Molecular electronics has made steady progress in recent years and components made from single molecules could overcome the limits of conventional, silicon-based microelectronics. However, many challenges still need to be overcome before molecular electronic devices can become a reality, including the fact that it is difficult to integrate them into ordered structures.

The photodiode fabricated by Kimura and colleagues consists of two types of helical peptide - long protein chains - each of which has a different light-absorbing end-group, or chromophore. The two peptides, which are about 1 nm across, were anchored on a gold substrate. Such molecules are good candidates for molecular devices because they can form highly ordered self-assembled monolayers.

The Kyoto team found that when one of the chromophores was excited with light of a certain wavelength it generated an anodic photocurrent. However, when the other chromphore was excited - with light of a different wavelength - the current flowed in the opposite direction, towards the cathode. The reason for this behaviour is that each helical peptide has a large intrinsic dipole moment that accelerates electron transfer in the same direction in which its dipole moment is pointing. Since the two peptides have dipole moments that point in opposite directions, the current is sent in opposite directions (see figure).

"The large dipole moment of helical peptides means they could be used as modulators in many types of electronic device made on the nanoscale," Kimura told PhysicsWeb. "They could thus be useful starting materials for the coming age of molecular electronics." The team now hopes to make a molecular transistor using such peptides.