Graphene Islands Research Articles

Spin-Dependent Electron Scattering at Graphene Edges on Ni(111)

We investigate the scattering of surface electrons by the edges of graphene islands grown on Ni(111). By combining local tunneling spectroscopy and ab initio electronic structure calculations we find that the hybridization between graphene and Ni states results in strongly reflecting graphene edges. Quantum interference patterns formed around the islands reveal a spin-dependent scattering of the Shockley bands of Ni, which we attribute to their distinct coupling to bulk states. Moreover, we find a strong dependence of the scattering amplitude on the atomic structure of the edges, depending on the orbital character and energy of the surface states.

Role of Au in Graphene Growth on a Ni Surface

Density functional calculations were performed to investigate the role of Au in graphene growth on the Ni(111) step. It was shown that armchair (AC) and zigzag (ZZ) graphene edge growths have nucleation selectivity, depending on the curvature of the stepped surface. The AC and ZZ pristine graphene edges are energetically more favorable than the Ni-terminated one, and the stabilities of Au-passivated graphene edges strongly depend on the Au concentration. Au modification on the Ni terrace lowers the energy barrier of C incorporation onto the AC/ZZ graphene edge process, in agreement with the experimental observation that graphene can be produced at the low temperature of ∼723 K with Au alloying. The growth rate of the AC graphene edge is always faster than the ZZ, leading to the ZZ edge’s dominating the circumference of the growing graphene islands. With a decrease in the temperature, the increase in the AC graphene edge growth ratio greatly exceeds that of ZZ, driving the edges to incorporate a zigzag geo...

DFT study on the atomic-scale nucleation path of graphene growth on the Cu(111) surface

The nucleation path of graphene growth on the Cu(111) surface is investigated by importing carbon atoms step-by-step using density functional theory (DFT) calculations. An overall path of graphene nucleation has been proposed based on configuration and energy analysis. At the very first stage, linear chains will be formed and dominate the copper surface. Then, Y-type (furcate) carbon species will be shaped when new carbon atoms are absorbed aside the linear chains. Finally, ring-containing carbon species and graphene islands will be formed stepwise, with energetic preference. We find that the Y-type and ring-containing carbon species are not likely formed directly at the initial stage of graphene nucleation, but should be formed starting from linear chains. The nucleation limiting step is the formation of the Y-type species, which must pass an energy barrier of about 0.25 eV. These underlying observations are instructive to stimulate future experimental efforts on graphene synthesis.

Polypyrrole decorated graphene nanostructure: Fabrication, depiction and anomalous dimensional crossover in electronic conduction

We had fabricated polypyrrole based graphene nanostructure by using CTAB (cetyl trimethylammonium bromide) surfactant with variation of graphene loading and characterized them by XRD, FT-IR, SEM, TEM, AFM, Raman etc. Variable range hopping (VRH) model had been applied for explication of temperature dependent D.C. conductivity scrutinized from 20 to 300K. A synergetic effect of π-electron flow in conducting polymer polypyrrole and 2D graphene layer was appeared to demonstrate the plausible charge-transport mechanism. Dramatically, two peculiar crossovers had been observed simultaneously in the composite samples with variation of temperature. A 3D–2D effective dimensional crossover had been observed at ≈100K with variation of hopping length which can be controlled by temperatures. Below ≈50K another cross-over between Mott and ES (Efrost & Shklovskii) type VRH mechanism had also been explored as resistivity increases with lowering of temperature. Here, charge transport occurred via VRH between intact graphene island and polypyrrole chain.

Deposition of thin silicon layers on transferred large area graphene

Physical vapor deposition of Si onto transferred graphene is investigated. At elevated temperatures, Si nucleates preferably on wrinkles and multilayer graphene islands. In some cases, however, Si can be quasi-selectively grown only on the monolayer graphene regions while the multilayer islands remain uncovered. Experimental insights and ab initio calculations show that variations in the removal efficiency of carbon residuals after the transfer process can be responsible for this behavior. Low-temperature Si seed layer results in improved wetting and enables homogeneous growth. This is an important step towards realization of electronic devices in which graphene is embedded between two Si layers.

Physical ambiguity of the notion of the solubility limit by the example of its determination using the formation of graphene on the surface in the Re-C system

Results obtained during the determination of the solubility limit of carbon in metal based on the detection of the time instant of the nucleation of the new phase, graphene, have been considered. Graphene is easily detected by the surface-sensitive methods of thermionic emission and electron Auger spectroscopy. It has been shown that the nucleation of the first graphene islands and the formation of the graphene monolayer covering the whole surface and even the multilayer graphite film on it can be used as the time instant of the phase transition depending on the sensitivity of the method. The solubility limits differ by many times depending on the determination method. This indicates the physical ambiguity of this physical characteristic of the double systems analogous in physical meaning to the pressure of the saturated vapor for the phase transitions in the one-component systems.

Influence of Chemisorbed Oxygen on the Growth of Graphene on Cu(100) by Chemical Vapor Deposition

Understanding the influence that copper substrate surface symmetry and oxygen impurities have on the growth of graphene by chemical vapor deposition is important for developing techniques for producing high-quality graphene. Therefore, we have studied the growth of graphene by catalytic decomposition of ethylene in an ultrahigh-vacuum chamber on both a clean Cu(100) surface and a Cu(100) surface predosed with a layer of chemisorbed oxygen. The crystal structure of the graphene films was characterized with in situ low energy electron diffraction. By heating the clean Cu(100) substrate from room temperature to the growth temperature in ethylene, epitaxial graphene films were formed. The crystal quality was found to depend strongly on the growth temperature. At 900 °C, well-ordered two-domain graphene films were formed. Predosing the Cu(100) surface with a chemisorbed layer of oxygen before graphene growth was found to adversely affect the crystal quality of the graphene overlayer by inducing a much higher degree of rotational disorder of the graphene grains with respect to the Cu(100) substrate. The growth morphology of the graphene islands during the initial stages of nucleation was monitored with ex situ scanning electron microscopy. The nucleation rate of the graphene islands was observed to drop by an order of magnitude by predosing the Cu(100) surface with a chemisorbed oxygen layer before growth. Therefore, the presence of oxygen during graphene growth affects both the relative orientation and average size of grains within the films grown on Cu(100) substrates.

Nucleation and growth of HfO2 layers on graphene by chemical vapor deposition

We investigate a seed layer-free growth of HfO2 on commercially available chemical vapor deposited (CVD) graphene from various suppliers. It is revealed that the samples of monolayer graphene transferred from Cu to SiO2/Si substrates have different coverage with bi- and multi-layer graphene islands. We find that the distribution and number of such islands impact the nucleation and growth of HfO2 by CVD. In particular, we show that the edges and surface of densely distributed bi-layer graphene islands provide good nucleation sites for conformal CVD HfO2 layers. Dielectric constant of 16 is extracted from measurements on graphene-HfO2-TiN capacitors.

Synthesis and application of graphene–silver nanowires composite for ammonia gas sensing

Graphene, consisting of a single carbon layer in a two-dimensional (2D) lattice, has been a promising material for application to nanoelectrical devices in recent years. In this study we report the development of a useful ammonia (NH3) gas sensor based on graphene–silver nanowires ‘composite’ with planar electrode structure. The basic strategy involves three steps: (i) preparation of graphene oxide (GO) by modified Hummers method; (ii) synthesis of silver nanowires by polyol method; and (iii) preparation of graphene and silver nanowires on two electrodes using spin and spray-coating of precursor solutions, respectively. Exposure of this sensor to NH3 induces a reversible resistance change at room temperature that is as large as ΔR/R0 ∼ 28% and this sensitivity is eight times larger than the sensitivity of the ‘intrinsic’ graphene based NH3 gas sensor (ΔR/R0 ∼ 3,5%). Their responses and the recovery times go down to ∼200 and ∼60 s, respectively. Because graphene synthesized by chemical methods has many defects and small sheets, it cannot be perfectly used for gas sensor or for nanoelectrical devices. The silver nanowires are applied to play the role of small bridges connecting many graphene islands together to improve electrical properties of graphene/silver nanowires composite and result in higher NH3 gas sensitivity.

Graphene attached to glassy carbon surfaces. From simple deposition to redox polymer cathodic generation

The electrochemical reduction (in aprotic polar solvents) of thin compact graphene layers mechanically deposited onto flat glassy carbon (GC) surfaces was studied. For the first time, cathodic voltammetry of graphene supported by GC was achieved. Furthermore, deposited graphene was reduced in the presence of α,ω-dihaloalkanes. These mixed electrolyses implied on one hand graphene moieties considered as redox blocks and/or free radical traps and on the other hand X–R–X as pro-links between graphene islands. Upon electron transfer, and depending on the applied potential, α,ω-dihaloalkanes generate reactive transients (e.g. bi-radicals or di-nucleophiles). Additionally, the electrode surface is involved in this process and contributes to create a redox polymer (poly-graphene) immobilized on the electrode that one may imagine highly chemically and electrochemically malleable. This first contribution outlines both the formation and electrochemical capacities of this 3D-electrode.

Crystal Structure Evolution of Individual Graphene Islands During CVD Growth on Copper Foil

Single-crystal percentage of graphene islands on Cu foil is associated with island sizes and shapes. In polycrystalline islands, certain grain boundary types are favored. There is no obvious relation between the number of lobes and grain orientations. An observed structure evolution and surface disorder of Cu grains can be possible factors for the formation of grain boundaries within graphene islands.

Simulating the conditions for the formation of graphene and graphene nanowalls by semiempirical quantum chemical methods

A “substrate-surface graphene island” is simulated by semiempirical quantum chemical methods for different substrates. It is established that such systems with various island sizes correspond to graphene growth on corresponding substrates when CVD is used. Carbon atoms can be incorporated into the island from either the side of the substrate or the side of the CVD reactor exposed to it. It is shown for a wide range of island sizes that the best configuration with respect to the substrate is the structure of a carbon nanowall oriented perpendicular to the substrate. It is emphasized that a transition to this configuration is possible in reality only if CVD is plasma-stimulated when there is a strong near-electrode field near the surface, and the preliminary scission of the carbon carrier material occurs simultaneously in the CVD reactor volume.

Electronic triple-dot transport through a bilayer graphene island with ultrasmall constrictions

A quantum dot has been etched in bilayer graphene connected by two small constrictions to the leads. We show that this structure does not behave like a single quantum dot but consists of at least three sites of localized charge in series. The high symmetry and electrical stability of the device allowed us to triangulate the positions of the different sites of localized charge and find that one site is located in the island and one in each of the constrictions. Nevertheless we measure many consecutive non-overlapping Coulomb-diamonds in series. In order to describe these findings, we treat the system as a strongly coupled serial triple quantum dot. We find that the non-overlapping Coulomb diamonds arise due to higher order cotunneling through the outer dots located in the constrictions. We extract all relevant capacitances, simulate the measured data with a capacitance model and discuss its implications on electrical transport.

Clean transfer of graphene on Pt foils mediated by a carbon monoxide intercalation process

Noble metals such as Pt are a perfect substrate for the catalytic growth of monolayer graphene. However, the requirements of the subsequent transfer process are not compatible with the traditional etching method. In this work, we find that the interaction of graphene with Pt foil can be weakened through the intercalation of carbon monoxide (CO) under ambient pressure. This intercalation process occurs on both hexagonal-shape graphene islands and irregular graphene patches on changing the CO partial pressure from 0 to 0.6 MPa, as observed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoemission spectroscopy. We demonstrate that, on a practical timescale, the intercalation ratio is proportional to the partial pressure of CO. Furthermore, we develop a clean transfer method of CO-intercalated graphene with water as a peeling agent. We show that this method enables the transfer of tens of micrometer-scale graphene patches onto SiO2/Si, which are free from metal or oxide particle contamination. This transfer method should be a significant step towards the clean transfer of graphene, as well as the recyclable use of noble metal substrates.

Photochemical Synthesis of Noble Metal (Ag, Pd, Au, Pt) on Graphene/ZnO Multihybrid Nanoarchitectures as Electrocatalysis for H2O2 Reduction

For the first time, a series of noble metal (Ag, Au, Pd, and Pt) nanoparticles (NPs) based on new functional graphene were successfully achieved via UV-assisted photocatalytic reduction by ZnO nanorods. The whole preparation strategy for constructing noble metal deposited graphene sheets/ZnO (GS/ZnO) was elucidated in detail in this work. First, graphene oxide based two-dimensional carbon nanostructures served as a support to disperse ZnO nanorods through a hydrothermal route. The ZnO nanorods were self-assembled onto the surface of graphene sheets, forming GS/ZnO nanocomposite, and the graphene oxide was reduced, yielding reduced graphene sheets in this synthetic procedure. Second, the GS/ZnO films were further employed as supporting materials for the dispersion of metal nanoparticles. Photogenerated electrons from UV-irradiated ZnO were transported across GS to stepwise and respectively reduce v μL metal ions (Ag(+), Pd(2+), AuCl4(-), PtCl6(2-), 20 mg/mL) into metal (Ag, Pd, Au, Pt) NPs at a location distinct from the ZnO anchored site, forming five graphene-based hybrid nanocomposites designated as GS/ZnO, GS/ZnO@Agv, GS/ZnO@Pdv, GS/ZnO@Auv, GS/ZnO@Ptv, respectively. The obtained mutihybrid nanoarchitectured materials were clearly characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). According to the diameters and distribution, the four metal NPs on GS/ZnO were divided into two categories: Ag&Au and Pd&Pt. Their difference was rooted in the rival abilities of gathering electron between graphene and different metal islands in the photochemical reduction process. The electrochemical behaviors of the five resultant hybrid nanocomposites were investigated in H2O2 as well as in potassium ferricyanide (Fe(CN)6(3-/4-)) and displayed distinct electrocatalytic activity.

Microscopic View on a Chemical Vapor Deposition Route to Boron-Doped Graphene Nanostructures

Single layer boron-doped graphene layers have been grown on polycrystalline copper foils by chemical vapor deposition using methane and diborane as carbon and boron sources, respectively. Any attempt to deposit doped layers in one-step has been fruitless, the reason being the formation of very reactive boron species as a consequence of diborane decomposition on the Cu surface, which leads to disordered nonstoichiometric carbides. However, a two-step procedure has been optimized: as a first step, the surface is seeded with pure graphene islands, while the boron source is activated only in a second stage. In this case, the nonstochiometric boron carbides formed on the bare copper areas between preseeded graphene patches can be exploited to easily release boron, which diffuses from the peripheral areas inward of graphene islands. The effective substitutional doping (of the order of about 1%) has been demonstrated by Raman and photoemission experiments. The electronic properties of doped layers have been char...

Influence of Gas Phase Equilibria on the Chemical Vapor Deposition of Graphene

We have investigated the influence of gas phase chemistry on the chemical vapor deposition of graphene in a hot wall reactor. A new extended parameter space for graphene growth was defined through literature review and experimentation at low pressures (≥0.001 mbar). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy, and dark field optical microscopy, with the latter showing promise as a rapid and nondestructive characterization technique for graphene films. The equilibrium gas compositions have been calculated across this parameter space. Correlations between the graphene films grown and prevalent species in the equilibrium gas phase revealed that deposition conditions associated with a high acetylene equilibrium concentration lead to good quality graphene deposition, and conditions that stabilize large hydrocarbon molecules in the gas phase result in films with multiple defects. The transition between lobed and hexagonal graphene islands was found to be linked to the concentration of the monatomic hydrogen radical, with low concentrations associated with hexagonal islands.

Graphene Growth Dynamics on Epitaxial Copper Thin Films

Graphene chemical vapor deposition on copper is phenomenologically complex, yielding diverse crystal morphologies, including lobes, dendrites, stars, and hexagons, of various orientations depending on conditions. We present a comprehensive study of the evolution of these morphologies as a function of the copper surface orientation, absolute pressure, hydrogen-to-methane ratio (H2:CH4), and nucleation density. Growth was studied on ultrasmooth, epitaxial copper films inside copper enclosures to minimize copper polycrystallinity and roughness and decrease the graphene nucleation density. At low pressure and low H2:CH4, circular graphene islands initially form. After exceeding ∼1.0 μm, Mullins-Sekerka instabilities evolve into dendrites extending hundreds of micrometers in the ⟨100⟩, ⟨111⟩, and ⟨110⟩ directions on Cu(100), Cu(110), and Cu(111), respectively, indicating mass transport limited growth. Twin boundaries perturb the preferential growth direction on Cu(111) and alter graphene morphology. Increasing H2:CH4 results in compact islands that reflect the copper symmetry. At atmospheric pressure and low H2:CH4, Mullins-Sekerka instabilities develop but with multiple preferred orientations. Increasing H2:CH4 results in more hexagonal islands. Every growth regime can be tuned to yield continuous monolayers with a D:G Raman ratio <0.1. The understanding gained from this study provides a roadmap to rationally tailor the structure, morphology, and orientation of graphene crystals.

Transformation of Carbon Monomers and Dimers to Graphene Islands on Co(0001): Thermodynamics and Kinetics

Although graphene on metal surfaces has been extensively investigated, the comprehensive and quantitative experimental results exploring the growth mechanism of graphene on metal surfaces are quite few. Employing in situ and time-resolved X-ray photoelectron spectroscopy measurements, we report the comprehensive and quantitative data for the thermodynamics and kinetics of both the C1-to-graphene transformation reaction on Co(0001) covered with C1 adatoms and graphene islands and the C2-to-graphene transformation reaction on Co(0001) covered with C2 dimers and graphene islands. Both transformation reactions proceed via carbon cluster attachment mechanism. The enthalpy and apparent activation energy were determined, respectively, to be ∼261 and ∼139 kJ/mol for the C1-to-graphene transformation reaction, and ∼287 and ∼280 kJ/mol for the C2-to-graphene transformation reaction. The C1-to-graphene transformation reaction on Co(0001) likely proceeds without the formation of C2 dimers. These experimental data pro...

Argon-assisted growth of epitaxial graphene on Cu(111)

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 elevated temperatures. During the initial stages of growth, a random distribution of rounded graphene islands is observed. The predominant rotational orientation of the islands is within $\pm1^\circ$ of the Cu(111) substrate lattice.