EELS IN THE STEM - DETERMINATION OF MATERIALS PROPERTIES ON THE ATOMIC-SCALE

By performing EELS in conjunction with the Z-contrast imaging techniqu e in the scanning transmission electron microscope (STEM), detailed in formation on the composition, chemistry and structure of materials can be obtained with atomic resolution and sensitivity. This unprecedente d resolution can be achieved by using an atomic resolution Z-contrast image to first identify structural features of interest and then to po sition the electron probe over the feature for spectral acquisition. T his method greatly limits the number of spectra that need to be acquir ed from a given specimen, thus reducing total acquisition time and spe cimen stability problems. In addition, a key advantage of this methodo logy is that the collection conditions for the spectrum can be tailore d to produce an incoherent, atomic resolution spectrum that can be cor related directly with the image. This means the image can be used as a reference atomic structure for theoretical modeling of the spectral f ine-structure. Using multiple scattering analysis, the reference struc ture can be modified to reproduce the experimental spectrum, thus givi ng a 3-dimensional structural determination that is sensitive to singl e atom vacancies and impurities. In this paper, the application of thi s combined EELS and Z-contrast technique to various materials problems is described. In the case of the MBE growth of CdTe on Si, the combin ed atomic resolution techniques allow the diffusion of Te into the sil icon substrate to be identified and a novel graphoepitaxial grow-th me chanism to be identified. For nanoscale iron particles used as a fuel additive to reduce/enhance soot formation during combustion, measureme nt of the distribution of iron oxidation states within the particles p ermits a mechanism for soot formation to be proposed. The application of the multiple scattering analysis techniques to the study of grain b oundaries is described for tilt boundaries in TiO2 and SrTiO3. In both cases, the multiple scattering analysis gives information not present in the image and allows the 3-dimensional structure of each boundary to be determined.