Scalable templated growth of graphene nanoribbons on SiC

Graphene nanoribbons grown in hBN stacks for high-performance electronics

Van der Waals encapsulation of two-dimensional materials in hexagonal boron nitride (hBN) stacks is a promising way to create ultrahigh-performance electronic devices1-4. However, contemporary approaches for achieving van der Waals encapsulation, which involve artificial layer stacking using mechanical transfer techniques, are difficult to control, prone to contamination and unscalable. Here we report the transfer-free direct growth of high-quality graphene nanoribbons (GNRs) in hBN stacks. The as-grown embedded GNRs exhibit highly desirable features being ultralong (up to 0.25 mm), ultranarrow (<5 nm) and homochiral with zigzag edges. Our atomistic simulations show that the mechanism underlying the embedded growth involves ultralow GNR friction when sliding between AA'-stacked hBN layers. Using the grown structures, we demonstrate the transfer-free fabrication of embedded GNR field-effect devices that exhibit excellent performance at room temperature with mobilities of up to 4,600 cm2 V-1 s-1 and on-off ratios of up to 106. This paves the way for the bottom-up fabrication of high-performance electronic devices based on embedded layered materials.

Quantum Confinement in Epitaxial Armchair Graphene Nanoribbons on SiC Sidewalls

The integration of graphene into devices necessitates large-scale growth and precise nanostructuring. Epitaxial growth of graphene on SiC surfaces offers a solution by enabling both simultaneous and targeted realization of quantum structures. We investigated the impact of local variations in the width and edge termination of armchair graphene nanoribbons (AGNRs) on quantum confinement effects using scanning tunneling microscopy and spectroscopy (STM, STS), along with density-functional tight-binding (DFTB) calculations. AGNRs were grown as an ensemble on refaceted sidewalls of SiC mesas with adjacent AGNRs separated by SiC(0001) terraces hosting a buffer layer seamlessly connected to the AGNRs. Energy band gaps measured by STS at the centers of ribbons of different widths align with theoretical expectations, indicating that hybridization of π-electrons with the SiC substrate mimics sharp electronic edges. However, regardless of the ribbon width, band gaps near the edges of AGNRs are significantly reduced. DFTB calculations successfully replicate this effect by considering the role of edge passivation, while strain or electric fields do not account for the observed effect. Unlike idealized nanoribbons with uniform hydrogen passivation, AGNRs on SiC sidewalls generate additional energy bands with non-pz character and nonuniform distribution across the nanoribbon. In AGNRs terminated with Si, these additional states occur at the conduction band edge and rapidly decay into the bulk of the ribbon. This agrees with our experimental findings, demonstrating that edge passivation is crucial in determining the local electronic properties of epitaxial nanoribbons.

Graphene Hybrid Metasurfaces for Mid-Infrared Molecular Sensors

We integrated graphene with asymmetric metal metasurfaces and optimised the geometry dependent photoresponse towards optoelectronic molecular sensor devices. Through careful tuning and characterisation, combining finite-difference time-domain simulations, electron-beam lithography-based nanofabrication, and micro-Fourier transform infrared spectroscopy, we achieved precise control over the mid-infrared peak response wavelengths, transmittance, and reflectance. Our methods enabled simple, reproducible and targeted mid-infrared molecular sensing over a wide range of geometrical parameters. With ultimate minimization potential down to atomic thicknesses and a diverse range of complimentary nanomaterial combinations, we anticipate a high impact potential of these technologies for environmental monitoring, threat detection, and point of care diagnostics.

Rapid production of kilogram-scale graphene nanoribbons with tunable interlayer spacing for an array of renewable energy

Graphene nanoribbons (GNRs) are widely recognized as intriguing building blocks for high-performance electronics and catalysis owing to their unique width-dependent bandgap and ample lone pair electrons on both sides of GNR, respectively, over the graphene nanosheet counterpart. However, it remains challenging to mass-produce kilogram-scale GNRs to render their practical applications. More importantly, the ability to intercalate nanofillers of interest within GNR enables in-situ large-scale dispersion and retains structural stability and properties of nanofillers for enhanced energy conversion and storage. This, however, has yet to be largely explored. Herein, we report a rapid, low-cost freezing-rolling-capillary compression strategy to yield GNRs at a kilogram scale with tunable interlayer spacing for situating a set of functional nanomaterials for electrochemical energy conversion and storage. Specifically, GNRs are created by sequential freezing, rolling, and capillary compression of large-sized graphene oxide nanosheets in liquid nitrogen, followed by pyrolysis. The interlayer spacing of GNRs can be conveniently regulated by tuning the amount of nanofillers of different dimensions added. As such, heteroatoms; metal single atoms; and 0D, 1D, and 2D nanomaterials can be readily in-situ intercalated into the GNR matrix, producing a rich variety of functional nanofiller-dispersed GNR nanocomposites. They manifest promising performance in electrocatalysis, battery, and supercapacitor due to excellent electronic conductivity, catalytic activity, and structural stability of the resulting GNR nanocomposites. The freezing-rolling-capillary compression strategy is facile, robust, and generalizable. It renders the creation of versatile GNR-derived nanocomposites with adjustable interlay spacing of GNR, thereby underpinning future advances in electronics and clean energy applications.

Electron-phonon interaction toward engineering carrier mobility of periodic edge structured graphene nanoribbons

Graphene nanoribbons have many extraordinary electrical properties and are the candidates for semiconductor industry. In this research, we propose a design of Coved GNRs with periodic structure ranged from 4 to 8 nm or more, of which the size is within practical feature sizes by advanced lithography tools. The carrier transport properties of Coved GNRs with the periodic coved shape are designed to break the localized electronic state and reducing electron-phonon scattering. In this way, the mobility of Coved GNRs can be enhanced by orders compared with the zigzag GNRs in same width. Moreover, in contrast to occasional zero bandgap transition of armchair and zigzag GNRs without precision control in atomic level, the Coved GNRs with periodic edge structures can exclude the zero bandgap conditions, which makes practical the mass production process. The designed Coved-GNRs is fabricated over the Germanium (110) substrate where the graphene can be prepared in the single-crystalline and single-oriented formants and the edge of GNRs is later repaired under "balanced condition growth" and we demonstrate that the propose coved structures are compatible to current fabrication facility.

Tunable growth of one-dimensional graphitic materials: graphene nanoribbons, carbon nanotubes, and nanoribbon/nanotube junctions

Graphene nanoribbons (GNRs) and carbon nanotubes (CNTs), two representative one-dimensional (1D) graphitic materials, have attracted tremendous research interests due to their promising applications for future high-performance nanoelectronics. Although various methods have been developed for fabrication of GNRs or CNTs, a unified method allowing controllable synthesis of both of them, as well as their heterojunctions, which could largely benefit their nano-electronic applications, is still lacking. Here, we report on a generic growth of 1D carbon using nanoparticles catalyzed chemical vapor deposition (CVD) on atomically flat hexagonal boron nitride (h-BN) substrates. Relative ratio of the yielded GNRs and CNTs is able to be arbitrarily tuned by varying the growth temperature or feeding gas pressures. The tunability of the generic growth is quantitatively explained by a competing nucleation theory: nucleation into either GNRs or CNTs by the catalysts is determined by the free energy of their formation, which is controlled by the growth conditions. Under the guidance of the theory, we further realized growth of GNR/CNT intramolecular junctions through changing H2 partial pressure during a single growth process. Our study provides not only a universal and controllable method for growing 1D carbon nanostructures, but also a deep understanding of their growth mechanism, which would largely benefit future carbon-based electronics and optoelectronics.

Recent Advances in Surface Modifications of Elemental Two-Dimensional Materials: Structures, Properties, and Applications

The advent of graphene opens up the research into two-dimensional (2D) materials, which are considered revolutionary materials. Due to its unique geometric structure, graphene exhibits a series of exotic physical and chemical properties. In addition, single-element-based 2D materials (Xenes) have garnered tremendous interest. At present, 16 kinds of Xenes (silicene, borophene, germanene, phosphorene, tellurene, etc.) have been explored, mainly distributed in the third, fourth, fifth, and sixth main groups. The current methods to prepare monolayers or few-layer 2D materials include epitaxy growth, mechanical exfoliation, and liquid phase exfoliation. Although two Xenes (aluminene and indiene) have not been synthesized due to the limitations of synthetic methods and the stability of Xenes, other Xenes have been successfully created via elaborate artificial design and synthesis. Focusing on elemental 2D materials, this review mainly summarizes the recently reported work about tuning the electronic, optical, mechanical, and chemical properties of Xenes via surface modifications, achieved using controllable approaches (doping, adsorption, strain, intercalation, phase transition, etc.) to broaden their applications in various fields, including spintronics, electronics, optoelectronics, superconducting, photovoltaics, sensors, catalysis, and biomedicines. These advances in the surface modification of Xenes have laid a theoretical and experimental foundation for the development of 2D materials and their practical applications in diverse fields.

An epitaxial graphene platform for zero-energy edge state nanoelectronics

Graphene's original promise to succeed silicon faltered due to pervasive edge disorder in lithographically patterned deposited graphene and the lack of a new electronics paradigm. Here we demonstrate that the annealed edges in conventionally patterned graphene epitaxially grown on a silicon carbide substrate (epigraphene) are stabilized by the substrate and support a protected edge state. The edge state has a mean free path that is greater than 50 microns, 5000 times greater than the bulk states and involves a theoretically unexpected Majorana-like zero-energy non-degenerate quasiparticle that does not produce a Hall voltage. In seamless integrated structures, the edge state forms a zero-energy one-dimensional ballistic network with essentially dissipationless nodes at ribbon-ribbon junctions. Seamless device structures offer a variety of switching possibilities including quantum coherent devices at low temperatures. This makes epigraphene a technologically viable graphene nanoelectronics platform that has the potential to succeed silicon nanoelectronics.

Graphene Incorporated Electrospun Nanofiber for Electrochemical Sensing and Biomedical Applications: A Critical Review

The extraordinary material graphene arrived in the fields of engineering and science to instigate a material revolution in 2004. Graphene has promptly risen as the super star due to its outstanding properties. Graphene is an allotrope of carbon and is made up of sp²-bonded carbon atoms placed in a two-dimensional honeycomb lattice. Graphite consists of stacked layers of graphene. Due to the distinctive structural features as well as excellent physico-chemical and electrical conductivity, graphene allows remarkable improvement in the performance of electrospun nanofibers (NFs), which results in the enhancement of promising applications in NF-based sensor and biomedical technologies. Electrospinning is an easy, economical, and versatile technology depending on electrostatic repulsion between the surface charges to generate fibers from the extensive list of polymeric and ceramic materials with diameters down to a few nanometers. NFs have emerged as important and attractive platform with outstanding properties for biosensing and biomedical applications, because of their excellent functional features, that include high porosity, high surface area to volume ratio, high catalytic and charge transfer, much better electrical conductivity, controllable nanofiber mat configuration, biocompatibility, and bioresorbability. The inclusion of graphene nanomaterials (GNMs) into NFs is highly desirable. Pre-processing techniques and post-processing techniques to incorporate GNMs into electrospun polymer NFs are precisely discussed. The accomplishment and the utilization of NFs containing GNMs in the electrochemical biosensing pathway for the detection of a broad range biological analytes are discussed. Graphene oxide (GO) has great importance and potential in the biomedical field and can imitate the composition of the extracellular matrix. The oxygen-rich GO is hydrophilic in nature and easily disperses in water, and assists in cell growth, drug delivery, and antimicrobial properties of electrospun nanofiber matrices. NFs containing GO for tissue engineering, drug and gene delivery, wound healing applications, and medical equipment are discussed. NFs containing GO have importance in biomedical applications, which include engineered cardiac patches, instrument coatings, and triboelectric nanogenerators (TENGs) for motion sensing applications. This review deals with graphene-based nanomaterials (GNMs) such as GO incorporated electrospun polymeric NFs for biosensing and biomedical applications, that can bridge the gap between the laboratory facility and industry.

Hybridization and localized flat band in the WSe2/MoSe2heterobilayer

Nearly localized moiré flat bands in momentum space, arising at particular twist angles, are the key to achieve correlated effects in transition-metal dichalcogenides. Here, we use angle-resolved photoemission spectroscopy (ARPES) to visualize the presence of a flat band near the Fermi level of van der Waals WSe₂/MoSe₂heterobilayer grown by molecular beam epitaxy. This flat band is localized near the Fermi level and has a width of several hundred meVs. By combining ARPES measurements with density functional theory calculations, we confirm the coexistence of different domains, namely the reference 2H stacking without layer misorientation and regions with arbitrary twist angles. For the 2H-stacked heterobilayer, our ARPES results show strong interlayer hybridization effects, further confirmed by complementary micro- Raman spectroscopy measurements. The spin-splitting of the valence band atKis determined to be 470 meV. The valence band maximum (VBM) position of the heterobilayer is located at the Γ point. The energy difference between the VBM at Γ and theKpoint is of -60 meV, which is a stark difference compared to individual single monolayer WSe₂and monolayer WSe₂, showing both a VBM atK.

Silica Particle-Mediated Growth of Single Crystal Graphene Ribbons on Cu(111) Foil

The authors report the growth of micrometer-long single-crystal graphene ribbons (GRs) (tapered when grown above 900 °C, but uniform width when grown in the range 850 °C to 900 °C) using silica particle seeds on single crystal Cu(111) foil. Tapered graphene ribbons grow strictly along the Cu<101> direction on Cu(111) and polycrystalline copper (Cu) foils. Silica particles on both Cu foils form (semi-)molten Cu-Si-O droplets at growth temperatures, then catalyze nucleation and drive the longitudinal growth of graphene ribbons. Longitudinal growth is likely by a vapor-liquid-solid (VLS) mechanism but edge growth (above 900 °C) is due to catalytic activation of ethylene (C₂ H₄ ) and attachment of C atoms or species ("vapor solid" or VS growth) at the edges. It is found, based on the taper angle of the graphene ribbon, that the taper angle is determined by the growth temperature and the growth rates are independent of the particle size. The activation enthalpy (1.73 ± 0.03 eV) for longitudinal ribbon growth on Cu(111) from ethylene is lower than that for VS growth at the edges of the GRs (2.78 ± 0.15 eV) and for graphene island growth (2.85 ± 0.07 eV) that occurs concurrently.

Graphene nanoribbons initiated from molecularly derived seeds

Semiconducting graphene nanoribbons are promising materials for nanoelectronics but are held back by synthesis challenges. Here we report that molecular-scale carbon seeds can be exploited to initiate the chemical vapor deposition (CVD) synthesis of graphene to generate one-dimensional graphene nanoribbons narrower than 5 nm when coupled with growth phenomena that selectively extend seeds along a single direction. This concept is demonstrated by subliming graphene-like polycyclic aromatic hydrocarbon molecules onto a Ge(001) catalyst surface and then anisotropically evolving size-controlled nanoribbons from the seeds along [Formula: see text] of Ge(001) via CH4 CVD. Armchair nanoribbons with mean normalized standard deviation as small as 11% (3 times smaller than nanoribbons nucleated without seeds), aspect ratio as large as 30, and width as narrow as 2.6 nm (tunable via CH4 exposure time) are realized. Two populations of nanoribbons are compared in field-effect transistors (FETs), with off-current differing by 150 times because of the nanoribbons' different widths.

Inducing a topological transition in graphene nanoribbon superlattices by external strain

Armchair graphene nanoribbons, when forming a superlattice, can be classified into different topological phases, with or without edge states. By means of tight-binding and classical molecular dynamics (MD) simulations, we studied the electronic and mechanical properties of some of these superlattices. MD shows that fracture in modulated superlattices is brittle, as for unmodulated ribbons, and occurs at the thinner regions, with staggered superlattices achieving a larger fracture strain than inline superlattices. We found a general mechanism to induce a topological transition with strain, related to the electronic properties of each segment of the superlattice, and by studying the sublattice polarization we were able to characterize the transition and the response of these states to the strain. For the cases studied in detail here, the topological transition occurred at ∼3-5% strain, well below the fracture strain. The topological states of the superlattice - if present - are robust to strain even close to fracture. The topological transition was characterized by means of the sublattice polarization of the states.

Evolution of Graphene Patterning: From Dimension Regulation to Molecular Engineering

The realization that nanostructured graphene featuring nanoscale width can confine electrons to open its bandgap has aroused scientists' attention to the regulation of graphene structures, where the concept of graphene patterns emerged. Exploring various effective methods for creating graphene patterns has led to the birth of a new field termed graphene patterning, which has evolved into the most vigorous and intriguing branch of graphene research during the past decade. The efforts in this field have resulted in the development of numerous strategies to structure graphene, affording a variety of graphene patterns with tailored shapes and sizes. The established patterning approaches combined with graphene chemistry yields a novel chemical patterning route via molecular engineering, which opens up a new era in graphene research. In this review, the currently developed graphene patterning strategies is systematically outlined, with emphasis on the chemical patterning. In addition to introducing the basic concepts and the important progress of traditional methods, which are generally categorized into top-down, bottom-up technologies, an exhaustive review of established protocols for emerging chemical patterning is presented. At the end, an outlook for future development and challenges is proposed.

Atomically precise graphene nanoribbons: interplay of structural and electronic properties

Graphene nanoribbons hold great promise for future applications in nanoelectronic devices, as they may combine the excellent electronic properties of graphene with the opening of an electronic band gap - not present in graphene but required for transistor applications. With a two-step on-surface synthesis process, graphene nanoribbons can be fabricated with atomic precision, allowing precise control over width and edge structure. Meanwhile, a decade of research has resulted in a plethora of graphene nanoribbons having various structural and electronic properties. This article reviews not only the on-surface synthesis of atomically precise graphene nanoribbons but also how their electronic properties are ultimately linked to their structure. Current knowledge and considerations with respect to precursor design, which eventually determines the final (electronic) structure, are summarized. Special attention is dedicated to the electronic properties of graphene nanoribbons, also in dependence on their width and edge structure. It is exactly this possibility of precisely changing their properties by fine-tuning the precursor design - offering tunability over a wide range - which has generated this vast research interest, also in view of future applications. Thus, selected device prototypes are presented as well.

Topological Surface State in Epitaxial Zigzag Graphene Nanoribbons

Protected and spin-polarized transport channels are the hallmark of topological insulators, coming along with an intrinsic strong spin-orbit coupling. Here we identified such corresponding chiral states in epitaxially grown zigzag graphene nanoribbons (zz-GNRs), albeit with an extremely weak spin-orbit interaction. While the bulk of the monolayer zz-GNR is fully suspended across a SiC facet, the lower edge merges into the SiC(0001) substrate and reveals a surface state at the Fermi energy, which is extended along the edge and splits in energy toward the bulk. All of the spectroscopic details are precisely described within a tight binding model incorporating a Haldane term and strain effects. The concomitant breaking of time-reversal symmetry without the application of external magnetic fields is supported by ballistic transport revealing a conduction of G = e²/h.

Strain-inducing photochemical chlorination of graphene nanoribbons on SiC (0001)

As different low-dimensional materials are sought to be incorporated into microelectronic devices, graphene integration is dependent on the development of band gap opening strategies. Amidst the different methods currently investigated, application of strain and use of electronic quantum confinement have shown promising results. In the present work, epitaxial graphene nanoribbons (GNR), formed by surface graphitization of SiC (0001) on crystalline step edges, were submitted to photochemical chlorination. The incorporation of Cl into the buffer layer underlying graphene increased the compressive uniaxial strain in the ribbons. Such method is a promising tool for tuning the band gap of GNRs.

Materials Science Challenges to Graphene Nanoribbon Electronics

Graphene nanoribbons (GNRs) have recently emerged as promising candidates for channel materials in future nanoelectronic devices due to their exceptional electronic, thermal, and mechanical properties and chemical inertness. However, the adoption of GNRs in commercial technologies is currently hampered by materials science and integration challenges pertaining to synthesis and devices. In this Review, we present an overview of the current status of challenges, recent breakthroughs toward overcoming these challenges, and possible future directions for the field of GNR electronics. We motivate the need for exploration of scalable synthetic techniques that yield atomically precise, placed, registered, and oriented GNRs on CMOS-compatible substrates and stimulate ideas for contact and dielectric engineering to realize experimental performance close to theoretically predicted metrics. We also briefly discuss unconventional device architectures that could be experimentally investigated to harness the maximum potential of GNRs in future spintronic and quantum information technologies.

Diversified Phenomena in Metal- and Transition-Metal-Adsorbed Graphene Nanoribbons

Adatom-adsorbed graphene nanoribbons (GNRs) have gained much attention owing to the tunable electronic and magnetic properties. The metal (Bi, Al)/transition metal (Ti, Fe, Co, Ni) atoms could provide various outermost orbitals for the multi-orbital hybridizations with the out-of-plane π bondings on the carbon honeycomb lattice, which dominate the fundamental properties of chemisorption systems. In this study, the significant similarities and differences among Bi-/Al-/Ti-/Fe-/Co-/Ni-adsorbed GNRs are thoroughly investigated by using the first-principles calculations. The main characterizations include the adsorption sites, bond lengths, stability, band structures, charge density distributions, spin- and orbital-projected density of states, and magnetic configurations. Furthermore, there exists a transformation from finite gap semiconducting to metallic behaviors, accompanied by the nonmagnetism, antiferromagnetism, or ferromagnetism. They arise from the cooperative or competitive relations among the significant chemical bonds, finite-size quantum confinement, edge structure, and spin-dependent many-body effects. The proposed theoretical framework could be further improved and generalized to explore other emergent 1D and 2D materials.

Ambipolar 2D Semiconductors and Emerging Device Applications

With the rise of 2D materials, new physics and new processing techniques have emerged, triggering possibilities for the innovation of electronic and optoelectronic devices. Among them, ambipolar 2D semiconductors are of excellent gate-controlled capability and distinctive physical characteristic that the major charge carriers can be dynamically, reversibly and rapidly tuned between holes and electrons by electrostatic field. Based on such properties, novel devices, like ambipolar field-effect transistors, light-emitting transistors, electrostatic-field-charging PN diodes, are developed and show great advantages in logic and reconfigurable circuits, integrated optoelectronic circuits, and artificial neural network image sensors, enriching the functions of conventional devices and bringing breakthroughs to build new architectures. This review first focuses on the basic knowledge including fundamental principle of ambipolar semiconductors, basic material preparation techniques, and how to obtain the ambipolar behavior through electrical contact engineering. Then, the current ambipolar 2D semiconductors and their preparation approaches and main properties are summarized. Finally, the emerging new device structures are overviewed in detail, along with their novel electronic and optoelectronic applications. It is expected to shed light on the future development of ambipolar 2D semiconductors, exploring more new devices with novel functions and promoting the applications of 2D materials.

One-dimensional confinement and width-dependent bandgap formation in epitaxial graphene nanoribbons

The ability to define an off state in logic electronics is the key ingredient that is impossible to fulfill using a conventional pristine graphene layer, due to the absence of an electronic bandgap. For years, this property has been the missing element for incorporating graphene into next-generation field effect transistors. In this work, we grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. Angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling spectroscopy measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. Furthermore, ARPES demonstrates an ideal one-dimensional electronic behavior that is realized in a graphene-based environment, consisting of well-resolved subbands, dispersing and non-dispersing along and across the ribbons respectively. Our experimental findings, coupled with theoretical tight-binding calculations, set the grounds for a deeper exploration of quantum confinement phenomena and may open intriguing avenues for new low-power electronics.

Graphene impregnated electrospun nanofiber sensing materials: a comprehensive overview on bridging laboratory set-up to industry

Owing to the unique structural characteristics as well as outstanding physio-chemical and electrical properties, graphene enables significant enhancement with the performance of electrospun nanofibers, leading to the generation of promising applications in electrospun-mediated sensor technologies. Electrospinning is a simple, cost-effective, and versatile technique relying on electrostatic repulsion between the surface charges to continuously synthesize various scalable assemblies from a wide array of raw materials with diameters down to few nanometers. Recently, electrospun nanocomposites have emerged as promising substrates with a great potential for constructing nanoscale biosensors due to their exceptional functional characteristics such as complex pore structures, high surface area, high catalytic and electron transfer, controllable surface conformation and modification, superior electric conductivity and unique mat structure. This review comprehends graphene-based nanomaterials (GNMs) (graphene, graphene oxide (GO), reduced GO and graphene quantum dots) impregnated electrospun polymer composites for the electro-device developments, which bridges the laboratory set-up to the industry. Different techniques in the base polymers (pre-processing methods) and surface modification methods (post-processing methods) to impregnate GNMs within electrospun polymer nanofibers are critically discussed. The performance and the usage as the electrochemical biosensors for the detection of wide range analytes are further elaborated. This overview catches a great interest and inspires various new opportunities across a wide range of disciplines and designs of miniaturized point-of-care devices.

Recent Advances in Graphene Patterning

As an emerging field of research, graphene patterning has received considerable attention because of its ability to tailor the structure of graphene and the respective properties, aiming at practical applications such as electronic devices, catalysts, and sensors. Recent efforts in this field have led to the development of a variety of different approaches to pattern graphene sheets, providing a multitude of graphene patterns with different shapes and sizes. These established patterning techniques in combination with graphene chemistry have paved the road towards highly attractive chemical patterning approaches, establishing a very promising and vigorously developing research topic. In this review, an overview of commonly used strategies is presented that are categorized into top-down and bottom-up routes for graphene patterning, focusing mainly on new advances. Other than the introduction of basic concepts of each method, the advantages/disadvantages are compared as well. In addition, for the first time, an overview of chemical patterning techniques is outlined. At the end, the challenges and future perspectives in the field are envisioned.

Quantum Confinement of Dirac Quasiparticles in Graphene Patterned with Sub-Nanometer Precision

Quantum confinement of graphene Dirac-like electrons in artificially crafted nanometer structures is a long sought goal that would provide a strategy to selectively tune the electronic properties of graphene, including bandgap opening or quantization of energy levels. However, creating confining structures with nanometer precision in shape, size, and location remains an experimental challenge, both for top-down and bottom-up approaches. Moreover, Klein tunneling, offering an escape route to graphene electrons, limits the efficiency of electrostatic confinement. Here, a scanning tunneling microscope (STM) is used to create graphene nanopatterns, with sub-nanometer precision, by the collective manipulation of a large number of H atoms. Individual graphene nanostructures are built at selected locations, with predetermined orientations and shapes, and with dimensions going all the way from 2 nm up to 1 µm. The method permits the patterns to be erased and rebuilt at will, and it can be implemented on different graphene substrates. STM experiments demonstrate that such graphene nanostructures confine very efficiently graphene Dirac quasiparticles, both in 0D and 1D structures. In graphene quantum dots, perfectly defined energy bandgaps up to 0.8 eV are found that scale as the inverse of the dot's linear dimension, as expected for massless Dirac fermions.

Molecular Interpretation of Pharmaceuticals’ Adsorption on Carbon Nanomaterials: Theory Meets Experiments

The ability of carbon-based nanomaterials (CNM) to interact with a variety of pharmaceutical drugs can be exploited in many applications. In particular, they have been studied both as carriers for in vivo drug delivery and as sorbents for the treatment of water polluted by pharmaceuticals. In recent years, the large number of experimental studies was also assisted by computational work as a tool to provide understanding at molecular level of structural and thermodynamic aspects of adsorption processes. Quantum mechanical methods, especially based on density functional theory (DFT) and classical molecular dynamics (MD) simulations were mainly applied to study adsorption/release of various drugs. This review aims to compare results obtained by theory and experiments, focusing on the adsorption of three classes of compounds: (i) simple organic model molecules; (ii) antimicrobials; (iii) cytostatics. Generally, a good agreement between experimental data (e.g. energies of adsorption, spectroscopic properties, adsorption isotherms, type of interactions, emerged from this review) and theoretical results can be reached, provided that a selection of the correct level of theory is performed. Computational studies are shown to be a valuable tool for investigating such systems and ultimately provide useful insights to guide CNMs materials development and design.

Non-Invasive Nanoscale Potentiometry and Ballistic Transport in Epigraphene Nanoribbons

The recent observation of non-classical electron transport regimes in two-dimensional materials has called for new high-resolution non-invasive techniques to locally probe electronic properties. We introduce a novel hybrid scanning probe technique to map the local resistance and electrochemical potential with nm- and μV resolution, and we apply it to study epigraphene nanoribbons grown on the sidewalls of SiC substrate steps. Remarkably, the potential drop is non-uniform along the ribbons, and μm-long segments show no potential variation with distance. The potential maps are in excellent agreement with measurements of the local resistance. This reveals ballistic transport, compatible with μm-long room-temperature electronic mean-free paths.

Raman 2D Peak Line Shape in Epigraphene on SiC

We measured a 2D peak line shape of epitaxial graphene grown on SiC in high vacuum, argon and graphene prepared by hydrogen intercalation from the so called buffer layer on a silicon face of SiC. We fitted the 2D peaks by Lorentzian and Voigt line shapes. The detailed analysis revealed that the Voigt line shape describes the 2D peak line shape better. We have determined the contribution of the homogeneous and inhomogeneous broadening. The homogeneous broadening is attributed to the intrinsic lifetime. Although the inhomogeneous broadening can be attributed to the spatial variations of the charge density, strain and overgrown graphene ribbons on the sub-micrometer length scales, we found dominant contribution of the strain fluctuations. The quasi free-standing graphene grown by hydrogen intercalation is shown to have the narrowest linewidth due to both homogeneous and inhomogeneous broadening.

Laser-Induced Graphene: En Route to Smart Sensing

The discovery of laser-induced graphene (LIG) from polymers in 2014 has aroused much attention in recent years. A broad range of applications, including batteries, catalysis, sterilization, and separation, have been explored. The advantages of LIG technology over conventional graphene synthesis methods are conspicuous, which include designable patterning, environmental friendliness, tunable compositions, and controllable morphologies. In addition, LIG possesses high porosity, great flexibility, and mechanical robustness, and excellent electric and thermal conductivity. The patternable and printable manufacturing process and the advantageous properties of LIG illuminate a new pathway for developing miniaturized graphene devices. Its use in sensing applications has grown swiftly from a single detection component to an integrated smart detection system. In this minireview, we start with the introduction of synthetic efforts related to the fabrication of LIG sensors. Then, we highlight the achievement of LIG sensors for the detection of a diversity of stimuli with a focus on the design principle and working mechanism. Future development of the techniques toward in situ and smart detection of multiple stimuli in widespread applications will be discussed.

A novel molecularly imprinted sensor based on PtCu bimetallic nanoparticle deposited on PSS functionalized graphene with peroxidase-like activity for selective determination of puerarin

In this work, PtCu bimetallic nanoparticle was deposited on poly (styrene sulfonate) (PSS) functionalized graphene (Gr) to form a nanocomposite PtCu/PSS-Gr and its enzyme-like activity was investigated. Benefiting from the synergistic effect from Pt and Cu monometal as well as the superior properties of PSS-Gr, such as large surface area, good dispersity, strong adsorption of substrate and additional peroxidase-like activity, the PtCu/PSS-Gr nanocomposite was demonstrated as an excellent peroxidase mimic to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂. Combined with molecularly imprinted polymer (MIP), a new colorimetric approach for puerarin detection was proposed with the linear range of 2 × 10^-5-6 × 10^-4 mol L^-1 and LOD of 1 × 10^-5 mol L^-1. The combination of MIP with PtCu/PSS-Gr nanocomposite not only endowed the determination of puerarin with high selectivity, but also realized the detection of small molecules which are neither substrate of the nanozyme nor substances with strong oxidizing or reducing activity by using peroxidase-like catalytic activity of nanozyme, expanding the application of nanozyme.

Paper-Based Flexible Electrode Using Chemically-Modified Graphene and Functionalized Multiwalled Carbon Nanotube Composites for Electrophysiological Signal Sensing

Flexible paper-based physiological sensor electrodes were developed using chemically-modified graphene (CG) and carboxylic-functionalized multiwalled carbon nanotube composites (f@MWCNTs). A solvothermal process with additional treatment was conducted to synthesize CG and f@MWCNTs to make CG-f@MWCNT composites. The composite was sonicated in an appropriate solvent to make a uniform suspension, and then it was drop cast on a nylon membrane in a vacuum filter. A number of batches (0%~35% f@MWCNTs) were prepared to investigate the performance of the physical characteristics. The 25% f@MWCNT-loaded composite showed the best adhesion on the paper substrate. The surface topography and chemical bonding of the proposed CG-f@MWCNT electrodes were characterized by scanning electron microscopy (SEM) and Raman spectroscopy, respectively. The average sheet resistance of the 25% CG-f@MWCNT electrode was determined to be 75 Ω/□, and it showed a skin contact impedance of 45.12 kΩ at 100 Hz. Electrocardiogram (ECG) signals were recorded from the chest and fingertips of healthy adults using the proposed electrodes. The CG-f@MWCNT electrodes demonstrated comfortability and a high sensitivity for electrocardiogram signal detection.

Graphene on Group-IV Elementary Semiconductors: The Direct Growth Approach and Its Applications

Since the first development of large-area graphene synthesis by the chemical vapor deposition (CVD) method in 2009, CVD-graphene has been considered to be a key material in the future electronics, energy, and display industries, which require transparent, flexible, and stretchable characteristics. Although many graphene-based prototype applications have been demonstrated, several important issues must be addressed in order for them to be compatible with current complementary metal-oxide-semiconductor (CMOS)-based manufacturing processes. In particular, metal contamination and mechanical damage, caused by the metal catalyst for graphene growth, are known to cause severe and irreversible deterioration in the performance of devices. The most effective way to solve the problems is to grow the graphene directly on the semiconductor substrate. Herein, recent advances in the direct growth of graphene on group-IV semiconductors are reviewed, focusing mainly on the growth mechanism and initial growth behavior when graphene is synthesized on Si and Ge. Furthermore, recent progress in the device applications of graphene with Si and Ge are presented. Finally, perspectives for future research in graphene with a semiconductor are discussed.

Alignment of semiconducting graphene nanoribbons on vicinal Ge(001)

Chemical vapor deposition of CH4 on Ge(001) can enable anisotropic growth of narrow, semiconducting graphene nanoribbons with predominately smooth armchair edges and high-performance charge transport properties. However, such nanoribbons are not aligned in one direction but instead grow perpendicularly, which is not optimal for integration into high-performance electronics. Here, it is demonstrated that vicinal Ge(001) substrates can be used to synthesize armchair nanoribbons, of which ∼90% are aligned within ±1.5° perpendicular to the miscut. When the growth rate is slow, graphene crystals evolve as nanoribbons. However, as the growth rate increases, the uphill and downhill crystal edges evolve asymmetrically. This asymmetry is consistent with stronger binding between the downhill edge and the Ge surface, for example due to different edge termination as shown by density functional theory calculations. By tailoring growth rate and time, nanoribbons with sub-10 nm widths that exhibit excellent charge transport characteristics, including simultaneous high on-state conductance of 8.0 μS and a high on/off conductance ratio of 570 in field-effect transistors, are achieved. Large-area alignment of semiconducting ribbons with promising charge transport properties is an important step towards understanding the anisotropic nanoribbon growth and integrating these materials into scalable, future semiconductor technologies.

Design and Fabrication of Heterogeneous, Deformable Substrates for the Mechanically Guided 3D Assembly

Development of schemes to form complex three-dimensional (3D) mesostructures in functional materials is a topic of broad interest, thanks to the ubiquitous applications across a diversity of technologies. Recently established schemes in the mechanically guided 3D assembly allow deterministic transformation of two-dimensional structures into sophisticated 3D architectures by controlled compressive buckling resulted from strain release of prestretched elastomer substrates. Existing studies mostly exploited supporting substrates made of homogeneous elastomeric material with uniform thickness, which produces relatively uniform strain field to drive the 3D assembly, thus posing limitations to the geometric diversity of resultant 3D mesostructures. To offer nonuniform strains with desired spatial distributions in the 3D assembly, this paper introduces a versatile set of concepts in the design of engineered substrates with heterogeneous integration of materials of different moduli. Such heterogeneous, deformable substrates can achieve large strain gradients and efficient strain isolation/magnification, which are difficult to realize using the previously reported strategies. Theoretical and experimental studies on the underlying mechanics offer a viable route to the design of heterogeneous, deformable substrates to yield favorable strain fields. A broad collection of 3D mesostructures and associated heterogeneous substrates is fabricated and demonstrated, including examples that resemble windmills, scorpions, and manta rays and those that have application potentials in tunable inductors and vibrational microsystems.

Facile and Green Synthesis of Graphene-Based Conductive Adhesives via Liquid Exfoliation Process

In this study, we report a facile and green process to synthesize high-quality and few-layer graphene (FLG) derived from graphite via a liquid exfoliation process. The corresponding characterizations of FLG, such as scanning electron microscopy (SEM), transmission electron microscope (TEM), atomic force microscopy (AFM) and Raman spectroscopy, were carried out. The results of SEM show that the lateral size of as-synthesized FLG is 1–5 μm. The results of TEM and AFM indicate more than 80% of graphene layers is <10 layers. The most surprising thing is that D/G ratio of graphite and FLG are 0.15 and 0.19, respectively. The result of the similar D/G ratio demonstrates that little structural defects were created via the liquid exfoliation process. Electronic conductivity tests and resistance of composite film, in terms of different contents of graphite/polyvinylidene difluoride (PVDF) and FLG/PVDF, were carried out. Dramatically, the FLG/PVDF composite demonstrates superior performance compared to the graphite/PVDF composite at the same ratio. In addition, the post-sintering process plays an important role in improving electronic conductivity by 85%. The composition-optimized FLG/PVDF thin film exhibits 81.9 S·cm⁻¹. These results indicate that the developed FLG/PVDF composite adhesives could be a potential candidate for conductive adhesive applications.

Ballistic tracks in graphene nanoribbons

High quality graphene nanoribbons epitaxially grown on the sidewalls of silicon carbide (SiC) mesa structures stand as key building blocks for graphene-based nanoelectronics. Such ribbons display 1D single-channel ballistic transport at room temperature with exceptionally long mean free paths. Here, using spatially-resolved two-point probe (2PP) measurements, we selectively access and directly image a range of individual transport modes in sidewall ribbons. The signature of the independently contacted channels is a sequence of quantised conductance plateaus for different probe positions. These result from an interplay between edge magnetism and asymmetric terminations at opposite ribbon edges due to the underlying SiC structure morphology. Our findings demonstrate a precise control of transport through multiple, independent, ballistic tracks in graphene-based devices, opening intriguing pathways for quantum information device concepts.

Salt and water co-assisted exfoliation of graphite in organic solvent for efficient and large scale production of high-quality graphene

Graphene has attracted enormous attention due to its unique physical properties and attractive applications in many fields. However, it is an ongoing challenge to develop a facile and low-cost method for the large scale preparation of high-quality graphene (HQGr). In this work, we have developed an improved liquid-phase exfoliation method to mass produce HQGr. This method is quite simple but efficient by exfoliation of graphite in organic solvent with the co-assistance of sodium citrate and water. Remarkably, the concentration of as-exfoliated HQGr was as high as 0.71 mg/mL under optimal conditions, while the oxygen content in HQGr was only 2.39%. After annealing at 500 °C for 2 h in argon atmosphere, the mean conductivity of annealed HQGr was as high as 1.4 × 10⁴ S m^-1. Therefore, this facile method for liquid-phase exfoliation of graphite has excellent potential in the industrial-scale production of HQGr for numerous applications in energy storage, optical and electronic fields.

Structure of graphene and its disorders: a review

Monolayer graphene exhibits extraordinary properties owing to the unique, regular arrangement of atoms in it. However, graphene is usually modified for specific applications, which introduces disorder. This article presents details of graphene structure, including sp2 hybridization, critical parameters of the unit cell, formation of σ and π bonds, electronic band structure, edge orientations, and the number and stacking order of graphene layers. We also discuss topics related to the creation and configuration of disorders in graphene, such as corrugations, topological defects, vacancies, adatoms and sp3-defects. The effects of these disorders on the electrical, thermal, chemical and mechanical properties of graphene are analyzed subsequently. Finally, we review previous work on the modulation of structural defects in graphene for specific applications.

Green and facile synthesis of few-layer graphene via liquid exfoliation process for Lithium-ion batteries

A green and facile method using jet cavitation (JC) was utilized to prepare few layer graphene (FLG) derived from artificial graphite delamination without adding any strong acids and oxidants. The JC method not only provides high quality FLG with high yield but also demonstrate excellent electrochemical performance as anode materials for Li-ion batteries. Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) as well as BET isotherms and XPS are carried out in this study. The results of atomic force microscopy (AFM) further revealed that up to 85% of the prepared FLG were less than 10 layers. This exfoliation process happened mainly due to the cavitation-induced intensive tensile stress acting on the layered materials. Electrochemical measurements demonstrate that graphite anode delivered only 240 mAh/g while FLG anode achieved more than 322 mAh/g at 5C rate test. These results indicate that JC method not only paves the way for cheaper and safer production of graphene but also holds great potential applications in energy-related technology.

3D Fabrication with Integration Molding of a Graphene Oxide/Polycaprolactone Nanoscaffold for Neurite Regeneration and Angiogenesis

Treating peripheral nerve injury faces major challenges and may benefit from bioactive scaffolds due to the limited autograft resources. Graphene oxide (GO) has emerged as a promising nanomaterial with excellent physical and chemical properties. GO has functional groups that confer biocompatibility that is better than that of graphene. Here, GO/polycaprolactone (PCL) nanoscaffolds are fabricated using an integration molding method. The nanoscaffolds exhibit many merits, including even GO nanoparticle distribution, macroporous structure, and strong mechanical support. Additionally, the process enables excellent quality control. In vitro studies confirm the advantages of the GO/PCL nanoscaffolds in terms of Schwann cell proliferation, viability, and attachment, as well as neural characteristics maintenance. This is the first study to evaluate the in vivo performance of GO-based nanoscaffolds in this context. GO release and PCL biodegradation is analyzed after long-term in vivo study. It is also found that the GO/PCL nerve guidance conduit could successfully repair a 15 mm sciatic nerve defect. The pro-angiogenic characteristic of GO is evaluated in vivo using immunohistochemistry. In addition, the AKT-endothelial nitric oxide synthase (eNOS)-vascular endothelial growth factor (VEGF) signaling pathway might play a major role in the angiogenic process. These findings demonstrate that the GO/PCL nanoscaffold efficiently promotes functional and morphological recovery in peripheral nerve regeneration, indicating its promise for tissue engineering applications.

Raman spectroscopy of graphene-based materials and its applications in related devices

Graphene-based materials exhibit remarkable electronic, optical, and mechanical properties, which has resulted in both high scientific interest and huge potential for a variety of applications. Furthermore, the family of graphene-based materials is growing because of developments in preparation methods. Raman spectroscopy is a versatile tool to identify and characterize the chemical and physical properties of these materials, both at the laboratory and mass-production scale. This technique is so important that most of the papers published concerning these materials contain at least one Raman spectrum. Thus, here, we systematically review the developments in Raman spectroscopy of graphene-based materials from both fundamental research and practical (i.e., device applications) perspectives. We describe the essential Raman scattering processes of the entire first- and second-order modes in intrinsic graphene. Furthermore, the shear, layer-breathing, G and 2D modes of multilayer graphene with different stacking orders are discussed. Techniques to determine the number of graphene layers, to probe resonance Raman spectra of monolayer and multilayer graphenes and to obtain Raman images of graphene-based materials are also presented. The extensive capabilities of Raman spectroscopy for the investigation of the fundamental properties of graphene under external perturbations are described, which have also been extended to other graphene-based materials, such as graphene quantum dots, carbon dots, graphene oxide, nanoribbons, chemical vapor deposition-grown and SiC epitaxially grown graphene flakes, composites, and graphene-based van der Waals heterostructures. These fundamental properties have been used to probe the states, effects, and mechanisms of graphene materials present in the related heterostructures and devices. We hope that this review will be beneficial in all the aspects of graphene investigations, from basic research to material synthesis and device applications.

Investigation on the growth mechanism of SiC whiskers during microwave synthesis

SiC whiskers with different morphologies are fabricated by microwave heating and the growth mechanisms of the SiC whiskers are simulated.

Effect of ribbon width on electrical transport properties of graphene nanoribbons

There has been growing interest in developing nanoelectronic devices based on graphene because of its superior electrical properties. In particular, patterning graphene into a nanoribbon can open a bandgap that can be tuned by changing the ribbon width, imparting semiconducting properties. In this study, we report the effect of ribbon width on electrical transport properties of graphene nanoribbons (GNRs). Monolayer graphene sheets and Si nanowires (NWs) were prepared by chemical vapor deposition and a combination of nanosphere lithography and metal-assisted electroless etching from a Si wafer, respectively. Back-gated GNR field-effect transistors were fabricated on a heavily p-doped Si substrate coated with a 300 nm-thick SiO₂ layer, by O₂ reactive ion etching of graphene sheets using etch masks based on Si NWs aligned on the graphene between the two electrodes by a dielectrophoresis method. This resulted in GNRs with various widths in a highly controllable manner, where the on/off current ratio was inversely proportional to ribbon width. The field-effect mobility decreased with decreasing GNR widths due to carrier scattering at the GNR edges. These results demonstrate the formation of a bandgap in GNRs due to enhanced carrier confinement in the transverse direction and edge effects when the GNR width is reduced.