Fullerene-like Carbon Nitride Research Articles

Nanotribological performance of fullerene-like carbon nitride films

a b s t r a c t Fullerene-like carbon nitride films exhibit high elastic modulus and low friction coefficient. In this study, thin CNx films were deposited on silicon substrate by DC magnetron sputtering and the tribological behavior at nanoscale was evaluated using an atomic force microscope. Results show that CNx films with fullerene-like structure have a friction coefficient (CoF ∼ 0.009-0.022) that is lower than amorphous CNx films (CoF ∼ 0.028-0.032). Analysis of specimens characterized by X-ray photoelectron spectroscopy shows that films with fullerene-like structure have a higher number of sp

Filtered pulsed cathodic arc deposition of fullerene-like carbon and carbon nitride films

Carbon and carbon nitride films (CNx, 0 ≤ x ≤ 0.26) were deposited by filtered pulsed cathodic arc and were investigated using transmission electron microscopy and X-ray photoelectron spectroscopy. A “fullerene-like” (FL) structure of ordered graphitic planes, similar to that of magnetron sputtered FL-CNx films, was observed in films deposited at 175 °C and above, with N2 pressures of 0 and 0.5 mTorr. Higher substrate temperatures and significant nitrogen incorporation are required to produce similar FL structure by sputtering, which may, at least in part, be explained by the high ion charge states and ion energies characteristic of arc deposition. A gradual transition from majority sp3-hybridized films to sp2 films was observed with increasing substrate temperature. High elastic recovery, an attractive characteristic mechanical property of FL-CNx films, is evident in arc-deposited films both with and without nitrogen content, and both with and without FL structure.

The Structure and Bonding State for Fullerene-Like Carbon Nitride Films with High Hardness Formed by Electron Cyclotron Resonance Plasma Sputtering

The Structure and Bonding State for Fullerene-Like Carbon Nitride Films with High Hardness Formed by Electron Cyclotron Resonance Plasma Sputtering

The Structure and Bonding State for Fullerene-Like Carbon Nitride Films with High Hardness Formed by Electron Cyclotron Resonance Plasma Sputtering

We studied pure carbon films and carbon nitride (CN) films by using electron cyclotron resonance (ECR) sputtering. The main feature of this method is high density ion irradiation during deposition, which enables the pure carbon films to have fullerene-like (FL) structures without nitrogen incorporation. Furthermore, without substrate heating, the ECR sputtered CN films exhibited an enhanced FL microstructure and hardness comparable to that of diamond at intermediate nitrogen concentration. This microstructure consisted of bent and cross-linked graphene sheets where layered areas remarkably decreased due to increased sp3 bonding. Under high nitrogen concentration conditions, the CN films demonstrated extremely low hardness because nitrile bonding not only decreased the covalent-bonded two-dimensional hexagonal network but also annihilated the bonding there. By evaluating lattice images obtained by transmission electron microscopy and the bonding state measured by X-ray photoelectron spectroscopy, we classified the ECR sputtered CN films and offered phase diagram and structure zone diagram.

Protein adsorption on thin films of carbon and carbon nitride monitored with in situ ellipsometry

Thin films of amorphous carbon and amorphous, graphitic and fullerene-like carbon nitride were deposited by reactive magnetron sputtering and optically characterized with spectroscopic ellipsometry. Complementary studies using scanning electron microscopy and atomic force microscopy were performed. The films were exposed to human serum albumin (HSA) and the adsorption was monitored in situ using dynamic ellipsometry. From the ellipsometric data the adsorbed amount of proteins was quantified in terms of surface mass density using de Feijter’s model. The results indicate larger adsorption of proteins onto the amorphous films compared to the films with a more textured structure. Complementary studies with 125I-labeled HSA showed an apparent protein adsorption up to six times larger compared to the ellipsometry measurement. In addition, the four types of films were incubated in blood plasma followed by exposure to anti-fibrinogen, anti-HMWK or anti-C3c, revealing the materials’ response to complement and contact activation. The amorphous and graphitic carbon nitride exhibit rather high immune activity compared to a titanium reference, whereas the amorphous carbon and the fullerene-like CNx show less immune complement deposition. Compared to the reference, all films exhibit indications of a stronger ability to initiate the intrinsic pathway of coagulation. Finally, the surfaces’ bone-bonding ability was investigated by examination of their ability to form calcium phosphate crystals in a simulated body fluid, with a-CNx depositing most calcium phosphate after 21days of incubation.

Synthesis, structure and mechanical properties of fullerene-like carbon nitride films deposited by DC magnetron sputtering

Carbon nitride (CNx) films were deposited by direct current magnetron sputtering at different N2 fractions and a substrate temperature of 450°C. The composition and microstructure of CNx films were investigated by high resolution transmission electron microscopy, X-ray absorption near-edge spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The CNx films change from amorphous to fullerene-like nanostructure as the N2 fraction is ranged from 0.05 to 0.1, and then to nitridized graphite nanocrystals in amorphous matrix at pure N2, and the curvature of the basal planes decreases with the increase of N content in CNx films. Meanwhile, the structural analysis provides the identification of spectral signatures related with the formation and promotion of FL arrangements, and the formation of C-sp3 sites has been generally considered as the cross-linking path in fullerene-like CNx. Their mechanical and tribological properties were also investigated in detail, aiming at illustrating excellent mechanical, friction and wear behaviours of the fullerene-like CNx film with respect to its high hardness and elastic recovery and less dependence on humidity and oxygen as compared with the amorphous CNx film.

Nanostructural C-Al-N thin films studied by x-ray photoelectron spectroscopy, Raman and high-resolution transmission electron microscopy

The effects of Al incorporation and post-deposition annealing on the structural properties of C-Al-N thin films prepared by reactive unbalanced dc-magnetron sputtering were investigated using x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). XPS studies demonstrated the presence of abundant Al-N bonds in addition to C-C and N-C bonds. At low incorporations of Al and N, the films were found to be essentially amorphous. By Raman and HRTEM, the formation of ∼5 nm fullerene-like carbon nitride (FL-CNx) nanostructures in an amorphous (C, CNx) matrix was evidenced with increasing Al content in the films. Crystalline improvement of FL-CNx nanostructures was seen, as well as the precipitation of ∼3–4 nm face centered cubic (fcc-) AlN nanograins by thermal annealing at 500 °C or above. Through these improvements, C-Al-N nanocomposite thin films were achieved. The effects of the incorporated Al and annealing on stabilizing fcc-AlN nanograins and FL-CNx nanostructures are elucidated and explained through the use of thermodynamic considerations.

Dangling bond energetics in carbon nitride and phosphorus carbide thin films with fullerene-like and amorphous structure

The energy cost for dangling bond formation in Fullerene-like Carbon Nitride (FL-CN x ) and Phosphorus carbide (FL-CP x ) as well as their amorphous counterparts: a-CN x , a-CP x , and a-C has been calculated within the framework of Density Functional Theory and compared with surface water adsorption measurements. The highest energy cost is found in the FL-CN x (about 1.37 eV) followed by FL-CP x compounds (0.62–1.04 eV).

Spectroscopic ellipsometry characterization of amorphous carbon and amorphous, graphitic and fullerene-like carbon nitride thin films

Carbon nitride (CN x ) and amorphous carbon (a-C) thin films are deposited by reactive magnetron sputtering onto silicon (001) wafers under controlled conditions to achieve amorphous, graphitic and fullerene-like microstructures. As-deposited films are analyzed by Spectroscopic Ellipsometry in the UV–VIS–NIR and IR spectral ranges in order to get further insight into the bonding structure of the material. Additional characterization is performed by High Resolution Transmission Electron Microscopy, X-ray Photoelectron Spectroscopy, and Atomic Force Microscopy. Between eight and eleven resonances are observed and modeled in the ellipsometrically determined optical spectra of the films. The largest or the second largest resonance for all films is a feature associated with C–N or C–C modes. This feature is generally associated with sp 3 C–N or sp 3 C–C bonds, which for the nitrogen-containing films instead should be identified as a three-fold or two-fold sp 2 hybridization of N, either substituted in a graphite site or in a pyridine-like configuration, respectively. The π→π⁎ electronic transition associated with sp 2 C bonds in carbon films and with sp 2 N bonds (as N bonded in pyridine-like manner) in CN x films is also present, but not as strong. Another feature present in all CN x films is a resonance associated with nitrile often observed in carbon nitrides. Additional resonances are identified and discussed and moreover, several new, unidentified resonances are observed in the ellipsometric spectra.

Water adsorption on fullerene-like carbon nitride overcoats

Humidity influences the tribological performance of the head-disk interface in magnetic data storage devices. In this work we compare the uptake of water of amorphous carbon nitride (a-CN x ) films, widely used as protective overcoats in computer disk drive systems, with fullerene-like carbon nitride (FL-CN x ) and amorphous carbon (a-C) films. Films with thickness in the range 10–300 nm were deposited on quartz crystal substrates by reactive DC magnetron sputtering. A quartz crystal microbalance placed in a vacuum chamber was used to measure the water adsorption. Electron paramagnetic resonance (EPR) has been used to correlate water adsorption with film microstructure and surface defects (dangling bonds). Measurements indicate that the amount of adsorbed water is highest for the pure a-C films and that the FL-CN x films adsorbed less than a-CN x . EPR data correlate the lower water adsorption on FL-CN x films with a possible lack of dangling bonds on the film surface. To provide additional insight into the atomic structure of defects in the FL-CN x , a-CN x and a-C compounds, we performed first-principles calculations within the framework of Density Functional Theory. Emphasis was put on the energy cost for formation of vacancy defects and dangling bonds in relaxed systems. Cohesive energy comparison reveals that the energy cost formation for dangling bonds in different configurations is considerably higher in FL-CN x than for the amorphous films. These simulations thus confirm the experimental results showing that dangling bonds are much less likely in FL-CN x than in a-CN x and a-C films.

Transmission electron microscopy studies and simulation of the indentation response of superelastic fullerenelike carbon nitride thin films

The highly elastic behavior of fullerenelike carbon nitride thin films on single crystal Si (001) substrates has been studied by nanoindentation. The films exhibit a microstructure of frequently bent and intersecting graphene sheets. Transmission electron microscopy of indented areas revealed no tendency to plastic deformation in the highly elastic and compliant film. Also, a unique deformation pattern in the Si substrate is observed, where a {111} facetted inverted pyramid of untransformed Si remained intact under the indenter with a retained CNx/Si(001) interface. Analytical approaches using the effective indenter method, supported by finite element methods, are employed to understand the nonlinear but fully elastic behavior of the coatings as well as to describe the critical parameters for the deformation and phase transformation of the Si substrate.

Water adsorption on lubricated fullerene-like CN x films

Humidity influences the tribological performance of the head–disk interface in magnetic data storage devices. In this work we compare the uptake of water of amorphous hydrogenated carbon (a-CH y ) and carbon nitride (a-CN x ) films, widely used as protective overcoats in computer disk drive systems, with two types of amorphous non-hydrogenated carbon (a-C and a-C sp 2 ) films, and fullerene-like carbon nitride (FL-CN x ) films. Carbon films were deposited on quartz crystal substrates by reactive dc magnetron sputtering in Ar/N 2 discharges. After deposition, some of the films were coated with a 2-nm-thick layer of Z-tetraol, a lubricant used in hard disk devices. A quartz crystal microbalance placed in a vacuum chamber was used to measure the adsorption of water at room temperature and at pressures of water corresponding to relative humidities in the range RH = 0 to 90%. Water adsorption and desorption is fast, indicating that equilibrium with ambient humidity is reached on time scales of minutes, much faster than the time scales for fluctuations in ambient humidity. The amount of water adsorbed on the non-lubricated amorphous carbon films is significantly higher than that on the fullerene-like films. The presence of the lubricant influences water adsorption but its impact differs on different carbon films.

First-principles calculations on the structural evolution of solid fullerene-like CP x

The formation and structural evolution of fullerene-like (FL) carbon phosphide (CP x ) during synthetic growth were studied by first-principles calculations. Geometry optimizations and comparison between the cohesive energies suggest stability for solid FL-CP x compounds. In comparison with fullerene-like carbon nitride, higher curvature of the graphene sheets and higher density of cross-linkages between them is predicted and explained by the different electronic properties of P and N. Cage-like and onion-like structures, both containing tetragons, are found to be typical for fullerene-like CP x . Segregation of P is predicted at fractions exceeding ∼20 at.%.

First-principles calculations on the curvature evolution and cross-linkage in carbon nitride

First-principles calculations were utilized to study the formation mechanisms and structural features of fullerene-like carbon nitride (FL CN x ). Cohesive energy comparisons reveal the energy cost for different defects arising from substitution of C for N as a function of the nitrogen concentration. In FL CN x , combinations of pentagons and heptagons compete in causing graphene sheet curvature during the addition of CN-precursors and single species. Also, cross-linkage between graphene layers in FL CN x can be explained by the bond rotation due to incorporated N atoms. The computational results agree with recent experimental observations from the growth of FL CN x thin films.

Correlation between bonding structure and microstructure in fullerenelike carbon nitride thin films

The bonding structure of highly ordered fullerenelike (FL) carbon nitride $(\mathrm{C}{\mathrm{N}}_{x})$ thin films has been assessed by x-ray absorption near-edge spectroscopy (XANES). Samples with different degrees of FL character have been analyzed to discern spectral signatures related to the FL microstructure. The XANES spectra of $\mathrm{FL}\text{\ensuremath{-}}\mathrm{C}{\mathrm{N}}_{x}$ films resemble that of graphitic $\mathrm{C}{\mathrm{N}}_{x}$, evidencing the $s{p}^{2}$ hybridization of both C and N atoms. The FL structure is achieved with the promotion of N in threefold positions over pyridinelike and cyanidelike bonding environments. In addition, the relative ${\ensuremath{\pi}}^{*}∕{\ensuremath{\sigma}}^{*}$ XANES intensity ratio at the $\mathrm{C}(1s)$ edge is independent of the FL character, while it decreases $\ensuremath{\sim}40%$ at the $\mathrm{N}(1s)$ edge with the formation of FL arrangements. This result indicates that there is no appreciable introduction of $\mathrm{C}\text{\ensuremath{-}}s{p}^{3}$ hybrids with the development of FL structures and, additionally, that a different spatial localization of $\ensuremath{\pi}$ electrons at C and N sites takes place in curved graphitic structures. The latter has implications for the elastic properties of graphene sheets and could, as such, explain the outstanding elastic properties of $\mathrm{FL}\text{\ensuremath{-}}\mathrm{C}{\mathrm{N}}_{x}$.

Comparative study of wear-resistant DLC and fullerene-like CN x coatings produced by pulsed laser and filtered cathodic arc depositions

Unhydrogenated amorphous diamond-like carbon (DLC) and fullerene-like carbon nitride (CN x, N/C ratio ∼0.2) are rival surface protection coatings with exceptional mechanical and tribological properties. A comparative study was made on their performance under sliding wear. Identical surface preparation procedures, coating architectures, thickness, and substrates were used to facilitate the comparison. Coatings were produced using pulsed laser deposition (PLD) and filtered cathodic arc deposition (FCAD). Samples of laser-DLC, laser-CN x, arc-DLC, and arc-CN x coatings were prepared and an extensive comparison of properties was completed: composition, structure, hardness, elastic modulus, stress, friction, and wear. The tribological properties were determined using steel and SiC balls in humid and dry environments. The comparison indicated that PLD and FCAD produce coatings with very similar compositions, structures, and properties. Independent of the growth technique, DLC coatings had hardnesses of 52–57 GPa and elastic moduli in the range of 490–560 GPa. The CN x coatings offered a reasonably high hardness of 28–30 GPa, while their elastic modulus was as low as 160 GPa. There was a clear difference in the tribological behavior of DLC and CN x, which depends on humidity. In humid air, DLC coatings had a coefficient of friction (c.o.f.) of 0.1, a very low wear rate, and formed a graphitic transfer film in friction contact. In the same tests, CN x coatings had a c.o.f. of 0.3–0.4, a higher wear rate, and did not form a transfer film. In this environment, the wear tracks on CN x coatings were polished by abrasive wear. In dry nitrogen, DLC coatings had a c.o.f. of approximately 0.15 and a higher wear rate, while CN x coatings had a c.o.f. of 0.03–0.04 and a lower wear rate with formation of a graphitic-like transfer film. The observed difference in mechanical response and tribological performance can be used to optimize the selection between DLC and CN x coatings, depending on application requirements.

First-principles calculations on the role of CN precursors for the formation of fullerene-like carbon nitride

First-principles calculations were utilized to study the formation mechanisms of the fullerene-like structure in carbon nitride compounds in the context of aligned incorporation of CN precursors along edges of evolving sheets during vapor phase synthesis. The incorporation of CN dimers and cyanogen molecules promotes an initial stabilization of pentagon defects, necessary for the observed bending of graphene multilayers in these compounds. The theoretical results corroborate recent experimental findings suggesting that preformed CxNy species, besides C and N atoms and ions, play an important role during the growth of FL CNx thin films by reactive magnetron sputtering.

Cryogenic deposition of carbon nitride thin solid films by reactive magnetron sputtering; suppression of the chemical desorption processes

Nanostructured fullerene-like carbon nitride (FL CNx) is commonly grown by reactive magnetron sputtering of carbon in a nitrogen-containing atmosphere. The film structure formation for this technique is presumably due to the existence of preformed molecular CxNy (x,y≤2) species in the deposition flux, which act as growth templates and enhance the selectivity of chemical desorption processes. In the present study, the extent of the desorption processes and the implications on the resulting film have been investigated in detail, addressing in particular the structure evolution and the origin of the incorporated nitrogen. This was studied by varying the N2-fraction in the discharge from 0 to 1 and the substrate temperature from 600 °C (873 K) down to minus 130 °C (143 K). The results show that the incorporation rate of carbon and nitrogen into the film increases substantially with an increased N2-content in the plasma and decreasing substrate temperature, thus indicating that the chemistry and magnitude of the arriving flux is substantially altered with the N2-fraction in the discharge. It is concluded that the chemically activated desorption in conjunction with the varying chemistry of the film-forming flux affects the sensitive structural balance, determined by incorporation and desorption of film forming CxNy (x,y≤2) species.

Correlation between plasma characterization and growth of fullerene-like CN x thin films deposited by pulsed laser ablation

Fullerene-like amorphous carbon nitride (CN x ) thin films were synthesized by pulsed laser ablation (PLA) of pyrolytic graphite (99.99%) target in nitrogen using a Nd:YAG (Neodimium Doped Yttrium Aluminum Garnet) -pulsed laser. The films were deposited onto silicon substrates at 300 °C in nitrogen atmosphere within the 0.7–13.0 Pa pressures. The composition and structure of these films were analyzed by means of reflection absorption infrared spectroscopy (RAIRS), X-ray photoemission spectroscopy (XPS), and Raman spectroscopy and the morphology of the films surfaces by atomic force microscopy (AFM). The XPS study of the N 1s bonding energy region provided typical spectra of the CN x materials. The spectra present the energy peaks at 400.8 ( P 2) and 398.4 ( P 3) eV. In our conditions for films deposited at 0.7 Pa, the P 2/ P 3 ratio is 1.4, indicating a fullerene-like structure. Raman analysis of films produced at different pressures shows the characteristic D and G peaks at 1360–1370 and 1580–1590 cm −1 relate to the bond length in sp 3 or sp 2 coordinated carbon, respectively. The optical emission spectra (OES) from the PLA–plume allowed to correlate the concentration and vibrational temperatures of CN and C 2 species present in the plasma with the fullerene-like CN x film composition and bonding determined by XPS, IR, and Raman spectroscopy.

Arrhenius-type temperature dependence of the chemical desorption processes active during deposition of fullerene-like carbon nitride thin films

The chemical desorption of carbon and nitrogen-containing species from the growth surface was investigated for the deposition of fullerene-like carbon nitride (FL CN x ) thin solid films by reactive magnetron sputtering of a carbon target in a N 2-containing atmosphere. The desorption of mainly C 2N 2 was suppressed by decreasing the substrate temperature for various N 2 fractions in the discharge stepwise from 873 K down to cryogenic temperatures of 153 K. This approach enabled us to quantify the film-forming flux by determining the carbon and nitrogen incorporation rates by elastic recoil detection. The incorporation of both, carbon and nitrogen, was found to increase substantially at lower substrate temperatures, whereas this effect is most pronounced for the higher N 2 fractions. In turn, a modified Arrhenius-type rate equation was applied to extrapolate the total flux of the elements as well as their respective activation energies of desorption for the series at higher N 2 fractions. The reasonable fit indicates that the desorption process is mainly determined by the surface diffusion rate of adsorbed C x N y species as well as their structure and total number. The extrapolated fluxes of carbon and nitrogen atoms arriving as preformed species at the growth surface scaled strongly with the availability of N 2 in the discharge, while the obtained activation energies of 0.05–0.17 eV point towards a saturation of the process at elevated temperatures. Furthermore, the constant C/N ratio found in the film-forming flux leads to the notion that most of the nitrogen incorporated originates from preformed species instead of N atoms or ions. The FL structure evolution has to be seen as a sensitive interplay between the type and magnitude of preformed C x N y species in the deposition flux and the selectiveness of the preferential etching by means of the chemical desorption.