Today’s featured image (full version below) shows four versions of a close-up view of one of the hexagonal alumina crystals we’ve been investigating over the past few weeks in microMondays. The upper-left version of the image is what you see in the standard SEM backscattered-electron view, while the other three are created using a special tool on the electron microscope called an energy dispersing spectrometer (EDS). The EDS tool makes it possible to “filter” images to show the spatial distributions of individual elements such as silicon (Si), aluminum (Al) and iron (Fe). Comparing the versions of the image we see clearly that the hexagonal platelet is high in alumina and low in silica, consistent with our discussion of the color optical microscope image from our very first post. If we are feeling generous we could look at the iron (Fe)-selective version of the image below and see evidence in support of the idea that hematite crystals form around the edges of the large alumina hexagon. However the spatial resolution of the EDS technique is not quite good enough to provide a clear picture of the edges. As we’ll see next week, it is possible to obtain a somewhat higher-resolution view using a more sophisticated instrument called a nanoSIMS.
K-12 STEAM Connections: (1) In addition to the dark hexagon in the center of the silicon (Si) image, there is a dark bar to the upper-left. What do the EDS images suggest this might be? (2) The basic principle of the EDS tool is to scan a focused electron beam over the region of interest, as in normal SEM imaging, but instead of backscattered electrons the EDS tool detects x-rays emitted by the sample. Each atomic element emits x-rays at a specific set of characteristic energies, hence, by discriminating the energies of the detected x-rays the EDS tool can discern what atoms are present in each “pixel” as the focused electron beam scans over the region of interest. High-school chemistry/physics students may be able to understand the basic reason why each element emits characteristic x-ray energies by referring to the shell model of atomic structure.
Acknowledgments: Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) of Stanford University.