Growth of highly oriented graphite films at room temperature by pulsed laser deposition using carbon-sulfur targets

Carbon-sulfur films were grown by pulsed laser deposition at room temperature using different graphite-sulfur mixtures as targets. The structure of the films was characterized by X-ray diffraction and transmission electron microscopy. The composition and the chemical bonds were analyzed by Rutherford-backscattering spectroscopy, X-ray photoelectron spectroscopy and energy dispersive X-ray analysis. The films were composed of amorphous carbon with sp²-, sp³- and S-C-C-S bonds and textured graphite on the top of the film. The thin graphite layer on top of the carbon-sulfur films is highly oriented, comparable to highly oriented pyrolytic graphite, and free of sulfur in the graphite lattice. The lateral size of the oriented graphite grains in the films was up to 8 μm. Magnetic measurements reveal that the films prepared under the conditions of our study show neither magnetic ordering nor superconductivity in the studied temperature range T > 2 K.     

Anomalous electrical transport properties of amorphous carbon films on Si substrates

Amorphous carbon (a-C) films are deposited on n-Si substrates at different temperatures using pulsed laser deposition. Some anomalous current-voltage (I-V) characteristics of the a-C/n-Si are reported. The films deposited at 27 °C have an apparent voltage-induced switch effect, and the value of the switch voltage decreases with increasing temperature. However, the I-V characteristics of the a-C/n-Si deposited at 300 °C and 500 °C are completely different from those deposited at 27 °C. The anomalous I-V characteristics should be of interest for various applications such as field effect devices. In addition, the magnetoresistance (MR) and the resistance of the a-C/n-Si have been studied. Finally, we interpret the anomalous I-V characteristics and MR observed by use of energy band theory.     

Structural, electrical and magnetic properties of carbon-nickel composite thin films

Carbon-nickel composite thin films (600 nm thick) were prepared by dc magnetron sputtering of Ni and C at several temperatures (25-800 °C) on oxidized silicon substrates. By transmission electron microscopy it was found that the composite consisted of Ni (or Ni₃C) nanoparticles embedded in a carbon matrix. The metallic nanoparticles were shaped in the form of globular grains or nanowires (of the aspect ratio as high as 1:60 in the sample prepared at 200 °C). The carbon matrix was amorphous, or graphite-like depending on deposition temperature. At low deposition temperatures T_S (25-400 °C) the Ni₃C nanoparticles were of hcp phase. Samples prepared at T_S ? 600 °C contained ferromagnetic fcc Ni nanoparticles. A correlation was found between the structural, electrical and magnetic properties of the composites. To characterise the films, dependences, such as resistivity vs. temperature, current vs. voltage, differential conductivity vs. bias voltage, and magnetoresistivity, were determined. For example, the tunneling effect was found in samples in which the metallic nanoparticles were separated by 2-3 nm thick amorphous carbon. When the metallic nanoparticles were connected by graphite-like carbon regions (having a metallic conductivity, in contrast to a-C), the temperature coefficient of the resistivity became slightly positive. An anisotropic magnetoresistivity of ∼0.1% was found in the sample that contained ferromagnetic columnar fcc Ni. Zero magnetoresistivity was found in the sample in which the metallic nanoparticles were of non-magnetic hcp phase.     

Characterization of La0.67Sr0.33MnOz thin films synthesized by metal-organic decomposition on different substrates

La_0.67Sr_0.33MnO_z (LSMO) thin films were synthesized by means of metal-organic decomposition on the substrates including amorphous quartz, (1 0 0) Si chip, (1 0 0) MgO single crystal and polycrystalline Al₂O₃ ceramic plate. The structure and magnetotransport properties of the films were characterized. X-ray diffraction spectra show that all samples are polycrystalline with (2 0 2) preferred orientation. All films present metal–insulator transition and enhanced magnetoresistance (MR) effect below metal–insulator transition peak temperature (T_p). At room temperature (RT) low-field magnetoresistance effect (LFMR) and linear change of resistivity under applied field are exhibited by all the films. These magnetotransport properties were first ascribed to the porous structural characteristics in the films observed by atomic force microscope. Furthermore, the LSMO film synthesized on (1 0 0) MgO substrate presents a bit different magnetic properties and magnetotransport from the other samples, including broad ferromagnetic–paramagnetic transition zone, lower T_p and weaker LFMR at RT. However, for the samples synthesized on the other substrates, the LFMR effect is very similar to each other and their MR ratio reaches near 5% under 10 kOe field. Thus the substrate effect of LSMO film on (1 0 0) MgO is more intensive than that of the other samples.     

AFM study of crystallization and melting of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} in ultrathin films

Crystallization and melting of a poly(ethylene oxide) (PEO) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) in ultrathin films have been studied using atomic force microscopy (AFM) coupled with a hot stage. The PEO and PMPCS block possess the number-average molecular weights (M_n) of 5300 and 2100 g/mol, respectively. The ultrathin films on the mica and glow-discharged carbon surfaces were obtained by static dilute solution casting at room temperature. Isothermal melt crystallization from ultrathin films always leads to flat-on lamellae. Selective area electron diffraction (SAED) experiments have demonstrated that the PEO blocks crystallize with a monoclinic structure identical to that of homo-PEO and the chain direction is perpendicular to the substrate. At T_c<44 °C, the monolayer crystals are dendrites. At T_c>48 °C, square-shaped crystals are formed with the (100) and (020) planes as the crystal edges. At 44 °C≤T_c≤48 °C, an intermediate monolayer morphology is observed. The monolayer thickness increases monotonically with increasing T_c. At the same T_c, the monolayer lamellae with the top and bottom amorphous layers contacting with the atmosphere and the substrate possess a significantly larger overall thickness than the long period of the crystals in bulk. For the spiral terraces induced by screw dislocation, the thickness of each terrace is close to that of the monolayer formed at the same T_c, and their melting is mainly determined by the terrace thickness.     

Kinetic study of the II-I phase transition of isotactic polybutene-1

Kinetics of the solid-solid II-I phase transition of isotactic polybutene-1 was investigated. The fraction W_I of phase I as a function of time t_tr during the phase transition was measured by X-ray diffraction at various temperatures T_tr. The Avrami indices n of the W_I-t_tr plots are approximately unity for T_tr > 288 K. A bell-shaped temperature dependence of the transition rate V with the maximum transition rate at 285 K was obtained. The V-T_tr curve and the Avrami index n = 1 suggest that the rate-determining process is primary nucleation. The dependence of V on T_tr for T_tr < 283 K is described by the William-Landel-Ferry (WLF) equation, which shows that the glass transition affects the transition rate. The Avrami index decreases to n < 1 for T_tr < 283 K, indicating a broadened distribution of the transition rate caused by the spatial heterogeneity of the amorphous state at low temperatures near the glass transition. Those evidences at low temperature clearly suggest that the solid-solid phase transition is influenced by the mobility of chain folding, tie chains and cilia in the amorphous between the stacks of lamellar crystals.     

Structural investigation and gas barrier performance of diamond-like carbon based films on polymer substrates

In this report, tetrahedral amorphous carbon (ta-C), hydrogenated amorphous carbon (a-C:H), silicon doped tetrahedral amorphous carbon (ta-C:Si:H), and silicon doped hydrogenated amorphous carbon (a-C:H:Si) films with thickness in the range 50-370 nm have been produced by PECVD (Plasma Enhanced Chemical Vapour Deposition) and FCVA ( Filtered Cathodic Vacuum Arc) techniques on Polyethylene terepthalate (PET) and polycarbonate (PC) substrates. The paper is concerned with exploring the links between the atomic structure, gas barrier performance in carbon based films deposited on polymer substrates. A range of techniques including XRR, NEXAFS, Raman, surface profilometry, nano-indentation and water vapour permeation analysis were used to analyze the microstructure and properties of the films. The intensity and area of π* peak at the C K (carbon) edge of the NEXAFS spectra was lower in the FCVA films in comparison to that of PECVD ones confirming the higher sp³ content of FCVA films. The surface of ta-C films showed a network of micro-cracks, which is detrimental for gas barrier application. However, the surfaces of both ta-C:H:Si and a-C:H:Si silicon-incorporated films were almost free of cracks. We also found that the incorporation of Si into both types of DLC films lead to a significant reduction of water vapour transmission rate.     

Cathodoluminescence of fluorine doped amorphous carbon nanoparticles deposited by a filtered cathodic arc plasma system

The fluorine doped amorphous carbon nanoparticles (a-C:F NPs) films with sizes 50-100 nm were deposited on polyethylene terephthalate in an atmosphere of CF₄ by a 90°-bend magnetic filtered cathodic arc plasma system. The surface morphology of a-C:F NPs films was observed by field emission scanning electron microscope and atomic force microscope. The microstructure and chemical bonding nature of the a-C:F NPs films were investigated by Raman, X-ray diffraction and X-ray photoelectron spectroscopy. This work presents cathodoluminescence (CL) spectra of a-C:F NPs films obtained at 1.9-2.4 eV and verifies luminescence from a-C:F NPs films in the visible region. The incorporation of fluorine into the carbon network results in orange emission (∼2.03 eV) due to the transitions between fluorine-related electron levels and σ* states, and the red emission (∼1.97 eV) results from the recombination of carriers in the valence π and conduction π* states. The peak at ∼2.10 eV may result from the defects of the structures in a-C:F NPs films.     

Amorphous carbon nanostructures from chlorination of ferrocene

The chlorination of ferrocene at different temperature conditions yields several carbon nanostructures, which were studied by means of transmission and scanning electron microscopies. Amorphous carbon nanotubes (α-CNTs) up to 10 μm long with thick walls and ∼15 nm of internal diameter were observed in a sample treated at 200 °C during 30 min. They consisted on ∼90% of carbon, while the remaining 10% consists on iron and chlorine. At this temperature, amorphous carbon bags and open-ended branches were also found. When chlorinating ferrocene at the same temperature but with longer reaction time (180 min), no α-CNTs were formed. At higher temperature (300 °C, 30 min), amorphous carbon bags were found, with lower content of residual chlorine and iron, and presenting thinner walls. In the sample treated at even higher temperature (900 °C, 30 min) the carbon nanobags (wall thickness ∼12 nm) were almost spherical and more graphitic, and without impurities.     

Experimental electrical resistivity of liquid carbon in the temperature range from 4800 to ∼20,000 K

Specimens of high density pyrolytic graphite (2.2 g/cm³) were placed inside thick-walled sapphire tubes and heated over several microseconds by an electric current of 68 kA. The electrical resistivity of the liquid carbon was measured in a constant volume heating process. The transition of liquid carbon from semi-metal properties (resistivity decreasing with increase of input energy) to metal-like behavior (resistivity increasing with increase of input energy) was obtained at a high input energy (25-75 kJ/g) and at a high, but not measured, pressure. The transition temperature, T, was roughly estimated through the C_V value (heat capacity under constant volume). The relationship between the density and the transition temperature is as follows: for 1.88 g/cm³ density, the transition temperature T = 6300 K, for 1.76 g/cm³, T = 10,100 K, and for 1.1 g/cm³, T = 13,500 K. The estimated temperature at the maximum input energy (75 kJ/g) for liquid metal-like carbon (just before the destruction of the sapphire tube) is 23,000 K, with a corresponding measured electrical resistivity of 3000 μΩ cm.     

Ignition of methane and propane in high-temperature oxidizers with various oxygen concentrations

In this investigation, ignition processes of methane (CH₄ > 98%) and propane (C₃H₈ > 95%) using a high-temperature oxidizer (T_oxi > T_ai) with a varying oxygen concentration (z_O2=0.05÷0.21), applying two types of experimental installations, viz. a constant-volume bomb (CVB) and a co-flow reactor (CFR) were investigated. The influence of the initial temperature of the oxidizer (for methane T_oxi = 960 ÷ 1234 K and for propane T_oxi = 803 ÷ 1055 K), the equivalence ratio and oxygen concentration in the oxidizer on ignition of gaseous fuels is analyzed and discussed. It is shown that in order to achieve an effective reaction of ignition (taking into account the minimum value of ignition delay time τ_ig and maximal value of the increment of temperature ΔT) the oxidizer temperature need not be maximized. There are optimal values of temperature of the oxidizer (for methane T_oxi ≈ 1100 K and for propane T_oxi ≈ 950 K) in which the parameters mentioned above reach their extreme values.     

Structure and mechanical properties of W incorporated diamond-like carbon films prepared by a hybrid ion beam deposition technique

W incorporated diamond-like carbon films were prepared on silicon(1 0 0) wafers using a hybrid deposition system composed of an end-Hall-type hydrocarbon ion gun and a tungsten DC magnetron sputter source. The W concentration in the films was controlled by changing the fraction of Ar in the Ar and C₆H₆ reaction gas. The chemical composition, atomic bond structure, and mechanical properties were investigated for W concentrations ranging from 0 to 8.6 at.%. When the W concentration was <2.8 at.%, the W atoms were dissolved in the amorphous carbon matrix without forming a WC_1−x phase. Amorphous and crystalline WC_1−x nano-particles appeared when the W concentration was >2.8 and >3.6 at.%, respectively. It was found that the hardness and elastic modulus were not sensitive to the W concentration in this concentration range. On the other hand, the residual compressive stress was strongly dependent on the chemical state of the incorporated W atoms. The change in mechanical properties is discussed in terms of the microstructural changes induced by W incorporation.     

Lignin-based carbon fibers: Oxidative thermostabilization of kraft lignin

The thermostabilization of lignin fibers used as precursors for carbon fibers was studied at temperatures up to 340 °C at various heating rates in the presence of air. The glass transition temperature (T_g) of the thermally treated lignin varied inversely with hydrogen content and was found to be independent of heating rate or oxidation temperature. A continuous heating transformation (CHT) diagram was constructed from kinetic data and used to predict the optimum heating rate for thermostabilization; a heating rate of 0.06 °C/min or lower was required in order to maintain T_g > T during thermostabilization. Elemental and mass analyses show that carbon and hydrogen content decrease during air oxidation at constant heating rates. The hydrogen loss is sigmoidal, which is consistent with autocatalytic processes. A net increase in oxygen occurs up to 200-250 °C; at higher temperatures, oxygen is lost. Spectroscopic analyses revealed the oxidation of susceptible groups within the lignin macromolecule to ketones, phenols and possibly carboxylic acids in the early stage of the reaction; the later stage involving the loss of CO₂ and water and the formation of anhydrides and possibly esters. Slower heating rates favored oxygen gain and, consequently, higher glass transition temperatures (T_g) as opposed to faster heating rates.     

Nanodiamond crystallites embedded in carbon films prepared by thermionic vacuum arc method

Nanostructured carbon films with thicknesses of 100 and 200 nm have been deposited from pure vapour carbon plasma using an original thermionic vacuum arc method. Silicon single crystalline wafers, glass and stainless steel held at 400 °C were used for substrates. The films consist of diamond nanoparticles of 5 nm diameter on the average embedded in a disordered graphite matrix as revealed by HRTEM, XPS and visible Raman measurements. The graphitic cluster diameters La range from 1.5 to 2.3 nm. Thicker films (200 nm) on stainless steel exhibit the largest clusters.     

Thermal molecular motion at surface of atactic polypropylene films

Surface molecular motion in atactic polypropylene (aPP) films was studied by scanning force microscopy. Glass transition temperature at the surface, T_g^s, was determined to be 251±1 K on the basis of temperature dependence of lateral force, whereas its bulk glass transition temperature, T_g^b, by dynamic viscoelastic and differential scanning calorimetric measurements was 262 K. In general, polyolefin is easily oxidized, and thus, many kinds of additives are mixed in it for practical use. Hence, effects of oxidation and additives on surface properties of the aPP surface were examined as well. To achieve oxidation, the aPP films were annealed at 428 K for 100 min under the ambient atmosphere. After this treatment, T_g^s decreased by approximately 15 K in comparison with the intact film due to degradation of surface chains. On the contrary, in the case of the aPP containing 10 wt% antioxidant, T_g^s was almost the same as that of the intact film before and even after the oxidation treatment.     

Computer simulation for storage of methane and capture of carbon dioxide in carbon nanoscrolls by expansion of interlayer spacing

We perform a molecular simulation study on methane and carbon dioxide storage in carbon nanoscrolls. The effects of temperature and pressure, interlayer spacing, VDW gap and innermost radius on the gas storage have been examined extensively. It is found that the adsorption of gases on pristine carbon nanoscrolls is relatively low. However, once the interlayer spacing is expanded, both adsorption capacities of methane and carbon dioxide exhibit a significant improvement. In particular, the excess uptake of methane reaches 13 mmol/g at p = 6.0 MPa and T = 298.15 K and VDW gap Δ = 1.1 nm, which is about 3.5 times of uptake of the pristine carbon nanoscrolls; while the uptake of carbon dioxide could also be raised by 294.9% at T = 298.15 K and p = 3.0 MPa and Δ = 1.5 nm, reaching 30.21 mmol/g at 6.0 MPa. This work demonstrates that carbon nanoscrolls with an expansion of interlayer spacing may be a suitable material for methane storage and carbon dioxide capture.     

Synthesis, characterization and optical properties of cross-linkable poly(phthalazinone ether ketone sulfone)

Cross-linkable poly(phthalazinone ether ketone sulfone) bearing tetrafluorostyrene groups (PPEKS-FSt) has been prepared by copolycondensation reaction for optical waveguide applications. The resulting amorphous polymer exhibits good solubility in some common polar organic solvents (e.g., N,N′-dimethylacetamide, N-methyl-2-pyrrolidinone, chloroform) at room temperature, and can be easily spin-coated into thin films with good optical quality. The glass transition temperature (T_g) and the temperature of 1% weight loss (1% T_d) are 261 °C and 494 °C, respectively, which could be further increased by 31 °C and 14 °C upon thermal cross-linking. The cross-linked polymer thin films exhibit high refractive index (∼1.65, TE mode), high thermo-optic coefficient value (dn/dT) (−1.455 × 10⁻⁴/°C, TE mode), low optical loss (less than 0.24 dB/cm at 1310 nm) and relatively low birefringence (∼0.007).     

Local structure of composite Cr-containing diamond-like carbon thin films

Composite Cr-containing hydrogenated amorphous diamond-like carbon (Cr-DLC) films were synthesized by a hybrid PVD/CVD plasma-assisted deposition process. In a recent study, it was found that Cr-DLC films with <∼12 at.% Cr possess excellent tribological properties. However, the role of Cr in inducing these characteristics is not clear. In the present report, the local structure around the Cr atoms in the latter films was studied as a function of Cr content by X-ray absorption spectroscopy. The Cr K-edge X-ray absorption near edge structure spectra show that Cr in DLC has a chemical state similar to that of chromium carbide. Analysis of the extended X-ray absorption fine structure spectra shows that at low Cr content (<0.4 at.% Cr), Cr is dissolved in the amorphous DLC matrix forming an atomic-scale composite. Simulation studies suggest that in the latter films, Cr tends to be present as very small atomic clusters of 2-3 Cr atoms. At higher Cr contents (>1.5 at.%), Cr is present as nanoparticles (<10 nm) of a defected carbide structure forming a nanocomposite.     

Morphology of melt-quenched poly(ε-caprolactone)-block-polyethylene copolymers

The morphology of a melt-quenched crystalline-crystalline diblock copolymer, poly(ε-caprolactone)-block-polyethylene (PCL-b-PE), was studied by small-angle X-ray scattering and transmission electron microscopy. The melting behavior of PCL-b-PE was also investigated by differential scanning calorimetry. The melting temperature of PCL blocks, T_m,PCL, was ca. 55 °C and that of PE blocks was ca. 96 °C. Therefore, the PE block always crystallized first during quenching from the microphase-separated melt into various temperatures T_c below T_m,PCL to yield an alternating structure composed of PE lamellae and amorphous layers (PE lamellar morphology), and subsequently the crystallization of PCL blocks started at T_c after some induction period. The PE lamellar morphology was preserved after the crystallization of PCL blocks at low crystallization temperatures (T_c<30 °C), that is, the PCL block crystallized within the PE lamellar morphology. At high crystallization temperatures (45 °C>T_c>30 °C), on the other hand, the crystallization of PCL blocks destroyed the PE lamellar morphology to result in a new lamellar morphology mainly consisting of PCL lamellae and amorphous layers (PCL lamellar morphology). The PE crystals were fragmentarily dispersed in the PCL lamellar morphology.     

Electrochemical behavior of Fe(II) in acetamide-urea-NaBr-KBr melt and magnetic properties of inductively codeposited Nd-Fe film

The electroreduction of Fe(II) and Nd(III) in MCl_x-acetamide-urea-NaBr-KBr were studied by cyclic voltammetry and chronoamperometry. The reduction of Fe(II) to Fe is an irreversible process, the value of αn_α of the electrode reaction was calculated to be 0.31 and the diffusion coefficient of Fe(II) was calculated to be 9.53 × 10⁻⁷ cm² s⁻¹ at 343 K. Nd(III) cannot be reduced alone in urea melt, but Nd-Fe can be codeposited by induced codeposition. The composition of Nd-Fe film varies with the Nd(III)/Fe(II) molar ratio, at the potential of −1.25 V the maximum content of Nd in Nd-Fe film is 60.4 wt%. The morphology of Nd-Fe film was investigated by SEM and AFM. Nd-Fe film comprises of nanoparticles with the size about 100-200 nm. X-ray diffraction (XRD) shows it is amorphous. After heat-treatment at 1173 K the crystal Nd₂Fe₁₇ phase can be formed. The magnetic properties of the Nd-Fe films were determined using hysteresis loops, at 5 K the coercive field H_c of Nd (62.6 wt%)-Fe amorphous film is 1225 Oe, the remanent magnetization M_R and the saturation magnetization M_S are 5.15 and 15.80 emu g⁻¹, respectively.