Atomic Column Resolved Electron Energy-Loss Spectroscopy

Abstract

Spatially resolved electron energy‐loss spectroscopy (EELS) is rapidly developing into a unique and powerful tool to characterize internal interfaces. Because atomic column resolved Z‐contrast imaging can be performed simultaneously with EELS in the scanning transmission electron microscope, this combination allows the atomic structure to be correlated with the electronic structure, and thus the local properties of interfaces or defects can be determined directly. However, the ability to characterize interfaces and defects at that level requires not only high spatial resolution but also the exact knowledge of the beam location, from where the spectrum is obtained. Here we discuss several examples progressing from cases where the limitation in spatial resolution is given by the microscopes or the nature of the sample, to one example of impurity atoms at a grain boundary, which show intensity and fine structure changes from atomic column to atomic column. Such data can be interpreted as changes in valence of the impurity, depending on its exact site in the boundary plane. Analysis of this nature is a valuable first step in understanding the macroscopic structural, optical and electronic properties of materials.

Nanoscale-SiC doping for enhancing Jc and Hc2 in superconducting MgB2

“…As we used relatively low sintering temperatures, 800-850°C, the C substitution for B is expected to be lower. Figure 8(a) is the Z-contrast image [17][18][19] for the nano-SiC-doped sample, which shows a typical MgB 2 crystal in the [100] orientation. Z-contrast imaging in scanning transmission electron microscopy mode utilizes electrons scattered at high angle ͑Ͼ25 mrad͒ to form an incoherent image, with an image intensity that is proportional to the square of the average atomic number (i.e., ϳZ 2 ).…”

“…However, it is rather difficult to distinguish a small C signal originating from within the lattice or from surface contamination, as the low signal-to-noise ratio of the C core loss for such low concentrations makes it nearly impossible to distinguish the near-edge fine structure. Due to the large variety of phases present in the SiC-doped sample, it is therefore possible that C substitution at a level of 1%-3%, which is believed to be quite reasonable in the framework of the literature on C substitution, [15][16][17][18][19] cannot be readily identified, and more careful analysis is needed.…”

Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO3

“…EELS provides complementary information for distinguishing atomic columns in addition to Zcontrast image, especially light atoms which are not detected in Z-contrast images. EELS is also sensitive to changes in electronic structure or valence induced by changes in local coordination [10]. …”

Interpreting atomic resolution EELS spectra using multiple scattering theory

“…For SrTiO3, the calculations were found to converge for clusters including atoms up to the eighth shell, i.e. r=6.173 A (Browning et al, 1998). …”

“…This information is provided by electron energy loss spectroscopy (EELS). Using a scanning transmission electron microscope (STEM) it has been demonstrated that spectra can be acquired with the same atomic resolution as the image (Browning et al, 1993;Batson 1993;Duscher et al, 1998).…”