SUNLIGHT could one day be harnessed to power nanobots, thanks to nanocables that turn light into electricity.
The nanocables work in a similar way to the semiconductors in solar panels, but on a much smaller scale. 鈥淭his is the first example of a photoconducting nanostructure,鈥� says Takanori Fukushita of the University of Tokyo, Japan, a member of the team that built the cables.
The cables are hollow, around 16 nanometres wide, and can grow several micrometres long. To build them, Fukushita and colleagues created a compound containing hexabenzocoronene (HBC), two 12-carbon chains and trinitrofluorenone (TNF). They placed this in a solution of tetrahydrofuran and bubbled methane though it, creating a chemical reaction that made the compound assemble itself into hollow cables. The HBC, which sheds electrons when hit by light, formed the inside of the cable wall, and the TNF, which readily accepts electrons, coated the outside (Science, vol 314, p1761).
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Each time photons hit the cable, they knock loose an electron from the inner layer, causing it to jump to the outer layer and leave behind a positively charged hole. In this way the electrons are corralled into creating a current. To test this, the researchers placed the nanocables on a silicon surface and applied a voltage across it. When they shone light onto the surface, a current began flowing down the cable, stopping when the light was switched off.
The voltage was needed to kick-start the process because TNF is not a very effective conductor of electricity. This means the cables cannot yet produce usable electricity from sunlight alone. So the next step, Fukushita says, is to modify the TNF outer layer, for example by attaching buckyballs, so that it acts as a semiconductor, and allows current to flow.
Once this has been achieved, the nanocables could be fitted to nano-sized robots and machines to power their movements, says Franz W眉rthner of the University of W眉rzburg in Germany. The cables are similar in size and function to the light-harvesting antennae used by some bacteria for photosynthesis. So it might also be possible to create hybrid devices by connecting the two, he says.
