ta-C Thin Films Research Articles

Nanodiamonds on tetrahedral amorphous carbon significantly enhance dopamine detection and cell viability

We hypothesize that by using integrated carbon nanostructures on tetrahedral amorphous carbon (ta-C), it is possible to take the performance and characteristics of these bioelectrodes to a completely new level. The integrated carbon electrodes were realized by combining nanodiamonds (NDs) with ta-C thin films coated on Ti-coated Si-substrates. NDs were functionalized with mixture of carboxyl and amine groups NDandante or amine NDamine, carboxyl NDvox or hydroxyl groups NDH and drop-casted or spray-coated onto substrate. By utilizing these novel structures we show that (i) the detection limit for dopamine can be improved by two orders of magnitude [from 10µM to 50nM] in comparison to ta-C thin film electrodes and (ii) the coating method significantly affects electrochemical properties of NDs and (iii) the ND coatings selectively promote cell viability. NDandante and NDH showed most promising electrochemical properties. The viability of human mesenchymal stem cells and osteoblastic SaOS-2 cells was increased on all ND surfaces, whereas the viability of mouse neural stem cells and rat neuroblastic cells was improved on NDandante and NDH and reduced on NDamine and NDvox. The viability of C6 cells remained unchanged, indicating that these surfaces will not cause excess gliosis. In summary, we demonstrated here that by using functionalized NDs on ta-C thin films we can significantly improve sensitivity towards dopamine as well as selectively promote cell viability. Thus, these novel carbon nanostructures provide an interesting concept for development of various in vivo targeted sensor solutions.

Nanoscale scanning electron microscopy based graphitization in tetrahedral amorphous carbon thin films

Electron beam lithography is a powerful maskless tool to fabricate structures on the nanometer scale. Here, we show that low-keV-electron beams enable a direct patterning of tetrahedral amorphous carbon (ta-C) thin films by inducing a local graphitization without the need for any resist or development process step. Irradiation with 4 keV electrons leads to a local decrease of the ta-C film’s electrical resistance and an increase of both the sp2/sp3-ratio and the material’s work function. We investigate the impact of electron exposure on ta-C by a variety of microscopy as well as spectroscopy methods including scanning tunneling microscope-based current-distance spectroscopy, conductive atomic force microscopy, spatially resolved ultraviolet and x-ray photo emission spectroscopy, and μ-Raman spectroscopy. The electron exposure has been performed under ultrahigh vacuum conditions to prevent from electron-induced deposition of contaminants which may obstruct the application of surface-sensitive analysis techniques to the modified ta-C films.

Conducting ion tracks generated by charge-selected swift heavy ions

Conducting ion tracks in tetrahedral amorphous carbon (ta-C) thin films were generated by irradiation with swift heavy ions of well-defined charge state. The conductivity of tracks and the surface topography of the films, showing characteristic hillocks at each track position, were investigated using conductive atomic force microscopy measurements. The dependence of track conductivity and hillock size on the charge state of the ions was studied using 4.6MeV/u Pb ions of charge state 53+, 56+ and 60+ provided by GANIL, as well as 4.8MeV/u Bi and Au ions of charge state from 50+ to 61+ and 4.2MeV/u 238U ions in equilibrium charge state provided by UNILAC of GSI. For the charge state selection at GSI, an additional stripper-foil system was installed at the M-branch that now allows routine irradiations with ions of selected charge states. The conductivity of tracks in ta-C increases significantly when the charge state increases from 51+ to 60+. However, the conductivity of individual tracks on the same sample still shows large variations, indicating that tracks formed in ta-C are either inhomogeneous or the conductivity is limited by the interface between ion track and Si substrate.

Correlation between sp3-to-sp2 Ratio and Surface Oxygen Functionalities in Tetrahedral Amorphous Carbon (ta-C) Thin Film Electrodes and Implications of Their Electrochemical Properties

Detailed information about the physicochemical properties of a given surface is important in order to understand and predict the performance of materials in electrochemical applications. Here we present a detailed X-ray absorption spectroscopy study of two different tetrahedral amorphous-carbon (ta-C) thin films and their subsequent electrochemical characterization. The results show marked differences in ta-C surface and bulk properties, namely differences in the amount of surface functional groups and their sp3/sp2 ratios, respectively. In particular, the variation in the oxygen content of the surface leads to significantly different behavior in electrochemical measurements, such as a 10-fold increase in sensitivity for dopamine and stronger response to ascorbic acid. Results of the surface properties were further analyzed by simulations carried out within the framework of density functional theory (DFT) as well as by utilizing Raman spectroscopy.

Vibrational spectroscopy of Ga+ ion implanted ta-C films

In the present work, low energy Ga+ ion beam implantation was used for the structural and optical properties modification of tetrahedral amorphous carbon (ta-C) thin films, using gallium (Ga+) as the ion species. Thin film samples (d∼40nm) of ta-C, deposited by filtered cathodic vacuum arc (FCVA), have been implanted with Ga+ at ion energy E = 20 keV and ion doses D=3.1014÷3.1015 cm-2. The Ga+ ion beam induced structural modification of the implanted material results in a considerable change of its optical properties, displayed in a significant shift of the optical absorption edge to lower photon energies as obtained from optical transmission measurements. This shift is accompanied by a considerable increase of the absorption coefficient (photo-darkening effect) in the measured photon energy range (0.5÷3.0 eV). These effects could be attributed both to additional defect introduction and increased graphitisation, as well as to accompanying formation of bonds between the implanted ions and the host atoms of the target, as confirmed by infra-red (IR) and Raman measurements. The optical contrast thus obtained (between implanted and unimplanted film material) could be made use of for information archiving, in the area of high-density optical data storage, while using focused Ga+ ion beams.

Structural morphology of carbon nanofibers grown on different substrates

We present a detailed microstructural study comparing conventional carbon nanofibers (CNFs) and novel carbon hybrid CNF materials. The hybrid consists of CNFs grown on top of tetrahedral amorphous carbon (ta-C) thin films on silicon with nickel catalyst and Ti adhesion layers. The conventional CNFs were grown on silicon with nickel catalyst and Cr layers. Even though CNFs can be grown in both systems by tip growth, the micro- and nanoscale features are very different in the two systems. The crystalline structure of the CNF in the hybrid case changes from horizontal alignment to near-vertical alignment from the root to the tip and no bamboo structure is observed. The results show that micro- and nanoscale properties of CNFs grown under the same process conditions can be readily altered by using a sacrificial ta-C layer below the metallic layer to prevent the alloying of Ni with carbide-forming metals used as adhesion promoters and to act as an additional carbon source during the pre-annealing stage. The experimental results are further rationalized with the aid of assessed thermodynamic data and simulations based on density functional theory (DFT) with van der Waals (vdW) corrections.

Electrochemical reactions of catechol, methylcatechol and dopamine at tetrahedral amorphous carbon (ta-C) thin film electrodes

The electrochemical reactions of dopamine, catechol and methylcatechol were investigated at tetrahedral amorphous carbon (ta-C) thin film electrodes. In order to better understand the reaction mechanisms of these molecules, cyclic voltammetry with varying scan rates was carried out at different pH values in H2SO4 and PBS solutions. The results were compared to the same redox reactions taking place at glassy carbon (GC) electrodes. All three catechols exhibited quasi-reversible behavior with sluggish electron transfer kinetics at the ta-C electrode. At neutral and alkaline pH, rapid coupled homogeneous reactions followed the oxidation of the catechols to the corresponding o-quinones and led to significant deterioration of the electrode response. At acidic pH, the extent of deterioration was considerably lower. All the redox reactions showed significantly faster electron transfer kinetics at the GC electrode and it was less susceptible toward surface passivation. An EC mechanism was observed for the oxidation of dopamine at both ta-C and GC electrodes and the formation of polydopamine was suspected to cause the passivation of the electrodes.

Multi-walled carbon nanotubes (MWCNTs) grown directly on tetrahedral amorphous carbon (ta-C): An interfacial study

In this paper we present the results on the microstructure of a novel carbon hybrid material consisting of multi-walled carbon nanotubes (MWCNTs) grown on the top of tetrahedral amorphous carbon (ta-C) thin films on Si substrates with Ti adhesion promotion layer on top. It is shown here that: (i) the synthesis of the MWCNTs on top of ta-C layer at low temperatures is possible by using Al/Co/Fe trimetallic catalyst, (ii) the catalytic particles formed and used in the growth of MWCNTs form an FeCo alloy during the activation step, (iii) in addition to the catalytic particles, Fe-silicides with variable amount of dissolved Co are formed on both sides of the Ti adhesion layer and therefore it can be concluded that all species (Co, Fe and Si) diffuse during the formation of the silicides, (iv) Ti adhesion layer remains almost unaffected by the growth process and in particular no significant TiSi2 or TiCx layer formation takes place, (v) the MWCNT that are grown on top of ta-C are not well oriented and appear to have high defect densities and (vi) ta-C layer is fully replaced by a graphitic layer during the growth of MWCNTs.

Laser interference induced nano-crystallized surface swellings of amorphous carbon for advanced micro tribology

We have investigated the direct laser interference patterning (DLIP) of tetrahedral amorphous carbon (ta-C) thin films in the nanometer region. In this paper we achieved texturing of ta-C from a surface swelling either by a phase change from sp3 to sp2 or thin film delamination. In contrast to surface ablation, the thin film is not removed from the silicon surface. With this technique a surface nano-swelling of ta-C with an area-speed of 1 m2 min−1 and nanometric resolution can be realized. The phase changes in the ta-C, due to DLIP material heating are undermined by thermal simulations of the patterning process. Additionally we investigated for the first time the influence of periodic nano-swelling on the coefficient of friction (COF). The COF of periodic graphitized surfaces was found to be lower (∼50%) compared to as grown ta-C. This reduction of the COF can be attributed to a reduced area of contact.

Stoichiometry behavior of TaN, TaCN and TaC thin films produced by magnetron sputtering

Thin films were synthesized in a magnetron sputtering system using a target of Ta with 99.99% purity and silicon substrates (111). The gases used for the film growth were (Ar+N2), (Ar+CH4+N2) and (Ar+CH4) mixtures for TaN, TaCN and TaC, respectively. The substrate temperature increased from room temperature to 500°C. The chemical composition and bonding configuration were examined using X-ray photoelectron spectroscopy (XPS), revealing TaN, TaCN, TaC and CC bonds. Moreover, the crystallographic structure was analyzed using X-ray diffraction (XRD), indicating the presence of (111) and (200) planes belonging to a face-centered cubic structure. The stoichiometry variation dependence on the CH4 and N2 flow was analyzed, and the influence of the substrate temperature on the coatings was investigated. Finally, scanning electron microscopy (SEM) was used to determine the evolution on the grain formation in the coatings as the substrate temperature increased.

Electroanalytical Performance of Nitrogen-Containing Tetrahedral Amorphous Carbon Thin-Film Electrodes

Tetrahedral amorphous carbon (ta-C) consists of a mixture of sp(3)- and sp(2)-bonded carbon ranging from 60 to 40% (sp(3)/sp(3)+sp(2)) depending on the deposition conditions. The physical, chemical, and electrochemical properties depend on the sp(2)/sp(3) bonding ratio as well as the presence of incorporated impurities, such as hydrogen or nitrogen. The ability to grow ta-C at lower temperatures (25-100 °C) on a wider variety of substrates as compared to CVD diamond is an advantage of this material. Herein, we report on the structural and electrochemical properties of nitrogen-incorporated ta-C thin films (ta-C:N). The incorporation of nitrogen into the films decreases the electrical resistivity from 613 ± 60 (0 sccm N(2)) to 1.10 ± 0.07 Ω-cm (50 sccm N(2)), presumably by increasing the sp(2)-bonded carbon content and the connectedness of these domains. Similar to boron-doped diamond, these materials are characterized by a low background voltammetric current, a wide working potential window (~ 3 V), and relatively rapid electron-transfer kinetics for aqueous redox systems, including Fe(CN)(6)(-3/-4) and Ru(NH(3))(6)(+3/+2), without conventional pretreatment. Additionally, there is weak molecular adsorption of polar molecules (methylene blue) on the ta-C surface. Overall, the properties of the ta-C and ta-C:N electrodes are such that they could be excellent new choices for electroanalytical measurements.

Fabrication and bio-functionalization of tetrahedral amorphous carbon thin films for bio sensor applications

Tetrahedral amorphous carbon (ta-C) thin films are a promising material for use as biocompatible interfaces in applications such as in-vivo biosensors. However, the functionalization of ta-C film surface, which is a pre-requisite for biosensors, remains a big challenge due to its chemical inertness. We have investigated the bio-functionalization of ta-C films fabricated under specific physical conditions through the covalent attachment of functional biomolecular probes of peptide nucleic acid (PNA) to ta-C films, and the effect of fabrication conditions on the bio-functionalization. The study showed that the functional bimolecular probes such as protected long-chain ω-unsaturated amine (TFAAD) can be covalently attached to the ta-C surface through a well-defined structure. With the given fabrication process, electrochemical methods can be applied to the detection of biomolecular interaction, which establishes the basis for the development of stable, label-free biosensors.

Surface modification of thin tetrahedral amorphous carbon films by means of UV direct laser interference patterning

In this work, direct laser interference patterning of hydrogen-free tetrahedral amorphous carbon (ta-C) thin films using an ultraviolet (UV) nanosecond pulsed laser is investigated. Using this method, line-like arrays with submicrometer resolution were fabricated. In particular, a 180 nm grating period was successfully produced. In addition, it was found that depending on the laser energy density, the tetrahedral carbon film either graphitizes or crystallizes locally at the interference maxima positions. Furthermore, the crystallization is accompanied with a delamination of the film and even the formation of carbon fiber-like structures. The electrical properties of graphitized and delaminated ta-C were measured.

Effect of 4MeV electron beam irradiation on carbon films

Pulsed KrF excimer laser is used to deposit tetrahedral amorphous carbon (ta-C) thin films on Si (1 1 1) single crystal substrates at room temperature under vacuum ∼10 −6 mbars. The pristine deposited films are then irradiated by 4 MeV electron beam at doses varying from 1000 to 4000 cGy. Analysis through AFM illustrates that the irradiation of electron has induced cluster formation on the film surface and increased the surface roughness. Optical properties ( n, α and Tauc optical band gap) measured by spectroscopic ellipsometry and electrical resistivity measured by four probe technique are found to depend strongly on electron dose. High electron doses cause significant alteration in order/disorder or sp 2 states in the film which is the main cause of modifying band gap in the carbon films. The electrical conductivity of the films also increases by increasing electron dose which is due to tunneling of charge carriers through neighbouring conductive chains. The present electron irradiation process at varying electron doses proved to be successful to modulate the optical and electrical properties of carbon films.

Zirconium-based coatings in highly stressed rolling contacts as alternative solution to DLC and ta-C coatings

Abstract The slip-rolling resistance of DLC, a-C and ta-C thin film coatings was improved considerably in the last years, but there is still room for improvements in relation to (a) the temperature stability and (b) initial surface roughness after deposition. This paper presents a novel coating-substrate system in comparison to recently developed DLC coatings in a bench-mark test procedure exerting slip-rolling conditions in the presence of liquid lubricants. One of the Zr-based thin film coatings can withstand at least 1 million cycles under initial Hertzian contact pressures of up to P 0max = 3500 MPa and oil temperatures of at least 120 °C associated with low coefficients of friction under mixed/boundary conditions or ten million of cycles under P 0max = 2940 MPa. In comparison, some of the newly developed DLC coatings are slip-rolling resistant for at least up to 10 million cycles at RT (some of them also at 120 °C oil temperature) under Hertzian contact pressures of P max = 2600/2940 MPa. In general, Zr-based thin film coatings do not require special formulated oil formulations and Zr(C,N) bear on a straight coating architecture suited for mass production with nanosized layers.

Thermal stability and rehybridization of carbon bonding in tetrahedral amorphous carbon

We perform a quantitative investigation of the energetics of thermally induced sp3→sp2 conversion of carbon-carbon bonds in tetrahedral amorphous carbon (ta-C) films by using near-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy. We investigate the evolution of the bonding configuration in ta-C thin films subjected to high temperature annealing in flowing Argon gas using a rapid thermal annealing furnace over the range of 200–1000 °C. We observe no substantial change in bonding structure below 600 °C. Changes in the NEXAFS and Raman spectra start to appear above 600 °C, and by 1000 °C a significant increase in the sp2 bonding in the film is observed. No oxygen bonding is detected in the NEXAFS spectra, but we do observe an isosbestic point, demonstrating that the thermally driven sp3→sp2 conversion reaction occurs without passing through an intermediate transition state. This allows us to use NEXAFS spectra of thermally annealed ta-C films to quantitatively determine that the activation energy for directly converting the sp3-bonded carbon to the sp2-bonded configuration is 3.5±0.9 eV.

Phase composition and morphology of TaC coating on carbon fibers by chemical vapor infiltration

Chemical vapor infiltration was applied to deposit TaC thin film on carbon fibers using TaCl 5–Ar–C 3H 6–H 2 source. The influence of temperature, pressure and addition of H 2 on phase composition and morphology of the coating had been studied. The results showed that the composition of the coating changes with temperature. The films are mainly composed of TaC and C when deposited at 800–900 °C and C phase disappears at 950 °C and 1000 °C. TaC grains are very small at 800 °C and get bigger at 900 °C; while at 1000 °C, a dense coating has developed. The influence of pressure on coating composition is much weaker when deposited at 850 °C and the films are consisted of TaC and C phases at 0.6–8.0 kPa; TaC grains formed at 0.6 kPa are smaller and more homogeneous than those obtained at 8.0 kPa. With 100 ml/min H 2 flow, the coating composition changes and turns into a mixture of TaC and Ta 2C with fine and homogeneous grains at 850 °C and coarse grains at 950 °C.

Epitaxy and recrystallization kinetics of TaC thin films on SiC for high temperature processing of semiconductor devices

The authors report the epitaxial growth of cubic TaC thin films on 4H-SiC (0001) substrates by pulsed laser deposition and their recrystallization kinetics. The growth temperature for epitaxy was found to be 1000°C. The activation energy for the recrystallization was 1.5eV. High temperature annealing produced changes in the surface morphology which caused grain growth and created pinholes through a recrystallization process in the films deposited at lower temperatures. The films deposited at higher temperatures remained virtually unchanged. Implications of these results are discussed in the context of the development of future high temperature and high-power electronic devices.

Magnetic Rare Earth (Gd) Implanted Tetrahedral Amorphous Carbon (ta-C)

ABSTRACTTetrahedrally bonded amorphous carbon (ta-C) thin films were prepared by mass selected ion beam deposition (MSIBD) using 100 eV carbon ions at room temperature. Gadolinium, a magnetic rare earth element, was implanted as a dopant into ta-C with two different fluences. The doping level of the ta-C:Gdx layers was in maximum 4 or 7 at.%, respectively. The Gd is believed to be electrically activated in as-implanted films, although contributions from the sp2 sites cannot be ruled out. A transition temperature (T') was found, below which there is a large negative magnetoresistance (MR) with a strong temperature dependence. Thermal annealing greatly increases the sample conductivity due to the increase of sp2 sites. However, the MR persists at least up to an annealing temperature of 500°C. Magnetically, the Gd dopants behave like non-interacting local moments. Similarities and differences in physical properties between the ta-C:Gdx films and other Gd doped amorphous semiconductors are compared.

Temperature dependence of the mechanical properties of tetrahedrally coordinated amorphous carbon thin films

The complete elastic properties of tetrahedrally coordinated amorphous carbon (ta-C) thin films have been measured in the temperature range of 300–873K. Flexural and torsional mechanical oscillators were fabricated from ta-C, and using the resonant frequency of the oscillators as a function of temperature, we calculated the temperature-dependent Young’s and shear moduli (658±24 and 271±6.6GPa, at 300K, respectively). From these values, we calculated the bulk modulus, Poisson’s ratio, and the elastic stiffness and compliance constants as a function of temperature. In addition, the temperature dependence of the coefficient of thermal expansion of ta-C was determined using a wafer curvature technique.