Formation of diamond-like carbon by fs laser irradiation of organic liquids

The high intensities generated in femtosecond (fs) laser interactions offers the possibility of novel formation routes for diamond and diamond-like carbon materials. Coulomb explosion, a common phenomenon in intensive fs irradiation, has recently been shown to lead to a direct graphite–diamond phase transition on the surface of graphite. In this paper we report the results of fs irradiation of a variety of liquid organic compounds at intensities in the Coulomb explosion regime. The products of laser-induced chemistry under these conditions have been studied using visual/surface enhanced Raman and transmission electron microscopy (TEM). Surface enhanced Raman spectra/TEM show that an intermediate diamond phase, trans-polyacetylene chains and amorphous carbon are present after fs irradiation of liquid alkanes. We also find that the diamond component can be enhanced by irradiation in the presence of certain transition metals; however the origin of this effect is still uncertain. Diamond films deposited in this way are found to exhibit a nano-assembled structure involving individual nanodiamonds extending over an area of about 1 cm². This process represents a wet chemical method for room temperature formation of diamond-like carbon films     

Improvement on the electrochemical characteristics of graphite anodes by coating of the pyrolytic carbon using tumbling chemical vapor deposition

The electrochemical characteristics of graphite coated with pyrolytic carbon materials using tumbling chemical vapor deposition (CVD) process have been studied for the active material of anodes in lithium ion secondary batteries. Coating of pyrolytic carbons on the surface of graphite particles, which tumble in a rotating reactor tube, was performed through the pyrolysis of liquid propane gas (LPG). The surface morphology of these graphite particles coated with pyrolytic carbon has been observed with scanning electron microscopy (SEM). The surface of graphite particles can well be covered with pyrolytic carbon by tumbling CVD. High-resolution transmission electron microscopy (HRTEM) image of these carbon particles shows that the core part is highly ordered carbon, while the shell part is disordered carbon. We have found that the new-type carbon obtained from tumbling CVD has a uniform core (graphite)-shell (pyrolytic carbon) structure. The electrochemical property of the new-type carbons has been examined using a charge-discharge cycler. The coating of pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity by 47.5%. Cyclability and rate-capability of theses carbons with the core-shell structure are much better than those of bare graphite. From electrochemical impedance spectroscopy (EIS) spectra, it is found that the coating of pyrolytic carbon on the surface of graphite causes the decrease of the contact resistance in the carbon electrodes, which means the formation of solid electrolyte interface (SEI) layer is suppressed. We suggest that coating of pyrolytic carbon by the tumbling CVD is an effective method in improving the electrochemical properties of graphite electrodes for lithium ion secondary batteries.     

Diamond nanoparticles to carbon onions transformation: X-ray diffraction studies

Carbon onions prepared by high temperature annealing of ultradispersed diamond nanoparticles of about 5 nm in average diameter have been studied by X-ray diffraction using synchrotron radiation. The X-ray diffraction patterns show transformation of the diamond nanoparticles with sp³ bonds into spherical carbon onions containing remaining diamond-like core and then into polyhedral onions with facets on their outer part and pure sp² graphitic bonds. The prepared onions form concentric-shell particles which comprise of about ten shells with an intershell distance of 0.35-0.36 nm. The large intershell distance suggests a considerable reduction in intershell interaction when compared to perfect graphite. The X-ray data are related to the previously performed studies by electron energy-loss spectroscopy and electron spin resonance.     

Dependence of Diamond Transition on Microtexture in Graphite Starting Materials under Shock Compression

The effect of microtexture on diamond transition was examined for graphite starting materials under shock compressions of 50 to 60 GPa and 80 to 90 GPa. Each of the starting materials used in the present study possessed a fully homogeneous microtexture. To distinguish the effect of microtexture from that of other experimental parameters, the shock conditions were standardized for all specimens tested. Three graphite materials—a glassy carbon, a carbon black, and a natural graphite—were selected and shock compressed using a quenching technique to generate conditions common to all samples. Detailed characterization by transmission electron microscopy and electron energy-loss spectroscopy revealed a clear tendency: The lower the crystallinity and crystallite size of the starting graphite, the more easily the graphite transformed to diamond when the transition mechanism was reconstructive.     

Shock compressibility and shock-induced phase transitions of C60 fullerite

Shock compressibility of C₆₀ fullerite and sound velocities in shock-compressed fullerite were experimentally studied at the pressures range up to 50 GPa. In our experiments, we used polycrystalline C₆₀ specimens with a density of 1.64 g/cc. The Hugoniot of C₆₀ fullerite had a set of peculiarities, which may be attributed to a series of polymorphic phase transitions. The jump of sound velocity in shocked C₆₀ at pressure 9 GPa indicates the formation of a rather hard carbon phase. It is possible to assume that it is a polymerized C₆₀ phase. In the region of pressures 9–25 GPa, destruction of this phase and formation of a graphite-like carbon occurs. With further growth of shock pressure, phase transition of the graphite-like carbon to a diamond-like phase is observed with a transition onset pressure 25 GPa. If shock pressures are higher than 33 GPa, Hugoniot of C₆₀ fullerite is determined by the thermodynamic properties of the diamond-like phase.     

Microstructural features and thermal stability of alumina-bonded nano-polycrystalline diamond synthesized by detonation sintering

In this work, alumina-bonded nano-polycrystalline diamond (NPD) was synthesized by detonation sintering in the temperature range of 3000–3500 K and pressure range of 15–25 GPa. The microstructures and thermal stability of the NPD detonation sintered at 3255.05 K and 24.51 GPa were studied, and are described herein. Transmission electron microscopy and electron diffraction revealed that the polycrystalline diamond has a unique formation process and no graphitization. Scanning electron microscopy indicated that the size of polycrystalline particles increased in samples 2, 3, and 4. And thermogravimetric analysis indicated that the thermal stability of the diamond particles was enhanced. The 18% mass loss of specimen corresponded to the oxidation and decomposition of the amorphous carbon and carbon-containing compounds synthesized by detonation. Finally, graphitization calculations showed that the graphitization probability of polycrystalline diamond produced at the temperature of 3255.05 K and pressure of 24.51 GPa was 15.04%.     

X-ray diffraction of multiwalled carbon nanotube under high pressure: Structural durability on static compression

The structural durability of multiwalled carbon nanotubes under hydrostatic and non-hydrostatic compression was examined by in situ X-ray powder diffraction at room temperature. No interlayer interaction such as sp³ hybridization that could lead to hexagonal diamond in graphite was observed under compression up to 52 GPa, even though the nanotubes were similar in compressibility to graphite. This result could be attributed to the nested structure, which makes the interlayer stacking of carbon atoms take on an irregular arrangement. Despite the history of non-hydrostatic compression, electron microscopic observation revealed that the structure remained nested tubular. This reversibility suggests the nanotubes have strong durability on non-hydrostatic compression under extreme pressures.     

Transition of graphite into diamond in a solid phase under the atmospheric pressure

Transition of small-size graphite particles into diamond has been experimentally detected under atmospheric pressure. The transformation in a solid phase occurs when graphite heated up to 2500 K is rapidly cooled. Electron and X-ray diffraction methods were applied to investigate the solid-phase transition products. The transition of graphite into diamond may be explained whether by the setting up of high pressures or by retention in quenching the structures that are characteristic for elevated temperatures.     

Nanosized carbon forms in the processes of pressure-temperature-induced transformations of hydrocarbons

The products of thermal conversions of naphthalene, anthracene, pentacene, perylene, and coronene at 8 GPa in the temperature range up to 1300 °C have been studied by scanning electron and high-resolution transmission electron microscopies. As a result, it has been established that various nanometer-sized carbon species (spherical and coalesced two-core onion-like carbon particles, faceted polyhedral particles, graphitic ribbons, graphitic folds, and nanocrystalline diamonds) are present in the conversion products together with micron-sized crystallites of graphite and diamond.     

高压下CL-20拉曼光谱和红外光谱相变研究

为了更好地认识和了解CL-20晶体结构演变规律和相变行为,利用金刚石对顶砧超高压实验技术,在0~50GPa下,研究了高压下ε-CL-20的原位拉曼光谱和红外光谱。结果表明,CL-20晶体在整个加压过程中存在两个相变,第一个相变发生在4.2~7.5GPa,认为是ε相到对称性更低的γ相转变,相变产生的原因是在压强的作用下,笼环外的硝基方向发生改变,电子云密度重置导致的分子构型转变;第二个相变发生在14.2~18.9GPa,属于γ相到ζ相的晶体结构转变;卸压后,拉曼和红外光谱恢复常压状态,表明CL-20晶体在研究压强范围内的相变过程是可逆的。     

A novel mechanical cleavage method for synthesizing few-layer graphenes

A novel method to synthesize few layer graphene from bulk graphite by mechanical cleavage is presented here. The method involves the use of an ultrasharp single crystal diamond wedge to cleave a highly ordered pyrolytic graphite sample to generate the graphene layers. Cleaving is aided by the use of ultrasonic oscillations along the wedge. Characterization of the obtained layers shows that the process is able to synthesize graphene layers with an area of a few micrometers. Application of oscillation enhances the quality of the layers produced with the layers having a reduced crystallite size as determined from the Raman spectrum. Interesting edge structures are observed that needs further investigation.     

Hydrogen-free deposition of diamond-like coatings from a graphite source

We successfully deposited diamond-like hydrogen-free carbon coatings by pulsed electron beam vapor deposition (by pseudospark discharges), which is a novel technique in this field. Transmission electron microscopy and electron diffraction demonstrate that these layers consist essentially of two components: (i) amorphous spherical particles with an average diameter of 50 nm, which form an amorphous particle network, and (ii) single crystalline diamond crystallites of up to 10 μm in diameter. The investigation of the amorphous regions by parallel electron energy loss spectroscopy permits to distinguish amorphous carbon and amorphous diamond.     

Surface characteristics of selected carbon materials exposed to supercritical water

Interactions of supercritical water (SCW) with several carbon (including polycrystalline graphite, highly ordered pyrolytic graphite (HOPG), pyrolytic carbon, and diamond) and selected ceramic materials under oxic or anoxic conditions are investigated. Wettability and surface/material properties of samples, before and after the SCW exposure, are characterized with sessile drop contact angle measurement, profilometry, XPS, XRD, and ToF-SIMS. All tested ceramic materials became more hydrophilic during the SCW exposure, mainly due to hydrolysis reactions. Carbon samples exposed to oxic SCW became more hydrophilic as a result of surface oxidation. Carbon materials, except HOPG and diamond, became more hydrophobic when exposed to anoxic SCW because of degradation of hydrophilic oxygen functionalities. HOPG and diamond became more hydrophilic after anoxic SCW exposure mainly due to the removal of hydrophobic hydrocarbon contaminants. Hydrophobicity of different carbon samples exposed to SCW are explained based on the abundance of surface hydrophilic sites (i.e., oxygen functionalities, reactive carbon dangling bonds or defects), hydrocarbon impurities, and surface roughness.     

Low-Temperature Synthesis of Diamond-like Carbon Films on Steel Substrates by Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition

Using microwave electron-cyclotron-resonance plasma-enhanced chemical vapor deposition, diamond-like carbon films were directly grown at low temperatures (lessthan equal to400°C) on Fe-based alloy substrates without diamond seeding or use of a template layer. A single, broad line in the Raman spectra was observed in the region of 1328-1335 cm^-1 for films grown in gas mixtures with a ratio of CH₄:H₂ greaterthan equal to 2%. In contrast, disordered carbon and graphite phases appeared in the spectra for film grown with a concentration of 20% CH₄ in hydrogen. Diamond nucleation with an amorphous carbon layer was observed in the initial growth stage, while many diamond particles with irregular morphological features were observed on the surface of thicker films. These growth features are a consequence of the catalytic nature of the Fe-based substrate.     

Nanoindentation of diamond,graphite and fullerene films

The recently developed method of nanoindentation is applied to various forms of carbon materials with different mechanical properties, namely diamond, graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nanoindentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force–depth curve the Young's modulus as well as the hardness were obtained reflecting a very large hardness of 95 GPa and 117 GPa for the {100} and {111} crystal surfaces, respectively. Investigation of a layered material such as highly oriented pyrolytic graphite again showed elastic deformation for small indentation depths but as the load increased, the induced stress became sufficient to break the layers after which again an elastic deformation occurred. The Young’s modulus was calculated to be 10.5 GPa for indentation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the softer material of a molecular crystalline solid formed by C₆₀ molecules. The hardness values of 0.24 GPa and 0.21 GPa for these films grown by layer epitaxy and island growth on mica and glass, respectively, vary with the morphology of the C₆₀ films. In addition to the experimental work, molecular dynamics simulations of the indentation process have been performed to see how the tip–crystal interaction turns into an elastic deformation of atomic layers, the creation of defects and nanocracks. The simulations are performed for both graphite and diamond but, because of computing power limitations, for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulations capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However, if the speed of indentation is increased, the simulations indicate that permanent displacements of atoms are possible and permanent deformation of the material takes place.     

Conversion of polycyclic aromatic hydrocarbons to graphite and diamond at high pressures

The polycyclic aromatic hydrocarbons (PAH): naphthalene, anthracene, pentacene, perylene, and coronene were submitted to temperatures up to 1500 °C at 8 GPa. To avoid catalytic action of metals on thermal conversion, graphite was used as container material. Moreover, graphite is very permeable to the gaseous products of thermal decomposition of PAH. The resulting thermal transformations and their evolution were studied by X-ray diffraction, Raman spectroscopy and scanning electron microscopy as a function of temperature for 60-s treatments. The nature of the initial compounds clearly affects the products of the different stages of carbonization and the first steps of graphitization. This becomes hardly discernible in the final stages of graphitization above 1000 °C. Above 1200 °C, graphite with high crystallinity forms in all cases. The temperature of the beginning of diamond formation does not seem to be influenced by the nature of the initial PAH and is equal to ∼1280 °C for all investigated compounds. Diamonds formed from the PAH are high-quality 5-40 μm single crystals. The p,T values of diamond formation here obtained are significantly lower than those previously known for direct graphite-diamond transformation.     

Ru-doped nanostructured carbon films

Pure and Ru-doped carbon films are deposited on Si (100) substrates by electron cyclotron resonance chemical vapor deposition. The films are characterized by transmission electron microscopy, electron energy loss spectroscopy, energy dispersive X-ray spectroscopy and atomic force microscopy. In both the pure and Ru-doped samples, diamond nanocrystallites are formed in amorphous carbon matrices. The Ru-doped film contains much smaller diamond nanocrystallites (approx. 3 nm) than the pure samples (approx. 11 nm). Lower surface roughnesses are observed in both pure and Ru-doped samples as compared to other reported nanocrystalline diamond films. The conductivity of the Ru-doped film is significantly higher than that of the pure film. The results show that Ru-doped nanocrystalline diamond films have unique structures and properties as compared to pure nanocrystalline diamond films or metal doped diamond-like carbon films, which may offer advantages for electrochemical, optical-window, field emission or tribological applications.     

Structural characterization and properties enhancement of diamond-like carbon films synthesized under low energy Ne bombardment

Diamond-like carbon (DLC) films were synthesized by Ar⁺ sputtering graphite with concurrent Ne⁺ bombardment. Transmission electron microscopy diffraction revealed that some diamond crystals were distributed in the amorphous matrix of DLC films synthesized under Ne⁺ bombardment at an energy of 200 eV and ion current density of 0.19 mA cm⁻². X-ray photo electron spectra showed that the valence band of the DLC films was similar to that of diamond, and the binding energy of electrons was 284.9 eV. The DLC films possessed a high hardness of 42.14 GPa and excellent wear resistance. It was confirmed that the wide atomic intermixed film-substrate interface meant that the DLC films would improve greatly the wear-resistant properties of AISI 52100 steel if the DLC films were coated on its surface.     

XRD and TEM study of high pressure treated single-walled carbon nanotubes and C60-peapods

In situ synchrotron X-ray diffraction measurements of single-walled carbon nanotube and C₆₀-peapod samples under high pressures up to 13 GPa and at high temperature were carried out. Anisotropical shrinkages of their bundle two-dimensional lattices by compression at room temperature were observed. It was found that the lattices recover original forms reversibly upon pressure release. It was also found that irreversible phase transformations occur by raising temperature at the highest pressure. The high pressure and high temperature treated samples were examined by X-ray diffraction, transmission electron microscope, and Raman measurements. It was indicated by transmission electron microscope observation that hexagonal diamond is able to be synthesized by high pressure and high temperature treatment of C₆₀-peapods.     

Comparative assessment of the effect of carbon-based material surfaces on blood clotting activation and haemolysis

This work presents results of haemolytic reaction and activation of blood clotting in contact with various carbon materials. Synthetic graphite (SG), glass-like carbon (GLC), pyrolytic carbon (LTI), pyrolytic graphite (PG) and diamond-like carbon (DLC) were investigated. Haemolytic reaction was determined by the assessment of haemolytic index (HI), haemolysis percentage and by the morphological evaluation of erythrocytes. The results indicated that, independently of the methods used and the materials studied, values of haemolysis and morphological appearances of erythrocytes were in the range of standards. It was found that LTI carbon surface prolongs the most effectively clotting activation among the carbon materials studied. The most distinct changes in haemolysis were noted for synthetic graphite, while the smallest ones for LTI carbon. Interfacial bonding energy between GLC surface and human fibrinogen was slightly lower than that for LTI carbon, whereas its total surface energy reached the highest value among the carbon materials studied. The LTI and GLC samples were shown to be the most effective in preventing thrombus formation and in prolonging the clotting time as compared with the other carbon surfaces.