Carbon Adatoms Research Articles

Self-assembly mechanisms of short atomic chains on single-layer graphene and boron nitride

Nucleation and growth mechanisms of short chains of carbon atoms on single-layer, hexagonal boron nitride (h-BN), and short BN chains on graphene are investigated using first-principles plane wave calculations. Our analysis starts with the adsorption of a single carbon ad-atom and examines its migrations. Once a C$_2$ nucleates on h-BN, the insertion of each additional carbon at its close proximity causes a short segment of carbon atomic chain to grow by one atom at at a time in a quaint way: The existing chain leaves its initial position and subsequently is attached from its bottom end to the top of the carbon ad-atom. The electronic, magnetic and structural properties of these chains vertically adsorbed to h-BN depend on the number of carbon atoms in the chain, such that they exhibit an even-odd disparity. An individual carbon chain can also modify the electronic structure with localized states in the wide band gap of h-BN. As a reverse situation we examined the growth of short BN atomic chains on graphene, which attribute diverse properties depending on whether B or N is the atom bound to the substrate. These results together with ab-initio molecular dynamics simulations of the growth process reveal the interesting self-assembly behavior of the grown chains. Furthermore, we find that these atomic chains enhance the chemical activity of h-BN and graphene sheets by creating active sites for the bonding of various ad-atoms and can act as pillars between two and multiple sheets of these honeycomb structures leaving wider spacing between them to achieve high capacity storage of specific molecules.

Size‐Dependent Evolution of Graphene Nanopores Under Thermal Excitation

Graphene nanopores expand when pore diameter is larger than membrane thickness after heat treatment; otherwise, nanopore size shrinks. Such size-dependent evolutionary mechanism of nanopores is considered as thermal-induced migration of uncombined carbon atoms. The amount of carbon adatoms determines the extent of diameter change. This could provide an applicable strategy for nanopore fabrication.

Characterization of the post-thermal annealing effect for p-GaAs/i-InGaAsN/n-GaAs hetero-junction solar cells

In this study, we demonstrated the fabrication and characterization of p-GaAs/i-InGaAsN/n-GaAs double hetero-junction solar cells (DHJSCs). The intrinsic InGaAsN absorption layer which is lattice-matched with GaAs substrates was grown by the metal-organic vapor phase epitaxy (MOVPE) system. The metal-organic sources used in the MOVPE growth induced unintentionally-doped carbon adatoms, and this in turn caused some carbon-related defects. This carbon pollution subsequently deteriorated the optical and electrical characteristics of the dilute nitride material. With ex-situ post-annealing being carried out in the furnace, it could not only modify the crystallinity of the InGaAsN material, but the deterioration in the performance of the dilute nitride solar cell due to the nitrogen incorporation could also be mitigated. After conducting the thermal annealing at 550°C for 1h in nitrogen ambience, the conversion efficiency of the DHJSCs was increased from 0.61% to 4.46% under Air Mass 1.5 Global (1000W/m2, AM1.5G) irradiance. The external quantum efficiency (EQE) spectrum further revealed that the efficiency of DHJSCs had been enhanced significantly owing to the crystalline improvement of the intrinsic InGaAsN absorption layer after the ex-situ post-thermal annealing was implemented. This improvement also led to a significant increase in the short-circuit current. The X-ray photoelectron spectroscopy (XPS) measurement result also showed that the ex-situ thermal annealing contributed to the improvement of crystal quality and the reduction of carbon related defects and clusters in the as-grown InGaAsN absorption layer.

Calculating the energy of vacancies and adatoms in a hexagonal SiC monolayer

It is noted that the development of semiconductor SiC-electronics is prevented by a low quality of grown silicon carbide single crystals. It is found that structural defects of a substrate penetrating into an epitaxial layer upon subsequent homoepitaxial growth can considerably degrade a device’s characteristics. We investigate the effect of the deformation of a hexagonal SiC monolayer on vacancy stability and material properties, and study the processes of silicon and carbon adatom migration over a surface of SiC.

Anchoring platinum on graphene using metallic adatoms: a first principles investigation

First principles calculations based on spin-polarized density functional theory were used to identify metallic adatoms that would strengthen the Pt(111)/graphene interface (with a low work of separation of 0.009 J m−2), when the adatom was placed between the Pt(111) and the graphene. It was shown that the strength of the Pt–adatom bond, which had a metallic character, increased with the amount of charge transferred from the adatom to the Pt. The carbon–adatom bond, on the other hand, had a mixed ionic and covalent character and was weaker than the Pt–adatom bond for each of the 25 elements considered. Consequently, the total Pt(111)/graphene interface strength and, hence, the anchoring effect of the adatom were controlled by the carbon–adatom bond strength. Metals with unfilled d orbitals increased the Pt/graphene interface strength to above 0.5 J m−2. The carbon–adatom bond strength was proportional to the ratio between the charge transferred from the adatom to the graphene (ΔZC) and the charge transferred to the Pt surface (ΔZPt); i.e., the ΔZC/ΔZPt ratio defined the ability of an adatom to anchor Pt to graphene. For Ir, Os, Ru, Rh and Re, ΔZC/ΔZPt > 1.0, making these elements the most effective adatoms for anchoring Pt to graphene.

Epitaxial kinetics with an intermediate polyatomic species

Graphene, a flat monolayer of carbon atoms packed tightly into a two dimensional hexagonal lattice, has unusual electronic properties which have many promising nanoelectronic applications. Recent Low Energy Electron Microscopy (LEEM) experiments show that the step edge velocity of epitaxially grown 2D graphene islands on Ru(0001) varies with the fifth power of the supersaturation of carbon adatoms. This suggests that graphene islands grow by the addition of clusters of five atoms rather than by the usual mechanism of single adatom attachment. We have carried out Kinetic Monte Carlo (KMC) simulations in order to further investigate the general scenario of epitaxial growth by the attachment of mobile clusters of atoms. We did not seek to directly replicate the Gr/Ru(0001) system but instead considered a model involving mobile tetramers of atoms on a square lattice. Our results show that the energy barrier for tetramer break up and the number of tetramers that must collide in order to nucleate an immobile island are the important parameters for determining whether, as in the Gr/Ru(0001) system, the adatom density at the onset of island nucleation is an increasing function of temperature. A relatively large energy barrier for adatom attachment to islands is required in order for our model to produce an equilibrium adatom density that is a large fraction of the nucleation density. A large energy barrier for tetramer attachment to islands is also needed for the island density to dramatically decrease with increasing temperature. We show that islands grow with a velocity that varies with the fourth power of the supersaturation of adatoms when tetramer attachment is the dominant process for island growth.

Is graphene aromatic?

We analyze the chemical bonding in graphene using a fragmental approach, the adaptive natural density partitioning method, electron sharing indices, and nucleus-independent chemical shift indices. We prove that graphene is aromatic, but its aromaticity is different from the aromaticity in benzene, coronene, or circumcoronene. Aromaticity in graphene is local with two π-electrons delocalized over every hexagon ring. We believe that the chemical bonding picture developed for graphene will be helpful for understanding chemical bonding in defects such as point defects, single-, double-, and multiple vacancies, carbon adatoms, foreign adatoms, substitutional impurities, and new materials that are derivatives of graphene.

Interface mediated control of microstructure and magnetic properties of FePt–C thin films

The microstructure and magnetic properties of FePt–C film deposited on the MgO/CrRu underlayer are studied in lieu of carbon doping in FePt and RF biasing assisted etching of the MgO underlayer. Increasing amounts of carbon doping in the FePt deteriorates the chemical ordering as a result of reduced kinetic energy of FePt adatoms due to frequent collision with carbon adatoms. This leads to a reduction in the magnetocrystalline anisotropy energy and hence the coercivity. RF etching of the MgO/CrRu underlayer before deposition of FePt helps to reduce the grain size and intergranular exchange interaction without adversely affecting the chemical ordering. Reduced intergranular exchange interaction leads to an increase in the coercivity.

Atomic-Scale Electron-Beam Sculpting of Near-Defect-Free Graphene Nanostructures

In order to harvest the many promising properties of graphene in (electronic) applications, a technique is required to cut, shape, or sculpt the material on the nanoscale without inducing damage to its atomic structure, as this drastically influences the electronic properties of the nanostructure. Here, we reveal a temperature-dependent self-repair mechanism that allows near-damage-free atomic-scale sculpting of graphene using a focused electron beam. We demonstrate that by sculpting at temperatures above 600 °C, an intrinsic self-repair mechanism keeps the graphene in a single-crystalline state during cutting, even though the electron beam induces considerable damage. Self-repair is mediated by mobile carbon ad-atoms that constantly repair the defects caused by the electron beam. Our technique allows reproducible fabrication and simultaneous imaging of single-crystalline free-standing nanoribbons, nanotubes, nanopores, and single carbon chains.

Novel Growth Mechanism of Epitaxial Graphene on Metals

Graphene, a hexagonal sheet of sp(2)-bonded carbon atoms, has extraordinary properties which hold immense promise for nanoelectronic applications. Unfortunately, the popular preparation methods of micromechanical cleavage and chemical exfoliation of graphite do not easily scale up for application purposes. Epitaxial graphene provides an attractive alternative, though there are many challenges, not least of which is the absence of any understanding of the complex atomistic assembly kinetics of graphene layers. Here, we present a simple rate theory of epitaxial graphene growth on close-packed metal surfaces. On the basis of recent low-energy electron-diffraction microscopy experiments, our theory supposes that graphene islands grow predominantly by the attachment of five-atom clusters. With optimized kinetic parameters, our theory produces a quantitative account of the measured time-dependent carbon adatom density. The temperature dependence of this density at the onset of nucleation leads us to predict that the smallest stable precursor to graphene growth is an immobile island composed of six five-atom clusters. This conclusion is supported by a recent study based on temperature-programmed growth of epitaxial graphene, which provides direct evidence of nanoclusters whose coarsening leads to the formation of graphene layers. Our findings should motivate additional high-resolution imaging experiments and more detailed simulations which will yield important input to developing strategies for the large-scale production of epitaxial graphene.

Effect of coverage by carbon on the possibility of forming an interstitial solid solution in Fe(001) and Fe(111) subsurface layers

The interaction between carbon adatoms as a function of the coverage of the Fe(001) and Fe(111) surfaces by carbon has been theoretically investigated using first-principles calculations in terms of the density functional theory. It has been established for the first time that the sequential filling of the upper surface layer by carbon atoms leads to the embedding of a part of atoms in the subsurface iron layer due to the their collective interaction, which provides the possibility of forming the interstitial solid solution. It has been demonstrated that the high coverage of the (001) surface by carbon leads to a considerable decrease in the energy barrier to the diffusion of carbon atoms into the subsurface layer as compared to the diffusion barrier for single atoms.

Adsorption of carbon adatoms to graphene and its nanoribbons

This paper investigates the adsorption of carbon adatoms on graphene and its nanoribbons using first-principles plane wave calculations within density functional theory. The stability at high carbon adatom coverage, migration, and cluster formation of carbon atoms are analyzed. Carbon adatoms give rise to important changes in electronic and magnetic properties even at low coverage. While bare graphene is nonmagnetic semimetal, it is metallized and acquires magnetic moment upon coverage of carbon adatoms. Calculated magnetic moments vary depending on the coverage of adatoms even for large adatom-adatom distances. Electronic and magnetic properties of hydrogen passivated armchair and zigzag nanoribbons show strong dependence on the adsorption site. We also predict a new type of carbon impurity defect in graphene, which has a small formation energy. Interactions between distant carbon adatoms imply a long ranged interaction.

A first-principles study on magnetic coupling between carbon adatoms on graphene

The weak ferromagnetism reported for graphite and related carbon nanostructures is frequently related to the magnetic coupling of point defects such as vacancies or hydrogen adatoms that interact with only one sublattice in the bipartite graphene lattice. In this paper, using density functional theory calculations we study the magnetic coupling between point defects, such as carbon adatoms, which form bonds with atoms located on both sublattices. We show that there is ferromagnetic coupling for small separations between the adatoms. Further, we demonstrate that it is energetically favorable for C adatoms to agglomerate. Our results indicate that the magnetism induced in graphite by irradiation can be explained as being due to the existence of adatom agglomerations and small graphene flakes formed from isolated adatoms rather than uniformly distributed point defects.

High reactivity of carbon nanotubes and fluorinated carbon nanotubes irradiated by Ar+ ions

AbstractPristine and fluorinated single‐wall carbon nanotubes (SWCNTs) irradiated by Ar+ ion beam with kinetic energy 1 keV have been studied using C K‐edge X‐ray absorption near‐edge spectroscopy (XANES). Theoretical spectra used for interpretation of the experiment were plotted based on density functional theory (DFT) calculations of graphene fragment contained different types of defects (single‐atom vacancy, carbon adatom, oxygen or fluorine functionalized vacancy) within Z + 1 approximation. Comparison between experimental and theoretical spectra shows that ion irradiation creates vacancies and boundary atoms of defects readily interact with oxygen yielding ether groups mainly.

Spontaneous edge-defect formation and defect-induced conductance suppression in graphene nanoribbons

We present a first-principles study of the migration and recombination of edge defects (carbon adatom and/or vacancy) and their influence on electrical conductance in zigzag graphene nanoribbons (ZGNRs). It is found that at room temperature, the adatom is quite mobile while the vacancy is almost immobile along the edge of ZGNRs. The recombination of an adatom-vacancy pair leads to a pentagon-heptagon ring defect structure having a lower energy than the perfect edge, implying that such an edge defect can be formed spontaneously. This edge defect can suppresses the conductance of ZGNRs drastically, which provides some useful hints for understanding the observed semiconducting behavior of the fabricated narrow GNRs.

Nucleation effect of Si a of 6H–SiC-(0 0 0 1)–(√3×√3)R30° surface: First-principles study

First-principles calculations have been performed to investigate the nucleation effect of Si adatoms (Si a) of the 6H–SiC-(0 0 0 1)–(√3×√3)R30° reconstruction surface. The results of C adatoms adsorption energy, Si a desorption energy and the substituting energy of carbon with Si a indicate that the Si a of Si-rich-(√3×√3)R30° reconstruction surface behave as nucleation center in favor of the formation of graphene buffer layer on this surface. The Si a atoms of the reconstruction surface are readily substituted by exotic C atoms, especially when there are carbon adatoms on the surface, which urges the growth of the graphene buffer-layer. The mechanism of the epitaxial growth of graphene on the 6H–SiC-(0 0 0 1) surface was also briefly discussed in our present work.

Ab initio investigation of the clustering of carbon adatoms on Fe(001) and Fe(111) surfaces

A theoretical investigation of the interaction between carbon adatoms on the Fe(001) and Fe( 111 ) surfaces is performed using ab initio calculations in terms of density functional theory. Calc ulations of the adsorption energy demonstrate the existence of a strong bonding between single carbon adatoms and the iron surface. An analysis of the calculated energies of the interaction between carbon adatoms reveals for the first time that the repulsion between the carbon adatoms located at the nearest neighbor sites on the Fe(001) surface occurs and that clusters with a looser packing are formed on the surface.

Reaction between Thiophene and Ni Nanoparticles Supported on SiO2or ZnO: In Situ Synchrotron X-ray Diffraction Study

Synchrotron X-ray powder diffraction (XRD) and mass spectrometry were applied in situ to study the interaction of thiophene with Ni0 particles (5−6 nm) deposited on inert (SiO2) or sulfidable (ZnO) supports. The reaction was performed in a hydrogen flow containing 20−40 mbar of thiophene at 280−360 °C for Ni/SiO2 and at 360 °C for Ni/ZnO. Surprisingly, in the first XRD pattern of the solid recorded after introduction of thiophene in the reactor, we observed a considerable increase of the Ni cell parameter: Δa = 0.06 Å for Ni/SiO2 and Δa = 0.15 Å for Ni/ZnO at 360 °C. The simultaneous presence of CH4 in the gas phase revealed that in this stage thiophene molecules undergo complete hydrogenolysis. During this reaction, carbon adatoms can also be produced and their diffusion into the bulk of Ni nanoparticles was supposed to provoke the observed lattice expansion. After this initial stage, the SiO2-supported Ni nanoparticles transformed progressively into a complex mixture of phases with the composition dependent on the reaction temperature: at 280 °C the main product was identified as Ni3S2 (cubic modification and heazelwoodite), while at 360 °C it was Ni3C. The behavior of ZnO-supported Ni is different. The carbon-modified dilated Ni nanoparticles, formed in the initial stage, remained intact until the end of ZnO sulfidation to be later transformed into Ni3C. On the basis of these results, we concluded that carbon-modified Ni particles play the role of an active phase during the reactive adsorption of thiophene on Ni/ZnO.

Photosensitive self-assembled nanoarchitectures containing surfactant-free Si nanocrystals produced by laser fragmentation in water

We report on a rapid and scalable method for the self-assembly of networks of photosensitive silicon nanocrystal (Si-nc) micrograin produced by nanosecond laser fragmentation in water. Electrical conductivity through the long range (>200 μm) self-assembled luminescent Si-nc based networks is shown. The process forms stable nanoarchitectures which are compatible with subsequent plasma and heat treatments allowing for the deposition of carbon adatoms on the self-assembled structure. This strategy is environmental-friendly and could be efficiently used to control the complex interactions at the Si-ncs surface promoting the growth of specific nanostructures that in turn would enable the formation of long-ranged assemblies.

Self-organized carbon connections between catalyst particles on a silicon surface exposed to atmospheric-pressure Ar + CH 4 microplasmas

Ag nanoparticles and Fe-coated Si micrograins were separately deposited onto Si(1 0 0) surfaces and then exposed to an Ar + CH 4 microplasma at atmospheric pressure. For the Ag nanoparticles, self-organized carbon nanowires, up to 400 nm in length were produced, whereas for the Fe-coated Si micrograins carbon connections with the length up to 100 μm were synthesized on the plasma-exposed surface area of about 0.5 mm 2. The experiment has revealed that long carbon connections and short nanowires demonstrate quite similar behavior and structure. While most connections/nanowires tended to link the nearest particles, some wires were found to ‘dissolve’ into the substrate without terminating at the second particle. Both connections and nanowires are mostly linear, but long carbon connections can form kinks which were not observed in the carbon nanowire networks. A growth scenario explaining the carbon structure nucleation and growth is proposed. Multiscale numerical simulations reveal that the electric field pattern around the growing connections/nanowires strongly affects the surface diffusion of carbon adatoms, the main driving force for the observed self-organization in the system. The results suggest that the microplasma-generated surface charges can be used as effective controls for the self-organized formation of complex carbon-based nano-networks for integrated nanodevices.