Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

A general ink formulation of 2D crystals for wafer-scale inkjet printing

Recent advances in inkjet printing of two-dimensional (2D) crystals show great promise for next-generation printed electronics development. Printing nonuniformity, however, results in poor reproducibility in device performance and remains a major impediment to their large-scale manufacturing. At the heart of this challenge lies the coffee-ring effect (CRE), ring-shaped nonuniform deposits formed during postdeposition drying. We present an experimental study of the drying mechanism of a binary solvent ink formulation. We show that Marangoni-enhanced spreading in this formulation inhibits contact line pinning and deforms the droplet shape to naturally suppress the capillary flows that give rise to the CRE. This general formulation supports uniform deposition of 2D crystals and their derivatives, enabling scalable and even wafer-scale device fabrication, moving them closer to industrial-level additive manufacturing.

Smart textile lighting/display system with multifunctional fibre devices for large scale smart home and IoT applications

Smart textiles consist of discrete devices fabricated from-or incorporated onto-fibres. Despite the tremendous progress in smart textiles for lighting/display applications, a large scale approach for a smart display system with integrated multifunctional devices in traditional textile platforms has yet to be demonstrated. Here we report the realisation of a fully operational 46-inch smart textile lighting/display system consisting of RGB fibrous LEDs coupled with multifunctional fibre devices that are capable of wireless power transmission, touch sensing, photodetection, environmental/biosignal monitoring, and energy storage. The smart textile display system exhibits full freedom of form factors, including flexibility, bendability, and rollability as a vivid RGB lighting/grey-level-controlled full colour display apparatus with embedded fibre devices that are configured to provide external stimuli detection. Our systematic design and integration strategies are transformational and provide the foundation for realising highly functional smart lighting/display textiles over large area for revolutionary applications on smart homes and internet of things (IoT).

Thermodynamically Stable Synthesis of Large-Scale and Highly Crystalline Transition Metal Dichalcogenide Monolayers and their Unipolar n-n Heterojunction Devices

Transition metal dichalcogenide (TMDC) monolayers are considered to be potential materials for atomically thin electronics due to their unique electronic and optical properties. However, large-area and uniform growth of TMDC monolayers with large grain sizes is still a considerable challenge. This report presents a simple but effective approach for large-scale and highly crystalline molybdenum disulfide monolayers using a solution-processed precursor deposition. The low supersaturation level, triggered by the evaporation of an extremely thin precursor layer, reduces the nucleation density dramatically under a thermodynamically stable environment, yielding uniform and clean monolayer films and large crystal sizes up to 500 µm. As a result, the photoluminescence exhibits only a small full-width-half-maximum of 48 meV, comparable to that of exfoliated and suspended monolayer crystals. It is confirmed that this growth procedure can be extended to the synthesis of other TMDC monolayers, and robust MoS₂ /WS₂ heterojunction devices are easily prepared using this synthetic procedure due to the large-sized crystals. The heterojunction device shows a fast response time (≈45 ms) and a significantly high photoresponsivity (≈40 AW^-1 ) because of the built-in potential and the majority-carrier transport at the n-n junction. These findings indicate an efficient pathway for the fabrication of high-performance 2D optoelectronic devices.

Technology progress on quantum dot light-emitting diodes for next-generation displays

Quantum dot light-emitting diodes (QD-LEDs) are widely recognised as great alternatives to organic light-emitting diodes (OLEDs) due to their enhanced performances. This focus article surveys the current progress on the state-of-the-art QD-LED technology including material synthesis, device optimization and innovative fabrication processes. A discussion on the material synthesis of core nanocrystals, shell layers and surface-binding ligands is presented for high photoluminescence quantum yield (PLQY) quantum dots (QDs) using heavy-metal free materials. The operational principles of several types of QD-LED device architectures are also covered, and the recent evolution of device engineering technologies is investigated. By exploring the fabrication process for pixel-patterning of QD-LEDs on an active-matrix backplane for full-colour display applications, we anticipate further improvement in device performance for the commercialisation of next-generation displays.

Truly form-factor-free industrially scalable system integration for electronic textile architectures with multifunctional fiber devices

An integrated textile electronic system is reported here, enabling a truly free form factor system via textile manufacturing integration of fiber-based electronic components. Intelligent and smart systems require freedom of form factor, unrestricted design, and unlimited scale. Initial attempts to develop conductive fibers and textile electronics failed to achieve reliable integration and performance required for industrial-scale manufacturing of technical textiles by standard weaving technologies. Here, we present a textile electronic system with functional one-dimensional devices, including fiber photodetectors (as an input device), fiber supercapacitors (as an energy storage device), fiber field-effect transistors (as an electronic driving device), and fiber quantum dot light-emitting diodes (as an output device). As a proof of concept applicable to smart homes, a textile electronic system composed of multiple functional fiber components is demonstrated, enabling luminance modulation and letter indication depending on sunlight intensity.

Ambipolar Deep-Subthreshold Printed-Carbon-Nanotube Transistors for Ultralow-Voltage and Ultralow-Power Electronics

The development of ultralow-power and easy-to-fabricate electronics with potential for large-scale circuit integration (i.e., complementary or complementary-like) is an outstanding challenge for emerging off-the-grid applications, e.g., remote sensing, "place-and-forget", and the Internet of Things. Herein we address this challenge through the development of ambipolar transistors relying on solution-processed polymer-sorted semiconducting carbon nanotube networks (sc-SWCNTNs) operating in the deep-subthreshold regime. Application of self-assembled monolayers at the active channel interface enables the fine-tuning of sc-SWCNTN transistors toward well-balanced ambipolar deep-subthreshold characteristics. The significance of these features is assessed by exploring the applicability of such transistors to complementary-like integrated circuits, with respect to which the impact of the subthreshold slope and flatband voltage on voltage and power requirements is studied experimentally and theoretically. As demonstrated with inverter and NAND gates, the ambipolar deep-subthreshold sc-SWCNTN approach enables digital circuits with complementary-like operation and characteristics including wide noise margins and ultralow operational voltages (≤0.5 V), while exhibiting record-low power consumption (≤1 pW/μm). Among thin-film transistor technologies with minimal material complexity, our approach achieves the lowest energy and power dissipation figures reported to date, which are compatible with and highly attractive for emerging off-the-grid applications.

Two-dimensional material-enhanced surface plasmon resonance for antibiotic sensing

Two-dimensional (2D) materials attract attention from the academic community due to their excellent properties, and their wide application in sensing is expected to revolutionize environmental monitoring, medical diagnostics, and food safety. In this work, we systematically evaluate the effects of 2D materials on the Au chip surface plasmon resonance (SPR) sensor. The results reveal that 2D materials cannot improve the sensitivity of intensity-modulated SPR sensors. However, there exists an optimal real part of RI of 3.5-4.0 and optimal thickness when choosing nanomaterials for sensitivity enhancement of SPR sensors in angular modulation. In addition, the smaller the imaginary part of the nanomaterial RI, the higher the sensitivity of the proposed Au SPR sensor. The 2D material's thickness needed for the highest sensitivity decreases with increasing real part and imaginary part of the RI. As a case study, we developed a 5 nm-thickness MoS₂-enhanced SPR biosensor, which exhibited a low sulfonamides (SAs) detection limit of 0.05 μg/L based on a group-targeting indirect competitive immunoassay, nearly 12-fold lower than that of the bare Au SPR system. The proposed criteria help to shed light on the 2D material-Au surface interaction, which has greatly promoted the development of novel SPR biosensing with outstanding sensitivity.

Nanofabrication of Conductive Metallic Structures on Elastomeric Materials

Existing techniques for patterning metallic structures on elastomers are limited in terms of resolution, yield and scalability. The primary constraint is the incompatibility of their physical properties with conventional cleanroom techniques. We demonstrate a reliable fabrication strategy to transfer high resolution metallic structures of <500 nm in dimension on elastomers. The proposed method consists of producing a metallic pattern using conventional lithographic techniques on silicon coated with a thin sacrificial aluminium layer. Subsequent wet etching of the sacrificial layer releases the elastomer with the embedded metallic pattern. Using this method, a nano-resistor with minimum feature size of 400 nm is fabricated on polydimethylsiloxane (PDMS) and applied in gas sensing. Adsorption of solvents in the PDMS causes swelling and increases the device resistance, which therefore enables the detection of volatile organic compounds (VOCs). Sensitivity to chloroform and toluene vapor with a rapid response (~30 s) and recovery (~200 s) is demonstrated using this PDMS nano-resistor at room temperature.

SC-CNNs for chaotic signal applications in secure communication systems

In this paper a CNNs based circuit for the generation of hyperchaotic signals is proposed. The circuit has been developed for applications in secure communication systems. An Saito oscillator has been designed by using a suitable configuration of a four-cells State-Controlled CNNs. A cryptography system based on the Saito oscillator has been implemented by using inverse system synchronization. The proposed circuit implementation and experimental results are given.

Double-Framed Thin Elastomer Devices

Elastomers and, in particular, polydimethylsiloxane (PDMS) are widely adopted as biocompatible mechanically compliant substrates for soft and flexible micro-nanosystems in medicine, biology, and engineering. However, several applications require such low thicknesses (e.g., <100 μm) that make peeling-off critical because very thin elastomers become delicate and tend to exhibit strong adhesion with carriers. Moreover, microfabrication techniques such as photolithography use solvents which swell PDMS, introducing complexity and possible contamination, thus limiting industrial scalability and preventing many biomedical applications. Here, we combine low-adhesion and rectangular carrier substrates, adhesive Kapton frames, micromilling-defined shadow masks, and adhesive-neutralizing paper frames for enabling fast, easy, green, contaminant-free, and scalable manufacturing of thin elastomer devices, with both simplified peeling and handling. The accurate alignment between the frame and shadow masks can be further facilitated by micromilled marking lines on the back side of the low-adhesion carrier. As a proof of concept, we show epidermal sensors on a 50 μm-thick PDMS substrate for measuring strain, the skin bioimpedance and the heart rate. The proposed approach paves the way to a straightforward, green, and scalable fabrication of contaminant-free thin devices on elastomers for a wide variety of applications.

Finite Element Simulation Model of Metallic Thermal Conductivity Detectors for Compact Air Pollution Monitoring Devices

Air pollution has been associated with several health problems. Detecting and measuring the concentration of harmful pollutants present in complex air mixtures has been a long-standing challenge, due to the intrinsic difficulty of distinguishing among these substances from interferent species and environmental conditions, both indoor and outdoor. Despite all efforts devoted by the scientific and industrial communities to tackling this challenge, the availability of suitable device technologies able to selectively discriminate these pollutants present in the air at minute, yet dangerous, concentrations and provide a quantitative measure of their concentrations is still an unmet need. Thermal conductivity detectors (TCDs) show promising characteristics that make them ideal gas sensing tools capable of recognising different gas analytes based on their physical fingerprint characteristics at the molecular level, such as their density, thermal conductivity, dynamic viscosity, and others. In this paper, the operation of TCD gas sensors is presented and explored using a finite element simulation of Joule heating in a sensing electrode placed in a gas volume. The results obtained show that the temperature, and hence, the resistance of the individual suspended microbridge sensor device, depends on the surrounding gas and its thermal conductivity, while the sensitivity and power consumption depend on the properties of the constitutive metal. Moreover, the electrode resistance is proven to be linearly dependent on the applied voltage.

Machine Learning-Assistant Colorimetric Sensor Arrays for Intelligent and Rapid Diagnosis of Urinary Tract Infection

Urinary tract infections (UTIs), which can lead to pyelonephritis, urosepsis, and even death, are among the most prevalent infectious diseases worldwide, with a notable increase in treatment costs due to the emergence of drug-resistant pathogens. Current diagnostic strategies for UTIs, such as urine culture and flow cytometry, require time-consuming protocols and expensive equipment. We present here a machine learning-assisted colorimetric sensor array based on recognition of ligand-functionalized Fe single-atom nanozymes (SANs) for the identification of microorganisms at the order, genus, and species levels. Colorimetric sensor arrays are built from the SAN Fe1-NC functionalized with four types of recognition ligands, generating unique microbial identification fingerprints. By integrating the colorimetric sensor arrays with a trained computational classification model, the platform can identify more than 10 microorganisms in UTI urine samples within 1 h. Diagnostic accuracy of up to 97% was achieved in 60 UTI clinical samples, holding great potential for translation into clinical practice applications.

Pathogen Detection via Impedance Spectroscopy-Based Biosensor

This paper presents the development of a miniaturized sensor device for selective detection of pathogens, specifically Influenza A Influenza virus, as an enveloped virus is relatively vulnerable to damaging environmental impacts. In consideration of environmental factors such as humidity and temperature, this particular pathogen proves to be an ideal choice for our study. It falls into the category of pathogens that pose greater challenges due to their susceptibility. An impedance biosensor was integrated into an existing platform and effectively separated and detected high concentrations of airborne pathogens. Bio-functionalized hydrogel-based detectors were utilized to analyze virus-containing particles. The sensor device demonstrated high sensitivity and specificity when exposed to varying concentrations of Influenza A virus ranging from 0.5 to 50 μg/mL. The sensitivity of the device for a 0.5 μg/mL analyte concentration was measured to be 695 Ω· mL/μg. Integration of this pathogen detector into a compact-design air quality monitoring device could foster the advancement of personal exposure monitoring applications. The proposed sensor device offers a promising approach for real-time pathogen detection in complex environmental settings.

Simultaneous Isotropic Omnidirectional Hypersensitive Strain Sensing and Deep Learning-Assisted Direction Recognition in a Biomimetic Stretchable Device

Omnidirectional strain sensing and direction recognition ability are features of the human tactile sense, essential to address the intricate and dynamic requirements of real-world applications. Most of the current strain sensors work by converting uniaxial strain into electrical signals, which restricts their use in environments with multiaxial strain. Here, the first device with simultaneous isotropic omnidirectional hypersensitive strain sensing and direction recognition (IOHSDR) capabilities is introduced. By mimicking the human fingers from three dimensions, the IOHSDR device realizes a novel heterogeneous substrate that incorporates the involute of a circle, resulting in isotropic behavior in the radial direction and anisotropic property in the involute direction for hypersensitive strain sensing. With the assistance of a deep learning-based model, the IOHSDR device accomplishes an impressive accuracy of 99.58% in recognizing 360° stretching directions. Additionally, it exhibits superior performance in the typical properties of stretchable strain sensors, with a gauge factor of 634.12, an ultralow detection limit of 0.01%, and outstanding durability exceeding 15 000 cycles. The demonstration of radial artery pulse and throat vibration applications highlights the IOHSDR's unique characteristics of isotropic omnidirectional sensing and precise direction detection unleashing new classes of wearable health monitoring devices.

A deep learning-enabled smart garment for accurate and versatile monitoring of sleep conditions in daily life

In wearable smart systems, continuous monitoring and accurate classification of different sleep-related conditions are critical for enhancing sleep quality and preventing sleep-related chronic conditions. However, the requirements for device-skin coupling quality in electrophysiological sleep monitoring systems hinder the comfort and reliability of night wearing. Here, we report a washable, skin-compatible smart garment sleep monitoring system that captures local skin strain signals under weak device-skin coupling conditions without positioning or skin preparation requirements. A printed textile-based strain sensor array responds to strain from 0.1 to 10% with a gauge factor as high as 100 and shows independence to extrinsic motion artifacts via strain-isolating printed pattern design. Through reversible starching treatment, ink penetration depth during direct printing on garments is controlled to achieve batch-to-batch performance variation <10%. Coupled with deep learning, explainable AI, and transfer learning data processing, the smart garment is capable of classifying six sleep states with an accuracy of 98.6%, maintaining excellent explainability (classification with low bias) and generalization (95% accuracy on new users with few-shot learning less than 15 samples per class) in practical applications, paving the way for next-generation daily sleep healthcare management.

Robust In-Zn-O Thin-Film Transistors with a Bilayer Heterostructure Design and a Low-Temperature Fabrication Process Using Vacuum and Solution Deposited Layers

We report on the design, fabrication, and characterization of heterostructure In-Zn-O (IZO) thin-film transistors (TFTs) with improved performance characteristics and robust operation. The heterostructure layer is fabricated by stacking a solution-processed IZO film on top of a buffer layer, which is deposited previously using an electron beam (e-beam) evaporator. A thin buffer layer at the dielectric interface can help to template the structure of the channel. The control of the precursors and of the solvent used during the sol-gel process can help lower the temperature needed for the sol-gel condensation reaction to proceed cleanly. This boosts the overall performance of the device with a significantly reduced subthreshold swing, a four-fold mobility increase, and a two-order of magnitude larger on/off ratio. Atomistic simulations of the a-IZO structure using molecular dynamics (both classical and ab initio) and hybrid density functional theory (DFT) calculations of the electronic structure reveal the potential atomic origin of these effects.

Graphene-Perovskite Fibre Photodetectors

The integration of optoelectronic devices, such as transistors and photodetectors (PDs), into wearables and textiles is of great interest for applications such as healthcare and physiological monitoring. These require flexible/wearable systems adaptable to body motions, thus materials conformable to non-planar surfaces, and able to maintain performance under mechanical distortions. Here, fibre PDs are prepared by combining rolled graphene layers and photoactive perovskites. Conductive fibres (~500 Ωcm-1) are made by rolling single-layer graphene (SLG) around silica fibres, followed by deposition of a dielectric layer (Al2O3 and parylene C), another rolled SLG as a channel, and perovskite as photoactive component. The resulting gate-tunable PD has a response time~9ms, with an external responsivity~22kAW-1 at 488nm for a 1V bias. The external responsivity is two orders of magnitude higher, and the response time one order of magnitude faster, than state-of-the-art wearable fibre-based PDs. Under bending at 4mm radius, up to~80% photocurrent is maintained. Washability tests show~72% of initial photocurrent after 30 cycles, promising for wearable applications.

Spray-lithography of hybrid graphene-perovskite paper-based photodetectors for sustainable electronics

Paper is an ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems. When combined with nanomaterial-based devices, it can be harnessed for various Internet-of-Things applications, ranging from wearable electronics to smart packaging. However, paper remains a challenging substrate for electronics due to its rough and porous nature. In addition, the absence of established fabrication methods is impeding its utilization in wearable applications. Unlike other paper-based electronics with added layers, in this study, we present a scalable spray-lithography on a commercial paper substrate. We present a non-vacuum spray-lithography of chemical vapor deposition (CVD) single-layer graphene (SLG), carbon nanotubes (CNTs) and perovskite quantum dots (QDs) on a paper substrate. This approach combines the advantages of two large-area techniques: CVD and spray-coating. The first technique allows for the growth of SLG, while the second enables the spray coating of a mask to pattern CVD SLG, electrodes (CNTs), and photoactive (QDs) layers. We harness the advantages of perovskite QDs in photodetection, leveraging their strong absorption coefficients. Integrating them with the graphene enhances the photoconductive gain mechanism, leading to high external responsivity. The presented device shows high external responsivity of ∼520 A W-1at 405 nm at <1 V bias due to the photoconductive gain mechanism. The prepared paper-based photodetectors (PDs) achieve an external responsivity of 520 A W-1under 405 nm illumination at <1 V operating voltage. To the best of our knowledge, our devices have the highest external responsivity among paper-based PDs. By fabricating arrays of PDs on a paper substrate in the air, this work highlights the potential of this scalable approach for enabling ubiquitous electronics on paper.

Coexistence of Contact Electrification and Dynamic p-n Junction Modulation Effects in Triboelectrification

The triboelectric effect occurs when two dissimilar materials are in physical contact, attributed to the combination of contact electrification (CE) and electrostatic induction. It has been extensively explored for the development of high-performance triboelectric nanogenerators (TENGs). In this paper, we report on, besides the CE-related charge generation, an additional charge generation phenomenon associated with the modulation of the p-n junction when two semiconductor materials [methylammonium lead iodide (MAPI) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)] are put in contact and separated dynamically. The electrical outputs generated by the CE effect are determined by the surface potential difference between the two friction materials, while the ones induced by the p-n junction modulation are determined by the dynamic variations in the depletion widths of the two semiconductor friction materials. The outputs generated by the CE effect and the p-n junction effect are well separated in time scale; the p-n junction modulation contributes ∼20% of the total charge generated and could be varied by changing the chemical composition of the semiconductors. The results may provide an alternative method for the development of high-performance TENGs by utilizing this additional p-n junction modulation effect.