Image credit: Pixabay 2017.
There is a large number of physical and chemical techniques available today for the analysis of substances. They provide the opportunity to gain information about the properties of atoms, molecules and ions. Infrared spectroscopy, for example, measures the vibrations of molecules. Mass spectrometry, which is widely known from TV crime dramas, gives information about the weight of molecules or ions. This information about mass can then be used to identify poisons, drugs and other substances.
However, most techniques cannot give us a direct picture of what atoms, molecules or ions really look like. Normally, conclusions about their looks are drawn from measured properties. Imagine you are drawing a picture of a house while somebody is telling you about its appearance, but you cannot see it yourself. This is how physicists and chemists use the information from their measurements to figure out what atoms, molecules and ions look like. The measurements are giving them information about substances, which corresponds to the information about the house, such as colours; size or where doors and windows are.
But are there any methods that can provide us with – let us say – a photo of an atom? The main problem is that atoms are extremely tiny. One atom in an apple is as small as the apple itself is compared to our entire Earth. Nevertheless, there are a few methods that can indeed capture pictures of atoms with the help of quantum mechanics and other cool science. A few of these methods are listed below.
Transmission electron microscopy (TEM)
Atoms cannot be seen under normal light microscopes. The reason is that the wavelength of visible light is much larger than atoms themselves are. To be able to see a sample in a microscope, the wavelength has to be smaller than the sample itself. For example, light waves are smaller than cells which is why cells observed in light microscopes. Transmission electron microscopes use electrons instead of visible light. Thanks to the wave-particle duality, electrons can behave as both waves and particles. As waves, they have a much smaller wavelength than light which makes it possible to see atoms in a transmission electron microscope. Besides the use of electrons, the working principle of a TEM is the same as that of a light microscope.
Scanning tunneling microscopy (STM)
Scanning tunneling microscopy is another microscopic technique. It is based on a quantum mechanical phenomenon called ”tunneling”. Electrons can ”tunnel”, or in other words transtition, from an atom on the tip of an extremely sharp needle to atoms on a sample surface. Needle and surface have to be at a very short distance from each other to enable tunneling. The probability of tunneling increases when the gap between the needle and the surface gets smaller. This means that electron tunneling is more likely to occur when the needle tip is above the center of an atom as compared to the tip being above the space between two atoms. In consequence, a topographic picture of the atoms on the sample surface can be obtained.
Atom probe tomography (APT)
Atom probe tomography is a very powerful technique that can provide three-dimensional images of a sample’s atomic structure. In APT, the magnification is caused by a highly curved electric field instead of electron properties like wavelength or tunneling. For this technique, atoms have to be removed from the sample surface and turned into ions, in other words they have to be ionized. To obtain three-dimensional information, the atoms are removed from the sample layer by layer, which means that, unlike the previous two methods, this technique is destructive.