Controlled Folding of Single Crystal Graphene

Toward Clean 2D Materials and Devices: Recent Progress in Transfer and Cleaning Methods

Two-dimensional (2D) materials have tremendous potential to revolutionize the field of electronics and photonics. Unlocking such potential, however, is hampered by the presence of contaminants that usually impede the performance of 2D materials in devices. This perspective provides an overview of recent efforts to develop clean 2D materials and devices. It begins by discussing conventional and recently developed wet and dry transfer techniques and their effectiveness in maintaining material "cleanliness". Multi-scale methodologies for assessing the cleanliness of 2D material surfaces and interfaces are then reviewed. Finally, recent advances in passive and active cleaning strategies are presented, including the unique self-cleaning mechanism, thermal annealing, and mechanical treatment that rely on self-cleaning in essence. The crucial role of interface wetting in these methods is emphasized, and it is hoped that this understanding can inspire further extension and innovation of efficient transfer and cleaning of 2D materials for practical applications.

Electronic structures and quantum capacitance of twisted bilayer graphene with defects based on three-band tight-binding model

Twisted bilayer graphene (tBLG) with C vacancies would greatly improve the density of states (DOS) around the Fermi level (EF) and quantum capacitance; however, the single-band tight-binding model only considering pz orbitals cannot accurately capture the low-energy physics of tBLG with C vacancies. In this work, a three-band tight-binding model containing three p orbitals of C atoms is proposed to explore the modulation mechanism of C vacancies on the DOS and quantum capacitance of tBLG. We first obtain the hopping integral parameters of the three-band tight-binding model, and then explore the electronic structures and the quantum capacitance of tBLG at a twisting angle of θ = 1.47° under different C vacancy concentrations. The impurity states contributed by C atoms with dangling bonds located around the EF and the interlayer hopping interaction could induce band splitting of the impurity states. Therefore, compared with the quantum capacitance of pristine tBLG (∼18.82 μF cm-2) at zero bias, the quantum capacitance is improved to ∼172.76 μF cm-2 at zero bias, and the working window with relatively large quantum capacitance in the low-voltage range is broadened in tBLG with C vacancies due to the enhanced DOS around the EF. Moreover, the quantum capacitance of tBLG is further increased at zero bias with an increase of the C vacancy concentration induced by more impurity states. These findings not only provide a suitable multi-band tight-binding model to describe tBLG with C vacancies but also offer theoretical insight for designing electrode candidates for low-power consumption devices with improved quantum capacitance.

Twisted van der Waals Quantum Materials: Fundamentals, Tunability, and Applications

Twisted van der Waals (vdW) quantum materials have emerged as a rapidly developing field of two-dimensional (2D) semiconductors. These materials establish a new central research area and provide a promising platform for studying quantum phenomena and investigating the engineering of novel optoelectronic properties such as single photon emission, nonlinear optical response, magnon physics, and topological superconductivity. These captivating electronic and optical properties result from, and can be tailored by, the interlayer coupling using moiré patterns formed by vertically stacking atomic layers with controlled angle misorientation or lattice mismatch. Their outstanding properties and the high degree of tunability position them as compelling building blocks for both compact quantum-enabled devices and classical optoelectronics. This paper offers a comprehensive review of recent advancements in the understanding and manipulation of twisted van der Waals structures and presents a survey of the state-of-the-art research on moiré superlattices, encompassing interdisciplinary interests. It delves into fundamental theories, synthesis and fabrication, and visualization techniques, and the wide range of novel physical phenomena exhibited by these structures, with a focus on their potential for practical device integration in applications ranging from quantum information to biosensors, and including classical optoelectronics such as modulators, light emitting diodes, lasers, and photodetectors. It highlights the unique ability of moiré superlattices to connect multiple disciplines, covering chemistry, electronics, optics, photonics, magnetism, topological and quantum physics. This comprehensive review provides a valuable resource for researchers interested in moiré superlattices, shedding light on their fundamental characteristics and their potential for transformative applications in various fields.

Angle-Resolved Optical Imaging of Interlayer Rotations in Twisted Bilayer Graphene

Twisted bilayer graphene (TBG) is a prototypical layered material whose properties are strongly correlated to interlayer coupling. The two stacked graphene layers with distinct orientations are investigated to generate peculiar optical and electronic phenomena. Thus, the rapid, reliable, and nondestructive twist angle identification technique is of essential importance. Here, we integrated the white light reflection spectra (WLRS), the Raman spectroscopy, and the transmission electron microscope (TEM) to propose a facile RGB optical imaging technique that identified the twist angle of the TBG in a large area intuitively with high efficiency. The RGB technique established a robust correlation between the interlayer rotation angle and the contrast difference in the RGB color channels of a standard optical image. The angle-resolved optical behavior is attributed to the absorption resonance matching with the separation of van Hove singularities in the density of states of the TBG. Our study thus developed a route to identify the rotation angle of stacked bilayer graphene by means of a straightforward optical method, which can be further applied in other stacked van der Waals layered materials.

Hierarchical Colloidal Self-Assembly on Lattice-Mismatched Moiré Patterns

Extending atomic epitaxy concepts to colloidal systems for realizing functional surface structures has recently piqued scientific interest. Akin to the growth of ordered metal clusters on graphene moiré, spatially ordered colloidal crystals have been realized on soft lithographically fabricated moiré patterns. In addition to moiré periodicity, lattice misfit strain can bring about a further level of hierarchy in colloidal self-assembly, although its role in self-organization remains unexplored. Here, we demonstrate the self-organized growth of micrometer-sized colloidal pyramid arrays with lateral order extending over millimeter length scales on lattice-mismatched moiré patterns. By probing the film growth dynamics with single-particle resolution, we uncovered the interplay between lattice misfit strain and topographically varying surface potential within the moiré unit cell, which significantly alters the nucleation process. We also show that the structural organization of colloids within moiré regions primarily depends on the moiré angle, and by tuning it, multiple levels of hierarchy can be achieved.

Engineering Grain Boundaries in Two-Dimensional Electronic Materials

Engineering the boundary structures in 2D materials provides an unprecedented opportunity to program the physical properties of the materials with extensive tunability and realize innovative devices with advanced functionalities. However, structural engineering technology is still in its infancy, and creating artificial boundary structures with high reproducibility remains difficult. In this review, various emergent properties of 2D materials with different grain boundaries, and the current techniques to control the structures, are introduced. The remaining challenges for scalable and reproducible structure control and the outlook on the future directions of the related techniques are also discussed.

Perfect optical absorption in a single array of folded graphene ribbons

Due to its one atom thickness, optical absorption (OA) in graphene is a fundamental and challenging issue. Practically, the patterned graphene-dielectric-metal structure is commonly used to achieve perfect OA (POA). In this work, we propose a novel scenario to solve this issue, in which POA is obtained by using free-standing folded graphene ribbons (FGRs). We show several local resonances, e.g. a dipole state (Mode-I) and a bound state in continuum (BIC, Mode-II), will cause very efficient OA. At normal incidence, by choosing appropriate folding angle θ, 50% absorptance by the two states is easily achieved; at oblique incidence, the two states will result in roughly 98% absorptance as incidence angle φ≈40^∘. It is also interesting to see that the system has asymmetric OA spectra, e.g. POA of the former (latter) state existing in reverse (forward) incidence, respectively. Besides the angles θ and φ, POA here can also be actively tuned by electrostatic gating. As increasing Fermi level, POA of Mode-I will undergo a gradual blueshift, while that of Mode-II will experience a rapid blueshift and then be divided into three branches, due to Fano coupling to two guided modes. In reality, the achieved POA is well maintained even the dielectric substrates are used to support FGRs. Our work offers a remarkable scenario to achieve POA, and thus enhance light-matter interaction in graphene, which can build an alternative platform to study novel optical effects in general two-dimensional (2D) materials. The folding, mechanical operation in out-of-plane direction, may emerge as a new degree of freedom for optoelectronic device applications based on 2D materials.

One Dimensional Twisted Van der Waals Structures Constructed by Self-Assembling Graphene Nanoribbons on Carbon Nanotubes

Twisted van der Waals heterostructures were recently found to possess unique physical properties, such as superconductivity in magic angle bilayer graphene. Owing to the nonhomogeneous stacking, the energy of twisted van der Waals heterostructures are often higher than their AA or AB stacking counterpart, therefore, fabricating such structures remains a great challenge in experiments. On the other hand, one dimensional (1D) coaxial van der Waals structures has less freedom to undergo phase transition, thus offer opportunity for fabricating the 1D cousin of twisted bilayer graphene. In this work, we show by molecular dynamic simulations that graphene nanoribbons can self-assemble onto the surface of carbon nanotubes driven by van der Waals interactions. By modifying the size of the carbon nanotubes or graphene nanoribbons, the resultant configurations can be controlled. Of particular interest is the formation of twisted double walled carbon nanotubes whose chiral angle difference can be tuned, including the 1.1° magic angle. Upon the longitudinal unzipping of such structures, twisted bilayer graphene nanoribbons can be obtained. As the longitudinal unzipping of carbon nanotubes is a mature technique, we expect the strategy proposed in this study to stimulate experimental efforts and promote the fast growing research in twistronics.

Designed growth of large bilayer graphene with arbitrary twist angles

The production of large-area twisted bilayer graphene (TBG) with controllable angles is a prerequisite for proceeding with its massive applications. However, most of the prevailing strategies to fabricate twisted bilayers face great challenges, where the transfer methods are easily stuck by interfacial contamination, and direct growth methods lack the flexibility in twist-angle design. Here we develop an effective strategy to grow centimetre-scale TBG with arbitrary twist angles (accuracy, <1.0°). The success in accurate angle control is realized by an angle replication from two prerotated single-crystal Cu(111) foils to form a Cu/TBG/Cu sandwich structure, from which the TBG can be isolated by a custom-developed equipotential surface etching process. The accuracy and consistency of the twist angles are unambiguously illustrated by comprehensive characterization techniques, namely, optical spectroscopy, electron microscopy, photoemission spectroscopy and photocurrent spectroscopy. Our work opens an accessible avenue for the designed growth of large-scale two-dimensional twisted bilayers and thus lays the material foundation for the future applications of twistronics at the integration level.

Folding and Fracture of Single-Crystal Graphene Grown on a Cu(111) Foil

A single-crystal graphene film grown on a Cu(111) foil by chemical vapor deposition (CVD) has ribbon-like fold structures. These graphene folds are highly oriented and essentially parallel to each other. Cu surface steps underneath the graphene are along the <110> and <211> directions, leading to the formation of the arrays of folds. The folds in the single-layer graphene (SLG) are not continuous but break up into alternating patterns. A "joint" (an AB-stacked bilayer graphene) region connects two neighboring alternating regions, and the breaks are always along zigzag or armchair directions. Folds formed in bilayer or few-layer graphene are continuous with no breaks. Molecular dynamics simulations show that SLG suffers a significantly higher compressive stress compared to bilayer graphene when both are under the same compression, thus leading to the rupture of SLG in these fold regions. The fracture strength of a CVD-grown single-crystal SLG film is simulated to be about 70 GPa. This study greatly deepens the understanding of the mechanics of CVD-grown single-crystal graphene and such folds, and sheds light on the fabrication of various graphene origami/kirigami structures by substrate engineering. Such oriented folds can be used in a variety of further studies.

A method for designing tunable chiral mechanical carbon networks for energy storage

A method is proposed for designing tunable chiral nano-networks using partly hydrogenated graphene ribbons and carbon nanotubes (CNTs). In the network, the hydrogenated graphene ribbons (HGRs) act as basic components, which connect each other via CNT joints. Each component contains two HGR segments and an internal graphene joint (G-J2) or CNT joint (CNT-J2). Since the two HGR segments are hydrogenated at opposite surfaces, they may wind in chiral about the internal joint to form a scroll (G-J2-scroll or CNT-J2-scroll) or about the two end joints to form CNT-J4-scrolls. In general, a G-J2-scroll is formed more easily than both a CNT-J4-scroll and a CNT-J2-scroll. Because of scrolling, the surface energy is reduced. This reduction is converted to and stored as deformation potential energy. By means of molecular-dynamics simulations, we studied the final configurations of two types of networks from the same components, the maximum shrinkage, and their capacity of energy storage for potential application of energy storage or as large-deformable components in a nano-device. The results indicate that the network reaches a stable state when the shrinkage reaches 70% of the two in-plane dimensions.

Single sheets of graphene for fabrication of fibers with enhanced mechanical properties

This paper reports the fabrication and mechanical properties of macroscale graphene fibers (diameters of 10 to 100 μm with lengths upwards of 2 cm) prepared from a single sheet of single-layer graphene grown via chemical vapor deposition (CVD). The breaking strength of these graphene fibers increased with consecutive tensile test measurements on a single fiber, where fiber fragments produced from a prior test exhibited larger breaking strengths. Additionally, we observed an overall reduction of surface folds and wrinkles, and an increase in their alignment parallel to the tensile direction. We propose that a foundation of this property is the plastic deformations within the fiber that accumulate through sequential tensile testing. Through this cyclic method, our best fiber produced a strength of 2.67 GPa with a 1 mm gauge length.

Tunnel barrier engineering of spin-polarized mild band gap vertical ternary heterostructures

The atomic and electronic structures and properties of advanced 2D ternary vertical spin-polarized semiconducting heterostructures based on mild band gap graphitic carbon nitride g-C₃N₄ and ferromagnetic single-layer CrI₃ fragments, namely CrI₃/g-C₃N₄/CrI₃ and g-C₃N₄/CrI₃/g-C₃N₄, were proposed and examined using the ab initio GGA PBE PBC technique. Both possible ferromagnetic (FM) and antiferromagnetic (AFM) spin ordering configurations of CrI₃/g-C₃N₄/CrI₃ were considered and found to be energetically degenerated, being significantly different in the density of states. Electronic structure calculations revealed that weak van der Waals interactions between the fragments are responsible for the main features of the atomic and electronic structures of both the types of heterostructures. The combination of flat valence and conduction bands and conductivity channels localized at spin-polarized semiconducting CrI₃ fragments makes proposed heterostructures as magnetic tunnel junctions for spin- and photo-related applications such as spintronics, magnetoresistive random-access memory, photocatalysis, and as elements for highly efficient spin-polarized photovoltaic nanodevices.

Monolayer Twisted Graphene-Based Schottky Transistor

The outstanding properties of graphene-based components, such as twisted graphene, motivates nanoelectronic researchers to focus on their applications in device technology. Twisted graphene as a new class of graphene structures is investigated in the platform of transistor application in this research study. Therefore, its geometry effect on Schottky transistor operation is analyzed and the relationship between the diameter of twist and number of twists are explored. A metal–semiconductor–metal twisted graphene-based junction as a Schottky transistor is considered. By employing the dispersion relation and quantum tunneling the variation of transistor performance under channel length, the diameter of twisted graphene, and the number of twists deviation are studied. The results show that twisted graphene with a smaller diameter affects the efficiency of twisted graphene-based Schottky transistors. Additionally, as another main characteristic, the I_D-V_GS is explored, which indicates that the threshold voltage is increased by diameter and number of twists in this type of transistor.

Self-folding and self-scrolling mechanisms of edge-deformed graphene sheets: a molecular dynamics study

Graphene-based nanofolds (GNFs) are edge-connected 2D stacked monolayers that originate from single-layer graphene. Graphene-based nanoscrolls (GNSs) are nanomaterials with geometry resembling graphene layers rolled up into a spiral (papyrus-like) form. Both GNS and GNF structures induce significant changes in the mechanical and optoelectronic properties of single-layer graphene, aggregating new functionalities in carbon-based applications. Here, we carried out fully atomistic reactive (ReaxFF) molecular dynamics simulations to study the self-folding and self-scrolling mechanisms of edge-deformed graphene sheets. We adopted initial armchair edge-scrolled graphene (AESG(φ, θ)) structures with similar (or different) twist angles (φ, θ) in each edge, mimicking the initial configuration that was experimentally developed to form biscrolled sheets. The results showed that AESG(0, 2π) and AESG(2π, 2π) evolved to single-folded and two-folded fully stacked morphologies, respectively. As a general trend, for twist angles higher than 2π, the self-deformation process of AESG morphologies yields GNSs. Edge twist angles lower than π are not enough for triggering the self-deformation processes. In the AESG(0, 3π) and AESG(3π, 3π) cases, after a relaxation period, their morphology transition towards GNSs occurred rapidly. In the AESG(3π, 3π) dynamics, a metastable biscroll was formed by the interplay between the left- and right-sided partial scrolling while forming a unique GNS. At high-temperature perturbations, the edge folding and scrolling transitions to GNFs and GNSs occurred within an ultrafast time-period. Remarkably, the AESG(2π, 3π) evolved to a dual state that combines folded and scrolled structures in a temperature-independent process.

Fabrication Strategies of Twisted Bilayer Graphenes and Their Unique Properties

Twisted bilayer graphene (tBLG) exhibits a host of innovative physical phenomena owing to the formation of moiré superlattice. Especially, the discovery of superconducting behavior has generated new interest in graphene. The growing studies of tBLG mainly focus on its physical properties, while the fabrication of high-quality tBLG is a prerequisite for achieving the desired properties due to the great dependence on the twist angle and the interfacial contact. Here, the cutting-edge preparation strategies and challenges of tBLG fabrication are reviewed. The advantages and disadvantages of chemical vapor deposition, epitaxial growth on silicon carbide, stacking monolayer graphene, and folding monolayer graphene methods for the fabrication of tBLG are analyzed in detail, providing a reference for further development of preparation methods. Moreover, the characterization methods of twist angle for the tBLG are presented. Then, the unique physicochemical properties and corresponding applications of tBLG, containing correlated insulating and superconducting states, ferromagnetic state, soliton, enhanced optical absorption, tunable bandgap, and lithium intercalation and diffusion, are described. Finally, the opportunities and challenges for fabricating high-quality and large-area tBLG are discussed, unique physical properties are displayed, and new applications inferred from its angle-dependent features are explored, thereby impelling the commercialization of tBLG from laboratory to market.

Preparation of Twisted Bilayer Graphene via the Wetting Transfer Method

Assembling monolayers into a bilayer system unlocks the rotational free degree of van der Waals (vdW) homo/heterostructure, enabling the building of twisted bilayer graphene (tBLG) which possesses novel electronic, optical, and mechanical properties. Previous methods for preparation of homo/heterstructures inevitably leave the polymer residue or hexagonal boron nitride (h-BN) mask, which usually obstructs the measurement of intrinsic mechanical and surface properties of tBLG. Undoubtedly, to fabricate the designable tBLG with clean interface and surface is necessary but challenging. Here, we propose a simple and handy method to prepare atomically clean twisted bilayer graphene with controllable twist angles based on wetting-induced delamination. This method can transfer tBLG onto a patterned substrate, which offers an excellent platform for the observation of physical phenomena such as relaxation of moiré pattern in marginally tBLG. These findings and insight should ultimately guide the designable packaging and atomic characterization of the two-dimensional (2D) materials.

Opportunities and Challenges in Twisted Bilayer Graphene: A Review

Two-dimensional (2D) materials exhibit enhanced physical, chemical, electronic, and optical properties when compared to those of bulk materials. Graphene demands significant attention due to its superior physical and electronic characteristics among different types of 2D materials. The bilayer graphene is fabricated by the stacking of the two monolayers of graphene. The twisted bilayer graphene (tBLG) superlattice is formed when these layers are twisted at a small angle. The presence of disorders and interlayer interactions in tBLG enhances several characteristics, including the optical and electrical properties. The studies on twisted bilayer graphene have been exciting and challenging thus far, especially after superconductivity was reported in tBLG at the magic angle. This article reviews the current progress in the fabrication techniques of twisted bilayer graphene and its twisting angle-dependent properties.

Doping effect in graphene-graphene oxide interlayer

Interlayer coupling in graphene-based van der Waals (vdW) heterostructures plays a key role in determining and modulating their physical properties. Hence, its influence on the optical and electronic properties cannot be overlooked in order to promote various next-generation applications in electronic and opto-electronic devices based on the low-dimensional materials. Herein, the optical and electrical properties of the vertically stacked large area heterostructure of the monolayer graphene transferred onto a monolayer graphene oxide film are investigated. An effective and stable p-doping property of this structure is shown by comparison to that of the graphene device fabricated on a silicon oxide substrate. Through Raman spectroscopy and density functional theory calculations of the charge transport characteristics, it is found that graphene is affected by sustainable p-doping effects induced from underneath graphene oxide even though they have weak interlayer interactions. This finding can facilitate the development of various fascinating graphene-based heterostructures and extend their practical applications in integrated devices with advanced functionalities.

Growth of Single-Layer and Multilayer Graphene on Cu/Ni Alloy Substrates

ConspectusGraphene, a one-atom-thick layer of carbon with a honeycomb lattice, has drawn great attention due to its outstanding properties and its various applications in electronic and photonic devices. Mechanical exfoliation has been used for preparing graphene flakes (from monolayer to multilayer with thick pieces also typically present), but with sizes limited typically to less than millimeters, its usefulness is limited. Chemical vapor deposition (CVD) has been shown to be the most effective technique for the scalable preparation of graphene films with high quality and uniformity. To date, CVD growth of graphene on the most commonly used substrates (Cu and Ni foils) has been demonstrated and intensively studied. However, a survey of the existing literature and earlier work using Cu or Ni substrates for CVD growth indicates that the bilayer and multilayer graphene over a large area, particularly single crystals, have not been obtained.In this Account, we review current progress and development in the CVD growth of graphene and highlight the important challenges that need to be addressed, for example, how to achieve large single crystal graphene films with a controlled number of layers. A single-layer graphene film grown on polycrystalline Cu foil was first reported by our group, and since then various techniques have been devoted to achieving the fast growth of large-area graphene films with high quality. Commercially available Cu/Ni foils, sputtered Cu/Ni thin films, and polycrystalline Cu/Ni foils have been used for the CVD synthesis of bilayer, trilayer, and multilayer graphene. Cu/Ni alloy substrates are particularly interesting due to their greater carbon solubility than pure Cu substrates and this solubility can be finely controlled by changing the alloy composition. These substrates with controlled compositions have shown the potential for the growth of layer-tunable graphene films in addition to providing a much higher growth rate due to their stronger catalytic activity. However, the well-controlled preparation of single crystal graphene with a defined number of layers on Cu/Ni substrates is still challenging.Due to its small lattice mismatch with graphene, a single crystal Cu(111) foil has been shown to be an ideal substrate for the epitaxial growth of graphene. Our group has reported the synthesis of large-size single crystal Cu(111) foils by the contact-free annealing of commercial Cu foils, and single crystal Cu/Ni(111) alloy foils have also been obtained after the heat-treatment of Ni-coated Cu(111) foils. The use of these single crystal foils (especially the Cu/Ni alloy foils) as growth substrates has enabled the fast growth of single crystal single-layer graphene films. By increase of the Ni content, single crystal bilayer, trilayer, and even multilayer graphene films have been synthesized. In addition, we also discuss the wafer-scale growth of single-layer graphene on the single crystalline Cu/Ni(111) thin films.Recent research results on the large-scale preparation of single crystal graphene films with different numbers of layers on various types of Cu/Ni alloy substrates with different compositions are reviewed and discussed in detail. Despite the remarkable progress in this field, further challenges, such as the wafer-scale synthesis of single crystal graphene with a controlled number of layers and a deeper understanding of the growth mechanism of bilayer and multilayer graphene growth on Cu/Ni substrates, still need to be addressed.

Twist-Dependent Raman and Electron Diffraction Correlations in Twisted Multilayer Graphene

Twisted multilayer graphene (tMLG), in contrast to twisted bilayer graphene, offers a range of angular rotations for tuning the properties of the system. In this work, a turbostratic graphene system with a high degree of two-dimensional (2D) crystallinity is chosen to represent tMLG. We have investigated the distribution and population of twist angles from distributed sextets in electron diffraction (SAED) patterns with the collective Raman behavior at the same locations. A descriptor, termed the turbostratic factor, was calculated on the basis of angular spacings in SAEDs, to account for their distribution; the greater the spread, the higher the turbostratic factor. Raman spectra have revealed that the turbostratic factor remains low (∼0°) for a graphitic region with a low 2D to G intensity ratio (I_2D/I_G) and increases rapidly at higher I_2D/I_G values, saturating at 60° for highly turbostratic systems. Relating the intensities associated with the sextets and I_2D/I_G values, we found the maximum achievable value of I_2D/I_G to be 17.92.

In Situ Friction-Induced Graphene Originating from Methanol at the Sliding Interface between the WC Self-Mated Tribo-Pair and Its Tribological Performance

Alcohols are reported to have superlubricity at low loads during sliding; however, their lubricity under high loads has rarely been reported. Meanwhile, the lubrication mechanism of alcohols under high loads is still not well understood. Here, we first report the lubricity of methanol under 98 N and 1450 rpm and demonstrate the formation of graphene and fullerene-like nanostructures induced by tribochemical reactions. Results show that the lubrication mechanism was mainly attributed to the friction-induced graphene under boundary lubrication condition. Besides that, the wear rate of a YG8 hard alloy ball mainly occurred at the run-in processes, and the friction-induced graphene effectively inhibited further wear after the run-in processes. The formation mechanism of graphene was well investigated, and the flash temperature rise and catalyst (WC, WO₂, and WO₃) were the major causes for the formation of graphene.

Electrical, Transport, and Optical Properties of Multifunctional Graphitic Films Synthesized on Dielectric Surfaces by Nickel Nanolayer-Assisted Pyrolysis

We demonstrate that predepositing a nanometrically thin nickel film on a dielectric surface is sufficient to transform an amorphous pyrolyzed photoresist film (PPF) into a graphitic film (GRF) enriched with nickel particles. The GRF shows 3 orders of magnitude higher carrier mobility than the amorphous PPF, whereas its electrical conductivity doubles after etching away the nickel remains. The pronounced 2D peak in the Raman spectrum, almost dispersionless absorbance in the spectral range of 750-2000 nm, and the saturable absorption coefficient indicate that GRF possesses a graphene-like band structure. The proposed cost-efficient and scalable synthesis route opens avenues toward fabrication of micron size patterned graphitic structures of any shape directly on a dielectric substrate. Having graphene-like transport and electrical properties at 20 times higher absorbance than the single-layer graphene, GRF is attractive for fabrication of fast modulators for optical radiation, bolometers, and other photonics and optoelectronic devices that require enhanced optical absorption.

The effects of confinement in pores built of folded graphene sheets on the equilibrium of nitrogen monoxide dimerisation reaction

In the current work we have used reactive Monte Carlo simulations to systematically study the effects of graphene folding on equilibria of NO dimerisation occurring at isolated surfaces and in porous networks built of corrugated graphene sheets. It has been demonstrated that the folding of isolated graphene sheets significantly improves the yield of reactions occurring on their surface. Then, it has also been shown that in slit-like pores formed by the folded graphene sheets the reaction yield depends on the corrugation and arrangement of the pore walls. It has been found that the reaction yield increases when the walls' corrugation is high because of the appearance of narrow regions and/or wedge-like regions in the pores. The condensation of reacting fluid in such places, where the bulges at both walls are close one to another, leads to much higher reaction yield than on the surface of isolated sheets. Thus, we recommended the highly corrugated graphene to control the chemical reactions.

Entropic selectivity in air separation via a bilayer nanoporous graphene membrane

Molecular dynamics simulations show that controlling the pore size and the pore shape via the bilayer nanoporous graphene membrane provides a novel way to enhance entropic selectivity for air separation via tumbling motion of the oxygen molecule.

Graphene Nanoribbons with Atomically Sharp Edges Produced by AFM Induced Self-Folding

The ability to create graphene nanoribbons with atomically sharp edges is important for various graphene applications because these edges significantly influence the overall electronic properties and support unique magnetic edge states. The discovery of graphene self-folding induced by traveling wave excitation through atomic force microscope scanning under a normal force of less than 15 nN is reported. Most remarkably, the crystallographic direction of self-folding may be either along a chosen direction defined by the scan line or along the zigzag or armchair direction in the presence of a pre-existing crack in the vicinity. The crystalline direction of the atomically sharp edge is confirmed via careful lateral force microscopy measurements. Multilayer nanoribbons with lateral dimensions of a few tens of nanometers are realized on the same graphene sheet with different folding types (e.g., z-type or double parallel). Molecular dynamics simulations reveal the folding dynamics and suggest a monotonic increase of the folded area with the applied normal force. This method may be extended to other 2D van der Waals materials and lead to nanostructures that exhibit novel edge properties without the chemical instability that typically hinders applications of etched or patterned graphene nanostructures.

High-yield scalable graphene nanosheet production from compressed graphite using electrochemical exfoliation

Electrochemical exfoliation is a promising bulk method for producing graphene from graphite; in this method, an applied voltage drives ionic species to intercalate into graphite where they form gaseous species that expand and exfoliate individual graphene sheets. However, a number of obstacles have prevented this approach from becoming a feasible production route; the disintegration of the graphite electrode as the method progresses is the chief difficulty. Here we show that if graphite powders are contained and compressed within a permeable and expandable containment system, the graphite powders can be continuously intercalated, expanded, and exfoliated to produce graphene. Our data indicate both high yield (65%) and extraordinarily large lateral size (>30 μm) in the as-produced graphene. We also show that this process is scalable and that graphene yield efficiency depends solely on reactor geometry, graphite compression, and electrolyte transport.

Confined states in graphene quantum blisters

Bilayer graphene samples may exhibit regions where the two layers are locally delaminated forming a so-called quantum blister in the graphene sheet. Electron and hole states can be confined in this graphene quantum blisters (GQB) by applying a global electrostatic bias. We scrutinize the electronic properties of these confined states under the variation of interlayer bias, coupling, and blister's size. The spectra display strong anti-crossings due to the coupling of the confined states on upper and lower layers inside the blister. These spectra are layer localized where the respective confined states reside on either layer or equally distributed. For finite angular momentum, this layer localization can be at the edge of the blister and corresponds to degenerate modes of opposite momenta. Furthermore, the energy levels in GQB exhibit electron-hole symmetry that is sensitive to the electrostatic bias. Finally, we demonstrate that confinement in GQB persists even in the presence of a variation in the inter-layer coupling.

One-pot synthesis of graphene- cobalt hydroxide composite nanosheets (Co/G NSs) for electrocatalytic water oxidation

We report a one-pot method for the preparation of graphene-cobalt hydroxide nanosheets (Co/G NSs) and their use as an effective elelctrocatalyst for water oxidation. Mechanical exfoliation of graphite via sonication produced graphene sheets, which were stabilized by the surface adsorption of a cationic surfactant (CTAB). In a subsequent step, varying amount of a cobalt complex [sodium hexanitrocobaltate(III)] was added which selectively bound with the positively charged head of surfactant. In the last step, cobalt complex was reduced with sodium borohydride to obtain Co/G NSs catalyst. The catalyst showed lower overpotential (280 mV) as compared to benchmark catalysts and decent stability and turnover frequency (TOF: 0.089 s⁻¹) for oxygen evolution reaction (OER).

Critical Annealing Temperature for Stacking Orientation of Bilayer Graphene

It is rarely reported that stacking orientations of bilayer graphene (BLG) can be manipulated by the annealing process. Most investigators have painstakingly fabricated this BLG by chemical vapor deposition growth or mechanical means. Here, it is discovered that, at ≈600 °C, called the critical annealing temperature (CAT), most stacking orientations collapse into strongly coupled or AB-stacked states. This phenomenon is governed (i) macroscopically by the stress generation and release in top graphene domains, evolving from mild ripples to sharp billows in certain local areas, and (ii) microscopically by the principle of minimal potential obeyed by carbon atoms that have acquired sufficient thermal energy at CAT. Conspicuously, evolutions of stacking orientations in Raman mappings under various annealing temperatures are observed. Furthermore, MoS₂ synthesized on BLG is used to directly observe crystal orientations of top and bottom graphene layers. The finding of CAT provides a guide for the fabrication of strongly coupled or AB-stacked BLG, and can be applied to aligning other 2D heterostructures.

Folding Large Graphene-on-Polymer Films Yields Laminated Composites with Enhanced Mechanical Performance

A folding technique is reported to incorporate large-area monolayer graphene films in polymer composites for mechanical reinforcement. Compared with the classic stacking method, the folding strategy results in further stiffening, strengthening, and toughening of the composite. By using a water-air-interface-facilitated procedure, an A5-size 400 nm thin polycarbonate (PC) film is folded in half 10 times to a ≈0.4 mm thick material (1024 layers). A large PC/graphene film is also folded by the same process, resulting in a composite with graphene distributed uniformly. A three-point bending test is performed to study the mechanical performance of the composites. With a low volume fraction of graphene (0.085%), the Young's modulus, strength, and toughness modulus are enhanced in the folded composite by an average of 73.5%, 73.2%, and 59.1%, respectively, versus the pristine stacked polymer films, or 40.2%, 38.5%, and 37.3% versus the folded polymer film, proving a remarkable mechanical reinforcement from the combined folding and reinforcement of graphene. These results are rationalized with combined theoretical and computational analyses, which also allow the synergistic behavior between the reinforcement and folding to be quantified. The folding approach could be extended/applied to other 2D nanomaterials to design and make macroscale laminated composites with enhanced mechanical properties.

Two- and three-dimensional self-folding of free-standing graphene by liquid evaporation

Two-dimensional (2-D) atomically thin graphene has exhibited overwhelming excellent properties over its bulk counterpart graphite, yet the broad applications and explorations of its unprecedented properties require a diversity of its geometric morphologies, beyond its inherently planar structures. In this study, we present a self-folding approach for converting 2-D planar free-standing graphene to 2-D and 3-D folded structures through the evaporation of its liquid solutions. This approach involves competition between the surface energy of the liquid, and the deformation energy and van der Waals energy of graphene. An energy-based theoretical model is developed to describe the self-folding process during liquid evaporation by incorporating both graphene dimensions and surface wettability. The critical elastocapillary length by liquid evaporation is extracted and exemplified by investigating three typical graphene geometries with rectangular, circular and triangular shapes. After the complete evaporation of the liquid, the critical self-folding length of graphene that can enable a stable folded pattern by van der Waals energy is also obtained. In parallel, full-scale molecular dynamics (MD) simulations are performed to monitor the evolution of deformation energies and folded patterns with liquid evaporation. The simulation results demonstrate the formation of 2-D folded racket-like and 3-D folded cone-like patterns and show remarkable agreement with theoretical predictions in both energy variations and folded patterns. This work offers quantitative guidance for controlling the self-folding of graphene and other 2-D materials into complex structures by liquid evaporation.

Camphor-Enabled Transfer and Mechanical Testing of Centimeter-Scale Ultrathin Films

Camphor is used to transfer centimeter-scale ultrathin films onto custom-designed substrates for mechanical (tensile) testing. Compared to traditional transfer methods using dissolving/peeling to remove the support-layers, camphor is sublimed away in air at low temperature, thereby avoiding additional stress on the as-transferred films. Large-area ultrathin films can be transferred onto hollow substrates without damage by this method. Tensile measurements are made on centimeter-scale 300 nm-thick graphene oxide film specimens, much thinner than the ≈2 μm minimum thickness of macroscale graphene-oxide films previously reported. Tensile tests were also done on two different types of large-area samples of adlayer free CVD-grown single-layer graphene supported by a ≈100 nm thick polycarbonate film; graphene stiffens this sample significantly, thus the intrinsic mechanical response of the graphene can be extracted. This is the first tensile measurement of centimeter-scale monolayer graphene films. The Young's modulus of polycrystalline graphene ranges from 637 to 793 GPa, while for near single-crystal graphene, it ranges from 728 to 908 GPa (folds parallel to the tensile loading direction) and from 683 to 775 GPa (folds orthogonal to the tensile loading direction), demonstrating the mechanical performance of large-area graphene in a size scale relevant to many applications.

Synthesis of a Helical Bilayer Nanographene

A rigid, inherently chiral bilayer nanographene has been synthesized as both the racemate and enantioenriched M isomer (with 93 % ee) in three steps from established helicenes. This folded nanographene is composed of two hexa-peri-hexabenzocoronene layers fused to a [10]helicene, with an interlayer distance of 3.6 Å as determined by X-ray crystallography. The rigidity of the helicene linker forces the layers to adopt a nearly aligned AA-stacked conformation, rarely observed in few-layer graphene. By combining the advantages of nanographenes and helicenes, we have constructed a bilayer system of 30 fused benzene rings that is also chiral, rigid, and remains soluble in common organic solvents. We present this as a molecular model system of bilayer graphene, with properties of interest in a variety of potential applications.

Geometric and electronic structure of multilayered graphene: synergy of the nondirective ripples and the number of layers

According to the Mermin-Wagner theorem, ripple deformation is ubiquitous in a two-dimensional (2D) free-standing sheet, influencing the electronic properties. However, the synergistic effects of the unrestricted ripples and the number of layers have still been a topic of extensive debate. To address this issue, we employed density functional theory including many-body van der Waals (vdW) correction to investigate the effects of the nondirective ripples on the geometric and electronic structures of multilayered graphene. We found that the many-body effects of vdW forces were essential for the binding of multilayered rippled graphene. The increase of curvature affects the electronic structures of rippled graphene by modifying stacking modes, while the increase in the number of layers can reduce band gap and work function directly. The coupling of these two effects can enhance the chemical activity of rippled graphene. Our results facilitate new insights into the geometric and electronic properties of rippled graphene, which can be generalized to other layered materials.

NanoVelcro: Theory of Guided Folding in Atomically Thin Sheets with Regions of Complementary Doping

Folding has been commonly observed in two-dimensional materials such as graphene and monolayer transition metal dichalcogenides. Although interlayer coupling stabilizes these folds, it provides no control over the placement of the fold, let alone the final folded shape. Lacking nanoscale "fingers" to externally guide folding, control requires interactions engineered into the sheets that guide them toward a desired final folded structure. Here we provide a theoretical framework for a general methodology toward this end: atomically thin 2D sheets are doped with patterns of complementary n-type and p-type regions whose preferential adhesion favors folding into desired shapes. The two-colorable theorem in flat-foldable origami ensures that arbitrary folding patterns are in principle accessible to this method. This complementary doping method can be combined with nanoscale crumpling (by, for example, passage of 2D sheets through holes) to obtain not only control over fold placements but also the ability to distinguish between degenerate folded states, thus attaining nontrivial shapes inaccessible to sequential folding.

Quantum transport across van der Waals domain walls in bilayer graphene

Bilayer graphene can exhibit deformations such that the two graphene sheets are locally detached from each other resulting in a structure consisting of domains with different van der Waals inter-layer coupling. Here we investigate how the presence of these domains affects the transport properties of bilayer graphene. We derive analytical expressions for the transmission probability, and the corresponding conductance, across walls separating different inter-layer coupling domains. We find that the transmission can exhibit a valley-dependent layer asymmetry and that the domain walls have a considerable effect on the chiral tunnelling properties of the charge carriers. We show that transport measurements allow one to obtain the strength with which the two layers are coupled. We perform numerical calculations for systems with two domain walls and find that the availability of multiple transport channels in bilayer graphene significantly modifies the conductance dependence on inter-layer potential asymmetry.

Ultrathin thermoresponsive self-folding 3D graphene

Graphene and other two-dimensional materials have unique physical and chemical properties of broad relevance. It has been suggested that the transformation of these atomically planar materials to three-dimensional (3D) geometries by bending, wrinkling, or folding could significantly alter their properties and lead to novel structures and devices with compact form factors, but strategies to enable this shape change remain limited. We report a benign thermally responsive method to fold and unfold monolayer graphene into predesigned, ordered 3D structures. The methodology involves the surface functionalization of monolayer graphene using ultrathin noncovalently bonded mussel-inspired polydopamine and thermoresponsive poly(N-isopropylacrylamide) brushes. The functionalized graphene is micropatterned and self-folds into ordered 3D structures with reversible deformation under a full control by temperature. The structures are characterized using spectroscopy and microscopy, and self-folding is rationalized using a multiscale molecular dynamics model. Our work demonstrates the potential to design and fabricate ordered 3D graphene structures with predictable shape and dynamics. We highlight applicability by encapsulating live cells and creating nonlinear resistor and creased transistor devices.