis to trunk elongation to meet the demand of axial elongation. The caudal neural tube (NT) is formed with neural progenitors (NPCs) derived from neuromesodermal progenitors localized at the tail tip. However, the molecular and cellular basis of elongating NT morphogenesis is yet elusive. Here, we provide evidence that caudal NPCs exhibit strong adhesion affinity that is gradually decreased along the anteroposterior (AP) axis in mouse embryonic spinal cord and human cellular models. Strong cell-cell adhesion causes collective migration, allowing AP alignment of NPCs depending on their birthdate. We further validated that this axial adhesion gradient is associated with the extracellular matrix and is under the control of graded Wnt signaling emanating from tail buds and antagonistic retinoic acid (RA) signaling. These results suggest that progressive reduction of NPC adhesion along the AP axis is under the control of Wnt-RA molecular networks, which is essential for a proper elongation of the spinal cord., Graphical Abstract, Highlights • NPCs along the spinal cord exhibit a gradient of cell adhesion (anterior < posterior) • Cell adhesion changes depending on the position and developmental stage of NPCs • Wnt-RA signaling acts as an upstream effector of NPCs • Microarray analysis revealed enrichment of ECM among posterior NPCs, In this article, Shaker and colleagues show that cell-cell adhesion and cell-migration properties of the NPCs in the elongating embryonic spinal cord were different depending on their anteroposterior position (anterior < posterior). Transcriptomic analysis revealed ECM enrichment in posterior NPCs, which is dependent on anteroposterior gradient of Wnt-RA signaling.

Separation of two-dimensional thin-film-materials into single layers had been considered very challenging both theoretically and experimentally until the mechanical exfoliation method was discovered. After the successful separation of single- and/or few-layer graphene, the possibility of wrinkle formation has been one of the main open topics because they were not readily observed in experiments. Here, we report experimental observations of different kinds of repetitive nanoscale deformations (ripples, wrinkles, and crumples) in folded single-layer graphene (SLG) on an SiO2 substrate. Using high-vacuum atomic force microscopy, we observed that SLG that was pre-transferred onto an SiO2 substrate was accidentally folded multiple times during the tip-scanning, resulting in the formation of bilayer and trilayer graphene (TLG). Through high-resolution tapping-mode scanning, we could observe the wrinkles, ripples, and crumples in TLG. Additionally, we observed a herringbone pattern that was attributed to an intermediate stage between the wrinkles and ripples; this intermediate state was labeled as wrinklon structure. In our analysis, to characterize the ripples, wrinkles, and crumples, we measured the spatial repetition and amplitude of each pattern using their average line profile and compared them.

We report the enhanced electrical performance of a MoS2 field-effect transistor (FET) by using a contact with a layered CrPS4. Our transport measurements revealed that MoS2 channel with CrPS4 junction showed higher mobility of 33.9 cm2/Vs than that without CrPS4 junction on SiO2/Si substrate. We also fabricated a MoS2 FET with a top gate insulator, CrPS4, which showed low leakage current of 10−11 A and high on/off ratio of 105. In a dual-gated FET with SiO2 bottom gate insulator and CrPS4 top gate insulator, much decreased sub-threshold swing of 0.70 V/dec was obtained.

The function of a neural circuit can be determined by the following: (1) characteristics of individual neurons composing the circuit, (2) their distinct connection structure, and (3) their neural circuit activity. However, prior research on correlations between these three factors revealed many limitations. In particular, profiling and modeling of the connectivity of complex neural circuits at the cellular level are highly challenging. To reduce the burden of the analysis, we suggest a new approach with simplification of the neural connection in an array of honeycomb patterns on 2D, using a microcontact printing technique. Through a series of guided neuronal growths in defined honeycomb patterns, a simplified neuronal circuit was achieved. Our approach allowed us to obtain the whole network connectivity at cellular resolution using a combination of stochastic multicolor labeling via viral transfection. Therefore, we were able to identify several types of hub neurons with distinct connectivity features. We also compared the structural differences between different circuits using three-node motif analysis. This new model system, iCANN, is the first experimental model of neural computation at the cellular level, providing neuronal circuit structures for the study of the relationship between anatomical structure and function of the neuronal network.

Next-generation electronic and optoelectronic devices require a high-quality channel layer. Graphene is a good candidate because of its high carrier mobility and unique ambipolar transport characteristics. However, the on/off ratio and photoresponsivity of graphene are typically low. Transition metal dichalcogenides (e.g., MoSe2) are semiconductors with high photoresponsivity but lower mobility than that of graphene. Here, we propose a graphene/MoSe2 barristor with a high-k ion-gel gate dielectric. It shows a high on/off ratio (3.3 × 104) and ambipolar behavior that is controlled by an external bias. The barristor exhibits very high external quantum efficiency (EQE, 66.3%) and photoresponsivity (285.0 mA/W). We demonstrate that an electric field applied to the gate electrode substantially modulates the photocurrent of the barristor, resulting in a high gate tuning ratio (1.50 μA/V). Therefore, this barristor shows potential for use as an ambipolar transistor with a high on/off ratio and a gate-tunable photodetector with a high EQE and responsivity. An approach that enhances control over charge carrier movement in graphene-based transistors may be useful in applications such as bendable circuits and light sensors. The difference in current flow when a transistor turns from on to off plays a critical role in determining device performance. Bae Ho Park from Konkuk University in Seoul, South Korea, and colleagues report that stacking graphene on top of another two-dimensional (2D) film of molybdenum selenide can lead to larger on/off current ratios than conventional graphene transistors. Optical and electrical characterizations revealed that the interface between the 2D films created a potential barrier that blocked current flow in the device’s off state without compromising graphene’s mechanical flexibility. Using an external voltage to modulate current flow through the barrier region also allowed the stacked device to serve as a high-efficiency photodetector. we report a field-effect device with a graphene/MoSe2 channel layer and high-k ion-gel gate dielectric. The device shows a high carrier mobility (~247 cm2/V ∙ s), a high on/off ratio (3.3 × 104), and ambipolar behavior that are controlled by an applied gate voltage. The strong gating effect of the device results in higher external quantum efficiency (EQE) (66.3%), photoresponsivity (285.0 mA/W), and gate-tuning ratio (1.50 μA/V) compared to pristine devices. Therefore, our graphene/MoSe2 barristor device can be a suitable candidate for use in ambipolar transistors and gate-tunable broad-area photodetectors.

Memristive electrochemical metallization (ECM) devices based on cation migration and electrochemical metallization in solid electrolytes are considered promising for neuromorphic computing systems. Two-dimensional (2D) layered materials are emerging as potential candidates for electrolytes in reliable ECM devices due to their two-dimensionally confined material properties. However, electrochemical metallization within a single-crystalline 2D layered material has not yet been verified. Here, we use transmission electron microscopy and energy-dispersive X-ray spectroscopy to investigate the resistive switching mechanism of an ECM device containing a single-crystalline 2D layered CrPS4 electrolyte. We observe the various conductive filament (CF) configurations induced by an applied voltage in an Ag/CrPS4/Au device in the initial/low-resistance/high-resistance/breakdown states. These observations provide concrete experimental evidence that CFs consisting of Ag metal can be formed inside single-crystalline 2D layered CrPS4 and that their configuration can be changed by an applied voltage. Density functional theory calculations confirm that the sulfur vacancies in single-crystalline CrPS4 can facilitate Ag ion migration from the active electrode layer. The electrically induced changes in Ag CFs inside single-crystalline 2D layered CrPS4 raise the possibility of a reliable ECM device that exploits the properties of two-dimensionally confined materials. A 2D layered material has provided critical insights toward improving the stability of devices that can emulate biological synapses. Researchers in Seoul, South Korea, led by Jae-Pyoung Ahn at the Korea Institute of Science and Technology and Bae Ho Park from Konkuk University have used high-resolution microscopy to investigate electrochemical metallization cells, a new type of non-volatile computer memory. These cells, made by infusing insulators with tiny metallic filaments, can mimic the low and high conductivity states of synapses. When the team looked at the structure of a chromium thiophosphate insulator composed of numerous 2D crystal layers, they found that the pathways the filaments follow are guided by specific crystal defects. These findings may help manufacturers produce more reliable electrochemical metallization cells by reducing the likelihood of random, non-uniform filament formation. We report on the formation and rupture of CFs through Ag ion migration inside a single-crystalline 2D van der Waals (vdW) solid electrolyte material within an ECM device structure. This study provides clear experimental evidence that CFs consisting of Ag can be formed inside single-crystalline 2D layered chromium thiophosphate (CrPS4) and their configuration can be changed by an applied voltage. Density functional theory calculations confirm that the Ag ion migration is an energetically favorable process. The electrically induced changes in Ag CFs inside single-crystalline CrPS4 raise the possibility of a reliable ECM device that exploits the properties of two-dimensionally confined materials.

In the era of "big data," the cognitive system of the human brain is being mimicked through hardware implementation of highly accurate neuromorphic computing by progressive weight update in synaptic electronics. Low-energy synaptic operation requires both low reading current and short operation time to be applicable to large-scale neuromorphic computing systems. In this study, an energy-efficient synaptic device is implemented comprising a Ni/Pb(Zr

Semiconductors have long been perceived as a prerequisite for solid-state transistors. Although switching principles for nanometer-scale devices have emerged based on the deployment of two-dimensional (2D) van der Waals heterostructures, tunneling and ballistic currents through short channels are difficult to control, and semiconducting channel materials remain indispensable for practical switching. In this study, we report a semiconductor-less solid-state electronic device that exhibits an industry-applicable switching of the ballistic current. This device modulates the field emission barrier height across the graphene-hexagonal boron nitride interface with ION/IOFF of 106 obtained from the transfer curves and adjustable intrinsic gain up to 4, and exhibits unprecedented current stability in temperature range of 15–400 K. The vertical device operation can be optimized with the capacitive coupling in the device geometry. The semiconductor-less switching resolves the long-standing issue of temperature-dependent device performance, thereby extending the potential of 2D van der Waals devices to applications in extreme environments., In field-effect transistors, a semiconducting channel is indispensable for device switching. Here, the authors demonstrate semiconductor-less switching via modulation of the field emission barrier height across a graphene-hBN interface with ON/OFF ratio of 106.

Resistive switching phenomena have been attracting much interest due to their various physical mechanisms and industrial potential as prospective candidates for next-generation non-volatile memories. Because diverse resistive switching behaviors have been observed in a wide variety of materials, various mechanisms and applications are proposed. In this paper, we appropriately categorize resistive switching behaviors based on their physical mechanisms, such as formation/rupture of conductive filament, Schottky barrier modulation, valence changing, and conductive bridging. Memristive behaviors of resistive switching materials are also addressed because they have potential for neuromorphic devices. In addition, we introduce several problems raised during materialization of cross point array of resistive switching memories and suggested solutions.

Two-dimensional (2D)-layered semiconducting materials with considerable band gaps are emerging as a new class of materials applicable to next-generation devices. Particularly, black phosphorus (BP) is considered to be very promising for next-generation 2D electrical and optical devices because of its high carrier mobility of 200–1000 cm2 V–1 s–1 and large on/off ratio of 104 to 105 in field-effect transistors (FETs). However, its environmental instability in air requires fabrication processes in a glovebox filled with nitrogen or argon gas followed by encapsulation, passivation, and chemical functionalization of BP. Here, we report a new method for reduction of BP-channel devices fabricated without the use of a glovebox by galvanic corrosion of an Al overlayer. The reduction of BP induced by an anodic oxidation of Al overlayer is demonstrated through surface characterization of BP using atomic force microscopy, Raman spectroscopy, and X-ray photoemission spectroscopy along with electrical measurement of a...

Two-dimensional (2D) van der Waals (vdW) materials have recently attracted considerable attention due to their excellent electrical and mechanical properties. TmPSx (where Tm = a transition metal), which is a new class of 2D vdW materials, is expected to show various physical phenomena depending on the Tm used. In this paper, the unprecedented synaptic behavior of a vertical Ag/CrPS4/Au capacitor structure, where CrPS4 is a single-crystalline 2D vdW layer, is reported. Multi-stable resistive states were obtained using an external voltage of less than 0.3 V. Both short-term plasticity and long-term potentiation were observed by controlling the interval of the external voltage pulse. Simple mechanical exfoliation was used to develop a synaptic device based on a very thin CrPS4 layer with a thickness of ~17 nm. Therefore, it was demonstrated that vertical Ag/CrPS4/Au capacitors could be promising inorganic synaptic devices compatible with next-generation, flexible neuromorphic technologies. A capacitor made of a chromium thiophosphate (CrPS4) layer sandwiched between a silver electrode and a gold electrode behaves like a synapse. A Korea-based team led by Bae Ho Park from Konkuk University, Seoul, peeled very thin, crystalline CrPS4 layers of different thicknesses from the bulk material and fabricated top and bottom electrodes to prepare a series of Ag/CrPS4/Au capacitors. Under electrical pulses of different amplitudes, durations and intervals, these structures mimicked short-term and long-term variations in synaptic strength—neural behaviors involved in learning and memory. The devices’ behavior is attributed to the electrochemically induced migration of silver ions across the CrPS4 layer, leading to the formation and rupture of a conducting filament between the electrodes. The Ag/CrPS4/Au structure is promising for computing technologies that imitate neural pathways in the nervous system. We suggest synaptic devices using cation migration along thickness direction in a new class of 2D layered materials. An electrochemically active metal, such as Ag and Cu, is used for the operation of the synaptic device and chromium thiophosphate (CrPS4) single crystal is used as an electrolyte material. Multi-stable resistive states, short-term plasticity, and long-term potentiation are observed by controlling external voltage pulse with height smaller than 0.3 V. Given that simple mechanical exfoliation can generate very thin CrPS4 layers, the vertical Ag/CrPS4/Au capacitor offers a promising inorganic synaptic device compatible with next-generation flexible neuromorphic technology.

We report the effect of the oxygen content of the LaAlO3 layer on the synaptic behavior in the Pt/LaAlO3/Nb-doped SrTiO3 memristor for neuromorphic applications. As the oxygen-content decreases, the current becomes larger and the spike time-dependent plasticity (STDP) becomes less sensitive to the time difference between pre- and post-synaptic spike voltage. In addition, the conduction mechanism, which was found to be a combination of thermionic and Poole-Frenkel emissions, and the effect of oxygen content are explained in association with the oxygen vacancy in the LaAlO3 layer. The trade-off between large current and efficient STDP can be controlled by the oxygen content. Furthermore, the results of extracting the synaptic strength-based model parameters indicate that the Pt/LaAlO3/Nb-doped SrTiO3 shows the efficient STDP characteristics in comparison to previously reported memristor materials.

Accurate and precise determination of mechanical properties of nanoscale materials is mandatory since device performances of nanoelectromechanical systems (NEMS) are closely related to the flexural properties of the materials. In this study, the intrinsic mechanical properties of highly stressed silicon nitride (SiN) beams of varying lengths are investigated using two different techniques: Dynamic flexural measurement using optical interferometry and quasi-static flexural measurement using atomic force microscopy. The resonance frequencies of the doubly clamped, highly stressed beams are found to be inversely proportional to their length, which is not usually observed from a beam but is expected from a string-like structure. The mass density of the SiN beams can be precisely determined from the dynamic flexural measurements by using the values for internal stress and Young’s modulus determined from the quasi-static measurements. As a result, the mass resolution of the SiN beam resonators was predicted to be a few attograms, which was found to be in excellent agreement with the experimental results. This work suggests that accurate and precise determination of mechanical properties can be achieved through combined flexural measurement techniques, which is a crucial key for designing practical NEMS applications such as biomolecular sensors and gas detectors.

The temperature ($\it T$) dependence of electrical resistivity in graphene has been experimentally investigated between 10 and 400 K for samples prepared on various substrates; HfO$_2$, SiO$_2$ and h-BN. The resistivity of graphene shows a linear $\it T$-dependence at low $\it T$ and becomes superlinear above a substrate-dependent transition temperature. The results are explained by remote interfacial phonon scattering by surface optical phonons at the substrates. The use of an appropriate substrate can lead to a significant improvement in the charge transport of graphene.

A fast-ion Dα (FIDA) diagnostics system was installed for core and edge measurements on KSTAR. This system has two tangential FIDA arrays that cover both blue- and redshifted Dα lines (cold: 656.09 nm) in active views along the neutral beam 1 A centerline. The spectral band is 647–662.5 nm, and it covers the Doppler shift of the emission from the maximum energy of the neutral beam (100 keV). A curved filter strip with a motorized stage adequately prevents saturation of the electron multiplying charge-coupled device signal by the cold Dα line from the plasma edge. From comparisons of the measured spectra and FIDASIM modeling code, the FIDA spectra are well matched quantitatively. Moreover, the first measurements show that the FIDA radiance agrees with the neutron rate in the time trace during external heating and perturbation. In addition, responses are observed in the core FIDA radiance during the edge-localized mode cycle.

Polycrystalline 0.67Bi1.05FeO3-0.33BaTiO3 (BF33BT) lead-free piezoelectric thin film is fabricated on Pt(111)/Ti/SiO2/Si(100) substrate by using pulsed laser deposition technique. The remnant polarization (2Pr) and coercive field (2EC) observed from polarization hysteresis (P-E) loop are 18 μC/cm2 and 208 kV/cm, respectively. The local piezoelectric constant (d33,PFM) of BF33BT thin film is 92.5 pm/V which is as high as lead-based piezoelectric thin film results. From the temperature dependent dielectric constant result, the Curie temperature is 405 °C. These results show that BF33BT thin film is a promising candidate for lead-free piezoelectric nano- and micro-devices with high Curie temperature.

The set process in a unipolar resistance switching Pt/NiO/Pt thin film was conducted in two different ways: the current-biased set process (current sweep mode) and the voltage-biased set process (voltage sweep mode). In the current-biased set process, a compliance current setting was not necessary for continuing stable resistance switching. The resistance of the low resistance state, the reset and the set switching parameters were compared in both modes of the set processes. The distributions of the set parameters were found to be effectively reduced in the current-biased set process. These intriguing properties can be attributed to the prevention of an overshoot current during the set transition.

SrRu 1− x Fe x O 3−δ ( x = 0.00, 0.05, 0.10, and 0.20) thin films were fabricated to study the intrinsic aspects of a “self spin valve”. Using epitaxial strain and high oxygen partial pressure during thin film growth, single phase thin films with negligible oxygen vacancies were successfully grown, and problems related to A-site disorder and grain boundaries were minimized. Under application of an external magnetic field of up to 9 T, the resistivity of all films decreased, resulting in large negative magnetoresistance (up to ∼14.4%), which was stronger at temperatures in the range 10–30 K. An abrupt metal-insulator transition at T ∼ 43 K was found in the x = 0.20 film, which was explained using a two-fluid model related to electron–electron interactions. From the model, two fitting parameters were found to be necessary for in-situ and homogenous defects, while three or unphysical fitting parameters were necessary for ex-situ and inhomogeneous defects.

Recently, various electronic components including transistor, barristor, memory, and transparent electrode were implemented using graphene. While integrated circuits were demonstrated by combining graphene transistors and passive components, system on chip (SoC) platform, state-of-the-art semiconductor technology, by combining transistors and memories on the same chip, has not yet been demonstrated. The main obstacle of the realization of SoC is the complexity of fabrication processes originated from the process differences between the transistors and the memories. In this study, using simple and clean atomic force microscope lithography, we fabricated both the switching devices and the memories by forming very thin graphene oxide (GO) barriers in mono-layer graphene at the controlled oxidation voltages. Formed with 7 V and 9 V, the lateral graphene/GO/graphene junction devices exhibit switching of Fowler-Nordheim tunneling current and resistive memory behavior, respectively. The combination of high on/off current ratio (∼1000) of the switching device and nonvolatility of the memory device fabricated by the same process demonstrates the possibility of graphene SoC platform.

Lead-free (100 − x)BiFeO3–xBaTiO3 (BFBTx, x = 0, 30, 33, 40 and 50) piezoelectric thin films were deposited on platinized silicon substrates by using a pulsed laser deposition method. The BFBTx thin films has a single-phase perovskite structure with (111) preferred orientations. Atomic force microscopy images showed the dense morphology, and large piezoresponses were observed in the piezoelectric force microscope measurements. The best local piezoelectric coefficient, value of 259 pm V−1, was observed in the BFBT40 thin film. This result is one of the best values in lead-free piezoelectric thin films with a simple geometry. The BFBT40 piezoelectric thin film can be considered as a strong candidate to replace lead-based piezoelectric thin films for piezoelectric-based microelectromechanical systems.

Graphene-based spintronics has attracted much attention owing to the weak spin–orbit interaction and long spin relaxation length in graphene. For implementation of practical and high-density graphene-based spintronic devices, we need to define nanoscale areas with room-temperature ferromagnetism on graphene. Here we report the room-temperature ferromagnetism observed in nanoscale functionalized (oxidized and hydrogenated) graphene using atomic force microscope lithography without involving potential surface contamination and chemical agents. By performing magnetic force microscope (MFM) measurements, we can clearly distinguish the local ferromagnetic signal of selectively functionalized graphene from that of surrounding non-magnetic pristine graphene. The nanoscale functionalized graphene shows experimental evidence of room-temperature ferromagnetism: (1) larger MFM signal than that of graphene; (2) repulsive and attractive interaction with an MFM tip the magnetization of which points into and out of the graphene, respectively; and (3) MFM signal reversal after applying a high magnetic field at an elevated temperature of 400 K. Our first-principles calculations reveal that unpaired spins are present at non-passivated dangling bonds of vacancies on functionalized graphene and the stable ferromagnetic exchange interactions between them are favored. Therefore, nanoscale functionalized graphene is a good candidate for use as the spin injector or detector of high-density graphene-based spintronic devices.

We investigate temperature-dependent electrical properties of Pt/Nb:SrTiO3 Schottky junctions by measuring current–voltage and capacitance–voltage curves at various temperatures (20 K–300 K). Interestingly, the Schottky junctions have shown different temperature dependences of resistance according to the polarity of bias voltage: insulating behaviors are displayed in the positive bias branch, while metallic behaviors are observed in the negative bias branch. These behaviors can be ascribed to Schottky barrier modulation that arises from the temperature and field dependences of dielectric permittivity of SrTiO3. The modulation of the barrier profile determines anomalous tunneling current in the negative bias branch as a function of temperature. These comprehensive analyses could not only reveal the rich physics of the Pt/Nb:SrTiO3 Schottky junction but also enhance our understanding for high dielectric materials.We investigate temperature-dependent electrical properties of Pt/Nb:SrTiO3 Schottky junctions by measuring current–voltage and capacitance–voltage curves at various temperatures (20 K–300 K). Interestingly, the Schottky junctions have shown different temperature dependences of resistance according to the polarity of bias voltage: insulating behaviors are displayed in the positive bias branch, while metallic behaviors are observed in the negative bias branch. These behaviors can be ascribed to Schottky barrier modulation that arises from the temperature and field dependences of dielectric permittivity of SrTiO3. The modulation of the barrier profile determines anomalous tunneling current in the negative bias branch as a function of temperature. These comprehensive analyses could not only reveal the rich physics of the Pt/Nb:SrTiO3 Schottky junction but also enhance our understanding for high dielectric materials.

The manipulation of local ionic behavior via external stimuli in oxide systems is of great interest because it can help in directly tuning material properties. Among external stimuli, mechanical force has attracted intriguing attention as novel stimulus for ionic modulation. Even though effectiveness of mechanical force on local ionic modulation has been validated in terms of static effect, its real-time i.e., dynamic, behavior under an application of the force is barely investigated in spite of its crucial impact on device performance such as force or pressure sensors. In this study, we explore dynamic ionic behavior modulated by mechanical force in NiO thin films using electrochemical strain microscopy (ESM). Ionically mediated ESM hysteresis loops were significantly varied under an application of mechanical force. Based on these results, we were able to investigate relative relationship between the force and voltage effects on ionic motion and, further, control effectively ionic behavior through combination of mechanical and electrical stimuli. Our results can provide comprehensive information on the effect of mechanical forces on ionic dynamics in ionic systems.

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 VOx, where x can be in the range of 1 < x < 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 t...

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.

We report the effect of the external electric field on the photoluminescence (PL) properties of ZnO films grown by a pulsed laser deposition method. The PL quenching of bound excitons under the electric field was attributed to a decrease in the capture cross section of the radiative centers. In addition, the change in the surface/grain boundaries charge induced a degradation of the 3.33 eV emission line over the whole sample, which remained even after voltage removal. Besides the PL degradation, this emission at 3.33 eV demonstrates the change in the thermal quenching process, where the activation energy of exciton detachment corresponds to its binding energy. All behaviors were restored to the initial state by application of the voltage with opposite polarity. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

A precise measurement of graphene thickness is required for the design and development of nano-devices based on the material. Many factors affect this measurement when using scanning tunneling microscope (STM) and atomic force microscope (AFM), including the interaction between the scanning tip and ripples on graphene; such effects have not previously been explored. To investigate this, we measure the sample rotation angle dependence of graphene thickness as determined by contact mode and tapping mode AFM. The graphene thickness as determined by contact mode AFM follows a cosine modulus function of sample rotation angle, while tapping mode AFM reveals a constant graphene thickness, independent of sample rotation angle. For comparison, the AFM torsion signal is measured and follows a sine function of the sample rotation angle. All the measured sample rotation angle dependences can be explained by the interaction between linearly aligned ripples on graphene and the AFM tip in contact with the graphene.

An oxide-based resistance memory is a leading candidate to replace Si-based flash memory as it meets the emerging specifications for future memory devices. The non-uniformity in the key switching parameters and low endurance in conventional resistance memory devices are preventing its practical application. Here, a novel strategy to overcome the aforementioned challenges has been unveiled by tuning the growth direction of epitaxial brownmillerite SrFeO2.5 thin films along the SrTiO3 [111] direction so that the oxygen vacancy channels can connect both the top and bottom electrodes rather directly. The controlled oxygen vacancy channels help reduce the randomness of the conducting filament (CF). The resulting device displayed high endurance over 106 cycles, and a short switching time of ∼10 ns. In addition, the device showed very high uniformity in the key switching parameters for device-to-device and within a device. This work demonstrates a feasible example for improving the nanoscale device performance by controlling the atomic structure of a functional oxide layer.

We report flexible resistive random access memory (ReRAM) arrays fabricated by using NiO