Epitaxial graphene on SiC{0001}: advances and perspectives

Polymer-Assisted Graphite Exfoliation: Advancing Nanostructure Preparation and Multifunctional Composites

Exfoliated graphite (ExG) embedded in a polymeric matrix represents an accessible, cost-effective, and sustainable method for generating nanosized graphite-based polymer composites with multifunctional properties. This review article analyzes diverse methods currently used to exfoliate graphite into graphite nanoplatelets, few-layer graphene, and polymer-assisted graphene. It also explores engineered methods for small-scale pilot production of polymer nanocomposites. It highlights the chemistry involved during the graphite intercalation and exfoliation process, particularly emphasizing the interfacial interactions related to steric repulsion forces, van der Waals forces, hydrogen bonds, π-π stacking, and covalent bonds. These interactions promote the dispersion and stabilization of the graphite derivative structures in polymeric matrices. Finally, it compares the enhanced properties of nanocomposites, such as increased thermal and electrical conductivity and electromagnetic interference (EMI) shielding applications, with those of neat polymer materials.

Position-resolved charge collection of silicon carbide detectors with an epitaxially-grown graphene layer

Silicon carbide (SiC) has outstanding physical properties therefore, diodes based on SiC are being considered for many radiation detection applications such as particle accelerator experiments and medical dosimetry. Moreover, by reducing the metal on the surface of the diode there is the potential to enhance its performance in some fields where the presence of metal is detrimental. To this end, SiC detectors with an epitaxially-grown graphene layer (EG), that substitutes the metallic contact, in the sensitive region were produced at IMB-CNM, profiting from the conductivity of the mono-atomic layer material. To isolate the effect of the graphene on the charge collection, samples without graphene were produced in parallel. In this paper, the effect of EG on Silicon Carbide p-in-n radiation detectors is studied in terms of charge collection with a radioactive source and by means of the transient current technique (TCT), which allows for position-dependent signal formation analysis. As a result of the former, we show the capability of the EG-SiC sensor for charge collection after signal integration, to a resolution close to that of a sensor fully metallised. Moreover, from the TCT studies, we observe uniform charge collection across the active region, as well as an up-to ∼ 40% transient amplitude damping which, compared with the ∼ 90% on the sample containing no metallic contact, proves that the presence of graphene benefits the performance of the device and that the technology is viable for radiation detection as an alternative to metal.

First use of silicon carbide detectors with graphene-enhanced contacts for medical dosimetry

Silicon Carbide (SiC) is a radiation hard wide bandgap semiconductor, which makes it an interesting alternative for radiation detector fabrication, with potential applications such as High Energy Physics, synchrotron and radiotherapy instrumentation. In addition, by reducing the amount of metal over the active area of said detectors (typically used for electrical connectivity with the implant of the pn-junction) unwanted effects from secondary interactions which can affect the accuracy of the measurement can be diminished, essential to meet the medical standards of precision. In this article, the use of epitaxially-grown graphene is explored as an alternative to metallic contacts with these prototypes. To this end, the first prototypes of SiC diodes with epitaxial graphene contacts were produced at IMB-CNM for radiation detection,along with reference devices. In order to characterise the feasibility of the technology in the medical application, the dose rate linearity of the SiC device with graphene was measured in a radiotherapy Linac in the dose rate range of 1-6 Gy/min. The response of the device was compared to that observed on devices with similar geometries reported elsewhere. To fully characterise the devices, the same exercise was repeated in a laboratory X-ray tube. Under the later set-up, the prototype is compared against a device with a fully metallised active region.

Implications of graphene-based materials in dentistry: present and future

Since the advent of nanoscience, nanobiomaterials have been applied in the dental industry. Graphene and its derivatives have attracted the most interest of all of them due to their exceptional look, biocompatibility, multiplication differential, and antibacterial capabilities. We outlined the most recent developments about their applications to dentistry in our review. There is discussion of the synthesis processes, architectures, and characteristics of materials based on graphene. The implications of graphene and its counterparts are then meticulously gathered and described. Finally, in an effort to inspire more excellent research, this paper explores the obstacles and potential of graphene-based nanomaterials for dental aspects.

Remote epitaxial interaction through graphene

The concept of remote epitaxy involves a two-dimensional van der Waals layer covering the substrate surface, which still enable adatoms to follow the atomic motif of the underlying substrate. The mode of growth must be carefully defined as defects, e.g., pinholes, in two-dimensional materials can allow direct epitaxy from the substrate, which, in combination with lateral epitaxial overgrowth, could also form an epilayer. Here, we show several unique cases that can only be observed for remote epitaxy, distinguishable from other two-dimensional material-based epitaxy mechanisms. We first grow BaTiO3 on patterned graphene to establish a condition for minimizing epitaxial lateral overgrowth. By observing entire nanometer-scale nuclei grown aligned to the substrate on pinhole-free graphene confirmed by high-resolution scanning transmission electron microscopy, we visually confirm that remote epitaxy is operative at the atomic scale. Macroscopically, we also show variations in the density of GaN microcrystal arrays that depend on the ionicity of substrates and the number of graphene layers.

Modification on Flower Defects and Electronic Properties of Epitaxial Graphene by Erbium

Manipulating the topological defects and electronic properties of graphene has been a subject of great interest. In this work, we have investigated the influence of Er predeposition on flower defects and electronic band structures of epitaxial graphene on SiC. It is shown that Er atoms grown on the SiC substrate actually work as an activator to induce flower defect formation with a density of 1.52 × 1012 cm-2 during the graphitization process when the Er coverage is 1.6 ML, about 5 times as much as that of pristine graphene. First-principles calculations demonstrate that Er greatly decreases the formation energy of the flower defect. We have discussed Er promoting effects on flower defect formation as well as its formation mechanism. Scanning tunneling microscopy (STM) and Raman and X-ray photoelectron spectroscopy (XPS) have been utilized to reveal the Er doping effect and its modification to electronic structures of graphene. N-doping enhancement and band gap opening can be observed by using angle-resolved photoemission spectroscopy (ARPES). With Er coverage increasing from 0 to 1.6 ML, the Dirac point energy decreases from -0.34 to -0.37 eV and the band gap gradually increases from 320 to 360 meV. The opening of the band gap is attributed to the synergistic effect of substitution doping of Er atoms and high-density flower defects.

Recent Advances in Moiré Superlattice Systems by Angle-Resolved Photoemission Spectroscopy

The last decade has witnessed a flourish in 2D materials including graphene and transition metal dichalcogenides (TMDs) as atomic-scale Legos. Artificial moiré superlattices via stacking 2D materials with a twist angle and/or a lattice mismatch have recently become a fertile playground exhibiting a plethora of emergent properties beyond their building blocks. These rich quantum phenomena stem from their nontrivial electronic structures that are effectively tuned by the moiré periodicity. Modern angle-resolved photoemission spectroscopy (ARPES) can directly visualize electronic structures with decent momentum, energy, and spatial resolution, thus can provide enlightening insights into fundamental physics in moiré superlattice systems and guides for designing novel devices. In this review, first, a brief introduction is given on advanced ARPES techniques and basic ideas of band structures in a moiré superlattice system. Then ARPES research results of various moiré superlattice systems are highlighted, including graphene on substrates with small lattice mismatches, twisted graphene/TMD moiré systems, and high-order moiré superlattice systems. Finally, it discusses important questions that remain open, challenges in current experimental investigations, and presents an outlook on this field of research.

New Approaches to Produce Large-Area Single Crystal Thin Films

Wafer-scale growth of single crystal thin films of metals, semiconductors, and insulators is crucial for manufacturing high-performance electronic and optical devices, but still challenging from both scientific and industrial perspectives. Recently, unconventional advanced synthetic approaches have been attempted and have made remarkable progress in diversifying the species of producible single crystal thin films. This review introduces several new synthetic approaches to produce large-area single crystal thin films of various materials according to the concepts and principles.

Graphene and Its Derivatives: Synthesis and Application in the Electrochemical Detection of Analytes in Sweat

Wearable sensors and invasive devices have been studied extensively in recent years as the demand for real-time human healthcare applications and seamless human-machine interaction has risen exponentially. An explosion in sensor research throughout the globe has been ignited by the unique features such as thermal, electrical, and mechanical properties of graphene. This includes wearable sensors and implants, which can detect a wide range of data, including body temperature, pulse oxygenation, blood pressure, glucose, and the other analytes present in sweat. Graphene-based sensors for real-time human health monitoring are also being developed. This review is a comprehensive discussion about the properties of graphene, routes to its synthesis, derivatives of graphene, etc. Moreover, the basic features of a biosensor along with the chemistry of sweat are also discussed in detail. The review mainly focusses on the graphene and its derivative-based wearable sensors for the detection of analytes in sweat. Graphene-based sensors for health monitoring will be examined and explained in this study as an overview of the most current innovations in sensor designs, sensing processes, technological advancements, sensor system components, and potential hurdles. The future holds great opportunities for the development of efficient and advanced graphene-based sensors for the detection of analytes in sweat.

Growth of 2D Materials at the Wafer Scale

Wafer-scale growth has become a critical bottleneck for scaling up applications of van der Waal (vdW) layered 2D materials in high-end electronics and optoelectronics. Most vdW 2D materials are initially obtained through top-down synthesis methods, such as exfoliation, which can only prepare small flakes on a micrometer scale. Bottom-up growth can enable 2D flake growth over a large area. However, seamless merging of these flakes to form large-area continuous films with well-controlled layer thickness and lattice orientation is still a significant challenge. This review briefly introduces several vdW layered 2D materials covering their lattice structures, representative physical properties, and potential roles in large-scale applications. Then, several methods used to grow vdW layered 2D materials at the wafer scale are reviewed in depth. In particular, three strategies are summarized that enable 2D film growth with a single-crystalline structure over the whole wafer: growth of an isolated domain, growth of unidirectional domains, and conversion of oriented precursors. After that, the progress in using wafer-scale 2D materials in integrated devices and advanced epitaxy is reviewed. Finally, future directions in the growth and scaling of vdW layered 2D materials are discussed.

Graphene-Based Nanomaterials for Dental Applications: Principles, Current Advances, and Future Outlook

With the development of nanotechnology, nanomaterials have been used in dental fields over the past years. Among them, graphene and its derivatives have attracted great attentions, owing to their excellent physicochemical property, morphology, biocompatibility, multi-differentiation activity, and antimicrobial activity. In our review, we summarized the recent progress about their applications on the dentistry. The synthesis methods, structures, and properties of graphene-based materials are discussed. Then, the dental applications of graphene-based materials are emphatically collected and described. Finally, the challenges and outlooks of graphene-based nanomaterials on the dental applications are discussed in this paper, aiming at inspiring more excellent studies.

Graphene as a Piezoresistive Material in Strain Sensing Applications

High accuracy measurement of mechanical strain is critical and broadly practiced in several application areas including structural health monitoring, industrial process control, manufacturing, avionics and the automotive industry, to name a few. Strain sensors, otherwise known as strain gauges, are fueled by various nanomaterials, among which graphene has attracted great interest in recent years, due to its unique electro-mechanical characteristics. Graphene shows not only exceptional physical properties but also has remarkable mechanical properties, such as piezoresistivity, which makes it a perfect candidate for strain sensing applications. In the present review, we provide an in-depth overview of the latest studies focusing on graphene and its strain sensing mechanism along with various applications. We start by providing a description of the fundamental properties, synthesis techniques and characterization methods of graphene, and then build forward to the discussion of numerous types of graphene-based strain sensors with side-by-side tabular comparison in terms of figures-of-merit, including strain range and sensitivity, otherwise referred to as the gauge factor. We demonstrate the material synthesis, device fabrication and integration challenges for researchers to achieve both wide strain range and high sensitivity in graphene-based strain sensors. Last of all, several applications of graphene-based strain sensors for different purposes are described. All in all, the evolutionary process of graphene-based strain sensors in recent years, as well as the upcoming challenges and future directions for emerging studies are highlighted.

High-Quality Few-Layer Graphene on Single-Crystalline SiC thin Film Grown on Affordable Wafer for Device Applications

Graphene is promising for next-generation devices. However, one of the primary challenges in realizing these devices is the scalable growth of high-quality few-layer graphene (FLG) on device-type wafers; it is difficult to do so while balancing both quality and affordability. High-quality graphene is grown on expensive SiC bulk crystals, while graphene on SiC thin films grown on Si substrates (GOS) exhibits low quality but affordable cost. We propose a new method for the growth of high-quality FLG on a new template named “hybrid SiC”. The hybrid SiC is produced by bonding a SiC bulk crystal with an affordable device-type wafer and subsequently peeling off the SiC bulk crystal to obtain a single-crystalline SiC thin film on the wafer. The quality of FLG on this hybrid SiC is comparable to that of FLG on SiC bulk crystals and much higher than of GOS. FLG on the hybrid SiC exhibited high carrier mobilities, comparable to those on SiC bulk crystals, as anticipated from the linear band dispersions. Transistors using FLG on the hybrid SiC showed the potential to operate in terahertz frequencies. The proposed method is suited for growing high-quality FLG on desired substrates with the aim of realizing graphene-based high-speed devices.

Long-lived charge separation following pump-wavelength-dependent ultrafast charge transfer in graphene/WS2 heterostructures

Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS₂ heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS₂ following photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WS₂ interfaces, we find that it occurs via photo-thermionic emission for sub-A-exciton excitations and direct hole transfer from WS₂ to the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the performance of optoelectronic devices, in particular photodetection.

Temperature-dependent phonon dynamics and anharmonicity of suspended and supported few-layer gallium sulfide

Phonons play a fundamental role in the electronic and thermal transport of 2D materials which is crucial for device applications. In this work, we investigate the temperature-dependence of A[Formula: see text] and A[Formula: see text] Raman modes of suspended and supported mechanically exfoliated few-layer gallium sulfide (GaS), accessing their relevant thermodynamic Grüneisen parameters and anharmonicity. The Raman frequencies of these two phonons soften with increasing temperature with different [Formula: see text] temperature coefficients. The first-order temperature coefficients θ of A[Formula: see text] mode is ∼ -0.016 cm^-1/K, independent of the number of layers and the support. In contrast, the θ of A[Formula: see text] mode is smaller for two-layer GaS and constant for thicker samples (∼ -0.006 2 cm^-1 K^-1). Furthermore, for two-layer GaS, the θ value is ∼ -0.004 4 cm^-1 K^-1 for the supported sample, while it is even smaller for the suspended one (∼ -0.002 9 cm^-1 K^-1). The higher θ value for supported and thicker samples was attributed to the increase in phonon anharmonicity induced by the substrate surface roughness and Umklapp phonon scattering. Our results shed new light on the influence of the substrate and number of layers on the thermal properties of few-layer GaS, which are fundamental for developing atomically-thin GaS electronic devices.

From 2D Graphene Nanosheets to 3D Graphene-based Macrostructures

The combination of exceptional functionalities offered by 3D graphene-based macrostructures (GBMs) has attracted tremendous interest. 2D graphene nanosheets have a high chemical stability, high surface area and customizable porosity, which was extensively researched for a variety of applications including CO₂ adsorption, water treatment, batteries, sensors, catalysis, etc. Recently, 3D GBMs have been successfully achieved through few approaches, including direct and non-direct self-assembly methods. In this review, the possible routes used to prepare both 2D graphene and interconnected 3D GBMs are described and analyzed regarding the involved chemistry of each 2D/3D graphene system. Improvement of the accessible surface of 3D GBMs where the interface exchanges are occurring is of great importance. A better control of the chemical mechanisms involved in the self-assembly mechanism itself at the nanometer scale is certainly the key for a future research breakthrough regarding 3D GBMs.

Enhancing Thermal Interface Conductance to Graphene Using Ni-Pd Alloy Contacts

To identify superior thermal contacts to graphene, we implement a high-throughput methodology that systematically explores the Ni-Pd alloy composition spectrum and the effect of Cr adhesion layer thickness on thermal interface conductance with monolayer graphene. Frequency domain thermoreflectance measurements of two independently prepared Ni-Pd/Cr/graphene/SiO₂ samples identify a maximum metal/graphene/SiO₂ junction thermal interface conductance of 114 ± (39, 25) MW/m² K and 113 ± (33, 22) MW/m² K at ∼10 at. % Pd in Ni-nearly double the highest reported value for pure metals and 3 times that of pure Ni or Pd. The presence of Cr, at any thickness, suppresses this maximum. Although the origin of the peak is unresolved, we find that it correlates with a region of the Ni-Pd phase diagram that exhibits a miscibility gap. Cross-sectional imaging by high-resolution transmission electron microscopy identifies striations in the alloy at this particular composition, consistent with separation into multiple phases. Through this work, we draw attention to alloys in the search for better contacts to two-dimensional materials for next-generation devices.

Graphene-Based Thermoacoustic Sound Source

Thermoacoustic (TA) effect has been discovered for more than 130 years. However, limited by the material characteristics, the performance of a TA sound source could not be compared with magnetoelectric and piezoelectric loudspeakers. Recently, graphene, a two-dimensional material with the lowest heat capacity per unit area, was discovered to have a good TA performance. Compared with a traditional sound source, graphene TA sound sources (GTASSs) have many advantages, such as small volume, no diaphragm vibration, wide frequency range, high transparency, good flexibility, and high sound pressure level (SPL). Therefore, graphene has a great potential as a next-generation sound source. Photoacoustic (PA) imaging can also be applied to the diagnosis and treatment of diseases using the photothermo-acoustic (PTA) effect. Therefore, in this review, we will introduce the history of TA devices. Then, the theory and simulation model of TA will be analyzed in detail. After that, we will talk about the graphene synthesis method. To improve the performance of GTASSs, many strategies such as lowering the thickness and using porous or suspended structures will be introduced. With a good PTA effect and large specific area, graphene PA imaging and drug delivery is a promising prospect in cancer treatment. Finally, the challenges and prospects of GTASSs will be discussed.

Complete aggregation pathway of amyloid β (1-40) and (1-42) resolved on an atomically clean interface

To visualize amyloid β (Aβ) aggregates requires an uncontaminated and artifact-free interface. This paper demonstrates the interface between graphene and pure water (verified to be atomically clean using tunneling microscopy) as an ideal platform for resolving size, shape, and morphology (measured by atomic force microscopy) of Aβ-40 and Aβ-42 peptide assemblies from 0.5 to 150 hours at a 5-hour time interval with single-particle resolution. After confirming faster aggregation of Aβ-42 in comparison to Aβ-40, a stable set of oligomers with a diameter distribution of ~7 to 9 nm was prevalently observed uniquely for Aβ-42 even after fibril appearance. The interaction energies between a distinct class of amyloid aggregates (dodecamers) and graphene was then quantified using molecular dynamics simulations. Last, differences in Aβ-40 and Aβ-42 networks were resolved, wherein only Aβ-42 fibrils were aligned through lateral interactions over micrometer-scale lengths, a property that could be exploited in the design of biofunctional materials.

Conductive Polyamide-Graphene Composite Fabric via Interface Engineering

Conductive fabrics have received significant attention because of their widespread applications from smart textiles to energy storage devices. Conductive colloidal materials are preferred as a coating on the fabric to achieve desirable electronic conductivity; however, obtaining a uniform coverage with a simple and effective route is a challenge. Herein, we report exfoliated graphene nanoplatelets (GNPs) in low boiling point solvents and their subsequent coating onto a polyamide fabric surface. Few-layered (average <7 layers) GNPs were obtained by optimizing solubility parameters of solvent mixtures and sonication time. Raman spectroscopy showed that the I_D/ I_G ratio changed from 0.33 to 0.38 in the GNP solution before and after the sonication, confirming an insignificant increase in defects on the basal plane of graphene after sonication treatment. Uniform coating of GNPs was obtained by optimizing concentration and sonication times. Scanning electron microscopy showed a uniform coverage of GNPs, and the surface resistivity of the polyamide fabric decreased from infinity to ∼40 kΩ after 4 h of coating. X-ray diffraction analysis confirmed the minimal effect on the fabric crystallinity during processing. This interface engineering approach is simple and scalable, and it is applicable for the coating of different polymeric fabrics with a great promise in electronic textiles.

Graphene Nanomaterials: Synthesis, Biocompatibility, and Cytotoxicity

Graphene, graphene oxide, and reduced graphene oxide have been widely considered as promising candidates for industrial and biomedical applications due to their exceptionally high mechanical stiffness and strength, excellent electrical conductivity, high optical transparency, and good biocompatibility. In this article, we reviewed several techniques that are available for the synthesis of graphene-based nanomaterials, and discussed the biocompatibility and toxicity of such nanomaterials upon exposure to mammalian cells under in vitro and in vivo conditions. Various synthesis strategies have been developed for their fabrication, generating graphene nanomaterials with different chemical and physical properties. As such, their interactions with cells and organs are altered accordingly. Conflicting results relating biocompatibility and cytotoxicity induced by graphene nanomaterials have been reported in the literature. In particular, graphene nanomaterials that are used for in vitro cell culture and in vivo animal models may contain toxic chemical residuals, thereby interfering graphene-cell interactions and complicating interpretation of experimental results. Synthesized techniques, such as liquid phase exfoliation and wet chemical oxidation, often required toxic organic solvents, surfactants, strong acids, and oxidants for exfoliating graphite flakes. Those organic molecules and inorganic impurities that are retained in final graphene products can interact with biological cells and tissues, inducing toxicity or causing cell death eventually. The residual contaminants can cause a higher risk of graphene-induced toxicity in biological cells. This adverse effect may be partly responsible for the discrepancies between various studies in the literature.

Intrinsic structural and electronic properties of the Buffer Layer on Silicon Carbide unraveled by Density Functional Theory

The buffer carbon layer obtained in the first instance by evaporation of Si from the Si-rich surfaces of silicon carbide (SiC) is often studied only as the intermediate to the synthesis of SiC supported graphene. In this work, we explore its intrinsic potentialities, addressing its structural and electronic properties by means of Density Functional Theory. While the system of corrugation crests organized in a honeycomb super-lattice of nano-metric side returned by calculations is compatible with atomic microscopy observations, our work reveals some possible alternative symmetries, which might coexist in the same sample. The electronic structure analysis reveals the presence of an electronic gap of ~0.7 eV. In-gap states are present, localized over the crests, while near-gap states reveal very different structure and space localization, being either bonding states or outward pointing p orbitals and unsaturated Si dangling bonds. On one hand, the presence of these interface states was correlated with the n-doping of the monolayer graphene subsequently grown on the buffer. On the other hand, the correlation between their chemical character and their space localization is likely to produce a differential reactivity towards specific functional groups with a spatial regular modulation at the nano-scale, opening perspectives for a finely controlled chemical functionalization.

Tip-enhanced Raman spectroscopy - from early developments to recent advances

An analytical technique operating at the nanoscale must be flexible regarding variable experimental conditions while ideally also being highly specific, extremely sensitive, and spatially confined. In this respect, tip-enhanced Raman scattering (TERS) has been demonstrated to be ideally suited to, e.g., elucidating chemical reaction mechanisms, determining the distribution of components and identifying and localizing specific molecular structures at the nanometre scale. TERS combines the specificity of Raman spectroscopy with the high spatial resolution of scanning probe microscopies by utilizing plasmonic nanostructures to confine the incident electromagnetic field and increase it by many orders of magnitude. Consequently, molecular structure information in the optical near field that is inaccessible to other optical microscopy methods can be obtained. In this general review, the development of this still-young technique, from early experiments to recent achievements concerning inorganic, organic, and biological materials, is addressed. Accordingly, the technical developments necessary for stable and reliable AFM- and STM-based TERS experiments, together with the specific properties of the instruments under different conditions, are reviewed. The review also highlights selected experiments illustrating the capabilities of this emerging technique, the number of users of which has steadily increased since its inception in 2000. Finally, an assessment of the frontiers and new concepts of TERS, which aim towards rendering it a general and widely applicable technique that combines the highest possible lateral resolution and extreme sensitivity, is provided.

Strain and Bond Length Dynamics upon Growth and Transfer of Graphene by NEXAFS Spectroscopy from First-Principles and Experiment

As the quest toward novel materials proceeds, improved characterization technologies are needed. In particular, the atomic thickness in graphene and other 2D materials renders some conventional technologies obsolete. Characterization technologies at wafer level are needed with enough sensitivity to detect strain in order to inform fabrication. In this work, NEXAFS spectroscopy was combined with simulations to predict lattice parameters of graphene grown on copper and further transferred to a variety of substrates. The strains associated with the predicted lattice parameters are in agreement with experimental findings. The approach presented here holds promise to effectively measure strain in graphene and other 2D systems at wafer levels to inform manufacturing environments.

High-quality PVD graphene growth by fullerene decomposition on Cu foils

We present a new protocol to grow large-area, high-quality single-layer graphene on Cu foils at relatively low temperatures. We use C₆₀ molecules evaporated in ultra high vacuum conditions as carbon source. This clean environment results in a strong reduction of oxygen-containing groups as depicted by X-ray photoelectron spectroscopy (XPS). Unzipping of C₆₀ is thermally promoted by annealing the substrate at 800ºC during evaporation. The graphene layer extends over areas larger than the Cu crystallite size, although it is changing its orientation with respect to the surface in the wrinkles and grain boundaries, producing a modulated ring in the low energy electron diffraction (LEED) pattern. This protocol is a self-limiting process leading exclusively to one single graphene layer. Raman spectroscopy confirms the high quality of the grown graphene. This layer exhibits an unperturbed Dirac-cone with a clear n-doping of 0.77 eV, which is caused by the interaction between graphene and substrate. Density functional theory (DFT) calculations show that this interaction can be induced by a coupling between graphene and substrate at specific points of the structure leading to a local sp³ configuration, which also contribute to the D-band in the Raman spectra.

Growth of low doped monolayer graphene on SiC(0001) via sublimation at low argon pressure

Silicon carbide (SiC) sublimation is the most promising option to achieve transfer-free graphene at the wafer-scale. We investigated the initial growth stages from the buffer layer to monolayer graphene on SiC(0001) as a function of annealing temperature at low argon pressure (10 mbar). A buffer layer, fully covering the SiC substrate, forms when the substrate is annealed at 1600 °C. Graphene formation starts from the step edges of the SiC substrate at higher temperature (1700 °C). The spatial homogeneity of the monolayer graphene was observed at 1750 °C, as characterized by Raman spectroscopy and magneto-transport. Raman spectroscopy mapping indicated an A_G-graphene/A_G-HOPG ratio of around 3.3%, which is very close to the experimental value reported for a graphene monolayer. Transport measurements from room temperature down to 1.7 K indicated slightly p-doped samples (p ≃ 10¹⁰ cm^-2) and confirmed both continuity and thickness of the monolayer graphene film. Successive growth processes have confirmed the reproducibility and homogeneity of these monolayer films.

Recent advances in bioactive 1D and 2D carbon nanomaterials for biomedical applications

One-dimensional (1D) carbon nanotubes (CNTs) and the two-dimensional (2D) graphene represent the most widely studied allotropes of carbon. Due to their unique structural, electrical, mechanical and optical properties, 1D and 2D carbon nanostructures are considered to be leading candidates for numerous applications in biomedical fields, including tissue engineering, drug delivery, bioimaging and biosensors. The biocompatibility and toxicity issues associated with these nanostructures have been a critical impediment for their use in biomedical applications. In this review, we present an overview of the various materials types, properties, functionalization strategies and characterization methods of 1D and 2D carbon nanomaterials and their derivatives in terms of their biomedical applications. In addition, we discuss various factors and mechanisms affecting their toxicity and biocompatibility.

Molecular assembly on two-dimensional materials

Molecular self-assembly is a well-known technique to create highly functional nanostructures on surfaces. Self-assembly on two-dimensional (2D) materials is a developing field driven by the interest in functionalization of 2D materials in order to tune their electronic properties. This has resulted in the discovery of several rich and interesting phenomena. Here, we review this progress with an emphasis on the electronic properties of the adsorbates and the substrate in well-defined systems, as unveiled by scanning tunneling microscopy. The review covers three aspects of the self-assembly. The first one focuses on non-covalent self-assembly dealing with site-selectivity due to inherent moiré pattern present on 2D materials grown on substrates. We also see that modification of intermolecular interactions and molecule-substrate interactions influences the assembly drastically and that 2D materials can also be used as a platform to carry out covalent and metal-coordinated assembly. The second part deals with the electronic properties of molecules adsorbed on 2D materials. By virtue of being inert and possessing low density of states near the Fermi level, 2D materials decouple molecules electronically from the underlying metal substrate and allow high-resolution spectroscopy and imaging of molecular orbitals. The moiré pattern on the 2D materials causes site-selective gating and charging of molecules in some cases. The last section covers the effects of self-assembled, acceptor and donor type, organic molecules on the electronic properties of graphene as revealed by spectroscopy and electrical transport measurements. Non-covalent functionalization of 2D materials has already been applied for their application as catalysts and sensors. With the current surge of activity on building van der Waals heterostructures from atomically thin crystals, molecular self-assembly has the potential to add an extra level of flexibility and functionality for applications ranging from flexible electronics and OLEDs to novel electronic devices and spintronics.

Toward fast growth of large area high quality graphene using a cold-wall CVD reactor

In this work we provide a detailed analysis on graphene synthesis by Chemical Vapor Deposition (CVD) using a cold wall CVD reactor to achieve fast production of large area high quality graphene.

Synthesis of Freestanding Graphene on SiC by a Rapid-Cooling Technique

Graphene has a negative thermal expansion coefficient; that is, when heated, the graphene lattice shrinks. On the other hand, the substrates typically used for graphene growth, such as silicon carbide, have a positive thermal expansion coefficient. Hence, on cooling graphene on SiC, graphene expands but SiC shrinks. This mismatch will physically break the atomic bonds between graphene and SiC. We have demonstrated that a graphenelike buffer layer on SiC can be converted to a quasifreestanding monolayer graphene by a rapid-cooling treatment. The decoupling of graphene from the SiC substrate was actually effective for reducing the electric carrier scattering due to interfacial phonons. In addition, the rapidly cooled graphene obtained in this way was of high-quality, strain-free, thermally stable, and strongly hole doped. This simple, classical, but quite novel technique for obtaining quasifreestanding graphene could open a new path towards a viable graphene-based semiconductor industry.

Van der Waals stacks of few-layer h-AlN with graphene: an ab initio study of structural, interaction and electronic properties

Graphite-like hexagonal AlN (h-AlN) multilayers have been experimentally manifested and theoretically modeled. The development of any functional electronics applications of h-AlN would most certainly require its integration with other layered materials, particularly graphene. Here, by employing vdW-corrected density functional theory calculations, we investigate structure, interaction energy, and electronic properties of van der Waals stacking sequences of few-layer h-AlN with graphene. We find that the presence of a template such as graphene induces enough interlayer charge separation in h-AlN, favoring a graphite-like stacking formation. We also find that the interface dipole, calculated per unit cell of the stacks, tends to increase with the number of stacked layers of h-AlN and graphene.

Amphiphilic Fluorinated Block Copolymer Synthesized by RAFT Polymerization for Graphene Dispersions

Despite the superior properties of graphene, the strong π–π interactions among pristine graphenes yielding massive aggregation impede industrial applications. For non-covalent functionalization of highly-ordered pyrolytic graphite (HOPG), poly(2,2,2-trifluoroethyl methacrylate)-block-poly(4-vinyl pyridine) (PTFEMA-b-PVP) block copolymers were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization and used as polymeric dispersants in liquid phase exfoliation assisted by ultrasonication. The HOPG graphene concentrations were found to be 0.260–0.385 mg/mL in methanolic graphene dispersions stabilized with 10 wt % (relative to HOPG) PTFEMA-b-PVP block copolymers after one week. Raman and atomic force microscopy (AFM) analyses revealed that HOPG could not be completely exfoliated during the sonication. However, on-line turbidity results confirmed that the dispersion stability of HOPG in the presence of the block copolymer lasted for one week and that longer PTFEMA and PVP blocks led to better graphene dispersibility. Force–distance (F–d) analyses of AFM showed that PVP block is a good graphene-philic block while PTFEMA is methanol-philic.

Synthesis and applications of carbon nanomaterials for energy generation and storage

The world is facing an energy crisis due to exponential population growth and limited availability of fossil fuels. Over the last 20 years, carbon, one of the most abundant materials found on earth, and its allotrope forms such as fullerenes, carbon nanotubes and graphene have been proposed as sources of energy generation and storage because of their extraordinary properties and ease of production. Various approaches for the synthesis and incorporation of carbon nanomaterials in organic photovoltaics and supercapacitors have been reviewed and discussed in this work, highlighting their benefits as compared to other materials commonly used in these devices. The use of fullerenes, carbon nanotubes and graphene in organic photovoltaics and supercapacitors is described in detail, explaining how their remarkable properties can enhance the efficiency of solar cells and energy storage in supercapacitors. Fullerenes, carbon nanotubes and graphene have all been included in solar cells with interesting results, although a number of problems are still to be overcome in order to achieve high efficiency and stability. However, the flexibility and the low cost of these materials provide the opportunity for many applications such as wearable and disposable electronics or mobile charging. The application of carbon nanotubes and graphene to supercapacitors is also discussed and reviewed in this work. Carbon nanotubes, in combination with graphene, can create a more porous film with extraordinary capacitive performance, paving the way to many practical applications from mobile phones to electric cars. In conclusion, we show that carbon nanomaterials, developed by inexpensive synthesis and process methods such as printing and roll-to-roll techniques, are ideal for the development of flexible devices for energy generation and storage - the key to the portable electronics of the future.

Exoelectrogens Leading to Precise Reduction of Graphene Oxide by Flexibly Switching Their Environment during Respiration

Reduced graphene oxide (RGO) has been prepared by a simple, cost-effective, and green route. In this work, graphene oxide (GO) has been reduced using Gram-negative facultative anaerobe S. dysenteriae, having exogenic properties of electron transfer via electron shuttling. Apparently, different concentrations of GO were successfully reduced with almost complete mass recovery. An effective role of lipopolysaccharide has been observed while comparing RGO reduced by S. dysenteriae and S. aureus. It was observed that the absence of lipopolysaccharide in Gram-positive S. aureus leads to a disrupted cell wall and that S.aureus could not survive in the presence of GO, leading to poor and inefficient reduction of GO, as shown in our results. However, S. dysenteriae having an outer lipopolysaccharide layer on its cell membrane reduced GO efficiently and the reduction process was extracellular for it. RGO prepared in our work has been characterized by X-ray diffraction, ζ potential, X-ray photoelectron spectroscopy, and Raman spectroscopy techniques, and the results were found to be in good agreement with those of chemically reduced GO. As agglomeration of RGO is the major issue to overcome while chemically reducing GO, we observed that RGO prepared by a bacterial route in our work has ζ potential value of -26.62 mV, good enough to avoid restacking of RGO. The role of exoelectrogens in electron transfer in the extracellular space has been depicted. Toxin released extracellularly during the process paves the way for reduction of GO due to its affinity towards oxygen.

Graphene growth and properties on metal substrates

Graphene-metal interface as one of the interesting graphene-based objects attracts much attention from both application and fundamental science points of view. This paper gives a timely review of the recent experimental works on the growth and the electronic properties of the graphene-metal interfaces. This work makes a link between huge amount of experimental and theoretical data allowing one to understand the influence of the metallic substrate on the electronic properties of a graphene overlayer and how its properties can be modified in a controllable way. The further directions of studies and applications of the graphene-metal interfaces are discussed.

The Covalent Functionalization of Graphene on Substrates

The utilization of grown or deposited graphene on solid substrates offers key benefits for functionalization processes, but especially to attain structures with a high level of control for electronics and "smart" materials. In this review, we will initially focus on the nature and properties of graphene on substrates, based on the method of preparation. We will then analyze the most relevant literature on the functionalization of graphene on substrates. In particular, we will comparatively discuss radical reactions, cycloadditions, halogenations, hydrogenations, and oxidations. We will especially address the question of how the reactivity of graphene is affected by its morphology (i.e., number of layers, defects, substrate, curvature, etc.).

Processes for non-destructive transfer of graphene: widening the bottleneck for industrial scale production

The exceptional charge-transport, mechanical, and barrier properties of graphene are well known. High-quality films of single-layer graphene produced over large areas, however, are extremely expensive. The high cost of graphene precludes its use in industries-such as transparent electrodes and flexible packaging-that might take full advantage of its properties. This minireview presents several strategies for the transfer of graphene from the substrates used for growth to substrates used for the final application. Each strategy shares the characteristic of being non-destructive: that is, the growth substrate remains reusable for further synthesis of new graphene. These processes have the potential to lower significantly the costs of manufacturing graphene, to increase production yields, and to minimize environmental impact. This article is divided into sections on (i) the synthesis of high-quality single-layer graphene and (ii) its non-destructive transfer to a host substrate. Section (ii) is further divided according to the substrate from which graphene is transferred: single-crystalline wafers or flexible copper foils. We also comment, wherever possible, on defects produced as a result of the transfer, and potential strategies to mitigate these defects. We conclude that several methods for the green synthesis and transfer of graphene have several of the right characteristics to be useful in industrial scale production.

Hybrids of cationic porphyrins with nanocarbons

In the review hybrids of cationic porphyrins (i.e. porphyrins functionalized by quaternary pyridinium groups) with nanocarbons such as fullerenes, carbon nanotubes and graphene are described. Selected examples of these species are characterized in regard of their properties and possible applications.

Synthesis of two-dimensional materials by selective extraction

CONSPECTUS: Two-dimensional (2D) materials have attracted much attention in the past decade. They offer high specific surface area, as well as electronic structure and properties that differ from their bulk counterparts due to the low dimensionality. Graphene is the best known and the most studied 2D material, but metal oxides and hydroxides (including clays), dichalcogenides, boron nitride (BN), and other materials that are one or several atoms thick are receiving increasing attention. They may deliver a combination of properties that cannot be provided by other materials. The most common synthesis approach in general is by reacting different elements or compounds to form a new compound. However, this approach does not necessarily work well for low-dimensional structures, since it favors formation of energetically preferred 3D (bulk) solids. Many 2D materials are produced by exfoliation of van der Waals solids, such as graphite or MoS2, breaking large particles into 2D layers. However, these approaches are not universal; for example, 2D transition metal carbides cannot be produced by any of them. An alternative but less studied way of material synthesis is the selective extraction process, which is based on the difference in reactivity and stability between the different components (elements or structural units) of the original material. It can be achieved using thermal, chemical, or electrochemical processes. Many 2D materials have been synthesized using selective extraction, such as graphene from SiC, transition metal oxides (TMO) from layered 3D salts, and transition metal carbides or carbonitrides (MXenes) from MAX phases. Selective extraction synthesis is critically important when the bonds between the building blocks of the material are too strong (e.g., in carbides) to be broken mechanically in order to form nanostructures. Unlike extractive metallurgy, where the extracted metal is the goal of the process, selective extraction of one or more elements from the precursor materials releases 2D structures. In this Account, in addition to graphene and TMO, we focused on MXenes as an example for the use of selective extraction synthesis to produce novel 2D materials. About 10 new carbides and carbonitrides of transition metals have been produced by this method in the past 3 years. They offer an unusual combination of metallic conductivity and hydrophilicity and show very attractive electrochemical properties. We hope that this Account will encourage researchers to extend the use of selective extraction to other layered material systems that in turn will result in expanding the world of nanomaterials in general and 2D materials in particular, generating new materials that cannot be produced by other means.

Effect of UV light-induced nitrogen doping on the field effect transistor characteristics of graphene

Control of electron concentration in graphene is achieved in the range of 10¹² to 10¹³ cm⁻² by nitrogen doping using photochemical reactions.