The large-scale crossbar array is a promising architecture for hardware-amenable energy efficient three-dimensional memory and neuromorphic computing systems. While accessing a memory cell with ...negligible sneak currents remains a fundamental issue in the crossbar array architecture, up-to-date memory cells for large-scale crossbar arrays suffer from process and device integration (one selector one resistor) or destructive read operation (complementary resistive switching). Here, we introduce a self-selective memory cell based on hexagonal boron nitride and graphene in a vertical heterostructure. Combining non-volatile and volatile memory operations in the two hexagonal boron nitride layers, we demonstrate a self-selectivity of 10
with an on/off resistance ratio larger than 10
. The graphene layer efficiently blocks the diffusion of volatile silver filaments to integrate the volatile and non-volatile kinetics in a novel way. Our self-selective memory minimizes sneak currents on large-scale memory operation, thereby achieving a practical readout margin for terabit-scale and energy-efficient memory integration.
Growth of 2D van der Waals layered single‐crystal (SC) films is highly desired not only to manifest the intrinsic physical and chemical properties of materials, but also to enable the development of ...unprecedented devices for industrial applications. While wafer‐scale SC hexagonal boron nitride film has been successfully grown, an ideal growth platform for diatomic transition metal dichalcogenide (TMdC) films has not been established to date. Here, the SC growth of TMdC monolayers on a centimeter scale via the atomic sawtooth gold surface as a universal growth template is reported. The atomic tooth‐gullet surface is constructed by the one‐step solidification of liquid gold, evidenced by transmission electron microscopy. The anisotropic adsorption energy of the TMdC cluster, confirmed by density‐functional calculations, prevails at the periodic atomic‐step edge to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming the SC film, regardless of the Miller indices. Growth using the atomic sawtooth gold surface as a universal growth template is demonstrated for several TMdC monolayer films, including WS2, WSe2, MoS2, the MoSe2/WSe2 heterostructure, and W1−xMoxS2 alloys. This strategy provides a general avenue for the SC growth of diatomic van der Waals heterostructures on a wafer scale, to further facilitate the applications of TMdCs in post‐silicon technology.
The atomic sawtooth Au surface allows the anisotropic adsorption energy of transition metal dichalcogenide (TMdC) clusters to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming a single‐crystal TMdC film, regardless of the Miller indices. Growth using the atomic sawtooth gold surface as a universal growth template is further demonstrated for several TMdC monolayer films, including WS2, WSe2, MoS2, the MoSe2/WSe2 heterostructure, and W1−xMoxS2 alloys.
Bonding geometry engineering of metal–oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, ...thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin‐film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO3/SrTiO3 (SRO/STO) superlattices. In particular, the propagation of RuO6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.
Propagation control of octahedral bonding geometry is established via atomic‐scale fabrication of artificial oxide superlattices. RuO6 octahedral distortion in SrRuO3 layers are systematically modulated by controlling the atomic unit cell thickness of adjacent cubic SrTiO3 layers, further stabilizing the macroscopic crystalline symmetry. The customized lattice degree of freedom provides selective controllability over the functionalities of the strongly correlated oxide.
Water- and acid-stable electride realizes a sustainable electrocatalytic reaction as an efficient solid-state electron source.
Electrides have emerged as promising materials with exotic properties, ...such as extraordinary electron-donating ability. However, the inevitable instability of electrides, which is caused by inherent excess electrons, has hampered their widespread applications. We report that a self-passivated dihafnium sulfide electride (Hf
2
S
2+
∙2e
−
) by double amorphous layers exhibits a strong oxidation resistance in water and acid solutions, enabling a persistent electrocatalytic hydrogen evolution reaction. The naturally formed amorphous Hf
2
S layer on the cleaved Hf
2
S
2+
∙2e
−
surface reacts with oxygen to form an outermost amorphous HfO
2
layer with ~10-nm thickness, passivating the Hf
2
S
2+
∙2e
−
electride. The excess electrons in the Hf
2
S
2+
∙2e
−
electride are transferred through the thin HfO
2
passivation layer to water molecules under applied electric fields, demonstrating the first electrocatalytic reaction with excellent long-term sustainability and no degradation in performance. This self-passivation mechanism in reactive conditions can advance the development of stable electrides for energy-efficient applications.
Surface oxygen vacancies in anatase-type Fe@TiO2 nanoparticles for superior photocatalytic activities can be readily controlled by simple pH treatment. A comprehensive analysis at the atomic to ...mesoscopic scale clearly reveals that the point defects are maximized and predominantly formed on the surface of the anatase NPs in basic pH condition, thereby resulting in electronically different core-shell nanostructures.
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•Surface oxygen vacancies in the nanoparticles are controlled via the pH treatment.•The surface defect population can be maximized by use of basic pH condition.•pH treatment affects catalytic properties of Fe-doped anatase TiO2 nanoparticles.•The nanoparticles treated at basic pH condition show the best catalytic properties.•Atomic analysis reveals that the anatase structure was kept in the whole pH range.
Introduction of defect structures into Fe-doped TiO2 nanoparticles (Fe@TiO2 NPs) has been shown to endow NPs with improved photocatalytic properties. However, current strategies for the preparation of defect-containing NPs require high temperature or complicated treatments, which can induce unwanted phase transition. In this paper, we report a facile method to introduce surface oxygen vacancies into anatase-type Fe@TiO2 NPs without altering the crystalline phase via simple pH treatments at moderate temperatures. Furthermore, we present the effects of pH on the formation of surface oxygen vacancies. The optimized treatment under basic conditions is revealed to promote the formation of oxygen vacancies on the surface of anatase Fe@TiO2 NPs and effectively reduces the particle size by more than 25%, thereby causing a significant enhancement in the photocatalytic activities of the NPs (e.g. ~3.5 times better in photocatalytic degradation rate of 4-CP as compared to acid-treated Fe@TiO2). Comprehensive structural and chemical characterizations reveal that the point defects are predominantly formed on the surface of anatase NPs, and their population can be maximized by use of basic pH conditions. Our results pave a way toward the facile and efficient engineering of surface defect structures on catalytic metal oxide NPs for the design of high-performance photocatalysts.
To realize ultrafast and energy-efficient electronic devices, reducing the switching voltage slope for ON and OFF states that scales the supply voltage and device dimensions is critical. Novel device ...architectures based on two-dimensional (2D) materials have overcome the fundamental thermionic limit of the switching slope (60 mV/dec); however, a versatile switching device required for highly integrated memory and neuromorphic applications has not been achieved with such exceptional switching slope characteristics. Here, we demonstrate a switching voltage slope down to 0.62 mV/dec in a threshold switching device based on a vertical heterojunction of silver/hexagonal boron nitride (h-BN)/graphene. The sub-1 mV/dec switching slope for the first time, maintaining a high ON/OFF ratio (up to 1010), originates from the unique coupling between the migrated silver atoms and the chemically-inert graphene electrode through the 2D insulating h-BN. Moreover, our original switching device enables the evolution from a conventional volatile (threshold switching) to non-volatile memristive state by adequate voltage spikes, which is ideal for selector applications in highly integrated crossbar array architecture and in a novel synaptic device for neuromorphic computing.
Ultralow (down to sub-1 mV/dec) switching voltage slopes maintaining extraordinary ON/OFF ratios beyond 1010 could be realized by using a van der Waals heterostructure, which provides an ideal component for crossbar array architecture and novel synaptic devices for neuromorphic computing. Display omitted
•A switching device based on a vertical heterojunction was realized with sub-1 mV/dec switching voltage slope.•The resistance switching ratio of our tunable memory cell reaches up to 1011.•Scanning transmission electron microscopy reveals a unique switching mechanism of our device.•The reliable memory switching transitions from volatile to non-volatile memory emulates biological synaptic operations.
Atomic-scale defects are essential for improving thermoelectric (TE) performance of most state-of-the-art materials by simultaneously tuning the electronic and thermal properties. However, because ...the plural atomic-scale defects are generally inherent and disordered in nanostructured TE materials, their complexity and ambiguity on determining TE performance remain a challenge to be solved. Furthermore, the thermal stability of atomic-scale defects in nanostructured TE materials has not been studied much so far. Herein, we report that the atomic-scale defect disorders are indispensable for high TE performance of nanostructured Ti1–xHfxNiSn1–ySby half-Heusler alloys, but gradually degraded at over 773 K, deteriorating the TE performance. It is found from the thermal annealing of nanostructured Ti0.5Hf0.5NiSn0.98Sb0.02 alloys that the annihilation of Ti,Hf/Sn antisite defects primarily reduces atomic-scale defect disorders and largely contributes to the increase of lattice thermal conductivity. Moreover, it is verified that the Ni interstitial defects mainly dominate the electronic transport properties, leading to the enhancement of power factor. Direct atomic structure observations clearly demonstrate the inherent Ni interstitial defects and the thermal vulnerability of Ti,Hf/Sn antisite defects. These results provide an important guide for the application of half-Heusler alloys with highly disordered atomic-scale defects.
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Polarity discontinuity across LaAlO
/SrTiO
(LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions ...characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (
) and disappear simultaneously above
with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (
) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with
distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.
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