Ultrathin Tetrahedral Amorphous Carbon Research Articles

Effect of Arc Current on Structure and Properties of Ultrathin Tetrahedral Amorphous Carbon Film

Effect of Arc Current on Structure and Properties of Ultrathin Tetrahedral Amorphous Carbon Film

Atomic-force-microscopy nanowriting on ultrathin tetrahedral amorphous carbon films

We report the atomic-force-microscopy (AFM)-based nanolithography writing of surface patterns on ultrathin ( 4 V, and (3) the contact pressures in the GPa range results in the formation of a novel carbon phase in a nm-thick surface layer characterized by the lower density, lower mechanical strength, lower electrical conductivity, and increased nanofriction as compared to the original film. The structure of the tip-modified nm-thick layer on the ta-C film is assumed to be a structure of graphite oxide which can be further modified in the presence of water under high contact pressures.

Deformation of Ultra-Thin Diamond-Like Carbon Coatings Under Combined Loading on a Magnetic Recording Head

Ultra-thin diamond-like carbon (DLC) coatings are used in precision engineering applications, such as magnetic storage devices, to protect intricate structures from wear and corrosion. A DLC coating typically consists of hard amorphous carbon in combination with an interlayer such as silicon (Si), to improve adhesion to the substrate material. Deformation and delamination of these coatings, even in part, could expose the substrate material and compromise its integrity and functionality. We have implemented a molecular dynamics model to quantify the strength of the interface between an ultra-thin tetrahedral amorphous carbon coating, a Si layer, and a permalloy (NiFe) substrate, under combined normal and tangential loading that mimics accidental contact between the recording head and the disk of a hard drive. We have evaluated the effect of the thickness of the different coating layers on deformation and interfacial strength of the coating during combined loading. The results indicate that deformation occurs primarily in the Si layer, and at the interface between the Ni–Si and the Si–C layers. Permanent separation of the Si and ta-C layers is observed, which gradually increases with multiple combined loading cycles. We find that increasing the Si and carbon layer thickness strengthens the DLC coating. However, increasing the carbon layer thickness has a larger effect on coating strength than increasing the Si layer thickness.

Ultrathin undoped tetrahedral amorphous carbon films: thickness dependence of the electronic structure and implications for their electrochemical behaviour.

In this paper we show that the electronic properties of ultrathin tetrahedral amorphous carbon (ta-C) films are heavily dependent on their thickness. By using scanning tunnelling spectroscopy, Raman spectroscopy, and conductive atomic force microscopy, it was found that a decrease of ta-C thickness from 30 to 7 nm leads to (i) the narrowing of the band gap; (ii) appearance of shallower monoenergetic traps as well as the increase of their concentration; (iii) the increase of the equilibrium concentration of free charge carriers and their mobility; which were caused by (iv) the increase in the sp(2) fraction. However, beyond a certain ta-C thickness (7 nm) the electronic properties of the studied samples start to deteriorate, which is highly likely related to titanium oxide formation at the Ti/ta-C interface. The same tendency is observed for the sample with beforehand air-formed native titanium oxide at the interface. With respect to the last point, it is suggested that the ta-C layer has no uniform coverage if its thickness is small enough (less than 7 nm). The experimental results were rationalized by detailed atomistic simulations. By using the so-called "Tauc plot" we introduce the possibility of the coexistence of bulk and surface band gaps originating from the large increase in sp(2) bonded carbon atoms in the surface region compared to that in the bulk ta-C. The results from the simulations were found to be consistent with the experimental measurements. The previously stated variation in the electronic properties of the layers as a function of their thickness was also exhibited in the electrochemical properties of the samples. It appears that the thinner ta-C layers had more facile electron transfer kinetics as determined with a ferrocenemethanol (FcMeOH) outer sphere redox system. However, if the ta-C layer thickness was reduced too much, the films were not stable anymore.

Thickness dependence of properties and structure of ultrathin tetrahedral amorphous carbon films: A molecular dynamics simulation

Molecular dynamics simulation was performed to investigate the effect of thickness on the properties and structure of ultrathin tetrahedral amorphous carbon films (ta-C). The present simulation showed that both the density and residual compressive stress decreased with increasing the thickness of ultrathin ta-C films, which was in agreement with the experimental results. The gradient of properties with the thickness was dependent on the incident kinetic energy of deposited atoms; when the incident kinetic energy was 10eV/atom, the slower gradient was observed. Further structural analysis indicated that the critical relaxation of highly distorted bond angles was responsible for the reduction of residual compressive stress in the films deposited at 10eV/atom, while the joint relaxation of both the distorted bond angles and CC bond length led to the fast release of residual compressive stress in the films deposited at 70eV/atom.

Stress reduction dependent on incident angles of carbon ions in ultrathin tetrahedral amorphous carbon films

We presented the combined experimental and simulation study on stress evolution as a function of incident angles of carbon ions for the ultrathin tetrahedral amorphous carbon films (ta-C). The residual stress was found about 3.6 +/- 0.1 GPa for the incident angle of C ions with range of 0 degrees-30 degrees, while it decreased significantly to 2.8 GPa with the incident angle of 60 degrees. Different with the previous reports, noted that in this case both the sp(3) content and mechanical properties of film were not deteriorated. Taking molecular dynamics simulation, it was in particularly concluded that the critical relaxation of distorted C-sp(3) bond lengths and bond angles played key role on the unusual stress reduction mechanism. The results provide a route to fabricate the ultrathin ta-C films with low stress and high hardness for the precision wear resistant applications. (C) 2014 AIP Publishing LLC.

Released Plasmonic Electric Field of Ultrathin Tetrahedral-Amorphous-Carbon Films Coated Ag Nanoparticles for SERS

We have demonstrated the plasmonic characteristics of an ultrathin tetrahedral amorphous carbon (ta-C) film coated with Ag nanoparticles. The simulation result shows that, under resonant and non-resonant excitations, the strongest plasmonic electric field of 1 nm ta-C coated Ag nanoparticle is not trapped within the ta-C layer but is released to its outside surface, while leaving the weaker electric field inside ta-C layer. Moreover, this outside plasmonic field shows higher intensity than that of uncoated Ag nanoparticle, which is closely dependent on the excitation wavelength and size of Ag particles. These observations are supported by the SERS measurements. We expect that the ability for ultrathin ta-C coated Ag nanoparticles as the SERS substrates to detect low concentrations of target biomolecules opens the door to the applications where it can be used as a detection tool for integrated, on-chip devices.

Si seed layer thickness effect on the structure of ultrathin tetrahedral amorphous carbon films

Tetrahedral amorphous carbon (ta-C) film is a promising materials serving as a protective coating layer due to its high sp3 content and its thickness up to 1–2nm. In order to improve the adhesion of ta-C on metal, a Si film is usually pre-coated as seed layer. However, the Si film will increase the total thickness of protective layer. How to reduce the thickness of Si film is a requirement of industry applications. Here, we have studied the Si film thickness effect on the structure of ultrathin ta-C film. In our experiment, the thickness of Si film varies from 5 to 30Å while the thickness of ta-C film is kept at about 2nm. The Raman result shows that the sp3 fraction of ta-C film with pre-coated 5Å Si film is a little lower than that of ta-C film without Si pre-coated. When the Si film becomes thicker from 5 to 30Å, the sp3 fraction of Si/ta-C composite films is almost no changed. The XPS analysis shows that a part of Si bonding changes to SiC bonding which can enhance the adhesion of ta-C on metal. It also confirms that the SiC bonding fraction of Si film has no change when the thickness of Si film increases from 5 to 30Å, which is consistent with the Raman analysis. Thus, an indication is that the fundamental limitation thickness of Si seed layer can be reduced to 5Å.

Developing an (Al,Ti)N x C y Interlayer to Improve the Durability of the ta-C Coating on Magnetic Recording Heads

In this study, a new surface pre-treatment technique has been developed to improve the durability of the ultra-thin tetrahedral amorphous carbon (ta-C) coating for magnetic tape drive read/write heads. In this technique, prior to the deposition of an 8-nm ta-C overcoat, a 2-nm thin TiN interlayer was deposited on the heads surface and bombarded with energetic Ar+ and C+ ions. X-ray photo-electron spectroscopy results revealed that this surface pre-treatment technique would lead to the formation of an atomically mixed (AlTi)N x C y interlayer which can chemically bond the interlayer to the overcoat and substrate. The effect of this atomically mixed interlayer on the wear resistance of the ta-C coating was investigated using ball-on-flat tests as well as a functional tape drive tester. According to the ball-on-flat test results, the application of the (AlTi)N x C y interlayer was able to improve the wear life of the ta-C overcoat by up to 3.3 times as compared to that of the conventional ta-C coating. The results of the wear tests in a real head/tape interface were in agreement with the ball-on-flat results, and showed that while the conventional ta-C film was completely removed from the head surface, the ta-C film with (AlTi)N x C y was able to protect the head surface for wear tests of about 1.6 million meters.

Thickness dependent electronic structure of ultra-thin tetrahedral amorphous carbon (ta-C) films

Microstructural properties of ultrathin (1–10nm) tetrahedral amorphous carbon (ta-C) films are investigated by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray Photoelectron Spectroscopy, Raman spectroscopy and Atomic Force Microscopy (AFM). The CK-edge NEXAFS spectra of 1nm ta-C films provided evidence of surface defects (C―H bonds) which rapidly diminish with increasing film thickness. A critical thickness for stabilization of largely sp3 matrix structure distorted by sp2 sites is observed via the change of π*C═C peak behavior. Meanwhile, an increase in the film thickness promotes an enhancement in sp3 content, the film roughness remains nearly constant as probed by spectroscopic techniques and AFM, respectively. The effect of thickness on local bonding states of ultrathin ta-C films proves to be the limiting factor for their potential use in magnetic and optical storage devices.

Silicon nitride gradient film as the underlayer of ultra-thin tetrahedral amorphous carbon overcoat for magnetic recording slider

There are higher technical requirements for protection overcoat of magnetic recording slider used in high-density storage fields for the future. In this study, silicon nitride (Si-N) composition-gradient films were firstly prepared by nitriding of silicon thin films pre-sputtered on silicon wafers and magnetic recording sliders, using microwave electron cyclotron resonance plasma source. The ultra-thin tetrahedral amorphous carbon films were then deposited on the Si-N films by filtered cathodic vacuum arc method. Compared with amorphous carbon overcoats with conventional silicon underlayers, the overcoats with Si-N underlayers obtained by plasma nitriding especially at the substrate bias of −150 V, can provide better corrosion protection for high-density magnetic recording sliders.

Effect of pre-treatment of the substrate surface by energetic C+ ion bombardment on structure and nano-tribological characteristics of ultra-thin tetrahedral amorphous carbon (ta-C) protective coatings

Depositing an ultra-thin tetrahedral amorphous carbon (ta-C) protective coating on the surface of the recording heads in magnetic tape drives can improve the tribological problems at the head/tape interface. In this work the effect of pre-treatment of the surface of AlTiC substrate (main bearing surface of head in contact with tape) by C+ ions of moderate energy (smaller than 400 eV) on the structural and tribo-mechanical behaviours of the coated surfaces is studied. Sample preparation consisted of two separate stages of surface pre-treatment and deposition of the protective film, and was done by means of filtered cathodic vacuum arc. Structure of the ta-C film and its interface with the substrate were studied by transmission electron microscopy and time-of-flight secondary ion mass spectrometry depth profiling. The results revealed the formation of a broader, dense atomically mixed layer at the ta-C film–substrate interface of the pre-treated samples comparing with that of the samples without pre-treatment. Chemical characterization of thin diamond-like carbon coatings was conducted by means of x-ray photoelectron spectroscopy and the surface pre-treatment was found to have a remarkable effect on increasing the sp3 hybridization fraction in the ta-C overcoat. Nano-tribological properties of the treated surfaces were examined using ball-on-flat wear test at very low load (20 mN). There was a good correlation between the surface and structure characteristics of the film, and the tribological results and the pre-treated surfaces presented a very low coefficient of friction and higher wear life. The experimental results demonstrate the effectiveness of bombardment of the surface with C+ ions of moderate ion energy to improve the structural and tribo-mechanical properties of the protective ta-C films on the magnetic head substrate material.

Effect of ECR-assisted microwave plasma nitriding treatment on the microstructure characteristics of FCVA deposited ultra-thin ta-C films for high-density magnetic storage applications

There are higher technical requirements for protecting layer of magnetic heads and disks used in future high-density storage fields. In this paper, ultra-thin (2 nm thickness) tetrahedral amorphous carbon (ta-C) films were firstly prepared by filtered cathodic vacuum arc (FCVA) method, then a series of nitriding treatments were performed with nitrogen plasma generated using electron cyclotron resonance (ECR) microwave source. Here it highlighted the influence of nitrogen flow and applied substrate bias voltage on the structural characteristics of ta-C films during the plasma nitriding process. The chemical compositions, element depth distribution profiles, physical structures and bonding configurations of plasma-nitrided ta-C films were investigated by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and UV–vis Raman spectroscopy. The experimental results show that the carbon nitride compounds (CN x ) are formed in nitrogenated ta-C films in which the N content and its depth distribution depends on bias voltage to large extent rather than N 2 flow. The N content of nitrogenated ta-C films can reach 16 at.% for a substrate bias of −300 V and a N 2 flow of 90 sccm. With increasing nitrogen content, there is less G peak dispersion and more ordering of structure. Furthermore, appropriate nitriding treatment (substrate bias: −100 V, N 2 flow: 150 sccm) can greatly increase the fraction of sp 3 and sp 3C–N bonds, but the values begin to fall when the N content is above 9.8 at.%. All these indicate that suitable ECR-assisted microwave plasma nitriding is a potential modification method to obtain ultra-thin ta-C films with higher sp 3 and sp 3C–N fractions for high-density magnetic storage applications.

Corrosion protection of ultra-thin ta-C films for recording slider applications at varied substrate bias

Ultra-thin tetrahedral amorphous carbon (ta-C) films have been prepared by filtered catholic vacuum arc system for recording slider applications. In order to study the corrosion behavior of the slider coatings deposited at different substrate bias, the micro-structure and surface properties of the ta-C films were respectively investigated in terms of Raman spectroscopy, atomic force microscopy, water contact angle and corrosion measurements. Results showed that a very smooth ta-C film with lowest surface energy and highest sp 3 bonding content was obtained at the medium bias voltage of 100 V. However, as to the protection against corrosion, the optimum bias was found to be greater than that giving the maximum diamond-like properties.

Thermal conductivity of ultrathin tetrahedral amorphous carbon films

We investigate the thermal conductivity of ultrathin tetrahedral amorphous carbon (ta-C) films on silicon, down to subnanometer thickness. For films with an initial sp3 content of 60%, the thermal conductivity reduces from 1.42to0.09W∕mK near room temperature as the thickness decreases from 18.5to∼1nm. The variation in ta-C film thickness is accompanied by changes in Young’s modulus, density, and sp3 content. The thermal resistance of the finite-thickness interface layer, which forms between ta-C and silicon, is ∼10−8m2K∕W near room temperature, thus producing a noticeable effect on thermal transport in ultrathin ta-C films.

Improving organic light-emitting devices by modifying indium tin oxide anode with an ultrathin tetrahedral amorphous carbon film

The characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum with an ultrathin tetrahedral amorphous carbon (ta-C) film on indium tin oxide have been investigated. The device with a 1.0-nm ta-C layer has the highest current and power efficiency. The current efficiency of a device with a ta-C layer thickness of 1.0nm is 3.7cd∕A at 20mA∕cm2, however, the current efficiency of a standard device without a ta-C layer is 2.56cd∕A at the same current density. The current efficiency is improved by 46% compared to the standard device. Although there is a 1.2-V increase in driving voltage for 100-cd∕m2 luminance, the power efficiency is still improved by 22% compared to that of the standard device. The improvement of the efficiency is due to smoothing indium tin oxide surface, blocking hole injection from anode and balancing hole and electron currents. The optimal thickness of ta-C layer for hole injection mechanics can be understood by tunneling.

Improvement of efficiency and stability of polymer light-emitting devices by modifying indium tin oxide anode surface with ultrathin tetrahedral amorphous carbon film

Poly(p-phenylene vinylene)-based polymer light-emitting devices using different thicknesses of tetrahedral amorphous carbon ultrathin films between indium tin oxide and polyethylenedioxythiophene hole transporting layer have been fabricated. The device with a 0.5nm tetrahedral amorphous carbon (ta-C) layer has the highest luminance and current efficiency compared to that of other devices. The current efficiency of a standard device without a ta-C layer is 1.1cd∕A at 5V, however, the current efficiency of a device with a ta-C layer thickness of 0.5nm is 2.7cd∕A; the current efficiency is improved about 2.5 times compared to the standard device. The improvement of the efficiency is due to blocking the hole injection from anode and balancing the hole and electron current. The lifetime of a device with a ta-C layer is significantly much longer than the standard device, and the device with a 0.5nm ta-C layer has the longest lifetime.

Surface properties of ultra-thin tetrahedral amorphous carbon films for magnetic storage technology

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Film thickness below 2 nm and roughness well below 1 nm are needed for storage density of 200 Gbit/inch 2. We use atomic force microscopy to study the roughness evolution vs. thickness of highly sp 3 hydrogen-free tetrahedral amorphous carbon (ta-C). The roughness of films r generally follows fractal scaling laws, increasing with film thickness h as r= ah β, where β is the growth exponent. For a fixed film thickness and scan length l, the roughness varies as l α, where α is the roughness exponent. We find α∼0.39 and β∼0–0.1. We performed Monte Carlo simulations, modelling the smoothing effects caused by the thermal spike. The simulation results closely match the experimental findings and define a new growth mechanism for ta-C films. The structural evolution of these ultra-thin films is monitored by Raman spectroscopy. A linear relation between G-peak dispersion and Young's modulus is found. The 2-nm-thick ta-C films are pin-hole free, corrosion resistant, have a Young's Modulus of ∼100 GPa, sp 3 content of ∼50% and roughness of ∼0.12 nm. So, data storage density of 1 Tbit/inch 2 could be achieved.

Ultra-thin tetrahedral amorphous carbon films with strong adhesion, as measured by nanoscratch testing

Amorphous carbon can form hard and dense films, which find applications for tooling components, magnetic recording, etc. The challenge is to prepare hard layers that have good adhesion to the substrate. Moreover, mechanical characterisation of sub-20 nm layer is still difficult and semi-quantitative. The paper is addressing these issues. We prepared 10-nm and 50-nm thick tetrahedral amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H) films, respectively, by filtered cathodic arc deposition (FCVA) and plasma enhanced chemical vapour deposition (PECVD). The films were characterised by nanoindentation and nano-scratching. With such hard films, blunting of the diamond tip can affect the measurements and we used a hardness slope ratio protocol, which gave more accurate results. The ta-C films were the hardest and more wear resistant, i.e. for the 50-nm thick ta-C film on Al 2O 3–TiC; H (25 nm)=51 GPa and residual depth=4 nm for a 5 mN scratch load. This is typical of the FCVA conditions, i.e. a relatively high energy, mono-energetic (approx. 25 eV) non-hydrogenated ion flux, resulting in formation of a dense sp 3 network. Ramping the scratch load to observe delamination events, we measured the highest critical loads (approx. 6 mN) again for the ta-C films. We used SEM/EDX thickness measurements to confirm these delaminations. Again, the ta-C films show the best adhesion. We believe that, the FCVA beam is sufficiently energetic to provide a dense and rigid network structure and a good ion beam mixing in the substrate but, appropriately, not enough to produce the large internal stress observed at −100 V substrate bias.

Surface roughness, mechanical and tribological properties of ultrathin tetrahedral amorphous carbon coatings from atomic force measurements

Atomic force microscope (AFM), lateral force microscope and AFM-based scratch and wear testing techniques were used to evaluate and compare the surface roughness, tribological and mechanical properties of thin (2.7–43 nm) tetrahedral amorphous carbon coatings prepared by pulsed cathodic arc discharge. It was found that surface roughness of ultrathin (2–8 nm) coatings was mainly determined by the roughness of the Si substrate and their average density strongly depended on their thickness. Poor friction, mechanical properties of thinner (2.7–15 nm) coatings can be associated with their low average density. The dense coatings (>15 nm) had lower friction coefficient, better scratch and wear resistance properties that were independent of their thickness. It appears that the over 15-nm coatings studied are feasible for some wear-resistant and tribological applications.