Influence of nitrogen ion energy on the Raman spectroscopy of carbon nitride films

Carbon nitride films were deposited by filtered cathode vacuum arc combined with radio frequency nitrogen ion beam source. Both visible Raman spectroscopy and UV Raman spectroscopy are used to study the bonding type and the change of bonding structure in carbon nitride films with nitrogen ion energy. Both C–N bonds and CN bonds can be directly observed from the deconvolution results of visible and UV Raman spectra for carbon nitride films. Visible Raman spectroscopy is more sensitive to the disorder and clustering of sp² carbon. The UV (244 nm) Raman spectra clearly reveal the presence of the sp³ C atoms in carbon nitride films. Nitrogen ion energy is an important factor that affects the structure of carbon nitride films. At low nitrogen ion energy (below 400 eV), the increase of nitrogen ion energy leads to the drastic increase of sp²/sp³ ratio, sp² cluster size and C---N bonds fraction. At higher nitrogen ion energy, increase leads to the slight increase of CN bonds fraction and sp² cluster size, slight decrease of C---N bonds fraction and sp²/sp³ ratio.     

Evaluation of diamond films by nuclear magnetic resonance and Raman spectroscopy

The quality of chemically vapor deposited diamond films was assessed in terms of sp²/sp³ content as determined by solid-state nuclear magnetic resonance (NMR) and Raman spectroscopy. While the results of the two techniques are in qualitative agreement, only the NMR spectra yield quantitative values for the sp²/sp³ ratio. Only sp³ carbon was observed in the NMR spectra of very high quality hot-filament, microwave plasma, and d.c. arc-jet chemically vapor deposited films. As expected, Raman spectroscopy is extremely sensitive to sp² bonded carbon, identifying small amounts below the detection limit of the NMR spectrometer. Comparison of the two techniques, however, indicates that Raman spectroscopy may be so sensitive to sp² bonded carbon that sp³ bonded carbon in films containing as much as 90% sp³ bonded material may remain undetected. NMR linewidths indicate that the sp³ carbon in such material shows more disorder than that found in high-quality polycrystalline films.     

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.     

Near-edge X-ray absorption fine structure spectroscopy of arcjet-deposited cubic boron nitride

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was employed to help determine the structure of boron nitride films grown by bias-enhanced chemical vapor deposition in a low-density supersonic arcjet flow. BN films containing 0.90% cubic boron nitride were analyzed by NEXAFS and compared with c-BN and h-BN reference spectra. The mainly cubic films have been shown previously to be nanocrystalline, which leads to the inability to obtain structural information from Raman scattering spectra. However, with NEXAFS, the nanocrystalline nature of the films does not strongly affect the structural interpretation. It is shown that films deposited with a bias of −75 V are primarily sp³ bonded. This high sp³ bonding character agrees with previous measrements based on infraredtransmission and reflectance spectroscopy, as well as X-ray photoelectron spectroscopy.     

Synthesis of amorphous carbon nitride using reactive ion beam sputtering deposition with grazing bombardment

Amorphous carbon nitride (a-C:N) thin films were synthesised on steel substrates using reactive ion beam sputtering deposition (RIBSD). A single ion beam is arranged to sputter the graphite target at 75° incidence and concurrently bombard the growing film at grazing incidence angles of the ion beam. Nanoindentation, Raman spectroscopy, FTIR, FT-Raman and XPS were employed to characterise the mechanical and structural properties of the films. It was found that grazing incident bombardment has a significant effect on film structure through an increase in nitrogen content and formation of nitrogen doped structure.     

Energy dependent structure changes in ion beam deposited a-C:H

The present study investigates the effect of ion energy in the form of acceleration potential on the structure of ion beam deposited a-C:H films using Raman spectroscopy, FTIR spectroscopy and profilometry. The results indicate that for low acceleration potentials (100–200 V), the sp² fraction of the film becomes more ordered as the acceleration potential increases. However, FTIR indicates that the hydrogen bonding in the film is unaffected. For mid-range acceleration potentials (200–800 V) the film structure remains stable, then, as the acceleration potential is increased above 800 V there is a further increase in the order of the sp² fraction in the films and a change in the hydrogen bonding in the film. These structure changes suggest that two separate energy dependent processes effect the structure of IBD a-C:H; a densification/relaxation process at low acceleration potentials and ion damage/sputtering processes at higher acceleration potentials.     

Synthesis of nanocrystalline nitrogen-rich carbon nitride powders at high pressure

Nitrogen-rich carbon nitride powders with composition close to C₄N_2.8–3.3 were synthesized by using a chemical reaction between sodium azide and hexachlorobenzene under high pressure (7.7 GPa) and a temperature of 500 °C for 30 min. The final black powders were relatively hard materials with high resistance to hot acids and common organic solvents. Analytical electron microscopy and X-ray diffraction studies showed that the powders are predominantly amorphous, containing nanometer-sized crystallites. Carbon and nitrogen K-edge structures obtained by electron energy-loss spectroscopy suggest the existence of chemical bonding between C and N, and that the amorphous carbon nitride matrix is primarily sp²-bonded. Raman and infrared features of the carbon nitride powders closely resemble those in carbon and diamond-like films and also characterize the powders as strongly disordered, sp²-bonded carbon nitride exhibiting graphite-like microdomains with dimensions of approximately 1 nm.     

Effect of microwave power on diamond-like carbon films deposited using electron cyclotron resonance chemical vapor deposition

Diamond-like carbon (DLC) films have been deposited using electron cyclotron resonance chemical vapor deposition (ECR-CVD) under various microwave power conditions. Langmuir probe measurement and optical emission spectroscopy (OES) were used to characterize the ECR plasma, while the films were characterized using Raman and infrared (IR) spectroscopies, hardness and optical gap measurements. It was found that the ion density and all signal peaks in the optical emission (OE) spectra increased monotonously following the increase in microwave power. Raman spectra and optical gap measurements indicate that the films become more graphitic with lower content of sp³-hybridized carbon atoms as the microwave power was increased. IR and hardness measurements indicate a reduction in hydrogen content and decrease in hardness for the film produced at relatively high microwave powers. A deposition mechanism is described which involved the ion bombardment of film surfaces and hydrogen–surface interactions. The deposition rate of DLC film is correlated to the ion density and CH₃ density.     

Nano-carbon nitride synthesis from a bio-molecular target for ion beam sputtering at low temperature

Nano-crystalline carbon nitride has been successfully synthesized at a temperature below 100 °C from an adenine(C₅N₅H₅) target sputtered by an Ar ion beam. Because adenine possesses a ring structure similar to the hypothetical β-C₃N₄ phase, the use of this bio-molecular compound as the target is believed to reduce the energy barrier of carbon-nitride growth. The effect of Ar ion-sputtering voltage on the film growth and the effect of extra-N-atom incorporation on the carbon-nitride film growth are examined in this study. Only a carbon film is formed with an ion energy of 500 V. For the ion-beam energy above 750 V, carbon nitride films are deposited, and there is some hydrogen incorporation in the films. The N/C composition ratio in the films could reach 1:1 and is independent on the ion beam voltage. The nitrogen is bonded with carbon within the films, as determined by the IR and XPS measurements. However, the films deposited at a higher ion voltage could possess some original functional groups of adenine. A strong and broad peak at a d-spacing of 0.32 nm, comparable to the calculated d-spacing of the β-C₃N₄(110), is observed in the XRD spectra of the carbon nitride films. The TEM results indicated that the film contained nano-crystalline grains. Several d values are also in good agreement with those of adenine and the calculated values of β-C₃N₄. The C/N ratios of the films are still kept at almost 1:1 with N atoms added during deposition. The XRD spectra and IR spectra of these films are all similar to the film deposited without nitrogen source.     

Characterization of diamond-like carbon films before and after heavy energetic ion implantations

Hydrogenated diamond-like carbon films were implanted by 110 keV Fe⁺ at doses ranging from 1 × 10¹³ to 5 × 10¹⁶ ions cm⁻². The film resistivities and the infra-red transmittances of the specimens were determined as functions of the implanted doses. Raman spectra and the infra-red transmittances of the film layers were used to characterize the structural changes of the implanted films. It was found that, when the implantation dose was higher than about 5 × 10¹⁴ or 1 × 10¹⁵ ions cm⁻², the film resistivity and the total infra-red transmittance of the specimens decreased significantly. However, when the dose was smaller than this value, the resistivity decreased firstly and then increased with dose and the measured values were higher than those of corresponding as-grown ones. The infra-red transmittance of the specimens was also improved to some extent under the lower dose range. By using structural characterization results, especially the infra-red transmittances of the film layers, we conclude that the electrical and optical property changes at doses higher than about 5 × 10¹⁴ or 1 × 10¹⁵ ions cm⁻² were due to the following changes, i.e., the decrease in the population of both sp² C-H and sp³ C-H bonds (compared with that of sp³ C-H bonds, the decrease in speed of sp² C-H bonds is smaller), the decrease of bond-angle disorder and the increased population of sp² C-C bonds. However, at doses between 1 × 10¹⁴ and 5 × 10¹⁴ or 1 × 10¹⁵ ions cm⁻², the implantation induced increase of C-H bonds was responsible for the observed property changes. Compared with the previous reports, the novelty of the present work is: the IR transmittance curves of the single film layers give us direct evidence for the changes of different C-H bonds with increasing ion dose and thus proved the transformation mechanism proposed previously.     

XPS studies of amorphous SiCN thin films prepared by nitrogen ion-assisted pulsed-laser deposition of SiC target

Amorphous SiCN films were prepared on Si (100) substrates by nitrogen ion-assisted pulsed-laser ablation of an SiC target. The dependence of the formed chemical bonds in the films on nitrogen ion energy and the substrate temperature was investigated by an X-ray photoelectron spectroscopy (XPS). The fractions of sp² CC, sp³ CC, and sp² CN bonds decreased, and that of NSi bonds increased when the nitrogen ion energy was increased without heating during the film preparation. The fraction of sp³ CN bonds was not changed by the nitrogen ion irradiation below 200 eV. Si atoms displaced carbon atoms in the films and the sp³ bonding network was made between carbon and silicon through nitrogen. This tendency was remarkable in the films prepared under substrate heating, and the fraction of sp³ CN bonds also decreased when the nitrogen ion energy was increased. Under the impact of high-energy ions or substrate heating, the films consisted of sp² CC bonds and SiN bonds, and the formation of sp³ CN bonds was difficult.     

Structural features of diamond layers photo-emitting at sub-band gap energies

The photoemission behaviour of a series of diamond-based polycrystalline films irradiated by the second (2.3 eV), third (3.5 eV) and fourth (4.7 eV) harmonics produced by a Q-switched-mode-locked Nd: Yag laser has been investigated and related to the structural and compositional characteristics of the layers. The materials were polycrystalline undoped diamond films as well as Nd- and N-containing diamond films grown by CVD techniques, diamond-like and amorphous carbon layers. The morphological and structural characteristics of the films were investigated by electron microscopy, Raman spectroscopy and electron diffraction. The analysis of the photoemission curves does not evidence any improvement of the emission efficiency in the case of Nd-containing films nor for the diamond films grown in the presence of N_2. The results evidence conversely a strong correlation between the characteristics of the photoemission process at sub-band gap energies and the presence of amorphous sp²-C patches located at the diamond film surfaces. The photoemitting properties of our samples are discussed and rationalized by considering charge emission occurring at the sp²-diamond-vacuum border and the emission process governed by the ratio of amorphous sp²-C to crystalline sp³-C. The rather high values of quantum efficiency measured in the course of the present research at 3.5 and 4.7 eV suggest that a proper distribution of amorphous carbon onto a good quality diamond surface is the key factor for the preparation of efficient and stable photocathode materials.     

Experimental comparison of N(1s) X-ray photoelectron spectroscopy binding energies of hard and elastic amorphous carbon nitride films with reference organic compounds

In this work, hard and elastic amorphous carbon nitride (a-CN_x) films were deposited by DC magnetron sputtering on heated Si(001) substrates at 400 °C. Nanoindentation results confirmed that the films were highly compliant and had high elastic recovery. X-ray photoelectron spectroscopy (XPS) was used to investigate nitrogen bonding by directly comparing the N(1s) spectra of a-CN_x with the N(1s) peak positions of a variety of organic compounds that were characterized in the same XPS system. The N(1s) XPS spectra of hard and elastic a-CN_x is resolved into two dominant intensity contributions at 398.5 and 400.6 eV. We show that the N(1s) spectra of a-CN_x do not conclusively support a film-structure model with nitrogens bonded to sp³ carbons. We offer an alternate interpretation based on the presented data and previous XPS, nuclear magnetic resonance (NMR), and computational work. Together, the data suggest that hard and elastic a-CN_x consists of an sp² carbon network and that single-atom vacancy defects, as found in a graphite layer, may be present in the material. This implies that the low binding energy N(1s) component at 398.5 eV may be due to pyridine-like nitrogen bonded at the perimeter of a vacancy defect.     

Carbon nitride layers created by laser deposition combined with RF discharge

The structure, composition and bonding of carbon nitride films created by pulsed laser deposition in combination with radio-frequency discharge for nitrogen activation were studied by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) and by Raman spectroscopy for various deposition conditions. XPS measurements revealed a maximum N/C of ∼0.5 and an increased number of N-sp³C bonds for lower N/C values. FTIR and Raman spectra indicate the presence of a polymeric phase.     

Room temperature synthesis of boron nitride thin films by dual-ion beam sputtering deposition

Boron nitride (BN) films are prepared by dual-ion beam sputtering deposition at room temperature (~25 °C). An assisting argon/nitrogen ion beam (ion energy E_i=0–300 eV) directly bombards the substrate surface to modify the properties of the BN films. The effects of assisting ion beam energy on the characteristics of BN films were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectra, atomic force microscopy, and optical transmittance. The density of the B–N bond in the film increased with the increase in assisting ion beam energy. The highest transmittance of more than 95% in the visible region was obtained under the assisting ion beam energy of 300 eV. The band gap of BN films increased from 5.54 eV to 6.13 eV when the assisted ion-beam energy increased from 0 eV to 300 eV.     

Evolution of the opto-electronic properties of amorphous carbon films as a function of nitrogen incorporation

The present work provides correlations between the optical, electronical and microstructural properties of amorphous carbon nitride films (a-CN_x) deposited by Direct Current (DC) magnetron sputtering technique versus the N₂/Ar + N₂ ratio. The microstructure of the films was characterized by Raman spectroscopy and optical transmission measurements. The evolution of both the density of states (DOS) located between the bandtail states and the density of states around the Fermi level N(E_f), have been investigated by electrical measurements versus temperature varying the N₂/Ar + N₂ ratio. The evolution of the microstructure versus N reveals a continuous structural ordering of the sp² phase, which is confirmed by the optical and the conductivity measurements. The conductivity variation was interpreted within the framework of the band structure model of the π electrons in a disordered carbon with the presence of localized states.     

木棉纤维改性氮化碳光催化降解有机污染物

采用一步热解法制备了木棉纤维(KF)改性的石墨相氮化碳(g-C₃N₄)催化剂,并考察了催化剂光催化降解有机污染物的性能。采用XRD、UV-Vis DRS、FT-IR、TEM、XPS、N₂吸附-脱附、PL表征对催化剂进行了结构、形貌、光学性能测试。结果表明,KF改性可以提高催化剂的比表面积,更大的比表面积可以提供更多的活性位点来参与光催化降解过程。UV-Vis DRS结果表明KF改性可以缩小催化剂的禁带宽度,提高催化剂对光能的吸收。在可见光下,KF改性的g-C₃N₄基催化剂对苯酚降解速率常数为0.259 h^-1,是纯g-C₃N₄的4.2倍,且具有优异的催化稳定性和结构稳定性。     

Kapok fiber modified carbon nitride photocatalytic degradation of organic pollutants

A kapok fiber (KF) modified graphite phase carbon nitride (g-C₃N₄) catalyst was prepared by a one-step pyrolysis method, and the photocatalytic degradation of organic pollutants was investigated. The structure and optical properties of KF-CN were characterized by XRD, UV-Vis DRS, TEM, PL, XPS, FT-IR and N₂ adsorption-desorption. The nitrogen adsorption-desorption isotherm results show that the presence of the mesoporous structure can improve the specific surface area of KF-CN. Elemental analysis characterization indicates the biochar modification is conductive to increase the C/N ratio of KF-CN. The XPS characterization also indicates that the introduction of carbon element can change the chemical environment of N element in the lattice of g-C₃N₄ and thus increases the electron density of N element. The photocatalytic degradation of phenol experiment was carried out to investigate the performance of as-prepared kapok fiber modified graphite carbon nitride photocatalysts by using high-pressure sodium lamp as visible light source. The results show the KF(5%)-CN(600) displayed the highest phenol degradation rate constant of 0.259 h^-1, which is 4.2 times of that of neat g-C₃N₄.The activity of KF(5%)-CN(600) does not decrease significantly after 5 cycles, hinting its excellent catalytic stability and structural stability. The possible reaction mechanism was proposed.     

Carbon and nitrogen 1s energy levels in amorphous carbon nitride systems: XPS interpretation using first-principles

A series of carbon nitride structures have been generated, by using a classical potential in a Metropolis Monte Carlo liquid quench procedure. The resulting structures are relaxed further using the density functional theory approach. Structures are generated with various mass densities and varying N/C ratios. X-ray photoelectron spectroscopy (XPS) calculations within the first-principles methodology are performed on the generated amorphous carbon nitride systems. Depending on the carbon bonding configuration, the carbon 1s energies are found to vary from 283 to 288 eV while those of nitrogen are found to range from 397 to 405 eV. Additional calculations on model crystalline systems like nitrogen substituted graphite (nitrogen in trigonal structure) and nitrogen substituted diamond support the finding that the C and N 1s energies are not only sensitive to the coordination number of the atom but also to the interatomic distances. Our calculations based on computer generated structures are a viable alternative for the analysis of XPS spectra and support the interpretation of the N 1s energy at 398.4 eV to correspond to two-coordinated N with at least one sp² C nearest neighbor.     

Internal stress of a-C:H(N) films deposited by radio frequency plasma enhanced chemical vapor deposition

Amorphous hydrogenated carbon nitride [a-C:H(N)] films were deposited from the mixture of C₂H₂ and N₂ using the radio frequency plasma enhanced chemical vapor deposition technique. The films were characterized by X-ray photon spectroscopy, infrared, and positron annihilation spectroscopy. The internal stress was measured by substrate bending method. Up to 9.09 at% N was incorporated in the films as the N₂ content in the feed gas was increased from 0 to 75%. N atoms are chemically bonded to C as C–N, CN and CN bond. Positron annihilation spectra shows that density of voids increases with the incorporation of nitrogen in the films. With rising nitrogen content the internal stress in the a-C:H(N) films decrease monotonically, and the rate of decrease in internal stress increase rapidly. The reduction of the average coordination number and the relax of films structure due to the decrease of H content and sp³/sp² ratio in the films, the incorporation of nitrogen atoms, and the increases of void density in a-C:H(N) films are the main factors that induce the reduction of internal stress.