Carbon nanotubes emit light more energetic than that absorbed, due to phonon interactions and dark exciton formation

https://scitechdaily.com/new-light-trick-in-carbon-nanotubes-could-boost-solar-power/

"up-conversion photoluminescence... exciton receives an energy boost from the material by interacting with vibrations in it known as phonons... occurs with high efficiency even in defect-free single-walled carbon nanotubes, higher temperature produced stronger effect... optoelectronic/ photonic devices, solar power, laser-cooling, biological imaging... future: cooling nanotube using laser illumination to remove thermal energy"