[MUSIC] Welcome, my name is Carrie Donley and I'm the Director of the Chapel Hill Analytical and Nanofabrication Lab, or a channel at UNC. Today we will be talking about scanning electron microscopy, often called SEM. And environmental scanning electron microscopy, often called ESEM. The SEM and ESEM are microscopes that produce images using electrons instead of visible light. Remember the wave length of light limits the resolution in an optical microscope. Today we're going to learn about using electrons which have a much shorter wave length than visible light for imaging purposes. This is a diagram of a light microscope. The basic components include a light source. A way to focus that light onto the sample. A way to collect the light that travels through the sample. And a way to detect that light. With a light microscope, glass lenses similar to magnifying glasses are used to focus the light and collect the light. An electron microscope has many of the same components as a light microscope. Instead of a light source, the electron microscope uses an electron gun to produce electrons. Electromagnetic lenses are used to focus the electrons and the detector is sensitive to electrons instead of visible light. Electrons can interact with the sample in a number of ways. Today, we will focus on the backscattered and secondary electrons that we can detect in an electron microscope. When an electron beam strikes a sample, some of the electrons are absorbed. Other electrons are backscattered and some sample electrons can be ejected as secondary electrons. If the number of electrons that strike the sample, is not equal to the number of electrons that leave the sample then the sample will build up a charge. This is called charging, and it negatively affects the quality of the resulting image. In order to prevent charging, many SEM samples are coated with a thin layer of metal. Most SEM images are produced by collecting secondary electrons. This image is a secondary electron image of zinc oxide nano-wires and nano-flowers. Secondary electron images show the surface features of a sample and therefore look very three dimensional. The scale bar at the bottom right of the image indicates that most of this nano-wires are a few microns long and only 50 to 100 nanometers wide. This is another secondary electron image showing cells that were cultured on top of manufactured pillars. Notice that the scale bar for this image is much larger than the previous image. Scanning electron microscopes can typically image features as small as 1 or 2 nanometers and as large as 1 or 2 millimeters. Backscatter SEM images show fewer surface features than secondary electron images. Often backscatter images look very flat. The contrast that we do see in a backscatter image is due to differences in average atomic number. Regions of a sample with higher atomic number, will produce more backscattered electrons and appear bright. This image is of a polymer sample with barium titanate particles embedded in it. Since the barium titanate has a much higher average atomic number, these particles appear much brighter than the polymer that they are embedded in. Many electron microscopes have both secondary and backscattered electron detectors, and acquiring both images on the same sample can illustrate the differences between them. This sample is a polymer resin circuit board with some soldered connections. The secondary electron image on the left shows the surface topology, while the backscattered image on the right shows the atomic number contrast. It is clear that the bright regions are from the higher atomic number solder, which is composed primarily of tin. Both types of images provide useful information. In a typical SEM, vacuum is required because the electrons that we use for imaging will scatter off gas molecules and prevent us from focusing the electron beam on the sample. As a result, we can only image dry samples in a atypical SEM. If we want to look at wet samples, we need to dry them out first. And this often distorts their shape. This image shows how there are very few residual gas molecules in the SEM chamber of a traditional SEM. And the beam spot on the samples is relatively small. An environmental SEM often called an ESEM allows the operator to introduce a controllable amount of water vapor into the SEM vacuum chamber. There is one disadvantage of using an environmental SEM. When you introduce water molecules into the chamber, the electrons that are travelling towards the sample, will hit these water molecules and scatter. The result is that the electron beam is not as tightly focused as in a traditional low pressure SEM. Thus the resolution of the images is not as good. The main advantage of an ESEM is that you can image "Wet Samples" without having to dry them out. These include many types of biological samples such as cells, bacteria and plants. In the ESEM we can keep these samples hydrated and image them in their natural state. In addition, we can use a small amount of water vapor to also prevent charging. This allows us to image non-conductive samples without the need for a conductive coating. This is an ESEM image of a bacterial bio film. The bacteria are the rod-shaped particles that are about 1 micron long. The ESEM is great for imaging these types of wet samples. Thank you for joining this discussion of scanning electron microscopy and environmental scanning electron microscopy.
