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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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