Photo-Assisted Ferroelectric Domain Control for α-In2Se3 Artificial Synapses Inspired by Spontaneous Internal Electric Fields.

α-In2Se3 semiconductor crystals realize artificial synapses by tuning in-plane and out-of-plane ferroelectricity with diverse avenues of electrical and optical pulses. While the electrically induced ferroelectricity of α-In2Se3 shows synaptic memory operation, the optically assisted synaptic plasticity in α-In2Se3 has also been preferred for polarization flipping enhancement. Here, the synaptic memory behavior of α-In2Se3 is demonstrated by applying electrical gate voltages under white light. As a result, the induced internal electric field is identified at a polarization flipped conductance channel in α-In2Se3/hexagonal boron nitride (hBN) heterostructure ferroelectric field effect transistors (FeFETs) under white light and discuss the contribution of this built-in electric field on synapse characterization. The biased dipoles in α-In2Se3 toward potentiation polarization direction by an enhanced internal built-in electric field under illumination of white light lead to improvement of linearity for long-term depression curves with proper electric spikes. Consequently, upon applying appropriate electric spikes to α-In2Se3/hBN FeFETs with illuminating white light, the recognition accuracy values significantly through the artificial learning simulation is elevated for discriminating hand-written digit number images.

Three-Dimensional Flower-like MoS2 Nanosheets Grown on Graphite as High-Performance Anode Materials for Fast-Charging Lithium-Ion Batteries.

The demand for fast-charging lithium-ion batteries (LIBs) with long cycle life is growing rapidly due to the increasing use of electric vehicles (EVs) and energy storage systems (ESSs). Meeting this demand requires the development of advanced anode materials with improved rate capabilities and cycling stability. Graphite is a widely used anode material for LIBs due to its stable cycling performance and high reversibility. However, the sluggish kinetics and lithium plating on the graphite anode during high-rate charging conditions hinder the development of fast-charging LIBs. In this work, we report on a facile hydrothermal method to achieve three-dimensional (3D) flower-like MoS2 nanosheets grown on the surface of graphite as anode materials with high capacity and high power for LIBs. The composite of artificial graphite decorated with varying amounts of MoS2 nanosheets, denoted as MoS2@AG composites, deliver excellent rate performance and cycling stability. The 20-MoS2@AG composite exhibits high reversible cycle stability (~463 mAh g-1 at 200 mA g-1 after 100 cycles), excellent rate capability, and a stable cycle life at the high current density of 1200 mA g-1 over 300 cycles. We demonstrate that the MoS2-nanosheets-decorated graphite composites synthesized via a simple method have significant potential for the development of fast-charging LIBs with improved rate capabilities and interfacial kinetics.

Atom-Scale Chemistry in Chalcopyrite-Based Photovoltaic Materials Visualized by Atom Probe Tomography.

Chalcopyrite-based materials for photovoltaic devices tend to exhibit complex structural imperfections originating from their polycrystalline nature; nevertheless, properly controlled devices are surprisingly irrelevant to them in terms of resulting device performances. The present work uses atom probe tomography to characterize co-evaporated high-quality Cu(In,Ga)Se2 (CIGS) films on flexible polyimide substrates either with or without doping with Na or doping with Na followed by K via a post-deposition treatment. The intent is to elucidate the unique characteristics of the grain boundaries (GBs) in CIGS, in particular the correlations/anti-correlations between matrix elements and the alkali dopants. Various compositional fluctuations are identified at GBs irrespective of the presence of alkali elements. However, [Cu-poor and Se/In,Ga-rich] GBs are significantly more common than [Cu-rich and Se/In,Ga-poor] ones. In addition, the anti-correlations between Cu and the other matrix elements are found to show not only regular trends among themselves but also the association with the degree of alkali segregation at GBs. The Na and K concentrations exhibited a correlation at the GBs but not in the intragrain regions. Density functional theory calculations are used to explain the compositional fluctuations and alkali segregation at the GBs. Our experimental and theoretical findings not only reveal the benign or beneficial characteristics of the GBs of CIGS but also provide a fundamental understanding of the GB chemistry in CIGS-based materials.

Chloroaluminate Anion Intercalation in Graphene and Graphite: From Two-Dimensional Devices to Aluminum-Ion Batteries.

A rechargeable aluminum-ion battery based on chloroaluminate electrolytes has received intense attention due to the high abundance and chemical stability of aluminum. However, the fundamental intercalation processes and dynamics in these battery systems remain unresolved. Here, the energetics and dynamics of chloroaluminate ion intercalation in atomically thin single crystal graphite are investigated by fabricating mesoscopic devices for charge transport and operando optical microscopy. These mesoscopic measurements are compared to the high-performance rechargeable Al-based battery consisting of a few-layer graphene-multiwall carbon nanotube composite cathode. These composites exhibit a 60% capacity enhancement over pyrolytic graphite, while an ∼3-fold improvement in overall ion diffusivity is also obtained exhibiting ∼1% of those in atomically thin single crystals. Our results thus establish the distinction between intrinsic and ensemble electrochemical behavior in Al-based batteries and show that engineering ion transport in these devices can yet lead to vast improvements in battery performance.

Low-Loss Organic Hyperbolic Materials in the Visible Spectral Range: A Joint Experimental and First-Principles Study.

Hyperbolic media strengthen numerous attractive applications in optics such as super-resolution imaging, enhanced spontaneous emission, and nanoscale waveguiding. Natural hyperbolic materials exist at visible frequencies; however, implementations of these materials suffer substantial compromises resulting from the high loss in the currently available candidates. Here, the first experimental and theoretical investigation of regioregular poly(3-alkylthiophenes) (rr-P3ATs), a naturally low-loss organic hyperbolic material (OHM) in the visible frequency range, is shown. These hyperbolic properties arise from a highly ordered structure of layered electron-rich conjugated thiophene ring backbones separated by insulating alkyl side chains. The optical and electronic properties of the rr-P3AT can be tuned by controlling the degree of crystallinity and alkyl side chain length. First-principles calculations support the experimental observations, which result from the rr-P3AT's structural and optical anisotropy. Conveniently, rr-P3AT-based OHMs are facile to fabricate, flexible, and biocompatible, which may lead to tremendous new opportunities in a wide range of applications.

Pulverization-Tolerance and Capacity Recovery of Copper Sulfide for High-Performance Sodium Storage.

Finding suitable electrode materials is one of the challenges for the commercialization of a sodium ion battery due to its pulverization accompanied by high volume expansion upon sodiation. Here, copper sulfide is suggested as a superior electrode material with high capacity, high rate, and long-term cyclability owing to its unique conversion reaction mechanism that is pulverization-tolerant and thus induces the capacity recovery. Such a desirable consequence comes from the combined effect among formation of stable grain boundaries, semi-coherent boundaries, and solid-electrolyte interphase layers. The characteristics enable high cyclic stability of a copper sulfide electrode without any need of size and morphological optimization. This work provides a key finding on high-performance conversion reaction based electrode materials for sodium ion batteries.

Effective CO2 and CO Separation Using [M2(DOBDC)] (M = Mg, Co, Ni) with Unsaturated Metal Sites and Excavation of Their Adsorption Sites.

Isostructural [M2(DOBDC)(EG)2] (M = Mg, Co, Ni) frameworks are first synthesized by controlling the pH* in the reaction medium. Coordinated ethylene glycols form a hexagonal OH cluster, which works as a template to grow single crystals with high crystallinity. After the liberation of solvated molecules, [M2(DOBDC)] shows notably higher surface areas than the reported values and completely different CO2 and CO separation properties depending on the kinds of unsaturated metal. Therefore, breakthrough experiments using a CO2/CO mixed gas show that Mg-MOF has a longer breakthrough time for CO2 than for CO, whereas Co/Ni-MOFs have longer breakthrough times for CO than for CO2. Apart from CO2 and CO, other gases such as CH4, H2, and N2 were almost not adsorbed at all in these materials at 298 K. To reveal the role of unsaturated metal sites, CO2 and CO adsorption sites are unequivocally determined by single-crystal X-ray diffraction analysis. One of very interesting discoveries is that there are two CO2 and CO adsorption positions (sites A and B) in the hexagonal channels. Site A is the unsaturated metal center working as Lewis acidic sites, and site B is the secondary adsorption site located between two A sites. A close inspection of crystal structures reveals that unsaturated Co(II) and Ni(II) sites adsorb both CO2 and CO, whereas the unsaturated Mg(II) sites strongly capture only CO2, not CO. Density functional theory calculations elucidate the discrepancy in CO affinity: Co(II) and Ni(II) form strong π-back-donating bonds with CO via electron transfer from the d orbitals of the transition metals to the antibonding molecular orbitals of CO, whereas Mg(II) does not participate in electron transfer or orbital overlap with CO. This observation provides new insight into the synthesis of novel functional materials with high CO2/CO separation performance.

High-throughput ab initio calculations on dielectric constant and band gap of non-oxide dielectrics.

High-k dielectrics, materials having a large band gap (Eg) and high dielectric constant (k) simultaneously, constitute critical components in microelectronic devices. Because of the inverse relationship between Eg and k, materials with large values in both properties are rare. Therefore, massive databases on Eg and k will be useful in identifying optimal high-k materials. While experimental and theoretical data on Eg and k of oxides are accumulating, corresponding information is scarce for non-oxide dielectrics with anions such as C, N, F, P, S, and Cl. To identify promising high-k dielectrics among these material groups, we screen 869 compounds of binary carbides, nitrides, sulfides, phosphides, chlorides, and fluorides, through automated ab initio calculations. Among these compounds, fluorides exhibit an Eg-k relation that is comparable to that of oxides. By further screening over ternary fluorides, we identify fluorides such as BiF3, LaF3, and BaBeF4 that could serve as useful high-k dielectrics.

Hydrogen Evolution Reaction at Anion Vacancy of Two-Dimensional Transition-Metal Dichalcogenides: Ab Initio Computational Screening.

The catalytic activity for the hydrogen evolution reaction (HER) at the anion vacancy of 40 2D transition-metal dichalcogenides (TMDs) is investigated using the hydrogen adsorption free energy (Δ GH) as the activity descriptor. While vacancy-free basal planes are mostly inactive, anion vacancy makes the hydrogen bonding stronger than clean basal planes, promoting the HER performance of many TMDs. We find that ZrSe2 and ZrTe2 have similar Δ GH as Pt, the best HER catalyst, at low vacancy density. Δ GH depends significantly on the vacancy density, which could be exploited as a tuning parameter. At proper vacancy densities, MoS2, MoSe2, MoTe2, ReSe2, ReTe2, WSe2, IrTe2, and HfTe2 are expected to show the optimal HER activity. The detailed analysis of electronic structure and the multiple linear regression results identifies the vacancy formation energy and band-edge positions as key parameters correlating with Δ GH at anion vacancy of TMDs.

Effect of Nb concentration on the spin-orbit coupling strength in Nb-doped SrTiO3 epitaxial thin films.

Several oxide materials have attracted much interest for the application in spintronic devices due to unusual properties originating from the strongly correlated orbital and spin degrees of freedom. One missing part in oxide spintronics is a good spin channel featured by strong spin-orbit coupling (SOC) which enables an efficient control of the electron's spin. We have systematically investigated the dependence of the SOC strength of Sr(Nb x Ti1-x)O3 thin films on Nb concentration (nNb = 2~20 at. %) as a deeper exploration of a recent finding of the strong SOC in a heavily Nb-doped SrTiO3 (Sr(Nb0.2Ti0.8)O3) epitaxial film. Apart from a finding of a proportionality of the SOC to nNb, we have observed an intriguing temperature dependence of the SOC strength and the anisotropic magnetoresistance (MR) in the intermediate nNb region. These phenomena are associated with the temperature dependence of Landé g-factor and the change of the band structure, which is consistent with the result of density functional theory (DFT) calculation.

Property database for single-element doping in ZnO obtained by automated first-principles calculations.

Throughout the past decades, doped-ZnO has been widely used in various optical, electrical, magnetic, and energy devices. While almost every element in the Periodic Table was doped in ZnO, the systematic computational study is still limited to a small number of dopants, which may hinder a firm understanding of experimental observations. In this report, we systematically calculate the single-element doping property of ZnO using first-principles calculations. We develop an automation code that enables efficient and reliable high-throughput calculations on thousands of possible dopant configurations. As a result, we obtain formation-energy diagrams for total 61 dopants, ranging from Li to Bi. Furthermore, we evaluate each dopant in terms of n-type/p-type behaviors by identifying the major dopant configurations and calculating carrier concentrations at a specific dopant density. The existence of localized magnetic moment is also examined for spintronic applications. The property database obtained here for doped ZnO will serve as a useful reference in engineering the material property of ZnO through doping.