Large-scale synthesis of free-standing N-doped graphene using microwave plasma

Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achieved using microwave plasmas at atmospheric pressure conditions. The synthesis is accomplished via a single step using ethanol and ammonia as carbon and nitrogen precursors. Tailoring of the high-energy density plasma environment results in a selective synthesis of N-graphene (~0.4% doping level) in a narrow range of externally controlled operational conditions, i.e. precursor and background gas fluxes, plasma reactor design and microwave power. Applying infrared (IR) and ultraviolet (UV) irradiation to the flow of free-standing sheets in the post-plasma zone carries out changes in the percentage of sp², the N doping type and the oxygen functionalities. X-ray photoelectron spectroscopy (XPS) revealed the relative extension of the graphene sheets π-system and the type of nitrogen chemical functions present in the lattice structure. Scanning Electron microscopy (SEM), Transmission Electron microscopy (TEM) and Raman spectroscopy were applied to determine morphological and structural characteristics of the sheets. Optical emission and FT-IR spectroscopy were applied for characterization of the high-energy density plasma environment and outlet gas stream. Electrochemical measurements were also performed to elucidate the electrochemical behavior of NG for supercapacitor applications.

Non-covalent control of spin-state in metal-organic complex by positioning on N-doped graphene

Nitrogen doping of graphene significantly affects its chemical properties, which is particularly important in molecular sensing and electrocatalysis applications. However, detailed insight into interaction between N-dopant and molecules at the atomic scale is currently lacking. Here we demonstrate control over the spin state of a single iron(II) phthalocyanine molecule by its positioning on N-doped graphene. The spin transition was driven by weak intermixing between orbitals with z-component of N-dopant (pz of N-dopant) and molecule (dxz, dyz, dz2) with subsequent reordering of the Fe d-orbitals. The transition was accompanied by an electron density redistribution within the molecule, sensed by atomic force microscopy with CO-functionalized tip. This demonstrates the unique capability of the high-resolution imaging technique to discriminate between different spin states of single molecules. Moreover, we present a method for triggering spin state transitions and tuning the electronic properties of molecules through weak non-covalent interaction with suitably functionalized graphene.

Size and Electronic Modulation of Iridium Nanoparticles on Nitrogen-Functionalized Carbon toward Advanced Electrocatalysts for Alkaline Water Splitting

Developing efficient catalytic materials for electrochemical water splitting is important. Herein, uniformly dispersed and size-controllable iridium (Ir) nanoparticles (NPs) were prepared using a nitrogen-functionalized carbon as the support (Ir/CN). We found that nitrogen functionalization can simultaneously modulate the size of Ir NPs to substantially enhance the catalytically active sites and adjust the electronic structure of Ir, thereby promoting electrocatalytic activity for water splitting. Consequently, the as-synthesized Ir/CN shows excellent electrocatalytic performance with overpotentials of 12 and 265 mV for hydrogen and oxygen evolution reactions in basic medium, respectively. These findings may pave the way for designing and synthesizing other similar materials as efficient catalysts for electrochemical water splitting.

Identifying the Key Role of Pyridinic-N-Co Bonding in Synergistic Electrocatalysis for Reversible ORR/OER

For many regenerative electrochemical energy-conversion systems, hybrid electrocatalysts comprising transition metal (TM) oxides and heteroatom-doped (e.g., nitrogen-doped) carbonaceous materials are promising bifunctional oxygen reduction reaction/oxygen evolution reaction electrocatalysts, whose enhanced electrocatalytic activities are attributed to the synergistic effect originated from the TM-N-C active sites. However, it is still ambiguous which configuration of nitrogen dopants, either pyridinic or pyrrolic N, when bonded to the TM in oxides, predominately contributes to the synergistic effect. Herein, an innovative strategy based on laser irradiation is described to controllably tune the relative concentrations of pyridinic and pyrrolic nitrogen dopants in the hybrid catalyst, i.e., NiCo₂ O₄ NPs/N-doped mesoporous graphene. Comparative studies reveal the dominant role of pyridinic-NCo bonding, instead of pyrrolic-N bonding, in synergistically promoting reversible oxygen electrocatalysis. Moreover, density functional theory calculations provide deep insights into the corresponding synergistic mechanism. The optimized hybrid, NiCo/NLG-270, manifests outstanding reversible oxygen electrocatalytic activities, leading to an overpotential different ΔE among the lowest value for highly efficient bifunctional catalysts. In a practical reversible Zn-air battery, NiCo/NLG-270 exhibits superior charge/discharge performance and long-term durability compared to the noble metal electrocatalysts.

Nitrogen Doping of Carbon Nanoelectrodes for Enhanced Control of DNA Translocation Dynamics

Controlling the dynamics of DNA translocation is a central issue in the emerging nanopore-based DNA sequencing. To address the potential of heteroatom doping of carbon nanostructures and for achieving this goal, herein, we carry out atomistic molecular dynamics simulations for single-stranded DNAs translocating between two pristine or doped carbon nanotube (CNT) electrodes. Specifically, we consider the substitutional nitrogen doping of capped CNT (capCNT) electrodes and perform two types of molecular dynamics simulations for the entrapped and translocating single-stranded DNAs. We find that the substitutional nitrogen doping of capCNTs facilitates and stabilizes the edge-on nucleobase configurations rather than the original face-on ones and slows down the DNA translocation speed by establishing hydrogen bonds between the N dopant atoms and nucleobases. Due to the enhanced interactions between DNAs and N-doped capCNTs, the duration time of nucleobases within the nanogap was extended by up to ∼300%. Given the possibility to be combined with the extrinsic light or gate voltage modulation methods, the current work demonstrates that the substitutional nitrogen doping is a promising direction for the control of DNA translocation dynamics through a nanopore or nanogap, based of carbon nanomaterials.

Preparation of graphene and its application in polycarbonate/acrylonitrile-butadiene-styrene composites

Graphene was prepared by the reduction of graphene oxide through chemical, thermal, and microwave methods. The morphology and structure of graphene obtained using different reduction processes have been characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectra. Polycarbonate/acrylonitrile-butadiene-styrene copolymers were modified with the addition of the as-prepared graphene. Electrical resistivity and tensile strength as well as thermal stability of composites have been investigated. The results reveal that graphene from chemical reduction is well compatible with composites and suitable for the enhancement of thermal stability. Graphene prepared from thermal reduction and microwave reduction are applicable for mechanical reinforcement and antistatic fields, respectively.

Encapsulating Chemically Doped Graphene via Atomic Layer Deposition

Controlling graphene's doping will be critically important for its incorporation into future electronic and optoelectronic devices. Noncovalent functionalization through adsorption of organic molecules on graphene's surface has proved to be a promising route for achieving p- or n-type doping. However, due to the poor adhesion of the molecules, these tend to desorb over time under standard environmental conditions or in the presence of certain solvents. The resulting reversal in the achieved chemical doping is a major obstacle to using organic molecules as noncovalent graphene dopants. In this work, we present a simple method for achieving long-term p- and n-doping of graphene devices through vapor phase evaporation of organic molecules, followed by encapsulation under an inert Al₂O₃ film. This film, grown via an optimized atomic layer deposition process, ensures long-term doping stability, as confirmed by electrical transport and Raman spectroscopy measurements. The doping is maintained even after storing the devices for six weeks in ambient conditions and immersing them in a dopant removing solvent, demonstrating that the film is as an effective barrier against environmental degradation of the doped devices.

Structural and Electronic Descriptors of Catalytic Activity of Graphene-Based Materials: First-Principles Theoretical Analysis

Characteristic features of the d-band in electronic structure of transition metals are quite effective as descriptors of their catalytic activity toward oxygen reduction reaction (ORR). With the promise of graphene-based materials to replace precious metal catalysts, descriptors of their chemical activity are much needed. Here, a site-specific electronic descriptor is proposed based on the p_z (π) orbital occupancy and its contribution to electronic states at the Fermi level. Simple structural descriptors are identified, and a linear predictive model is developed to precisely estimate adsorption free energies of OH (ΔG_OH ) at various sites of doped graphene, and it is demonstrated through prediction of the most optimal site for catalysis of ORR. These structural descriptors, essentially the number of ortho, meta, and para sites of N/B-doped graphene sheet, can be extended to other doped sp² hybridized systems, and greatly reduce the computational effort in estimating ΔG_OH and site-specific catalytic activity.

Thickness dependent Raman spectra and interfacial interaction between Ag and epitaxial graphene on 6H-SiC(0001)

Graphene as the thinnest material has an extremely large specific surface area, and thus the physical properties of graphene based devices should be sensitively dependent on the contacted metals. Moreover, the interfacial interaction between graphene and metals is complicated and it is difficult to probe. In this paper, epitaxial graphene is prepared by thermal decomposition of 6H-SiC(0001), and then Ag is deposited on it. It is found that the morphology and distribution of Ag particles on graphene domains are independent of the graphene thickness. The Ag particles induce the surface enhanced Raman scattering (SERS) effect and the doping effect in epitaxial graphene. The enhancement factor of SERS as well as the splitting of the G band and the shift of the 2D band decreases with increasing graphene thickness, which can be ascribed to the weakened interaction between Ag and EG. This is confirmed by the charge transfer between the Ag atom and epitaxial graphene on 6H-SiC predicted by first-principles calculations. The results are helpful to the design and development of graphene-based composites and devices.

Predicting Two-Dimensional C3B/C3N van der Waals p-n Heterojunction with Strong Interlayer Electron Coupling and Enhanced Photocurrent

The interlayer coupling in 2D van der Waals (vdW) heterostructures (HTS) plays the main role in generating new physics. However, the interlayer coupling is often weak, and little information on the strength of interlayer interaction in HTS is available. On the basis of density functional theory, we demonstrate that an effective electron coupling can be created in 2D C₃B/C₃N vdW HTS. The experimentally synthesized monolayers C₃B and C₃N are p- and n-type doped large gap semiconductors, respectively. However, the formed vdW HTS exhibits novel Dirac fermion. The strong interlayer electron coupling results in a large interlayer built-in electric field and improved optical properties of the 2D C₃B/C₃N vdW HTS. Moreover, a simple tight-binding model of C₃B/C₃N HTS with the nonzero interlayer hopping parameters captures the physical picture of interlayer coupling. Our results demonstrate the importance of interlayer electron coupling in the modulation of materials properties of 2D vdW HTS.

Electronic and thermoelectric properties of atomically thin C3Si3/C and C3Ge3/C superlattices

The nanostructuring of graphene into superlattices offers the possibility of tuning both the electronic and thermal properties of graphene. Using classical and quantum mechanical calculations, we have investigated the electronic and thermoelectric properties of the atomically thin superlattice of C₃Si₃/C (C₃Ge₃/C) formed by the incorporation of Si (Ge) atoms into graphene. The bandgap and phonon thermal conductivity of C₃Si₃/C (C₃Ge₃/C) are 0.54 (0.51) eV and 15.48 (12.64) W m^-1 K^-1, respectively, while the carrier mobility of C₃Si₃/C (C₃Ge₃/C) is 1.285 × 10⁵ (1.311 × 10⁵) cm² V^-1 s^-1 at 300 K. The thermoelectric figure of merit for C₃Si₃/C (C₃Ge₃/C) can be optimized via the tuning of carrier concentration to obtain the prominent ZT value of 1.95 (2.72).

Tuning the Doping Types in Graphene Sheets by N Monoelement

The doping types of graphene sheets are generally tuned by different dopants with either three or five valence electrons. As a five-valence-electrons element, however, nitrogen dopants in graphene sheets have several substitutional geometries. So far, their distinct effects on electronic properties predicted by theoretical calculations have not been well identified. Here, we demonstrate that the doping types of graphene can be tuned by N monoelement under proper growth conditions using chemical vapor deposition (CVD), characterized by combining scanning tunneling microscopy/spectroscopy, X-ray/ultraviolet photoelectron spectroscopy, Hall effect measurement, Raman spectroscopy, and density functional theory calculations. We find that a relatively low partial pressure of CH₄ (mixing with NH₃) can lead to the growth of dominant pyridinic N substitutions in graphene, in contrast with the growth of dominant graphitic N substitutions under a higher partial pressure of CH₄. Our results unambiguously confirm that the pyridinic N leads to the p-type doping, and the graphitic N leads to the n-type doping. Interestingly, we also find that the pyridinic N and the graphitic N are preferentially separated in different domains. Our findings shed light on continuously tuning the doping level of graphene monolayers by using N monoelement, which can be very convenient for growth of functional structures in graphene sheets.

Recent advances in three-dimensional graphene based materials for catalysis applications

This review presents recent theoretical and experimental progress in the construction, properties, and catalytic applications of 3D graphene-based materials.

Spotting the differences in two-dimensional materials – the Raman scattering perspective

This review discusses the Raman spectroscopic characterization of 2D materials with a focus on the “differences” from primitive 2D materials.

Creation of Triple Hierarchical Micro-Meso-Macroporous N-doped Carbon Shells with Hollow Cores Toward the Electrocatalytic Oxygen Reduction Reaction

A series of triple hierarchical micro-meso-macroporous N-doped carbon shells with hollow cores have been successfully prepared via etching N-doped hollow carbon spheres with CO₂ at high temperatures. The surface areas, total pore volumes and micropore percentages of the CO₂-activated samples evidently increase with increasing activation temperature from 800 to 950 °C, while the N contents show a contrary trend from 7.6 to 3.8 at%. The pyridinic and graphitic nitrogen groups are dominant among various N-containing groups in the samples. The 950 °C-activated sample (CANHCS-950) has the largest surface area (2072 m² g^-1), pore volume (1.96 cm³ g^-1), hierarchical micro-mesopore distributions (1.2, 2.6 and 6.2 nm), hollow macropore cores (~91 nm) and highest relative content of pyridinic and graphitic N groups. This triple micro-meso-macropore system could synergistically enhance the activity because macropores could store up the reactant, mesopores could reduce the transport resistance of the reactants to the active sites, and micropores could be in favor of the accumulation of ions. Therefore, the CANHCS-950 with optimized structure shows the optimal and comparable oxygen reduction reaction (ORR) activity but superior methanol tolerance and long-term durability to commercial Pt/C with a 4e^--dominant transfer pathway in alkaline media. These excellent properties in combination with good stability and recyclability make CANHCSs among the most promising metal-free ORR electrocatalysts reported so far in practical applications.

Nitrogen doped high quality CVD grown graphene as a fast responding NO2 gas sensor

Nitrogen doped high quality CVD grown graphene is demonstrated for application in a high performance NO₂ gas sensor.

Computational investigation of double nitrogen doping on graphene

Density functional theory (DFT) calculations were performed to study doping of two nitrogen atoms at different positions on a finite-sized graphene model of C₈₂H₂₄. We examined 21 structures of double nitrogen doped graphene to calculate their relative stabilities. The structure with two nitrogen atoms located apart is the most stable among the positional isomers considered in this study. For double nitrogen doping within a six-membered ring, the 1,4-position is more preferred than 1,3- or 1,2-positions for the finite-sized single layer graphene sheet. Our computational study supports the experimental observation of two nitrogen atoms at the 1,3- and 1,4-positions in a single six-membered ring of graphene. Furthermore, the structures with N-N bond are the least stable among two nitrogen doped graphene structures. The effects of nitrogen doping and the positions of two nitrogen atoms on the HOMO-LUMO energy gap of pristine graphene were analyzed.

Chemical Ni-C Bonding in Ni-Carbon Nanotube Composite by a Microwave Welding Method and Its Induced High-Frequency Radar Frequency Electromagnetic Wave Absorption

In this work, a microwave welding method has been used for the construction of chemical Ni-C bonding at the interface between carbon nanotubes (CNTs) and metal Ni to provide a different surface electron distribution, which determined the electromagnetic (EM) wave absorption properties based on a surface plasmon resonance mechanism. Through a serial of detailed examinations, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectrum, the as-expected chemical Ni-C bonding between CNTs and metal Ni has been confirmed. And the Brunauer-Emmett-Teller and surface zeta potential measurements uncovered the great evolution of structure and electronic density compared with CNTs, metal Ni, and Ni-CNT composite without Ni-C bonding. Correspondingly, except the EM absorption due to CNTs and metal Ni in the composite, another wide and strong EM absorption band ranging from 10 to 18 GHz was found, which was induced by the Ni-C bonded interface. With a thinner thickness and more exposed Ni-C interfaces, the Ni-CNT composite displayed less reflection loss.

NanoVelcro: Theory of Guided Folding in Atomically Thin Sheets with Regions of Complementary Doping

Folding has been commonly observed in two-dimensional materials such as graphene and monolayer transition metal dichalcogenides. Although interlayer coupling stabilizes these folds, it provides no control over the placement of the fold, let alone the final folded shape. Lacking nanoscale "fingers" to externally guide folding, control requires interactions engineered into the sheets that guide them toward a desired final folded structure. Here we provide a theoretical framework for a general methodology toward this end: atomically thin 2D sheets are doped with patterns of complementary n-type and p-type regions whose preferential adhesion favors folding into desired shapes. The two-colorable theorem in flat-foldable origami ensures that arbitrary folding patterns are in principle accessible to this method. This complementary doping method can be combined with nanoscale crumpling (by, for example, passage of 2D sheets through holes) to obtain not only control over fold placements but also the ability to distinguish between degenerate folded states, thus attaining nontrivial shapes inaccessible to sequential folding.

Resolving the Spatial Structures of Bound Hole States in Black Phosphorus

Understanding the local electronic properties of individual defects and dopants in black phosphorus (BP) is of great importance for both fundamental research and technological applications. Here, we employ low-temperature scanning tunnelling microscope (LT-STM) to probe the local electronic structures of single acceptors in BP. We demonstrate that the charge state of individual acceptors can be reversibly switched by controlling the tip-induced band bending. In addition, acceptor-related resonance features in the tunnelling spectra can be attributed to the formation of Rydberg-like bound hole states. The spatial mapping of the quantum bound states shows two distinct shapes evolving from an extended ellipse shape for the 1s ground state to a dumbbell shape for the 2p_x excited state. The wave functions of bound hole states can be well-described using the hydrogen-like model with anisotropic effective mass, corroborated by our theoretical calculations. Our findings not only provide new insight into the many-body interactions around single dopants in this anisotropic two-dimensional material but also pave the way to the design of novel quantum devices.

Imaging the potential distribution of individual charged impurities on graphene by low-energy electron holography

While imaging individual atoms can routinely be achieved in high resolution transmission electron microscopy, visualizing the potential distribution of individually charged adsorbates leading to a phase shift of the probing electron wave is still a challenging task. Low-energy electrons (30 - 250 eV) are sensitive to localized potential gradients. We employed low-energy electron holography to acquire in-line holograms of individual charged impurities on free-standing graphene. By applying an iterative phase retrieval reconstruction routine we recover the potential distribution of the localized charged impurities present on free-standing graphene.

Controlled Chemical Synthesis in CVD Graphene

Due to the unique properties of graphene, single layer, bilayer or even few layer graphene peeled off from bulk graphite cannot meet the need of practical applications. Large size graphene with quality comparable to mechanically exfoliated graphene has been synthesized by chemical vapor deposition (CVD). The main development and the key issues in controllable chemical vapor deposition of graphene has been briefly discussed in this chapter. Various strategies for graphene layer number and stacking control, large size single crystal graphene domains on copper, graphene direct growth on dielectric substrates, and doping of graphene have been demonstrated. The methods summarized here will provide guidance on how to synthesize other two-dimensional materials beyond graphene.

Electron-Hole Symmetry Breaking in Charge Transport in Nitrogen-Doped Graphene

Graphitic nitrogen-doped graphene is an excellent platform to study scattering processes of massless Dirac Fermions by charged impurities, in which high mobility can be preserved due to the absence of lattice defects through direct substitution of carbon atoms in the graphene lattice by nitrogen atoms. In this work, we report on electrical and magnetotransport measurements of high-quality graphitic nitrogen-doped graphene. We show that the substitutional nitrogen dopants in graphene introduce atomically sharp scatters for electrons but long-range Coulomb scatters for holes and, thus, graphitic nitrogen-doped graphene exhibits clear electron-hole asymmetry in transport properties. Dominant scattering processes of charge carriers in graphitic nitrogen-doped graphene are analyzed. It is shown that the electron-hole asymmetry originates from a distinct difference in intervalley scattering of electrons and holes. We have also carried out the magnetotransport measurements of graphitic nitrogen-doped graphene at different temperatures and the temperature dependences of intervalley scattering, intravalley scattering, and phase coherent scattering rates are extracted and discussed. Our results provide an evidence for the electron-hole asymmetry in the intervalley scattering induced by substitutional nitrogen dopants in graphene and shine a light on versatile and potential applications of graphitic nitrogen-doped graphene in electronic and valleytronic devices.

A Kinetic Pathway toward High-Density Ordered N Doping of Epitaxial Graphene on Cu(111) Using C5NCl5 Precursors

Pristine graphene possesses high electrical mobility, but its low charge carrier density severely limits its technological significance. Past efforts to increase graphene's carrier density via chemical doping have shown limited successes, accompanied by substantial reductions in the mobility caused by disordered dopants. Here, based on first-principles calculations, we propose to grow graphene on Cu(111) via self-assembly of C₅NCl₅ molecular precursors to achieve high-density (1/6) and highly ordered nitrogen doping. Such a process relies on the elegant concerted roles played by the London dispersion, chemical, and screened Coulomb repulsive forces in enhancing molecular adsorption, facilitating easy dechlorination, and dictating the overall orientation of the C₅N radicals, respectively. Further growth from the orientationally correlated graphene islands is accompanied by significantly minimized density of grain boundaries as the grains coalesce to form larger N-doped graphene sheets, which are further shown to possess superb electronic properties for future device applications. Initial kinetic processes involved in N-doped graphene growth using C₅NH₅ precursors are also investigated and contrasted with that of C₅NCl₅.

Controlled Synthesis of Nitrogen-Doped Graphene on Ruthenium from Azafullerene

The controlled synthesis of high-quality nitrogen (N) doped single layer graphene on the Ru(0001) surface has been achieved using the N-containing sole precursor azafullerence (C₅₉NH). The synthesis process and doping properties have been investigated on the atomic scale by combining scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements. We find for the first time that the concentration of N-related defects on the N-doped graphene/Ru(0001) surface is tunable by adjusting the dosage of sole precursor and the number of growth cycles. Two primary types of N-related defects have been observed. The predominant bonding configuration of N atoms in the obtained graphene layer is pyridinic N. Our findings indicate that the synthesis from heteroatom-containing sole precursors is a very promising approach for the preparation of doped graphene materials with controlled doping properties.

Highly Crumpled Hybrids of Nitrogen/Sulfur Dual-Doped Graphene and Co9S8 Nanoplates as Efficient Bifunctional Oxygen Electrocatalysts

A bifunctional electrocatalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly attractive for the manufacture of clean energy conversion devices. In this work, highly crumpled hybrid of nitrogen and sulfur dual-doped graphene and quasi-hexagonal Co₉S₈ nanoplates (Co₉S₈/NSG_g-C3N4) is fabricated via a facile ionic assembly approach. The unique structure of Co₉S₈/NSG_g-C3N4 renders it high specific surface area (288.3 m² g^-1) and large pore volume (1.32 cm³ g^-1). As the electrocatalyst for ORR, Co₉S₈/NSG_g-C3N4 demonstrates excellent performance with the onset potential of -0.02 V vs Ag/AgCl and the limited current density of 6.05 mA cm^-2 at -0.9 V vs Ag/AgCl. Co₉S₈/NSG_g-C3N4 also presents outstanding catalytic activity toward OER by delivering a limited current density of 48 mA cm^-2 at 1 V vs Ag/AgCl. The bifunctional catalytic behaviors of Co₉S₈/NSG_g-C3N4 enable the assembly of a rechargeable Zn-air battery with it as the cathode catalyst, which exhibits stable discharge/charge voltage plateaus upon long time cycling over 50 h.

Atomic Defects in Two-Dimensional Materials: From Single-Atom Spectroscopy to Functionalities in Opto-/Electronics, Nanomagnetism, and Catalysis

Two-dimensional layered graphene-like crystals including transition-metal dichalcogenides (TMDs) have received extensive research interest due to their diverse electronic, valleytronic, and chemical properties, with the corresponding optoelectronics and catalysis application being actively explored. However, the recent surge in two-dimensional materials science is accompanied by equally great challenges, such as defect engineering in large-scale sample synthesis. It is necessary to elucidate the effect of structural defects on the electronic properties in order to develop an application-specific strategy for defect engineering. Here, two aspects of the existing knowledge of native defects in two-dimensional crystals are reviewed. One is the point defects emerging in graphene and hexagonal boron nitride, as probed by atomically resolved electron microscopy, and their local electronic properties, as measured by single-atom electron energy-loss spectroscopy. The other will focus on the point defects in TMDs and their influence on the electronic structure, photoluminescence, and electric transport properties. This review of atomic defects in two-dimensional materials will offer a clear picture of the defect physics involved to demonstrate the local modulation of the electronic properties and possible benefits in potential applications in magnetism and catalysis.

Efficient Nitrogen Doping of Single-Layer Graphene Accompanied by Negligible Defect Generation for Integration into Hybrid Semiconductor Heterostructures

While doping enables application-specific tailoring of graphene properties, it can also produce high defect densities that degrade the beneficial features. In this work, we report efficient nitrogen doping of ∼11 atom % without virtually inducing new structural defects in the initial, large-area, low defect, and transferred single-layer graphene. To shed light on this remarkable high-doping-low-disorder relationship, a unique experimental strategy consisting of analyzing the changes in doping, strain, and defect density after each important step during the doping procedure was employed. Complementary micro-Raman mapping, X-ray photoelectron spectroscopy, and optical microscopy revealed that effective cleaning of the graphene surface assists efficient nitrogen incorporation accompanied by mild compressive strain resulting in negligible defect formation in the doped graphene lattice. These original results are achieved by separating the growth of graphene from its doping. Moreover, the high doping level occurred simultaneously with the epitaxial growth of n-GaN micro- and nanorods on top of graphene, leading to the flow of higher currents through the graphene/n-GaN rod interface. Our approach can be extended toward integrating graphene into other technologically relevant hybrid semiconductor heterostructures and obtaining an ohmic contact at their interfaces by adjusting the doping level in graphene.

Graphene as a flexible template for controlling magnetic interactions between metal atoms

Metal-doped graphene produces magnetic moments that have potential application in spintronics. Here we use density function theory computational methods to show how the magnetic interaction between metal atoms doped in graphene can be controlled by the degree of flexure in a graphene membrane. Bending graphene by flexing causes the distance between two substitutional Fe atoms covalently bonded in graphene to gradually increase and these results in the magnetic moment disappearing at a critical strain value. At the critical strain, a carbon atom can enter between the two Fe atoms and blocks the interaction between relevant orbitals of Fe atoms to quench the magnetic moment. The control of interactions between doped atoms by exploiting the mechanical flexibility of graphene is a unique approach to manipulating the magnetic properties and opens up new opportunities for mechanical-magnetic 2D device systems.

A hybrid MBE-based growth method for large-area synthesis of stacked hexagonal boron nitride/graphene heterostructures

Van der Waals heterostructures combining hexagonal boron nitride (h-BN) and graphene offer many potential advantages, but remain difficult to produce as continuous films over large areas. In particular, the growth of h-BN on graphene has proven to be challenging due to the inertness of the graphene surface. Here we exploit a scalable molecular beam epitaxy based method to allow both the h-BN and graphene to form in a stacked heterostructure in the favorable growth environment provided by a Ni(111) substrate. This involves first saturating a Ni film on MgO(111) with C, growing h-BN on the exposed metal surface, and precipitating the C back to the h-BN/Ni interface to form graphene. The resulting laterally continuous heterostructure is composed of a top layer of few-layer thick h-BN on an intermediate few-layer thick graphene, lying on top of Ni/MgO(111). Examinations by synchrotron-based grazing incidence diffraction, X-ray photoemission spectroscopy, and UV-Raman spectroscopy reveal that while the h-BN is relaxed, the lattice constant of graphene is significantly reduced, likely due to nitrogen doping. These results illustrate a different pathway for the production of h-BN/graphene heterostructures, and open a new perspective for the large-area preparation of heterosystems combining graphene and other 2D or 3D materials.

Chemical functionalization and characterization of graphene-based materials

This review offers an overview on the chemical functionalization, characterization and applications of graphene-based materials.

Metal-catalyst-free growth of graphene on insulating substrates by ammonia-assisted microwave plasma-enhanced chemical vapor deposition

We study the metal-catalyst-free growth of uniform and continuous graphene on different insulating substrates by microwave plasma-enhanced chemical vapor deposition (PECVD) with a gas mixture of C₂H₂, NH₃, and H₂ at a temperature of 700–750 °C.

Structural, electronic, and magnetic properties of non-planar doping of BeO in graphene: a DFT study

The non-planar molecular doping of BeO is more efficient in inducing a band gap relative to its planar doping with no magnetic effect realization.

Nitrogen doping for facile and effective modification of graphene surfaces

Nitrogen-doped graphene (N-graphene) was prepared by exposing the graphene transferred to different substrates to atomic nitrogen plasma.

Effect of Heteroatoms in Ordered Microporous Carbons on Their Electrochemical Capacitance

Micropores play a more important role in enhancing the electrochemical capacitance than mesopores and macropores; therefore, the effect of heteroatom doping into micropores on the electrochemical behavior is interesting. However, heteroatom doping into porous carbon materials would potentially change their pore structures and pore sizes, which also affect their electrochemical capacitive behaviors. To gain insight into the intrinsic effects of heteroatoms on the electrochemical capacitive behaviors, zeolite-templated carbon (ZTC) may be the most suitable candidate. ZTC is an ordered microporous carbon with a uniform micropore size of 1.2 nm, a high surface area, and a large micropore volume. In this work, a series of ZTCs containing oxygen, nitrogen, or boron as heteroatoms, with an ordered pore structure and the same pore size, are prepared. By examining their electrochemical capacitive behaviors in an organic electrolyte, the effect of heteroatom doping can be isolated and discussed without considering the effects of pore structure and pore size. Acid anhydride groups are found to generate pseudocapacitance in two potential ranges, -1.0 to -0.3 V (vs Ag/AgClO₄) and -0.2 to 0.4 V. B is introduced into the ZTC framework solely as -B(OH)₂, which is found to be an electrochemically inert species. N is introduced as pyridine (3.0%), pyridone/pyrrole (23.8%), quaternary (66.6%), and oxidized N (6.6%), and these species exhibit noticeable pseudocapacitance in the microporous carbon.

Purely substitutional nitrogen on graphene/Pt(111) unveiled by STM and first principles calculations

Nitrogen doping of graphene can be an efficient way of tuning its pristine electronic properties. Several techniques have been used to introduce nitrogen atoms on graphene layers. The main problem in most of them is the formation of a variety of C-N species that produce different electronic and structural changes on the 2D layer. Here we report on a method to obtain purely substitutional nitrogen on graphene on Pt(111) surfaces. A detailed experimental study performed in situ, under ultra-high vacuum conditions with scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) of the different steps on the preparation of the sample, has allowed us to gain insight into the optimal parameters for this growth method, that combines ion bombardment and annealing. This experimental work is complemented by first-principles calculations and STM simulations that provide the variation of the projected density of states due to both the metallic substrate and the nitrogen atoms. These calculations enlighten the experimental findings and prove that the species found are graphitic nitrogen. This easy and effective technique leads to the possibility of playing with the amount of dopants and the metallic substrate to obtain the desired doping of the graphene layer.

Atomic resolution of nitrogen-doped graphene on Cu foils

Atomic-level substitutional doping can significantly tune the electronic properties of graphene. Using low-temperature scanning tunneling microscopy and spectroscopy, the atomic-scale crystalline structure of graphene grown on polycrystalline Cu, the distribution of nitrogen dopants and their effect on the electronic properties of graphene were investigated. Both the graphene sheet growth and nitrogen doping were performed using microwave plasma-enhanced chemical vapor deposition. The results indicated that the nitrogen dopants preferentially sit at the grain boundaries of the graphene sheets and confirmed that plasma treatment is a potential method to incorporate foreign atoms into the graphene lattice to tailor the graphene's electronic properties.