Field emission characteristics of Diamond-like Carbon (DLC) films prepared using a Magnetron Sputter-type Negative Ion Source (MSNIS)

The field emission characteristics of Diamond-like Carbon (DLC) films prepared using a MSNIS were studied. Due to Negative Electron Affinity (NEA), the high hardness, high thermal conductivity and chemical inertness, DLC films are considered as good electron emitters for Field Emission Displays. However, the very smooth DLC films showed poor field emission characteristics regardless of their sp³ contents or density. After reviewing the emitted spots using Atomic Force Microscope (AFM), a metal-diamond-vacuum triple-junction type emission model was tried. Triple-junction type patterns were fabricated using KrF excimer-laser drilling method. Without fabricating spindt type tips, stable field emission was observed with the low turn on voltages (∼2 V/μm). The FE (Field emission) characteristics such as I-V characteristic curves, turn on voltages, and emission lifetime data were obtained using a diode type FE measurement system. Overall results show significantly enhanced performance of field emission characteristics can be achieved at the laser-patterned DLC films.     

Diamond-like carbon films: electron spin resonance (ESR) and Raman spectroscopy

Tetrahedral diamond-like carbon (ta-C) films and hydrogenated a-C:H films were deposited onto Si substrates using filtered cathodic vacuum arc (FCVA) process and direct ion beam deposition from CH₄/C₂H₄ plasma, respectively. Stress of deposited films was varied in the range 2.8–8.5 GPa depending on deposition conditions. Stationary and pulse electron spin resonance (ESR), and Raman spectroscopy techniques were used to analyze sp² related defects in pseudo-gap of undoped as deposited and annealed 20–100 nm thick films.¹ High density of ESR active paramagnetic centers (PC) N_s=(1.0–4.5)×10²¹ cm⁻³ at g=2.0025 was observed in the films. The dependence of ESR line width and line shape vs. deposition conditions and internal film stress were investigated. The several actual mechanisms for ESR line width broadening were considered: spin–spin dipole–dipole and exchange interactions, super-hyperfine interaction (SHFI) with ¹H (for a-C:H), averaging of SHFI due to electron jumps between PC positions with different SHFI values, and broadening due to Mott's electron hopping process. Three types of samples were revealed depending on relative contribution of these mechanisms. Effects of annealing on mechanical and paramagnetic properties of films were studied. An electrical resistance anisotropy at room temperature for ta-C films and g-value anisotropy at low temperature (T<77 K) for both ta-C and a-C:H films were found for the first time. Nature and distribution details of paramagnetic defects in DLC films, anisotropy effects and Raman spectroscopy data are discussed.     

Biaxial elastic modulus of very thin diamond-like carbon (DLC) films

The biaxial elastic modulus of very thin diamond-like carbon (DLC) films was measured by the recently suggested free overhang method. The DLC films of thickness ranging from 33 to 1100 nm were deposited on Si wafers by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) or by the filtered vacuum arc (FVA) process. Because the substrate was partially removed to obtain sinusoidal free overhang of the DLC film, this method has an advantage over other methods in that the measured value is not affected by the mechanical properties of the substrate. This advantage is more significant for a very thin film deposited on a substrate with a large difference in mechanical properties. The measured biaxial elastic moduli were reasonable values as can be judged from the plane strain modulus of thick films measured by nanoindentation. The biaxial elastic modulus of the film deposited by r.f.-PACVD was 90±3 GPa and that of the film deposited by FVA process was 600±50 GPa. While the biaxial elastic modulus of the film deposited by FVA is independent of the film thickness, the film deposited by r.f.-PACVD exhibited decreased elastic modulus with decreasing film thickness when the film is thinner than 500 nm. Although the reason for the different behavior could not be clarified at the present state, differences in structural evolution during the initial stage of film growth seem to be the reason.     

Instability of diamond-like carbon (DLC) films during sliding in aqueous environment

The instability of diamond-like carbon (DLC) film deposited on Ti-6Al-4V alloy substrate using the r.f.-PACVD method was investigated under sliding conditions in an aqueous environment. Significant adhesive wear was observed when tested in this environment, while normal abrasive wear occurred in an ambient air of relative humidity about 25%. A critical test was performed to elucidate the reason for the instability which limits the biomedical applications of the DLC coating. By employing a multi-step coating process, it was shown that the instability is closely related to the penetration of water molecules to the interface via through-film defects or pinholes. These results suggest that the stability of DLC film in aqueous environment can be improved by controlling the through-film defects in the DLC coating layer.     

Diamond-like carbon films synthesized under atmospheric pressure synthesized on PET substrates

Diamond-like carbon films were synthesized under atmospheric pressure (AP-DLC) and their gas barrier properties and hardness were measured. The AP-DLC films were uniformly obtained by RF-plasma CVD method at room temperature with a size of 450 mm². The growth rate increased as a function of C₂H₂ concentration and the average growth rate was around 12 μm/min. The maximum deposition rate was ~ 1 μm/s, which is approximately 2000 times larger than that by low-pressure plasma CVD of 1–2 μm/h. The gas barrier properties of AP-DLC films, ~ 1 μm thick, were 5–10 times larger than those of uncoated PET substrates. The microhardness of AP-DLC films was around 3 GPa, measured by the nano-indentation method. The issue lies in the removal of macro-particles of the films to improve the microhardness and the surface roughness.In this paper, we report the physical properties of DLC films synthesized under atmospheric pressure by the radio-frequency CVD method. We also summarize a brief history of PET bottle coating by vacuum-DLC films, as well as that of the development of atmospheric pressure technology and related DLC films, focused on gas barrier properties and micro-hardness.     

Diamond-like carbon films with End-Hall ion source enhanced chemical vapour deposition

A custom-designed End-Hall ion source was used to deposit diamond-like carbon (DLC) films in a plasma enhanced chemical vapour deposition (PECVD) mode. The deposition system was characterised and optimised for infrared transmission enhancement applications and large area deposition onto silicon or germanium substrates. Ion bombardment energy (in eV) on substrate was found to scale about 60% of the discharge voltage. Uniformity was about 2.5% and 5% for substrate diameters of 20 cm and 40 cm respectively. For the infrared enhancement applications the optimised ion bombardment energy was about 54 eV with a high deposition rate approximate 30 nm/min. Coating the DLC onto a single side of double-sided polished silicon wafers resulted in a transmission of 69.5% in the wavelength of about 4 μm, very close to the ideal value. Mechanical and reliability properties of the DLC films on silicon wafers were analysed at different environmental conditions. It was found that the DLC films produced in the ion source PECVD deposition system were satisfied with the requirements for the infrared transmission enhancement applications.     

Diamond-like carbon films in multilayered interference coatings for IR optical elements

In the majority of modern IR interference multilayer coatings (MLC), conventional film-forming materials (FFM) of fluoride and chalcogenide types are used. Such coatings are characterized by relatively low mechanical strength and stability against enhanced humidity and, therefore, require surface protection. Our present results support the view that mechanical strength of these MLCs can be improved by applying a diamond-like carbon (DLC) film as an external layer. Nanoindentation measurements show that the addition of a DLC film to ZnSe/BaF₂/Y₂O₃ IR antireflection MLC increases the combined hardness of the coatings from 0.5 to 3.6 GPa. The formation of an indent on the upper and subsequent layers of MLC has been studied by SEM and X-ray spectrum microanalysis. The resistance of DLC films applied onto MLC against light irradiation, organic solvents as well as against environmental factors was also studied. Atomic force microscopy (AFM) was used to study variations of the surface morphology of the initial MLC components before and after DLC film deposition.     

The influence of nitrogen on the dielectric constant and surface hardness in diamond-like carbon (DLC) films

In this work a carbon target was sputtered by a methane/argon/nitrogen plasma in order to produce nitrogenated diamond-like carbon films (a-C:H:N). As the N₂ content in the sputtering gas was increased, the deposition rate increased markedly. Rutherford backscattering spectrometry (RBS) was used to investigate the chemical composition of the films. This nitrogen incorporation modifies the chemical bonding structure of the films, as shown by the analysis of the Raman spectra, including the occurrence of two extra peaks at approximately 2200 and 690 cm⁻¹. Electrical properties were measured through capacitance–voltage (C–V) curves. The hardness of the films decreased with the N content as shown by measurements performed by indentation method. A correlation among the Raman studies, the N content in the films, the dielectric constant and the surface hardness is presented.     

Effects of reactors on the deposition of DLC films using liquid electrochemical technique

Carbon films were deposited on silicon substrates by liquid electrochemical technique at low temperature (60 °C) in ambient atmosphere. Glass reactor, glass reactor with PTFE-coating inside, glass reactor with quartz-coating inside and quartz reactor were used with the same experimental setup to compare the effects of reactors on the deposition of carbon films. The applied potential, the distance between anode and substrate and the deposition time were fixed at 900 V (4.2 kHz, 50%), 6 mm and 5 h, respectively. The morphology and microstructure of the films were analyzed by scanning electron microscopy (SEM) and Raman spectroscopy. Energy Dispersive X-ray Spectrometry (EDX) was used to measure the composition of the films. The SEM observations showed that the films deposited using glass reactor were composed of crystals of several micrometers which contained nearly 10 at.% of Ca. Raman spectroscopy analysis confirmed that DLC films have been deposited, but with an obvious sharp peak at 1085 cm^− 1 which is assigned to calcium carbonate (CaCO₃) crystals. The glass reactor is the possible source of Ca because the electrolyte was composed of analytically pure acetone and deionized water with the proportion of Ca below the determination of AAS (atomic absorption emission spectrophotometer AA-6200). Using glass reactor with PTFE-coating inside could successfully avoid the impurity of Ca from the glass reactor, but new non-metallic impurities coming from the PTFE-coating made the films rough. Continuous and smooth films were deposited by using a glass reactor with quartz-coating inside and quartz reactor, which could avoid both Ca (< 1 at.%) and other impurities. Raman spectroscopy analysis confirmed typical DLC films without CaCO₃. It can be concluded that the materials of the reactors could play an important role not only in the composition, but also the morphology and microstructure of films deposited by liquid electrochemical technique.     

Fabrication and electrochemical properties of carbon nanotube film electrodes

Carbon nanotube (CNT) film electrodes were fabricated by a novel process involving the electrostatic spray deposition (ESD) of a CNT solution. Acid treated CNTs were dispersed in an aqueous solvent through sonication and then the CNT solution was electrostatically sprayed onto a metallic substrate by the ESD method. The CNT film electrodes showed well-entangled and interconnected porous structures with good adherence to the substrate. A specific capacitance of 108 F/g was achieved for the electrodes in 1 M H₂SO₄. In addition, the CNT film electrode showed good high rate capability.     

An electrochemical study of reduced ilmenite carbon paste electrodes

The anodic dissolution of reduced ilmenite has been studied using the carbon paste electrode technique. To understand the polarization behaviour of reduced ilmenite, the polarization behaviour of iron oxide and iron powder carbon paste electrodes has also been investigated. It was shown that synthetic rutile and reduced ilmenite promoted the anodic dissolution of iron.     

Electrophoretic deposition of carbon nanotubes films as counter electrodes of dye-sensitized solar cells

Carbon nanotubes (CNTs) films have been successfully fabricated by electrophoretic deposition (EPD) technique and used as counter electrodes of dye-sensitized solar cells (DSSCs). The CNTs counter electrodes consisting of a large number of bamboo-like structures with defect-rich edge planes exhibit a highly interconnected network structure with high electrical conductivity and good catalytic activity. A high photovoltaic conversion efficiency of 7.03% is achieved for DSSCs based on the CNTs counter electrodes, which is comparable to the cell based on conventional Pt counter electrode at one sun (AM 1.5G, 100 mW cm⁻²). The results suggest that the present synthetic strategy provides a potential feasibility for the fabrication of low-cost flexible counter electrodes of DSSCs using a facile deposition technique from an environmentally “friendly” solution at low temperature.     

Porous carbon thin films for electrochemical capacitors

Activation effects on carbon films, derived from commercial aromatic polyimide films (Kapton, DuPont), in CO₂ atmosphere at 1203 K on capacitance properties were studied. Two thicknesses of polyimide films were used: 7 and 25 μm. Pore formation during the activation process progresses in two steps due to the existence of a denser surface layer and a more porous core material. In the first step micropores are opening in the dense surface region of the material with average pore diameter smaller than 1 nm. During the second step, mesopores start opening, while micropore volume remains constant with the average micropore diameter of over 1 nm, producing bimodal texture. The first step finishes after 30 min for the thinner samples while for the thicker samples it finishes after 60 min of activation. As a consequence of such textural changes during activation, the thicker sample has a maximum areal capacitance of 0.35 F/cm². The thinner sample activated for 30 min has a maximum volumetric capacitance of 220 F/cm³ and achieves a maximum gravimetric capacitance of 240 F/g when the texture becomes bimodal after 240 min of activation. These results confirm that activation of carbonized Kapton films gives promising electrode materials for supercapacitors.     

Powdered activated carbon and carbon paste electrodes: comparison of electrochemical behaviour

Commercial activated carbon (Norit R3ex), de-mineralised with conc. HF and HCl, was oxidised (conc. HNO₃) and heat-treated at various temperatures (180, 300 and 420 °C). The physicochemical properties of the samples obtained were characterised by selective neutralisation and pH-metric titration of surface functional groups (acid–base properties), thermogravimetry (thermal stability—TG), FTIR spectroscopy (chemical structure) and low-temperature nitrogen adsorption (BET surface area). Thermal treatment of the carbon materials caused the surface functional groups to decompose; in consequence, the chemical properties of the carbon surfaces changed. Cyclic voltammetric studies were carried out on all samples using a powdered activated carbon electrode (PACE) and a carbon paste electrode (CPE), as were electrochemical measurements in aqueous electrolyte solutions (0.1 M HNO₃ or NaNO₃) in the presence of Cu²⁺ ions acting as a depolariser. The shapes of the cyclic voltammograms varied according to the form of the electrodes (powder or paste) and to the changes in the surface chemical structure of the carbons. The electrochemical behaviour of the carbons depended on the presence of oxygen-containing surface functional groups. The peak potentials and their charge for the redox reactions of copper ions depended on their interaction with the carbon surface.     

Diamond-like carbon thin films prepared by ECR argon plasma assisted pulsed laser deposition

Diamond-like carbon films were prepared by pulsed laser ablation of graphite target in argon plasma produced from electron cyclotron resonance (ECR) microwave discharge and analyzed by Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The analysis shows that the films prepared with argon plasma assistance have different chemical structure compared with the films prepared in vacuum without plasma assistance. The structure of the films prepared with plasma assistance depends strongly on the bias voltages applied on the substrate. Surface morphology observation shows that the films prepared with argon plasma assistance have a smoother surface than the films prepared without plasma assistance. The re-sputtering of the growing film due to the bombardment of the plasma stream results in reduction of the deposition rate. The ablation plumes during film preparation with and without plasma assistance were examined through optical emission spectroscopy. In vacuum, emission lines from mono-atomic carbons and carbon ions dominate the plume emission. In argon plasma, the plume emission exhibits different behavior in its temporal and spatial evolution. It is initially dominated by strong lines from mono-atomic carbons and carbon ions and then evolves to consist mainly of emissions from C₂ molecules superposed on a featureless continuum. It is also found that the emission intensity of the C₂ molecules as well as the continuum varies with the bias voltages.     

液相法沉积DLC的表征及交流阻抗谱

分别以乙醇和甲醇为电解液,采用电化学沉积法,在Si(100)表面得到碳薄膜。通过拉曼光谱和傅立叶变换红外光谱分析,证实所制备的薄膜具有类金刚石(DLC)结构特征。利用扫描电子显微镜对不同工艺制备的薄膜进行了表面形貌分析,并研究了样品在0.5mol/LH2SO4溶液中的电化学交流阻抗谱。结果表明,与在乙醇体系中制得的DLC膜相比,在甲醇体系中制得的DLC膜具有更好的表面形貌和更好的抗腐蚀能力。     

Nanotribological study of PECVD DLC and reactively sputtered Ti containing carbon films

Amorphous carbon film, also known as DLC film, is a promising material for tribological application. It is noted that properties relevant to tribological application change significantly depending on the method of preparation of these films. These properties are also altered by the compositions of these films. DLC films are well known for their self-lubricating properties, as well. In view of this, the objective of the present work is to compare the tribological properties of diamond like carbon (DLC) film obtained by plasma enhanced chemical vapour deposition (PECVD) with the Ti containing nanocrystalline carbon (Ti/a-C:H) film obtained by unbalanced magnetron sputter deposition (UMSD) in nN load range. Towards that purpose, DLC and Ti/a-C:H films are deposited on silicon substrate by PECVD and UMSD processes respectively. The microstructural features and the mechanical properties of these films are determined by scanning electron microscope (SEM), transmission electron microscope (TEM) and nano indenter. The surface topographies and the friction force surfaces of these films are evaluated by means of an atomic force microscope (AFM). The results show that although PECVD DLC film has higher elastic modulus and higher hardness than UMSD Ti/a-C:H film, the surface roughness and the friction coefficient of PECVD film is significantly higher than that of UMSD Ti/a-C:H film.     

Super-aligned carbon nanotube films as aligning layers and transparent electrodes for liquid crystal displays

Super-aligned carbon nanotube (CNT) film is a transparent conductive film composed of numerous parallel CNTs connected to each other by the van der Waals force. We have found that such films, attached to glass, can readily align liquid crystal molecules. A prototype liquid crystal display (LCD) using the film as the aligning layer has been fabricated. The on-off ratio of the as-prepared display cell is 85 when 24 V is the applied voltage. Because of its high transparency, the film can simultaneously be used as a transparent electrode material, in place of expensive indium tin oxide. The resistance of the film makes it a suitable built-in heating layer, extending the working temperature range of LCDs.