Dangling Carbon Bonds Research Articles

Temperature effects in EPR spectra and optical features of plastically deformed natural IaAB, IaB, and low-nitrogen diamonds

In this study, the three natural IaB, IaAB, and low-nitrogen diamond crystals from Yakutian province with plastic deformation characteristic features have been investigated. It is shown that for low-nitrogen and type IaB diamonds, plastic deformation does not lead to brown coloration raising the question about the nature of the color. The crystal of the IaAB type is brown and contains from 53 to 64 % of the total nitrogen in the form of A-aggregates. In electron paramagnetic resonance (EPR), this sample reveals the W7 centers which are formed from the A-centers as a result of plastic deformation and the 490.7 single line related to dangling bonds in the dislocation cores. For this brown plastically deformed diamond the narrowing of the 490.7 EPR line and the dynamical broadening of the W7 lines are observed with a temperature increase from 77 to 300 K. It is assumed that the charge transfer from the W7 defects to dangling bonds in the dislocation core takes place. The obtained changes in the linewidths of EPR spectra can be explained by the mobility of electrons localized on dangling carbon bonds where the varying polarization of the W7 orbitals leads to the modulation of the nitrogen hyperfine structure. In this case, the charge-transfer complexes formed can be responsible for the appearance of the 550 nm band in the optical absorption spectra and, as a consequence, for the brown color of the plastically deformed IaAB diamond sample.

Photoinduced Dynamics of Spin Centers in Carbon-Modified Titanium Dioxide Nanotubes

Arrays of titanium dioxide (TiO2) nanotubes with different chemical compositions have been synthesized; their structural properties have been studied, and the characteristics of spin centers (defects) have been determined. All samples have appeared to contain carbon. It has been established that the main type of spin centers in TiO2 nanotubes are dangling carbon bonds, and their concentration correlates with the carbon content in the obtained structures. Under illumination, a reversible increase in the concentration of defects occurs, which is caused by their photoinduced recharging in the process of impurity absorption. This process is accompanied by an increase in the concentration of photoexcited electrons in the conduction band. The originality and novelty of the work are determined by the development of a method for controlling the density of defects and, accordingly, the concentration of photoinduced electrons by thermal treatment of samples under various conditions. The results open up new possibilities for the development of photocatalysts based on titanium dioxide nanotubes with a controlled electron concentration in the conduction band that function in the visible range of the spectrum.

Friction-induced reconstruction of sliding interface and low friction mechanism of WC/a-C films

Tungsten carbide reinforced amorphous carbon (WC/a-C) films exhibit significantly lower coefficient of friction (COF) than typical a-C films in the atmospheric environment; thus, they are promising candidates for use as solid lubricants. However, the low friction mechanism of WC/a-C films remains poorly understood. Here, we revealed that the friction-induced formation of WO3-rich transfer film changed the real sliding interface between the initial friction pair materials into an interface between a WO3-rich transfer film and carbon-rich worn film surface (WO3/C interface). First-principles calculations indicated that the WO3/C interface showed weaker interactions than the C/C interface of the a-C film under the same conditions, particularly when the dangling carbon bonds at the sliding interface were not completely passivated. This resulted in a reduced COF of the WO3/C interface. Thus, friction-induced reconstruction of the sliding interface to an intrinsically weak-interacting interface is proposed as a new path to further improve the tribological performance of a-C-based films.

Pt-Sandwiched Graphene Ultra-Durable and Highly Active Catalyst for Oxygen Reduction Reaction

Fuel cells are viable replacements as clean energy sources. Main obstacles towards technology-spread are low performance and high cost. Oxygen reduction (ORR) is a typical reaction at cathode side with sluggish kinetics, where state of the art catalyst is based on Pt supported on commercial carbon. Main objective is to overcome poor stability due to carbon corrosion and/or Pt agglomeration.2D materials (e.g. Graphene) possess tremendous chemical and mechanical stability. Sandwiching catalyst material between Graphene enables a robust support, providing a template dictating Pt growth as well. In addition, masking Pt with another graphene layer is plausible to enhance catalyst lifetime, while ORR activity is intact. Accordingly, the investigated structure is based on Pt nanoparticles sandwiched between two separate graphene sheets (GR/Pt/GR sandwich structure). The thickness of top graphene sheet varied between 1, 3 and 5 layers thick, where their respective effect on electrocatalytic activity and stability towards oxygen reduction reaction was evaluated. Ultra-small Pt nanoparticles were sputtered on tri-layer thick graphene films. TEM analysis showed particles monodispersity with an average particle size of 2.1 nm. Particles grew on graphene following either (111) or (110) crystallographic orientation. STEM imaging showed cloud of single atomically dispersed Pt atoms around the peripheral of all Pt nanoparticles. Accelerated durability testing (ADT) was performed to track electrochemical active surface area, and catalyst stability in particular. Samples with graphene coverage (GR/Pt/GR) showed higher stability compared to state of the art commercial catalyst (Pt/carbon produced by JM company). GR/Pt/GR with 3-layers graphene thick as a top cover showed interesting phenomenon with an increase of ECSA up to 5K ADT cycles. Afterwards, ECSA was negatively affected by the presence of point defects introduced with graphene (as shown by Ex-situ Raman analysis). After 15K, samples with 3-layers thick graphene survived ~84% of ECSA. Samples with 5-layers graphene thick, showed no ECSA at the beginning, however ECSA signal was increased systematically with ADT cycling up to 30K cycles. Electrocatalytic activity was tested using rotating disk electrode (RDE) setup. Samples with 3-layers graphene coverage showed 7 folds enhancement of mass activity compared to state of the art commercial Pt/carbon catalyst (measured at 0.9V vs RHE). Pt adatoms covalently bonded to C atoms of graphene enhancing adatom stability. In addition, absence of hydrogen waves during cyclic voltammetry scans of 5-layers thick graphene during ADT indicating that ORR reaction for less graphene-thick samples (i.e. 1 and 3 layers thick) occurred on top of graphene, where both graphene and Pt forming a hybrid catalyst. ADT introduces point defects within graphene mask. Single Pt atoms hop through defects between layers towards surface. DFT calculations showed Pt single-atom favors trapping at point defects, bonded to dangling carbon bonds. Pt lateral diffusion (release from defect site) activated with Pt clustering right underneath the defect site. Migrated Pt adatom travels between graphene layers till trapped in a defect site of an upper layer to hop above to the surface systematically. Herein, we are reporting an optimization study of using graphene as a protective shield to suppress Pt catalyst dissolution. Results showed tremendous stability enhancement without any penalty of ORR activity, but boosting ORR electrocatalytic activity for GR/Pt/GR over state of the art commercial catalyst due to compressive strain induced on Pt metal adatoms by graphene. This shifts Pt d-band center position away from the fermi level. As a consequence, oxygen adsorption to Pt surface becomes weaker which facilitate the release of ORR reaction intermediates from Pt surface.

Defects in graphene nanoplatelets and their interface behavior to reinforce magnesium alloys

Defects in graphene nanoplatelets (GNPs) and their interface behavior to reinforce magnesium alloys was studied by using two types of GNPs, one with high integrity and the other with multiple defects, as reinforcements in ZK60 alloy. Herein, GNPs prepared by chemical exfoliation exhibited multiple defects, such as carbon atom vacancies, dangling carbon bonds, oxygen-containing functional groups and wrinkles. During the composite fabrication, Mg atoms restored the sp2 structure of graphene and reacted with the oxygen-containing groups on graphene, leading to a strong interfacial bonding. Meanwhile, graphene defects scattering in nanoscale triggered nucleation of Mg nanograins, which connect the “hard” GNPs and “soft” Mg matrix for effective transfer of stress and strain. The defective GNPs (0.05 wt%) reinforced ZK60 alloy with large tensile yield strength enhancement (65%) and especially remarkable elongation improvement (44%). This work highlights the structural feature of GNPs to reach their full strengthening potential as reinforcements in metal matrix composites.

Structure and electronic properties of biomorphic carbon matrices and SiC ceramics prepared on their basis

Biomorphic carbon matrices (BCMs) were produced by pyrolysis from wood species of different forest and garden trees, after which the as-prepared BCMs were converted to SiC ceramics through their impregnation with liquid silicon and further heat-treatment. Both types of obtained samples were studied by scanning electron microscopy (SEM), Raman scattering (RS), and electron spin resonance (ESR) methods. The SEM data reveal that all BCM samples contain large (10–50 μm) and small (1–5 μm) micro-pores with surface densities ∼109 m−2 and 1011 m−2, respectively. Analysis of RS allowed to estimate carbon cluster sizes of about 5–11 nm depending on the sample type. The study of the electronic structure using ESR spectroscopy is carried out for BCM and SiC ceramics samples. Using theoretical analysis of the ESR spectra, it was found that spin resonance in BCMs is due to the contribution of three spin systems: free electron spins, “pseudo-free” electron spins from the tail of density states below the conduction band, and localized spins at dangling carbon bonds (DCBs). Their contributions depend on the ratio of different structural phases such as sp2-hybridized graphite-like carbon network and amorphous carbon phase. For most BCM samples, the large ESR line width is dramatically narrowed when samples are pumped out due to the exclusion of the broadening effect of molecular oxygen. The transformation of BCM into SiC by impregnation with liquid silicon can be clearly traced in the Raman spectra and in the ESR spectra. It is established that the electronic properties of synthesized SiC ceramics are due to the presence of residual graphite-like carbon nanoclusters.

Атомная и электронная структура поверхности 3C-SiC(111)-(2sqrt(3)sqrt

AbstractThe theoretical studies and ab initio calculations have been carried out for the first time for atomic and electronic structure of four variants of 3 C -SiC(111)-( $$2\sqrt 3 \times 2\sqrt 3 $$ 2 3 × 2 3 )- R 30° surface with Si- terminated surface: initial, relaxed, reconstructed, and relaxed after reconstruction. The surface was simulated in the layered superlattice approximation by a system of thin films (slabs) with a thickness of 12 atomic layers separated by vacuum gaps of ~16 Å. In order to close the dangling carbon bonds, the opposite side of the film was complemented with 12 atoms of hydrogen. Ab initio calculations were performed by means of QUANTUM ESPRESSO program based on the density functional theory. It is shown that the reconstruction makes the atomic layers split. Our previous works and experimental data show that these splits are observed at reconstructions of surface (111) in crystals with the sphalerite structure. The band structures of four alternative slabs are calculated and analyzed. Total and layer-by-layer valence electron state densities are calculated for four top layers. It is shown that the real surface has a metallic conductivity.

Microwave plasma induced surface modification of diamond-like carbon films

Tailoring the surface of diamond-like carbon (DLC) film is technically relevant for altering the physical and chemical properties, desirable for useful applications. A physically smooth and sp3 dominated DLC film with tetrahedral coordination was prepared by plasma-enhanced chemical vapor deposition technique. The surface of the DLC film was exposed to hydrogen, oxygen and nitrogen plasma for physical and chemical modifications. The surface modification was based on the concept of adsorption–desorption of plasma species and surface entities of films. Energetic chemical species of microwave plasma are adsorbed, leading to desorbtion of the surface carbon atoms due to energy and momentum exchange. The interaction of such reactive species with DLC films enhanced the roughness, surface defects and dangling bonds of carbon atoms. Adsorbed hydrogen, oxygen and nitrogen formed a covalent network while saturating the dangling carbon bonds around the tetrahedral sp3 valency. The modified surface chemical affinity depends upon the charge carriers and electron covalency of the adsorbed atoms. The contact angle of chemically reconstructed surface increases when a water droplet interacts either through hydrogen or van dear Waals bonding. These weak interactions influenced the wetting property of the DLC surface to a great extent.

Effects of charge and surface defects of multi-walled carbon nanotubes on the disruption of model cell membranes

The direct contact between multi-walled carbon nanotubes (MWCNTs) and cell membranes causes membrane disruption, potentially leading to cytotoxicity. However, the role of electrostatic forces and MWCNT properties is still open to debate. In this study, the influences of charge and MWCNT surface defects on membrane disruption were investigated by microscopy and a quartz crystal microbalance with dissipation monitoring (QCM-D). Positively/negatively charged giant unilamellar vesicles (GUVs) and supported lipid bilayers (SLBs) were made as model cell membranes. Negatively charged MWCNTs disrupted the GUVs containing positively charged lipids, which confirmed the electrostatically mediated interaction. However, the mass loss was detected from the negatively charged SLBs after MWCNT exposure, which suggests the extraction of phospholipids. The defect degree of MWCNTs correlated with their adhesion amount on the membranes. Both the oxygenated functional groups and unoxidized dangling carbon bonds were active sites for MWCNT-membrane interactions. The MWCNTs were observed to be engulfed inside the GUVs. The results clearly demonstrate that phospholipid extraction by MWCNTs could occur in electrostatically repulsive conditions, and MWCNT defects were active binding sites whether or not they were oxygenated. Our findings should be helpful in the design and safe applications of carbon nanomaterials.

Role of Defects in Carbon Nanotube Walls in Deposition of CdS Nanoparticles from a Chemical Bath

Multiwall carbon nanotubes (MWCNTs), as-prepared and annealed in an inert atmosphere, have been used as templates for growth of CdS nanoparticles from an aqueous ammonium solution of cadmium chloride and thiourea. Transmission electron microscopy and Raman spectroscopy revealed that formation of CdS nanocrystals on the raw MWCNT surface proceeds markedly faster than on the annealed one. X-ray photoelectron spectroscopy showed a strong difference in the chemical state of the deposited CdS depending on the defect density in the MWCNTs. An interaction of Cd2+ ions and mixed Cd(II) complexes with a defective graphene fragment was studied using density functional theory. The calculations indicated that, for attachment of Cd-containing species to the graphitic surface, defects with the dangling carbon bonds are needed. By combining the experimental and theoretical results, a model of CdS nucleation on the MWCNTs was proposed.

Quantum Chemical Simulation of Carbon Nanotube Nucleation on Al2O3 Catalysts via CH4 Chemical Vapor Deposition.

We present quantum chemical simulations demonstrating how single-walled carbon nanotubes (SWCNTs) form, or "nucleate", on the surface of Al2O3 nanoparticles during chemical vapor deposition (CVD) using CH4. SWCNT nucleation proceeds via the formation of extended polyyne chains that only interact with the catalyst surface at one or both ends. Consequently, SWCNT nucleation is not a surface-mediated process. We demonstrate that this unusual nucleation sequence is due to two factors. First, the π interaction between graphitic carbon and Al2O3 is extremely weak, such that graphitic carbon is expected to desorb at typical CVD temperatures. Second, hydrogen present at the catalyst surface actively passivates dangling carbon bonds, preventing a surface-mediated nucleation mechanism. The simulations reveal hydrogen's reactive chemical pathways during SWCNT nucleation and that the manner in which SWCNTs form on Al2O3 is fundamentally different from that observed using "traditional" transition metal catalysts.

Influence of hydrogen functionalization on the fracture strength of graphene and the interfacial properties of graphene–polymer nanocomposite

Using molecular dynamics and classical continuum concepts, we investigated the effects of hydrogen functionalization on the fracture strength of graphene and also on the interfacial properties of graphene–polymer nanocomposite. Moreover, we developed an atomistic model to assess the temperature and strain rate dependent fracture strength of functionalized graphene along various chiral directions. Results indicate that hydrogen functionalization at elevated temperatures highly degrade the fracture strength of graphene. The functionalization also deteriorates the interfacial strength of graphene–polymer nanocomposite. Near-crack-tip stress distribution depicted by continuum mechanics can be successfully used to investigate the impact of hydrogen passivation of dangling carbon bonds on the strength of graphene. We further derived a continuum-based model to characterize the non-bonded interaction of graphene–polymer nanocomposite. These results indicate that classical continuum concepts are accurate even at a scale of several nanometers. Our work provides a remarkable insight into the fracture strength of graphene and graphene–polymer nanocomposites, which are critical in designing experimental and instrumental applications.

What Does Annealing Do to Metal–Graphene Contacts?

Annealing is a postprocessing treatment commonly used to improve metal-graphene contacts with the assumption that resist residues sandwiched at the metal-graphene contacts are removed during annealing. Here, we examine this assumption by undertaking a systematic study to understand mechanisms that lead to the contact enhancement brought about by annealing. Using a soft shadow-mask, we fabricated residue-free metal-graphene contacts with the same dimensions as lithographically defined metal-graphene contacts on the same graphene flake. Both cases show comparable contact enhancement for nickel-graphene contacts after annealing treatment signifying that removal of resist residues is not the main factor for contact enhancement. It is found instead that carbon dissolves from graphene into the metal at chemisorbed Ni- and Co-graphene interfaces and leads to many end-contacts being formed between the metal and the dangling carbon bonds in the graphene, which contributes to much smaller contact resistance.

Rolling up a Graphene Sheet

Carbon Nanotubes, CNTs, have been described as rolled-up graphene layers. Matching this concept to experiments has been a great experimental challenge for it requires a method to exfoliate graphite, generate ordered and stable dangling carbon bonds, and roll up the layer without affecting the unpaired electrons of the dangling bonds that finally have to zip up in an orderly fashion: A tall order for any synthetic strategy. The combined use of ultrasonication of graphite in dimethylformamide and addition of ferrocene aldehyde just does it!

Thermodynamics of a Potts-like model for a reconstructed zigzag edge in graphene nanoribbons

We construct a three-color Potts-like model for the graphene zigzag edge reconstructed with Stone-Wales carbon rings, in order to study its thermal equilibrium properties. We consider two cases which have different ground-states: the edge with non-passivated dangling carbon bonds and the edge fully passivated with hydrogen. We study the concentration of defects perturbing the ground-state configuration as a function of the temperature. The defect concentration is found to be exponentially dependent on the effective parameters that describe the model at all temperatures. Moreover, we analytically compute the domain size distribution of the defective domains and conclude that it does not have fat-tails. In an appendix, we show how the exchange parameters of the model can be estimated using density functional theory results. Such equilibrium mechanisms place a lower bound on the concentration of defects in zigzag edges, since the formation of such defects is due to non-equilibrium kinetic mechanisms.

High-temperature transformation of Fe-decorated single-wall carbon nanohorns to nanooysters: a combined experimental and theoretical study

The processes by which single-wall carbon nanohorns are transformed by iron nanoparticles at high temperatures to form "nanooysters", hollow graphene capsules containing metal particles that resemble pearls in an oyster shell, are examined both experimentally and theoretically. Quantum chemical molecular dynamics (QM/MD) simulations based on the density-functional tight-binding (DFTB) method were performed to investigate their growth mechanism. The simulations suggest that the nanoparticles self-encapsulate to form single-wall nanooysters (SWNOs) by assisting the assembly of dangling carbon bonds, accompanied by migration of the metal particle inside the carbon structure. These calculations indicate that the structure of the oyster consists primarily of hexagons along with a few pentagons that are predominantly formed near the former nanohorn edges as a result of their fusion. Experimental observations of large diameter nanoparticles inside multiwall carbon shells indicate that migration and coalescence of many iron particles must occur, perhaps by the convergence of smaller SWNOs or carbon-coated Fe-nanoparticles, whereby the void space is generated by the corresponding increase in the carbon shell surface area to metal nanoparticle volume.

Effects of oxygen bonding on defective semiconducting and metallic single-walled carbon nanotube bundles

We report the effects of oxygen doping on the electrical properties of defective metallic and semiconducting single-walled carbon nanotube bundles. Carbon vacancies are generated by electron beam knock-out process. The carbon nanotube bundles are placed on top of suspended electrodes produced on a through-hole chip. This allows a physical correlation to be established for transmission electron microscopy inspection and electrical characterization. The dangling carbon bonds of the vacancies are very active and can easily adsorb oxygen molecules. In terms of the semiconducting bundles, oxygen bonding lowers the bandgap and the original p-type bundles thereby modifying them to become bi-polar. For the metallic bundles, a hysteretic bi-stable state in gate-voltage cycling is observed; this is attributed to the electrically controlled dipole field of the oxygen molecules.

Oxidation of monovacancies in graphene by oxygen molecules

We study the oxidation of monovacancies in graphene by oxygen molecules using first principles calculations. In particular, we address the local magnetic moments which develop at monovacancies and show that they remain intact when a molecule is adsorbed such that the dangling carbon bonds are not fully saturated. However, the lowest energy configuration does not maintain dangling bonds and is found to be semiconducting. Our data can explain the experimentally observed behavior of graphene under exposure to an oxygen plasma.

The n-type conduction and microstructural properties of phosphorus ion implanted nanocrystalline diamond films

Phosphorus ions are implanted into nanocrystalline diamond (NCD) films followed by being annealed at different temperatures. The results show that the samples exhibit good n-type conductivity when annealing temperature is increased to 800 ℃ and above. Raman spectroscopy and electron paramagnetic resonance measurements display that the sample with a larger quantity of diamond phase with better lattice perfection has a lower resistivity. It is indicated that nano-sized diamond grains make contributions to the n-type conductivity in the films. After 1000 ℃ annealing, the amorphous carbon grain boundaries become more ordered, which leads the dangling carbon bonds to decrease and the resistivity of the film to increases. It is revealed that the amorphous carbon grain boundaries supply a conduction path to the n-type phosphorus ion implanted nanocrystalline diamond grains.

Ab initio calculation of adhesion and potential corrugation of diamond (001) interfaces

The system consisting of two diamond (001) surfaces in contact was studied by means of plane-wave/pseudopotential density functional calculations. Different hydrogen coverages, ranging from fully hydrogenation to bare surfaces, were considered. The adhesion energy was calculated as a function of both the separation and the lateral displacement of the two surfaces. The effects of dangling carbon bonds on the adhesion and potential corrugation are quantitatively discussed.