A highly sensitive, highly transparent, gel-gated MoS2 phototransistor on biodegradable nanopaper

Tungsten Nitride (W5N6): An Ultraresilient 2D Semimetal

Two-dimensional transition metal nitrides offer intriguing possibilities for achieving novel electronic and mechanical functionality owing to their distinctive and tunable bonding characteristics compared to other 2D materials. We demonstrate here the enabling effects of strong bonding on the morphology and functionality of 2D tungsten nitrides. The employed bottom-up synthesis experienced a unique substrate stabilization effect beyond van-der-Waals epitaxy that favored W5N6 over lower metal nitrides. Comprehensive structural and electronic characterization reveals that monolayer W5N6 can be synthesized at large scale and shows semimetallic behavior with an intriguing indirect band structure. Moreover, the material exhibits exceptional resilience against mechanical damage and chemical reactions. Leveraging these electronic properties and robustness, we demonstrate the application of W5N6 as atomic-scale dry etch stops that allow the integration of high-performance 2D materials contacts. These findings highlight the potential of 2D transition metal nitrides for realizing advanced electronic devices and functional interfaces.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Sulfurization engineering of single-zone CVD vertical and horizontal MoS2 on p-GaN heterostructures for self-powered UV photodetectors

Molybdenum disulfide (MoS2) has been attracting considerable attention due to its excellent electrical and optical properties. We successfully grew high-quality, large-area and uniform few-layer (FL)-MoS2 on p-doped gallium nitride (p-GaN) using a simplified sulfurization technique by the single-zone CVD of a Mo seed layer via E-beam evaporation. Tuning the sulfurization parameters, namely temperature and duration, has been discovered to be an effective strategy for improving MoS2 orientation (horizontally aligned and vertically aligned) and quality, which affects photodetector (PD) performance. The increase in the sulfurization temperature to 850 °C results in improved structural quality and crystallite size. However, a prolonged sulfurization duration of 60 minutes caused the degradation of the film quality. The close lattice match between p-GaN and MoS2 contributes to the excellent quality growth of deposited MoS2. Following this, an n-MoS2/p-GaN heterostructure PD was successfully built by a MoS2 position-selectivity method. We report a highly sensitive and self-powered GaN/MoS2 p-n heterojunction PD with a relatively high responsivity of 14.3 A W-1, a high specific detectivity of 1.12 × 1013 Jones, and a fast response speed of 8.3/13.4 μs (20 kHz) under a UV light of 355 nm at zero-bias voltage. Our PD exhibits superior performance to that of the previously reported MoS2/GaN p-n PD. Our findings suggest a more efficient and straightforward approach to building high-performance self-powered UV PDs.

Ionic Liquid-Gated Near-Infrared Polymer Phototransistors and Their Persistent Photoconductivity Application in Optical Memory

Organic phototransistors (OPTs) based on polymers have attracted substantial attention due to their excellent signal amplification, significant noise reduction, and solution process. Recently, the near-infrared (NIR) detection becomes urgent for OPTs with the increased demand for biomedicine, medical diagnostics, and health monitoring. To achieve this goal, a low working voltage of the OPTs is highly desirable. Therefore, the traditional dielectric gate can be replaced by an electrolyte gate to form electrolyte-gated organic phototransistors (EGOPTs), which are not only able to work at voltages below 1.0 V but also are biocompatible. PCDTPT, one of the most popular narrow band gap donor-acceptor copolymer, has been rarely studied in EGOPTs. In this work, an organic NIR-sensitive EGOPT based on PCDTPT is demonstrated with the detectivity of 7.08 × 10¹¹ Jones and the photoresponsivity of 3.56 A/W at a low operating voltage. In addition, an existing persistent photoconductivity (PPC) phenomenon was also observed when the device was exposed to air. The PPC characteristic of the EGOPT in air has been used to achieve a phototransistor memory, and the gate bias can directly eliminate the PPC as an erasing operation. This work reveals the underlying mechanism of the electrolyte-gated organic phototransistor memories and broadens the application of the EGOPTs.

Highly Tunable, Broadband, and Negative Photoresponse MoS2 Photodetector Driven by Ion-Gel Gate Dielectrics

Revealing the light-matter interaction of molybdenum disulfide (MoS₂) and further improving its tunability facilitate the construction of highly integrated optoelectronics in communication and wearable healthcare, but it still remains a significant challenge. Herein, polyvinylidene fluoride and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PVDF-EMIM-TFSI) ion-gel are employed to replace the oxide to fabricate a MoS₂-based phototransistor. The high capacitance enables a large tunability of the carrier concentration that results in ambipolar transport of MoS₂. It is found that the photoelectrical effect of the MoS₂ ion-gel phototransistor can be greatly tuned by the gate voltage including its photoresponsivity, detectivity, and response wavelength. An abnormal negative photoelectrical effect in both the electron branch and the hole branch is observed which is due to the adsorption/desorption of the C₂F₆NO₄S₂^- ion. By tuning the carrier concentration, the photoresponse can be extended from the visible region to the short infrared region. At 1200 nm, the photoresponse and detectivity can be tuned as large as 0.90 A/W and 1.88 × 10¹¹ Jones, respectively. Ultimately, by combining the tunability of gate voltage and wavelength, it is demonstrated that the photoelectrical effect is dominated by the photogating effect in the hole carrier, while it is coregulated by a photogating and photothermal effect in electron carrier. This study provides new insights for developing a highly tunable broadband photodetector with low consumption.

Flexible Low-Temperature Ammonia Gas Sensor Based on Reduced Graphene Oxide and Molybdenum Disulfide

Owing to harsh working environments and complex industrial requirements, traditional gas sensors are prone to deformation damage, possess a limited detection range, require a high working temperature, and display low reliability, thereby necessitating the development of flexible and low-temperature gas sensors. In this study, we developed a low-temperature polyimide (PI)-based flexible gas sensor comprising a reduced graphene oxide (rGO)/MoS2 composite. The micro-electro-mechanical system technology was used to fabricate Au electrodes on a flexible PI sheet to form a “sandwiched” sensor structure. The rGO/MoS2 composites were synthesized via a one-step hydrothermal method. The gas-sensing response was the highest for the composite comprising 10% rGO. The structure of this material was characterized, and a PI-based flexible gas sensor comprising rGO/MoS2 was fabricated. The optimal working temperature of the sensor was 141 °C, and its response-recovery time was significantly short upon exposure to 50–1500 ppm NH3. Thus, this sensor exhibited high selectivity and a wide NH3 detection range. Furthermore, it possessed the advantages of low power consumption, a short response-recovery time, a low working temperature, flexibility, and variability. Our findings provide a new framework for the development of pollutant sensors that can be utilized in an industrial environment.

Recent Progress of Flexible Image Sensors for Biomedical Applications

Flexible image sensors have attracted increasing attention as new imaging devices owing to their lightness, softness, and bendability. Since light can measure inside information from outside of the body, optical-imaging-based approaches, such as X-rays, are widely used for disease diagnosis in hospitals. Unlike conventional sensors, flexible image sensors are soft and can be directly attached to a curved surface, such as the skin, for continuous measurement of biometric information with high accuracy. Therefore, they are expected to gain wide application to wearable devices, as well as home medical care. Herein, the application of such sensors to the biomedical field is introduced. First, their individual components, photosensors, and switching elements, are explained. Then, the basic parameters used to evaluate the performance of each of these elements and the image sensors are described. Finally, examples of measuring the dynamic and static biometric information using flexible image sensors, together with relevant real-world measurement cases, are presented. Furthermore, recent applications of the flexible image sensors in the biomedical field are introduced.

Strategy and Future Prospects to Develop Room-Temperature-Recoverable NO2 Gas Sensor Based on Two-Dimensional Molybdenum Disulfide

Nitrogen dioxide (NO2), a hazardous gas with acidic nature, is continuously being liberated in the atmosphere due to human activity. The NO2 sensors based on traditional materials have limitations of high-temperature requirements, slow recovery, and performance degradation under harsh environmental conditions. These limitations of traditional materials are forcing the scientific community to discover future alternative NO2 sensitive materials. Molybdenum disulfide (MoS2) has emerged as a potential candidate for developing next-generation NO2 gas sensors. MoS2 has a large surface area for NO2 molecules adsorption with controllable morphologies, facile integration with other materials and compatibility with internet of things (IoT) devices. The aim of this review is to provide a detailed overview of the fabrication of MoS2 chemiresistance sensors in terms of devices (resistor and transistor), layer thickness, morphology control, defect tailoring, heterostructure, metal nanoparticle doping, and through light illumination. Moreover, the experimental and theoretical aspects used in designing MoS2-based NO2 sensors are also discussed extensively. Finally, the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS2. Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO2 gas sensors for environmental monitoring.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

Ultrasensitive Phototransistor Based on WSe2-MoS2 van der Waals Heterojunction

Band engineering using the van der Waals heterostructure of two-dimensional materials allows for the realization of high-performance optoelectronic devices by providing an ultrathin and uniform PN junction with sharp band edges. In this study, a highly sensitive photodetector based on the van der Waals heterostructure of WSe₂ and MoS₂ was developed. The MoS₂ was utilized as the channel for a phototransistor, whereas the WSe₂-MoS₂ PN junction in the out-of-plane orientation was utilized as a charge transfer layer. The vertical built-in electric field in the PN junction separated the photogenerated carriers, thus leading to a high photoconductive gain of 10⁶. The proposed phototransistor exhibited an excellent performance, namely, a high photoresponsivity of 2700 A/W, specific detectivity of 5 × 10¹¹ Jones, and response time of 17 ms. The proposed scheme in conjunction with the large-area synthesis technology of two-dimensional materials contributes significantly to practical photodetector applications.

Organic field-effect transistor-based flexible sensors

Flexible transistors are the next generation sensing technology, due to multiparametric analysis, reduced complexity, biocompatibility, lightweight with tunable optoelectronic properties. We summarize multitude of applications realized with OFETs.

Nanocellulose applications in sustainable electrochemical and piezoelectric systems: A review

Recent studies advocate the use of cellulose nanomaterials (CNs) as a sustainable carbohydrate polymer in numerous innovative electronics for their quintessential features such as flexibility, low thermal expansion and self-/directed assembly within multiphase matrices. Herein, we review the contemporary advances in CN-built electrochemical systems and highlight the constructive effects of these nanoscopic entities once engineered in conductive composites, proton exchange membranes (PEMs), electrochromics, energy storage devices and piezoelectric sensors. The adopted strategies and designs are discussed in view of CN roles as copolymer, electrolyte reservoir, binder and separator. Finally, physiochemical attributes and durability of resulting architectures are compared to conventional materials and the possible challenges/solutions are delineated to realize the promising capabilities. The volume of the up-to-present literature in the field indeed implies to nanocellulose overriding importance and the presented angles perhaps shed more lights on prospect of the biosphere's most dominant biomaterial in the energy-related arena that deserve attention.

Application of Biodegradable and Biocompatible Nanocomposites in Electronics: Current Status and Future Directions

With the continuous increase in the production of electronic devices, large amounts of electronic waste (E-waste) are routinely being discarded into the environment. This causes serious environmental and ecological problems because of the non-degradable polymers, released hazardous chemicals, and toxic heavy metals. The appearance of biodegradable polymers, which can be degraded or dissolved into the surrounding environment with no pollution, is promising for effectively relieving the environmental burden. Additionally, biodegradable polymers are usually biocompatible, which enables electronics to be used in implantable biomedical applications. However, for some specific application requirements, such as flexibility, electric conductivity, dielectric property, gas and water vapor barrier, most biodegradable polymers are inadequate. Recent research has focused on the preparation of nanocomposites by incorporating nanofillers into biopolymers, so as to endow them with functional characteristics, while simultaneously maintaining effective biodegradability and biocompatibility. As such, bionanocomposites have broad application prospects in electronic devices. In this paper, emergent biodegradable and biocompatible polymers used as insulators or (semi)conductors are first reviewed, followed by biodegradable and biocompatible nanocomposites applied in electronics as substrates, (semi)conductors and dielectrics, as well as electronic packaging, which is highlighted with specific examples. To finish, future directions of the biodegradable and biocompatible nanocomposites, as well as the challenges, that must be overcome are discussed.

Flexible Photodetectors Based on Novel Functional Materials

Flexible photodetectors have attracted a great deal of research interest in recent years due to their great possibilities for application in a variety of emerging areas such as flexible, stretchable, implantable, portable, wearable and printed electronics and optoelectronics. Novel functional materials, including materials with zero-dimensional (0D) and one-dimensional (1D) inorganic nanostructures, two-dimensional (2D) layered materials, organic semiconductors and perovskite materials, exhibit appealing electrical and optoelectrical properties, as well as outstanding mechanical flexibility, and have been widely studied as building blocks in cost-effective flexible photodetection. Here, we comprehensively review the outstanding performance of flexible photodetectors made from these novel functional materials reported in recent years. The photoresponse characteristics and flexibility of the devices will be discussed systematically. Summaries and challenges are provided to guide future directions of this vital research field.

Transparent Large-Area MoS2 Phototransistors with Inkjet-Printed Components on Flexible Platforms

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have gained considerable attention as an emerging semiconductor due to their promising atomically thin film characteristics with good field-effect mobility and a tunable band gap energy. However, their electronic applications have been generally realized with conventional inorganic electrodes and dielectrics implemented using conventional photolithography or transferring processes that are not compatible with large-area and flexible device applications. To facilitate the advantages of 2D TMDCs in practical applications, strategies for realizing flexible and transparent 2D electronics using low-temperature, large-area, and low-cost processes should be developed. Motivated by this challenge, we report fully printed transparent chemical vapor deposition (CVD)-synthesized monolayer molybdenum disulfide (MoS₂) phototransistor arrays on flexible polymer substrates. All the electronic components, including dielectric and electrodes, were directly deposited with mechanically tolerable organic materials by inkjet-printing technology onto transferred monolayer MoS₂, and their annealing temperature of <180 °C allows the direct fabrication on commercial flexible substrates without additional assisted-structures. By integrating the soft organic components with ultrathin MoS₂, the fully printed MoS₂ phototransistors exhibit excellent transparency and mechanically stable operation.

Review of recent research on flexible multifunctional nanopapers

Traditional paper and papermaking have struggled with a declining market during the last few decades. However, the incorporation of nanotechnology into papermaking has brought possibilities to develop low-cost, biocompatible and flexible products with sophisticated functionalities. The functionality of nanopapers emerges from the intrinsic properties of the nanofibrous network, the additional loading of specific nanomaterials (NMs), or the additional deposition and patterning of thin films of nanomaterials on the paper surface. A successful development of functional nanopapers requires understanding how the nanopaper matrix, nanofillers, nanocoating pigments, nanoprinting inks, processing additives and manufacturing processes all interact to provide the intended functionality. This review addresses the emerging area of functional nanopapers. This review discusses flexible and multifunctional nanopapers, NMs being used in nanopaper making, manufacturing techniques, and functional applications that provide new important possibilities to utilize papermaking technology. The interface where NM research meets traditional papermaking has important implications for food packaging, energy harvesting and energy storage, flexible electronics, low-cost devices for medical diagnostics, and numerous other areas.

New insights and perspectives into biological materials for flexible electronics

Materials based on biological materials are becoming increasingly competitive and are likely to be critical components in flexible electronic devices.

Development and applications of transparent conductive nanocellulose paper

Increasing attention has been paid to the next generation of 'green' electronic devices based on renewable nanocellulose, owing to its low roughness, good thermal stability and excellent optical properties. Various proof-of-concept transparent nanopaper-based electronic devices have been fabricated; these devices exhibit excellent flexibility, bendability and even foldability. In this review, we summarize the recent progress of transparent nanopaper that uses different types of nanocellulose, including pure nanocellulose paper and composite nanocellulose paper. The latest development of transparent and flexible nanopaper electronic devices are illustrated, such as electrochromic devices, touch sensors, solar cells and transistors. Finally, we discuss the advantages of transparent nanopaper compared to conventional flexible plastic substrate and the existing challenges to be tackled in order to realize this promising potential.