Photoconduction at the nanoscale

Scientists have for the first time created highly conductive nanotubes that could be used for specialised photovoltaic applications, and enable a new technology based on self-assembled nanometre-scale electronic circuits.

Photoconductive nanotube

In a recent issue of Science (subscription required), Yohei Yamamoto and colleagues at the Aida Nanospace Project in Tokyo report on the controlled self-assembly of coaxial nanowires via the bubbling of methane through a solution containing a mixture of hexabenzocoronene (HBC) and trinitrofluorenone (TNF). HBC forms the inside of the cable walls, while TNF coats the outside. The 16-nm diameter nanowires are insulators in the dark, but generate a current when illuminated with visible or ultraviolet light.

Most attempts to create supramolecular organic photoconductors have focused on liquid crystal materials. These high-mobility bulk materials may be used to replace existing low-molecular weight organic photoconductors, or in other applications such as field-effect transistors.

The incentive behind the work on photoconductive nanotubes, on the other hand, comes from a desire to create functional supramolecular entities similar to those found in nature. For example, photosynthesis involves self-assembled units interacting to achieve light harvesting, charge separation and water oxidation, all within the confined space of a membrane. Yamamoto’s photoconductive nanotubes bear a striking similarity to the light-harvesting antennae of green sulphur bacteria.

When asked about the advantage of nanoscale conductive polymers over other types, research team leader Takanori Fukushima replied: ‘The distinct advantage of our nanotubes is the perfect segregation of electron donor and acceptor moieties within a single nano-object. Such a highly organised nanostructure with a redox couple has not been achieved so far.’

Fukushima and his group plan next to develop a complete nanoscale photovoltaic system, but to do this they must first achieve unidirectional alignment of the coaxial nanotubes, and better contact with the electrodes. They will also study the incorporation of electron-accepting functionalities other than TNF into the nanotube architecture.

Figure: Coaxial supramolecular nanowire formed through the self-assembly of HBC (red) and TNF (blue) molecules. Illumination with visible or UV light leads to the generation of electrons (e-) and holes (h+), followed by hole transport through the HBC layer. Taken from Science (subscription required).

Article first published in Nanomaterials News.