Enhanced room-temperature positive magnetoresistance of a-C:Fe film

Fe-doped amorphous carbon (a-C:Fe) film and pure amorphous carbon (a-C) film were synthesized on n-Si substrate using pulsed laser deposition. The a-C:Fe film has a positive magnetoresistance (15% at magnetic field B = 1 T) at room temperature, while it has a negative magnetoresistance below 260 K. The electrical conduction in a-C:Fe film is one order of magnitude higher than that in pure a-C film. It is found that a-C:Fe film has very different conduction mechanism from that of pure a-C film. The activation energy of electron conduction in a-C:Fe film could be tuned significantly by magnetic field. The magnetoresistance effect of the a-C:Fe film seems difficult to explain by known magnetoresistance mechanisms.     

The growth of transparent amorphous carbon thin films from coal

Large-area, uniform, transparent amorphous carbon (a-C) thin films were synthesized through simple chemical vapor deposition using coal as the solid carbon source. The atomic force microscopy characterization showed that the synthesized carbon thin film has ∼5 nm thickness with ∼0.55 nm surface roughness. The optical transmittance spectrum showed that the carbon thin film has >96% optical transmittance over the spectral range from 350 nm to 900 nm. The carbon thin films can be transferred to various substrates, which show promise for applications in solar cell, optical and magnetic storage disks, light emitting diodes, photodiodes, and biomedical implants.     

Cathodoluminescence and electron field emission of boron-doped a-C:N films

The optical and electrical properties of so-called carbon nitride films (a-C:N) and boron doped so-called carbon nitride films (a-C:N:B) are studied with cathodoluminescence (CL) spectroscopy and electron field emission measurement. The a-C:N films were first deposited on Si by a filtered cathodic arc plasma system, and then boron ions (∼1 × 10¹⁶ cm⁻²) were implanted into the a-C:N films to form a-C:N:B films by a medium current implanter. The structural and morphological properties of a-C:N and a-C:N:B films were then analyzed using secondary ion mass spectrometer, X-ray photoelectron spectroscopy, FT-IR spectra, Raman spectroscopy and atomic force microscopy. The a-C:N film exhibits luminescence of blue light (∼2.67 eV) and red light (∼1.91 eV), and the a-C:N:B film displays luminescence of blue light (∼2.67 eV) in CL spectra measured at 300 K. Furthermore, the incorporated boron atoms change the electron field emission property, which shows a higher turn on field for the a-C:N:B film (3.6 V/μm) than that for the a-C:N film (2.8 V/μm).     

Effects of pulse duration in laser processing of diamond-like carbon films

Experimental studies of the interaction between amorphous hydrogenated carbon (a-C:H) film and short and ultrashort laser pulses in the near-infrared and visible spectral ranges (150 ns and 1064 nm, 10 ns and 1078 nm, 300 ps and 1078 nm, 220 ps and 539 nm, 100 fs and 800 nm.) are reported. The influence of the irradiation conditions (pulse duration, wavelength, laser intensity and the number of laser shots) on the structure and thickness of the laser-induced graphitized layer has been investigated. The effects of heat dissipation on the annealing duration and on the graphitized layer thickness are discussed for the case of laser processing with short pulses. It was found that the resulting morphology of the irradiated a-C:H film surface was determined by the concurrence of three processes: change of the mass density induced by structural transformations, multiple spallations of thin layers, and material evaporation. The laser-induced spallation is asserted to be the main factor limiting the laser microprocessing reproducibility for the examined a-C:H film; its effects were found to increase dramatically for longer (150 ns) laser pulses. The ablation (evaporation) rates of the a-C:H films and glassy carbon were revealed to be similar for femtosecond and picosecond pulses, but they essentially differed for nanosecond pulses. The ablation process demonstrated the same main features for both materials: (i) increase of the ablation rate with the pulse duration, and (ii) saturation of the ablation rate with fluence for picosecond and nanosecond pulses.     

Correlation of film structure and molecular oxygen reduction at nitrogen doped amorphous carbon thin film electrochemical electrodes

Nitrogen doped amorphous carbon (a-C:N) thin film electrodes with a range of film structures have been deposited using a filtered cathodic vacuum arc system. The correlation between film structure and electro-reduction of molecular oxygen in aqueous media at the electrodes has been explored. In aqueous 0.1 M NaOH, dioxygen reduction is inhibited at all the a-C:N electrodes compared with that at glassy carbon electrodes. The potential of the dioxygen reduction current peak shifts negatively at a-C:N electrodes as the sp³ C fraction in the a-C:N materials increases, while the current peak height decreases simultaneously. The a-C:N electrodes possess high sensitivity for investigating the mechanism of dioxygen reduction. It was found that the catalytic H₂O₂ reduction to H₂O on carbon materials is attributed to oxygen species at sp² C sites.     

Novel hybrid facing targets sputtered amorphous carbon overcoat for ultra-high density hard disk media

Amorphous carbon films form a critical protective layer on magnetic hard disk media. A novel in-house configured hybrid facing targets sputtering (HyFTS) utilizing a special magnetron arrangement was used to deposit amorphous carbon (a-C) overcoats. The corrosion inhibition ability and mechanical property were investigated and compared with that of the a-C overcoats deposited by conventional magnetron sputtering (CMS). Studies were done using electrochemical techniques and nano-scratch analysis to determine how corrosion inhibition ability changes at thickness of 2 nm and 5 nm and the scratch resistance of the two types of a-C overcoats, respectively. Electrochemical impedance spectroscopy and potentiodynamic polarization investigations have shown that a-C films deposited by HyFTS have good corrosion resistance even at 2 nm thickness. But at 2 nm, CMS deposited a-C overcoat shows sign of corrosion. Nano-scratch test has also shown improved scratch resistance of the HyFTS deposited a-C. This may be attributed to the better quality film with higher sp³ content deposited by HyFTS as a result of it having a higher amount of ionized species generated.     

Synthesis and evaluation of several high absorbance black pigments for spacecraft thermal control coatings

Using black coatings and materials with high light absorbance that are capable of absorbing photons at visible and longer wavelengths is a very effective way to reduce unwanted stray light, also known as optical noise, within optical equipment. These lights can be greatly reduced to a reasonable level by functional and performable black coatings that are modified to absorb incident light as much as possible by their specific pigments. In the present work, several carbonaceous pigments were synthesized for the first time from wasteful materials and their optical properties in the visible and near‐infrared ranges studied. First, MCM‐48 and SBA‐15 were synthesized at different conditions and were then used as templates for carbonaceous products. SSS‐1 (the carbonic pigment synthesized by the mixture of sucrose and sodium silicate), SSS‐2 (the carbonic pigment synthesized by the mixture of sawdust and sodium silicate), and mesoporous carbon pigments (CMK‐3 and CMK‐1 with different levels of saturations) were synthesized. Finally, their structure, morphology, and optical properties were investigated by X‐ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE‐SEM), and Diffuse Reflectance Spectroscopy (DRS). The results indicated that the SSS‐1 pigment had a lower reflectance (below 1%) than carbon black (about 2.5%) in the visible region despite it being more cost‐effective than carbon black. The mesoporous pigments showed very high light absorbance in the visible region (about 2.5%). Compared with other black pigments, the CMK‐1 was the blackest synthesized material with a very low reflectance (about 0.05% in visible region), making it an ideal candidate as a super black pigment for reducing unwanted stray light within optical equipment.     

Investigation of the initial growth of ultrananocrystalline diamond films by multiwavelength Raman spectroscopy

The initial growth phase of ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) has been investigated by scanning electron microscopy, atomic force microscopy and especially Raman spectroscopy. As due to resonance effects Raman spectra of carbon materials strongly depend on the excitation wavelength, a multiwavelength analysis has been performed with λ_exc ranging from the UV region (325 nm) over the visible range (488 and 514 nm) to the IR region (785 nm). In addition, a set of measurements has been performed with a confocal Raman microscope, i.e. depth resolved, with a wavelength of 532 nm. The samples investigated were deposited with constant parameters, the deposition time being the only parameter varied, resulting in film thicknesses from 100 to 500 nm. It turned out that the diamond fraction and also the grain boundary material do not vary during that stage whereas there are slight but distinct changes of the nature of the amorphous matrix which reflect, among others, in a shift of the graphite-related G band to higher wavenumbers and in an increase of the ratio of D and G bands with increasing film thickness. These changes are discussed in terms of the above mentioned resonance effects; the major changes are a transition of hydrogen containing sp² chains to hydrogen-free condensed sp² rings when the material is no longer in the surface region of the films but becomes incorporated within the film bulk.     

Permeable porous 1–3 nm thick overoxidized polypyrrole films on nanostructured carbon fiber microdisk electrodes

Ionically conducting 1–3 nm thick porous films of overoxidized polypyrrole (OPPY) were electrodeposited on nanostructured 7 μm diameter carbon fiber microdisk electrodes. The microdisk electrodes were fabricated from two types of polyacrylonitrile (PAN) carbon fibers, PAN T650 and PAN HCB. The electrodes were nanostructured by electrochemical etching of the microdisk electrode surface. Ultrathin porous polypyrrole (PPY) films were electrodeposited by the electropolymerization of pyrrole (PY) to PPY by a short (10 ms) single potential pulse. During the electropolymerization, the polymer “precipitated” on the nanostructured surface producing ultrathin porous film. OPPY films were fabricated by constant potential overoxidation of PPY.In steady-state voltammetry of ferricyanide, the nanostructured electrodes behave as a random array of microscopic nodules and pores. At potential scan rates of 0.050 V s⁻¹ diffusion fields at the 300–600 nm nodules on the 7 μm diameter microdisk electrode overlap. The surface area of the electroactive nanofeatures decreases after deposition of insulating OPPY. Kinetics of ferricyanide at bare and OPPY-coated nanostructured electrodes reflect the electrode surface area, as predicted by the model for charge transfer at a partially blocked surface. A model reflecting the 58–94% coverage of the nanostructured electrodes by OPPY was developed to address the high permeability of the porous OPPY-coated microdisk electrodes.     

The role of pressure on thin amorphous carbon films deposited using microwave surface wave plasma CVD

We report the effects of gas composition pressure (GCP) on the optical, structural and electrical properties of thin amorphous carbon (a-C) films grown on p-type silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (MW SWP CVD). The films, deposited at various GCPs ranging from 50 to 110 Pa, were studied by UV/VIS/NIR spectroscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and current–voltage characteristics. The optical band gap of the a-C film was tailored to a relatively high range, 2.3–2.6 eV by manipulating GCPs from 50 to 110 Pa. Also, spin density strongly depended on the band gap of the a-C films. Raman spectra showed qualitative structured changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). The photovoltaic measurements under light illumination (AM 1.5, 100 mW/cm²) show that short-circuit current density, open-circuit voltage, fill factor and photo-conversion efficiency of the film deposited at 50 Pa were 6.4 μA/cm², 126 mV, 0.164 and 1.4 × 10^− 4% respectively.     

Photovoltaic properties of an amorphous carbon/fullerene junction

This paper demonstrates all-carbon photovoltaic devices made of amorphous carbon (a-C) and C₆₀ thin films. C₆₀ film is deposited by the sublimation in vacuum and a-C film is synthesized by exposing N₂ radicals to C₆₀ during the deposition. C₆₀ is converted into a-C when the rf power is larger than 150 W and the optical band gap decreases with increasing the power. Photovoltaic properties of device with the structure of Al/C₆₀/a-C/indium tin oxide/glass are presented. It is shown that the present cell has a strong spectral response in the wavelength range shorter than 550 nm and a small response at around 620 nm.     

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.     

Microwave-hydrothermal synthesis of extremely high specific surface area anatase for decomposing NOx

Apparently C-doped and undoped or pure nanoparticles of anatase were synthesized using a microwave hydrothermal process in the temperature range of 140–180 °C for 1 h from several Ti precursors, such as Ti ethoxide, Ti isopropoxide and Ti oxysulfate. Nanoparticles of anatase samples were characterized by powder X-ray diffraction, transmission electron microscopy (TEM) and photocatalytic activity measurements. Results showed that nanoparticles in the size range of 4–17 nm of anatase were obtained in all cases with surface areas in the range of 151–267 m²/g. The photocatalytic activity of the prepared titanias was measured using methylene blue (MB) and NO_x molecules. Because MB has very strong adsorption on the samples, photocatalytic degradation under either solar light or black light irradiation was found to be very limited. However, the DeNO_x abilities of carbon-doped titanias were higher than those of Degussa P25 commercial titania sample and undoped or pure titanias especially under irradiation by long wavelength or visible light (>500 nm).     

Synthesis and characterization of iron-impregnated porous carbon spheres prepared by ultrasonic spray pyrolysis

Porous carbon microspheres impregnated with iron-based nanoparticles are prepared in a single step, continuous process using ultrasonic spray pyrolysis (USP). Precursor solutions containing a carbon source, an inorganic salt, and an iron salt are ultrasonically aerosolized and pyrolyzed. Solutions containing nitrate or chloride salts are examined. During pyrolysis, sucrose is dehydrated to carbon, and the metal salt is converted to crystalline or non-crystalline iron species, depending on processing conditions. The product’s porosity is generated from: (1) aromatization of carbon around an in situ template, (2) in situ gasification of isolated carbon, or (3) in situ chemical activation of the carbon precursor. Porous carbon spheres (0.5–3 μm diameter) containing well-dispersed iron oxide nanoparticles (4–90 nm diameter), referred to here as Fe–C, are prepared. Iron loadings between 1 and 35 wt.% are achieved while maintaining well-dispersed Fe nanoparticles with as-produced surface areas up to 800 m²/g. Post-pyrolysis heat and hydrogen treatments increase the surface area of the materials while reducing iron species. USP Fe–C materials may have useful catalytic applications due to their potential for high-loading of well-dispersed metal nanoparticles. Despite negligible surface Fe content, chromium reduction tests indicate that internal Fe sites are catalytically active.     

Polyaniline-carbon composite films as supports of Pt and PtRu particles for methanol electrooxidation

Vulcan XC-72 carbon black particles (average size: ca. 50 nm) was incorporated into polyaniline (PANI) matrix by an electrochemical codeposition technique during the electropolymerization process. The doping by carbon particles leads to a higher polymeric degree and a lower defect density in the PANI structure. Furthermore, the incorporation of carbon particles not only increases the electrochemical accessible surface areas (S_a) and electron conductivity of the PANI film, but also decreases charge transfer resistance at PANI/electrolyte interfaces. Therefore, as expected, a fabricated PANI + C composite film with dispersed Pt and PtRu particles exhibited excellent electrocatalytic activity for methanol oxidation due to better Pt dispersion and utilization. The PANI + C composite film is more promising as a support material in electrocatalysis than a PANI film. Meanwhile, a new application for regular carbon black as a doping material into conducting polymer for electrocatalysis was thus demonstrated.     

Superhydrophobic carbon nanotube/amorphous carbon nanosphere hybrid film

Fabrication of superhydrophobic surfaces has been widely investigated due to their wide range of applications. Here, synthesis of self-assembled aligned carbon nanotubes (ACNT)/amorphous carbon (a-C) nanosphere hybrid film is reported. Carbon plasma produced by FCVA was used to deposit a-C nanospheres on the ACNT films fabricated by PECVD. The superhydrophobic properties of the surface was investigated by static contact angle (CA) measurement. It is found that the surface morphology of the film which depends on the size of the a-C nanospheres, has a great influence on the hydrophobic properties of the surface. The hydrodynamic properties of the surface is discussed in terms of both Cassie and Wenzel mechanisms. The microstructure of the films is also investigated by XPS and HRTEM. It is shown that the bombardment of the CNTs with high energy carbon ions will damage the crystalline structure of the CNT walls as well.     

Enhanced cycling stability of Ni-MH battery by depositing amorphous carbon film on the Ti1.4V0.6Ni negative electrode

Amorphous carbon (a-C) films with various thicknesses depending on the reaction time are deposited on the surface of Ti_1.4V_0.6Ni alloy electrodes for Ni-MH (nickel-metal hydride) battery by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). With the increasing deposition time, the Raman spectra show a gradually disordered sp²-bonding change of the films and the changing trend of sp²/sp³ is obtained by X-ray photoelectron spectroscopy. The a-C film of depositing for 30 min with the thickness of 400 nm shows a favorable stability in alkaline electrolyte, the capacity is enhanced by 36.2% after 50 cycles than the bare electrode, and the charge voltage is 80 mV lower than the bare one. The a-C film with high sp²-bonded carbon content effectively reduces the charge transfer resistance, and as a coating layer, the dissolution of V of the alloy is also inhibited. In particular, to get a proper discharge voltage and a stable capacity simultaneously, covering completely and an appropriate thickness of the a-C film are crucial for an expected performance.     

High Tunability in (111)‐Oriented Relaxor Pb0.8Ba0.2ZrO3 Thin Film with Antiferroelectric and Ferroelectric Two‐Phase Coexistence

Using a sol‐gel method Pb_0.8Ba_0.2ZrO₃ (PBZ) thin film with a thickness of ~320 nm was fabricated on Pt(111)/TiO_x/SiO₂/Si substrate. The analysis results of XRD, SEM, and dielectric properties revealed that this thin film is a (111)‐oriented nano‐scaled antiferroelectric and ferroelectric two‐phase coexisted relaxor. Calculations of dielectric tunability (η) and figure‐of‐merit (FOM) at room temperature display a maximum value of 75% at E = 560 kV/cm and ~236, respectively. High‐temperature stability (η > 75% and FOM > 230 at 560 kV/cm in the range from 300 to 380 K) and high breakdown dielectric strength (leakage current < 1 nA at 598 kV/cm) make the PBZ thin film to be an attractive material for applications of tunable devices.     

Ultraviolet photodetectors based on ZnO nanoparticles

Ultraviolet (UV) photodetectors based on ZnO nanoparticles (NPs) were fabricated and their optoelectronic properties were examined. The dominant photoluminescence (PL) peak of the ZnO NPs was located at a wavelength of 380 nm under the illumination of 325-nm wavelength light. The direct bandgap transition of the charge carriers at λ = 380 nm contributed to the photocurrent. The ratio of the photocurrent to the dark current (on/off ratio) was as high as 10⁶, which is favorable for photodetectors. The decay time constant in the photoresponse was relatively small, while the rise time constant was relatively large. The reasons for the high on/off ratio and photoresponse characteristics are discussed in this paper.     

Giant photoconductivity induced by plasmonic Co nanoparticles in Co-doped amorphous carbon/silicon heterostructures

A giant photoconductivity was observed in heterostructures of Co-doped amorphous carbon/silicon (Co–C/Si), which were fabricated by growing Co–C films on n-type Si substrates through pulsed laser deposition (PLD). When the sample was illuminated by a monochromatic light with a small density of 1 mW/cm², the obtained photosensitivity, defined as the ratio of the photoconductivity σ_p to the dark conductivity σ_d, was as large as ∼10⁵. Spectral dependence of the photoconductivity demonstrated significant photosensitivity in a large spectrum range and exhibited a peak photosensitivity around ∼710 nm. Transmission electron microscopy (TEM) revealed self-assembled cobalt nanoparticles in the amorphous carbon layer. In comparison with Co–C/Si heterostructures, the pure amorphous carbon (a-C)/Si heterostructure without Co nanoparticles was also fabricated and no significant photoconductivity was found. The mechanism of giant photoconductivity might be attributed to the surface plasmon resonance (SPR) absorption of Co nanoparticles.