See at the Nano Level with Electron Microscopy
In the tiny world of nanotechnology, the first need for researchers is to see what they’re working with. Microscopes being used in nanotech labs today do just that. They come with handy acronyms such as TEM and SEM and cost anywhere from one hundred thousand to several million dollars.
For looking at nano-sized objects, the microscopes are often bigger than you are. Instead of towering over the equipment as you stand at a lab bench, you sit in front of these desk-sized microscopes and they tower over you.
An electron microscope uses electrons to visualize nano objects. The electron gun in an electron microscope, is large because you have to accelerate the electrons before you illuminate the sample with a beam of electrons, and all that running around requires some space.
An electron microscope also uses an electric field of several thousand volts to shoot electrons at a sample. The higher voltage accelerates the electrons, resolving the smaller features in the sample.
Electron microscopes come in a few varieties. The most common is a scanning electron microscope (SEM). In an SEM, electrons are accelerated in the electron gun and run through a scanning coil that applies an electric field to scan the beam of electrons over the sample. (All this happens in a vacuum because the electrons don’t get far moving through air.)
These electrons excite other electrons in the sample, which are picked up by the detector. A computer turns the signal from the detector into a picture on the monitor screen.
SEMs are found in labs that investigate nanomaterials. The nice thing about SEMs is that it’s easy to prepare a sample.
Some SEMs are capable of resolving features as small as a few nanometers. SEMs have very good depth of field, meaning that they can produce sharp images of rough samples.
If you’re dealing in a really tiny realm and need to get details of a sample down to a few tenths of a nanometer (the size of an atom) to see the internal structure of the sample, you can use a transmission electron microscope (TEM). The term transmission in its name means that the electrons pass through the sample.
Because the image is formed by the electrons passing through the sample, the sample has to be very thin. To make such a sample, you must slice and polish it; therefore, sample preparation is typically much more involved for a TEM than an SEM.
To produce images with a resolution of a few tenths of a nanometer, a TEM uses a much higher voltage to accelerate electrons than an SEM, which makes the sticker price and installation cost of a TEM much higher.
Given the higher equipment cost, more difficult sample prep, and more extensive training required to use TEMs, many more SEMs are in use than TEMs. Think of SEMs as the workhorses of nanolabs and TEMs as thoroughbreds. Only those who truly need the capability to view things at an extremely small scale have a TEM in their lab.