Direct imaging of lattice atoms and topological defects in graphene membranes

Controllable defects in monolayer graphene induced by hydrogen and argon plasma

Graphene has attracted wide attentions since its successfully exfoliation. Honeycombsp2carbon lattice and Dirac semi-metal band structure make graphene a promising material with excellent mechanical strength, thermal conductivity, and carrier mobility. However, the absence of intrinsic bandgap limits its application in semiconductor. Defects in graphene is supposed to modify its band structure and lead to an opened bandgap. Many methods have been demonstrated to introduce defects into graphene, such as chemical reaction, plasma, electron beam, and laser. However, the species of defects are mostly uncontrollable in most treatment processes. In this study, we report three kinds of defects can be controllably induced in graphene via hydrogen (H2) and argon (Ar) plasma. With different parameter and feeding gas, hydrogenated graphene, graphene nanomesh and graphene with vacancies can be well obtained. The defect density can be precisely controlled by tuning plasma power and irradiation time. Morphological, spectroscopic, and electrical characterizations are performed to systematically investigate the defect evolution. Graphene nanomesh and graphene with vacancies show obvious difference for roughness and coverage, whereas the morphology of hydrogenated graphene remains similar with that of as-prepared graphene. For hydrogenated graphene, an opened bandgap of ∼20 meV is detected. For graphene nanomesh and graphene with vacancies, the semiconductive on/off behaviors are observed. We believe this work can provide more details of plasma-induced defects and assist the application of graphene in semiconductor industry.

Effects of Intrinsic Defects in Pt-Based Carbon Supports on Alkaline Hydrogen Evolution Reaction

Carbon material has widely been utilized in the synthesis of efficient carbon-supported Pt (Pt/C) catalysts, in which the structural properties greatly influence the electrocatalytic performances of Pt/C catalysts. However, the effects of intrinsic defects in carbon supports on the performance of the alkaline hydrogen evolution reaction (HER) have not been systematically investigated. Herein, porous carbon supports with different degrees of intrinsic defects were prepared by a simple template-assisted strategy, and the resulting samples were systematically studied by various analytical methods. The results suggested that the presence of abundant intrinsic defects (vacancy and topological defects) in the carbon support was advantageous in terms of favoring the dispersion and anchoring of Pt species, promoting electron transfer between Pt atoms and the carbon support, and tuning the electronic states of Pt species. These features improved the HER performance of Pt/C catalysts. Compared to the nontemplate-assisted carbon-supported Pt catalyst (Pt/NTC) with an overpotential of 178 mV, the optimized template-assisted carbon-supported Pt catalyst (Pt/TC) exhibited a lower overpotential of 58 mV at 10 mA cm-2. Besides, the Pt/TC catalyst displayed better HER durability than the Pt/NTC catalyst owing to its strong metal-support interaction. The DFT calculations confirmed the important role played by intrinsic defects (vacancy and topological defects) in stabilizing Pt atoms, with Pt-C3 coordination identified as the most favorable structure for improving the HER performance of Pt. Overall, novel insights on the significant contribution of intrinsic defects in porous carbon supports on the HER performances of Pt/C catalysts were provided, useful for future design and fabrication of advanced carbon-supported catalysts or other carbon-based electrode materials.

Enhancing the energetic and magnetic stability of atomic hydrogen chemisorbed on graphene using (non)compensated B-N pairs

In this pioneering study for identifying atomic scale magnetic moment, a single hydrogen atom chemisorbed on pristine graphene exhibits distinct spin polarization. Using first-principles calculations and analyses, we demonstrate that the binding between a H adsorbate and a C substrate is substantially enhanced via compensated B-N pairs embedded into graphene. Surprisingly, the interaction can be further enhanced via non-compensated B-N pair doping. Our established prototype of orbital intercoupling between H 1s and hybridized pz of gapped band edges gives an insight into the enhancing mechanism. For compensated B-N doping, the conduction band minimum (CBM) is pushed upward, which induces stronger interaction between the H 1s and hybridized pz orbitals of the CBM. For non-compensated B-N doping, the orbital interaction occurs between H 1s and hybridized pz orbitals of valence band maximum, thus further lowering the resulting bonding energy due to the enlarged gap. This significantly enhanced interaction between H and C atoms agrees well with the results of charge localization at the gapped band edges. More importantly, the corresponding magnetic moments can be well maintained or even enhanced in both doping; here, one more H atom is needed for non-compensated doping, where its electron occupies the empty CBM. Our findings might provide an effective and practical way to enhance the energetic and magnetic stability of atomic scale magnetic moment on graphene and extensively expand the conception of non-compensated doping.

Toward the Production of Super Graphene

The quality requirements of graphene depend on the applications. Some have a high tolerance for graphene quality and even require some defects, while others require graphene as perfect as possible to achieve good performance. So far, synthesis of large-area graphene films by chemical vapor deposition of carbon precursors on metal substrates, especially on Cu, remains the main way to produce high-quality graphene, which has been significantly developed in the past 15 years. However, although many prototypes are demonstrated, their performance is still more or less far from the theoretical property limit of graphene. This review focuses on how to make super graphene, namely graphene with a perfect structure and free of contaminations. More specially, this study focuses on graphene synthesis on Cu substrates. Typical defects in graphene are first discussed together with the formation mechanisms and how they are characterized normally, followed with a brief review of graphene properties and the effects of defects. Then, the synthesis progress of super graphene from the aspects of substrate, grain size, wrinkles, contamination, adlayers, and point defects are reviewed. Graphene transfer is briefly discussed as well. Finally, the challenges to make super graphene are discussed and a strategy is proposed.

The Formation and Transformation of Low-Dimensional Carbon Nanomaterials by Electron Irradiation

Low-dimensional materials based on graphene or graphite show a large variety of phenomena when they are subjected to irradiation with energetic electrons. Since the 1990s, electron microscopy studies, where a certain irradiation dose is unavoidable, have witnessed unexpected structural transformations of graphitic nanoparticles. It is recognized that electron irradiation is not only detrimental but also bears considerable potential in the formation of new graphitic structures. With the availability of aberration-corrected electron microscopes and the discovery of techniques to produce monolayers of graphene, detailed insight into the atomic processes occurring during electron irradiation became possible. Threshold energies for atom displacements are determined and models of different types of lattice vacancies are confirmed experimentally. However, experimental evidence for the configuration of interstitial atoms in graphite or adatoms on graphene remained indirect, and the understanding of defect dynamics still depends on theoretical concepts. This article reviews irradiation phenomena in graphene- or graphite-based nanomaterials from the scale of single atoms to tens of nanometers. Observations from the 1990s can now be explained on the basis of new results. The evolution of the understanding during three decades of research is presented, and the remaining problems are pointed out.

Undercoordination Chemistry of Sulfur Electrocatalyst in Lithium-Sulfur Batteries

Undercoordination chemistry is an effective strategy to modulate the geometry-governed electronic structure and thereby regulate the activity of sulfur electrocatalysts. Efficient sulfur electrocatalysis is requisite to overcome the sluggish kinetics in lithium-sulfur (Li-S) batteries aroused by multi-electron transfer and multi-phase conversions. Recent advances unveil the great promise of undercoordination chemistry in facilitating and stabilizing sulfur electrochemistry, yet a related review with systematicness and perspectives is still missing. Herein, it is carefully combed through the recent progress of undercoordination chemistry in sulfur electrocatalysis. The typical material structures and operational strategies are elaborated, while the underlying working mechanism is also detailly introduced and generalized into polysulfide adsorption behaviors, polysulfide conversion kinetics, electron/ion transport, and dynamic reconstruction. Moreover, perspectives on the future development of undercoordination chemistry are further proposed.

Toward three-dimensionally ordered nanoporous graphene materials: template synthesis, structure, and applications

Precise template synthesis will realize three-dimensionally ordered nanoporous graphenes (NPGs) with a spatially controlled seamless graphene structure and fewer edges. These structural features result in superelastic nature, high electrochemical stability, high electrical conductivity, and fast diffusion of gases and ions at the same time. Such innovative 3D graphene materials are conducive to solving energy-related issues for a better future. To further improve the attractive properties of NPGs, we review the template synthesis and its mechanism by chemical vapor deposition of hydrocarbons, analysis of the nanoporous graphene structure, and applications in electrochemical and mechanical devices.

Periodic Defect Engineering of Iron-Nitrogen-Carbon Catalysts for Nitrate Electroreduction to Ammonia

Iron-nitrogen-carbon single atom catalyst (SAC) is regarded as one of the promising electrocatalysts for NO3 - reduction reaction (NO3 RR) to NH3 due to its high activity and selectivity. However, synergistic effects of topological defects and FeN4 active moiety in Fe-N-C SAC have rarely been investigated. By performing density functional theory (DFT) calculations, 13 defective graphene FeN4 with 585, 484, and 5775 topological line defects are constructed, yielding 585-68-FeN4 with optimal NO3 RR catalytic activity, high selectivity, as well as robust anti-dissolution stability. The high NO3 RR activity on 585-68-FeN4 is well explained by the high valence state of Fe center as well as asymmetric charge distribution on FeN4 moiety influenced by 5- and 8-member rings. This DFT work provides theoretical guidance for engineering NO3 RR performance of iron-nitrogen-carbon catalysts by modulating periodic topological defects.

Ab initiostudy of the processes of nitrogen functionalisation in graphene

Nitrogen functionalisation of graphene is studied with the help ofab initioelectronic structure methods. Both static formation energies and energy barriers obtained from nudged elastic band calculations are considered. If carbon defects are present in the graphene structure, low energy barriers on the order of 0.5 eV were obtained to incorporate nitrogen atoms inside the sheet. For defect-free graphene, much larger barriers in the range of 3.70-4.38 eV were found, suggesting an external energy source is required to complete this type of incorporation.

Bespoke Tailoring of Graphenoid Sheets: A Rippled Molecular Carbon Comprising Cyclically Fused Nonbenzenoid Rings

The incorporation of nonbenzenoid rings into the hexagonal networks of graphenoid nanostructures is of immense importance for electronic, magnetic, and mechanical properties, but the underlying mechanisms of nonbenzenoid ring fusion are rather unexplored. Here, we report the synthesis and characterization of a rippled C84 molecular carbon, which contains 10 nonbenzenoid rings (five-, seven-, and eight-membered rings) that are contiguously fused to give a cyclic geometry. The fused nonbenzenoid rings impart high solubility, configurational stability, multiple reversible redox behaviors, unique aromaticity, and a narrow band gap to the system. Moreover, this carbon nanostructure allows for further functionalization via electrophilic substitution and metalation reactions, enabling access to finely tuned derivatives. Interestingly, both the bowl-shaped and planar conformations of the core in molecular carbon are observed in the solid state. Additionally, this molecular carbon displays ambipolar transport characteristics.

Topological line defects in hexagonal SiC monolayer

Defect engineering of two-dimensional (2D) materials offers an unprecedented route to increase their functionality and broaden their applicability. In light of the recent synthesis of the 2D Silicon Carbide (SiC), a deep understanding of the effect of defects on the physical and chemical properties of this new SiC allotrope becomes highly desirable. This study investigates 585 extended line defects (ELDs) in hexagonal SiC considering three types of interstitial atom pairs (SiSi-, SiC-, and CC-ELD) and using computational methods like Density Functional Theory, Born-Oppenheimer Molecular Dynamics, and Kinetic Monte-Carlo (KMC). Results show that the formation of all ELD systems is endothermic, with the CC-ELD structure showing the highest stability at 300 K. To further characterize the ELDs, simulated scanning tunneling microscopy (STM) is employed, and successfully allow identify and distinguish the three types of ELDs. Although pristine SiC has a direct band gap of 2.48 eV, the presence of ELDs introduces mid-gap states derived from the pz orbitals at the defect sites. Furthermore, our findings reveal that the ELD region displays enhanced reactivity towards hydrogen adsorption, which was confirmed by KMC simulations. Overall, this research provides valuable insights into the structural, electronic, and reactivity properties of ELDs in hexagonal SiC monolayers and paves the way for potential applications in areas such as catalysis, optoelectronics, and surface science.

High-Level Multireference Investigations on the Electronic States in Single-Vacancy (SV) Graphene Defects Using a Pyrene-SV Model

The nonplanar character of graphene with a single carbon vacancy (SV) defect is investigated utilizing a pyrene-SV model system by way of complete-active-space self-consistent field theory (CASSCF) and multireference configuration interaction singles and doubles (MR-CISD) calculations. Planar structures were optimized with both methods, showing the 3B1 state to be the ground state with three energetically close states within an energy range of 1 eV. These planar structures constitute saddle points. However, following the out-of-plane imaginary frequency yields more stable (by 0.22 to 0.53 eV) but nonplanar structures of Cs symmetry. Of these, the 1A' structure is the lowest in energy and is strongly deformed into an L shape. Following a further out-of-plane imaginary frequency in the nonplanar structures leads to the most stable but most deformed singlet structure of C1 symmetry. In this structure, a bond is formed between the carbon atom with the dangling bond and a carbon of the cyclopentadienyl ring. This bond stabilizes the structure by more than 3 eV compared to the planar 3B1 structure. Higher excited states were calculated at the MR-CISD level, showing a grouping of four states low in energy and higher states starting around 3 eV.

Structural Dynamics in the Presence of Water of Graphene Bilayers with Defects

The dynamics of a bilayer of graphene containing one mono-vacancy in the top layer has been investigated in the framework of DFTB in the absence and in the presence of water. Due to the speed of the code, we can describe details of the behavior, which are not directly accessible experimentally and cannot be treated by DFT or classical molecular dynamics. The presence of water enhances the displacement of carbon atoms in the perpendicular direction to the surface. Our results explain very well a variety of experimental findings. In particular, the stabilization of the Jahn-Teller distortion by hydrogenation of one of the carbon atoms at the edge of a mono-vacancy has been elucidated. This work is the first analysis of the behavior of a graphene vacancy at room temperature in contact with water based on a quantum mechanical molecular dynamics method, where both graphene and solvent are treated at the same level.

Ab initio study of water anchored in graphene pristine and vacancy-type defects

CONTEXT

In this paper, we have addressed two issues that are relevant to the interaction of water in pristine and vacant graphene through first-principles calculations based on the Density Functional Theory (DFT). The results showed that for the interaction of pristine graphene with water, the DOWN configuration (with the hydrogen atoms facing downwards) was the most stable, presenting binding energies in the order of -13.62 kJ/mol at a distance of 2.375 Å in the TOP position. We also evaluated the interaction of water with two vacancy models, removing one carbon atom (Vac-1C) and four atoms (Vac-4C). In the Vac-1C system, the most favourable system was the DOWN configuration, with binding energies ranging from -20.60 kJ/mol to -18.41 kJ/mol in the TOP and UP positions, respectively. A different behaviour was observed for the interaction of water with Vac-4C; regardless of the configuration of the water, it is always more favourable for the interaction to occur through the vacancy centre, with binding energies ​​between -13.28 kJ/mol and -20.49 kJ/mol. Thus, the results presented open perspectives for the technological development of nanomembranes as well as providing a better understanding of the wettability effects of graphene sheets, whether pristine or with defects.

METHOD

We evaluated the interaction of pristine and vacant graphene with the water molecule, through calculations based on Density Functional Theory (DFT); implemented by the SIESTA program. The electronic, energetic, and structural properties were analyzed by solving self-consistent Kohn-Sham equations. In all calculations, a double ζ plus a polarized function (DZP) was used for the numerical baise set. Local Density Approximation (LDA) with the Perdew and Zunger (PZ) parameterisation along with a basis set superposition error (BSSE) correction were used to describe the exchange and correlation potential (Vxc). The water and isolated graphene structures were relaxed until the residual forces were less than 0.05 eV/Å^-1 in all atomic coordinates.

Electron irradiation effects in transmission electron microscopy: Random displacements and collective migrations

Electron beam damage in transmission electron microscopy (TEM) is complicated because the damage phenomena can be the result of random atomic displacements or collective migrations. The former is categorized as the primary beam effects and the latter is the secondary beam effects. The mechanisms for these two distinguishing atomic processes of damage are different. The primary beam effects can be caused by the mechanisms of knock-on and/or radiolysis, while the secondary effects must be driven by a field that is induced by electron irradiation. One such field has been identified to be the electric field produced by the accumulated charges due to the ejection of secondary and Auger electrons from the irradiated region. One convincing example is the electron irradiation-induced domain switch in ferroelectric materials, in which the collective cation displacements are driven by the induced electric field. A detailed interpretation is given in this review. The sintering of metal NPs under electron irradiation is a secondary beam effect and is most likely also caused by the induced electric fields. The interactions between the charged NP and substrate, and between charged NPs, result in NP motion. Interchanging atoms between NPs during the sintering may also be driven by the electric fields. Although many beam-damage phenomena in C nanotubes and layered materials, such as graphene, BN, and transition metal dichalcogenides, are caused by the primary beam effects and have been well studied experimentally and theoretically in the literature, some phenomena from the secondary beam effects have also been identified in this review. These phenomena are sensitive to electron current density, the shape and orientation of the specimen, and even the illumination mode (i.e., TEM or STEM). Unfortunately, the mechanisms responsible for these phenomena still need to be clarified.

Localized magnetic moment induced by boron adatoms chemisorbed on graphene

Inducing local spin-polarization in pristine graphene is highly desirable and recent experiment shows that boron adatom chemical attachment to graphene exhibits local high spin state. Using hybrid exchange-correlation functional, we show that boron (B) monomer chemisorbed on the bridge site of graphene is energically favorable, and indeed induces a weak local spin-polarization ∼0.56μB. The localized magnetic moment can be attributed to the charge transfer from boron atom to graphene, resulting in local spin charge dominantly surrounding to the adsorbed B and neighboring carbon (C) atoms. We also surprisingly find that boron dimer can even much more stable upright anchor the same site of graphene, giving rise to sizable spin magnetic moment 2.00μB. Although the apparent spin state remains mainly contributed by Bpand Cporbitals as the case of boron monomer, the delicate and substantial charge transfer of theintra-dimerplays a fundamental role in producing such sizable local spin-polarization. We employed various van der Waals corrections to check and confirm the validity of appeared local spin-polarization. In terms of the almost identical simulated scanning tunneling microscope between boron monomer and dimer, we might tend to support the fact that boron dimer can also be chemisorbed on graphene with much larger and stable localized spin magnetic moment.

Nanoresonator vibrational behaviour analysis of single- and double-layer graphene with atomic vacancy and pinhole defects

CONTEXT

Nanosensors and actuators are frequently made of graphene. Any defect in the graphene's manufacturing has an impact on its sensing performance and on its dynamic behaviour. Using a molecular dynamics technique, the influence of pinhole defects and atomic defects on the performance parameters of single-layer graphene sheets (SLGSs) and double-layer graphene sheets (DLGSs) with various boundary conditions and lengths is explored. In contrast to the perfect nanostructure of a graphene sheet, defects are described as holes formed by atomic vacancies. As the number of defects increases, the simulation results show that the presence of defects has the greatest impact on the resonance frequency of SLGSs and DLGSs. The influence of pinhole defect (PD) and atomic vacancy defect (AVD) on armchair, zigzag, and chiral SLGSs and DLGSs was investigated in this article using molecular dynamics simulation. The influence of both types of defects is largest when it is adjacent to the fixed support for all three different types of graphene sheets, i.e. armchair, zigzag, and chiral.

METHODS

The structure of the graphene sheet has been created using ANSYS APDL software. In the structure of the graphene sheet, atomic and pinhole defects have been generated. SLG and DLG sheets are modelled using a space frame structure that is identical to a three-dimensional beam. Dynamic analysis of single-layer and double-layer graphene sheets performed with different lengths using the atomistic finite element method. The interlayer separation in the form of Van der Waals interaction is modelled using characteristic spring element (Combin14). The upper and lower sheets of DLGSs are described as elastic beams connected by a spring element. With atomic vacancy defect for the bridged boundary condition, the highest frequency of 2.86 × 10⁸ Hz was found for zigzag DLG (20 0) and with same boundary condition for pinhole defect 2.79 × 10⁸ Hz frequency achieved. In a single-layer graphene sheet with an atomic vacancy and cantilever boundary condition, the maximum efficiency was 4.13 × 10³ Hz for SLG (20 0), while in a pinhole defect, it produced 2.73 × 10⁷ Hz. Moreover, the elastic parameters of beam components are calculated using the mechanical properties of covalent bonds between carbon atoms in the hexagonal lattice. The model has been tested against previous research. The focus of this research is to develop a mechanism for determining how defects affect graphene frequency band in application as nano resonators.

Synthesis and Functionalization of Graphene Materials for Biomedical Applications: Recent Advances, Challenges, and Perspectives

Since its discovery in 2004, graphene is increasingly applied in various fields owing to its unique properties. Graphene application in the biomedical domain is promising and intriguing as an emerging 2D material with a high surface area, good mechanical properties, and unrivalled electronic and physical properties. This review summarizes six typical synthesis methods to fabricate pristine graphene (p-G), graphene oxide (GO), and reduced graphene oxide (rGO), followed by characterization techniques to examine the obtained graphene materials. As bare graphene is generally undesirable in vivo and in vitro, functionalization methods to reduce toxicity, increase biocompatibility, and provide more functionalities are demonstrated. Subsequently, in vivo and in vitro behaviors of various bare and functionalized graphene materials are discussed to evaluate the functionalization effects. Reasonable control of dose (<20 mg kg^-1 ), sizes (50-1000 nm), and functionalization methods for in vivo application are advantageous. Then, the key biomedical applications based on graphene materials are discussed, coupled with the current challenges and outlooks of this growing field. In a broader sense, this review provides a comprehensive discussion on the synthesis, characterization, functionalization, evaluation, and application of p-G, GO, and rGO in the biomedical field, highlighting their recent advances and potential.

Graphene-Based Nanocomposites as Antibacterial, Antiviral and Antifungal Agents

Over the past decade, there have been many interesting studies in the scientific literature about the interaction of graphene-based polymeric nanocomposites with microorganisms to tackle antimicrobial resistance. These studies have reported variable intensities of biocompatibility and selectivity for the nanocomposites toward a specific strain, but it is widely believed that graphene nanocomposites have antibacterial, antiviral, and antifungal activities. Such antibacterial activity is due to several mechanisms by which graphene nanocomposites can act on cells including stimulating oxidative stress; disrupting membranes due to sharp edges; greatly changing core structure mechanical strength and coarseness. However, the underlying mechanisms of graphene nanocomposites as antiviral and antifungal agents remain relatively scarce. In this review, recent advances in the synthesis, functional tailoring, and antibacterial, antiviral, and antifungal applications of graphene nanocomposites are summarized. The synthesis of graphene materials and graphene-based polymeric nanocomposites with techniques such as pressurized gyration, electrospinning, chemical vapor deposition, and layer-by-layer self-assembly is first introduced. Then, the antimicrobial mechanisms of graphene membranes are presented and demonstrated typical in vitro and in vivo studies on the use of graphene nanocomposites for antibacterial, antiviral, and antifungal applications. Finally, the review describes the biosafety, current limitations, and potential of antimicrobial graphene-based nanocomposites.

Iron(II) Phthalocyanine Adsorbed on Defective Graphenes: A Density Functional Study

The adsorptions of iron(II) phthalocyanine (FePc) on graphene and defective graphene were investigated systematically using density functional theory. Three types of graphene defects covering stone-wales (SW), single vacancy (SV), and double vacancy (DV) were taken into account, in which DV defects included DV(5-8-5), DV(555-777), and DV(5555-6-7777). The calculations of formation energies of defects showed that the SW defect has the lowest formation energy, and it was easier for DV defects to form compared with the SV defect. It is more difficult to rotate or move FePc on the surface of defective graphenes than on the surface of graphene due to bigger energy differences at different sites. Although the charge analysis indicated the charge transfers from graphene or defective graphene to FePc for all studied systems, the electron distributions of FePc on various defective graphenes were different. Especially for FePc@SV, the d _xy orbital of Fe in the conduction band moved toward the Fermi level about 1 eV, and the d _xz of Fe in the valence band for FePc@SV also moved toward the Fermi level compared with FePc@graphene and other FePc@defective graphenes. Between the planes of FePc and defective graphene, the electron accumulation occurs majorly in the position of the FePc molecular plane for FePc@SW, FePc@DV(5-8-5), and FePc@DV(5555-6-7777) as well as FePc@graphene. However, electrons were accumulated on the upper and lower surfaces of the FePc molecular plane for FePc@SV and FePc@DV(555-777). Thus, the electron distribution of FePc can be modulated by introducing the interfaces of different defective graphenes.

Exploring the configurational space of amorphous graphene with machine-learned atomic energies

Two-dimensionally extended amorphous carbon ("amorphous graphene") is a prototype system for disorder in 2D, showing a rich and complex configurational space that is yet to be fully understood. Here we explore the nature of amorphous graphene with an atomistic machine-learning (ML) model. We create structural models by introducing defects into ordered graphene through Monte-Carlo bond switching, defining acceptance criteria using the machine-learned local, atomic energies associated with a defect, as well as the nearest-neighbor (NN) environments. We find that physically meaningful structural models arise from ML atomic energies in this way, ranging from continuous random networks to paracrystalline structures. Our results show that ML atomic energies can be used to guide Monte-Carlo structural searches in principle, and that their predictions of local stability can be linked to short- and medium-range order in amorphous graphene. We expect that the former point will be relevant more generally to the study of amorphous materials, and that the latter has wider implications for the interpretation of ML potential models.

Strain-Tuneable Magnetism and Spintronics of Distorted Monovacancies in Graphene

The electronic and spintronic properties of the monovacancies in freestanding and isotopically compressed graphene are investigated using hybrid exchange density functional perturbation theory. When the effects of electronic self-interaction are taken into account, an integer magnetic moment of 2 μ_B is identified for a Jahn-Teller reconstructed V₁(5-9) monovacancy in freestanding graphene. For graphene with stable ripples induced by a compressive strain of 5%, a bond reconstruction produces a V₁(55-66) structure for the monovacancy, which is localized at the saddle points of the ripple. The sizeable local distortion induced by reconstruction modifies both the geometric and electronic properties of rippled graphene and quenches the magnetic moment of the vacancy due to the sp³ hybridization of the central atom. The nonmagnetic V₁(55-66) structure is found to be stable on rippled structures, with the formation energy ∼2.3 eV lower than that of the metastable distorted V₁(5-9) structures localized at sites other than the saddle points. The electronic ground state of distorted V₁(5-9) corresponds to a wide range of fractional magnetic moments (0.50-1.25 μ_B). The computed relative stabilities and the electronic and magnetic properties of the V₁(5-9) structures are found to be closely related to their local distortions. This analysis of the fundamental properties of defective graphene under compression suggests a number of strategies for generating regular defect patterns with tuneable magnetic and electronic properties and may, therefore, be used as a novel technique to achieve more precise control of graphene electronic structure for various application scenarios such as transistors, strain sensors, and directed chemisorption.

Identifying and manipulating single atoms with scanning transmission electron microscopy

The manipulation of individual atoms has developed from visionary speculation into an established experimental science. Using focused electron irradiation in a scanning transmission electron microscope instead of a physical tip in a scanning probe microscope confers several benefits, including thermal stability of the manipulated structures, the ability to reach into bulk crystals, and the chemical identification of single atoms. However, energetic electron irradiation also presents unique challenges, with an inevitable possibility of irradiation damage. Understanding the underlying mechanisms will undoubtedly continue to play an important role to guide experiments. Great progress has been made in several materials including graphene, carbon nanotubes, and crystalline silicon in the eight years since the discovery of electron-beam manipulation, but the important challenges that remain will determine how far we can expect to progress in the near future.

Graphene and Its Derivatives: Synthesis and Application in the Electrochemical Detection of Analytes in Sweat

Wearable sensors and invasive devices have been studied extensively in recent years as the demand for real-time human healthcare applications and seamless human-machine interaction has risen exponentially. An explosion in sensor research throughout the globe has been ignited by the unique features such as thermal, electrical, and mechanical properties of graphene. This includes wearable sensors and implants, which can detect a wide range of data, including body temperature, pulse oxygenation, blood pressure, glucose, and the other analytes present in sweat. Graphene-based sensors for real-time human health monitoring are also being developed. This review is a comprehensive discussion about the properties of graphene, routes to its synthesis, derivatives of graphene, etc. Moreover, the basic features of a biosensor along with the chemistry of sweat are also discussed in detail. The review mainly focusses on the graphene and its derivative-based wearable sensors for the detection of analytes in sweat. Graphene-based sensors for health monitoring will be examined and explained in this study as an overview of the most current innovations in sensor designs, sensing processes, technological advancements, sensor system components, and potential hurdles. The future holds great opportunities for the development of efficient and advanced graphene-based sensors for the detection of analytes in sweat.

Fabrication and Characterization of Visible to Near-Infrared Photodetector Based on Multilayer Graphene/Mg2Si/Si Heterojunction

In this investigation, p-Mg₂Si/n-Si heterojunction photodetector (PD) is fabricated by magnetron sputtering and low vacuum annealing in the absence of argon or nitrogen atmosphere. Multilayer Graphene (MLG)/Mg₂Si/Si heterojunction PD is first fabricated by transferring MLG to Mg₂Si/Si heterojunction substrate using the suspended self-help transfer MLG method. After characterizing the phase composition, morphology and detection properties of Mg₂Si/Si and MLG/Mg₂Si/Si heterojunction PDs, the successful fabrication of the Mg₂Si/Si and MLG/Mg₂Si/Si heterojunction PDs are confirmed and some detection capabilities are realized. Compared with the Mg₂Si/Si heterojunction PD, the light absorption and the ability to effectively separate and transfer photogenerated carriers of MLG/Mg₂Si/Si heterojunction PD are improved. The responsivity, external quantum efficiency (EQE), noise equivalent power (NEP), detectivity (D*), on/off ratio and other detection properties are enhanced. The peak responsivity and EQE of the MLG/Mg₂Si/Si heterojunction PD are 23.7 mA/W and 2.75%, respectively, which are better than the previous 1-10 mA/W and 2.3%. The results illustrate that the fabrication technology of introducing MLG to regulate the detection properties of the Mg₂Si/Si heterojunction PD is feasible. In addition, this study reveals the potential of MLG to enhance the detection properties of optoelectronic devices, broadens the application prospect of the Mg₂Si/Si-based heterojunction PDs and provides a direction for the regulation of optoelectronic devices.

Topological Stone-Wales Defects Enhance Bonding and Electronic Coupling at the Graphene/Metal Interface

Defects play a critical role for the functionality and performance of materials, but the understanding of the related effects is often lacking, because the typically low concentrations of defects make them difficult to study. A prominent case is the topological defects in two-dimensional materials such as graphene. The performance of graphene-based (opto-)electronic devices depends critically on the properties of the graphene/metal interfaces at the contacting electrodes. The question of how these interface properties depend on the ubiquitous topological defects in graphene is of high practical relevance, but could not be answered so far. Here, we focus on the prototypical Stone-Wales (S-W) topological defect and combine theoretical analysis with experimental investigations of molecular model systems. We show that the embedded defects undergo enhanced bonding and electron transfer with a copper surface, compared to regular graphene. These findings are experimentally corroborated using molecular models, where azupyrene mimics the S-W defect, while its isomer pyrene represents the ideal graphene structure. Experimental interaction energies, electronic-structure analysis, and adsorption distance differences confirm the defect-controlled bonding quantitatively. Our study reveals the important role of defects for the electronic coupling at graphene/metal interfaces and suggests that topological defect engineering can be used for performance control.

Various defects in graphene: a review

Pristine graphene has been considered one of the most promising materials because of its excellent physical and chemical properties. However, various defects in graphene produced during synthesis or fabrication hinder its performance for applications such as electronic devices, transparent electrodes, and spintronic devices. Due to its intrinsic bandgap and nonmagnetic nature, it cannot be used in nanoelectronics or spintronics. Intrinsic and extrinsic defects are ultimately introduced to tailor electronic and magnetic properties and take advantage of their hidden potential. This article emphasizes the current advancement of intrinsic and extrinsic defects in graphene for potential applications. We also discuss the limitations and outlook for such defects in graphene.

High Hydrogen Isotope Separation Efficiency: Graphene or Catalyst?

Single-layer graphene has been demonstrated to be a high-efficiency hydrogen isotope sieving membrane in the electrochemical hydrogen pumping system. In this work, we transferred this membrane to proton exchange membrane water electrolysis (PEMWE), which has wide industrial applications. Two membrane electrode assemblies with decorated Pt and ink-coated Pt were investigated. The graphene with the decorated Pt scheme acquired the reported highest proton-to-tritium separation factor of 19.50 in PEMWE. However, rather than graphene, the decorated catalyst was demonstrated to be responsible for this remarkable separation efficiency. Previous studies from Geim's group underestimated the enhanced separation efficiency of decorated Pt over ink-coated Pt, resulting in an exaggerated separation efficiency for graphene. The behavior of proton transfer with hydrogen isotope separation through graphene was interpreted by a serial-parallel circuit model, which suggested that hydrogen isotope separation occurs at defect sites. The limited separation efficiency for graphene was also well understood by a density functional theory (DFT) calculation using an SW 55-77 model and the transition state theory for the kinetic isotope effect. This research provides a thorough understanding of proton transfer with hydrogen isotope separation through graphene.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

A novel sensor for visual and selective detection of Hg2+ based on functionalized doped quantum dots

In this paper, a novel analytical platform for the visual, sensitive and reliable analysis of mercury ions (Hg²⁺) is fabricated based on functionalized doped quantum dots. We synthesized a new specific nano-material, zinc dithiothreitol combined with graphene quantum dots (ZnNCs-NGQDs), by a simple and convenient method which, as an efficient luminophore, was then applied to construct an electrochemiluminescence (ECL) system for the first time. Under optimized conditions, the ECL sensor showed an excellent response for Hg²⁺ in the linear range of 1.0 mM to 10 pM, with a low detection limit of 3 pM. Moreover, the proposed method demonstrated satisfactory selectivity, stability and acceptable reproducibility for the detection of Hg²⁺. The recovery of tap water and lake water samples ranged from 96% to 105%, indicating the potential applicability of the proposed method for monitoring environmental water samples. Meanwhile, visual attempts for mercury ion detection by using doped quantum dots have also obtained satisfactory results. Importantly, our research revealed a viable method for improving the sensitivity and convenience of target studies in sensing fields derived from functional material design.

STEM Image Analysis Based on Deep Learning: Identification of Vacancy Defects and Polymorphs of MoS2

Scanning transmission electron microscopy (STEM) is an indispensable tool for atomic-resolution structural analysis for a wide range of materials. The conventional analysis of STEM images is an extensive hands-on process, which limits efficient handling of high-throughput data. Here, we apply a fully convolutional network (FCN) for identification of important structural features of two-dimensional crystals. ResUNet, a type of FCN, is utilized in identifying sulfur vacancies and polymorph types of MoS₂ from atomic resolution STEM images. Efficient models are achieved based on training with simulated images in the presence of different levels of noise, aberrations, and carbon contamination. The accuracy of the FCN models toward extensive experimental STEM images is comparable to that of careful hands-on analysis. Our work provides a guideline on best practices to train a deep learning model for STEM image analysis and demonstrates FCN's application for efficient processing of a large volume of STEM data.

Two-Dimensional Ultrathin Silica Films

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO₂ stoichiometry. It consists of precisely two layers of tetrahedral [SiO₄] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

Molecular Dynamics Analysis of Graphene Nanoelectromechanical Resonators Based on Vacancy Defects

Due to the limitation of graphene processing technology, the prepared graphene inevitably contains various defects. The defects will have a particular influence on the macroscopic characteristics of the graphene. In this paper, the defect-based graphene nanoresonators are studied. In this study, the resonant properties of graphene were investigated via molecular dynamic simulations. The effect of vacancy defects and hole defects at different positions, numbers, and concentrations on the resonance frequency of graphene nanoribbons was studied. The results indicated that single monatomic vacancy has no effect on graphene resonant frequency, and the concentration of the resonant frequency of graphene decreases almost linearly with the increase of single-atom vacancy concentration. When the vacancy concentration is 5%, the resonance frequency is reduced by 12.77% compared to the perfect graphene. Holes on the graphene cause the resonance frequency to decrease. As the circular hole defect is closer to the center of the graphene nanoribbon, not only does its resonant frequency increase, but the tuning range is also expanded accordingly. Under the external force of 10.715 nN, the resonant frequency of graphene reaches 429.57 GHz when the circular hole is located at the center of the graphene nanoribbon, which is 40 GHz lower than that of single vacancy defect graphene. When the circular hole is close to the fixed end of graphene, the resonant frequency is 379.62 GHz, which is 90 GHz lower than that of single vacancy graphene. When the hole defect is at the center of nanoribbon, the frequency tunable range of graphene reaches 120 GHz. The tunable frequency range of graphene is 100.12 GHz when the hole defect is near the fixed ends of the graphene nanoribbon. This work is of great significance for design and performance optimization of graphene-based nanoelectro-mechanical system (NEMS) resonators.

Benchmarking Noise and Dephasing in Emerging Electrical Materials for Quantum Technologies

As quantum technologies develop, a specific class of electrically conducting materials is rapidly gaining interest because they not only form the core quantum-enabled elements in superconducting qubits, semiconductor nanostructures, or sensing devices, but also the peripheral circuitry. The phase coherence of the electronic wave function in these emerging materials will be crucial when incorporated in the quantum architecture. The loss of phase memory, or dephasing, occurs when a quantum system interacts with the fluctuations in the local electromagnetic environment, which manifests in "noise" in the electrical conductivity. Hence, characterizing these materials and devices therefrom, for quantum applications, requires evaluation of both dephasing and noise, although there are very few materials where these properties are investigated simultaneously. Here, the available data on magnetotransport and low-frequency fluctuations in electrical conductivity are reviewed to benchmark the dephasing and noise. The focus is on new materials that are of direct interest to quantum technologies. The physical processes causing dephasing and noise in these systems are elaborated, the impact of both intrinsic and extrinsic parameters from materials synthesis and devices realization are evaluated, and it is hoped that a clearer pathway to design and characterize both material and devices for quantum applications is thus provided.

High-Resolution Ion-Flux Imaging of Proton Transport through Graphene|Nafion Membranes

In 2014, it was reported that protons can traverse between aqueous phases separated by nominally pristine monolayer graphene and hexagonal boron nitride (h-BN) films (membranes) under ambient conditions. This intrinsic proton conductivity of the one-atom-thick crystals, with proposed through-plane conduction, challenged the notion that graphene is impermeable to atoms, ions, and molecules. More recent evidence points to a defect-facilitated transport mechanism, analogous to transport through conventional ion-selective membranes based on graphene and h-BN. Herein, local ion-flux imaging is performed on chemical vapor deposition (CVD) graphene|Nafion membranes using an "electrochemical ion (proton) pump cell" mode of scanning electrochemical cell microscopy (SECCM). Targeting regions that are free from visible macroscopic defects (e.g., cracks, holes, etc.) and assessing hundreds to thousands of different sites across the graphene surfaces in a typical experiment, we find that most of the CVD graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at ≈20-60 localized sites across a ≈0.003 mm² area of the membrane (>5000 measurements total). When localized proton transport occurs, it can be a highly dynamic process, with additional transmission sites "opening" and a small number of sites "closing" under an applied electric field on the seconds time scale. Applying a simple equivalent circuit model of ion transport through a cylindrical nanopore, the local transmission sites are estimated to possess dimensions (radii) on the (sub)nanometer scale, implying that rare atomic defects are responsible for proton conductance. Overall, this work reinforces SECCM as a premier tool for the structure-property mapping of microscopically complex (electro)materials, with the local ion-flux mapping configuration introduced herein being widely applicable for functional membrane characterization and beyond, for example in diagnosing the failure mechanisms of protective surface coatings.

Influence of Defects and Heteroatoms on the Chemical Properties of Supported Graphene Layers

A large and growing number of theoretical papers report the possible role of defects and heteroatoms on the chemical properties of single-layer graphene. Indeed, they are expected to modify the electronic structure of the graphene film, allow for chemisorption of different species, and enable more effective functionalisation. Therefore, from theoretical studies, we get the suggestion that single and double vacancies, Stone–Wales defects and heteroatoms are suitable candidates to turn nearly chemically inert graphene into an active player in chemistry, catalysis, and sensoristics. Despite these encouraging premises, experimental proofs of an enhanced reactivity of defected/doped graphene are limited because experimental studies addressing adsorption on well-defined defects and heteroatoms in graphene layers are much less abundant than theoretical ones. In this paper, we review the state of the art of experimental findings on adsorption on graphene defects and heteroatoms, covering different topics such as the role of vacancies on adsorption of oxygen and carbon monoxide, the effect of the presence of N heteroatoms on adsorption and intercalation underneath graphene monolayers, and the role of defects in covalent functionalisation and defect-induced gas adsorption on graphene transistors.

A Quantitative Chemical Method for Determining the Surface Concentration of Stone-Wales Defects for 1D and 2D Carbon Nanomaterials

A quantitative method is proposed to determine Stone–Wales defects for 1D and 2D carbon nanostructures. The technique is based on the diene synthesis reaction (Diels–Alder reaction). The proposed method was used to determine Stone–Wales defects in the few-layer graphene (FLG) nanostructures synthesized by the self-propagating high-temperature synthesis (SHS) process in reduced graphene oxide (rGO) synthesized based on the method of Hammers and in the single-walled carbon nanotubes (SWCNT) TUBAL trademark, Russia. Our research has shown that the structure of FLG is free of Stone–Wales defects, while the surface concentration of Stone–Wales defects in TUBAL carbon nanotubes is 1.1 × 10⁻⁵ mol/m² and 3.6 × 10⁻⁵ mol/m² for rGO.

SiC monolayers as promising substrates for the development of highly stable single atom catalysts (Pd1/SiC): A density functional theory study

Single-atom catalysts have been touted as highly efficient catalysts, but the catalytic single-atom sites are unstable and tends to aggregate into nanoparticles during chemical reactions. In this study, we show that SiC monolayers are promising substrates for the development of highly stable single-atom catalysts (Pd 1 /SiC) within the density functional theory. In the presence of Si-vacancy, the diffusion barrier energy of Pd 1 atom embedded on SiC monolayer is substantially enhanced from 2.3 to 7.8 eV, which is much higher than the reported diffusion barrier energies of graphene, boron nitride and defective MgO of the same catalytic system. Ab initio molecular dynamic at 500K also confirms the enhanced stability of Pd1/SiC monolayer (Si-vacancy) such that the Pd 1 atom remains embedded in the vacancy. Additionally, the Pd 1 /SiC monolayer (Si-vacancy) catalysts show a ~34% reduction of activation barrier energy for CO oxidation as compared to the pristine catalysts. This work implies that nanostructured SiC materials are potential substrates for the synthesis of highly stable single-atom catalysts.

One-step functionalization of mildly and strongly reduced graphene oxide with maleimide: an experimental and theoretical investigation of the Diels-Alder [4+2] cycloaddition reaction

For large-scale graphene applications, such as the production of polymer-graphene nanocomposites, exfoliated graphene oxide (GO) and its reduced form (rGO) are presently considered to be very suitable starting materials, showing enhanced chemical reactivity with respect to pristine graphene, in addition to suitable electronic properties (i.e., tunable band gap). Among other chemical processes, a suitable way to obtain surface decoration of graphene is through a direct one-step Diels-Alder (DA) reaction, e.g. through the use of dienophile or diene moieties. However, the feasibility and extent of decoration largely depends on the specific graphene microstructure that in the case of rGO sheets is not easy to control and generally presents a high degree of inhomogeneity owing to various on-plane functionalization (e.g., epoxide and hydroxyl groups) or in-plane lattice defects. In an effort to gain some insights into the covalent functionalization of variably reduced GO samples, we present a combined experimental and theoretical study on the DA cycloaddition reaction of maleimide, a dienophile functional unit well-suited for chemical conjugation of polymers and macromolecules. In particular, we considered both mildly and strongly reduced GOs. Using thermogravimetry, Raman and X-Ray photoelectron spectroscopy, and elemental analysis we show evidence of variable chemical reactivity of rGO as a function of the residual oxygen content. Moreover, from quantum mechanical calculations carried out at the DFT level on different graphene reaction sites, we provide a more detailed molecular view to interpret experimental findings and to assess the reactivity series of different graphene modifications.

Influence of Defect Number, Distribution Continuity and Orientation on Tensile Strengths of the CNT-Based Networks: A Molecular Dynamics Study

Networks based on carbon nanotube (CNT) have been widely utilized to fabricate flexible electronic devices, but defects inevitably exist in these structures. In this study, we investigate the influence of the CNT-unit defects on the mechanical properties of a honeycomb CNT-based network, super carbon nanotube (SCNT), through molecular dynamics simulations. Results show that tensile strengths of the defective SCNTs are affected by the defect number, distribution continuity and orientation. Single-defect brings 0 ~ 25% reduction of the tensile strength with the dependency on defect position and the reduction is over 50% when the defect number increases to three. The distribution continuity induces up to 20% differences of tensile strengths for SCNTs with the same defect number. A smaller arranging angle of defects to the tensile direction leads to a higher tensile strength. Defective SCNTs possess various modes of stress concentration with different concentration degrees under the combined effect of defect number, arranging direction and continuity, for which the underlying mechanism can be explained by the effective crack length of the fracture mechanics. Fundamentally, the force transmission mode of the SCNT controls the influence of defects and the cases that breaking more force transmission paths cause larger decreases of tensile strengths. Defects are non-negligible factors of the mechanical properties of CNT-based networks and understanding the influence of defects on CNT-based networks is valuable to achieve the proper design of CNT-based electronic devices with better performances.

Electrochemical Growth of High-Strength Carbon Nanocoils in Molten Carbonates

The reported mechanical strength of carbon nanocoils (CNCs) obtained from traditional preparation of catalytic acetylene pyrolysis is far below its theoretical value. Herein, we report a molten salt electrolysis method that employs CO₃^2- as feedstock to grow CNCs without using metal catalyst. We meticulously mediate the alkalinity of molten carbonate to tune the electrochemical reduction of CO₃^2- on graphite electrode to selectively grow CNCs in Li₂CO₃-Na₂CO₃-K₂CO₃-0.001 wt %Li₂O. Graphite substrate, current density, and alkalinity of molten salt dictate the growth of CNCs. In addition, the electrolytic CNCs shows a spring constant of 1.92-39.41 N/m and a shear modulus of 21-547 GPa, which are 10-200 times that of CNCs obtained from catalyst-assisted gas-to-solid conversions. Overall, this paper opens up an electrochemical way to prepare CNCs through liquid-to-solid conversion without using catalysts and acetylene, providing new perspectives on green synthesis of 1D carbon nanomaterials with high mechanical strength.

Two-dimensional carbon materials with an anisotropic Dirac cone: high stability and tunable Fermi velocity

A new 2D Dirac carbon allotrope is proposed, which has unique self-doping properties and a tunable Fermi velocity via the applied strain.

The Adsorption Behavior of Gas Molecules on Co/N Co-Doped Graphene

Herein, we have used density functional theory (DFT) to investigate the adsorption behavior of gas molecules on Co/N3 co-doped graphene (Co/N3-gra). We have investigated the geometric stability, electric properties, and magnetic properties comprehensively upon the interaction between Co/N3-gra and gas molecules. The binding energy of Co is -5.13 eV, which is big enough for application in gas adsorption. For the adsorption of C2H4, CO, NO2, and SO2 on Co/N-gra, the molecules may act as donors or acceptors of electrons, which can lead to charge transfer (range from 0.38 to 0.7 e) and eventually change the conductivity of Co/N-gra. The CO adsorbed Co/N3-gra complex exhibits a semiconductor property and the NO2/SO2 adsorption can regulate the magnetic properties of Co/N3-gra. Moreover, the Co/N3-gra system can be applied as a gas sensor of CO and SO2 with high stability. Thus, we assume that our results can pave the way for the further study of gas sensor and spintronic devices.

High-Throughput Screening of Two-Dimensional Planar sp2 Carbon Space Associated with a Labeled Quotient Graph

The configurational space of two-dimensional planar sp² carbon has been systematically scanned by a random strategy combined with group and graph theory, and 1114 new carbon allotropes have been identified. These allotropes are energetically more favorable than most of the previously predicted 120 carbon allotropes. By fitting the HSE06 band structures of six old structures, we optimize the parameters for a general and transferable tight-binding model for high-throughput band structure calculations. We identified that there are 190 Dirac semimetals, 241 semiconductors, and 683 normal metals among the new allotropes. Interestingly, several stable low-energy carbon systems with exotic electronic properties are proposed, such as type III, type I/II mixed, and type I/III mixed semimetals, which are very rare in planar carbon systems. In particular, one nodal-line semimetal has been discovered among these thousands of allotropes, which is the first nodal-line semimetal in sp² carbon systems. Our discoveries greatly enrich our knowledge of the structures and electronic properties of the two-dimensional carbon family.

The Defect Chemistry of Carbon Frameworks for Regulating the Lithium Nucleation and Growth Behaviors in Lithium Metal Anodes

Carbon materials have been widely considered as the frameworks in lithium (Li) metal anodes due to their lightweight, high electrical conductivity, and large specific surface area. Various heteroatom-doping strategies have been developed to enhance the lithiophilicity of carbon frameworks, thus rendering a uniform Li nucleation in working Li metal batteries. The corresponding lithiophilicity chemistry of doping sites has been comprehensively probed. However, various defects are inevitably introduced into carbon materials during synthesis and their critical role in regulating Li nucleation and growth behaviors is less understood. In this contribution, the defect chemistry of carbon materials in Li metal anodes is investigated through first-principles calculations. The binding energy towards a Li atom and the critical current density are two key descriptors to reveal the defect chemistry of carbon materials. Consequently, a diagram of designing carbon frameworks with both high lithiophilicity and a large critical current density is built, from which the Stone-Wales defect is predicted to possess the best performance for delivering a uniform Li deposition. This work uncovers the defect chemistry of carbon frameworks and affords fruitful insights into defect engineering for achieving dendrite-free Li metal anodes.

Dirac Fermions in Graphene with Stacking Fault Induced Periodic Line Defects

The exploration of carbon phases with intact massless Dirac fermions in the presence of defects is critical for practical applications to nanoelectronics. Here, we identify by first-principles calculations that the Dirac cones can exist in graphene with stacking fault (SF) induced periodic line defects. These structures are width (n)-dependent to graphene nanoribbon and are thus termed as (SF)_n-graphene. The electronic properties reveal that the semimetallic features with Dirac cones occur in (SF)_n-graphene with n = 3m + 1, where m is a positive integer, and then lead to a quasi-one-dimensional conducting channel. Importantly, it is found that the twisted Dirac cone in the (SF)₄-graphene is tunable among type-I, type-II, and type-III through a small uniaxial strain. The further stability analysis shows that (SF)_n-graphene is thermodynamic stable. Our findings provide an artificial avenue for exploring Dirac Ffermions in carbon-allotropic structures in the presence of defects.

Emerging properties of carbon based 2D material beyond graphene

Graphene turns out to be the pioneering material for setting up boulevard to a new zoo of recently proposed carbon based novel two dimensional (2D) analogues. It is evident that their electronic, optical and other related properties are utterly different from that of graphene because of the distinct intriguing morphology. For instance, the revolutionary emergence of Dirac cones in graphene is particularly hard to find in most of the other 2D materials. As a consequence the crystal symmetries indeed act as a major role for predicting electronic band structure. Since tight binding calculations have become an indispensable tool in electronic band structure calculation, we indicate the implication of such method in graphene's allotropes beyond hexagonal symmetry. It is to be noted that some of these graphene allotropes successfully overcome the inherent drawback of the zero band gap nature of graphene. As a result, these 2D nanomaterials exhibit great potential in a broad spectrum of applications, viz nanoelectronics, nanooptics, gas sensors, gas storages, catalysis, and other specific applications. The miniaturization of high performance graphene allotrope based gas sensors to microscopic or even nanosized range has also been critically discussed. In addition, various optical properties like the dielectric functions, optical conductivity, electron energy loss spectra reveal that these systems can be used in opto-electronic devices. Nonetheless, the honeycomb lattice of graphene is not superconducting. However, it is proposed that the tetragonal form of graphene can be intruded to form new hybrid 2D materials to achieve novel superconducting device at attainable conditions. These dynamic experimental prospects demand further functionalization of these systems to enhance the efficiency and the field of multifunctionality. This topical review aims to highlight the latest advances in carbon based 2D materials beyond graphene from the basic theoretical as well as future application perspectives.

Defect-Dependent Corrugation in Graphene

Graphene's intrinsically corrugated and wrinkled topology fundamentally influences its electronic, mechanical, and chemical properties. Experimental techniques allow the manipulation of pristine graphene and the controlled production of defects which allows one to control the atomic out-of-plane fluctuations and thus tune graphene's properties. Here, we perform large scale machine learning-driven molecular dynamics simulations to understand the impact of defects on the structure of graphene. We find that defects cause significantly higher corrugation leading to a strongly wrinkled surface. The magnitude of this structural transformation strongly depends on the defect concentration and specific type of defect. Analyzing the atomic neighborhood of the defects reveals that the extent of these morphological changes depends on the preferred geometrical orientation and the interactions between defects. While our work highlights that defects can strongly affect graphene's morphology, it also emphasizes the differences between distinct types by linking the global structure to the local environment of the defects.