Carbon nanotubes release more energetic light than they receive, could help solar power

A team of scientists from Japan’s RIKEN Center for Advanced Photonics have revealed how carbon nanotubes emit more energetic light than the light they absorb. Since materials typically emit less energetic light than they take in, this discovery could hold important ramifications for the solar energy industry.

To understand the significance of the research, consider glow-in-the-dark paints or fluorescent lights. They absorb high-energy light (like UV) and then emit lower-energy light which is perceived as a neon glow, in what is referred to as photoluminescence.

However, some materials exhibit the opposite effect. Dubbed as up-conversion photoluminescence (UCPL), the phenomenon involves a material absorbing low-energy light (like infrared) and giving out higher-energy brighter light. It’s like charging your phone with a AAA battery and getting it to run a hairdryer, which obviously is not the norm.

Up-conversion in pristine nanotubes

To get to the bottom of UCPL in nanotubes, Yuichiro Kato and his team at RIKEN set out to uncover the mechanism in single-walled carbon nanotubes. For the unversed, carbon nanotubes are ultra-thin, straw-like structures only few nanometers wide and made of carbon.

While earlier theories proposed that UCPL required defects in the nanotube structure to trap excitons, the RIKEN researchers observed the phenomenon occurring efficiently even in pristine nanotubes. This pointed to a different, intrinsic mechanism at play.

According to the team, this phenomenon works when incoming infrared light hits a carbon nanotube. When this happens, an electron gets excited, forming something called an exciton (an electron + the ‘hole’ it left behind).

Usually, this exciton would fall back down, giving off less energetic light. But here, the exciton absorbs extra energy from phonons, which are quantum vibrations (like tiny sound waves in the material).

What results is a ‘dark exciton’ state. Eventually, upon loosing some energy, the exciton emits light. But now it does so with more energy than the original infrared light.

Because phonons (vibrations) are more active in hot enviornments, more vibrations equate to more energy available to boost the exciton. This, in turn, equals a more substantial up-conversion effect. “Phonons are more abundant at higher temperatures, enhancing the likelihood of phonon-mediated transitions,” explained Kato.

Implications for solar energy research

This is a big deal because if you can efficiently turn low-energy light into a high-energy beam, you can boost solar panel performance, converting ‘wasted’ infrared light into ‘usable’ visible light.

The mechanism can also create better biological imaging tools to see deep inside tissues using safer infrared light. It could even cool materials with lasers, by removing thermal energy through UCPL.

“By establishing an intrinsic model of UCPL in single-walled carbon nanotubes, we hope to open up new possibilities for designing advanced optoelectronic and photonic devices,” added Kato.

What the RIKEN scientists have essentially discovered is that one does not need structural defects for up-conversion in carbon nanotubes. Instead, phonons and dark excitons do the trick. This opens up cleaner, more efficient, and more flexible designs for future energy and photonic technologies.

The study has been published in the journal Physical Review B.