Electronic Structure Of Graphene Research Articles

The Interplay of Solvation and Polarization Effects on Ion Pairing in Nanoconfined Electrolytes.

The nature of ion-ion interactions in electrolytes confined to nanoscale pores has important implications for energy storage and separation technologies. However, the physical effects dictating the structure of nanoconfined electrolytes remain debated. Here we employ machine-learning-based molecular dynamics simulations to investigate ion-ion interactions with density functional theory level accuracy in a prototypical confined electrolyte, aqueous NaCl within graphene slit pores. We find that the free energy of ion pairing in highly confined electrolytes deviates substantially from that in bulk solutions, observing a decrease in contact ion pairing but an increase in solvent-separated ion pairing. These changes arise from an interplay of ion solvation effects and graphene's electronic structure. Notably, the behavior observed from our first-principles-level simulations is not reproduced even qualitatively with the classical force fields conventionally used to model these systems. The insight provided in this work opens new avenues for predicting and controlling the structure of nanoconfined electrolytes.

Scope and Limitations of Using Microemulsions for the Covalent Patterning of Graphene.

Patterning of graphene (functionalizing some areas while leaving others intact) is challenging, as all the C atoms in the basal plane are identical, but it is also desirable for a variety of applications, like opening a bandgap in the electronic structure of graphene. Several methods have been reported to pattern graphene, but most of them are very technologically intensive. Recently, we reported the use of microemulsions as templates to pattern graphene at the μm scale. This method is very simple and in principle tunable, as emulsions of different droplet size and composition can be prepared easily. Here, we explore in detail the scope of this methodology by applying it to all the combinations of four different emulsions and three different organic reagents, and characterizing the resulting substrates exhaustively through Raman, SEM and AFM. We find that the method is general, works better when the reactive species are outside the micelles, and requires reactive species that involve short reaction times.

Enhancing CO2 adsorption capacity of hydroxypyridine-based ionic liquids using fluorinated graphene as carrier Material: A density functional theory study

Graphene carriers can improve the properties of ionic liquids and enhance their CO2 capture performance. Density functional theory calculations were employed to evaluate the feasibility of utilizing graphene as a carrier material to enhance the CO2 adsorption capacity of hydroxypyridine-based ionic liquids (ILs). The results indicated that BN co-doped fluorinated graphene represented a promising carrier material capable of weakening the interaction between cations and anions while enhancing the charges of O and N in the anionic species of ILs, thereby increasing the adsorption capacity of ionic liquids for CO2. Furthermore, the microstructure and properties of the graphene-ILs composites were explored. Doping atoms in graphene were found to induce alterations in the distribution of surface electrostatic potential, influencing the adsorption sites of ionic liquids and their properties. Compared to hydrogen-terminated graphene, fluorinated graphene was a more stable carrier material due to its higher binding energy with hydroxypyridine-based ILs. The higher binding energy was not derived from the direct interaction between doped atoms and ILs, but rather from the impact of doped atoms on the electronic structure of graphene. In methanol solvent, the adsorption of ILs on various graphene surfaces was a spontaneous exothermic process, indicating the feasibility of material preparation.

In-Plane Topological-Defect-Enriched Graphene as an Efficient Metal-Free Catalyst for pH-Universal H2 O2 Electrosynthesis.

Developing efficient metal-free catalysts to directly synthesize hydrogen peroxide (H2 O2 ) through a 2-electron (2e) oxygen reduction reaction (ORR) is crucial for substituting the traditional energy-intensive anthraquinone process. Here, in-plane topological defects enriched graphene with pentagon-S and pyrrolic-N coordination (SNC) is synthesized via the process of hydrothermal and nitridation. In SNC, pentagon-S and pyrrolic-N originating from thiourea precursor are covalently grafted onto the basal plane of the graphene framework, building unsymmetrical dumbbell-like S─C─N motifs, which effectively modulates atomic and electronic structures of graphene. The SNC catalyst delivers ultrahigh H2 O2 productivity of 8.1, 7.3, and 3.9mol gcatalyst -1 h-1 in alkaline, neutral, and acidic electrolytes, respectively, together with long-term operational stability in pH-universal electrolytes, outperforming most reported carbon catalysts. Theoretical calculations further unveil that defective S─C─N motifs efficiently optimize the binding strength to OOH* intermediate and substantially diminish the kinetic barrier for reducing O2 to H2 O2 , thereby promoting the intrinsic activity of 2e-ORR.

Stable room-temperature ferromagnetism and gate-tunable quantum anomalous Hall effect of two-dimensional 5d transition-metal trihalide OsX3 (X = Cl, Br, I) monolayers.

5d transition-metal compounds are usually not expected to exhibit distinct magnetic ordering owing to their substantial binding energy associated with 5d electrons. In this study, we demonstrate that two-dimensional (2D) 5d transition-metal Os trihalide OsX3 monolayers can exhibit room-temperature ferromagnetism and quantum anomalous Hall effect (QAHE) by utilizing density functional theory and Monte Carlo simulation. Our calculation results of coexisting Raman and infrared activities of lattice vibration reveal the structural stability of 2D OsX3 (X = Cl, Br, I) and structural instability of 2D OsX3 (X = F). Furthermore, all 2D OsX3 trihalides (X = Cl, Br, I) are half-metals, and their ferromagnetism remains stable under ambient temperature, where 2D OsCl3 and OsBr3 have an in-plane easy axis while 2D OsI3 has an out-of-plane easy axis. Notably, when spin-orbit coupling is included, the gate-tunable QAHE could emerge in ferromagnetic 2D OsI3, while 2D OsCl3 and OsBr3 are topologically trivial. Additionally, the magnon bands of 2D OsX3 (X = Cl, Br, I) possess two spin-wave branches with dispersion similar to that of the Dirac cone in the electronic structure of graphene, which are attributed to the unique ferromagnetic honeycomb sublattice of osmium atoms.

Room Temperature Ferromagnetism in Graphene/SiC(0001) System Intercalated by Fe and Co

The utilization of graphene on silicon carbide (SiC) substrates holds substantial promise for advancements in spintronics and nanoelectronics. Furthermore, incorporating magnetic metals provides an optimal framework for probing fundamental physical phenomena. The approach to developing such systems is in situ intercalation of graphene with magnetic metals. Herein, the electronic structure is analyzed and the magnetic properties of the system are synthesized by the thermal decomposition of 6H‐SiC(0001) surface and subsequent intercalation of graphene with cobalt (Co) and iron (Fe) atoms. X‐ray photoemission spectroscopy and low‐energy electron diffraction are employed to control the synthesis and metal intercalation processes. The morphological characteristics of the synthesized system are studied by means of atomic force microscopy. The findings derived from magneto‐optic Kerr effect measurements reveal a homogeneous ferromagnetic ordering at room temperature. Angle‐resolved photoemission spectroscopy is used to ascertain the impact of intercalation on graphene's electronic structure. The results of this study are essential for the development of graphene‐based spintronics and nanoelectronic devices as well as for fundamental studies in magnetic graphene systems.

A Density Functional Theory (DFT) Perspective on Optical Absorption of Modified Graphene Interacting with the Main Amino Acids of Spider Silk.

We investigated the possible adsorption of each of the main building blocks of spider silk: alanine, glycine, leucine, and proline. This knowledge could help develop new biocompatible materials and favors the creation of new biosensors. We used ab initio density functional theory methods to study the variations in the optical absorption, reflectivity, and band structure of a modified graphene surface interacting with these four molecules. Four modification cases were considered: graphene with vacancies at 5.55% and fluorine, nitrogen, or oxygen doping, also at 5.55%. We found that, among the cases considered, graphene with vacancies is the best candidate to develop optical biosensors to detect C=O amide and differentiate glycine and leucine from alanine and proline in the visible spectrum region. Finally, from the projected density of states, the main changes occur at deep energies. Thus, all modified graphene's electronic energy band structure undergoes only tiny changes when interacting with amino acids.

Ionic Liquid–Graphene Interface: Effect of Anions on the Fermi Level

Since energy conversion and storage processes take place at the electrolyte–electrode interface, it is important to develop experimental and theoretical procedures to understand the interfacial nanostructure in graphene-based electrochemical storage devices where ionic liquids (ILs) are used as electrolytes. In this contribution, the impact of the anions of imidazolium-based ILs on the IL–graphene interface as well as on the electronic structure of graphene is investigated. Raman spectroscopy unveils that 1-butyl-3-methylimidazolium ILs having smaller anions induce n-type doping, while ILs with larger anions have a negligible effect on the doping. Molecular modeling simulations reveal that changes in the electrostatic potential at the IL–graphene interface are responsible for the n-type doping.

Spin and orbital metallic magnetism in rhombohedral trilayer graphene

We provide a complete theoretical interpretation of the metallic broken spin/valley symmetry states recently discovered in ABC trilayer graphene (ABC) perturbed by a large transverse displacement field. Our conclusions combine insights from ABC trilayer graphene electronic structure models and mean field theory, and are guided by precise magneto-oscillation Fermi-surface-area measurements. We conclude that the physics of ABC trilayer graphene is shaped by the principle of momentum-space condensation, which favors Fermi surface reconstructions enabled by broken spin/valley flavor symmetries when the single-particle bands imply thin annular Fermi seas. We find one large outer Fermi surface enclosed majority-flavor states and one or more small inner hole-like Fermi surfaces enclosed minority-flavor states that are primarily responsible for nematic order. The smaller surfaces can rotate along a ring of van-Hove singularities or reconstruct into multiple Fermi surfaces with little cost in energy. We propose that the latter property is responsible for the quantum oscillation frequency fractionalization seen experimentally in some regions of the carrier-density/displacement-field phase diagram.

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 μB 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 μB). 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.

Spatial segregation of substitutional B atoms in graphene patterned by the moiré superlattice on Ir(111)

Fabrication of ordered structures at the nanoscale limit poses a cornerstone challenge for modern technologies. In this work we show how naturally occurring moiré patterns in Ir(111)-supported graphene template the formation of 2D ordered arrays of substitutional boron species. A complementary experimental and theoretical approach provides a comprehensive description of the boron species distribution, bonding configurations, interfacial interaction with Ir(111) and the impact on graphene's electronic structure. Atomically-resolved scanning tunnelling microscopy images and density functional theory calculations reveal that boron preferably forms small aggregates of substitutional defects in geometrically low regions of the moiré superlattice of graphene, by inducing local bending of graphene towards the underlying Ir(111). Surprisingly, calculations reveal that the incorporation of electron deficient boron does not lead to an enhanced p-type doping, as the local rippling of the graphene layer prompts electron uptake from the iridium substrate that compensates the initial electron loss due to substitutional boron. Scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy measurements corroborate that the arrays of boron species do not modify the electronic structure of graphene near the Fermi level, hence preserving the slight p-type doping induced by Ir(111).

Electronic structure of partially fluorinated graphene: The impact of adsorption patterns and dynamic stability

The impact of fluorination on the stability and electronic structure of graphene is investigated by employing density functional theory calculations. It is observed that both the cell symmetry and dynamical stability (phonon dispersion spectra) are necessary for the identification of energetically optimum configurations. In general, an increase in band gap is observed with fluorination, which is in good agreement with the experimental studies. These results underscore the importance of employing key stability (energy, dynamical and thermal) filters for correct assessment of electronic properties of partially fluorinated graphene and may help to explain the inconsistencies of previous theoretical and experimental findings.

The electronic structure of graphene like C20H10CdN6O8.5 metal–organic nanotube (MONT) based on FP-LAPW: for optoelectronic and thermoelectric devices

The electronic structure of graphene like C20H10CdN6O8.5 metal–organic nanotube (MONT) based on FP-LAPW: for optoelectronic and thermoelectric devices

Influence of Hexagonal Boron Nitride on Electronic Structure of Graphene.

By performing first-principles calculations, we studied hexagonal-boron-nitride (hBN)-supported graphene, in which moiré structures are formed due to lattice mismatch or interlayer rotation. A series of graphene/hBN systems has been studied to reveal the evolution of properties with respect to different twisting angles (21.78°, 13.1°, 9.43°, 7.34°, 5.1°, and 3.48°). Although AA- and AB-stacked graphene/hBN are gapped at the Dirac point by about 50 meV, the energy gap of the moiré graphene/hBN, which is much more asymmetric, is only about several meV. Although the Dirac cone of graphene residing in the wide gap of hBN is not much affected, the calculated Fermi velocity is found to decrease with the increase in the moiré super lattice constant due to charge transfer. The periodic potential imposed by hBN modulated charge distributions in graphene, leading to the shift of graphene bands. In agreement with experiments, there are dips in the calculated density of states, which get closer and closer to the Fermi energy as the moiré lattice grows larger.

Electronic and Magnetic Properties of the Graphene/Y/Co(0001) Interfaces: Insights from the Density Functional Theory Analysis

The effect of Y intercalation on the atomic, electronic, and magnetic properties of the graphene/Co(0001) interface is studied using state-of-the-art density functional theory calculations. Different structural models of the graphene/Y/Co(0001) interface are considered: (i) graphene/Y/Co(0001), (ii) graphene/1ML-YCo2/Co(0001), and (iii) graphene/bulk-like-YCo2(111). It is found that the interaction strength between graphene and the substrate is strongly affected by the presence of Y at the interface and the electronic structure of graphene (doping and the appearance of the energy gap) is defined by the Y concentration. For the Co-terminated interfaces between graphene and the metallic support in the considered systems, the electronic structure of graphene is strongly disturbed, leading to the absence of the linear dispersion for the graphene π band; in the case of the Y-terminated interfaces, a graphene layer is strongly n-doped, but the linear dispersion for this band is preserved. Our calculations show that the magnetic anisotropy for the magnetic atoms at the graphene/metal interface is strongly affected by the adsorption of a graphene layer, giving a possibility for one to engineer the magnetic properties of the graphene/ferromagnet systems.

Electronic Structure of Graphene on the Hexagonal Boron Nitride Surface: A Density Functional Theory Study

Poor electron-related cutting current in graphene-based field-effect transistors (FETs) can be solved by placing a graphene layer over a hexagonal boron nitride (BN) substrate, as established by Giovannetti et al. and other researchers. In order to produce high-quality results, this investigation uses 2 × 2 cells (~2.27% mismatch), given that larger cells lead to more favourable considerations regarding interactions on cell edges. In this case, the substrate-induced band gap is close to 138 meV. In addition, we propose a new material based on graphene on BN in order to take advantage of the wonderful physical properties of both graphene and BN. In this new material, graphene is rotated with respect to BN, and it exhibits a better mismatch, only ~1.34%, than the 1 × 1-graphene/1 × 1-BN; furthermore, it has a very small bandgap, which is almost zero. Therefore, in the bands, there are electronic states in cone form that are like the Dirac cones, which maintain the same characteristics as isolated graphene. In the first case (2 × 2-graphene/2 × 2-BN), for example, the resulting band gap of 138 meV is greater than Giovannetti’s value by a factor of ~2.6. The 2 × 2-graphene/2 × 2-BN cell is better than the 1 × 1-graphene/BN one because a greater bandgap is an improvement in the cutting current of graphene-based FETs, since the barrier created by the bandgap is larger. The calculations in this investigation are performed within the density functional theory (DFT) theory framework, by using 2 × 2-graphene/2 × 2-BN and 13 × 13-graphene/23 × 23-(0001) BN cells. Pseudopotentials and the generalized gradient approximation (GGA), combined with the Perdew–Burke–Ernzerhof parametrization, were used. Relaxation is allowed for all atoms, except for the last layer of the BN substrate, which serves as a reference for all movements and simulates the bulk BN.

Effect of gold intercalation on the electronic structure of graphene on Co-Si/SiC(0001)

Graphene grown on an ultrathin CoSi/CoSi2 silicide layer atop a SiC(0001) substrate and intercalated by Au was studied. Intercalation of Au was found to occur at a temperature of 500oC, and it is accompanied by the formation of gold silicide under graphene with a stoichiometry close to Au2Si. The study of the electronic structure of the system in the region of the (K) point of the surface Brillouin zone revealed the quasi-freestanding character of graphene with a linear spectrum of π states and a Dirac point near the Fermi level. Photoelectron microscopy revealed the homogeneity of the work function along the surface of the sample on a micrometer scale. Keywords: Graphene, magneto-spin-orbit graphene, intercalation, photoelectron spectroscopy, electronic structure.

Prediction of functionalized graphene as potential catalysts for overall water splitting

The two-dimensional catalyst for water splitting is a hot topic in this decade, since searching for low-cost candidates to replace rare Pt-based catalysts is the key to solving the energy issue. In this work, the electronic structure of graphene is well adjusted with hybridized halogen and oxyacid radical (SO4, SeO4, MoO4) groups, which improves its catalytic activity for water splitting. Especially, the G-SO4 shows high activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Moreover, the catalytic activity of functionalized graphene is strongly dependent on the interfacial charge transfer and pz band center of carbon active sites. Generally, our work provides a new idea for predicting and designing potential carbon-based catalysts for overall water splitting.

Spin-orbit coupling and interactions in quantum Hall states of graphene/ WSe2 heterobilayers

We use magnetotransport measurements to probe quantum Hall ground states in graphene/${\mathrm{WSe}}_{2}$ heterobilayers. Compared to pristine graphene, inter-Landau level (LL) gaps at half-filled quartets away from filling factor $\ensuremath{\nu}=0$ show significantly weaker dependence on the magnetic field $B$, while odd $\ensuremath{\nu}$ fillings show a stronger dependence. We interpret this behavior using a model incorporating Ising and Rashba spin-orbit coupling (SOC) along with Coulomb interactions within the self-consistent Hartree-Fock framework. A model fit yields Ising SOC in range $\ensuremath{\sim}1--2 \mathrm{meV}$, Rashba $\ensuremath{\sim}10 \mathrm{meV}$, and the in-plane dielectric constant $\ensuremath{\sim}12$, in agreement to previously found values. In the zeroth LL quartet, the $\ensuremath{\nu}=0$ gap as a function of magnetic field exhibits a plateau near 5 T, compared to $\ensuremath{\sim}20--25 \mathrm{T}$ for pristine graphene. This behavior is in agreement with a model in which the SOC causes a phase transition from a canted antiferromagnetic state to a ferromagnetic state to occur at a much lower field. Our studies demonstrate how the interplay of SOC and electronic interactions affect graphene's electronic structure.

Controllable Schottky barriers by ferroelectric switching in graphene/In2Te3 heterostructures

Van der Waals (vdWs) heterostructures have attracted significant research attention due to their tunable electronic structures and potential for device applications. In this study, the interfacial properties of graphene/In2Te3 heterostructures are systematically explored by employing first principles calculations. It is revealed that the electronic structures of graphene and In2Te3 monolayer are well preserved upon contact. The transition between the n-type Ohmic and p-type Schottky contacts can be realized for the graphene/In2Te3 heterostructures by switching the polarity of In2Te3. The findings reported in this study are not only vital for basic theoretical research, but also provide a feasible strategy to control the Schottky barriers for vdWs heterostructures.