tThe chemical bonding in the carbide core and the surface chemistry in a new group of transition-metalcarbides Tin+1Cn-Tx(n = 1,2) called MXenes have been investigated by surface-sensitive valence bandX-ray photoelectron spectroscopy. Changes in band structures of stacked nano sheets of different thick-nesses are analyzed in connection to known hybridization regions of TiC and TiO2that affect elastic andtransport properties. By employing high excitation energy, the photoelectron cross-section for the C 2s– Ti 3d hybridization region at the bottom of the valence band is enhanced. As shown in this work, theO 2p and F 2p bands strongly depend both on the bond lengths to the surface groups and the adsorptionsites. The effect of surface oxidation and Ar+sputtering on the electronic structure is also discussed.

We present a study on the high-Tc superconductor (HTSC) YBa2Cu3O7-x(YBCO) using polarization-dependent X-ray absorption and resonant inelastic X-ray scattering. High-resolution measurementsusing synchrotron-radiation are compared with calculations using a quasi-atomic multiplet approachperformed at the Cu 2p3/2-edge of YBCO. We use a multiplet approach within the single impurity Ander-son model to reproduce and understand the character of the localized low-energy excitations in YBCO.We observe a distinct peak at about 0.5 eV in O K RIXS. This peak shows dependence on doping, incidentenergy, and momentum transfer that suggests that it has a different origin than the previously discussedcuprate bimagnons. Therefore, we assign it to bimagnon excitations within the Zhang Rice bands and/orthe Upper Hubbard bands, respectively.

The variation in local atomic structure and chemical bonding of ZrH_x (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 (a-phase) to 3.395 Å in the understoichiometric d-ZrH_x film (CaF₂-type structure) with x=1.16 that largely resembles that of bulk d-ZrH₂. For yet lower hydrogen contents, the structures are mixed a- and d-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 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.

We investigate the amorphous structure, chemical bonding, and electrical properties ofmagnetron sputtered Fe1−xCx (0.21 < x < 0.72) thin films. X-ray, electron diffraction andtransmission electron microscopy show that the Fe1−xCx films are amorphousnanocomposites, consisting of a two-phase domain structure with Fe-rich carbidic FeCy , and acarbon-rich matrix. Pair distribution function analysis indicates a close-range order similar tothose of crystalline Fe3C carbides in all films with additional graphene-like structures at highcarbon content (71.8 at% C). From x-ray photoelectron spectroscopy measurements, we findthat the amorphous carbidic phase has a composition of 15–25 at% carbon that slightlyincreases with total carbon content. X-ray absorption spectra exhibit an increasing number ofunoccupied 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-transfereffects at the domain-size-dependent carbide/matrix interfaces. The four-point proberesistivity of the Fe1−xCx films increases exponentially with carbon content from ∼200μcm(x = 0.21) to ∼1200μcm (x = 0.72), and is found to depend on the total carbon contentrather than the composition of the carbide. Our findings open new possibilities for modifyingthe resistivity of amorphous thin film coatings based on transition metal carbides through thecontrol of amorphous domain structures.

The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr2GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal di erences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr2GeC. The Cr L2;3, C K, and Ge M1, M2;3 emission spectra are interpreted with rst-principles density-functional theory (DFT) including core-tovalence 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 a ect the thermal expansion coecient 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.

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 about10–20 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 thatthe 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.

The interplay between the quasi 1-dimensional CuO-chains and the 2-dimensional CuO2 planes ofYBa2Cu3O61x (YBCO) has been in focus for a long time. Although the CuO-chains are known to beimportant as charge reservoirs that enable superconductivity for a range of oxygen doping levels in YBCO,the understanding of the dynamics of its temperature-driven metal-superconductor transition (MST)remains a challenge. We present a combined study using x-ray absorption spectroscopy and resonantinelastic x-ray scattering (RIXS) revealing how a reconstruction of the apical O(4)-derived interplanarorbitals during theMSTof optimally dopedYBCOleads to substantial hole-transfer from the chains into theplanes, i.e. self-doping. Our ionic model calculations show that localized divalent charge-transferconfigurations are expected to be abundant in the chains of YBCO. While these indeed appear in the RIXSspectra from YBCO in the normal, metallic, state, they are largely suppressed in the superconducting stateand, instead, signatures of Cu trivalent charge-transfer configurations in the planes become enhanced. Inthe quest for understanding the fundamental mechanism for high-Tc-superconductivity (HTSC) inperovskite cuprate materials, the observation of such an interplanar self-doping process in YBCO opens aunique novel channel for studying the dynamics of HTSC.

The nitrogen 1s near edge X-ray absorption fine structure (NEXAFS) of gallium nitride (GaN) shows astrong natural linear dichroism that arises from its anisotropic wurtzite structure. An additional spectro-scopic variation arises from lattice strain in epitaxially grown GaN thin films. This variation is directlyproportional to the degree of strain for some spectroscopic features. This strain variation is interpretedwith the aid of density functional theory calculations.

Herein, we report X-ray Photoelectron Spectroscopy (XPS) analysis for cold pressed exfoliated 2D nanocrystals of transition metal carbides, MXenes. MXenes are a recently discovered family of 2D materials produced by selective chemical etching of the A element from MAX phases which are ternary metal carbides and nitrides. The latter has the formula of M_n+1AX_n, where M is an early transition metal, A is an A-group element, and X is C and/or N. This study is a comparison between two MXenes, Ti₃C₂T_x and Ti₂CT_x, where T_x stands for surface termination groups such as –O, –OH, and –F. Ti₃C₂T_x and Ti₂CT_x were prepared by immersion of Ti₃AlC₂ and Ti₂AlC powders in 50% conc. HF. A thorough XPS analysis was performed through peak fitting of high resolution XPS spectra and valence band, VB, spectra analysis. The effect of Ar sputtering as well as the number of layers n was the primarily interest of this study. According to the peak fitting analysis, both phases contain the following species, Ti–C, C–C, Ti–F, Ti–O and Ti–OH resulting in the following chemical formulas: Ti₃C₂(OH)_x(O)_y(F)_z and Ti₂C(OH)_x(O)_y(F)_z. Comparing the VB spectra with the DOS calculations show the valance band spectra is actually a mixture of MXene with various terminations of OH, O and F. Before Ar⁺ sputtering both phases show a large percentage of fluorinated-TiO₂ which is due to MXene surface oxidation as well as CH_x, C-O and COO groups arising from either surface contaminations or due to drying the etched powders in ethanol after washing the powder of the HF acid. According to the VB spectra, it is shown that the fluorinated TiO₂ is actually a mixture of anatase and rutile. The number of layers, _n, also plays a role; the lower _n, the more the MXene is prone to oxidation.