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X-ray absorption fine structure spectroscopy (XAFS)

Brief description of the Method

The usage of synchrotron radiation has the advantage of its tuneability, providing  the possibility to analyze the X-ray absorption edges in particular the near edge region (Near-Edge X-ray Absorption Fine Structure – NEXAFS). NEXAFS spectroscopy analyzes the density of the unoccupied electronic states within the band structure. This method allows for a reliable determination of the chemical binding state by the specific fine structure and the chemical shift.

In contrast to most other works in this field, we are measuring in fluorescence mode, which has the advantage of varying the information depth by the control of the penetration depth. Diverse sample systems can be addressed: It ranges from contamination and nano particles on flat surfaces to deeply buried structures like layers, implantation profiles and interfaces, and bulk-like structures.

In the framework of a DFG project the combined method GIXRF-NEXAFS has been developed and validated, allowing for a depth dependent chemical speciation of interfaces and thin layers.

For in-situ experiments in liquids or inert gas, various measuring cells with thin Si3N4 windows are available. It allows for chemical speciation of organics and other materials.

Application

  • Chemical speciation of bulk materials
  • Chemical speciation of surface contaminations, deposited nanoparticles or other types of samples
  • Species depth profiling of complex nanolayered structures including buried nanolayers and interfaces

Research Topics

  • Application of GIXRF-NEXAFS for a variety of nano structures, e.g. photovoltaics, semiconductor and materials
  • Application of NEXAFS to in-situ processes, e.g. battery, biological and medical devices and materials

Specification

  • K- and L-absorption edges of elements with Z ≥ 5 measureable
  • Energy resolution ≈ 0.2 eV (varies with the photon energy)
  • Measurement in fluorescence mode with energy dispersive detector

Research Highlights

  • Chemical speciation of buried layers and interfaces [1,2]
  • Characterization of organic molecules and nanoparticles [3]
  • Chemical speciation in in-situ or inert gas conditions [4]
Fig. 1: Left: Sketch of the experiment. Right: Example for the procedure for the interface speciation (REFERENZ AC 2013).

References

  1. Opens external link in new windowNon-destructive speciation depth profiling of complex TiOx nanolayer structures by GIXRF-NEXAFS, Anal. Chem. (2015) 87(15), 7705-7711
  2. Opens external link in new windowGIXRF-NEXAFS investigations on buried ZnO/Si interfaces: A first insight in changes of chemical states due to annealing of the specimen, Nucl. Instrum. Meth. B (2010) 268, 370-373
  3. Opens external link in new windowA comparison between self-assembled monolayers on gold, germanium employing grazing incidence X-ray absorption spectrometry GIXRF-NEXAFS, ECS Transactions (2009) 25, 433-439
  4. Opens external link in new windowSulfur X-ray absorption fine structure in porous Li-S cathode films measured under argon atmospheric conditions, Spectrochim. Acta B (2014) 94-95, 22-26