High-pressure synthesis of a novel form of endohedral Li diamond from Li graphite intercalation compound

A novel form of hexagonal diamond containing Li atoms in the open rooms surrounded by sp³-bonded carbon atoms was successfully synthesized from a Li graphite intercalation compound under high pressure, as had been predicted by theoretical studies. High-pressure experiments with LiC₆ were performed in the pressure range from 0.1 MPa to 43 GPa using a diamond-anvil cell. In situ X-ray diffractometry and optical microscopy revealed that LiC₆ was transformed to a hexagonal-diamond form without losing Li atoms. The c-axis of the hexagonal-diamond form was considerably longer than that of the hexagonal diamond transformed from pure graphite, which was consistent with the predicted structure of the endohedral Li diamond. The observed high-pressure form exhibited a golden metallic gloss, which was also consistent with the calculated metallic band structure.     

Modification of X-ray excited photoelectron and C KVV Auger spectra during radiative carbonization of poly(vinylidene fluoride)

Modifications of the photoelectron and C KVV Auger spectra during the long-term surface degradation of partially crystalline PVDF under simultaneous soft X-ray and electron followed by ion irradiation are reported. Deep radiative carbonization brings about the formation of carbynoid structures (chain-like carbon) in the surface, while the number of interchain cross-links is insignificant. As a result, the shape of the electron emission spectra of carbon in the carbonized sample essentially differs from that of graphite and PVDF. The ion bombardment of the carbonized sample destroys one-dimensional structure due to the formation of cross-links. Thus, carbon atoms transit into sp²-hybrid state and, therefore, the photoelectron and Auger spectra show features characteristic for microcrystalline graphite.     

Percolative transition in carbon-ion implanted type II<Emphasis Type="Italic">a</Emphasis> diamond

Experimental data for the conductivity of type IIa diamond specimens implanted at low temperatures with carbon ions, followed by high temperature annealing, have been analyzed using hopping and percolation theories in the vicinity of the insulator-metal transition. Near the transition it appears that conductivity occurs viasp ²-bonded graphitic clusters which are randomly distributed in thesp ³-bonded diamond matrix. A conductivity crossover between the Mott and Efros-Shklovskii VRH laws has been observed on the insulating side of the transition.     

Liquid methane at extreme temperature and pressure: Implications for models of Uranus and Neptune

We present large scale electronic structure based molecular dynamics simulations of liquid methane at planetary conditions. In particular, we address the controversy of whether or not the interior of Uranus and Neptune consists of diamond. In our simulations we find no evidence for the formation of diamond, but rather sp ²-bonded polymeric carbon. Furthermore, we predict that at high temperature hydrogen may exist in its monoatomic and metallic state. The implications of our finding for the planetary models of Uranus and Neptune are in detail discussed.     

ELECTRONIC STRUCTURE STUDIES OF THE C60 FILM CONDENSED ON 2H-MoS2(0001) SURFACE

The electronic structure and vibrational spectrum of the C₆₀ film condensed on a 2H- MoS₂(0001) surface have been investigated by X-ray photoelectron spectroscopy (XPS), ul-traviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES) and infrared high-resolution electron-energy-loss spectroscopy (HREELS). AES analysis showed that at low energy side of the main transition, C₆₀ contains a total of three peaks just like that of graphite. However, the energy position of the KLL main Auger transition of C₆₀ looks like that of diamond, indicating that the hybridization of the carbon atoms in C₆₀ is not strictly in sp²- bonded state but that the curvature of the molecular surface introduces some sp²pz- bonded character into the molecular orbitals. XPS showed that the C 1s binding energy in C₆₀ was 285.0eV, and its main line was very symmetric and offered no indication of more than a single carbon species. In UPS measurement the valence band spectrum of C₆₀ within 10eV below the Fermi level (E_F) shows a very distinct five-band structure that character-izes the electronic structure of the C₆₀ molecule. HREEL results showed that the spectrum obtained from the C₆₀ film has very rich vibrational structure. At least, four distinct main loss peaks can be identified below 200 meV. The most intense loss was recorded at 66 meV, and relatively less intense losses were recorded at 95, 164 and 197meV at a primary energy of electron beam E_P = 2.0eV. The other energy-loss peaks at 46, 136, 157 and 186meV in HREEL spectrum are rather weak. These results have been compared to infrared spectrum data of the crystalline solid C₆₀ taken from recent literatures.     

Study of diamond film by dynamic nuclear polarization-enhanced13C nuclear magnetic resonance spectroscopy

Three chemical vapor deposited diamond films were studied by dynamic nuclear polarization (DNP)-enhanced high-resolution solid-state¹³C nuclear magnetic resonance (NMR) spectroscopy. Enhanced¹³C direct-polarization spectra of diamond films were obtained by irradiating the samples with microwaves at or near electron spin resonance Larmor frequency of carbon center free radicals. No NMR signal for sp² hybridized carbons could be observed. From the curve of the DNP enhancement as a function of frequency, it is found that the dominant DNP mechanism is the solid-state effect. The¹³C cross-polarization spectrum, which is an evidence for existence of the proton defect in the lattice of diamond films, is much broader than the¹³C single pulse spectrum. The reason is discussed shortly.     

Substrate effects on the microstructure of hydrogenated amorphous carbon films

In this work, plasma enhanced chemical vapour deposition was used to prepare hydrogenated amorphous carbon films (a-C:H) on different substrates over a wide range of thickness. In order to observe clear substrate effect the films were produced under identical growth conditions. Raman and near edge X-ray absorption fine structure (NEXAFS) spectroscopies were employed to probe the chemical bonding of the films. For the films deposited on silicon substrates, the Raman I_D/I_G ratio and G-peak positions were constant for most thickness. For metallic and polymeric substrates, these parameters increased with film thickness, suggesting a change from a sp³-bonded hydrogenated structure to a more sp² network, NEXAFS results also indicate a higher sp² content of a-C:H films grown on metals than silicon. The metals, which are poor carbide precursors, gave carbon films with low adhesion, easily delaminated from the substrate. The delamination can be decreased/eliminated by deposition of a thin (∼10 nm) silicon layer on stainless steel substrates prior to a-C:H coatings. Additionally we noted the electrical resistivity decreased with thickness and higher dielectric breakdown strength for a-C:H on silicon substrate.     

The M4,5- VV Auger spectrum of silver: An interpretation based on quasi-atomic final states

The M_4,5- VV Auger spectrum of silver has been studied under high-resolution conditions. The relative energies and intensifies of the Auger lines calculated for a 4 d⁸ final state configuration account well for the experimental spectrum. This quasi-atomic behaviour is due to the fact that the effective two-hole Coulomb interaction is larger than the bandwidth. The influence of the solid state on the width of the Auger lines is discussed.     

A new study on the catalytic mechanism of Fe-based alloys in diamond formation by Mössbauer spectroscopy

M?ssbauer spectroscopy has been used to systemically study the catalytic mechanism of Fe-based alloys in diamond formation at high temperature–high pressure (HTHP) for the first time. M?ssbauer spectra reveal the magnetic state of the 3d electrons of a Fe atom in the Fe-based alloy catalyst during diamond formation at HTHP. During carburization at lower temperatures than that required for diamond formation and diamond formation in the diamond-stability region using Fe-based alloys as a catalyst, both the quadrupole splitting QS and the isomer shift IS change from negative to positive, especially reaching a state in which they are zero. It was indicated that the state of the 3d-shell electrons of the iron atom changes greatly during carburization and diamond formation and that the incomplete 3d sub-bands of Fe atoms in the catalyst alloys could be filled up in proper order by electrons of interstitial carbon atoms. During diamond formation, the unpaired 3d-shell electrons of an iron atom in the Fe-based alloy absorb and interact with 2P_z electrons of the carbon atoms. There exist a Fe–C bonding and an electron charge transfer stage. The 2P_z electrons of the carbon atoms could be dragged into the metal atoms in the catalyst alloy and would make a transition of triangular (sp²π) hybridization of valence electrons to tetrahedral (sp³) hybridization of valence electrons (a transition of sp²π bonds of graphite to sp³ bonds of diamond), resulting in a transition of graphite structure to diamond. Although the conclusion of this study is strictly applicable only to Fe-based alloy catalysts, it could be considered more general because of the chemical similarities between the transition elements used as solvent catalysts for diamond synthesis. Received: 2 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Simulations of structures of amorphous SixC1−x films

Amorphous Si_xC_1−x films possess the potential to improve wear performance in humid atmospheres and at higher temperatures. But some experimental work on the films showed that silicon contents greatly influenced their microstructures and mechanical properties. Therefore, simulations of molecular dynamics were carried out to predict structures of the Si_xC_1−x films at different silicon contents. The results show that the sp³/sp² ratio of all the films increases, but the stiffness of the films is decreasing with an increase in silicon contents. Moreover, silicon atoms are almost surrounded by carbon atoms, which is in agreement with the experiments.     

Substitutional impurity in the graphene quantum dots

The process of formation of the localized defect states due to substitutional impurity in sp²-bonded graphene quantum dot is considered using a simple tight-binding-type calculation. We took into account the interaction of the quantum dot atoms surrounding the substitutional impurity from the second row of elements. To saturate the external dangling sp² orbitals of the carbon additionally 18 hydrogen atoms were introduced. The chemical formula of the quantum dot is H₁₈C₅₁X, where X is the symbol of substitutional atom. The position of the localized levels is determined relative to the host-atoms (C) ε_p energies. We focused on the effect of substitutional doping by the B, N and O on the eigenstate energies and on the total energy change of the graphene dots including for O the effect of lattice distorsion. We conclude that B, N, and O can form stable substitutional defects in graphene quantum dot.     

Characterization of doped diamond-like carbon films deposited by hot wire plasma sputtering of graphite

Diamond-like carbon (DLC) films doped with nitrogen and oxygen were deposited on silicon(100) and polytetrafluoroethylene (PTFE) substrates by hot wire plasma sputtering of graphite. The morphology and chemical composition of deposited films has been characterized by scanning electron microscopy, XPS, Auger, FTIR spectroscopy and micro-Raman scattering. Plasmon loss structure accompanying the XPS C 1s peak and electron energy loss spectroscopy (EELS) in reflection mode was used to study the fraction of sp³ bonded C atoms and the density of valence electrons. Raman spectra show two basic C–C bands around 1575 cm^-1 (G line) and 1360 cm^-1 (D line) . Auger depth profiling spectroscopy was used to measure the spatial distributions of C, N and O atoms in the surface layer of DLC films. The fraction of sp³ bonded atoms of about 40% was detected in DLC films by XPS plasmon loss and EELS techniques. Nitrile and iso-nitrile groups observed in FTIR spectra demonstrated the existence of sp bonded carbon in doped DLC films. The typical for DLC films specific density 1.7–1.8 g/cm³ was obtained from EELS and XPS data. PACS 52.77.Dq; 81.65.-b; 82.80.Pv     

金刚石表面无定形碳氢薄膜生长的分子动力学模拟

利用分子动力学模拟方法研究了CH₂基团轰击金刚石(111)面所形成的无定形碳氢薄膜(a-C:H)的生长过程. 结构分析表明, 得到的无定形碳氢薄膜中碳原子的局域结构(如C–C第一近邻数)与其中氢原子的含量密切相关. CH₂ 基团入射能量的增加会导致得到的薄膜的氢含量降低, 从而改变薄膜中类sp³成键碳原子的比例.     

Diamond deposition on siliconized stainless steel

Silicon diffusion layers in AISI 304 and AISI 316 type stainless steels were investigated as an alternative to surface barrier coatings for diamond film growth. Uniform 2 μm thick silicon rich interlayers were obtained by coating the surface of the steels with silicon and performing diffusion treatments at 800 °C. Adherent diamond films with low sp² carbon content were deposited on the diffused silicon layers by a modified hot filament assisted chemical vapor deposition (HFCVD) method. Characterization of as-siliconized layers and diamond coatings was performed by energy dispersive X-ray analysis, scanning electron microscopy, X-ray diffraction and Raman spectroscopy.     

Chemical state of carbon atoms on the surface of nanodiamond particles

Auger electron spectroscopy study of the chemical state of carbon atoms on the surface of nanodiamond particles is performed. Auger spectra of nanodiamond particles indicate that carbon atoms in nanodiamond are in the same state as those in graphite, i.e., in the σ _s ¹ σ _p ² π¹ state, but the π band is displaced 1 eV in energy below the Fermi level. The surface of nanodiamond particles is inert with respect to the ambient medium.     

Electrochemical properties of undoped CVD diamond films

The electrochemical properties of undoped diamond polycrystalline films grown on tungsten wire substrates using methanol as a precursor are described. The diamond film quality was changed by introducing sp²-bonded non-diamond carbon impurity through adjustment of the methanol-to-hydrogen (C/H) source gas ratio used for diamond growth.The electrodes were characterized by Raman spectroscopy, scanning electronic microscopy (SEM) and cyclic voltammetry (CV).Diamond coated tungsten wires were then used as a working electrode to ascertain their electrochemical behavior in electrolytic medium. Electrochemical windows of these films were found to be suitable in the potential range of [−2.5 V, +2.2 V] vs. Ag/AgCl in acid medium (0.1 M KCl).The electrochemical behavior was evaluated also using the Fe(CN)₆^3−/4−redox couple.The results demonstrate that the grain boundaries and sp²-hybridized carbon impurity can have a significant influence on electrochemical window of undoped diamond electrodes. It was observed that with increasing sp² carbon impurity concentration the electrochemical window decreases.     

Evaluation of bacterial adhesion on Si-doped diamond-like carbon films

Diamond-like carbon (DLC) films as biomaterial for medical devices have been attracting great interest due to their excellent properties such as hardness, low friction and chemical inertness. It has been demonstrated that the properties of DLC films can be further improved by the addition of silicon into DLC films, such as thermal stability, compressive stress, etc. However no research work on anti-bacterial properties of silicon-doped diamond-like carbon films has been reported. In this paper the surface physical and chemical properties of Si-doped diamond-like carbon films with various Si contents on 316 stainless steel substrate prepared by a magnetron sputtering technique were investigated, including surface topography, surface chemistry, the sp³/sp² ratio, contact angle, surface free energy, etc. Bacterial adhesion to Si-doped DLC films was evaluated with Pseudomonas aeruginosa, Staphylococcus epidermidis and Staphylococcus aureus which frequently cause medical device-associated infections. The experimental results showed that bacterial adhesion decreased with increasing the silicon content in the films. All the Si-doped DLC films performed much better than stainless steel 316L on reducing bacterial attachment.     

Ultraviolet Raman analysis of the formation of diamond from C60

The ultraviolet (257 nm) Raman spectrum of C₆₀ compressed to 30 GPa in a Mao–Bell diamond anvil cell with no pressure transmitting medium at ambient temperature indicates the formation of diamond after release of pressure. Previously, more extreme non-hydrostatic compression was reported to be required to form diamond from C₆₀. These results provide confirmation of the transformation of C₆₀ to diamond upon non-hydrostatic compression at room temperature and illustrate the utility of UV Raman spectroscopy for the analysis of carbon phases containing both sp² and sp³ bonding.     

Effect of titanium incorporation on the structural, mechanical and biocompatible properties of DLC thin films prepared by reactive-biased target ion beam deposition method

Amorphous diamond like carbon (DLC) and titanium incorporated diamond like carbon (Ti-DLC) thin films were deposited by using reactive-biased target ion beam deposition method. The effects of Ti incorporation and target bias voltage on the microstructure and mechanical properties of the as-deposited films were investigated by means of X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy and nano-indentation. It was found that the Ti content in Ti-DLC films gets increased with increasing target bias voltage. At about 4.2 at.% of Ti, uniform sized well dispersed nanocrystals were seen in the DLC matrix. Using FFT analysis, a facility available in the TEM, it was found that the nanocrystals are in cubic TiC phase. Though at the core, the incorporated Ti atoms react with carbon to form cubic TiC; most of the surface exposed Ti atoms were found to react with the atmospheric oxygen to form weakly bonded Ti-O. The presence of TiC nanocrystals greatly modified the sp³/sp² hybridized bonding ratio and is reflected in mechanical hardness of Ti-DLC films. These films were then tested for their biocompatibility by an invitro cell culturing test. Morphological observation and the cell proliferation test have demonstrated that the human osteoblast cells well attach and proliferate on the surface of Ti incorporated DLC films, suggesting possible applications in bone related implant coatings.     

Effect of nickel on the structure and bond type in amorphous diamond-like carbon films

Experimental data are presented from studies of the structure and bond type of carbon atoms in amorphous carbon-nickel films deposited from pulsed vacuum-arc discharge plasma sources. X-ray photoelectron spectroscopy was used. The characteristics of the plasmon loss spectra depend significantly on the deposition parameters. Carbon exists in a mixed sp²+sp³ hybridized state in the carbon–nickel films. The ratio of sp³/sp² carbon bonds increases when the nickel content is reduced (from 5.5 to 1.0 atomic %) and the deposition angle is increased. The structure closest to that of diamond was with a substrate bias voltage of –80 to –100 V and a deposition angle of 90°.