Programmable Liquid Metal Microstructures for Multifunctional Soft Thermal Composites

Soft, elastically deformable composites can enable new generations of multifunctional materials for electronics, robotics, and reconfigurable structures. Liquid metal (LM) droplets dispersed in elastomer matrices represent an emerging material architecture that has shown unique combinations of soft mechanical response with exceptional electrical and thermal functionalities. These properties are strongly dependent on the material composition and microstructure. However, approaches to control LM microdroplet morphology to program mechanical and functional properties are lacking. Here, this limitation is overcome by thermo‐mechanically shaping LM droplets in soft composites to create programmable microstructures in stress‐free materials. This enables LM loadings up to 70% by volume with prescribed particle aspect ratios and orientation, enabling control of microstructure throughout the bulk of the material. Through this microstructural control in soft composites, a material which simultaneously achieves a thermal conductivity as high as 13.0 W m^?1 K^?1 (>70 × increase over polymer matrix) with low modulus (<1.0 MPa) and high stretchability (>750% strain) is demonstrated in stress‐free conditions. Such properties are required in applications that demand extreme mechanical flexibility with high thermal conductivity, which is demonstrated in soft electronics, wearable robotics, and electronics integrated into 3D printed materials.     

Recent Progress of Flexible Photodetectors Based on Low-Dimensional II–VI Semiconductors and Their Application in Wearable Electronics

Flexible photodetectors exhibit many advantages such as a good bendability, foldability, and even stretchability as well as weight light, which have triggered a widely concerned in wearable electronics including wearable monitoring, wearable image sensing, self-powered integrated electronics, etc. Recently, various II–VI semiconductor nanostructures have become promising candidates in flexible photodetectors due to their unique characteristics, such as direct bandgap semiconductors, excellent optical and electric properties, high quantum efficiency, and inherent mechanical flexibility. Herein, the most recent progress on low-dimensional (0D, 1D, 2D, and related heterostructures) II–VI semiconductors based flexible photodetectors and their application in wearable electronic is reviewed. First, a brief introduction of the main sensing mechanisms and key figures of merits for photodetectors is presented. Then, the recent progresses on flexible photodetectors are provided, in which the functional materials synthesis methods are also discussed. More importantly, the applications of the flexible photodetectors are summarized, including wearable monitoring sensors, image sensors, and self-powered integrated wearable electronics. Finally, the challenges and the future research direction of the flexible photodetectors are discussed, meanwhile the outlook for the development of flexible photodetectors in the future integration of wearable electronic is also provided.     

Fishing Net-Inspired Mutiscale Ionic Organohydrogels with Outstanding Mechanical Robustness for Flexible Electronic Devices

Mechanically robust and electrically conductive organohydrogels/hydrogels are increasingly required in flexible electronic devices, but it remains a challenge to achieve organohydrogels/hydrogels with integrated high performances. Herein, inspired by the geometric deformability and robustness of fishing nets, multiscale ionic organohydrogels with outstanding isotropic mechanical robustness are developed. The organohydrogels are prepared by introducing polyacrylamide (PAM) hydrogel, Zn²⁺ and a binary solvent of glycerol-water into a crosslinked fibrous mat which is electrospun from poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA). Because of the unique structure, the resultant organohydrogels, being mentioned as PAA-PVA/PAM/Zn²⁺ organohydrogels, exhibit outstanding tensile strength (9.45 MPa), high stretchability, excellent anti-fatigue property, skin-like mechanical behaviors and ionic conductivity. Importantly, the organohydrogels are promising in flexible electronic devices capable of operating properly over a wide temperature range and under harsh mechanical conditions, such as mechanical-electrical signal transducing materials in flexible mechanosensors and robust electrolytes in zinc ion hybrid supercapacitors. Not only the multiscale design strategy will provide a clue to improve the mechanical properties of soft materials, but also the organohydrogels offer promising materials for future flexible electronic devices.     

Bioinspired Self-healing Soft Electronics

Inspired by nature, various self-healing materials that can recover their physical properties after external damage have been developed. Recently, self-healing materials have been widely used in electronic devices for improving durability and protecting the devices from failure during operation. Moreover, self-healing materials can integrate many other intriguing properties of biological systems, such as stretchability, mechanical toughness, adhesion, and structural coloration, providing additional fascinating experiences. All of these inspirations have attracted extensive research on bioinspired self-healing soft electronics. This review presents a detailed discussion on bioinspired self-healing soft electronics. Firstly, two main healing mechanisms are introduced. Then, four categories of self-healing materials in soft electronics, including insulators, semiconductors, electronic conductors, and ionic conductors, are reviewed, and their functions, working principles, and applications are summarized. Finally, human-inspired self-healing materials and animal-inspired self-healing materials as well as their applications, such as organic field-effect transistors (OFETs), pressure sensors, strain sensors, chemical sensors, triboelectric nanogenerators (TENGs), and soft actuators, are introduced. This cutting-edge and promising field is believed to stimulate more excellent cross-discipline works in material science, flexible electronics, and novel sensors, accelerating the development of applications in human motion monitoring, environmental sensing, information transmission, etc.     

The future of bipolar power transistors

Silicon based bipolar power transistor (BPT) as a switching power transistor has been replaced by other superior power devices in the past two decades. This transformation is primarily due to the poor performance of the BPT. Among many problems of the BPT, low current gain and small safe operation area (SOA) caused by the second breakdown have been most detrimental to silicon BPT's fate. However, BPT performance based on newer materials, such as wide bandgap semiconductors, has not been previously studied. This paper systematically compares the BPTs based on wide bandgap semiconductor materials. Device figures-of-merit for conduction and switching losses are proposed. Comparison of the BPT based on total power loss is then provided. Based on this work, it is concluded that BPTs based on wide bandgap materials overcome the critical disadvantages of silicon BPTs, and are capable of switching power operation at several hundred kilohertz frequencies at very high current densities and voltages. Therefore, BPTs based on wide bandgap materials are still very attractive switching power devices for the future     

High-temperature electronics - a role for wide bandgap semiconductors?

The fact that wide bandgap semiconductors are capable of electronic functionality at much higher temperatures than silicon has partially fueled their development, particularly in the case of SiC. It appears unlikely that wide bandgap semiconductor devices will find much use in low-power transistor applications until the ambient temperature exceeds approximately 300/spl deg/C, as commercially available silicon and silicon-on-insulator technologies are already satisfying requirements for digital and analog VLSI in this temperature range. However practical operation of silicon power devices at ambient temperatures above 200/spl deg/C appears problematic, as self-heating at higher power levels results in high internal junction temperatures and leakages. Thus, most electronic subsystems that simultaneously require high-temperature and high-power operation will necessarily be realized using wide bandgap devices, once they become widely available. Technological challenges impeding the realization of beneficial wide bandgap high ambient temperature electronics, including material growth, contacts, and packaging, are briefly discussed.     

SiC and GaN transistors - is there one winner for microwave power applications?

Wide bandgap semiconductors show promise for high-power microwave electronic devices. Primarily due to low breakdown voltage, it has not been possible to design and fabricate solid-state transistors that can yield radio-frequency (RF) output power on the order of hundreds to thousands of watts. This has severely limited their use in power applications. Recent improvements in the growth of wide bandgap semiconductor materials, such as SiC and the GaN-based alloys, provide the opportunity to now design and fabricate microwave transistors that demonstrate performance previously available only from microwave tubes. The most promising electronic devices for fabrication in wide bandgap semiconductors for these applications are metal-semiconductor field-effect transistors (MESFETs) fabricated from the 4H-SiC polytype and heterojunction field-effect transistors (HFETs) fabricated using the AlGaN/GaN heterojunction. These devices can provide RF output power on the order of 5-6 W/mm and 10-12 W/mm of gate periphery, respectively. 4H-SiC MESFETs should produce useful performance at least through X band and AlGaN/GaN HFETs should produce useful performance well into the millimeter-wave region, and potentially as high as 100 GHz.     

Ⅲ族氮化物及其二维电子气输运特性的研究进展

Ⅲ族氮化物半导体具有宽禁带和直接带隙 ,导带能谷间距大 ,强场输运特性好。Al Ga N/Ga N异质结产生高密度的二维电子气 ,屏蔽了杂质和缺陷的散射 ,改善了低场输运性能。它弥补了宽禁带半导体输运性能差的缺点 ,已研制成大功率的 HFET。利用强场下的速度过冲有望消除阴极端的速度凹坑 ,显著改进器件性能。电子气的强二维性使输运特征依赖于器件结构和工作状态 ,器件设计变为一个剪裁电子状态和输运特性的复杂工程。文中综述了 族氮化物及其二维电子气的输运特性 ,讨论了从输运特性出发 ,优化 HFET性能的问题。     

3D Printing of Hydrogels for Stretchable Ionotronic Devices

In the booming development of flexible electronics represented by electronic skins, soft robots, and human–machine interfaces, 3D printing of hydrogels, an approach used by the biofabrication community, is drawing attention from researchers working on hydrogel-based stretchable ionotronic devices. Such devices can greatly benefit from the excellent patterning capability of 3D printing in three dimensions, as well as the free design complexity and easy upscale potential. Compared to the advanced stage of 3D bioprinting, 3D printing of hydrogel ionotronic devices is in its infancy due to the difficulty in balancing printability, ionic conductivity, shape fidelity, stretchability, and other functionalities. In this review, a guideline is provided on how to utilize the power of 3D printing in building high-performance hydrogel-based stretchable ionotronic devices mainly from a materials’ point of view, highlighting the systematic approach to balancing the printability, printing quality, and performance of printed devices. Various 3D printing methods for hydrogels are introduced, and then the ink design principles, balancing printing quality, printed functions, such as elastic conductivity, self-healing ability, and device (e.g., flexible sensors, shape-morphing actuators, soft robots, electroluminescent devices, and electrochemical biosensors) performances are discussed. In conclusion, perspectives on the future directions of this exciting field are presented.     

GaN and other materials for semiconductor spintronics

Existing semiconductor electronic and photonic devices use the charge on electrons and holes to perform their specific functionality, such as signal processing or light emission. The field of semiconductor spintronics seeks to exploit the spin of charge carriers in new generations of transistors, lasers, and integrated magnetic sensors. The use of such devices depends on the availability of materials with practical magnetic-ordering temperatures. Here, we summarize recent progress in the development of GaN and other wide bandgap semiconductors that retain ferromagnetic properties above room temperature.     

Fundamental physics of infrared detector materials

The fundamental parameters of IR photon detection are discussed relevant to the meaningful comparison of a wide range of proposed IR detecting materials systems. The thermal generation rate of the IR material is seen to be the key parameter that enables this comparison. The simple materials physics of 1) intrinsic direct bandgap semiconductors; 2) extrinsic semiconductors; 3) quantum well devices, including types I, II, and III superlattices; 4) Si Schottky barriers; and 5) high temperature superconductors, will be examined with regard to the potential performance of these materials as IR detectors, utilizing the thermal generation rate as a differentiator. The possibility of room temperature photon detection over the whole IR spectral range is discussed, and comparisons made with uncooled thermal detection.     

Tunable Assembly of sp3 Cross‐Linked 3D Graphene Monoliths: A First‐Principles Prediction

One of the biggest challenges in graphene applications is how one can fabricate 3D architectures comprising graphene sheets in which the resulting architectures have inherited graphene's excellent intrinsic properties but have overcome its shortcomings. Two series of 3D graphene monoliths (GMs) using zigzag or armchair graphene nanoribbons as building blocks and sp³ carbon chains as junction nodes are constructued, and calculations based on first principles are performed in order to predict their mechanical and electronic properties. The perfect match between sp² nanoribbons and sp³ linkers results in favorable energy and mechanical/dynamic stability. Owing to their tailored motifs, wine‐rack‐like pores, and rigid sp³ linkers, these GMs possess high surface areas, appreciable mechanical strength, and tunable band gaps. Negative linear compressibilities in a wide range are found for the zigzag GMs. By solving the problems of zero gap and dimensionality of graphene sheets simultaneously, these GMs offer a viable strategy towards many applications, e.g., microelectronic devices, energy storage, molecular sieves, sensitive pressure detectors, and telecommunication line systems.     

Stretchable Supercapacitor with Adjustable Volumetric Capacitance Based on 3D Interdigital Electrodes

Reduced graphene oxide (rGO)‐based materials have shown good performance as electrodes in flexible energy storage devices owing to their physical properties, high specific surface area, and excellent electrical conductivity. Here, a novel road is reported for fabricating high‐performance supercapacitors based on 3D rGO electrodes and solid electrolyte multilayers via pressure spray printing and machine coating. These supercapacitors demonstrate high and adjustable volumetric capacitance, excellent flexibility, and stretchability. The results show that this commercial strategy has its essential merits such as low‐cost, inexpensive, and simple fabrication for large area production. These properties are in the favor of fabricating high‐performance supercapacitor to meet the practical energy demands in devices, especially flexible electronic devices. Furthermore, this novel 3D interdigital electrode concept can be widely applied to other energy devices for enhancing performances and to other micro devices for reducing cost.     

宽禁带半导体关键设备技术及发展

宽禁带半导体设备技术是宽禁带半导体器件的支撑和重要基础。简要介绍了宽禁带半导体器件发展面临的设备问题,重点介绍了碳化硅晶体生长炉、碳化硅外延生长炉、碳化硅离子注入机和氮化镓MOCVD四种制约我国宽禁带半导体器件技术发展的关键设备,指出了宽禁带半导体设备技术的未来发展趋势。     

A Generic Soft Encapsulation Strategy for Stretchable Electronics

Recent progress in stretchable forms of inorganic electronic systems has established a route to new classes of devices, with particularly unique capabilities in functional biointerfaces, because of their mechanical and geometrical compatibility with human tissues and organs. A reliable approach to physically and chemically protect the electronic components and interconnects is indispensable for practical applications. Although recent reports describe various options in soft, solid encapsulation, the development of approaches that do not significantly reduce the stretchability remains an area of continued focus. Herein, a generic, soft encapsulation strategy is reported, which is applicable to a wide range of stretchable interconnect designs, including those based on two‐dimensional (2D) serpentine configurations, 2D fractal‐inspired patterns, and 3D helical configurations. This strategy forms the encapsulation while the system is in a prestrained state, in contrast to the traditional approach that involves the strain‐free configuration. A systematic comparison reveals that substantial enhancements (e.g., ≈6.0 times for 2D serpentine, ≈4.0 times for 2D fractal, and ≈2.6 times for 3D helical) in the stretchability can be achieved through use of the proposed strategy. Demonstrated applications in highly stretchable light‐emitting diodes systems that can be mounted onto complex curvilinear surfaces illustrate the general capabilities in functional device systems.     

Self-Healing,Robust, and Stretchable Electrode by Direct Printing on Dynamic Polyurea Surface at Slightly Elevated Temperature

Printed electronics on elastomer substrates have found wide applications in wearable devices and soft robotics. For everyday usage, additional requirements exist for the robustness of the printed flexible electrodes, such as the ability to resist scratching and damage. Therefore, highly robust electrodes with self-healing, and good mechanical strength and stretchability are highly required and challenging. In this paper, a cross-linking polyurea using polydimethylsiloxane as the soft segment and dynamic urea bonds is prepared and serves as a self-healing elastomer substrate for coating and printing of silver nanowires (AgNWs). Due to the dynamic exchangeable urea bond at 60 °C, the elastomer exhibits dynamic exchange of the cross-linking network while retaining the macroscopic shape. As a result, the AgNWs are partially embedded in the surface of the elastomer substrate when coated or printed at 60 °C, forming strong interfacial adhesion. As a result, the obtained stretchable electrode exhibits high mechanical strength and stretchability, the ability to resist scratching and sonication, and self-healing. This strategy can be applied to a variety of different conducting electrode materials including AgNWs, silver particles, and liquid metal, which provides a new way to prepare robust and self-healing printed electronics.     

Properties of semiconductors useful for sensors

Different types of solid state sensors demand different basic properties of the semiconductor materials from which they are made. The best transistor material is not necessarily the best material for a certain sensor. One must choose a semiconductor for a particular sensor on the basis of its fundamental properties, such as energy band structure. For example, piezoresistance sensors are made of silicon or germanium mainly because a large effect occurs only in semiconductors, such as these, having complex band edge structures. On the other hand, an intrinsic infrared photon detector requires an energy gap Egcorresponding to the longest wavelength λmto be detected, [Eg(eV)=1/λm(µm)] as well as a direct gap. Compatibility with integrated circuitry may sometimes be a consideration also. A review is given of the materials requirements of effects and devices useful in solid state sensing, and of the present state of development of semiconductors important for sensor applications. Materials discussed include Si, Ge, III-V compounds, II-VI compounds, IV-VI compounds, and others. Problems and prospects for future development are indicated.     

半导体材料研究的新进展

首先对作为现代信息社会的核心和基础的半导体材料在国民经济建设、社会可持续发展以及国家安全中的战略地位和作用进行了分析,进而介绍几种重要半导体材料如,硅材料、GaAs和InP单晶材料、半导体超晶格和量子阱材料、一维量子线、零维量子点半导体微结构材料、宽带隙半导体材料、光学微腔和光子晶体材料、量子比特构造和量子计算机用材料等目前达到的水平和器件应用概况及其发展趋势作了概述.最后,提出了发展我国半导体材料的建议.本文未涉及II-VI族宽禁带与II-VI族窄禁带红外半导体材料、高效太阳电池材料Cu(In,Ga)Se.CuIn(Se,S)等以及发展迅速的有机半导体材料等.     

3D Printed Multifunctional,Hyperelastic Silicone Rubber Foam

Highly elastic silicone foams, especially those with tunable properties and multifunctionality, are of great interest in numerous fields. However, the liquid nature of silicone precursors and the complicated foaming process hinder the realization of its three‐dimensional (3D) printability. Herein, a series of silicone foams with outstanding performance with regards to elasticity, wetting and sensing properties, multifunctionality, and tunability is generated by direct ink writing. Viscoelastic inks are achieved from direct dispersion of sodium chloride in a unique silicone precursor solution. The 3D‐architectured silicone rubber exhibits open‐celled trimodal porosity, which offers ultraelasticity with hyper compressibility/cycling endurance (near‐zero stress/strain loss under 90% compression or 1000 compression cycles), excellent stretchability (210% strain), and superhydrophobicity. The resulting foam is demonstrated to be multifunctional, such that it can work as an oil sorbent with super capacity (1320%) and customizable soft sensor after absorption of carbon nanotubes on the foam surface. The strategy enables tunability of mechanical strength, elasticity, stretchability, and absorbing capacity, while printing different materials together offers property gradients as an extra dimension of tunability. The first 3D printed silicone foam, which serves an important step toward its application expansion, is achieved.     

Applying Quantitative Microstructure Control in Advanced Functional Composites

Microstructure control in functional materials draws from a historical reserve rich in established theory and experimental observation of metallurgy. Methods such as rapid solidification, eutectoid reaction, and nucleation and growth precipitation have all proven to be effective means to produce microstructure relevant for a wide array of applications. Here, the available parameters to control structure morphology, size, and spacing are discussed using thermoelectric composites as an example. Moreover, exploiting different aspects of a material system's phase diagram enables a controlled introduction of nanostructures. While much of this discussion is pertinent to the rapidly developing field of thermal conductivity control in thermoelectric composites, these techniques can be applied to a variety of other material systems where their use may lead to novel electrical, optical, as well as thermal properties of semiconductors and insulators as it has in the past for the mechanical properties of metals.