Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

Preparation of tetrahedral amorphous carbon films by filtered cathodic vacuum arc deposition

Tetrahedrally bonded amorphous carbon (ta-C) and nitrogen doped (ta-C:N) films were obtained at room temperature in a filtered cathodic vacuum arc (FCVA) system incorporating an off-plane double bend (S-bend) magnetic filter. The influence of the negative bias voltage applied to substrates (from −20 to −350 V) and the nitrogen background pressure (up to 10⁻³ Torr) on film properties was studied by scanning electron microscopy (SEM), electron energy loss spectroscopy (EELS), Raman spectroscopy, X-ray photoemission spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and X-ray reflectivity (XRR). The ta-C films showed sp³ fractions between 84% and 88%, and mass densities around 3.2 g/cm³ in the wide range of bias voltage studied. In contrast, the compressive stress showed a maximum value of 11 GPa for bias voltages around −90 V, whereas for lower and higher bias voltages the stress decreased to 6 GPa. As for the ta-C:N films grown at bias voltages below −200 V and with N contents up to 7%, it has been found that the N atoms were preferentially sp³ bonded to the carbon network with a reduction in stress below 8 GPa. Further increase in bias voltage or N content increased the sp² fraction, leading to a reduction in film density to 2.7 g/cm³.     

New electrochemically improved tetrahedral amorphous carbon films for biological applications

Carbon based materials have been frequently used to detect different biomolecules. For example high sp³ containing hydrogen free diamond-like carbon (DLC) possesses many properties that are beneficial for biosensor applications. Unfortunately, the sensitivities of the DLC electrodes are typically low. Here we demonstrate that by introducing topography on the DLC surface and by varying its layer thickness, it is possible to significantly increase the sensitivity of DLC thin film electrodes towards dopamine. The electrode structures are characterized in detail by atomic force microscopy (AFM) and conductive atomic force microscopy (C-AFM) as well as by transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS). With cyclic voltammetry (CV) measurements we demonstrate that the new improved DLC electrode has a very wide water window, but at the same time it also exhibits fast electron transfer rate at the electrode/solution interface. In addition, it is shown that the sensitivity towards dopamine is increased up to two orders of magnitude in comparison to the previously fabricated DLC films, which are used as benchmarks in this investigation. Finally, it is shown, based on the cyclic voltammetry measurements that dopamine exhibits highly complex behavior on top of these carbon electrodes.     

Heat treatment of tetrahedral amorphous carbon films grown by filtered cathodic vacuum-arc technique

Tetrahedral amorphous carbon (ta-C) is a potential low-cost substitute for diamond in certain applications, but little is known of the temperature range over which its desirable properties are retained. The thermal stability of tetrahedral amorphous carbon (ta-C) films has been investigated by heat treatment of the films at temperatures from room temperature to 450°C in high vacuum, low vacuum and oxygen ambient. It was found that heat treatment in oxygen ambient leads to a much more prominent variation in film thickness, stress and hardness than in both low and high vacuum. Raman studies also show an increase of the G-band frequency to higher values, an increase of the integrated intensity ratio and a narrowing of the G bands for films annealed in oxygen ambient with increasing temperature. By contrast, ta-C films exhibit a high resistance to degradation during treatment in low and high vacuum. They sustain their structure, thickness, stress and hardness for temperatures up to 400°C.     

Microstructure effect on field emission from tetrahedral amorphous carbon films annealed in nitrogen and acetylene ambient

Tetrahedral amorphous carbon (ta-C) films have been deposited by filtered cathodic vacuum arc technique. The samples were then annealed at various temperatures in nitrogen and acetylene ambient. The surface morphologies and microstructure of the films were characterized using atomic force microscopy, scanning electron microscopy, visible and ultraviolet Raman spectroscopy. A thin layer of amorphous carbon was deposited on the surface of the ta-C films after annealed at 700 and 800 °C while submicro crystalline pyrolytic graphite was formed on the surface of the ta-C film annealed at 900 °C. The surface layer was found to enhance the sp² clustering of the underlying ta-C layer. Field emission results reveal that the sp² cluster size plays an important role in electron field emission properties. The threshold field decreases as the sp² cluster size increases. For the film annealed at 800 °C, the lowest threshold field and the largest cluster size concurred.     

Formation and characteristics of undoped and doped tetrahedral amorphous carbon films

Synthesis of undoped and doped tetrahedral amorphous carbon (ta-C) films has been achieved using magnetic field filtered plasma stream system in an ambient gas of pure Ar and Ar with N₂, respectively. The optical and electrical properties of these films as a function of the substrate bias voltages (V_b) or nitrogen partial pressures (P_N) have been studied using UV-visible optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and measurements of electrical conductivity. The results show that ta-C films with a high sp³ fraction were formed when the V_b was in the range of −10 to −50 V. The optical band gap of such ta-C films was found to be larger than 3 eV. The incorporation of nitrogen into the ta-C films deposited at low P_N (P_N<25%), results in a slight drop in activation energy, which indicates that there is evidently some doping effect of nitrogen. The configurations of N atoms in ta-C network are identified and discussed.     

Chemical analysis and vibrational properties of boronated tetrahedral amorphous carbon films

Boronated tetrahedral amorphous carbon (ta-C:B) films were prepared by filtered cathodic vacuum arc technique using boron mixed graphite targets. The effect of boron content on the chemical bonding and vibrational properties of these films has been investigated by X-ray photoelectron spectroscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. It has been found that boron atoms are predominantly configured in a graphitic network, while the carbon atoms in the ta-C:B films are mainly in sp³ hybridization which tend to decrease as boron content increases. The Raman and infrared spectra of ta-C:B films both show prominent features in the regions of 1100–1900 cm^− 1 and 900–1600 cm^− 1 respectively. It was identified that the Raman parameters are strongly correlated with the boron content which is due to the clustering of sp² domains induced by B introduction. The activation of infrared spectrum of ta-C:B film is a consequence of heteroatomic (C–B) vibration combined with changes in the sp² carbon configuration. And the enhanced infrared absorption of ta-C:B with increased boron incorporation results from the increased effective charges in the delocalized sp² carbon phase.     

Field emission properties from aligned carbon nanotube films with tetrahedral amorphous carbon coatings

Tetrahedral amorphous carbon (ta-C) film was coated on aligned carbon nanotube (CNT) films via filtered cathodic vacuum arc (FCVA) technique. Field electron emission properties of the CNT films and the ta-C/CNT films were measured in an ultra high vacuum system. The I–V measurements show that, with a thin ta-C film coating, the threshold electric field (E_thr) of CNTs can be significantly decreased from 5.74 V/μm to 2.94 V/μm, while thick ta-C film coating increased the E_thr of CNTs to around 8.20 V/μm. In addition, the field emission current density of CNT films reached 14.9 mA/cm² at 6 V/μm, while for CNTs film coated with thin ta-C film only 3.1 V/μm of applied electric field is required to reach equal amount of current density. It is suggested that different field emission mechanisms should be responsible for the distinction in field emission features of CNT films with different thickness of ta-C coating.     

Field emission from tetrahedral amorphous carbon films with various surface morphologies

Field emission properties of tetrahedral amorphous carbon films prepared by filtered cathodic vacuum arc technique have been compared with different surface morphologies. With fewer cycles of conditioning, field emission from relatively rough granular ta-C films on nickel-coated silicon substrates was routinely improved, due to a local field enhancement resulting from both a ‘protrusion-on-protrusion’ geometry and a relatively high sp² content in the film. A 2-MeV ion implantation machine was also employed to intentionally produce local graphitic channels in smooth ta-C films with a high fraction of sp³ content on bare silicon. A relatively low threshold field was obtained from the ta-C film implanted at a dose of 10¹² cm⁻², which still remained an extremely smooth surface. However, for the highly graphitic sample implanted with a higher dose of over 4×10¹³ cm⁻², no electron field emission was observed, even under a very high electric field of 40 V μm⁻¹. Therefore, a suitable sp² content in an sp³ matrix, resulting in graphitic conductive channels in amorphous carbon films to produce a local field enhancement, may be the main factor in obtaining low threshold fields. Furthermore, protrusive structures could further increase the field enhancement factor, due to a ‘protrusion-on-protrusion’ geometry.     

Preparation and microstructure properties of tetrahedral amorphous carbon films by pulsed laser deposition using camphoric carbon target

The properties of tetrahedral amorphous carbon (ta-C) films grown by pulsed laser deposition (PLD) using camphoric carbon (CC) target and their respective effects of diamond percentages by weight in the target (Dwt.%) are discussed. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Visible-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses indicated that the Dwt.% noticeably modified the sp³ bonds content and the morphology of the ta-C films. The optical gap (E_g) and electrical resistivity (ρ) increase with Dwt.% up to 1.6 eV and 5.63×10⁷ (Ω cm), respectively, for the ta-C films deposited using target with higher of 50 Dwt.%. We found that the Dwt.% has modified the surface morphological, structural, bonding and physical properties of the camphoric carbon films.     

Direct Laser Interference Patterning of tetrahedral amorphous carbon films for tribological applications

In this work, the influence of surface topography and micro structural changes on the tribological properties of tetrahedral amorphous carbon coatings (ta-C) structured using a holographic technique the direct laser interference pattering (DLIP) is investigated. By utilizing a nanosecond pulsed UV-laser (wavelength 355 nm), both ablation and graphitization thresholds were determined as a function of the pulse number. Incubation effects for the ablation threshold ( ~ 205 mJ cm^− 2) were found to be negligible. However, for the graphitization of the film thresholds varying from 47 to 74 mJ cm^− 2 were observed depending on the number of laser pulses utilized (from 1 to 30) and thus obtaining an incubation factor of 1.13. Using two- and three-beam interference setups, dot- and line-like periodic arrays were fabricated. The tribological performance of these patterns was investigated under reciprocating sliding with a ball on disk method under non-lubricated conditions showing that coefficient of friction can be reduced from ~ 0.089 (un-patterned) to ~ 0.055 patterned ta-C ( ~ 30% reduction). The results can be explained based on the reduction of surface contact area combined with high hardness as well as the good intrinsic tribological properties of the ta-C films.     

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm³) and sp³ content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Young's modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Young's modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm², producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.     

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.     

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.     

Nanotribological and nanomechanical properties of 5–80 nm tetrahedral amorphous carbon films on silicon

Nanoscratch testing has been used to investigate the tribological behaviour of 5, 20, 60 and 80 nm tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The nanoscratch behaviour of the films was found to depend on the film thickness, with 60 and 80 nm films undergoing border cracking and then at higher critical load a dramatic delamination event. 5 and 20 nm films have a lower critical load for onset of border cracks but do not undergo a clear dramatic failure, and instead are increasingly worn/ploughed through until film removal as confirmed by microscopic analysis. This is consistent with the thinner films having lower stress and reduced load-carrying ability. Nanoindentation confirms that the thicker films have enhanced load support and higher measured composite (film + substrate) hardness. The 80 nm film in particular can retain appreciable load support whilst deformed during indentation, as shown by its ability to alter the critical loads for contact-induced phase transformations in the Silicon substrate during unloading.     

Properties of tetrahedral amorphous carbon prepared by vacuum arc deposition

The structural, optical, electrical and physical properties of amorphous carbon deposited from the filtered plasma stream of a vacuum arc were investigated. The structure was determined by electron diffraction, neutron diffraction and energy loss spectroscopy and the tetrahedral coordination of the material was confirmed. The measurements gave a nearest neighbour distance of 1.53 Å, a bond angle of 110 and a coordination number of four. A model is proposed in which the compressive stress generated in the film by energetic ion impact produces pressure and temperature conditions lying well inside the region of the carbon phase diagram within which diamond is stable. The model is confirmed by measurements of stress and plasmon energy as a function of ion energy. The model also predicts the formation of sp²-rich materials on the surface owing to stress relaxation and this is confirmed by a study of the surface plasmon energy. Some nuclear magnetic resonance, infrared and optical properties are reported and the behaviour of diodes using tetrahedral amorphous carbon is discussed.     

Investigation on the change in structure of tetrahedral amorphous carbon by a large amount of nitrogen incorporation

The change of the structure of carbon films after nitrogen incorporation is a topic of extensive discussion. Concerning this topic, tetrahedral amorphous carbon (ta-C) prepared by filtered cathodic arc deposition was chosen for the present investigations with up to 29 at.% nitrogen incorporated into the films. Studies on the film microstructure in a high-resolution transmission electron microscope (TEM) showed nanocrystalline structures of nitrogenated carbon from the films with a high nitrogen concentration. The variation of the microstructure of the films was thoroughly emphasized from carbon and nitrogen K-edges using electron energy loss spectroscopy (EELS) as well as near-edge X-ray absorption fine structure (NEXAFS), spectroscopy. In addition, NEXAFS spectra were used to find out the most probable molecular structure of the CN system and have been shown to be consistent with results obtained from EELS.     

The influence of the surface texture of hydrogen-free tetrahedral amorphous carbon films on their wear performance

Hydrogen-free and predominantly tetrahedrally bonded amorphous carbon thin films (ta-C) are excellent coatings to protect surfaces from wear due to their low coefficient of friction and high hardness. Since these coatings may be several times harder than common engineering materials counterpart wear can be significant. Therefore the surface texture of the ta-C coating is critical to wear applications. While the surface roughness is an important factor, the paper shows that other surface texture parameters have to be considered as well to predict the wear performance of the coating. Wear data are compared of as deposited, polished and brushed ta-C coatings. The results show that typically referenced average values for the surface roughness such as R_a and R_z may prove insufficient to reliably predict the wear behavior of the coating. Additional parameters describing the surface texture such as the “Skewness” (R_sk) and “Kurtosis” (R_ku) can provide relevant information. For example, a brushed ta-C surface with an average roughness of R_a = 31 nm showed a tenfold improved wear performance over a polished ta-C surface with an average roughness of R_a = 10 nm. This phenomenon is explained by analyzing the R_sk and R_ku data, which prove to more closely capture the post-treatment specific changes to the surface texture of the coatings.     

Evolution of compressive stress and electrical conductivity of tetrahedral amorphous carbon films with phosphorus incorporation

Successful modification of stress and conductivity for tetrahedral amorphous carbon (ta-C) films is realized by phosphorus incorporation via filtered cathodic vacuum arc technique with PH₃ as the impurity source. By establishing the structure as a function of phosphorus content, it is found that phosphorus fraction in phosphorus incorporated ta-C (ta-C:P) films increases with varying levels of PH₃ from 3 to 30 sccm, and that all samples retain their amorphous structures without remarkable changes, just exhibiting the clustering of sp² sites and the evolution of structural ordering. Furthermore, the addition of phosphorus causes the compressive stress relaxation in terms of the rearrangement in atomic bonding structures. The increased number of localized electronic π and π⁎ states as hopping sites after phosphorus incorporation results in several orders of magnitude increase in the conductivity, and the films represent the hopping conduction in band tail states in the temperature range of 293–463 K. However, more H induced by excessive PH₃ may saturate some defects and compensate the hopping sites, leading to a slight drop in the conductivity. The nature of ta-C:P films as n-type semiconductors is proved from the features of rectifying current–voltage cures.