Vacancy Graphene Research Articles

Topologically Frustrated Graphene Antidot Lattice Semiconductors with Room-Temperature Magnetism

Realizing graphene spintronics is intriguing due to the weak spin–orbital coupling; however, developing intrinsic room-temperature magnetic semiconductors in graphene is still a great challenge. Graphene antidot lattices (GALs), as a type of regular vacancy graphene, exhibit topology-dependent magnetism and offer an ideal platform to achieve room-temperature magnetic semiconductors. Recently, on-surface-synthesized open-shell [n]triangulene polymers as topologically frustrated graphene nanoflakes (GNFs) are the new building blocks to construct topologically frustrated GALs with robust magnetism. Herein, on the basis of the density functional theory calculations, we report seven magnetic GAL semiconductors by assembling two types of open-shell GNFs with topological frustration, that is, isomeric π-extended heptauthrene (cis triangulene dimer) and heptazethrene (trans triangulene dimer). Our results demonstrate that topologically frustrated GALs are semiconductors with either bipolar ferromagnetism or antiferromagnetism, inheriting the topologically frustrated magnetism from their building blocks. In particular, three ferromagnetic and two antiferromagnetic GALs exhibit above room-temperature magnetic order with their Curie or Néel temperatures varying from 507 to 527 or 366 to 391 K, respectively. This study provides a feasible route to obtain topologically frustrated GAL semiconductors with the above room-temperature magnetism from open-shell GNFs for graphene spintronics applications.

Ab Initio Investigation of the Adsorption of CO2 Molecules on Defect Sites of Graphene Surfaces: Role of Local Vacancy Structures.

An in-depth investigation into the adsorption of CO2 on graphene vacancies is essential for the understanding of their applications in various industries. Herein, we report an investigation of the effects of vacancy defects on CO2 gas adsorption behavior on graphene surfaces using the density functional theory. The results show that the formation of vacancies leads to various deformations of local carbon structures, resulting in different adsorption capabilities. Even though most carbon atoms studied can only trigger physisorption, there are also carbon sites that are energetically favored for chemisorption. The general order of the adsorption capabilities of the local carbon atoms is as follows: carbon atoms with dangling bonds > carbon atoms shared by five- and six-membered rings and a vacancy > carbon atoms shared by two six-membered rings and a vacancy. A stronger interaction in the adsorption process generally corresponds to more obvious changes in the partial density of states and a larger amount of transferred charge.

An Atomistic View of Platinum Cluster Growth on Pristine and Defective Graphene Supports.

Density functional theory (DFT) is used to systematically investigate the electronic structure of platinum clusters grown on different graphene substrates. Platinum clusters with 1 to 10 atoms and graphene vacancy defect supports with 0 to 5 missing C atoms are investigated. Calculations show that Pt clusters bind more strongly as the vacancy size increases. For a given defect size, increasing the cluster size leads to more endothermic energy of formation, suggesting a templating effect that limits cluster growth. The opposite trend is observed for defect-free graphene where the formation energy becomes more exothermic with increasing cluster size. Calculations show that oxidation of the defect weakens binding of the Pt cluster, hence it is suggested that oxygen-free graphene supports are critical for successful attachment of Pt to carbon-based substrates. However, once the combined material is formed, oxygen adsorption is more favorable on the cluster than on the support, indicating resistance to oxidative support degradation. Finally, while highly-symmetric defects are found to encourage formation of symmetric Pt clusters, calculations also reveal that cluster stability in this size range mostly depends on the number of and ratio between PtC, PtPt, and PtO bonds; the actual cluster geometry seemssecondary.

Thermal Conductance of Copper-Graphene Interface: A Molecular Simulation.

Copper is often used as a heat-dissipating material due to its high thermal conductivity. In order to improve its heat dissipation performance, one of the feasible methods is to compound copper with appropriate reinforcing phases. With excellent thermal properties, graphene has become an ideal reinforcing phase and displays great application prospects in metal matrix composites. However, systematic theoretical research is lacking on the thermal conductivity of the copper-graphene interface and associated affecting factors. Molecular dynamics simulation was used to simulate the interfacial thermal conductivity of copper/graphene composites, and the effects of graphene layer number, atomic structure, matrix length, and graphene vacancy rate on thermal boundary conductance (TBC) were investigated. The results show that TBC decreases with an increase in graphene layers and converges when the number of graphene layers is above five. The atomic structure of the copper matrix affects the TBC, which achieves the highest value with the (011) plane at the interface. The length of the copper matrix has little effect on the TBC. As the vacancy rate is between 0 and 4%, TBC increases with the vacancy rate. Our results present insights for future thermal management optimization based on copper matrix composites.

Long-Term Evolution of Vacancies in Large-Area Graphene.

Devices based on two-dimensional (2D) materials such as graphene and molybdenum disulfide have shown extraordinary potential in physics, nanotechnology, and electronics. The performances of these applications are heavily affected by defects in utilized materials. Although great efforts have been spent in studying the formation and property of various defects in 2D materials, the long-term evolution of vacancies is still unclear. Here, using a designed program based on the kinetic Monte Carlo method, we systematically investigate the vacancy evolution in monolayer graphene on a long-time and large spatial scale, focusing on the variation of the distribution of different vacancy types. In most cases, the vacancy distribution remains nearly unchanged during the whole evolution, and most of the evolution events are vacancy migrations with a few being coalescences, while it is extremely difficult for multiple vacancies to dissolve. The probabilities of different categories of vacancy evolutions are determined by their reaction rates, which, in turn, depend on corresponding energy barriers. We further study the influences of different factors such as the energy barrier for vacancy migration, coalescence, and dissociation on the evolution, and the coalescence energy barrier is found to be dominant. These findings indicate that vacancies (also subnanopores) in graphene are thermodynamically stable for a long period of time, conducive to subsequent characterizations or applications. Besides, this work provides hints to tune the ultimate vacancy distribution by changing related factors and suggests ways to study the evolution of other defects in various 2D materials.

Pd12Ag1 nanoalloy on dendritic CNFs catalyst for boosting formic acid oxidation

In this study, dendritic carbon nanofibers (CNFs) loaded with Pd12Ag1 nanoalloy were prepared as a catalyst using a one-pot method. The average particle size of the Pd12Ag1 nanoalloy is 4.2 nm, which is uniformly dispersed over the surface of the CNFs. The Pd12Ag1/CNFs catalyst has the best performance for formic acid oxidation (FAO), with catalytic activity (2825.0 mA mgPd−1) and durability after 7200 s (70.6 mA mgPd−1) up to 5.25 and 10.87 times, respectively, compared with commercial Pd/C. The improved electrocatalytic performance of the Pd12Ag1/CNFs catalyst is attributed to the appropriate Ag doping, which changes the electronic state around the Pd atom, forming two new active sites (Pd-Pd and Pd-Ag); on the other hand, the special structure and rough surface of CNFs expose more active sites to enhance the dehydrogenation process of FAO. Furthermore, Pd13, Ag13, and Pd12Ag1 clusters loaded on the surface of defect-free graphene (DG) and single vacancy defective graphene (SVG) were simulated using density functional theory (DFT) to confirm the effect of different carbon materials and doped metals on the overall catalyst at the atomic level.

Novel nanostructures suspended in graphene vacancies, edges and holes

Under electron beam irradiation, graphene is inclined to form defects (such as vacancies and holes) that can trap foreign atoms to form new structures. The interactions between these structures and graphene have garnered considerable research interest as they can yield exciting properties. This review focuses on the fabrication and characterization of free-standing nanostructures suspended in graphene using transmission electron microscopy, which enables the observations with atomic resolution and investigations of the dynamic behavior of atoms/structures in such materials. Additionally, the review discusses the influence of novel metal/nonmetal dopants in graphene vacancies with varying bond configurations and the catalytic activities of single atoms/clusters located at the graphene edges. Moreover, the dynamic forming process of freestanding single-atom-thick two-dimensional (2D) clusters/metal/metallenes and 2D clusters/metal/metallenes oxides is discussed. Understanding the behavior, stabilities, and macroscopic effects of these nanostructures is vital for the practical deployment of novel atom/molecule scale nanotechnology. Overall, accumulative evidence confirms the growing number of these novel nanostructures, implying a bright future for further exciting discoveries.

A DFT study of H2S, H2O, SO2 and CH4 Adsorption Behavior on Graphene Surface Decorated with Alkaline Earth Metals

The adsorption of CH4, H2S, SO2 and H2O by alkaline earth metal (AEM) decorated double vacancy graphene (DVG) was investigated with the first principles method. The most stable adsorption configurations, adsorption energy, density of states, and charge density distributions of CH4, H2S, SO2 and H2O on AEM_DVG have been discussed. The calculated ΔEads values of all gas molecules on AEM_DVG show that there was a weak interaction between CH4 and AEM_DVG, while the other molecules present favorable interaction with AEM_DVG. Notably, the Ba_DVG is demonstrated to have a more practical CH4 desorption temperature, as well as a broader window for the selective adsorption of CH4 over H2S, SO2 and H2O. This considerable promotion is ascribed to charge transfer and weak covalent interaction between Ba and gas molecules.

Theoretical investigation of chemical reaction kinetics of CO catalytic combustion over NiNx-Gr

Carbon monoxide (CO) produced by automobile exhaust emissions and fuel combustion is one of the domain pollutants severely causing experimental and human health problems, which catalytic combustion at low temperature is an effective way to remove. Here, a Ni and N co-doped double vacancy graphene catalyst (NiNx-Gr) was established for the combustion of CO at low temperature by regulating the number of N atoms. Based on the density function theory (DFT) method, the reaction mechanisms of CO catalytic combustion on different surface were developed, and the rate constants were also calculated by the means of transition state theory (TST). All NiNx-Gr catalysts exhibit excellent stability and catalytic performance, among which NiN3-Gr is the most outstanding. Molecular dynamics (MD) simulation results show that NiN3-Gr can still exist stably at the temperature of 1000 K. The calculation results of the potential energy surface show that CO is most advantageously oxidized by the TER mechanism on NiN3-Gr with a reaction energy barrier of only 1.71 eV. Furthermore, the results of the rate constants also indicate that NiN3-Gr is the most effective catalyst for CO combustion. All these results demonstrate that NiN3-Gr may serve as a high-performance material for catalytic CO combustion, which may offer some design insights for the experiments. This work not only proposes a catalyst for CO combustion, but also depicts a reasonable reaction mechanism in detail, which serves as a solid starting point for the further understanding of catalytic combustion kinetics of CO under low temperature.

Local multiplet formation around a single vacancy in graphene: An effective Anderson model analysis based on the block-Lanczos DMRG method

To better understand the electronic structure of a single vacancy in graphene, we study the ground state property of an effective Anderson model, consisting of three dangling $sp^2$ orbitals of the surrounding carbon atoms around the vacancy and the $\pi$ orbitals of carbon atoms that form the honeycomb lattice with a single vacancy. Employing the block-Lanczos density-matrix renormalization group method, we show that there are two phases in the relevant parameter space, i.e., a nonmagnetic phase in the weak coupling region and a free magnetic moment phase in the realistic parameter region. The systematic analysis finds that, in the free magnetic moment phase, local multiplets of the doubly degenerate irreducible representation of the $\mathcal{C}_3$ point group with spin 1 become dominant in the ground state, and approximately half of this local spin 1 is screened by electrons in the surrounding $\pi$ orbitals, indicating the emergence of the residual spin-1/2 free magnetic moment. Furthermore, we find that the emergence of the free magnetic moment is robust against carrier doping, which is in sharp contrast to the case of graphene with an adatom, thus explaining the qualitative difference observed experimentally in these two classes of systems. We also find the enhancement of the spin correlation function between $\pi$ electrons around the vacancy and those in the conduction band away from the vacancy in the undoped case, as compared to that in the doped case, while the spin correlation function between the $\sigma$ electrons in the $sp^2$ dangling orbitals around the vacancy and the $\pi$ electrons in the conduction band remains large in both undoped and doped cases. Our calculations thus support qualitatively the previous experiment that suggests the emergence of free magnetic moment with two distinct origins.

Ab initio investigations of Fe(110)/graphene interfaces

Interfacial bonding of three different semi-coherent bcc-Fe(110)/graphene interfaces is investigated using the plane-wave pseudopotential method within density functional theory. The analysis of bond lengths, charge densities, charge transfer, magnetic moments and densities of states shows that interfacial adhesion can be understood from the electronic structure. Graphene is considered on Fe(110) surfaces as well as embedded in (110) planes of bulk Fe. Moreover, the influence of single vacancies in graphene is studied in case of graphene on the Fe(110) surfaces. It is found that a single vacancy in graphene leads to a strong increase of interfacial adhesion. The most important contribution to the adhesion is covalent bonding with hybridization of Fe states and C states which is most pronounced for neighboring atoms of the vacancy.

Molecular Dynamics Analysis of Graphene Nanoelectromechanical Resonators Based on Vacancy Defects.

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

Frontispiece: Demonstrating and Unraveling a Controlled Nanometer‐Scale Expansion of the Vacancy Defects in Graphene by CO2

Graphene In their Research Article (e202200321), Kumar Varoon Agrawal et al. demonstrate CO2 as a promising etchant for the controlled manipulation of graphene edges and vacancy defects.

Effect of Ni Doping and Vacancy Defects on the Sensing Characteristics of Graphene for NO<sub>2</sub> and CO Detection: A DFT Study

The sensing characteristics of pristine, Ni-doped, and C-vacancy graphene towards CO and NO2 gas molecules were studied using density functional theory (DFT). The adsorption energies, electronic properties, charge transfer, and stable geometries were calculated to evaluate the gas-surface interaction mechanisms. Both pristine and vacancy graphene have smaller CO and NO2 adsorption energies and charge transfer than the Ni-doped graphene, whereas the adsorption energy on Ni-doped vacancy graphene is higher than that of Ni-doped graphene. The results indicate that both CO and NO2 gas molecules only attach to pristine graphene through weak physical adsorption. Stronger chemisorption occurs when the gas molecules adsorb on the surface of vacancy, Ni-doped, and Ni-doped vacancy graphene. Additionally, the results demonstrated that Ni-doped vacancy graphene has higher sensitivity and selectivity towards the NO2.

The dehydrogenation of butane on metal-free graphene

The dehydrogenation of alkane feedstock to produce alkenes is a significant and energy intensive industrial process, generally occurring on metals and metal oxides. Here, we investigate a catalytic mechanism for the dehydrogenation of butane on single-layer, metal-free graphene using a combination of ab initio quantum chemical calculations and adsorption microcalorimetry. Dispersion-corrected Density Functional Theory (DFT) is employed to calculate transition states and energy minima that describe the reaction pathways connecting butane to the two possible products, but-1-ene and but-2-ene. The deprotonations occur with moderate energy barriers in the 0.54 eV-0.69 eV range. A strong agreement is observed between the results of the adsorption energies calculated by DFT (0.40 eV) and the measured differential heat of adsorption of n-butane on a graphitic overlayer. We conclude that the active-site for this catalytic reaction is a metal-free graphene vacancy, created by removing a carbon atom from a single-layer graphene sheet.

Hydrogen storage by spillover on Ni4 cluster embedded in three vacancy graphene. A DFT and dynamics study

An ab initio dynamics study was performed with the aim of understanding better the hydrogen adsorption mechanism, stability and capacity with the temperature on Ni4 cluster embedded in a three vacancy graphene substrate. Additionally, the hydrogen migration to the substrate was analyzed observing an outstanding improvement for the activation energy compared with the systems without vacancies. Moreover, a suitable reduction in the hydrogen migration energy barriers was achieved with the subsequent hydrogen adsorption of more molecules on the cluster. Consequently, in the light of these new insights hydrogen spillover on Ni4 can be highly influenced by both, vacancy defects and the saturation with hydrogen molecules.

Demonstrating and Unraveling a Controlled Nanometer‐Scale Expansion of the Vacancy Defects in Graphene by CO2

AbstractA controlled manipulation of graphene edges and vacancies is desired for molecular separation, sensing and electronics applications. Unfortunately, available etching methods always lead to vacancy nucleation making it challenging to control etching. Herein, we report CO2‐led controlled etching down to 2–3 Å per minute while completely avoiding vacancy nucleation. This makes CO2 a unique etchant for decoupling pore nucleation and expansion. We show that CO2 expands the steric‐hindrance‐free edges with an activation energy of 2.71 eV, corresponding to the energy barrier for the dissociative chemisorption of CO2. We demonstrate the presence of an additional configurational energy barrier for nanometer‐sized vacancies resulting in a significantly slower rate of expansion. Finally, CO2 etching is applied to map the location of the intrinsic vacancies in the polycrystalline graphene film where we show that the intrinsic vacancy defects manifest mainly as grain boundary defects where intragrain defects from oxidative etching constitute a minor population.

Demonstrating and Unraveling a Controlled Nanometer-Scale Expansion of the Vacancy Defects in Graphene by CO2.

A controlled manipulation of graphene edges and vacancies is desired for molecular separation, sensing and electronics applications. Unfortunately, available etching methods always lead to vacancy nucleation making it challenging to control etching. Herein, we report CO2‐led controlled etching down to 2–3 Å per minute while completely avoiding vacancy nucleation. This makes CO2 a unique etchant for decoupling pore nucleation and expansion. We show that CO2 expands the steric‐hindrance‐free edges with an activation energy of 2.71 eV, corresponding to the energy barrier for the dissociative chemisorption of CO2. We demonstrate the presence of an additional configurational energy barrier for nanometer‐sized vacancies resulting in a significantly slower rate of expansion. Finally, CO2 etching is applied to map the location of the intrinsic vacancies in the polycrystalline graphene film where we show that the intrinsic vacancy defects manifest mainly as grain boundary defects where intragrain defects from oxidative etching constitute a minor population.

Long-range in-plane elastic displacement fields of double vacancies in graphene

An analytical model based on the force-dipole approximation for an elastic 2D continuum is examined for a description of long-range in-plane elastic displacement fields of point defects in graphene. Through atomistic numerical calculations with a particular interatomic potential (PPBE-G) the model is adjusted to a particular point defect (double vacancy). Both atomistic and continuum calculations are carried out with periodic boundary conditions. The displacement field of a single divacancy in an infinite graphene sheet is then obtained and is shown to be highly anisotropic. It is established that the elasticity theory approximates the atomistic simulations with the accuracy better than 0.3% at more than 10 interatomic distances from the defect. • The divacancy displacement field can be described by the continuum elasticity theory. • A simulation box of a few thousand atoms ensures a numerically stable result. • The divacancy displacement field in graphene is highly anisotropic.

Role of hydrogen in the dissociation of CH4 on different graphene by DFT study

The effect of hydrogen on the growth mechanism of pyrocarbon has attracted much attention. The influence of hydrogen on the dissociation from CH4 to C2H2 on pristine graphene, N-doped graphene and vacancy graphene have been investigated by using density functional theory. There are two kinds of heterogeneous reaction pathways when the hydrogen is involved, i.e., dehydrogenation reactions and H-abstraction reactions. The transition state calculations were performed to acquire the reaction pathways on each substrate after obtaining the most stable adsorption configurations of the reactants and products. The results indicate that the adsorptions of reactants are not affected by hydrogen. The dehydrogenation reactions are more favored on vacancy graphene, and the H-abstraction reactions are more favored on the pristine graphene and N-doped graphene. The dehydrogenation reactions on the vacancy graphene are most favored among all these reactions in favor of the deposition of pyrocarbon. The dehydrogenation reactions on these three substrates are affected by hydrogen.