NANO AT HOME: An Experiment That You Can Try
PLEASE NOTE: The Center for Nano- and Molecular Science and Technology (CNM) at The University of Texas at Austin (UT-Austin) cannot guarantee the accuracy or the safety of these activities. Some of these activities might pose safety hazards for young children, and all activities should be performed under the supervision of a responsible parent, teacher or adult. The CNM and UT-Austin do not assume any responsibility for these activities or their results. If you have questions, corrections, or comments please do not hesitate to contact the CNM.
Model an Electron Microscope
An electron microscope uses electrons, rather than light (photons), to obtain magnified images of a sample. There are many different electron microscopy techniques, including scanning electron microscopy (SEM). In SEM, a beam of electrons is scanned back and forth across a surface, producing a variety of electrons and X-rays that can be used to analyze the structure of that surface.
One of the most common SEM techniques involves detecting electrons that have been bumped away from sample atoms by the microscope electron beam. These secondary electrons, as they are called, are produced both at the surface and under the surface of the sample. On areas of the sample that face the electron beam, only the secondary electrons produced at the surface escape the sample and are detected. At the edges of the sample that do not directly face the electron beam, the secondary electrons produced under the surface can also escape the sample and are also detected. Increased numbers of electrons reaching the detector in SEM become brighter areas of the sample image that the microscope produces. The resulting sample images show features that appear to be lit from the side – brighter at their edges than where they face the electron beam.
This edge effect can be modeled with simple household items:
- firm, flat surface (like a table top)
- black paper
- adhesive tape
- flat, rigid objects with well-defined edges (coins and metal washers work well)
1. Tape your coin or other rigid object to one side of the black paper. This prevents the coin from shifting during the remaining steps.
2. After you’ve taped down the coin, flip the paper over, so that the coin is on the bottom. Place the paper on the flat, firm surface, coin-side down.
3. Rub the chalk lightly over the paper, over the area where the coin is taped. The chalk dust will cover all of the paper to some extent, but will be heaviest at the edges of the object under the paper, similar to secondary electron emission being greatest at the edges of samples in the SEM.
In this demonstration, the piece of chalk represents the electron beam of the microscope scanning across a sample surface. The chalk dust on the paper represents the secondary electrons produced by the sample, and the image on the black paper resembles an image that would be produced by an electron microscope.
Another common SEM technique involves detecting beam electrons that bounce directly off the sample atoms. These backscattered electrons, as they are called, tend to reflect more effectively from heavier atoms. In other words, the areas of a sample that contain heavier elements will appear brighter when the microscope is operated in backscatter mode.
Light can be used as an analogy for both backscattered and secondary electrons, again with inexpensive items:
- flat, dark surface (like a table top)
- flat, transparent, fluorescent plastic object (like a ruler or a clipboard)
- light source that will excite the fluorescence in the plastic object (for example, a handheld blacklight)
To perform this demonstration, first place the fluorescent plastic object on the flat surface. Move the light source close over the fluorescent object. The reflection of the light source from the top surface of the fluorescent object represents backscattered electrons. The light that fluoresces from the plastic represents secondary electron emission. When the light source is near one of the edges of the object, fluorescent light is emitted significantly from that edge, similar to secondary electron emission being significant at the edges of samples in the SEM.