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Contribution Oral

Budker INP - Conference Hall
X-ray spectroscopy

XAFS spectroscopy - a useful tool for determining structure parameters and the electronic state of various nanosystems

Speakers

  • Dr. Simon ERENBURG

Primary authors

  • Dr. Simon ERENBURG (Nikolaev Institute of Inorganic Chemistry SB RAS, Budker Institute of Nuclear Physics SB RAS)

Co-authors

Content

Some of the EXAFS spectra above GaK-and GeK- edges were measured at the VEPP-3 storage ring (beamline 8) of Siberian Synchrotron and Terahertz Radiation Center (SSTRC) of the Budker Institute of Nuclear Physics (BINP SB RAS, Novosibirsk, Russia). Other part of the spectra was measured at the ESRF (Grenoble, France). The high energy resolution fluorescence detected HERFD-XANES and EXAFS experiments have been performed at 12K at the beamline ID26 or ID20 equipped by 5-analyzer x-ray emission spectrometer.

1. Electronic and spatial structure of "calibrated" gold clusters in the cavities of cucurbit[6, 7] uril molecules.

The AuL$_{III}$ X-ray absorption fine structure (XAFS) spectra have been measured for samples containing calibrated gold nanoparticles D ≤ 1nm nm in cavities of cucurbituril molecules (CB[6, 7]). It has been found that gold clusters are characterized by smaller 0.03 Å interatomic distances and a considerable reduction in the Au-Au coordination numbers (N ≤ 8) as compared with bulk gold. No visible shifts of AuL$_{III}$ absorption edges (±0.02 eV) and differences in the Au charge state for Au@СВ[6, 7] samples containing small Au clusters were detected in comparison with bulk Au metal. A threefold and more increase in the Debye-Waller factor at 12K was found for the nanoparticles in comparison with bulk metal and correspondingly, a substantial increase in the structural disorder. It has been found that special chemical (catalytic) and physical (optical) properties of small gold particles are likely to be attributed to this structure changes and the size effect with the appearance of a band gap between the occupied and unoccupied electronic states, rather than changes in the charge state of gold clusters and presence of strongly pronounced vertices and edges in structure of nanoparticles.

2. Microstructure of multilayered heterosystems GaNAlN.

A minimal drop (~0.01 Å) in the interatomic Ga–Ga distances R(Ga) relative to a thick solid film was obtained from the analysis of GaK EXAFS spectra for multilayered GaN/AlN samples with "thick" (550–850 nm) superlattices, agreeing with the numerous dislocations found in them and the corresponding stress relaxation in the GaN layers. The interatomic Ga–Ga distances R(Ga) for samples with fewer layers and "thin" (80–150 nm) superlattices fell more substantially (by $sim{0.03}$ Å), corresponding to the more substantial deformations and stresses indicated by our earlier results for GaN quantum dots in a AlN host. The influence of the growth conditions and the thickness of the superlattices on mixing in the near boundary layers and the optical properties of the GaN/AlN superlattices were revealed. It was established that Ga–Al mixing occurs only in the layer nearest to the interface. Anomalously long Ga–Al distances ($sim{3.25}$ Å) were observed for samples with thick superlattices. This effect can be explained by a nonequilibrium transition from GaN growth to AlN growth and the more substantial stresses at the interface of such superlattices.

3. Structural characteristics of K-Bi citrate (De-Nol) and its clusters in aqueous solutions.

BiL$_{III}$ EXAFS spectra of an amorphous solid Bi complex with citrate (De-Nol) and its aqueous solutions in a wide concentration range are measured. For the solutions good agreement is revealed between their structural parameters and the averaged interatomic distances and coordination numbers of 12-nuclear Bi clusters. So, it is found that droplets of the colloidal solution have a structure close to the solid $Bi_{12}O_{22}$ cluster structure. When the concentrated solution is diluted the cluster structure is somewhat modified, it remaining similar to the structure of the $Bi_{12}O_{22}$ cluster and even at a tenfold dilution and the nearest (oxygen) spheres of the Bi environment changing insignificantly.

This work was supported by the Russian Foundation for Basic Research (project no. 16-02-00175a)