Experimental and theoretical study of the electronic structure of Fe, Co, and Ni aluminides with the B2 structure

Carbon K edge spectra of functionalized molybdenum-based MXenes, Mo2CT2 (T = F, OH and O), from first-principles calculations

In this study, the effect of the termination groups (T = F, OH, O) on the energy loss near edge structure (ELNES) of carbon K edge in Mo2C MXene at orientation-independent conditions has been investigated using first-principles calculations based on the full-potential linearized augmented plane wave (FP-LAPW) method. The results show that within the YS-PBE0 functional, the Mo2CF2 is a semiconductor with an indirect band gap of 0.723 eV. For Mo2CO2, the indirect band gap increases to 0.17 eV within the screened hybrid functional. The calculation results of ELNES spectra with the affection of core-hole show that, in comparison to pristine Mo2C, as a fingerprint of termination groups, the spectral structures in Mo2CT2 are reproduced at higher energies. Moreover, the spectral features of Mo2CT2 are sensitive to the chemical nature and the location of the T groups on the pristine Mo2C MXene surface. When going from T = O to T = F and, further, to T = OH, the energy separation between the main peaks increases, which is a sign of decreasing the Mo-C bond length, respectively, from T = O to T = F and to T = OH. The comparison of ELNES spectra and the unoccupied densities of states (DOS) reveal that, the origin of the first structure at the carbon K edge of Mo2CT2 is mostly result from electron transition to pz state, while in pristine Mo2C, mainly due to the transition to px + py state. Other structures at higher energies mainly arise from electron transitions to px + py state and partially to pz state. The spectral decomposition of the ELNES into in-plane (l' = 1, m' = ± 1) and out-of-plane (l' = 1, m' = 0) components also confirms these results. Generally, in both Mo2C and Mo2CT2, the contribution of in-plane element in most of the structures is more considerable.

Femtosecond X-ray induced changes of the electronic and magnetic response of solids from electron redistribution

Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. With increasing X-ray intensity, the X-ray absorption strength is expected to become nonlinear. Here, we report the onset of such a nonlinearity in the resonant X-ray absorption of magnetic Co/Pd multilayers near the Co L[Formula: see text] edge. The nonlinearity is directly observed through the change of the absorption spectrum, which is modified in less than 40 fs within 2 eV of its threshold. This is interpreted as a redistribution of valence electrons near the Fermi level. For our magnetic sample this also involves mixing of majority and minority spins, due to sample demagnetization. Our findings reveal that nonlinear X-ray responses of materials may already occur at relatively low intensities, where the macroscopic sample is not destroyed, providing insight into ultrafast charge and spin dynamics.

Robust theoretical modelling of core ionisation edges for quantitative electron energy loss spectroscopy of B- and N-doped graphene

Electron energy loss spectroscopy (EELS) is a powerful tool for understanding the chemical structure of materials down to the atomic level, but challenges remain in accurately and quantitatively modelling the response. We compare comprehensive theoretical density functional theory (DFT) calculations of 1s core-level EEL K-edge spectra of pure, B-doped and N-doped graphene with and without a core-hole to previously published atomic-resolution experimental electron microscopy data. The ground state approximation is found in this specific system to perform consistently better than the frozen core-hole approximation. The impact of including or excluding a core-hole on the resultant theoretical band structures, densities of states, electron densities and EEL spectra were all thoroughly examined and compared. It is concluded that the frozen core-hole approximation exaggerates the effects of the core-hole in graphene and should be discarded in favour of the ground state approximation. These results are interpreted as an indicator of the overriding need for theorists to embrace many-body effects in the pursuit of accuracy in theoretical spectroscopy instead of a system-tailored approach whose approximations are selected empirically.

NMR signature of evolution of ductile-to-brittle transition in bulk metallic glasses

The mechanical properties of monolithic metallic glasses depend on the structures at atomic or subnanometer scales, while a clear correlation between mechanical behavior and structures has not been well established in such amorphous materials. In this work, we find a clear correlation of (27)Al NMR isotropic shifts with a microalloying induced ductile-to-brittle transition at ambient temperature in bulk metallic glasses, which indicates that the (27)Al NMR isotropic shift can be regarded as a structural signature to characterize plasticity for this metallic glass system. The study provides a compelling approach for investigating and understanding the mechanical properties of metallic glasses from the point of view of electronic structure.

Measuring and relating the electronic structures of nonmodel supported catalytic materials to their performance

Identifying structure-performance relationships is critical for the discovery and optimization of heterogeneous catalysts. Recent theoretical contributions have led to the development of d-band theory, which relates the calculated electronic structure of a metal to its chemical and catalytic activity. While there are many contributions where quantum-chemical calculations have been utilized to validate the d-band theory, experimental examples relating the electronic structures of commercially relevant nonmodel catalysts to their performance are lacking. We show that even small changes in the near-Fermi-level electronic structures of nonmodel supported catalysts, induced by the formation of surface alloys, can be measured and related to the chemical and catalytic performance of these materials. We demonstrate that critical shifts in the d-band center in alloys are related to the formation of new electronic states in response to alloying rather than to charge redistribution among constitutive alloy elements, i.e., the number of d holes and d electrons localized on the constitutive alloy elements is constant. On the basis of the presented results, we provide a simple, physically transparent framework for predicting shifts in the d-band center in response to alloying and relating these shifts to the chemical characteristics of the alloys.

First-principles calculations of x-ray absorption near edge structure and energy loss near edge structure: present and future

Computational methods for theoretical x-ray absorption near edge structure (XANES) and energy loss near edge structure (ELNES) are classified into a few groups. Depending on the absorption (or excitation) edge, required accuracy and desired information, one needs to select the most suitable method. In this paper, after providing a map of available computational methods, some examples of first-principles calculations of XANES/ELNES for selected wide gap materials are given together with references. For ZnO, for example, experimental spectra at three edges, Zn K, L(3), and O K, including their orientation dependence, are well reproduced by the supercell calculations with a core hole. Good agreement between theoretical and experimental spectra of ZnO alloys can also be seen. Theoretical fingerprints are satisfactorily obtained in this way. However, there are remaining issues beyond 'good agreements' which need to be solved in the future.

Study of changes in L32 EELS ionization edges upon formation of Ni-based intermetallic compounds

EELS L32 ionization edges in several Ni-based intermetallic compounds have been studied and interpreted in terms of the distribution of electrons in the valence d-bands. It is demonstrated that the integral EELS cross-sections change only slightly upon the formation of intermetallic compounds and therefore the charge transfer between atoms is negligible. On the other hand, the changes in the fine energy-loss near-edge structure (ELNES) of the Ni L3 edge can be readily detected indicating an important redistribution of d-electrons at the Ni site with alloying. These features are well reproduced by ab-initio calculations with a FLAPW method in its WIEN97 implementation. In contrast to the drastic effect of chemical environment, structural transformations in the investigated intermetallics result in smaller ELNES changes, which can be detected by only exceptional instruments with a higher energy resolution.

The orientation-dependent simulation of ELNES

We describe a program that allows the simulation of energy-loss near edge structure (ELNES). As an extension to the WIEN97 package (a full potential linearized augmented plane wave package for calculating crystal properties) [1] it permits to separate different contributions to the inelastic scattering cross section according to the character of the final state, explicitly taking into account projection onto scattering vector and integration over collection and convergence angle. Thus the program facilitates analysis of ELNES under precisely defined experimental conditions, and allows the investigation of anisotropic effects in ELNES from crystal structures. Dipole-allowed as well as dipole-forbidden transitions can be analyzed with this program.