Rosin-enabled ultraclean and damage-free transfer of graphene for large-area flexible organic light-emitting diodes

Thermal boundary resistance at graphene-pentacene interface explored by a data-intensive approach

As the machinery of artificial intelligence matures in recent years, there has been a surge in applying machine learning (ML) techniques for material property predictions. Artificial neural network (ANN) is a branch of ML and has gained increasing popularity due to its capabilities of modeling complex correlations among large datasets. The interfacial thermal transport plays a significant role in the thermal management of graphene-pentacene based organic electronics. In this work, the thermal boundary resistance (TBR) between graphene and pentacene is comprehensively investigated by classical molecular dynamics simulations combined with the ML technique. The TBR values along thea,bandcdirections of pentacene at 300 K are 5.19 ± 0.18 × 10^-8m²K W^-1, 3.66 ± 0.36 × 10^-8m²K W^-1and 5.03 ± 0.14 × 10^-8m²K W^-1, respectively. Different architectures of ANN models are trained to predict the TBR between graphene and pentacene. Two important hyperparameters, i.e. network layer and the number of neurons are explored to achieve the best prediction results. It is reported that the two-layer ANN with 40 neurons each layer provides the optimal model performance with a normalized mean square error loss of 7.04 × 10^-4. Our results provide reasonable guidelines for the thermal design and development of graphene-pentacene electronic devices.

Interface Engineering of a Silicon/Graphene Heterojunction Photodetector via a Diamond-Like Carbon Interlayer

Silicon/graphene nanowalls (Si/GNWs) heterojunctions with excellent integrability and sensitivity show an increasing potential in optoelectronic devices. However, the performance is greatly limited by inferior interfacial adhesion and week electronic transport caused by the horizontal buffer layer. Herein, a diamond-like carbon (DLC) interlayer is first introduced to construct Si/DLC/GNWs heterojunctions, which can significantly change the growth behavior of the GNWs film, avoiding the formation of horizontal buffer layers. Accordingly, a robust diamond-like covalent bond with a remarkable enhancement of the interfacial adhesion is yielded, which notably improves the complementary metal oxide semiconductor compatibility for photodetector fabrication. Importantly, the DLC interlayer is verified to undergo a graphitization transition during the high-temperature growth process, which is beneficial to pave a vertical conductive path and facilitate the transport of photogenerated carriers in the visible and near-infrared regions. As a result, the Si/DLC/GNWs heterojunction detectors can simultaneously exhibit improved photoresponsivity and response speed, compared with the counterparts without DLC interlayers. The introduction of the DLC interlayer might provide a universal strategy to construct hybrid interfaces with high performance in next-generation optoelectronic devices.

Tailored Graphene Micropatterns by Wafer-Scale Direct Transfer for Flexible Chemical Sensor Platform

2D materials, such as graphene, exhibit great potential as functional materials for numerous novel applications due to their excellent properties. The grafting of conventional micropatterning techniques on new types of electronic devices is required to fully utilize the unique nature of graphene. However, the conventional lithography and polymer-supported transfer methods often induce the contamination and damage of the graphene surface due to polymer residues and harsh wet-transfer conditions. Herein, a novel strategy to obtain micropatterned graphene on polymer substrates using a direct curing process is demonstrated. Employing this method, entirely flexible, transparent, well-defined self-activated graphene sensor arrays, capable of gas discrimination without external heating, are fabricated on 4 in. wafer-scale substrates. Finite element method simulations show the potential of this patterning technique to maximize the performance of the sensor devices when the active channels of the 2D material are suspended and nanoscaled. This study contributes considerably to the development of flexible functional electronic devices based on 2D materials.

Laser-induced graphene for bioelectronics and soft actuators

Laser-assisted process can enable facile, mask-free, large-area, inexpensive, customizable, and miniaturized patterning of laser-induced porous graphene (LIG) on versatile carbonaceous substrates (e.g., polymers, wood, food, textiles) in a programmed manner at ambient conditions. Together with high tailorability of its porosity, morphology, composition, and electrical conductivity, LIG can find wide applications in emerging bioelectronics (e.g., biophysical and biochemical sensing) and soft robots (e.g., soft actuators). In this review paper, we first introduce the methods to make LIG on various carbonaceous substrates and then discuss its electrical, mechanical, and antibacterial properties and biocompatibility that are critical for applications in bioelectronics and soft robots. Next, we overview the recent studies of LIG-based biophysical (e.g., strain, pressure, temperature, hydration, humidity, electrophysiological) sensors and biochemical (e.g., gases, electrolytes, metabolites, pathogens, nucleic acids, immunology) sensors. The applications of LIG in flexible energy generators and photodetectors are also introduced. In addition, LIG-enabled soft actuators that can respond to chemicals, electricity, and light stimulus are overviewed. Finally, we briefly discuss the future challenges and opportunities of LIG fabrications and applications.

Pushing the conductance and transparency limit of monolayer graphene electrodes for flexible organic light-emitting diodes

Graphene has emerged as an attractive candidate for flexible transparent electrode (FTE) for a new generation of flexible optoelectronics. Despite tremendous potential and broad earlier interest, the promise of graphene FTE has been plagued by the intrinsic trade-off between electrical conductance and transparency with a figure of merit (σDC/σOp) considerably lower than that of the state-of-the-art ITO electrodes (σDC/σOp <123 for graphene vs. ∼240 for ITO). Here we report a synergistic electrical/optical modulation strategy to simultaneously boost the conductance and transparency. We show that a tetrakis(pentafluorophenyl)boric acid (HTB) coating can function as highly effective hole doping layer to increase the conductance of monolayer graphene by sevenfold and at the same time as an anti-reflective layer to boost the visible transmittance to 98.8%. Such simultaneous improvement in conductance and transparency breaks previous limit in graphene FTEs and yields an unprecedented figure of merit (σDC/σOp ∼323) that rivals the best commercial ITO electrode. Using the tailored monolayer graphene as the flexible anode, we further demonstrate high-performance green organic light-emitting diodes (OLEDs) with the maximum current, power and external quantum efficiencies (111.4 cd A-1, 124.9 lm W-1 and 29.7%) outperforming all comparable flexible OLEDs and surpassing that with standard rigid ITO by 43%. This study defines a straightforward pathway to tailor optoelectronic properties of monolayer graphene and to fully capture their potential as a generational FTE for flexible optoelectronics.

Layer-engineered large-area exfoliation of graphene

The competition between quality and productivity has been a major issue for large-scale applications of two-dimensional materials (2DMs). Until now, the top-down mechanical cleavage method has guaranteed pure perfect 2DMs, but it has been considered a poor option in terms of manufacturing. Here, we present a layer-engineered exfoliation technique for graphene that not only allows us to obtain large-size graphene, up to a millimeter size, but also allows selective thickness control. A thin metal film evaporated on graphite induces tensile stress such that spalling occurs, resulting in exfoliation of graphene, where the number of exfoliated layers is adjusted by using different metal films. Detailed spectroscopy and electron transport measurement analysis greatly support our proposed spalling mechanism and fine quality of exfoliated graphene. Our layer-engineered exfoliation technique can pave the way for the development of a manufacturing-scale process for graphene and other 2DMs in electronics and optoelectronics.

New Growth Frontier: Superclean Graphene

The last 10 years have witnessed significant progress in chemical vapor deposition (CVD) growth of graphene films. However, major hurdles remain in achieving the excellent quality and scalability of CVD graphene needed for industrial production and applications. Early efforts were mainly focused on increasing the single-crystalline domain size, large-area uniformity, growth rate, and controllability of layer thickness and on decreasing the defect concentrations. An important recent advance was the discovery of the inevitable contamination phenomenon of CVD graphene film during high-temperature growth processes and the superclean growth technique, which is closely related to the surface defects and to the peeling-off and transfer quality. Superclean graphene represents a new frontier in CVD graphene research. In this Perspective, we aim to provide comprehensive understanding of the intrinsic growth contamination and the experimental solution of making superclean graphene and to provide an outlook for future commercial production of high-quality CVD graphene films.

Superclean Growth of Graphene Using a Cold-Wall Chemical Vapor Deposition Approach

Chemical vapor deposition (CVD) has become a promising approach for the industrial production of graphene films with appealing controllability and uniformity. However, in the conventional hot-wall CVD system, CVD-derived graphene films suffer from surface contamination originating from the gas-phase reaction during the high-temperature growth. Shown here is that the cold-wall CVD system is capable of suppressing the gas-phase reaction, and achieves the superclean growth of graphene films in a controllable manner. The as-received superclean graphene film, exhibiting improved optical and electrical properties, was proven to be an ideal candidate material used as transparent electrodes and substrate for epitaxial growth. This study provides a new promising choice for industrial production of high-quality graphene films, and the finding about the engineering of the gas-phase reaction, which is usually overlooked, will be instructive for future research on CVD growth of graphene.

Hybrid nanostructured plasmonic electrodes for flexible organic light-emitting diodes

Improved performance in flexible organic light-emitting diodes (OLEDs) is demonstrated by using a hybrid nanostructured plasmonic electrode consisting of silver nanowires (AgNWs) decorated with silver nanoparticles (AgNPs) and covered by exfoliated graphene sheets. Such all-solution processed electrodes show high optical transparency and electrical conductivity. When integrated in an OLED with super yellow polyphenylene vinylene as the emissive layer, the plasmon coupling of the NW-NP hybrid plasmonic system is found to significantly enhance the fluorescence, demonstrated by both simulations and photoluminescence measurements, leading to a current efficiency of 11.61 cd A^-1 and a maximum luminance of 20 008 cd m^-2 in OLEDs. Stress studies reveal a superior mechanical flexibility to the commercial indium-tin-oxide (ITO) counterparts, due to the incorporation of exfoliated graphene sheets. Our results show that these hybrid nanostructured plasmonic electrodes can be applied as an effective alternative to ITO for use in high-performance flexible OLEDs.

Cera alba-assisted ultraclean graphene transfer for high-performance PbI2 UV photodetectors

Large polymer residues introduced by the graphene transfer process is still a major obstacle limiting the integration of chemical vapor deposition (CVD)-grown graphene into next-generation electronic and photoelectronic devices. Here we use cera alba, a natural and environmental-friendly material that derives from honeycomb, as the supporting layer for ultraclean graphene transfer. The transferred graphene has a low surface roughness with a surface height fluctuation within 5 nm and an only 80.08% average sheet resistance of the polymethyl methacrylate (PMMA)-transferred graphene. Further, the ultraclean graphene is used as electrodes for the PbI₂-based UV photodetector and enables a 135% improvement on responsivity. The cera alba assisted transfer method reported here could achieve clean and damage-free graphene transfer, promoting the application of CVD-grown two-dimensional (2D) materials in large-area thin-film electronic and optoelectronic devices.

Facile Morphological Qualification of Transferred Graphene by Phase-Shifting Interferometry

Post-growth graphene transfer to a variety of host substrates for circuitry fabrication has been among the most popular subjects since its successful development via chemical vapor deposition in the past decade. Fast and reliable evaluation tools for its morphological characteristics are essential for the development of defect-free transfer protocols. The implementation of conventional techniques, such as Raman spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy in production quality control at an industrial scale is difficult because they are limited to local areas, are time consuming, and their operation is complex. However, through a one-shot measurement within a few seconds, phase-shifting interferometry (PSI) successfully scans ≈1 mm² of transferred graphene with a vertical resolution of ≈0.1 nm. This provides crucial morphological information, such as the surface roughness derived from polymer residues, the thickness of the graphene, and its adhesive strength with respect to the target substrates. Graphene samples transferred via four different methods are evaluated using PSI, Raman spectroscopy, and AFM. Although the thickness of the nanomaterials measured by PSI can be highly sensitive to their refractive indices, PSI is successfully demonstrated to be a powerful tool for investigating the morphological characteristics of the transferred graphene for industrial and research purposes.

Effect of Metal-Ligand Coordination Complexes on Molecular Dynamics and Structure of Cross-Linked Poly(dimethylosiloxane)

Poly(dimethylosiloxane) (PDMS) cross-linked by metal-ligand coordination has a potential functionality for electronic devices applications. In this work, the molecular dynamics of bipyridine (bpy)–PDMS-MeCl₂ (Me: Mn²⁺, Fe²⁺, Ni²⁺, and Zn²⁺) are investigated by means of broadband dielectric spectroscopy and supported by differential scanning calorimetry and density functional theory calculations. The study of molecular motions covered a broad range of temperatures and frequencies and was performed for the first time for metal-ligand cross-linked PDMS. It was found that the incorporation of bpy moieties into PDMS chain prevents its crystallization. The dielectric permittivity of studied organometallic systems was elevated and almost two times higher (ε′ ~4 at 1 MHz) than in neat PDMS. BpyPDMS-MeCl₂ complexes exhibit slightly higher glass transition temperature and fragility as compared to a neat PDMS. Two segmental type relaxations (α and α_ac) were observed in dielectric studies, and their origin was discussed in relation to the molecular structure of investigated complexes. The α_ac relaxation was observed for the first time in amorphous metal-ligand complexes. It originates from the lower mobility of PDMS polymer chains, which are immobilized by metal-ligand coordination centers via bipyridine moieties.

Review of fabrication methods of large-area transparent graphene electrodes for industry

Graphene is a two-dimensional material showing excellent properties for utilization in transparent electrodes; it has low sheet resistance, high optical transmission and is flexible. Whereas the most common transparent electrode material, tin-doped indium-oxide (ITO) is brittle, less transparent and expensive, which limit its compatibility in flexible electronics as well as in low-cost devices. Here we review two large-area fabrication methods for graphene based transparent electrodes for industry: liquid exfoliation and low-pressure chemical vapor deposition (CVD). We discuss the basic methodologies behind the technologies with an emphasis on optical and electrical properties of recent results. State-of-the-art methods for liquid exfoliation have as a figure of merit an electrical and optical conductivity ratio of 43.5, slightly over the minimum required for industry of 35, while CVD reaches as high as 419.

Towards large-scale graphene transfer

The transfer process is crucial for obtaining high-quality graphene for its large-scale industrial application. In this review, graphene transfer methods are systematically classified along with an analysis of the contamination or impurity of graphene that is introduced during the transfer process. Two key processes are emphasized, the substrate removal process and the direct/indirect transfer of graphene. Based on the efficiency and cost factors of industrial scale production, various transfer methods are summarized and evaluated. Potential transfer technologies and future research directions for industrial application are prospected.

An Improved Rosin Transfer Process for the Reduction of Residue Particles for Graphene

In this work, an improved rosin transfer process is initiated. An anisole coating is introduced based on the rosin transfer process to reduce the residue particles on the surface of transferred graphene. Rosin/graphene and anisole/rosin/graphene samples are handled without baking and with baking at different temperatures, i.e., 100 °C, 150 °C, and 200 °C. Atomic force microscopy (AFM) and Raman spectroscopy are employed to characterize the surface properties of transferred graphene. The removal of the protective rosin layer and anisole/rosin layers without baking is found to be more effective and beneficial compared to the conventional PMMA transfer process. Furthermore, better results in terms of reduced surface roughness and residue particles are accomplished by introducing anisole in the improved rosin transfer process. Uniform and low sheet resistance (R_sh) is also observed across transferred graphene using this improved process.

Emerging Self-Emissive Technologies for Flexible Displays

Featuring a combination of ultrathin and lightweight properties, excellent mechanical flexibility, low power-consumption, and widely tunable saturated emission, flexible displays have opened up a new possibility for optoelectronics. The demands for flexible displays are growing on a continual basis due not only to their successful commercialization but, more importantly, their endless possibilities for wearable integrated systems. Up to now, self-emissive technologies for displays, flexible active-matrix organic light-emitting diodes (flex-AMOLED), flexible quantum dot light-emitting diodes (flex-QLEDs), and flexible perovskite light-emitting diodes (flex-PeLEDs) have been widely reported, but despite the significant progress made in these technologies, enormous obstacles and challenges remain for the vision of truly wearable applications, in particular with flex-QLEDs and flex-PeLEDs. Here, a review of the recent progress of all three self-emissive technologies for flexible displays is conducted, including the emissive active materials, device structures and approaches to manufacturing, the flexible substrates, and conductive electrodes, as well as the encapsulation techniques. The fast-paced improvement made to the efficiency of flexible devices in recent years is also summarized. The review concludes by making suggestions on the future development in this area, and is expected to help researchers in gaining a comprehensive understanding about the newly emerging technologies for flexible displays.

Laser Fabrication of Graphene-Based Flexible Electronics

Recent years have witnessed the rise of graphene and its applications in various electronic devices. Specifically, featuring excellent flexibility, transparency, conductivity, and mechanical robustness, graphene has emerged as a versatile material for flexible electronics. In the past decade, facilitated by various laser processing technologies, including the laser-treatment-induced photoreduction of graphene oxides, flexible patterning, hierarchical structuring, heteroatom doping, controllable thinning, etching, and shock of graphene, along with laser-induced graphene on polyimide, graphene has found broad applications in a wide range of electronic devices, such as power generators, supercapacitors, optoelectronic devices, sensors, and actuators. Here, the recent advancements in the laser fabrication of graphene-based flexible electronic devices are comprehensively summarized. The various laser fabrication technologies that have been employed for the preparation, processing, and modification of graphene and its derivatives are reviewed. A thorough overview of typical laser-enabled flexible electronic devices that are based on various graphene sources is presented. With the rapid progress that has been made in the research on graphene preparation methodologies and laser micronanofabrication technologies, graphene-based electronics may soon undergo fast development.

A vertical silicon-graphene-germanium transistor

Graphene-base transistors have been proposed for high-frequency applications because of the negligible base transit time induced by the atomic thickness of graphene. However, generally used tunnel emitters suffer from high emitter potential-barrier-height which limits the transistor performance towards terahertz operation. To overcome this issue, a graphene-base heterojunction transistor has been proposed theoretically where the graphene base is sandwiched by silicon layers. Here we demonstrate a vertical silicon-graphene-germanium transistor where a Schottky emitter constructed by single-crystal silicon and single-layer graphene is achieved. Such Schottky emitter shows a current of 692 A cm⁻² and a capacitance of 41 nF cm⁻², and thus the alpha cut-off frequency of the transistor is expected to increase from about 1 MHz by using the previous tunnel emitters to above 1 GHz by using the current Schottky emitter. With further engineering, the semiconductor-graphene-semiconductor transistor is expected to be one of the most promising devices for ultra-high frequency operation.

Graphene-base transistors were originally proposed for high-frequency applications, but the height of the emitter potential barrier limits the transistor performance towards the THz range. Here, the authors fabricate a vertical silicon-graphene-germanium transistor with a Schottky emitter enabling a transition from MHz to GHz operation.

A Force-Engineered Lint Roller for Superclean Graphene

Contamination is a major concern in surface and interface technologies. Given that graphene is a 2D monolayer material with an extremely large surface area, surface contamination may seriously degrade its intrinsic properties and strongly hinder its applicability in surface and interfacial regions. However, large-scale and facile treatment methods for producing clean graphene films that preserve its excellent properties have not yet been achieved. Herein, an efficient postgrowth treatment method for selectively removing surface contamination to achieve a large-area superclean graphene surface is reported. The as-obtained superclean graphene, with surface cleanness exceeding 99%, can be transferred to dielectric substrates with significantly reduced polymer residues, yielding ultrahigh carrier mobility of 500 000 cm² V^-1 s^-1 and low contact resistance of 118 Ω µm. The successful removal of contamination is enabled by the strong adhesive force of the activated-carbon-based lint roller on graphene contaminants.

A Double Support Layer for Facile Clean Transfer of Two-Dimensional Materials for High-Performance Electronic and Optoelectronic Devices

Clean transfer of two-dimensional (2D) materials grown by chemical vapor deposition is critical for their application in electronics and optoelectronics. Although rosin can be used as a support layer for the clean transfer of graphene grown on Cu, it has not been usable for the transfer of 2D materials grown on noble metals or for large-area transfer. Here, we report a poly(methyl methacrylate) (PMMA)/rosin double support layer that enables facile ultraclean transfer of large-area 2D materials grown on different metals. The bottom rosin layer ensures clean transfer, whereas the top PMMA layer not only screens the rosin from the transfer conditions but also improves the strength of the transfer layer to make the transfer easier and more robust. We demonstrate the transfer of monolayer WSe₂ and WS₂ single crystals grown on Au as well as large-area graphene films grown on Cu. As a result of the clean surface, the transferred WSe₂ retains the intrinsic optical properties of the as-grown sample. Moreover, it does not require annealing to form good ohmic contacts with metal electrodes, enabling high-performance field effect transistors with mobility and ON/OFF ratio ∼10 times higher than those made by PMMA-transferred WSe₂. The ultraclean graphene film is found to be a good anode for flexible organic photovoltaic cells with a high power conversion efficiency of ∼6.4% achieved.

Transfer-Medium-Free Nanofiber-Reinforced Graphene Film and Applications in Wearable Transparent Pressure Sensors

Graphene exhibits properties of atomic thickness, high transparency, and high carrier mobility, which is highly desirable for a flexible transparent conductive material. However, the electronic properties of large-area chemical vapor deposition grown graphene film suffer from insulated polymer contaminations introduced by the transfer process and the easily cracked nature. Here, we report a preparation method of a transfer-medium-free large-area nanofiber-reinforced graphene (a-PAN/G) film simply by annealing the electrostatically spun polyacrylonitrile (PAN) nanofibers on the graphene film. The film could be free-standing on water and suspended in air with high transparency and enhanced electrical and mechanical properties compared to that of a monolayer graphene film. The flexible transparent a-PAN/G films were demonstrated as active materials for sensitive pressure sensors. The obtained pressure sensors demonstrate high sensitivity (44.5 kPa^-1 within 1.2 kPa), low operating voltage (0.01-0.5 V), and excellent stability for 5500 loading-unloading cycles, revealing promising potential applications in wearable electronics.

Remote heteroepitaxy of atomic layered hafnium disulfide on sapphire through hexagonal boron nitride

Two-dimensional (2D) heterostructures have attracted a great deal of attention due to their novel phenomena arising from the complementary properties of their constituent materials, and provide an ideal platform for exploring new fundamental research and realizing technological innovation. Here, for the first time, we report the formation of high quality HfS2/h-BN heterostructures by the remote heteroepitaxy technique, in which the large-area single-crystal HfS2 layers were epitaxially grown on c-plane sapphire through a polycrystalline h-BN layer via chemical vapor deposition. It is found that c-sapphire substrates can penetrate monolayer and bilayer h-BN to remotely handle the epitaxial growth of HfS2. Benefitting from the high crystal quality of HfS2 epilayers and the weak interface scattering of HfS2 on h-BN, the HfS2 photodetectors demonstrate excellent performance with a high on/off ratio exceeding 105, an excellent photoresponsivity up to 0.135 A W-1 and a high detectivity of over 1012 Jones. Furthermore, the HfS2/h-BN heterostructures prepared by the remote epitaxy can be rapidly released and transferred to a substrate of interest, which opens a new pathway for large-area advanced wearable electronics applications.

Evaporable Glass-State Molecule-Assisted Transfer of Clean and Intact Graphene onto Arbitrary Substrates

Graphene and its clean transfer methods have gathered growing interest and concern in recent decades. Here, we develop a novel large-scale intact transferring technology of paraffin wax onto arbitrary substrates. The wax will then be removed by thermal evaporation, avoiding uncontrollable reactions and leaving no residues. For characterizations, we adopt Raman, FT-IR, XPS, and DRS to measure the optical reflection difference on various surfaces and the thickness of graphene accurately. All the results demonstrate transferred surfaces' cleanliness and our method's validity. This technique allows for an effective transfer of graphene and enables a wider range of applications in many fields.

Copper-Containing Carbon Feedstock for Growing Superclean Graphene

Chemical vapor deposition (CVD) enables the large-scale growth of high-quality graphene film and exhibits considerable potential for the industrial production of graphene. However, CVD-grown graphene film contains surface contamination, which in turn hinders its potential applications, for example, in electrical and optoelectronic devices and in graphene-membrane-based applications. To solve this issue, we demonstrated a modified gas-phase reaction to achieve the large-scale growth of contamination-free graphene film, i.e., superclean graphene, using a metal-containing molecule, copper(II) acetate, Cu(OAc)₂, as the carbon source. During high-temperature CVD, the Cu-containing carbon source significantly increased the Cu content in the gas phase, which in turn suppressed the formation of contamination on the graphene surface by ensuring sufficient decomposition of the carbon feedstock. The as-received graphene with a surface cleanness of about 99% showed enhanced optical and electrical properties. This study opens a new avenue for improving graphene quality with respect to surface cleanness and provides new insight into the mechanism of graphene growth through the gas-phase reaction pathway.

Advances in Rosin-Based Chemicals: The Latest Recipes, Applications and Future Trends

A comprehensive review of the publications about rosin-based chemicals has been compiled. Rosin, or colophony, is a natural, abundant, cheap and non-toxic raw material which can be easily modified to obtain numerous useful products, which makes it an excellent subject of innovative research, attracting growing interest in recent years. The last extensive review in this research area was published in 2008, so the current article contains the most promising, repeatable achievements in synthesis of rosin-derived chemicals, published in scientific literature from 2008 to 2018. The first part of the review includes low/medium molecule weight compounds: Especially intermediates, resins, monomers, curing agents, surfactants, medications and biocides. The second part is about macromolecules: mainly elastomers, polymers for biomedical applications, coatings, adhesives, surfactants, sorbents, organosilicons and polysaccharides. In conclusion, a critical evaluation of the publications in terms of data completeness has been carried out with an indication of the most promising directions of rosin-based chemicals development.

Towards super-clean graphene

Impurities produced during the synthesis process of a material pose detrimental impacts upon the intrinsic properties and device performances of the as-obtained product. This effect is especially pronounced in graphene, where surface contamination has long been a critical, unresolved issue, given graphene's two-dimensionality. Here we report the origins of surface contamination of graphene, which is primarily rooted in chemical vapour deposition production at elevated temperatures, rather than during transfer and storage. In turn, we demonstrate a design of Cu substrate architecture towards the scalable production of super-clean graphene (>99% clean regions). The readily available, super-clean graphene sheets contribute to an enhancement in the optical transparency and thermal conductivity, an exceptionally lower-level of electrical contact resistance and intrinsically hydrophilic nature. This work not only opens up frontiers for graphene growth but also provides exciting opportunities for the utilization of as-obtained super-clean graphene films for advanced applications.

MoS2 Liquid Cell Electron Microscopy Through Clean and Fast Polymer-Free MoS2 Transfer

Two dimensional (2D) materials have found various applications because of their unique physical properties. For example, graphene has been used as the electron transparent membrane for liquid cell transmission electron microscopy (TEM) due to its high mechanical strength and flexibility, single-atom thickness, chemical inertness, etc. Here, we report using 2D MoS₂ as a functional substrate as well as the membrane window for liquid cell TEM, which is enabled by our facile and polymer-free MoS₂ transfer process. This provides the opportunity to investigate the growth of Pt nanocrystals on MoS₂ substrates, which elucidates the formation mechanisms of such heterostructured 2D materials. We find that Pt nanocrystals formed in MoS₂ liquid cells have a strong tendency to align their crystal lattice with that of MoS₂, suggesting a van der Waals epitaxial relationship. Importantly, we can study its impact on the kinetics of the nanocrystal formation. The development of MoS₂ liquid cells will allow further study of various liquid phenomena on MoS₂, and the polymer-free MoS₂ transfer process will be implemented in a wide range of applications.

Electronic and Thermal Properties of Graphene and Recent Advances in Graphene Based Electronics Applications

Recently, graphene has been extensively researched in fundamental science and engineering fields and has been developed for various electronic applications in emerging technologies owing to its outstanding material properties, including superior electronic, thermal, optical and mechanical properties. Thus, graphene has enabled substantial progress in the development of the current electronic systems. Here, we introduce the most important electronic and thermal properties of graphene, including its high conductivity, quantum Hall effect, Dirac fermions, high Seebeck coefficient and thermoelectric effects. We also present up-to-date graphene-based applications: optical devices, electronic and thermal sensors, and energy management systems. These applications pave the way for advanced biomedical engineering, reliable human therapy, and environmental protection. In this review, we show that the development of graphene suggests substantial improvements in current electronic technologies and applications in healthcare systems.

Paraffin-enabled graphene transfer

The performance and reliability of large-area graphene grown by chemical vapor deposition are often limited by the presence of wrinkles and the transfer-process-induced polymer residue. Here, we report a transfer approach using paraffin as a support layer, whose thermal properties, low chemical reactivity and non-covalent affinity to graphene enable transfer of wrinkle-reduced and clean large-area graphene. The paraffin-transferred graphene has smooth morphology and high electrical reliability with uniform sheet resistance with ~1% deviation over a centimeter-scale area. Electronic devices fabricated on such smooth graphene exhibit electrical performance approaching that of intrinsic graphene with small Dirac points and high carrier mobility (hole mobility = 14,215 cm² V⁻¹ s⁻¹; electron mobility = 7438 cm² V⁻¹ s⁻¹), without the need of further annealing treatment. The paraffin-enabled transfer process could open realms for the development of high-performance ubiquitous electronics based on large-area two-dimensional materials.

The transfer process of as-grown graphene limits its electrical performance and reliability. Here, the authors develop a transfer approach using paraffin as a support layer and obtain wrinkle-reduced and clean large-area graphene retaining high mobility.

Nano-arrayed OLEDs: enhanced outcoupling efficiency and suppressed efficiency roll-off

Organic light-emitting diodes (OLEDs) with an enhanced outcoupling efficiency and a suppressed efficiency roll-off were fabricated by inserting a nanosize pixel-defining layer (nPDL) that defines the OLED emission region as an array of nanoholes. The outcoupling efficiency of the nano-arrayed OLEDs was increased through the reduced surface plasmon polariton loss caused by the wavy diffraction grating at the metal-organic interface, and their efficiency roll-off was suppressed through the diffusive exciton outside the exciton-formation zone. As a result, the nano-arrayed OLEDs exhibited enhancements of 148.7% in the power efficiency and 137.0% in the external quantum efficiency at 1000 cd m^-2 compared with a reference device. Furthermore, the critical current density (J₀) where the external quantum efficiency decreased to half of its initial value was improved by a factor of 2.5.

Secondary-Transferring Graphene Electrode for Stable FOLED

In this work, sharp wrinkles on graphene films, caused by graphene duplicating the grain boundary cracks of copper foil during the preparation process, were carefully explored. A secondary-transferring graphene film process was proposed to re-transform the "Peak" morphology of graphene surface into "Valley" form. The process we have developed is highly effective and almost nondestructive to the graphene through testing the surface morphology and photo-electric properties before and after the secondary-transferring process. Flexible organic light-emitting device (FOLED) with PEDOT:PSS/SLG/NOA63 framework as a targeted application was fabricated to illustrate the value of our proposed method in fabricating stable devices, the maximum luminance can reach about 35000 cd/m², and the maximum current efficiency was 16.19 cd/A. This method can also be applied to the roll-to-roll preparation of large area high-quality graphene.

Efficient Deep-Blue Electrofluorescence with an External Quantum Efficiency Beyond 10

The design of blue fluorescent materials combining both deep-blue emission (CIE_y<0.06) and high-efficiency climbing over the typically limited exciton production efficiency of 25% is a challenge for organic light-emitting diodes (OLEDs). In this work, we have synthesized two blue luminogens, trans-9,10-bis(2-butoxyphenyl)anthracene (BBPA) and trans-9,10-bis (2,4-dimethoxyphenyl)anthracene with high photoluminescence quantum yields (PLQYs) of 89.5% and 87.0%, respectively. Intriguingly, we have proposed a strategy to avoid aggregation-caused quenching, which can effectively reduce the undesirable excimeric emission by introducing two host matrices with twisted molecular structure, 9,10-di(naphth-2-yl) anthracene and 10,10′-bis-(4-fluorophenyl)-3,3′-dimethyl-9,9′-bianthracene (MBAn-(4)-F), in the BBPA emission layer. The device containing the EML of BBPA-doped MBAn-(4)-F exhibited a high external quantum efficiency of 10.27% for deep-blue emission with the Commission International de L'Eclairage CIE coordinates of (0.15, 0.05) via the steric effect. Importantly, this represents an advance in deep-blue-emitting fluorescent OLED architectures and materials that meet the requirements of high-definition display.

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Highly efficient deep-blue luminogens BBPA and DMPA are synthesized

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Low-efficiency roll-off deep-blue OLEDs with CIE coordinate Y < 0.06

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Record-high external quantum efficiency of 10.27% for deep-blue fluorescent OLEDs

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Host matrix of twisted structure showing steric effect reduces intermolecular aggregation

Blister-based-laser-induced-forward-transfer: a non-contact, dry laser-based transfer method for nanomaterials

We show that blister-based-laser-induced forward-transfer can be used to cleanly desorb and transfer nano- and micro-scale particles between substrates without exposing the particles to the laser radiation or to any chemical treatment that could damage the intrinsic electronic and optical properties of the materials. The technique uses laser pulses to induce the rapid formation of a blister on a thin metal layer deposited on glass via ablation at the metal/glass interface. Femtosecond laser pulses are advantageous for forming beams of molecules or small nanoparticles with well-defined velocity and narrow angular distributions. Both fs and ns laser pulses can be used to cleanly transfer larger nanoparticles including relatively fragile monolayer 2D transition metal dichalcogenide crystals and for direct transfer of nanoparticles from chemical vapour deposition growth substrates, although the mechanisms for inducing blister formation are different.

Bridging the Gap between Reality and Ideal in Chemical Vapor Deposition Growth of Graphene

Graphene, in its ideal form, is a two-dimensional (2D) material consisting of a single layer of carbon atoms arranged in a hexagonal lattice. The richness in morphological, physical, mechanical, and optical properties of ideal graphene has stimulated enormous scientific and industrial interest, since its first exfoliation in 2004. In turn, the production of graphene in a reliable, controllable, and scalable manner has become significantly important to bring us closer to practical applications of graphene. To this end, chemical vapor deposition (CVD) offers tantalizing opportunities for the synthesis of large-area, uniform, and high-quality graphene films. However, quite different from the ideal 2D structure of graphene, in reality, the currently available CVD-grown graphene films are still suffering from intrinsic defective grain boundaries, surface contaminations, and wrinkles, together with low growth rate and the requirement of inevitable transfer. Clearly, a gap still exits between the reality of CVD-derived graphene, especially in industrial production, and ideal graphene with outstanding properties. This Review will emphasize the recent advances and strategies in CVD production of graphene for settling these issues to bridge the giant gap. We begin with brief background information about the synthesis of nanoscale carbon allotropes, followed by the discussion of fundamental growth mechanism and kinetics of CVD growth of graphene. We then discuss the strategies for perfecting the quality of CVD-derived graphene with regard to domain size, cleanness, flatness, growth rate, scalability, and direct growth of graphene on functional substrate. Finally, a perspective on future development in the research relevant to scalable growth of high-quality graphene is presented.

Laser speckle formed disordered micro-meander structures for light extraction enhancement of flexible organic light-emitting diodes

A new approach for efficiently recovering the wasted light energy in conventional flexible organic light-emitting diodes (FOLEDs) is developed by implementing disordered micro-meander structures (DMMs) via laser speckle holography technology. Compared to conventional flat device architecture, the structured FOLEDs with DMMs result in substantial improvement of the device efficiency and superior angular color stability. The resulting current efficiency (CE) and external quantum efficiency (EQE) are 1.31 and 1.39 times that of a common flat structure, respectively. Moreover, the proposed DMMs micro-structure simultaneously offers the unique characteristics of angular color stability with a wide viewing angle, which is usually considered as the criteria of the high-quality lighting applications. We hope that the demonstrated method could provide an alternative way for the development of high efficiency flexible OLEDs.

Asymmetric Two-Terminal Graphene Detector for Broadband Radiofrequency Heterodyne- and Self-Mixing

Graphene, a single atomic layer of covalently bonded carbon atoms, has been investigated intensively for optoelectronics and represents a promising candidate for high-speed electronics. Here, we present a microwave mixer constructed as an asymmetrically contacted two-terminal graphene device based on the thermoelectric effect. We report a 50 GHz (minimum) mixer bandwidth as well as 130 V/W (163 mA/W) extrinsic direct-detection responsivity. Anomalous second-harmonic generation due to self-mixing in our graphene detector is also observed. Careful investigation of the responsivity from four different approaches gives consistent results, confirming the exceptional performance of our zero-bias device operating at room temperature. The 50 GHz bandwidth indicates an extremely fast response time and our experimental results represent an encouraging advance toward practical graphene microwave devices with anticipated future applications extended through millimeter wave and terahertz frequencies.

Charge Carrier Scattering in Polymers: A New Neutral Coupled Soliton Channel

The dynamical scattering of two oppositely charged bipolarons in non-degenerate organic semiconducting lattices is numerically investigated in the framework of a one-dimensional tight-biding-Hubbard model that includes lattice relaxation. Our findings show that it is possible for the bipolaron pair to merge into a state composed of a confined soliton-antisoliton pair, which is characterized by the appearance of states within less than 0.1 eV from the Fermi level. This compound is in a narrow analogy to a meson confining a quark-antiquark pair. Interestingly, solitons are quasi-particles theoretically predicted to arise only in polymer lattices with degenerate ground state: in the general case of non-degenerate ground state polymers, isolated solitons are not allowed.

Two-In-One Method for Graphene Transfer: Simplified Fabrication Process for Organic Light-Emitting Diodes

Graphene as one of the most promising transparent electrode materials has been successfully applied in organic light-emitting diodes (OLEDs). However, traditional poly(methyl methacrylate) (PMMA) transfer method usually results in hardly removed polymeric residues on the graphene surface, which induces unwanted leakage current, poor diode behavior, and even device failure. In this work, we proposed a facile and efficient two-in-one method to obtain clean graphene and fabricate OLEDs, in which the poly(9,9-di-n-octylfluorene-alt-(1,4-phenylene-(4-sec-butylphenyl)imino)-1,4-phenylene) (TFB) layer was inserted between the graphene and PMMA film both as a protector during the graphene transfer and a hole-injection layer in OLEDs. Finally, green OLED devices were successfully fabricated on the PMMA-free graphene/TFB film, and the device luminous efficiency was increased from 64.8 to 74.5 cd/A by using the two-in-one method. Therefore, the proposed two-in-one graphene transfer method realizes a high-efficient graphene transfer and device fabrication process, which is also compatible with the roll-to-roll manufacturing. It is expected that this work can enlighten the design and fabrication of the graphene-based optoelectronic devices.

Flexible Graphene-Based Wearable Gas and Chemical Sensors

Wearable electronics is expected to be one of the most active research areas in the next decade; therefore, nanomaterials possessing high carrier mobility, optical transparency, mechanical robustness and flexibility, lightweight, and environmental stability will be in immense demand. Graphene is one of the nanomaterials that fulfill all these requirements, along with other inherently unique properties and convenience to fabricate into different morphological nanostructures, from atomically thin single layers to nanoribbons. Graphene-based materials have also been investigated in sensor technologies, from chemical sensing to detection of cancer biomarkers. The progress of graphene-based flexible gas and chemical sensors in terms of material preparation, sensor fabrication, and their performance are reviewed here. The article provides a brief introduction to graphene-based materials and their potential applications in flexible and stretchable wearable electronic devices. The role of graphene in fabricating flexible gas sensors for the detection of various hazardous gases, including nitrogen dioxide (NO₂), ammonia (NH₃), hydrogen (H₂), hydrogen sulfide (H₂S), carbon dioxide (CO₂), sulfur dioxide (SO₂), and humidity in wearable technology, is discussed. In addition, applications of graphene-based materials are also summarized in detecting toxic heavy metal ions (Cd, Hg, Pb, Cr, Fe, Ni, Co, Cu, Ag), and volatile organic compounds (VOCs) including nitrobenzene, toluene, acetone, formaldehyde, amines, phenols, bisphenol A (BPA), explosives, chemical warfare agents, and environmental pollutants. The sensitivity, selectivity and strategies for excluding interferents are also discussed for graphene-based gas and chemical sensors. The challenges for developing future generation of flexible and stretchable sensors for wearable technology that would be usable for the Internet of Things (IoT) are also highlighted.

Selenium-Doped Cathodes for Lithium-Organosulfur Batteries with Greatly Improved Volumetric Capacity and Coulombic Efficiency

For the first time a new strategy is reported to improve the volumetric capacity and Coulombic efficiency by selenium doping for lithium-organosulfur batteries. Selenium-doped cathodes with four sulfur atoms and one selenium atom (as the doped heteroatom) in the confined structure are designed and synthesized; this structure exhibits greatly improved volumetric/areal capacities, and a Coulombic efficiency of almost 100% for highly stable lithium-organosulfur batteries. The doping of Se significantly enhances the electronic conductivity of battery electrodes by a factor of 6.2 compared to pure sulfur electrodes, and completely restricts the production of long-chain lithium polysulfides. This allows achievement of a high gravimetric capacity of 700 mAh g^-1 close to its theoretical mass capacity, an exceptional volumetric capacity of 2457 mAh cm^-3 , and excellent capacity retention of 92% after 400 cycles. Shuttle effect is efficiently weakened since no long-chain polysulfides are detected from in situ UV/vis results throughout the entire cycling process arising from selenium doping, which is theoretically confirmed by density functional theory calculations.