Defect-free Graphene Research Articles

Reversible Deposition of Lithium Particles Enabled by Ultraconformal and Stretchable Graphene Film for Lithium Metal Batteries.

Originating from inhomogeneous Li deposition and dissolution, the formation of dendritic and/or dead Li lies as a fundamental barrier to the practical implementation of Li metal anodes for high-energy Li-ion batteries. Here, an ultraconformal and stretchable solid-electrolyte interphase (SEI) composed of parallelly stacked few-layer defect-free graphene nanosheets, which can deform to remain ultraconformal during the expansion and shrinkage of micro-sized Li metal particles is reported. The shape-adaptive graphene protective skin is prepared via a facile mechanical method followed by Li stripping, which enables fast Li-ion diffusion, and inhibits Li dendrites and Li pulverization. The interlayer slips and wrinkles of the graphene film endow the robust protective skin with high stretchability. This work represents a unique strategy of building ultraconformal and stretchable 2D-materials-based protective skins on the surface of Li metal toward high-energy, long-life, and safe Li metal batteries.

Growth Phase Diagram of Graphene Grown Through Chemical Vapor Deposition on Copper

The phase diagram for graphene growth was obtained to understand the physics of the growth mechanism and control the layer number or coverage of graphene deposited on copper via low-pressure chemical vapor deposition (LPCVD). Management of the number of graphene layers and vacancies is essential for producing defect-free monolayer graphene and engineering multilayered functionalized graphene. In this work, the effects of the CH4 and H2 flow rates were investigated to establish the phase diagram for graphene growth. Using this phase diagram, we selectively obtained fully covered and partially grown monolayer graphene, graphene islands through Volmer–Weber growth, and multilayer graphene through Stranski–Krastanov-like growth. The layer numbers and coverage were determined using optical microscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy. The growth modes were determined by the competition between catalytic growth with CH4 and catalytic etching with H2 on the copper surface during CVD growth. Intriguingly, this phase diagram showed that multilayer graphene flakes can be grown via LPCVD even with low CH4 and H2 flows.

Mechanism of CO Intercalation through the Graphene/Ni(111) Interface and Effect of Doping.

Molecules intercalate at the graphene/metal interface even though defect-free graphene is impermeable to any atomic and molecular species in the gas and liquid phase, except hydrogen. The mechanism of molecular intercalation is still a big open question. In this Letter, by means of a combined experimental (STM, XPS, and LEED) and theoretical (DFT) study, we present a proof of how CO molecules succeed in permeating the graphene layer and get into the confined zone between graphene and the Ni(111) surface. The presence of N-dopants in the graphene layer is found to highly facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude, through the stabilization of multiatomic vacancy defects that are the open doors to the bidimensional nanospace, with crucial implications for the catalysis under cover and for the graphene-based electrochemistry.

Graphene: an exotic condensed matter and its impact on technology

Graphene is a (2 + 1)-dimensional quantum electrodynamic system. The carriers have a vanishing mass and a relativistic velocity of 106 m/s, obeying charge-conjugate parity time symmetry. Graphene exhibits the Klein paradox, due to which, through spatial confinement, a bandgap can be opened in zigzag graphene nanoribbons for logic applications. The high Debye temperature of 2800 K ensures that phonons are frozen out and lattice scattering is suppressed at 300 K. The perfect crystallinity ensures that defect/impurity scattering is suppressed. The two together give very high electrical conductivity and electric mobility. This permits a current density of 108 A/cm2, about 100 times greater than that in copper. Single-layer graphene has a thermal conductivity of 3000–5000 (W/m)/K at 300 K, but graphite has K = 2000 (W/m)/K. This may open up few-layer graphene applications in thermal management of nanoelectronics. The half-integer quantum Hall effect and non-zero Berry phase have been verified in the laboratory. These magneto-transport properties are a result of the exceptional topology of the graphene band structure. Ballistic transport observable up to 300 K makes graphene an ideal replacement for silicon (Si) electronics. The optical response of graphene is determined by the fine-structure constant over a wide band of the visible spectrum, and hence, it is ideal for high-speed optical modulators. This paper describes a commercially significant process for synthesizing large-area fold-free and defect-free graphene.

Composites Based on Nanodispersed Nickel, Graphene-Like Carbon, and Aerosil for Catalytic Hydrogenation of Furfural and Quinoline

It is shown that a composite containing nickel nanoparticles and carboneous multi-layer graphene-like particles on aerosil, prepared by pyrolysis of deposited nickel complex with melamine, catalyzes hydrogenation of furfural to furfuryl alcohol as well as hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline. The highest yield of hydrogenation products (27% of furfuryl alcohol at 100°C, 30 atm H2, and 25% of quinoline at 100°C, 100 atm H2) is achieved with the use of composites with higher nickel content (8.4%) and higher size of defect-free graphene areas (ratio of band intensities ID/IG in Raman spectrum 0.9).

Interface between graphene and liquid Cu from molecular dynamics simulations.

Controllable synthesis of defect-free graphene is crucial for applications since the properties of graphene are highly sensitive to any deviations from the crystalline lattice. We focus here on the emerging use of liquid Cu catalysts, which have high potential for fast and efficient industrial-scale production of high-quality graphene. The interface between graphene and liquid Cu is studied using force field and ab initio molecular dynamics, revealing a complete or partial embedding of finite-sized flakes. By analyzing flakes of different sizes, we find that the size-dependence of the embedding can be rationalized based on the energy cost of embedding vs bending the graphene flake. The embedding itself is driven by the formation of covalent bonds between the under-coordinated edge C atoms and the liquid Cu surface, which is accompanied by a significant charge transfer. In contrast, the central flake atoms are located around or slightly above 3 Å from the liquid Cu surface and exhibit weak van der Waals-bonding and much lower charge transfer. The structural and electronic properties of the embedded state revealed in our work provide the atomic-scale information needed to develop effective models to explain the special growth observed in experiments where various interesting phenomena such as flake self-assembly and rotational alignment, high growth speeds, and low defect densities in the final graphene product have been observed.

Oxygen intercalation in PVD graphene grown on copper substrates: A decoupling approach

We investigate the intercalation process of oxygen in-between a PVD-grown graphene layer and different copper substrates as a methodology for reducing the substrate-layer interaction. This growth method leads to an extended defect-free graphene layer that strongly couples with the substrate. We have found, by means of X-ray photoelectron spectroscopy, that after oxygen exposure at different temperatures, ranging from 280 °C to 550 °C, oxygen intercalates at the interface of graphene grown on Cu foil at an optimal temperature of 500 °C. The low energy electron diffraction technique confirms the adsorption of an atomic oxygen adlayer on top of the Cu surface and below graphene after oxygen exposure at elevated temperature, but no oxidation of the substrate is induced. The emergence of the 2D Raman peak, quenched by the large interaction with the substrate, reveals that the intercalation process induces a structural undoing. As suggested by atomic force microscopy, the oxygen intercalation does not change significantly the surface morphology. Moreover, theoretical simulations provide further insights into the electronic and structural undoing process. This protocol opens the door to an efficient methodology to weaken the graphene-substrate interaction for a more efficient transfer to arbitrary surfaces.

Proton and Li-Ion Permeation through Graphene with Eight-Atom-Ring Defects.

Defect-free graphene is impermeable to gases and liquids but highly permeable to thermal protons. Atomic-scale defects such as vacancies, grain boundaries, and Stone-Wales defects are predicted to enhance graphene's proton permeability and may even allow small ions through, whereas larger species such as gas molecules should remain blocked. These expectations have so far remained untested in experiment. Here, we show that atomically thin carbon films with a high density of atomic-scale defects continue blocking all molecular transport, but their proton permeability becomes ∼1000 times higher than that of defect-free graphene. Lithium ions can also permeate through such disordered graphene. The enhanced proton and ion permeability is attributed to a high density of eight-carbon-atom rings. The latter pose approximately twice lower energy barriers for incoming protons compared to that of the six-atom rings of graphene and a relatively low barrier of ∼0.6 eV for Li ions. Our findings suggest that disordered graphene could be of interest as membranes and protective barriers in various Li-ion and hydrogen technologies.

(Invited) Exotic Electronic Excitation to Manipulate Electrochemical Potential of Electrons and Holes at Liquid/Solid Interfaces

Spectroelectrochemistry is powerful tool to investigate electronic sates and geometrical structures of electrified interfaces in-situ under the conditions where photonic and electronic energy conversion functionalities emerge. One can use plasmon-powered processes to expand the ability of the spectroscopy regarding sensitivities in space, time, and energy resolutions. As their characteristics on localization of surface plasmons into molecule scale, huge intensity gradient of electromagnetic field would break the limit of light-matter interactions via the modification of the selection rule of the electronic excitation process. I have discussed about the possibility for the surface-enhanced-Raman scattering (SERS) as the probe to monitor the exotic electronic excitation processes[1-7]. Based on the observation of a small number of molecules at the plasmonic metal nanostructures by SERS, possibilities leading to the future innovative photo-functionalities were demonstrated by showing improved properties at optical manipulation of electrons and molecules, and ions. In this presentation, we will show that the localized surface plasmons of the Au nanodimer structures prepared on defect-free graphene provide highly-confined electromagnetic field to generate holes and electrons of the electrochemical potentials beyond the limit of far field light illumination [8]. Novel non-zero wavevector excitation results in the generation of highly reductive and oxidative powers for electrochemical reactions. The present findings point to a novel approach to breaking the limit of opto-electronic interactions and photochemical reactions using various materials.

Optical Absorption Spectrum of Small Graphene Fragments with Topological Defects

Energy spectra of small graphene fragments with and without topological defects (vacancies, nanopores and Stone–Wales defects) are obtained using the Hubbard model based on the approximation of static fluctuations. Modeling of optical absorption spectra is performed for both defect-free and defective small graphene fragments of the lattice. The analysis is given to optical absorption spectra changing with increasing number of vacancies. It is shown how the energy gap depends on the defect configuration and concentration.

Limits on gas impermeability of graphene.

Despite being only one-atom thick, defect-free graphene is considered to be completely impermeable to all gases and liquids1-10. This conclusion is based on theory3-8 and supported by experiments1,9,10 that could not detect gas permeation through micrometre-size membranes within a detection limit of 105 to 106 atoms per second. Here, using small monocrystalline containers tightly sealed with graphene, we show that defect-free graphene is impermeable with an accuracy of eight to nine orders of magnitude higher than in the previous experiments. We are capable of discerning (but did not observe)permeation of just a few helium atoms per hour, and this detection limit is also valid for all other gasestested (neon, nitrogen, oxygen, argon, krypton and xenon), except for hydrogen. Hydrogen shows noticeable permeation, even though its molecule is larger than helium and should experience a higher energy barrier. This puzzling observation is attributed to a two-stage process that involves dissociation of molecular hydrogen at catalytically active graphene ripples, followed by adsorbed atoms flipping to the other side of the graphene sheet with a relatively low activation energy of about 1.0electronvolt, a value close to that previously reported for proton transport11,12. Our work provides a key reference for the impermeability of two-dimensional materials and is important from a fundamental perspective and for their potential applications.

High-Yield Production of Aqueous Graphene for Electrohydrodynamic Drop-on-Demand Printing of Biocompatible Conductive Patterns.

Presented here is a scalable and aqueous phase exfoliation of graphite to high yield and quality of few layer graphene (FLG) using Bovine Serum Albomine (BSA) and wet ball milling. The produced graphene ink is tailored for printable and flexible electronics, having shown promising results in terms of electrical conductivity and temporal stability. Shear force generated by steel balls which resulted in 2–3 layer defect-free graphene platelets with an average size of hundreds of nm, and with a concentration of about 5.1 mg/mL characterized by Raman spectroscopy, atomic force microscopy (AFM), transmittance electron microscopy (TEM) and UV-vis spectroscopy. Further, a conductive ink was prepared and printed on flexible substrate (Polyimide) with controlled resolution. Scanning electron microscopy (SEM) and Profilometry revealed the effect of thermal annealing on the prints to concede consistent morphological characteristics. The resulted sheet resistance was measured to be for prints as long as 100 mm. Printable inks were produced in volumes ranging from 20 mL to 1 L, with potential to facilitate large scale production of graphene for applications in biosensors, as well as flexible and printable electronics.

Stable, concentrated, biocompatible, and defect-free graphene dispersions with positive charge.

The outstanding properties of graphene offer high potential for biomedical applications. In this framework, positively charged nanomaterials show better interactions with the biological environment, hence there is strong interest in the production of positively charged graphene nanosheets. Currently, production of cationic graphene is either time consuming or producing dispersions with poor stability, which strongly limit their use in the biomedical field. In this study, we made a family of new cationic pyrenes, and have used them to successfully produce water-based, highly concentrated, stable, and defect-free graphene dispersions with positive charge. The use of different pyrene derivatives as well as molecular dynamics simulations allowed us to get insights on the nanoscale interactions required to achieve efficient exfoliation and stabilisation. The cationic graphene dispersions show outstanding biocompatibility and cellular uptake as well as exceptional colloidal stability in the biological medium, making this material extremely attractive for biomedical applications.

On Fracture of Pseudo-Graphenes

Abstract—Molecular dynamics is used to study the fracture of pseudo-graphenes, namely, allotropic forms of carbon that are modifications of graphene with a high density of periodically distributed disclinations with zero total charge. Disclinations in pseudo-graphenes are associated with improper carbon rings, that is, rings having 4, 5, 7, or 8 members, in contrast to the proper 6-member carbon rings constituting the ideal crystal lattice of graphene. Pseudo-graphenes, which have a significant excess of energy relative to graphene, show a significant (up to 50%) decrease in the critical tensile stress compared to defect-free graphene.

Synergistic Effect of a Defect-Free Graphene Nanostructure as an Anode Material for Lithium Ion Batteries.

Graphene nanosheets have been among the most promising candidates for a high-performance anode material to replace graphite in lithium ion batteries (LIBs). Studies in this area have mainly focused on nanostructured electrodes synthesized by graphene oxide (GO) or reduced graphene oxide (rGO) and surface modifications by a chemical treatment. Herein, we propose a cost-effective and reliable route for generating a defect-free, nanoporous graphene nanostructure (df-GNS) through the sequential insertion of pyridine into a potassium graphite intercalation compound (K-GIC). The as-prepared df-GNS preserves the intrinsic property of graphene without any crystal damage, leading to micro-/nano-porosity (microporosity: ~10–50 µm, nanoporosity: ~2–20 nm) with a significantly large specific surface area. The electrochemical performance of the df-GNS as an anode electrode was assessed and showed a notably enhanced capacity, rate capability, and cycle stability, without fading in capacity or decaying. This is because of the optimal porosity, with perfect preservation of the graphene crystal, allowing faster ion access and a high amount of electron pathways onto the electrode. Therefore, our work will be very helpful for the development of anode and cathode electrodes with higher energy and power performance requirements.

Exploring Voltage Mediated Delamination of Suspended 2D Materials as a Cause of Commonly Observed Breakdown

Two-dimensional (2D) barrier materials such as graphene, boron nitride, and molybdenum disulfide hold great promise for important applications such as DNA sequencing, desalination, and biomolecular sensing. The 2D materials commonly span pores through an insulating membrane, and electrical fields are applied to drive cross-barrier transport of charged solvated species. While the low-voltage transmembrane transport is well-understood and controllable, high-voltage phenomena are uncontrolled and result in the apparent breakdown of the 2D material's critical insulating properties. Here we use suspended graphene over a 50 nm silicon nitride nanopore as a model system and show that delamination of the 2D material occurs at higher voltages and can directly cause a number of the puzzling high-voltage transport observations. We confirm the occurrence of delamination and observe via atomic force microscopy measurement a micron-scale delaminated patch in a system using chemical vapor deposition graphene. Furthermore, we show that the conductivity of the same system is strongly correlated to the area of delamination via coincident current measurements and optical imaging of the delaminated area. Finally, we demonstrate that delamination alone can cause a dramatic breakdown of barrier function through observation of a reversible increase in conductance of samples prepared with pristine defect-free graphene. These findings should have a great impact on the design and interpretation of 2D barrier material for both experiments and applications.

Preparation of ultrathin defect-free graphene sheets from graphite via fluidic delamination for solid-contact ion-to-electron transducers in potentiometric sensors

In this study, ultrathin and defect-free graphene (Gr) sheets were prepared through a fluid dynamics–induced shear exfoliation method using graphite. The high hydrophobicity and surface area of Gr make it attractive as a solid-contact ion-to-electron transducer for potentiometric K+ sensors, in which the electrodes are fabricated through a screen-printing process. The electrochemical characterization demonstrates that Gr solid contact results in a high double-layer capacitance, potential stability, and strong resistance against water layer, gases, and light. The Gr-based K+ sensors showed a Nernstian slope of 53.53 mV/log[K+] within a linear concentration range of 10−1–10−4 M, a low detection limit of 10−4.28 M, a fast response time of ~8 s, good repeatability, and excellent long-term stability. Moreover, the Gr-based K+ sensors provided accurate ion concentrations in actual samples of human sweat and sports drinks.

Effect of structural defects on the strength of adsorption of La and Lu species on graphene

By using DFT calculations, we addressed the influence of topological defects on the bonding strength and some electronic parameters for lanthanide atoms La0 and Lu0, ions La3+ and Lu3+ interacting with graphene. The results obtained for the cluster models containing isolated pentagon (5), Stone-Wales (SW) defect and pyracylene unit (5665), as well as for C60 fullerene, were compared to those for a defect-free graphene model (G). We found that topological defects are always preferential sites for Lnn adsorption, as compared to pristine graphene. Particular orders of complex stability depend on lanthanide species, carbon cluster model, the Lnn position when the cluster has positive curvature (‘endo’, inside, or ‘exo’, outside), and are as follows: for La0, 5665_endo > SW > 5_endo > 5_exo > 5665_exo > C60 > G; for La3+, SW > G > 5665_endo > 5665_exo > 5_endo > 5_exo > C60; for Lu0, 5665_endo > 5_endo > 5_exo > SW > 5665_exo > C60 > G; for Lu3+, SW > G > 5665_endo > 5_endo > 5665_exo > 5_exo > C60. Ln…C distances, frontier orbital energies, HOMO, LUMO and spin density plots, as well as charge and spin on Lnn species from Mulliken population analysis are presented.

Visualization of subnanometric phonon modes in a plasmonic nano-cavity via ambient tip-enhanced Raman spectroscopy

Phonons provide information on the physicochemical properties of a crystalline lattice from the material’s vibrational spectrum. Optical phonons, in particular, can be probed at both micrometre and nanometre scales using light-based techniques, such as, micro-Raman and tip-enhanced Raman spectroscopy (TERS), respectively. Selection rules, however, govern the accessibility of the phonons and, hence, the information that can be extracted about the sample. Herein, we simultaneously observe both allowed and forbidden optical phonon modes of defect-free areas in monolayer graphene to study nanometre scale strain variations and plasmonic activation of the Raman peaks, respectively, using our home-built TERS system in ambient. Through TERS imaging, strain variations and nanometre-sized domains down to 5 nm were visualised with a spatial resolution of 0.7 nm. Moreover, such subnanometric confinement was found to activate not only the D and D’ forbidden phonon modes but also their D + D’ combination mode. With our TERS in ambient system, the full phonon characterisation of defect-free graphene and other 2D nanomaterials is now possible, which will be useful for subnanometre strain analysis and exploring the inherent properties of defect-free materials.

Nonzero Wavevector Excitation of Graphene by Localized Surface Plasmons.

Electrochemical surface-enhanced Raman scatteringmeasurements of single layer graphene provide unique information on resonant excitation induced by localized surface plasmons under controlled electron or hole doping. The highly confined electromagnetic field from the LSPs of the Au nanodimer structures prepared on defect-free graphene can generate holes and electrons of the electrochemical potentials beyond the limit of far-field light illumination. The electrochemical in situ SERS spectra prove nonzero wavevector excitation through the observation of normally forbidden Raman bands in graphene. The present findings point to a novel approach to breaking the limit of optoelectronic interactions and photochemical reactions of graphene and other semiconductors.