Spectroscopic imaging in electron microscopy
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A historic era for electron microscopy The introduction of analytical devices within the columns of transmission electron microscopes (TEMs) has dramatically extended the field of information they can bring for the local exploration of materials and nanostructures. The primary electron beam in the electron microscope generates different signals when traveling through the specimen prepared as an ultrathin foil. The richness of the modern instruments operating in the scanning mode, in which a finely focused electron probe is rastered over the specimen (scanning transmission electron microscopy [STEM]), is to collect several complementary signals in parallel, giving birth to a multi-signal approach. A key success in recent years is to efficiently combine structural imaging and spectroscopic imaging in parallel in one electron microscope.1–3 Spectroscopic imaging is a general technique in which a multi-dimensional dataset is obtained, two of which would be spatial, while the third dimension would either be a full energy-loss spectrum,4 or could equally well comprise an x-ray or cathodoluminescence (CL) spectrum, see Figure 1. The figure compares spectral imaging in STEM, where the energy dimension (E) is obtained in parallel and the spatial positions sequentially, with energy-filtered imaging in TEM (EFTEM), where the spatial image is obtained in parallel and
the energy dependence sequentially (E1 to E2). It is important to note that in STEM, the spectrum image can be recorded simultaneously with other signals, such as the Z-contrast image obtained using an annular detector to collect scattered electrons, allowing pixel-by-pixel correlation of spectroscopic information with atomic structure. Spectroscopic imaging is valuable not only in electron microscopy but also in other microscopies, for example, scanning probe microscopy (SPM)5 or 3D atom probe microanalysis.6,7 It can also be used in higher dimensions, for example introducing a time dimension as in 4D electron microscopy,8 or recording a diffraction pattern at each probe position in STEM or a hysteresis loop at each probe position in SPM. Features in the electron energy-loss spectroscopy (EELS) spectrum carry information on the elemental composition through the appearance of characteristic core losses, as well as on the electronic properties through fine structure on the core loss edges and also from low-loss features. The origin and nature of these spectral features are discussed more fully in the contribution by Botton in this issue. It is essential to point out the direct correlation of these features with the position of the probe on the specimen, which can be defined with great accuracy relative to the structures to be analyzed. In Figure 2, this immediate correlation is demonstrated for different
Stephen J. Pennycook, Oak Ridge National Laboratory; [email protected] Christian Colliex, Université Paris Sud, Orsay (France); [email protected] DOI: 10.1557/mrs.2011.332
© 2012 Materials Research Society
MRS BULLETIN • VOLUME 37 • JANUARY 2012 • www.mrs.org/bu
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