Article

Electronic structure and chemical bonding of nanocrystalline-TiC/amorphous-C nanocomposites

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

The electronic structure of nanocrystalline (nc-) TiC/amorphous C nanocomposites has been investigated bysoft x-ray absorption and emission spectroscopy. The measured spectra at the Ti 2p and C 1s thresholds of thenanocomposites are compared to those of Ti metal and amorphous C. The corresponding intensities of theelectronic states for the valence and conduction bands in the nanocomposites are shown to strongly depend onthe TiC carbide grain size. An increased charge transfer between the Ti 3d-eg states and the C 2p states hasbeen identified as the grain size decreases, causing an increased ionicity of the TiC nanocrystallites. It issuggested that the charge transfer occurs at the interface between the nanocrystalline-TiC and the amorphous-Cmatrix and represents an interface bonding which may be essential for the understanding of the properties ofnc-TiC/amorphous C and similar nanocomposites.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... The sensitivity of X-ray spectroscopy enables us to probe the orbital directional occupations to investigate the bonding structure at the internal interfaces that determines the bond characteristics (symmetry and orbital directions at interfaces between cubic and hexagonal crystals) and local chemistry that affects conductivity and hardness [13]. The cubic-AlN phase that may form has a bandgap of about 6 eV [14] while it is 3 eV for ZrN [15]. ...
... Previous spectroscopic investigations of ZrAlN thin films have utilized surface-sensitive X-ray photoemission spectroscopy (XPS) where electrons were detected at the Zr 3d levels [18] [19]. However, XPS is not ideal for probing the electronic structure and chemical bonding in the bulk of the material's interior and to distinguish the bonding structure between semi-alloys, compounds and nanocomposites [13] [20]. These types of materials require deep probing techniques, which X-ray spectroscopy that detects photons can provide. ...
... In the cubic octahedral symmetry, the crystal-field of the Zr 4d orbitals are divided into t2g and eg states containing the dxy, dxz,yz, and dz2, dx2-y2 orbitals, respectively. Cubic systems containing alternating metal-nonmetal elements are known to have metallic states with t2g symmetry (p-bonding) between the metal atoms and covalent (ligand-field) bonding with eg symmetry (s-bonding) between the metal and non-metal atoms [13]. ...
Preprint
Full-text available
The electronic structure, chemical bonding and interface component in ZrN-AlN nanocomposites formed by phase separation during thin film deposition of metastable Zr1-xAlxN (x=0.0, 0.12, 0.26, 0.40) is investigated by resonant inelastic X-ray scattering/X-ray emission and X-ray absorption spectroscopy and compared to first-principles calculations including transitions between orbital angular momentum final states. The experimental spectra are compared with different interface-slab model systems using first-principle all electron full-potential calculations where the core states are treated fully relativistic. As shown in this work, the bulk sensitivity and element selectivity of X-ray spectroscopy enables to probe the symmetry and orbital directions at interfaces between cubic and hexagonal crystals. We show how the electronic structure develop from local octahedral bond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results in three different kinds of bonding originating from semi-coherent interfaces with segregated ZrN and lamellar AlN nanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes place with increasing Al content and affects the bond strength and increases resistivity.
... The sensitivity of X-ray spectroscopy enables us to probe the orbital directional occupations to investigate the bonding structure at the internal interfaces that determines the bond characteristics (symmetry and orbital directions at interfaces between cubic and hexagonal crystals) and local chemistry that affects conductivity and hardness [13]. The cubic-AlN phase that may form has a bandgap of about 6 eV [14] while it is 3 eV for ZrN [15]. ...
... Previous spectroscopic investigations of ZrAlN thin films have utilized surface-sensitive X-ray photoemission spectroscopy (XPS) where electrons were detected at the Zr 3d levels [18] [19]. However, XPS is not ideal for probing the electronic structure and chemical bonding in the bulk of the material's interior and to distinguish the bonding structure between semi-alloys, compounds and nanocomposites [13] [20]. These types of materials require deep probing techniques, which X-ray spectroscopy that detects photons can provide. ...
... In the cubic octahedral symmetry, the crystal-field of the Zr 4d orbitals are divided into t2g and eg states containing the dxy, dxz,yz, and dz2, dx2-y2 orbitals, respectively. Cubic systems containing alternating metal-nonmetal elements are known to have metallic states with t2g symmetry (p-bonding) between the metal atoms and covalent (ligand-field) bonding with eg symmetry (s-bonding) between the metal and non-metal atoms [13]. ...
Article
Full-text available
The electronic structure, chemical bonding, and interface component in ZrN-AlN nanocomposites formed by phase separation during thin film deposition of metastable Zr 1−x Al x N (x = 0.0, 0.12, 0.26, 0.40) are investigated by resonant inelastic x-ray scattering, x-ray emission, and x-ray absorption spectroscopy and compared to first principles calculations including transitions between orbital angular momentum final states. The experimental spectra are compared with different interface-slab model systems using first principles all-electron full-potential calculations where the core states are treated fully relativistically. As shown in this work, the bulk sensitivity and element selectivity of x-ray spectroscopy enables one to probe the symmetry and orbital directions at interfaces between cubic and hexagonal crystals. We show how the electronic structure develops from local octahedral bond symmetry of cubic ZrN that distorts for increasing Al content into more complex bonding. This results in three different kinds of bonding originating from semicoherent interfaces with segregated ZrN and lamellar AlN nanocrystalline precipitates. An increasing chemical shift and charge transfer between the elements takes place with increasing Al content and affects the bond strength and increases resistivity.
... Fig. 3 shows examples of θ-2θ X-ray diffractograms (XRD) from MAX phase films of Ti 3 AlC 2 , Ti 3 SiC 2 , and Ti 3 GeC 2 that are used to extract the lattice constants (a and c) by applying Bragg's law. In all three cases, predominantly Ti 3 AC 2 (000l) reflections are present from the films together with the TiC(III) reflections [22] and α-Al 2 O 3 (000l) substrate peaks (S) indicating strongly-oriented growth [23]. The observed caxis lattice parameters as presented in Fig. 3 correlate well with the reported data of sintered bulk materials [2]. ...
... As the XAS technique requires an intense (and often polarized) X-ray beam in a range of photon energies in the vicinity of a core-level energy (1s, 2p, 3p-shells) that depends on the element of interest, XAS is nowadays performed at synchrotron radiation sources by scanning the photon energy over the absorption edge [34]. The measurements can be performed either in surface-sensitive Total Electron-Yield (TEY) mode or in a more bulk-sensitive Total Fluorescence Yield (TFY) mode [22] at different incidence angles. ...
... The Coster-Kronig decay from the 2p 1/2 core-level to the 2p 3/2 level that precedes the X-ray emission process, not only leads to a higher L 3 /L 2 branching ratio but also to a shorter lifetime and a larger Lorentzian width for the 2p 1/2 core state than for the 2p 3/2 state [44]. The trend in XES branching ratios (L 3 /L 2 or M 3 /M 2 ) in the transition-metal compounds is a signature of the degree of metallicity or ionicity in the systems [22,49]. A lower branching ratio is thus an indication of higher ionicity (resistivity) in the material. ...
Article
Full-text available
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong M\ \C bonds in high-density MC slabs, and relatively weak M\ \A bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity , elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... Fig. 3 shows examples of θ-2θ X-ray diffractograms (XRD) from MAX phase films of Ti 3 AlC 2 , Ti 3 SiC 2 , and Ti 3 GeC 2 that are used to extract the lattice constants (a and c) by applying Bragg's law. In all three cases, predominantly Ti 3 AC 2 (000l) reflections are present from the films together with the TiC(III) reflections [22] and α-Al 2 O 3 (000l) substrate peaks (S) indicating strongly-oriented growth [23]. The observed caxis lattice parameters as presented in Fig. 3 correlate well with the reported data of sintered bulk materials [2]. ...
... As the XAS technique requires an intense (and often polarized) X-ray beam in a range of photon energies in the vicinity of a core-level energy (1s, 2p, 3p-shells) that depends on the element of interest, XAS is nowadays performed at synchrotron radiation sources by scanning the photon energy over the absorption edge [34]. The measurements can be performed either in surface-sensitive Total Electron-Yield (TEY) mode or in a more bulk-sensitive Total Fluorescence Yield (TFY) mode [22] at different incidence angles. ...
... The Coster-Kronig decay from the 2p 1/2 core-level to the 2p 3/2 level that precedes the X-ray emission process, not only leads to a higher L 3 /L 2 branching ratio but also to a shorter lifetime and a larger Lorentzian width for the 2p 1/2 core state than for the 2p 3/2 state [44]. The trend in XES branching ratios (L 3 /L 2 or M 3 /M 2 ) in the transition-metal compounds is a signature of the degree of metallicity or ionicity in the systems [22,49]. A lower branching ratio is thus an indication of higher ionicity (resistivity) in the material. ...
Article
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
... This concludes that the chemical environment of carbon exists in different states. The main peak at the binding energy of 284-285.5 eV is attributed to C-C bonding [12,16,25,26]. In as-deposited TiC (a) film, the peak at 281.4 eV occurs due to C-Ti bonding [12,16,25,26]. ...
... The main peak at the binding energy of 284-285.5 eV is attributed to C-C bonding [12,16,25,26]. In as-deposited TiC (a) film, the peak at 281.4 eV occurs due to C-Ti bonding [12,16,25,26]. This peak is almost disappeared in C þ ion implanted films. ...
... Oxidation of TiC (a) and TiC (c) films are well evident from O1s peak which shows large amount of oxygen and this could be atmospheric one which was introduced during deposition process and film exposure to atmosphere (Fig. 6b). In these films, the region of O1s spectra can be mainly deconvoluted into two components arises from the bulk O 2 oxygen atoms, usually forming TiO 2 phase due to the covalent interaction of O-Ti bonding [26,27]. Shift in oxygen peak is direct evidence of carbon-oxygen interaction. ...
... A less intensity and slightly higher binding energy is assigned to a distorted TiC x phase [39]. The chemical shift in C-H/C-C peak is observed towards less binding energy is attributed to valence charge transfer from carbide phase to a more electronegative carbon matrix phase [40]. The chemical shift is directly related to the presence of carbon matrix phase surrounded by carbide and metal phases. ...
... In this aspect, a large amount of charge transfer from TiC (3d) to a-C (2p) is possible in sample (c) leading to the chemical shift of the peak. Theoretical ab-initio simulations indicate that such a charge-transfer is plausible and in agreement with the soft X-ray studies, indicating that the charge transfer occurs from Ti 3d to C 2p orbitals which is reasonably considering that the Ti 3d orbitals are highest in energy [38,40]. The Ti 3d orbitals in the TiC x are partially overlapping and responsible for the metallic bonding within the carbide phase [41]. ...
... For a larger TiC grain, the total loss of electrons will be negligible because of a less interaction volume with carbon matrix phase. In TiC, the highest occupied electron states are bonding and significant electron deficiency is expected to weaken the Ti-C bond within the carbide phase leading to lattice expansion [40][41][42]. With a decrease in grain-size, the electrondeficiency per Ti atom in the grain increases, and hence gradually weakens the Ti-C bond. ...
... Each subspectrum further split in double sub-peak due to ligand field splitting marked as II and IV in Fig. 4. Such splitting has been observed in several transition metal-carbides [34,[47][48][49]. The spectra show two characteristic change with T s (i) peak become narrow, and (ii) the total integrated intensity of the feature is decreasing as shown in the inset of Fig. 4. ...
... In addition, the shoulder a can be solely due to formation of metal carbide [34,[47][48][49]. The intensity of this shoulder is faint and does not show any significant change. ...
Article
We studied the structural and magnetic properties of Fe0.8C0.2 thin films deposited by co-sputtering of Fe and C targets in a direct current magnetron sputtering (dcMS) process at a substrate temperature (Ts) of 300, 523, and 773 K. The structure and morphology were measured using x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES) at Fe L and C K edges and atomic/magnetic force microscopy (AFM, MFM). An ultrathin (3-nm) Fe0.857C0.2 layer, placed between relatively thick Fe0.8C0.2 layers was used to estimate Fe self-diffusion taking place during growth at different Ts using depth profiling measurements. Such Fe0.857C0.2 layer was also used for Fe57 conversion electron Mössbauer spectroscopy (CEMS) and nuclear resonance scattering (NRS) measurements, yielding the magnetic structure of this ultrathin layer. We found from XRD measurements that the structure formed at low Ts (300 K) is analogous to Fe-based amorphous alloy and at high Ts (773 K), predominantly a Fe3C phase has been formed. Interestingly, at an intermediate Ts (523 K), a clear presence of Fe4C (along with Fe3C and Fe) can be seen from the NRS spectra. The microstructure obtained from AFM images was found to be in agreement with XRD results. MFM results also agree well with NRS as the presence of multi-magnetic components can be clearly seen in the sample grown at Ts = 523 K. The information about the hybridization between Fe and C, obtained from Fe L- and C K-edge XANES also supports the results obtained from other measurements. In essence, from this work, a possibility for experimental realization of Fe4C has been demonstrated. It can be anticipated that by further fine-tuning of the deposition conditions, even single-phase Fe4C can be realized which hitherto remains an experimental challenge.
... Each subspectrum further split in double sub-peak due to ligand field splitting marked as II and IV in Fig. 4. Such splitting has been observed in several transition metal-carbides [34,[47][48][49]. The spectra show two characteristic change with T s (i) peak become narrow, and (ii) the total integrated intensity of the feature is decreasing as shown in the inset of Fig. 4. ...
... In addition, the shoulder a can be solely due to formation of metal carbide [34,[47][48][49]. The intensity of this shoulder is faint and does not show any significant change. ...
Preprint
We studied the structural and magnetic properties of \FeC~thin films deposited by co-sputtering of Fe and C targets in a direct current magnetron sputtering (dcMS) process at a substrate temperature (\Ts) of 300, 523 and 773\,K. The structure and morphology was measured using x-ray diffraction (XRD), x-ray absorption near edge spectroscopy (XANES) at Fe $L$ and C $K$-edges and atomic/magnetic force microscopy (AFM, MFM), respectively. An ultrathin (3\,nm) $^{57}$\FeC~layer, placed between relatively thick \FeC~layers was used to estimate Fe self-diffusion taking place during growth at different \Ts~using depth profiling measurements. Such $^{57}$\FeC~layer was also used for $^{57}$Fe conversion electron M\"{o}ssbauer spectroscopy (CEMS) and nuclear resonance scattering (NRS) measurements, yielding the magnetic structure of this ultrathin layer. We found from XRD measurements that the structure formed at low \Ts~(300\,K) is analogous to Fe-based amorphous alloy and at high \Ts~(773\,K), pre-dominantly a \tifc~phase has been formed. Interestingly, at an intermediate \Ts~(523\,K), a clear presence of \tefc~(along with \tifc~and Fe) can be seen from the NRS spectra. The microstructure obtained from AFM images was found to be in agreement with XRD results. MFM images also agrees well with NRS results as the presence of multi-magnetic components can be clearly seen in the sample grown at \Ts~= 523\,K. The information about the hybridization between Fe and C, obtained from Fe $L$ and C $K$-edges XANES also supports the results obtained from other measurements. In essence, from this work, experimental realization of \tefc~has been demonstrated. It can be anticipated that by further fine-tuning the deposition conditions, even single phase \tefc~phase can be realized which hitherto remains an experimental challenge.
... In cases of materials without configurational disorder, such as, e.g., structurally disordered transition metal carbides, tuneable [20] tribological properties have been demonstrated, as well as functionality for energy storage applications [21]. In many cases such disordered carbides are not amorphous at the atomic scale but have a composite structure of nanocrystalline metal carbide particles embedded in amorphous graphite [22][23][24][25][26]. The addition of weak carbide-forming alloying elements aids in the full amorphization of these materials [22]. ...
Article
High entropy materials (HEMs) are of great interest for their mechanical, chemical and electronic properties. In this paper we analyse (TaNbHfTiZr)C, a carbide type of HEM, both in crystalline and amorphous phases, using density functional theory (DFT). We find that the relaxed lattice volume of the amorphous phase is larger, while its bulk modulus is lower, than that of its crystalline counterpart. Both phases are metallic with all the transition metals contributing similarly to the density of states close to the Fermi level, with Ti and Nb giving the proportionally largest contribution of states. We confirm that despite its great structural complexity, 2 × 2 × 2 supercells are large enough for reliable simulation of the presented mechanical and electronic properties by DFT.
... Exemplarily, the formation of a two-phase nanocomposite structure in such systems is favored due to limited solubility of carbon in the lattice of a transition metal carbide, i.e., TiC, or when a maximum Ti vacancy concentration is exceeded in the TiC lattice. The constitution, chemical bonding, microstructural design and control, as well as the tuning of mechanical and tribological properties of TiC(:H)/a-C(:H) thin films have been systematically studied [9][10][11][12][13][14][15][74][75][76]. It is common understanding that both their friction and wear properties (like mechanical properties in general) are clearly dependent on the Ti:C concentration ratio, the volume fraction of both the nanocrystalline and amorphous phases, the grain size of the carbide phase and the thin films hardness, surface roughness and further characteristics, for example H/E and H 3 /E 2 ratios [1][2][3][4][5][9][10][11][12][13][14][15]. ...
Article
Full-text available
Friction and wear performance of non-reactively magnetron-sputtered hydrogen-free TiC/a-C coatings were characterized under lubricated oscillating sliding conditions against 100Cr6 steel. The friction mediators, isooctane, ethanol and distilled water, were chosen to address the actual trend of environmentally friendly green technologies in mobility and the potential use of carbon-based nanocomposite thin film materials for tribocomponents in contact with gasoline and alternative biofuels. Sliding pairs of the TiC/a-C coatings showed significantly reduced friction and wear compared to the reference materials under both unlubricated and lubricated conditions (when using the aforementioned media isooctane, ethanol and distilled water). Quasi-stationary friction coefficient of the TiC/a-C sliding pairs after running-in was almost independent of test conditions and could be traced back to self-lubrication as a result of the formation of a transfer layer on the steel counter body. Wear of the coatings based on micro-abrasion and tribochemical reaction was significantly influenced by the environmental conditions. Lowest wear was measured after tests in non-polar isooctane whereas highest wear was measured after tests in water.
... The binding energy of C1s can be deconvoluted into three parts. The peak located at 285.1 ± 0.1 eV, 282 ± 0.1 eV and 289.1 ± 0.1 eV is attributed to C-C, Ti-C and O-C=O, respectively, which are the main functional groups on the surface of a-C:H coatings [33]. The Ti-C bond is observed in the XPS spectrum, demonstrating the strong adhesion force between the DLC film and the substrate. ...
Article
Full-text available
The amorphous hydrogenated (a-C:H) film-coated titanium, using different CH4/H2 and deposition times, was prepared by the ion beam deposition (IBD) method, which has the advantage of high adhesion because of the graded interface mixes at the atomic level. The chemical characterizations and corrosion behaviors of a-C:H film were investigated and evaluated by SEM, AFM, Raman spectroscopy, EPMA, TEM and XPS. An a-C:H film-coated titanium was corroded at 0.8 V, 90 °C in a 0.5 mol/L H2SO4 solution for 168 h. The metal ion concentration in the H2SO4 corrosion solution and the potentiodynamic polarization behavior were evaluated. Results indicate that a higher CH4/H2 of 1:0 and a deposition time of 12 h can result in a minimum ID/IG ratio of 0.827, Ra of 5.76 nm, metal ion concentration of 0.34 ppm in the corrosion solution and a corrosion current of 0.23 µA/cm2. The current density in this work meets the DOE’s 2020 target of 1 µA/cm2. Electrical conductivity is inversely proportional to the corrosion resistance. The significant improvement in the corrosion resistance of the a-C:H film was mainly attributed to the increased sp3 element and nanocrystalline TiC phase in the penetration layer. As a result, the a-C:H film-coated titanium at CH4/H2 = 1:0 with improved anti-corrosion behavior creates a great potential for PEMFC bipolar plates.
... The average of these measurements is indicated by a vertical dotted line at 283.5 eV in Fig. 2b. and potential C-W* bonding [40]. In addition, the C 1s spectra show no evidence of C-O bonding in the deposited films as no peak is visible at ~292 eV [34]. ...
Preprint
Full-text available
W-B-C films were deposited on Si(100) substrates held at elevated temperature by reactive sputtering from a W target in Kr/trimethylboron (TMB) plasmas. Quantitative analysis by X-ray photoelectron spectroscopy (XPS) shows that the films are W-rich between ~ 73 and ~ 93 at.% W. The highest metal content is detected in the film deposited with 1 sccm TMB. The C and B concentrations increase with increasing TMB flow to a maximum of ~18 and ~7 at.%, respectively, while the O content remains nearly constant at 2-3 at.%. Chemical bonding structure analysis performed after samples sputter-cleaning reveals C-W and B-W bonding and no detectable W-O bonds. During film growth with 5 sccm TMB and 500 oC or with 10 sccm TMB and 300-600 oC thin film X-ray diffraction shows the formation of cubic 100-oriented WC1-x with a possible solid solution of B. Lower flows and lower growth temperatures favor growth of W and W2C, respectively. Depositions at 700 and 800 oC result in the formation of WSi2 due to a reaction with the substrate. At 900 oC, XPS analysis shows ~96 at.% Si in the film due to Si interdiffusion. Scanning electron microscopy images reveal a fine-grained microstructure for the deposited WC1-x films. Nanoindentation gives hardness values in the range from ~23 to ~31 GPa and reduced elastic moduli between ~220 and 280 GPa in the films deposited at temperatures lower than 600 oC. At higher growth temperatures the hardness decreases by a factor of 3 to 4 following the formation of WSi2 at 700-800 oC and Si-rich surface at 900 oC.
... The average of these measurements is indicated by a vertical dotted line at 283.5 eV in Fig. 2b. and potential C-W* bonding [40]. In addition, the C 1s spectra show no evidence of C-O bonding in the deposited films as no peak is visible at ~292 eV [34]. ...
Article
W-B-C films were deposited on Si(100) substrates held at elevated temperature by reactive sputtering from a W target in Kr/trimethylboron (TMB) plasmas. Quantitative analysis by Xray photoelectron spectroscopy (XPS) shows that the films are W-rich between ~ 73 and ~ 93 at.% W. The highest metal content is detected in the film deposited with 1 sccm TMB. The C and B concentrations increase with increasing TMB flow to a maximum of ~18 and ~7 at.%, respectively, while the O content remains nearly constant at 2-3 at.%. Chemical bonding structure analysis performed after samples sputter-cleaning reveals C-W and B-W bonding and no detectable W-O bonds. During film growth with 5 sccm TMB and 500 oC or with 10 sccm TMB and 300-600 oC thin film X-ray diffraction shows the formation of cubic 100-oriented WC1-x with a possible solid solution of B. Lower flows and lower growth temperatures favor growth of W and W2C, respectively. Depositions at 700 and 800 oC result in the formation of WSi2 due to a reaction with the substrate. At 900 oC, XPS analysis shows ~96 at.% Si in the film due to Si interdiffusion. Scanning electron microscopy images reveal a fine-grained microstructure for the deposited WC1-x films. Nanoindentation gives hardness values in the range from ~23 to ~31 GPa and reduced elastic moduli between ~220 and 280 GPa in the films deposited at temperatures lower than 600 oC. At higher growth temperatures the hardness decreases by a factor of 3 to 4 following the formation of WSi2 at 700-800 oC and Si-rich surface at 900 oC.
... %. In addition, there is a signature of C-C bonds as indicated by a dotted vertical line in Fig. 2(a), bottom panel, at 284.6 eV, 48 most pronounced for the Ti 3 SiC 2 target. In Fig. 2(b) (top panel), the binding energies of the Si 2p peaks are higher in the films seen from 99.85 eV for the film deposited with 300 W sputtering power at RT condition, 100.15 eV for the film deposited with 50 W sputtering power at 850°C, and 100.15 eV for the film deposited with 300 W sputtering power at 850°C compared to that of the Ti 3 SiC 2 compound target at 99.1 eV. ...
Article
We investigate sputtering of a Ti3SiC2 compound target at temperatures ranging from RT (no applied external heating) to 970 oC as well as the influence of the sputtering power at 850 oC for the deposition of Ti3SiC2 films on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight energy elastic recoil detection analysis shows an excess of carbon in all films, which is explained by differences in angular distribution between C, Si and Ti, where C scatters the least during sputtering. The oxygen content is 2.6 at.% in the film deposited at RT and decreases with increasing deposition temperature, showing that higher temperatures favor high purity films. Chemical bonding analysis by X-ray photoelectron spectroscopy shows C-Ti and Si-C bonding in the Ti3SiC2 films and Si-Si bonding in the Ti3SiC2 compound target. X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be deposited from a Ti3SiC2 compound target at substrate temperatures above 850 oC and with growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures. High-resolution scanning transmission electron microscopy shows epitaxial growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 oC. Four-point probe resistivity measurements give values in the range 120 to 450 mucro-Ohm-cm and with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and Ti7Si2C5.
... This gives rise to a change in the spectra weight toward lower R-value in Figure 7. Furthermore, from XPS and ToF-ERDA we note that this sample (x = 0.30) has somewhat higher oxygen content than the other samples (see Figure 4c in ref 13) that might lead to a more columnar microstructure and strain. As the α-Zr grain size becomes smaller, the interface contribution increases, 44 and the relative scattering intensity from the half-diagonal coordination increases their spectral weight. The reason for strain might be an effect of changes in the stoichiometry, given the high flux of sputtered material applied during synthesis. ...
Article
The variation in local atomic structure and chemical bonding of ZrHx (x=0.15, 0.30, 1.16) magnetron sputtered thin films are investigated by Zr K-edge (1s) X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopies. A chemical shift of the Zr K-edge towards higher energy with increasing hydrogen content is observed due to charge-transfer and an ionic or polar covalent bonding component between the Zr 4d and the H 1s states with increasing valency for Zr. We find an increase in the Zr-Zr bond distance with increasing hydrogen content from 3.160 Å in the hexagonal closest-packed metal (α-phase) to 3.395 Å in the understoichiometric -ZrHx film (CaF2-type structure) with x=1.16 that largely resembles that of bulk δ-ZrH2. For yet lower hydrogen contents, the structures are mixed α and δ-phases, while sufficient hydrogen loading (x>1) yields a pure δ-phase that is understoichiometric, but thermodynamically stable. The change in the hydrogen content and strain is discussed in relation to the corresponding change of bond lengths, hybridizations, and trends in electrical resistivity.
... The C1s peak at~284.5 eV is attributed to carbon from surface contaminations [67], the other peak at~282 eVto carbon atoms substituting for oxygen atoms in the TiO 2 lattice and forming O -Ti -C bonds. The spectra of the Ti 1-x Fe x O (2-x/2)-y C y (х ¼ 0.1) samples produced by the annealing of Ti 1-x Fe x (OCH 2 CH 2 O) 2-x/2 precursor in air at 375 (a) and 450 С (b) for 2 h contain two peaks at 284.6 eV and 281.9 eV, which are due to the C-C-H bonds from the surface sorbed hydrocarbons and the Ti -C bonds in the titanium carbide structure [68], respectively. Fig. 9 demonstrates the XPS spectra for the C1s core level of Ti 1-x Fe x O (2-x/2)-y C y samples before and after etching with Ar þ ions. ...
... Mo 2 BC, characteristic of borocarbides [31] and transition-metal carbides [26,[32][33][34][35]. The second one localized at 285.00 eV that corresponds to C non-reacted. ...
Article
The structural, elastic, vibrational, thermodynamic and electronic properties of the Mo2 B intermetallic under pressure are assessed using first-principles calculations based on the generalized gradient approximation (GGA) proposed by Perdew-Wang (PW91). Our results show that the calculated structural parameters at a pressure of zero GPa are in good agreement with the available experimental data. The effect of high pressures on the lattice constants shows that the compression along the c-axis and along the a-axis are similar. The elastic constants were calculated using the static finite strain technique, and the bulk shear moduli are derived from the ideal polycrystalline aggregate. We find that the elastic constants, elastic modulus and hardness monotonically increase as a function of pressure; consequently, the structure is dynamically stable and tends from brittle to ductile behavior under pressure. The Debye temperature θD increases and the so-called Gr neisen constant γ decreases due to stiffening of the crystal structure. The phonon dispersion curves were obtained using the direct method. Additionally, the internal energy (△ E), the Helmholtz free energy (△ F), the entropy (S) and the lattice contribution to the heat capacity Cv were calculated and analyzed with the help of the phonon dispersion curves. The N(EF) and the electron transfer between the B and Mo atoms increase as a function of pressure.
... However, the theoretical C-C bond distances are strikingly different in amorphous carbides (∼1.5 Å) compared to crystalline carbide materials (∼3 Å), which are closer to the second C-C coordination shell. As discussed in a previous work, 20 these results qualitatively agree well with the relative distribution of carbon in the carbidic, CrC x phase and the amorphous carbon phase as reported in ref. 18,19 For comparison, the C-C bond lengths are ∼1.5 Å in a-C, graphite, and diamond. 47−49 The experimentally observed C-C bond lengths (∼2 Å) 20 are larger than the theoretical, but significantly shorter than the bonds for the CrC crystallites in Table 3, which are similar to previous calculations. ...
Article
The local structure and chemical bonding in two-phase amorphous Cr1−xCx nanocomposite thin films are investigated by Cr K-edge (1s) X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies in comparison to theory. By utilizing the computationally efficient stochastic quenching (SQ) technique, we reveal the complexity of different Cr-sites in the transition metal carbides, highlighting the need for large scale averaging to obtain theoretical XANES and EXAFS spectra for comparison with measurements. As shown in this work, it is advantageous to use ab initio theory as an assessment to correctly model and fit experimental spectra and investigate the trends of bond lengths and coordination numbers in complex amorphous materials. With sufficient total carbon content (≥ 30 at%), we find that the short-range coordination in the amorphous carbide phase exhibit similarities to that of a Cr7C3±y structure, while excessive carbons assemble in the amorphous carbon phase.
... The C 1s and P 2p peak contributions were similar to those obtained at pH = 0, although samples pretreated with HF ( Figure 3b) showed also a C 1s peak contribution at B.E. 283.0 eV, which was assigned to the presence of carbons in carbide form, probably Na−C or Ti−C bonds. 39 Therefore, the XPS results obtained pointed out a high relevance of the pH on the covalent bonding of IP6 to Ti, since only the samples prepared at pH 0 showed a P−O−Ti peak contribution on their XPS O 1s high-resolution spectra. As indicated by the presence of P 2p and Na 1s on the surfaces and the high-resolution spectra obtained, at higher pHs the IP6 molecule was adsorbed on the surface, but the covalent bonding of IP6 to the TiO 2 layer did not occur. ...
Article
Myo-inositol hexaphosphate, also called phytic acid or phytate (IP6), is a natural molecule abundant in vegetable seeds and legumes. Among other functions, IP6 inhibits bone resorption. It is adsorbed on the surface of hydroxyapatite, inhibiting its dissolution, and decreasing the progressive loss of bone mass. We present here a method to directly functionalize Ti surfaces covalently with IP6, without using a crosslinker molecule, through the reaction of the phosphate groups of IP6 with the TiO2 layer of Ti substrates. The grafting reaction consisted of an immersion in an IP6 solution to allow the physisorption of the molecules onto the substrate, followed by a heating step to obtain its chemisorption, in an adaptation of the T-Bag method. The reaction was highly dependent on the IP6 solution pH, only achieving a covalent Ti-O-P bond at pH 0. We evaluated two acidic pre-treatments of the Ti surface, to increase its hydroxylic content, HNO3 30% and HF 0.2%. The structure of the coated surfaces was characterized by XPS, ToF-SIMS and ellipsometry. The stability of the IP6 coating after three months of storage and after sterilization with gamma irradiation was also determined. Then, we evaluated the biological effect of Ti-IP6 surfaces in vitro on MC3T3-E1 osteoblastic cells, showing an osteogenic effect. Finally, the effect of the surfaces on the adhesion and biofilm viability of oral microorganisms S. mutans and S. sanguinis was also studied, and found that Ti-IP6 surfaces decreased the adhesion of S. sanguinis. A surface that actively improves osseointegration while decreases bacterial adhesion could be suitable for use in bone implants.
... The Mo 2 BC XPS survey spectrum shows the C 1s core-level, the high-resolution spectrum is shown in Fig. 5. The signal is fitted assuming two components; the first one located at 282.95 eV corresponds to Mo 2 BC, characteristic of borocarbides [31] and transition-metal carbides [26,[32][33][34][35]. The second one localized at 285.00 eV that corresponds to C non-reacted. ...
Article
Full-text available
The electronic structure of the Mo(Formula presented.)BC and Mo(Formula presented.)B compounds was investigated by X-ray photoelectron spectroscopy. The Mo 3d, C 1s, and B 1s core levels are identified. For the Mo(Formula presented.)BC, the core-level binding energies corresponding to Mo 3d(Formula presented.), B 1s, and C 1s are localized at 227.90, 187.94, and 282.95 eV, respectively, whereas for the Mo(Formula presented.)B, the Mo 3d(Formula presented.), and B 1s are localized at 228.09 and 188.06 eV, respectively. Core-level binding energies shifts are observed in both compounds using the charge-potential model. The electronic density of states was calculated for Mo(Formula presented.)B and Mo(Formula presented.)BC using GGA approximation. Our results show that the electronic density of states at the Fermi level in the Mo(Formula presented.)B is higher than that in the Mo(Formula presented.)BC. The dominance of the Mo 4d states down to 8 eV below the Fermi level is found. The calculated total DOS was consistent with the XPS valence band spectra. Finally, within the BCS theory framework, the presence of superconductivity in both compounds can not be explained only as a function of the electronic density of states at the Fermi level. The electron-phonon coupling constant ((Formula presented.)) was calculated using the McMillan equation; the obtained values were 0.75 for Mo(Formula presented.)BC and 0.70 for Mo(Formula presented.)B. These values indicate that both compounds are intermediate coupled superconductors.
... Intensity of the sharp peak at 285.4 eV decreases with increasing atomic-oxygen fluence. On the other hand, intensity of the peak at 289 eV, which was assigned to C-O bonds [39], increases. In other words, the C-C bonds are changed to C-O bonds by irradiation with atomic oxygen. ...
Article
Full-text available
The effect of irradiation by a hyperthermal-atomic-oxygen beam on hydrogenated titanium-doped diamond-like carbon (hydrogenated Ti-DLC) films, applied as a solid lubricant for equipment used in low-earth orbit was investigated. Unlike the film thickness of hydrogenated non-doped DLC films, that of hydrogenated Ti-DLC films was found to be constant after the films were exposed to atomic oxygen. In addition, bulk composition of the hydrogenated Ti-DLC film stayed constant, and in particular, hydrogen content in the film did not decrease. These results indicate that a hydrogenated Ti-DLC film can keep its low friction properties under vacuum. Surface chemical analysis showed that a titanium-oxide layer is form on the film by exposure to atomic oxygen. The thickness of the titanium oxide layer was estimated to be about 5 nm from the element distribution in the depth direction of the hydrogenated Ti-DLC films. The titanium-oxide layer was interpreted to protect the bulk film from erosion by hyperthermal atomic oxygen.
... The XAS and XES measurements were performed at room temperature (300 K) at the undulator beamline I511-3 at MAX II (MAX-IV Laboratory, Lund, Sweden), comprising a 49-pole undulator, and a modified SX-700 plane grating monochromator [21,22]. The measurements were made at a base pressure lower than 6.7*10 −7 Pa. ...
Article
The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr$_{2}$GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr$_{2}$GeC. The Cr $L_{2,3}$, C $K$, and Ge $M_{1}$, $M_{2,3}$ emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge $4s$ states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow $3d$ core levels to the $4s$ valence band explain the large Ge density of states at the Fermi level.
Article
A one-step approach to synthesize ultrafine transition metal particles (size < 5 nm) in carbon substrates is highly desirable for fabricating electrodes for energy devices. Herein, cobalt ion implantation into amorphous carbon films (a:C) and hydrogenated amorphous carbon films (a:CH) was explored, with the aim of synthesizing ultrafine metallic cobalt nanoparticles at room temperature. Co ions of 30 keV energy were implanted into the carbon films to achieve a Co areal density of 1.0 ± 0.1 × 1017 atoms cm-2. Rutherford backscattering measurements revealed that hydrogenated amorphous carbon films gave a broader Co depth distribution compared to the amorphous carbon films. Further, cross-sectional TEM analysis revealed that hydrogenated carbon films suppressed metallic Co nanoparticle aggregation, leading to the creation of ultrafine Co nanoparticles (size < 5 nm). Co L-edge X-ray absorption spectroscopy measurements confirmed the formation of predominantly metallic Co nanoparticles by ion implantation. Results conclusively demonstrate that the presence of hydrogen (~ 28 at.%) in the carbon matrix facilitates the synthesis of ultrafine metallic Co nanoparticles during Co ion implantation.
Article
The films in Ti-B-C system have been deposited onto Si (100) substrates by dual direct current magnetron sputtering of Ti-B and graphite targets. During deposition, the sputtering parameters at the Ti-B target were unchanged, and the carbon content in films was controlled by variation of sputtering current at the graphite target in the range of 0-250 mA. The films were characterized in terms of their structure, composition, and mechanical properties. The X-ray diffraction measurements showed that all the films contain only one TiB2 crystalline phase of prominent (001) and (002) textures. The film hardness first increased with the increased carbon content due to the reduced grain size reaching maximum value of about 51.5 GPa for films deposited with 200 mA sputtering current, and then it is started to decrease due to the increased content of a softer amorphous phase. The friction coefficient steadily decreased with increasing film hardness. First-principles calculations together with the experimental results have made it possible to assume that the deposited Ti-B-C films consist of nanocrystals of solid solution TiB2-xCx surrounded by amorphous B-C-O phase.
Article
Titanium carbide (TiC)-based electrodes are attractive in supercapacitor due to their ultra-high density and pseudocapacitive charge storage mechanism. However, TiC films with horizontal alignment of flakes or random nanostructures limit the high rate of charge transfer and hinder the migration of ions to redox active sites. In this work, the TiC nanotubes and three-dimensional interconnected nanoflakes are synthesized by electrodeposition and carbothermal treatment of carbon nanotube (CNT) film and graphite, respectively. To study the capacitance mechanism of TiC nanotube-interconnected branch (NTIB) films, the in-situ Raman spectrums of the TiC-NTIB negative electrode during the charge/discharge processes in H2SO4 show that hydronium is bonded to the terminal O during discharge, and debonding occurs during charging. The integrated TiC NTIB electrode is capable of operating at rates faster than that of carbon, conductive polymers or transition metal oxides, but still delivers a specific capacitance of 273 F g⁻¹ at 10 A g⁻¹ after repeating 2000 cycles at current densities of 1, 3, 5 and 10 A g⁻¹. The symmetric supercapacitor composed of the TiC NTIB electrodes delivers an energy density of 64.4 Wh kg⁻¹ (at 892.3 W kg⁻¹) and a power density of 9.5 kW kg⁻¹ (at 55.6 Wh kg⁻¹), and a good cycle stability (≈86.7% retention after 15000 cycles).
Article
The prismatic and octahedral type layered cathode materials can provide enhanced performance in sodium‐ion batteries, due to low potential barrier during the intercalation. Among them, the prismatic layered Na‐Ni‐Mn‐O system is the most preferable electrode with a high theoretical capacity of 173 mAhg⁻¹. The P2‐type phase could be attained only by satisfying the conditions of alkali, and transition metal ratio 0.5 ≤ x ≤ 1. The P2‐type layered structure in the Na‐Ni‐Mn‐O system has been studied via optimizing the alkali and transition metals in two phases. Here, the vacuum assisted solid‐state preparation method was carried out to avoid NiMnO4 impurities. From diffraction analysis and refinement data, the structural changes were analyzed, then the optimal ratio for perfect P2‐structure has been confirmed. The perfect P2‐structure was obtained only for the samples Na0.66Ni0.3Mn0.7O2, Na0.66Ni0.33Mn0.67O2. The prepared materials showed the initial discharge capacity of 194 mAhg⁻¹ at 0.1C. Highlights • Prismatic type layered Na‐Ni‐Mn‐O electrodes synthesized via simple solid‐state reaction. • The prepared electrode delivered initial capacity of 194 mAh g⁻¹ at 0.1 C. • Mn richness maintains the low potential barrier for Na‐ion during cycling. • This enhances the electrochemical performances of the material. • Increment of Nickel concentration enhance the unite cell volume.
Article
Full-text available
Single‐crystalline {100} faceted TiC is of great significance in theory to a large number of engineering applications owing to its extraordinary catalytic properties. However, the {111} planes are prevalent in conventional TiC powders given their favorable thermodynamic stability during the initial low stoichiometric growth stage. Herein, a disproportionation–decomposition strategy is developed to directly produce Ti and C atoms to synthesize single‐crystalline {100} faceted TiC powders. Outstanding electrochemical performance of TiC {100} crystal planes in terms of the hydrogen evolution reaction is evidenced by an overpotential of 392 mV at 10 mA cm−2, which is 52% lower than that of randomly faceted TiC counterparts (815 mV). Single‐crystalline {100} faceted TiC nanocubes, which theoretically have extraordinary catalytic properties, are successfully synthesized. The formation of the single‐crystalline {100} faceted TiC can be ascribed to the atomic stacking process that avoids the presence of {111} planes. The single‐crystalline {100} faceted TiC displays outstanding performance in hydrogen evolution reaction, which is comparable to the emerging pristine MXene.
Article
Alkaline fuel cells are considered as promising energy conversion devices for future application, and oxygen reduction reaction (ORR) catalyst is one of the critical factors in determining fuel cells' performance. However, the application of conventional Pt-based catalysts is limited by its high cost and insufficient reserve. Here we synthesized TiC/C nanofibers as ORR catalyst through electrospinning. The morphology, TiC content, and interface between TiC and carbon in TiC/C nanofibers could be adjusted by changing preparation conditions. The changes in material properties affect the catalytic performance due to the synergistic enhanced effect between TiC and carbon. The best TiC/C sample (TCNFs/C) exhibited the ORR performance with the onset potential, peak potential, peak current density, and charge transfer number of 0.941 V vs. RHE, 0.812 V vs. RHE, 0.742 mA cm⁻², and 3.29, respectively, as well as good catalytic durability and excellent methanol tolerance. The results demonstrate the great research value and application potential of TiC/C nanofibers as ORR catalysts.
Chapter
Similarly to other transition metals of group 4 – hafnium and zirconium, titanium forms with carbon, practically, the only one chemical compound (see also section C – Ti in Table I-2.13) – titanium monocarbide TiC1–x, having the broadest homogeneity range compared to all other refractory carbides of groups 4-5 [1-14]; although some low-temperature (< 600-1100 °C) ordered and metastable structures such as Ti2±xC (Fd(–3)m, R(–3)m, P3121, Pnma, P3m1, Pbcn, I41/amd, P4/mmm), Ti8C5±x (R(–3)m), Ti3C2±x (C2/m, I4/mmm, Immm, P(–3)m1, C2221), Ti4C3±x(P(–3)m1, C2/c, Pm(–3)m, I4/mmm), Ti5C4±x (P(–1), C2/m, I4/m), Ti6C5±x (C2/m, C2/c, P3112, P31, C2), Ti7C6±x (R(–3)) and Ti8C7±x (P4332, Fm(–3)m) [1, 14-41, 157, 445-450, 3297] and various molecular clusters Ti8C12 (Ti2C3), Ti8C13, Ti13C22 (TiC~2), C60Tix, C70Tix, including endohedral Ti2@C80, (Ti2C2)@C78, Ti@C28, C@Ti8C12, C2@Ti8C12 and nanocrystal Ti14C13, Ti17C19, Ti22C35, are also described in literature [42-57, 451-461, 491], they have not become attractive for any technical applications and often – not confirmed sufficiently.
Article
The paper presents an interaction of Ti/Ti-oxides with: Si(100), HOPG(0001) and graphene/4H-SiC(0001) substrates. A thin layer (~3 nm) of Ti was deposited by means of DC sputtering technique on all the considered substrates. XPS, AFM and Raman spectroscopy were applied to find out the differences in interaction of Ti/Ti-oxides with selected substrates under UHV annealing. In the case of Si(100), substrate, apart from the expected TiO2, the presence of TiSi2 and TiSiOx components and the easiest reduction towards Ti2O3 was observed under UHV annealing. The sample exhibited also substantial evolution of surface morphology without the change of surface roughness which was attributed to formation of TiSi2 and TiSiOx components. This was in contrast with HOPG substrate where an annealing-induced agglomeration of the deposited material increased the surface roughness substantially. However, in the case of Ti/Ti-oxides deposited on graphene/4H-SiC(0001), UHV annealing caused no noticeable change of surface morphology. Agglomerates were not formed which was attributed to degradation of graphene (confirmed by Raman spectra) and supressing van der Waals' interactions responsible for easy surface diffusion. The differences in morphology were also discussed in the context of the surface energy of selected substrates.
Article
Full-text available
A fast and large scalable process to produce TiC powder was obtained using a horizontal rotary ball mill within 60 min via a mechanically induced self-propagating reaction (MSR). The as-synthesized TiC particles have well-defined crystals structure and the median diameter of ∼1 μ m with less impurity due to short duration of milling. The mechanism for the acceleration of MSR was investigated in detail. The graphene nano-platelets are exfoliated from graphite by ultra-high-energy of the horizontal rotary ball mill after less than 30 min of milling, which plays a crucial role in formation of leaf-like nanoplatelets with Ti lamella by ball collision. At the same time, the ultra-high-energy of ball milling can boost self-prorogating reaction in a short duration of milling.
Article
The chemical interaction between Ti and graphene is of significant interest for engineering low resistance electrical contacts. To study interface chemistry, sequential depositions of Ti are performed on both as-received and UHV-annealed CVD-grown graphene samples. In-situ X-ray photoelectron spectroscopy (XPS) reveals no experimental evidence for the reaction of Ti with graphene at room temperature or after heating to 500 °C. The presence of the TiC chemical state is instead attributed to reactions between Ti and background gases in the UHV chamber as well as adventitious carbon on the surface of the graphene sample. We find that surface contamination can be substantially reduced by annealing in UHV. The deposition of Ti on graphene results in n-type doping which manifests in core-level shifts and broadening of the graphene C 1s peak. Annealing the sample following the deposition of Ti reverses the n-type doping. Raman spectroscopy results are in agreement with XPS analysis, which together provide insight into the possible mechanisms driving the changes in graphene doping.
Article
Hard multilayer coatings are technologically promising materials for reducing wear of tribological parts. Multilayer coatings with a systematic alternation of the pair [(TiC x /Ti/C)÷(a-C)] were deposited on stainless and tool steel by the PVD technique. Hardness (H), elasticity modulus (E) and critical cracking load (P cr ) were determined by the nanoindentation method. Nanofrictional wear test was conducted under multipass sliding of a diamond indenter (Ø 50 nm) under constant load. The specific coefficient of nanofrictional wear of [(TiC x /Ti/C)÷(a-C)] n with different composition of titanium-containing layers was determined. The nanofrictional wear rate of [(TiC x /Ti/C)÷a-C] n depends on the elastic and plastic characteristics of multilayer coating as a whole. Coatings having H ³ /E ² > 0.12 and P cr > 58 mN demonstrate low wear rate.
Article
Hydrogen free Ti-doped amorphous carbon layers were prepared by dual beam pulsed laser deposition using two excimer lasers. The air-exposed surfaces were analyzed by high-energy resolved and angular-resolved core-level photoelectron spectroscopy, and were then step-by-step sputtered with an argon gas cluster ion beam (ArGCIB), which is known to be a very gentle technique with respect to changes in surface chemistry. The results show that the top surface of the sample and its sub-surface region differ in composition and in bonding. The top surface is enriched by oxygen-bearing species. Carbon-bearing species located on the surface are mostly in sp³ hybridization. Titanium carbide clusters, TiC, are not directly exposed at the surface. They are embedded in a carbon network with dominating C sp² hybridization. Their interface is formed by a distinct carbon-rich titanium carbide with stoichiometry close to TiC3. The surface damage induced by ArGCIB was found to be minimal, verifiably affecting carbon atoms in the carbon network.
Article
Full-text available
Nanocomposites (CNTi) with different mass ratios of carbon nitride (C3N4) and TiO2 nanoparticles were prepared hydrothermally. Different characterization techniques were used including X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), transmission electron spectroscopy (TEM) and Brunauer-Emmett-Teller (BET). UV-Vis DRS demonstrated that the CNTi nanocomposites exhibited absorption in the visible light range. A sun light-simulated photoexcitation source was used to study the kinetics of phenol degradation and its intermediates in presence of the as-prepared nanocomposite photocatalysts. These results were compared with studies when TiO2 nanoparticles were used in the presence and absence of H2O2 and/or O3. The photodegradation of phenol was evaluated spectrophotometrically and using the total organic carbon (TOC) measurements. It was observed that the photocatalytic activity of the CNTi nanocomposites was significantly higher than that of TiO2 nanoparticles. Additionally, spectrophotometry and TOC analyses confirmed that degraded phenol was completely mineralized to CO2 and H2O with the use of CNTi nanocomposites, which was not the case for TiO2 where several intermediates were formed. Furthermore, when H2O2 and O3 were simultaneously present, the 0.1% g-C3N4/TiO2 nanocomposite showed the highest phenol degradation rate and the degradation percentage was greater than 91.4% within 30 min.
Article
We report how the total carbon content and the amorphous carbon (a-C) phase fraction in transition metal carbide/a-C nanocomposite coatings can be obtained using optical methods, which are much more practical for industrial use than conventional X-ray photoelectron spectroscopy. A large set of carbon-containing nanocomposite coatings deposited using different magnetron sputtering techniques were analyzed by X-ray photoelectron spectroscopy, reflectance spectrophotometry, and spectroscopic ellipsometry. The chemical composition and the a-C phase fraction were determined by X-ray photoelectron spectroscopy for each coating and results are presented for the TiC, CrC, and NbC systems. The composition and the a-C phase fraction are correlated to optical reflectance in the visible range, by parametrization in L*a*b* color space, and by ellipsometry primary data. Results show that it is possible to rapidly estimate the composition and the a-C fraction using these optical methods. We propose that optical methods have promising use in the industry as a cost-efficient technique for characterization of carbide-based coatings.
Article
The TiSiC coating was deposited on Ti6Al4V substrate material by multi-arc ion plating. Microstructures and mechanical properties of the TiSiC coating were characterized by SEM, XRD, XPS and nanoindentation. The tribological behaviors of the samples were studied in atmosphere, deionized water and seawater by tribometer. The experimental results showed that the coating exhibited a typical amorphous/nanocrystalline structure. The hardness of the TiSiC coating was up to 28 GPa, which was almost 5 times higher than that of the substrate material. The friction coefficients of the TiSiC coating decreased by 2.3%, 25.6%, 27.0%, respectively, compared with the substrate material in atmosphere, deionized water and seawater environments. The wear rates of the TiSiC coating in atmosphere, deionized water and seawater environments were about 1.72 × 10⁻⁵mm³N⁻¹m⁻¹ 8.61 × 10⁻ mm³ N⁻¹ m⁻¹,6.78 × 10⁻⁶ mm³ N⁻¹ m⁻¹ respectively, which significantly lower than that of the substrate materials, this mainly due to the high hardness and the lubrication of graphite structure sp²C-C phase in the coating.
Article
Full-text available
V–Al–C–N hard coatings with high carbon content were deposited by reactive radio-frequency magnetron sputtering using an experimental combinatorial approach, deposition from a segmented sputter target. The compositiondependent coexisting phases within the coating were analysed using the complementary methods of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS). For the analysis of the X-ray absorption near-edge spectra, a new approach for evaluation of the pre-edge peak was developed, taking into account the selfabsorption effects in thin films. Within the studied composition range, a mixed face-centred cubic (V,Al)(C,N) phase coexisting with a C–C-containing phase was observed. No indication of hexagonal (V,Al)(N,C) was found. The example of V–Al–C–N demonstrates how important a combination of complementary methods is for the detection of coexisting phases in complex multi-element coatings.
Article
The mechanism of mechanically induced self-propagating reaction (MSR) of titanium with graphite to produce TiC during reactive ball milling of elemental powders was investigated using high resolution electron microscopy, and Raman and X-ray photoelectron spectroscopies. Prior to the exothermic ignition, off-stoichiometric Ti(1 + x)C(1 − x) nuclei are formed by reaction at the interface between the severely deformed Ti particles and graphite milling debris. After ignition, both rapid growth of existing nuclei, and nucleation and growth of additional off-stoichiometric Ti(1 + x)C(1 − x) occurs. Local product morphologies after the ignition varied, depending local restrictions on the rate of diffusion of carbon into the TiC as it grows.
Article
Ti-Si-B-C-N film was deposited by DC magnetron sputtering at different argon and nitrogen ratios such as N2/Ar=1:5, 2:4, 3:3, 4:1 and 5:0. The formation of TiN and TiB phases was observed because of incorporation of nitrogen. The hardness, modulus, microstructure, structure and bond formation with different nitrogen contents during the deposition were studied by nanoindentation, scanning electron microscope, X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The oxidation kinetics of Ti-Si-B-C-N was investigated. The nitrogen incorporation during deposition influences different properties of the coating. Hardness and modulus decreased, and microstructure showed very fine grain presence, and film changes to fully amorphous because of incorporation of nitrogen in the film.
Article
In this paper, we report a new, low-cost, and facile solvothermal approach to synthesize visible-light-active S-doped TiO2 (S-TiO2) by using dimethyl sulfoxide (DMSO) as both the S source and the solvent. Energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) solidly confirmed the presence of S element in the final product. The as-prepared S-TiO2 nanoparticles exhibited excellent and long-term stable photocatalytic performance for the degradation of organic pollutants under visible and indoor sunlight illumination. The catalyst still maintained high photoactivity even after several months of exposure to the indoor sunlight irradiation. This result suggests a new approach to achieve stable and highly efficient solar light driven photocatalysts for water purification.
Article
Full-text available
Abstract Vanadium-doped and vanadium- and carbon-doped nanostructured anatase with threedimensional architecture of aggregates has been synthesized by the developed precursor method. Glycolate Ti1-xVx(OCH2CH2O)2 (0.01 ≤ x ≤ 0.05) was used as a precursor. The obtained samples of the compositions Ti1-xVxO2, Ti1-xVxO2-yCy and Ti1-xVxO2-yCy·nC were characterized by X-ray diffraction, scanning electron microscopy, absorption spectroscopy in ultraviolet and visible regions and by X-ray photoelectron spectroscopy methods. It was established that the synthesized materials exhibit photocatalytic activity in the oxidation reaction in aqueous solution of hydroquinone under ultraviolet and visible light irradiation. First principle calculations of the electronic band structure and optical absorption of vanadium- and carbon-doped anatase have been performed. The calculations showed that doping gives rise to the impurity states inside the band gap, which are responsible for the appearance of absorption, and consequently, visible light photocatalytic activity.
Article
Full-text available
Deposition of titanium carbide (TiC) thin film on 304 stainless steel was investigated by the plasma enhanced chemical vapor deposition (PECVD) method using a 5% C2H2 + 95% Ar gas mixture as a reactant gases and carbon source with titanium plate as a target. Physical and mechanical properties of the synthesized TiC thin film on 304 stainless steel were studied to determine its potential application as a protective layer for first wall of Tokamaks. X-ray diffraction (XRD) showed that the crystalline structures of TiC (111 and 220) for sample without annealing and TiC (111, 220 and 311) for sample with annealing at 1000 • C for 1 h, was formed. The XPS Ti 2p spectrum of sample shows two peaks related to the binding energies of 455.3 eV (the photoelectron peaks of Ti 2p 3/2) and 460.9 eV (the photoelec-tron peaks of Ti 2p 1/2). Also, the C 1s spectra with the binding energy of 281.3 eV was seen in XPS analysis. TiC film of about 10.9 μm thickness was observed on scanning electron microscopy (SEM) image. The morphological feature like statistical parameters, the motifs analysis and the watershed segmentation of the thin film 3D surface of TiC films were investigated by the AFM analysis and MountainsMap Premium 7.2 (64-bit version) and Gwyddion softwares. Also, the effect of annealing on Minkowski functionals of TiC samples were studied. At the end, we can conclude that TiC NPs synthesized in this work by PECVD method have uniformly deposition. All results of AFM analysis provide general insight of nanostructures and properties of these thin films and can be used to achieve to a new topography model of the thin film 3D surface.
Article
The aim of this study was to investigate the effect of Zr as alloying element to carbon films, particularly in respect to film structure and mechanical properties. The films were deposited by magnetron sputtering in reactive (Ar + CH4) and non-reactive (Ar) atmosphere with different Zr contents (from 0 to 14 at.%) in order to achieve a nanocomposite based films. With an increase of Zr content a broad peak was observed in X-ray diffraction spectra suggesting the presence of nanocrystalline (nc) ZrC phase for the coatings with Zr content higher than 4 at.%. The application of Scherrer formula yielded a grain sizes with a dimension of 1.0-2.2 nm. These results were supported by X-ray photoelectron spectroscopy showing typical charge transfer at Zr C nanograins and carbon matrix interface. The nc-ZrC phase was also observed by transmission electron microscopy. The hardness of the coatings was approximately independent of Zr content. However, the Young modulus increased linearly. The residual stress of the coatings was strongly improved by the presence of nc-ZrC phase embedded in the a-C matrix. Finally, the incorporation of H into the matrix led to denser and harder films.
Article
C-doped TiO2 nanoplates (CTNP) with exposed {001} facets were successfully synthesized by hydrothermal treatment of TiC powder in a HF–HNO3 mixed aqueous solution for the first time. The effects of hydrothermal temperature (140, 160, 180 and 200 °C) on the crystal phase, morphology, specific surface area and porous properties, surface element composition, optical response properties of the resultant samples were investigated in detail, and the photocatalytic activities of these obtained CTNP samples for the degradation of methylene blue (MB) were evaluated under visible light irradiation. The results showed that precursor cubic TiC was transformed into anatase TiO2 with the morphology of well-defined nanoplate completely after hydrothermal treatment for 30 h. The specific surface area of CTNP was significantly improved in comparison with that of micrometer sized C-doped TiO2 plates (CTP) due to smaller particle size. Due to C-doping, CTNP presented red-shift absorption edge, which supported it with strong visible-light response. All these factors led to as-prepared CTNP having more excellent visible-light photocatalytic activity in comparison with micrometer sized CTP. The CTNP synthesized at 180 °C presented the optimal visible-light photocatalytic activity with a high reaction rate constant (0.03692 min−1) among the four CTNP samples. Based on the results of the present study, a reasonable mechanism of photocatalysis on CTNP under visible light was proposed.
Article
To investigate the influence of temperature on nucleation and growth of Ti3SiC2, Ti–Si–C thin films were deposited by magnetron sputtering from elemental targets of Ti, Si and C on Si(100) and Al2O3 substrates at temperature <200°C. Subsequently, the as deposited films were annealed in vacuum at 800, 950, 1100 and 1200°C respectively. The as deposited films consisted of amorphous TiC, amorphous Si and free C, as determined by X-ray diffraction and X-ray photoelectron spectroscopy. Annealing in vacuum <950°C resulted in improved crystallinity of TiC and formation of SiCx and Ti5Si3 phases. However, the Ti3SiC2 phase forms in films at 1100°C owing to the increase in Si diffusion coefficient. Moreover, the evolution of hardness and elastics modulus with annealing temperatures was determined by nanoindentation. The results showed a continuous decline of film hardness with increasing annealing temperature due to the formation of Ti3SiC2 and Ti5Si3 phases.
Article
Titanium carbide (TiC) is an electrically conducting refractory interstitial compound possessing several unique properties. A cost-effective, efficient and non-Pt electrocatalyst based on TiC is explored and the multi-functionality of TiC towards various electrochemical reactions that are of significant interest in low temperature fuel cells is studied. Ameliorated activities towards oxygen reduction reaction (ORR) and borohydride oxidation are observed with TiC-carbon composites. High sensitivity and selectivity towards ORR have been demonstrated with very good methanol tolerance. The charge transfer interactions between TiC and carbon seem to play a vital role in the improved activity as compared to their individual counterparts. The present study opens up a way to realize completely Pt-free borohydride fuel cell architecture.
Article
Nanocomposite coatings consisting of Ag and TiCx (x < 1) crystallites in a matrix of amorphous SiC were deposited by high-rate magnetron sputtering from Ti–Si–C–Ag compound targets. Different target compositions were used to achieve coatings with a Si content of ∼13 at.%, while varying the C/Ti ratio and Ag content. Electron microscopy, helium ion microscopy, X-ray photoelectron spectroscopy and X-ray diffraction were employed to trace Ag segregation during deposition and possible decomposition of amorphous SiC. Eutectic interaction between Ag and Si is observed, and the Ag forms threading grains which coarsen with increased coating thickness. The coatings can be tailored for conductivity horizontally or vertically by controlling the shape and distribution of the Ag precipitates. Coatings were fabricated with hardness in the range 10–18 GPa and resistivity in the range 77–142 μΩ cm.
Article
Full-text available
The same method used to determine {sigma}{sup *} excitations in the x-ray-absorption spectra of organic molecules is applied to analyze the structure of the {sigma}{sup *} band in the x-ray absorption spectra of amorphous carbon (a-C) films. The analysis assumes that only a {sigma} bond interaction to first neighbors is relevant to explain the structure of the {sigma}{sup *} band in a-C films. This is justified by the local character of the x-ray absorption probe and the short- range order existing in these films. The identification of the different {sigma}{sup *} components is based on the dependence of the {sigma}{sup *} binding energy with bond distance. The {sigma}{sup *} band is built up by summing the components resulting from the possible different types of {sigma} bonds in a-C. This method serves to separate the {pi}{sup *} states from the {sigma}{sup *} states and to identify the kind of chemical bonds existing between carbon atoms. This analysis yields a proportion of sp{sup 3}-bonded atoms of 60% in a film with a density of about 2.9 g/cm{sup 3}, which is a value closer to what is expected from theoretical calculations. The analysis identifies a component at about 288.5 eV which is associated to strained {sigma} bonds of the type of the existing in sp{sup 3} bonded rings like cyclopropene. Raman and photoemission spectroscopies help in the interpretation of the x-ray absorption spectra and the assignment of the {sigma}{sup *} components.
Article
Full-text available
The electronic structure and chemical bonding of the nitrides and carbides of Ti, V, Zr, and Nb are studied. The augmented plane wave method is used and results are discussed in terms of energy bands and density of states. (AIP)
Article
Full-text available
Nanocrystalline TiC/amorphous carbon (a-C) composite films were synthesized at near room temperature with a hybrid process combining laser ablation of graphite and magnetron sputtering of titanium. Film microstructure was investigated by x-ray photoelectron spectroscopy, x-ray diffraction analyses, and transmission electron microscopy. Mechanical properties were evaluated from nanoindentation, scratch, and friction tests. The films consisted of 10 nm sized TiC crystallites encapsulated in a sp3 bonded a-C matrix. They had a hardness of about 32 GPa and a remarkable plasticity (40% in indentation deformation) at loads exceeding their elastic limit. They were also found to have a high scratch toughness in addition to a low (about 0.2) friction coefficient. The combination of hardness and ductility was correlated with film phase composition and structural analyses, using concepts of nanocomposite mechanics. The properties of the TiC/a-C composites make them beneficial for surface wear and friction protection. © 1997 American Institute of Physics.
Article
Full-text available
Carbon K-edge NEXAFS and XPS measurements were performed on a-C and 7.5 at% a-C:Ti films annealed in-situ up to 1300 K. During annealing of a-C, the sp2 content increased from ~80 to ~95 %, associated with the development of a peak at 285.4 eV in the NEXAFS spectrum, which is assigned to sp2 carbon atoms in a 'graphite-like' local order. After annealing of the a-C:Ti film to 470 K, an additional peak shows up, indicating the creation of carbonyl functionalities. This is explained by the increased reactivity of the carbon phase by Ti-doping. Oxygen contents of ~30 at% are determined by XPS up to 900 K. For a-C at temperatures >420 K, the C 1s peak width (FWHM) decreased continuously with annealing temperature, which is not the case for a-C:Ti. This is explained by oxidised carbon and by Ti diffusion during carbide crystallite formation, partly disturbing the ordering of the carbon phase.
Article
Full-text available
The electronic structures of epitaxially grown films of Ti(3)AlC(2), Ti(3)SiC(2), and Ti(3)GeC(2) have been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured high-resolution Ti L, C K, Al L, Si L, and Ge M emission spectra are compared with ab initio density-functional theory including core-to-valence dipole matrix elements. A qualitative agreement between experiment and theory is obtained. A weak covalent Ti-Al bond is manifested by a pronounced shoulder in the Ti L emission of Ti(3)AlC(2). As Al is replaced with Si or Ge, the shoulder disappears. For the buried Al and Si layers, strongly hybridized spectral shapes are detected in Ti(3)AlC(2) and Ti(3)SiC(2), respectively. As a result of relaxation of the crystal structure and the increased charge-transfer from Ti to C, the Ti-C bonding is strengthened. The differences between the electronic structures are discussed in relation to the bonding in the nanolaminates and the corresponding change of materials properties.
Article
Full-text available
Carbon and C:V, C:Co, C:Cu nanocomposite films grown by ion beam cosputtering in the temperature range from room temperature (RT) to 500 degrees C are investigated. Soft X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) have been used to determine electronic structure of the occupied and unoccupied electronic states of the coexisting carbon and transition metal (TM) constituents. The results from the spectroscopy are. supplemented by the film composition data and TM inclusion phase structural information obtained by elastic recoil detection analysis and X-ray diffraction, respectively. The TM(2p) XAS shows that V (Cu) is in carbidic (metallic) state over the whole temperature range, while Co shows a transition from a carbidic toward a metallic state when the growth temperature increases from RT to 500 degrees C. The C(1s) XAS demonstrates that the increase in the growth temperature favors the formation of graphite-like structures in carbon films. On the other hand, the TM metal incorporation strongly promotes the sp(3) admixture in the surrounding carbon phase which manifests itself through a significant increase in the intensity of a feature in the C(1s) XAS spectra positioned at similar to 291 eV resulting from 1s ->sigma* transitions. In addition, the codeposition of TM atoms with carbon enhances the formation of carbon structures with the prominent peak between pi* and sigma* regions in the C(1s) XAS spectra positioned at similar to 288.5 eV. The effect is independent of the TM tendency to form carbides or TM state (carbidic metallic) while its magnitude increases concomitantly with the TM content and decreases when the crystallinity degree of the inclusion phase increases. The results are discussed on the basis of the nanoparticle imposed curvature on the surrounding carbon network and interactions at the atomic level at the C-TM interfaces.
Article
Full-text available
Advances in laser-assisted deposition have enabled the production of hard composites consisting of nanocrystalline and amorphous materials. Deposition conditions were selected to produce super-tough coatings, where controlled formation of dislocations, nanocracks and microcracks was permitted as stresses exceeded the elastic limit. This produced a self-adjustment in the composite deformation from hard elastic to quasiplastic, depending on the applied stress, which provided coating compliance and eliminated catastrophic failure typical of hard and brittle materials. The load-adaptive concept was used to design super-tough coatings consisting of nanocrystalline (10–50 nm) TiC grains embedded in an amorphous carbon matrix (about 30 vol%). They were deposited at near room temperature on steel surfaces and studied using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, Raman spectroscopy, nanoindentation and scratch tests. Design concepts were verified using composition–structure–property investigations in the TiC–amorphous carbon (a-C) system. A fourfold increase in the toughness of hard (32 GPa) TiC–a-C composites was achieved in comparison with nanocrystalline single-phase TiC.
Article
Full-text available
The non-metal K-absorption edge of TiCx (1.0 ≥ x ≥ 0.6) TiNx (1.0 ≥ x ≥ 0.8) and VNx (1.0 ≥ x ≥ 0.8) have been measured by high resolution electron-energy-loss spectroscopy. The spectra are interpreted in terms of existing band-structure calculations. Changes of the unoccupied density of states due to non-metal vacancies are observed.
Article
Full-text available
Integral breadth methods for line profile analysis are reviewed, including modifications of the Williamson-Hall method recently proposed for the specific case of dislocation strain broadening. Two cases of study, supported by the results of a TEM investigation, are considered in detail: nanocrystalline ceria crystallized from amorphous precursors and highly deformed nickel powder produced by extensive ball milling. A further application concerns a series of Fe-Mo powder specimens that were ball milled for increasing time. Traditional and modified Williamson-Hall methods confirm their merits for a rapid overview of the line broadening effects and possible understanding of the main causes. However, quantitative results are generally not reliable. Limits in the applicability of integral breadth methods and reliability of the results are discussed in detail.
Article
Full-text available
We measure 2p --> 3d --> 2p resonant X-ray emission spectra (RXES) of d(0) system (ScF3 and TiO2) and analyze them by means of MX6 (M=Sc or Ti, X=F or O, respectively) cluster model with full multiplet effects. We treat the whole RXES process as a coherent second order optical process and take the polarization dependence into account. The strong polarization dependence of the inelastic peak is clearly seen both in the experimental and calculated results, In order to clarity it, we investigate the detailed mechanism of the polarization dependent RXES by the total energy level diagram and the group theoretical consideration.
Article
Metal containing a-C:H films are prepared by a combined PVD-CVD process. The electrical conductivity of these films is measured as a function of metal content and temperature. The varying conductivity behavior for films containing noble metals (Au and Cu) on the one hand and carbide forming metals (Nb and Ta) on the other is correlated to their microstructure.
Article
TiCx films with a wide range of C/Ti ratios have been prepared by plasma based ion implantation. The bonding states and structure were investigated as functions of the relative carbon content to titanium. The results of Rutherford backscattering spectroscopy showed that there was more than 20 at.% hydrogen contained in the films. By the deconvolution of X-ray photoelectron core level spectra, the excess carbon was suggested to lead to the formation of interstitial carbon, amorphous hydrogenated carbon (or graphite), polymer-like carbon, and organic compound of titanium. The cross-sectional transmission electron microscopy (XTEM) displayed that the TiC0.81 films mainly consisted of rod-shaped TiC crystal arranged along the growth direction, while the TiC1.55 films contained a relatively random microstructure. XTEM also provided evidence for the existence of amorphous carbon in carbon-rich films, not graphite.
Article
Titanium carbide thin films show attractive mechanical properties for engineering applications. Thin films of TiC were deposited on a 〈100〉 silicon substrate by RF sputtering from a TiC target. Various sputtering pressures were carried out in order to observe the influence of this parameter on structural and mechanical properties. The sputtering pressure was varied from 0.35 to 1 Pa at a sputtering power of 300 W. Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD) and atomic force microscopy (AFM) were used to characterize TiC thin films. Hardness was obtained by nanoindentation. Residual stress was determined by radius of curvature measurements. Lower pressures induce the formation of a distorted titanium carbide and a dense structure. In correlation to the lower pressure, large residual stress was measured and changed the TiC texture in XRD results. Both the compressive stress and the hardness exhibited a maximum value at a process pressure using pure argon at 0.35 Pa with a pressure of 1 Pa necessary to obtain TiC films with 〈111〉 texture.
Article
Information about the local structure of amorphous carbon (a-C) films was obtained by comparing soft x-ray absorption spectra in the CK region of sputtered a-C films to reference carbon compounds such as hydrofullerene (C60H36), highly oriented pyrolytic graphite (HOPG), and carbon black. The spectra of the a-C films exhibited fine structures, which consisted of at least different six portions. Comparing the spectral features of the a-C films with the reference compounds and the spectral analysis by discrete variational (DV)-Xα molecular orbital calculations indicated that the fine structures of the a-C films are due to a hybrid of sp2 and sp3 carbon atoms.
Article
The electronic structure of the nanolaminated transition metal carbide Ti2AlC has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Ti L, C K, and Al L emission spectra are compared with calculated spectra using ab initio density-functional theory including dipole matrix elements. The detailed investigation of the electronic structure and chemical bonding provides increased understanding of the physical properties of this type of nanolaminates. Three different types of bond regions are identified: The relatively weak Ti 3d–Al 3p bond 1 eV below the Fermi level and the Ti 3d–C 2p and Ti 3d–C 2s bonds which are stronger and deeper in energy are observed around 2.5 and 10 eV below the Fermi level, respectively. A strongly modified spectral shape of the 3s final states in comparison to pure Al is detected for the intercalated Al monolayers indirectly reflecting the Ti 3d–Al 3p hybridization. The differences between the electronic and crystal structures of Ti2AlC, Ti3AlC2, and TiC are discussed in relation to the number of Al layers per Ti layer in the two former systems and the corresponding change of the unusual materials properties.
Article
Extensive density-functional calculations are performed for chemisorption of atoms in the three first periods (H, B, C, N, O, F, Al, Si, P, S, and Cl) on the polar TiC(111) surface. Calculations are also performed for O on TiC(001), for full O(1×1) monolayer on TiC(111), as well as for bulk TiC and for the clean TiC(111) and (001) surfaces. Detailed results concerning atomic structures, energetics, and electronic structures are presented. For the bulk and the clean surfaces, previous results are confirmed. In addition, detailed results are given on the presence of C-C bonds in the bulk and at the surface, as well as on the presence of a Ti-based surface resonance (TiSR) at the Fermi level and of C-based surface resonances (CSR’s) in the lower part of the surface upper valence band. For the adsorption, adsorption energies Eads and relaxed geometries are presented, showing great variations characterized by pyramid-shaped Eads trends within each period. An extraordinarily strong chemisorption is found for the O atom, 8.8 eV/adatom. On the basis of the calculated electronic structures, a concerted-coupling model for the chemisorption is proposed, in which two different types of adatom-substrate interactions work together to provide the obtained strong chemisorption: (i) adatom-TiSR and (ii) adatom-CSR’s. This model is used to successfully describe the essential features of the calculated Eads trends. The fundamental nature of this model, based on the Newns-Anderson model, should make it apt for general application to transition-metal carbides and nitrides and for predictive purposes in technological applications, such as cutting-tool multilayer coatings and MAX phases.
Article
Thin film nanocomposites of nanocrystalline TiC embedded in a matrix of amorphous carbon have been prepared by nonreactive unbalanced dc-magnetron sputtering. These samples have been tested as coating materials for electrical contacts and show great potential as an alternative to traditional metallic coatings for contacts subjected to wear and friction. Through variation of composition and deposition temperature different microstructures have been attained. The coatings have been characterized using x-ray diffraction, x-ray photoelectron spectroscopy, and Raman spectroscopy. The performance of the coatings has been coupled to the microstructure whereby tuning and optimization possibilities have been identified.
Article
Amorphous and nanostructured carbon films were grown by using two different techniques: ion sputtering and cluster beam deposition. The films were studied by near-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy. Depending on the precursors, atoms, or clusters, the films are characterized by a different sp2/sp3 ratio which influences the mechanical and the electronic properties. Due to the sensitivities of NEXAFS (local order) and Raman (medium-range order), we have characterized and compared the structure of the films over different length scales. The complementarity of NEXAFS and Raman techniques for the characterization of disordered forms of carbon is here presented and discussed. We also present an original method of NEXAFS spectra calibration allowing a better determination of peak positions.
Article
Soft X-ray emission and absorption spectra in the C K region of amorphous carbon films systematically deposited by RF, ion-beam, and ECR sputtering under various deposition conditions were measured using highly brilliant synchrotron radiation. A broad main peak and a high-energy shoulder were observed in the emission spectra, and a fine structure consisting of at least five peaks was observed in the absorption spectra. By analogy to the occupied/unoccupied C2p-DOS obtained by DV-Xα molecular orbital calculations of simple cluster models composed of sp2 and sp3 carbon atoms, we approximately explained the spectral features in the measured X-ray emission spectra by σ and π bonds and estimated the fine structures in the absorption spectra by the hybridized unoccupied molecular orbitals formed by the local-structures composed of sp2 and sp3 carbon atoms.
Article
In this study, nanocomposite nc-TiC/a-C coatings were deposited on polished stainless steel substrates and silicon wafers by co-sputtering of graphite and titanium targets at a low temperature of 150 8C. Atomic force microscopy (AFM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to investigate the morphology and structure of the coatings. The hardness and tribological properties were assessed using Nanoindentation and Tribometer. The coatings consisted of two phases: nanocrystalline of TiC embedded in amorphous matrix of a-C. With Ti content of less than 8 at.%, the coating appeared X-ray amorphous. As Ti increased to 16 at.%, nanocrystalline phase (nc-TiC) was detected. The crystallite size was in the range of 5 to 16 nm as Ti increased from 16 to 48 at.%. The maximum hardness of 31 GPa was obtained at 36 at.%Ti. In dry tribotests against stainless steel ball, the coefficient of friction was less than 0.24 as Ti was less than 36 at.%, but abruptly increased to 0.39 as Ti reached 42 at.%. Extremely low coefficient of friction of 0.046 was obtained with oil lubrication. D 2004 Elsevier B.V. All rights reserved.
Article
ARXPS (angle resolved X-ray photoelectron spectroscopy) measurements are used to obtain informations about surfaces and grain boundaries. Data acquired from nanocrystalline carbidic hard coatings have been employed to establish structural models. Magnetron-sputtered coatings of TiC, SiC and TiC/SiC were examined. In such coatings, partly defective TiC nanocrystallites are surrounded by interfacial carbide. This excess carbon shows a binding state similar to that of doped graphite or fullerenes. X-ray amorphous SiC is found in the residue. On top of sputtered SiC coatings, less oxide and graphite is found as compared to TiC/SiC or TiC coatings.
Article
TiC, SiC and Ti0.5Si0.5C layers have been deposited by magnetron sputtering in Argon at bias voltages between 0 and 1500 V. AES and ARXPS analyses show that TiC and Ti0.5Si0.5C, at bias voltages below 1000 V, are C-rich (+20%) and contain TiC crystallites of diameter below about 10 nm and have a metal-like resistance of about mcm. The excess C segregates to the surface of TiC nanocrystallites showing an XPS C 1s level shift similar to Li-graphite or doped fullerenes. The doped carbon (carbidic) interface layer, higher deposition rate and better mechanical strength seem to be interrelated. Magnetron sputtered SiC is X-ray amorphous and insulating, grows more slowly, has reduced mechanical strength and does not contain excess C. The ARXPS analysis of Ti0.5Si0.5C layers allows the modelling of the TiC nanocrystallites embedded in interfacial carbon and defective SiC.
Article
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
Article
The applications of resonant soft X-ray emission spectroscopy on a variety of carbon systems have yielded characteristic fingerprints. With high resolution monochromatized synchrotron radiation excitation, resonant inelastic X-ray scattering has emerged as a new source of information about electronic structure and excitation dynamics. The selectivity of this excitation, in terms of energy and polarization, has also facilitated studies of emission anisotropy. Various features observed in resonant emission spectra have been identified and studied. In the ordered wide-band systems (e.g., diamond and graphite), momentum conservation of resonant inelastic X-ray scattering has been observed, and the aspect of using this for studies of band dispersion is pointed out. In the molecule-like systems, such as C60, C70, benzene and derivatives, conjugated polymers, symmetry selection rules have been established, and deviations from the symmetry selection rules observed have been interpreted in terms of dynamical symmetry breaking. In the studies of π-conjugated polymers, it has been found that the benzene-ring has a strong identity as a building block in some of the conjugated polymers. Isomeric dependence was able to be clarified in the studies of polypyridine. The doping induced protonation effect has been observed as the depletion of the upmost occupied valence band in the doped polyaniline.
Article
X-ray photoelectron spectroscopy (XPS) spectra of sputter-etched nc-TiC/a-C nanocomposite thin films published in literature show an extra feature of unknown origin in the C1s region. This feature is situated between the contributions of carbide and the carbon matrix. We have used high kinetic energy XPS (HIKE-XPS) on magnetron-sputtered nc-TiC/a-C thin films to show that this feature represents a third chemical environment in the nanocomposites, besides the carbide and the amorphous carbon. Our results show that component is present in as-deposited samples, and that the intensity is strongly enhanced by Ar+-ion etching. This third chemical environment may be due to interface or disorder effects. The implications of these observations on the XPS analysis of nanocomposites are discussed in the light of overlap problems for ternary carbon based systems.
Article
The new undulator beamline I511 at MAX-lab, now under commissioning, has been optimized for X-ray emission and photoelectron spectroscopies. Using an SX-700 high flux monochromator the accessible photon energy range is from 90 eV to about 1500 eV. The performance of the undulator agrees very well with the specifications, as shown by measurements using a photodiode. The energy resolution of the monochromator has been checked using absorption measurements in a gas cell. It was found to meet the expectations and exceeds a resolving power of 10 000 at 244 eV. The photon flux as a function of energy has been recorded as well and gives a maximum flux of 3×1013 photons/s/100 mA/0.1% BW. Beamlines I511 and I411 will be the first synchrotron beamlines making use of a so-called beam waist phenomenon, known from laser physics. We show results of ray-tracing calculations to determine the ultimate spot size on the sample location. The endstations to be used at this new beamline and their capabilities will be discussed as an example of the future use of this facility.
Article
The objective of nanocomposite coatings combining hard and lubricant phases is the development of advanced multi-functional protective thin films showing abrasion resistance, and simultaneously, low friction. Up to now, no clear relation between constitution, microstructural properties and performance of such nanocomposite coatings based on dry lubricants like carbon or MoS2 has been evaluated. Deposition techniques, constitution, properties and performance of magnetron-sputtered nanocomposite coatings in the TiCC system are presented. The Vickers hardness could be optimized to values of polycrystalline TiC thin films, and at the same time, low friction coefficients against steel, similar to diamond-like amorphous carbon, could be realized. The mechanical properties and the tribological behavior of these thin films are related to the chemical composition and the microstructure of these advanced materials, characterized by electron microprobe analysis, Auger electron spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy.
Article
X-ray photoelectron spectroscopy (XPS) is a standard method of determining chemical bonding in e.g. nanocomposites. We demonstrate that sputter-cleaning of the sample prior to analysis can substantially alter the attained information. We present an in-depth analysis of sputter damage on binary and ternary TiC-based coatings in the Ti–Ni–C system. XPS was performed after sputter etching with different ion energies (0.15–4 keV). Results are compared to data from the bulk of undamaged samples attained using high kinetic energy XPS. We observe substantial sputter damage, strongly dependent on sputter energies and coating stability. Metastable samples exhibit severe sputter damage after etching with 4 keV. Additional samples from other Ti–Me–C (Me = Al, Fe, Cu or Pt) systems were also examined, and notable sputter damage was observed. This suggests that accurate analysis of any metastable nanocomposite requires careful consideration of sputter damages.
Article
The experimental X-ray emission spectra of titanium carbide, nitride and oxide have been obtained. Quantum-chemical calculations of the electronic structure of clusters in TiC, TiN and TiO have been carried out by the semiempirical Mulliken-Wolfsberg-Helmholtz method with self-consistency on charges and configurations. The results of these calculations are in good agreement with the X-ray spectroscopy data and offer a reasonable explanation of the experimental spectra. Chemical bonding and electronic structure of the compounds are discussed. Ionicity is shown to increase from TiC to TiO according to the electronegativity principle, the calculated charges on the metal ions being close to experimental estimates. The role of metal-metal and metal-nonmetal interactions in the chemical bonding is analysed. Vacancy models for TiO and their effect on the X-ray emission spectra are investigated. By the CNDO method with configurational interactions the optical spectrum of titanium carbide has been calculated. It is shown that this spectrum may be interpreted from the results for the [TiC6] cluster, without introducing the Lye-Logothetis band scheme with negative charge on the metal ion.
Article
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Article
Titanium carbide/amorphous-carbon (TiC/a-C:H) nanocomposite coatings deposited by pulsed unbalanced reactive magnetron sputtering have been investigated in terms of structure, chemical and phase composition by AFM, TEM, XPS and XRD analyses. Subject to total carbon content, metallic titanium, titanium carbide and amorphous-carbon phases were found in the deposited coatings, which contributed to the observed microstructures and morphologies. The specific resistivity of nanocomposite coatings scales up with increasing amount of matrix-forming carbon. Hardness profiles of the different compositions revealed that nearly stoichiometric TiC films with average crystallite size of 70 nm exhibit the maximum hardness, whereas the lowest friction coefficient (μ<0.1) was found in films rich in amorphous-carbon and containing smaller TiC nanocrystallites (〈d〉 ∼ 10 nm).
Article
Thin films of titanium carbide and amorphous hydrogenated carbon at various compositions have been deposited by unbalanced reactive magnetron sputtering from a metallic titanium target in the presence of argon and acetylene. XRD probed the presence of nanocrystalline TiC and, at high titanium concentrations, of metallic titanium. The XPS examinations allowed one to determine the amount of TiC produced at any concentration of titanium. Raman spectroscopy proved the presence of a-C:H up to 38 at.% of titanium. The coatings have a pronounced hardness maximum of 35 GPa at a composition of approximately 80% TiC and 20% a-C:H. The hardness at 60% TiC and 40% a-C:H as well as that of 100% TiC does not exceed 18 GPa. The mean separation of the crystallites, whose diameter is approximately 4 nm, amounts to a few atomic distances. At the maximum hardness a coefficient of friction of 0.25–0.3 is obtained. The coatings thus provide, at the optimum composition, high hardness at low friction.
Article
X-ray fluorescence spectra of Ti Kα for various titanium compounds are reported. Peak shifts, linewidths, intensity ratios (), and line spacings between kα1 and kα2 due to changes in chemical state are reported. The chemical shifts and line spacings are calculated by an atom Hartree-Fock-Slater method and compared with the experimental results. The kα2 linewidth of TiB2 is the narrowest of all the compounds. The origin of the linewidth modifications is clarified by quantum chemical calculations using the discrete variational (DV) Hartree-Fock-Slater (Xa) method with cluster molecular-orbital approximation.