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Quentin Ramasse (UK): From phonons to core losses in real and momentum spaces with sub 20 meV monochromated STEM-EELS

Monday, 24. April 2017, 17:30

Location:  Lise-Meitner-Lecture hall, Strudlhofgasse 4, 1st floor

Vortrag im Rahmen des Wiener Physikalischen Kolloquiums

A new generation of electron beam monochromators has recently pushed the energy resolution of (scanning) transmission electron microscopes deep into the sub 20 meV range, allowing to explore the phonon region of the EELS spectrum. This will be illustrated on a number of systems studied using a Nion UltraSTEM100MC ‘Hermes’ instrument recently installed at the SuperSTEM Laboratory. The different phonon excitations in EELS for the cubic and hexagonal phases of boron nitride can be better understood by observing the dependence of the phonon peak under different optical conditions and mapping the energy of the peaks in momentum space.
Similarly, the low loss spectrum of carbon nanotubes contains a variety of characteristic losses which are of great importance to understand their electronic behavior. The band gap of semi-conductors is known to be strongly affected by structural variations. Local compositional changes in Cu(InGa)Se2 solar cell absorber layers lead to subtle band gap changes, and we show how the increased energy resolution can provide an increased precision on these nm-scale measurements. We also highlight how peaks associated with excitonic effects in MoS2 exhibit a clear spatial dependence at step edges and on terraces of varying thicknesses.

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