The memristor, a composite word of memory and resistor, has become one of the most important electronic components for brain‐inspired neuromorphic computing in recent years. This device has the ...ability to control resistance with multiple states by memorizing the history of previous electrical inputs, enabling it to mimic a biological synapse in the neural network of the human brain. Among many candidates for memristive materials, including metal oxides, organic materials, and low‐dimensional nanomaterials, 2D layered materials have been widely investigated owing to their outstanding physical properties and electrical tunability, low‐power‐switching capability, and hetero‐integration compatibility. Hence, a large number of experimental demonstrations on 2D material‐based memristors have been reported showing their unique memristive characteristics and novel synaptic functionalities, distinct from traditional bulk‐material‐based systems. Herein, an overview of the latest advances in the structures, mechanisms, and memristive characteristics of 2D material‐based memristors is presented. Additionally, novel strategies to modulate and enhance the synaptic functionalities of 2D‐memristor‐based artificial synapses are summarized. Finally, as a foreseeing perspective, the potentials and challenges of these emerging materials for future neuromorphic electronics are also discussed.
The recent progress regarding memristors based on 2D materials for neuromorphic electronics is reviewed. They are categorized by their various materials, structures, and mechanisms. Owing to their outstanding synaptic characteristics, neuromorphic devices based on 2D materials offer excellent performance, energy‐efficient operation, and modifiable synaptic functions applicable to complex learning.
Nanoscale dynamic processes such as the diffusion of ions within solid-state structures are critical for understanding and tuning material properties in a wide range of areas, such as energy storage ...and conversion, catalysis, and optoelectronics. In the generation of new types of nanocrystals (NCs), diffusion-mediated ion exchange reactions have also been proposed as one of the most effective transformational strategies. However, retaining the original morphology and crystal structure of metal oxide NCs has been challenging because of Kirkendall void formation, and there has been no success, especially for anion exchange. Here we show that with the aid of an oxygen extracting reagent (OER), anion diffusion is dramatically accelerated and morphology-conserving anion exchange without Kirkendall void formation is possible. In the case of the conversion of Fe3O4 to Fe3S4, oxygen extraction and subsequent formation of the amorphous phase facilitate the migration of incoming sulfur anions by approximately 100-fold, which is close to the level of the outgoing cation diffusivity. We also demonstrate that the working principle of the morphology-conserving non-Kirkendall anion exchange is operative for metal oxide NCs with different shapes and crystal structures.
We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) using graphene-layered sheets. Heteroepitaxial nitride thin films were grown on graphene layers by using ...high-density, vertically aligned zinc oxide nanowalls as an intermediate layer. The nitride thin films on graphene layers show excellent optical characteristics at room temperature, such as stimulated emission. As one of the examples for device applications, LEDs that emit strong electroluminescence emission under room illumination were fabricated. Furthermore, the layered structure of a graphene substrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic.
Metabolic reprogramming during macrophage polarization supports the effector functions of these cells in health and disease. Here, we demonstrate that pyruvate dehydrogenase kinase (PDK), which ...inhibits the pyruvate dehydrogenase-mediated conversion of cytosolic pyruvate to mitochondrial acetyl-CoA, functions as a metabolic checkpoint in M1 macrophages. Polarization was not prevented by PDK2 or PDK4 deletion but was fully prevented by the combined deletion of PDK2 and PDK4; this lack of polarization was correlated with improved mitochondrial respiration and rewiring of metabolic breaks that are characterized by increased glycolytic intermediates and reduced metabolites in the TCA cycle. Genetic deletion or pharmacological inhibition of PDK2/4 prevents polarization of macrophages to the M1 phenotype in response to inflammatory stimuli (lipopolysaccharide plus IFN-γ). Transplantation of PDK2/4-deficient bone marrow into irradiated wild-type mice to produce mice with PDK2/4-deficient myeloid cells prevented M1 polarization, reduced obesity-associated insulin resistance, and ameliorated adipose tissue inflammation. A novel, pharmacological PDK inhibitor, KPLH1130, improved high-fat diet-induced insulin resistance; this was correlated with a reduction in the levels of pro-inflammatory markers and improved mitochondrial function. These studies identify PDK2/4 as a metabolic checkpoint for M1 phenotype polarization of macrophages, which could potentially be exploited as a novel therapeutic target for obesity-associated metabolic disorders and other inflammatory conditions.
Ni‐rich layered LiNixCoyMn1−x−yO2 (LNCM) with Ni content over >90% is considered as a promising lithium ion battery (LIB) cathode, attributed by its low cost and high practical capacity. However, ...Ni‐rich LNCM inevitably suffers rapid capacity fading at a high state of charge due to the mechanochemical breakdown; in particular, the microcrack formation has been regarded as one of the main culprits for Ni‐rich layered cathode failure. To address these issues, Ni‐rich layered cathodes with a textured microstructure are developed by phosphorous and boron doping. Attributed by the textured morphology, both phosphorous‐ and boron‐doped cathodes suppress microcrack formation and show enhanced cycle stability compared to the undoped cathode. However, there exists a meaningful capacity retention difference between the doped cathodes. By adapting the various analysis techniques, it is shown that the boron‐doped Ni‐rich layered cathode displays better cycle stability not only by its ability to suppress microcracks during cycling but also by its primary particle morphology that is reluctant to oxygen evolution. The present work reveals that not only restraint of particle cracks but also suppression of oxygen release by developing the oxygen stable facets is important for further improvements in state‐of‐the‐art Li ion battery Ni‐rich layered cathode materials.
Herein, the effect of boron doping on oxygen stability in LiNi0.92Co0.04Mn0.04O2 (LNCM) lithium ion battery cathodes is systematically investigated using various measurements. The boron‐doped LNCM produces the textured microstructure with more oxygen stabilized facets, thus not only aiding in restraining the particle cracks but also effectively suppressing the oxygen evolution to improve the cycle stability.
Abstract
Although the thermoelectric effect was discovered around 200 years ago, the main application in practice is thermoelectric cooling using the traditional Bi
2
Te
3
. The related studies of ...new and efficient room-temperature thermoelectric materials and modules have, however, not come to fruition yet. In this work, the electronic properties of n-type Mg
3.2
Bi
1.5
Sb
0.5
material are maximized via delicate microstructural design with the aim of eliminating the thermal grain boundary resistance, eventually leading to a high
zT
above 1 over a broad temperature range from 323 K to 423 K. Importantly, we further demonstrated a great breakthrough in the non-Bi
2
Te
3
thermoelectric module, coupled with the high-performance p-type α-MgAgSb, for room-temperature power generation and thermoelectric cooling. A high conversion efficiency of ~2.8% at the temperature difference of 95 K and a maximum temperature difference of 56.5 K are experimentally achieved. If the interfacial contact resistance is further reduced, our non-Bi
2
Te
3
module may rival the long-standing champion commercial Bi
2
Te
3
system. Overall, this work represents a substantial step towards the real thermoelectric application using non-Bi
2
Te
3
materials and devices.
The development of energy‐efficient artificial synapses capable of manifoldly tuning synaptic activities can provide a significant breakthrough toward novel neuromorphic computing technology. Here, a ...new class of artificial synaptic architecture, a three‐terminal device consisting of a vertically integrated monolithic tungsten oxide memristor, and a variable‐barrier tungsten selenide/graphene Schottky diode, termed as a ‘synaptic barrister,’ are reported. The device can implement essential synaptic characteristics, such as short‐term plasticity, long‐term plasticity, and paired‐pulse facilitation. Owing to the electrostatically controlled barrier height in the ultrathin van der Waals heterostructure, the device exhibits gate‐controlled memristive switching characteristics with tunable programming voltages of 0.2−0.5 V. Notably, by electrostatic tuning with a gate terminal, it can additionally regulate the degree and tuning rate of the synaptic weight independent of the programming impulses from source and drain terminals. Such gate tunability cannot be accomplished by previously reported synaptic devices such as memristors and synaptic transistors only mimicking the two‐neuronal‐based synapse. These capabilities eventually enable the accelerated consolidation and conversion of synaptic plasticity, functionally analogous to the synapse with an additional neuromodulator in biological neural networks.
A synaptic barristor, an artificial synaptic architecture formed by monolithically integrating a memristor and a barristor using phase‐engineered 2D heterostructures, is presented. This three‐terminal artificial synapse could implement fundamental synaptic functions with external gate controllability. Such unique capabilities enable the accelerated consolidation and conversion of synaptic plasticity, functionally analogous to the synapse with an additional neuromodulator in biological neural networks.
Perovskite solar cells (PSCs) based on organic monovalent cation (methylammonium or formamidinium) have shown excellent optoelectronic properties with high efficiencies above 22%, threatening the ...status of silicon solar cells. However, critical issues of long‐term stability have to be solved for commercialization. The severe weakness of the state‐of‐the‐art PSCs against moisture originates mainly from the hygroscopic organic cations. Here, rubidium (Rb) is suggested as a promising candidate for an inorganic–organic mixed cation system to enhance moisture‐tolerance and photovoltaic performances of formamidinium lead iodide (FAPbI3). Partial incorporation of Rb in FAPbI3 tunes the tolerance factor and stabilizes the photoactive perovskite structure. Phase conversion from hexagonal yellow FAPbI3 to trigonal black FAPbI3 becomes favored when Rb is introduced. The authors find that the absorbance and fluorescence lifetime of 5% Rb‐incorporated FAPbI3 (Rb0.05FA0.95PbI3) are enhanced than bare FAPbI3. Rb0.05FA0.95PbI3‐based PSCs exhibit a best power conversion efficiency of 17.16%, which is much higher than that of the FAPbI3 device (13.56%). Moreover, it is demonstrated that the Rb0.05FA0.95PbI3 film shows superior stability against high humidity (85%) and the full device made with the mixed perovskite exhibits remarkable long‐term stability under ambient condition without encapsulation, retaining the high performance for 1000 h.
Partial substitution of inorganic rubidium cation (Rb+) for formamidinium lead iodide (FAPbI3) perovskite suppresses nonperovskite phase formation and increases fluorescence lifetime. Introduction of the smaller monovalent cation in FAPbI3 renders the perovskite more tolerant to high humidity. These lead to enhanced photovoltaic performances and long‐term stability of perovskite solar cells based on Rb‐mixed FAPbI3.
Unit‐cell‐thick MoS2 is a promising electrocatalyst for the hydrogen evolution reaction (HER) owing to its tunable catalytic activity, which is determined based on the energetics and molecular ...interactions of different types of HER active sites. Kinetic responses of MoS2 active sites, including the reaction onset, diffusion of the electrolyte and H2 bubbles, and continuation of these processes, are important factors affecting the catalytic activity of MoS2. Investigating these factors requires a direct real‐time analysis of the HER occurring on spatially independent active sites. Herein, the H2 evolution and electrolyte diffusion on the surface of MoS2 are observed in real time by in situ electrochemical liquid‐phase transmission electron microscopy (LPTEM). Time‐dependent LPTEM observations reveal that different types of active sites are sequentially activated under the same conditions. Furthermore, the electrolyte flow to these sites is influenced by the reduction potential and site geometry, which affects the bubble detachment and overall HER activity of MoS2.
In situ electrochemical liquid‐phase transmission electron microscopy (LPTEM) facilitates real‐time observation of the H2 evolution reaction (HER) on MoS2 monolayer. Time‐series LPTEM shows sequential activation, H2 bubble formation, and electrolyte flow on different types of catalytic active sites. Directionality of H2 bubbling and competitive wetting between the bubbles and electrolyte on catalyst surface significantly affect the HER activity of the active sites.
Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential in optoelectronic applications due to their direct band gap and exceptional quantum yield. ...However, TMD‐based light‐emitting devices have shown low external quantum efficiencies as imbalanced free carrier injection often leads to the formation of non‐radiative charged excitons, limiting practical applications. Here, electrically confined electroluminescence (EL) of neutral excitons in tungsten diselenide (WSe2) light‐emitting transistors (LETs) based on the van der Waals heterostructure is demonstrated. The WSe2 channel is locally doped to simultaneously inject electrons and holes to the 1D region by a local graphene gate. At balanced concentrations of injected electrons and holes, the WSe2 LETs exhibit strong EL with a high external quantum efficiency (EQE) of ≈8.2 % at room temperature. These experimental and theoretical results consistently show that the enhanced EQE could be attributed to dominant exciton emission confined at the 1D region while expelling charged excitons from the active area by precise control of external electric fields. This work shows a promising approach to enhancing the EQE of 2D light‐emitting transistors and modulating the recombination of exciton complexes for excitonic devices.
This work demonstrates electrically confined electroluminescence of neutral excitons in WSe2 light‐emitting transistors (LETs). By balancing injected electrons and holes, and electrically confining neutral excitons, WSe2 LETs exhibit strong electroluminescence with a high external quantum efficiency of ≈8.2 % at room temperature. This work shows a promising approach to enhancing efficiency and modulating the recombination of exciton complexes for 2D excitonic devices.
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