Untitled - IAP/TU Wien - Technische Universität Wien

71st IUVSTA Workshop
Mapping the surface structural and electronic properties of individual
nanoparticles with the tiny beam of a Scanning Transmission Electron
Microscope (STEM)
Christian Colliex
Laboratoire de Physique des Solides (UMR CNRS 8502)
Bldg. 510, Université Paris-Sud 11, 91405 Orsay, France
christian.colliex@u-psud.fr
Aberration-corrected scanning transmission electron microscopes deliver Angström-sized
electron probes of typically 40 to 200 kV kinetic energy, on individual nanoparticles, outside of them,
on the apex of one of their external surfaces or through them. They constitute very powerful tools to
investigate both the structural and electronic properties of the selected targets at the finest spatial
resolution. As a result of the strong interactions between probe and matter, many different signals can
be picked simultaneously (see figure below). Electrons scattered at large angles are collected by
annular detectors and they deliver signals which are of use for reconstructing, atom by atom, the 3D
structure of individual nanocrystals. In parallel, electron energy-loss spectroscopy (EELS) records the
electronic excitation spectrum of the specimen over a very broad domain, typically from 1 to 1000 eV,
with a high level of spatial and energy resolution. When using the core-loss signals, single atom
sensitivity has been demonstrated and elemental maps, atomic column by atomic column, of
crystallized nanoparticles or across interfaces, are now routinely available. In some cases, taking
benefit of the fine structures appearing on the characteristic edges, the bonding state of the atoms
across nanostructures and at their surfaces is also accessible. In the 1 to 5 eV range encompassing the
visible spectral domain, one maps the distribution in energy and in intensity of the surface plasmon
modes at metal surfaces with a spatial resolution typically down to 1 nm and an energy resolution of
100 meV. It thus constitutes a very efficient technique to explore the sub-wavelength spatial variation
of the electro-magnetic fields associated to surface plasmons. This is therefore a powerful alternative
to near-field optical microscopy, with much increased spatial resolution, for investigating a broad
range of photonic modes in very diversified geometries. Quite recently, spectrum-imaging of the
emitted photons under the primary electron beam and the spectacular introduction of time-resolved
techniques down to the fs-time domain, have constituted innovative keys for the development and use
of a brand new multi-dimensional and multi-signal electron microscopy.
Illustration of the multisignal strategy in a
modern STEM instrument, displaying two
channels of parallel information : ADF, EELS
and EDX elemental mapping of the atomic
structure and composition in SrTiO3 (left) ;
ADF, EELS plasmon map and cathodeluminescence
(CL = photon emission
spectroscopy) on a single Ag nanoplatelet
(right)
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