How Scanning Tunneling Microscope Works
Scanning Tunneling Microscope (STM) is a novel apparatus developed in 1980s with capabilities in the atomic scale. It can be used to manipulate, image and identify atoms on surfaces. In simple words, STM operates by measuring the electric current between a conducting tip and a surface. When a conducting tip is brought very close to a surface, application of a voltage difference between the tip and surface can produce an electric current whose magnitude will be a function of properties of the surface like local electron densities beside type and relative position of the tip and magnitude of applied voltage. One important feature is the fact that when the tip gets closer to an atom, the magnitude of current increases and vice versa. Such dependencies of electric current make it possible to gather information about atoms on the examined surface. For example, when the tip is moved across the surface, the tip will pass higher atoms of the surface at a closer distance than the lower ones and as a result, the magnitude of electric current will be higher at the location of higher atoms where such information can be analysed to provide an image of the surface with a resolution in the atomic scale.
In addition to the location of atoms on the surface, their type can be identified where such process is usually referred as scanning tunneling spectroscopy (STS). In scanning tunneling spectroscopy, the type of atom close to the tip is identified by measuring electric current at different applied voltages where dependency of electric current to applied voltage will be a function of the type of the examined atom and can be used to identify the atom. Beside image acquisition and spectroscopy, STM can also be used to manipulate atoms by using the tip as a tool for moving atoms on the surface. For example, it was these atom manipulation and image acquisition capabilities of STM that allowed IBM scientists to create “IBM in atoms” in 1989, a picture in which xenon atoms on a nickel surface create the three letter initialism of the company, and also “A Boy And His atom” in 2013, the world’s smallest stop-motion film.
STM uses the concept of quantum tunneling. In STM, although the tip is very close to the atoms of the examined surface (for example at a distance in the 0.4-0.7 nm range), vacuum exists between the tip and the closest atoms. At such conditions, the pass of current between the tip and surface requires overcoming potential barriers where based on classic physics, it will not be possible for the current to pass at voltages typically applied in STM. However, a concept of quantum physics, quantum tunneling, allows the pass of current to happen. Overcoming a potential barrier or hill based on classic physics requires a minimum of energy determined by the height of the hill. However, in some phenomena explained by quantum physics, it is possible to overcome the potential barrier by an amount of energy lower than the minimum one determined by the height of the hill. In simple words, this is considered as tunneling through the potential hill as a tunnel can provide a way to pass a hill by lower amount of energy. As a result, the scanned electric current between the tip and surface is considered as tunneling current.
Interested readers are recommended to visit Wikipedia page of scanning tunneling microscope to find more about it (at https://en.wikipedia.org/wiki/Scanning_tunneling_microscope).