Absorption properties of Iron and Sulfur doped Graphene Carbon Quantum Dots

Graphene Carbon Quantum Dots (GCQDs) are carbon nanostructures that have great optoelectronic properties and low toxicity which makes them excellent candidates for relevant technologies. Most studies on GCQDs report high absorption in the UV range and weak or none in the VIS range. The latter limits the use of GCQDs for applications such as solar cells, which require strong absorption in this range. A solution to this problem is to dope the carbon nanostructure. Extensive research is being performed to find suitable doping atoms or molecules to extend the absorption range of GCQDs. In this work, we report the doping of CQDs by Iron (Fe) and Sulfur (S) atoms (S-Fe-GCQDs) by pyrolysis of citric acid. Their optical properties were studied experimentally, by absorption and Raman spectroscopy, and theoretically by DFT. The measured absorption spectra show absorption bands around the UV and green-yellow part of the VIS range. From the Density of States these observed bands in the VIS region could be associated with electronic transitions between the n-π* (S-C) and n-π* (Fe-C) states. Raman peaks associated with C=C-S and Fe-C-O vibrations support the successful doping of S and Fe atoms in the GCQD structures.