Nanometer‐Scale Phase Transformation Determines Threshold and Memory Switching Mechanism

Creation of nanometer‐scale conductive filaments in resistive switching devices makes them appealing for advanced electrical applications. While in situ electrical probing transmission electron microscopy promotes fundamental investigations of how the conductive filament comes into existence, it does not provide proof‐of‐principle observations for the filament growth. Here, using advanced microscopy techniques, electrical, 3D compositional, and structural information of the switching‐induced conductive filament are described. It is found that during in situ probing microscopy of a Ag/TiO₂/Pt device showing both memory‐ and threshold‐switching characteristics, a crystalline Ag‐doped TiO₂ forms at vacant sites on the device surface and acts as the conductive filament. More importantly, change in filament morphology varying with applied compliance currents determines the underlying switching mechanisms that govern either memory or threshold response. When focusing more on threshold switching features, it is demonstrated that the structural disappearance of the filament arises at the end of the constricted region and leads to the spontaneous phase transformation from crystalline conductive state into an initial amorphous insulator. Use of the proposed method enables a new pathway for observing nanosized features in a variety of devices at the atomic scale in three dimensions.     

Nanoscale Plasmon‐Enhanced Spectroscopy in Memristive Switches

Resistive switching memories are nonvolatile memory cells based on nano‐ionic redox processes and offer prospects for high scalability, ultrafast write and read access, and low power consumption. In two‐terminal cation based devices a nanoscale filament is formed in a switching material by metal ion migration from the anode to the cathode. However, the filament growth and dissolution mechanisms and the dynamics involved are still open questions, restricting device optimization. Here, a spectroscopic technique to optically characterize in situ the resistive switching effect is presented. Resistive switches arranged in a nanoparticle‐on‐mirror geometry are developed, exploiting the high sensitivity to morphological changes occurring in the tightly confined plasmonic hotspot within the switching material. The focus is on electrochemical metallization and the optical signatures detected over many cycles indicate incomplete removal of metal particles from the filament upon RESET and suggest that the filament can nucleate from different positions from cycle to cycle. The technique here is nondestructive and the measurements can be easily performed in tunable ambient conditions and with realistic cell geometries.     

Controllable Organic Resistive Switching Achieved by One‐Step Integration of Cone‐Shaped Contact

Conductive filaments (CFs)‐based resistive random access memory possesses the ability of scaling down to sub‐nanoscale with high‐density integration architecture, making it the most promising nanoelectronic technology for reclaiming Moore's law. Compared with the extensive study in inorganic switching medium, the scientific challenge now is to understand the growth kinetics of nanoscale CFs in organic polymers, aiming to achieve controllable switching characteristics toward flexible and reliable nonvolatile organic memory. Here, this paper systematically investigates the resistive switching (RS) behaviors based on a widely adopted vertical architecture of Al/organic/indium‐tin‐oxide (ITO), with poly(9‐vinylcarbazole) as the case study. A nanoscale Al filament with a dynamic‐gap zone (DGZ) is directly observed using in situ scanning transmission electron microscopy (STEM) , which demonstrates that the RS behaviors are related to the random formation of spliced filaments consisting of Al and oxygen vacancy dual conductive channels growing through carbazole groups. The randomicity of the filament formation can be depressed by introducing a cone‐shaped contact via a one‐step integration method. The conical electrode can effectively shorten the DGZ and enhance the localized electric field, thus reducing the switching voltage and improving the RS uniformity. This study provides a deeper insight of the multiple filamentary mechanisms for organic RS effect.     

3D Porous Hydrogel/Conducting Polymer Heterogeneous Membranes with Electro‐/pH‐Modulated Ionic Rectification

Heterogeneous membranes composed of asymmetric structures or compositions have enormous potential in sensors, molecular sieves, and energy devices due to their unique ion transport properties such as ionic current rectification and ion selectivity. So far, heterogeneous membranes with 1D nanopores have been extensively studied. However, asymmetric structures with 3D micro‐/nanoscale pore networks have never been investigated. Here, a simple and versatile approach to low‐costly fabricate hydrogel/conducting polymer asymmetric heterogeneous membranes with electro‐/pH‐responsive 3D micro‐/nanoscale ion channels is introduced. Due to the asymmetric heterojunctions between positively charged nanoporous polypyrrole (PPy) and negatively charged microscale porous hydrogel poly (acrylamide‐co‐acrylic acid) (P(AAm‐co‐AA)), the membrane can rectify ion transmembrane transport in response to both electro‐ and pH‐stimuli. Numerical simulations based on coupled Poisson and Nernst–Plank equations are carried out to explain the ionic rectification mechanisms for the membranes. The membranes are not dependent on elaborately fabricated 1D ion channel substrates and hence can be facilely prepared in a low‐cost and large‐area way. The hybridization of hydrogel and conducting polymer offers a novel strategy for constructing low‐cost, large‐area and multifunctional membranes, expanding the tunable ionic rectification properties into macroscopic membranes with micro‐/nanoscale pores, which would stimulate practical applications of the membranes.     

Self‐Assembly of Functional Molecules into 1D Crystalline Nanostructures

Self‐assembled functional nanoarchitectures are employed as important nanoscale building blocks for advanced materials and smart miniature devices to fulfill the increasing needs of high materials usage efficiency, low energy consumption, and high‐performance devices. One‐dimensional (1D) crystalline nanostructures, especially molecule‐composed crystalline nanostructures, attract significant attention due to their fascinating infusion structure and functionality which enables the easy tailoring of organic molecules with excellent carrier mobility and crystal stability. In this review, we discuss the recent progress of 1D crystalline self‐assembled nanostructures of functional molecules, which include both a small molecule‐derived and a polymer‐based crystalline nanostructure. The basic principles of the molecular structure design and the process engineering of 1D crystalline nanostructures are also discussed. The molecular building blocks, self‐assembly structures, and their applications in optical, electrical, and photoelectrical devices are overviewed and we give a brief outlook on crucial issues that need to be addressed in future research endeavors.     

Nano‐Architectured Composite Anode Enabling Long‐Term Cycling Stability for High‐Capacity Lithium‐Ion Batteries

Failure mechanisms associated with silicon‐based anodes are limiting the implementation of high‐capacity lithium‐ion batteries. Understanding the aging mechanism that deteriorates the anode performance and introducing novel‐architectured composites offer new possibilities for improving the functionality of the electrodes. Here, the characterization of nano‐architectured composite anode composed of active amorphous silicon domains (a‐Si, 20 nm) and crystalline iron disilicide (c‐FeSi₂, 5–15 nm) alloyed particles dispersed in a graphite matrix is reported. This unique hierarchical architecture yields long‐term mechanical, structural, and cycling stability. Using advanced electron microscopy techniques, the nanoscale morphology and chemical evolution of the active particles upon lithiation/delithiation are investigated. Due to the volumetric variations of Si during lithiation/delithiation, the morphology of the a‐Si/c‐FeSi₂ alloy evolves from a core‐shell to a tree‐branch type structure, wherein the continuous network of the active a‐Si remains intact yielding capacity retention of 70% after 700 cycles. The root cause of electrode polarization, initial capacity fading, and electrode swelling is discussed and has profound implications for the development of stable lithium‐ion batteries.     

Robust Electrodes Based on Coaxial TiC/C–MnO2 Core/Shell Nanofiber Arrays with Excellent Cycling Stability for High‐Performance Supercapacitors

A coaxial electrode structure composed of manganese oxide‐decorated TiC/C core/shell nanofiber arrays is produced hydrothermally in a KMnO₄ solution. The pristine TiC/C core/shell structure prepared on the Ti alloy substrate provides the self‐sacrificing carbon shell and highly conductive TiC core, thus greatly simplifying the fabrication process without requiring an additional reduction source and conductive additive. The as‐prepared electrode exhibits a high specific capacitance of 645 F g^?1 at a discharging current density of 1 A g^?1 attributable to the highly conductive TiC/C and amorphous MnO₂ shell with fast ion diffusion. In the charging/discharging cycling test, the as‐prepared electrode shows high stability and 99% capacity retention after 5000 cycles. Although the thermal treatment conducted on the as‐prepared electrode decreases the initial capacitance, the electrode undergoes capacitance recovery through structural transformation from the crystalline cluster to layered birnessite type MnO₂ nanosheets as a result of dissolution and further electrodeposition in the cycling. 96.5% of the initial capacitance is retained after 1000 cycles at high charging/discharging current density of 25 A g^?1. This study demonstrates a novel scaffold to construct MnO₂ based SCs with high specific capacitance as well as excellent mechanical and cycling stability boding well for future design of high‐performance MnO₂‐based SCs.     

Nanowire‐Intensified Metal‐Enhanced Fluorescence in Hybrid Polymer‐Plasmonic Electrospun Filaments

Hybrid polymer‐plasmonic nanostructures might combine high enhancement of localized fields from metal nanoparticles with light confinement and long‐range transport in subwavelength dielectric structures. Here, the complex behavior of fluorophores coupling to Au nanoparticles within polymer nanowires, which features localized metal‐enhanced fluorescence (MEF) with unique characteristics compared to conventional structures, is reported. The intensification effect when the particle is placed in the organic filaments is remarkably higher with respect to thin films of comparable thickness, thus highlighting a specific, nanowire‐related enhancement of MEF effects. A dependence on the confinement volume in the dielectric nanowire is also indicated, with MEF significantly increasing upon reduction of the wire diameter. These findings are rationalized by finite element simulations, predicting a position‐dependent enhancement of the quantum yield of fluorophores embedded in the fibers. Calculation of the ensemble‐averaged fluorescence enhancement unveils the possibility of strongly enhancing the overall emission intensity for structures with size twice the diameter of the embedded metal particles. These new, hybrid fluorescent systems with localized enhanced emission, and the general nanowire‐enhanced MEF effects associated to them, are highly relevant for developing nanoscale light‐emitting devices with high efficiency and intercoupled through nanofiber networks, highly sensitive optical sensors, and novel laser architectures.     

Metal‐Polymer Nanocomposites for Functional Applications

Nanocomposites combine favorable features of the constituents on the nanoscale to obtain new functionalities. The present paper is concerned with the preparation of polymer‐based nanocomposites consisting of metal nanoparticles in a polymer matrix and the resulting functional properties. Emphasis is placed on vapor phase deposition which inter alia allows the incorporation of alloy clusters with well defined composition and tailored filling factor profiles. Examples discussed here include optical composites with tuned particle surface plasmon resonances for plasmonic applications, magnetic high frequency materials with cut‐off frequencies well above 1 GHz, sensors that are based on the dramatic change in the electronic properties near the percolation threshold, and antibacterial coatings which benefit from the large effective surface of nanoparticles and the increased chemical potential which both strongly enhance ion release.     

Observation of Resistive Switching Behavior in Crossbar Core–Shell Ni/NiO Nanowires Memristor

The crossbar structure of resistive random access memory (RRAM) is the most promising technology for the development of ultrahigh‐density devices for future nonvolatile memory. However, only a few studies have focused on the switching phenomenon of crossbar RRAM in detail. The main purpose of this study is to understand the formation and disruption of the conductive filament occurring at the crossbar center by real‐time transmission electron microscope observation. Core–shell Ni/NiO nanowires are utilized to form a cross‐structure, which restrict the position of the conductive filament to the crosscenter. A significant morphological change can be observed near the crossbar center, which results from the out‐diffusion and backfill of oxygen ions. Energy dispersive spectroscopy and electron energy loss spectroscopy demonstrate that the movement of the oxygen ions leads to the evolution of the conductive filament, followed by redox reactions. Moreover, the distinct reliability of the crossbar device is measured via ex situ experiments. In this work, the switching mechanism of the crossbar core–shell nanowire structure is beneficial to overcome the problem of nanoscale minimization. The experimental method shows high potential to fabricate high‐density RRAM devices, which can be applied to 3D stacked package technology and neuromorphic computing systems.     

Eigenschaften amorpher transparenter leitfähiger Oxide (a‐TCOs)

Properties of amorphous transparent conducting oxides (a‐TCOs) In this contribution the optical and electrical properties of amorphous transparent conductive oxides (a‐TCOs) are analysed by means of atomistic material modelling. The investigation of model structures by computer simulations allows a deeper understanding of the physical phenomena. In the case of the a‐TCOs deep energy levels in the electronic band structure, that reduce for example the optical transparency, can be linked to structural defects on the atomic scale. This allows a well‐directed search for strategies which anneal these defects.     

Highly Solid‐State Emissive Pyridinium‐Substituted Tetraphenylethylene Salts: Emission Color‐Tuning with Counter Anions and Application for Optical Waveguides

In this paper seven salts of pyridinium‐substituted tetraphenylethylene with different anions are reported. They show typical aggregation‐induced emission. Crystal structures of three of the salts with (CF₃SO₂)₂N^?, CF₃SO₃ ^?, and SbF₆ ^? as the respective counter anions, are determined. The emission behavior of their amorphous and crystalline solids is investigated. Both amorphous and crystalline solids, except for the one with I^?, are highly emissive. Certain amorphous solids are red‐emissive with almost the same quantum yields and fluorescence life‐times. However, some crystalline solids are found to show different emission colors varying from green to yellow. Thus, their emission colors can be tuned by the counter anions. Furthermore, certain crystalline solids are highly emissive compared to the respective amorphous solids. Such solid‐state emission behavior of these pyridinium‐substituted tetraphenylethylene salts is interpreted on the basis of their crystal structures. In addition, optical waveguiding behavior of fabricated microrods is presented.     

Stimulus‐Responsive Anti‐Oxidizing Drug Crystals and their Ecological Implication

Various antioxidants are being used to neutralize the harmful effects of reactive oxygen species (ROS) overproduced in diseased tissues and contaminated environments. Polymer‐directed crystallization of antioxidants has attracted attention as a way to control drug efficacy through molecular dissolution. However, most recrystallized antioxidants undertake continuous dissolution independent of the ROS level, thus causing side‐effects. This study demonstrates a unique method to assemble antioxidant crystals that modulate their dissolution rate in response to the ROS level. We hypothesized that antioxidants recrystallized using a ROS‐labile polymer would be triggered to dissolve when the ROS level increases. We examined this hypothesis by using catechin as a model antioxidant. Catechin was recrystallized using polyethylenimine cross‐linked with ROS‐labile diselanediylbis‐(ethane‐2,1‐diyl)‐diacrylate. Catechin crystallized with the ROS‐labile polymer displays accelerated dissolution proportional to the H₂O₂ concentration. The ROS‐responsive catechin crystals protect vascular cells from oxidative insults by activating intracellular glutathione peroxidase expression and, in turn, inhibiting an increase in the intracellular oxidative stress. In addition, ROS‐responsive catechin crystals alleviate changes in the heart rate of Daphnia magna in oxidative media. We propose that the results of this study would be broadly useful for improving the therapeutic efficacy of a broad array of drug compounds.     

High‐Quality‐Factor Mid‐Infrared Toroidal Excitation in Folded 3D Metamaterials

With unusual electromagnetic radiation properties and great application potentials, optical toroidal moments have received increasing interest in recent years. 3D metamaterials composed of split ring resonators with specific orientations in micro‐/nanoscale are a perfect choice for toroidal moment realization in optical frequency considering the excellent magnetic confinement and quality factor, which, unfortunately, are currently beyond the reach of existing micro‐/nanofabrication techniques. Here, a 3D toroidal metamaterial operating in mid‐infrared region constructed by metal patterns and dielectric frameworks is designed, by which high‐quality‐factor toroidal resonance is observed experimentally. The toroidal dipole excitation is confirmed numerically and further demonstrated by phase analysis. Furthermore, the far‐field radiation intensity of the excited toroidal dipoles can be adjusted to be predominant among other multipoles by just tuning the incident angle. The related processing method expands the capability of focused ion beam folding technologies greatly, especially in 3D metamaterial fabrication, showing great flexibility and nanoscale controllability on structure size, position, and orientation.     

Highly Deformable and See‐Through Polymer Light‐Emitting Diodes with All‐Conducting‐Polymer Electrodes

Despite the high expectation of deformable and see‐through displays for future ubiquitous society, current light‐emitting diodes (LEDs) fail to meet the desired mechanical and optical properties, mainly because of the fragile transparent conducting oxides and opaque metal electrodes. Here, by introducing a highly conductive nanofibrillated conducting polymer (CP) as both deformable transparent anode and cathode, ultraflexible and see‐through polymer LEDs (PLEDs) are demonstrated. The CP‐based PLEDs exhibit outstanding dual‐side light‐outcoupling performance with a high optical transmittance of 75% at a wavelength of 550 nm and with an excellent mechanical durability of 9% bending strain. Moreover, the CP‐based PLEDs fabricated on 4 µm thick plastic foils with all‐solution processing have extremely deformable and foldable light‐emitting functionality. This approach is expected to open a new avenue for developing wearable and attachable transparent displays.     

Highly Uniform Resistive Switching Properties of Solution‐Processed Silver‐Embedded Gelatin Thin Film

The silver‐embedded gelatin (AgG) thin film produced by the solution method of metal salts dissolved in gelatin is presented. Its simple fabrication method ensures the uniform distribution of Ag dots. Memory devices based on AgG exhibit good device performance, such as the ON/OFF ratio in excess of 10⁵ and the coefficient of variation in less of 50%. To further investigate the position of filament formation and the role of each element, current sensing atomic force microscopy (CSAFM) analysis as well as elemental line profiles across the two different conditions in the LRS and HRS are analyzed. The conductive and nonconductive regions in the current map of the CSAFM image show that the conductive filaments occur in the AgG layer around Ag dots. The migration of oxygen ions and the redox reaction of carbon are demonstrated to be the driving mechanism for the resistive switching of AgG memory devices. The results show that dissolving metal salts in gelatin is an effective way to achieve high‐performance organic–electronic applications.     

High‐Throughput Near‐Field Optical Nanoprocessing of Solution‐Deposited Nanoparticles

The application of nanoscale electrical and biological devices will benefit from the development of nanomanufacturing technologies that are high‐throughput, low‐cost, and flexible. Utilizing nanomaterials as building blocks and organizing them in a rational way constitutes an attractive approach towards this goal and has been pursued for the past few years. The optical near‐field nanoprocessing of nanoparticles for high‐throughput nanomanufacturing is reported. The method utilizes fluidically assembled microspheres as a near‐field optical confinement structure array for laser‐assisted nanosintering and nanoablation of nanoparticles. By taking advantage of the low processing temperature and reduced thermal diffusion in the nanoparticle film, a minimum feature size down to ≈100 nm is realized. In addition, smaller features (50 nm) are obtained by furnace annealing of laser‐sintered nanodots at 400 °C. The electrical conductivity of sintered nanolines is also studied. Using nanoline electrodes separated by a submicrometer gap, organic field‐effect transistors are subsequently fabricated with oxygen‐stable semiconducting polymer.     

Nanoscale Charge Percolation Analysis in Polymer‐Sorted (7,5) Single‐Walled Carbon Nanotube Networks

The current percolation in polymer‐sorted semiconducting (7,5) single‐walled carbon nanotube (SWNT) networks, processed from solution, is investigated using a combination of electrical field‐effect measurements, atomic force microscopy (AFM), and conductive AFM (C‐AFM) techniques. From AFM measurements, the nanotube length in the as‐processed (7,5) SWNTs network is found to range from ≈100 to ≈1500 nm, with a SWNT surface density well above the percolation threshold and a maximum surface coverage ≈58%. Analysis of the field‐effect charge transport measurements in the SWNT network using a 2D homogeneous random‐network stick‐percolation model yields an exponent coefficient for the transistors OFF currents of 16.3. This value is indicative of an almost ideal random network containing only a small concentration of metallic SWNTs. Complementary C‐AFM measurements on the other hand enable visualization of current percolation pathways in the xy plane and reveal the isotropic nature of the as‐spun (7,5) SWNT networks. This work demonstrates the tremendous potential of combining advanced scanning probe techniques with field‐effect charge transport measurements for quantification of key network parameters including current percolation, metallic nanotubes content, surface coverage, and degree of SWNT alignment. Most importantly, the proposed approach is general and applicable to other nanoscale networks, including metallic nanowires as well as hybrid nanocomposites.     

Toward Tailorable Zn‐Ion Textile Batteries with High Energy Density and Ultrafast Capability: Building High‐Performance Textile Electrode in 3D Hierarchical Branched Design

Rechargeable Zn‐ion batteries are promising candidates for wearable energy storage devices. However, their performance is severely restricted by the low conductivity and inferior mass loading. Herein, a new type of the textile based electrodes with 3D hierarchical branched design is reported. Both Ni nanoparticles and carbon nanotubes are used to build conductive coatings on the textiles. The 3D hierarchical nanostructures, consisting of the vertical‐aligned nanosheets and the fluffy‐like small flakes, grow on the conductive textiles to form the self‐supported electrodes. It ensures fast electron/ion transport and high mass loading, and maintains the structure stability during cycling. Two textile electrodes with the NiCo hydroxide and MnO₂ self‐branched nanostructures are constructed. Their faster kinetics, higher capacity and better rate capability than the solitary nanosheets based counterpart demonstrate the superiority of the hierarchical architecture. Moreover, the solid‐state Zn‐MnO₂ and Zn‐NiCo batteries are fabricated based on the textile electrodes and the polymer electrolytes. The high energy density, superior power density and good long‐term cycling stability confirm their excellent energy storage ability and fast charge/discharge capability. Particularly, the high safety under various conditions enable them promising candidates for wearable electronics.     

Superwettability‐Based Interfacial Chemical Reactions

Superwetting interfaces arising from the cooperation of surface energy and multiscale micro/nanostructures are extensively studied in biological systems. Fundamental understandings gained from biological interfaces boost the control of wettability under different dimensionalities, such as 2D surfaces, 1D fibers and channels, and 3D architectures, thus permitting manipulation of the transport physics of liquids, gases, and ions, which profoundly impacts chemical reactions and material fabrication. In this context, the progress of new chemistry based on superwetting interfaces is highlighted, beginning with mass transport dynamics, including liquid, gas, and ion transport. In the following sections, the impacts of the superwettability‐mediated transport dynamics on chemical reactions and material fabrication is discussed. Superwettability science has greatly enhanced the efficiency of chemical reactions, including photocatalytic, bioelectronic, electrochemical, and organic catalytic reactions, by realizing efficient mass transport. For material fabrication, superwetting interfaces are pivotal in the manipulation of the transport and microfluidic dynamics of liquids on solid surfaces, leading to the spatially regulated growth of low‐dimensional single‐crystalline arrays and high‐quality polymer films. Finally, a perspective on future directions is presented.