Carbon Nanotube 'Trions' Keep the Tube's Handedness, Opening Door to Electric Control of Light
Tokyo researchers found that adding a charge to an exciton on a single-walled nanotube preserves its chirality, so voltage could tune how the tube treats circularly polarized light.
Scientists at Tokyo Metropolitan University have shown that charged particle states on carbon nanotubes keep the same "handedness" as the nanotube itself, a result that could let engineers control how devices respond to light using electricity. The work was published in Nature Communications and was led by Professor Kazuhiro Yanagi.
Chirality is a feature of the material world in which an object and its mirror image can never be matched. A spiral staircase, a wrung towel, double-stranded DNA and many pharmaceutical molecules all have it. Light has a version too. Circularly polarized light has an electromagnetic field that rotates either left or right as it travels, and it can interact differently with materials whose own structure is left- or right-handed. That difference is called circular dichroism, and researchers think it is key to manipulating light and electric current in nanotechnology.
Single-walled carbon nanotubes, ultrathin rolled-up cylinders of carbon atoms, are among the front-runners for building chirality into future devices. Like a twisted rolled-up newspaper, a tube's surface can be left- or right-handed depending on how its atoms are arranged. Those tubes can host excitons, which are pairs of a negatively charged electron and a positively charged hole. Excitons were already known to show circular dichroism that follows the chirality of the underlying tube, and they are stable at room temperature, which makes them attractive for applications.
Excitons are only the beginning of the states a nanotube can hold. A pair can pick up an extra electron or hole, forming a charged state called a trion. Trions are potentially useful, but a key question remained: would the extra charge disturb the chirality that the exciton inherited from the tube?
To find out, the team used specially separated samples of left-handed and right-handed nanotubes. They generated trions electrochemically, by injecting electrons or holes, and measured their circular dichroism with unprecedented precision. The trions matched the chirality of the excitons they came from, accurately reflecting the handedness of the tube. The strength of the effect was also roughly the same as for the original excitons.
Because trions are easy to create by injecting charge into a nanotube, the finding points to a way of electrically controlling how strongly the tube interacts with circularly polarized light. The researchers say this may lead to technologies such as light amplification, detection and modulation.
The result is a materials-science step rather than a finished device. It answers a basic question that had stood in the way, whether adding charge scrambles the chirality, and the answer is that it does not. That clears one obstacle for optoelectronic components that would use voltage, rather than mechanical changes, to switch how a nanotube handles polarized light.
