Electronic structure and chemical bonding of amorphous chromium carbide thin films (original) (raw)
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Electronic Structure and Chemical Bonding of morphous Chromium Carbide Thin Films
The microstructure, electronic structure, and chemical bonding of chromium carbide thin films with different carbon contents have been investigated with high-resolution transmission electron microscopy, electron energy loss spectroscopy and soft x-ray absorption-emission spectroscopies. Most of the films can be described as amorphous nanocomposites with non-crystalline CrC x in an amorphous carbon matrix. At high carbon contents, graphene-like structures are formed in the amorphous carbon matrix. At 47 at% carbon content, randomly oriented nanocrystallites are formed creating a complex microstructure of three components. The soft x-ray absorption-emission study shows additional peak structures exhibiting non-octahedral coordination and bonding.
The local structure and chemical bonding in two-phase amorphous Cr 1−x C x 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 Cr 7 C 3 ± y structure, while excessive carbons assemble in the amorphous carbon phase.
Structure and bonding in amorphous iron carbide thin films
We investigate the amorphous structure, chemical bonding, and electrical properties of magnetron sputtered Fe1−xCx (0.21 < x < 0.72) thin films. X-ray, electron diffraction and transmission electron microscopy show that the Fe1−xCx films are amorphous nanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeCy , and a carbon-rich matrix. Pair distribution function analysis indicates a close-range order similar to those of crystalline Fe3C carbides in all films with additional graphene-like structures at high carbon content (71.8 at% C). From x-ray photoelectron spectroscopy measurements, we find that the amorphous carbidic phase has a composition of 15–25 at% carbon that slightly increases with total carbon content. X-ray absorption spectra exhibit an increasing number of unoccupied 3d states and a decreasing number of C 2p states as a function of carbon content. These changes signify a systematic redistribution in orbital occupation due to charge-transfer effects at the domain-size-dependent carbide/matrix interfaces. The four-point probe resistivity of the Fe1−xCx films increases exponentially with carbon content from ∼200μOhmcm (x = 0.21) to ∼1200μOhmcm (x = 0.72), and is found to depend on the total carbon content rather than the composition of the carbide. Our findings open new possibilities for modifying the resistivity of amorphous thin film coatings based on transition metal carbides through the control of amorphous domain structures.
First-principles study of structural, elastic, and electronic properties of chromium carbides
Applied Physics Letters, 2008
The structural, elastic and electronic properties of Ti 2 InC and Ti 2 InN compounds have been calculated using the full-potential linear muffin-tin orbital (FP-LMTO) method. The exchange and correlation potential is treated by the local density approximation (LDA). The calculated ground state properties, including, lattice constants, internal parameters, bulk modulus and the pressure derivative of the bulk modulus are in reasonable agreement with the available data. The effect of pressure, up to 40 GPa, on the lattice constants and the internal parameters is also investigated. Using the total energy-strain technique, we have determined the elastic constants C ij , which have not been measured yet. The band structure and the density of states (DOS) show that both materials have a metallic character and Ti 2 InN is more conducting than Ti 2 InC. The analysis of the site and momentum projected densities shows that the bonding is achieved through a hybridization of Ti-atom d states with C (N)-atom p states. Otherwise, it has been shown that TiC and TiN bonds are stronger than Ti-In bonds.
The crystal structure and chemical bonding of magnetron-sputtering deposited nickel carbide Ni1−xCx (0.05≤x≤0.62) thin films have been investigated by high-resolution x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, and soft x-ray absorption spectroscopy. By using x-ray as well as electron diffraction, we found carbon-containing hcp-Ni (hcp-NiCy phase), instead of the expected rhombohedral-Ni3C. At low carbon content (4.9 at%), the thin film consists of hcp-NiCy nanocrystallites mixed with a smaller amount of fcc-NiCx . The average grain size is about 10–20 nm. With the increase of carbon content to 16.3 at%, the film contains single-phase hcp-NiCy nanocrystallites with expanded lattice parameters. With a further increase of carbon content to 38 at%, and 62 at%, the films transform to x-ray amorphous materials with hcp-NiCy and fcc-NiCx nanodomain structures in an amorphous carbon-rich matrix. Raman spectra of carbon indicate dominant sp2 hybridization, consistent with photoelectron spectra that show a decreasing amount of C–Ni phase with increasing carbon content. The Ni 3d–C 2p hybridization in the hexagonal structure gives rise to the salient double-peak structure in Ni 2p soft x-ray absorption spectra at 16.3 at% that changes with carbon content. We also show that the resistivity is not only governed by the amount of carbon, but increases by more than a factor of two when the samples transform from crystalline to amorphous.
Gadau Journal of Pure and Allied Sciences
Transition metal carbides are recently considered to be potential substitutes to platinum counter electrode due to their low cost, high catalytic activity and good thermal stability. In this paper, we investigate the structural, electronics, and optical properties of one of the transition metal carbides (Cr3C). The structural and electronic properties were computed using the first principles approach, a generalized gradient approximation (PBE-GGA) as exchange correlation functional within density functional theory (DFT) was used for the calculations. The optimized lattice parameters of the compound were = 4.525 Ǻ, = 5.186 Ǻ, = 6.659 Ǻ and ∝ = β = γ = 90.00which are in agreement with number of theoretical and experimental results. The calculated band gap and density of state (DOS) of this compound reveals the presence of Valence Band Maximum (VBM) and Conduction Band Minimum (CBM) at different symmetry point with the intersection of several energy bands crossing the Fermi level, th...
Journal of The European Ceramic Society, 2011
Nanosized chromium carbide has been prepared by metal–organic chemical vapour deposition (MOCVD) method in a fluidized bed and carburized in the mixture of CH4/H2 atmosphere in temperature range 700–850 °C. The carburization process involves carbon deposition on the outer surface of the Cr2O3 powder, followed by carbon diffusion into the powder, leading to formation of metastable Cr3C2−x phase and stable Cr3C2. The phase transformation from Cr2O3 to Cr3C2 via an intermediate state Cr3C2−x has been identified using electron-energy loss spectroscopy (EELS) and micro-Raman spectroscopy. We could hypothesize that the formation of carbon nanofilms surrounding the carbide crystallites provides the stress and assist the phase transformation from metastable Cr3C2−x to stable Cr3C2.
Effect of the bias voltage on the structure of nc-CrC/a-C:H coatings with high carbon content
Nanocomposite coatings consisting of a hard nanocrystalline carbide phase and a-C:H amorphous matrix are the focus of many investigations because of their mechanical and tribological properties such as high hardness, low friction coefficient and high resistance to wear. In this work, nanocomposite coatings of nanocrystalline chromium carbide embedded in an amorphous matrix (nc-CrC/a-C:H) were deposited onto silicon substrates by cathodic vacuum arc deposition using a Cr target in an Ar/C 2 H 2 gas mixture atmosphere. A linear magnetic shield was employed to reduce the macroparticle content in the films. A range of negative bias voltages from 50 to 450 V was applied to substrates during deposition. X-ray diffraction (XRD) analysis showed that amorphous or nanocrystalline thin films were formed in all cases. The hydrogen bonding in the material was studied by Fourier transform infrared spectroscopy (FTIR). X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and electron energy loss spectroscopy (EELS) were used to determine the carbon bonding and to study the presence of different forms of amorphous carbon. High resolution transmission electron microscopy (HRTEM) revealed a nanocomposite structure with chromium carbide nanocrystallites embedded in an amorphous hydrocarbon matrix. It was observed that the negative bias voltage significantly affected the carbon bonding and the hydrogen content.