Secondary electrons are electrons that get dislodged from the sample itself by the impacting electron beam.  When talking with students in the lab, I describe this as being like taking a baseball (or cricket ball) and throwing it really hard into the dry sand on a beach.  The ball is like the high energy electrons in the beam.  The sand gets flung up into the air by the impact is analogous to the secondary electrons.  The amount of sand that flies into the air depends on 1) how hard you throw the ball, 2) the angle the ball hits the sand, and 3) how dry the sand is.  In electron microscopy terms, these factors correspond to 1) energy of the electron beam, 2) shape of the sample surface, and 3) how many electrons are loosely held by the atoms in the sample.

Because secondary electrons come from the outermost surface of the sample, they are best for seeing the shapes of the surfaces of things like this ant's head.  This image was taken by students in my mineralogy class during their initial training.  (2.2 Mb original file)

Secondary electron image of ant head.  Image is 1.85 mm wide. (2.2 Mb file)

Just like in life, there is more than one way to to see things.  Our SEM has three different ways to "see" the secondary electrons flying off the sample:

Everhart-Thornley SE2 detector - for seeing 3-D views and lower-magnification big pictures.

In-lens SE detector - for our highest magnification needs (0.8 nanometer resolution)

Variable Pressure Secondary Electron detector - for imaging challenging samples that build up static electric charge, and also for cathodoluminescence imaging.

This image of the stinger of a wasp was made by an extraordinarily smart high school student.  She learned to operate the SEM very proficiently in about 20 minutes.  I'm always so impressed and optimistic for the future when I meet young people like her.  You can see the barbs near the end of the stinger very clearly in the original-resolution image (original 7.2 Mb image)

Wasp stinger imaged with Everhart-Thornley secondary electron detector (7.2 Mb image)

Everhart-Thornley SE2 detector - this is the typical detector that's in almost every scanning electron microscope.  It is mounted inside the SEM sample chamber to the side of the sample.  Just as we see longer shadows and more contrast when the sun is low than when it is directly overhead, so do we see more shadowing and thus 3-D effect with the side-mounted SE2 detector.  

This image of a partial hopper crystal of halite (table salt) was taken by a student in my SEM course.  When ions dissolved in fluids have extremely high concentrations and begin to crystallize, the crystals can grow fastest along their edges rather than on their flat faces (as the Bravais rule would normally predict).  The resulting crystals have stepped holes in their faces rather than being simple cubes.  Halite and bismuth crystals are both famous for this tendency.  (3.2 Mb original file)

Table salt crystals with hopper forms imaged with Everhard-Thornley secondary electron detector (3.2 Mb file)

In-lens SE detector - Located in the electron column above the sample chamber, we use this for our highest magnification needs (0.8 nanometer resolution).

The in-lens SE detector is very powerful because it uses the electric fields inside the main column to amplify the signal.  The strength of this detector is its super magnification power.  The drawback to this detector is that many images with this detector have less shadowing (because the detector is located overhead, like the noontime sun), which makes images look less 3-D.

This is an image of the surface of a solar cell that Dr. Justin Smoyer (our experimental/engineering physicist) was studying.  The width of the entire image is 8.85 micrometers (about one three-thousandths of an inch).  The bright spots on the top surface are crystals of a heavy element. (10.2 Mb original image)

Solar power cell imaged by Dr. Justin Smoyer using in-lens secondary electron detector (10.2 Mb file)

Variable Pressure Secondary Electron detector - for imaging challenging samples that build up static electric charge, and also for cathodoluminescence imaging.

This image shows gold nanoparticles stuck to a carbon substrate.  The smallest particles in this image are 2-5 nanometers in diameter.  This is unimaginably small.  I could tell you that a gold atom is 0.166 nanometers, so a 2 nanometer gold particle is about 12 atoms wide, but that's abstract.  Alternatively, lining up 500,000 of those particles would be about as wide as the period at the end of this sentence, but who can imagine 500,000 anything?  Sometimes when we study geology, we work with time frames beyond our ability to comprehend; when we study astronomy with telescopes, our attentions span distances beyond imagination; and when we view the world with microscopes, we find awe and abstract worlds on the smallest grains of dust.  I understand where Dr. Seuss got the idea for Horton Hears a Who!

Gold nanoparticles imaged using the variable pressure secondary electron detector (VPSE) (0.5 Mb file)