By Alex Rettie, The University of Texas at Austin, Materials Chemistry for Energy Group and MRS Student Chapter
When it comes to discussing carbon in the materials science community, graphene, fullerenes and carbon nanotubes tend to dominate conversation. Antonios Kelarakis highlights a new class of molecular carbons: carbon dots (C-dots) in a recent review in MRS Energy and Sustainability.
“C-dots” is a general term for carbon nanoparticles which can range from nanometer-scale disks of graphene (highly graphitic C-dots) to those that are completely unordered (amorphous C-dots). These materials exhibit photoluminescence (PL) and visible light absorption, which, combined with simple syntheses and non-toxicity make them low-cost candidates for bioimaging, photocatalysis, and solar cell applications. Indeed, amorphous C-dots have been synthesized by burning coffee grounds, eggs, and barbeque char! The author makes it clear that the promise of C-dots is proven, but the origins of light absorption and emission are under debate. Hopefully, we will see an application-driven push for fundamental understanding of these fascinating materials. MRS Energy and Sustainability is available from Cambridge Journals Online for free.
The PL observed from C-dots is unconventional based on our understanding of semiconductor PL—graphene has no band gap and does not absorb or emit light. A combination of quantum confinement effects (applicable only to very small particles: less than 10 nm) and surface defects has been proposed, with surface passivation usually required to achieve strong PL from C-dots. Substitution of other elements for carbon and the creation of composite metal oxide/C-dot systems have also been successful in tailoring properties for catalysis and photodegradation of dyes and toxic gases. The sheer number of systems to explore means C-dots could have impact in unexpected areas, but guidance from theoretical modelling will be vital.