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Department of Chemistry

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Calculation for XANES and XAFS

 

Some helpful terminology...

1. Fabrication of Nanostructures using Soft and Hard Templates

We recently developed techniques to prepare metal and semiconductor nanoparticles using soft and hard nano templates. Using synchrotron and electron microscopy techniques, we have shown that capping nanoclusters with molecules of desired properties (soft templates) is a convenient way to tune the size and properties of metal nanostructures and that we can prepare metallic nano systems using substrates such as porous silicon and nanowires (hard templates).
Ongoing Research: Preparation of novel nanoscale binary systems as well as metallic and semiconductor nanowires, and heterostructures.

 

2. Synchrotron Light Induced Luminescence of Nanoscaled Systems, Organic Light Emitting Materials and Molecular Beacons

We use XEOL (X-ray Excited Optical Luminescence) capabilities at the Canadian Synchrotron Radiation Facility (CSRF) to investigate the optical properties of light emitting materials. XEOL is a photon-in photon out technique in which the optical luminescence excited by tunable x-rays from a synchrotron light source is monitored. When the excitation is tuned across an absorption edge, XEOL can be site and excitation channel specific. We have been able to identify the origin of luminescence from nanoscaled systems, OLED (Organic Light Emitting Device) materials and devices and molecular beacons tagged on proteins.
Ongoing Research: XEOL of mixed-colour nanoscaled systems and soft matters; Time-Resolved X-ray Excited Optical Luminescence (TRXEOL) at APS and CLS.

 

3. Preparation, Structure and Electronic Properties of One-dimensional Nano Materials

The thermal evaporation system in our laboratory has produced a number of novel one-dimensional nanomaterials of Si, CdS, ZnS, ZnO, CdTe, etc. (nanowires and nanoribbons). The morphology, structure and properties of these systems can be tuned by varying experimental condition. The properties of these materials can be further modified by surface chemistry.
Ongoing Research: Exploration of desirable conditions in the controlled synthesis and characterization of one-dimensional materials.

 

4. X-ray Absorption Spectroscopy and Related Techniques

Our group has been engaged in the development of synchrotron techniques and application for more than two decades and has made contributions to a number of techniques such as photo-fragmentation of molecules, photoconductivity XAFS of liquids, x-ray excited optical luminescence, EXAFS study of dynamics of metal complex in solution and layer resolved photoemission spectroscopy. In addition to XEOL, recent works include:
a. Sub-lifetime partial Auger yield technique (circumventing core-hole lifetime broadening/uncertainty principle) using the Auger channel of Resonant X-ray Auger Raman can be used to obtain high resolution X-ray Absorption Near Edge Structure (XANES) chemical systematic.
b. Photoconductivity XAFS of liquids.
c. X-ray Magnetic Circular Dichroism capabilities at the Canadian Double Crystal monochromator beamline at CSRF. This capability provides new opportunities for MXCD of 4d and 5d metals.
d. Resonant X-ray Inelastic Scattering studies of Ce mixed valence systems.
Ongoing Research: Development of time-resolved XEOL and high-resolution resonance spectroscopy at the Canadian Light Source ( http://www.lightsource.ca/experimental/). These facilities will provide high quality photons in the energy range of 5 eV to 5 keV.

 

5. Electronic Structure of Bimetallic (Bulk, Surface, Interface, Thin films and Nanostructures)

We use photoemission and x-ray absorption spectroscopy to reveal the electronic properties concerning bulk, overlayers and two-dimensional alloying. This has been one of the ongoing programs, which evolves to include materials in low dimensions and nano structures.
Ongoing Research: Comparison of structure and electronic properties of bulk, surface and nanostructures of metallic, metal silicide and compound semiconductors.

 

6. X-ray Microscopy Studies of Structure, Bonding and Distribution of Metal in Tissues

The availability of micro x-ray beam has greatly facilitated the microanalysis of materials. We have conducted a series of preliminary experiments at the PNC-CAT Beamline of the APS looking at the chemical identity and distribution of iron in hemochromatosis liver and copper in mice kidney tissues in connection with a study of diabetes (in collaboration with Paul Adams and Subrata Chakrabarti of the Department of Medicine and Pathology, respectively).
Ongoing Research: Systematic studies of normal and homochromatic liver tissues using the micro-spectroscopy and spectromicroscopy capabilities (KB mirror and multi element fluorescence detector) at APS and CLS.

 

Terminology:

Synchrotron Radiation:

Synchrotron radiation is electron magnetic radiation emitted by (near speed of light) electrons travelling through magnetic fields (bending magents, wigglers, undulators) in a storage ring. Synchrotron radiation is emitted over the entire range of the electromagnetic spectrum, tangential to the orbit of the electrons and is collected by a beamline. The beamline includes optical devices which control the wavelength, photon flux, beam dimensions, focus, and collimation of the rays. The optical devices include slits, attenuators, crystal monochromators, and mirrors. At the end of the beamline is the experimental end-station, where samples are placed in the path of the radiation, and detectors are positioned to measure the resulting absorption, diffraction, scattering or secondary radiation.

Additional resources: http://www-ssrl.slac.stanford.edu/primer.pdf

 

X-ray Absorption Fine Structure:

X-ray absorption fine structure (XAFS) is a specific structure observed in X-ray absorption spectroscopy. XAFS is a spectroscopic technique that uses X-rays to probe the physical and chemical structure of matter at an atomic scale. By analyzing the XAFS, information can be acquired on the local structure and on the unoccupied electronic states. XAFS is element-specific, in that X-rays are chosen to be at or above the binding energy of a particular core electronic level of a particular atomic species therefore an energy-tunable X-ray source like a synchrotron is needed for XAFS measurements.

The X-ray absorption spectra exhibit a steep rise in the absorption coefficient at the core-level binding energy of X-ray absorbing atoms and attenuate gradually with the X-ray energy. The XAFS spectra are usually divided into three energy regions: 1) the edge region, 2) the X-ray Absorption Near Edge Structure (XANES); 3) the Extended X-ray Absorption Fine Structure (EXAFS). The absorption peaks at the absorption edge region ~5eV are due to electronic dipole transitions from a core-level to an unoccupied orbital or band above the Fermi level. The oscillatory structure extending for hundreds of eV past the absorption edge is the EXAFS, resulting from the interference in the single scattering process of the excited photoelectron scattered by neighbouring atoms and provides information on the local structure. The energy region of XANES (extending over a range of about 100 eV) between the edge region and the EXAFS region has been assigned to multiple scattering resonances and provide information on the geometry of the local structure. In the case of organic molecules this energy region has been later called near-edge X-ray absorption fine structure (NEXAFS), but NEXAFS is synonymous with XANES.

Additional Resources: http://xafs.org/Tutorials?action=AttachFile&do=get&target=Newville_xas_fundamentals.pdf

X-ray Excited Optical Luminescence:

X-ray excited optical luminescence (XEOL) monitors the luminescence from a light emitting material, by measuring a specific de-excitation channel associated with the energy redistribution by a system upon the absorption of an energetic photon. The absorption of an X-ray photons and the decay leads to the production of photoelectrons, Auger electrons, and fluorescence X-ray photons. These processes and associated secondary processes result in the formation of thermalized holes in the valence band and electrons in the conduction band of the luminescent solid. The radiative recombination of holes and electrons produces luminescence. Optical photons are the product of electron-hole recombination between the conduction and valence bands, or from defect energy levels in the band gap. Essentially, XEOL is an X-ray photon in, optical photon out technique. XEOL has the added advantage of being element, site and excitation channel specific, which is achieved by tuning the photon (excitation) energy to a particular absorption edge of an element of which the local electronic structure is effectively coupled to the luminescence channel, thereby exciting preferentially, those sites responsible for the optical emission. The optical or photoluminescence yield (PLY), in turn, can be used to monitor the absorption; this technique is sometimes called optical-XAFS.

Additional Resources: A. Rogalev, J. Goulon, X-ray Excited Optical Luminescence Spectroscopies, in Chemical Applications of Synchrotron Radiation, Part II: X-ray Applications, editor: T.K. Sham, River Edge, NJ: World Scientific, 2002, Vol. 12B, pp. 707-760.

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