Properties Of Tetrahedral Amorphous Carbon Research Articles

Electron transport determines the electrochemical properties of tetrahedral amorphous carbon (ta-C) thin films

Amorphous carbon based electrodes are very promising for electrochemical sensing applications. In order to better understand their structure-function relationship, the effect of film thickness on the electrochemical properties of tetrahedral amorphous carbon (ta-C) electrodes was investigated. ta-C thin films of 7, 15, 30, 50 and 100nm were characterized in detail with Raman spectroscopy, transmission electron microscopy (TEM), conductive atomic force microscopy (c-AFM), scanning tunneling spectroscopy (STS) and X-ray absorption spectroscopy (XAS) to assess (i) the surface properties of the films, (ii) the effect of film thickness on their structure and electrical properties and (iii) the subsequent correlation with their electrochemistry. The electrochemical properties were investigated by cyclic voltammetry (CV) using two different outer-sphere redox probes, Ru(NH3)63+/2+ and FcMeOH, and by electrochemical impedance spectroscopy (EIS). Computational simulations using density functional theory (DFT) were carried out to rationalize the experimental findings. The characterization results showed that the sp2/sp3 ratio increased with decreasing ta-C film thickness. This correlated with a decrease in mobility gap value and an increase in the average current through the films, which was also consistent with the computational results. XAS indicated that the surface of the ta-C films was always identical and composed of a sp2-rich layer. The CV measurements indicated reversible reaction kinetics for both outer-sphere redox probes at 7 and 15nm ta-C films with a change to quasi-reversible behavior at a thickness of around 30nm. The charge transfer resistance, obtained from EIS measurements, decreased with decreasing film thickness in accordance with the CV results. Based on the characterization and electrochemical results, we conclude that the reaction kinetics in the case of outer-sphere redox systems is determined mainly by the electron transport through the films and not the electron transfer between the redox probe and the electrode surface, since the surface region is expected to be identical in all the ta-C films. Finally, it was shown that a titanium interlayer between the Si substrate and ta-C significantly affected the electrical and electrochemical properties of the electrodes, further emphasizing the role of electron transport through the film in determining the electrochemical behavior of ta-C.

Effects of structure of zinc dialkyldithiophosphates on tribological properties of tetrahedral amorphous carbon f ilm under boundary lubrication

Effects of the structure of zinc dialkyldithiophosphates (ZDDPs) on the tribological properties of a tetrahedral amorphous carbon (ta-C) film were investigated. Friction tests were performed at diamond-like carbon (DLC)/DLC and steel/steel contacts under lubrication with poly-alpha olefin (PAO) and PAO containing six types of ZDDPs with different alkyl groups. The tribological properties of the ta-C film depended on the chemical composition of ZDDP-derived tribofilms on the outermost surface, which depended on the abundance ratio of neutral-ZDDP and basic-ZDDP in the ZDDP solution. Thus, the ZDDP forms play important roles in the tribological properties of the ta-C film. All these results suggest that the ZDDP forms can determine the tribological properties of the ta-C film lubricated with the ZDDP solution.

Magnetoelectronic properties of Gd-implanted tetrahedral amorphous carbon

The structural, electronic, magnetic, and magnetoelectronic properties of tetrahedral amorphous carbon ($ta$-C) thin films doped with gadolinium via ion implantation ($ta$-C${}_{1\ensuremath{-}x}:$Gd${}_{x}$, $x$ = 0.02 \ensuremath{\sim} 0.20) have been studied, both as prepared and after annealing, with Xe-implanted samples as control samples. Gd implantation causes significant increases in electrical conductivity, showing that Gd adds carriers as in other rare earth--semiconductor systems. Gd also provides a large local moment from its half-filled $f$ shell. Carrier-mediated Gd-Gd interactions are strong but very frustrated, causing a spin-glass state 10 K for higher $x$. An enormous negative magnetoresistance (about \ensuremath{-}10${}^{3}$ at 3 K in a 70-kOe field for $x$ = 0.088) is observed at low $T$ (30 K), an indication of carrier--moment interactions that cause magnetic disorder--induced localization and consequent magnetic field--induced delocalization as Gd moments align with the magnetic field. Gd implantation causes substantial changes in Raman intensity, associated with conversion of C-C bonds into Raman inactive bonds, which induce further graphitization after annealing. The changing nature of the C-C bonding with increasing x or with annealing causes the electrical transport properties to depend on Gd concentration $x$ with a nonmonotonic dependence. Systematic but nonmonotonic trends are seen on comparing the magnetic and magnetotransport properties of Gd-doped $a$-C, $a$-Si, and $a$-Ge matrices, suggesting that electron concentration and band gap play separate important roles.

The importance of rare events in thin film deposition: a molecular dynamics study of tetrahedral amorphous carbon

The transition of diamond-like materials from an sp3 to sp2 rich state is of particular interest because of the desirable properties of tetrahedral amorphous carbon (ta-C). Previous works indicate that infrequent processes may dominate this transition, but simulation of these processes presents significant difficulties, since the infrequent processes are activated on the millisecond scale. In this molecular dynamics study the environment dependent interaction potential is used to simulate the thin film deposition of ta-C. Infrequent processes occurring between energetic impacts were activated on the picosecond scale using elevated temperatures. The simulations reveal an abrupt transition in which the ta-C films transform into graphite-like sheets. A similar transformation, albeit at much higher temperatures, is also observed when the films are heated without energetic impacts. These results are found to be in good agreement with published experimental data.

Surface atomic properties of tetrahedral amorphous carbon

The surface energy and surface atomic structure of tetrahedral amorphous carbon has been calculated by an ab-initio method. The surface atoms are found to reconstruct into sp 2 sites often bonded in graphitic rings. Placing the dangling bonds on adjacent surface atoms lower their energy by π-bonding and this is the source of the low surface energy. The even lower surface energy of hydrogenated amorphous carbon (a-C:H) is due to the hydrogenation of all broken surface bonds.

Electrode Properties of Tetrahedral Amorphous Carbon

Impedance spectra in a 2.5 M H2SO4 solution and kinetic characteristics of reactions in the Fe(CN) 6 3−/4− redox system are measured for thin-film electrodes of tetrahedral amorphous carbon (ta-C). After an anneal in a vacuum at 700 to 900°C or implantation of C+ ions (1015 to 1017 ion/cm2), ta-C films acquire electrochemical activity, which can be explained by an increased content of sp 2 carbon.

Growth processes and surface properties of diamondlike carbon films

In this study, we compare the deposition processes and surface properties of tetrahedral amorphous carbon (ta-C) films from filtered pulsed cathodic arc discharge (PCAD) and hydrogenated amorphous carbon (a-C:H) films from electron cyclotron resonance (ECR)-plasma source ion implantation. The ion energy distributions (IEDs) of filtered-PCAD at various filter inductances and Ar gas pressures were measured using an ion energy analyzer. The IEDs of the carbon species in the absence of background gas and at low gas pressures are well fitted by shifted Maxwellian distributions. Film hardness and surface properties show a clear dependence on the IEDs. ta-C films with surface roughness at an atomic level and thin (0.3–0.9 nm) graphitelike layers at the film surfaces were deposited at various filter inductances in the highly ionized plasmas with the full width at half maximum ion energy distributions of 9–16 eV. The a-C:H films deposited at higher H∕C ratios of reactive gases were covered with hydrogen and sp3 bonded carbon-enriched layers due to the simultaneous interaction of hydrocarbon species and atomic hydrogen. The effects of deposited species and ion energies on film surface properties were analyzed. Some carbon species have insufficient energies to break the delocalized π(nC) bonds at the graphitelike film surface, and they can govern film formation via surface diffusion and coalescence of nuclei. Dangling bonds created by atomic hydrogen lead to uniform chemisorption of hydrocarbon species from the ECR plasmas. The deposition processes of ta-C and a-C:H films are discussed on the basis of the experimental results.

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

The properties of tetrahedral amorphous carbon (ta-C) films grown by pulsed laser deposition (PLD) using camphoric carbon (CC) target and their respective effects of diamond percentages by weight in the target (Dwt.%) are discussed. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Visible-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses indicated that the Dwt.% noticeably modified the sp 3 bonds content and the morphology of the ta-C films. The optical gap ( E g) and electrical resistivity ( ρ) increase with Dwt.% up to 1.6 eV and 5.63×10 7 (Ω cm), respectively, for the ta-C films deposited using target with higher of 50 Dwt.%. We found that the Dwt.% has modified the surface morphological, structural, bonding and physical properties of the camphoric carbon films.

Calculation of the bandgap and of the type of interband transitions in tetrahedral amorphous carbon using electron energy loss spectroscopy

A fundamental understanding of the electronic properties of materials and their potential for application in new devices requires detailed knowledge of their electronic structure. Electron energy loss spectra can provide a continuous measurement of collective electron plasma oscillations and single electron excitations in the energy range 0.3–100 eV. In contrast to conventional optical spectroscopy, which has an intrinsic upper information limit of approximately 6 eV due to absorption of light from the optical components, electron energy loss spectroscopy (EELS) offers electronic excitation data over an energy range sufficient for accurate application of Kramers–Kronig calculations. In addition, the incident electron probe offers an extremely high spatial resolution. Furthermore, the incident fast electrons can transfer both energy and momentum to the valence electrons to trigger both direct and indirect band electronic transitions. In this paper, we have calculated the electronic properties of tetrahedral amorphous carbon (ta-C). The imaginary part of the dielectric function, ε 2, experimentally derived from the EELS data has been used to calculate the energy dependence of the M 2 ρ product, where M is the matrix element and ρ is the joint density of states (JDOS). The shape of the M 2 ρ curve has been used to calculate the bandgap value and the type of transition (direct or indirect). Using ta-C as an example, the ability to calculate the semiconducting properties of materials at the nanoscale is demonstrated.

Evaluating the fracture properties and fatigue wear of tetrahedral amorphous carbon films on silicon by nano-impact testing

A repetitive contact technique, nano-impact testing, has been used to investigate the fracture properties of tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The impact test has shown clear differences in the resistance to impact wear of ta-C films with their thickness, for film thicknesses between 5 and 80 nm. The resistance to impact-induced fracture decreases as the film thickness increases. This may be due to the maximum shear stress being closer to the film–substrate interface, or reflect reduced toughness in the thicker films that are less able to deform as the substrate deforms plastically during the repetitive contact test. The mechanism of impact-induced failure on these thin films is (i) the initial impact stage where plastic deformation causes cracks to nucleate sub-surface, (ii) fatigue—further nucleation and growth of sub-surface cracks (with little or no change in probe depth) and (iii) crack coalescence and film fracture leading to a rapid change in probe depth as the film fails. The influence of impact load on fracture probability has been investigated. Fracture probability increases sharply as the impact load is increased from 100 to 300 μN. The greater load provides the stresses necessary to nucleate and propagate the sub-surface cracks; at low load the driving force for the cracks to coalesce and spall the coating is much reduced, so failure is less likely to occur within the test duration.

Nanoscale manipulation of tetrahedral amorphous carbon films

The outstanding mechanical, chemical and tribological properties of tetrahedral amorphous carbon (ta-C) films have attracted much attention. For a wide variety of applications, a great effort should be focused on the nanoscale structure control. In the present work, we have adopted a novel technique for nanoscale manipulation of ta-C films; incorporating nano Ni dots at the interface between the ta-C film and the substrate. For Ni dot pretreatment, the Ni thin film was deposited and annealed prior to the ta-C films deposition. TEM and Raman spectrum analysis shows that the nano Ni dots at the interface between the film and the substrate results in nanoscale graphitic phase embedded in hard ta-C matrix. Mechanical and electrical properties were strongly dependent on the changes of the Ni-induced second phase. The reduction of mechanical properties and electrical resistivity with increasing the size of Ni dots could be understood in view of the local increase of sp 2 bonds in hard ta-C matrix.

Influence of the incident angle of energetic carbon ions on the properties of tetrahedral amorphous carbon (ta-C) films

Tetrahedral amorphous carbon (ta-C) films have been grown on Ar+-beam-cleaned silicon substrates by changing the incident angle of energetic carbon ions produced in the plasma of pulsed cathodic vacuum arc discharge. Their surface roughness, deposition rate, composition, and mechanical and frictional properties as a function of the incident angle of energetic carbon ions were reported. The substrate holder can be rotated, and so an angle of deposition was defined as the angle of ion flux with respect to the substrate surface. While the deposition angle is varied from 20° to 59°, the root-mean-square (rms) roughness decreases from 0.5 to 0.1 nm, then it turns to increase at a slow rate when the deposition angle is over 77°. The variation correlates well with the one of hardness with the deposition angle and the films with lower rms roughness exhibit the higher hardness. The soft graphite-like surface layers existing at the surfaces of these films were revealed by atomic force microscopy-based nanowear tests and their thickness increases from 0.35 to 2.9 nm with the deposition angle decreasing from 90° to 30°. The soft surface layer thickness can have a great effect on the sp3 contents measured by x-ray photoelectron spectra. Nanoscale friction coefficient measurements were performed from lateral force microscopy by using a V-shaped Si3N4 cantilever. The low friction coefficients (0.076–0.093) of ta-C films can be attributed to their graphite-like surface structure. The implications of these results on the mechanisms proposed for the film formation were discussed.

The effect of the surface layer of tetrahedral amorphous carbon films on their tribological and electron emission properties investigated by atomic force microscopy

The nanowear resistance, tribological, and field emission properties of tetrahedral amorphous carbon (ta-C) films have been analyzed by atomic force microscope (AFM)-based wear testing technique, lateral force microscope, and conducting AFM. The ta-C films grown by filtered pulsed cathodic arc discharge were found to have soft surface layers, 1.1±0.1 nm thick, which contribute to an improvement of their field emission properties. The low friction coefficient between the nanotip and film surface is correlated to one or two graphite-like atomic layers at the ta-C film surfaces. The analysis of Fowler–Nordheim tunneling currents indicates the formation of filament-like emission channels in amorphous carbon films.

Properties Oftetrahedralamorphous Carbon Films Deposited By the Filtered Cathodicarc Method

ABSTRACTFirst, we studied the relation between the sp3 bond ratio and the hardness of 100-nm-thick tetrahedral amorphous carbon (ta-C) films deposited by a Filtered Cathodic Arc (FCA) system at different substrate bias voltages. For comparison, sputtered amorphous carbon (a-C) films and Highly Oriented Pyrolytic Graphite (HOPG) were also analyzed. According to the results, ta-C film deposited at a -70 volt substrate bias voltage had high hardness and high sp3 bond ratio of up to 88 GPa and 85%, respectively, whereas those of sputtered a-C were 29 GPa and 28%. Furthermore, we found that the hardness of carbon films, including sputtered a-C and HOPG, increased with increasing sp3 bond ratio. Based on this relation, the chemical bond structures of carbon films are considered to be closely related to their mechanical properties. Secondly, we investigated the relation between sp3 bond ratio and ta-C film thickness, over the range from 1 to 10 nm. The measurements showed that a 2-nm-thick initial layer grown on the surface of CoCrPt magnetic film had low sp3 bond ratios. It is suggested that this reduction in sp3 bond ratio in the initially grown layer seriously degrades the film's performance as a hard disk media overcoat. Further efforts to improve ta-C film processing will be required to improve its mechanical properties.

Influence of the heterojunction on the field emission from tetrahedral amorphous carbon on Si

In order to study the influence of the back barrier on the electron emission properties of tetrahedral amorphous carbon (ta-C), we have deposited identical films on p and n+ Si. The valence and conduction band offsets were measured for ta-C on p and n+ Si using in situ x-ray photoelectron spectroscopy and optical spectroscopy. From the band measurements it is shown that there is a substantial back barrier to emission. We show that for films having very similar properties, the electron emission can be influenced by the ta-C/Si heterojunction.

Time and temperature-dependent changes in the structural properties of tetrahedral amorphous carbon films

The time and temperature-dependent changes in the structural properties of tetrahedral amorphous carbon (ta-C) films were accessed continuously by Raman spectroscopy. It has been found that a film of 70-nm thickness remains structurally stable after annealing in air at up to 300°C for 4 h. Although some degree of graphitization was observed on a film annealed at 400°C, the film begins to oxidize and lose thickness only after annealing at 500°C for more than 2 h. This reflects the high thermal stability of the films. In general, annealing results in a narrowing and an upshifting of the G-band together with an increase in the I D/ I G ratio. Most of these changes were observed during the first 2 h of annealing, after which the structure of the film appears to stabilize, with the exception at 500°C in which the film deteriorates further as oxidation occurs. It was also observed that thin film has better thermal stability against graphitization than thick film.

Influence of nitrogen and temperature on the deposition of tetrahedrally bonded amorphous carbon

The effect of nitrogen addition on the properties of tetrahedral amorphous carbon (ta-C) has been studied. The ta-C is deposited by a filtered cathodic vacuum arc. The effect of introducing nitrogen on its plasma was measured by a retarding field analyzer and optical emission spectroscopy. The ta-C:N films were studied as a function of nitrogen content, ion energy, and deposition temperature. The incorporation of nitrogen was measured over the range of 10−2–10 at. % by secondary ion mass spectrometry and elastic recoil detection analysis. The N content was found to vary slightly sublinearly with the N2 partial pressure during deposition. A doping regime was found for N contents of up to 0.4 at. %, in which the conductivity changes while the sp3 content and the optical band gap remain constant. For 0.4%–8% N, the sp3 fraction remains above 80% but the optical gap closes due to a clustering of sp2 sites. Only above about 10% N, the sp3 fraction falls. The influence of nitrogen on the a-C was found to be independent of ion energies between 20 and 220 eV. Deposition above 200 °C causes a sudden loss of sp3 bonding. Raman and optical gap data show however that existing sp2 sites begin to cluster below this temperature.

Calculated electronic density of states and structural properties of tetrahedral amorphous carbon

Abstract A series of tetrahedral amorphous carbon structures with different microscopic mass densities was generated by calculations based on the density functional theory with a local density approximation and using a method of melting–cooling cycles. A detailed investigation of the properties of the simulated structures has been carried out. Particularly, the short-range order, nearest neighbour distances, fractions of sp 1 , sp 2 and sp 3 sites, average C C C bond angles and electronic density of states have been analysed. The simulated structures and calculated properties are in good agreement with those obtained by others and with the experimental data. An unexpected observation is the presence of planar structures, which are typical for graphite, in the sample with low density. In addition, the nearest neighbour distance in the sample with mass density 3.54 g/cm 3 is different from those reported previously. Possibilities to compare the density of states obtained from the simulations with the experimental results from scanning tunnelling spectroscopy and X-ray near edge spectrum are discussed.

Vibrational properties of tetrahedral amorphous carbon from first principles

We study the vibrational properties of hydrogenated tetrahedral amorphous carbon from first principles. Our results reproduce the C density of states (DOS) measured by electron energy loss. We decompose the theoretical DOS in terms of vibrational modes of microscopic units. These partial DOS have large overlaps in frequency. Only the region between 1350 and 1600 cm−1 is covered by a single type of stretching (sp2–sp2), whose DOS reproduces the experimental visible Raman spectra. We find a linear correlation between CH stretch frequency and CH bond length, and we show that the spreading of CH distances due to the constraints imposed by the C network is an important source of broadening of the CH stretching spectrum.

Electronic properties of tetrahedral amorphous carbon investigated by scanning tunneling microscopy

Scanning tunneling spectroscopy (STS) has been used to investigate the electronic structure of tetrahedral amorphous carbon (ta-C) films. The density of states (DOS) was derived from the normalized conductivity spectra at fixed tip heights. Band edges were defined by extrapolating the generalized conductivity (dI/dV) within ±2 eV of the Fermi level. The resulting band gap was found to be similar to that measured optically for ta-C films of different sp3 content. The Fermi level of undoped ta-C was found to lie below midgap, confirming ta-C to be a weakly p-type semiconductor. The valence band tail is found to be steeper than the conduction band tail, the opposite of the situation in a-Si:H. With nitrogen addition, the Fermi level is found to move above midgap, confirming that n-type doping is occurring. The band gap is found to decline at higher N contents. The DOS at larger tip heights and as a function of surface etching shows evidence of a surface layer with a narrower band gap, which we identify as the sp2-rich surface layer seen previously by cross-sectional electron energy loss spectroscopy. The STS DOS shows peaks which we attribute to states of the surface layer having more graphitic bonds. The scanning tunneling microscopy images show that ta-C deposited on Si has a root mean square surface roughness of 1.7 Å and this increases to about 10 Å for films deposited on metals such as Ti.