Tetrahedral Amorphous Carbon Thin Films Research Articles

Development of smart molecularly imprinted tetrahedral amorphous carbon thin films for in vitro dopamine sensing

This study investigates how varying the thickness of tetrahedral amorphous carbon (ta-C) thin films and incorporating a titanium adhesion layer influences the structural and electrochemical properties of molecularly imprinted ta-C thin film-based sensing platforms, aiming to develop a molecularly imprinted ta-C electrochemical sensor for dopamine (DA) detection with physiologically relevant sensitivity. This electrochemical sensing platform was designed by integrating ta-C with molecularly imprinted polymers (MIPs). The process involved depositing a ta-C thin film onto boron-doped p-type silicon wafers through a filtered cathodic vacuum arc (FCVA) system. Subsequently, the ta-C sensing platforms were electrochemically coated with the MIP layer (DA-imprinted polypyrrole). We evaluated three configurations: (i) a 15 nm ta-C layer, (ii) a 7 nm ta-C layer with a 20 nm titanium adhesion layer, and (iii) a 15 nm ta-C layer with a 20 nm titanium adhesion layer. Comprehensive structural and electrochemical characterization was performed to understand how these modifications affect sensor performance. The optimized MIP/ta-C sensor demonstrated a sensitivity of 0.16 μA μM−1 cm−2 and a limit of detection (LOD) of 48.6 nM, suitable for detecting DA at physiological levels. Leveraging the synergistic effects of ta-C coatings and molecular imprinting, as well as its compatibility with common complementary metal–oxide–semiconductor (CMOS) processes underlines its potential for integration into microanalytical systems, paving the way for miniaturized and high-throughput sensing platforms.

The inner workings of sp2 clusters in nitrogen-incorporated tetrahedral amorphous carbon thin films according to optical analyses

The conduction mechanism in nitrogen-incorporated tetrahedral amorphous carbon (ta-C: N) thin films is mainly attributed to the conjugation in sp2 clusters. Here, ta-C:N thin films (about 100 nm) were deposited by laser controlled pulsed cathodic vacuum arc (Laser-Arc) at different nitrogen gas flow rates to investigate the electrical properties of such films. Mechanical and structural properties indicate an increase in sp2 fraction with a rise in nitrogen incorporation into the deposited films. However, after an initial decrease in electrical resistivity, it plateaus at higher flow rates while the sp2 content is still rising. Hence, to gain better understanding of the electronic properties, the dielectric constants ε1 and ε2 were obtained from the optical constants [n] and [k], which were numerically determined from reflectance [R] and transmittance [T] measurements in the wavelength range from 190 to 2500 nm (0.5 to 6.5 eV). The dielectric constant ε1 was used to calculate the dielectric volume of the atom, which is interpreted as a measure of the conjugated π electron system. Furthermore, modeling of the dielectric constants yielded the number of conjugated electrons per atom. Through the dielectric volume and number of conjugated electrons, it was discovered that the electrical conductivity does not only depend on the sp2 content and cluster size, but on how many electrons are conjugated and how much space these electrons occupy. The ta-C:N samples with the highest electrical conductivity, were either characterized by the largest occupied volume or the highest number of conjugated electrons.

Filtered cathodic vacuum arc deposition of tetrahedral amorphous carbon thin films on surgical blades and its corrosion resistance in phosphate buffer solution (PBS) at pH 7.4

Surgical blades are prone to corrosion in a body fluid environment, increasing the risk of surgical infection and inflammation. In this paper, the ta-C thin films was deposited on surgical blades by filtered cathode vacuum arc (FCVA) method. Modern analytical methods such as SEM, Raman, surface profiler and electrochemical corrosion were used for the test and analysis. The results show that the corrosion potential and corrosion current density of uncoated surgical blades are −507 mV and 2.43 μA/cm2 during surgery operation in a simulated human environment (pH 7.4 PBS solution), and the corrosion potential of the ta-C thin films surgical blades immersed in 0.25 h, 0.5 h and 1 h are −395 mV, −431 mV, −445 mV, and the corrosion current density are 1.11 μA/cm2, 1.46 μA/cm2, 1.58 μA/ cm2. The results show that the ta-C thin films surgical blades are excellent corrosion resistance during surgery operation in a simulated human environment. SEM shows that the ta-C thin films surgical blades have a smooth surface with a compact and uniform films structure, and the thicknesses of the ta-C thin films and Ti transition layer are 2.2 µm and 2.4 µm respectively. The surface profiler shows that the average roughness of the surgical blades and the ta-C thin films surgical blades are 295.979 nm and 477.417 nm. Raman shows that ta-C thin films is of high purity, without any impurity phase, and does not cause other adverse reactions during surgery operation.

An ultra-sensitive dopamine measurement platform based on molecularly imprinted polymer-carbon hybrid nanomaterials for in vitro use

In the present study, we designed an ultrasensitive sensing platform for the evaluation of the physiologically relevant values of basal dopamine (DA) in a culture medium as a complex biological environment. The proposed sensing platform was fabricated via the integration of molecular imprinting technology with carbon hybrid nanomaterials. Carbon nanofibers (CNFs) were grown by using plasma-enhanced chemical vapor deposition (PECVD) on tetrahedral amorphous carbon (ta-C) thin films on silicon wafers. The prepared ta-C/CNFs sensing platforms were electrochemically coated with DA-imprinted polypyrrole as the molecularly imprinted polymer (MIP) or "artificial receptors". The three-dimensional MIP receptors were able to determine trace values of DA in phosphate-buffered saline solution (PBS) pH 7.4 (LOD = 5.43 nM) as well as in the absolute culture media such as DMEM/F-12 medium (LOD = 39 nM), DMEM/F-12 medium supplemented with 15% horse serum and 2.5% fetal bovine serum (LOD = 53.26 nM), and F-12 K cell culture medium (LOD = 62.57 nM), with highly physiologically relevant sensitivity and free of interference by other coexisting biomolecules and biological compounds. As all the fabrication steps of the composite electrode are compatible with common microsystem technology processes, the present results pave the way for integrating these ultra-sensitive electrodes to microelectrode arrays (MEA) platforms used for human dopaminergic neurons studies in vitro and enable continuous measurement of the basal DA concentration in real-time for instance in organoid studies.

Investigation of Vacuum Arc-Deposited ta-C and ta-C:N Thin Films on Silicon and Stainless-Steel Foil Substrates Using Raman Spectroscopy

In this paper, we investigated the vacuum arc-deposited tetrahedral amorphous carbon (ta-C) and nitrogen-doped ta-C (ta-C:N) thin films grown on silicon and stainless-steel foil substrates by the visible Raman Spectroscopy. We found that carbon films grown on silicon surfaces prefer more sp3 hybridized carbon compared to the stainless-steel foil, which prefer more sp2 hybridized carbon even though the films are grown concurrently under the same conditions. The impact energy of plasma ions modifies the interfacial layer formation, giving a strong dependence of the sp2/sp3 ratio with the substrate bias, behaving differently for different substrates. However, nitrogen doping in amorphous carbon thin films helps to increase sp2 contents uniformly on both the surfaces for a fixed substrate bias. Understanding such interfaces are of interest for many electronic, optoelectronic, and energy storage devices as these interfaces get buried and can influence the properties significantly.

Electrochemical Generation of Hot Electrons in Fully Aqueous Solutions at Tetrahedral Amorphous Carbon Thin Film Electrodes and Electrochemiluminescence Immunoassay of Serum Amyloid A

AbstractElectrochemistry of hot electrons in fully aqueous solutions at tetrahedral amorphous carbon thin film electrodes is discussed. The generation of these highly reducing chemical species was confirmed by normal pulse voltammetry and several electrochemiluminescent systems. Electron transfer into pre‐existing solvent cavities was observed at approximately −2.65 V vs. Ag/AgCl (sat.). Electrogenerated hot electrons were utilized as chemiluminescent mediators in heterogeneous sandwich immunoassay of Serum Amyloid A. The calibration curve was linear over four orders of magnitude and the detection limit was 85 ng L−1 that demonstrates the efficiency of hot electron generation at this electrode material.

What Determines the Electrochemical Properties of Nitrogenated Amorphous Carbon Thin Films?

Linking structural and compositional features with the observed electrochemical performance is often ambiguous and sensitive to known and unknown impurities. Here an extensive experimental investigation augmented by computational analyses is linked to the electrochemical characterization of in situ nitrogen-doped tetrahedral amorphous carbon thin films (ta-C:N). Raman spectroscopy combined with X-ray reflectivity shows nitrogen disrupting the sp3 C–C structure of the reference ta-C, supported by the observations of graphitic nitrogen substitution in X-ray absorption spectroscopy. The surface roughness also increases, as observed in atomic force microscopy and atomic-level computational analyses. These changes are linked to significant increases in the hydrogen and oxygen content of the films by utilizing time-of-flight elastic recoil detection analysis. The conductivity of the films increases as a function of the nitrogen content, which is seen as a facile reversible outer-sphere redox reaction on ta-C:N electrodes. However, for the surface-sensitive inner-sphere redox (ISR) analytes, it is shown that the electrochemical response instead follows the oxygen and hydrogen content. We argue that the passivation of the required surface adsorption sites by hydrogen decreases the rates of all of the chemically different ISR probes investigated on nitrogenated surfaces significantly below that of the nitrogen-free reference sample. This hypothesis can be used to readily rationalize many of the contradictory electrochemical results reported in the literature.

Modification of graphene-like, hydrogenated amorphous, hydrogenated tetrahedral amorphous carbon and amorphous carbon thin films by UV-C light

The study aims to explore the effect of low fluence UV-C radiation on the structural quality of thin carbon films. We have modified single to few-layered nano-sized graphene-like films deposited by pulsed laser deposition (PLD) on ~300 nm SiO2/Si substrates and additionally different hydrogenated amorphous carbon a-C:H, tetrahedral amorphous carbon (ta-C:H) and amorphous carbon (a-C) thin carbon films. The modification was carried out by irradiation of the samples with UV-C lamps (Hg lamps λ= 254 nm wavelength and fluence of about 2 × 10−3 W/cm−2) for 5- 30 min in air-atmosphere for graphene-like and up to 60 min for thin carbon films. The irradiated graphene-like films were oriented either at about 2 arcdeg to the impinging light, i.e. the light was almost parallel to the honey-comb plane of graphene film or perpendicular to the UV-C light. The thin carbon films were treated only by a light directed perpendicularly to the films' surface for 60 min. Films were studied before and right after UV-C modification by ellipsometry or profilometry, X-ray photoelectron and Raman spectroscopies to clarify the influence of the UV-C treatment. The most pronounced influence of the UV-C irradiation on the structural quality of the films was established on a-C:H and ta-C:H films where the sp3 and the oxygen-containing radicals content decrease moderately and drops significantly, respectively. The influence of the UV-C irradiation directed almost parallel to the films’ surface on the structural quality of the graphene-like films was slightly higher than that directed perpendicularly. No significant influence on the quality of a-C films synthesized or a-C:H films annealed at high temperatures (1020–1050) °C was observed.

High-Pressure Tetrahedral Amorphous Carbon Synthesized by Compressing Glassy Carbon at Room Temperature

Tetrahedral amorphous carbon(ta-C) thin films with high sp3 fraction have extraordinary mechanical properties and wide applications. Despite intensive effort to increase the thickness of ta-C thin ...

Stress relaxation through thermal gradient structure of tetrahedral amorphous carbon thin film deposited on Ge–Se–Sb-based chalcogenide glass

Surface thermal gradient effects, which can improve interfacial adhesion between chalcogenide glasses and tetrahedral amorphous carbon (ta-C) films, are used to relax intrinsic stress at the ta-C deposition temperature. The intrinsic stress of ta-C is difficult to control through substrate bias voltage applied to the insulating substrate; however, under increased deposition temperatures, thermal stress is generated by the different thermal expansion coefficients of the ta-C film and the chalcogenide glass. Here, an ion source was used to increase the substrate temperature at the initial stage of deposition to simultaneously relax the thermal and intrinsic stresses. The 100-nm-thick ta-C coating film was applied by filtered cathodic vacuum arc deposition. The ta-C coating was deposited at varying surface temperatures. At the effective surface temperature of 80 °C, the adhesive strength exceeded 12.5 N/mm. In Raman and infrared (IR) transmittance spectral results, the I(D)/I(G) ratio was increased and the IR transmittance was decreased by 2%.

In-situ functionalization of tetrahedral amorphous carbon by filtered cathodic arc deposition

Modification of the surface chemistry of carbon-based nanomaterials is often necessary in order to embrace their full potential. A wide variety of different post-fabrication treatments, such as acid, oxidizing plasma and heat treatments have been described in the literature. However, their specific effects on the materials surface chemistry is typically only vaguely disclosed. Here we report an in-situ method to functionalize tetrahedral amorphous carbon (ta-C) thin films by introducing high purity oxygen into the vacuum chamber during the film fabrication. Additionally, we analyze and compare the material properties of the resulting thin films to those of nitric acid and oxygen plasma treated as well as those with no treatment at all. Using x-ray absorption spectroscopy (XAS), we show that in-situ functionalizing decreases the sp2 content of the surface and increases the amount of carboxyl-like functionalities. Subsequent oxygen plasma treatment further decreases the sp2 fraction and ketone/aldehyde content as well as increases the amount of carboxyl groups. The same trends are observed with the reference ta-C exposed to oxygen plasma treatment. For both materials, a concentrated nitric acid treatment has only a subtle effect on the surface chemistry. Capitalizing on this knowledge, we can selectively produce materials with higher surface loading of specific functional groups, paving the way for application specific material fabrication.

Fabrication of Micro- and Nanopillars from Pyrolytic Carbon and Tetrahedral Amorphous Carbon.

Pattern formation of pyrolyzed carbon (PyC) and tetrahedral amorphous carbon (ta-C) thin films were investigated at micro- and nanoscale. Micro- and nanopillars were fabricated from both materials, and their biocompatibility was studied with cell viability tests. Carbon materials are known to be very challenging to pattern. Here we demonstrate two approaches to create biocompatible carbon features. The microtopographies were 2 m or 20 m pillars (1:1 aspect ratio) with three different pillar layouts (square-grid, hexa-grid, or random-grid orientation). The nanoscale topography consisted of random nanopillars fabricated by maskless anisotropic etching. The PyC structures were fabricated with photolithography and embossing techniques in SU-8 photopolymer which was pyrolyzed in an inert atmosphere. The ta-C is a thin film coating, and the structures for it were fabricated on silicon substrates. Despite different fabrication methods, both materials were formed into comparable micro- and nanostructures. Mouse neural stem cells were cultured on the samples (without any coatings) and their viability was evaluated with colorimetric viability assay. All samples expressed good biocompatibility, but the topography has only a minor effect on viability. Two m pillars in ta-C shows increased cell count and aggregation compared to planar ta-C reference sample. The presented materials and fabrication techniques are well suited for applications that require carbon chemistry and benefit from large surface area and topography, such as electrophysiological and -chemical sensors for in vivo and in vitro measurements.

Control of residual stress of tetrahedral amorphous carbon thin film deposited on dielectric material by filtered cathodic vacuum arc source by using mid-frequency pulse bias voltage

It was shown that the residual stress of the tetrahedral amorphous carbon (ta-C) thin film deposited on insulating material by filtered cathodic vacuum arc (FCVA) source can be controlled with mid-frequency pulse bias voltage. The residual stress of ta-C thin film decreased monotonically with the substrate pulse bias when the base material to be coated was insulating. There was not the apparent peak of residual stress about the substrate pulse bias which appeared for the electrically conducting base material. It was observed that the residual stress increased with the arc current and dielectric thickness at some pulse voltage range (100 V–1000 V). The sp3 contents calculated by using Raman spectra and the surface hardness values as a function of substrate pulse voltage showed the similar trend with the residual stress values. For ta-C thin film deposited on the polished alumina sample, it was shown that the critical load in the scratch test decreased with the residual stress. The residual stress of ta-C on the insulating base material was explained with the change of the sheath voltage in the equivalent circuit model.

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.

Partially Reduced Graphene Oxide Modified Tetrahedral Amorphous Carbon Thin-Film Electrodes as a Platform for Nanomolar Detection of Dopamine

In this study we present for the first time tetrahedral amorphous carbon (ta-C)—a partially reduced graphene oxide (PRGO) hybrid electrode nanomaterial platform for electrochemical sensing of dopamine (DA). Graphene oxide was synthesized with the modified Hummer’s method. Before modification of ta-C by drop casting, partial reduction of the GO was carried out to improve electrochemical properties and adhesion to the ta-C thin film. A facile nitric acid treatment that slightly reoxidized the surface and modified the surface chemistry was subsequently performed to further improve the electrochemical properties of the electrodes. The largest relative increase was seen in carboxyl groups. The HNO3 treatment increased the sensitivity toward DA and AA and resulted in a cathodic shift in the oxidation of AA. The fabricated hybrid electrodes were characterized with scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray...

Electron transport determines the electrochemical properties of tetrahedral amorphous carbon (ta-C) thin films

Amorphous carbon based electrodes are very promising for electrochemical sensing applications. In order to better understand their structure-function relationship, the effect of film thickness on the electrochemical properties of tetrahedral amorphous carbon (ta-C) electrodes was investigated. ta-C thin films of 7, 15, 30, 50 and 100nm were characterized in detail with Raman spectroscopy, transmission electron microscopy (TEM), conductive atomic force microscopy (c-AFM), scanning tunneling spectroscopy (STS) and X-ray absorption spectroscopy (XAS) to assess (i) the surface properties of the films, (ii) the effect of film thickness on their structure and electrical properties and (iii) the subsequent correlation with their electrochemistry. The electrochemical properties were investigated by cyclic voltammetry (CV) using two different outer-sphere redox probes, Ru(NH3)63+/2+ and FcMeOH, and by electrochemical impedance spectroscopy (EIS). Computational simulations using density functional theory (DFT) were carried out to rationalize the experimental findings. The characterization results showed that the sp2/sp3 ratio increased with decreasing ta-C film thickness. This correlated with a decrease in mobility gap value and an increase in the average current through the films, which was also consistent with the computational results. XAS indicated that the surface of the ta-C films was always identical and composed of a sp2-rich layer. The CV measurements indicated reversible reaction kinetics for both outer-sphere redox probes at 7 and 15nm ta-C films with a change to quasi-reversible behavior at a thickness of around 30nm. The charge transfer resistance, obtained from EIS measurements, decreased with decreasing film thickness in accordance with the CV results. Based on the characterization and electrochemical results, we conclude that the reaction kinetics in the case of outer-sphere redox systems is determined mainly by the electron transport through the films and not the electron transfer between the redox probe and the electrode surface, since the surface region is expected to be identical in all the ta-C films. Finally, it was shown that a titanium interlayer between the Si substrate and ta-C significantly affected the electrical and electrochemical properties of the electrodes, further emphasizing the role of electron transport through the film in determining the electrochemical behavior of ta-C.

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.

Correction: Evaluation of a nitrogen-incorporated tetrahedral amorphous carbon thin film for the detection of tryptophan and tyrosine using flow injection analysis with amperometric detection.

Correction for 'Evaluation of a nitrogen-incorporated tetrahedral amorphous carbon thin film for the detection of tryptophan and tyrosine using flow injection analysis with amperometric detection' by Romana Jarošová, et al., Analyst, 2016, DOI: .