Slow hot-carrier relaxation in colloidal graphene quantum dots

Reducing hot-carrier relaxation rates is of great significance in overcoming energy loss that fundamentally limits the efficiency of solar energy utilization. Semiconductor quantum dots are expected to have much slower carrier cooling because the spacing between their discrete electronic levels is much larger than phonon energy. However, the slower carrier cooling is difficult to observe due to the existence of many competing relaxation pathways. Here we show that carrier cooling in colloidal graphene quantum dots can be 2 orders of magnitude slower than in bulk materials, which could enable harvesting of hot charge carriers to improve the efficiency of solar energy conversion.