Electroluminescence from pure resonant states in hBN-based vertical tunneling junctions.

Defect centers in wide-band-gap crystals have garnered interest for their potential in applications among optoelectronic and sensor technologies. However, defects embedded in highly insulating crystals, like diamond, silicon carbide, or aluminum oxide, have been notoriously difficult to excite electrically due to their large internal resistance. To address this challenge, we realized a new paradigm of exciting defects in vertical tunneling junctions based on carbon centers in hexagonal boron nitride (hBN).

2D Electrolytes: Theory, Modeling, Synthesis, and Characterization.

A class of compounds sharing the properties of 2D materials and electrolytes, namely 2D electrolytes is described theoretically and demonstrated experimentally. 2D electrolytes dissociate in different solvents, such as water, and become electrically charged. The chemical and physical properties of these compounds can be controlled by external factors, such as pH, temperature, electric permittivity of the medium, and ionic concentration.

Mechanical Properties of Atomically Thin Tungsten Dichalcogenides: WS, WSe, and WTe.

Two-dimensional (2D) tungsten disulfide (WS), tungsten diselenide (WSe), and tungsten ditelluride (WTe) draw increasing attention due to their attractive properties deriving from the heavy tungsten and chalcogenide atoms, but their mechanical properties are still mostly unknown. Here, we determine the intrinsic and air-aged mechanical properties of mono-, bi-, and trilayer (1-3L) WS, WSe, and WTe using a complementary suite of experiments and theoretical calculations. High-quality 1L WS has the highest Young's modulus (302.

In situ manipulation of van der Waals heterostructures for twistronics.

In van der Waals heterostructures, electronic bands of two-dimensional (2D) materials, their nontrivial topology, and electron-electron interactions can be markedly changed by a moiré pattern induced by twist angles between different layers. This process is referred to as twistronics, where the tuning of twist angle can be realized through mechanical manipulation of 2D materials. Here, we demonstrate an experimental technique that can achieve in situ dynamical rotation and manipulation of 2D materials in van der Waals heterostructures.

Piezoelectricity in Monolayer Hexagonal Boron Nitride.

2D hexagonal boron nitride (hBN) is a wide-bandgap van der Waals crystal with a unique combination of properties, including exceptional strength, large oxidation resistance at high temperatures, and optical functionalities. Furthermore, in recent years hBN crystals have become the material of choice for encapsulating other 2D crystals in a variety of technological applications, from optoelectronic and tunneling devices to composites. Monolayer hBN, which has no center of symmetry, is predicted to exhibit piezoelectric properties, yet experimental evidence is lacking.

A Force-Engineered Lint Roller for Superclean Graphene.

Contamination is a major concern in surface and interface technologies. Given that graphene is a 2D monolayer material with an extremely large surface area, surface contamination may seriously degrade its intrinsic properties and strongly hinder its applicability in surface and interfacial regions. However, large-scale and facile treatment methods for producing clean graphene films that preserve its excellent properties have not yet been achieved.

The Worldwide Graphene Flake Production.

There are hundreds of companies worldwide claiming to produce "graphene," showing a large variation in its properties. A systematic and reliable protocol is developed to test graphene quality using electron microscopy, atomic force microscopy, Raman spectroscopy, elemental analysis, X-ray photoelectron spectrometry, and scanning and transmission electron microscopy, which is used to study graphene from 60 producers. The statistical nature of the liquid-phase exfoliation of graphite is established.

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe.

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere.

Ultrasensitive gas detection of large-area boron-doped graphene.

Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce.

Hydrogenation of Graphene by Reaction at High Pressure and High Temperature.

The chemical reaction between hydrogen and purely sp(2)-bonded graphene to form graphene's purely sp(3)-bonded analogue, graphane, potentially allows the synthesis of a much wider variety of novel two-dimensional materials by opening a pathway to the application of conventional chemistry methods in graphene. Graphene is currently hydrogenated by exposure to atomic hydrogen in a vacuum, but these methods have not yielded a complete conversion of graphene to graphane, even with graphene exposed to hydrogen on both sides of the lattice. By heating graphene in molecular hydrogen under compression to modest high pressure in a diamond anvil cell (2.

A Facile Route for Patterned Growth of Metal-Insulator Carbon Lateral Junction through One-Pot Synthesis.

Precise graphene patterning is of critical importance for tailor-made and sophisticated two-dimensional nanoelectronic and optical devices. However, graphene-based heterostructures have been grown by delicate multistep chemical vapor deposition methods, limiting preparation of versatile heterostructures. Here, we report one-pot synthesis of graphene/amorphous carbon (a-C) heterostructures from a solid source of polystyrene via selective photo-cross-linking process.

Electron transfer kinetics on natural crystals of MoS2 and graphite.

Here, we evaluate the electrochemical performance of sparsely studied natural crystals of molybdenite and graphite, which have increasingly been used for fabrication of next generation monolayer molybdenum disulphide and graphene energy storage devices. Heterogeneous electron transfer kinetics of several redox mediators, including Fe(CN)6(3-/4-), Ru(NH3)6(3+/2+) and IrCl6(2-/3-) are determined using voltammetry in a micro-droplet cell. The kinetics on both materials are studied as a function of surface defectiveness, surface ageing, applied potential and illumination.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems.

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries.

Growth dynamics and gas transport mechanism of nanobubbles in graphene liquid cells.

Formation, evolution and vanishing of bubbles are common phenomena in nature, which can be easily observed in boiling or falling water, carbonated drinks, gas-forming electrochemical reactions and so on. However, the morphology and the growth dynamics of the bubbles at nanoscale have not been fully investigated owing to the lack of proper imaging tools that can visualize nanoscale objects in the liquid phase. Here, we demonstrate for the first time that the nanobubbles in water encapsulated by graphene membrane can be visualized by in-situ ultra-high vacuum transmission electron microscopy.

Electron transfer kinetics on mono- and multilayer graphene.

Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized. Here we use voltammetric determination of the electron transfer rate for three redox mediators, ferricyanide, hexaammineruthenium, and hexachloroiridate (Fe(CN)(6)(3-), Ru(NH3)(6)(3+), and IrCl(6)(2-), respectively), to measure the reactivity of graphene samples prepared by mechanical exfoliation of natural graphite. Electron transfer rates are measured for varied number of graphene layers (1 to ca.

Electromechanical sensing of substrate charge hidden under atomic 2D crystals.

The functionality of graphene and other two-dimensional materials in electronic devices is highly influenced by the film-substrate charge transfer affecting local carrier density. We demonstrate that charges buried under the few layer graphene on/in the insulating substrate can be detected using electromechanical actuation of the conductive atomically thin layers, allowing measurements of areal density of film-substrate transferred charges under few layer graphene and MoS2 suspended films.

Thermal properties of graphene-copper-graphene heterogeneous films.

We demonstrated experimentally that graphene-Cu-graphene heterogeneous films reveal strongly enhanced thermal conductivity as compared to the reference Cu and annealed Cu films. Chemical vapor deposition of a single atomic plane of graphene on both sides of 9 μm thick Cu films increases their thermal conductivity by up to 24% near room temperature. Interestingly, the observed improvement of thermal properties of graphene-Cu-graphene heterofilms results primarily from the changes in Cu morphology during graphene deposition rather than from graphene's action as an additional heat conducting channel.

Control of radiation damage in MoS(2) by graphene encapsulation.

Recent dramatic progress in studying various two-dimensional (2D) atomic crystals and their heterostructures calls for better and more detailed understanding of their crystallography, reconstruction, stacking order, etc. For this, direct imaging and identification of each and every atom is essential. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are ideal and perhaps the only tools for such studies.

Graphene reknits its holes.

Nanoholes, etched under an electron beam at room temperature in single-layer graphene sheets as a result of their interaction with metal impurities, are shown to heal spontaneously by filling up with either nonhexagon, graphene-like, or perfect hexagon 2D structures. Scanning transmission electron microscopy was employed to capture the healing process and study atom-by-atom the regrown structure. A combination of these nanoscale etching and reknitting processes could lead to new graphene tailoring approaches.

Direct experimental evidence of metal-mediated etching of suspended graphene.

Atomic resolution high angle annular dark field imaging of suspended, single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al, and Au atoms had been deposited, was carried out in an aberration-corrected scanning transmission electron microscope. In combination with electron energy loss spectroscopy, employed to identify individual impurity atoms, it was shown that nanoscale holes were etched into graphene, initiated at sites where single atoms of all the metal species except for gold come into close contact with the graphene. The e-beam scanning process is instrumental in promoting metal atoms from clusters formed during the original metal deposition process onto the clean graphene surface, where they initiate the hole-forming process.

Scanning tunnelling microscopy of suspended graphene.

Suspended graphene has been studied by STM for the first time. Atomic resolution on mono- and bi-layer graphene samples has been obtained after ridding the graphene surface of contamination via high-temperature annealing. Static local corrugations (ripples) have been observed on both types of structures.

Interaction of Metals with Suspended Graphene Observed by Transmission Electron Microscopy.

In this Perspective, we present an overview of how different metals interface with suspended graphene, providing a closer look into the metal-graphene interaction by employing high-resolution transmission electron microscopy, especially using high-angle dark field imaging. All studied metals favor sites on the omnipresent hydrocarbon surface contamination rather than on the clean graphene surface and present nonuniform distributions, which never result in continuous films but instead in clusters or nanocrystals, indicating a weak interaction between the metal and graphene. This behavior can be altered to some degree by surface pretreatment (hydrogenation) and high-temperature vacuum annealing.

Metal-graphene interaction studied via atomic resolution scanning transmission electron microscopy.

Distributions and atomic sites of transition metals and gold on suspended graphene were investigated via high-resolution scanning transmission electron microscopy, especially using atomic resolution high angle dark field imaging. All metals, albeit as singular atoms or atom aggregates, reside in the omni-present hydrocarbon surface contamination; they do not form continuous films, but clusters or nanocrystals. No interaction was found between Au atoms and clean single-layer graphene surfaces, i.

Interaction between metal and graphene: dependence on the layer number of graphene.

The interaction between graphene and metal was investigated by studying the G band splitting in surface-enhanced Raman scattering (SERS) spectra of single-, bi-, and trilayer graphene. The Ag deposition on graphene induced large enhancement of the Raman signal of graphene, indicating SERS of graphene. In particular, the G band was split into two distinct peaks in the SERS spectrum of graphene.