Room 357, John Hodgins Building

Materials Science and Engineering
McMaster University
1280 Main Street West
Hamilton, Ontario L8S 4L7
Canada

Tel: 905 525 9140 x24767

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Advanced Microscopy

Key words: Electron channeling, beam damage, momentum-dependent EELS, STEM-HAADF, ELNES fine structures, atomic-resolution imaging & spectroscopy, core-hole distribution, strain mapping...

Contributors: Alex Pofeliski, Shaobo Chen, Matthieu Bugnet, David Rossouw, Steffi Woo, Nicolas Gauquelin, Guozhen Zhu... Contact us >

Selected Publications

Following the development of new electron sources, stable electron microscopes, aberration correctors, and commercial monochromators, the applications of EELS in the two worlds of materials analysis and materials physics are now finally converging. 
 
The capability of mapping at the atomic scale is not simply revealing local changes in composition but it allows us to identify the termination of the surface of substrates, and the chemical species that are in direct contact with the substrate. For the particular case of La2/3Ca1/3MnO3 (LCMO) grown on YBa2Cu3O7-d (YBCO), our work has shown that the last atomic plane in YBCO is Ba, while the first atomic plane in LCMO is Mn. Similarly, it is also possible to use EELS mapping to provide unambiguous information on the site preference of transition metals in oxides whereby, in a Ca2FeMnO5, Mn is found on octahedral sites and Fe on tetrahedral sides. In disordered systems, this approach has allowed us to directly map the distribution of implanted Pr atoms in SrTiO3 and has demonstrated the expected statistical distribution of atoms that cannot be resolved based on purely Z-contrast imaging. For surfaces, we have been able to determine the valence state of surface atoms in reconstructed SrTiO3. With the improved energy resolution, we have demonstrated that, simultaneous intense light irradiation (in-situ) and acquisition of energy-loss spectra, there is a significant change in the excitonic peak portion of the σ* fine structure. This high-sensitivity demonstrates that the local heating and the charge carriers generated by infra-red photons significantly modify the electronic structure of the nanotubes. At very low energy losses, we have also shown the detection of energy loss features down to 0.17eV, the lowest feature ever reported. In addition, using numerical deconvolution, we have been able to show improvements in the effective energy resolution of spectra down to a FWHM of 0.01eV.
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Heterostructures & Intefaces

Key words: Grain-boundary segregation, dopants, semiconductor materials, thin films, Internal oxidation, twin-boundaries, defects, voids, alloy clustering, atomic-ordering, heat-treatments, atomic-resolution imaging and chemical mapping....  

Contributors: Alex Pofeliski, Shaobo Chen, Steffi Woo, Sharzhad Hosseini, Matthieu Bugnet, Guozhen Zhu, Karleen Dudeck, Sagar Prabhudev.... Contact us >

Selected Publications

The development of aberration correctors for the scanning transmission electron microscope has revolutionized the field of electron microscopy and dramatically improved the analytical “toolkit” of materials scientists. In particular, when combined with electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM) makes it possible to detect compositional and spectroscopic changes at the atomic level that can be used to understand the structure, and ultimately the performance of materials.
 
Beyond the “simple” deduction of the distribution of elements in nanostructures from maps, quantification is essential to understand the detailed structure of defects and correlate compositional measurement with, for instance, the optical response of materials. The detailed quantification of the atomic position of a defect, for example, a so-called {311} defect [see Karleen Dudeck's work] generated by the implantation of ions in Si shows that an excellent agreement is obtained between the experimental atomic positions and molecular dynamics simulations with an accuracy of better than 0.05nm for more than 100 atomic columns. Similarly, quantitative analysis of SiGe alloys deposited on Si has allowed us to deduce compositional fluctuations and interdiffusion in proximity of interfaces with the substrate. Here a quantitative analysis with EELS can also be carried out, e.g. in the measurement of the composition of InGaN quantum dots in GaN nanowires. These deductions can be further supported with emission wavelengths measured from cathodoluminescence in STEM and photoluminescence measurements [see Steffi Woo's works].
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Plasmonics

Key words: Fractal geometries, lithography, high-energy resolution EELS, plasmon edge modes and gap modes, coupling, monochromator excitation, silver nanostructures, plasmon hybridization, surface plasmons

Contributors: Edson Bellido, Isobel Bicket, Viktor Kapetanovic, Alex Pofeliski, David Rossouw, Matthieu Bugnet, Steffi Woo... Contact us >

Selected Publications

The Monochromated electron beam of the aberration-corrected microscopes has allowed the detailed study of plasmonic response of nanostructures. Achieving 0.06eV energy resolution with a sub-nm size probe, the CCEM instruments have been used to study surface plasmon resonances in metallic nanoscale wires that have potential use in photonic information transfer. Using the electron beam, surface plasmon polaritons are excited and a standing wave pattern is formed which can be simultaneously detected with the same electrons when analyzed with a high-resolution spectrometer. This electron excitation is directly related to the photonic density of states of the sample and is very effectively probed with an electron beam smaller than 1nm.
 
As demonstrated by Rossouw et al, in Physical Review Letters, the energy loss spectrum shows multiple excitations in energy range down to 0.17eV, the lowest energy features ever observed with an electron beam. By selecting the energy window for a given energy loss, the spatial resolution of the standing wave, of a given energy has also been mapped. The results show that, using the CCEM instrumentation, we can resolve optical excitation down to the mid-infra red regime and this by spatially resolving energy modes extended in space by few 10’s of nm. These features would hence be not detectable with photon-based techniques because some of the excitations would not couple to light (i.e. they are dark-modes) and would be significantly smaller than the free-space wavelength of light.
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Energy Materials

Key words: Fuel Cells, Li-ion Batteries, Machine Learning, CO oxidation, Electrocatalysis, Pt-alloy catalysts, Liquid-cell microscopy, In situ Annealing, Electron Tomography, Time-evolution studies, Durability tests, Charging/Discharging cycles, single-atom catalysis...

Contributors: Hanshuo Liu, Jie Yang, Sagar Prabhudev, Mike Chatzidakis, Sam Stambula... Contact us >

Selected Publications

The development of aberration correctors for the scanning transmission electron microscope has revolutionized the field of electron microscopy and dramatically improved the analytical “toolkit” of materials scientists. In particular, when combined with electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM) makes it possible to detect compositional and spectroscopic changes at the atomic level that can be used to understand the structure, and ultimately the performance of materials.
 
Using an FEI Titan (80-300 Cubed) microscope equipped with a monochromator and EELS spectrometer (Quantum 966) we have been able to image single Pt atoms on multilayer graphene nanosheets (GNS) and demonstrate that single Pt atoms are stabilized during atomic layer deposition on N-doped GNS. Similarly, quantitative STEM images have been used to detect atomic displacements on PtFe intermetallic core-shell nanoparticles that exhibit very high specific activity compared to pure Pt. Not only is elemental mapping at the atomic scale possible, but the high beam current and fast spectrometers also allow the acquisition of these maps with large sampling of the nanostructure. For instance, in the study of PtRu nanocatalysts used in fuel cells the Ru core-Pt shell structures can be very clearly mapped. In the same system, we have also shown that it is possible to obtain maps from the Ti K edge (4970eV loss) and Pt M45 edge (2120eV) using the dual-EELS capabilities of the spectrometer. Further studies with EELS demonstrate the detection of valence changes and mapping of valence in Li-ion battery materials.
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Botton's group focuses on the application and development of electron microscopy techniques to study materials at high spatial resolution. The core research is based on transmission electron microscopy but we also use a myriad of other characterization techniques to provide information on the structure and electronic properties of materials. 

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