Week 4 - SEM Imaging

The SEM produces 6 different signals which include:  secondary electrons, back-scattered electrons (BSE), characteristic X-rays, light (cathodoluminescence), specimen current and transmitted electrons. The SEM that will be used, transmits secondary electron detector signals. There are 3 different types of microscopes that can be used which range from LM, TEM, SEM as shown in figure 5 [12].



Figure 5. Functionality of LM, TEM, SEM [12]

 

All three microscopes begin with the light source at the top illuminating to a condenser lens. During the next step the LM and TEM microscopes vary from the SEM because the specimen is between the first condenser lens and the objective lens. In the SEM microscope there is a second condenser lens for a better image. The LM microscope requires you to use your eye to look at the projection lens while the TEM uses a fluorescent screen [13]. The SEM makes a crisper image because it has the specimen after the objective lens, and instead the specimen then emits a signal to a signal detector which creates a crisp image as shown below.  



Figure 6. Difference between optical & SEM of a small marine organism [13]

The SEM imaging is the ideal choice in this case because as shown in figure 6 the image is more detailed than an average optical micrograph. It uses the electron's wavelength to send a signal to the specific specimen being observed. The wavelength bounces off and the resultant signal is transmitted, filtered, and an image is forced. This process is different from optical microscopes and results in less focus because the wavelength is much higher than an electron's [12]. In the experiment the SEM imaging is used to measure the diameter, and count the nanofibers in the electrospun strand. Some important details about the SEM microscope are: working distance, depth-of-focus, and the secondary electron. The working distance is important because it is the distance between the lower surface of the objective lens and the surface of the specimen and altering this distance will change the clarity of the picture. The depth-of-focus is plainly the focus of the SEM image. Finally, the last variable  is the secondary electron which are the electrons ejected by the specimen durring the inelastic scattering of the energetic beam electrons. The more electrons ejected the clearer the image. In week 4, Evan and Tristan are using the SEM to complete these viewing tasks.