CNx Films Research Articles

Aminosilanized Interface Promotes Electrochemically Stable Carbon Nitride Films with Fewer Trap States on FTO for (Photo)electrochemical Systems.

We have demonstrated the direct growth of a CNx layer on a plasma-cleaned and aminosilanized F-doped SnO2 (FTO) electrode to study the CNx|FTO interface that is critical for (photo)electrocatalytic systems. The (3-aminopropyl)triethoxysilane (APTES) was chosen as a bifunctional organosilane, with the amino end incorporating into CNx and the silane end connecting to the hydroxylated FTO surface. Plasma cleaning and aminosilanization resulted in a highly hydrophilic surface, which leads to better contact of melted thiourea to the aminosilanized FTO (p-FTONH2) during CNx polymer condensation, thus generating a thinner and more compact CNx layer. The modification at the interface was shown to influence the CNx growth on length scales of tens of micrometers. We grew CNx thin films on p-FTONH2 (CNx/p-FTONH2) and nonaminosilanized p-FTO (CNx/p-FTO). CNx/p-FTONH2 had a smaller density of trap states and passed 2.4 times the charges before failure compared to CNx/p-FTO. Additionally, a 40% decrease in interfacial charge transfer resistance at the CNx|electrolyte interface was measured for CNx/p-FTONH2 compared to CNx/p-FTO under -0.5 V vs RHE in 0.1 M Na2SO4. Furthermore, with the CNx surface coated with a Pt cocatalyst, Pt/CNx/p-FTONH2 exhibited faster hydrogen evolution rates and larger current densities than Pt/CNx/p-FTO. The highest Faraday efficiency toward electrochemical hydrogen evolution (FEH2) in 0.1 M Na2SO4 (pH = 7) was 46.1%, 37.3%, 57.7%, and 70.5% for Pt/CNx/p-FTONH2, Pt/CNx/p-FTO, CNx/p-FTONH2, and CNx/p-FTO, respectively. The increase in hydrogen evolution rate did not follow the magnitude of the current density change, indicating electrochemical processes other than proton reduction. Overall, we have carefully investigated the CNx|FTO interface and suggested potential solutions to make CNx films better (photo)electrodes for (photo)electrochemical systems.

Rational Design of Carbon Nitride Photoelectrodes with High Activity Toward Organic Oxidations.

Carbon nitride (CNx ) is a light-absorber with excellent performance in photocatalytic suspension systems, but the activity of CNx photoelectrodes has remained low. Here, cyanamide-functionalized CNx (NCN CNx ) was co-deposited with ITO nanoparticles on a 1.8 Å thick alumina-coated FTO electrode. Transient absorption spectroscopy and impedance measurements support that ITO acts as a conductive binder and improves electron extraction from the NCN CNx , whilst the alumina underlayer reduces recombination losses between the ITO and the FTO glass. The Al2 O3 |ITO : NCN CNx film displays a benchmark performance for CNx -based photoanodes with an onset of -0.4 V vs a reversible hydrogen electrode (RHE), and 1.4±0.2 mA cm-2 at 1.23 V vs RHE during AM1.5G irradiation for the selective oxidation of 4-methylbenzyl alcohol. This assembly strategy will improve the exploration of CNx in fundamental and applied photoelectrochemical (PEC) studies.

Tailoring microstructure, mechanical and tribological properties of amorphous CNx films by incorporating Fe

Amorphous carbon nitride (α-CNx) films are perfect protective materials for advanced tribological application due to low coefficients of friction and chemical stability. However, their lower nano-hardness and poor degree of graphitization during sliding, result in the limited lubrication effect and inevitable wear, which greatly limits its wider potential for protective films. Herein, α-CNx/Fe films were constructed at different deposition temperatures by magnetron sputtering. The results indicated that the α-CNx/Fe film deposited at 600 °C has the highest hardness of ~27.3 GPa (2.13 times that of the pure α-CNx film), which is mainly attributed to the high sp3/sp2 carbon ratio. Besides, the FeN bond of the film can induce the formation of self-lubricating tribofilm on the worn surface, which leads to low friction coefficient (μ = 0.18) of the α-CNx/Fe film during sliding, only 46 % of pure α-CNx films. High-resolution transmission electron microscopy, Raman spectra, and Electron energy loss spectra further confirmed that most of the tribofilm on the worn surface is composed of graphitized onion-like carbon structure scrolls, which is the critical factor for the decrease of friction coefficient. These results demonstrated that the obtained CNx/Fe films have excellent lubrication characteristics, providing guidance for CNx-based films in advanced precision machining and tribological applications.

Amorphous carbon nitride dual-function anti-reflection coating for crystalline silicon solar cells

Crystalline silicon (c-Si) solar cells have dominated the photovoltaic industry for decades. However, due to high reflectivity and the presence of numerous types of surface contaminants, the solar cell only absorbs a limited amount of the incident solar radiation. To improve the efficiency of the solar cell, anti-reflection and self-cleaning coatings must be applied to the surface. The main objective of this work is to synthesize an amorphous carbon nitride CNx thin film as a novel dual-function anti-reflection coating (ARC) for c-Si solar cells. The CNx film was synthesized by the RF magnetron sputtering technique and characterized by different chemical, structural, and optical analysis techniques. The performance of CNx film was investigated via measuring the reflectance, photoelectric conversion efficiency, and external quantum efficiency. The minimum reflectance was 0.3% at 550 nm wavelength, and the external quantum efficiency achieved was more than 90% within the broad wavelength range. The open circuit voltage and short circuit current density that have been achieved are 578 mV and 33.85 mAcm−2, respectively. Finally, a photoelectric conversion efficiency of 13.05% was achieved with the coated c-Si solar cell in comparison with 5.52% for the uncoated c-Si solar cell. This study shows that CNx films have promising application potential as an efficient ARC for c-Si solar cells as compared to traditional ARC materials.

In Situ Analyses of Surface-Layer Composition of CxNy Thin Films Using Methods Based on Penning Ionization Processes-Introductory Investigations.

Carbon nitride materials have received much attention due to their excellent tribological, mechanical and optical properties. It was found that these qualities depend on the N/C ratio; therefore, the possibility to control it in situ in the sputtered film is of high importance. The plasma-electron spectroscopy method based on the Penning ionization process analysis is developed here to control this ratio in CNx films produced by plasma-sputtering in a pulsed-periodic regime of glow discharge. The electron energy distribution function is determined by the means of a single Langmuir probe placed in the center of the discharge tube. The mixture N2:CH4:He was used in the process of sputtering. The applied concentrations of CH4 varied in the range of 2–8%, and He concentration was 80–90%. The gas pressure in the discharge tube used for sputtering varied between 1 and 10 Torr, and the current was between 10 and 50 mA. It was shown that the proposed method enables the extraction of information on the composition of the surface layer of the investigated film and the development of an on-line inspection, without extracting the film from the sputtering chamber.

Applications insight into the plasmochemical state and optical properties of amorphous CNx films deposited by gas injection magnetron sputtering method

This manuscript reports the results of carbon nitride (CNx) film deposition carried out using the gas injection magnetron sputtering method. In this experiment, a pulsed gas injection was used to arrange the varying neon/nitrogen (Ne/N2) plasma discharge. This approach influences on the transition between two excitation mechanisms of plasma particles, which changed from Penning ionization to electron impact excitation, as the share of N2 pressure was increased in the range of 0.05–0.45 Pa. In relation to that outcome, the CNx (0.04 ≤ x ≤ 0.21) films presented a smoothly fluctuated chemical state, which varied from bonds hybridized in the sp1 (N ≡ C) nitrile groups, sp2 (N = C) graphite sheets, to disordered sp3 (N – C) chain structures as proved by the study of the N1s and C1s core-level spectra. These were tied up in amorphous structure arrangement, and consisted of either oxidized graphitic sheet or β-C3N4 phase, as revealed in the fast Fourier transform analysis. In conclusion, on the basis of an investigation of indiscrete optical band gap (E1g < 1.1 eV and 3.2 < E2g < 4.3 eV), the CNx were emphasized as a highly transparent (~90%) film, proving its application potential for the near-infrared devices.

Sustained Photovoltaic Effect from Nitrogen Rich Carbon Nitride (CNx) Prepared by Reactive Magnetron Sputtering

Nitrogen rich carbon nitride (CNx) materials were prepared using reactive magnetron sputtering from a pure nitrogen plasma at elevated deposition pressures and low deposition powers. Solid-state photovoltaic devices utilizing such materials were prepared and characterized. As-prepared CNx materials showed photoconductive behaviour and no photovoltaic effect. This was attributed to the p-doping effect of oxygen present in as-prepared CNx films resulting in formation of a compensated semiconductor. However, upon thermal annealing, CNx materials showed a pronounced photovoltaic effect consistent with the n-type semiconductor behaviour. This result demonstrates that nitrogen rich carbon nitride is a promising photovoltaic material for all-carbon photovoltaic solar cells.

Study of the effect of normal load on friction coefficient and wear properties of CNx thin films

The present study reports on the systematic analysis of the friction and the wear behaviors of carbon nitride (CNx) thin films deposited by direct current unbalanced magnetron sputtering. We focus on the normal load dependence on the friction coefficient and wear rate for the sliding friction of the SUJ2 ball with the CNx thin film. The friction coefficient was monitored using a ball-on-disc apparatus with various loads in ambient air. The friction coefficient (μ) increases linearly from ∼0.05 to 0.28 with increasing normal load, while the specific wear rate decreases with an increase in load. The wear rates of the SUJ2 ball ∼1.5 × 10−7 mm3/Nm for μ = 0.05 and ∼5.5 × 10−8 mm3/Nm for μ = 0.28 were observed in the deposited CNx films with the minimum and maximum normal load. The friction and wear mechanisms are discussed in relation to the surface morphology and microstructure of the CNx coatings.

Comparison of TiN and CNx coatings on orthodontic stainless steel: Tribological and biological evaluation

Surface modification of orthodontic appliance materials has contributed to the development and popularization of orthodontic treatment. Important characteristics of modified materials include the coefficient of friction, antimicrobial activity, and biocompatibility. To investigate these characteristics, we coated a stainless steel (SS) substrate, which is used to manufacture orthodontic appliances, with a titanium nitride (TiN) or carbon nitride (CNx) film. The coating thickness, elements, and surface morphology were characterized by scanning electron microscopy and energy-dispersive spectrometry. The coefficient of friction was measured using a universal micro-tribotester; the density of Streptococcus mutans was used to assess antimicrobial ability; and the viability of human periodontal ligament fibroblasts implanted on the samples was used to evaluate the biocompatibility of the different films. We found that the CNx film exhibited a lower friction force, effective antimicrobial activity, and favorable biocompatibility compared to the uncoated and TiN-coated SS. In conclusion, a CNx film is promising for modifying archwires to decrease friction and improve antimicrobial activity and biocompatibility.

Interface defects and stress relief of super hard Ti+C/a-CNx gradient multilayer films

Ti + C/a-CNx gradient multilayer film (GMF) are prepared using ion-beam assisted magnetron sputtering. The hardness of as-deposited Ti + C/a-CNx-GMF reach 29 GPa, which is obviously higher that of pure CNx film. Transmission electron microscopy (TEM) and optical microscope (OM) were used to examine the structure and morphologies of Ti + C/a-CNx-GMF films. Investigation results indicated that there was a Ti + C gradient layer zone with gradual Ti lattice parameter existed in this super hard Ti + C/a-CNx-GMF, and interface zones are mainly composed of nanocrystallie TiC and Ti grains and amorphous matrix, the growth of interlayer depends on compressive stress. As deposited GMFs voluntarily buckling and peeling along the interfacial defect zone (Ti-Si-O), due to the existence of high internal stress in films, lead to adhesion failure. After undergoing thermal treatment, Ti + C gradient multilayer (GM) zone present a semi-coherent type with some misfit, which could relax the internal stress of GMFs.

Carbon nitride transparent counter electrode prepared by magnetron sputtering for a dye-sensitized solar cell

Carbon nitride (CNx) films supported on fluorine-doped tin oxide (FTO) glass are prepared by radio frequency magnetron sputtering, in which the film thicknesses are 90–100 nm, and the element components in the CNx films are in the range of x = 0.15–0.25. The as-prepared CNx is for the first time used as counter electrode for dye-sensitized solar cells (DSSCs), and show a preparation-temperature dependent electrochemical performance. X-ray photoelectron spectroscopy (XPS) demonstrates that there is a higher proportion of sp2 CC and sp3 CN hybridized bonds in CNx-500 (the sample treated at 500 °C) than in CNx-RT (the sample without a heat treatment). It is proposed that the sp2 CC and sp3 C–N hybridized bonds in the CNx films are helpful for improving the electrocatalytic activities in DSSCs. Meanwhile, Raman spectra also prove that CNx-500 has a relatively high graphitization level that means an increasing electrical conductivity. This further explains why the sample after the heat treatment has a higher electrochemical performance in DSSCs. In addition, the as-prepared CNx counter electrodes have a good light transmittance in the visible light region. The results are meaningful for developing low-cost metal-free transparent counter electrodes for DSSCs.

Investigation of silicon carbon nitride nanocomposite films as a wear resistant layer in vitro and in vivo for joint replacement applications

Silicon-contained CNx nanocomposite films were prepared using the ion beam assisted magnetron sputtering under different nitrogen gas pressure. With increase of the nitrogen pressure, silicon and nitrogen content of the CNx films drastically increase, and is saturated as the PN2 reach about 40%. Surface roughness and the contact angle are increase, while the friction coefficient decreased. The CNx film with 5.7at.% Si content possess the lowest friction coefficient of only 0.07, and exhibited the best tribological properties. The impact of CNx films with different silicon content on the growth and the activation of osteoblasts were compared to that of Ti6Al4V. The incorporation of silicon in the CNx film also showed an increase cell adhesion. Bonding structure and surface energy were determined to be the factors contributing to the improved biocompatibility. Macrophages attached to 5.7at.% Si contained CNx films down regulated their production of cytokines and chemokines. Moreover, employed with Si contained CNx coated joint replacements, which were implanted subcutaneously into Sprague-Dawley mice for up to 36days, the tissue reaction and capsule formation was significantly decreased compared to that of Ti6Al4V. A mouse implantation study demonstrated the excellent in vivo biocompatibility and functional reliability of wear resist layer for joint replacements with a Si doped a-CNx coating for 36days.

Comparative study of macro- and microtribological properties of carbon nitride thin films deposited by HiPIMS

The macro- and microtribological properties of carbon nitride thin films deposited by high power impulse magnetron sputtering at different substrate bias voltages (Vb) were investigated. Vb of -100, -150, -200, and -300 V were used. A Hysitron Triboindenter TI950 and a reciprocating Tribotechnic tribometer with diamond counterparts were used in order to assess the tribological performance of the films at the micro- and macroscale, respectively. Initial Hertzian contact pressures of 2.5GPa, 3.3GPa and 3.9GPa were chosen for the comparative measurements at both scales.At the macroscale, films with higher initial roughness present an increased wear. Debris creation and asperity deformation takes place causing abrasive wear. At the microscale, compression of the surface material occurs. The run-in friction shows similar trends at both scales; an initial decrease and an increase thereafter. Steady-state friction is not reached at the microscale, attributed to the absence of a graphitic tribolayer in the contact. At the macroscale, all films show abrasive wear and debris creation. Here, the changes in friction coefficients are attributed to the debris loss from the contact during the tribotests. The CNx film tested at 2.5GPa shows a continuous increase of friction, due to the continuous loss of debris from the contact. The other films reach a steady-state friction coefficient, since most of the debris is lost before the end of the tribotests.

Interpretation of X-ray photoelectron spectra of carbon-nitride thin films: New insights from in situ XPS

We report on angular-resolved x-ray photoelectron spectroscopy (XPS) studies of magnetron sputtered CNx thin films, first in situ (without air exposure), then after air exposure (for time periods ranging from minutes to several years), and finally after Ar ion etching using ion energies ranging from 500 eV to 4 keV. The as-deposited films typically exhibit two strong N1s peaks corresponding to pyridine-like, and graphite-like, at ∼398.2 eV and ∼400.7 eV, respectively. Comparison between in situ and air-exposed samples suggests that the peak component at ∼402–403 eV is due only to quaternary nitrogen and not oxidized nitrogen. Furthermore, peak components in the ∼399–400 eV range cannot only be ascribed to nitriles or pyrrolic nitrogen as is commonly done. We propose that it can also be due to a polarization shift in pyridinic N, induced by surface water or hydroxides. Argon ion etching readily removes surface oxygen, but results also in a strong preferential sputtering of nitrogen and can cause amorphization of the film surface. The best methods for evaluating and interpreting the CNx film structure and composition with ex-situ XPS are discussed.

Rolling contact fatigue of bearing components coated with carbon nitride thin films

Bearing rollers were coated with CNx films using high power impulse magnetron sputtering deposition in order to reduce their rolling-contact fatigue as investigated using a Micro-Pitting Rig tribometer under poly-alpha-olefin lubricated conditions. Coated rollers with a ~15nm thick W adhesion layer to the substrate, exhibit the best performance, presenting mild wear and no fatigue after 700 kcycles. The steady-state friction coefficient was ~0.05 for both uncoated and coated rollers. Uncoated rollers show run-in friction in the first 50 kcycles, because of steel-to-steel contact, which is absent for coated rollers. Analytical transmission electron microscopy and X-ray photoelectron spectroscopy show that the presence of a CNx coating prevents steel-to-steel contact of the counterparts, prior to the elastohydrodynamic lubrication, reducing their wear and increasing the lifetime expectancy.

Growth feature of ionic nitrogen doped CNx bilayer films with Ti and TiN interlayer by pulse cathode arc discharge

Using nano-scaled Ti and TiN as interlayer, ionic nitrogen doped carbon (CNx (N+)) bilayer films were prepared at various pulse frequencies by cathode arc technique. Elemental distribution at the interface, bonding compositions, microstructure, and mechanical properties of CNx (N+) bilayer films were investigated in dependence of interlayer and pulse frequency by Auger electron spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, nanoindentation, and surface profilometer. The results showed that the diffusion extent of C atoms at the interface of CNx (N+) bilayers is higher than for the α-C and CNx (N2) bilayers with the same interlayer. Nitrogen atoms could diffuse throughout the pre-deposited Ti and TiN layers into the Si substrate for all CNx (N+) bilayers. Ti interlayer facilitates the introduction of N atoms into the CNx (N+) films and exhibits a certain catalytic effect on the coordination of N atoms with sp2- and sp3-C binding. More nitrogenated and intense CN bonding configurations (mainly graphite-like N) form in the TiN/CNx (N+) bilayer. Ti/CNx (N+) bilayer prepared at low frequency possesses small size and disordering of Csp2 clusters but TiN interlayer weakens the formation of Csp2 bonding and increases the disordering of Csp2 clusters in the films. The residual stress in the bilayer is lower than for CNx (N+) monolayer. The higher hardness and the lower residual stress are present in the TiN/CNx (N+, 10Hz) bilayer.

Study on re-sputtering during CNx film deposition through spectroscopic diagnostics of plasma

A nitrogen-carbon plasma was generated during the deposition of carbon nitride (CNx) thin films by pulsed laser ablation of a graphite target in a discharge nitrogen plasma, and the optical emission of the generated nitrogen-carbon plasma was measured for the diagnostics of the plasma and the characterization of the process of CNx film deposition. The nitrogen-carbon plasma was recognized to contain various species including nitrogen molecules and molecular ions excited in the ambient N2 gas, carbon atoms and atomic ions ablated from the graphite target and CN radicals. The temporal evolution and spatial distribution of the CN emission and their dependence on the substrate bias voltage show two groups of CN radicals flying in opposite directions. One represents the CN radicals formed as the products of the reactions occurring in the nitrogen-carbon plasma, revealing the reactive deposition of CNx film due to the reactive expansion of the ablation carbon plasma in the discharge nitrogen plasma and the effective formation of gaseous CN radicals as precursors for CNx film growth. The other one represents the CN radicals re-sputtered from the growing CNx film by energetic plasma species, evidencing the re-sputtering of the growing film accompanying film growth. And, the re-sputtering presents ion-induced sputtering features.

Exploring the low-friction mechanism of CNx films in inert atmospheres by first-principles calculations

Amorphous CNx films show low friction coefficients in inert atmospheres, but the mechanism is still not well understood. Here we find that, by first-principles calculations, the dangling bonds on a diamond (111) surface are replaced with lone-pairs when nitrogen atoms are doped in the 1st or the 2nd top layer, and an ‘electron transfer passivation mechanism’ is identified. Then, two feasible ways for nitrogen atoms to present in the surface layer during the process of wear are suggested. On the basis of the insights gained, we propose a mechanism that highlights the positive role of the nitrogen lone-pairs in the formation of a weak-interacting interface between the friction-induced ‘graphitized’ layer and the intact below part, which is believed to be the reason for the low-frictions of CNx films in inert atmospheres.

Low-temperature growth of low friction wear-resistant amorphous carbon nitride thin films by mid-frequency, high power impulse, and direct current magnetron sputtering

The potential of different magnetron sputtering techniques for the synthesis of low friction and wear resistant amorphous carbon nitride (a-CNx) thin films onto temperature-sensitive AISI52100 bearing steel, but also Si(001) substrates was studied. Hence, a substrate temperature of 150 °C was chosen for the film synthesis. The a-CNx films were deposited using mid-frequency magnetron sputtering (MFMS) with an MF bias voltage, high power impulse magnetron sputtering (HiPIMS) with a synchronized HiPIMS bias voltage, and direct current magnetron sputtering (DCMS) with a DC bias voltage. The films were deposited using a N2/Ar flow ratio of 0.16 at the total pressure of 400 mPa. The negative bias voltage, Vs, was varied from 20 to 120 V in each of the three deposition modes. The microstructure of the films was characterized by high-resolution transmission electron microscopy and selected area electron diffraction, while the film morphology was investigated by scanning electron microscopy. All films possessed an amorphous microstructure, while the film morphology changed with the bias voltage. Layers grown applying the lowest substrate bias of 20 V exhibited pronounced intercolumnar porosity, independent of the sputter technique. Voids closed and dense films are formed at Vs ≥ 60 V, Vs ≥ 100 V, and Vs = 120 V for MFMS, DCMS, and HiPIMS, respectively. X-ray photoelectron spectroscopy revealed that the nitrogen-to-carbon ratio, N/C, of the films ranged between 0.2 and 0.24. Elastic recoil detection analysis showed that Ar content varied between 0 and 0.8 at. % and increased as a function of Vs for all deposition techniques. All films exhibited compressive residual stress, σ, which depends on the growth method; HiPIMS produces the least stressed films with values ranging between −0.4 and −1.2 GPa for all Vs, while CNx films deposited by MFMS showed residual stresses up to −4.2 GPa. Nanoindentation showed a significant increase in film hardness and reduced elastic modulus with increasing Vs for all techniques. The harder films were produced by MFMS with hardness as high as 25 GPa. Low friction coefficients, between 0.05 and 0.06, were recorded for all films. Furthermore, CNx films produced by MFMS and DCMS at Vs = 100 and 120 V presented a high wear resistance with wear coefficients of k ≤ 2.3 × 10−5 mm3/Nm. While all CNx films exhibit low friction, wear depends strongly on the structural and mechanical characteristics of the films. The MFMS mode is best suited for the production of hard CNx films, although high compressive stresses challenge the application on steel substrates. Films grown in HiPIMS mode provide adequate adhesion due to low residual stress values, at the expense of lower film hardness. Thus, a relatively wide mechanical property envelope is presented for CNx films, which is relevant for the optimization of CNx film properties intended to be applied as low friction and wear resistant coatings.

Soft X-ray emission spectroscopy used for the characterization of a-C and CNx thin films

We present the results of a soft X-ray emission spectroscopy study of a-C and CNx films on a Si(100) substrate. Also for the characterization of the homogeneity in depth of these films electron energy loss spectroscopy measurements with localization better than 4nm were carried out. In case of CNx films the highest diamond-like modification occurs in the region close to the Si(100) substrate. The film density decreases with increasing distance from the substrate and becomes almost constant in range of thicknesses more than ~2nm.