Nicolas Weibel, Sergio Grunder and Marcel Mayor, University of Basel, Switzerland look at functional molecules in electronic circuits

The miniaturization trend known as Moore's law is only driven by the prospect of reducing the price per unit - more chips per silicon wafer reduces production costs. The exponential increase in the cost of semiconductor production will most likely stop this miniaturization trend before its physical limits are reached. The growing interest in alternative concepts, like the integration of molecules as carriers of an electronic function, is not surprising in view of the expected restrictions. Mainly driven by the greed for fundamental knowledge, the increasing availability of investigative tools and the hope for appealing solutions at lower cost, molecular electronics has developed to a mature research area in the past few years. Alongside central contributions from physics, electronic engineering, nanotechnology and other applied sciences, only physical and synthetic chemistry provide the desired feedback mechanisms required for a successful development of molecular structures for electronic devices. Together, they can judiciously correlate molecular structure with physical properties, and design and synthesize tailor-made functional molecules.

With numerous examples of molecules integrated into electronic circuits, our recent Perspective article1 illustrates the promising potential of the concept but also the remaining challenges and limitations. From the point of view of a synthetic chemist, the focus is set on molecular structure as the origin of electronic function. Particularly, examples of systems providing rectification and switching are considered, as the combination of rectification with hysteretic switching paves the way to future molecule-based memory devices.

Rectification, the very first electronic function suggested to be provided by a single molecule, has meanwhile been achieved in devices comprising different numbers of molecules, ranging from large self-assembled monolayers down to individual molecules. While the integrated molecules were confirmed to be the origin of the observed current rectification, the present performances of these molecular rectifiers can barely compete with those of their semiconductor fellows.

Switching between two current levels, triggered by an external stimulus such as light or an electrochemical potential, has been realized in several prototype set-ups consisting of integrated molecules. While these studies are of fundamental interest to investigate the underlying mechanisms and the potential of molecules as switches, the integration of neither a light source nor an electrolyte into an integrated circuit of the future is very likely to happen. However, a few examples of hysteretic switches, which are of particular interest as potential memory devices, have been achieved. Interestingly, these switches are triggered by an applied potential. However, the underlying mechanisms are still under investigation.

In summary, the future of molecular electronics is not to supplant but rather to complement and enrich existing electronic circuits. The advent of hybrid devices profiting from molecules as functional units integrated into pre-existing platforms will most likely permit fundamental scientific advances and technological achievements. Overcoming the size gap between electrodes and molecules as well as bringing in new concepts for massive parallel contacting of molecular devices are still major challenges in the field of nanotechnology. We are convinced that numerous unpredicted solutions and applications will arise from the fascinating and challenging interdisciplinary research in the area of molecular electronics.

Read the full Perspective article 'Functional molecules in electronic circuits' in issue 15 of Organic & Biomolecular Chemistry