Fundamentals on Synchrotron Radiation and X-ray Methods

<p>Slides for the PhD Academy "Preserving and Safeguarding the Beauty of Cultural Heritage"</p> <p>Venice International University, Isola di San Servolo, Venice, Nov 8 2023.</p><p>In this lecture the fundamental aspects of Synchrotron Radiation are presented as well as the basic theory of matter-radiation interaction with particular emphasis on absorption.</p><p>Then, the X-ray absorption technique is presented from a practical point of view.</p&gt

Local structure of [(GeTe)₂/(Sb₂Te₃)_m]_n super-lattices by X-ray Absorption Spectroscopy

Herein, the local structure of [(GeTe)2/(Sb2Te3)m]n chalcogenide super-lattices (SLs), which are at the basis of emerging interfacial Phase-Change Memory (iPCM), is studied by X-ray Absorption Spectroscopy (XAS) at the Ge-K edge. The quantitative analysis of the first coordination shells reveal that the SLs appear to possess a structure very similar to that of thin film of the canonical Ge2Sb2Te5 (GST225) phase-change alloy. By comparing experimental data with ab initio Molecular Dynamics simulations of the EXAFS spectra, we show that chemical disorder is mandatory in order to reproduce the experimental data in the full spectral range. As a result, we can unambiguously conclude that Ge/Sb intermixing resulting from inter-diffusion of the GeTe and Sb2Te3 layers within SLs is inherent to SLs and is not induced by sample preparation method nor by interaction with the electron beam of electron microscopes used in all the previous studies that were suggesting such a phenomenon. We further evidence that the short Ge-Te distance is the same in GeTe and GST225 films, as well as in SLs. The main difference is the impact of disorder in GST225 and SLs. Intermixing being definitively present in [(GeTe)2/(Sb2Te3)m]n SLs, this parameter must be considered in future models aiming at going further in the understanding and the development of iPCM technology. This seems mandatory in order to allow such technology to emerge in the near future on the non-volatile memory market

The fate of CdS Quantum Dots in plants as revealed by Extended X-ray Absorption Fine Structure (EXAFS) analysis

Use of Quantum Dots (QDs) is widespread and as such, the potential risk associated with their dispersion in the environment has stimulated research on interaction with potential sensitive receptors. To this end, the model plant Arabidopsis thaliana wild type (wt) and two mutant lines known to be tolerant to cadmium-based CdS QDs but not CdSO4 were exposed to CdS QDs or CdSO4 at sub-inhibitory concentrations for 20 days. X-ray Absorption Spectroscopy (XAS) was employed to investigate cadmium speciation in the cellular environment of the plants after treatment. After exposure to CdS QDs and CdSO4, differences in biomass were observed between wt and mutants, but the form of Cd in the treatment had a marked influence on cadmium atomic environment. The spectra of whole plant samples were found compatible with a mixed O/S coordination: while Cd-S distances did not show ample variations, Cd-O distances varied from ≈2.16 Å in samples grown with QDs to ≈2.22 Å in those grown on CdSO4. In addition, the amount of Cd-S bonds in plants grown with QDs was higher than Cd-O bonds. XAS data showed that CdS QDs were bio-transformed after their uptake; the particle original structure was modified but not totally eliminated, Cd atoms were not released as Cd(II) ions. These findings show the nanoscale specific response of plants to QDs, provide important insight to understanding nanoparticle fate in plants and in the environment, and have implications for both risk assessment and design of appropriate remediation strategies