Hi, I'm Toby Tung, a research scholar with the Analytical Instrumentation Facility or AIF at NC State. The transmission electron microscope or TEM, is a microscope that can give us nanoscale resolution images. This is possible because instead of using light for imaging, we use electrons. Electrons much like light can behave as waves. The wavelengths are much shorter than light on the order of one trillionth of meter or about one tenth of a diameter of an atom. Visible light has a range of wavelengths in the hundreds of nanometers or thousands of atomic diameters. It is these short wavelengths that enable imaging at the atomic scale. Here is TEM image of a metal alloy. Note that, we are able to resolve individual atoms. Since we are observing things at the atomic level it is very important to image in a controlled environment. Even small changes to the environment like vibrations from someone talking or an increase in temperature by one degree can affect imaging dramatically. Thus, TEMs are usually housed in basement rooms that have thick concrete walls or are on floating tables that will automatically compensate for any building vibrations. As you can see in this image, radiative cooling panels can be used to keep the temperature constant. Water flows through these panels to help control the rooms temperature. As the name indicates, in transmission electron microscopy, electrons travel through the sample. They will interact with the sample in different ways. Those that are transmitted and continue down the column can be used to generate an image. For this to occur, samples must be very, very thin. If samples are too thick the electrons will be unable to pass through. The necessary thickness is a function of the material itself and the type of analysis being performed. On average, samples should be less than 150 nanometers thick. Most of the time, samples are not at an appropriate thickness for to TEM. They must be processed in some way to generate a thin sample. We process samples in different ways depending on what they are and how the process will affect the sample. Ideally, the technique will minimally affect the sample itself or aspects of the sample that are under analysis. Here's a diagram of the TEM. At the top of the instrument is the electron gun. There are two types of methods to generate an electron beam. The first is thermionic emission and the second is field emission. With thermionic emission, the electron gun works similarly to an incandescent light bulb. A filament, the cathode, is the source of electrons. It is usually a hairpin shaped tungsten wire or LaB6 crystal. The filament is heated to a very high temperature and electrons boil off from its tip. Field emission guns use a sharp typically tungsten tip as an emitter with an applied voltage to extract electrons. This emitter generates more electrons than traditional thermionic emission guns. Both emission types have an anode located below the filament. The electrons accelerate from the cathode toward the anode to create an electron beam. Once the beam is generated it travels down the column. These electrons are at high energy around 80 to 300 KEV. Before imaging a sample, this beam is aligned to ensure optimal energy. The beam is focused using different lenses. These are not traditional light microscope or eyeglass lenses. These lenses are electromagnetic lenses and move the beam using magnetic fields. The electron beam also travels through a small opening or aperture, that helps focus the beam by blocking electrons that are off course. As the electrons travel through the sample, they will interact with it in a variety of ways. Some may be transmitted. Some may be scattered. In TEM we are typically concerned with the electrons that are transmitted. The transmitted electrons and the electrons energy gives us information about the sample and help generate the image. The transmitted electrons will travel through another set of lenses and apertures and be projected onto a phosphorescent screen. This screen produces photons when the beam hits it which allows us to see the sample. In order to capture an image of the sample, the screen must be removed so the electrons can be collected using a piece of film or digitally using a camera. Though TEM enables atomic scale resolution, it does have its drawbacks which are important to note. The TEM is a two-dimensional projection of a three-dimensional sample thus artifacts may be produced. It's often difficult to distinguish what is real from what is an artifact. For comparison, look at this image. It appears to show a two-headed rhino. It is obvious to us that this is an imaging artifact. However, in TEM these types of artifacts often exist and may not be so obvious. This can lead to a misinterpretation of results. To appropriately interpret results, complementary surface analysis technique should also be performed. In TEM only a small volume of the samples are imaged. Thus, it is important to ensure that the sample you are selecting for imaging is truly representative. This is another reason why it is best to supplement TEM experiments with other characterization techniques. In addition, aberrations or imperfections in the electromagnetic lenses lead to a distorted image. This makes it difficult to get clean crisp images at a high resolution. Luckily, many newer microscopes such as the analytical instrumentation facilities FEI Titan TEM have an aberration corrector. This type of advance and instrumentation makes collecting very high-resolution images possible. Speaking of the Titan, this microscope is also unique because of its ability to run in situ experiments. For example, researchers can change the temperature in the chamber to observe effects on the sample in real time. We continue to learn more about TEMs capabilities and new instrumentation techniques develop every day. Who knows, maybe one day we may be able to see things even smaller than we can now.
