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.     

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.     

The effect of nitrogen addition on field emission of diamond-like carbon films

Diamond-like carbon (DLC) films and nitrogen-doped DLC (N-DLC) films were deposited on a molybdenum-coated ceramic substrate using the pulsed laser deposition technique. The structure and surface morphology of the films were examined using X-ray diffraction, Raman spectroscopy, Auger electron spectroscopy and scanning electron microscopy. Field emission measurements were carried out, with the DLC or the N-DLC films as the cathode and ITO-coated glass as the anode. The field emission measurements indicated that the nitrogen doping could lower the turn-on field and increase the current density. It was believed that the interface at the molybdenum–N-DLC film plays an important role in improving the field emission performance of the N-DLC film.     

Linear sweep anodic stripping voltammetry of heavy metals from nitrogen doped tetrahedral amorphous carbon thin films

Nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films were deposited on p-Si (1 1 1) substrates (1 × 10⁻³ to 6 × 10⁻³ Ω cm) by a filtered cathodic vacuum arc technique with different nitrogen flow rates (3 and 20 sccm). The ta-C:N film coated samples were used as working electrodes to detect trace heavy metals such as zinc (Zn), lead (Pb), copper (Cu) and mercury (Hg) by using linear sweep anodic stripping voltammetry in 0.1 M KCl solutions (pH 1). The influence of nitrogen flow rate on the sensitivity of the films to the metal ions was investigated. The results showed that the current response of the ta-C:N film electrodes was significant to differentiate all the tested trace metal ions (Zn²⁺, Pb²⁺, Cu²⁺, and Hg²⁺) and the three ions (Pb²⁺ + Cu²⁺ + Hg²⁺) could be simultaneously identified with good stripping peak potential separations.     

Substrate bias effect on structure of tetrahedral amorphous carbon films by Raman spectroscopy

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Now tetrahedral amorphous carbon films (ta–C) thickness of 2 nm are becoming the preferred means due to the highly sp³ content. In this paper, Raman spectra at visible and ultraviolet excitation of ta–C films have been studied as a function of substrate bias voltage. The spectra show that the sp³ content of 70 nm thick DLC films increases with higher substrate bias, while sp³ content of 2 nm ultra-thin films falls almost linearly with bias increment. And this is also consistent with the hardness measurement of 70 nm thick films. We proposed that substrate bias enhances mixing between the carbon films and either the Si films or Al₂O₃TiC substrate such that thin films contain less sp³ fraction. These mixing bonds are longer than C–C bonds, which inducing the hardness decreasing of ultra-thin DLC films with bias. But for 70 nm DLC, the effect of mixing layer can be negligible by compared to bias effect with higher carbon ion energy. So sp³ content will increase for thick films with substrate bias.     

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.     

The effect of hydrogen plasma chemical annealing on electron field emission of amorphous carbon films

Layer-by-layer deposition method, in which nanometer-thick film deposition and hydrogen plasma annealing processes were alternatively repeated, was applied to fabricate hydrogenated amorphous carbon films in our present work. It was found that the hydrogen plasma treatment changed the sp²/sp³ ratio due to chemical etching. Consequently, a stable vacuum electron emission with a low threshold field was achieved compared with that from conventionally deposited a-C films. The threshold electric field is as low as 2 V/μm. The influence of the hydrogen plasma chemical annealing on the field emission behavior was systematically investigated. The improvement of field emission characteristics can be attributed to the large field enhancement effect due to the inhomogeneous distribution of nanometer scale sp² clusters.     

Field emission from metal-containing amorphous carbon composite films

Metal-containing (Co, Al and Ti) amorphous carbon composite films (a-C:Me) have been prepared by the filtered cathodic arc technique using metal-containing graphite targets at room temperature. Field emission properties of the heat-treated a-C:Me films were improved and were found to be dependent on the metal content and variety of metals. After heat-treatment at 550°C in a mixture of acetylene and nitrogen gases, the field emission properties of a-C:Co films were significantly improved, in which Co acted as catalysts to enhance graphitization as well as formation of carbon nanotubes during heat-treatment. A threshold electric field of less than 2 V/μm was obtained from the heat-treated a-C:Co composite films without conditioning. The heat-treated a-C:Al and a-C:Ti films, though the conditioning step could be avoided and relatively low threshold fields could be obtained, exhibited relatively low emission site densities, however. The a-C:Me films, which can be deposited with a high rate at room temperature and require a relatively low temperature, heat-treatment process to enhance electron emission, are promising for practical applications in field emission display.     

Thickness dependency of field emission in amorphous and nanostructured carbon thin films

Thickness dependency of the field emission of amorphous and nanostructured carbon thin films has been studied. It is found that in amorphous and carbon films with nanometer-sized sp² clusters, the emission does not depend on the film thickness. This further proves that the emission happens from the surface sp² sites due to large enhancement of electric field on these sites. However, in the case of carbon films with nanocrystals of preferred orientation, the emission strongly depends on the film thickness. sp²-bonded nanocrystals have higher aspect ratio in thicker films which in turn results in higher field enhancement and hence easier electron emission.     

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.     

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.     

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.     

Ring-shaped field emission patterns from carbon nanotube films

Highly symmetric ring-shaped field emission patterns were observed from broad-area flat cathodes prepared by growing a film of vertically aligned carbon nanotubes (CNTs) on TiN coated Si substrates. The images were obtained utilizing a luminescent screen of a specially designed triode cell composed of parallel electrodes. The emission rings sporadically appeared during voltage scans in which the emission patterns and cathode currents were recorded. The fine structure and stability of the rings suggests that their formation is due to an emission state of an individual CNT. The observed patterns are consistent with models that predict the formation of emission rings produced by the inhomogeneous electron emission from CNTs. The macroscopic value of the electric field when the rings were observed was between 0.7 and 2.5 V/μm, and the emission current corresponding to individual rings was estimated to be in the range of 2–4 μA. Numerical simulation of electron trajectories for sidewall emission from similar shaped metallic structures is in qualitative and quantitative agreement with the experimentally observed ring-shaped field emission patterns. The results also appear consistent with a recent model [Marchand M, Journet C, Adessi C, Purcell ST. Phys Rev B 2009;80:245425] based on thermal-field emission due to Joule heating.     

Effects of acetylene addition on mechanical and dielectric properties of amorphous carbon films

The para-xylene added with acetylene from 15% to 50% was plasma polymerized at 50 to 150 W to deposit the a-C:H films. After the films were annealed from 200 to 400 °C, the network structure, hardness and dielectric constant of films were analyzed by FT-IR, Raman, nanoindentor and capacitance–voltage plot, respectively. Those measured results suggest that hydrocarbon bonds and oxygen related bonds of the a-C:H film effectively reduce and the number of ordered aromatic rings increases with decreasing the deposition power after annealing at 400 °C. In addition, both the dielectric constant and the hardness, respectively, increase up to 2.82 and 2.37 GPa, but the adhesion strength decreases with increasing the C₂H₂ concentration and deposition power. Therefore, the a-C:H films not only have a lower dielectric constant, but also have enough mechanical strength for the IC processing.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

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.     

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.     

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.     

Field emission enhancement of amorphous carbon films by nitrogen-implantation

Effect of nitrogen-implantation on electron field emission properties of amorphous carbon films has been examined. Raman and X-ray photoelectron spectroscopy measurements reveal different types of C-N bonds formed upon nitrogen-implantation. The threshold field is lowered from 14 to 4 V/μm with increasing the dose of implantation from 0 to 5 × 10¹⁷ cm⁻² and the corresponding effective work function is estimated to be in the range of 0.01-0.1 eV. From the perspective of tetrahedron bond formation, a mechanism for the nitrogen-lowered work function is proposed, suggesting that both the nitrogen nonbonding (lone pair) and the lone-pair-induced carbon antiboding (dipole) states are responsible for lowering the work function and hence the threshold field.