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S3 Seminar: A. Zobelli

Foto: Zobelli (© https://www.stem.lps.u-psud.fr/users/zobelli)


Modena - 15.06.2017 - S3 Seminar: A. Zobelli
Alberto Zobelli, Laboratoire de Physique des Solides, Université Paris Sud, Orsay (France)

15/6/2017 – 15.00 Seminar Room, S3 Cnr Nano, Physics Building, Unimore
Host: Stefano Ossicini

New generation of spectro-microscopies in scanning transmission electron microscopes (STEM) can provide a closer look to the topology, chemistry and optics response of individual nanostructures. Bidimensional materials are the ideal systems to test ultimate performance since their reduced dimensionality permits a relatively easy access to heterogeneities down to the atomic scale. In this talk I will present some of our recent results on 2D materials which illustrate novel capabilities in STEM spectro-microscopy. Individual dopant atoms and vacancies can be imaged through Z contrast in high angle annular dark field images. As an example, we analyze the effect of Cr doping on WSe2 crystals demonstrating that individual Cr atoms occupy the metal sublattice and promote the appearance of Se vacancies in neighboring sites [1]. Finally the valence of individual Cr atoms has then measured by core electron energy loss spectroscopy (EELS).
The use of fast electrons gives access to such high resolutions in transition metal dichalcogenides but these performance can not be achieved on irradiation sensitive materials. We have recently overcome this restriction by developing material-specific acquisition protocols for the study of archetypical fragile nanosystems such as graphene oxide (GO) and reduced graphene oxide (RGO). By core EELS spectroscopy we have then derived oxygen maps of (R)GO at an unprecedented nanometric spatial resolution. On the basis also of EELS fine structures analysis, we have revised the existing models for the type and distribution of oxygen functional groups on these materials [2].
Layered and 2D materials may also present peculiar optical properties. h-BN is for instance a very promising material for optical application due to its strong emission in the far UV. Very recently, our team has developed an original cathodoluminescence (CL) detection system integrated within a scanning transmission electron microscope which provides a full emission spectra with a resolution as low as few tens of meV associated with an electron probe size of one nanometer. Thanks to this unique experimental set-up we provided a microscopic explanation for the high energy emission spectrum of h-BN, relating local changes in the layer stacking order at crystal folds to the appearance of additional excitons [3]. Furthermore, by combining nanometric resolved CL with HBT interferometry, we mapped the spatial localization of emission signal due to individual point¬ defects and identified a new bright and stable single photon source in the UV [4]. To conclude, I will illustrate how original illumination protocols for irradiation sensitive materials and innovative CL methods can be combined to study the luminescence of individual molecular dyes.

[1] C-H. Ho et al. Submitted
[2] A. Tararan et al. Chemistry of Materials 28, 3741 (2016)
[3] R Bourrellier et al, ACS Photonics 1,857 (2014)
[4] R Bourrellier et al, Nano Lett. 16, 4317 (2016)

Info stefano.ossicini@unimore.it

Link https://www.stem.lps.u-psud.fr/users/zobelli


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