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Yasmin Ahmed Salem, M.A.
Yasmin Ahmed Salem
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Phone: +49 211 6792 722
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Scientific Events

Scientific Events

Month:

Aberration-corrected STEM and ultra-high energy resolution EELS

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Aberration-corrected STEM and ultra-high energy resolution EELS

Electron microscopy has advanced very significantly in the last two decades. Electron-optical correction of aberrations, which we introduced for the scanning transmission electron microscope (STEM) in 1997, has allowed STEMs to reach sub-Å resolution from 2002 on. It has led to new STEM capabilities, such as atomic-resolution elemental mapping, and determining the type of single atoms by electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDXS). More recently, we have focused on Ultra-High Energy Resolution EELS (UHERE). We have developed a monochromator and a spectrometer that use multipolar optics similar to the optics of aberration correctors, plus several stabilization methods, and we have reached <5 meV energy resolution at 30 keV primary energy. This has opened up a new field: vibrational spectroscopy in the electron microscope. When collecting large-angle scattering events, vibrational spectroscopy can lead to sub-nm spatial resolution, and when collecting small-angle scattering angle events, it can produce EEL spectra with the electron beam positioned tens of nm away from the probed area. The second geometry has led to a powerful new technique: aloof vibrational analysis of materials, which avoids significant radiation damage. Even more recently, we have focused on combining the analytical techniques with in-situ sample treatment. Our progress includes cooling the sample to liquid N2 temperature in a side-entry holder capable of reaching better than 1 Å resolution. My talk will review these developments, and illustrate them by application examples. [more]

18319 1555331518

Making quantum transport visible in thermoelectric Bi2Te3 nanoparticles

Bi2Te3, Sb2Te3, and Bi2Se3, well established thermoelectric materials, are also three-dimensional (3D) topological insulators (TI) exhibiting a bulk bandgap and highly conductive, robust, gapless surface states. While the transport properties of 3D TIs are of utmost importance for potential applications, they are difficult to characterize. The reason is that transport in those materials is always dominated by bulk carriers. Still, the signature of the nontrivial electronic band structure on the thermoelectric transport properties can be evidenced in transport experiments using nanostructures with a high surface-to-volume ratio. Using a nanoparticle-based materials’ design, the highly porous macroscopic sample features a carrier density of the surface states in a comparable order of magnitude as the bulk carrier density. Further, the sintered nanoparticles impose energetic barriers for the transport of bulk carriers (hopping transport), while the connected surfaces of the nanoparticles provide a 3D percolation path for surface carriers. Within this work, I will discuss the nanoparticle processing as well as the transport properties of these combined thermoelectric and 3D TI samples. [more]

Nanoindentation based investigations of PLC-type plastic instability

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Nanoindentation based investigations of PLC-type plastic instability

Portevin Le-Chatelier (PLC) effect is a type of plastic instability that results in severe strain localization, reduction in ductility and formation of surface striations during forming operations. Understanding the underlying microscopic mechanism(s) that govern it requires detailed experimental investigations of the relationships between the phenomenon and local microstructural constituents. Most current models of PLC, both phenomenological and theoretical, are based on descriptions of mesoscopic observations and global responses observed in stress-strain curves. More predictive (or physically based) models will require investigations at the microstructural length-scales. In this talk, it will be shown that the gap in understanding of the microscopic origins and macroscopic manifestations of PLC can be bridged by nanoindentation testing. Specifically, it will be shown that by exploiting the high resolution of force and displacement measurements and the site-specific capabilities of the nanoindenter, coupled with complimentary microstructural characterization techniques, we are able to gain new insight into critical aspects of the PLC effect, including its anisotropy, underlying governing mechanisms and associated activation parameters. [more]

The Heusler System (For Thermoelectric Application): How You Can Use the periodic table As A Lego Box To Build The States You Are Interested In

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The Heusler System (For Thermoelectric Application): How You Can Use the periodic table As A Lego Box To Build The States You Are Interested In

The periodic table becomes one hundred years old just this year. The family of Heusler compounds uses nearly all the elements in the Periodic Table to allow for the design of materials with all sorts of properties. These include: hard and soft magnets, shape memory and magnetocaloric metals, thermoelectric semiconductors, topological insulators, and Weyl semimetals. These are just a few examples of more than 1000 known members of this remarkable class of materials that can display such a wide range of extraordinary multifunctional and tunable properties. Many more remain to be discovered! Just like a box of Lego bricks we can put together certain atoms (valence electrons), arranged in a particular symmetry, to achieve a desired electronic energy band structure. A necessary precondition for such a straightforward approach is a single particle picture: this allows for the prediction of many properties in this versatile class of materials, and equally enables “inverse design”. In my talk I will discuss the simple rules that we have learned to date and what the future might portend for further additions to the large and ever-growing Heusler family. [more]

HEA symposium "High entropy and compositionally complex alloys" at DPG Spring Meeting 2019 in Regensburg

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HEA symposium "High entropy and compositionally complex alloys" at DPG Spring Meeting 2019 in Regensburg

[more]

4th International Conference on Medium and High Manganese Steels

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4th International Conference on Medium and High Manganese Steels

[more]

TEM Studies on Materials with a Negative Poisson’s Ratio

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TEM Studies on Materials with a Negative Poisson’s Ratio

[more]

 
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