Recent Progress in the Transfer of Graphene Films and Nanostructures

Observation of reversible and irreversible charge transfer processes in dye-monolayer graphene systems using Raman spectroscopy as a tool

Herein, we report the Raman spectroscopy of crystal violet (CV) and IR-780 Iodide molecules dispersed on the monolayer graphene film (MGF). In the CV-MGF system, the enhancement in the Raman scattering of CV molecules is observed irrespective of the location probed during the spectral measurements. This enhancement is due to the charge transfer from the MGF to CV molecules. However, in the case of the IR-780 Iodide - MGF system, the enhancement of Raman scattering of dye molecules or MGF is observed strongly depending upon the probed location. These observations indicate that the charge transfer is irreversible and reversible in the CV-MGF and IR-780 Iodide-MGF systems, respectively. Importantly, for the first time, this experimental study revealed that enhancing the Raman scattering of MGF is possible through the "chemical mechanism" with suitable dye molecules apart from the "electromagnetic mechanism" with plasmonic hot spots of the metal nanoparticles and photonic nanojets of single dielectric microparticles.

Real-Time Monitoring of Cell Adhesion onto a Soft Substrate by a Graphene Impedance Biosensor

Soft substrates are interesting for many applications, ranging from mimicking the cellular microenvironment to implants. Conductive electrodes on such substrates allow the realization of flexible, elastic, and transparent sensors. Single-layer graphene as a candidate for such electrodes brings the advantage that the active area of the sensor is transparent and conformal to the underlying substrate. Here, we overcome several challenges facing the routine realization of graphene cell sensors on a canonical soft substrate, namely, poly(dimethylsiloxane) (PDMS). We have systematically studied the effect of surface energy before, during, and after the transfer of graphene. Thus, we have identified a suitable support polymer, optimal substrate (pre)treatment, and an appropriate solvent for the removal of the support. Using this procedure, we can reproducibly obtain stable and intact graphene sensors on a millimeter scale on PDMS, which can withstand continuous measurements in cell culture media for several days. From local nanomechanical measurements, we infer that the softness of the substrate is slightly affected after the graphene transfer. However, we can modulate the stiffness using PDMS with differing compositions. Finally, we show that graphene sensors on PDMS can be successfully used as soft electrodes for real-time monitoring of the cell adhesion kinetics. The routine availability of single-layer graphene electrodes on a soft substrate with tunable stiffness will open a new avenue for studies, where the PDMS-liquid interface is made conducting with minimal alteration of the intrinsic material properties such as softness, flexibility, elasticity, and transparency.

Atomically Substitutional Engineering of Transition Metal Dichalcogenide Layers for Enhancing Tailored Properties and Superior Applications

Transition metal dichalcogenides (TMDs) are a promising class of layered materials in the post-graphene era, with extensive research attention due to their diverse alternative elements and fascinating semiconductor behavior. Binary MX2 layers with different metal and/or chalcogen elements have similar structural parameters but varied optoelectronic properties, providing opportunities for atomically substitutional engineering via partial alteration of metal or/and chalcogenide atoms to produce ternary or quaternary TMDs. The resulting multinary TMD layers still maintain structural integrity and homogeneity while achieving tunable (opto)electronic properties across a full range of composition with arbitrary ratios of introduced metal or chalcogen to original counterparts (0-100%). Atomic substitution in TMD layers offers new adjustable degrees of freedom for tailoring crystal phase, band alignment/structure, carrier density, and surface reactive activity, enabling novel and promising applications. This review comprehensively elaborates on atomically substitutional engineering in TMD layers, including theoretical foundations, synthetic strategies, tailored properties, and superior applications. The emerging type of ternary TMDs, Janus TMDs, is presented specifically to highlight their typical compounds, fabrication methods, and potential applications. Finally, opportunities and challenges for further development of multinary TMDs are envisioned to expedite the evolution of this pivotal field.

Design and Scalable Fabrication of Liquid Metal and Nano-Sheet Graphene Hybrid Phase Change Materials for Thermal Management

Here, a paraffin/liquid metal (LM)/graphene hybrid thermal composite material with a high thermal-conductivity as well as  high latent heat is developed. The paraffin is encapsulated in calcium alginate, which produces leakage-free phase change material (PCM) capsules. LM is filled among the gaps of PCM capsules to enhance overall heat conduction. Graphene nano-sheets coating attains efficient heat dissipation because of its high spectral emissivity (>91%) in the spectrum of the mid-infrared region. The developed material is verified to have strong compatibility and durable stability. The composite is utilized as a thermal buffer (TB) for central processing unit thermal management to demonstrate the synergy of these superior thermal properties. In certain cases, active cooling normally used could be replaced by the developed TB without any energy consumption for thermal management, demonstrating a completely passive cooling strategy. Compared to traditional heat sink active cooling, general energy savings of 10.4-26.3% could thus be achieved by the developed composite in wider operating conditions, proving its potential for more efficient and sustainable data center cooling alongside thermal management of other ground-based electrical/electronic equipment.

A Modified Wet Transfer Method for Eliminating Interfacial Impurities in Graphene

Graphene has immense potential as a material for electronic devices owing to its unique electrical properties. However, large-area graphene produced by chemical vapor deposition (CVD) must be transferred from the as-grown copper substrate to an arbitrary substrate for device fabrication. The conventional wet transfer technique, which uses FeCl₃ as a Cu etchant, leaves microscale impurities from the substrate, and the etchant adheres to graphene, thereby degrading its electrical performance. To address this limitation, this study introduces a modified transfer process that utilizes a temporary UV-treated SiO₂ substrate to adsorb impurities from graphene before transferring it onto the final substrate. Optical microscopy and Raman mapping confirmed the adhesion of impurities to the temporary substrate, leading to a clean graphene/substrate interface. The retransferred graphene shows a reduction in electron-hole asymmetry and sheet resistance compared to conventionally transferred graphene, as confirmed by the transmission line model (TLM) and Hall effect measurements (HEMs). These results indicate that only the substrate effects remain in action in the retransferred graphene, and most of the effects of the impurities are eliminated. Overall, the modified transfer process is a promising method for obtaining high-quality graphene suitable for industrial-scale utilization in electronic devices.

Biocompatible Material-Based Flexible Biosensors: From Materials Design to Wearable/Implantable Devices and Integrated Sensing Systems

Human beings have a greater need to pursue life and manage personal or family health in the context of the rapid growth of artificial intelligence, big data, the Internet of Things, and 5G/6G technologies. The application of micro biosensing devices is crucial in connecting technology and personalized medicine. Here, the progress and current status from biocompatible inorganic materials to organic materials and composites are reviewed and the material-to-device processing is described. Next, the operating principles of pressure, chemical, optical, and temperature sensors are dissected and the application of these flexible biosensors in wearable/implantable devices is discussed. Different biosensing systems acting in vivo and in vitro, including signal communication and energy supply are then illustrated. The potential of in-sensor computing for applications in sensing systems is also discussed. Finally, some essential needs for commercial translation are highlighted and future opportunities for flexible biosensors are considered.

Reducing Carbonaceous Salts for Facile Fabrication of Monolayer Graphene

Novel methods and mechanisms for graphene fabrication are of great importance in the development of materials science. Herein, a facile method to directly convert carbonaceous salts into high-quality freestanding graphene via a simple one-step redox reaction, is reported. The redox couple can be a combination of sodium borohydride (reductant) and sodium carbonate (oxidant), which can readily react with each other when evenly mixed/calcined and yield gram-scale, high-quality, contamination-free, micron-sized, freestanding graphene. More importantly, this method is applicable to a variety of input reductants and oxidants that are low cost and easily accessible. An in-depth investigation reveals that the carbonaceous oxidants can not only provide reduced carbon mass for graphene formation but also act as a self-template to guide the polymerization of carbon atoms following the pattern of the monolayer, six-carbon rings. In addition, the direct formation of graphene exhibits theoretically lower energy barriers than that of other allotropes such as fullerene and carbon nanotube. This facile, low-cost, scalable, and applicable method for mass production of high-quality graphene is expected to revolutionize graphene fabrication technology and boost its practical application to the industry level.

Structural Design and Fabrication of Multifunctional Nanocarbon Materials for Extreme Environmental Applications

Extreme environments represent numerous harsh environmental conditions, such as temperature, pressure, corrosion, and radiation. The tolerance of applications in extreme environments exemplifies significant challenges to both materials and their structures. Given the superior mechanical strength, electrical conductivity, thermal stability, and chemical stability of nanocarbon materials, such as carbon nanotubes (CNTs) and graphene, they are widely investigated as base materials for extreme environmental applications and have shown numerous breakthroughs in the fields of wide-temperature structural-material construction, low-temperature energy storage, underwater sensing, and electronics operated at high temperatures. Here, the critical aspects of structural design and fabrication of nanocarbon materials for extreme environments are reviewed, including a description of the underlying mechanism supporting the performance of nanocarbon materials against extreme environments, the principles of structural design of nanocarbon materials for the optimization of extreme environmental performances, and the fabrication processes developed for the realization of specific extreme environmental applications. Finally, perspectives on how CNTs and graphene can further contribute to the development of extreme environmental applications are presented.

Graphene Synthesis Techniques and Environmental Applications

Graphene is fundamentally a two-dimensional material with extraordinary optical, thermal, mechanical, and electrical characteristics. It has a versatile surface chemistry and large surface area. It is a carbon nanomaterial, which comprises sp² hybridized carbon atoms placed in a hexagonal lattice with one-atom thickness, giving it a two-dimensional structure. A large number of synthesis techniques including epitaxial growth, liquid phase exfoliation, electrochemical exfoliation, mechanical exfoliation, and chemical vapor deposition are used for the synthesis of graphene. Graphene prepared using different techniques can have a number of benefits and deficiencies depending on its application. This study provides a summary of graphene preparation techniques and critically assesses the use of graphene, its derivates, and composites in environmental applications. These applications include the use of graphene as membrane material for the detoxication and purification of water, active material for gas sensing, heavy metal ions detection, and CO₂ conversion. Furthermore, a trend analysis of both synthesis techniques and environmental applications of graphene has been performed by extracting and analyzing Scopus data from the past ten years. Finally, conclusions and outlook are provided to address the residual challenges related to the synthesis of the material and its use for environmental applications.

Ultrasensitive Antibiotic Perceiving Based on Aptamer-Functionalized Ultraclean Graphene Field-Effect Transistor Biosensor

Antibiotics are powerful tools to treat bacterial infections, but antibiotic pollution is becoming a severe threat to the effective treatment of human bacterial infections. The detection of antibiotics in water has been a crucial research area for bioassays in recent years. There is still an urgent need for a simple ultrasensitive detection approach to achieve accurate antibiotic detection at low concentrations. Herein, a field-effect transistor (FET)-based biosensor was developed using ultraclean graphene and an aptamer for ultrasensitive tetracycline detection. Using a newly designed camphor-rosin clean transfer (CRCT) scheme to prepare ultraclean graphene, the carrier mobility of the FET is found to be improved by more than 10 times compared with the FET prepared by the conventional PMMA transfer (CPT) method. Based on the FET, aptamer-functionalized transistor antibiotic biosensors were constructed and characterized. A dynamic detection range of 5 orders of magnitude, a sensitivity of 21.7 mV/decade, and a low detection limit of 100 fM are achieved for the CRCT-FET biosensors with good stability, which are much improved compared with the biosensor prepared by the CPT method. The antibiotic sensing and sensing performance enhancement mechanisms for the CRCT-FET biosensor were studied and analyzed based on experimental results and a biosensing model. Finally, the CRCT-FET biosensor was verified by detecting antibiotics in actual samples obtained from the entrances of Bohai Bay.

Developing Graphene Grids for Cryoelectron Microscopy

Cryogenic electron microscopy (cryo-EM) single particle analysis has become one of the major techniques used to study high-resolution 3D structures of biological macromolecules. Specimens are generally prepared in a thin layer of vitrified ice using a holey carbon grid. However, the sample quality using this type of grid is not always ideal for high-resolution imaging even when the specimens in the test tube behave ideally. Various problems occur during a vitrification procedure, including poor/nonuniform distribution of particles, preferred orientation of particles, specimen denaturation/degradation, high background from thick ice, and beam-induced motion, which have become important bottlenecks in high-resolution structural studies using cryo-EM in many projects. In recent years, grids with support films made of graphene and its derivatives have been developed to efficiently solve these problems. Here, the various advantages of graphene grids over conventional holey carbon film grids, functionalization of graphene support films, production methods of graphene grids, and origins of pristine graphene contamination are reviewed and discussed.

In situ growth of laser-induced graphene micro-patterns on arbitrary substrates

In this article we report a new laser processing method, combining the in situ graphitization of polyimide with simultaneous transfer of the graphene patterns to arbitrary substrates. The synthesis conditions are similar to those normally used for the well-known laser-induced graphene method. The approach is based on the enclosure of polyimide sheets between microscope glass slides. Graphene patterns have been successfully generated on glass and on PDMS, as well as graphene decorated with metals and oxides. In order to illustrate the usefulness of the proposed approach, the method was applied to the fabrication of hybrid supercapacitors, which exhibited very good electrochemical performance.

Transport properties of MoS2/V7(Bz)8 and graphene/V7(Bz)8 vdW junctions tuned by bias and gate voltages

The MoS2/V7(Bz)8 and graphene/V7(Bz)8 vdW junctions are designed and the transport properties of their four-terminal devices are comparatively investigated based on density functional theory (DFT) and the nonequilibrium Green's function (NEGF) technique. The MoS2 and graphene nanoribbons act as the source-to-drain channel and the spin-polarized one-dimensional (1D) benzene-V multidecker complex nanowire (V7(Bz)8) serves as the gate channel. Gate voltages applied on V7(Bz)8 exert different influences of electron transport on MoS2/V7(Bz)8 and graphene/V7(Bz)8. In MoS2/V7(Bz)8, the interplay of source and gate bias potentials could induce promising properties such as negative differential resistance (NDR) behavior, output/input current switching, and spin-polarized currents. In contrast, the gate bias plays an insignificant effect on the transport along graphene in graphene/V7(Bz)8. This dissimilarity is attributed to the fact that the conductivity follows the sequence of MoS2 < V7(Bz)8 < graphene. These transport characteristics are examined by analyzing the conductivity, the currents, the local density of states (LDOS), and the transmission spectra. These results are valuable in designing multi-terminal nanoelectronic devices.