Structure and electronic properties of closed-ring defects in epitaxial graphene

The density functional tight binding approach (DFTB) is well adapted for the study of point and line defects in graphene based systems. After briefly reviewing the use of DFTB in this area, we present a comparative study of defect structures, energies, and dynamics between DFTB results obtained using the dftb+ code, and density functional results using the localized Gaussian orbital code, AIMPRO. DFTB accurately reproduces structures and energies for a range of point defect structures such as vacancies and Stone–Wales defects in graphene, as well as various unfunctionalized and hydroxylated graphene sheet edges. Migration barriers for the vacancy and Stone–Wales defect formation barriers are accurately reproduced using a nudged elastic band approach. Finally we explore the potential for dynamic defect simulations using DFTB, taking as an example electron irradiation damage in graphene.magnified image DFTB‐MD derived sputtering energy threshold map for a carbon atom in a graphene plane.

Graphene and carbon nanotubes have extraordinary mechanical and electronic properties. Intrinsic line defects such as local non-hexagonal reconstructions or grain boundaries, however, significantly reduce the tensile strength, but feature exciting electronic properties. Here, we address the properties of line defects in graphene from first-principles on the level of full-potential density-functional theory, and assess doping as one strategy to strengthen such materials. We carefully disentangle the global and local effect of doping by comparing results from the virtual crystal approximation with those from local substitution of chemical species, in order to gain a detailed understanding of the breaking and stabilization mechanisms. We find that n-type doping or local substitution with nitrogen increases the ultimate tensile strength significantly. In particular, it can stabilize the defects beyond the ultimate tensile strength of the pristine material. We therefore propose this as a key strategy to strengthen graphenic materials. Furthermore, we find that doping and/or applying external stress lead to tunable and technologically interesting metal/semi-conductor transitions.

Topological defects can affect the physical properties of graphene in unexpected ways. Harnessing their influence may lead to enhanced control of both material strength and electrical properties. Here we present a new class of topological defects in graphene composed of a rotating sequence of dislocations that close on themselves, forming grain boundary loops that either conserve the number of atoms in the hexagonal lattice or accommodate vacancy/interstitial reconstruction, while leaving no unsatisfied bonds. One grain boundary loop is observed as a "flower" pattern in scanning tunneling microscopy (STM) studies of epitaxial graphene grown on SiC(0001). We show that the flower defect has the lowest energy per dislocation core of any known topological defect in graphene, providing a natural explanation for its growth via the coalescence of mobile dislocations.

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.

We present a systematic study of 3d transition-metal (TM) atomic chains adsorbed on a line defect consisting of octagonal and pentagonal carbon rings embedded in a perfect graphene sheet. The results suggest that Sc, Ti, V, Cr and Mn atoms can be spontaneously adsorbed at octagonal sites and form an atomic chain along the line defect in graphene, while Fe, Co and Ni atomic chains are adsorbed at pentagonal sites. Moreover, the Sc, Ti, V, Cr, Mn and Fe chains embedded in line defect of graphene structures show half-metallic characters. These results propose a new route to explore the potential applications of the system in spintronic devices.

Global air quality has deteriorated significantly in recent years due to large emissions from the transformation industry and combustion vehicles. This issue requires the development of portable, highly sensitive, and selective gas sensors. Nanostructured materials, including defective graphene, have emerged as promising candidates for such applications. In this work, we investigated the B-doped topological line defect in graphene as a sensing material for various gas molecules (CO, CO2, NO, and NH3) based on a combination of density functional theory and the non-equilibrium Green's function method. The electronic transport calculations reveal that the electric current can be confined to the line defect region by gate voltage control, revealing highly reactive sites. The B-doped topological line defect is metallic, favoring the adsorption of NO and NH3 over CO and CO2 molecules. We notice changes in the conductance after gas molecule adsorption, producing a sensitivity of 50% (16%) for NO (NH3). In addition, the recovery time for nitride gases was calculated for different temperatures and radiation frequencies. At 300 K the ultraviolet (UV) has a fast recovery time compared to the visible (VIS) one by about two orders of magnitude. This study gives an understanding of how engineering transport properties at the microscopic level (by topological line defect and chemical B-doping) leads to promising nanosensors for detecting nitride gas.

The application of graphene, a one atom-thick honeycomb lattice of carbon atoms with superlative properties, such as electrical conductivity, thermal conductivity and strength, has already shown that it can be used to benefit metrology itself as a new quantum standard for resistance. However, there are many application areas where graphene and other 2D materials, such as molybdenum disulphide (MoS2) and hexagonal boron nitride (h-BN), may be disruptive, areas such as flexible electronics, nanocomposites, sensing and energy storage. Applying metrology to the area of graphene is now critical to enable the new, emerging global graphene commercial world and bridge the gap between academia and industry. Measurement capabilities and expertise in a wide range of scientific areas are required to address this challenge. The combined and complementary approach of varied characterisation methods for structural, chemical, electrical and other properties, will allow the real-world issues of commercialising graphene and other 2D materials to be addressed. Here, examples of metrology challenges that have been overcome through a multi-technique or new approach are discussed. Firstly, the structural characterisation of defects in both graphene and MoS2 via Raman spectroscopy is described, and how nanoscale mapping of vacancy defects in graphene is also possible using tip-enhanced Raman spectroscopy (TERS). Furthermore, the chemical characterisation and removal of polymer residue on chemical vapour deposition (CVD) grown graphene via secondary ion mass spectrometry (SIMS) is detailed, as well as the chemical characterisation of iron films used to grow large domain single-layer h-BN through CVD growth, revealing how contamination of the substrate itself plays a role in the resulting h-BN layer. In addition, the role of international standardisation in this area is described, outlining the current work ongoing in both the International Organization of Standardization (ISO) and the International Electrotechnical Commission (IEC).

Using first‐principles and analytical potential atomistic simulations, production of defects in epitaxial graphene (EG) on SiC upon ion irradiation for ion types and energies accessible in helium‐ion microscope is studied. Graphene‐SiC systems consisting of the buffer (zero) graphene layer and SiC substrate, as well as one (monolayer) and two (bilayer) additional graphene layers, are focused on. The probabilities for single, double, and more complex vacancies to appear upon impacts of energetic ions in each graphene layer as functions of He‐ and Ne‐ion energies are calculated and the data are compared with those obtained for free‐standing graphene. The results indicate that the role of the substrate is minimal for He‐ion irradiation with energies above 5 keV, which can be associated with a low sputtering yield from this system upon ion irradiation, as compared with the common Si/SiO2 substrate. In contrast, SiC substrate has a significant effect on defect production upon Ne‐ion irradiation. The results can serve as a guide to the experiments on ion irradiation of EG to choose the optimum ion beam parameters for defect‐mediated engineering of such systems, for example, for creating nucleation centers to grow other 2D materials, such as h‐BN, on top of the irradiated EG.

Thermal decomposition of SiC at high temperature usually brings about excessively fast Si sublimation and a very rough surface. In order to fabricate high-quality homogeneous epitaxial graphene on a SiC(0001) substrate, highly reactive erbium atoms are employed in this work. Scanning tunneling microscopy and Raman spectroscopy have been utilized to investigate the modulations of Er atoms on graphitization evolution and structural defects for graphene after annealing durations. Experimental results show that Er atoms pre-deposited on clean substrates can definitely enhance the surface graphitization of SiC and make graphene grow in a controllable way. The existence of Er layer is believed to break Si–C bonds at low temperature and to decrease the Si sublimate rate. It is also demonstrated that Er atoms can modify the type of structural defects in graphene, and the areal density of flower defects increases to 1.22 × 1012 cm−2, quadrupling that in pristine graphene. This work puts forward a fabrication method for epitaxial graphene with flower defects in high density and will enlighten some future applications of graphene in nanoelectronics, electron energy filtering, and chemical catalysis.

A major difficulty in implementing carbon-based electrode arrays with high device-packing density is to ensure homogeneous and high sensitivities across the array. Overcoming this obstacle requires quantitative microscopic models that can accurately predict electrode sensitivity from its material structure. Such models are currently lacking. Here, it is shown that the sensitivity of graphene electrodes to dopamine and serotonin neurochemicals in fast-scan cyclic voltammetry measurements is strongly linked to point defects, whereas it is unaffected by line defects. Using the physics of point defects in graphene, a microscopic model is introduced that explains how point defects determine sensitivity. The predictions of this model match the empirical observation that sensitivity linearly increases with the density of point defects. This model is used to guide the nanoengineering of graphene structures for optimum sensitivity. This approach achieves reproducible fabrication of miniaturized sensors with extraordinarily higher sensitivity than conventional materials. These results lay the foundation for new integrated electrochemical sensor arrays based on nanoengineered graphene.

Two-dimensional material graphene has proven to have remarkable electrical and photonic properties, opening the door to a wide range of uses, including employment under the harsh conditions of space. The creation of graphene on various substrate types is known to be possible via a number of approaches, including direct deposition and the substrate transfer process. In this work, we used an argon plasma, methane as a carbon source, and a nanoCVD-8G graphene reactor to deposit monolayer graphene (MLG) on transition metal substrates for studying the effects of gamma irradiation on the physical and electronic properties. Graphene's crystalline structure is investigated utilizing Raman and X-ray Photo Electron Spectroscopy (XPS) techniques before and after gamma irradiation. The results show that point defects predominate in the damage following gamma irradiation. The defective structure and electronic properties are connected in light of density functional theory (DFT) simulations of pristine and defective graphene.

Single Ru atom supported on defective graphene for water splitting: DFT and microkinetic investigation

By processing graphene quantum dot, an ideal semiconductor material with suitable band gap and higher electron mobility can be obtained. Thus, it has a broad prospect in the application of photoelectric response materials. Here, a graphene defect with porphyrin-like structure is selected to achieve the controlable light absorption. The double five-membered-ring parallel vacancy are based on self-healing properties of popular graphene defects. Aimed to separate exciton and hole more effectively and achieve higher photoelectric conversion efficiency, the occupied orbital and unoccupied orbital of the quantum dot with objected defect structure is taken as orderly dispersion to form an obvious charge separation state under the demonstration with first principles calculation. Most importantly, a real time real space charge separation is calculated by time-dependent ab-initio quantum dynamics based on numerical atomic basis sets. The result shows the specific graphene defects can form an efficient pure graphene photoelectric response medium like porphyrin skeleton, and the vacancy will induce to adjust and control the specific wavelength of the response light and charge separated state manipulably with odevity of number of peripheral carbon rings by the calculation of ultrafast process.

Single-atom catalyst (SAC) has an electronic structure that is very different from its bulk counterparts, and has shown unexpectedly high specific activity with a significant reduction of noble metal usages although physical origins of such performance enhancements are still poorly understood. Herein, by means of density functional theory calculations, we for the first time investigate the great potential of single atom catalyst for CO2 electroreduction applications. In particular, we study a single transition metal atom anchored on the defective graphene with single or double vacancies, denoted as M@sv-Gr or M@dv-Gr, where M are various transition metals, as a CO2 reduction catalyst. Many SACs are indeed shown to be highly selective for CO2 reduction reaction over a competitive H2 evolution reaction due to a favorable adsorption of carboxyl or formate over hydrogen on the catalysts. On the basis of free energies, we found that the Pt@dv-Gr catalyst shows a remarkable reduction in the limiting potential for CH3OH production compared to any existing catalysts, synthesized or predicted (Figure 1). We focus on the Pt@dv-Gr to investigate the origin of improvement on the SACs compared to the transition metal. As shown in Figure 1, with the Pt@dv-Gr catalyst, all the intermediates are destabilized compared to those on Pt (211), but most importantly, the destabilization of *CO (0.98 eV) is much more noticeable than that of *CHO (0.42 eV), leading to a 0.49 V reduction in the limiting potential. We also observe that with SACs, the conventional scaling relation between the *CO binding and *CHO binding established for the bulk transition metal significantly deviates from linearity (Figure 2). We discuss features of SACs which contribute to the breakdown of scaling relation between *CO and *CHO, namely, the lack of atomic ensemble for adsorbates binding and the metal-support interactions that lead to the electronic structures conducive to the catalysis. Atomic Ensemble: The optimized geometries of the bare catalysts are shown for Pt@dv-Gr and Pt (211) in Figure 3. One can see that, for Pt (211), two surface Pt atoms are involved in the *CO bonding while only one Pt atom is bonding with *CHO, leading to a large destabilization in the relative free energies when going from *CO to *CHO. On the other hand, for Pt@dv-Gr, only one Pt atom is utilized for both *CO and *CHO binding, resulting in a much more moderate destabilization in relative free energies compared to Pt (211). Thus, the lack of Pt ensemble in the Pt@dv-Gr is responsible for a significantly weaker binding of *CO on the Pt@dv-Gr compared to the Pt (211) surface. Electronic Structure: The strong metal-support interaction affects the electronic structure of a metal atom in the SACs greatly. In Figure 4A, the Pt 5d density of states (DOS) in Pt@dv-Gr shows a significant orbital overlap with the C 2p orbitals of the graphene. The electron density isosurfaces (Figure 4B) illustrate that electron clouds of four carbon atoms surrounding the Pt atom are significantly hybridized with the Pt atom. The differential charge density map (Figure 4C) between the defective graphene and Pt@dv-Gr also suggests that the Pt atom is positively charged by electron transfer from the Pt atom to the defective graphene support. In this paper, we investigated the single atom catalysts (SACs) as a promising CO2 electroreduction catalyst using DFT calculations. The main findings of this work are as follows. (i) By comparing the free energies of the initial protonation steps for the CRR and HER, we found that all SACs can selectively reduce CO2 rather than producing H2. In particular, the predicted limiting potential for Pt@dv-Gr (-0.27 V) for CH3OH production is considerably less negative than the conventional transition metal catalysts. (ii) To understand the origin of the improvements in SACs, we investigated two aspects of the Pt@dv-Gr that affect the relative stability of *CO vs. *CHO. A one-fold bonding of *CO on Pt@dv-Gr due to a lack of atomic ensemble, as compared to the two-fold *CO bonding on the Pt (211) is responsible for the significant weakening of *CO binding on the Pt@dv-Gr. (iii) We investigated the electronic structure of Pt atom in the SAC to find the origin of the deviation of SACs from the conventional scaling relation of transition metals, which arises from the d-band center theory. We suggest that the strong electronic interaction between the d-orbital of metal atom and the p-orbital of graphene is responsible for the different behavior from the transition metal surfaces, as evidenced by the electron transfer and the overlap in the DOS. Figure 1

Patterning SiC substrates with focused ion beam for growth of confined graphene nanostructures is interesting for fabrication of graphene devices. However, by imposing an ion beam, the morphology of illuminated SiC substrate surface is inevitably damaged, which imposes significant effects on the subsequent growth of graphene. By using confocal Raman spectroscopy, we investigate the effects of ion beam illumination on the quality of graphene layers that are grown on 6H-SiC (0001) substrates with two different growth methods. With the first method, the 6H-SiC (0001) substrate is flash annealed in ultra-high vacuum. Prominent defects in graphene grown on illuminated areas are revealed by the emergence of Raman D peak. Significant changes in D peak intensity are observed with Ga+ ion fluence as low as 105 μm−2. To eliminate the damage from the ion beam illumination, hydrogen etching is employed in the second growth method, with which prominent improvement in the quality of crystalline graphene is revealed by its Raman features. The defect density is significantly reduced as inferred from the disappearance of D peak. The Raman shift of G peak and 2D peak indicates strain-released graphene layers as grown in such a method. Such results provide essential information for patterning graphene nano-devices.