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Register a new userElectronic structure and chemical bonding in Ti4SiC3 investigated by soft x-ray emission spectroscopy and first principle theory
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The electronic structure in the new transition metal carbide Ti4SiC3 has been investigated by bulk-sensitive soft x-ray emission spectroscopy and compared to the well-studied Ti3SiC2 and TiC systems. The measured high-resolution Ti L, C K and Si L x-ray emission spectra are discussed with ab initio calculations based on density-functional theory including core-to-valence dipole matrix elements. The detailed investigations of the Ti-C and Ti-Si chemical bonds provide increased understanding of the physical properties of these nanolaminates. A strongly modified spectral shape is detected for the buried Si monolayers due to Si 3p hybridization with the Ti 3d orbitals. As a result of relaxation of the crystal structure and the charge-transfer from Ti (and Si) to C, the strength of the Ti-C covalent bond is increased. The differences between the electronic and crystal structures of Ti4SiC3 and Ti3SiC2 are discussed in relation to the number of Si layers per Ti layer in the two systems and the corresponding change of materials properties.
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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 hybridization 1 eV below the Fermi level, and the Ti 3d - C 2p and Ti 3d - C 2s hybridizations which are stronger and deeper in energy are observed around 2.5 eV 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 buried 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.
The electronic structures of epitaxially grown films of Ti3AlC2, Ti3SiC2 and Ti3GeC2 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 Ti3AlC2. 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 Ti3AlC2 and Ti3SiC2, 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.
The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallisation scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high temperature events or its behaviour when exposed to air. Here, a combined soft and hard X-ray photoelectron spectroscopy (XPS) characterisation approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behaviour of a 300~nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the precursor titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft X-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO$_2$, WO$_3$ and a WO$_3$ oxide coordinated with a titanium environment. This combinatorial characterisation approach provides valuable insights into the thermal and oxidation stability of TiW alloys from two depth perspectives, aiding the development of future device technologies.
Chemical interaction and changes in local electronic structure of Cr, Fe, Co, Ni and Cu transition metals (TMs) upon formation of an $Al_{8}Co_{17}Cr_{17}Cu_{8}Fe_{17}Ni_{33}$ compositionally complex alloy (CCA) have been studied by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. It was found that upon CCA formation, occupancy of the Cr, Co and Ni 3d states changes and the maximum of the occupied and empty Ni 3d states density shifts away from Fermi level ($E_f$) by 0.5 and 0.6 eV, respectively, whereas the Cr 3d empty states maximum shifts towards $E_f$ by 0.3 eV, compared to the corresponding pure metals. The absence of significant charge transfer between the elements was established, pointing to the balancing of the 3d states occupancy change by involvement of delocalized 4s and 4p states into the charge redistribution. Despite the expected formation of strong Al-TMs covalent bonds, the Al role in the transformation of the TMs 3d electronic states is negligible. The work demonstrates a decisive role of Cr in the Ni local electronic structure transformation and suggests formation of directional Ni-Cr bonds with covalent character. These findings can be helpful for tuning deformation properties and phase stability of the CCA.
Motivated by the recent synthesis of Ba$_2$CuO$_{3+delta}$ (BCO), a high temperature superconducting cuprate with putative $d_{3z^2-r^2}$ ground state symmetry, we investigated its electronic structure by means of Cu $L_3$ x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge on a polycrystalline sample. We show that the XAS profile of BCO is characterised by two peaks associated to inequivalent Cu sites, and that its RIXS response features a single, sharp peak associated to crystal-field excitations. We argue that these observations are only partially compatible with the previously proposed crystal structure of BCO. Based on our spectroscopic results and on previously published powder diffraction measurements, we propose a crystalline structure characterized by two inequivalent Cu sites located at alternated planes along the $c$ axis: nominally trivalent Cu(1) belonging to very short Cu-O chains, and divalent Cu(2) in the oxygen deficient CuO$_ {1.5}$ planes. We also analyze the low-energy region of the RIXS spectra to estimate the magnitude of the magnetic interactions in BCO and find that in-plane nearest neighbor superexchange exceeds 120~meV, similarly to that of other layered cuprates. Although these results do not support the pure $d_{3z^2-r^2}$ ground state scenario, they hint at a significant departure from the common quasi-2D electronic structure of superconducting cuprates of pure $d_{x^2-y^2}$ symmetry.
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