Seeing and manipulating atoms

The scanning tunnelling microscope (STM)

Tunnelling is not merely a phenomenon we are subjected to: we exploit it to build one of the most extraordinary instruments ever invented, the scanning tunnelling microscope (STM, invented in 1981, Nobel Prize 1986). It makes it possible to see atoms one by one.

The principle

An ultra-fine metal tip (ideally ending in a single atom) is brought to within a few tenths of a nanometre of the surface to be observed, without touching it. At this distance, electrons can cross the vacuum between the tip and the surface by tunnelling, creating a small tunnelling current.

       pointe (1 atome au bout)
             \\
              \\  <- courant tunnel (electrons traversant le vide)
               v  ~~~~~~
   ============o============   <- surface a atomes
   (o) (o) (o) (o) (o) (o)         chaque bosse = un atome

The key: exponential hypersensitivity

Remember: the tunnelling current depends exponentially on the distance. A variation as tiny as a hundredth of a nanometre strongly alters the current. This is what gives the STM its atomic resolution:

   pointe au-dessus d'un atome (creux plus proche) -> courant fort
   pointe au-dessus d'un vide (plus loin)          -> courant faible

   en balayant la surface et en enregistrant le courant,
   on reconstruit une CARTE des atomes, bosse par bosse

By moving the tip line by line over the surface and measuring the current, we build up an image of the relief at the atomic scale. For the first time in history, individual atoms could be seen.

Manipulating atoms one by one

Better still: with the tip, individual atoms can be moved, like marbles. In 1989, researchers at IBM wrote the "IBM" logo with 35 xenon atoms. We went from observation to construction atom by atom — the beginning of nanotechnology.

Another everyday application: flash memory

Tunnelling is also in your pocket. Flash memory (USB drives, SSDs, memory cards) stores information by trapping electrons in an isolated "floating gate". To get electrons in or out, they are forced to cross a thin insulating layer by tunnelling:

   ecrire un bit  : on fait passer des electrons par effet tunnel
                    dans la grille flottante (piegee) -> "1"
   effacer        : on les fait ressortir par effet tunnel      -> "0"

Every time you save a photo on your phone, billions of electrons cross a barrier by tunnelling. An "esoteric" quantum phenomenon... at the heart of everyday electronics.

In summary

The scanning tunnelling microscope exploits the exponential hypersensitivity of the tunnelling current to distance in order to map — and even move — atoms one by one. Flash memory uses tunnelling to store information by trapping electrons. Far from being a curiosity, tunnelling is a tool and a ubiquitous technology: we are subjected to it in radioactivity, and we exploit it in our instruments and our devices.