Carbon Adatoms Research Articles

Adsorption and surface reactions of C2H2 and C2H4 on Co(0001)

In this paper we have studied adsorption and surface reactions of acetylene and ethylene on Co(0001) in detail by temperature desorption spectrum (TDS), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). C2H4 adsorption at 130 and 300 K followed by subsequent heating mainly forms C2 clusters and graphitic carbon, respectively, while C2H4 decomposes at 400 and 500 K to form dominant graphitic carbon and carbon adatoms, respectively. C2H2 molecularly adsorbs at 130 K but exclusively dehydrogenates upon heating. The resulting C2H2(a) species at low coverages remains stable up to 400 K and then exclusively dehydrogenates into C2 clusters, while the resulting C2H2(a) species at high coverages remains stable up to 310 K and then majorly dehydrocyclizates into (C2H)n intermediates with ring structures at 340 K which further dehydrogenates into graphitic carbon, and minorly dehydrogenates into C2 clusters. Exposed at 370 K, C2H2 dehydrocyclizates into (C2H)n intermediates with ring structures. These temperature and coverage dependent surface reactions of C2H2 and C2H4 on Co(0001) greatly enrich our fundamental understanding of Co-catalyzed F-T synthesis reaction.

Inhibiting the Laydown of Polymeric Carbon and Simultaneously Promoting Its Facile Burn-Off during the Industrial-Scale Production of Hydrogen with Nickel-Based Catalysts: Insights from Ab Initio Calculations.

This paper presents a computational study of the mechanistic models for the laydown of carbon species on nickel surface facets and the burn-off models for their gasification mechanism in methane steam reforming based on density functional theory. Insights into catalyst design strategies for achieving the simultaneous inhibition of the laydown of polymeric carbon and the promotion of its burn-off are obtained by investigating the influence of single atom dopants on nickel surfaces. The effects of single atom dopants on adsorption energies are determined at both low and high carbon coverages on nickel and used to introduce appropriate thermodynamic descriptors of the associated surface reactions. It is found that the critical size of the nucleating polymeric carbon adatom contains three atoms, i.e., C3. The results show that the burn-off reaction of a polymeric carbon species is thermodynamically limited and hard to promote when the deposited carbon cluster grows beyond a critical size, C4. The introduction of single atom dopants into nickel surfaces is found to modify the structural stability and adsorption energies of carbon adatom species, as well as the free energy profiles of surface reactions for the burn-off reactions when CH4, H2O, H2, and CO species react to form hydrogen. The results reveal that materials development strategies that modify the sub-surface of the catalyst with potassium, strontium, or barium will inhibit carbon nucleation and promote burn-off, while surface doping with niobium, tungsten, or molybdenum will promote the laydown of polymeric carbon. This study provides underpinning insights into the reaction mechanisms for the coking of a nickel catalyst and the gasification routes that are possible for the recovery of a nickel catalyst during the steam reforming of methane for large-scale production of hydrogen.

The role of temperature on defect diffusion and nanoscale patterning in graphene

Graphene is of great scientific interest due to a variety of unique properties such as ballistic transport, spin selectivity, the quantum hall effect, and other quantum properties. Nanopatterning and atomic scale modifications of graphene are expected to enable further control over its intrinsic properties, providing ways to tune the electronic properties through geometric and strain effects, introduce edge states and other local or extended topological defects, and sculpt circuit paths. The focused beam of a scanning transmission electron microscope (STEM) can be used to remove atoms, enabling milling, doping, and deposition. Utilization of a STEM as an atomic scale fabrication platform is increasing; however, a detailed understanding of beam-induced processes and the subsequent cascade of aftereffects is lacking. Here, we examine the electron beam effects on atomically clean graphene at a variety of temperatures ranging from 400 to 1000 °C. We find that temperature plays a significant role in the milling rate and moderates competing processes of carbon adatom coalescence, graphene healing, and the diffusion (and recombination) of defects. The results of this work can be applied to a wider range of 2D materials and introduce better understanding of defect evolution in graphite and other bulk layered materials.

Indirect measurement of the carbon adatom migration barrier on graphene

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300–1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 ± 0.03) eV.

Indirect Measurement of the Carbon Adatom Migration Barrier on Graphene

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300-1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 \pm 0.03) eV.

Promoting propane dehydrogenation via strain engineering on iridium single-atom catalyst

The rational design of active and selective catalysts is a challenge for heterogeneous catalysis, also true for propane dehydrogenation (PDH). Owing to the separated active sites, Single-atom catalysts (SACs) exhibit promising PDH applications with high propylene selectivity. Here, by means of density functional theory and the energetic span model, we systemically investigate the effect of strain on PDH over the nitrogen-coordinated iridium SAC (IrN4). We find that the thermodynamics and kinetics of each elementary step in PDH process exhibit linear relationship with applied strain (from −4.0% to 4.0%), as well as the geometry and electronic structure of the intermediates. We show that the compression strain can promote the first dehydrogenation while tension strain hinders it, which originates from the enhanced electron-donating of IrN4 site under compression strain and more bonding orbitals filling between Ir and carbon adatom (Ir-C). The first dehydrogenation and the second dehydrogenation are presented opposite energy trends under applied strain. This is attributed to strain-induced opposite changes in the energy levels of the d-band center of the IrN4 site relative to the p-band and s-band center of the intermediates. Further, using the energetic span model, we show that the first dehydrogenation dominates the activity of the PDH process, where the overall turnover frequency (TOF) of PDH can be greatly improved by stabilizing the first dehydrogenation intermediate (C3H7 + H)*. Thereby, compressive strain is suggested to improve PDH performance. Moreover, the selectivity of PDH over IrN4 SAC to propylene can be not affected by the applied strain qualitatively, which is due to the propylene-π adsorption mode at the single-atom site.

Temperature-dependent displacement cross section of graphene and its impurities: measuring the carbon adatom migration barrier

Temperature-dependent displacement cross section of graphene and its impurities: measuring the carbon adatom migration barrier

Coulomb blockade in field electron emission from carbon nanotubes

We report the observation of Coulomb blockade in electron field emission (FE) from single-wall carbon nanotubes (SWCNTs), which is manifested as pronounced steps in the FE current–voltage curves and oscillatory variations in the energy distribution of emitted electrons. The appearance of the Coulomb blockade is explained by the formation of nanoscale protrusions at the apexes of SWCNTs due to the electric field-assisted surface diffusion of adsorbates and carbon adatoms. The proposed adsorbate-assisted FE mechanism is substantially different from the well-known resonant tunneling associated with discrete electronic states of adsorbed atoms. The simulations based on the Coulomb blockade theory are in excellent agreement with the experimental results. The SWCNT field emitters controlled by the Coulomb blockade effect are expected to be used to develop on-demand coherent single-electron sources for advanced vacuum nanoelectronic devices.

Healing of a Hole in a Carbon Nanotube under Electron Irradiation in High-Resolution Transmission Electron Microscopy

Healing of a hole in a carbon nanotube under electron irradiation in HRTEM at room temperature is demonstrated using molecular dynamics simulations with the CompuTEM algorithm. Formation of an amorphous patch is observed in all simulation runs. The amorphous patch is formed in the absence of external carbon adatoms only via reconstruction of the carbon bond network. It consists mainly of 5-, 6- and 7-membered rings and causes a small bottleneck. In addition, further growth of the initial amorphous patch under electron irradiation takes place. Two-coordinated atoms are found to play a crucial role in the latter process, analogous to autocatalisys of rearrangements of rings in fullerenes. The principal rearrangements in the presence of two-coordinated atoms can be described as generalized sp-defect migration: a bond is broken between two three-coordinated atoms and one of them forms a new bond with a nearby two-coordinated atom. If the new and former two-coordinated atoms are not bonded, the reaction leads both to displacement of the sp defect and changes in rings of the sp$^2$ carbon structure. Migration by hopping of two-coordinated atoms and other reactions involving simultaneous breakage of two bonds are also detected but much rarely. Long-living two-coordinated atoms in the patch structure and related fast growth of the patch are observed in more than half of the simulation runs. Since the amorphous patch and bottleneck affect the electronic properties of the nanotube, such nanotubes can be perspective for nanoelectronic applications.

Paving the way to dislocation reduction in Ge/Si(001) heteroepitaxy using C-based strained layer superlattices

Epitaxial Ge films were grown on Si(001) substrates by molecular beam epitaxy. During epitaxial growth, two carbon interlayers were deposited at varying substrate temperatures (140−620°C) and with varying C quantity (0−1.5monolayers). The influence of the second carbon interlayer on in-plane strain was investigated using high-resolution x-ray diffraction and transmission electron microscopy (TEM). All samples exhibited compressive strain, which was attributed to substitutional incorporation of carbon atoms. In-plane strain decreases with increasing substrate temperature during carbon deposition, indicating that enhanced surface mobility of carbon adatoms leads to formation of carbon clusters. This was confirmed by cross-sectional TEM investigations. Variation of C quantity at 180°C reveals maximum strain at an intermediate quantity of 0.8 monolayers. Omission of the second C interlayer results in much lower strain, indicating a mismatch between the two Ge layers separated by a C interlayer. This could be used to enforce dislocation filtering following the principle of strained layer superlattices. An upper estimate of 1×10−3 was found for the mismatch strain, resulting in a critical thickness for dislocation filtering of hc=153nm. A sample just exceeding hc exhibited a clear dislocation reduction effect as shown by TEM.

Activation and surface reactions of CO and H2 on ZnO powders and nanoplates under CO hydrogenation reaction conditions

Abstract Activation and surface reactions of CO and H2 on ZnO powders and nanoplates under CO hydrogenation reaction conditions were (quasi) in situ studied using temperature programmed surface reaction spectra, diffuse reflectance Fourier transform infrared spectroscopy, inelastic neutron scattering spectroscopy and electron paramagnetic resonance. CO undergoes disproportion reaction to produce gaseous CO2 and surface carbon adatoms, and adsorbs to form surface formate species. H2 adsorption forms dominant irreversibly-adsorbed surface hydroxyl groups and interstitial H species and very minor surface Zn-H species. Surface formate species and hydroxyl groups react to produce CO2 and H2, while surface carbon adatoms are hydrogenated by surface Zn-H species sequentially to produce CH(a), CH2(a), CH3(a) and eventually gaseous CH4. The ZnO nanoplates, exposing a higher fraction of Zn-ZnO(0001) and O-ZnO(000–1) polar facets, are more active than the ZnO powders to catalyze CO hydrogenation to CH4. These results provide fundamental understanding of the reaction mechanisms and structural effects of CO hydrogenation reaction catalyzed by ZnO-based catalysts.

Carbon dangling-bond center (carbon Pb center) at 4H-SiC(0001)/SiO2 interface

We identify a carbon dangling-bond center intrinsically formed at thermally oxidized 4H-SiC(0001)/SiO2 interfaces. Our electrically detected-magnetic-resonance spectroscopy and first-principles calculations demonstrate that this center, which we name “the PbC center,” is formed at a carbon adatom on the 4H-SiC(0001) honeycomb-like structure. The PbC center (Si3≡C-, where “-” represents an unpaired electron) is determined to be a just carbon version of the famous Pb center (Si dangling-bond center, Si3≡Si-) at Si(111)/SiO2 interfaces because we found close similarities between their wave functions. The PbC center acts as one of the major interfacial traps in 4H-SiC(0001) metal-oxide-semiconductor field-effect transistors (MOSFETs), which decreases the free-carrier density and the field-effect mobility of 4H-SiC(0001) MOSFETs. The formation of the PbC centers has the role of reducing the oxidation-induced strain, similar to the case of the formation of the Pb centers.

Formation of defects during fullerene bombardment and repair of vacancy defects in graphene

The use of high-energy ions or clusters to bombard graphene is emerging as a new method for the theoretical study of graphene properties. In this study, using molecular dynamics simulations and empirical potentials, the behaviors of graphene after bombardment by C60 under different initial velocities from 13.7 to 15.7 km/s were investigated. The simulations showed four types of defects: Stone–Wales defects, single vacancy defects, multiple vacancy defects, and out-of-plane carbon adatoms. The self-healing phenomenon of defective graphene was observed. In the low-speed region (< 14.5 km/s), the self-healing ability of graphene is enhanced at higher temperature. However, the effect of temperature is not obvious in the high-speed region, where velocity dominates. To repair vacancy defects in graphene, a physical method was proposed. The initial positions of lost atoms were traced, and then, the atoms were slowly dropped into the vacancy defects to effect repair. The simulations provide a fundamental understanding of bombardment between graphene and C60 and propose a new method for repairing vacancy defects in graphene.

On the Decoration of Zigzag Edges of an Epitaxial Graphene Nanoribbon

A double-chain model of an epitaxial graphene nanoribbon, the zigzag edges of which are decorated with foreign adparticles, has been proposed. The substrate is assumed to be a metal. Analytical expressions for the Green’s functions of carbon adatoms and adparticles are obtained. The band spectrum for the free state is determined, and the approximation of the density of states is proposed. Analytical expressions for the occupation numbers in the mode of tight binding between the adsorption complex and the substrate are presented. A chain of carbon adatoms decorated with adparticles (epicarbyne) is considered.

Gate-tunable magnetism of C adatoms on graphene

We have performed density functional theory calculations of graphene decorated with carbon adatoms, which bind at the bridge site of a C--C bond. Earlier studies have shown that the C adatoms have magnetic moments and have suggested the possibility of ferromagnetism with high Curie temperature. Here we propose to use a gate voltage to fine tune the magnetic moments from zero to 1$\mu_B$ while changing the magnetic coupling from antiferromagnetism to ferromagnetism and again to antiferromagnetism. These results are rationalized within the Stoner and RKKY models. When the SCAN meta-GGA correction is used, the magnetic moments for zero gate voltage are reduced and the Stoner band ferromagnetism is slightly weakened in the ferromagnetic region.

О декорировании зигзагообразных краев наноленты эпитаксиального графена

AbstractA double-chain model of an epitaxial graphene nanoribbon, the zigzag edges of which are decorated with foreign adparticles, has been proposed. The substrate is assumed to be a metal. Analytical expressions for the Green’s functions of carbon adatoms and adparticles are obtained. The band spectrum for the free state is determined, and the approximation of the density of states is proposed. Analytical expressions for the occupation numbers in the mode of tight binding between the adsorption complex and the substrate are presented. A chain of carbon adatoms decorated with adparticles (epicarbyne) is considered.

On Decoration of a Zigzag Edge of Epitaxial Graphene

Two approaches are proposed to the problem of the coupling of adsorbed particles with atoms of a zigzag edge of graphene formed on a metal substrate. The first approach is based on the Kalkstein and Soven scheme, which makes it possible to determine the electronic structure of a semi-infinite graphene sheet. The second approach is based on a cluster model of a zigzag edge. Analytical expressions are obtained for the local densities of the system’s states and the occupation numbers of a carbon adatom and an adparticle. The case of an isolated adparticle is considered in detail, and a method of taking into account the dipole–dipole interaction of particles aligned along the edge is proposed.

О декорировании зигзагообразной кромки эпитаксиального графена

AbstractTwo approaches are proposed to the problem of the coupling of adsorbed particles with atoms of a zigzag edge of graphene formed on a metal substrate. The first approach is based on the Kalkstein and Soven scheme, which makes it possible to determine the electronic structure of a semi-infinite graphene sheet. The second approach is based on a cluster model of a zigzag edge. Analytical expressions are obtained for the local densities of the system’s states and the occupation numbers of a carbon adatom and an adparticle. The case of an isolated adparticle is considered in detail, and a method of taking into account the dipole–dipole interaction of particles aligned along the edge is proposed.

Altered superatomic properties of U@C28 by the electron rearrangement via adatom defects

First-principles calculations show that when a carbon adatom has been introduced to the surface of U@C28 superatom, the neutral U@C29 has (cage)1(adatom)1 electronic state instead of (cage)2 state in U@C28. The valence orbitals of U atom keep fully occupied, while the origin of 32-electron principle is changed due to electron rearrangement via introducing the adatom. Furthermore, there exist two transition states when adatom slipping on the surface of U@C28, indicating that it is a two-step process from [5: 5] to [6: 6] adsorption sites.

Surface Chemistry of CH2I2 on Clean, Hydrogen- and Carbon Monoxide-Covered Co(0001) Surfaces

Fundamental understanding of complex FT synthesis is of great interest. We have employed X-ray photoelectron spectroscopy and temperature-programmed desorption to comparatively investigate CH2I2 adsorption and reactions on clean, hydrogen- and CO-covered Co(0001) surfaces. Surface chemistry of CH2I2 was demonstrated to sensitively depend on available vacant surface sites on Co(0001). Upon adsorption on clean Co(0001) surface at 110 K, CH2I2 undergoes stepwise decomposition reactions to produce carbon adatoms, CH(a) and CH2(a) species at small coverages, and chemisorbs both dissociatively and molecularly at large coverages. Upon heating, CH2(a) species facilely undergoes surface reactions to produce CH4, C3H6, and C2H4 in gas phase and CH(a) species on the surface at low temperatures. CH(a) species undergoes surface reactions to produce CH4 in gas phase and C2H2(a) species on the surface at higher temperatures, and both CH(a) and C2H2(a) species undergo further surface reactions to produce H2 in gas phase and carbon species on the surface. Coadsorbed H adatoms and CO molecules were found to strongly affect surface chemistry of CH2I2 and the resulting CHx species on Co(0001) via suppressing the decomposition reactions and promoting the carbon–carbon bond coupling reactions. These results add novel insights in fundamental understanding of complex FT synthesis.