Pristine Single-layer Graphene Research Articles

Blue Luminescence from N-doped Graphene

Abstract Blue luminescence (BL) has been observed in various astrophysical environments, such as the inner region of the Red Rectangle Nebula. It has been previously linked to small polycyclic aromatic hydrocarbons. In this study, we explore the potential of N- and O-doped graphene material as carriers of BL. Using vacuum ultraviolet irradiation, pristine single-layer graphene films were exposed to solid N2 and O2 at low temperatures, resulting in the formation of N- or O-doped graphene. The photoluminescence (PL) spectrum of the synthesized N-doped graphene exhibits an asymmetrical peak at 378 nm, which is nearly identical to the BL observed in the Red Rectangle. In contrast, the synthesized O-doped graphene shows a symmetrical PL band at 414 nm. Further Raman and X-ray photoelectron spectroscopy analyses reveal that nitrogen atoms substitute carbon atoms in the graphene lattice, introducing structural defects and sp2 regions responsible for the observed luminescence. The simplicity of the bonding structures in the N-doped graphene, dominated by pyrrolic-N and pyridinic-N groups, further supports its potential as a model system for studying BL.

Effect of structural defects on the physiochemical properties of supportive single-layer graphene in a sliding electrical contact interface under ambient conditions

We investigated the physiochemical properties of single-layer graphene (SLG) with controlled structural defects that were used in sliding electrical contact. Conductive atomic force microscopy (cAFM) probes applied with electric biases were used to rub the SLG surface under ambient conditions. Structural and chemical properties of rubbed SLG were investigated using Raman spectroscopy, photothermal infrared atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. The work function (WF) and conductivity were determined using Kelvin probe force microscopy and cAFM. Adhesive properties were examined using AFM in the force-volume mode. Biases applied to the cAFM probe resulted in the tunneling triboelectric effect (TTE), which modulated the WF of SLG. The formation of water menisci around the cAFM probe–SLG contact will be electrolyzed, leading to the surface oxidation of SLG, which also impacts the WF of SLG. Compared with pristine SLG with minimum defects, more-defective SLG enhanced the TTE and surface functionalization during the rubbing process, substantially affecting the morphological, electronic, chemical, and adhesive properties of rubbed defective SLG. Our results demonstrated that the physiochemical properties of SLG in a sliding contact are considerably affected by the presence of structural defects in SLG, which should be considered when SLG is used for electrode applications.

Experimental observation of spin−split energy dispersion in high-mobility single-layer graphene/WSe2 heterostructures

Proximity-induced spin–orbit coupling in graphene has led to the observation of intriguing phenomena like time-reversal invariant {{mathbb{Z}}}_{2} topological phase and spin-orbital filtering effects. An understanding of the effect of spin–orbit coupling on the band structure of graphene is essential if these exciting observations are to be transformed into real-world applications. In this research article, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin–orbit coupling. We achieve this in high-mobility hexagonal boron nitride (hBN)-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a band gap opening and band inversion in the SLG. Further, we establish that the deviation of the low-energy band structure from pristine SLG is determined primarily by the valley-Zeeman SOC and Rashba SOC, with the Kane–Mele SOC being inconsequential. Despite robust theoretical predictions and observations of band-splitting, a quantitative measure of the spin-splitting of the valence and the conduction bands and the consequent low-energy dispersion relation in SLG was missing—our combined experimental and theoretical study fills this lacuna.

Bidirectional Modulation of Neuronal Cells Electrical and Mechanical Properties Through Pristine and Functionalized Graphene Substrates.

In recent years, the quest for surface modifications to promote neuronal cell interfacing and modulation has risen. This course is justified by the requirements of emerging technological and medical approaches attempting to effectively interact with central nervous system cells, as in the case of brain-machine interfaces or neuroprosthetic. In that regard, the remarkable cytocompatibility and ease of chemical functionalization characterizing surface-immobilized graphene-based nanomaterials (GBNs) make them increasingly appealing for these purposes. Here, we compared the (morpho)mechanical and functional adaptation of rat primary hippocampal neurons when interfaced with surfaces covered with pristine single-layer graphene (pSLG) and phenylacetic acid-functionalized single-layer graphene (fSLG). Our results confirmed the intrinsic ability of glass-supported single-layer graphene to boost neuronal activity highlighting, conversely, the downturn inducible by the surface insertion of phenylacetic acid moieties. fSLG-interfaced neurons showed a significant reduction in spontaneous postsynaptic currents (PSCs), coupled to reduced cell stiffness and altered focal adhesion organization compared to control samples. Overall, we have here demonstrated that graphene substrates, both pristine and functionalized, could be alternatively used to intrinsically promote or depress neuronal activity in primary hippocampal cultures.

Investigation of plasma-assisted functionalization of pristine single layer graphene

Investigations on the oxygen plasma treatment of graphene have been already reported in literature, but in this paper, as a novelty, we present a structural conversion of single layer graphene by O2 plasma treatment. Graphene Oxide is a chemically controlled graphene with hydroxyl, epoxy, carboxyl and carbonyl groups. Low O2 plasma treatment has been evidenced to be an efficient method to introduce defects and active groups in graphene structure with significantly contribution in biosensor area for detecting pathogens and biomolecules. Single Layer Graphene was grown on copper substrate by Chemical Vapor Deposition method and then transferred by wet chemical technique on gold substrate. Pristine Graphene was exposed to a downstream oxygen plasma and we have investigated the modification of the pristine graphene using Scanning Electron Microscopy (SEM), Raman Spectroscopy, Fourier Transform-Infrared Spectroscopy (FT-IR), X-Ray Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS) to examine the quality and structure modification of pristine graphene.

Mechanical properties of graphene, defective graphene, multilayer graphene and SiC-graphene composites: A molecular dynamics study

In this article, using molecular dynamics (MD) simulation, several mechanical properties of graphene were calculated for pristine single layer graphene sheet (SLGS), defective SLGS, multilayer graphene sheets (MLGS) without defects and for composites of SiC and graphene. Firstly, the room temperature elastic moduli (Young's, shear and bulk), constants and Poison's ratios were simulated for the above structures and then their properties such as stress vs strain under tensile and compressive loading were simulated within the temperature range of 300K–2000K. The simulations were performed in both zigzag and armchair directions of graphene surface and differences were observed in the mechanical properties. Simulations revealed that the location of defects as well as their type influence the elastic moduli and constants at room temperature and that their tensile and compressive properties vary with temperature. For both MLGS, and SiC-graphene composites, the properties improved with increase in number of continuous graphene layers.

Pristine Graphene as a Racemization Catalyst for Axially Chiral BINOL.

Despite versatile applications of functionalized graphene in catalysis, applications of pure, unfunctionalized graphene in catalysis are in their infancy. This work uses both computational and experimental approaches to show that single-layer graphene can efficiently catalyze the racemization of axially chiral BINOL in solution. Using double-hybrid density functional theory (DHDFT) we calculate the uncatalyzed and catalyzed Gibbs free reaction barrier heights in a number of representative solvents of varying polarity: benzene, diphenyl ether, dimethylformamide (DMF), and water. These calculations show that (i) graphene can achieve significant catalytic efficiencies (▵▵G≠ cat ) varying between 47.2 (in diphenyl ether) and 60.7 (in DMF) kJ mol-1 . An energy decomposition analysis reveals that this catalytic activity is driven by electrostatic and dispersion interactions. Based on these computational results, we explore the graphene-catalyzed racemization of axially chiral BINOL experimentally and show that single-layer graphene can efficiently catalyze this process. Whilst the uncatalyzed racemization requires high temperatures of over 200 °C, a pristine single-layer graphene catalyst makes it accessible at 60 °C.

Remarkable thermal rectification in pristine and symmetric monolayer graphene enabled by asymmetric thermal contact

Thermal rectification is a nonreciprocal thermal transport phenomenon, which typically takes place in asymmetric structures or hetero-junctions. In this work, we propose a new route to achieve remarkable thermal rectification even in pristine single-layer graphene without asymmetry by engineering the thermal contacts at the two ends. When setting a fixed long thermal contact at one end and varying the length of thermal contact at the other end, our molecular dynamics simulations demonstrate that notable thermal rectification efficiency can be achieved with very short thermal contact, which vanishes in the limit of long thermal contact. Such a strategy of asymmetric thermal contact can provide a significant enhancement of thermal rectification efficiency, achieving around 920% thermal rectification in the short sample with a length of 200 nm and around 110% thermal rectification in the micrometer scale sample. Phonon participation ratio analysis reveals that the strong localization of low-frequency acoustic phonons is induced by the short thermal contact in the backward direction, leading to a significant temperature jump at the short thermal contact in the backward direction and thus the thermal rectification in pristine single-layer graphene. Our study provides a new path to achieve notable thermal rectification even in the symmetric structures by engineering the thermal contact.

In-plane and out-of-plane tensile behaviour of single-layer graphene sheets: a new interatomic potential

This paper compares simple interatomic potentials for carbon nanostructures with hexagonal lattice, by investigating the in-plane and the out-of-plane tensile behaviour of single-layer graphene sheets. Attention is given both to potentials already considered in the literature and to a new one, which we call the damped DREIDING potential, in which damping functions are added to the DREIDING potential. For each potential, a calibration of its parameters and a focus on its performance are carried out in the in-plane context, by comparison with ab initio results of the rigidities and of the tensile limit properties, under periodic conditions. In addition, the damped DREIDING potential is used to perform in-plane tensile tests on both pristine and perforated single-layer graphene sheets of finite size. In the out-of-plane context, the calibration from ab initio results is only possible with reference to the rigidity. For the damped DREIDING potential, a sensitivity analysis, applied to a nanoindentation problem, on a pristine single-layer graphene sheet of finite size is provided. In doing so, a narrow range of value of the force needed to remove an atom from a sheet is given.

A Charge-Transfer-Resistance Model for Proton Transmission through CVD Single-Layer Graphene in Proton-Exchange-Membrane Cells

High-rate proton transmission through single-layer graphene has been reported by several research groups but the mechanism by which it occurs is still not clear. Low activation energies for proton transmission have been experimentally obtained but are difficult to reconcile with computational modeling studies that predict high activation energies and low transmission rates for protons passing through pristine single-layer graphene at ambient temperatures. It seems likely that a reaction coordinate with an activated complex structure involving proton passage through graphene is involved, though the structural details of such a complex are still not clear. This lecture will review some of our recent experimental results on proton transmission rates through CVD single-layer graphene at variable temperature, with special attention to a kinetics model whereby experimentally-obtained proton-transfer resistances are interpreted as proton-transfer resistances that are linked to heterogeneous proton—transfer rate constants, in a manner that is formally similar to the Butler-Volmer model used to treat electron-transfer rate data in the low overpotential limit. The heterogeneous proton-transfer rate constants are then interpreted using a pre-equilibrium model for protons approaching graphene, coupled to a first-order rate constant for proton transmission through graphene from the pre-equilibrium complex. This first-order proton-transmission rate constant has fundamental significance and may be interpreted in terms of an activation energy and frequency factor for proton transmission. In applying such an analysis to our proton transmission rate data, we obtain a proton-transmission frequency factor that is within a factor of two of the vibrational frequency of the O-H bond in water, suggesting that O-H bond vibration is involved in the rate-determining step of proton transmission.

Combined effect of point defects and layer number on the adsorption of benzene and toluene on graphene

Understanding the adsorption properties of organic molecules on graphene-based substrates is important for such applications as air and water filters. Pristine graphene is often the model substrate used in the theoretical investigations of this problem. While useful, pristine single-layer graphene is however an idealized model. In this work, we assess the effect of the presence of point defects (single vacancy, divacancy, and the Stone-Wales defect) in single-layer and bilayer graphene on the energetics of adsorption of benzene and toluene. Our calculations benchmark three different dispersion-corrected DFT schemes, namely PBE-D2, vdW-DF1, and vdW-DF2-C09. Whereas the presence of the single vacancy and the double vacancy does not appear to alter the adsorption energies of the aromatic molecules by an appreciable amount, the Stone-Wales defect and the addition of a second graphene layer stabilizes their interaction with the substrate by several tens of meV.

Mechanical Characterization of Reduced Graphene Oxide Using AFM

Nanoindentation coupled with Atomic Force Microscopy was used to study stiffness, hardness, and the reduced Young’s modulus of reduced graphene oxide. Oxygen reduction on the graphene oxide sample was performed via LightScribe DVD burner reduction, a cost-effective approach with potential for large scale graphene production. The reduction of oxygen in the graphene oxide sample was estimated to about 10 percent using FTIR spectroscopic analysis. Images of the various samples were captured after each reduction cycle using Atomic Force Microscopy. Elastic and spectroscopic analyses were performed on the samples after each oxygen reduction cycle in the LightScribe, thus allowing for a comparison of stiffness, hardness, and the reduced Young’s modulus based on the number of reduction cycles. The highest values obtained were after the fifth and final reduction cycle, yielding a stiffness of 22.4 N/m, a hardness of 0.55 GPa, and a reduced Young’s modulus of 1.62 GPa as compared to a stiffness of 22.8 N/m, a hardness of 0.58 GPa, and a reduced Young’s modulus of 1.84 GPa for a commercially purchased graphene film made by CVD. This data was then compared to the expected values of pristine single layer graphene. Furthermore, two RC circuits were built, one using a parallel plate capacitors made of light scribed graphene on a kapton substrate (LSGC) and a second one using a CVD deposited graphene on aluminum (CVDGC). Their RC time constants and surface charge densities were compared.

Nanographene growing on free-standing monolayer graphene

Contamination on graphene is a critical problem that has limited the applications of graphene despite its innate versatility. However, the structures of such contamination remain unclear. In this study, we investigated the structure of a contaminated region growing on free-standing monolayer graphene in situ at room temperature using an aberration-corrected scanning transmission electron microscope. Energy-dispersive X-ray spectroscopy showed that carbon atoms were the dominant element of the growing contamination. We quantitatively confirmed that the growing contamination consisted of several layers of nanoscale graphene flakes on pristine single-layer graphene. Electron energy-loss spectroscopy indicated that the hexagonal carbon lattice deforms considerably from the pristine one at the higher layers. Analysis of the three-dimensional atomic structure revealed that the atoms in the third layer were the suppliers of the second layer at the step edge. In addition, we found that the lattice constant of pristine graphene in the region of the first layer was larger than that of the bulk graphite crystals, and that it gradually decreased as the second layer grew on the first layer. The findings of this paper will be helpful not only for the control of nanographene but also for a basic understanding of the intrinsic features of graphene.

Tunable intrinsic magnetic phase transition in pristine single-layer graphene nanoribbons

In this paper, we report on the interesting phenomenon of magnetic phase transitions (MPTs) observed under the combined influence of an electric field (E) and temperature (T) leading to a thermo-electromagnetic effect on the pristine single-layer zigzag graphene nanoribbon (szGNR). Density functional theory-based first principles calculations have been deployed for this study on the intrinsic magnetic properties of graphene. Interestingly, by tuning electric field (E) and temperature (T), three distinct magnetic phase behaviors, para-, ferro- and antiferromagnetic are exhibited in pristine szGNR. We have investigated the unrivaled positional parameters of these MPTs. MPT occurring in the system also follows a positional trend and the change in these positional parameters with regard to the size of the szGNR along with the varied E and T is studied. We propose a bow-tie schematic to induce the intrinsic magnetism in graphene and present the envisaged model of the processor application with the reported intrinsic MPT in szGNR. This fundamental insight into the intrinsic MPTs in graphene is an essential step towards developing graphene-based spin-transfer torque magnetoresistive random access memory, quantum computing devices, magnonics and spintronic memory application.

Structural Modification of Pristine Graphene Network Towards Nanoporous Graphene Membrane: A Review

A single graphene layer is superior many ways preferably in electronic devices application. However, mild modification of the graphene network could open a new potential to the ultrathin graphene membrane. Moreover, recent studies demonstrated that a few simple techniques could generate and control the nanopores size on single layer graphene sheet simultaneously. This review paper will discuss all potential techniques that are capable to generate nanopores structure on the pristine single layer graphene network.

Experimental study on thermal conductivity of free-standing fluorinated single-layer graphene

We measured the thermal conductivity of free-standing fluorinated single-layer graphene (FSLG) using a precise T-type method. Pristine graphene was fluorinated and suspended above the substrate using xenon difluoride gas. Compared with the thermal conductivity of pristine single-layer graphene (SLG) (∼2000 W/mK) previously measured by the same T-type method for the same original SLG, the FSLG exhibited a much lower thermal conductivity (∼80 W/mK) and a weak dependence of the thermal conductivity on nanohole defects. The experimental results suggest that the fluorine atoms and sp3 bonding in the FSLG strongly contributed to phonon scattering. The phonon scattering by the fluorine atoms and sp3 bonding has a dominant effect on the thermal conductivity decrease over the phonon scattering by nanohole defects. This study lays a foundation for the thermal measurement of 2D fluorinated materials and benefits future applications of fluorinated graphene.

Synergistic effect of temperature and point defect on the mechanical properties of single layer and bi-layer graphene

The present study reports a comprehensive molecular dynamics simulation of the effect of a) temperature (300–1073 K at intervals of every 100 K) and b) point defect on the mechanical behaviour of single (armchair and zigzag direction) and bilayer layer graphene (AA and AB stacking). Adaptive intermolecular reactive bond order (AIREBO) potential function was used to describe the many-body short-range interatomic interactions for the single layer graphene sheet. Moreover, Lennard Jones model was considered for bilayer graphene to incorporate the van der Waals interactions among the interlayers of graphene. The effect of temperature on the strain energy of single layer and bilayer graphene was studied in order to understand the difference in mechanical behaviour of the two systems. The strength of the pristine single layer graphene was found to be higher as compared to bilayer AA stacked graphene at all temperatures. It was observed at 1073 K and in the presence of vacancy defect the strength for single layer armchair sheet falls by 30% and for bilayer armchair sheet by 33% as compared to the pristine sheets at 300 K. The AB stacked graphene sheet was found to have a two-step rupture process. The strength of pristine AB sheet was found to decrease by 22% on increase of temperature from 300 K to 1073 K.

Phonon and Defect Induced Transparencies in the Mid-Infrared Spectrum of Grafted Single Layer Graphene

The Drude-like response of graphene in the terahertz and infrared region of the spectrum has made it attractive for optoelectronic applications in this range, because the response can be controlled by gating and doping. [1] Graphene infrared response can further be tailored for photonics and plasmonics, as the patterned material harbors low energy plasmon modes. [2] However, the infrared spectrum of pristine single layer graphene (SLG) is monotonous; in contrast to Raman, there are no infrared-active phonon modes, while bilayer graphene displays a Fano resonance in the infrared at ~1600 cm-1. [3] In a first time, we show experimentally that grafting SLG with halogenophenyl moieties induces optical transparencies at two specific energies: 1250 and 1600 cm-1 [4], in close similarity to the bands that can also be observed in carbon nanotubes as Fano resonances. [5] Unlike bands caused by the absorption of light by vibrational modes, these antiresonances show a decrease of the absorbance, an optical transparency effect. Moreover, we show that the amplitude of the transparencies can be modulated by changing the charge carrier density through doping, and by the defect density through controlled grafting. In as second time, we will present a theory based on quantum mechanics to calculate the optical conductivity of grafted SLG. [4] The model puts into play phonon modes with momenta different from Γ that can be addressed through scattering on defects. Numerical simulations reproduce the experimental data with good agreement. The theory also captures the dependence of the signal on charge carrier density and defect density. Our findings bring a new understanding for the physics behind the infrared activity of nanostructures, while opening new capabilities for tailoring the optical spectrum of nanomaterials.

Ultra-low thermal conductivity in graphene nanomesh

Graphene nanomesh (GNM), a new nanostructure of graphene, has attracted extensive interest recently due to the promising chemical, electronic and photonic applications. In this paper, another important property – thermal conductivity is systematically investigated by using molecular dynamics simulations. The thermal conductivity (κ) is found to be extremely low, up to more than 3 orders lower than the pristine single layer graphene. Roughly, κ decreases exponentially with increasing porosity and linearly with decreasing neck width, and is not temperature sensitive in the range of 300 K–700 K. κ of GNMs is found to be even up to 200-fold lower than the graphene nanoribbons (GNR), a potential thermoelectric material, of the same neck width and boundary-to-area ratio. The extremely low κ in the GNM makes it a potential candidate for thermoelectrics. The phonon participation spectra show that the low κ in GNM is due to the localization and phonon back scattering around the nanopores. We also find that the phonon coherence in two dimensional superlattice GNM indeed exists, but is not as important as in the one dimensional superlattices. The isotope effect is negligible. The thermal conductivity reduction by edge passivation increases with increasing neck width and porosity.

Ultrahigh-throughput exfoliation of graphite into pristine 'single-layer' graphene using microwaves and molecularly engineered ionic liquids.

Graphene has shown much promise as an organic electronic material but, despite recent achievements in the production of few-layer graphene, the quantitative exfoliation of graphite into pristine single-layer graphene has remained one of the main challenges in developing practical devices. Recently, reduced graphene oxide has been recognized as a non-feasible alternative to graphene owing to variable defect types and levels, and attention is turning towards reliable methods for the high-throughput exfoliation of graphite. Here we report that microwave irradiation of graphite suspended in molecularly engineered oligomeric ionic liquids allows for ultrahigh-efficiency exfoliation (93% yield) with a high selectivity (95%) towards 'single-layer' graphene (that is, with thicknesses <1 nm) in a short processing time (30 minutes). The isolated graphene sheets show negligible structural deterioration. They are also readily redispersible in oligomeric ionic liquids up to ~100 mg ml(-1), and form physical gels in which an anisotropic orientation of graphene sheets, once induced by a magnetic field, is maintained.