Defect Engineering Of Graphene Research Articles

Atomic-scale characterization of defects in oxygen plasma-treated graphene by scanning tunneling microscopy

Defects in graphene are important nanoscale pathways for metal atoms to enter the interface between epitaxial graphene and SiC in order to form stable ultrathin metal layers with new exotic physical properties. However, the atomic-scale details of defects that mainly govern the intercalation process remain modest. In this work, we present the first atomic investigation of point defects generated by oxygen plasma treatment on epitaxial graphene grown on SiC using low-temperature scanning tunneling microscopy, corroborated by density functional theory calculations. We found a broad spectrum of point defects that varies in size, shape, and symmetry and is dominated by triangular species. Tunneling spectroscopy identified defect-induced states in the vicinity of the Fermi level that significantly perturb the graphene electronic properties at the defect site. Based on the well-defined defect symmetry, we simulated the local density of states of the triangular defects and their corresponding scanning tunneling microscopy images which further helped us to identify the exact atomic configurations of monovacancy defects. The combination of atomic-scale scanning tunneling microscopy experiments and reliable density functional theory simulations provides ultimate microscopic details and opens a new way to identify the atomic configurations of defects in oxygen plasma-treated graphene. Our work might shed light on precise control of defect engineering in graphene for metal intercalation by controlling the defect types based on a deep understanding of each configuration.

Inverse kinetic isotope effect of proton and deuteron permeation through pyridinic N-doped graphene

Graphene is becoming an increasingly popular platform for developing high-efficiency hydrogen isotope separation technologies operated at room temperature. However, its performance can be strongly affected by contributions from atomic defects. Understanding the influence of defects on the hydrogen isotope selectivity of graphene is thus imperative but remains elusive. In this work, we investigated the electrically driven proton/deuteron permeation through nitrogen-doped graphene membranes with varying N concentrations and configurations. The results show that N-doping significantly enhances the proton and deuteron permeability of graphene (by one order of magnitude). More importantly, deuterons are found to permeate faster through pyridinic N-containing defects than protons, demonstrating a pronounced inverse kinetic isotope effect (KIE) with kH/kD of 0.75. Conversely, pyrrolic N-containing defects exhibit a normal KIE (kH/kD = 1.56). The exclusive contribution of the pyridinic N to the inverse KIE is confirmed by the reversed KIE factor of the pyridinic N-dominated sample subjected to modification. Theoretical calculations indicate that the explicit dependence of KIE on N configuration originates from the different leading mechanisms of proton/deuteron permeation through these N-containing defects, which is governed by the size and the chemisorption imparted by defects. These findings provide fundamental insights into the correlation between defect characteristics and proton/deuteron transport in graphene, and contribute to developing economical hydrogen isotope separation technologies through the defect engineering of graphene.

Dual Heteroatoms-Induced defect engineering in graphene towards metacomposites with epsilon-near-zero response

In this paper, the effect of diatomic doping on negative permittivity and epsilon-near-zero (ENZ) properties of graphene (GR)/polyvinylidene difluoride (PVDF) composites were investigated for the first time. Nitrogen (N) and phosphorus (P) atoms were doped into the GR lattice, and the doping sites and ratio of N and P atoms in the GR lattice were studied in detail. The results showed that introducing N and P atoms controlled the composite's percolation structure and carrier concentration. The dual atom synergy modulated the value of negative permittivity, and a stable weak negative permittivity between −30 and −40 was obtained. With further regulation of the diatomic doping ratio, an ENZ response in the kHz region was successfully acquired under synergistic effects. The ENZ response appeared at 170 kHz and exhibited a zero-crossing behavior. After the permittivity shifted to negative, the maximum negative permittivity was only −11.92 up to 1 MHz. The negative permittivity obtained for N/P co-doped GR/PVDF composites could all be perfectly fitted by Lorentz and/or Drude model. Further analysis revealed that plasma oscillation and dielectric resonance work in concert were necessary to produce a steady ENZ response in the kHz range.

Lateral Controlled Doping and Defect Engineering of Graphene by Ultra-Low-Energy Ion Implantation

In this paper, the effectiveness of ultra-low-energy ion implantation as a means of defect engineering in graphene was explored through the measurement of Scanning Kelvin Probe Microscopy (SKPM) and Raman spectroscopy, with boron (B) and helium (He) ions being implanted into monolayer graphene samples. We used electrostatic masks to create a doped and non-doped region in one single implantation step. For verification we measured the surface potential profile along the sample and proved the feasibility of lateral controllable doping. In another experiment, a voltage gradient was applied across the graphene layer in order to implant helium at different energies and thus perform an ion-energy-dependent investigation of the implantation damage of the graphene. For this purpose Raman measurements were performed, which show the different damage due to the various ion energies. Finally, ion implantation simulations were conducted to evaluate damage formation.

Porous reduced graphene oxide for ultrasensitive detection of nitrogen dioxide

The defect engineering in graphene plays a significant role for the application of gas sensors. In this work, we proposed an efficient method to prepare ultrasensitive gas sensors based on the porous reduced graphene oxide (PRGO). Photo-Fenton etching was carried out on GO nanosheets in a controlled manner to enrich their vacancy defects. The resulting porous graphene oxide (PGO) was then drop-coated on interdigital electrodes and hydrothermal reduced at 180 °C. Controllable reduction was achieved by varying the water amount. The gas sensor based on PRGO-5 min-6 h exhibited superior sensing and selective performance toward nitrogen dioxide (NO2), with an exceptional high sensitivity up to 12 ppm−1. The theoretical limit of detection is down to 0.66 ppb. The excellent performance could be mainly attributed to the typical vacancy defects of PRGO. Some residue carboxylic groups on the edges could also facilitate the adsorption of polar molecules. The process has a great potential for scalable fabrication of high-performance NO2 gas sensors.

Defect engineering of graphene using electron-beam chemistry with radiolyzed water

Defect engineering of graphene is attractive for a wide range of applications. Here, we present a mask-less, resist-free, and fully reversible process to engineer defects in graphene using electron-beam (e-beam) chemistry with radiolyzed water. This process was performed inside a variable pressure scanning electron microscope by generating radiolysis products using reactions between the e-beam and water vapor, which in turn reacted with the graphene at the location of the probe. These reactions enabled controlled chemistry on the graphene surface at a resolution of ∼60 nm and hence created defects in precise locations defined by the e-beam. Detailed characterization and theoretical analyses suggested the presence of sp3-type defects, the density of which was tuned by varying the e-beam dose. In addition, these sp3-type defects were cycled in and out of graphene by alternating e-beam chemistry and thermal annealing. This reversibility promises future applications of e-beam chemistry in reconfigurable plasmonics and electronics.

Electronic structure of electron-irradiated graphene and effects of hydrogen passivation

We report results for the electronic structure of irradiated and irradiation-induced amorphized graphene based on first-principles density functional theory calculations, using models of irradiated graphene sheets that were initially relaxed structurally according to molecular-dynamics simulations. We find that localized states appear at the Fermi level upon irradiation damage and the corresponding local density of states increases with increasing defect density. Electronic structure calculations show that band flattening occurs due to electron localization in the vicinity of irradiation-induced defects and reduces the charge carrier mobility. This band flattening effect becomes stronger with increasing defect density due to a greater degree of carrier localization at irradiation-induced carbon dangling bonds. Passivating these dangling bonds with hydrogen atoms delocalizes the charge density, reduces the density of states at the Fermi level, and increases the band dispersion. Hydrogen passivation also has the additional effect of quenching any localized magnetic moments at dangling bonds. Our studies show that defect engineering of graphene—even at a gross level without atomic-scale precision—can be employed to tune its properties for additional electronic functionality.

Dislocation Shielding of a Nanocrack in Graphene: Atomistic Simulations and Continuum Modeling

Combining atomistic simulations and continuum modeling, we studied dislocation shielding of a nanocrack in monolayer graphene under mode-I loading. Different crack-dislocation configurations were constructed and the shielding effects on the threshold stress intensity for crack propagation were examined. Excellent agreement between simulation results and linear-elastic fracture mechanics (LEFM) predictions was achieved. As the separation between the crack-tip and dislocation, that is, rR, varies (with respect to the crack size a), the shielding effect exhibits two different dependences on rR, scaling as 1/rR 1/2 for rR/a ≪ 1 (near-tip), whereas 1/rR for rR/a ≫ 1 (far-field), respectively. Particularly, the far-field 1/rR scaling was shown to be a direct manifestation of the stress field of dislocation in graphene. Our work presents a systematic study of nanoscale crackdislocation interactions in graphene, providing valuable information on defect engineering of graphene.

First-Principles Study of Dislocation Slips in Impurity-Doped Graphene

Employing density-functional theory (DFT) calculations, the generalized-stacking-fault energy (GSFE) curves along two crystallographic slips, glide and shuffle, for both pristine graphene and impurity of boron (B) or nitrogen (N) doped graphene were examined. The effects of B and N doping on the GSFE were clarified and correlated with local electron interactions and bonding configurations. The GSFE data were then used to analyze dislocation dipole and core structure, and subsequently combined with the Peierls-Nabarro (P-N) model to examine the role of doping on several key characteristics of dislocations in graphene. We showed that the GSFE curve may be significantly altered by the presence of dopants, which subsequently leads to profound modulations of dislocation properties, such as increasing spontaneous pair-annihilation distance and reducing resistance to dislocation slip. Our results indicate that doping can play an important role in controlling dislocation density and microscopic plasticity in graphene, thereby providing critical insights for dopant-mediated defect engineering in graphene.

AuCl3 chemical doping on defective graphene layer

This study investigated the effects of defects on chemical doping of graphene layer. Graphene grown by the chemical vapor deposition method on copper foil was subjected to ultraviolet treatments to introduce defects, including sp3 bonding and vacancies. The chemical doping process was performed using a gold chloride (AuCl3)/nitromethane solution at a concentration of 20 mM. Raman spectroscopy and four-point probe measurement were used to analyze the effects of AuCl3 doping on the electrical and optical properties of defective graphene. AuCl3 doping was effective for lowering sheet resistance even for the highly damaged graphene. Additionally, the 2D-peak of the defective graphene was partially recovered after AuCl3-based chemical doping. The authors believe that the defect engineering in graphene can enhance the electrical properties and the long-term stability of chemical doping.

Atom-by-Atom STEM Investigation of Defect Engineering in Graphene

Atom-by-Atom STEM Investigation of Defect Engineering in Graphene

A General Route Towards Defect and Pore Engineering in Graphene

Defect engineering in graphene is important for tailoring graphene's properties thus applicable in various applications such as porous membranes and ultra-capacitors. In this paper, we report a general route towards defect- and pore- engineering in graphene through remote plasma treatments. Oxygen plasma irradiation was employed to create homogenous defects in graphene with controllable density from a few to ≈10(3) (μm(-2)). The created defects can be further enlarged into nanopores by hydrogen plasma anisotropic etching with well-defined pore size of a few nm or above. The achieved smallest nanopores are ≈2 nm in size, showing the potential for ultra-small graphene nanopores fabrication.

Graphene-Assisted Controlled Growth of Highly Aligned ZnO Nanorods and Nanoribbons: Growth Mechanism and Photoluminescence Properties

We demonstrate graphene-assisted controlled fabrication of various ZnO 1D nanostructures on the SiO2/graphene substrate at a low temperature (540 °C) and elucidate the growth mechanism. Monolayer and a few layer graphene prepared by chemical vapor deposition (CVD) and subsequently coated with a thin Au layer followed by rapid thermal annealing is shown to result in highly aligned wurtzite ZnO nanorods (NRs) with clear hexagonal facets. On the other hand, direct growth on CVD graphene without a Au catalyst layer resulted in a randomly oriented growth of dense ZnO nanoribbons (NRBs). The role of in-plane defects and preferential clustering of Au atoms on the defect sites of graphene on the growth of highly aligned ZnO NRs/nanowires (NWs) on graphene was established from micro-Raman and high-resolution transmission electron microscopy analyses. Further, we demonstrate strong UV and visible photoluminescence (PL) from the as-grown and post-growth annealed ZnO NRs, NWs, and NRBs, and the origin of the PL emission is correlated well with the X-ray photoelectron spectroscopy analysis. Our results hint toward an epitaxial growth of aligned ZnO NRs on graphene by a vapor-liquid-solid mechanism and establish the importance of defect engineering in graphene for controlled fabrication of graphene-semiconductor NW hybrids with improved optoelectronic functionalities.

Properties of Strained Structures and Topological Defects in Graphene

Strain and defect engineering of graphene can modify the topological features of electronic states, leading to novel properties such as pseudomagnetism in bubbles and metallicity in extended topological defects. A consequence of graphene being a soft membrane is that it can be strain-engineered to become highly corrugated by modifying its adhesion to the substrate. Extended grain boundaries in graphene can be constructed from periodic combinations of nonhexagonal rings (5-7 pairs). However, a controlled method of producing these defects is not currently available. In this Perspective, we discuss some of the recent advances in studying the properties and formation mechanisms of strained structures and defects in graphene, extending across both physics and chemistry.

Mechanical properties of irradiated single-layer graphene

The mechanical properties of irradiated single-layer graphene sheets are determined as a function of inserted vacancy concentration. We find that the vacancy-induced crystalline-to-amorphous transition is accompanied by a brittle-to-ductile transition in the failure response of irradiated graphene sheets for inserted vacancy concentrations of 8%–12%. While point defects and larger voids appreciably degrade the strength of pristine graphene, we find that even heavily damaged samples (∼20% vacancies) exhibit tensile strengths of ∼30 GPa, in significant excess of those typical of engineering materials. Our results suggest that defect engineering of graphene is feasible without incurring a complete loss of its desirable mechanical properties.

Tunable Kondo effect in graphene with defects

Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer1 and a large number of electronic phenomena have been demonstrated in graphene, from the fractional2, 3 quantum Hall effect to superconductivity4. However, the coupling of conduction electrons to local magnetic moments5, 6, a central problem of condensed matter physics, has not been realized in graphene, and, given carbon's lack of d or f electrons, magnetism in graphene would seem unlikely. Nonetheless, magnetism in graphitic carbon in the absence of transition-metal elements has been reported7-10, with explanations ranging from lattice defects11 to edge structures12, 13 to negative curvature regions of the graphene sheet14. Recent experiments suggest that correlated defects in highly-ordered pyrolytic graphite (HOPG) induced by proton irradiation9 or native to grain boundaries7, can give rise to ferromagnetism. Here we show that point defects (vacancies) in graphene15 are local moments which interact strongly with the conduction electrons through the Kondo effect6, 16-18 providing strong evidence that defects in graphene are indeed magnetic. The Kondo temperature TK is tunable with carrier density from 30-90 K; the high TK is a direct consequence of strong coupling of defects to conduction electrons in a Dirac material18. The results indicate that defect engineering in graphene could be used to generate and control carrier-mediated magnetism, and realize all-carbon spintronic devices. Furthermore, graphene should be an ideal system in which to probe Kondo physics in a widely tunable electron system.