Low‐Dimensional Lead‐Free Inorganic Perovskites for Resistive Switching with Ultralow Bias

3D organic–inorganic and all‐inorganic lead halide perovskites have been intensively pursued for resistive switching memories in recent years. Unfortunately, instability and lead toxicity are two foremost challenges for their large‐scale commercial applications. Dimensional reduction and composition engineering are effective means to overcome these challenges. Herein, low‐dimensional inorganic lead‐free Cs₃Bi₂I₉ and CsBi₃I₁₀ perovskite‐like films are exploited for resistive switching memory applications. Both devices demonstrate stable switching with ultrahigh on/off ratios (≈10⁶), ultralow operation voltages (as low as 0.12 V), and self‐compliance characteristics. 0D Cs₃Bi₂I₉‐based device shows better retention time and larger reset voltage than the 2D CsBi₃I₁₀‐based device. Multilevel resistive switching behavior is also observed by modulating the current compliance, contributing to the device tunability. The resistive switching mechanism is hinged on the formation and rupture of conductive filaments of halide vacancies in the perovskite films, which is correlated with the formation of AgI_x layers at the electrode/perovskite interface. This study enriches the library of switching materials with all‐inorganic lead‐free halide perovskites and offers new insights on tuning the operation of solution‐processed memory devices.     

Direct Transcription of Two‐Dimensional Colloidal Crystal Arrays into Three‐Dimensional Photonic Crystals

A simple protocol for the fabrication of three‐dimensional (3D) photonic crystals in silicon is presented. Surface structuring by nanosphere lithography is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, sequential passivation reactive ion etching, is a one‐step processing protocol relying on sequential passivation and reactive ion etching reactions using C₄F₈ and SF₆ plasma chemistries. The diffusion of fresh reactants and etch product species inside the etched channels is found to play an important role affecting the structural uniformity of the designed structures and the etch rate drift is corrected by adjusting the reaction times. High quality photonic crystals are thus obtained by adding the third dimension to the two‐dimensional (2D) colloidal crystal assemblies through SPRIE. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. 3D photonic crystal lattices exhibiting optical stop‐bands in the infrared spectral region are demonstrated, proving the potential of SPRIE for fast, simple, and large‐scale fabrication of photonic structures.     

Three‐dimensional Printing for Catalytic Applications: Current Status and Perspectives

Three‐dimensional (3D) printing, also known as additive manufacturing, is a fabrication method that has recently received worldwide attention. It provides a convenient and economical way to prepare 3D structures in designable ways. As the technology has developed and the operational costs have decreased, the applications of 3D printing have greatly expanded. Catalyst fabrication is a promising area for 3D printing. Printing processes result in better control of catalyst structures and catalyst distribution. In this perspective, a general overview of the commonly available 3D printing methods that are feasible for the preparation of heterogeneous catalysts is given. Additionally, recent works on printing strategies and new materials for catalysts are discussed. Future development is also addressed.     

High‐Resolution Patterning of Hydrogels in Three Dimensions using Direct‐Write Photofabrication for Cell Guidance

The development of three‐dimensional, spatially defined neuronal cultures that mimic chemical and physical attributes of native tissue is of considerable interest for various applications, including the development of tailored neuronal networks and clinical repair of damaged nerves. Here, the use of multiphoton excitation to photocrosslink protein microstructures within three‐dimensional, optically transparent hydrogel materials, such as those based on hyaluronic acid, is reported. Multiphoton excitation confines photocrosslinking to a three‐dimensional voxel with submicron spatial resolution, enabling fabrication of protein matrices with low‐ to sub‐micrometer feature sizes by scanning the focus of a laser relative to the reagent solution. These methods can be used to create complex three‐dimensional architectures that provide both chemical and topographical cues for cell culture and guidance, providing for the first time a means to direct cell adhesion and migration on size scales relevant to in vivo environments. Using this approach, guidance of both dorsal root ganglion cells (DRGs) and hippocampal neural progenitor cells (NPCs) along arbitrary, three‐dimensional paths is demonstrated.     

Low‐Temperature‐Grown Transition Metal Oxide Based Storage Materials and Oxide Transistors for High‐Density Non‐volatile Memory

An effective stacked memory concept utilizing all‐oxide‐based device components for future high‐density nonvolatile stacked structure data storage is developed. GaInZnO (GIZO) thin‐film transistors, grown at room temperature, are integrated with one‐diode (CuO/InZnO)–one‐resistor (NiO) (1D–1R) structure oxide storage node elements, fabricated at room temperature. The low growth temperatures and fabrication methods introduced in this paper allow the demonstration of a stackable memory array as well as integrated device characteristics. Benefits provided by low‐temperature processes are demonstrated by fabrication of working devices over glass substrates. Here, the device characteristics of each individual component as well as the characteristics of a combined select transistor with a 1D–1R cell are reported. X‐ray photoelectron spectroscopy analysis of a NiO resistance layer deposited by sputter and atomic layer deposition confirms the importance of metallic Ni content in NiO for bi‐stable resistance switching. The GIZO transistor shows a field‐effect mobility of 30 cm² V⁻¹ s⁻¹, a V_th of +1.2 V, and a drain current on/off ratio of up to 10⁸, while the CuO/InZnO heterojunction oxide diode has forward current densities of 2 × 10⁴ A cm⁻². Both of these materials show the performance of state‐of‐the‐art oxide devices.     

High‐Density Hotspots Engineered by Naturally Piled‐Up Subwavelength Structures in Three‐Dimensional Copper Butterfly Wing Scales for Surface‐Enhanced Raman Scattering Detection

Very recently, wing scales of natural Lepidopterans (butterflies and moths) manifested themselves in providing excellent three dimensional (3D) hierarchical structures for surface‐enhanced Raman scattering (SERS) detection. But the origin of the observed enormous Raman enhancement of the analytes on 3D metallic replicas of butterfly wing scales has not been clarified yet, hindering a full utilization of this huge natural wealth with more than 175 000 3D morphologies. Herein, the 3D sub‐micrometer Cu structures replicated from butterfly wing scales are successfully tuned by modifying the Cu deposition time. An optimized Cu plating process (10 min in Cu deposition) yields replicas with the best conformal morphologies of original wing scales and in turn the best SERS performance. Simulation results show that the so‐called “rib‐structures” in Cu butterfly wing scales present naturally piled‐up hotspots where electromagnetic fields are substantially amplified, giving rise to a much higher hotspot density than in plain 2D Cu structures. Such a mechanism is further verified in several Cu replicas of scales from various butterfly species. This finding paves the way to the optimal scale candidates out of ca. 175 000 Lepidopteran species as bio‐templates to replicate for SERS applications, and thus helps bring affordable SERS substrates as consumables with high sensitivity, high reproducibility, and low cost to ordinary laboratories across the world.     

Self‐Powered Trace Memorization by Conjunction of Contact‐Electrification and Ferroelectricity

Triboelectric nanogenerator (TENG) is a newly invented technology that can effectively harvest ambient mechanical energy from various motions with promising applications in portable electronics, self‐powered sensor networks, etc. Here, by coupling TENG and a thin film of ferroelectric polymer, a new application is designed for TENG as a self‐powered memory system for recording a mechanical displacement/trace. The output voltage produced by the TENG during motion can polarize the dipole moments in the ferroelectric thin film. Later, by applying a displacement current measurement to detect the polarization density in the ferroelectric film, the motion information of the TENG can be directly read. The sliding TENG and the single‐electrode TENG matrix are both utilized for realizing the memorization of the motion trace in one‐dimensional and two‐dimensional space, respectively. Currently, the ferroelectric thin film with a size of 3.1 mm² can record a minimum area changing of 30 mm² and such resolution can still be possibly improved. These results prove that the ferroelectric polymer is an effective memory material to work together with TENG and thereby the fabricated memory system can potentially be used as a self‐powered displacement monitor.     

3D Printed High‐Loading Lithium‐Sulfur Battery Toward Wearable Energy Storage

Wearable electronic devices are the new darling of consumer electronics, and energy storage devices are an important part of them. Here, a wearable lithium‐sulfur (Li‐S) bracelet battery using three‐dimensional (3D) printing technology (additive manufacturing) is designed and manufactured for the first time. The bracelet battery can be easily worn to power the wearable device. The “additive” manufacturing characteristic of 3D printing provides excellent controllability of the electrode thickness with much simplified process in a cost‐effective manner. Due to the conductive 3D skeleton providing interpenetrating transmission paths and channels for electrons and ions, the 3D Li‐S battery can provide 505.4 mAh g^?1 specific capacity after 500 cycles with an active material loading as high as 10.2 mg cm^?1. The practicality is illustrated by wearing the bracelet battery on the wrist and illuminating the red light‐emitting diode. Therefore, the bracelet battery manufactured by 3D printing technology can address the needs of the wearable power supply.     

A Die‐Selection Method Using Search‐Space Conditions for Yield Enhancement in 3D Memory

Three‐dimensional (3D) memories using through‐silicon vias (TSVs) as vertical buses across memory layers will likely be the first commercial application of 3D integrated circuit technology. The memory dies to stack together in a 3D memory are selected by a die‐selection method. The conventional die‐selection methods do not result in a high‐enough yields of 3D memories because 3D memories are typically composed of known‐good‐dies (KGDs), which are repaired using self‐contained redundancies. In 3D memory, redundancy sharing between neighboring vertical memory dies using TSVs is an effective strategy for yield enhancement. With the redundancy sharing strategy, a known‐bad‐die (KBD) possibly becomes a KGD after bonding. In this paper, we propose a novel die‐selection method using KBDs as well as KGDs for yield enhancement in 3D memory. The proposed die‐selection method uses three search‐space conditions, which can reduce the search space for selecting memory dies to manufacture 3D memories. Simulation results show that the proposed die‐selection method can significantly improve the yield of 3D memories in various fault distributions.     

Investigation of Time–Dependent Resistive Switching Behaviors of Unipolar Nonvolatile Organic Memory Devices

Organic resistive memory devices are one of the promising next‐generation data storage technologies which can potentially enable low‐cost printable and flexible memory devices. Despite a substantial development of the field, the mechanism of the resistive switching phenomenon in organic resistive memory devices has not been clearly understood. Here, the time–dependent current behavior of unipolar organic resistive memory devices under a constant voltage stress to investigate the turn‐on process is studied. The turn‐on process is discovered to occur probabilistically through a series of abrupt increases in the current, each of which can be associated with new conducting paths formation. The measured turn‐on time values can be collectively described with the Weibull distribution which reveals the properties of the percolated conducting paths. Both the shape of the network and the current path formation rate are significantly affected by the stress voltage. A general probabilistic nature of the percolated conducting path formation during the turn‐on process is demonstrated among unipolar memory devices made of various materials. The results of this study are also highly relevant for practical operations of the resistive memory devices since the guidelines for time‐widths and magnitudes of voltage pulses required for writing and reading operation can be potentially set.     

Three‐Dimensional Heteroatom‐Doped Carbon Nanofiber Networks Derived from Bacterial Cellulose for Supercapacitors

Recently, heteroatom‐doped three‐dimensional (3D) nanostructured carbon materials have attracted immense interest because of their great potential in various applications. Hence, it is highly desirable to exploit a simple, renewable, scalable, multifunctional, and general strategy to engineer 3D heteroatom‐doped carbon nanomaterials. Herein, a simple, eco‐friendly, general, and effective way to fabricate 3D heteroatom‐doped carbon nanofiber networks on a large scale is reported. Using this method, 3D P‐doped, N,P‐co‐doped, and B,P‐co‐doped carbon nanofiber networks are successfully fabricated by the pyrolysis of bacterial cellulose immersed in H₃PO₄, NH₄H₂PO₄, and H₃BO₃/H₃PO₄ aqueous solution, respectively. Moreover, the as‐prepared N,P‐co‐doped carbon nanofibers exhibit good supercapacitive performance.     

Oxygen Ion Drift‐Induced Complementary Resistive Switching in Homo TiOx/TiOy/TiOx and Hetero TiOx/TiON/TiOx Triple Multilayer Frameworks

Developing a means by which to compete with commonly used Si‐based memory devices represents an important challenge for the realization of future three‐dimensionally stacked crossbar‐array memory devices with multifunctionality. Therefore, oxide‐based resistance switching memory (ReRAM), with its associated phenomena of oxygen ion drifts under a bias, is becoming increasingly important for use in nanoscalable crossbar arrays with an ideal memory cell size due to its simple metal–insulator–metal structure and low switching current of 10–100 μA. However, in a crossbar array geometry, one single memory element defined by the cross‐point of word and bit lines is highly susceptible to unintended leakage current due to parasitic paths around neighboring cells when no selective devices such as diodes or transistors are used. Therefore, the effective complementary resistive switching (CRS) features in all Ti‐oxide‐based triple layered homo Pt/TiO_x/TiO_y/TiO_x/Pt and hetero Pt/TiO_x/TiON/TiO_x/Pt geometries as alternative resistive switching matrices are reported. The possible resistive switching nature of the novel triple matrices is also discussed together with their electrical and structural properties. The ability to eliminate both an external resistor for efficient CRS operation and a metallic Pt middle electrode for further cost‐effective scalability will accelerate progress toward the realization of cross‐bar ReRAM in this framework.     

2D Metal–Organic Framework Nanosheets with Time‐Dependent and Multilevel Memristive Switching

Metal–organic framework (MOF) nanosheets have attracted significant interests for sensing, electrochemical, and catalytic applications. Most significantly, 2D MOF with highly accessible sites on the surface is expected to be applicable in data storage. Here, the memory device is first demonstrated by employing M‐TCPP (TCPP: tetrakis(4‐carboxyphenyl)porphyrin, M: metal) as resistive switching (RS) layer. The as‐fabricated resistive random access memory (RRAM) devices exhibit a typical electroforming free bipolar switching characteristic with on/off ratio of 10³, superior retention, and reliability performance. Furthermore, the time‐dependent RS behaviors under constant voltage stress of 2D M‐TCPP–based RRAMs are systematically investigated. The properties of the percolated conducting paths are revealed by the Weibull distribution by collecting the measured turn‐on time. The multilevel information storage state can be gotten by setting a series of compliance current. The charge trapping assisted hopping is proposed as operation principle of the MOF‐based RRAMs which is further confirmed by atomic force microscopy at electrical modes. The research is highly relevant for practical operation of 2D MOF nanosheet–based RRAM, since the time widths, magnitudes of pulses, and multilevel‐data storage can be potentially set.     

3D‐Printed All‐Fiber Li‐Ion Battery toward Wearable Energy Storage

Conventional bulky and rigid power systems are incapable of meeting flexibility and breathability requirements for wearable applications. Despite the tremendous efforts dedicated to developing various 1D energy storage devices with sufficient flexibility, challenges remain pertaining to fabrication scalability, cost, and efficiency. Here, a scalable, low‐cost, and high‐efficiency 3D printing technology is applied to fabricate a flexible all‐fiber lithium‐ion battery (LIB). Highly viscous polymer inks containing carbon nanotubes and either lithium iron phosphate (LFP) or lithium titanium oxide (LTO) are used to print LFP fiber cathodes and LTO fiber anodes, respectively. Both fiber electrodes demonstrate good flexibility and high electrochemical performance in half‐cell configurations. All‐fiber LIB can be successfully assembled by twisting the as‐printed LFP and LTO fibers together with gel polymer as the quasi‐solid electrolyte. The all‐fiber device exhibits a high specific capacity of ≈110 mAh g^?1 at a current density of 50 mA g^?1 and maintains a good flexibility of the fiber electrodes, which can be potentially integrated into textile fabrics for future wearable electronic applications.     

Highly Stable Contact Doping in Organic Field Effect Transistors by Dopant‐Blockade Method

In organic device applications, a high contact resistance between metal electrodes and organic semiconductors prevents an efficient charge injection and extraction, which fundamentally limits the device performance. Recently, various contact doping methods have been reported as an effective way to resolve the contact resistance problem. However, the contact doping has not been explored extensively in organic field effect transistors (OFETs) due to dopant diffusion problem, which significantly degrades the device stability by damaging the ON/OFF switching performance. Here, the stability of a contact doping method is improved by incorporating “dopant‐blockade molecules” in the poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) film in order to suppress the diffusion of the dopant molecules. By carefully selecting the dopant‐blockade molecules for effectively blocking the dopant diffusion paths, the ON/OFF ratio of PBTTT OFETs can be maintained over 2 months. This work will maximize the potential of OFETs by employing the contact doping method as a promising route toward resolving the contact resistance problem.     

Monolithic Heterogeneous Integration of 3D Radio Frequency L−C Elements by Self‐Rolled‐Up Membrane Nanotechnology

This work reports a three‐dimensional (3D) radio frequency L?C filter network enabled by a CMOS‐compatible two‐dimensional (2D) fabrication approach, which combines inductive (L) and capacitive (C) self‐rolled‐up membrane (S‐RuM) components monolithically into a single L?C network structure, thereby greatly reducing the on‐chip area footprint. The individual L?C elements are fabricated in‐plane using standard semiconductor processing techniques, and subsequently triggered by the built‐in stress to self‐assemble and roll into cylindrical air‐core architectures. By designing the planar structure geometry and constituent layer properties to achieve a specific number of turns with a desired inner diameter when the device is rolled up, the electrical characteristics can be engineered. The network layouts of the L and C components are also reconfigurable by selecting appropriate input, output, and ground contact routing topographies. The devices demonstrated here operate over the range of ≈1?10 GHz. Their area and volume footprints are ≈0.09 mm² and ≈0.01 mm³, respectively, which are ≈10× smaller than most of the comparable conventional filter designs. These S‐RuM‐enabled 3D microtubular L?C filter networks represent significant advancement for miniaturization and integration of passive electronic components for applications in mobile connectivity and other frequency range.     

Two‐Dimensional Nanostructured Materials for Gas Sensing

Two‐dimensional (2D) nanostructures are highly attractive for fabricating nanodevices due to their high surface‐to‐volume ratio and good compatibility with device design. In recent years 2D nanostructures of various materials including metal oxides, graphene, metal dichalcogenides, phosphorene, BN and MXenes, have demonstrated significant potential for gas sensors. This review aims to provide the most recent advancements in utilization of various 2D nanomaterials for gas sensing. The common methods for the preparation of 2D nanostructures are briefly summarized first. The focus is then placed on the sensing performances provided by devices integrating 2D nanostructures. Strategies for optimizing the sensing features are also discussed. By combining both the experimental results and the theoretical studies available, structure‐properties correlations are discussed. The conclusion gives some perspectives on the open challenges and future prospects for engineering advanced 2D nanostructures for high‐performance gas sensors devices.     

Three‐dimensional radiation pattern measurement strategy for ground station antennas

This article describes how to measure a three‐dimensional antenna radiation pattern and the most effective method to achieve this. We present a new automated three‐dimensional measurement scheme, inspired by the seed pattern of a sunflower seed head, where the seed placements are arranged in a way so that rotational symmetry is obtained to compromise the circular shape of a dish antenna radiation pattern in the three‐dimensional space. This new AU–TUM Antenna Radiation Measurement Strategy has produced superior results. It has been shown by this scheme that for instance the three‐dimensional transmission pattern of a dish antenna can be measured automatically and easily checked against present regulations. The generic AU–TUM Antenna Radiation Measurement Strategy was verified on a 4.8‐m Ka‐band dish antenna. It was concluded that a three‐dimensional radiation pattern can be measured two to thirteen times faster than the best performing current strategies. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimal Planar Array Architecture for Full‐Dimensional Multi‐user Multiple‐Input Multiple‐Output with Elevation Modeling

Research interest in three‐dimensional multiple‐input multiple‐output (3D‐MIMO) beamforming has rapidly increased on account of its potential to support high data rates through an array of strategies, including sector or user‐specific elevation beamforming and cell‐splitting. To evaluate the full performance benefits of 3D and full‐dimensional (FD) MIMO beamforming, the 3D character of the real MIMO channel must be modeled with consideration of both the azimuth and elevation domain. Most existing works on the 2D spatial channel model (2D‐SCM) assume a wide range for the distribution of elevation angles of departure (eAoDs), which is not practical according to field measurements. In this paper, an optimal FD‐MIMO planar array configuration is presented for different practical channel conditions by restricting the eAoDs to a finite range. Using a dynamic network level simulator that employs a complete 3D SCM, we analyze the relationship between the angular spread and sum throughput. In addition, we present an analysis on the optimal antenna configurations for the channels under consideration.     

Highly Luminescent and Stable Green Quasi‐2D Perovskite‐Embedded Polymer Sheets by Inkjet Printing

Perovskite materials serve as promising candidates for display and lighting due to their excellent optical properties, including tunable bandgaps and efficient luminescence. However, their efficiency and stability must be improved for further application. In this work, quasi‐two‐dimensional (quasi‐2D) perovskites embedded in different polymers are prepared by inkjet printing to construct any luminescent patterns/pictures on the polymer substrates. The optimized quantum yield reaches over 65% by polyvinyl‐chloride‐based quasi‐2D perovskite composites. In addition, as‐fabricated perovskite?polymer composites with patterns show excellent resistance to abrasion, moisture, light irradiation, and chemical erosion by various solvents. Both quantum yield and lifetime are superior to those reported to date. These achievements are attributed to the introduction of the PEA⁺ cations to improve the luminance and stability of perovskite. This patterned composite can be useful for color‐conversion films with low cost and large‐scale fabrication.