Topmost Graphene Layer Research Articles

Twisted Bilayer Graphene Induced by Intercalation.

Twisted bilayer graphene (tBLG) has gained significant attention due to its unique physical and electronic properties. However, efficient fabrication of high-quality tBLG with diverse twist angles is crucial to expedite research on angle-dependent physics and potential applications. In this study, an intercalation strategy utilizing organic molecules, such as 1,2-dichloroethane, is developed to weaken the interlayer interaction and induce slide or rotation of the topmost graphene layer for tBLG fabrication. The proportion of tBLGs in the resulting 1,2-dichloroethane-treated BLG (dtBLG) reaches up to 84.4% for twist angles ranging from 0° to 30°, surpassing previously reported methods using chemical vapor deposition (CVD). Moreover, the twist angle distribution is not uniform and tends to concentrate in the ranges of 0-10° and 20-30°. This facile and rapid intercalation-based methodology provides a practical solution for studying angle-dependent physics and advancing the utilization of twisted two-dimensional materials.

Growth of High-Purity and High-Quality Turbostratic Graphene with Different Interlayer Spacings.

Turbostratic graphene is a multilayer graphene, which has exotic electrical properties similar to those of monolayer graphene due to the low interlayer interaction. Additionally, the stacking structure of the turbostratic multilayer graphene can decrease the effect of attachment of charge impurities and surface roughness. This paper explores the growth of high-purity and high-quality turbostratic graphene with different interlayer spacings by calcining ferric chloride and sucrose at 1000 °C for 1 h under an argon atmosphere. X-ray diffraction patterns and Raman results imply that the turbostratic graphene contains two different interlayer spacings: 3.435 and 3.55 Å. The 3.55 Å turbostratic graphene is on top of the 3.435 Å turbostratic graphene, and there is an AB stacking pattern between the topmost graphene layer of 3.435 Å turbostratic graphene and the first graphene layer of the 3.55 Å turbostratic graphene, with an interlayer spacing of 3.35 Å. The two different interlayer spacings of turbostratic graphene arise from different cooling rates between the higher temperature ranges (>700 °C) and lower temperatures (<700 °C).

Correlating surface structures and nanoscale friction of CVD Multi-Layered graphene

Surface structures on chemical vapor deposition (CVD) grown graphene determine its functional properties, and thus, in-depth understanding about the relationship between the structures and the properties is essential to investigate CVD graphene in a variety of scientific and engineering applications. Here, we correlate surface structures with nanoscale friction of a multi-layered graphene island. By cleaning graphene surface utilizing mechanical scratching of polymeric surface contamination, we unveiled the surface structures such as small-scale and large-scale (folded) wrinkles on graphene using atomic force microscopy (AFM) and investigated their effect on nanoscale friction. The formation of such surface structures induced twisting of the top-most graphene layer as confirmed by Raman spectroscopic analysis, and the twisted top-most graphene layer influenced the layer dependency of nanoscale friction depending on the applied normal load as revealed by friction force microscopy (FFM) measurements. No identifiable layer-dependency was observed at the low normal load due to the increased interlayer distance between the twisted top-most graphene layer and the underlying AB-stacked layers. Increasing the normal load resulted in the decrease of the interlayer distance and made the contribution of different lateral bending stiffness more dominant to nanoscale friction, resulting in the strong layer-dependency of nanoscale friction on multi-layered graphene.

Epitaxial Graphene on 4H-SiC (0001) as a Versatile Platform for Materials Growth: Mini-Review

Material growth on a dangling-bond-free interface such as graphene is a challenging technological task, which usually requires additional surface pre-treatment steps (functionalization, seed layer formation) to provide enough reactive sites. Being one of the most promising and adaptable graphene-family materials, epitaxial graphene on SiC, due to its internal features (substrate-induced n-doping, compressive strain, terrace-stepped morphology, bilayer graphene nano-inclusions), may provide pre-conditions for the enhanced binding affinity of environmental species, precursor molecules, and metal atoms on the topmost graphene layer. It makes it possible to use untreated pristine epitaxial graphene as a versatile platform for the deposition of metals and insulators. This mini-review encompasses relevant aspects of magnetron sputtering and electrodeposition of selected metals (Au, Ag, Pb, Hg, Cu, Li) and atomic layer deposition of insulating Al2O3 layers on epitaxial graphene on 4H-SiC, focusing on understanding growth mechanisms. Special deliberation has been given to the effect of the deposited materials on the epitaxial graphene quality. The generalization of the experimental and theoretical results presented here is hopefully an important step towards new electronic devices (chemiresistors, Schottky diodes, field-effect transistors) for environmental sensing, nano-plasmonics, and biomedical applications.

Effect of Ambient Chemistry on Friction at the Basal Plane of Graphite.

Graphite is widely used as a solid lubricant due to its layered structure, which enables ultralow friction. However, the lubricity of graphite is affected by ambient conditions and previous studies have shown a sharp contrast between frictional behavior in vacuum or dry environments compared to humid air. Here, we studied the effect of organic gaseous species in the environment, specifically comparing the adsorption of phenol and pentanol vapor. Atomic force microscopy experiments and reactive molecular dynamics simulations showed that friction was larger with phenol than with pentanol. The simulation results were analyzed to test multiple hypotheses to explain the friction difference, and it was found that mechanically driven chemical bonding between the tip and phenol molecules plays a critical role. Bonding increases the number of phenol molecules in the contact, which increases the adhesion as well as the number of atoms in registry with the topmost graphene layer acting as a pinning site to resist sliding. The findings of this research provide insight into how the chemistry of the operating environment can affect the frictional behavior of graphite and layered materials more generally.

Linear-scaling self-consistent field theory based molecular dynamics: application to C60buckyballs colliding with graphite

ABSTRACTIn this work, we investigate the collision of a C fullerene with graphite using large-scale molecular dynamics simulations, where the interatomic forces are computed ‘on-the-fly’ by means of self-consistent tight-binding calculations. This method is based on an exact decomposition of the grand-canonical potential for independent fermions suitable for linear-scaling electronic structure calculations. We observe that at lower collision velocities, the buckyball is rebound from the graphite surface, but that starting from 50 km/s chemisorption processes are occurring that causes the buckyball to sticks to the topmost graphene layer. These outcomes are in stark contrast to previous simulations using empirical interaction potentials, but in very good agreement with experimental measurements.

Origin of hydration lubrication of zwitterions on graphene.

Formation of a hydration layer on charge sites can support normal pressure, and meanwhile it retains excellent fluidity to provide efficient boundary lubrication; however, it is limited to the sliding system between two similarly charged surfaces. In the present study, we report extremely low friction as the zwitterions in a lipid bilayer slide on the topmost graphene layer of graphite across pure water, with the friction coefficient falling to the level of 0.001, which provides direct evidence that hydration lubrication is effective even between such dissimilar surfaces. The origin of hydration lubrication on graphene was studied by atomic force microscopy and molecular dynamics simulation simultaneously. It reveals that a subnanometer hydration layer is confined between zwitterions and graphene, which remains as a liquid phase under normal pressure. The shear occurs between water molecules and graphene because of the extremely low shear strength of the water/graphene interface, which contributes to extremely low friction. Our finding demonstrates that the formation of a hydration layer is possible to lubricate layered materials efficiently, which has potential implications for designing efficient boundary lubrication with layered materials.

Effects of Reduced Dimensional Scales on the Electrochemical Reversible Li-Storage and Associated Integrity of Well-Ordered Graphitic/Graphenic Carbon

Even though graphitic carbon is the most commonly used anode material for Li-ion battery, it possesses low gravimetric capacity and rate capability; which has stimulated research towards alternate anode materials. Recent investigations have indicated that graphene, the two dimensional building block of graphitic carbon, may possess enhanced Li-storage capacity, along with improved rate capability. However, the mechanism of reversible Li-storage and mechanical/structural integrity of graphene upon repeated Li-insertion/removal are yet to be understood; with too many contradictory reports are available in the literature concerning the Li-storage reversible capacity/behavior and cyclic stability of graphenic carbon [1, 2].Therefore, using fairly pristine and well-ordered CVD-grown few layers graphene (FLG) film as model material and performing extensive electrochemical/structural investigations, also involving in-situ studies, we have been able to develop better insights into the inter-related phenomena concerning lithiation/delithiation mechanisms leading to higher Li-capacity, stress developments at different states of charges and mechanical integrity upon lithiation/delithiation of such nanoscaled ‘graphitic’ carbon. Presence of distinct features corresponding to formation of Li-intercalated graphite compounds below 0.3 V against Li/Li+ in chronopotentiograms (viz., potential plateaus) and cyclic voltammograms (viz., current peaks) obtained with FLG (~7 graphene layers), similar to thicker (~450 nm) bulk graphite (TBG) film, indicates that Li-intercalation/de-intercalation in the interlayer spaces does take place even for such reduced (nano)dimensional form of ‘graphitic’ carbon. Furthermore, the capacity obtained for FLG within just the same lower potential window (i.e., below 0.3 V) and even after subtracting entire possible contribution from Cu substrate underneath, is ~3-4 times greater than the overall specific capacity obtained with TBG (i.e., ~380 mAh/g) [3]. Analysis based on the electrochemical response indicated that this excess capacity in the case of FLG (or Li-storage in FLG below 0.25 V) has contributions from both diffusion controlled ‘classical’ bulk processes of Li-storage, as well as surface phenomena. In this context, simulation based on density functional theory indicated that the excess Li-capacity of graphene (beyond the LiC6 composition) is associated with additional stable Li-storage on the surface of the outer or topmost graphene layer in the form of two Li-layers (but different from Li-plating) and segregation of Li close to the ‘stepped’ exposed edges of the inner graphene layers; the relative contributions from which would go down with increase in dimensional scale [3]. Even though the nanoscaled FLG possess higher specific capacity, as well as improved rate capability, with respect to the thicker TBG, cyclic stability of FLG over the full range of lithiation/delithiation (i.e., between 2 and 0.001 V against Li/Li+) was observed to be inferior w.r.t. TBG. Interestingly, the cyclic stability, when compared with respect to the capacities obtained just within the lower potential window (i.e., below 0.25 V), was not inferior to that for TBG. In order to gain further insights into this, in-plane stress developments in the FLG was monitored in-situ during galvanostatic cycling. The net in-plane stress development upon full lithiation in FLG agrees reasonably well with the stress magnitude expected based on the net dilation along graphite a-axis upon ‘classical’ bulk Li-intercalation up to LiC6 composition. This tends to additionally indicate that the mechanisms of excess Li-storage (i.e., other than the ‘classical’ bulk Li-intercalation) leads to much lesser in-plane dimensional change (if any). Another interesting observation was that of ‘stress release’ in the stress-time profile of FLG, just within the potential window of 0.5-0.25 V (against Li/Li+) [4] where pristine graphene/graphite, dilute stages I and IV co-exist. Based on the evidences obtained from structural characterizations (including changes in Raman spectra after cycling within different potential windows) and geometrical modelling, such stress release is believed to be associated with mechanical/structural degradation taking place primarily during the initial stages of lithiation and later stages of delithiation as a result of stretching of individual graphene layers beyond fracture strain around intercalated Li at interfaces between the concerned co-existing stages [4]. Accordingly, the observed ‘stress release’ within the concerned potential window above 0.25 V and the as-believed mechanism for mechanical/structural degradation in the case of FLG tends to agree well with the fading of the overall capacity.

Band gap engineering for single-layer graphene by using slow Li+ ions

In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li+ ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li+ ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li+ ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li+ ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices.

The thermal behavior of Pd on graphene/Ru(0001)

The thermal behavior of various amounts of palladium deposited onto graphene/Ru(0001) at room temperature was investigated by X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRAS) of adsorbed CO. IRAS spectra of adsorbed CO on palladium deposited onto the graphene layer reveal the formation of defect-rich, highly-stepped and/or nanoparticle-typed palladium clusters. Annealing to temperatures below ~900K, palladium clusters mainly experience agglomeration on the graphene layer, but at higher temperatures intercalation of palladium between the graphene layer and Ru substrate takes place. Eventually, palladium completely desorbs and the graphene layer dissolves into the ruthenium substrate at 1400K. Even though the annealing induces the intercalation and desorption of palladium, the topmost graphene layer stays intact without any physical damage at and below 1300K.

Copper intercalation at the interface of graphene and Ir(111) studied by scanning tunneling microscopy

We report on the intercalation of a submonolayer of copper at 775 K underneath graphene epitaxially grown on Ir(111) studied by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) at 77 K. Nucleation and growth dynamics of Cu below graphene have been investigated, and, most importantly, the intercalation mechanism has been identified. First, LEED patterns reveal the pseudomorphic growth of Cu on Ir under the topmost graphene layer resulting in a large Cu in-plane lattice parameter expansion of about 6% compared to Cu(111). Second, large-scale STM topographs as a function of Cu coverage show that Cu diffusion on Ir below graphene exhibits a low energy barrier resulting in Cu accumulation at Ir step edges. As a result, the graphene sheet undergoes a strong edges reshaping. Finally, atomically-resolved STM images reveal a damaged graphene sheet at the atomic scale after metal intercalation. Point defects in graphene were shown to be carbon vacancies. According to these results, a Cu penetration path beneath graphene is proposed to occur via metal aided defect formation with no or poor self healing of the graphene sheet. This work illustrates the fact that Cu intercalation is harmful for graphene grown on Ir(111) at the atomic scale.

Direct imaging of monovacancy-hydrogen complexes in a single graphitic layer

Understanding how foreign chemical species bond to atomic vacancies in graphene layers can advance our ability to tailor the electronic and magnetic properties of defective graphenic materials. Here, we use ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and density functional theory to identify the precise structure of hydrogenated single atomic vacancies in a topmost graphene layer of graphite and establish a connection between the details of hydrogen passivation and the electronic properties of a single atomic vacancy. Monovacancy-hydrogen complexes are prepared by sputtering of the graphite surface layer with low-energy ions and then exposing it briefly to an atomic hydrogen environment. High-resolution experimental UHV-STM imaging allows us to determine unambiguously the positions of single missing atoms in the defective graphene lattice and, in combination with the ab initio calculations, provides detailed information about the distribution of low-energy electronic states on the periphery of the monovacancy-hydrogen complexes. We found that a single atomic vacancy where each \ensuremath{\sigma} dangling bond is passivated with one hydrogen atom shows a well-defined signal from the nonbonding \ensuremath{\pi} state, which penetrates into the bulk with a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}$ periodicity. However, a single atomic vacancy with full hydrogen termination of \ensuremath{\sigma} dangling bonds and additional hydrogen passivation of the extended \ensuremath{\pi} state at one of the vacancy's monohydrogenated carbon atoms is characterized by complete quenching of low-energy localized states. In addition, we discuss the migration of hydrogen atoms at the periphery of the monovacancy-hydrogen complexes, which dramatically change the vacancy's low-energy electronic properties, as observed in our low-bias, high-resolution STM imaging.

Effects of surface compliance and relaxation on the frictional properties of lamellar materials

We describe the results of atomic-level stick-slip friction measurements performed on chemically-modified graphite, using atomic force microscopy (AFM). Through detailed molecular dynamics simulations, coarse-grained simulations, and theoretical arguments, we report on complex indentation profiles during AFM scans involving local reversible exfoliation of the top layer of graphene from the underlying graphite sample and its effect on the measured friction force during retraction of the scanning tip. In particular, we report nearly constant lateral stick-slip magnitudes at decreasing loads, which cannot be explained within the standard framework based on continuum mechanics models for the contact area. We explain this anomalous behavior by introducing the effect of local compliance of the topmost graphene layer, which varies when interaction with the AFM tip is enhanced. Such behavior is not observed for non-lamellar materials. We extend our discussion toward the more general understanding of the effects of the top layer relaxation on the friction force under pushing and pulling loads. Our results may provide a more comprehensive understanding of the effectively negative coefficient of friction recently observed on chemically-modified graphite.

Capacitance enhancement via electrode patterning

The necessity of increasing the energy density in electric double layer capacitors to meet current demand is fueling fundamental and applied research alike. We report here molecular dynamics simulation results for aqueous electrolytes near model electrodes. Particular focus is on the effect of electrode patterning on the structure of interfacial electrolytes, and on the potential drop between the solid electrodes and the bulk electrolytes. The latter is estimated by numerically integrating the Poisson equation using the charge densities due to water and ions accumulated near the interface as input. We considered uniform and patterned electrodes, both positively and negatively charged. The uniformly charged electrodes are modeled as graphite. The patterned ones are obtained by removing carbon atoms from the top-most graphene layer, yielding nanoscopic squares and stripes patterns. For simplicity, the patterned electrodes are effectively simulated as insulators (the charge remains localized on the top-most layer of carbon atoms). Our simulations show that the patterns alter the structure of water and the accumulation of ions at the liquid-solid interfaces. Using aqueous NaCl solutions, we found that while the capacitance calculated for three positively charged electrodes did not change much, that calculated for the negatively charged electrodes significantly increased upon patterning. We find that both water structure and orientation, as well as ion accumulation affect the capacitance. As electrode patterning affects differently water structure and ion accumulation, it might be possible to observe ion-specific effects. These results could be useful for advancing our understanding of electric double layer capacitors, capacitive desalination processes, as well as of fundamental interfacial electrolytes properties.

Graphene ripples generated by grain boundaries in highly ordered pyrolytic graphite

Atomic scale periodic ripples that extend for several nanometers on the surface of adjacent graphitic grains have been observed for the first time on highly ordered pyrolitic graphite by UHV-STM. The ripples emanate from a grain boundary, and are explained in terms of a mechanical deformation due to the elastic strain accumulated along the GB, which is relieved out-of-plane in the topmost graphene layer. We present a molecular dynamics model that accounts for the formation of similar ripples as result of the lattice mismatch induced by two different grain orientations.

Structural Deformation of Graphene–Nanotube Contacts: First-Principles Simulations

By using first-principles simulations, we studied the geometry deformation of few-layer graphene and single-walled carbon nanotube (SWNT) contact. With SWNT on the graphene, the graphene interlayer spacing reduces. Moreover, there is a dip appears in the topmost graphene layer due to the interaction with SWNT. The depth of this dip increases with SWNT diameter. And geometry deformation is also observed in thin SWNT. We demonstrate that even for very thin SWNT, the geometry deformation and flexibility of both graphene layer and SWNT cannot be ignored, which will lead to atomic scale locking of the nanotube on graphene. Thus the contact between SWNT and graphene is more stable than expected.

An exchange intercalation mechanism for the formation of a two-dimensional Si structure underneath graphene

A two-dimensional (2D) Si film can form between a graphene overlayer and a Ru(0001) substrate through an intercalation process. At the graphene/2D-Si/Ru(0001) surface, the topmost graphene layer is decoupled from the Ru substrate and becomes quasi-freestanding. The interfacial Si layers show high stability due to the protection from the graphene cover. Surface science measurements indicate that the surface Si atoms can penetrate through the graphene lattice, and density functional theory calculations suggest a Si-C exchange mechanism facilitates the penetration of Si at mild temperatures. The new mechanism may be involved for other elements on graphene, if they can bond strongly with carbon. This finding opens a new route to form 2D interfacial layers between graphene and substrates.

Interaction of Au16 Nanocluster with Defects in Supporting Graphite: A Density-Functional Study

Soft-landed adsorption of Au16 on bilayered graphene is investigated using density functional theory. The orientation of the Au16 cluster and number of neighboring surface vacancies affect the overall structural and electronic properties of the cluster. The results of the PBE, vdW-DF, and vdW-DF2 exchange-correlation functionals are compared for the cluster-substrate interaction for systems with and without defects. In the presence of defects size two and greater, an Au atom adsorbs into the topmost graphene layer; this strongly influences the binding energy (>3 eV), while inducing substantial bending in the carbon plane and altering electronic properties of the system. Though the Td-symmetry and electronegative properties of the Au16 structure change in the presence of greater neighboring defects, elements of the cagelike starting structure remain throughout. The electron localization function shows that the in-plane Au–C bonds are of delocalized (metallic) nature and there is a local charge transfer to ...

Angular Intensity Distribution of a Molecular Oxygen Beam Scattered from a Graphite Surface

The scattering of the oxygen molecule from a graphite surface has been studied using a molecular beam scattering technique. The angular intensity distributions of scattered oxygen molecules were measured at incident energies from 291 to 614 meV with surface temperatures from 150 to 500 K. Every observed distribution has a single peak at a larger final angle than the specular angle of 45° which indicates that the normal component of the translation energy of the oxygen molecule is lost by the collision with the graphite surface. The amount of the energy loss by the collision has been roughly estimated as about 30-41% based on the assumption of the tangential momentum conservation during the collision. The distributions have also been analyzed with two theoretical models, the hard cubes model and the smooth surface model. These results indicate that the scattering is dominated by a single collision event of the particle with a flat surface having a large effective mass. The derived effective mass of the graphite surface for the incoming oxygen is 9-12 times heavier than that of a single carbon atom, suggesting a large cooperative motion of the carbon atoms in the topmost graphene layer.

Wide-range temperature dependence of epitaxial graphene growth on 4H-SiC (0 0 0 −1): A study of ridge structures formation dynamics associated with temperature

We investigated the temperature dependence (1200–2100°C) of morphological features in the growth of epitaxial graphene on 4H-SiC (000−1) C-face in ultra high vacuum (UHV). As a SiC initial surface preparation technique prior to the epitaxial graphene growth, Si-vapor etching was conducted to ensure a uniform step-terrace structure at 4H-SiC (000−1) consisting of 0.5nm step height. The epitaxial graphene growth has been performed in our original experimental setup with a TaC container inside an all metal chamber with radiative heating system, which allows to increase the graphitization temperature up to 2100°C in UHV. The surface morphology of the epitaxially grown multilayer graphene, characterized by atomic force microscopy (AFM), exhibited a drastic change as a function of temperature. Ridge structures of the topmost graphene layer(s) appeared above 1600°C, where the well-ordered step-terrace initial structure of the SiC vanished. In this high temperature range, the network of the ridges spread over the surface with an increase in temperature. Raman topography mapping for characterizing site dependent 2D-spectrum implies that rotational stacking disorder of graphene layers is accompanied by relatively small ridge network on the surface in a specific temperature range.