Friction and wear property of a-CNx coatings sliding against ceramic and steel balls in water

The amorphous carbon nitride coatings (a-CN_x) were deposited on Si₃N₄ disks using ion beam assisted deposition (IBAD), and their composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS). The a-CN_x coatings' hardness was measured by nano-indentation and the friction and wear property of the a-CN_x coatings sliding against Si₃N₄, SiC, Al₂O_3, SUS440C and SUJ2 balls in water were investigated by using ball-on-disk tribo-meter. The worn surfaces were observed using optical microscopy and analyzed by XPS. The results of XPS analysis showed that the a-CN_x coatings contained 12 at.% nitrogen and the major chemical bonding was sp² C = N and sp³C–N. The nano-hardness of the a-CN_x coatings was 29 GPa, higher than those of balls. Among five kinds of tribo-systems, the lowest friction coefficient was obtained in the range of 0.01 to 0.02 for the tribo-systems with SiC and Si₃N₄ balls, the largest wear rate of the a-CN_x coating of 1.77 × 10^− 7 mm³/Nm was obtained as sliding against Al₂O₃ ball, while the smallest wear rate of a-CN_x coating of 1.44 × 10^− 8 mm³/Nm was gotten as sliding against Si₃N₄ ball. However, SUJ2 ball showed the highest wear rate of 7.0 × 10^− 7 mm³/Nm, whereas Al₂O₃ ball exhibited the lowest wear rate of ball of 3.55 × 10^− 9 mm³/Nm. The XPS analysis on the worn surface for the a-CN_x coatings displayed that the nitrogen concentration decreased and the sp²-bonding-rich structure was formed after sliding tests in water.     

Real-time monitoring,growth kinetics and properties of carbon based materials deposited by sputtering

The nucleation stages of growth of carbon-based materials such as amorphous carbon (a-C) and carbon nitride (a-CN_x) films control the bonding configuration, sp² and sp³ formation, N concentration in a-CN_x films and consequently their final properties. Thus, in order to obtain insights into the deposition mechanisms, the study of growth kinetics by applying real-time monitoring is required. We present here, the development of a-C and a-CN_x thin-films by conventional and unbalanced magnetron sputtering, studied by in situ spectroscopic ellipsometry (SE) and multi-wavelength ellipsometry (MWE) in the energy region 1.5–5.5 eV. SE and MWE were used to monitor the deposition processes and to study the films properties. The MWE provides the ability for simultaneous acquisition of the dielectric function, in 16 different wavelengths distributed in the VIS-UV energy region and to investigate the nucleation and growth stages, the deposition rate and films composition, as well as their optical properties during deposition. The results deduced either by real-time MWE monitoring, or by the analysis of in situ SE and MWE data, for the carbon based materials prepared by the above techniques, were certified by X-ray reflectivity studies and were correlated to various mechanisms taking place during film growth (e.g. diffusion, surface adsorption, chemical reactions, etc.), which were investigated by applying a phenomenological model of growth kinetics.     

A comparative study of elastic recoil detection analysis (ERDA), electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) for structural analysis of amorphous carbon nitride films

A set of carbon and carbon nitride films has been prepared by ion-beam-assisted filtered cathodic vacuum arc deposition. The films were examined by elastic recoil detection analysis (ERDA), electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS). ERDA provides the stoichiometry and mass density of the samples. This study covers the range of 0 up to 29 at.% N and 2.9 g cm⁻³ down to 2.0 g cm⁻³ respectively. The carbon K-edge and the plasmon loss were examined by EELS. The C 1s and N 1s core level spectra were recorded by XPS. The main result is that for significant nitrogen contents the mass density, and hence the plasmon energy and very likely the sp³-fraction, is reduced considerably. The C 1s signals of the carbon nitride layers show a multi-peak structure but the peaks are rather broad and an accurate determination of peak positions is not possible. The splitting of the N 1s signal in four contributions at about 398, 400, 400.6 and 402 eV is observed only for the low density carbon nitrides. A CN_x sample with a density of 2.6 g cm⁻³ and equal amounts of sp²- and sp³-bonded carbon shows only a single peak. We show that the components in the N 1s multi-peak structure can be related to atomic arrangements in a fully sp²-bonded network. In particular, the component at 398 eV does not necessarily indicate the presence of a material similar to β-C₃N₄.     

The study of bonding composition of CNx film by thermal degradation method

Dynamic thermal degradation of amorphous carbon nitride (a-CN_x) thin films was studied in N₂ and air. The films were magnetron-sputtered at two different pressures to yield various C-C and C-N ratios. Typically, the films consist of interstitial and weakly-bonded carbon-nitrogen molecules or clusters and nitrogen-substituted graphitic-carbons. The carbon and nitrogen atoms are mainly bonded in sp, sp² and sp³ configurations that may contain N charged defects. The relative amount of nitrogen-substituted graphitic-carbons is significantly increased with plasma energy. The decomposition generally occurs in two steps. The first, initiating at ∼200 °C in air and with apparent activation energy (E_a) of ∼111 kJ/mol, relates to the oxidation and release of interstitial and weakly-bonded carbon-nitrogen molecules. The second, starting at ∼520 °C and having E_a of ∼140 kJ/mol, can be ascribed to the dissociation of nitrogen-substituted graphitic-carbons.     

The effect of processing parameters on amorphous carbon nitride layer properties

Surface morphology, deposition rate (DR), bonding composition, structural, optical and electrical properties of pulsed laser deposited amorphous carbon nitride (a-CN_x) layers using camphoric carbon (C₁₀H₁₆O) target precursor as a function of substrate temperatures (STs), laser fluences (LFs) and target to substrate distances (TSDs) are reported. At fixed LF and TSD, surface roughness, particle density and particle size increase, whereas the DR decreases with higher ST. When the TSD and ST are fixed, surface roughness, particle density, deposition rate and nitrogen (N) content increase, whereas particle size decreases with higher LF. While when the LF and ST are fixed, decreasing TSD results in an increase in the irregular small particle size, particle density, surface roughness, DR and N content. The N content in a-CN_x layers is found to increase with higher ST up to 400 °C and decrease thereafter. While the increase of LF and decrease of TSD result in an increase in the N content. We found that the amorphous structure of a-CN_x layers and the ratio of sp² trihedral component to sp³ tetrahedral component are strongly dependent on ST, LF and TSD. The a-CN_x layers with high N content have relatively high electrical resistivity.     

Structural and electronic properties of highly photoconductive amorphous carbon nitride

We have studied highly photoconductive amorphous carbon nitride (a-CN_x) films prepared by RF-reactive magnetron sputtering. a-CN_x is attractive as a potentially high performance photoconductive semiconductor suited to apply for such as an electron photography, ultraviolet (UV) detector and electron emitter, etc. In this paper, it is reported about the effects of high magnetic field on the sputtering process of a-CN_x films. The experimental results show that films prepared in the higher magnetic field made by samarium cobalt magnets have less CN triple bonding states, less electron spin density N_S=10¹⁸ cm⁻³ and smoother surface morphology than that made by the usual magnetron sputtering using ferrite magnets. From these results, high magnetic field is effective to prepare hard and stable a-CN_x films. We will also report a model of electronic density of states of a-CN_x especially stressed on the Urbach energy region. A preliminary experiment on electroluminescence is also reported.     

Reversible photo-induced deformation of amorphous carbon nitride thin films

A reversible photo-induced deformation was found in amorphous carbon nitride (a-CN_x) thin films prepared by reactive radio frequency magnetron sputtering method. The a-CN_x films were deposited on a rectangular shaped ultrathin Si substrate at different temperatures in the range of room temperature (RT) to 600 °C. A deflection of a-CN_x/Si bilayer system was measured using optical cantilever technique with laser light. The bending signal indicates contraction of the film under illumination. The deflection increased with increasing the intrinsic stress of a-CN_x films. An increase the ratio of deflection to the intrinsic stress corresponds to an expansion of optical band gap. As a result of Raman spectra, the photo-induced deformation was found to be inhibited with increasing sp² cluster size.     

XPS spectra of thin CNx films prepared by chemical vapor deposition

Carbon nitride (CN_x) thin films with an N/C ratio of 0.605:0.522 have been synthesized using different sources as a ksilol, CCl₄, N₂ and NH₃ by PECVD (plasma enhanced chemical vapor deposition) and hot filament-CVD reactors. X-ray photoelectron spectroscopy (XPS) analyses, which give C_1s peaks with a maximum at 285.7 eV and 287 eV, typical for C–N bonds and sp² hybridization and CN bonds and sp³ hybridization, respectively. The observed and N_1s peaks with a maximum at about 399 eV suggest the existence of different C–N bonds and polycrystallite structure in the amorphous carbide matrix. The concentration of the different CN bonds varies, depending on the deposition technique.     

Effect of deposition voltage on the microstructure of electrochemically deposited hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films were deposited on Si substrates by electrolysis in a methanol solution at ambient pressure and a low temperature (50 °C), using various deposition voltages. The influence of deposition voltage on the microstructure of the resulting films was analyzed by visible Raman spectroscopy at 514.5 nm and X-ray photoelectron spectroscopy (XPS). The contents of sp³ bonded carbon in the various films were obtained by the curve fitting technique to the C1s peak in the XPS spectra. The hardness and Young’s modulus of the a-C:H films were determined using a nanoindenter. The Raman characteristics suggest an increase of the ratio of sp³/sp² bonded carbon with increasing deposition voltage. The percentage of sp³-bonded carbon is determined as 33–55% obtained from XPS. Corresponding to the increase of sp³/sp², the hardness and Young’s modulus of the films both increase as the deposition voltage increases from 800 V to 1600 V.     

Possibility of graphene growth by close space sublimation

Carbon films on the Si/SiO₂ substrate are fabricated using modified method of close space sublimation at atmospheric pressure. The film properties have been characterized by micro-Raman and X-ray photoelectron spectroscopy and monochromatic ellipsometry methods. Ellipsometrical measurements demonstrated an increase of the silicon oxide film thickness in the course of manufacturing process. The XPS survey spectra of the as-prepared samples indicate that the main elements in the near-surface region are carbon, silicon, and oxygen. The narrow-scan spectra of C1s, Si2p, O1s regions indicate that silicon and oxygen are mainly in the SiO _x (x ≈ 2) oxide form, whereas the main component of C1s spectrum at 284.4 eV comes from the sp²-hybridized carbon phase. Micro-Raman spectra confirmed the formation of graphene films with the number of layers that depended on the distance between the graphite source and substrate.     

Clean graphene surface through high temperature annealing

The cleanliness of the surface of graphene is important for its proper functioning in devices and sensors. Impurities including residual poly(methyl methacrylate) (PMMA) and hydrocarbon contaminants can alter its electronic and chemical properties. In this study, we used two surface-sensitive techniques, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to monitor the chemical composition of the surface of graphene after washing it with acetone and annealing at high temperatures. The concentration of residual PMMA and hydrocarbon contaminants decreased as the annealing temperature increased. The atomic ratio of sp³ carbons to sp² carbons of a clean graphene surface determined using XPS can be used to estimate the amounts of sp³ defects in graphene. ToF-SIMS spectra indicate that residual PMMA was removed from the surface of graphene at 400 °C, while hydrocarbon contaminants required a higher temperature of 500 °C to remove. In ToF-SIMS spectra obtained at 500 °C, the characteristic ions for graphene, which are related to cleavage of ring structure, include C_x⁺ (x = 1, 2, 3…), C_xH⁺ and C_xH₂⁺ as well as C_x⁻ and C_xH⁻. For the first time, we developed a process to produce a very clean graphene surface which was verified by ToF-SIMS and XPS analyses.     

Evolution of TiOx-SiOx nano-composite during annealing of ultrathin titanium oxide films on Si substrate

This paper discusses the formation of the TiO_x-SiO_x nano-composite phase during annealing of ultrathin titanium oxide films (~27 nm). The amorphous titanium oxide films are deposited on silicon substrates by sputtering. These films are important for high-k dielectrics and sensing applications. Annealing of these films at 750 °C in the O₂ environment (for 15–60 min) resulted in the polycrystalline rutile phase. The films exhibit Raman peaks at 150 cm⁻¹ (B_1g), 435 cm⁻¹ (E_g), and 615 cm⁻¹ (A_1g) confirming the rutile phase. The signature TO (1078 cm⁻¹) and LO (1259 cm⁻¹) infrared active vibrational modes of Si–O–Si bond confirms the presence of silicon-oxide. The X-ray photoelectron spectra of the TiO_x films show multiple peaks corresponding to Ti metal (453.8 eV); Ti⁴⁺ state (458.3 eV (Ti 2p_3/2) and 464 eV (Ti 2p_1/2)); and Ti³⁺ state (456.4 eV (Ti 2p_3/2) and 460.8 eV (Ti 2p_1/2)). The O1s XPS spectra peaks at 530–533 eV can be attributed to Ti–O and Si–O bonds of the TiO_x-SiO_x nano-composite phase in the annealed films. The depth profiling XPS study shows that the top surface of the annealed film is mainly TiO_x and the amount of SiO_x increases with the depth.     

Bias enhanced diamond nucleation onto 3C–SiC(100) surfaces studied by high resolution X-ray photoelectron and high resolution electron energy loss spectroscopies

The nature of hydrogen and carbon bonding configuration formed onto 3C–SiC(100) surfaces by the diamond bias enhanced nucleation process consisting of stabilization and biasing stages were investigated by high resolution electron energy loss spectroscopy and high resolution X-ray photoelectron spectroscopy. During the stabilization stage a sp³-CH_x bonded carbonaceous mono-layer is formed onto a hydrogenated 3C–SiC(100)–C–H terminated surfaces. After the biasing stage a hydrogenated nano-diamond film is formed. It was determined that hydrogen is strongly bonded to these nano-diamond surfaces and boundaries in sp³-C–H and sp²-C–H mono-hydride configuration. In addition, CH_x (x > 1) weakly bonded surface or sub-surface species were detected. Regions which are not fully covered by the nano-diamond film expose the SiC surface covered with a very thin carbonaceous layer.     

Nitrogenated amorphous carbon films synthesized by electron cyclotron resonance plasma enhanced chemical vapor deposition

Nitrogenated amorphous carbon (a-CN_x:H) films were investigated as protective overcoats for industrial applications. Thin a-CN_x:H films have been deposited on silicon by electron cyclotron resonance plasma-enhanced chemical vapor deposition. The substrate bias was found to play an important role in determining the chemical compositions and mechanical properties of the films. The surface roughness and hardness of the films can reach 1.4 Å and 20 GPa, respectively. The influence of mechanical properties by hydrogen was studied. A correlation exists between the background slope of Raman spectra and the hydrogen content as determined by elastic recoil detection analysis.     

Influence of Ti ions valence states on the optical and magnetic properties of GdCr1-xTixO3 (0≤x≤0.05)

The vacancy defects, optical and magnetic properties of GdCr_1-xTi_xO₃ (0≤x ≤ 0.05) polycrystalline samples were investigated in this research. The crystal structural analyses show that the orthorhombic perovskite structure exists in all the samples. The positron annihilation spectra reveal that the vacancy defects and local electron density vary with the change of Ti ions valence states. The magnetic measurements show that the antiferromagnetic transition is shifted to lower temperature with the replacement of Ti ions in Cr sites, and the value of spin reorientation caused by the exchange interaction between Gd³⁺ ion and Cr³⁺ ion can be influenced by the change of Ti ions valence states, which rises from x = 0 to 0.04 and then decreases at x = 0.05 sample. Simultaneously, the optical band gap decreases from 2.7 eV to 0.8 eV with the introduction of Ti ions into the lattice, the result can provide a reference for improving optical activity of rare-earth chromium oxides.     

X-Ray photoelectron spectroscopy reference data for identification of the C3N4 phase in carbon–nitrogen films

The β-C₃N₄ phase should have a tetrahedral (sp³-bonded) structure resulting in C1s and N1s XPS peaks with only one feature at a position defined by the electronegativity of four CN bonds. In this work we determined the binding energy of the C1s and N1s XPS peaks in melamine (C₃N₆H₆). In this compound the carbon atoms have four bonds with nitrogen atoms (double and two single); the nitrogen atoms have two chemical states: CNC and CNH₂. Since the total number of chemical bonds in this compound is the same as in the hypothetical β-C₃N₄ compound, this compound is more suitable as a C1s XPS reference for the β-C₃N₄ phase. The binding energy of the C1s and N1s XPS peaks in melamine was determined to be equal to 287.9 and 399.1 eV, respectively. The binding energies were determined relative to the C1s XPS peak for carbon contamination (adventitious carbon).     

Dependence of the composition and bonding structure of carbon nitride films deposited by direct current plasma assisted pulsed laser ablation on the deposition temperature

Carbon nitride films were deposited by direct current plasma assisted pulsed laser ablation of a graphite target under nitrogen atmosphere. Atomic force microscopy (AFM), Fourier transform infrared (FTIR), Raman, and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface morphology, bonding structure, and composition of the deposited films. The influence of deposition temperature in the range 25–400 °C on the composition and bonding structure of carbon nitride films was systematically studied. AFM images show that surface roughness and cluster size increase monotonically with deposition temperature. XPS, FTIR, and Raman spectra indicate directly the existence of CN, CN, and CN bonds in the deposited films. The increase of deposition temperature results in a drastic decrease in the N/C ratio, the content of CN bond and N atoms bonded to sp³ C atoms, in addition to the increase in the content of disorder sp² C atoms and N atoms bonded to sp² C atoms in the deposited films. Raman spectra show that the intensity ratio of D peak over G peak increases with increasing deposition temperature to 200 °C, then decreases with the further increase of deposition temperature, which results from the continuous growth of sp² cluster in the films.     

Hard and sp2-rich amorphous carbon structure formed by ion beam irradiation of fullerene,a-C and polymeric a-C:H films

In this work, we present a comparative study of the ion irradiation effect on the mechanical and optical properties of fullerene, amorphous carbon (a-C) and polymeric hydrogenated amorphous carbon (a-C:H) films, irradiated with N ions at 400 keV in the fluence range from 10¹³ to 3×10¹⁶ N cm⁻². Modifications in the carbon structure, as function of the irradiation fluence, were investigated using the Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), Fourier transform infrared (FTIR), Raman spectroscopy, UV–Vis–near infrared (NIR) spectrophotometry and nanoindentation techniques. After high fluence, the three carbon samples are transformed into very similar hard (≈14 GPa) and non-hydrogenated amorphous carbon layers with very low optical gap (≈0.2 eV) and an unusual sp²-rich bonded atomic network. The mechanical properties of the irradiated films correlated with the bonding topologies of this new sp² carbon phase are investigated through the constraint-counting model. The results show that the structural modifications and the unusual rigidity were achieved by the distortion of the sp² carbon bond angles, giving origin to a constrained three-dimensional sp² carbon bonded network.     

Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

Hydrogen incorporation,bonding and stability in nanocrystalline diamond films

The hydrogen concentration in hot filament and microwave plasma CVD nanocrystalline diamond films is analysed by secondary ion mass spectrometry and compared to the film grain size. The surface and bulk film carbon bonds are analysed respectively by X-ray photoelectron spectroscopy (XPS) and ultra-violet Raman spectroscopy. XPS results show the presence of the hydrogenated p-type surface conductive layer. The respective intensities of the 1332 cm^− 1 diamond peak, of the G and D bands related to sp² phases, and of the 3000 cm^− 1 CH_x stretching mode band, are compared on Raman spectra. The samples are submitted to thermal annealing under ultra-high vacuum in order to get hydrogen out-diffusion. XPS analysis shows the surface desorption of hydrogen. Thermal annealing modifies the sp² phase structure as hydrogen out diffuses.