Today’s featured image was obtained using a Scanning Electron Microscope (SEM). It shows a patch of micro/nano-crystals found on the etched surface of the sample discussed in last week’s post. For scale, the edge of the hexagonal platelet located just to the right of and above center is about ten microns (one one-hundredth of a millimeter). Those not familiar with electron microscopy may wonder why the image is monochrome and why we use electron microscopes (as opposed to regular optical, or light microscopes) if they can’t really show us color. To answer the second question first, we use electron microscopes because they can provide much higher imaging magnification than an optical microscope. For physical reasons having to do with the wave nature of light, images from an optical microscope cannot clearly resolve features below the scale of a few tenths of a micron. As we’ll see in future posts, with an SEM the resolution can approach the scale of a few nanometers — that’s about a hundred-fold improvement. As we’ll also see in future posts, electron microscopes can be used to obtain information about the elemental composition of sample surfaces (that is, what kinds of atoms are present).
SEM images do not correspond to anything you could see with your eye by looking through a magnifying lens. Rather, they are images generated by a computer from data collected by scanning a focused electron beam over the surface of a sample and detecting electrons that scatter back from it. The featured image in this post was acquired using the “secondary electron” imaging mode of our SEM. In secondary electron mode the brightness scale of the image corresponds to something like the sample surface topography or relief. To provide a rough analogy, suppose someone gave you a carved ceramic tile and asked you to examine it only by running a finger over its surface without looking at it. Based on what you felt as your finger scanned over the surface you could try to reproduce the surface profile of the tile as a drawing. You couldn’t really say anything about its colors, however!
In future posts we may have occasions to discuss the “backscattered electron” imaging mode of an SEM and/or images obtained using a Transmission Electron Microscope (TEM). If you’re already curious about the differences, there are some pretty good explanations on the web from electron microscope manufacturers and on Wikipedia.
K-12 STEAM Connections: The focus of today’s post is mainly to introduce scanning electron microscopy, as we’ll look at a number of SEM images in the coming weeks. Here we suggest some related questions about this topic that you might discuss with your students: What are electrons and what do we mean by a focused beam of electrons? What are some prominent scientific uses of SEM? Can you think of other examples of “images” that are generated from data, which do not correspond to anything you could directly see with your eyes? What is the diffraction limit in optical microscopy?
Acknowledgments: Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) of Stanford University.