Flexible electronics and optoelectronics of 2D van der Waals materials

Flexible electronics and optoelectronics exhibit inevitable trends in next-generation intelligent industries,including healthcare and wellness,electronic skins,the automotive industry,and foldable or rollable displays.Traditional bulk-material-based flexible devices considerably rely on lattice-matched crystal structures and are usually plagued by unavoidable chemical disorders at the interface.Two-dimensional van der Waals materials(2D VdWMs)have exceptional multifunctional properties,including large specific area,dangling-bond-free interface,plane-to-plane van der Waals interactions,and excellent mechanical,electrical,and optical properties.Thus,2D VdWMs have considerable application potential in functional intelligent flexible devices.To utilize the unique properties of 2D VdWMs and their van der Waals heterostructures,new designs and configurations of electronics and optoelectronics have emerged.However,these new designs and configurations do not consider lattice mismatch and process incompatibility issues.In this review,we summarized the recently reported 2D VdWM-based flexible electronic and optoelectronic devices with various functions thoroughly.Moreover,we identified the challenges and opportunities for further applications of 2D VdWM-based flexible electronics and optoelectronics.

Recent advances in nanofiber-based flexible transparent electrodes

Flexible and stretchable transparent electrodes are widely used in smart display,energy,wearable devices and other fields.Due to the limitations of flexibility and stretchability of indium tin oxide electrodes,alternative electrodes have appeared,such as metal films,metal nanowires,and conductive meshes.However,few of the above electrodes can simultaneously have excellent flexibility,stretchability,and optoelectronic properties.Nanofiber(NF),a continuous ultra-long one-dimensional conductive material,is considered to be one of the ideal materials for high-performance transparent electrodes with excellent properties due to its unique structure.This paper summarizes the important research progress of NF flexible transparent electrodes(FTEs)in recent years from the aspects of NF electrode materials,preparation technology and application.First,the unique advantages and limitations of various NF materials are systematically discussed.Then,we summarize the preparation technology of various advanced NF FTEs,and point out the future development trend.We also discuss the application of NFs in solar cells,supercapacitors,electric heating equipments,sensors,etc,and analyze its development potential in flexible electronic equipment,as well as problems that need to be solved.Finally,the challenges and future development trends are proposed in the wide application of NF FTEs in the field of flexible optoelectronics.

Flexible optoelectronic neural transistors with broadband spectrum sensing and instant electrical processing for multimodal neuromorphic computing

A flexible optoelectronic neural transistor(OENT)that consists of a one‐step spin‐coated tri‐blend film composed of 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene(C8‐BTBT),poly(3‐hexylthiophene‐2,5‐diyl)(P3HT),and poly(methyl methacrylate)(PMMA)is demonstrated.The C8‐BTBT and P3HT phases in the film partially segregate into distinct domains,which combine to provide broadband spectrum sensing,and instant electrical‐processing capabilities dominated by C8‐BTBT.The OENT is sensitive to solar radiation from the near‐ultraviolet(NUV)and to visible(Vis)radiation from blue to red.When exposed to NUV radiation,the OENT responds sensitively and retains the memory of the exposure for over 10^(3 )s.The OENT provides a warning of excessive chronic exposure to harmful NUV.These properties allow high‐pass filtering with different cut‐off frequencies fc that can restrict the reception of blue,green,or red.These switchable fc enables sensitive image reconstruction and multitarget monitoring.The device combined with a chitosan gel achieves strictly defined short‐range plasticity of<1 s that can achieve diverse instant‐computing applications such as spatiotemporally correlated coding and logic functions.Stable real‐time signal processing facilitates the realization of a Morse‐code recognition system constructed using neuro‐morphological hardware,achieving highly accurate character recognition.This study provides a useful resource that can have applications in wearable biomedical electronics and multimodal neuromorphic computing.

Wafer-scale synthesis of monolayer WS2 for high-performance flexible photodetectors by enhanced chemical vapor deposition

Two-dimensional （2D） nanomaterials have recently attracted considerable attention due to their promising applications in next-generation electronics and optoelectronics. In particular, the large-scale synthesis of high-quality 2D materials is an essential requirement for their practical applications. Herein, we demonstrate the wafer-scale synthesis of highly crystalline and homogeneous monolayer WS2 by an enhanced chemical vapor deposition （CVD） approach, in which precise control of the precursor vapor pressure can be effectively achieved in a multi-temperature zone horizontal furnace. In contrast to conventional synthesis methods, the obtained monolayer WS2 has excellent uniformity both in terms of crystallinity and morphology across the entire substrate wafer grown （e.g., 2 inches in diameter）, as corroborated by the detailed characterization. When incorporated in typical rigid photodetectors, the monolayer WS2 leads to a respectable photodetection performance, with a responsivity of 0.52 mA/W, a detectivity of 4.9 × 10^9 Jones, and a fast response speed （〈 560μs）. Moreover, once fabricated as flexible photodetectors on polyimide, the monolayer WS2 leads to a responsivity of up to 5 mA/W. Importantly, the photocurrent maintains 89% of its initial value even after 3,000 bending cycles. These results highlight the versatility of the present technique, which allows its applications in larger substrates, as well as the excellent mechanical flexibility and robustness of the CVD-grown, homogenous WS2 monolayers, which can promote the development of advanced flexible optoelectronic devices.

Materials and devices for flexible and stretchable photodetectors and light-emitting diodes

Recently,significant efforts have been directed at overcoming the limitations of conventional rigid optoelectronic devices,particularly their poor mechanical stability under bending,folding,and stretching deformations.One of major approaches for rendering optoelectronic devices mechanically deformable is to replace the conventional electronic/optoelectronic materials with functional nanomaterials or organic materials that are intrinsically flexible/stretchable.Further,advanced device designs and unconventional fabrication methods have also contributed to the development of soft optoelectronic devices.Accordingly,new devices such as bio-inspired curved image sensors,wearable light emitting devices,and deformable bio-integrated optoelectronic devices have been developed.In this review,recent progress in the development of soft optoelectronic materials and devices is outlined.First,various materials such as nanomaterials,organic materials,and their hybrids that are suitable for developing deformable photodetectors,are presented.Then,the nanomaterials and organic/polymeric materials that are applicable in deformable light-emitting diodes are described.Finally,representative system-level applications of flexible and stretchable photodetectors and light-emitting diodes are reviewed,and future prospects are discussed.

Structural design of light-emitting fibers and fabrics for wearable and smart devices

Flexible light-emitting fibers and fabrics serve to bridge human–machine interactions. The desire for practical applications and the commercialization of flexible light-emitting fibers has accelerated structural progress and improvements. This review focuses on the structural design of light-emitting fibers and fabrics, starting with a summary of design principles, emission mechanisms, and structural evolution of coaxial structured light-emitting fibers. Subsequently, we explore recent advances in the helical structure design strategies that boost the mechanical sensitivity of light-emitting fibers. Following that, we analyze continuous preparation processes and the development of large-area intelligent light-emitting fabrics based on interwoven structures. Examples based on stiff and rigid inorganic-based lightemitting diodes integrated into flexible systems are also presented. Finally, we discuss the current challenges and future opportunities for light-emitting applications in the field of wearable and smart devices.

Controllable growth and flexible optoelectronic devices of regularly-assembled Bi2S3 semiconductor nanowire bifurcated junctions and crosslinked networks

Regularly assembled structures of nanowires, such as aligned arrays, junctions and interconnected networks, have great potential for the applications in logical circuits, address decoders, photoelectronic devices and transparent electrodes. However, for now it is still lack of effective approaches for constructing nanowire bifurcated junctions and crosslinked networks with ordered orientations and high quality. Herein, we report the controlled growth of Bi2S3 semiconductor nanowire bifurcated junctions and crosslinked networks with well-aligned directions and high crystalline degree by utilizing the proportional lattice match between nanowires and substrates. Taking advantages of the “tip-to-stem splice” assembly of individual nanowires, the precise orientation alignments of Bi2S3 semiconductor nanowire bifurcated junctions and crosslinked networks were successfully realized. The controlled growth mechanism and structural evolution process have been elucidated by detailed atomic structure characterizations and modeling. The highly crystal quality and direct energy bandgap of as-assembled photodetectors based on individual bismuth sulfide nanowires enabled high photoresponsivity and fast switch time under light illumination. The three-terminal devices based on nanowire bifurcated junctions present rapid carrier transport across the junction. The flexible photodetectors based on nanowire crosslinked networks show very minimal decay of photocurrent after long-term bending test. This work may provide new insights for the guided construction and regular assembly of low-dimensional ordered functional nanostructures towards advanced nanotechnologies.

Recent advances in flexible and wearable organic optoelectronic devices

Flexible and wearable optoelectronic devices have been developing to a new stage due to their unique capacity for the possibility of a variety of wearable intelligent electronics, including bendable smartphones, foldable touch screens and antennas, paper-like displays, and curved and flexible solid-state lighting devices. Before extensive commercial applications, some issues still have to be solved for flexible and wearable optoelectronic devices. In this regard, this review concludes the newly emerging flexible substrate materials, transparent conductive electrodes, device architectures and light manipulation methods. Examples of these components applied for various kinds of devices are also summarized. Finally, perspectives about the bright future of flexible and wearable electronic devices are proposed.