A Stable Bridge of Molecules

A molecular bridge based on graphene electrodes that is stable at room temperature promises further miniaturization of electrical circuits.

With the help of such a bridge, nanoscale electronic systems with completely new functions could be built. Now, Empa researchers, together with partners from Switzerland, the Netherlands, Israel and Great Britain, have succeeded for the first time in constructing such a bridge between the graphene electrodes, as reported in the latest issue of "Nature Nanotechnology". This is a decisive step because without such bridges, which are mechanically and electronically stable at room temperature, it would not be possible to assemble switching elements from nanoscale molecules.

The greatest difficulty lies in ensuring efficient electronic transport between the two graphene electrodes. The researchers chose the approach of building a molecular bridge to control the current. The bridge must be mechanically and electronically robust in order to avoid fluctuations - and this at room temperature. In order for the system to actually be used in the future, its features must also be reproducible.

The problem: Mechanical and electronic stability place different demands on the properties of the bridge. "A weak coupling between the orbitals provides an interesting electronic connection between the two graphene electrodes and makes the connection properties less sensitive to local electronic fluctuations of the electrodes. However, this strategy leads to mechanically unstable connections," explains Maria El Abbasi, the first author of the study. If, on the other hand, molecules are used that form a solid chemical bond with the graphene electrodes, the system becomes mechanically more stable. However, the transport properties of the bridge are poorly defined due to the lack of control over the electrode geometry and electrode edges. This means that the electronic properties vary greatly.

The researchers have now succeeded in building reproducible bridge elements that combine both properties: mechanical and electronic stability. The molecules consist of three components: a silane group, a head group and a separating alkane chain. The task of the silane group is to mechanically anchor the molecules to the silicon oxide substrate; they bond to the substrate via a strong, covalent bond. The silanization process offers another advantage: a protective layer is formed on the silicon oxide.

The second and most important part of the molecule is the head group. Its task is to trace the electrons a path between the two graphene electrodes. This takes place in a quantum mechanical process: the so-called pi-orbitals of the adjacent molecules overlap with each other and with those of the two graphene electrodes. This creates an extended orbital over the connection, which acts as a bridge between the graphene electrodes. The last part of the molecule forms the alkane chain. It electronically isolates the silane anchor and the head group from each other. The molecules formed in this way are stacked between the two graphene electrodes to form a controllable conductive element.

For the main group, the researchers investigated three different chemical compounds. The first main group (CH3) served as a control. When it was used, the components showed stable and reproducible behaviour, but at the same time only a limited electronic compound. A second group, N-carbazole, proved to be less than ideal, as an electronic bridge was formed, but it did not offer sufficient stability. The third candidate was biphenyl-N-carbazole. It led to a significantly stronger overlap of the orbitals and thus to a stable connection with a current that was several orders of magnitude higher than that of the control molecule CH3.

The researchers were also able to show that the electronic properties of the bridge construction are stable from only 20 degrees above absolute zero to room temperatures. "We have thus developed a simple but effective strategy for integrating molecule-based functions into nanoelectronic systems in the future," said Calame. The researchers at Empa's "Transport at Nanoscale Interfaces" laboratory under the direction of Michel Calame are working on the project together with partners from the universities of Basel and Bern, the Swiss Nanoscience Institute, Delft University of Technology in the Netherlands, Lancaster University and the University of Warwick in Great Britain, and Hebrew University in Jerusalem.