Pulsed laser deposition of glass-like cluster assembled carbon films

Carbon films have been synthesized at room temperature in helium atmosphere, at high pressure, on (1 0 0) Si substrates by pulsed KrF excimer laser ablation of highly oriented pyrolitic graphite. By changing laser power density (from 8.5 to 19 MW mm⁻²) and gas pressure (from 0.6 Pa to 2 kPa), nanometer sized cluster assembled films were obtained. Film morphology, as studied by scanning electron microscopy, changes with increasing helium pressure, from dense columns, to node-like morphology, then to an open dendritic structure. Carbon coordination was studied by visible Raman spectroscopy in all films. They are structurally disordered, sp² coordinated and belong to the family of glass-like carbons. The deduced film coherence length agrees with the average size of carbon aggregates that build up the films, as measured by transmission electron microscopy in representative samples. The average number of carbon atoms per cluster, that depends on helium (high) pressure, was obtained by a simple model.     

Orientation of graphitic planes during annealing of “dip deposited” amorphous carbon film: A carbon K-edge X-ray absorption near-edge study

Annealing effect of amorphous carbon thin films on Si(1 0 0) substrates is studied by normal incidence and angle dependent carbon K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The angle dependence of the XANES signal shows that the graphitic basal planes are oriented perpendicular to the surface when the film is annealed at 1000 °C. Micro-Raman spectroscopy reveals two well-separated bands the D band at 1355 cm⁻¹ and G band at ∼1600 cm⁻¹, and their I_D/I_G intensity ratio indicates the formation of more graphitic film at higher annealing temperatures. X-ray diffraction pattern of 1000 °C temperature annealed film confirms the formation of graphite structure.     

Fourier transform infrared spectroscopic study of nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films, which possess n-type conduction with enhanced electrical conductivities, were prepared by pulsed laser deposition and they were structurally studied by Fourier transform infrared (FTIR) spectroscopy. The film with a nitrogen content of 7.9 at.% possessed n-type condition with an electrical conductivity of 18 S/cm at 300 K. The FTIR spectra revealed peaks due to nitrogen impurities, C = N, C-N, and CH_n (n = 1, 2, 3) bands. The sp²-CH_n/(sp²-CH_n + sp³-CH_n), estimated from the area-integration of decomposed peaks, were 24.5 and 19.4% for undoped and 7.9 at.% doped films, respectively. The nitrogen-doping not only form the chemical bonds between carbon and nitrogen atoms such as C = N and C-N bonds but also facilitate the formation of both sp² and sp³ bonds, in particular, the sp³-CH_n bond is preferentially formed. From the analysis of the FTIR spectra, it was found that the hydrogen content in the film is increased with an increase in the nitrogen content. The increased hydrogen content might be owing to the enhanced volume of grain boundaries (GBs) between UNCD grains, and those between UNCD grains and an a-C:H matrix, which is caused by a reduction in the UNCD grain size. The CH_n peaks predominantly come from an a-C:H matrix and GBs. Since the nitrogen-doping for a-C:H has been known to be hardly effective, the n-type conduction with the enhanced electrical conductivities might be attributed to the sp²-CH_n formation at the GBs.     

Local structural analysis of a-SiCx:H films formed by decomposition of tetramethylsilane in microwave discharge flow of Ar

Hydrogenated amorphous silicon carbide (a-SiC_x:H) films were prepared by the decomposition of tetramethylsilane (TMS) with microwave discharge flow of Ar. When radio-frequency (RF) bias voltage (− V_RF) was applied to the substrate, the film hardness increased as (2.39 ± 1.12)-(9.15 ± 0.55) GPa for − V_RF = 0-100 V. The a-SiC_x:H films prepared under various − V_RF conditions were analyzed by the carbon-K near edge X-ray absorption fine structure (NEXAFS), by the elastic recoil detection analysis (ERDA), and by the X-ray photoelectron spectroscopy (XPS). From a quantitative analysis of NEXAFS, the sp²/(sp²+ sp³) ratios of C atoms were evaluated as 67.9 ± 2.0, 55.4 ± 2.7, and 51.7 ± 0.7% for − V_RF = 0, 60, and 100 V, respectively. From ERDA, hydrogen content of the film prepared under the condition of − V_RF = 100 V was found to decrease 28% comparing with that under − V_RF = 0 V. It is suggested that the cause of the increase of the film hardness when applying − V_RF is predominantly the growth of the sp³-hybridized structure of C atoms accompanied by the decrease of hydrogen terminations.     

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.     

Electrochemical impedance spectroscopy of poly[carbazole-co-N-p-tolylsulfonyl pyrrole] on carbon fiber microelectrodes,equivalent circuits for modelling

Polycarbazole (PCz) and copolymerization of carbazole (Cz) and N-p-tolylsulfonyl pyrrole (pTsp), P(Cz-co-pTsp), thin films have been cyclovoltammetrically coated onto carbon fiber electrodes as an active functionalized microelectrode in sodium perchlorate (NaClO₄)/acetonitrile (ACN) medium. The resulting thin films of homopolymer and copolymer were characterised by using Fourier transform infrared reflectance spectroscopy (ATR-FTIR), energy dispersive X-ray (EDX) point analysis, scanning electron microscopy (SEM) and atomic force microscopy (AFM). An electrical impedance study on the prepared electrodes is reported in the present paper under different feed ratios of [pTsp]₀/[Cz]₀ during electrochemical impedance spectroscopic (EIS) measurements. Specific capacitance (C_sp) were calculated, P(Cz-co-pTsp) in feed ratio of [pTsp]₀/[Cz]₀ = 200 has preserved more capacitive behavior especially at lower frequency (C_sp = ∼156 mF g⁻¹) than polycarbazole (C_sp = ∼2.1 mF g⁻¹. The electrochemical impedance data fitted to three different equivalent models were used to find out numerical values of the proposed components.     

Low-temperature synthesis of multiwalled carbon nanotubes by graphite antenna CVD in a hydrogen-free atmosphere

Multiwalled carbon nanotubes (MWCNTs) were synthesized at 390 °C in a hydrogen-free atmosphere by graphite antenna chemical vapor deposition, which provides carbon radicals by the etching of the antenna itself in a He- or Ar-based noble gas plasma. An increase in the number of defects in the graphite layers of the CNTs was observed with increasing hydrogen partial pressure. The results of X-ray photoelectron spectroscopy analysis suggested that these defects were caused by the transformation from an sp² to an sp³ structure in the graphite layers of CNTs due to hydrogen radicals.     

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.     

Sol–gel-deposition of thin TiO2:Eu thermographic phosphor films

A relatively new promising method for surface temperature measurement is the use of thermographic phosphors. For this application, the temperature-dependent luminescence properties of europium (III)-doped anatase (TiO₂:Eu³⁺) thin films were studied. The films were prepared by the sol–gel method using dip coating. The structures and the morphology of the films were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electron dispersive X-ray spectroscopy (EDX) was used to verify the europium concentration within the films. For using the films as temperature sensors the optical properties are the main concern. Therefore, the emission spectra of the films were measured after ultraviolet laser excitation (355 nm). They indicate that the red characteristic emission (617 nm) of TiO₂:Eu³⁺ due to the ⁵D₀ → ⁷F₂ electric dipole transition is the strongest. The decay time constant of the exponential emission decay under UV excitation with a Nd:YAG laser (355 nm, f = 10 Hz) is strongly temperature dependent in the range from 200 °C up to 400 °C; making it useful for temperature evaluation. The temperature dependence was measured for the emission line at 617 nm; the results demonstrate that anatase doped europium (III) can be used as a thermographic phosphor.     

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.     

Interaction of H (D) atoms with surfaces of glassy carbon: adsorption, abstraction, and etching

The interaction of thermal (2000 K) H and D atoms with glassy carbon (GC) surfaces was investigated in ultrahigh vacuum environment using thermal desorption and reaction kinetics mass spectroscopy. Virgin GC surfaces (not previously subjected to stationary etching by H atoms) exhibit a remarkably low reactivity with respect to adsorption of D. The saturation coverage of D on GC is about a factor of ten smaller than on (0 0 0 1) graphite surfaces (HOPG or natural single crystal). Thermal desorption spectra indicate that D atoms on virgin GC surfaces are adsorbed on distorted graphite basal planes, i.e. the recombinative desorption features of D on GC between 400 and 600 K are broadened as compared to those measured on graphite. Upon heating of D-covered virgin GC surfaces, CD₃ surface groups desorb between 500 and 1000 K. Stationary etching of GC by a flux of H atoms is most efficient around 600 K, as was previously observed on other carbon materials, a-C:H thin films and graphite, and as expected from the etching mechanism on C substrates. GC surfaces repeatedly etched by H exhibit an increasing density of C atoms located at edge sites which are capable to adsorb D via formation of spⁿ C-D bonds. D from these sites desorbs recombinatively around 830 K and competitive desorption of C₂ deuterocarbons at 780 K occurs. The graphite-like fraction of the surface is unaffected by etching. Abstraction of D on virgin GC by H exhibits the same phenomenology as on graphite: Eley-Rideal mechanism and large abstraction cross-section at small D coverages.     

Surface modification of multi-wall carbon nanotubes by nitrogen attachment

Commercial multiwall carbon nanotubes (MWCNT-s) were treated by RF activated N₂ gas plasma at (nominally) room temperature. Treatment time of 5 to 10 min was applied at negative bias varying in the 0-300 V range. Surface chemical alterations were followed by X-ray photoelectron spectroscopy (XPS). All the applied treatments resulted in a significant build-up of nitrogen in the surface of MWCNT-s. The amount of nitrogen varied between 19 and 25 at.% depending on the treatment time and, in a lesser extent, also on biasing conditions. Interestingly, the nitrogen attachment was also significant (20 at.%) when the treatment commenced without bias. Evaluating the high-resolution N1s XP spectral region, typically three different chemical bonding states of the nitrogen was delineated. Peak component at 398.3 ± 0.3 eV is assigned to CNC type, at 399.7 ± 0.3 eV to sp² N in melamine-type ring structure and at 400.9 ± 0.3 eV to N substituting carbon in a graphite-like environment. Identical chemical bonding of the nitrogen was detected on the surface of highly oriented pyrolytic graphite (HOPG) and on microcrystalline graphite surfaces treated in the same way for comparison. Estimating the penetration depth of the nitrogen atoms by the SRIM program it was concluded that at the applied DC bias energy range the implanted nitrogen is incorporated in the top 2-4 monoatomic layers of the samples. A model for the distribution of the chemically bonded nitrogen on the outer walls of the MWCNT-s is proposed.     

The infrared characteristics investigation of carbon nitride films

Two series of a-C(N):H films, with diamond-like character and graphite-like character respectively, are prepared. Without N incorporation, the two kinds of films have very close IR bands in the range of 1000–1800 cm^− 1. However, the difference in IR activity of the two series films became dramatical as N was introduced into both kinds of carbon films, which is attributed to purely electronic effect, the electronegativity of N. The N incorporated in carbon films is able to induce bond dipole of CC bonds in sp² graphite cluster, leading to a degree of dipoles for all the aromatic sp² CC bonds, a permanent electric dipole effect arises and this could lead to the increase in IR activity of the sp² clusters. As the N content exceeds 20 at.% in the carbon films, strong conjugation of C≡N bonds with aromatic graphite rings can induce conjugated π bond dipole too.     

A simple method to produce almost perfect graphene on highly oriented pyrolytic graphite

A simple and effective stepwise-method has been developed to remove defects from the top graphene layers of highly orientated pyrolytic graphite. Using a combination of ozone exposure and moderately high temperature we have shown that a defect-rich graphite surface can be modified to generate a graphene-like surface containing a negligible amount of oxygen, hydrogen and sp³ carbon. We report definitive X-ray photoelectron and X-ray absorption spectroscopy analysis after each stage of the process, suggest a mechanism by which the modification occurs and propose it as a route towards the preparation or manipulation of pristine graphene samples.     

Electronic and bonding properties of Fe- and Ni based hydrogenated amorphous carbon thin films by X-ray absorption, valence-band photoemission and Raman spectroscopy

Electronic and bonding properties of Me-based hydrogenated amorphous carbon (a-CH:Me, Me = Fe, Ni) thin films have been studied by X-ray absorption near-edge structure (XANES), valence-band photoemission (VB-PES) and Raman spectroscopy. Raman and XANES results show enhancement of the content of sp³-rich diamond-like carbon (DLC) by doping with Fe and Ni. The VB-PES spectrum of a-CH:Fe shows emergence of a prominent feature due to states of sp³-bonded clusters, indicating that a-CH:Fe induced enhancement of DLC structure. The nano-indentation measurement reveals that a-CH:Fe has a greatly enhanced hardness, while electrical resistance measurement shows that a-CH:Me reduces resistivity.     

Improvement of field emission performance on nitrogen ion implanted ultrananocrystalline diamond films through visualization of structure modifications

The relationship between the electron field emission properties and structure of ultra-nanocrystalline diamond (UNCD) films implanted by nitrogen ions or carbon ions was investigated. The electron field emission properties of nitrogen-implanted UNCD films and carbon-implanted UNCD films were pronouncedly improved with respect to those of as-grown UNCD films, that is, the turn-on field decreased from 23.2 V/μm to 12.5 V/μm and the electron field emission current density increased from 10E−5 mA/cm² to 1 × 10E−2 mA/cm². The formation of a graphitic phase in the nitrogen-implanted UNCD films was demonstrated by Raman microscopy and cross-sectional high-resolution transmission electron microscopy. The possible mechanism is presumed to be that the nitrogen ion irradiation induces the structure modification (converting sp³-bonded carbons into sp²-bonded ones) in UNCD films.     

Processing of highly oriented (K,Na)NbO3 thin films using a tailored metal-alkoxide precursor solution

Highly oriented lead-free K_0.5Na_0.5NbO₃ (KNN) thin films were synthesized by chemical solution deposition. The analysis of the KNN precursor solution revealed that the KNN precursor consists of complex metal alkoxides of potassium and sodium hexaalkoxy niobates with highly symmetric Nb-O octahedra. KNN precursor films were crystallized in a perovskite phase with a (1 0 0) preferred orientation on Pt(1 0 0)/MgO(1 0 0) substrates at 650 °C. The three-dimensional relationship between KNN(1 0 0) and Pt(1 0 0)/MgO(1 0 0) is confirmed by the fourfold symmetry of the pole figure. The fourfold symmetry indicates that the synthesized films are oriented in both the c and a, b directions on the Pt(1 0 0) surface. Although the insulating resistance was not sufficiently high at room temperature, the (1 0 0)-oriented KNN thin films showed potentially larger polarizations compared to the KNN thin films with no preferred orientation at low temperatures. The 2P_r and 2E_c of the oriented KNN films at −190 °C were 41.0 μC/cm² and 90 kV/cm, respectively.     

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.     

Evaluation on the optical properties of Ga2O3−x thin films co-doped with Tb and transition metals (Mn,Cr) prepared by a photochemical route

Gallium β-diketonate complexes were studied as precursors for the photochemical deposition of amorphous thin films of gallium oxide doped with terbium and co-doped with chromium or manganese. Solutions of the inorganic complexes were spin coated on Si(100) and quartz substrates and photolyzed at room temperature using 254 nm UV light. The photolysis of these films induces the fragmentation of the complexes and the partial reduction of the metal ion together with the release of volatile organic compounds as sub-products. When the metallic complexes are irradiated under air, the products of the reactions are metal oxide thin films. The photochemical reactivity of these films was monitored by UV–vis spectroscopy, followed by a post-annealing treatment. The obtained films were characterized by X-ray photoelectron spectroscopy and X-ray diffraction. The optical properties of the films showed that these are highly transparent in the visible spectrum but decrease significantly in doped and co-doped films. Under UV light excitation (254 nm) the doped films (Ga₂O_3−x/Tb) show the characteristic emissions at 486, 530, 542 and 610 nm associated to ⁵D₄→⁷F_J (J=6,5,4,3) transitions of Tb⁺³ ion. However, these emissions decrease and deteriorate in the co-doped films (Ga₂O_3−x/Tb/M, where M=Mn or Cr). A possible emission mechanism and energy transfer have been proposed.     

Diamond nanoparticles to carbon onions transformation: X-ray diffraction studies

Carbon onions prepared by high temperature annealing of ultradispersed diamond nanoparticles of about 5 nm in average diameter have been studied by X-ray diffraction using synchrotron radiation. The X-ray diffraction patterns show transformation of the diamond nanoparticles with sp³ bonds into spherical carbon onions containing remaining diamond-like core and then into polyhedral onions with facets on their outer part and pure sp² graphitic bonds. The prepared onions form concentric-shell particles which comprise of about ten shells with an intershell distance of 0.35-0.36 nm. The large intershell distance suggests a considerable reduction in intershell interaction when compared to perfect graphite. The X-ray data are related to the previously performed studies by electron energy-loss spectroscopy and electron spin resonance.