Aram L. Bugaev

Optimized Finite Difference Method for the Full-Potential XANES Simulations: Application to Molecular Adsorption Geometries in MOFs and Metal–Ligand Intersystem Crossing Transients

Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal-organic frameworks and (ii) light induced dynamics of spin crossover in metal-organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code [Phys. Rev. B, 2001, 63, 125120] we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni-CO configuration was found to be linear, while Ni-NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous [Fe(bpy)3](2+) with a 100 ps delay we identified the Fe-N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.

Progress in the Characterization of the Surface Species in Activated Carbons by means of INS Spectroscopy Coupled with Detailed DFT Calculations

Activated carbons are materials with relevance in different industrial applications. Due to the inherent complexity and heterogeneity of their structures, an easy assignment of the species present on their surface has a challenging result. Only recently, with the possibility to collect well-resolved inelastic neutron spectra and to simulate by DFT methods more or less extended graphitic clusters, this task is starting to become feasible. Here we report our investigation on a steam activated carbon and we show that different vibrations in the region of out-of-plane C-H bending modes are specifically connected to hydrogen terminations belonging to extended and regular borders or to short and defective ones. Furthermore, simulations including heteroatoms such as oxygen allowed us to point out spectral regions with a contribution from carboxyl species.

Atomic and electronic structure of CdS-based quantum dots

The ab initio computer design of the CdS-based quantum dots and the cobalt doped CdS quantum dots is carried out. The characteristics features of the atomic and electronic structures of semiconductor colloidal quantum dots on CdS of different sizes are studied, and the effect of cobalt impurity atoms is estimated. We have proved the sensitivity of the X-ray absorption near-edge structure (XANES) method for the verification of the nanosized atomic structural parameters calculated by the methods of computer modeling for small-scale quantum dots of the CdS family, and for the determination of the local environment parameters of the cobalt atom in the quantum dot.

Unravelling the Redox-catalytic Behavior of Ce4+ Metal–Organic Frameworks by X-ray Absorption Spectroscopy

The introduction of Ce4+ as a structural cation has been shown to be a promising route to redox active metal-organic frameworks (MOFs). However, the mechanism by which these MOFs act as redox catalysts remains unclear. Herein, we present a detailed study of the active site in [Ce6 O4 (OH)4 ]-based MOFs such as Ce-UiO-66, involved in the aerobic oxidation of benzyl alcohol, chosen as a model redox reaction. X-ray absorption spectroscopy (XAS) data confirm the reduction of up to one Ce4+ ion per Ce6 cluster with a corresponding outwards radial shift due to the larger radius of the Ce3+ cation, while not compromising the structural integrity of the framework, as evidenced by powder X-ray diffraction. This unambiguously demonstrates the involvement of the metal node in the catalytic cycle and explains the need for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a redox mediator to bridge the gap between the one-electron oxidation of the Ce4+ /Ce3+ couple and the two-electron alcohol oxidation. Finally, an improved catalytic system with Ce-MOF-808 and TEMPO was developed which outperformed all other tested Ce4+ -MOFs.

Structural studies of magnetic nanoparticles doped with rare-earth elements

Magnetic nanoparticles and those doped with rare-earth metal ions having spinel structure were synthesized, possessing the average particles size of 11.3-13.4 nm. According to Mössbauer spectroscopy data it can be concluded that prepared iron oxide nanoparticles are γ-Fe2O3. For materials containing rare-earth elements the decrease of octahedral component surface was observed in comparison to non-doped material, what can be explained by Eu3+, Sm3+ и Gd3+ ions occupying the octahedral position.

Finite difference method accelerated with sparse solvers for structural analysis of the metal-organic complexes

Finite difference method (FDM) implemented in the FDMNES software [Phys. Rev. B, 2001, 63, 125120] was revised. Thorough analysis shows, that the calculated diagonal in the FDM matrix consists of about 96% zero elements. Thus a sparse solver would be more suitable for the problem instead of traditional Gaussian elimination for the diagonal neighbourhood. We have tried several iterative sparse solvers and the direct one MUMPS solver with METIS ordering turned out to be the best. Compared to the Gaussian solver present method is up to 40 times faster and allows XANES simulations for complex systems already on personal computers. We show applicability of the software for metal-organic [Fe(bpy)3]2+ complex both for low spin and high spin states populated after laser excitation.

The Role of Hydrogen in Formation of Pd XANES in Pd-Nanoparticles

The effects of hydrogen in Pd nanoparticles on the features of Pd K-edge X-ray absorption near edge structure (XANES) are studied. Simulations indicate a linear dependence of the intensities of XANES maxima upon the concentration of hydrogen in the bulk of Pd clusters at low H-concentrations and stepwise increase of the first near edge peak at high concentrations. The effects of the diffusion of H atoms over the interstitial sites are considered. The application of the obtained results to the experimental spectra measured in PdH nanoparticles of sizes from 1.3nm to 10.5nm indicates low H-concentrations and high mobility of H-atoms in the bulk of the nanoparticles.

X-ray absorption spectroscopy determination of the products of manganese borohydride decomposition upon heating

Manganese borohydride Mn(BH4)2 powder is heated in a hydrogen atmosphere in vacuum. The long-range order in the structure is monitored in situ by means of X-ray absorption spectroscopy and X-ray diffraction; short-range order, via Mg K-edge X-ray absorption near-edge structure spectroscopy. Above 120°C, the X-ray diffraction pattern disappears and an irreversible phase transition occurs, accompanied by sample amorphization and profuse hydrogen desorption. In the hydrogen atmosphere, the phase transition occurs at a temperature of ∼110°C. The standard scheme of borohydride decomposition suggests hydrogen desorption and the formation of metallic manganese and boron. However, a theoretical analysis of X-ray absorption spectra shows that the most likely products of Mn(BH4)2 decomposition are manganese borides.

Doped CdTe-based quantum dots

Colloidal semiconductor CdTe-based quantum dots are investigated. An ab initio computer design of quantum dots based on nanoparticles of CdTe and CdTe doped with atoms of transition elements (Co, Mn) is executed. Partial densities of the electron states of the investigated quantum dots are calculated. The sensitivity of X-ray absorption near edge structure (XANES) spectroscopy for verifying the parameters of the nanoscale atomic structure of small quantum dots based on CdTe, and for determining the parameters of the local environment around cadmium atoms and doping atoms in quantum dots was proved.

Pd hydride and carbide studied by means of Pd K-edge X-ray absorption near-edge structure analysis

A Pd K-edge X-ray absorption near edge structure (XANES) analysis for palladium hydride and carbide nanoparticles is presented. It is shown that the presence of H and C atoms changes the Pd unoccupied p-electronic states, affecting the near-edge fine structure. Quantitative XANES analysis is performed using a multidimensional interpolation approach. Theoretical models of PdH and PdC clusters are developed using Monte Carlo methods. The proposed technique allow us to determine hydrogen concentrations in palladium nanoparticles and correctly reproduce all experimental features of XANES spectra for palladium hydride and carbide nanoparticles.