October 16, 2009 – Researchers led by a team at Arizona State U. have created a single-molecule diode that could pave the way to creation of new chemical sensors, and ultimately capabilities that complement and extend those in silicon-based electronic devices.

Diodes enable electrical current to flow in one direction around a circuit but not another — they’re critical and ubiquitous components in various electronics applications including power conversion, logic gates, photodetectors, and LEDs. A molecule with this capability needs to be asymmetric, with its ends forming covalent bonds with the anode (negative) and cathode (positive) contacts.

Work on using molecule-based components has been ongoing for decades, but most of it has focused on groups of molecules (e.g., molecular thin films). Challenges include bridging a single molecule to at least two electrodes, and in proper orientation of the molecule in the device, the researchers note.

The group, led by ASU’s N.J. Tao with participation from scientists at the U. of Chicago and U. of South Florida, came up with a technique relying on AC modulation, applying "a little periodically varying mechanical perturbation to the molecule" to tell if there’s a molecule bridged across two electrodes. They used conjugated molecules incorporating alternating single and multiple bonds, which display large electrical conductivity and have asymmetrical ends that can spontaneously form the needed covalent bonds with metal electrodes, they note.

Schematic for molecular diode. The symmetric molecule (top) allows for two-way current. The asymmetrical molecule (bottom) permits current in one direction only and acts as a single-molecule diode. (Source: ASU Biodesign Institute)

From the abstract of their paper, published in Nature Chemistry:

The diblock molecule exhibits pronounced rectification behaviour compared with its homologous symmetric block, with current flowing from the dipyrimidinyl to the diphenyl moieties. This behaviour is interpreted in terms of localization of the wave function of the hole ground state at one end of the diblock under the applied field. At large forward current, the molecular diode becomes unstable and quantum point contacts between the electrodes form.

Application for a single-molecule diode includes new chemical sensors; eventually they could offer electronic, mechanical, optical, and other properties that complement silicon-based technologies, Tao noted.