Graphene Islands Research Articles

FeN4 complexes in graphene match Pt for fuel cell catalysts

Researchers at the Helmholtz-Zentrum Berlin for Materials and Energy (HZB), the Technische Universität (TU) Darmstadt, and the Freie Universität Berlin in Germany have produced a cost-effective catalyst for fuel cells, using a new preparation process. It consists of iron-nitrogen complexes embedded in nm-diameter islands of graphene. It is only these FeN4 centres that provide the excellent catalytic efficiency, which approaches that of platinum.

QM/MD studies on graphene growth from small islands on the Ni(111) surface.

Quantum chemical molecular dynamics simulations of graphene growth from small island precursors in different carbon nucleation densities on the Ni(111) surface at high temperatures have been conducted. The results indicate that small islands are not static, i.e. lateral diffusion and vertical fluctuation are frequently observed. In the case of low carbon nucleation density, carbon atoms or small carbon patches diffuse and attach to the edge of the nuclei to expand the size of the growing carbon network. The growth of graphene precursors is accompanied by the corresponding changes in the bonding of nickel atoms with the precipitation of subsurface carbon atoms. This is because the carbon-carbon interaction is stronger than the nickel-carbon interaction. In the case of high carbon nucleation densities, the dominant ripening mechanism depends on different growth stages. In the initial stage, the coalescence of carbon islands takes place via the Smoluchowski ripening mechanism. In the later stage the Smoluchowski ripening process is damped owing to the higher diffusion barrier of larger clusters and the restriction of movement by self-assembled nickel step edges. The cross-linking mechanism eventually takes over by the coalescence of extended polyyne chains between graphene islands. In either case, the Ostwald ripening process is not found in our molecular dynamics simulations due to the stability of carbon-carbon bonds within the islands. These investigations should be instructive to the control of graphene growth in experiments.

Ethylene decomposition on Ir(111): initial path to graphene formation.

The complete mechanism behind the thermal decomposition of ethylene (C2H4) on Ir(111), which is the first step of graphene growth, is established for the first time employing a combination of experimental and theoretical methods. High-resolution X-ray photoelectron spectroscopy was employed, along with calculations of core level binding-energies, to identify the surface species and their evolution as the surface temperature is increased. To understand the experimental results, we have developed a reaction sequence between the various CnHm species, from ethylene to C monomers and dimers, based on ab initio density functional calculations of all the energy barriers and the Arrhenius prefactors for the most important processes. The resulting temperature evolution of all species obtained from the simulated kinetics of ethylene decomposition agrees with photoemission measurements. The molecular dissociation mechanism begins with the dehydrogenation of ethylene to vinylidene (CH2C), which is then converted to acetylene (CHCH) by the removal and addition of an H atom. The C-C bond is then broken to form methylidyne (CH), and in the same temperature range a small amount of ethylidyne (CH3C) is produced. Finally methylidyne dehydrogenates to produce C monomers that are available for the early stage nucleation of the graphene islands.

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.

Facet-dependent study of efficient growth of graphene on copper by ethanol-CVD

The growth of graphene by chemical vapour deposition (CVD) on copper is the most promising scalable method for high-quality graphene. The use of ethanol, an economic and safe precursor, for the fast growth of graphene on copper by a home-built CVD set-up was analysed. Full coverage of uniform single-layer graphene with high crystalline quality was found on 〈100〉 textured Cu foils in just 30 s. The nucleation density of graphene islands was found to be independent of facets but the island shape showed facet dependence. Diamond-like islands were observed on Cu(100) facets while random shaped islands were seen on other facets. The last observation is discussed in terms of a competition between graphene-island growth and its relaxation rate on different facets. On Cu(100) slower island growth as compared to its relaxation leads to equilibrium shapes as opposed to other facets. Further, an observed evolution in graphene contrast in electron micrographs with time on different facets was discussed in terms of oxygen diffusion between graphene and Cu.

Theoretical modeling of the plasma-assisted catalytic growth and field emission properties of graphene sheet

A theoretical modeling for the catalyst-assisted growth of graphene sheet in the presence of plasma has been investigated. It is observed that the plasma parameters can strongly affect the growth and field emission properties of graphene sheet. The model developed accounts for the charging rate of the graphene sheet; number density of electrons, ions, and neutral atoms; various elementary processes on the surface of the catalyst nanoparticle; surface diffusion and accretion of ions; and formation of carbon-clusters and large graphene islands. In our investigation, it is found that the thickness of the graphene sheet decreases with the plasma parameters, number density of hydrogen ions and RF power, and consequently, the field emission of electrons from the graphene sheet surface increases. The time evolution of the height of graphene sheet with ion density and sticking coefficient of carbon species has also been examined. Some of our theoretical results are in compliance with the experimental observations.

Formation of Pt and Rh Nanoclusters on a Graphene Moiré Pattern on Cu(111)

The formation of Pt and Rh nanoclusters on a graphene moire pattern on Cu(111) was studied with ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Isolated graphene islands with different periodicities were successfully grown on the Cu surface. As a result of weak coupling to the Cu substrate, different graphene rotational domains were observed including one with a periodicity of 4 nm that has not been previously reported. Furthermore, our results have shown that Pt and Rh form dispersed nanoclusters on graphene/Cu(111) with Pt forming more regular structures trapped mostly at the hollow sites. This variation in growth behavior is mainly attributed to differences in Pt–carbon and Rh–carbon interaction energies. Thermal stability experiments were also performed, and Pt clusters were found to be structurally stable up to 700 K.

Polyhydrogenated Graphene: Excited State Dynamics in Photo- and Electroactive Two-Dimensional Domains

Understanding the phenomenon of intense photoluminescence in carbon materials such as hydrogenated graphene, graphene nanoribbons, and so forth is at the forefront of investigations. In this study, six different types of hydrogenated graphene (phG) produced from different starting materials were fully characterized in terms of structure and optical spectroscopy. Comprehensive photoluminescence lifetime analyses of phGs were conducted by combining time-correlated single-photon counting with steady-state fluorescence spectroscopy and femtosecond transient absorption spectroscopy. The conclusion drawn from these assays is that graphene islands with diameters in the range from 1.1 to 1.75 nm reveal band gap photoluminescence between 450 and 800 nm. As a complement, phGs were implemented in hybrids with water-soluble electron accepting perylenediimides (PDIs). By virtue of mutual π-stacking and charge transfer interactions with graphene islands, PDIs assisted in stabilizing aqueous dispersion of phG. Implicit in these ground state interactions is the formation of 300 ps lived charge separated states once photoexcited.

The effect of graphene intercalation with alkali metal atoms on the field electron emission

The influence of adsorption and intercalation of alkali metal atoms (Cs, K, Na) on the field-electron emission from an iridium point emitter with surface graphene islands has been studied. It is established that the intercalation of graphene islands with alkali metal atoms does not lead to significant changes in the emission current. In the case of adsorption of Cs, K, and Na atoms on the emitter surface with graphene islands, the most pronounced decrease in the work function and localization of electron emission are observed for islands situated on the (100) and (111) faces of iridium.

Piezoresistive effects in controllable defective HFTCVD graphene-based flexible pressure sensor.

In this work, the piezoresistive effects of defective graphene used on a flexible pressure sensor are demonstrated. The graphene used was deposited at substrate temperatures of 750, 850 and 1000 °C using the hot-filament thermal chemical vapor deposition method in which the resultant graphene had different defect densities. Incorporation of the graphene as the sensing materials in sensor device showed that a linear variation in the resistance change with the applied gas pressure was obtained in the range of 0 to 50 kPa. The deposition temperature of the graphene deposited on copper foil using this technique was shown to be capable of tuning the sensitivity of the flexible graphene-based pressure sensor. We found that the sensor performance is strongly dominated by the defect density in the graphene, where graphene with the highest defect density deposited at 750 °C exhibited an almost four-fold sensitivity as compared to that deposited at 1000 °C. This effect is believed to have been contributed by the scattering of charge carriers in the graphene networks through various forms such as from the defects in the graphene lattice itself, tunneling between graphene islands, and tunneling between defect-like structures.

Surface melting of copper during graphene growth by chemical vapour deposition

Evidence for the presence of a liquid phase on a copper surface during graphene growth by chemical vapour deposition at temperatures of 1000 and 1050 °C has been experimentally obtained. It was established that, first, the liquid phase forms a groove structure and bubble-like nanoobjects on the copper surface. Second, the liquid phase promotes the propagation of the wavy relief of a growing graphene film to the adjacent copper, and, third, the surface flows of the liquid phase can influence the waviness and orientation of the graphene islands. From the obtained data, it was concluded that surface melting plays an important role in graphene formation by chemical vapour deposition at temperatures below the melting point of copper.

Real-time observation of graphene oxidation on Pt(111) by low-energy electron microscopy

A monolayer of graphene was prepared by thermal decomposition of ethylene gas on Pt(111). The graphene can be readily removed by dosing O2 at pressures in 10−8mbar range and surface temperatures (Ts) near 1000K. Residual gas analysis during the oxygen treatment of graphene layer detected CO to be the only formed product. The oxidation process has been continuously imaged by Low-energy Electron Microscope (LEEM) operated in mirror-electron mode. LEEM observations revealed that the oxidation of graphene on Pt(111) occurs simultaneously at the outer island perimeter and in the interior of the graphene island. Symmetric hexagonal pits were observed to form continuously within graphene sheets, the pits proceeded isotropically. The etch rate was determined to be equal for both modes and independent of the surface environment with the exception of areas above Pt step edges. The pit growth rate at constant oxygen pressure was found to increase exponentially with respect to temperature over the investigated Ts range of 927–1014K, yielding an apparent activation energy of 479kJ/mol.

Optimisation of copper catalyst by the addition of chromium for the chemical vapour deposition growth of monolayer graphene

The effect of adding chromium (Cr) to the copper catalyst for chemical vapour deposition growth of graphene is investigated. We observe a suppression of the formation of multilayer islands of graphene in the Cr-rich regions. This is shown with optical microscopy, scanning electron microscopy and scanning Raman spectroscopy. In addition, carbon isotope labelling is employed to elucidate the mechanism by which the formation of multilayer islands is minimised. The use of mixed catalysts is an important step in the optimisation of catalytic growth of graphene.

On the rotational alignment of graphene domains grown on Ge(110) andGe(111)

AbstractWe have used low-energy electron diffraction and microscopy to compare the growth of graphene on hydrogen-free Ge(111) and Ge(110) from an atomic carbon flux. Growth on Ge(110) leads to significantly better rotational alignment of graphene domains with the substrate. To explain the poor rotational alignment on Ge(111), we have investigated experimentally and theoretically how the adatom reconstructions of Ge interact with graphene. We find that the ordering transition of the Ge(111) adatom reconstruction is not significantly perturbed by graphene. Density functional theory calculations show that graphene on reconstructed Ge(110) has large-amplitude corrugations, whereas it is remarkably flat on reconstructed Ge(111). We argue that the absence of corrugations prevents graphene islands from locking into a preferred orientation.

Improving the ammonia sensing of reduced graphene oxide film by using metal nano-materials

Gas sensing is one of the most promising applications for reduced graphene oxide (rGO). High surface-to-volume ratio in conjunction with remaining reactive oxygen functional groups translates into sensitivity to molecular on the rGO surface. The response of the rGO based devices can be further improved by functionalizing its surface with metal nano-materials. In this paper, we report the ammonia (NH3) sensing behavior of rGO based sensors functionalized with nano-structured metal: silver (Ag) or platinum (Pt) or gold (Au) in air at room temperature and atmospheric pressure. The gas response is detected by the monitoring changes in electrical resistance of the rGO/metal hybrids due to NH3 gas adsorption. Compared to bare rGO, significantly improved NH3 sensitivity is observed with the addition of nano-structured metals. These materials are applied to play the small bridges role connecting many graphene islands together to improve electrical conduction of hybrids while maintaining the inherent advantage of rGO for NH3 gas sensitivity.

Field desorption of Na and Cs from graphene on iridium

Field electron and desorption microscopy has been used to study specific features of the field desorption of sodium and cesium ions adsorbed on the surface of iridium with graphene. It was found that adsorbed sodium atoms most strongly reduce the work function on graphene islands situated over densely packed faces of iridium. A strong electric field qualitatively similarly affects the sodium and cesium desorption processes from a field emitter to give two desorption phases and has no noticeable effect on the disintegration of the graphene layer.

Cover Image, Volume 36, Issue 23

Journal of Computational ChemistryVolume 36, Issue 23 p. i-ii Cover ImageFree Access Cover Image, Volume 36, Issue 23 First published: 21 July 2015 https://doi.org/10.1002/jcc.24024AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract On page 1719 (DOI: 10.1002/jcc.23949), X. W. Zhou, D. K. Ward, and M. E. Foster present a new parameterization of Pettifor's bond order potential for carbon, enabling direct molecular dynamics simulations of graphene growth on copper. In the image, the big bronze balls are copper atoms, red small balls are carbon atoms in an initial graphene island, and small blue balls are deposited carbon atoms. Traces of some selected depositing carbon atoms are shown by small gray balls at time-resolved locations. Without any assumptions regarding the graphene structures, other than the random addition of carbon adatoms, MD simulations have enabled high fidelity studies of defect formation during graphene growth. Volume36, Issue23September 5, 2015Pages i-ii RelatedInformation

Micron Scale Cathodically Coated Graphene Impedes Corrosion on Ti, Cube, and Stainless Steel

Graphene has been studied with great interest ever since its unique mechanical and electronic properties were reported a little more than a decade ago. Common experiments involve coating non-carbon surfaces such as paper,1 metal oxide nanoparticles,2 or nickel3in order to either increase their conductivity, or to make them resistant to corrosion. Coating processes often involve mechanical transfer of graphene or chemical reduction of graphene oxide to graphene, but sometimes electrochemical cathodic reduction of the graphene oxide is used in the coating process. Here we report the first instance of cathodically coated Ti metal along with cathodically coated stainless steel and copper-beryllium. We report Raman microscope images to verify complete surface coverage of Ti, and partial coverage of ~2 micron islands of graphene on both stainless steel and CuBe. We also report Tafel plotted voltammograms to show that the presence of graphene on Ti shifts the potential of zero current, normally called the “corrosion potential,” in 3.5% NaCl(aq) to a value positive of the formal potential for the O2/H2O redox couple in sea water. We also present Tafel plots which show that the corrosion potential of graphene coated CuBe has shifted positive by 0.1V and the corrosion current decreased by nearly an order of magnitude, and that the corrosion potential of graphene coated stainless steel has shifted positive by 0.3V. 1. Parvez, Khaled; Wu, Zhong-Shuai; Li, Rongjin; Liu, Xianjie; Graf, Robert; Feng, Xinliang; and Mullen, Klaus. “Exfoliation of Graphite into Graphene in Aqueous Solutions of Inorganic Salts”. Journal of the American Chemical Society (2014) vol 136, iss 16, pp 6083-6091. 2. Nguyen-Phan, Thuy-Duong; Pham, Viet Hung; Shin, Eun Woo; Pham, Hai-Dinh; Kim, Sunwook; Chung, Jin Suk; Kim, Eui Jung; and Hur, Seung Hyun. “The role of graphene oxide content on the adsorption-enhanced photocatalysis of titanium dioxide/graphene oxide composites”. Chemical Engineering Journal (2011) vol 170, iss 1, pp 226-232. 3. Prasai, Dhiraj; Tuberquia, Juan Carlos; Harl, Robert R.; Jennings, G. Kane; and Lolotin, Kirill I. “Graphene: Corrosion-Inhibiting Coating”. ACS Nano (2012) vol 6, iss 2, pp 1102-1108.

Improving Graphene Conductivity through Selective Atomic Layer Deposition

One of the most promising approaches for massive production of transparent conducting electrodes (TCE) is roll-to-roll printing of graphene flakes solution. However, graphene flakes inevitably contain structural defects like grain boundaries and dislocations, together with macroscopic defects such as wrinkles and cracks during transfer. In addition, the printed flakes can’t align with each other perfectly. All these defects and the gaps between detached flakes hinder electrical transport and limit the mechanical strength and reliability of the graphene. Here we report a technique to electrically heal and connect graphene flakes via selective atomic layer deposition (ALD) of conductive materials. We have demonstrated selective growth of ALD aluminum-doped zinc oxide, ruthenium and platinum on pristine graphene surface prepared from exfoliated highly ordered pyrolytic graphite (HOPG). ALD materials nucleate on graphene cracks and edges but exhibit a significant nucleation barrier on intact basal planes due to the chemical inertness from delocalized π-orbitals. We also present a high-throughput fabrication method to create graphene patterns via metal shadow mask and O2 plasma. This model system of patterned CVD graphene islands demonstrates electrical connection of separated graphnene flakes by selective ALD processes. In addition, shadow-mask-patterned CVD graphene field effect transistors (FETs) are used to monitor graphene carrier density and mobility variation after ALD growth. We found considerable carrier density and conductivity increase of a single graphene flake after V2O5 ALD, which can be explained by p-doping of graphene due to the high work function V2O5.

Overtone and combination features of G and D peaks in resonance Raman spectroscopy of the C78H26 polycyclic aromatic hydrocarbon

We have studied the Raman spectra of C78H26, a polycyclic aromatic hydrocarbon with D2h symmetry point group resembling a longitudinally confined graphene ribbon (or a graphene island) with armchair edge. The experimental spectra recorded with several excitation laser lines have been compared with the results from a theoretical analysis of the resonant Raman response based on density functional theory calculations. Compared to previous investigation the spectra show better signal‐to‐noise ratio, which allows determining previously unresolved weak spectroscopic features. We have extended our analysis to the overtone and combination region (i.e. above 2000 cm−1) demonstrating the presence of signals attributable to 2G, G + D, 2D, Dj + Dk and G + acoustic‐like modes. Moreover, we have measured the temperature dependence of the G peak position, which turns out to show a similar behavior with respect to that of graphene/graphite. Copyright © 2015 John Wiley & Sons, Ltd.