Graphene‐Based Electrodes

Graphene, the thinnest two dimensional carbon material, has become the subject of intensive investigation in various research fields because of its remarkable electronic, mechanical, optical and thermal properties. Graphene‐based electrodes, fabricated from mechanically cleaved graphene, chemical vapor deposition (CVD) grown graphene, or massively produced graphene derivatives from bulk graphite, have been applied in a broad range of applications, such as in light emitting diodes, touch screens, field‐effect transistors, solar cells, supercapacitors, batteries, and sensors. In this Review, after a short introduction to the properties and synthetic methods of graphene and its derivatives, we will discuss the importance of graphene‐based electrodes, their fabrication techniques, and application areas.     

Bioinspired Graphene‐Based Nanocomposites and Their Application in Flexible Energy Devices

Graphene is the strongest and stiffest material ever identified and the best electrical conductor known to date, making it an ideal candidate for constructing nanocomposites used in flexible energy devices. However, it remains a great challenge to assemble graphene nanosheets into macro‐sized high‐performance nanocomposites in practical applications of flexible energy devices using traditional approaches. Nacre, the gold standard for biomimicry, provides an excellent example and guideline for assembling two‐dimensional nanosheets into high‐performance nanocomposites. This review summarizes recent research on the bioinspired graphene‐based nanocomposites (BGBNs), and discusses different bioinspired assembly strategies for constructing integrated high‐strength and ‐toughness graphene‐based nanocomposites through various synergistic effects. Fundamental properties of graphene‐based nanocomposites, such as strength, toughness, and electrical conductivities, are highlighted. Applications of the BGBNs in flexible energy devices, as well as potential challenges, are addressed. Inspired from the past work done by the community a roadmap for the future of the BGBNs in flexible energy device applications is depicted.     

Graphene‐Based Fibers: A Review

Motivated by their unique structure and excellent properties, significant progress has been made in recent years in the development of graphene‐based fibers (GBFs). Potential applications of GBFs can be found, for instance, in conducting wires, energy storage and conversion devices, actuators, field emitters, solid‐phase microextraction, springs, and catalysis. In contrast to graphene‐based aerogels (GBAs) and membranes (GBMs), GBFs demonstrate remarkable mechanical and electrical properties and can be bent, knotted, or woven into flexible electronic textiles. In this review, the state‐of‐the‐art of GBFs is summarized, focusing on their synthesis, performance, and applications. Future directions of GBF research are also proposed.     

Progress in miRNA Detection Using Graphene Material–Based Biosensors

MicroRNAs (miRNAs) are short, endogenous, noncoding RNAs that play critical roles in physiologic and pathologic processes and are vital biomarkers for several disease diagnostics and therapeutics. Therefore, rapid, low‐cost, sensitive, and selective detection of miRNAs is of paramount importance and has aroused increasing attention in the field of medical research. Among the various reported miRNA sensors, devices based on graphene and its derivatives, which form functional supramolecular nanoassemblies of π‐conjugated molecules, have been revealed to have great potential due to their extraordinary electrical, chemical, optical, mechanical, and structural properties. This Review critically and comprehensively summarizes the recent progress in miRNA detection based on graphene and its derivative materials, with an emphasis on i) the underlying working principles of these types of sensors, and the unique roles and advantages of graphene materials; ii) state‐of‐the‐art protocols recently developed for high‐performance miRNA sensing, including representative examples; and iii) perspectives and current challenges for graphene sensors. This Review intends to provide readers with a deep understanding of the design and future of miRNA detection devices.     

Electrochemically Exfoliated High‐Quality 2H‐MoS2 for Multiflake Thin Film Flexible Biosensors

2D molybdenum disulfide (MoS₂) gives a new inspiration for the field of nanoelectronics, photovoltaics, and sensorics. However, the most common processing technology, e.g., liquid‐phase based scalable exfoliation used for device fabrication, leads to the number of shortcomings that impede their large area production and integration. Major challenges are associated with the small size and low concentration of MoS₂ flakes, as well as insufficient control over their physical properties, e.g., internal heterogeneity of the metallic and semiconducting phases. Here it is demonstrated that large semiconducting MoS₂ sheets (with dimensions up to 50 µm) can be obtained by a facile cathodic exfoliation approach in nonaqueous electrolyte. The synthetic process avoids surface oxidation thus preserving the MoS₂ sheets with intact crystalline structure. It is further demonstrated at the proof‐of‐concept level, a solution‐processed large area (60 × 60 µm) flexible Ebola biosensor, based on a MoS₂ thin film (6 µm thickness) fabricated via restacking of the multiple flakes on the polyimide substrate. The experimental results reveal a low detection limit (in femtomolar–picomolar range) of the fabricated sensor devices. The presented exfoliation method opens up new opportunities for fabrication of large arrays of multifunctional biomedical devices based on novel 2D materials.     

Prospects of Supercritical Fluids in Realizing Graphene‐Based Functional Materials

Supercritical‐fluids science and technology predate all the approaches that are currently established for graphene production by several decades in advanced materials design. However, it has only recently been proposed as a plausible approach for graphene processing. Since then, supercritical fluids have emerged into contention as an alternative to existing technologies because of their scalability and versatility in processing graphene materials, which include composites, aerogels, and foams. Here, an overview is presented of such materials prepared through supercritical fluids from an advanced materials science standpoint, with a discussion on their fundamental properties and technological applications. The benefits of supercritical‐fluid processing over conventional liquid‐phase processing are presented. The benefits include not only better performances for advanced applications but also environmental issues associated with the synthesis process. Nevertheless, the limitations of supercritical‐fluid processing are also stressed, along with challenges that are still faced toward the achievement of the great expectations from graphene materials.     

Recent Progress in Graphene‐Based Noble‐Metal Nanocomposites for Electrocatalytic Applications

The fast industrialization process has led to global challenges in the energy crisis and environmental pollution, which might be solved with clean and renewable energy. Highly efficient electrochemical systems for clean‐energy collection require high‐performance electrocatalysts, including Au, Pt, Pd, Ru, etc. Graphene, a single‐layer 2D carbon nanosheet, possesses many intriguing properties, and has attracted tremendous research attention. Specifically, graphene and graphene derivatives have been utilized as templates for the synthesis of various noble‐metal nanocomposites, showing excellent performance in electrocatalytic‐energy‐conversion applications, such as the hydrogen evolution reaction and CO₂ reduction. Herein, the recent progress in graphene‐based noble‐metal nanocomposites is summarized, focusing on their synthetic methods and electrocatalytic applications. Furthermore, some personal insights on the challenges and possible future work in this research field are proposed.     

Theoretical Efficiency of Nanostructured Graphene‐Based Photovoltaics

Graphene‐based organic photovoltaics (OPVs) have the potential for single‐cell efficiencies exceeding 12% (and 24% in a stacked structure). A generalized equivalent circuit for OPVs is proposed and the validation of the proposed models is verified by simulation. The simulated short‐circuit photocurrent density (computed using the simulated incident photon flux density and quantum yield), simulated current–voltage curve, and simulated 3D surface and 2D contour plots of solar‐power‐conversion efficiency versus carrier mobility and photoactive layer thickness are in good agreement with experimental observations. The results suggest that graphene renders a credible material for the construction of next‐generation flexible solar‐energy‐conversion devices that are low‐cost, high‐efficiency, thermally stable, environmentally friendly, and lightweight.     

Single‐Molecule Electrical Biosensors Based on Single‐Walled Carbon Nanotubes

Interactions between biological molecules are fundamental to biology. Probing the complex behaviors of biological systems at the molecular level provides new opportunities to uncover the wealth of molecular information that is usually hidden in conventional ensemble experiments and address the “unanswerable” questions in the physical, chemical and biological sciences. Nanometer‐scale materials are particularly well matched with biomolecular interactions due to their biocompatibility, size comparability, and remarkable electrical properties, thus setting the basis for biological sensing with ultrahigh sensitivity. This brief review aims to highlight the recent progress of the burgeoning field of single‐molecule electrical biosensors based on nanomaterials, with a particular focus on single‐walled carbon nanotubes (SWNTs), for better understanding of the molecular structure, interacting dynamics, and molecular functions. The perspectives and key issues that will be critical to the success of next‐generation single‐molecule biosensors toward practical applications are also discussed, such as the device reproducibility, system integration, and theoretical simulation.     

Intracellular Imaging with a Graphene‐Based Fluorescent Probe

Graphene is an increasingly important nanomaterial that has shown great promise in the area of nanotechnology. In this study, fluorescein‐functionalized graphene oxide (GO) is synthesized via a polyethylene glycol (PEG) bridge and its application in intracellular imaging is explored. GO is an oxide form of graphene that provides an ideal platform to prepare graphene‐based functional nanomaterials via chemical modification. The PEG bridge was introduced to prevent GO‐induced quenching of conjugated fluorescein. The fluorescein–PEG–GO conjugate thus prepared exhibits excellent pH‐tunable fluorescent properties and, more significantly, can be efficiently taken up by cells and serve as a fluorescent nanoprobe for intracellular imaging.     

Graphene–Graphene Oxide Floating Gate Transistor Memory

A novel transparent, flexible, graphene channel floating‐gate transistor memory (FGTM) device is fabricated using a graphene oxide (GO) charge trapping layer on a plastic substrate. The GO layer, which bears ammonium groups (NH₃⁺), is prepared at the interface between the crosslinked PVP (cPVP) tunneling dielectric and the Al₂O₃ blocking dielectric layers. Important design rules are proposed for a high‐performance graphene memory device: i) precise doping of the graphene channel, and ii) chemical functionalization of the GO charge trapping layer. How to control memory characteristics by graphene doping is systematically explained, and the optimal conditions for the best performance of the memory devices are found. Note that precise control over the doping of the graphene channel maximizes the conductance difference at a zero gate voltage, which reduces the device power consumption. The proposed optimization via graphene doping can be applied to any graphene channel transistor‐type memory device. Additionally, the positively charged GO (GO–NH₃⁺) interacts electrostatically with hydroxyl groups of both UV‐treated Al₂O₃ and PVP layers, which enhances the interfacial adhesion, and thus the mechanical stability of the device during bending. The resulting graphene–graphene oxide FGTMs exhibit excellent memory characteristics, including a large memory window (11.7 V), fast switching speed (1 μs), cyclic endurance (200 cycles), stable retention (10⁵ s), and good mechanical stability (1000 cycles).     

3D Graphene‐Infused Polyimide with Enhanced Electrothermal Performance for Long‐Term Flexible Space Applications

Polyimides (PIs) have been praised for their high thermal stability, high modulus of elasticity and tensile strength, ease of fabrication, and moldability. They are currently the standard choice for both substrates for flexible electronics and space shielding, as they render high temperature and UV stability and toughness. However, their poor thermal conductivity and completely electrically insulating characteristics have caused other limitations, such as thermal management challenges for flexible high‐power electronics and spacecraft electrostatic charging. In order to target these issues, a hybrid of PI with 3D‐graphene (3D‐C), 3D‐C/PI, is developed here. This composite renders extraordinary enhancements of thermal conductivity (one order of magnitude) and electrical conductivity (10 orders of magnitude). It withstands and keeps a stable performance throughout various bending and thermal cycles, as well as the oxidative and aggressive environment of ground‐based, simulated space environments. This makes this new hybrid film a suitable material for flexible space applications.