ta-C Films Research Articles

Effect of structure on the secondary electron emission of tetrahedral amorphous carbon films

Tetrahedral amorphous carbon (ta-C) films are appealing for electronic device due to its unique electronic and optical properties. In this work, ta-C films were prepared by filtered cathodic vacuum arc (FCVA) method which is developed by adopting a focusing-deflection system for adjusting the ion beam current density and increasing the deposition field size. The surface morphology of ta-C films prepared by this developed FCVA was presented by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results reveal that the roughness of these ta-C films is only ∼0.4 nm and bias voltages almost unaffected to the surface morphology of the films. Moreover, secondary electron emission properties of ta-C films were investigated with various bias voltages. Compared with bare stainless steel substrate, the secondary electron yield (SEY) of ta-C films decreases significantly. Furthermore, the films deposited at the bias voltages in the range of 110–155 V possess the highest maximum secondary electron emission coefficient (δmax) compared to others, which matches well with the variation of sp3content in the films from Raman spectra analyses. These results indicate that the SEY can be tailored by the sp3/sp2 ratio of ta-C films.

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%.

Wetting behavior investigation of a complex surface prepared by laser processing combined with carbon films coating

To solve blood adhesion problems on the surface of medical devices, such as scalpel blades and implants, a hydrophobic and serum-repellent surface was prepared and investigated. First, porous ablated surface on stainless steel (SS) was fabricated via laser processing. Then, the laser-ablated SS (LA-SS) surface was covered by thin tetrahedral amorphous carbon (ta-C) films using a filtered cathodic vacuum arc deposition (FCVAD) technique. As such, a complex surface, i.e., a porous LA-SS surface coated with ta-C films (named as LAC-SS surface), was obtained. The morphologies of the LA-SS surface were tailored by adjusting the scanning rate and laser fluence during laser processing. By decreasing scanning rate, an ablated layer with high-aspect-ratio cavities and large roughness can be fabricated. The wetting tests of water and serum droplets indicated that the LAC-SS surface exhibited excellent hydrophobicity and serum-repellency. Under optimal processing parameters, the wetting behaviors of droplets obey the Cassie-Baxter wetting model. An optimal LAC-SS surface was obtained with a water contact angle of 151.4 ± 1.4° and a serum contact angle of 143.0 ± 1.3°. The optimal surface also demonstrates great anti-adhesive properties with a water sliding angle of 14.1 ± 2.2° and a serum sliding angle of 21.7 ± 2.8°. This optimized complex surface can be used in medical devices to address blood adhesion problems.

Microstructure and mechanical properties of ta-C films by pulse-enhanced cathodic arc evaporation: Effect of pulsed current

The ta-C films were fabricated using direct-current cathodic arc evaporation (DC-CAE) and pulse-enhanced cathodic arc evaporation (PE-CAE). The effect of peak currents (Ip) on arc discharge, microstructure and mechanical properties of the films was focused on. The enhanced discharge process, microstructure and mechanical properties of the ta-C films were investigated by means of optical emission spectra (OES), Raman spectroscopy, Nano-indentation, etc. With the same average current, the Ip of 300 A of PE-CAE produced the substrate current of 1.14 A, but a markedly low current (0.82A) was observed for the DC-CAE. The plasma emission intensity of Ar II, C I and C II also increased by about 86%, 66% and 630% at pulse mode. This evidently indicated an enhanced arc discharge by PE-CAE. The highest ratio of CII/ArII (2.98) and electron excitation temperature (1.38 eV) was observed. DC-CAE led to a lower concentration of sp3 bonding (47.3%, ID/IG = 0.39) of ta-C films with a thickness of 171 nm. In contrast, the ta-C films fabricated by PE-CAE (Ip = 300 A) possessed the smallest ID/IG (0.22) as well as the highest concentration of sp3 bonding (65.1%). The thickness increased by about 30% in comparison with that by DC-CAE. The largest thickness (330 nm) was obtained at the peak current of 200A, mainly relating to the increased evaporation of carbon atom/ions. The hardness evaluated from the ta-C films fabricated by DC-CAE was about 29 GPa. However, the hardness increased significantly to 57 GPa by PE-CAE (Ip = 300 A), and the preferable toughness (H/E* of 0.11 and H3/E*2 of 0.68) was simultaneously achieved.

Enhanced tribological and corrosion properties of multilayer ta-C films via alternating sp3 content

Multilayer tetrahedral amorphous carbon (ta-C) films comprising alternating layers of different sp3 contents have been fabricated by periodically adjusting the bias voltage of filtered cathodic vacuum arc (FCVA) system. The microstructure, tribological and corrosion behaviors of the multilayer ta-C films deposited on YG6 cemented carbide were systematically investigated. Results showed that the multilayer structure effectively reduced the residual stress and restrained spalling of ta-C film, which frequently occurred in single-layer structure. The hardness and Young's modulus of multilayer ta-C films decreased with diminishing sp3 content of soft sublayer. Compared to the YG6 substrate, multilayer ta-C films exhibited much lower friction coefficient between 0.06 and 0.08 and effectively reduced the wear rate. Multilayer ta-C film deposited at bias voltage alternating between −50 V and –150 V (M-1 film) had the lowest friction coefficient of 0.06 and the lowest wear rate of 2.87 × 10−7 mm3/N m. The difference in anti-wear performance among the multilayer ta-C films was associated with the ratio of hardness to elastic modulus (H/E ratio). The electrochemical corrosion test confirmed the effective corrosion protection provided by the multilayer ta-C films, and the M-1 film obtained the highest polarization resistance. Judging from electrochemical impedance tests, the existence of some extent of microdefects could be the main factor limiting the electrochemical corrosion performance of the multilayer ta-C films.

Performance and Durability Enhancement of Tungsten Carbide Micro-Drills by Ti/ta-C Coatings

This paper presents results of the application of Ti/ta-C films to micro drilling operation for machining. Tetrahedral amorphous carbon (ta-C) films were successfully deposited on WC-Co substrates by a filtered cathodic vacuum arc (FCVA) system. The mechanical and flexion properties of Ti/ta-C films were systematically investigated. The experimental results show that the Ti/ta-C coated micro drills have the excellent microhardness, adhesion and flexion properties and represent the optimal coatings for micro drilling applications. The role of the Ti –sublayer on a sintered powder tungsten carbide substrate is not only limited by the adhesion improvement, but it is mainly used to neutralise the grain boundary microcracks on a surface. The results of drilling tests carried out on PCB boards showed that the durability and drilling efficiency of tools coated by Ti/ta-C films are significantly higher than that of uncoated ones.

The effect of surface treatment on the corrosion behavior of pure Ta sheet in an equimolar NaCl-KCl melt at 850℃ in air, part 2: Diamond film, TaC film, Ta-Si coating and Ta-Si-Al coating

The diamond film, TaC film, Ta-Si coating and Ta-Si-Al coating were developed on Ta sheet surface and their corrosion behavior were investigated in an equimolar NaCl-KCl melt at 850℃ in air, additionally, their corrosion behavior were concluded. The diamond film could prevent the corrosion mediums at beginning, while had no benefit in later period due to the corrosion product CO2 having no protective ability. The TaC film has good corrosion resistance mainly because of its low activity. The corrosion of Ta-Si and Ta-Si-Al coating could form continuous SiO2 amorphous film on surface to protect the un-corroded material.

Failure mechanism and protective role of ultrathin ta-C films on Si (100) during cyclic nano-impact

Complex mechanical behavior with phase transformation and high brittleness limits the reliability of silicon-based microelectromechanical systems. Although very hard ultra-thin films are being considered as protective overcoats to improve the service life of substrate materials, their resistance to fatigue can be at least as important as hardness when exposed to cyclic loading. In this study repetitive nano-impact tests with a spherical diamond probe have been used to investigate the fatigue behavior and protective role of 5 and 80 nm tetrahedral amorphous carbon (ta-C) films on silicon. At the lowest load there was delamination of the 80 nm film but not for the 5 nm film. At higher loads failure involved lateral cracking of the silicon substrate. Single impact tests showed that this was preceded by ring and radial cracking. Changing contact pressure during the test provided further support for the degradation mechanism and the influence of phase transformation in the Silicon substrate. Under repetitive contact the thin film systems showed lower impact depth and greater impact cycles before substrate fracture than the uncoated Silicon. This is related to their enhanced load support which affects phase transformation in the substrate, with potentially delamination providing an additional impact energy dissipation mechanism.

Self-supporting tetrahedral amorphous carbon films consisting of multilayered structure prepared using filtered arc deposition

Self-supporting 110-nm-thick tetrahedral amorphous carbon (ta-C) films with a multilayered structure were fabricated as a carbon film target for the laser-driven ion acceleration. The self-supporting ta-C films consisted of three layers with the thicknesses of 35 nm, 40 nm, and 35 nm thick, and the film density of 3.0 g/cm3, 2.7 g/cm3, and 3.0 g/cm3, respectively. The multilayered ta-C film was fabricated using the T-shape filtered arc deposition method on a Si substrate coated water-soluble material. Silk fibroin and dextran were used as the water-soluble material. The water-soluble material formed between a ta-C film and a Si substrate was dissolved, and then, the ta-C film released from the substrate. Thick single-layer ta-C films partially peeled off on the water-soluble material and broke during the dissolving process. Self-supporting ta-C films were obtained by scooping the released ta-C film on a perforated substrate. The laser was irradiated on the self-supporting ta-C films, and the ta-C film with a higher film thickness and/or film density showed a higher laser irradiation tolerance.

Nanostructured TaC film deposited by reactive magnetron sputtering: Influence of gas concentration on structural, mechanical, wear and corrosion properties

TaC coatings were applied on the surface of the AISI316L stainless steel by reactive magnetron sputtering, and the effects of CH4 concentrations (CCH4=0,5,10,20 and 30) on the structural, mechanical, wear and corrosion behavior were investigated. According to the dominant phase structure, deposited films classified into three categories; namely, metallic (Ta phase), ceramic (TaC phase) and amorphous (amorphous carbon phase). Increasing the CCH4 from 0% to 20%, hardness (H) and elastic modulus (E) of the coatings increased from 6.2 and 115.6 GPa to 27.6 and 274.2 GPa, respectively. Further increase in CCH4 led to decrease H and E to 1.5 and 16.3 GPa, respectively. Based on nanoindentation and pin on disk test results, in ceramic coatings due to high coating to substrate hardness ratio, the resistance of the coating against elastic (H/E) and plastic (H3/E2) deformation are the key factors in the determination of wear behavior. It is to say, the coating deposited at CCH4=20% showing the H/E=0.101 and H3/E2=0.28 had the highest wear resistance, while the coating deposited at CCH4=10% with the H/E=0.068 and H3/E2=0.084 showed the lowest wear resistance. Tafel and electrochemical impedance spectroscopy (EIS) tests were also used to investigate the corrosion behavior of the samples. Results showed that the films deposited at CCH4=5 and 10% display the best protective properties, which was attributed to lower porosity index.

Wear acceleration mechanism of diamond-like carbon (DLC) films lubricated with MoDTC solution: Roles of tribofilm formation and structural transformation in wear acceleration of DLC films lubricated with MoDTC solution

Abstract The wear acceleration mechanism of DLC films lubricated with MoDTC solution was investigated by using in-situ and ex-situ analytical techniques: Raman spectroscopy, XPS, ToF-SIMS and nano-indenter. Especially, to reveal the role of ZDDPs and ta-C films for inhibiting the wear acceleration, friction tests were carried out using a ball-on-disk setup at a-C:H/steel and ta-C/steel contacts lubricated with PAO, PAO + MoDTC and PAO + MoDTC + ZDDP. All our results suggest that the wear acceleration is caused by hard Mo-carbide containing tribofilms. Therefore, wear acceleration did not occur for the harder DLC, i.e., the ta-C films. Our results also suggest that ZDDPs play important roles in inhibiting the carburization of Mo-compounds involving DLC surfaces at the tribological contact.

Effective passivation of Ag nanowire network by transparent tetrahedral amorphous carbon film for flexible and transparent thin film heaters

We developed effective passivation method of flexible Ag nanowire (NW) network electrodes using transparent tetrahedral amorphous carbon (ta-C) film prepared by filtered cathode vacuum arc (FCVA) coating. Even at room temperature process of FCVA, the ta-C passivation layer effectively protect Ag NW network electrode and improved the ambient stability of Ag NW network without change of sheet resistance of Ag NW network. In addition, ta-C coated Ag NW electrode showed identical critical inner and outer bending radius to bare Ag NW due to the thin thickness of ta-C passivation layer. The time-temperature profiles demonstrate that the performance of the transparent and flexible thin film heater (TFH) with the ta-C/Ag NW network is better than that of a TFH with Ag NW electrodes due to thermal stability of FCVA grown ta-C layer. In addition, the transparent and flexible TFHs with ta-C/Ag NW showed robustness against external force due to its high hardness and wear resistance. This indicates that the FCVA coated ta-C is promising passivation and protective layer for chemically weak Ag NW network electrodes against sulfur in ambient.

Sculpting Extreme Electromagnetic Field Enhancement in Free Space for Molecule Sensing.

A strongly confined and enhanced electromagnetic (EM) field due to gap-plasmon resonance offers a promising pathway for ultrasensitive molecular detections. However, the maximum enhanced portion of the EM field is commonly concentrated within the dielectric gap medium that is inaccessible to external substances, making it extremely challenging for achieving single-molecular level detection sensitivity. Here, a new family of plasmonic nanostructure created through a unique process using nanoimprint lithography is introduced, which enables the precise tailoring of the gap plasmons to realize the enhanced field spilling to free space. The nanostructure features arrays of physically contacted nanofinger-pairs with a 2 nm tetrahedral amorphous carbon (ta-C) film as an ultrasmall dielectric gap. The high tunneling barrier offered by ta-C film due to its low electron affinity makes an ultranarrow gap and high enhancement factor possible at the same time. Additionally, its high electric permittivity leads to field redistribution and an abrupt increase across the ta-C/air boundary and thus extensive spill-out of the coupled EM field from the gap region with field enhancement in free space of over 103 . The multitude of benefits deriving from the unique nanostructure hence allows extremely high detection sensitivity at the single-molecular level to be realized as demonstrated through bianalyte surface-enhanced Raman scattering measurement.

Unmodified and multi-walled carbon nanotube modified tetrahedral amorphous carbon (ta-C) films as in vivo sensor materials for sensitive and selective detection of dopamine

Unmodified and multi-walled carbon nanotube (MWCNT) modified tetrahedral amorphous carbon (ta-C) films of 15 and 50 nm were investigated as potential in vivo sensor materials for the detection of dopamine (DA) in the presence of the main interferents, ascorbic acid (AA) and uric acid (UA). The MWCNTs were grown directly on ta-C by chemical vapor deposition (designated as ta-C+CNT) and were characterized with X-ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Electroanalytical sensitivity and selectivity were determined with cyclic voltammetry. Biocompatibility of the materials was assessed with cell cultures of mouse neural stem cells (mNSCs). The detection limits of DA for both ta-C and ta-C+CNT electrodes ranged from 40 to 85 nM, which are well within the required range for in vivo detection. The detection limits were lower for both ta-C and ta-C+CNT electrodes with 50 nm of ta-C compared to 15 nm. The ta-C electrodes showed a large dynamic linear range of 0.01–100 µM but could not resolve between the oxidation peaks of DA, AA and UA. Modification with MWCNTs, however, resulted in excellent selectivity and all three analytes could be detected simultaneously at physiologically relevant concentrations using cyclic voltammetry. Based on cell culture of mNSCs, both ta-C and ta-C+CNT exhibited good biocompatibility, demonstrating their potential as in vivo sensor materials for the detection of DA.

The effect of nitrogen implantation on resistive switching of tetrahedral amorphous carbon films

We report the effect of nitrogen implantation on the resistance switching of tetrahedral amorphous carbon (ta-C) films. Both unimplanted and implanted films show resistive switching, with a characteristic threshold voltage required to switch the films from the high-resistance to the low-resistance state. The switching voltages for the unimplanted films are between 7 and 10 V for ta-C films of thickness 15 to 40 nm. These are significantly reduced upon implantation by up to 60% when using an implantation dose ~3 × 1015 cm−2. We attribute this to increased sp2 bonding and clustering in the implanted films. This demonstrates the importance of sp2 clustering for resistive-switching in sp3-rich ta-C films.

Growth Mechanism and Origin of High sp^{3} Content in Tetrahedral Amorphous Carbon.

We study the deposition of tetrahedral amorphous carbon (ta-C) films from molecular dynamics simulations based on a machine-learned interatomic potential trained from density-functional theory data. For the first time, the high sp^{3} fractions in excess of 85% observed experimentally are reproduced by means of computational simulation, and the deposition energy dependence of the film's characteristics is also accurately described. High confidence in the potential and direct access to the atomic interactions allow us to infer the microscopic growth mechanism in this material. While the widespread view is that ta-C grows by "subplantation," we show that the so-called "peening" model is actually the dominant mechanism responsible for the high sp^{3} content. We show that pressure waves lead to bond rearrangement away from the impact site of the incident ion, and high sp^{3} fractions arise from a delicate balance of transitions between three- and fourfold coordinated carbon atoms. These results open the door for a microscopic understanding of carbon nanostructure formation with an unprecedented level of predictive power.

15.2: Invited Paper: Organic LCDs on TAC film: low‐cost, area‐scalable flexible displays with glass‐like optical performance

Flexible Organic LCD (OLCD) is a new display technology, developed by FlexEnable, based on organic thin‐film transistors (OTFTs) on plastic substrates. With optical performance being one of the most important attributes of any display, achieving glass‐like optical performance on plastic is a key requirement for mass market adoption.We explain how the use of low temperature processes (below 100°C) not only allows the use of plastic substrates (TAC) with near ideal optical properties, but also brings cost, yield and scalability advantages in manufacturing compared to flexible display approaches using inorganic TFTs that require much higher processing temperatures.

Structural, hardness and toughness evolution in Si-incorporated TaC films

Ta–Si-C films were deposited by DC magnetron co-sputtering using TaC and Si targets in an Ar-discharge atmosphere. Increasing the current of Si target from 0.0 to 0.5 A led to a continuous increase of Si content from 0.0 to 30.8 at%. The effects of Si content on microstructure were systematically investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). At low Si content (≤ 5.6 at%), Si occupied C vacancies to form a solid solution Ta(C, Si). Further increasing the Si content, some Si atoms bonded with C atoms resulting in the formation of amorphous phase (a-C:Si), and the films presented a nanocomposite structure consisting of solid solution Ta(C, Si) surrounded by a-C:Si matrix. At the highest Si content (30.8 at%), the film exhibited finally X-ray amorphous structures. The hardness (H) and fracture toughness (Kf) were observed to initially increase and then decrease as the Si content was increased. At 5.6 at% Si, the film exhibited a maximum in H (44.9 ± 2.7 GPa) and Kf (3.61 ± 0.16 MPa m1/2), which can be ascribed to the formation of a solid solution. On further increasing the Si content, an amorphous phase gradually appeared, leading to a decrease in H and Kf.

Carbon 3D microelectrode arrays for neuron and brain slice measurements

Carbon 3D microelectrode arrays for neuron and brain slice measurements

Quantifying Macroscopic Friction of Diamond-like Carbon Films by Microscopic Adsorption and Removal of Water Molecules.

The adsorption and desorption of water molecules, which affect the physical and chemical properties of the sliding interface, are critical for the friction behaviors of two solid contacts in atmosphere environment. The amount of water adsorbed on the open surface is a function of gas pressure according to an adsorption equation. However, for a confined sliding interface, the variation of surface fraction covered by gas molecules with water vapor pressure and its induced effects on friction have not been figured out. In this work, the macroscopic friction of diamond-like carbon (DLC) films in a water vapor atmosphere is quantified on the basis of microscopic adsorption and removal of water molecules. The studies correlate the fraction of water molecules adsorbed on the interface of self-mated DLC films with water vapor pressure to illustrate the direct relationship between friction coefficient and gas pressure by first-principles calculations and model fitting. The calculated results revealed that chemisorption and physisorption of water molecules occur on the surfaces of hydrogen-free DLC films (ta-C) and hydrogenated DLC films (HCF). Then, the relation between friction and gas pressure was built by employing a fractional coverage model based on the linear adsorption equation and gas removal. The obtained model agrees well with the typical experimental results about the steady-state friction coefficient of both highly hydrogenated DLC film (HCF) and tetrahedral amorphous carbon (ta-C) film during sliding at various water vapor pressures. In addition, it gave the curves of fractional surface coverage as a function of water vapor pressure. These results show that the frictional coefficient of DLC films could be predicted on the basis of fractional surface coverage as well as the intrinsic characters on surface chemistry. We suggest that the model may be thus extended to understand and predict the friction of DLC films under a specific gas pressure at a low load and speed.