Monolayer Graphene Sheet Research Articles

Geometric stability, electronic structure, and intercalation mechanism of Co adatom anchors on graphene sheets

We perform a systematic study of the adsorption of Co adatom on monolayer and bilayer graphene sheets, and the calculated results are compared through the van der Waals density functional (vdW-DF) and the generalized gradient approximation of Perdew, Burke and Ernzernhof (GGA + PBE) methods. For the single Co adatom, its adsorption energy at vacancy site was found to be larger than at the high-symmetry adsorption sites. For the different vdW corrections, the calculated adsorption energies of Co adatom on graphene substrates are slightly changed to some extent, but they do not affect the most preferable adsorption configurations. NEB calculations prove that the Co adatom has smaller energy barrier within pristine bilayer graphene (PBG) than that on the upper layer, indicating the high mobility of Co atom anchors at overlayer and easily aggregates. For the PBG substrate, the Co adatom intercalates into graphene sheets with a large energy barrier (9.29 eV). On the bilayer graphene with a single-vacancy (SV), the Co adatom can easily be trapped at the SV site and intercalates into graphene sheets with a much lower energy barrier (2.88 eV). These results provide valuable information on the intercalation reaction and the formation mechanism of metal impurity in graphene sheets.

Mechanical stability of substrate-bound graphene in contact with aqueous solutions

We report on the damage caused to mechanically exfoliated monolayer graphene, bound to silicon dioxide substrate, upon contact with liquids. This phenomenon is of significant importance for a wide range of applications where monolayer graphene sheets are used with liquids, especially as an electrode material in electrochemical applications such as energy storage and conversion. Liquid-induced damage to SiO2-bound graphene was previously observed with a range of solvents. A recently developed microdroplet system, used for a detailed examination of this behaviour, reveals that few-layer graphene flakes down to a bi-layer are stable with respect to aqueous electrolyte droplet formation, but the stability of these droplets is significantly reduced on monolayer graphene and irreversible rupture of the underlying graphene flake occurs. This damage, which we attribute to the presence of nanoscale defects and high adhesion between the graphene and the substrate, seems specific to plasma-cleaned SiO2 substrates and is not observed on flakes transferred to other substrates. Furthermore, the introduction of impurities, in the form of both polymer residues and native impurities between the flake and the SiO2 substrate, significantly enhance graphene’s immunity to external strain as shown by optical microscopy, atomic force microscopy, and Raman spectroscopy.

Nonlinear resonant behaviors of graphene sheet affixed on an elastic medium considering scale and thermal effects

An analytical method is presented to investigate the nonlinear vibration behaviors of graphene sheet affixed on an elastic medium, where small-scale and thermal effects are considered. Based on von Kármán geometrical nonlinear relationship and nonlocal elasticity theory, the nonlinear thermo-vibration equations of monolayer graphene sheet affixed on an elastic medium is derived. Utilizing the global residue harmonic balance method to solve the nonlinear thermo-vibration equations, the nonlinear thermo-resonant frequencies of graphene sheets affixed on an elastic medium with three different boundary conditions are obtained. Results show that small-scale effect has a significant impact on the nonlinear resonant frequency of monolayer graphene sheet and the impact of scale effect gradually decreases with the size of graphene sheet increases. Moreover the nonlinear thermal resonant frequency of monolayer graphene sheet nonlinearly grows with its initial displacement, and the nonlinear characteristics of thermal resonant frequency appear in larger growth rate as thermal loading increases. In addition, the effect of equivalent Winkler modulus and shear modulus of elastic medium on the nonlinear thermal resonant frequency is also described and discussed.

Atomically thin epitaxial template for organic crystal growth using graphene with controlled surface wettability.

A two-dimensional epitaxial growth template for organic semiconductors was developed using a new method for transferring clean graphene sheets onto a substrate with controlled surface wettability. The introduction of a sacrificial graphene layer between a patterned polymeric supporting layer and a monolayer graphene sheet enabled the crack-free and residue-free transfer of free-standing monolayer graphene onto arbitrary substrates. The clean graphene template clearly induced the quasi-epitaxial growth of crystalline organic semiconductors with lying-down molecular orientation while maintaining the "wetting transparency", which allowed the transmission of the interaction between organic molecules and the underlying substrate. Consequently, the growth mode and corresponding morphology of the organic semiconductors on graphene templates exhibited distinctive dependence on the substrate hydrophobicity with clear transition from lateral to vertical growth mode on hydrophilic substrates, which originated from the high surface energy of the exposed crystallographic planes of the organic semiconductors on graphene. The optical properties of the pentacene layer, especially the diffusion of the exciton, also showed a strong dependency on the corresponding morphological evolution. Furthermore, the effect of pentacene-substrate interaction was systematically investigated by gradually increasing the number of graphene layers. These results suggested that the combination of a clean graphene surface and a suitable underlying substrate could serve as an atomically thin growth template to engineer the interaction between organic molecules and aromatic graphene network, thereby paving the way for effectively and conveniently tuning the semiconductor layer morphologies in devices prepared using graphene.

Atomistic Simulation on Buckling Behavior of Monolayer Graphene

The buckling behavior of monolayer graphene sheets with simple-supported, clamped-free and clamped-clamped boundary conditions is investigated by the atomic-scale finite method (AFEM). The initial static equilibrium state of monolayer graphene sheet is obtained in the simulation as a waved configuration which is close to the real graphene observed in experiments. With the increase of compressive displacement, the force displays three stages: linear increasing, nonlinear increasing and decreasing slowly after a sudden drop. Different from the prediction by classical theory, the critical buckling loads of graphene sheets with different boundary conditions are similar, which is attributed to the initial waved configuration of the monolayer graphene sheets.

Coulomb impurity effect on electrically induced Dirac bound states in graphene

Using the massless Dirac–Weyl model for monolayer graphene sheet, we study the low-lying spectra of a single Dirac electron system bound to an on-center positively charged Coulomb impurity, under both electrostatic potential and magnetic field. Numerical results obtained from diagonalization show that, the increase of the electrostatic potential causes the low-lying states to evolve from one Landau-type plateau to the higher ones, and the whole spectra exhibit similar features with slight shifts in eigenenergies when the on-center impurity is considered. Electrostatic-potential dependent optical spectra with their corresponding absorption coefficients as functions of incident photon energies for transitions between low-lying states are presented.

Molecular dynamics study of ripples in graphene monolayer on silicon surface

By using the classical molecular dynamics and the simulated annealing techniques, the evolutions of the rippled morphology in single atomic graphenes placed on the Si (100), Si (111) and Si (211) surfaces respectively are performed at an atomic level. Our results show that the monolayer graphene sheets on the different Si surfaces form atomic scale rippled structures. A graphene monolayer prepared on Si surface forms rippled structure due to the relative lattice mismatch between graphene and Si substrate. The rippled morphology of graphene sheet on Si surface is strongly dependent on the annealing temperature. Such ripples will directly affect the adhesion strength between graphene and Si substrate. These findings are useful for understanding the structural morphology and stability of graphene on the semiconductor Si substrate, which will provide an analysis reference for further applications of graphene.

NiFe2O4/graphene nanocomposites with tunable magnetic properties

Novel NiFe2O4/graphene nanocomposites were synthesized via facile, one-pot solvothermal route, and the effects of processing conditions and composition on their magnetic properties have been studied. The nanocomposites consisted of monolayer graphene sheets decorated with uniformly dispersed NiFe2O4 nanoparticles of 6 nm in diameter. Increases in solvothermal temperature and time gave rise to improved crystallinity of NiFe2O4 nanoparticles and thus enhanced magnetic properties, while a high NiFe2O4 content resulted in a similar ameliorating effect on saturation magnetization, demonstrating tailored functional properties. A magnetic interaction between NiFe2O4/grahene was observed.

Incremental oxidation of the surface of monolayer and bilayer graphene: A computational study

We report the binding energies for a monolayer and bilayer graphene sheet coated with up to 24 oxygen atoms added sequentially to one surface of a monolayer and bilayer. Our graphene/graphite system consists of an arrangement of 3×5 phenyl rings or 48 carbon atoms in the monolayer and 96 carbon atoms in the bilayer. Geometries were energy optimized using the RM1 semiempirical method employing Periodic Boundary Conditions (PBC) followed by single point PBE and HSE06, all with the 6-31g⁎ basis and PBC. Results indicate that the first O atom bound to pure graphene has a binding energy 2.16eV on the monolayer and 2.14eV on the bilayer. As O atoms are added the binding energy increases to 2.61eV when the surface coverage on the monolayer reaches 45.8% or 11 O atoms on the unit cell, and for the bilayer this maximum occurs with 45.8% coverage or 11 O atoms on the top of the bilayer. The binding energy then gradually declines to 2.41eV for the monolayer and to 1.93eV for the bilayer with 100% coverage or 24 O atoms covering the top of the surface. Oxygen atoms added in close proximity to one another have a greater binding energy than O atoms added with larger separations relative to the unit cell. The difference in O binding energy between the monolayer system and the bilayer system is on average 0.02eV less for the bilayer, the second, and as the number of O atoms are increased, the binding energy between the graphene layers falls to zero after 45.8% coverage or with 11 O atoms.

Plasmonic Bloch oscillations in monolayer graphene sheet arrays.

We investigate the spatial plasmonic Bloch oscillations (BOs) in the monolayer graphene sheet arrays (MGSAs) as the surface plasmon polaritons (SPPs) between graphene in the arrays experience weak coupling. In order to realize BOs, linear gradient of the potential is introduced by changing the chemical potentials of individual graphene sheets or the interlayer space between graphene. Numerical simulations show that the complete plasmonic BOs can be observed in the former MGSAs. However, only harmonic oscillations occur in the latter of varying interlayer space. Theoretical analysis based on the coupled-mode theory agrees well with the numerical simulations.

Mechanics of nanoscale wrinkling of graphene on a non-developable surface

As a two-dimensional crystal, the graphene sheet is often used with substrate materials because the freestanding graphene tends to corrugate and can hardly display its extraordinary properties in devices. However, the substrate is rarely perfectly-flat, but has microscopic roughness. Whether or not the graphene sheet can conform fully to a substrate with nano- and micro-roughness is essential for the performance of the graphene-based devices. In this paper, a theoretical model is developed to predict the morphology of a monolayer graphene sheet attaching to the substrate with microscopic non-developable roughness by an energy-based analysis. The final graphene morphology is revealed to result from the competition between two energy terms: the adhesion energy between graphene and substrate, and the strain energy stored in the graphene due to the deformations. Thus, by accounting for these two parts of energy, the critical condition to predict the morphology conversion from full conformation to wrinkling is established, which agrees well with the results of molecular dynamics simulations. This study has significant meanings for design and fabrication of high quality nanostructured coatings on substrates with complex surfaces, and can offer a guide for designing new functional graphene electronical devices such as nano-sensors and nano-switches as well.

Tunable plasmonic band gap and defect mode in one-dimensional photonic crystal covered with graphene

We study the transmission characteristics of surface plasmons at the interface of a one-dimensional photonic crystal (1DPC) and a monolayer graphene sheet. The composite structure with a plasmonic band gap (PGB) was obtained by covering a graphene sheet on the lateral side of the 1DPC. Defect modes in these structures appeared when defect layers were inserted in the 1DPC. On the condition that the chemical potential of graphene is much larger than , the PBG and defect mode shift to higher frequencies as the chemical potential increases. As the carrier density in graphene can be controlled by the gate voltage, both the PBG and defect mode can be easily tuned by the chemical potential, which is determined by the carrier density.

Plasmon-negative refraction at the heterointerface of graphene sheet arrays.

We demonstrate negative refraction of surface plasmon polaritons (SPPs) at the heterointerface of two monolayer graphene sheet arrays (MGSAs) with different periods. The refraction angle is specifically related to the period ratio of the two MGSAs. By varying the incident Bloch momentum, the SPPs might be refracted in the direction normal to the heterointerface. Moreover, both positive and negative refraction could appear simultaneously. Because of the linear diffraction relation, the incident and refracted SPP beams experience diffraction-free propagation. The heterostructures composed of the MGSAs may find great applications in deep-subwavelength spatial light modulators, optical splitters, and switches.

Mechanical properties of fully hydrogenated graphene sheets

Graphane is a two-dimensional structure consisting of a flat monolayer graphene sheet fully covered with hydrogen atoms attached to its carbon atoms in an alternating pattern. The unique properties of graphane make it suitable for different applications. In this paper, the mechanical properties of the most stable conformer of graphane, the so-called chair-like, are extensively investigated using density functional theory (DFT) scheme within the framework of the generalized gradient approximation (GGA) and the well-known Perdew–Burke–Ernzerhof (PBE) exchange correlation. It is shown that the hydrogenation has significant influences on the mechanical properties of graphene sheet. In particular, it is found that the elastic, bulk and shear moduli and Poisson’s ratio of the chair–graphane are significantly smaller than those of graphene.

Broadband modeling surface plasmon polaritons in optically pumped and curved graphene structures with an improved leapfrog ADI-FDTD method

An improved one-step leapfrog alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method is proposed to study surface plasmon polaritons (SPPs) in optically pumped and electrostatic applied curved graphene structures, with their intraband and interband surface conductivities modeled with the vector fitting technique. Such a method can quickly capture the broadband characteristics of SPPs propagating along monolayer, bilayer and trilayer graphene sheets of arbitrary shapes. Numerical stability and accuracy are validated through characterizing field distributions of the SPPs supported in the 180°-curved and spiral graphene sheet waveguides and higher computational efficiency are achieved in comparison with the conventional FDTD method.

Precessed electron beam electron energy loss spectroscopy of graphene: Beyond channelling effects

The effects of beam precession on the Electron Energy Loss Spectroscopy (EELS) signal of the carbon K edge in a 2 monolayer graphene sheet are studied. In a previous work, we demonstrated the use of precession to compensate for the channeling-induced reduction of EELS signal when in zone axis. In the case of graphene, no enhancement of EELS signal is found in the usual experimental conditions, as graphene is not thick enough to present channeling effects. Interestingly, though it is found that precession makes it possible to increase the collection angle, and, thus, the overall signal, without a loss of signal-to-background ratio.

Three-dimensional bicomponent supramolecular nanoporous self-assembly on a hybrid all-carbon atomically flat and transparent platform.

Molecular self-assembly is a versatile nanofabrication technique with atomic precision en route to molecule-based electronic components and devices. Here, we demonstrate a three-dimensional, bicomponent supramolecular network architecture on an all-carbon sp(2)-sp(3) transparent platform. The substrate consists of hydrogenated diamond decorated with a monolayer graphene sheet. The pertaining bilayer assembly of a melamine-naphthalenetetracarboxylic diimide supramolecular network exhibiting a nanoporous honeycomb structure is explored via scanning tunneling microscopy initially at the solution-highly oriented pyrolytic graphite interface. On both graphene-terminated copper and an atomically flat graphene/diamond hybrid substrate, an assembly protocol is demonstrated yielding similar supramolecular networks with long-range order. Our results suggest that hybrid platforms, (supramolecular) chemistry and thermodynamic growth protocols can be merged for in situ molecular device fabrication.

Effect of graphene on photoluminescence properties of graphene/GeSi quantum dot hybrid structures

Graphene has been discovered to have two effects on the photoluminescence (PL) properties of graphene/GeSi quantum dot (QD) hybrid structures, which were formed by covering monolayer graphene sheet on the multilayer ordered GeSi QDs sample surfaces. At the excitation of 488 nm laser line, the hybrid structure had a reduced PL intensity, while at the excitation of 325 nm, it had an enhanced PL intensity. The attenuation in PL intensity can be attributed to the transferring of electrons from the conducting band of GeSi QDs to the graphene sheet. The electron transfer mechanism was confirmed by the time resolved PL measurements. For the PL enhancement, a mechanism called surface-plasmon-polariton (SPP) enhanced absorption mechanism is proposed, in which the excitation of SPP in the graphene is suggested. Due to the resonant excitation of SPP by incident light, the absorption of incident light is much enhanced at the surface region, thus leading to more exciton generation and a PL enhancement in the region. The results may be helpful to provide us a way to improve optical properties of low dimensional surface structures.

The seeded growth of graphene.

In this paper, we demonstrate the seeded growth of graphene under a plasma chemical vapor deposition condition. First, we fabricate graphene nanopowders (~5 nm) by ball-milling commercial multi-wall carbon nanotubes. The graphene nanoparticles were subsequently subject to a direct current plasma generated in a 100 Torr 10%CH4 - 90%H2 gas mixture. The plasma growth enlarged, over one hour, the nuclei to graphene sheets larger than one hundred nm2 in area. Characterization by electron and X-ray diffraction, high-resolution transmission electron microscopy images provide evidence for the presence of monolayer graphene sheets.

Mechanical properties and failure mechanisms of graphene under a central load.

By employing molecular dynamics simulations, the evolution of deformation of a monolayer graphene sheet under a central transverse loading are investigated. Dependence of mechanical responses on the symmetry (shape) of the loading domain, on the size of the graphene sheet, and on temperature, is determined. It is found that the symmetry of the loading domain plays a central role in fracture strength and strain. By increasing the size of the graphene sheet or increasing temperature, the tensile strength and fracture strain decrease. The results have demonstrated that the breaking force and breaking displacement are sensitive to both temperature and the symmetry of the loading domain. In addition, we find that the intrinsic strength of graphene under a central load is much smaller than that of graphene under a uniaxial load. By examining the deformation processes, two failure mechanisms are identified namely, brittle bond breaking and plastic relaxation. In the second mechanism, the Stone-Wales transformation occurs.