Enhanced chemical reactions of oxygen at grain boundaries in polycrystalline graphene

Highly Stretchable Graphene Scrolls Transistors for Self-Powered Tribotronic Non-Mechanosensation Application

Owing to highly desired requirements in advanced disease diagnosis, therapy, and health monitoring, noncontact mechanosensation active matrix has drawn considerable attention. To satisfy the practical demands of high energy efficiency, in this report, combining the advantage of multiparameter monitoring, high sensitivity, and high resolution of active matrix field-effect transistor (FET) with triboelectric nanogenerators (TENG), we successfully developed the tribotronic mechanosensation active matrix based on tribotronic ion gel graphene scrolls field-effect transistors (GSFET). The tribopotential produced by TENG served as a gate voltage to modulate carrier transport along the semiconductor channel and realized self-powered ability with considerable decreased energy consumption. To achieve high spatial utilization and more pronounced responsivity of the dielectric of this transistor, ion gel was used to act as a triboelectric layer to conduct friction and contact electrification with external materials directly to produce triboelectric charges to power GFET. This tribopotential-driving device has excellent tactile sensing properties with high sensitivity (1.125 mm^-1), rapid response time (~16 ms), and a durability operation of thousands of cycles. Furthermore, the device was transparent and flexible with the capability of spatially mapping touch stimuli and monitoring real-time temperature. Due to all these unique characteristics, this novel noncontact mechanosensation GSFET active matrix provided a new method for self-powered E-skin with promising potential for self-powered wearable devices and intelligent robots.

Grain-Boundary-Induced Ultrasensitive Molecular Detection of Graphene Film

Graphene has been considered a promising platform for molecular detection due to the graphene-enhanced Raman scattering (GERS) effect. However, the GERS performance of pristine graphene is limited by a low chemically active surface and insufficient density of states (DOS). Although diverse defects have been introduced, it remains a great challenge to improve the enhancement performance. Here, we show that graphene grain boundaries (GBs) possess stronger adsorption capacity and more abundant DOS. Thus, GERS performance increases with the atomic percentage of GBs, which makes nanocrystalline graphene (NG) film a superior GERS substrate. For R6G as a probe molecule, a low detection limit of 3 × 10^-10 M was achieved. Utilizing the high chemical activity of GBs, we also fabricated NG film decorated with Au particles using a one-step quenching strategy, and this hybrid film exhibits an extremely low limit of detection down to 5 × 10^-11 M, outperforming all the reported graphene-based systems.

Flexible Full-Surface Conformal Encapsulation for Each Fiber in Graphene Glass Fiber Fabric against Thermal Oxidation

Encapsulation for carbon-based electronic devices against oxidation can enhance their long-term working stability. Graphene glass fiber fabric (GGFF), as an advanced flexible electrothermal material, also struggles with graphene oxidation. The flexible, full-surface, conformal encapsulation for each fiber in the large-area fabric puts forward high requirements for encapsulating materials and techniques. Herein, the nanometer-thick h-BN layer was in situ grown on cambered surfaces of each fiber in GGFF with the chemical vapor deposition method. Stable heating duration (500 °C) of h-BN-encapsulated GGFF (h-BN/GGFF) was increased by 1 order of magnitude without compromising the electrothermal performances and flexibility. Theoretical simulations revealed that the enhanced oxidation resistance of h-BN/GGFF was attributed to the decreased interaction and adsorption life of oxygen. The proposed flexible, full-surface, conformal encapsulation technique targeting the fiber-shaped graphene electrothermal device is scalable and can be extended to the other carbon materials, even devices with intricate shapes, which will promote the development of flexible electronics.

Growth and Grain Boundaries in 2D Materials

Grain boundaries (GBs) are a kind of lattice imperfection widely existing in two-dimensional materials, playing a critical role in materials' properties and device performance. Related key issues in this area have drawn much attention and are still under intense investigation. These issues include the characterization of GBs at different length scales, the dynamic formation of GBs during the synthesis, the manipulation of the configuration and density of GBs for specific material functionality, and the understanding of structure-property relationships and device applications. This review will provide a general introduction of progress in this field. Several techniques for characterizing GBs, such as direct imaging by high-resolution transmission electron microscopy, visualization techniques of GBs by optical microscopy, plasmon propagation, or second harmonic generation, are presented. To understand the dynamic formation process of GBs during the growth, a general geometric approach and theoretical consideration are reviewed. Moreover, strategies controlling the density of GBs for GB-free materials or materials with tunable GB patterns are summarized, and the effects of GBs on materials' properties are discussed. Finally, challenges and outlook are provided.

Ultrafast Transition of Nonuniform Graphene to High-Quality Uniform Monolayer Films on Liquid Cu

It is essentially important to synthesize uniform graphene films with a controlled number of layers because their properties strongly depend on the number of layers. Although chemical vapor deposition (CVD) on Cu has been widely used to synthesize large-area graphene films, the growth on solid and liquid Cu (L-Cu) suffers from poor thickness uniformity with a great number of adlayers and difficulty in forming continuous films even after a long growth time of hours, respectively. Here, we found that nonuniform graphene films initially grown on solid Cu (S-Cu) foil can rapidly transform into continuously uniform monolayer graphene film on L-Cu within 3 min. Moreover, the films obtained show larger grain size, higher quality, better optical and electrical properties, and better performance in organic light-emitting diode applications than the original films grown on S-Cu foil. By using carbon isotope labeling, we revealed that the multilayer-to-monolayer transition of graphene on L-Cu experiences etching-"self-aligning"-coalescence processes. This two-step CVD method not only opens up a new way for the rapid growth of uniform monolayer graphene films but also provides helpful information for the controlled growth of uniform monolayers of other 2D materials such as monolayer h-BN.

The impact of substrate surface defects on the properties of two-dimensional van der Waals heterostructures

The recent emergence of vertically stacked van der Waals (vdW) heterostructures provides new opportunities for these materials to be employed in a wide range of novel applications. Understanding the interlayer coupling in the stacking geometries of the heterostructures and its effect on the resultant material properties is particularly important for obtaining materials with desirable properties. Here, we report that the atomic bonding between stacked layers and thereby the interlayer properties of the vdW heterostructures can be well tuned by the substrate surface defects using WS2 flakes directly grown on graphene. We show that the defects of graphene have no significant effect on the crystal structure or the quality of the grown WS2 flakes; however, they have a strong influence on the interlayer interactions between stacked layers, thus affecting the layer deformability, thermal stability, and physical and electrical properties. Our experimental and computational investigations also reveal that WS2 flakes grown on graphene defects form covalent bonds with the underlying graphene via W atomic bridges (i.e., formation of larger overlapping hybrid orbitals), enabling these flakes to exhibit different intrinsic properties, such as higher conductivity and improved contact characteristics than heterostructures that have vdW interactions with graphene. This result emphasizes the importance of understanding the interlayer coupling in the stacking geometries and its correlation effect for designing desirable properties.

Graphene as a thin-film catalyst booster: graphene-catalyst interface plays a critical role

Due to its extreme thinness, graphene can transmit some surface properties of its underlying substrate, a phenomenon referred to as graphene transparency. Here we demonstrate the application of the transparency of graphene as a protector of thin-film catalysts and a booster of their catalytic efficiency. The photocatalytic degradation of dye molecules by ZnO thin films was chosen as a model system. A ZnO thin film coated with monolayer graphene showed greater catalytic efficiency and long-term stability than did bare ZnO. Interestingly, we found the catalytic efficiency of the graphene-coated ZnO thin film to depend critically on the nature of the bottom ZnO layer; graphene transferred to a relatively rough, sputter-coated ZnO thin film showed rather poor catalytic degradation of the dye molecules while a smooth sol-gel-synthesized ZnO covered with monolayer graphene showed enhanced catalytic degradation. Based on a systematic investigation of the interface between graphene and ZnO thin films, we concluded the transparency of graphene to be critically dependent on its interface with a supporting substrate. Graphene supported on an atomically flat substrate was found to efficiently transmit the properties of the substrate, but graphene suspended on a substrate with a rough nanoscale topography was completely opaque to the substrate properties. Our experimental observations revealed the morphology of the substrate to be a key factor affecting the transparency of graphene, and should be taken into account in order to optimally apply graphene as a protector of catalytic thin films and a booster of their catalysis.

Oxidation behavior of graphene-coated copper at intrinsic graphene defects of different origins

The development of ultrathin barrier films is vital to the advanced semiconductor industry. Graphene appears to hold promise as a protective coating; however, the polycrystalline and defective nature of engineered graphene hinders its practical applications. Here, we investigate the oxidation behavior of graphene-coated Cu foils at intrinsic graphene defects of different origins. Macro-scale information regarding the spatial distribution and oxidation resistance of various graphene defects is readily obtained using optical and electron microscopies after the hot-plate annealing. The controlled oxidation experiments reveal that the degree of structural deficiency is strongly dependent on the origins of the structural defects, the crystallographic orientations of the underlying Cu grains, the growth conditions of graphene, and the kinetics of the graphene growth. The obtained experimental and theoretical results show that oxygen radicals, decomposed from water molecules in ambient air, are effectively inverted at Stone–Wales defects into the graphene/Cu interface with the assistance of facilitators.

Graphene holds promise as a protective coating; however, lattice defects may hinder its practical applicability. Here, the authors investigate the oxidation behavior of graphene-coated copper foils and unveil the interplay between structural defects and oxygen radicals from water molecules in ambient air.

Ultratransparent and stretchable graphene electrodes

Two-dimensional materials, such as graphene, are attractive for both conventional semiconductor applications and nascent applications in flexible electronics. However, the high tensile strength of graphene results in fracturing at low strain, making it challenging to take advantage of its extraordinary electronic properties in stretchable electronics. To enable excellent strain-dependent performance of transparent graphene conductors, we created graphene nanoscrolls in between stacked graphene layers, referred to as multilayer graphene/graphene scrolls (MGGs). Under strain, some scrolls bridged the fragmented domains of graphene to maintain a percolating network that enabled excellent conductivity at high strains. Trilayer MGGs supported on elastomers retained 65% of their original conductance at 100% strain, which is perpendicular to the direction of current flow, whereas trilayer films of graphene without nanoscrolls retained only 25% of their starting conductance. A stretchable all-carbon transistor fabricated using MGGs as electrodes exhibited a transmittance of >90% and retained 60% of its original current output at 120% strain (parallel to the direction of charge transport). These highly stretchable and transparent all-carbon transistors could enable sophisticated stretchable optoelectronics.

Continuous and ultrathin platinum films on graphene using atomic layer deposition: a combined computational and experimental study

Integrating metals and metal oxides with graphene is key in utilizing its extraordinary material properties that are ideal for nanoelectronic and catalyst applications. Atomic layer deposition (ALD) has become a key technique for depositing ultrathin, conformal metal(oxide) films. ALD of metal(oxide) films on graphene, however, remains a genuine challenge due to the chemical inertness of graphene. In this study we address this issue by combining first-principles density functional theory (DFT) simulations with ALD experiments. The focus is on the Pt ALD on graphene, as this hybrid system is very promising for solar and fuel cells, hydrogen technologies, microreactors, and sensors. Here we elucidate the surface reactions underpinning the nucleation stage of Pt ALD on pristine, defective and functionalized graphenes. The employed reaction mechanism clearly depends on (a) the available surface groups on graphene, and (b) the ligands accompanying the metal centre in the precursor. DFT calculations also indicate that graphene oxide (GO) can afford a stronger adsorption of MeCpPtMe₃, unlike Pt(acac)₂, as compared to bare (non-functionalized) graphene, suggesting that GO monolayers are effective Pt ALD seed layers. Confirming the latter, we evince that wafer-scale, continuous Pt films can indeed be grown on GO monolayers using a thermal ALD process with MeCpPtMe₃ and O₂ gas. Besides, the current in-depth atomistic insights are of practical use for understanding similar ALD processes of other metals and metal oxides on graphene.

Bright-field Nanoscopy: Visualizing Nano-structures with Localized Optical Contrast Using a Conventional Microscope

Most methods for optical visualization beyond the diffraction limit rely on fluorescence emission by molecular tags. Here, we report a method for visualization of nanostructures down to a few nanometers using a conventional bright-field microscope without requiring additional molecular tags such as fluorophores. The technique, Bright-field Nanoscopy, is based on the strong thickness dependent color of ultra-thin germanium on an optically thick gold film. We demonstrate the visualization of grain boundaries in chemical vapour deposited single layer graphene and the detection of single 40 nm Ag nanoparticles. We estimate a size detection limit of about 2 nm using this technique. In addition to visualizing nano-structures, this technique can be used to probe fluid phenomena at the nanoscale, such as transport through 2D membranes. We estimated the water transport rate through a 1 nm thick polymer film using this technique, as an illustration. Further, the technique can also be extended to study the transport of specific ions in the solution. It is anticipated that this technique will find use in applications ranging from single-nanoparticles resolved sensing to studying nanoscale fluid-solid interface phenomena.

Native defects in ultra-high vacuum grown graphene islands on Cu(1 1 1)

We present a scanning tunneling microscopy (STM) study of native defects in graphene islands grown by ultra-high vacuum decomposition of ethylene on Cu(1 1 1). We characterize these defects through a survey of their apparent heights, atomic-resolution imaging, and detailed tunneling spectroscopy. Bright defects that occur only in graphene regions are identified as C site point defects in the graphene lattice and are most likely single C vacancies. Dark defect types are observed in both graphene and Cu regions, and are likely point defects in the Cu surface. We also present data showing the importance of bias and tip termination to the appearance of the defects in STM images and the ability to achieve atomic resolution. Finally, we present tunneling spectroscopy measurements probing the influence of point defects on the local electronic landscape of graphene islands.

Evolution, kinetics, energetics, and environmental factors of graphene degradation on silicon dioxide

Recent studies have qualitatively shown that the oxidative stability of monolayer graphene integrated on oxides is relatively poor. Here, the evolution, kinetics, and energetics of this degradation are quantified. Specifically, the deterioration of graphene on SiO2 is studied in grain interiors and at grain boundaries in ambient air, dry air and nitrogen between 473 and 673 K, using spatially and temporally resolved in situ Raman spectroscopy in addition to electron microscopy and charge transport measurements. The grain interiors of chemical vapor deposition (CVD) grown graphene monolayers oxidize with an activation energy of 0.63 ± 0.05 eV in ambient air (15,000 ppm H2O). This energy increases to 1.85 ± 0.17 eV in dry air, whereas degradation is immeasurable in nitrogen and for multilayers even in ambient air. Gasification at grain boundaries in a CVD monolayer proceeds at a rate of (1.08 ± 0.02) × 10(-1) nm s(-1) at 673 K with an activation energy E(A) = 1.14 ± 0.10 eV in ambient air. The more facile degradation of the monolayer grain interiors in ambient air indicates the role of the substrate in decreasing the stability against oxidation. The electrical transport mobility decays with an activation rate similar to that of grain interiors. These results can be used to quantitatively predict graphene oxidation and gasification on SiO2 in different environments and temperatures.

Selective metal deposition at graphene line defects by atomic layer deposition

One-dimensional defects in graphene have a strong influence on its physical properties, such as electrical charge transport and mechanical strength. With enhanced chemical reactivity, such defects may also allow us to selectively functionalize the material and systematically tune the properties of graphene. Here we demonstrate the selective deposition of metal at chemical vapour deposited graphene's line defects, notably grain boundaries, by atomic layer deposition. Atomic layer deposition allows us to deposit Pt predominantly on graphene's grain boundaries, folds and cracks due to the enhanced chemical reactivity of these line defects, which is directly confirmed by transmission electron microscopy imaging. The selective functionalization of graphene defect sites, together with the nanowire morphology of deposited Pt, yields a superior platform for sensing applications. Using Pt-graphene hybrid structures, we demonstrate high-performance hydrogen gas sensors at room temperature and show its advantages over other evaporative Pt deposition methods, in which Pt decorates the graphene surface non-selectively.