Students and the SEM: The First Encounter

Abstract

The first demonstration of an SEM to students is a rewarding experience. Whether it is the beginning of a course in instrumentation or just an illustration of the abilities of an SEM for "prospective users", whether students have engineering or biological backgrounds, both graduate and undergraduate students are thrilled to see a "real electron microscope". We thus have an easy task of only having to choose the right specimens to fulfill their anticipation and to present the right amount of information that will be easily absorbed and remembered for a long time.Insects are the prima donnas of SEM imaging. We usually start our demonstrations with insects ( Fig.1). Their multifaceted eyes, terrifying (at proper magnifications) mouths, exoskeletons, jointed limbs, and segmented bodies look so astonishingly alien under the microscope, so remarkably different from their familiar (to the naked eye) appearance, that insects unfailingly get the students thrilled and prepare them to absorb information. Most insects do not require any special preparation. A sun-dried insect collected on a deck or a driveway in dry weather makes a nice specimen.While observing insects at various magnifications, we print an image on a video printer attached to the microscope and bring the students' attention to the fact that now we can see two similar images: one on an SEM monitor, which shows magnification, for example 1000x, and one on a small print, where the magnification is 373x. Surprisingly, most students are puzzled by the difference in magnifications, even though many of them have already used some type of microscope. Somehow the wrong idea that magnification is some instrumental constant gets imprinted i n s t u d e n t s' minds. Therefore, we have to explain the interrelation between the size of the field of view on the specimen surface and the size of the final picture, and that by increasing the size of the picture we are increasing the magnification proportionally. We stress that as a result practically all shown magnifications for pictures in publications are wrong, and the courteous author will present pictures with a micron bar.If our first specimens require some special specimen collection (usually outside a microscopy lab), our second set of specimens is available in any lab: it is two pieces of paper, writing paper and filter paper. We are now shifting the students' attention from creatures that are part of nature to man-made seemingly dull objects: plain, white, and featureless. After all the excitement with insects, students have pretty low expectations when we are switching to paper specimens and they are consequently surprised to see the complicated microstructure of paper. A tangle of pulp fibers looks pretty good on a micrograph of filter paper (Fig.2), and its morphology looks very different from the morphology of writing paper (Fig. 3 a). Switching accelerating voltage from 2 kV (see Fig. 3a) to 15 kV (Fig. 3b) transforms the image dramatically. With this example we can explain the importance of choosing the rig...