Nikolay Smolentsev

High energy resolution core-level X-ray spectroscopy for electronic and structural characterization of osmium compounds

A comprehensive study of the bulk solid OsCl3 and the molecular ion [Os(bpy)2(CO)Cl](+) is presented illustrating the application of RIXS and HERFD XANES spectroscopies to the investigation of both bulk materials and molecular complexes. In order to analyze the experimental results, DFT simulations were performed taking into account spin-orbit interaction. Calculations for both compounds resulted in good agreement with the experimental RIXS and HERFD XANES data, shedding light on the details of their local atomic and electronic structure. In particular, the spatial distribution of molecular orbitals was obtained, which allowed the determination of the origin of the absorption peaks. It was shown that for materials containing heavy atoms, only the application of advanced RIXS and HERFD XANES spectroscopies makes it possible to extract the information on local atomic and electronic structure details from XANES data.

Spin-polarized electronic structure of the core–shell ZnO/ZnO:Mn nanowires probed by X-ray absorption and emission spectroscopy

The combination of X-ray spectroscopy methods complemented with theoretical analysis unravels the coexistence of paramagnetic and antiferromagnetic phases in the Zn0.9Mn0.1O shell deposited onto array of wurtzite ZnO nanowires. The shell is crystalline with orientation toward the ZnO growth axis, as demonstrated by X-ray linear dichroism. EXAFS analysis confirmed that more than 90% of Mn atoms substituted Zn in the shell while a fraction of secondary phases was below 10%. The value of manganese spin magnetic moment was estimated from the Mn Kβ X-ray emission spectroscopy to be 4.3 μB which is close to the theoretical value for substitutional MnZn. However the analysis of L2,3 X-ray magnetic circular dichroism data showed paramagnetic behaviour with saturated spin magnetic moment value of 1.95 μB as determined directly from the spin sum rule. After quantitative analysis employing atomic multiplet simulations such difference was explained by a coexistence of paramagnetic phase and local antiferromagnetic coupling of Mn magnetic moments. Finally, spin-polarized electron density of states was probed by the spin-resolved Mn K-edge XANES spectroscopy and consequently analyzed by band structure calculations.

Application Ce L1 HERFD XAS to determine the atomic structure of CeO2 based nano-catalysts under working conditions

Ce L1-edge high-energy-resolution fluorescence detected (HERFD) X-ray absorption spectroscopy (XAS) was applied to unravel the electronic structure of Pt-promoted CeO2 nanoparticles under different redox conditions. The Ce Lγ3 emission line was chosen for partial fluorescence yield detection to improve the XAS spectral resolution. Oxygen vacancies were formed on the surface of ceria nanoparticles upon reducing in 5% CO in He. Theoretical Ce L1-edge simulations showed good agreement with the experimental data.

Determination of the local structure in metal-complexes by combining XAS and XES

Nanoscale local atomic structure determines most of unique properties of novel materials without long range order. To study its fine details one has to use both computer nanodesign and advanced experimental methods for nanodiagnostics. The status of modern theoretical analysis of the experimental x-ray absorption spectra to extract structural parameters is presented. Novel in-situ technique for nanodiagnostics extracting of 3D structure parameters on the basis of advanced quantitative analysis of X-ray absorption near edge structure (XANES) has been developed. The possibility to extract information on bond angles and bond-lengths (with accuracy up to 0.002 nm) is demonstrated and it opens new perspectives of quantitative XANES analysis as a 3D local structure probe for any type of materials without long range order in atoms positions (all nanostructured materials and free clusters belong to this class of materials). Even more possibilities are opening by using simultaneously several experimental synchrotron based techniques: XANES and XES and/or RIXS. In the framework of these approaches, the results of recent studies of local atomic structure for several types of nanostructures including nanoclusters with different types of chemical bonding, core-shell nanoneedles and thin films of dilute magnetic semiconductors, 5d-transition metal-organic complexes, Cu1+ and Cu2+ binding sites in amyloid-β peptide, Co aqua complexes in aqueous solution, nanostructured materials for hydrogen storage and nanocatalysts based on zeolites and MOF are reported. Along with the calculations of conventional XANES and XES, we show a possibility to simulate core-to-core and valenceto-core RIXS as well. Molecular orbitals (or DOS) of metal complexes can be directly related to the peaks in XES spectra in RIXS maps. This information is essential for understanding of electronic structure of metal complexes and design of novel materials.