Molecular electronics is a growing research area where scientists study electrical properties of the molecules with a chemically programmed function. Molecules can function as diodes, switches and transistors, all with a typical length of few nanometers. An international group of scientists from University of Bern, Leiden University, Delft University of Technology, and Chuo University has developed the first switchable molecular diode.

Diodes are basic electronic components which conduct electric current in a single direction. Researchers from the Department of Chemistry and Biochemistry of the University of Bern (Dr. Veerabhadrarao Kaliginedi, PD Dr. Peter Broekmann) and Leiden University (Prof. Sense Jan van der Molen's Group) together with colleagues from TU Delft (Prof. Joseph M. Thijssen's group), and Chuo University (Prof. Masa-aki Haga's group) have now demonstrated the first switchable molecular diode. This molecular diode can be turned on and off through humidity and vice versa it is a humidity sensor at nanoscale. The results now have been published in Nature Nanotechnology.

Humidity makes the difference

The team of scientists discovered that the electric conductivity of the dinuclear ruthenium organometallic complex molecule "2-Ru-N" depends on humidity. Under dry conditions, the same amount of current flows through the molecule under positive or negative voltage. This changes dramatically in a humid environment. In that case, only a positive voltage induces a current. With this, the researchers have created a molecular circuit that works as a unique combination of a switch and a diode: a switchable molecular diode that can be turned on and off with humidity. Vice versa the molecule serves as a humidity sensor based on the structure of a specific molecule.

Asymmetry induced by water molecules

The tiny diode works by means of an asymmetry caused by water molecules at the molecular junction interface. At around 60% humidity, water molecules lump together at tip side of the molecular layer and cause an imbalance between the energy levels on both sides. This strongly limits the flow of electrons. Under dry circumstances, the molecule's symmetry doesn't break and the diode behavior disappears.

Applications

"The whole principle is based on symmetry, so it doesn't exclusively apply to water," says Sense Jan van der Molen. "In theory this concept also works with for alcohol or toxic gases, for example." This means the discovery does not just relate to measuring moisture in the air. The main advantage associated with the molecule based sensors is their small size (this means device size can be extremely small) and chemical tunability for different sensing applications. "If scientists find a suitable molecule in the future consisting of two symmetric halves with energetically coupled functional groups sensitive to other external stimuli, just as "2-Ru-N," the principle enables other sensors too, like a molecular alcohol test or toxic or explosive chemical detector," says Veerabhadrarao Kaliginedi.