Abstract Summary: Some recent instrumental and conceptual advances in STEM EELS and CL are presented and illustrated with several examples from the fields of material science (oxide heterostructures, graphene and graphene oxide) and nano-optics (plasmonic nanostructures, semiconducting nanostructures and single photon emitters)
Introduction: The field of electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has recently achieved a succession of impressive successes linked with the development of aberration correctors, enabling atomically-resolved spectroscopy, which are now spreading worldwide. A new generation of monochromators is emerging, providing improvements in energy resolution of at least one order of magnitude and giving unprecedented access to low energy-loss ranges. Similarly, recent progress in the collection of visible-range photons emitted by a sample illuminated by a focused beam has enabled novel cathodo-luminescence (CL) experiments in STEM. In addition, new ways of exploiting fast electron beams, including combining them with beams of photons, have opened up the field of nano-optics, providing a high-spatial resolution alternative to more conventional optical techniques..
Results: Some of these new possibilities will be illustrated. Various strategies will be described for the acquisition of spatially-resolved core-level excitations signals in relation with the quantitative measurement of electron densities (charge accumulation) or 2D electron gases at interfaces in oxide-based nanodevices or of functional molecular groups in graphene oxide (GO) and reduced GO [3]. Recent developments in EELS and CL for reaching plasmon signatures in the visible and down to the IR spectral range will be described, allowing the mapping of eigen modes in plasmonic nanostructures and a deep understanding of the physics of these excitation. In particular, recent EELS experiments at 20 meV resolution as well as experiments combining EELS and CL will be presented, demonstrating how the usual macroscopic concepts such as extinction, absorption, and scattering cross-sections have to be adapted to describe optical phenomena at the nanoscale [4].
Finally, new possibilities for exploring the intimate link between a crystal structure (h-BN), its defects and its optical properties as revealed by nano-CL [5] will be discussed, together with some perspectives for entering the field of quantum nano-optics [6].
Acknowledgements
This work has been supported by the Agence Nationale de la Recherche (ANR), program of future investment TEMPOS-CHROMATEM (No. ANR-10-EQPX-50). The work has also received funding from the European Union in Seventh Framework Programme (No. FP7/2007 -2013) under Grant Agreement No. n312483 (ESTEEM2).