Atomic Graphene Research Articles

Scalable bottom-up synthesis of Co-Ni-doped graphene.

Introducing heteroatoms into graphene is a powerful strategy to modulate its catalytic, electronic, and magnetic properties. At variance with the cases of nitrogen (N)- and boron (B)-doped graphene, a scalable method for incorporating transition metal atoms in the carbon (C) mesh is currently lacking, limiting the applicative interest of model system studies. This work presents a during-growth synthesis enabling the incorporation of cobalt (Co) alongside nickel (Ni) atoms in graphene on a Ni(111) substrate. Single atoms are covalently stabilized within graphene double vacancies, with a Co load ranging from 0.07 to 0.22% relative to C atoms, controllable by synthesis parameters. Structural characterization involves variable-temperature scanning tunneling microscopy and ab initio calculations. The Co- and Ni-codoped layer is transferred onto a transmission electron microscopy grid, confirming stability through scanning transmission electron microscopy and electron energy loss spectroscopy. This method holds promise for applications in spintronics, gas sensing, electrochemistry and catalysis, and potential extension to graphene incorporation of similar metals.

Rare-earth metal neodymium anchored into graphene as a promising CO2 reduction electrocatalyst by regulating the coordination environment

Carbon-based single atom catalysts (SACs) are promising electrocatalysts in the field of carbon dioxide reduction reactions (CO2RR) due to their high efficiency and environmental friendliness, in which the coordination environment is the key factor determining their intrinsic catalytic activity. Furthermore, rare-earth-based SACs have shown great potential on CO2RR in recent years. Meanwhile, various studies have focused on combining metals with N-doped graphene, which together form potential Mx-Ny-C active sites. This work systematically investigates the impact of varying N/C coordination numbers on Nd atoms in graphene (Nd-NxC6-x, x = 0–5) on the CO2RR reaction mechanism and catalytic performance through density functional theory methods. Detailed Gibbs free energy calculation results indicate that most catalysts undergo a two-electron reduction pathway. For Nd-N3C3, Nd-N3C3–1, Nd-N3C3–2, Nd-N4C2, Nd-N4C2–1, Nd-N4C2–2, and Nd-N5C, HCOOH is the main product, with low UL values of -0.18, -0.17, -0.03, -0.10, -0.11, -0.09, and -0.10 V, respectively. In summary, our research results not only indicate that N atoms with different coordination numbers can improve the product selectivity and catalytic activity of catalysts, but also may provide valuable theoretical insights for studying the application of rare-earth-based SACs.

Achieving High Substitutional Incorporation in Mn-Doped Graphene.

Despite its broad potential applications, substitution of carbon by transition metal atoms in graphene has so far been explored only to a limited extent. We report the realization of substitutional Mn doping of graphene to a record high atomic concentration of 0.5%, which was achieved using ultralow-energy ion implantation. By correlating the experimental data with the results of ab initio Born-Oppenheimer molecular dynamics calculations, we infer that direct substitution is the dominant mechanism of impurity incorporation. Thermal annealing in ultrahigh vacuum provides efficient removal of surface contaminants and additional implantation-induced disorder, resulting in Mn-doped graphene that, aside from the substitutional Mn impurities, is essentially as clean and defect-free as the as-grown layer. We further show that the Dirac character of graphene is preserved upon substitutional Mn doping, even in this high concentration regime, making this system ideal for studying the interaction between Dirac conduction electrons and localized magnetic moments. More generally, these results show that ultralow energy ion implantation can be used for controlled functionalization of graphene with substitutional transition-metal atoms, of relevance for a wide range of applications, from magnetism and spintronics to single-atom catalysis.

Reinforcing cementitious composites with nitrogen-doped graphene: insights into molecular interactions and interfacial behavior

Graphene materials are useful functional fillers in cementitious materials. They are essential in creating intelligent cementitious materials, enabling the construction of internal conductive networks and the addition of thermoelectric properties. The success of this process hinges on the functional filler's compatibility with the cement matrix, which must be excellent to ensure optimal results. In this study, we delved into the interactions between nitrogen-doped graphene and calcium silicate hydrate (C-S-H), which is the principal binding phase in cementitious materials, employing Density Functional Theory (DFT) calculations. In this study, we explored diverse nitrogen doping configurations and levels, while concurrently analyzing the influence of calcium cations at the C-S-H matrix interface. The findings indicate that nitrogen atoms in doped graphene interact considerably with nucleophilic oxygen atoms on silicate chains, forming covalent N-O bonds with high bonding energies (length 1.4 A, −107.81 kcal/mol) during graphitic nitrogen doping. The presence of calcium ions further strengthens the interaction energy between C-S-H and graphene, suggesting a synergistic effect between different types of nitrogen doping. The binding and total bond energies under the synergistic effect were −479.52 kcal/mol and −183.23 kcal/mol, respectively. These values were 1.5 times higher than those of single nitrogen doping. These findings offer significant insights into the molecular-level interaction between CSH and carbon nanoparticles.

Effect of interlayer bonded bilayer graphene on friction

Abstract We study the friction properties of interlayer bonded bilayer graphene by simulating the movement of slider on the surface of bilayer graphene using molecular dynamics. The results show that the presence of the interlayer covalent bonds due to the local sp3 hybridization of carbon atoms in the bilayer graphene seriously reduced the frictional coefficient of the bilayer graphene surface to 30%, depending on the coverage of interlayer sp3 bonds and normal loads. For a certain coverage of interlayer sp3 bonds, when the normal load of the slider reaches a certain value, the surface of this interlayer bonded bilayer graphene will lose the friction reduction effect on the slider. Our findings provide guidance for the regulation and manipulation of the frictional properties of bilayer graphene surfaces through interlayer covalent bonds, which may be useful for the application of friction related graphene based nanodevices.

Oxygen reduction reaction activity of Co3O4-modified Pt-graphene electrocatalysts

Abstract In this paper, the ORR activity of Pt/Co3O4-modified N-doped graphene electrocatalysts was investigated based on DFT theory. The Dmol3 module in Materials Studio software was used to optimize the construction of NG models under different doping conditions and to calculate the adsorption and desorption properties of oxygen molecules by optimizing the conformation of Pt13 clusters and substrates. A comparative analysis revealed that the Co3O4 modification reduced the oxygen adsorption energy of the Pt/G catalyst. However, after N doping, the oxygen adsorption energy of Pt/Co3O4/NG was superior to that of Pt/NG. After theoretical analysis, it was determined that it was because the doping of N atoms in graphene could promote the release of oxygen vacancies in Co3O4 and strengthen the adsorption interaction between Pt and graphene carriers to improve the oxygen adsorption energy.

First-principles study of Ni clusters growth on graphene with a vacancy

In this work, we performed first-principles calculations to study the interactions of Nin clusters (n = 2-5) with graphene. Nin clusters were adsorbed on pristine graphene and graphene with a vacancy. We observed that the adsorption energy is significantly lowered when graphene has a single vacancy. The single vacancy gives place to a charge redistribution that favors chemisorption. The dangling bonds of the carbon atoms produced by the vacancy increase the magnetic moment of the C atoms around the hole induced by the Ni clusters. We found that the situation is very different when the Ni cluster is adsorbed with atoms on both sides of the vacancy. We consider the cases in which one of the Ni atoms is located on one side of the graphene sheet and the rest on the other, Nin,1. The adsorption energy for Ni3,1 and Ni4,1 clusters are slightly larger than when the whole cluster is chemisorbed on one side only, but the charge redistribution, occurring on both sides of the graphene, increases the possibility to adsorb other molecules. For all cases, we analyzed the electronic density of states to account for the electronic changes on the graphene sheet as a function of the size of the Nin cluster. Finally, the Bader effective charges were computed to follow the charge transfer between Nin clusters and the graphene atoms.

Micromechanical effects of substrate hardness on graphene nano-cutting quality

In the nanocutting process of graphene, the mechanism on the atomic scale of the effect of substrate hardness on the edge quality of graphene nanoribbons has not been thoroughly investigated. In this paper, we construct a diamond probe (radius of 5 Å) to cut graphene (190 Å × 113 Å) models with different substrates (silicon/sapphire/silicon carbide), molecular dynamics simulations were used to study the effect of substrate hardness and cutting depth on the atomic structure of graphene nanoribbon edges by observing the atomic morphology, the number of C-C bonds, the radial distribution function and friction. The results show that during the nanoindentation stage, when graphene is in the critical damage state, the substrate with high hardness has high deformation strength, the substrate deformation is small, and graphene undergoes deformation is also small, but the force Fz on the probe, the critical graphene stress σz and the equivalent stress are all large, and the number of C atoms with CN=3 is large; At the nano-cutting stage, the breakage of C-C bonds and the generation of edge defects are caused by local stress concentration, and the graphene stress σxx along the x-direction has a clear directionality. With the same cutting depth (critical damage depth of silicon substrate), silicon substrate hardness is small, the graphene nanoribbon obtained has less number of circular defects generated at the edge, less total number of defective atoms, more number of CN=3 atoms, large peak of radial distribution function of graphene after cutting, and graphene integrity is better. The hardness of the silicon carbide substrate is large, and the number of C-C bond breaks in graphene during the cutting process is high. Each substrate is cut at its own critical damage depth, and the substrate hardness has no significant effect on the number of C-C bond breaks, the number of circular defects, the total number of defective atoms, and the number of CN=3 atoms in graphene. When the silicon substrate was used, the stable force of the probe is small, the edge structure of graphene has little impact, the edge effect obtained is good. The cutting depth and substrate hardness have little effect on the graphene temperature during the nano-cutting process. In this paper, the atomic structure of graphene edges after cutting is analysed and investigated on the atomic scale, and the above results are combined to provide theoretical guidance for obtaining efficient and high-quality and low-damage graphene nanoribbon cutting.

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.

DFT assessments of optical and thermoelectric characteristics of (III/V)-doped elements into graphene sheets

First-principles simulations are conducted to explore the structural stability, electronic properties, and optical responses of pristine and boron- or nitrogen-doped monolayers graphene. The computed electronic density of states revealed that the substitutional doping of boron impurity atoms on monolayer graphene (MLG) shifts the Dirac point upward, although the substitution of nitrogen impurity atoms in graphene pushes the Dirac point downward the Fermi level. This could exhibit that upon the doping of MLG with boron or nitrogen, respectively, p-type or n-type semimetal is acquired. The overall optical spectral properties of the substituted graphene with boron or nitrogen atoms are simulated and compared with the optical spectra results of pure graphene. The optical features of pristine and doped MLG are determined by taking the interband and intra-band transitions into account ranging from the far-infrared to the ultraviolet regime of the electromagnetic radiation. A remarkable red shift in the optical spectra of the doped MLG towards the visible regime of radiation is established. An enhanced reflectivity illustrated that concentration-dependent optical properties of boron and nitrogen-doped MLG happen at lower electromagnetic radiation regimes. In addition, we explored the thermoelectric behaviors of the pristine/doped graphene monolayers with [Formula: see text] supercells. We found a significant improvement in the electrical conductivity of graphene when doped with boron or nitrogen impurities. However, an increase in the electrical conductivity has textured a decrease in the Seebeck coefficients. Improvement in the electrical conductivity is attributed to an interesting effect on the graphene monolayers’ power factor (PF). These findings indicate a positive impact of the dopants on the thermoelectric properties of graphene monolayers and reveal that they are potential materials for thermoelectric applications and nanodevices.

Cooperation of Mn and P atoms in graphene for efficiently catalyzing CO oxidation at low temperature

Computational studies show that the co-doping of Mn and P atoms in graphene (MnP4-Gr) can provide high CO oxidation performance at room temperature. Results suggest that the configuration (CO+O2)* can be easily obtained due to the better adsorption of CO on the Mn atom and O2 on the P atom. (CO+O2)* is further found to undergo a fast Langmuir−Hinshelwood (LH) process and then left one atomic oxygen on P which is difficult to remove. Therefore, due to the more favorable adsorption of CO on the Mn atom and O2 on other P atoms, coupled with the speed of the LH process, the configuration (CO+4O)* of one atomic oxygen adsorbed on each P atom and CO on Mn atom is finally obtained. The whole reaction is determined by the atomic oxygen removal in (CO+4O)* with an energy barrier of 0.80 eV. It is predicted that the cooperation of Mn and P atoms is conducive to the fast LH process of MnP4-Gr and also the weaker binding between P and O in MnP4-Gr. Moreover, thermodynamic and kinetics analysis indicates that increasing temperature can accelerate CO oxidation and facilitate reaction in thermodynamics. Therefore, the insights reported in this paper will provide a new catalyst for low-temperature CO oxidation.

Tunable hybridized plasmons-phonons in a graphene/mica-nanofilm heterostructure.

Graphene plasmons exhibit significant potential across diverse fields, including optoelectronics, metamaterials, and biosensing. However, the exposure of all surface atoms in graphene makes it susceptible to surrounding interference, including losses stemming from charged impurity scattering, the dielectric environment, and the substrate roughness. Thus, designing a dielectric environment with a long lifetime and tunability is essential. In this study, we created a van der Waals (vdW) heterostructure with graphene nanoribbons and mica nano-films. Through Fourier-transform infrared spectroscopy, we identified hybrid modes resulting from the interaction between graphene plasmons and mica phonons. By doping and manipulating the structure of graphene, we achieved control over the phonon-plasmon ratio, thereby influencing the characteristics of these modes. Phonon-dominated modes exhibited stable resonant frequencies, whereas plasmon-dominated modes demonstrated continuous tuning from 1140 to 1360 cm-1 in resonance frequency, accompanied by an increase in extinction intensity from 0.1% to 1.2%. Multiple phonon couplings limited frequency modulation, yielding stable resonances unaffected by the gate voltage. Mica substrates offer atomic level flatness, long phonon lifetimes, and dielectric functionality, enabling hybrid modes with high confinement, extended lifetimes (up to 1.9 picoseconds), and a broad frequency range (from 750 cm-1 to 1450 cm-1). These properties make our graphene and mica heterostructure promising for applications in chemical sensing and integrated photonic devices.

Correction: Deciphering the chemical bonding of the trivalent oxygen atom in oxygen doped graphene

Correction for ‘Deciphering the chemical bonding of the trivalent oxygen atom in oxygen doped graphene’ by Andoni Ugartemendia et al., Chem. Sci., 2024, 15, 6151–6159, https://doi.org/10.1039/D4SC00142G.

Simulation of Interaction Processes of C20 Fullerene with Graphene

Graphene, a carbon sheet one atom thick, with carbon atoms arranged in a two-dimensional honeycomb configuration, has a number of intriguing properties. Fullerenes are a promising material for creating electro-active elements in solar cells and active layers in thin-film organic transistors. A computer model of the C20 fullerene molecule was constructed using the energy minimization method with the second-generation Brenner potential (REBO). A computer model of "infinite" defect-free graphene was built, designed to consider the process of adsorption of a C20 fullerene molecule on its surface. To study adsorption process computer models of fullerene and "infinite" graphene were approached to the required distance with a different set of geometric arrangement of fullerene with respect to the graphene surface. It has been established that the adsorption of fullerene C20 on the surface of graphene can be carried out in three different ways, differing in the number of interacting fullerene and graphene atoms. The binding energies and adsorption lengths for C20 fullerene molecules adsorbed on the graphene surface in different ways are calculated. The way of adsorption corresponding to the highest binding energy and the shortest adsorption length was revealed.

Electronic and vibrational spectra of substitutional pair-defects of Boron and Nitrogen atoms in graphene and graphane

The substitutional doping of graphene and related materials with Boron (B) and/or Nitrogen (N) atoms can be effectively used to tailor their properties for specific applications. In this paper, we investigate the energetics, electronic structures and vibrational spectra of substitutional pair-defects in graphene and graphane. The structural distortions in the vicinity of BN pair-defects are small as compared to the distortions in the vicinity of BB or NN pair-defects. The BB (NN) pair-defects in graphene and graphane transform them into p (n) semiconductors. However, the BN pair-defects doped graphene and graphane are intrinsic semiconductors. The Raman and IR spectra of above mentioned pair-defects have unique features, which can be used to identify these defects in graphene as well as graphane structures.

A novel computational multi-scale modeling of randomly-distributed-graphene/epoxy nanocomposites with interfacial interactions

This article introduces a novel multi-scale modeling methodology that incorporates randomly distributed graphene/epoxy nanocomposites, accounting for interfacial interactions. The objective is to forecast the elastic characteristics and mechanical performance of nanocomposites reinforced with single-layer graphene. We developed a new multi-scale scheme by randomly distributing the triple-structure formed by graphene, interface material and their Van Der Walls interactions in epoxy with an improved algorithm by using the finite element method. The number of single-layer graphene affecting the random distribution method and the volume fraction of nanocomposites were investigated. A developed methodology was proposed for producing graphene-VanDerWalls interactions-interface structures, avoiding overlapping desired geometric dimensions and numbers in a representative control volume and determining random positions. In order to model Van Der Walls interactions between graphene and interface material atoms by finite element method and to apply them to randomly distributed graphene, a different algorithm was required. Performing numerical analyzes of graphene nanoparticles by embedding them into epoxy with the real dimensions is not an appropriate task today. In particular, it is impossible to analyze these real graphene nanoparticles as multiple by randomly distributing them in epoxy. Therefore, in this research, a new approach has been developed to overcome this problem. The result of the model is in acceptable agreement with the result of conducted experimental results from the literature.

Effect of nitrogen and transition-metal co-doping on quantum capacitance enhancement of graphene as supercapacitor electrodes: A density functional theory study

Quantum capacitance plays a crucial role in determining the energy density of graphene-based supercapacitors. In this study, density functional theory calculations were conducted to investigate the enhancement of quantum capacitance and surface storage charge density through the co-doping of transition metals (TM) such as Mn, Fe, Co, Ni, Cu, Zn, Cr, V, Ti, and N atoms in graphene (TMN3-G). At a doping concentration of 2 at% (atomic percentage), all types of metal doped graphene exhibited an increase in quantum capacitance compared to pristine graphene. The highest increase was observed in CrN3-G, where the maximum quantum capacitance increased from 19.2 μF/cm2 for pristine graphene to 122.8 μF/cm2, due to the increased density of states around the Fermi level. The maximum surface storage charge density also increased from 6.4 μC/cm2 for pristine graphene to 44.4 μC/cm2 for CrN3-G. Moreover, the study investigated the effect of different doping concentrations on quantum capacitance performance by changing the doping concentration for VN3-G. It was found that the quantum capacitance for VN3-G increased with higher doping concentration, reaching up to 282 μF/cm2 when the doping concentration was increased to 12.5 at% from 2 at%. Based on our calculation results, ZnN3-G, NiN3-G, CrN3-G, and TiN3-G are suitable for the anode of supercapacitors, while CuN3-G and CoN3-G are suitable for the cathode. FeN3-G, MnN3-G, and VN3-G are suitable for both electrodes. These findings provide valuable insights for designing high-capacity graphene-based supercapacitors.

Edge-pinning effect of graphene nanoflakes sliding atop graphene

Edge effect is one of the detrimental factors preventing superlubricity in laminar solid lubricants. Separating the friction contribution from the edge atom and inner atom is of paramount importance for rational design of ultralow friction across scales in van der Waals heterostructures. To decouple these contributions and provide the underlying microscopic origin at the atomistic level, we considered two contrast models, namely, graphene nanoflakes with dimerized and pristine edges sliding on graphene monolayer based on extensive ab initio calculations. On one hand, we found edge pinning effect by dimerization is obvious for misaligned contact. This case providing local commensuration along edges is reminiscent of Aubry’s pinned phase. The contribution of per dimerized edge carbon atom to the sliding potential energy corrugation is even 1.5 times more than that of an atom in bilayer graphene under commensurate contact. Thus, the dynamic and random edge dimerization during sliding will impact the tribological properties significantly and may be the important sources to account for the marked discrepancies in measured friction parameters [Qu et al., Phys. Rev. Lett. 2020, 125, 126102]. On the other hand, we found the edge contribution to friction is lattice orientation dependent and suppressed in aligned contact. This rationalizes the experimental finding where friction is dominated by interior atoms rather than edge pinning [Liao et al., Nat. Mater. 2022, 21, 47–53]. To eliminate the undesirable edge effects, we adopt strain engineering and edge fluorination to the tribological system constructed here. However, dimerized edges as high frictional pinning sites are robust to both approaches. We hope the detailed atomic information identified here would help for improving superlubric systems.

Graphene coating reduces the heat transfer performance of water vapor condensation on copper surfaces: A molecular simulation study

Water vapor condensation is a key process for many applications. Existing studies in water vapor heterogeneous condensation have used different methods to modify the hydrophobicity of the water vapor condensation surface. One way to modify the surface properties is to use graphene coating. On a macroscale, surface modification using graphene coating can improve water vapor condensation heat transfer of various substrates. However, on the molecular scale, its effect is poorly understood. This work investigated the effect of graphene coating on water vapor condensation using molecular dynamics simulations (MDS). We examined the water vapor condensation on bare copper surfaces considering the effects of initial temperature difference, water model, and surface size, parameters that have not been investigated by previous studies employing MDS for the same water-surface configuration. One water model was then used to simulate the condensation on a copper surface with and without graphene coating. We then investigated the effect of graphene defect, the energy and vibration of graphene atoms, and the interaction between graphene and copper layers. The surface size notably influenced the condensation rate and heat transfer performance. The condensation rate and heat transfer performance were significantly reduced when the copper surface was coated by graphene. The number of water molecules condensed was 1253 molecules/ns on the bare copper surface, compared to 587 molecules/ns on the graphene-coated copper surface. Moreover, the water molecules condensed on the graphene-coated copper surface tended to return to the bulk vapor phase. Other important results are also provided. This study gives an insight into the water vapor condensation on graphene-coated copper surfaces, useful to pursuit the design and optimization of graphene-coated copper surfaces for applications that need efficient water vapor condensation, such as for industrial applications, like thermal, chemical, and nuclear.

Exploring spin states by hybrid functional methods to define correct trends in electrocatalytic activity of SACs embedded in N-doped graphene

Embedding transition metal atoms in graphene is a promising strategy to make it an effective electrocatalyst for the oxygen evolution (OER) and reduction (ORR) reactions, which are crucial processes for the success of the energy transition. In this regard, theoretical investigations can be valuable complementary tools to experimental work, but only if they are able to adequately describe the system under examination. Transition metal atoms trapped in graphene pose serious challenges due to the presence of strongly correlated d-electrons, which can lead to complex spin configurations to be explored. In this work, by hybrid density functional theory (DFT) calculations we investigated several first-row transition metal atoms embedded in N-doped graphene as potential electrocatalysts for the OER and ORR. A detailed spin-dependent search has been performed to define the lowest energy spin state configurations for all intermediates along the associative path, finding Ni and Fe as the most promising systems for the OER and ORR, based on overpotential values (η) of 0.58 V and 0.52 V, respectively. Interestingly, if the study is limited to low-spin configurations, the resulting overpotential values differ within the range of 0.5 V, changing the order of activity between the various electrocatalytic systems. The most striking case is that of Fe, where according to the low-spin state reaction path it would suggest this to be the worst electrocatalyst for ORR. Thus, this work conveys a very important message and general warning to the community on trusting only data that come from a full spin optimization.