Behaviors Of Carbon Atoms Research Articles

Nanoscale precision welding-enabled quasi-3D conductive carbon blacks for fast-charging and long-lasting secondary batteries

Robust three-dimensional (3D) conductive networks are vital for enhancing the utilization, rate capability, and lifespan of battery electrode materials, which largely relies on the selection and synergy of various conductive additives. As the spatial dimension expands from granular to tubular or lamellar, the conductivity of conductive additives continues to increase, but the cost and dispersion difficulty will sharply rise. This determines that zero-dimensional carbon black (CB) spherical nanoparticles remain the dominant component of current composite conductive additives. Herein, we disclosed a transient high-temperature welding technique that can precisely regulate the “rearrangement-fusion” behavior of carbon atoms at specific temperatures, accurately splicing the CB nanospheres into quasi-3D dendritic aggregates with appropriate spatial extension. This not only enhances the powder conductivity of CBs by 205.1 %, but also improves the processability through reducing the specific surface area. The quasi-3D CBs upgrade their contact mode with active materials from point-to-point to line-to-point, while significantly enhancing the inter-particle connection of active materials. More encouragingly, the modified CBs greatly boost the reaction kinetics, rate and cycling performance of LiFePO4 cathode and SiO@C anode, and derives fast-charging and long-lasting SiO@C//LiFePO4 full battery. Our work provides innovative solutions for upgrading the spatial conductive network within future secondary batteries.

Simulation research on nucleation mechanism of graphene deposition assisted by diamond grain boundary

The growth of high-quality graphene is always a focused issue in the field of two-dimensional materials, and the growth of graphene on brand new substrates has received considerable attention from scholars especially. The research on the nucleation mechanism of graphene deposited on a polycrystalline diamond substrate is of significance in the large-scale preparation of graphene in practice. Here in this work, the direct growth without transfer process of graphene on a diamond substrate is used to obtain the high-quality graphene. The reactive molecular dynamics simulation technology is adopted to imitate the process of graphene deposition and growth on bi-crystal diamond assisted by nickel catalyzed at an atomic level. The effect of the bi-crystal diamond grain boundary on the dynamic behavior of graphene nucleation and growth process is studied. The results demonstrate that the grain boundary carbon atoms can be used as a supplementary carbon source to diffuse into the nickel free surface and participate in the nucleation and growth of graphene. Furthermore, the effect of temperature on the diffusion behavior of carbon atoms is explored, finding that high temperature facilitates the dissociation of atoms in the grain boundary. When the deposition temperature equals 1700 K, it is most conducive to the diffusion of grain boundary carbon atoms in the nickel lattice, which effectively enhances the nucleation density of graphene. Besides, the effect of the deposition carbon source flow rate on the surface quality of graphene is explored, finding that the high-quality graphene surface can be obtained by adopting a lower carbon deposit rate of 1 ps<sup>–1</sup> at 1700 K. In brief, the research results obtained not only provide an effective theoretical model and analysis of the mechanism for diamond grain boundary assisted graphene deposition and growth, but also reveal the regular pattern of influence of deposition temperature and deposition carbon source flow rate on the surface quality of synthesized graphene. The present study can lay a theoretical foundation for the fabrication and application of new functional graphene-polycrystalline diamond heterostructures in the fields of ultra-precision manufacturing and microelectronics.

Behaviors of Carbon Atoms during Plasma Oxidation of 4H-SiC(0001) Surfaces near Room Temperature

The behavior of C atoms during the thermal oxidation of a SiC surface has been a subject of controversy. In this study, we oxidize a SiC surface by atmospheric-pressure plasma treatment near room temperature. We find that more C atoms are included in the SiO2 film after plasma oxidation than after conventional thermal oxidation in O2 ambient at 1000°C, which is probably caused by the difference in oxidation temperature. We demonstrate that plasma oxidation followed by HF etching results in clusters of C atoms remaining at the SiO2/SiC interface. We speculate from X-ray photoelectron spectroscopy results that the C clusters are mixtures of C-C, CH2 and C-O. These C clusters allow us to obtain graphene with few pits because they are an additional C source for forming a homogeneous buffer layer over a SiC surface at elevated temperatures in ultrahigh vacuum.

Behaviors of Carbon Atoms during Plasma Oxidation of 4H-SiC(0001) Surfaces near Room Temperature

Epitaxial graphene on silicon carbide (SiC) is promising for future electronic devices. A well-known scheme for the epitaxial growth of graphene is to heat a Si-face SiC(0001) surface to 1100-1300°C in ultrahigh vacuum (UHV), which results in the sublimation of Si. However, this simple process produces a SiC surface with a pitted morphology, which degrades the transport properties of graphene on it.The pitted morphology originates from the rapid and random sublimation of Si from the terraces that are not covered by a carbon-rich buffer layer. Thus, a strategy to avoid the pitted morphology is to form a homogeneous buffer layer on the entire SiC surface. For this purpose, a key technique is the deposition of carbon at the monolayer level on a SiC substrate in a controllable manner. Researchers have reported various methods to enrich carbon concentration on a SiC surface with a Si face. Some examples are the direct deposition of carbon onto SiC, the rapid annealing of SiC surfaces, and the enhanced removal of silicon atoms by atomic hydrogen, all of which are based on sophisticated UHV techniques.In this study, we demonstrate that carbon atoms are segregated at the SiO2/SiC interface at the monolayer level when a SiC surface, atomically flattened by a solution-based process[1-3], is oxidized by an atmospheric-pressure plasma (He-based plasma with a small amount of O and OH species) at near room temperature. The behavior of carbon atoms has been an object of study in thermal oxidation of SiC. We discuss the mechanism of carbon aggregation at the SiO2/SiC interface occurring in the plasma oxidation at room temperature. After the plasma oxidation, the sample was etched in HF solution to strip off the oxide layer, which leaves a carbon source for graphene growth on the SiC substrate. And we show the carbon-rich surface yields a surface morphology with reduced pits on graphene when it is treated at elevated temperatures in vacuum. We speculate this is achieved by the additional carbon atoms assisting the formation of the buffer layer more uniformly over the surface prior to graphene growth. [1] K. Arima, K. Endo, K. Yamauchi, K. Hirose, T. Ono, and Y. Sano, J. Phys.: Condens. Matter 23, 394202 (2011).[2] A.N. Hattori, T. Okamoto, S. Sadakuni, J. Murata, K. Arima, Y. Sano, K. Hattori, H. Daimon, K. Endo and K. Yamauchi, Surf. Sci. 605, 597 (2011).[3] K. Nishitani, H. Sakane, A.N. Hattori, T. Okamoto, K. Kawai, J. Uchikoshi, Y. Sano, K. Yamauchi, M. Morita and K. Arima, ECS Transactions, 41(6), 241 (2011).

Ab initio Investigations of Carbon Atoms Adsorbed on α-Al2O3 Surfaces in Relation to Graphene Growth

Adsorption behaviors of carbon atoms on Al2O3(0001) are investigated to understand the graphene growth in its initial stage. For the most stable structure of carbon on sapphire, C–O binding is analyzed by examining projected density of states. Increased carbon atoms are found to form cluster structures such as chain-like or distorted graphene-like one. Due to strong C–O binding, at least one carbon atom of the cluster structures binds an oxygen atom of the sapphire surface. Combined with C–C interaction, this result may explain that carbon atoms on sapphire form nanocrystalline graphitic structures within a limited area, observed in experiment.

Graphene Growth and Carbon Diffusion Process during Vacuum Heating on Cu(111)/Al2O3 Substrates

In this study, the behavior of carbon atoms in the annealing/cooling process of graphene/Cu(111) substrates is investigated using photoelectron spectroscopy and secondary ion mass spectroscopy. After the growth of graphene on Cu(111) surfaces, Cu2O was formed at the graphene/Cu interface during transportation through air atmosphere. The Cu2O layer completely disappeared by vacuum annealing at 500 °C. Graphene was decomposed and carbon atoms diffused into the Cu substrate by further elevation of annealing temperature to 950 °C. When the sample was cooled down, the carbon atoms did not segregate on the surface and remained in the Cu substrate. This result indicates the carbon atoms easily diffuse into Cu substrates in vacuum annealing while the amount of diffused carbon atoms in the thermal chemical vapor deposition (CVD) process is smaller, suggesting that the barrier layer, which prevents the diffusion of C atoms, exists on Cu surfaces in the graphene CVD growth.

Diamond-Like Carbon Thin Films with Extremely High Compressive Stress (>8~12GPa) for Advanced CMOS Strain Engineering

ABSTRACTDiamond-like carbon (DLC) films as a new strain-capping material with compressive stress up to 12GPa for strained silicon technology were fabricated by filtered cathodic vacuum arc (FCVA) deposition system. The films’ compositions and bonding structures were characterized using multi-wavelength Raman spectroscopy. The relationship between intrinsic stress and G peak dispersion of the films’ Raman spectra were discussed. The results showed that the bias voltage applied to substrate during deposition determines films’ sp3 bonding content and intrinsic stress. Process compatibility of the DLC films with standard CMOS technology was confirmed by using WDXRF measurement. Also diffusion behavior of carbon atoms in DLC films with copper and silicon was studied with a Cu(200nm)/DLC(40nm)/silicon multilayer structure annealed at 500℃ in N2 atmosphere for an hour. At last, stress induced on silicon surface by DLC strips was characterized using surface sensitive UV-Raman spectroscopy. The results showed that DLC films with extremely high compressive stress have potential application in future CMOS strain engineering.

MD investigation of the collective carbon atom behavior of a (17, 0) zigzag single wall carbon nanotube under axial tensile strain

The collective dynamic behavior of carbon atoms of a (17, 0) zigzag single wall carbon nanotube is investigated under tensile strains by molecular dynamics (MD) simulations. The “slip vector” parameter is used to study the collective motion of a group of atoms and the deformation behavior in three different directions (axial, radial, and tangential) of a (17, 0) carbon nanotube. The variations of radial slip vectors indicate almost all carbon atoms of the (17, 0) carbon nanotube will stay on the cylindrical surface before the yielding of the single wall carbon nanotube (SWNT). Furthermore, the tangential vectors show kinking deformation for the (17, 0) zigzag tube only rarely appears when the crack occurs. Non-symmetrical deformation around a carbon atom along the axial direction also can be found. The variations in the slip vector values of each atom display a symmetrical crack along the horizontal direction and normal to the tube axis. Chain-like structures with 3–4 atoms can be observed, with the number of chain-like structures decreasing before the breakage of the SWNT. The mechanical properties and dynamic behavior of a (17, 0) zigzag SWNT under tensile strain are also compared with that of a (10, 10) armchair tube in our previous study (Weng et al. 2009).

Infrared absorption study on carbon and oxygen behavior in Czochralski silicon crystals

The behavior of carbon atoms in relation to oxygen precipitation in Czochralski silicon crystals subjected to various heat treatments is investigated by means of infrared (IR) spectroscopy. It is shown that carbon atoms enhance and modify oxygen precipitation both at 750 and at 1000 °C. Perturbed [Oi−Ci] C(3) centers, which are unstable at a high temperature (1250 °C), have been observed in the specimen subjected to 750 °C heat treatment by IR spectroscopy. The enhancement effect of carbon atoms is explained in two ways: (i) C(3) centers act as effective heterogeneous seeding sites for oxygen precipitation, and (ii) carbon atoms play a catalytic role by modifying interfacial energies or the point defect ambient at the oxygen precipitate surface.

The co-precipitation of vacancies and carbon atoms in quenched platinum

The geometry of secondary defect structures observed in quenched platinum containing various amounts of carbon is shown to be consistent with a simple model based on the premise of a strong impurity (carbon) atom/vacancy binding energy. When the ratio of carbon atoms to vacancies ( C c / C v ) is large, co-precipitation as platelets on {100} planes occurs; whereas when C c / C v is small, the effects of carbon are still manifest; the defect geometry is dominated by the vacancy behavior, and loops on {111} planes form. Consideration of the mechanism of defect formation on {100} planes leads to conclusions about the structure of the carbon atom/vacancy complex, its migration and stability. An electron microscopy analysis of the {100} defects is in excellent accord with the proposed model. Implications concerning the likely behavior of carbon atoms in a radiation environment are considered, and an interstitial impurity solute segregation effect to vacancy sinks is predicted.

Martensite crystal lattice, mechanism of austenite-martensite transformation and behavior of carbon atoms in martensite

Results concerning the crystal lattice of freshly formed martensite with abnormally low and abnormally high axial ratioc/a as well as the results of neutron diffraction studies of the positions of carbon atoms are reviewed. Mechanisms of the austenite to martensite transformation are considered, which can explain the formation of martensite with differentc/a for the same carbon content in the initial austenite. Occurrence of (011)m transformation twins explains both a lowering ofc/a and an orthorhombic distortion of the martensite lattice. It follows from analysis of the experimental results that the well known dependence of martensite lattice parameters and axial ratio on the carbon content relate to a partly disordered distribution of carbon atoms between three sublattices of the octahedral interstitial sites (ois). Using new values of the concentration coefficients of linear expansion of the martensite lattice, calculated for the case of carbon atoms distribution only in a single sublattice of the ois, leads to some corrections of the previous temperature and concentration dependences of order-disorder processes in martensite. Phenomena such as the reversible change of axial ratio due to the redistribution of carbon atoms between their normal positions and “traps” in irradiated martensite are described.