柔性磁性薄膜材料与器件研究进展

近年来,随着物联网、仿生机器人、移动式医疗健康等领域的兴起,柔性电子材料和器件受到广泛关注.基于磁性材料构建的传感器和存储器是电子器件的重要组成部分.随着柔性薄膜材料制备技术的发展,人们已经制备出高质量的柔性乃至可拉伸的磁性金属和氧化物薄膜,它们展现的不仅是更强的变形能力,还有新的物理效应与响应规律.研究结果表明,柔性磁电子器件在非接触传感、高灵敏应变探测、超分辨触觉感知等方面展现出独特的优势,具有广阔的应用前景.本文主要从柔性磁性材料的制备、物性调控规律和器件应用方面综述这一新兴领域的发展动态,并对其未来的发展趋势进行展望.     

柔性电子技术中的半导体材料性能调控概述

利用柔性电子技术对半导体材料性能调控研究具有重大的科学意义及应用价值.该研究一方面突破了传统应变工程中受限于无机材料硬而脆的特性,且引入应变多为固定值的局限;另一方面也为基于无机功能材料的可延展柔性电子器件在大变形环境下的性能评估提供了理论基础.因此,柔性电子技术为针对半导体材料或其他功能材料的应变调控提供了一种新方法,将有望应用在诸多需要材料性能周期性改变的新颖领域之中.本文将首先简介柔性无机电子技术,并对其中的两大关键技术:基于纳米金刚石颗粒的减薄及转印技术进行重点阐述,并探究两大关键技术对半导体电子器件性能的影响;随后介绍半导体材料近些年在应变-能带结构耦合关系方向的研究成果,并以基于屈曲砷化镓纳米薄膜条带及量子阱结构的研究为例,阐明柔性电子技术运用于半导体材料性能调控的独特优势;最后展望应变调控半导体特性的应用方向与发展前景.     

柔性冷阴极新进展

由于在场致电子发射平板显示器、电子枪、X射线放射治疗、微波放大器件等真空微电子器件中具有重要的运用,因此基于准一维纳米材料冷阴极的研究引起了人们极大的研究兴趣。其中,由于在可卷曲场致电子发射平板显示器和柔性电子器件中具有重要应用前景,     

可拉伸导体的最新进展

可拉伸导体因能够适应较大的变形以及与三维不规则表面实现无缝接触,受到了广泛关注,在信息、能源、医疗、国防等领域具有广阔的应用前景.在过去的几十年中,人们开发出了很多性能优异的导电纳米材料,如金属纳米线、碳纳米管、石墨烯和导电聚合物等.将导电纳米填料均匀分散到聚合物基质中是制备弹性导体的一种有效方法,可以实现导电性和拉伸性;另一种方法则是对导电复合物进行结构设计,引入可拉伸结构(如褶皱,网型,蛇形等),实现大形变下的性能稳定.本文主要总结了近五年来在弹性导体领域的最新进展,并指出了当前弹性导体领域存在的挑战.另外还讨论了一些柔性电子器件,如发光二极管、传感器、加热器等的研究现状,指明了柔性电子器件的发展趋势.     

柔性压力传感器的原理及应用

柔性压力传感器作为一种新型的电子器件,它在人机交互、医疗健康、机器人触觉等应用领域具有比刚性传感器更大的优势,但也对材料提出了更严格的要求.例如,它要求构成器件的材料很薄、较软,在某些情况下可贴合于人体皮肤表面或者植入体内,这进一步要求材料具有良好的生物相容性,并能与生物组织实现良好的力学匹配.在器件性能方面,柔性压力传感器的设计主要关注于灵敏度、响应时间、检测限、稳定性等性能的提高.最近,研究者们又将目光拓展到了器件的压力响应范围、压力分辨率、空间分辨率及拉伸性能等,使得传感器具有更广阔的应用前景.本篇综述介绍了近年来柔性压力传感器研究的进展,主要包括柔性压力传感器的传感原理、传感性能及应用前景,并最后对该类器件的发展进行了展望.     

用于触觉感知的自供能可拉伸压电橡胶皮肤电子器件

柔软、轻薄的皮肤电子器件是当今的研究热点之一,在健康监测、疾病预防及治疗方面有诸多应用.但是目前柔性能量供给器件从厚度和尺寸上仍然无法满足皮肤电子的集成要求.本文主要阐述一种应用压电橡胶复合材料的柔性皮肤电子器件,采用压电陶瓷材料锆钛酸铅(PZT)、聚二甲基硅氧烷(PDMS)和石墨烯的混合体系组成柔性传感器.混合后的压电橡胶材料柔软,轻薄,延展性良好,结合优化力学性能的蛇形结构叉指电极以及PDMS柔性基底,使该器件柔软、可拉伸,能与皮肤无缝紧密贴合.该触觉感知柔性皮肤电子器件不仅可以准确测量2.84—11.72 kPa范围的压力,且能够适应弯曲、拉伸等机械变化,并且在不同方向上的拉伸应变达到20%时都仍然能保持良好的压电性能,证明该器件可以在人体日常动作造成的皮肤应变下保持良好的稳定性能.该器件可以贴于体表以监测并区分触摸、点按、轻拍、敲击等动作,输出的电能足够点亮15个LED灯泡,在自供能皮肤电子领域有巨大发展和应用潜力.     

柔性有机/无机杂化钙钛矿太阳电池研究进展

柔性太阳电池具有重量轻、可卷对卷连续生产、可卷曲、不易破碎、便于携带和可穿戴等特点,可在多种领域为人们提供电力,具有非常广泛的应用前景。近年来,在基于刚性衬底的有机/无机杂化钙钛矿太阳电池（PSC）展示了出色的功率转换效率之后,柔性PSC研究也受到了人们的广泛关注。目前,柔性PSC的转换效率已经达到了18.1%。本文介绍了近年来柔性PSC领域的相关研究工作,综述了已应用于柔性PSC的柔性基底、透明电极和界面传输层等关键材料近来的发展,并探讨了这些材料应用于柔性PSC时的优势和面临的主要问题,最后对柔性PSC未来的发展进行了展望。     

柔性电子专题编者按

正柔性电子技术是利用柔性或可伸缩器件及其集成系统发展起来的一种新兴电子技术.这种柔性器件或系统是基于在柔性基底上集成大面积、大规模的不同材料和不同功能部件,实现具有可变形、重量轻和功能可重构的特点.这种新技术可以像传统的刚性电子器件一样实现数据的采集、处理、传输和显示.基于其卓越的整合性,可以实现"万物互联",为"物联网"提供强大的工具,将给信息、能源、医疗等领域带来应用革命.该技术为医疗保健、环境监测、显示和人机交互、能源、通信和无线互联网等领域开辟了新的前景.     

低维光电材料与器件专刊序

随着后摩尔时代的来临以及移动互联网、可穿戴柔性电子等领域的高速发展,人们对超小尺寸、超高速、超高效率、超低功耗光电子技术的需求日益迫切,对光电材料与器件的性能也提出了更高的要求.近年来,一系列新型低维光电材料如量子点、纳米线、二维材料、微纳光子结构、有机无机杂化钙钛矿等相继涌现,并展示出良好的性能.     

基于摩擦纳米发电机的可穿戴能源器件

随着电子器件向着小型化、功能化的方向迈进,可穿戴电子器件受到越来越多的关注,但是可穿戴电子器件的能源供给问题目前仍亟待解决.基于摩擦起电与静电感应耦合效应的摩擦纳米发电机具有成本低、选材广、柔性等特点,可以收集人体的低频、不规律能量并高效地转化为电能,在可穿戴带能源器件领域有着巨大的发展潜力.本文将首先介绍摩擦纳米发电机的四种基本工作模式以及摩擦起电机理的最新研究,然后从贴敷于人体皮肤的直接式能源收集与附着于衣物、鞋子等人体附属物的间接式能源收集两个部分详细综述基于摩擦纳米发电机的可穿戴能源器件的研究进展.最后,对用于驱动电子器件的能量管理模块进行系统介绍,分析讨论目前可穿戴能源器件发展中的问题和瓶颈,探讨未来的发展方向.     

Wavy structures for stretchable energy storage devices: Structural design and implementation

The application of wavy structures in stretchable electrochemical energy storage devices is reviewed. First, the mechanical analysis of wavy structures, specific to flexible electronics, is introduced. Second, stretchable electrochemical energy storage devices with wavy structures are discussed. Finally, the present problems and challenges are reviewed, and possible directions for future research are outlined.     

Mechanics of flexible and stretchable piezoelectrics for energy harvesting

As rapid development in wearable/implantable electronic devices benefit human life in daily health monitoring and disease treatment medically, all kinds of flexible and/or stretchable electronic devices are booming, together with which is the demanding of energy supply with similar mechanical property. Due to its ability in converting mechanical energy lying in human body into electric energy, energy harvesters based on piezoelectric materials are promising for applications in wearable/implantable device's energy supply in a renewable, clean and life-long way. Here the mechanics of traditional piezoelectrics in energy harvesting is reviewed, including why piezoelectricity is the choice for minor energy harvesting to power the implantable/wearable electronics and how. Different kinds of up to date flexible piezoelectric devices for energy harvesting are introduced, such as nanogenerators based on Zn O and thin and conformal energy harvester based on PZT. A detailed theoretical model of the flexible thin film energy harvester based on PZT nanoribbons is summarized, together with the in vivo demonstration of energy harvesting by integrating it with swine heart. Then the initial researches on stretchable energy harvesters based on piezoelectric material in wavy or serpentine configuration are introduced as well.     

Review of flexible and transparent thin-film transistors based on zinc oxide and related materials

Flexible and transparent electronics enters into a new era of electronic technologies.Ubiquitous applications involve wearable electronics,biosensors,flexible transparent displays,radio-frequency identifications(RFIDs),etc.Zinc oxide(ZnO) and relevant materials are the most commonly used inorganic semiconductors in flexible and transparent devices,owing to their high electrical performances,together with low processing temperatures and good optical transparencies.In this paper,we review recent advances in flexible and transparent thin-film transistors(TFTs) based on ZnO and relevant materials.After a brief introduction,the main progress of the preparation of each component(substrate,electrodes,channel and dielectrics) is summarized and discussed.Then,the effect of mechanical bending on electrical performance is highlighted.Finally,we suggest the challenges and opportunities in future investigations.     

An overview of healthcare monitoring by flexible electronics

Flexible electronics integrated with stretchable/bendable structures and various microsensors that monitor the temperature,pressure, sweat, bioelectricity, body hydration, etc., have a wide range of applications in the human healthcare sector. The science underlying this technology draws from many research areas, such as information technology, materials science, and structural mechanics, to efficiently and accurately monitor technology for various signals. In this paper, we make a classification and comb to the designs, materials, structures and functions of numerous flexible electronics for signal monitoring in the human healthcare sector. Some perspectives in this field are discussed in the concluding remarks.     

Systematic study on the mechanical and electric behaviors of the nonbuckling interconnect design of stretchable electronics

Recently, we developed a nonbuckling interconnect design that provides an effective approach to simultaneously achieving high elastic stretchability, easiness for encapsulation, and high electric performance for stretchable electronics. This paper aims to systematically study its mechanical and electric behaviors, including comparisons of the nonbuckling and buckling interconnect designs on stretchability, effects of the thickness on electric performance, and modeling and experimental investigations on the finite deformation mechanics. It is found that the results on stretchability depend on the layouts. Long straight segments and small arc radii for nonbuckling interconnects yield an enhancement of stretchability, which is much better than that of buckling designs. On the other hand, shorter straight segments or thicker interconnects are better to lower the resistances of interconnects.Therefore, optimization of the designs needs to balance the requirements of both the mechanical and electric performances. The finite deformation of interconnects during stretching is analyzed. The established analytic model is well validated by both the finite element modeling and experimental investigations. This work is key for providing the design guidelines for nonbucklingbased stretchable electronics.     

Mechanical properties of kirigami phosphorene via molecular dynamics simulation

The newly discovered two-dimensional phosphorene suffers low stretchability which limits its application in flexible devices. Herein we employ kirigami technique to overcome this limitation. Molecular dynamics simulation is employed to investigate the mechanical properties of kirigami-phosphorene under shear and tensile loadings. Our simulation results show that loading type, intrinsic structural anisotropy, and the height of middle cuts are three key factors that govern the mechanical response of kirigami-phosphorene. Under the tensile loading along the armchair direction, phosphorene exhibits a considerable increase in its tensile strain. By contrast, phosphorene is too weak to stand any structural modification induced by kirigami in the zigzag direction. Under shear loading, there is merely no improvement in the shear properties of kirigami-phosphorene. Our results demonstrate the prospective applications of kirigami-phosphorene along the armchair direction in modern wearable, and stretchable electronics and optoelectronics devices.     

Ultra-slim flexible glass for roll-to-roll electronic device fabrication

As displays and electronics evolve to become lighter, thinner, and more flexible, the choice of substrate continues to be critical to their overall optimization. The substrate directly affects improvements in the designs, materials, fabrication processes, and performance of advanced electronics. With their inherent benefits such as surface quality, optical transmission, hermeticity, and thermal and dimensional stability, glass substrates enable high-quality and long-life devices. As substrate thicknesses are reduced below 200 μm, ultra-slim flexible glass continues to provide these inherent benefits to high-performance flexible electronics such as displays, touch sensors, photovoltaics, and lighting. In addition, the reduction in glass thickness also allows for new device designs and high-throughput, continuous manufacturing enabled by R2R processes. This paper provides an overview of ultra-slim flexible glass substrates and how they enable flexible electronic device optimization. Specific focus is put on flexible glass’ mechanical reliability. For this, a combination of substrate design and process optimizations has been demonstrated that enables R2R device fabrication on flexible glass. Demonstrations of R2R flexible glass processes such as vacuum deposition, photolithography, laser patterning, screen printing, slot die coating, and lamination have been made. Compatibility with these key process steps has resulted in the first demonstration of a fully functional flexible glass device fabricated completely using R2R processes.     

Fabrication,properties, and applications of flexible magnetic films

Flexible magnetic devices, i.e., magnetic devices fabricated on flexible substrates, are very attractive in applications such as detection of magnetic field in an arbitrary surface, non-contact actuators, and microwave devices, due to their stretchable, biocompatible, light-weight, portable, and low cost properties. Flexible magnetic films are essential for the realization of various functionalities of flexible magnetic devices. To give a comprehensive understanding for flexible magnetic films and related devices, recent advances in the study of flexible magnetic films are reviewed, including fabrication methods, magnetic and transport properties of flexible magnetic films, and their applications in magnetic sensors, actuators, and microwave devices. Our aim is to foster a comprehensive understanding of these films and devices. Three typical methods have been introduced to prepare the flexible magnetic films, by deposition of magnetic films on flexible substrates, by a transfer and bonding approach or by including and then removing sacrificial layers. Stretching or bending the magnetic films is a good way to apply mechanical strain to them, so that magnetic anisotropy, exchange bias, coercivity, and magnetoresistance can be effectively manipulated. Finally, a series of examples is shown to demonstrate the great potential of flexible magnetic films for future applications.     

Buckling of a stiff thin film on a compliant substrate under anisotropic biaxial prestrain

The structure of stretchable electronics is based on the buckling of a thin film on a compliant substrate. Under anisotropic biaxial prestrains, this structure may buckle into several patterns, including cylindrical, checkerboard, and undulating patterns. The displacement and energy of each pattern are deduced analytically. By comparing their minimum potential energies, the critical buckling condition of each pattern is determined. After secondary bifurcation, the checkerboard pattern occurs just above the critical prestrains, but the undulating pattern dominates other regions. The buckling amplitude and wavenumber of the undulating pattern are shown under biaxial prestrains. Even if the structure is under equi-biaxial prestrains, it may buckle into an asymmetric undulating pattern.     

Synthesis of atomically thin GaSe wrinkles for strain sensors

A wrinkle-based thin-film device can be used to develop optoelectronic devices, photovoltaics, and strain sensors. Here, we propose a stable and ultrasensitive strain sensor based on two-dimensional (2D) semiconducting gallium selenide (GaSe) for the first time. The response of the electrical resistance to strain was demonstrated to be very sensitive for the GaSe-based strain sensor, and it reached a gauge factor of –4.3, which is better than that of graphene-based strain sensors. The results show us that strain engineering on a nanoscale can be used not only in strain sensors but also for a wide range of applications, such as flexible field-effect transistors, stretchable electrodes, and flexible solar cells.