Physicists at the Technical University of Munich have created a nano switch based on a single porphyrin ring. If one of two protons from the inside of the ring is removed, the remaining proton can take on any one of four positions, initiated by a single tunnel electron from the tip of a scanning tunneling microscope. Image: Knud Seufert/Technische Universitaet Muenchen |
For
a long time miniaturization has been the magic word in electronics. Dr.
Willi Auwaerter and Professor Johannes Barth, together with their team
of physicists at the Technische Universitaet Muenchen (TUM), have now
presented a novel molecular switch in the journal Nature Nanotechnology.
Decisive for the functionality of the switch is the position of a
single proton in a porphyrin ring with an inside diameter of less than
half a nanometer. The physicists can set four distinct states on demand.
Porphyins
are ring-shaped molecules that can flexibly change their structure,
making them useful for a wide array of applications.
Tetraphenylporphyrin is no exception: It likes to take on a saddle shape
and is not limited in its functionality when it is anchored to a metal
surface. The molecule holds has a pair of hydrogen atoms that can change
their positions between two configurations each. At room temperature
this process takes place continuously at an extremely rapid rate.
In
their experiment, the scientists suppressed this spontaneous movement
by cooling the sample. This allowed them to induce and observe the
entire process in a single molecule using a scanning tunneling
microscope. This kind of microscope is particularly well suited for the
task since—in contrast to other methods—it can be used not only to
determine the initial and final states, but also allows the physicists
to control the hydrogen atoms directly. In a further step they removed
one of the two protons from the inside of the porphyrin ring. The
remaining proton could now take on any one of four positions. A tiny
current that flows through the fine tip of the microscope stimulates the
proton transfer, setting a specific configuration in the process.
Although
the respective positions of the hydrogen atoms influence neither the
basic structure of the molecule nor its bond to the metallic surface,
the states are not identical. This small but significant difference,
taken together with the fact that the process can be arbitrarily
repeated, forms the basis of a switch whose state can be changed up to
500 times per second. A single tunneled electron initiates the proton
transfer.
The
molecular switch has a surface area of only one square nanometer,
making it the smallest switch implemented to date. The physicists are
thrilled by their demonstration and are also very happy about new
insights into the mechanism behind the proton transfer resulting from
their study. Knud Seufert played a key role with his experiments: “To
operate a four-state switch by moving a single proton within a molecule
is really fascinating and represents a true step forward in nano-scale
technologies.”
This
research was funded by the European Research Council (ERC Advanced
Grant MolArt, No. 247299), the Excellence Cluster Munich-Centre for
Advanced Photonics (MAP) and the Institute for Advanced Study of the TU
Muenchen.
A surface-anchored molecular four-level conductance switch based on single proton transfer
