The extreme sensitivity of 2D materials to defects and nanostructure requires precise imaging techniques to verify presence of desirable and absence of undesirable features in the atomic geometry. Helium-ion beams have emerged as a promising materials imaging tool, achieving up to 20 times higher resolution and 10 times larger depth-of-field than conventional or environmental scanning electron microscopes. Here, we offer first-principles theoretical insights to advance ion-beam imaging of atomically thin materials by performing real-time time-dependent density functional theory simulations of single impacts of 10-200 keV light ions in free-standing graphene. We predict that detecting electrons emitted from the back of the material (the side from which the ion exits) would result in up to 3 times higher signal and up to 5 times higher contrast images, making 2D materials especially compelling targets for ion-beam microscopy. We also find that the charge induced in the graphene equilibrates on a sub-fs time scale, leading to only slight disturbances in the carbon lattice that are unlikely to damage the atomic structure for any of the beam parameters investigated here.
At a single atom thick, it is challenging to distinguish graphene from its substrate using conventional techniques. In this paper we show that friction force microscopy (FFM) is a simple and quick technique for identifying graphene on a range of samples, from growth substrates to rough insulators. We show that FFM is particularly effective for characterising graphene grown on copper where it can correlate the graphene growth to the three-dimensional surface topography and map the crystallographic orientation of the graphene nondestructively, reproducibly and at high resolution. We expect FFM t…
Non-destructive and rapid evaluation of graphene directly on the growth substrate (Cu foils) by dark field (DF) optical microscopy is demonstrated. Without any additional treatment, graphene on Cu foils with various coverages can be quickly identified by DF imaging immediately after chemical vapor deposition growth with contrast comparable to scanning electron microscopy. The improved contrast of DF imaging compared to bright field optical imaging was found to be due to Rayleigh scattering of light by the copper steps beneath graphene. Indeed, graphene adlayers are readily distinguished, due t…
For the first time, the magnetic force microscopy (MFM) is used to characterize the mechanically-exfoliated single- and few-layer MoS2 and graphene nanosheets. By analysis of the phase and amplitude shifts, the magnetic response of MoS2 and graphene nanosheets exhibits the dependence on their layer number. However, the solution-processed single-layer MoS2 nanosheet shows the reverse magnetic signal to the mechanically-exfoliated one, and the graphene oxide nanosheet has not shown any detectable magnetic signal. Importantly, graphene and MoS2 flakes become nonmagnetic when they exceed a certain…
Graphene grown epitaxially on SiC has been proposed as a material for carbon-based electronics. Understanding the interface between graphene and the SiC substrate will be important for future applications. We report the ability to image the interface structure beneath single-layer graphene using scanning tunneling microscopy. Such imaging is possible because the graphene appears transparent at energies of 1 eV above or below the Fermi energy. Our analysis of calculations based on density functional theory shows how this transparency arises from the electronic structure of a graphene layer on a…
We present numerical simulations of scattering-type Scanning Near-Field Optical Microscopy (s-SNOM) of 1D plasmonic graphene junctions. A comprehensive analysis of simulated s-SNOM spectra is performed for three types of junctions. We find conditions when the conventional interpretation of the plasmon reflection coefficients from s-SNOM measurements does not apply. Our results are applicable to other conducting 2D materials and provide a comprehensive understanding of the s-SNOM techniques for probing local transport properties of 2D materials.
We report measurements of the electronic structure and surface morphology of exfoliated graphene on an insulating substrate using angle-resolved photoemission and low energy electron diffraction. Our results show that although exfoliated graphene is microscopically corrugated, the valence band retains a massless fermionic dispersion, with a Fermi velocity of ~10^6 m/s. We observe a close relationship between the morphology and electronic structure, which suggests that controlling the interaction between graphene and the supporting substrate is essential for graphene device applications.
The graphene moire structures on metals, as they demonstrate both long (moire) and short (atomic) scale ordered structures, are the ideal systems for the application of scanning probe methods. Here we present the complex studies of the graphene/Ir(111) system by means of 3D scanning tunnelling and atomic force microscopy/spectroscopy as well as Kelvin-probe force microscopy. All results clearly demonstrate a variation of the moire and atomic scale contrasts as a function of the bias voltage as well as the distance between the scanning probe and the sample, allowing to discriminate between topo…
We have used scanning tunneling microscopy to study the structure of graphene islands on Au(111) grown by deposition of elemental carbon at 950°C. Consistent with low-energy electron microscopic observations, we find that the graphene islands have dendritic shapes. The islands tend to cover depressed regions of the Au surface, suggesting that Au is displaced as the graphene grows. If small tunneling currents are used, it is possible to image simultaneously the graphene/Au moiré and the Au herringbone reconstruction, which forms underneath the graphene on cooling from the growth temperature. Th…
In electron microscopy, charging of non-conductive biological samples by focused electron beams hinders their high-resolution imaging. Gold or platinum coatings have been commonly used to prevent such sample charging, but it disables further quantitative and qualitative chemical analyses by energy dispersive spectroscopy (EDS). Here we report that graphene-coating on biological samples enables non-destructive high-resolution imaging by scanning electron microscopy (SEM) as well as chemical analysis by EDS, utilizing graphene's transparency to electron beams, high conductivity, outstanding mech…
Graphene based sheets have stimulated great interest due to their superior mechanical, electrical and thermal properties. A general visualization method that allows quick observation of these single atomic layers would be highly desirable as it can greatly facilitate sample evaluation and manipulation, and provide immediate feedback to improve synthesis and processing strategies. Here we report that graphene based sheets can be made highly visible under a fluorescence microscope by quenching the emission from a dye coating, which can be conveniently removed afterwards by rinsing without disrup…
CVD grown graphene used in (scanning) transmission electron microscopy ((S)TEM) studies must undergo a careful transfer of the one-atom-thick membrane from the growth surface (typically a Cu foil) to the TEM grid. During this transfer process, the graphene invariably becomes contaminated with foreign material. This contamination proves to be very problematic in the (S)TEM because often >95% of the graphene is obscured and imaging of the pristine areas results in e-beam-induced hydrocarbon deposition which further acts to obscure the desired imaging area. In this article, we examine two cleanin…
Tailoring the interlayer twist angle of bilayer graphene (BLG) has a significant influence on its electronic properties, including superconductivity, topological transitions, ferromagnetic states and correlated insulating states. These exotic electronic properties are sensitively dependent on the work functions of bilayer graphene samples. Here, the twisted angle-dependent work functions of CVD-grown twisted bilayer graphene (tBLG) are detailed investigated by Kelvin Probe Force Microscopy (KPFM) in combination with Raman spectra. The thickness-dependent surface potentials of Bernal-stacked mu…
We present an investigation of the atomic and electronic structure of graphene monolayer islands on the 6H-SiC(000-1)(3x3) (SiC(3x3)) surface reconstruction using scanning tunneling microscopy (STM) and spectroscopy (STS). The orientation of the graphene lattice changes from one island to the other. In the STM images, this rotational disorder gives rise to various superlattices with periods in the nm range. We show that those superlattices are moiré patterns (MPs) and we correlate their apparent height with the stacking at the graphene/SiC(3x3) interface. The contrast of the MP in STM images c…
Graphene oxide (GO) is one of chemically modified graphenes and has been extensively investigated worldwide. A monolayer sheet of GO can be chemically produced in solution and it has been expected to be adaptable to a wide variety of applications. Most of works have focused on fabrication and reduction processes of GO, while its intrinsic property has not been explored in detail. We have succeeded in fabricating large scale GO nanosheets reproducibly. In the present work, we observed GO nanosheets by conductive atomic force microscopy. The current image is useful to monitor the GO thickness as…
Epitaxial graphene grown on transition metal surfaces typically exhibits a moiré pattern due to the lattice mismatch between graphene and the underlying metal surface. We use both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) experiments to probe the electronic and topographic contrast of the graphene moiré on the Ir(111) surface. While STM topography is influenced by the local density of states close to the Fermi energy and the local tunneling barrier height, AFM is capable of yielding the 'true' surface topography once the background force arising from the van der Waa…
Graphene on copper is a system of high technological relevance, as Cu is one of the most widely used substrates for the CVD growth of graphene. However, very little is known about the details of their interaction. One approach to gain such information is studying the superlattices emerging due to the mismatch of the two crystal lattices. However, graphene on copper is a low-corrugated system making both their experimental and theoretical study highly challenging. Here, we report the observation of a new rotational Moire superlattice of CVD graphene on Cu (111), characterized by a periodicity o…
We report the synthesis of single and bi layer graphene films by low pressure chemical vapor deposition technique on Cu and Au substrates. The as grown films were characterized by transmission electron microscopy, scanning electron microscopy and Raman spectroscopy techniques. The large lateral area graphene deposited on Cu can easily be transferred on Si SiO2. In the case of Au substrate both the adsorption and diffusion-precipitation leads to the growth of graphene.
Atomically resolved images of monolayer organic crystals have only been obtained with scanning probe methods so far. On the one hand, they are usually prepared on surfaces of bulk materials, which are not accessible by (scanning) transmission electron microscopy. On the other hand, the critical electron dose of a monolayer organic crystal is orders of magnitudes lower than the one for bulk crystals, making (scanning) transmission electron microscopy characterization very challenging. In this work we present an atomically resolved study on the dynamics of a monolayer CuPcCl\textsubscript{16} cr…
Due to its ultrahigh electron transmissivity in a wide electron energy range, molecular impermeability, high electrical conductivity and excellent mechanical stiffness the suspended graphene membranes appear to be a nearly ideal window material for in situ (in vivo) environmental electron microscopy of nano- and mesoscopic objects (including bio-medical samples) immersed in liquids and/or in dense gaseous media. In this communication, taking advantage of little modification of the graphene transfer protocol on to metallic and SiN supporting orifices, the reusable environmental cells with excha…
We report on detailed microscopy studies of graphene and few-layer-graphene produced by mechanical exfoliation on various semi-conducting substrates. We demonstrate the possibility to prepare and analyze graphene on (001)-GaAs, manganese p-doped (001)-GaAs and InGaAs substrates. The morphology of graphene on these substrates was investigated by scanning electron and atomic force microscopy and compared to layers on silicon oxide. It was found that graphene sheets strongly follow the texture of the sustaining substrates independent on doping, polarity or roughness. Furthermore resist residues e…
We have developed a method for depositing graphene monolayers and bilayers with minimum lateral dimensions of 2-10 nm by the mechanical exfoliation of graphite onto the Si(100)-2x1:H surface. Room temperature, ultra-high vacuum (UHV) tunnelling spectroscopy measurements of nanometer-sized single-layer graphene reveal a size dependent energy gap ranging from 0.1-1 eV. Furthermore, the number of graphene layers can be directly determined from scanning tunnelling microscopy (STM) topographic contours. This atomistic study provides an experimental basis for probing the electronic structure of nano…
Transmission electron microscopy has witnessed rampant development and surging point resolution over the past few years. The improved imaging performance of modern electron microscopes shifts the bottleneck for image contrast and resolution to sample preparation. Hence, it is increasingly being realized that the full potential of electron microscopy will only be realized with the optimization of current sample preparation techniques. Perhaps the most recognized issues are background signal and noise contributed by sample supports, sample charging and instability. Graphene provides supports of …
Photoelectron emission microscopy PEEM is a powerful tool to spectroscopically image dynamic surface processes at the nanoscale but is traditionally limited to ultra high or moderate vacuum conditions. Here, we develop a novel grapheme capped multichannel array sample platform that extends the capabilities of photoelectron spectromicroscopy to routine liquid and atmospheric pressure studies with standard PEEM setups. Using this platform, we show that graphene has only a minor influence on the electronic structure of water in the first few layers and thus will allow for the examination of minim…
Amorphous carbon films have been routinely used to enhance the preparation of frozen-hydrated transmission electron microscopy (TEM) samples, either in retaining protein concentration, providing mechanical stability or dissipating sample charge. However, strong background signal from the amorphous carbon support obstructs that of the sample, and the insulating properties of amorphous carbon films preclude any efficiency in dispersing charge. Graphene addresses the limitations of amorphous carbon. Graphene is a crystalline material with virtually no phase or amplitude contrast and unparalleled,…
We examine the transfer of graphene grown by chemical vapor deposition (CVD) with polymer scaffolds of poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), and poly(bisphenol A carbonate) (PC). We find that optimally reactive PC scaffolds provide the cleanest graphene transfers without any annealing, after extensive comparison with optical microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Comparatively, films transferred with PLA, PPA, and PMMA have a two-fold higher roughness and a five-fold higher chemi…
Magnetic force microscopy (MFM) signals have recently been detected from whole pieces of mechanically exfoliated graphene and molybdenum disulfide (MoS2) nanosheets and magnetism of the two nanomaterials was claimed based on these observations. However, non-magnetic interactions or artefacts are commonly associated with MFM signals, which makes the interpretation of MFM signals not straightforward. A systematic investigation has been done to examine possible sources of the MFM signals from graphene and MoS2 nanosheets and whether the MFM signals can be correlated with magnetism. It is found th…
Graphene, a two-dimensional (2D) material with unique electronic properties, appears to be an ideal object for the application of surface-science methods. Among them, a family of scanning probe microscopy methods (STM, AFM, KPFM) and the corresponding spectroscopy add-ons provide information about the structure and electronic properties of graphene on the local scale (from inline imagem to atoms). This review focuses on the recent applications of these microscopic/spectroscopic methods for the investigation of graphene on metals (interfaces, intercalation-like systems, graphene nanoribbons, an…
We have performed scanning gate microscopy (SGM) on graphene field effect transistors (GFET), using a biased metallic nanowire coated with a dielectric layer as a contact mode tip and local top gate. Electrical transport through graphene at various back gate voltages is monitored as a function of tip voltage and tip position. Near the Dirac point, the dependence of graphene resistance on tip voltage shows a significant variation with tip position. SGM imaging reveals mesoscopic domains of electron-doped and hole-doped regions. Our measurements indicate a substantial spatial fluctuation (on the…
We demonstrate the application of graphene as a support for imaging individual biological molecules in transmission electron microscope (TEM). A simple procedure to produce free-standing graphene membranes has been designed. Such membranes are extremely robust and can support practically any sub-micrometer object. Tobacco mosaic virus has been deposited on graphene samples and observed in a TEM. High contrast has been achieved even though no staining has been applied.
Low-energy electron microscopy (LEEM) reveals a new mode of graphene growth on Ru(0001) in which Ru atoms are etched from a step edge and injected under a growing graphene sheet. Based on density functional calculations, we propose a model wherein injected Ru atoms form metastable islands under the graphene. Scanning tunneling microscopy (STM) reveals that dislocation networks exist near step edges, consistent with some of the injected atoms being incorporated into the topmost Ru layer, thereby increasing its density.
Using transient absorption (TA) microscopy as a hot electron thermometer we show disorder-assisted acoustic-phonon supercollisions (SCs) best describes the rate-limiting relaxation step in graphene over a wide range of lattice temperatures ($T_l=$5-300 K), Fermi energies ($E_F=\pm0.35$ eV), and optical probe energies (~0.3 - 1.1 eV). Comparison with simultaneously collected transient photocurrent, an independent hot electron thermometer, confirms the rate-limiting optical and electrical response in graphene are best described by the SC-heat dissipation rate model, $H=A(T^3_e- T^3_l)$. Our data…
Population and distribution of defects is one of the primary parameters controlling materials functionality, are often non-ergodic and strongly dependent on synthesis history, and are rarely amenable to direct theoretical prediction. Here, dynamic electron beam-induced transformations in Si deposited on a graphene monolayer are used to create libraries of the possible Si and carbon vacancy defects. Automated image analysis and recognition based on deep learning networks is developed to identify and enumerate the defects, creating a library of (meta) stable defect configurations. The electronic…
Atomic Force Microscopy (AFM) in the tapping (intermittent contact) mode is a commonly used tool to measure the thickness of graphene and few layer graphene (FLG) flakes on silicon oxide surfaces. It is a convenient tool to quickly determine the thickness of individual FLG films. However, reports from literature show a large variation of the measured thickness of graphene layers. This paper is focused on the imaging mechanism of tapping mode AFM (TAFM) when measuring graphene and FLG thickness and we show that at certain measurement parameters significant deviations can be introduced in the me…
We study the local density of states at and around a substituting impurity and use these results to compute current versus bias characteristic curves of Scanning Tunneling Microscopy (STM) experiments done on the surface of graphene. This allow us to detect the presence of substituting impurities on graphene. The case of vacancies is also analyzed. We find that the shape and magnitude of the STM characteristic curves depend on the position of the tip and on the nature of the defect, with the strength of the binging between the impurity and the carbon atoms playing an important role. Also the n…
The growth of graphene by catalytic decomposition of ethylene on Cu(111) in an ultra-high vacuum system was investigated with low energy electron diffraction, low energy electron microscopy, and atomic force microscopy. Attempts to form a graphene overlayer using ethylene at pressures as high as 10 mTorr and substrate temperatures as high as 900 $^\circ$C resulted in almost no graphene growth. By using an argon overpressure, the growth of epitaxial graphene on Cu(111) was achieved. The suppression of graphene growth without the use of an argon overpressure is attributed to Cu sublimation at el…
Graphene nanoribbons are prepared inside boron nitride nanotubes by liquid phase encapsulation and subsequent annealing of 1,2,4-trichlorobenzene. The product is imaged with high resolution transmission electron microscopy, and characterized by optical absorption and Raman spectroscopy. Carbon-containing material is detected inside the boron nitride nanotubes with energy-dispersive x-ray spectroscopy (EDS) and scanning transmission electron microscopy (STEM). The observed structures twist under the electron beam and the characteristic features of nanoribbons appear in the Raman spectra.
Using scanning tunneling microscopy (STM) in ultra high vacuum and atomic force microscopy, we investigate the corrugation of graphene flakes deposited by exfoliation on a Si/SiO2 (300 nm) surface. While the corrugation on SiO2 is long-range with a correlation length of about 25 nm, some of the graphene monolayers exhibit an additional corrugation with a preferential wave length of about 15 nm. A detailed analysis shows that the long range corrugation of the substrate is also visible on graphene, but with a reduced amplitude, leading to the conclusion that the graphene is partly freely suspend…
Twist, as a simple and unique degree of freedom, could lead to enormous novel quantum phenomena in bilayer graphene. A small rotation angle introduces low-energy van Hove singularities (VHSs) approaching the Fermi level, which result in unusual correlated states in the bilayer graphene. It is reasonable to expect that the twist could also affect the electronic properties of few-layer graphene dramatically. However, such an issue has remained experimentally elusive. Here, by using scanning tunneling microscopy/spectroscopy (STM/STS), we systematically studied a twisted trilayer graphene (TTG) w…
Observing the individual building blocks of matter is one of the primary goals of microscopy. The invention of the scanning tunneling microscope [1] revolutionized experimental surface science in that atomic-scale features on a solid-state surface could finally be readily imaged. However, scanning tunneling microscopy has limited applicability due to restrictions, for example, in sample conductivity, cleanliness, and data aquisition rate. An older microscopy technique, that of transmission electron microscopy (TEM) [2, 3] has benefited tremendously in recent years from subtle instrumentation a…
We investigated the effect of surfactants and their concentration (Csur) on the final graphene concentration (CG) via the liquid-phase exfoliation method. Six typical surfactants including ionic and non-ionic ones were explored and the optimized Csur for each surfactant was suggested. For ionic surfactants, CG increases with Csur before reaching its maximum and then maintains the high level. The different mechanisms of ionic and non-ionic surfactants in stabilizing graphene dispersions are explained by the theory for colloidal stability. The as-prepared graphene sheets are verified to be highl…
Recently, the rhombohedral trilayer graphene (r-TLG) has attracted much attention because of its low-energy flat bands, which are predicted to result in many strongly correlated phenomena. Here, we demonstrate that it is possible to probe the stacking order and electronic spectrum of the r-TLG directly with a scanning tunneling microscopy around a monoatomic step edge of the top graphene layer. The tunneling spectra of the r-TLG exhibit four adjacent peaks, which are generated by the low-energy flat bands, flanking the charge neutrality point. Based on these spectra, the true energy gap and th…
Irradiation with high-energy ions has been widely suggested as a tool to engineer properties of graphene. Experiments show that it indeed has a strong effect on its transport, magnetic and mechanical characteristics. However, to use ion irradiation as an engineering tool requires understanding of the type and detailed characteristics of the produced defects which is still lacking, as the use of high-resolution transmission electron microscopy (HRTEM) - the only technique allowing direct imaging of atomic-scale defects - often modifies or even creates defects during imaging, thus making it impo…
We investigate nanoscale local anodic oxidation (LAO) on hydrogen-intercalated graphene grown by controlled sublimation of silicon carbide (SiC). Scanning probe microscopy (SPM) was used as a lithographic and characterization tool in order to investigate the local properties of the nanofabricated structures. The anomalous thickness observed after the graphene oxidation process is linked to the impact of LAO on the substrate. Micro-Raman spectroscopy was employed to demonstrate the presence of two oxidation regimes depending on the applied bias. We show that partial and total etching of monolay…
Using the recently developed technique of microsoldering, we perform a systematic transport study of the influence of PMMA on graphene flakes revealing a doping effect of up to 3.8x10^12 1/cm^2, but a negligible influence on mobility and gate voltage induced hysteresis. Moreover, we show that the microsoldered graphene is free of contamination and exhibits a very similar intrinsic rippling as has been found for lithographically contacted flakes. Finally, we demonstrate a current induced closing of the previously found phonon gap appearing in scanning tunneling spectroscopy experiments, strongl…
We have used Low Energy Electron Microscopy (LEEM) and Photo Emission Electron Microscopy (PEEM) to study and improve the quality of graphene films grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated temperature already yields graphene sheets that are uniform and of monatomic thickness. Besides domains that are aligned with respect to the substrate, other rotational variants grow. Cyclic growth exploiting the faster growth and etch rates of the rotational variants, yields films that are 99 % composed of aligned domains. Precovering the substrate with a high density of graph…
Machine learning (ML) methods are extraordinarily successful at denoising photographic images. The application of such denoising methods to scientific images is, however, often complicated by the difficulty in experimentally obtaining a suitable expected result as an input to training the ML network. Here, we propose and demonstrate a simulation-based approach to address this challenge for denoising atomic-scale scanning tunneling microscopy (STM) images, which consists of training a convolutional neural network on STM images simulated based on a tight-binding electronic structure model. As mo…
Intercalation is an established technique for tailoring the electronic structure of epitaxial graphene. Moreover, it enables the synthesis of otherwise unstable two-dimensional (2D) layers of elements with unique physical properties compared to their bulk versions due to interfacial quantum confinement. In this work, we present uniformly Pb-intercalated quasi-freestanding monolayer graphene on SiC, which turns out to be essentially charge neutral with an unprecedented $p$-type carrier density of only $(5.5\pm2.5)\times10^9$ cm$^{-2}$. Probing the low-energy electronic structure throughout the …
Li metal has been deposited on the surface of a Ru(0001) single crystal containing patches of monolayer-thick epitaxial graphene islands. The use of low-energy electron microscopy and diffraction allowed us to {\em in situ} monitor the process by measuring the local work function as well as to study the system in real and reciprocal space, comparing the changes taking place on the graphene with those on the bare Ru(0001) surface. It is found that Li deposition decreases the work function of the graphene islands but to a much smaller degree than of the Ru(0001) surface, as corresponds to its in…
We report room temperature scanning tunneling microscopy and spectroscopy study of bilayer graphene prepared by mechanical exfoliation on SiO$_2$/Si surface and electrically contacted with gold pads using a mechanical mask. The bulk conductivity shows contribution from regions of varying electron density indicating significant charge inhomogeneity. Large scale topographic images show ripple like structures with a roughness of $\sim$1nm while the small scale atomic resolution images show graphite-like triangular lattice. Local dI/dV-V tunnel spectra have an asymmetric V-shape with the minima lo…
Graphene, consisting of an inert, thermally stable material with an atomically flat, dangling bond-free surface is by essence an ideal template layer for van der Waals heteroepitaxy of two-dimensional materials such as silicene. However, depending on the synthesis method and growth parameters, graphene (Gr) substrates could exhibit, on a single sample, various surface structures, thicknesses, defects, and step heights. These structures noticeably affect the growth mode of epitaxial layers, e.g. turning the layer-by-layer growth into the Volmer-Weber growth promoted by defect-assisted nucleatio…
Elemental phosphorous is believed to have several stable allotropes that are energetically nearly degenerate, but chemically reactive. To prevent chemical degradation under ambient conditions, these structures may be capped by monolayers of hexagonal boron nitride ({\em h}-BN) or graphene. We perform {\em ab initio} density functional calculations to simulate scanning tunneling microscopy (STM) images of different layered allotropes of phosphorus and study the effect of capping layers on these images. We find that protective monolayers of insulating {\em h}-BN allow to distinguish between the …
Using atomistic computer simulations, we study how ion irradiation can be used to alter the morphology of a graphene monolayer by introducing defects of specific type, and to cut graphene sheets. Based on the results of our analytical potential molecular dynamics simulations, a kinetic Monte Carlo code is developed for modelling morphological changes in a graphene monolayer under irradiation at macroscopic time scales. Impacts of He, Ne, Ar, Kr, Xe and Ga ions with kinetic energies ranging from tens of eV to 10 MeV and angles of incidence between 0\circ and 88\circ are studied. Our results pro…
The interaction of metals with carbon materials, specifically with graphene, is of importance for various technological applications. In particular, intercalation of alkali metals is believed to provide a means for tuning the electronic properties of graphene for device applications. While the macroscopic effects of such intercalation events can readily be studied, following the related processes at an atomic scale in detail and under well-defined experimental conditions constitutes a challenge. Here, we investigate in situ the adsorption and nucleation of the alkali metals K, Cs, and Li on fr…
We report that graphene films with thickness ranging from 1 to 7 layers can be controllably synthesized on the surface of polycrystalline copper by a chemical vapour deposition method. The number of layers of graphene is controlled precisely by regulating the flow ratio of CH4 and H2, the reaction pressure, the temperature and the reaction time. The synthesized graphene films were characterized by scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction and Raman spectroscopy. In addition, the graphene films transferred from copper t…
The fundamental properties of graphene are making it an attractive material for a wide variety of applications. Various techniques have been developed to produce graphene and recently we discovered the synthesis of large area graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also showed that graphene growth on Cu is a surface-mediated process and the films were polycrystalline with domains having an area of tens of square microns. In this paper we report on the effect of growth parameters such as temperature, and methane flow rate and partial pressure on the growth rate, d…
Developing applicable two-dimensional (2D) electrode materials with high performance, especially with high ion storage capacity, has become an ever more obsessive quest in recent years. Based on first-principles calculations, we report that T-graphene, a new carbon-based 2D material, has a record-high Li/Na storage capacity. The capacity of T-graphene is as high as 2233.2 mA h g-1 for Li, and can reach 2357.2 mA h g-1 for Na, which are 6 times as much as that of the commercial graphite and are the highest among 2D anode materials identified so far. We demonstrate that the ultrahigh storage cap…
The application of imaging techniques based on ensembles of nitrogen-vacancy (NV) sensors in diamond to characterise electrical devices has been proposed, but the compatibility of NV sensing with operational gated devices remains largely unexplored. Here we fabricate graphene field-effect transistors (GFETs) directly on the diamond surface and characterise them via NV microscopy. The current density within the gated graphene is reconstructed from NV magnetometry under both mostly p- and n-type doping, but the exact doping level is found to be affected by the measurements. Additionally, we obse…
Low-energy ion beam modification was proposed to create graphene on the top of the insulated diamond-like carbon films. In such low-temperature fabrication process the surface of the amorphous carbon could crystallize to graphene as a result of point defect creation and enhanced diffusion caused by the ion bombardment. In the experiment 130 eV argon ion irradiation was used. After the modification the resistivity of the sample surface drops. Raman spectra of the samples measured at 633 nm showed partial crystallization and were similar to the spectra of defected graphene. This result is very e…
Moiré superlattices in van der Waals heterostructures are gaining increasing attention because they offer new opportunities to tailor and explore unique electronic phenomena when stacking 2D materials with small twist angles. Here, we reveal local surface potentials associated with stacking domains in twisted double bilayer graphene (TDBG) moiré superlattices. Using a combination of both lateral Piezoresponse Force Microscopy (LPFM) and Scanning Kelvin Probe Microscopy (SKPM), we distinguish between Bernal (ABAB) and rhombohedral (ABCA) stacked graphene and directly correlate these stacking co…
