Amorphous Carbon Thin Films Research Articles

Metal (Ag/Ti)-Containing Hydrogenated Amorphous Carbon Nanocomposite Films with Enhanced Nanoscratch Resistance: Hybrid PECVD/PVD System and Microstructural Characteristics.

This study aimed to develop hydrogenated amorphous carbon thin films with embedded metallic nanoparticles (a–C:H:Me) of controlled size and concentration. Towards this end, a novel hybrid deposition system is presented that uses a combination of Plasma Enhanced Chemical Vapor Deposition (PECVD) and Physical Vapor Deposition (PVD) technologies. The a–C:H matrix was deposited through the acceleration of carbon ions generated through a radio-frequency (RF) plasma source by cracking methane, whereas metallic nanoparticles were generated and deposited using terminated gas condensation (TGC) technology. The resulting material was a hydrogenated amorphous carbon film with controlled physical properties and evenly dispersed metallic nanoparticles (here Ag or Ti). The physical, chemical, morphological and mechanical characteristics of the films were investigated through X-ray reflectivity (XRR), Raman spectroscopy, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and nanoscratch testing. The resulting amorphous carbon metal nanocomposite films (a–C:H:Ag and a–C:H:Ti) exhibited enhanced nanoscratch resistance (up to +50%) and low values of friction coefficient (<0.05), properties desirable for protective coatings and/or solid lubricant applications. The ability to form nanocomposite structures with tunable coating performance by potentially controlling the carbon bonding, hydrogen content, and the type/size/percent of metallic nanoparticles opens new avenues for a broad range of applications in which mechanical, physical, biological and/or combinatorial properties are required.

Density and microstructure of a-C thin films

In this work, we studied amorphous carbon (a-C) thin films deposited using direct current (dc) and high power-impulse magnetron sputtering (HiPIMS) techniques. The microstructure and electronic properties reveal subtle differences in a-C thin films deposited by two techniques. While films deposited with dcMS have a smooth texture typically found in a-C thin films, those deposited with HiPIMS consist of dense hillocks surrounded by a porous microstructure. The density of a-C thin films is a decisive parameter to judge their quality. Often, x-ray reflectivity (XRR) has been used to measure the density of carbon thin films. From the present work, we find that the determination of density of carbon thin films, especially those with a thickness of few tens of nm, may not be accurate with XRR due to a poor scattering contrast between the film and substrate. By utilizing neutron reflectivity in time of flight mode, it has been shown that the density of carbon films can be measured more accurately. Prime novelty statementIn this work, we have studied amorphous carbon (a-C) thin films prepared by direct current and high power impulse magnetron sputtering (dcMS and HiPIMS). We did precise density measurements using time of flight (ToF) neutron reflectivity (NR), a technique seldom used to study carbon thin films. We amply demonstrate limitation of x-ray reflectivity technique generally used for determination of density of thin carbon films. It is expected that our demonstration of NR in density determination of carbon thin film will turn out to be landmark for carbon film community and will certainly be helpful in enriching the related research work.

Effect of Sublimation Temperature on the Photovoltaic Properties of Amorphous Carbon Thin Films from Fullerene

This paper presents the effects of sublimation temperature of C60 fullerene on the photovoltaic properties of amorphous carbon (a-C) films synthesized by remote plasma cracking. Here, we show that the deposition rate increases rapidly,Raman peak intensity corresponding to disordered fullerene becomes strong and the optical band gap increases with increasing the sublimation temperature. The photovoltaic devices with structure of Al/C60/a-C/ITO glass are fabricated with different sublimation temperatures and therelationship between the photovoltaic properties and the material properties of a-C films are discussed. It is shown that the improvementof power conversion efficiency is explained by low component of disorderedC60in amorphous carbon at lower sublimation temperature.

Hydrogen plasma-enhanced atomic layer deposition of hydrogenated amorphous carbon thin films

Hydrogenated amorphous carbon (a-C:H) thin films were prepared by hydrogen plasma-enhanced atomic layer deposition (PE-ALD). The a-C:H thin films were grown at low temperatures in the range of 150–350 °C using CBr4 as the precursor and hydrogen plasma as the reactant. Raman spectroscopy, secondary ion mass spectrometry, X-ray photoelectron spectroscopy and Fourier transform infrared measurements showed that the a-C:H films consist of hydrogenated nanocrystalline sp3 diamond, disordered sp3 carbon and sp2-hybridized graphitic carbon incorporated with oxygen as a main contaminant. Moreover, the incorporation of bromine and oxygen in the a-C:H films was significantly reduced upon increasing the growth temperature from 200 to 300 °C. Surface hydroxylation and precursor exposure pretreatments were employed to saturate the adsorption of CBr4 precursors and enhance the initial nucleation of carbon during the deposition of the a-C:H thin film by the PE-ALD process. In addition, the conformal growth of a-C:H thin films on three-dimensional structures was confirmed.

Hydrogenated nanostructure boron doped amorphous carbon films by DC bias

Hydrogenated nanostructure-boron doped amorphous carbon thin film carbon was deposited at different negative bias using custom-made deposition bias assisted-CVD. Solid of boron and palm oil were used as dopant and carbon source, respectively. The hydrogenated nanostructure amorphous films were characterized by Field emission scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, while the photo-response studies of thin film is done by I-V measurement under light measurement. The results showed the carbon film were in nanostructure with hydrogen and boron might be incorporated in the film. The Raman spectra observed the increase of upward shift of D and G peaks as negative bias increased which related to the structural change as boron incorporated in carbon network. These structural changes were further correlated with photo-response study and the results obtained are discussed and compared.

Semitransparent conductive carbon films synthesized by sintering spin-coated sp3-based network polymer

We synthesized semitransparent conducting thin films of amorphous carbon from sp3-rich network polymer. The films showed a reasonable optical transparency (58–73% transmission in the wavelength range of 380–2200 nm), a low electric resistivity (6.7 × 10−3 Ω cm), and durability against corrosive chemical reagents. The sintering of the amorphous films results in the formation of a carbon honeycomb lattice in the films.

Neon ion beam induced pattern formation on amorphous carbon surfaces

We investigate the ripple pattern formation on amorphous carbon surfaces at room temperature during low energy Ne ion irradiation as a function of the ion incidence angle. Monte Carlo simulations of the curvature coefficients applied to the Bradley-Harper and Cater-Vishnyakov models, including the recent extensions by Harrison-Bradley and Hofsäss predict that pattern formation on amorphous carbon thin films should be possible for low energy Ne ions from 250 eV up to 1500 eV. Moreover, simulations are able to explain the absence of pattern formation in certain cases. Our experimental results are compared with prediction using current linear theoretical models and applying the crater function formalism, as well as Monte Carlo simulations to calculate curvature coefficients using the SDTrimSP program. Calculations indicate that no patterns should be generated up to 45° incidence angle if the dynamic behavior of the thickness of the ion irradiated layer introduced by Hofsäss is taken into account, while pattern formation most pronounced from 50° for ion energy between 250 eV and 1500 eV, which are in good agreement with our experimental data.

Influence of Nitrogen on Hydrogenated Amorphous Carbon Thin Films Deposited by Plasma Focus Device

To carry out our research, a plasma focus device is used to deposit thin films of nitrogen doped hydrogenated amorphous carbon (a-C:H:N) onto the stainless steel-AISI-304 substrates at room temperature. Thin films are deposited with the same numbers of focus shots, at the same distance from the anode tip and with different partial pressures of nitrogen in the mixtures of acetylene/nitrogen as working gas. The nitrogen contents of deposited films are studied using nuclear reaction analysis (NRA) techniques. The results prove that nitrogen contents of the samples do not increase significantly by increasing partial pressure of nitrogen of the working gas for both sets of the samples. Moreover, NRA results exhibit the limitation of nitrogen incorporated into the samples, when this experimental setup is used. G-peak position and peak intensity ratio of the D-band to G-band (ID/IG) are used to investigate the diamond characters. Also, they show that sp2 clustering is highly dependent on the nitrogen atomic contents and angular position of the samples. Scanning electron microscopy (SEM) shows the granular surface morphology of the films. Furthermore, it shows that angular position of the samples with respect to the anode axis plays an important role in the grain size of the surface of the samples. The thickness of the films decreases significantly by increasing angular position of the samples, while it decreases slightly by increasing partial pressure of nitrogen of the working gas. The Vickers surface hardness of the thin films exhibits significant dependency on the sp2 clustering.

Characterization and electrochemical properties of iron-doped tetrahedral amorphous carbon (ta-C) thin films.

Iron-doped tetrahedral amorphous carbon thin films (Fe/ta-C) were deposited with varying iron content using a pulsed filtered cathodic vacuum arc system (p-FCVA). The aim of this study was to understand effects of iron on both the physical and electrochemical properties of the otherwise inert sp3-rich ta-C matrix. As indicated by X-ray photoelectron spectroscopy (XPS), even ∼0.4 at% surface iron had a profound electrochemical impact on both the potential window of ta-C in H2SO4 and KOH, as well as pseudocapacitance. It also substantially enhanced the electron transport and re-enabled facile outer sphere redox reaction kinetics in comparison to un-doped ta-C, as measured with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) using outer-sphere probes Ru(NH3)6, IrCl6, and FcMeOH. These increases in surface iron loading were linked to increased surface oxygen content and iron oxides. Unlike few other metals, an iron content even up to 10 at% was not found to result in the formation of sp2-rich amorphous carbon films as investigated by Raman spectroscopy. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) investigations found all films to be amorphous and ultrasmooth with Rq values always in the range of 0.1–0.2 nm. As even very small amounts of Fe were shown to dominate the electrochemistry of ta-C, implications of this study are very useful e.g. in carbon nanostructure synthesis, where irregular traces of iron can be readily incorporated into the final structures.

Towards superlubricity in nanostructured surfaces: the role of van der Waals forces.

Hydrogenated amorphous carbon (a-C:H) thin films have a unique combination of properties that are fundamental in mechanical and electromechanical devices aimed at energy efficiency issues. The literature brings a wealth of information about the ultra-low friction (superlubricity) mechanism in a-C:H thin films. However, there is persistent controversy concerning the physicochemical mechanisms of contact mechanics at the atomic/molecular level and the role of electrical interactions at the sliding interface is still a matter of debate. We find that the hydrogenation of the outermost nanostructured surface atomic layers of a-C:H thin films is proportional to the surface potential and also to the friction forces arising at the sliding interface. A higher hydrogen-to-carbon ratio reduces the surface potential, directly affecting frictional forces by a less effective long-term interaction. The structural ultra-low friction (superlubricity) is attributed to a lower polarizability at the outermost nanostructured layer of a-C:H thin films due to a higher hydrogen density, which renders weaker van der Waals forces, in particular London dispersion forces. More hydrogenated nanodomains at the surface of a-C:H thin films are proposed to be used to tailor superlubricity.

Formation of carbon quantum dots and nanodiamonds in laser ablation of a carbon film

We have experimentally shown that nanosecond near-IR pulsed laser ablation of a thin amorphous carbon film produces carbon quantum dots with a graphite structure and nanodiamonds with a characteristic size of 20 – 500 nm on the substrate surface. The formation of these nanostructures is confirmed by electron microscopic images, luminescence spectra and Raman spectra. The mechanisms explaining the observed effects are proposed.

Properties of amorphous carbon thin films grown by ion beam sputtering

The electrical performance of amorphous carbon thin films obtained by the ion beam sputtering of a carbon target in argon has been investigated. It has been shown by the Raman spectroscopy method that these films have a graphite-like structure. It has also been found by conductivity and thermopower studies that the hopping mechanism of conductivity with a variable length of hops over localized states near the Fermi level changes to the mechanism of hopping over the nearest neighbors as the temperature rises from 77 to 190 K. Near room temperature, electrotransport is provided by variable-length hops over localized states at the tail of the valence band.

Correlation study of microstructure and photoluminescence properties of a-C:H thin films deposited by plasma-enhanced chemical vapor deposition technique

Hydrogenated amorphous carbon (a-C:H) thin films are deposited by plasma-enhanced chemical vapor deposition (PE-CVD) technique with different frequency (RF) powers at a low substrate temperature (fixed at 250°C). In this work, the composition, the types of chemical banding, carbon hybridization structure and optical properties of the a-C:H thin films have been studied by FTIR, XPS, UV–Vis spectrum and PL spectrum testing techniques. The results show that the hydrogen content and carbon hybridization structure play an important role in the evolution of photoluminescence (PL) properties with increasing RF power. It is found that with the decrease of sp3/sp2 ratio, the peak energy for main PL emissions peaks (mode 2) showed a red shift. PL emission intensity of the a-C:H thin films is influenced by the hydrogen content and carbon sp2 cluster. The enlargement of sp2 rich clusters and the increase of the hydrogen content will lead to an enhancement of PL emission intensity, which is a consequence that the domain of sp2 rich clusters creates more radiative recombination centers with the increase of RF power. Furthermore, different full width at half maximum (FWHM) of PL emissions peaks can be attributed to band-to-band recombination in various size sp2 rich clusters.

Charging of carbon thin films in scanning and phase-plate transmission electron microscopy

A systematic study on charging of carbon thin films under intense electron-beam irradiation was performed in a transmission electron microscope to identify the underlying physics for the functionality of hole-free phase plates. Thin amorphous carbon films fabricated by different deposition techniques and single-layer graphene were studied. Clean thin films at moderate temperatures show small negative charging while thin films kept at an elevated temperature are stable and not prone to beam-generated charging. The charging is attributed to electron-stimulated desorption (ESD) of chemisorbed water molecules from the thin-film surfaces and an accompanying change of work function. The ESD interpretation is supported by experimental results obtained by electron-energy loss spectroscopy, hole-free phase plate imaging, secondary electron detection and x-ray photoelectron spectroscopy as well as simulations of the electrostatic potential distribution. The described ESD-based model explains previous experimental findings and is of general interest to any phase-related technique in a transmission electron microscope.

Amorphous carbon thin film electrodes with intrinsic Pt-gradient for hydrogen peroxide detection

Nanoscale amorphous carbon thin films with intrinsic Pt gradient show great promise as new electrode materials for electrochemical detection of hydrogen peroxide. Embedding the Pt particles in the carbon matrix during the fabrication process allows tighter integration than, for example, adding them after the fabrication on top of the substrate. Especially, this approach can offer excellent electrochemical properties combined with CMOS compatibility, which is crucial for further device development. Here we provide extensive in depth electrochemical and physicochemical characterization of these novel materials by cyclic voltammetry (CV), chronoamperometry (CA), rotating disk electrode (RDE) experiments, transmission electron microscopy (TEM), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Equipped with these detailed results on these materials we proceed to present some suggestions how the physicochemical properties correlate with the results from electrochemical measurements. (i) It is shown that coarsening of the initially very finely dispersed structure occurs both under electron bombardment during TEM imaging as well as during cyclic voltammetry in H2SO4. (ii) Further, it is shown that OH is adsorbed on small Pt islands much more strongly compared to the bulk Pt, which may heavily influence hydrogen peroxide redox reactions on these Pt-containing amorphous carbon films. (iii) Finally, we proceed to demonstrate that despite these complications, these materials show linear response for hydrogen peroxide reduction in neutral phosphate buffered saline combined with very fast response times.

Characterization and evaluation of amorphous carbon thin film (ACTF) for sodium ion adsorption

The removal of sodium ions from aqueous solutions by adsorption onto amorphous carbon thin film (ACTF) has been studied in batch mode. In this work, the ACTF as new adsorbent was synthesized based on rice straw, then its structure and properties were taken into consideration to study its ability to adsorb sodium ions from synthetic water. The influence of pH, contact time, and temperature of the ion adsorption on ACTF was also studied using batch tests. We found that the contact time of sodium adsorption and its isothermal adsorption studied were described by pseudo-second-order kinetic model and Langmuir isotherm, respectively. Our results indicated that the adsorption of sodium ions on ACTF become be stronger and depends on pH, furthermore, the maximum adsorption capacities of sodium on ACTF recorded 107, 120 and 135 mg g−1 at 35, 45, and 65 °C. The thermodynamic parameters explain that the adsorption of sodium ions on ACTF is a spontaneous process and endothermic reaction. According to adsorption studies, we found that the ACTF can be used effectively for ion chromatography or desalinate sodium ion using ion exchange process in the hybrid desalination process with insignificant loss of adsorption capacity. However, the ACTF has better properties than any other carbon materials obtained from an agricultural byproduct.

The dependence of the structure and mechanical properties of thin ta-C coatings deposited using electromagnetic venetian blind plasma filter on their thickness

In this paper, we present the results of studies determining the optimum thickness of amorphous tetrahedral carbon (ta-C) thin films for industrial applications. ta-C coatings from 50 to 1000nm thickness were synthesized by a pulsed vacuum-arc method using a water-cooled electromagnetic Venetian blind plasma filter. Systematic studies of the thickness dependence of the microstructure and mechanical properties showed that the content of sp3 bonds in all the deposited coatings are equal (about 68%), but the 300nm thick coatings exhibited the best mechanical properties (hardness H>50GPa, adhesion Lc2=48N). These coatings were deposited on industrial cutting taps with 2 and 2.5mm diameters and tested in industrial conditions. The results of the threading tests performed with Inox stainless steel showed that the durability of the ta-C coated tools were significantly higher compared with the uncoated ones. These real-world industrial tests confirmed the usefulness of the electromagnetic Venetian blind plasma filter for synthesizing thin ta-C coatings with excellent physico-mechanical characteristics.

Improved electrochemical performance of nitrocarburised stainless steel by hydrogenated amorphous carbon thin films for bone tissue engineering

In this study, hydrogenated amorphous carbon thin films, structurally similar to diamond-like carbon (DLC), were deposited on the surface of untreated and plasma nitrocarburised (Nitrocarburizing-treated) stainless steel medical implants using a plasma-enhanced chemical vapour deposition method. The deposited DLC thin films on the nitrocarburising-treated implants (CN+DLC) exhibited an appropriate adhesion to the substrates. The results clearly indicated that the applied DLC thin films showed excellent pitting and corrosion resistance with no considerable damage on the surface in comparison with the other samples. The CN+DLC thin films could be considered as an efficient approach for improving the biocompatibility and chemical inertness of metallic implants.

Structure dependent negative and positive magnetoresistance of amorphous carbon films

Amorphous carbon thin films with a negative magnetoresistance (MR) of 13% and a positive MR of 31% at 2 K under a magnetic field of 7 T were fabricated through chemical vapor deposition (CVD) and pulsed laser deposition (PLD), respectively. Graphitic like carbon and amorphous structures of the thin films were observed by high-resolution transmission electron microscopy and X-ray diffraction. The MR decreases rapidly with the increase in temperature and vanishes after 40 K for the PLD grown sample, whereas for the CVD grown sample, the MR is observed up to 300 K. The negative MR may be due to the ordered graphitic like structures and its mechanism is explained by the weak localization theory for a lower temperature range of 2–50 K and the grain boundary scattering model for a higher temperature range of 50–300 K, whereas the positive MR may be due to its disordered amorphous structure and its mechanism is explained by the Efros-Shklovskii-type variable range hopping model.

Large magnetoresistance of amorphous carbon films

Magnetoresistance (MR) of pure amorphous carbon thin films deposited by pulsed laser deposition at various deposition temperatures was studied. Maximum MR of 46% was observed at 2 K under the magnetic field of 7 T for the sample deposited at temperature of 500 °C. No tendency of MR saturation was observed up to 7 T. The MR decreases rapidly with the increase in measurement temperature and vanishes after 40 K. The transport mechanism of all the samples follow Efros-Shklovskii variable range hopping model. The characteristics temperature decreasing from 2540 K to 1290 K and localization length increasing from 5.3 nm to 10.7 nm with increasing fraction of C(sp2) from 72% to 84%. The lower disorder degree may results in higher MR.